New Conducting Salts for Rechargeable Lithium-Ion Batteries

New Conducting Salts for Rechargeable Lithium-Ion Bateries

INAUGURALDISSERTATION

zur Erlangung des Doktorgrads der Fakultät für Chemie und Pharmazie der Albert-Ludwigs-Universität Freiburg im Breisgau

vorgelegt von

Michael Rohde aus Freiburg im Breisgau 2014

Vorsitzender des Promotionsausschusses: Prof. Dr. Torsten Koslowski Referent: Prof. Dr. Ingo Krossing Korreferent: Prof. Dr. Caroline Röhr

Datum der mündlichen Prüfung: 30.03.2014

Diese Arbeit wurde im Zeitraum von Oktober 2010 bis Februar 2014 am Freiburger Materialforschungszentrum der Albert-Ludwigs-Universität in Freiburg im Breis- gau unter Anleitung von Prof. Dr. Ingo Krossing angefertigt. Teile dieser Arbeit enthalten Ergebnisse, die im Rahmen von Bachelorarbeiten (Alexander Norow 08.2011, Verena Leppert 08.2012, Lutz Miensopust 05.2013 und Marcel Schorpp 07.2013) im Arbeitskreis von Prof. Krossing unter meiner Anleitung angefertigt wurden. „achtzehn?“ „weg.“ „weg!“ „Karo.“ „Kontra!“

VII

Danksagung

Ich danke allerherzlichst meinem Doktorvater Prof. Ingo Krossing für das Bereitstellen des spannenden Temas innerhalb dieser Industriekooperation, seine ausgezeichnete Betreuung über all die Jahre, seine Motivationsfähigkeit und die bei dieser Arbeit ge- lassenen Freiheiten. Ich möchte mich sehr herzlich bedanken bei der Korreferentin Prof. Caroline Röhr für die Übernahme der Zweitkorrektur dieser Arbeit, dem Dritprüfer Prof. Michael Fiederle, meinen Laborkollegen Michel Panzer und Christoph Schulz für die geniale Zeit, dem Gruppenleiter Philipp Eiden für seine Geduld bei meinen vielen Fragen, den Bachelorstudenten Alexander Norow, Verena Leppert, Lutz Miensopust und Marcel Schorpp für die Unterstützung im Labor, den Partnern von der BASF Dr. Michael Schmidt, Dr. Arnd Garsuch und Dr. Günter Semrau für die produktiven Projektrefen und angenehme Zusammenarbeit, Stefanie Kuhl für Hilfe bei allen organisatorischen Dingen, die tolle Zeit auf den Sci- ence-Days und die Zeit im FMF oder auch außerhalb. Ich danke allen anderen Mitgliedern der Arbeitsgruppe Krossing für die Unterstützung in jedweger Art: Dr. Anke Hofmann, Dr. Alexander Higelin, Alexander Rupp, Dr. Anne Kraf, Benedikt Burgenmeister, Brigite Jörger, Boumahdi Benkmil, Carola Sturm, Dr. Daniel Himmel, Dr. Daniel Kratzert, Dr. Elias Frei, Fadime Bitgül, Florian Kirschenmann, Florian Stahl, Franziska Scholz, Hannes Böhrer, Dr. Harald Scherer, Jennifer Beck, Dr. Julia Schäfer, Josephine Possart, Katharina Pütz, Dr. Lucia Alvarez Hernandez, Dr. Mara Bürchner, Maribel Sierra, Dr. Marina Artamonova, Mario Sander, Martin Lichtenthaler, Martin Lieder, Dr. Mathias Hill, Mathias Kamp, Dr. Mathias Keßler, Melanie Sanchez, Michael Hog, Mira Schulz, Miriam Schwab, Dr. Nils Trapp, Olaf Petersen, Pengcheng Zhang, Petra Klose, Philippe Weis, Dr. Przemek Malinowski, Dr. Safak Bulut, Sarah Keller, Dr. Sascha Goll, Stefanie Reininger, Tobias Engesser, Valentin Dybbert, Dr. Valentin Radtke, Vera Brucksch, Dr. Werner Deck. Ein besonderes Dankeschön an Dr. Philipp Eiden und Dr. Daniel Himmel bei der Unterstützung von quanten- chemischen Rechnungen, Dr. Harald Scherer für die Hilfe bei der Auswertung von NMR Spektren, Fadime Bitgül, Hannes Böhrer, Dr. Harald Scherer, Dr. Mara Bürchner und Dr. Mathias Keßler für das Messen der unzähligen NMR Spektren, den Glasbläsern Tim Lecke und Martin Walter für ihre Spitzenleistung, den Mitarbeitern der Chemikalienausgabe mit Frau Völker, Herrn Jerg und Herrn Lenhard, VIII

dem Werkstatteam um Herrn Melder für die vielen Reparaturen und Installationen im BASF Labor, der Verwaltung des FMF mit Frau Dr. Stefanie Meisen, Stefanie Kuhl, Nicola Weis, Daniel Frauzem, Assiyear Joers, Beate Gloderer, Leonhard Falk, Klaus Hasis und Elena König für die wunderbare Organisation und die unermüdliche Hilfsbereitschaf, meinen Beachvolleyballfreunden Burkhart Pössel, Nils Blumenthal, Jan Krause, Marc Neininger, Valentin Radtke, Tim Rabaschus und Leander Poocza. meinen Freundinnen und Freunden Marlene, Nikolas, Micha, Jonathan, Anthea, Burk- hart, Katrin, Christoph, Daniel, Liza, Laura, Teresa, Bar, Johanna, Marc, Sto, Martin, Rafael, Lisa, Nathalie, Dounia, Franzi und Tim für die spaßige Zeit jenseits der Uni.

Der aller größte Dank geht an meine Familie, Nicole, Alisha, Florian, Nadine, Mat- thias, Rike, Henry, Marie-Luise, Opa Werner und Oma Lore, meine Eltern, Donna, Kurt, Marlene und ihrer Familie. IX

Contents

1/INTRODUCTION ...... 1

1.1/Outline ...... 1 1.2/Principles of Operation ...... 2 1.3/Termodynamics ...... 3 1.3.1.!Cell voltage ...... 3 1.3.2.!Teoretical specifc charge capacity ...... 3 1.3.3.!Teoretical specifc energy or energy density ...... 3 1.4/Lithium Ion Bateries ...... 4 1.4.1.!General characteristics and performance ...... 4 1.4.2.!Anode materials ...... 5 1.4.3.!Cathode materials ...... 6 1.4.4.!Separators ...... 6 1.5/Liquid Nonaqueous Electrolytes ...... 7 1.5.1.!General requirements ...... 7 1.5.2.!Conducting salts ...... 8 1.5.3.!Solvents ...... 9 1.5.4.!Additives ...... 10 1.6/Lithium Sulfur Bateries ...... 10 1.6.1.!Introduction, fundamental chemistry and their problems . . . . . 10 1.6.2.!Solutions ...... 12 1.7/ References ...... 13

2/MOTIVATION ...... 17

2.1/References ...... 18

3/LITHIUM BOROXINATES ...... 19

3.1/Introduction and Overview ...... 19 3.2/Syntheses from fuoroalkyl borates and boron oxide ...... 21 3.3/Syntheses of fuoroalkoxy boroxines from alkoxy boroxine ...... 24 3.4/Fluorination of trimethoxyboroxine ...... 25 3.5/Conclusion and Summary ...... 25 3.6/Experimental Section ...... 27 3.6.1.!Chemicals ...... 27 3.6.2.!Synthesis of fuoroalkyl borates ...... 27 3.6.3.!Syntheses of fuoroalkyl boroxine ...... 29 3.6.4.!Substitutions at alkyl boroxines ...... 31 X

3.6.5.!Fluorination of trimethoxy boroxine ...... 33 3.7/References ...... 35

4/LITHIUM BIS(TRIFLUOROACETYL)PHOSPHATE ...... 37

4.1/Introduction and Overview ...... 37 4.2/Syntheses from phosphorus pentoxide ...... 39 4.2.1.!NMR characterisation ...... 39 4.3/Syntheses from phosphoryl chloride ...... 41 4.3.1.!Overview and thermodynamic considerations ...... 41 4.3.2.!Syntheses from POCl3 with LiOAcF in dimethyl carbonate . . . .43 4.3.3.!Syntheses in acetonitrile and tetrahydrofuran ...... 51 4.3.4.!Summary ...... 55 4.4/Syntheses from lithium dihydrogenphosphate ...... 55 4.4.1.!NMR characterization ...... 56 4.4.2.!IR characterisation ...... 57 4.4.3.!Purifcation of the product ...... 59 4.4.4.!Stability and purity considerations of the product ...... 59 4.4.5.!Conclusion ...... 62 4.5/Synthesis from lithium difuorophosphate ...... 62 4.6/Summary...... 63 4.7/Experimental Section ...... 65 4.7.1.!Chemicals ...... 65 4.7.2.!Syntheses from P4O10 ...... 65 4.7.3.!Syntheses from POCl3 ...... 67 4.7.4.!Syntheses from LiH2PO4 ...... 77 4.7.5.!Synthesis from Li[PO2F2] ...... 89 4.8/References ...... 91

5/LITHIUM TETRAKIS(TRIFLUOROETHOXY)BORATE ...... 93

5.1/Introduction and Overview ...... 93 5.2/Synthesis and Characterisation ...... 94 5.2.1.!Synthesis ...... 94 5.2.2.!NMR characterisation ...... 95 5.2.3.!Vibrational spectroscopic characterization ...... 98 5.3/Ionic Conductivity ...... 103 5.4/Termal stability ...... 105 5.5/Electrochemical analysis ...... 106 5.5.1.!Cyclic voltammetry...... 106 XI

5.5.2.!Electrochemical cycling ...... 108 5.6/Summary...... 109 5.7/Experimental Section ...... 110 5.7.1.!Chemicals ...... 110 5.7.2.!Preliminary remark ...... 110 5.7.3.!Syntheses in toluene ...... 111 5.7.4.!Synthesis in 1,2-dimethoxyethane ...... 113 5.7.5.!Synthesis in toluene with 4 eq. 1,2-dimethoxyethane ...... 114 5.7.6.!Synthesis in toluene with 2 eq. 1,2-dimethoxyethane ...... 115 5.8/References ...... 116

6/SUMMARY ...... 119

7/GENERAL EXPERIMENTAL AND THEORETICAL PROCEDURES . 123

7.1/Techniques ...... 123 7.2/Analytic methods ...... 123 7.2.1.!Infrared and Raman measurements ...... 123 7.2.2.!NMR measurements ...... 123 7.2.3.!Mass spectrometry ...... 123 7.2.4.!DSC measurements ...... 124 7.2.5.!Cyclic Voltammetry ...... 124 7.2.6.!Conductivity measurements ...... 124 7.2.7.!Qantum chemical calculations ...... 124 7.3/References ...... 126

8/LIST OF ABBREVIATIONS ...... 127

ABSTRACT ...... 131

KURZZUSAMMENFASSUNG ...... 132

1!Introduction

1.1!Outline Te storage of energy plays an essential role in our universe. Energy exists in many forms, and depending on the form of energy, there are numerous feasible ways of stor- ing it: Termal energy can be stored in heated mater, mechanical energy can be stored in kinetic energy, e. g. wind, or as gravitational potential energy in pump storage power stations. Te light of our sun is nuclear energy (stored in binding nucleons in an atom), which has been converted into electromagnetic radiation. Biological systems can transform this radiation and store it in chemical energy, e. g. glucose. Electrical energy can be stored in small quantities in capacitors or, only afer transformation into other forms of energy like chemical energy in rechargeable bateries. Te conversion of chemical energy into mechanical or electrical energy plays an important role in the development of the human society [1], especially considering that the rapid development of the global economy in the last centuries has depended on the consumption of biomass and fossil fuels, like coal, oil and natural gas. Meanwhile, the combustion of these fuels causes air pollution and global warming with unforeseeable consequences [2]. Modern civilizations have become dependent on these fnite fossil fuels. Tis makes nations, which depend on fossil-fuels imports more vulnerable [3]. One way out of this dilemma is the sustainable exploitation of renewable energy sources, such as solar, wind, tid- al and geothermal energy. However, these renewable sources of electricity production fuctuate over time or are restricted in location [4]. Terefore, these power sources re- quire energy storage, like stationary rechargeable bateries. Another important application of rechargeable bateries is electrical propulsion. Gasoline is used in combustion engines, because of its very high chemical specifc energy (theoretically 46 .9 MJ kg−1 [5] of which only ~3 MJ kg−1 are efectively used as mechanic/electric energy [7]) and because of its good portability, which makes it the most convenient form of energy for propulsion. In contrast, commonly used lithium-ion bateries have by orders of magni- tude lower energy densities. For example, a representative commercial lithium-ion cell made by Samsung (ICR18650-30A) has a specifc energy of 0.85 MJ kg−1 ([6], 1 Wh = 3600 J). To overcome the predominance of petrol for propulsion, rechargeable bateries have to contain more energy [8][9]. Tis is a great challenge for engineers and scientists, considering that the development of stationary bateries or bateries for electric vehicle should meet additional requirements [10][11] to prevail against fossil fuels: • long shelf life • prolonged cycle life • much larger energy density than present bateries • low cost • safety • rate capability • be non-toxic and environmentally harmless Rechargeable lithium-ion bateries have revolutionized mobile energy supply, since they were commercialized by Sony in 1991 [12]. Tese bateries already provide portable chemical energy with the ability to convert this energy into electric energy with high efciency [5]. 2 Introduction

Due to their characteristics [6] of • high open-circuit voltage, • high Coulombic and energy efciency, • low self-discharge rate, • long cycle life, • low weight sealed cells, • high energy density, in comparison to other commercial bateries, they have made possible portable electronic devices, like cell phones, laptop computers, mobile music players, iPads and digital cameras [13][14][15]. Analysts have forecast the growth of the lithium-ion batery market up to nine billion dollars per year in 2015 [16]. Especially the need for lithium-ion bateries in electric vehicle and hybrid cars will push this development, and the need to meet the requirements mentioned above. Terefore, scientists develop new materials to realize bateries with higher energy density. One of the limiting materials in commonly used lithium-ion bateries are the cathode materials (transition metal oxides and phosphates), due to their low capacity (~300 mAh g−1) [17]. One promising candidate on the horizon, as low cost cathode material, is sulfur. In combination with lithium in lithium-sulfur cells they have a theoretical specifc energy of 9.36 MJ kg−1 [18], which is only a ffh of the energy density of petrol but comparable to the usable energy of combustion engines! But before the lithium-sulfur batery can be commercialized, the scientifc-technological community has to solve the obstacle of the batery’s poor cycle life [19][20][21][22]. Tis brief introduction has given a general overview of the importance of energy storage. Te following subchapters provide a closer look on the principles of operation of bateries, such as thermodynamic background and components of cells. Tis is followed by a detailed overview on the lithium-ion batery and a short discussion of the lithium-sulfur batery. Te main focus in the last subchapter is on the design of non-aqueous electrolytes for lithium and lithium-ion bateries.

1.2!Principles of Operation Bateries are power sources, which consist of one or more electrochemical cells. Tese cells produce a current by converting chemical energy into electrical energy. Tis current is caused by a spontaneous electrochemical oxidation-reduction (redox) reaction in a cell between the anode and the cathode. Tis leads electrons to fow from the anode to the cathode through an external electric circuit. An electrochemical cell contains a negative electrode (anode), where the oxidation process takes place during the discharge. Te reducing material at the anode, gives of electrons (oxidation) to the positive electrode (cathode). Te oxidizing material in the cathode accepts the electrons (reduction) from the negative electrode, which are delivered from the external circuit. Between the anode and cathode there is an electrolyte system (liquid, gel or solid) that transfers only the ionic charge between the electrodes. During the discharge the positive ions difuse from the anode to the cathode. To prevent internal short-circuiting, which would be caused by a direct contact of the electrodes, a separator is used, which is permeable for the ions in the electrolyte. Rechargeable cells can be recharged, because the electrochemical redox reaction is reversible. An external power source must be connected to the cell so that the current can fow from the cathode to the anode. Thermodynamics 3

1.3!Thermodynamics

1.3.1.!Cell voltage

Tere are several important parameters which are used to defne batery performance, such as voltage, electrical energy and capacity. All these parameters depend on the active materials in the electrodes and their corresponding redox reaction. Te theoretical standard cell voltage, E°cell can be determined from the tabulated values of the electrochemical series and is given by the diference between the standard electrode potential at the cathode, E°cathode and the standard electrode potential at the anode, E°anode as

E°cell = E°cathode – E°anode >[V] Eq.1

Te theoretical standard cell potential of i.e. a commonly used lithium ion batery can be calculated:

E°(Li0.55CoO2) – E°(LiC6) = 1.25 V −(−2.8 V) = 4.05 V Eq.2

But in batery technology it is interesting to estimate the operating voltage, which is produced from the cell. Tis operational voltage depends on several thermodynamic and kinetic factors, which lower the theoretical cell voltage.

1.3.2.!Theoretical specific charge capacity

Te capacity is the amount of electrical charge that is stored in the active material in the cell, and it is expressed in Ah (1 Ah = 3600 C). But a more practical way to defne the capacity is the specifc charge capacity, which is expressed in Ah per 1 gram. Te theoretical value of this specifc charge capacity for all active materials can be determined with following Eq.3:

Cspecific = mactive⋅z⋅F/Mactive Eq.3

Where m is 1 gram equivalent weight of the active material which can deliver 1 F (F is the Faraday constant), or 26.8 Ah, in a z electron transfer process. Ten the value is divided by the molar mass M of the active material to obtain the specifc capacity. Terefore the theoretical specifc charge capacity of LiCoO2 (M[LiCoO2] = 98 g mol−1 and z = 1) is

Ccathode = 1 g⋅1⋅26.8 Ah/98 g mol−1 = 0.273 Ah g−1 Eq.4

But in reality the full theoretical charge capacity is never realized, because of kinetic limitations in the electrode process, temperature, electrode design and operating rates.

1.3.3.!Theoretical specific energy or energy density

Outgoing from the standard cell voltage and the specifc capacity, the theoretical specifc energy is given by Eq.5.

specifc energy = E°cell⋅(Ccathode−1 + Canode−1)−1⋅1000 [Wh kg−1] Eq.5 4 Introduction

specifc energy(Li-ion) = 4.05⋅(0.372−1 + 0.273−1)−1⋅1000 = 586 Wh kg−1 Eq.6

In Eq.6 the theoretical specifc energy of a common lithium-ion batery is calculated [23]. Because it is a mathematical product, it can be seen that to increase the energy of a batery it is necessary to increase the voltage of the cell, with suitable active materials or by increasing the capacity (in Ah), which inherently depends on the electrode materials too! As the isolated factors are dependent on numerous variables, the specifc energy is dependent on parameters such as temperature, manufacture, material purity, etc. Te practical specifc energy is always much lower than the theoretical value.

1.4!Lithium Ion Bateries

1.4.1.!General characteristics and performance

In the frst subsection the principle of operation of a rechargeable electrochemical cell, like a lithium-ion batery, was described, followed by brief thermodynamic considerations, which support the understanding of the characteristic values of batery technology. Tis section focuses on the features of lithium-ion bateries.

A fundamental characteristic of the main electrochemical cell reaction is the reversible intercalation–deintercalation of lithium ions in layered active materials. Tis feature is one reason why lithium-ion bateries are so successful. Tese topotacic reaction of ions and electrons go along with only small changes of the solid electrode structure [17]. According to the mentioned anode and cathode materials, the cathode reaction (Eq.7) of a lithium-ion batery can be writen as

LiCoO2>⇌>0.5 Li+ + 0.5 e− + Li0.5CoO2 >V = 4.05 vs. Li/Li+ Eq.7

A fully lithiated metal oxide cathode complies with the discharged state of the lithium ion batery. During the frst charging of the cell the LiCoO2 electrode gets oxidized and delithiated. On the other cell side (Eq.8) the graphite electrode intercalates reversibly lithium to form LiC6:

C6 + Li+ + e−>⇌>LiC6>V = 0.2 vs. Li/Li+ Eq.8

During the frst charging of the graphite anode, a passivating flm on the electrode is formed. Tereby some lithium from the electrolyte is irreversibly consumed for the reduction of electrolyte species [3]. Tis passivating flm, the so called solid electrolyte interphase (SEI), is permeable for lithium ions [24][25]. Te apparent negative loss of capacity, due to the consumption of lithium, is acceptable or even necessary, because the advantages of the SEI overcompensate the capacity loss. Te SEI ensures that no more other irreversible processes occur during the cycling [25]. Additionally, the SEI protects the layer structure of the graphite anode against exfoilation [26]. To maintain and support the SEI formation divers additives were used. A famous one is ethylene carbonate [25][30]. Related passivation phenomena are observed at the cathode side [27]. Because Lithium Ion Bateries 5 of the high voltage at this side of around 4 V vs. Li/Li+ it is a highly oxidative milieu. And for a further increase of the energy density, cathode materials are chosen with even higher redox potentials exceeding 5 V. Tus, oxidation of the electrolyte and cathode material has to be avoided at any state of the charge, fortunately these passivation flms seem to inhibit massive solvent oxidation [28][29]. On the basis of the importance of the electrode passivation [8], it can be concluded that the SEI formation is another reason for the success of high voltage lithium ion bateries!

1.4.2.!Anode materials

One possibility to increase the energy density (specifc energy) is given with the choice of the electrode material with a high specifc capacity. Tus, the most atractive material for lithium rechargeable bateries is lithium metal, with one of the lowest standard potentials of all materials, −3.01 V vs. SHE [31] and its high specifc capacity of 3.86 Ah g−1. Additionally, lithium is the lightest metal with the lowest density (0.54 g mL−1). Lithium metal has been widely used as an anode material in primary lithium bateries [32], which were used as power sources in watches, calculator or implantable medical devices [13]. However, afer prolonged deposition/dissolution cycling, lithium metal dendrite formation occurs, which led to explosions [13]. Due to this serious safety problem and the inferior cycleability, lithium metal was replaced by lithium alloys with Sn, Si, Al, Bi, Pb and In to resolve the dendrite deposition of metallic lithium [33]. Promising anode materials are lithium rich alloys Li4.4Si and Li4.4Sn, which have a high specifc capacity of 4.2 Ah g−1 and 0.996 Ah g−1 [34]. Te disadvantages of these alloys are their colossal reversible volume changes of the electrode of over 100 % during the charge-discharge processes, which limits the cycle life and dampen their commercialization [35]. Te volume expansion during the lithiation and contraction during the delithiation led to fractures, which forms 'inactive' material, which has no longer electrical contact and indicates a massive capacity loss [35]. Lithium titanate (Li4TiO12; LTO) is also able to insert reversibly lithium up to a stoichiometry of Li6−7Ti5O12 [36]. One advantage of LTO is the relatively high redox potential against lithium of 1.55 V [37], because the reduction potential of most electrolytes are lower, and a SEI for stabilization of the active material is not necessary [8][38]. Te absence of a passivation flm increases the charging rate, due to the decreased impedance at the anode-electrolyte interphase. And the high potential extends the cycle life. Resulting from the low capacity of around 0.150-0.160 Ah g−1 and the low voltage LTO has a lower energy density than graphite anodes [8]. But LTO bateries are interesting for applications, where an extremely long cycle life is required [37]. Graphite is currently the most popular anode material. With an extremely low redox potential of the lithiated graphite, LiC6, at around −2.8 V vs. SHE, which is close to the standard redox potential of lithium metal, it is suitable for high voltage lithium ion bateries [31]. But due to the small potential diference of 0.2 V vs. Li/Li+, a fast charging is not allowed, because of lithium plating [3][39]. But a safe and fast charging rate is the wish of every electric vehicle driver. With a theoretical specifc capacity of 0.372 Ah g−1, which is higher than the cathode capacity, it is not the limiting factor of the total capacity of a commercial lithium ion batery. Tere are many forms of graphite, but to obtain the maximum specifc capacity, graphite with the highest degree of order should be used, because lithium cannot be inserted in graphite sheets, which are shifed or rotated [40]. But parameters like particle size, tap density and specifc surface area play a role 6 Introduction

by the lithium intercalation. A small specifc surface is striven, because the capacity loss during the surface reactions (SEI) are proportional to the surface. However, a too small surface means large particles, but that increases the lithium difusion path length [40]. As mentioned before, during the frst charging the lithium ion permeable SEI is formed, which passivates the graphite anode and protects the copper collector against atack of electrolyte and of further electrolyte decomposition.

1.4.3.!Cathode materials

Te goal to achieve high energy density lithium ion bateries is related to the properties and characteristics of the intercalation cathode material (LiyMyXz) [41]. Tus, a high redox potential against lithium is aspired. Tis can be achieved with transition metal ions in a high oxidation state, which are stabilized with oxide counteranions. To increase the capacity, the cathode material should be able to intercalate a large number of lithium ions. Tis intercalating process should also be reversible, without structural change, which enhances the cycle life. A good conductivity for lithium ions and electrons increase the charge/discharge rates [41]. Many diferent commercial cathode materials are available, but they can be classifed in three structural types: layered structures (LiCoO2 [42], LCO and NMC: Li[NiMnCo]O2 [43]), spinel-type (LiMn2O4 [44]) and olivine-type structure (LiFePO4 [45]). LiCoO2 is widely used as cathode material and has a good cycle performance and is cheap in the synthesis. But cobalt is expensive. One drawback of the material occurs at a delithiation of more than 0.5 lithium ions from LiCoO2 [46] [47]. Tis leads to an oxidation of the oxide and O2 is released from the cell [48]. Tis indicates a dangerous safety problem. Tus, an over-charging has to be managed with

electronic systems. Te NMC [49][44][50] with LiNi1/3Mn1/3Co1/3O2 has the same layered structure and with a theoretical capacity of 0.163 Ah g−1 is a litle beter than of LiCoO2 [51]. Depending on the content of cobalt, the cost can be reduced. Te spinel LiMn2O4 convinces with the cheap manganese, high safety and higher redox potential of over 4 V vs. lithium compared with the layered structures. Te drawback of the LiMn2O4 is the low specifc capacity of only 0.120 Ah g−1 and its poor temperature stability [52]. Te other mainly used cathode material is LiFePO4 (0.160 Ah g−1) [45], which is inexpensive, environmentally harmless and very safe [3]. Te disadvantages of LiFePO4 are the low average voltage of 3.45 V vs. lithium and the poor electronic conductivity, which limits the rate of charge and discharge [3].

1.4.4.!Separators

To prevent the electric contact of the active materials a separator is put between. A separator should be an electric isolator but has to be highly permeable for ions. Other- wise it should be inert and long-lasting under the harsh conditions of a lithium ion bat- tery. Microporous polyolefn materials are widely used in all liquid electrolyte lithium ion bateries, due to their low cost and outstanding processability. Te pore size should be less than 0.1 µm. [53] Liquid Nonaqueous Electrolytes 7

1.5!Liquid Nonaqueous Electrolytes

1.5.1.!General requirements

An additional key issue to the performance of lithium ion bateries is the choice of the electrolyte. Te electrolyte consists mainly of a solvent (ethers, esters, alkyl carbonate) and the conducting salt (Li[PF6], Li[BF4], Li[NTf2], Li[BOB]) and a small share of additives. Because of the high operation voltage of >4 V versus lithium of modern lithium ion bateries, aqueous electrolyte are not usable: water is electrolysed into hydrogen and oxygen gas at a theoretical potential of 1.23 V versus SHE [31]. So it is highly important that nonaqueous electrolytes are stable in the electrochemical window (~4 V, in future: ~5 V) of the operation potential, which is given from the electrode materials. Tis means, the electrolyte (salt respectively anion and solvent) ought to be thermodynamically stable at the high oxidation potentials of the cathode, which is generally ascertained with low HOMO energies. On the other side high LUMO energies increase the stability against reduction at the anode side. Terefore, the classes of solvents can be classifed by their oxidation potential: alkyl carbonate > esters > ethers [54]. Tat is why the alkyl carbonates (DMC: dimethyl carbonate, EC: ethylene carbonate, EMC: ethyl methyl carbonate, DEC: diethyl carbonate, see Table 2) and the salt lithium hexafuorophosphate (LiPF6) evolved as the main standard electrolytes used [8][55][56]. But even these alkyl carbonates are not stable at potentials >3.5 V vs. Li/Li+ at gold electrodes [29]. However, due to the passivation phenomena [57] at the cathode side they are stable up to >4.8 V [58][59]. High stability at oxidation potentials requires a high oxidations state of the solvent atoms and of the anion atoms. But on the other hand a high oxidation state means a higher tendency for reducibility at the anode side—and just as a reminder, an anode potential of ~0.2 V versus lithium (LiC6) is extremely reductive. As mentioned above, the SEI formation at the anode and the passivation flm at the cathode, are the key factors that high voltage lithium-ion bateries in carbonate based electrolytes are possible. In short, the electrochemical stability of the electrolyte is realized not because of the thermodynamic stability, but for kinetic (passivation) reasons [25]. In addition 8 Introduction

to the essential requirements of an electrolyte for high voltage lithium ion bateries, thermodynamic stability of the electrolyte and sufcient electrode passivation, there are other desirable properties [60][57]:

• high conductivity of 3 to 20 mS cm−1 over a wide temperature range • liquid within −40 to 70 ℃ • low vapour pressure • good transport properties (solvation of ions, low viscosity, etc.) • good thermal stability (of salt, solvent, passivation flms) • chemical stability and inertness towards cell components (separator, collector, electrodes, etc.) • low toxicity • high safety • low cost • low molecular weight of the lithium salt (<400 g mol−1) • high purity (H2O < 20 ppm, HF < 40 ppm)

1.5.2.!Conducting salts

Suitable lithium salts, which meet the most preferable requirements, contain weakly coordination anions. Because of the small radius of the lithium ion and the high charge density, lithium salts have a high latice energy due to high coulombic interactions and a bad solubility in low dielectric media [25]. But with weakly coordinating counter anions, the conditions are diferent. Te singly negative charge of a weakly coordination anion (i.e. [PF6]−) is delocalized over the entire anion, through a groups of atoms. Additionally, it is very important that basic sites (like oxygen) should be absent in the periphery of the anion. Tis can be achieved with fuorine atoms, that are only available for the lithium ion, thus many weakly coordinating anions contain bulky fuorinated substituents [61][62], which are chemically inert and stable against oxidation. Table 1

Table 1!A short survey of some conducting salts and their conductivity.

Mol. σ at 25 ℃ Al Salt weight Ref. [mS cm−1] corrosion [g mol−1] 10.7 Li[PF6] 151.9 N [75] (1 mol L−1 in EC:DMC) 4.9 Li[BF4] 93.9 N [25] (1 mol L−1 in EC:DMC) 4.4 Li[BOx2] 193.7 N [74] (0.8 mol L−1 in PC:EC: EMC, 1:1:3) 10.7 Li[NTf2] 286.9 Y [77] (1 mol L−1 in EC:DMC) 6.3 Li[Al(Ohfp)4] 545.0 N [76] (0.6 mol L−1 in EC:DMC) 4.9 Li[B(Ohfp)4] 528.8 N [78] (0.9 mol L−1 in EC:DMC) Liquid Nonaqueous Electrolytes 9 lists some selected lithium salts and Table 2 shows organic solvents with some relevant data. Te properties and drawbacks of the most commonly used conducting salt, lithium hexafuorophosphate Li[PF6], is representatively described in the following.

Lithium hexafluorophosphate

It is not possible to fulfl all requirements of an ideal conducting salt, but the properties of Li[PF6] are the best compromise of all demands. Te good ionic conductivity of 10.7 mS cm−1 in 1 mol L−1 EC/DMC is remarkable and the actual benchmark. Lithium hexafuorophosphate forms a very efective passivation at the aluminium collector of the cathode, which is stable up to 5 V versus lithium [63]. Because the aluminium is the most relevant collector material for the cathode material, a corrosion of aluminium through the conducting salt is an exclusion criterion for every conducting salt [8]. Te disadvantages of Li[PF6] are its tendency of decomposition at 50-60 ℃ [64][65][66] into LiF and PF5 [67], which is a strong Lewis acid [68] and can atack the ethylene carbonate under formation of poly(ethylenoxid) and CO2 [69][70]. Furthermore the PF5 can react with water traces under formation of highly toxic hydrofuoric acid, which can also lead to further solvent decomposition and gas generation [71][72][73]. HF also atacks the manganese spinel cathode materials, so a moisture-free production of solvent and batery has to be guaranteed [74].

1.5.3.!Solvents

Appropriate batery solvents can be found in Table 2. It can be distinguished between cyclic (EC, PC) and open chain carbonates (DMC, DEC, EMC) and chelating non-cyclic ethers (DME, DMM) and cyclic ethers (DOL, THF). To combine the properties of the diferent solvents and to increase the liquid range ofen binary mixtures were used, such as 1:1 wt.-% mixture of EC:DMC for lithium-ion bateries or a 1:1 wt.-% mixture of DME:DOL for lithium-sulfur bateries.

Table 2!Organic carbonates and ethers as electrolyte solvents with physical properties ([25] and references therein).

Acro- Mol.weight ε Solvent m.p. [℃] b.p. [℃] nym [g mol−1] at 25 ℃ Ethylene carbonate EC 88 36.4 248 89.78 Propylene carbonate PC 102 −48.8 242 64.92 Dimethyl carbonate DMC 90 4.6 91 3.107 Diethyl carbonate DEC 118 −74.3 126 2.805 Ethyl methyl carbonate EMC 104 −53 110 2.958 Dimethoxy methane DMM 76 −105 41 2.7 Dimethoxy ethane DME 90 −58 84 7.2 1,3-Dioxolane DOL 74 −95 78 7.1 Tetrahydrofuran THF 72 −109 66 7.4 10 Introduction

1.5.4.!Additives

Every combination of solvent, conduction salt, electrode material, collectors and other cell components have fragilities which lead to capacity loss, lower cycle life or safety problems. To improve the lithium ion batery performance and safety litle amounts of less than 5 wt.-% additives are typically used in batery electrolytes [79]. Many additives were used as secret ingredients from the manufactures, [80] but additives can be divided in the following categories [79][8]:

• SEI forming improver (e. g. LiBOB, EC, boron based anion receptors). • cathode protecting agents, • Li[PF6] salt stabilizer, • safety protecting agents (overcharge protection, fre retarder), • HF and H2O scavengers, • aluminium corrosion inhibitors, • ionic solvation enhancers (e. g. boron based anion receptors).

1.6!Lithium Sulfur Bateries

1.6.1.!Introduction, fundamental chemistry and their problems

Te high interest in efcient energy storage for electric vehicles and renewable energy sources draws the atention of researches on sulfur: the promising cathode material. Te enormous sulfur deposits and the low price, non-toxicity of sulfur and the high specifc capacity of 1675 Ah kg−1 make it to the preferential choice [81]. A lithium-sulfur batery (Li/S) has a tremendous theoretical energy density of 2600 Wh kg−1 [82]. Te cell voltage is ~2.2 V versus Li/Li+ due to the redox couple of lithium and sulfur. But the commercialization of the Li/S batery is hindered, due to the short cycle life, capacity fading, high self-discharge and poor safety [82]. Tese problems originate from the several chemical species (polysulfdes), which are formed during the charge and discharge cycles (Eq.9). Te sulfur S8 is reduced in several steps during the discharging:

(2n-4) Li

Li2S8–n n Li2S2 + 2 Li S8 + 2 Li Li2S8 + 2 Li (2n-2) Li Eq.9 Li2Sn n Li2S ~2.3 V ~2.1 V insoluble soluble soluble n>2 insoluble

With the formation of the soluble lithium polysulfdes the problem arises: the dissolution of highly soluble and reactive polysulfdes in the liquid electrolyte leads to the 'polysulfde shutle' [81]. Te formation of the polysulfdes is inevitable and otherwise necessary for the batery performance, but sulfur and the lithium polysulfdes are electrically insulating [17]. Te dissolution of polysulfdes threatens the essentially Lithium Sulfur Bateries 11 intimate contact with the electrically conductive electrode material. Once the sulfur species have no more contact to the cathode, the electrochemical reduction cannot be maintained, instead the polysulfdes can migrate (if not hindered) to the lithium anode and undergo a parasitic chemical redox reaction, under formation of lower polysulfdes. Once the insoluble Li2S and Li2S2 are formed, they precipitate on the surface of the lithium anode. Following polysulfdes can react with the insoluble sulfdes to form lower polysulfdes again [83][84][85]. Tese can 'shutle' back to the cathode, where they are electrochemically oxidized to Li2S6 and Li2S8 [83]. In the next cycle the shut- tle mechanism is repeated. At the end, all lower polysulfdes are reduced to Li2S and Li2S2, which are deposited as insoluble precipitate on the cathode without electronic contact. Tis means a constant loss of active material during every charging, which re- duces markedly the coulombic efciency. Additionally, the insoluble sulfde flm on the cathode increases the cell resistance [86]. Besides the reactive polysulfde anions can react with the esters, carbonates and phosphates [87], therefore appropriate solvents are ethers, like 1,2-dimethoxyethan (DME) and 1,3-dioxolane (DOL). Te combination of both ethers has demonstrated a good efect on the performance of the lithium-sulfur batery [88][8]. However, the solvent molecules can react by and by with the lithium anode and the polysulfdes forming insoluble sulfur and gaseous species, which afect the safety of the batery [82]. Moreover the widely used conducting lithium salts, like LiPF6, LiBF4, LiBOB and LiDFOB are not suitable for lithium sulfur bateries, because of the nucleophilic atack of the polysulfde anion at the boron or phosphorus atom under formation of insoluble lithium fuoride [82], and their incompatibility with DOL, which gets polymerized. Li[NTf2] has become the standard conducting salt in Li/S bateries, but Li[NTf2] is known to corrode the aluminium collector [25].

Te problems of the lithium-sulfur batery can be summarized as [82]: • Te chemical reaction of polysulfdes with the lithium anode and their reaction with the electrolyte means a constant loss of active material, which leads to self-discharge, capacity retention and safety problems. • Te polysulfde shutle corrodes the lithium anode and decreases the coulombic efciency. • Te commonly used conducting salt Li[NTf2] causes a aluminium corrosion at the collector. With this in mind, several solutions based on the suppression of the polysulfde dif- fusion from the cathode and the protection of the lithium anode. Additionally, the electrolyte plays a highly important role. Solvents ought to be low viscous, chemically stable against the active sulfur species and stable against the lithium anode. Te same holds for the conducting lithium salt: high solubility and conductivity in ether solvents and chemical inertness against the polysulfdes. Tere is an inherent dilemma with the right choice of the electrolyte. Te high solubility of the polysulfdes in liquid electrolytes assist the polysulfde shutle, but the use of highly viscous, gel or even solid electrolytes were plagued with poorer ionic conductivities and retarded kinetics [89]. 12 Introduction

1.6.2.!Solutions

Sulfur cathode composites

Te non-conducting sulfur has to be loaded on to electrically conductive material. Car- bon materials were established in the batery technology, especially for the sulfur cathode [83]. Such sulfur-carbon composites are designed to trap the dissolved polysulfdes to suppress the difusion out of the cathode. Tis can be achieved with micro-porous carbon [17] which are impregnated with sulfur. Te channels of the nano structure combined with the high pore volume, encapsulate the sulfur. Te morphology of the used CMK-3 carbon with short rod-like nanofbers adsorbs and absorbs the polysulfde species and inhibits the difusion [90]. Terefore, a full conversion of the active species can be achieved, due to the permanent electric contact. Te initial capacity of 1320 mAh g−1 at the frst cycle and remaining capacity of 1100 mAh g−1 afer the 20th cycle indicates a suppressed shutle mechanism [17]. Further minimizing of the difusion of polysulfdes out of the mesoporous carbon matrix was achieved by coating of poly(3,4-ethylenedi- oxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) onto impregnated carbon matrix. Afer surface coating, coulomb efciency of the sulfur electrode was improved to 93- 97 % [91].

Lithium anode

Tere are two concepts for solving the problems involved with the parasitic side reactions at the anode: • Protection of the anode to prevent the redox shutle, reducing of the reactivity of the lithium anode and to increase the safety. Terefore, passivation layers can be formed due to the reaction of a fuorinated polymer with the lithium anode [92]. Te lithium corrosion can be avoided with a coating of a gel polymer electrolyte layer [88]. • And on the other side the replacement of the lithium anode with silicon and the sulfur cathode with Li2S [93][94]. Tis method bypasses the thermal runaway of polysulfdes with lithium at higher temperature—a serious safety issue [82].

Electrolyte additives

Te use of additives in liquid electrolytes ought to protect the lithium anode, enhancing the solubility and reducing the viscosity. One major invention was the use of lithium nitrate [95], which inhibits the redox shutle due to a passivation of the lithium electrode. Terefore LiNO3 acts as a oxidizing agent for the polysulfde moieties, and the reduction products from an efective SEI [96][97]. 13

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2!Motivation

Because of the importance of energy storage by rechargeable bateries and the associat- ed need for new materials for their performance improvement, a collaboration project was launched between our working group and Merck KGaA in September 2010 and than continued by BASF SE since April 2012. Te main focus of the project is the design of new lithium conducting salts for lithium-ion bateries. Te Krossing group is advanced in synthetic inorganic molecular chemistry. In particular, the applied and fundamental science of weakly coordinating anions is investigated in-depth. Many publications of the applications of weakly coordinating anions, such as catalysis chemistry [98][99][100], ionic liquids [101][102][103][104] or polymeri- zation [105][106][107][108] have been reported. More featuring aspects of weakly coordinating anions are, for instance, the ability to stabilize reactive cations like P9+ [111], CCl3+ [110], Ag(C2H4)3+ [109], without decomposition of the anions and their low coordination ability. A lithium cation is a strong Lewis acid and its chemistry is dominated by strong ion-ion interactions, which results in high latice energy in simple two atomic lithium salts (e.g. LiF, LiCl). But in combination with weakly coordinating anions the ion-ion interactions of the salt are minimized. Tis leads to an increased conductivity in non-aqueous batery solvents, and therefore the use of weakly coordinating anions is preferred. A further advantage is the chemical inertness of these kind of anions, which can resist the strong infuence of the cation. Tis thesis was dedicated to the search and synthesis of novel lithium conducting salts for lithium-ion bateries. Since lithium necessarily is the cation, the main task is the design of the weakly coordination counter-ion. Te general requirements to conducting salts in lithium-ion bateries are high conductivity, high solubility in appropriate batery solvents, high chemical and thermal stability. According to these requirements, there are additional demands of our industry collaboration partners which should be considered for the search and synthesis: Te novelty, low cost of the salt, non-toxicity and high purity. Te herein described design of new lithium salts orientates along these demands which includes to fnd unknown and non-patented lithium salts. Within a feasibility study of suitable candidates the anions were modelled with thermodynamic calculations and the syntheses were atempted by using simple accessible chemical routes, outgoing from cheap and high purity starting materials. In the following chapters the results of the development of three classes of lithium salts with new counter-ions are described: Lithium boroxinates, lithium phosphates and lithium borate. 18 Motivation

2.1!References

[98]:M. R. Lichtenthaler, A. Higelin, A. Kraf, S. Hughes, A. Stefani, D. A. Platner, J. M. Slatery, I. Krossing, Organometallics 2013, 32, 6725-6735. [99]:M. Rohde, L. O. Müller, D. Himmel, H. Scherer, I. Krossing, Chem. – Eur. J. 2014, 20, 1218-1222. [100]:T. Rudolph, K. Kempe, S. Croty, R. M. Paulus, U. S. Schubert, I. Krossing, F. H. Schacher, Polym. Chem. 2013, 4, 495-505. [101]:S. Bulut, P. Klose, M.-M. Huang, H. Weingaertner, P. J. Dyson, G. Laurenczy, C. Frie- drich, J. Menz, K. Kuemmerer, I. Krossing, Chem. - Eur. J. 2010, 16, 13139-13154. [102]:S. Bulut, M. A. Ab Rani, T. Welton, P. D. Lickiss, I. Krossing, ChemPhysChem 2012, 13, 1802-1805. [103]:F. Scholz, D. Himmel, H. Scherer, I. Krossing, Chem. - Eur. J. 2013, 19, 109-116. [104]:M. Buerchner, A. M. T. Erle, H. Scherer, I. Krossing, Chem. - Eur. J. 2012, 18, 2254- 2262. [105]:M. Klahn, C. Fischer, A. Spannenberg, U. Rosenthal, I. Krossing, Tetrahedron Let. 2007, 48, 8900-8903. [106]:N. Hildebrandt, H. M. Koenig, P. Hanefeld, K. Muehlbach, I. Krossing, G. Steinfeld, BASF SE, Germany, 2010, p. 35pp. [107]:I. Krossing, Angew. Chem., Int. Ed. 2011, 50, 11576-11578. [108]:A. Kraf, N. Trapp, D. Himmel, H. Boehrer, P. Schlueter, H. Scherer, I. Krossing, Chem. - Eur. J. 2012, 18, 9371-9380. [109]:A. Reisinger, N. Trapp, C. Knapp, D. Himmel, F. Breher, H. Ruegger, I. Krossing, Chem. - Eur. J. 2009, 15, 9505-9520. [110]:A. J. Lehner, N. Trapp, H. Scherer, I. Krossing, Dalton Trans. 2011, 40, 1448-1452. [111]:T. Koechner, T. A. Engesser, H. Scherer, D. A. Platner, A. Stefani, I. Krossing, Angew. Chem., Int. Ed. 2012, 51, 6529-6531. Introduction and Overview 19

3!Lithium boroxinates

3.1!Introduction and Overview In the course for our search of novel lithium salts for lithium bateries the class of lithium boroxinate salts (Li[B3O3X4], Figure 1) was not described in the literature yet. But the idea appeared quiet obvious: Te related class of borate or boronic ester (borate or boric acid ester: B(OX)3, boronic acid ester: R–B(OX)2) are well investigated as anion receptors for many important applications, including as additives in lithium batteries [112][113][114]. Anion receptors are used in nonaqueous electrolytes to increase the ionic conductivity and to improve the solid–electrolyte interphase (SEI) formation [115]. And the use of boroxines on the other side (B3O3X3 : boroxines are sometimes termed metaborate or triborate) is also described as additive in polymer electrolytes [116][117][118]. But lithium boroxinates represents a hybrid of a lithium salt and an anion receptor. Te synthesis of the tetrafuorob- F oroxinate ([B3O3F4]−, Figure 1) was not verifed O O O F O [119], but 2008 Finze & Reiss reported of the frst Li difuorodihydroxy boroxinate [B3O3F2(OH)2]− B B [120]. Te group of Jansen synthesised in 2009 O O O O the frst lithium fuorooxoborate (LiB6O9F, see B B B B Figure 2) with a solid state reaction. Consists O O O O O of two boroxine rings bridged by an oxygen Figure 2!Repetition unit of the LiB6O9F atom. But this lithium fuorooxoborate is an in- structure, reproduced from [121]. soluble solid in nonaqueous solvents, but was classifed as a solid ionic conductor [121]. Tis chapter describes the atempts to synthesize novel lithium fuoroalkoxy boroxinates in solution (Scheme 1). Terefore it was started with the clean and published synthesis of the tris(2,2,2-trifuoro-

F F X ORF B B O O O O

B B B B F O F RFO O ORF F – F X = [OR ], F ; R = –CH2CF3, –CH(CF3)3

Figure 1!Lewis formulas of tetrafluoroboroxinate (lef top) and fluoroalkoxyboroxinate (right top) anions and electrostatic potentials (calculated at the RI-BP86/def2-TZVP level) of tetrafluoroboroxinate (right down), tris(hexafluoroisopropoxy)monofluoro boroxinate (middle down) and tris(trifluoroethoxy)monofluoro boroxinate (right down) projected on an isomap of electron density (0.01 e Å−1). 20 Lithium boroxinates

F B(OR )3 + B2O3

addition LiX F F B3O3(OR )3 Li[B3O3(OR )3X]

substitution

– B3O3(OR)3 [B3O3F4] fluorination Scheme 1!Overview of the synthetic routes of lithium fluoroalkoxyboroxinates and tetrafluoroboroxinate anion.

ethyl)borate (B(OTfe)3) [122] and tris(1,1,1-3,3,3-hexafuoroisopropyl)borate (B(Ohfp)3) [123]. Te further addition of these borates with boron oxide to the Lewis acidic boroxines is described by Beckett [124]. Tis boroxine should be converted with addition of a lithium salt, like lithium trifuoroacetate, lithium fuoride or lithium fuoroalcoholate, to the new class of anions: lithium boroxinates. Another route was investigated to obtain the boroxinate precursor. Starting from trimethoxyboroxine, several experiments have been done to substitute the methoxy group of trimethoxy boroxine and triisopropoxy boroxine with the fuorinated alkoxy groups. Another promising candidate as a low weight anion is tetrafuoroboroxinate (M = 156.43 g mol−1), the corresponding lithium salt is hitherto unknown. But the sodium and potassium salts of the hexafuoroboroxinate ([B3O3F6]3−) were described and characterized in the literature [130][131]. But lithium salts of triply charged anions are not interesting for nonaqueous electrolytes. Te underlying neutral precursor compound, trifuoroboroxine (B3O3F3), is commercially unavailable. And the synthesis of gaseous trifuoroboroxine from boron trifuoride and boron oxide and its handling is difcult [125][126]. Moreover the studies about the stability of trifuoroboroxine from the re- searchers of the 20th century difers drastically: Goubeau & Keller reported the substance is stable only above 250 ℃ [127], Fisher et al. published 1960 that B3O3F3 is only stable below −135 ℃ [128]. Above this temperature a disproportion occurs into boron trifuoride and boron oxide. One year later, Magee argued that it was stable over days at room temperature and melted with decomposition at 80 ℃ [129]. For that reasons, there are concerns on the formation of fuoroboroxinates via the gaseous intermediate trifuoroboroxine. So our frst experiments aimed on the mild fuoridation of trimethoxyboroxine to assess the accessibility of a new class of mono charged boroxinates: [B3O3X4]−. Tese boroxinates can delocalize the negative charge to the boroxine ring system and to the fuorinated alkyl groups, which can be seen in Figure 1. Additionally

Table 3!Comparison of molecular orbital energies of tetrafluoroboroxinate, tris(trifluoroethoxy) monofluoro boroxinate and tris(hexafluoroisopropoxy)monofluoro boroxinate with other weakly coordinating anions (RI-BP86/def2-TZVPP).

MO [B3O3F4]− [B3O3(OTfe)3F]− [B3O3(Ohfip)3F]− [PF6]– [NTf2]− HOMO −3.2887 eV −2.7320 eV −3.5337 eV −3.3598 eV −2.8531 eV LUMO +3.8783 eV +2.8744 eV +2.3036 eV +5.5247 eV +3.1723 eV Gap +7.1670 eV +5.6064 eV +5.8373 eV +8.8845 eV +6.0254 eV Syntheses from fluoroalkyl borates and boron oxide 21 the boroxinates feature low HOMO energies Table 3, which are comparable to Li[PF6] and Li[NTf2] and suggest resistance against oxidation. Te tetrafuoroboroxinate anion has the highest LUMO energy of the other boroxinates, which indicates a good electrochemical stability against reduction.

3.2!Syntheses from fluoroalkyl borates and boron oxide Te synthesis of the fuoroalkyl boroxines was investigated with two diferent strategies. Te frst strategy was the addition of fuoroalkyl borates with boron oxide at temperature in a vessel (Eq.10):

F F B(OR )3 + B2O3 B3O3(OR )3 pressure, 200°C Eq.10

Te fuoroalkoxy borates (B(OTfe)3 and B(Ohfp)3) were synthesized from BH3⋅SMe2 and the corresponding alcohol in multi gram scale in high purity. For the addition of the borate with the solid B2O3 we used a Tefon based hydrothermal synthesis reactor. Te vessel was charged with a milled mixture of the reactants, which was heated for 24 h in an oven at 200 ℃. Afer the reaction the mixture was routinely analysed by hetero nuclear magnetic resonance. In Table 4 can be seen the results from the NMR measurements of the several experiments. A representative 11BZNMR spectrum of the reaction of B(Ohfp)3 with B2O3 is illustrated in Figure 3. As can be seen in the spectrum there are two signals in the chemical shif range of triplanar coordinated borates. Te 11BZNMR signal of the B(Ohfp)3 is down feld at 17.7 ppm and up feld at 15.0 ppm the resonance of the tris(1,1,1-3,3,3-hexafuoroisopropoxy)boroxine. Other signals were not detected. In all spectra, the parent borate was the main signal. Te product distribution of unreacted borate and the boroxine was determined due to integration of the signals

B(Ohfip)3 B(OTfe)3

B3O3(OTfe)3

B3O3(Ohfip)3

25 20 15 10 5 ppm 25 20 15 10 5 ppm Figure 3!11B,NMR spectrum of the reaction Figure 4!11B,NMR spectrum of the reaction of of B(Ohfip)3 with B2O3 (MR-031) (128.39 MHz in B(OTfe)3 with 1 eq. B2O3 (MR-041) (128.39 MHz CDCl3 at r.t.). in CDCl3 at r.t.). 22 Lithium boroxinates

Table 4!List of observed NMR signals of the reaction of tris(1,1,1-3,3,3-hexafluoroisopropyl) borate or tris(2,2,2-trifluoroethly)borate with boron oxide. All samples were measured at the Bruker Avance 400 II+ at r.t. The product samples were disssolved in diethyl ether (Et2O = 1.2 ppm) and toluene-D8 was used as external lock. The ratio of several boron products was determined due to integration of the signals in the 11B,NMR NMR spectra. (br: broad; sh: shoulder)

1H(NMR 19F(NMR 11B(NMR Exp. J or J or ratio δ δ δ ∆1 2 assignment no ∆1 2 ∆1 2 / [%] [ppm] / [ppm] / [ppm] [Hz] [Hz] [Hz]

MR-031 5.34 sep, 5.50 −76.31 d, 5.50 17.7 br, 68 B(Ohfp)3 80

5.09 br, 40 −76.24 br, sh 15.0 br, 108 B3O3(Ohfp)3 20

MR-033 5.20 sep, 5.50 −76.85 d, 5.50 18.1 br B(Ohfp)3 —a

4.95 br −76.79 br, 31 15.4 br B3O3(Ohfp)3 —a

—a —a —a —a 5.0 br [B(Ohfp)4]– —

MR-034 5.14 sep, 5.32 −76.87 d, 5.32 17.6 s, br B(Ohfp)3 —a

5.05 sep, 5.89 −77.06 d, 5.89 18.5 s, br B3O3(Ohfp)3 —a

4.41 sep, 6.10 −77.17 d, 6.10 — — HOhfp —

MR-035 5.34 sep, 5.60 −76.30 d, 5.60 17.7 br, 64 B(Ohfp)3 93

—a —a —a —a 14.9 br, 110 B3O3(Ohfp)3 7

MR-036 5.27 sep, 5.50 −76.67 d, 5.50 17.7 br, 55 B(Ohfp)3 90

4.75 br −76.80 br 15.1 br, 115 B3O3(Ohfp)3 10

MR-037 5.30 sep, br −76.57 br 17.7 s B(Ohfp)3 88

5.11 br −76.48 br 15.1 s B3O3(Ohfp)3 12

MR-038 5.32 sep, 5.50 −76.61 d, 5.50 17.7 s B(Ohfp)3 86

5.03 br −76.39 d, 5.54 15.1 s B3O3(Ohfp)3 14

MR-039 5.29 sep, 5.50 −76.61 d, 5.50 17.7 s B(Ohfp)3 86

5.03 br −76.50 d, 5.58 15.1 s B3O3(Ohfp)3 14

MR-040 5.31 sep, 5.50 −76.48 d, 5.50 17.7 s B(Ohfp)3 91

5.05 br −76.39 d, 5.54 15.1 s B3O3(Ohfp)3 9

MR-041 4.41 q, 8.41 −77.38 t, 8.50 192 br, B(OTfe)3 85b

4.38 q, 8.45 −77.74 t, 8.48 18.0 br, sh B3O3(OTfe)3 15b

MR-048 4.41 q, 8.41 −77.38 t, 8.41 19.2 br B(OTfe)3 —a

4.38 q, 8.44 −77.74 t, 8.44 18.0 br, sh B3O3(OTfe)3 —a a: could not determined, due to superimposing signals.3b: determined via EI-MS

in the 11BZNMR. In some cases the 11BZNMR signals were superimposed and an integration was not possible. Te maximum yield which was obtained due to the reaction was only 20 %. In the other experiments it was around 10 %. Most of the unreacted B(Ohfp)3 was regained from the reaction mixture, by sublimation. Due to the limited volume of the Tefon vessel (max. 25 mL) it was not possible to synthesize the boroxine in larger amounts. And because of the bad yield, not enough substance for the further reactions was obtained. To increase the yield of the reaction, it was atempted to increase the stoichiometric amount of the borate. In the experiment MR-033 1.5 equivalents of B(Ohfp)3 Syntheses from fluoroalkyl borates and boron oxide 23 was used. Te amount of boroxine could not be determined, because of superimposing 11B resonances. Additionally low intensive signals of tetra-coordinated borates were observed in the 11BZNMR spectrum. Te occurrence of these anionic species allows to sus- pect that moisture impurities are responsible. Probably it is due to a leakage of the hydrothermal vessel. In the experiments MR-035 and MR-036 two equivalents of B(Ohfp)3 were used, but it has no increasing efect on the yield. On the contrary, the amounts of tris(1,1,1-3,3,3-hexafuoroisopropyl)boroxine were 7 % in MR-035, respectively 10 % in MR-036. As a further atempt to optimize the yield, the alcohol (hexafuoro-2-propanol) was used to increase the pressure in the reactor. Terefore f ve equivalents alcohol, with respect to B2O3, were added. But the 11BZNMR spectrum indicated, that the borate was still the main product. Te variation of the stoichiometry had no efect on the yield of the reaction. And because of the limitations of the Tefon vessel it was not possible to heat to higher temperatures and to increase the reaction rate. Also the increase of the pressure due to addition of alcohol, did not lead to a higher yield. Te vessel had only a volume of 25 mL, and so it was not possible to synthesize large amounts of tris(1,1,1-3,3,3-hexafuoroisopropoxy)boroxine, despite the synthesis was published in the literature. And further conversions of the boroxines to the lithium boroxinates were not feasible! And also the synthesis of tris(2,2,2-trifuoroethly)boroxine (MR-041 and MR-048) was not with high yield. In the 11BZNMR spectrum (Figure 4) can be seen one main signal at 19.2 ppm, which was assigned to B(OTfe)3. Te signals exhibits at high feld shoulder with a second less intensive signal at 18.0 ppm, which is the resonance of tris(2,2,2-trifuoroethly)boroxine. For verifcation of the existence of the metaborate a mass spectrum was measured. Te mass spectrum showed only two peaks, which

Table 5!List of the observed 11B,NMR signals from of the reaction of trimethoxyboroxine or tri(isoproply)boroxine with three equivalents of hexafluoro-2-propanol. All samples were measured at the Bruker Avance 400 II+ at r.t.

11B(NMR Exp No. reactant assignment δ [ppm] MR-005.1 trimethoxyboroxine 19.2a B3O3(OMe)3c

18.7a B(OMe)3d 18.0a B3O3(Ohfp)x(OMe)y MR-005.2 trimethoxyboroxine 19.2a B3O3(OMe)3 18.7a B(OMe)3 18.0a B3O3(Ohfp)x(OMe)y MR-005.3 trimethoxyboroxine 19.1a B3O3(OMe)3 18.7a B(OMe)3 18.0a B3O3(Ohfp)x(OMe)y MR-017 triisopropylboroxine 20-18b very broad signals MR-019 triisopropylboroxine 18.7b B3O3(OiPr)3e 18.4b B3O3(Ohfp)x(OiPr)y 17.4b B3O3(Ohfp)y(OiPr)x a: in CDCl33b: in CD3CN3c: δ (11B) = 19.2 ppm in CDCl33d: δ (11B) = 18.5 ppm in CDCl33 e: δ (11B) = 18.9 ppm in CDCl3 24 Lithium boroxinates

were unambiguously assigned to B3O3(OTfe)3 and B(OTfe)3, out of there the amount of B3O3(OTfe)3 was determined to 15 %. Due to the limitations of the reactor and the low yield of the reaction in the hydrothermal reactor no more experiments were undertaken. But with the access to high performance reactors the synthesis should be repeated.

3.3!Syntheses of fluoroalkoxy boroxines from alkoxy boroxine Another approach of the synthesis of fuoroalkyl boroxines is the substitution of the alkoxy group in an existing boroxine (B3O3(OR)3) through the fuorinated alcohol, hexafuoro-2-propanol (Eq.11). For this trimethoxy boroxine and triisopropoxy boroxine were used. Te principle of the reaction is based on the diferent pKa values of the corresponding alcohols. Te used hexafuoro-2-propanol has a pKa of 9.75 and should be acidic enough to protonate the methoxy (methanol: pKa = 15.5 [133]) and isopropoxy (isopropanol: pKa = 17.1 [133]) group in the boroxine and substitute the groups with formation of methanol or isopropanol. So initial reactions of one equivalent of the corresponding boroxine with three equivalents of hexafuoro-2-propanol were done. Te conversion of the reaction was assessed by hetero nuclear magnetic resonance.

B3O3(OR)3 + 3 HOhfip B3O3(Ohfip)3 + 3 HOR R = Me, iPr Eq.11

On the basis of 11BZNMR reference spectra of the starting materials, the 11BZNMR shifs of the obtained products were evaluated. In the Table 5 the 11BZNMR signals of the several reactions are listed. In all substitutions reactions, with trimethoxy boroxine as reactant, the main signal was trimethoxy boroxine and trimethyl borate (as a main impurity of the trimethoxy boroxine). Te same observation was made with the reaction of B3O3(OiPr)3. Te resonance of the desired B3O3(Ohfp)3 was never observed. Te occurrence of the new 11BZNMR signals at high feld (18.0 ppm, reactions from trimethoxy boroxine or 18.4/17.4 from the reactions with B3O3(OiPr)3) are probably signals of boroxines with mixed substituents. Also the many broadened 1HZNMR and 19FZNMR signals of the corresponding groups (–OMe, –OiPr, –Ohfp) indicated a continuously exchange of the substituents. Tis facts showed that an easy and clean synthesis of tris(1,1,1-3,3,3-hexafuoroisopropoxy)boroxine is not realistic, due to the obtained mixture of boroxines. Even the atempts of substitution with lithium hexafuoro-2-propanolat do not lead to the boroxine, instead the boroxine ring was cleaved under formation of a vast range of tetra coordinated borates. Tereupon all further experiments were discontinued, because of the litle prospect of success to obtain clean and practical amounts of fuoroalkoxy boroxine. Fluorination of trimethoxyboroxine 25

3.4!Fluorination of trimethoxyboroxine Te f rst atempts to get access to the fuoroboroxinate system was the fuorination of trimethoxy boroxine with mild fuorinations reagents, like Olah’s reagent (Pyri- dine-HF) or potassium bifuoride (Eq.12).

[F-H-F]– – B3O3(OMe)3 [B3O3(OMe)3F] Eq.12 –HF

But instead of a fuorination of the boroxine ring to the boroxinates, the formation of tetrafuoroborate was observed. With Pyridine-HF, the main product was the tetrafuoroborate and additionally some fuoromethoxyborates ([BFxOMey]−) were observed in solution, but it was confrmed that the boron atom is not part of the boroxine ring. Te reaction products of trimethoxy boroxine with K[HF2] were also tetrafuoro borate, which could be detected in solution. Tis indicated a cleavage of the boroxine ring. Also a solid residue was obtained, which was insoluble in organic solvents and was analysed with ATR-IR. Te Infrared spectrum was in good agreement to the spectrum of a hexafuoroboroxinate, [B3O3F6]3−. Obviously the fuorination of trimethoxy boroxine does not stop with the formation of the mono anion. During the fuorination the thermodynamically stable tetrafuoroborates and the hexafuoroboroxinates were formed. Te accessibility of the alkali hexafuoroboroxinate opened a new chance to get the singly charged boroxinates, due to the defuorination with strong Lewis acids. Hence, some experiments of the reaction of Na3[B3O3F6] with aluminium chloride, trimethylsilyl chloride, trimethylsilyl trifate and TMS-F-Al{O[C(CF3)3]3} [132] were performed. But a defuorination was never observed.

3.5!Conclusion and Summary Te feasibility study of the synthesis of lithium fuoroalkoxy boroxinates in solution has demonstrated that the synthesis sufers from problems with the synthesis of the precursor, fuoroalkoxy boroxines (Scheme 2). Te reaction of borate with boron oxide to this precursor could not be obtained in sufciently large amounts to investigate further steps. In future with adequate synthesis reactor it may be possible. Te other investigated route to obtain the precursor, was the conversion of trimethoxy boroxine

+ 3 HORF F F B(OR )3 + B2O3 B3O3(OR )3 B3O3(OR)3 low yield –3 HOR ?

Li[B3O3(OMe)3F]

[F-H-F]– [F-H-F]– – 3– – BF4 and B3O3F6 B3O3(OMe)3 [B3O3(OMe)3F] –HF Scheme 2!Investigated syntheses during the feasibility study of lithium boroxinates. 26 Lithium boroxinates

with hexafuoro-2-propanol, which led to a mixture of boroxines, with no success in separation. Generally, problematic mixtures have to be avoided in every synthesis step, considering that batery grade materials have to be high in purity! An important understanding was found from the fuorination of trimethoxy boroxine with mild fuorination reagents: Te reactions in solution have shown that the thermodynamically stable products, like tetrafuoroborate and hexafuoroboroxinate, were the main products and therefore preferred. Tis indicated in particular a cleavage of the boroxine ring. Finally a singly charged boroxinate was never observed. Te idea of a singly charge lithium boroxinate is still atractive. And further approaches in synthetic inorganic chemistry will turn out if the realization of this salt is possible. Experimental Section 27

3.6!Experimental Section

3.6.1.!Chemicals

Chemical Manufacturer Qality CAS no. Purification before use borane dime- 1 mol L−1 in thyl sulfde Sigma Aldrich 13292-87-0 none CH2Cl2 complex 2,2,2-trifuor- for triple condensing on 3Å molecular Merck KGaA 75-89-8 ethanol synthesis sieve; water content <40 ppm 1,1,1-3,3,3-hex- distilling from P2O5 water content afuoroisopro- — NMR pure 920-66-1 <2 ppm panol

dried at 250 ℃ under vacuum for 7 boron oxide Merck KGaA +99 % 1303-86-2 days

trimethoxy- ABCR +95 % 102-24-9 distilling boroxine hydrogen fuor- 65 % HF Sigma Aldrich 62778-14-4 direct id pyridine solution

potassium Sigma Aldrich 99 % 7789-29-9 dried under vacuum bifuoride

triisopropoxy- Sigma Aldrich +95 % 10298-87-0 direct boroxine

3.6.2.!Synthesis of fluoroalkyl borates

Synthesis of tris(1,1,1-3,3,3-hexafluoroisopropyl)borate (MR-029) In a three-neked fask a solution of borane dimethyl sulfde complex in dichloromethane (1 mol L−1, 136 mL, 0.136 mol, 1 eq.) was cooled to −30 ℃. Under stirring hexafuoro-2-propanol (56.0 mL, 90.7 g, 0.54 mol, 4 eq.) was added within 4 h to the cooled solution via a dropping funnel. A gas evolution was observed. Ten the reaction mixture was allowed to warm up to room temperature over night. Afer this the methylene chloride and surplus alcohol was distilled of. Te residue was a turbid liquid. A NMR sample was taken. For removal of remaining traces of starting material the crude product was carefully removed in a condensate trap. Te clear residue crystallised out. For further purifcation the clear residue was sublimed. A clear crystalline product was obtained (50.50 g, yield: 73 %). m.p.: 31.5 ℃, b.p.: 117 ℃ 28 Lithium boroxinates

Crystalline product in Et2O: 1H(NMR (400.17 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = 5.31 (sep, 3J(1H,19F) = 5.53 Hz, B(OCH(CF3)2)3) ppm 19F(NMR (376.54 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = −76.50 ppm (d, 3J(1H,19F) = 5.53 Hz, B(OCH(CF3)2)3) ppm 11B(NMR (128.39 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = 17.7 (s, ∆1/2 = 64 Hz, B(OCH(CF3)2)3) ppm ATR-IR ν [cm−1] = 541 (w), 640 (sh), 644 (w), 691 (m), 871 (m), 906 (m), 1105 (s), 1200 (vs), 1220 (sh), 1245 (m), 1269 (s), 1282 (s), 1379 (s), 1407 (m), 1452 (w), 2974 (vw)

Synthesis of tris(2,2,2-trifluoroethyl)borate (MR-030)

A boran dimethyl sulfde solution in dichloromethane (1 mol L−1, 80 mL, 0.080 mol, 1 eq.) was cooled to −10 ℃. Ten 2,2,2-trifuoroethanol (23.2 ml, 32.0 g, 0.32 mol, 4 eq.) was added to the solution under stirring via a dropping funnel within 2.5 h. A gas evolution was observed. Ten the reaction mixture was allowed to warm up to room temperature overnight. Afer this the methylene chloride and surplus alcohol was distilled of. A clear liquid was obtained (22.4 g, yield: 91 %).

Clear liquid in CH2Cl2: 1H(NMR (400.17 MHz, CH2Cl2 = 5.31 ppm, toluene-D8 external lock, r.t.) δ = 4.31 (q, 3J(1H,19F) = 8.40 Hz, B(OCH2CF3)3) ppm 19F(NMR (376.54 MHz, CH2Cl2 = 5.31 ppm, toluene-D8 external lock, r.t.) δ = −78.95 ppm (t, 3J(1H,19F) = 8.40 Hz, B(OCH2CF3)3) ppm 11B(NMR (128.39 MHz, CH2Cl2 = 5.31 ppm, toluene-D8 external lock, r.t.) δ = 18.0 (s, ∆1/2 = 112 Hz, B(OCH2CF3)3) ppm ATR-IR ν [cm−1] = 520 (vw), 535 (vw), 548 (vw), 581 (w), 648 (w), 677 (w), 841 (w), 962 (m), 1078 (m), 1155 (vs), 1263 (s), 1287 (w), 1298 (w), 1389 (s), 1430 (m), 1474 (w), 1483 (sh), 2919 (vw), 2976 (w) Experimental Section 29

3.6.3.!Syntheses of fluoroalkyl boroxine

Synthesis of tris(1,1,1-3,3,3-hexafluoroisopropyl)boroxine

A hydrothermal synthesis reactor was flled with a milled mixture of boron oxide and tris(1,1,1-3,3,3-hexafuoro) borate (see following table for exact amounts) inside of a argon glove box. Te vessel was closed and the reactor was put in an oven for several hours at 150-200 ℃. Afer fnishing the heating the reactor was allowed to cool down to room temperature and was opened in the glove box. An ATR-IR spectrum was measured from the reaction mixture and a NMR sample was taken. Te unreacted borate was then recycled by sublimation. Te residue was suspended in diethyl ether and fltered. Afer this the solvent was removed in a condensate trap. A brownish liquid was obtained.

Heating Exp. no. Educt m [g] n [mol] Eq. T [℃] duration [h]

MR-031 B2O3 0.157 0.0023 1 150 24

B(Ohfp)3 1.156 0.0023 1

MR-033 B2O3 0.202 0.0029 1 150 24 B(Ohfp)3 2.227 0.059 1.5 MR-034 B2O3 0.136 0.002 1 180 24 B(Ohfp)3 1.015 0.002 1 HOhfp 1.648 0.010 5 MR-035 B2O3 3.303 0.047 1 200 24 B(Ohfp)3 12.064 0.024 2 MR-036 B2O3 3.301 0.047 1 200 24 B(Ohfp)3 12.075 0.024 2 MR-037 B2O3 0.270 0.0039 1 200 24 B(Ohfp)3 2.000 0.0039 1 MR-038 B2O3 0.410 0.0058 1 200 24 B(Ohfp)3 3.002 0.0058 1 MR-039 B2O3 4.013 0.0078 1 200 24 B(Ohfp)3 0.542 0.0078 1 MR-040 B2O3 0.682 0.0098 1 200 24 B(Ohfp)3 5.000 0.0098 1 30 Lithium boroxinates

Reaction mixture MR-033: ATR-IR ν [cm−1] = 457 (vw), 497 (vw), 612 (vw), 638 (w), 648 (w), 692 (m), 722 (w), 732 (w), 739 (w), 745 (w), 872 (w), 906 (m), 1105 (s), 1142 (s), 1200 (vs), 1220 (sh), 1246 (m), 1269 (s), 1280 (s), 1379 (s), 1398 (m), 1434 (sh), 1503 (vw), 2971 (vw) Raman ν [cm−1] = 432 (vw), 540 (m), 650 (vw), 732 (vs), 860 (s), 902 (m), 1113 (w), 1214 (w), 1288 (w), 1386 (s), 2985 (s) +EI-MS m/z (%): 49.2 (9.7), 51.2 (31.7), 69.1 (41.6) [+CF3], 78.1 (3.2), 79.1 (100), 80.1 (5.9), 82.1 (4.4), 97.1 (7.1), 99.1 (68.8), 101.1 (47.1), 129.1 (62.5), 132.0 (10.2), 151.1 (55.8) [C3HF6+], 175.0 (41.3), 177.0 (55.5), 299.1 (37.7), 345.1 (66.8) [C6H2BF12O2+], 512.1 (10.5), 513.1 (14) [C8H3B3F15O6+]

Syntheses of tris(2,2,2-trifluoroethyl)boroxine A hydrothermal synthesis reactor was flled with a milled mixture of boron oxide and tris(2,2,2-trifuoroethyl) borate (see following table for exact amounts) inside of a argon glove box. Te vessel was closed and the reactor was put in an oven for several hours at 150 ℃. Afer fnishing the heating, the reactor was allowed to cool down to room temperature and was opened in the glove box. An ATR-IR spectrum was measured from the reaction mixture and a NMR sample was taken. Te unreacted borate was then recycled due to sublimation. Te residue was suspended in diethyl ether and fltered. Afer this the solvent was removed in a condensate trap. A brownish liquid was obtained (1 g).

Heating Exp. no. Educt m [g] n [mol] Eq. T [℃] duration [h]

MR-041 B2O3 1.390 0.020 1 150 24 B(OTfe)3 1.998 0.020 1

MR-048 B2O3 1.390 0.020 1 150 24 B(OTfe)3 2.001 0.020 1

Reaction mixture MR-048: ATR-IR ν [cm−1] = 469 (vw), 491 (vw), 536 (vw), 588 (w), 668 (w), 720 (w), 840 (w), 904 (vw), 964 (m), 1078 (sh), 1113 (m), 1153 (vs), 1273 (s), 1299 (w), 1298 (w), 1389 (s), 1430 (m), 1469 (sh), 1515 (vw), 2977 (w) +EI-MS m/z (%): 306.9 (16), 308.2 (100), 309.0 (54) [B(OTfe)3], 377.1 (25), 378.1 (1.3) [B3O3(OTfe)3] Experimental Section 31

3.6.4.!Substitutions at alkyl boroxines

General

Several experiments on the conversion of trimethoxy boroxine or triisopropoxyboroxine with lithium hexafuoro-2-propanolat or hexafuoro-2-propanol were undertaken. Te procedures are described below. From the reaction mixtures or the residues NMR samples were taken. A exact assignment of all the NMR signals in the 1HZNMR or 19FZNMR was not possible or useful, because many and broad signals of the respective chemical groups were observed. Te NMR signals were compared with the blind NMR spectra of the starting materials, which are describe in the following. Te observed 11BZNMR resonances of the experiments are listed in the table below.

1H(NMR 11B(NMR δ [ppm] δ [ppm]

B3O3(OMe3) 3.60 s 19.2 s

4.55 sep, J = 6.15 Hz B3O3(O Pr)3 18.9 s i 1.20 d, J = 6.15 Hz

B(OMe)3 3.45 s 18.5 s

Conversion of triisopropoxyboroxine with hexafluoro-2-propanol

MR-017 A 5 mm NMR tube with Produran valve was charged with triisopropoxyboroxine (77 mg, 0.3 mmol, 1 eq.) and CD3CN (0.6 mL) and hexafuoro-2-propanol (0.08 mL, 130 mg, 0.9 mmol, 3 eq.) were added. Te NMR tube was treated in an ultrasonic bath, but the tris(isopropyl)metaborate was not completely dissolved.

1H(NMR (400.17 MHz, CD3CN = 1.94 ppm, r.t.) δ = 5.66 (s, CH(CF3)2), 4.69 (sep, 3J(1H,1H) = 6.45 Hz, CH(CH3)2), 4.48 (br, ∆1/2 = 27 Hz, CH(CH3)2), 4.33 (sep, 3J(1H,1H) = 6.30 Hz, CH(CH3)2), 1.19 (br, 6 H, ∆1/2 = 17 Hz, CH(CH3)2) ppm 19F(NMR (376.54 MHz, CD3CN = 1.94 ppm, r.t.) δ = −76.55 (d, 3J(1H,19F) = 6.45 Hz, CH(CF3)2), −76.27 (d, 3J(1H,19F) = 6.30 Hz, CH(CF3)2) ppm 11B(NMR (128.39 MHz, CD3CN = 1.94 ppm, r.t.) δ = 19.2 (br, ∆1/2 = 230 Hz, B3O3(OiPr)3) ppm 32 Lithium boroxinates

MR-019 A 5 mm NMR tube with Produran valve was charged with triisopropoxyboroxine (20 mg, 0.08 mmol, 1 eq.) and CD2Cl2 (1.0 mL). Te boroxine was not completely dissolved. Ten hexafuoro-2-propanol (0.024 mL, 40 mg, 0.024 mmol, 3 eq.) were added. Te NMR tube was treated in a ultrasonic bath, but the triisopropoxyboroxine was not complete dissolved.

1H(NMR (400.17 MHz, CD2Cl2 = 5.32 ppm, r.t.) δ = 5.02 (m, CH(CF3)2), 5.00 (m, CH(CF3)2), 4.52 (sep, 3J(1H,1H) = 6.2 Hz, CH(CH3)2), 4.42 (sep, 3J(1H,1H) = 6.20 Hz, CH(CH3)2), 3.82 br, ∆1/2 = 40 Hz, CH(CH3)2), 1.22 (d, 3J(1H,1H) = 6.20 Hz, CH(CH3)2), 1.17 (d, 3J(1H,1H) = 6.20 Hz, CH(CH3)2) ppm 19F(NMR (376.54 MHz, CD2Cl2 = 5.32 ppm, r.t.) δ = −76.55 (d, 3J(1H,19F) = 6.45 Hz, CH(CF3)2), −76.27 (d, 3J(1H,19F) = 6.30 Hz, CH(CF3)2) ppm 11B(NMR (128.39 MHz, CD2Cl2 = 5.32 ppm, r.t.) δ = 18.7 (B3O3(OiPr)3), 18.4 (br, unknown), 17.4 (br, B-Ohfp) ppm

Conversion of trimethoxyboroxine with hexafluoro-2-propanol

Several experiments of the conversion of tris(methyl)metaborate (trimethoxyboroxine) with lithium hexafuoro-2-propanol were undertaken. Terefore a mixture of the educts (see table below) was stirred and refuxed for three hours and a cryostat refux condenser (−20 ℃) was used. Ten the solvent and volatile starting materials were removed under the vacuum. Te obtained solid was colourless and a NMR sample (in CDCl3) was taken. A exact assignment of all the NMR signals in the 1HZNMR or 19FZNMR was not possible or useful, because of the many broad signals of the respective chemical groups (–OMe, –Ohfp). Te NMR signals were compared with the blind NMR spectra of the starting materials. Te observed 11BZNMR resonances are listed in Table 5.

Exp. no. Educt V [mL] m [g] n [mol] eq. solvent [mL] MR-005.1 B2O3(OMe)3 10.0 12.19 0.071 1 50 HOhfp 22.0 35.64 0.21 3 MR-005.2 B2O3(OMe)3 2.30 2.80 0.016 1 20 HOhfp 5.1 8.26 0.05 3.1 MR-005.2 B2O3(OMe)3 3.0 3.66 0.021 1 30 HOhfp 8.0 13.0 0.077 3.7 Experimental Section 33

Conversion of trimethoxyboroxine with lithium hexafluoro-2-propanolat

MR-009

Lithium hexafuoro-2-propanolat (1.37 g, 0.0079 mol, 3 eq.) was suspended in acetonitrile (40 mL). At 50 ℃ and under stirring trimethoxyboroxine (0.37 mL, 0.45 g, 2.61 mmol, 1 eq.) was dropwise added. Afer addition the reaction mixture was stirred for one hour. Ten a NMR sample were taken. Te mixture became yellowish over the weekend. Afer removal of the solvent, the concentrated residue was viscous and yellow.

1H(NMR (400.17 MHz, CD3CN = 1.94 ppm, r.t.) δ = 5.33 (br, CH(CF3)2), 5.11 (br, CH(CF3)2), 4.06 (br, OCH3), 3.81 (br, OCH3), 3.75 (br, OCH3), 3.70 (br, OCH3) ppm 19F(NMR (376.54 MHz, CD3CN = 1.94 ppm, r.t.) δ = −74.2 (d, 3J(1H,19F) = 6.2 Hz, CH(CF3)2), −74.6 (d, 3J(1H,19F) = 6.80 Hz, CH(CF3)2), −74.6 (br, ∆1/2 = 40 Hz, CH(CF3)2) ppm 11B(NMR (128.39 MHz, CD3CN = 1.94 ppm, r.t.) δ = 3.6 (s, [B(OX)4]–), 3.0 (s, [B(OX)4]–), 2.7 (s, [B(OX)4]–), 2.0 (s, [B(OX)4]–) ppm 7Li(NMR (155.52 MHz, CD3CN = 1.94 ppm, r.t.) δ = 0.62 (s), 0.23 (s), −0.69 (s) ppm

3.6.5.!Fluorination of trimethoxy boroxine

MR-013

Trimethoxyboroxine (1.77 mL, 0.12 g, 0.014 mol, 1 eq.) was dissolved in acetonitrile (15 mL). Te solution was cooled with an ice bath to 0 ℃. Ten Pyr⋅(HF)x (65 % HF in pyridine, 1 mL, 1.14 g, 42 mmol, 3 eq.) was added due to a syringe. Te reaction mixture was stirred over night. A NMR sample was taken from the clear mixture.

1H(NMR (400.17 MHz, CD3CN = 1.94 ppm, r.t.) δ = 3.51 (s, B3O3(OCH3)3), 3.48 (s, [BFx(OCH3)y]−), 3.42 (s, B(OCH3)3) ppm 19F(NMR (376.54 MHz, CD3CN = 1.94 ppm, r.t.) δ = −149.1 (s, [BF4]–), −149.1 (s, [BFxOMey]−) ppm 11B(NMR (128.39 MHz, CD3CN = 1.94 ppm, r.t.) δ = 19.2 (br, B3O3(OMe)3), 18.5 (s, B(OMe)3), –0.7 (s, [BFxOMey]−), −1.2 (s, [BF4]−) ppm 34 Lithium boroxinates

MR-016 Trimethoxyboroxine (2.4 mL, 2.9 g, 0.17 mol, 1 eq.) was dissolved in toluene (35 mL). Ten potassium bifuoride (4.00 g, 51 mmol, 3 eq.) was added. Te reaction mixture was stirred and refuxed for 72 h. Te reaction mixture was concentrated to the half of the volume and a NMR sample was taken. Afer this the solvent was removed due to distillation and a colourless powder was obtained (2 g).

Concentrated reaction mixture: 1H(NMR (400.17 MHz, CD3CN = 1.94 ppm, r.t.) δ = 3.61 (s, B3O3(OCH3)3), 3.47 (s, B(OCH3)3) ppm 19F(NMR (376.54 MHz, CD3CN = 1.94 ppm, r.t.) δ = −152.0 (s, [BF4]–) ppm 11B(NMR (128.39 MHz, CD3CN = 1.94 ppm, r.t.) δ = 19.2 (br, B3O3(OMe)3), 18.5 (s, B(OMe)3), −1.2 (s, [BF4]−) ppm

Powder: ATR-IR ν [cm−1] = 576 (w), 775 (vs), 918 (m), 1054 (m), 1194 (vs), 1392 (m), 1836 (w) Raman ν [cm−1] = 305 (vw), 331 (vw), 522 (vw), 589 (vw), 811 (w), 1003 (vs) References 35

3.7!References

[112]kH. S. Lee, X. Q. Yang, C. L. Xiang, J. McBreen, L. S. Choi, J. Electrochem. Soc. 1998, 145, 2813-2818. [113]kL. F. Li, H. S. Lee, H. Li, X. Q. Yang, K. W. Nam, W. S. Yoon, J. McBreen, X. J. Huang, J. Power Sources 2008, 184, 517-521. [114]kN. G. Nair, M. Blanco, W. West, F. C. Weise, S. Greenbaum, V. P. Reddy, J. Phys. Chem. A 2009, 113, 5918-5926. [115]kY. Qin, Z. Chen, H. S. Lee, X. Q. Yang, K. Amine, J. Phys. Chem. C 2010, 114, 15202- 15206. [116]kM. A. Mehta, T. Fujinami, Chem. Let. 1997, 915-916. [117]kM. A. Mehta, T. Fujinami, T. Inoue, J. Power Sources 1999, 81-82, 724-728. [118]kM. A. Mehta, T. Fujinami, S. Inoue, K. Matsushita, T. Miwa, T. Inoue, Electrochimica Acta 2000, 45, 1175-1180. [119]kH. B. Schmidbaur, Benno; Steigelmann Oliver Beruda Holger, Chemische Berichte 1992, 125, 2705-2710. [120]kM. Finze, G. J. Reiss, Eur. J. Inorg. Chem. 2008, 2321-2325. [121]kG. Cakmak, J. Nuss, M. Jansen, Z. Anorg. Allg. Chem. 2009, 635, 631-636. [122]kM. Aoki, H. Mimura, K. Kono, H. Eguchi, Tosoh Finechem Corporation, Japan 2012, 14. [123]kH. S. Lee, X. Q. Yang, C. L. Xiang, J. McBreen, L. S. Choi, J. Electrochem. Soc. 1998, 145, 2813-2818. [124]kM. A. Becket, M. P. Rugen-Hankey, G. C. Strickland, K. S. Varma, Phosphorus, Sulfur Silicon Relat. Elem. 2001, 168-169, 437-440. [125]kP. Baumgartner, W. Bruns, Ber. 1939, 72B, 1753. [126]kP. Baumgartner, W. Bruns, Ber. 1941, 74B, 1232. [127]kJ. Goubeau, H. Keller, Z. Anorg. Allg. Chem. 1951, 267, 1-26. [128]kH. D. Fisher, W. J. Lehmann, I. Shapiro, J. Phys. Chem. 1961, 65, 1166-1168. [129]kE. M. Magee, J. Inorg. Nucl. Chem. 1961, 22, 155-156. [130]kI. G. Ryss, Dokl. Akad. Nauk SSSR 1954, 97, 691-693. [131]kG. Cakmak, Dissertation thesis, Max-Planck-Institut für Festkörperforschung (Stutgart), 2009. [132]kM. Rohde, L. O. Müller, D. Himmel, H. Scherer, I. Krossing, Chem. – Eur. J. 2014, 20, 1218-1222. [133]kD. R. Lide, in CRC Handbook of Chemistry and Physics, 89th ed., CRC Press/Taylor and Francis, Boca Raton, FL, 2009.

4!Lithium bis(trifluoroacetyl)phosphate

4.1!Introduction and Overview Te idea to use lithium bis(trifuoroacetyl)phosphate (Figure 5) as a new phosphorus based salt in electrolytes for lithium-ion bateries was born 2011 in our laboratory. With a molecular weight of 295.94 g mol−1 the new salt is litle heavier than lithium bis(trifuoromethylsulfonyl)imide (M[LiNTf2] = 280.15 g mol) and just about the double of the molecular weight of lithium hexafuorophosphate (M[LiPF6] = 144.96 g mol−1). Te principle idea to introduce two trifuoroacetyl groups to the phosphate is, that they are free mov- ing and should be able to chelate the lithium cation due to the carbonyl oxygen’s. Tis should prevent a strong coordination to the oxygen’s at the phosphorus atom to the lithium cation. As well, the trifuoroacetyl groups are strongly electron withdrawing

F3C O Li O P O O

F3C O

Figure 5! Lewis formula of Li[PO2(OAcF)2]. Figure 6! Electrostatic potential (calculated at the RI-BP86/def2-TZVP level) of Li[PO2(OAcF)2] projected on an isomap of electron density (0.01 e Å−1). groups, which reduce the negative charge at the oxygen. Calculations of the electrostatic potential of the new phosphate anion visualize the charge distribution (Figure 6). It can be seen that there is an evenly distributed negative charge all over the anion, a re- quirement for a weakly coordinating anion. Furthermore the molecular orbital energies (MO) have been calculated (Table 1). Te large HOMO-LUMO gap of 3.96 eV indicates good electrochemical stability, and the energies are slightly beter with that of lithium bis(oxalato)borate. On the other side, the trifuoroacetate group should be thermally stable. For instance, the thermal decomposition of the lithium trifuoroacetate begins at 250 ℃ and indicates a high thermal stability [134]. Te introduction of the trifuoroacetyl groups as ligand in lithium phosphates is neither published nor patented, which

Table 6!Comparison of molecular orbital energies from bis(trifluoroacetyl)phosphate and bis(oxalato)borate (RI-BP86/def2-TZVPP).

MO [PO2(OAcF)2]– [BOB]– HOMO −2.9147 eV −2.9576 eV LUMO +1.0526 eV +0.6044 eV Gap +3.9673 eV +3.5620 eV 38 Lithium bis(trifluoroacetyl)phosphate

makes the lithium bis(trifuoroacetyl)phosphate more atractive for the use as conducting salt. To the best of my knowledge only four anions are known, which contain a negatively single charged central atom of main group elements with several trifuoroacetate groups: Harris & Milne reported 1971 the frst synthesis of Cs[B(OAcF)4] and H[B(OAcF)4] [135]. One year later Sartori et al. mentioned Cs[Al(OAcF)4] as the complex of Al(OAcF)3 with CsOAcF [136]. Only Na[Bi(OAcF)4] and Na[AsO(OAcF)2] were published from Radheshwar in 1972 [137], whereas all tris(trifuoracetyl) compounds of Al, Ga, In [136], P, As, Sb and Bi [138] are known for the trivalent state. Related compounds of the novel lithium bis(trifuoroacetyl)phosphate are the salts of the acetylphosphates, which were investigated from chemists in the 1950s and 1960s. Tey analysed the dehydration of alkali dihydrogenphosphates in acetic anhydride [139][140][141][142][143]. One of the scientists reported in 1982 the synthesis of the non-fuorinated Li[PO2(OAc)2] and the 31PVNMR shif in acetic acid/acetanhydride [144]. Te only reference of a bis(trifuoroacetyl)phosphoric acid was published by Smyth & Corby in 1998 [145]. Tey characterized via heteronuclear NMR the reaction of H3PO4 in a excess of trifuoroacetic anhydride, while investigating alternative and clean Friedel-Crafs acetylations. Tey reported that the “reaction of acyl trifuoroacetates with phosphoric acid in the presence of trifuoroacetic anhydride (TFAA) leads to the ready formation of acyl bis(trifuoroacetyl)phosphates, which are powerful acylating agents.” But the bis(trifuoroacetyl) phosphoric acid was never isolated, but investigated in-situ. Here in the section the atempts to synthesize and isolate the new lithium salt are described. According to the requirements of a new conducting salt for lithium-ion bateries, which should be afordable, the new lithium bis(trifuoroacetyl)phosphate should be easily synthesised from cheap, available and highly purely precursors, like phosphorus pentoxid (P4O10), lithium dihydrogenphosphate (LiH2PO4), phosphoryl chloride (POCl3), lithium difuorophosphate (Li[PO2F2]), trifuoroacetic anhydride (TFAA) and lithium trifuoroacetate (LiOAcF) which opens up a number of diferent synthesis routes to obtain the desired lithium bis(trifuoroacetyl)phosphate (Li[PO2(OAcF)2]). All reactions were investigated and are described in the following subchapters. Syntheses from phosphorus pentoxide 39

4.2!Syntheses from phosphorus pentoxide

Te f rst synthesis of lithium bis(trifuoroacetyl)phosphate and characterization via heteronuclear magnetic resonance in solution was achieved through the reaction of phosphorus pentoxide (P4O10), lithium trifuoroacetate (LiOAcF) and trifuoroacetic anhydride (TFAA) in various carbonate-based solvents like ethylene carbonate/dimethyl carbonate (1:1), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) under refux for several hours (Eq.13). Trough the similar solubility of the lithium trifuoroacetate and lithium bis(trifuoroacetyl)phosphate in various solvents an easy and practical isolation of the new lithium salt could not be achieved. So the product could only characterized via NMR of the reaction mixture.

F F P4O10 + 4 LiOAc + 2 TFAA carbonate, reflux 4 Li[PO2(OAc )2] Eq.13

4.2.1.!NMR characterisation

In the 19FVNMR (Figure 7) a broad resonance from lithium trifuoroacetate and trifluoroacetic anhydride can be seen at −76.53 ppm. At −77.24 ppm is the signal from the Li[PO2(OAcF)2], which is split to a doublet due to the coupling constant 4J(19F,31P) = 1.3 Hz and that disappears in a 31P decoupled 19FVNMR spectrum (Figure 8). In the 31PVNMR spectrum there is one signal at −21.99 ppm with a half width of 5.7 Hz; therefore the septet is not resolved (Figure 9). But the half width decreases to 2.7 Hz in the 19FVdecou- pled 31PVNMR spectrum (Figure 10). From this follows that the new phosphorus com-

LiOTfa/TFAA LiOTfa/TFAA

−77.2 ppm −77.2 ppm Li[PO2(OAcf)2] Li[PO2(OAcf)2]

−80 −100 −120 ppm −80 −100 −120 ppm Figure 7!19F+NMR spectrum of reaction mix- Figure 8!19F{31P}+NMR spectrum of reaction ture from P4O10, LiOAcF and TFAA in EMC mixture from P4O10, LiOAcF and TFAA in EMC (376.54 MHz in EMC at r.t., external toluene-D8 (376.54 MHz in EMC at r.t., external toluene-D8 lock). lock). 40 Lithium bis(trifluoroacetyl)phosphate

pound contains fuorine, which come from the trifuoroacetate groups. However from a synthesis in another solvent ethylene carbonate/dimethyl carbonate (1:1) a 31PVNMR spectrum could be measured with a signal which is split to a septet due to the coupling 4J(19F,31P) = 1.1 Hz. Tis made clear that two trifuoroacetyl groups a tethered to the phosphorus atom. All experiments indicated only a small yield of 8% (via NMR). Induced by the similar solubility of the lithium trifuoroacetate and lithium bis(trifuoroacetyl) phosphate, an easy and practical isolation of the new lithium salt in all tested solvent combinations could not be achieved. So there was a need for an alternative way of the synthesis of lithium bis(trifuoroacetyl)phosphate.

−23.3 −23.4 ppm −23.2 −23.3 ppm

Figure 9!31P+NMR spectrum of reaction mix- Figure 10!31P{1H}+NMR spectrum of reaction ture from P4O10, LiOAcF and TFAA in EC and mixture from P4O10, LiOAcF and TFAA in EC DMC (188.31 MHz at r.t., external toluene-D8 and DMC (188.31 MHz at r.t., external tolu- lock). ene-D8 lock). Syntheses from phosphoryl chloride 41

4.3!Syntheses from phosphoryl chloride

4.3.1.!Overview and thermodynamic considerations

Te next strategy to synthesize Li[PO2(OAcF)2] based on the preparation of a neutral precursor compound bis(trifuoroacetyl)phosphoryl chloride, POCl(OAcF)2. Tis precursor should be available through the reaction between phosphoryl chloride and lithium trifuoroacetate or trifuoroacetic acid (Eq.14, Eq.15). In the next step POCl(OAcF)2 should be hydrolysed to bis(trifuoroacetyl)phosphoric acid, H[PO2OAcF)2] (Eq.16). In the last step H[PO2(OAcF)2] could be deprotonated with lithium hydride to the Li[PO2(OAcF)2] (Eq.17). Te idea of this route is to avoid diferent lithium salts, which cannot be isolated from each other through their similar solubility. But to isolate and purify a neutral precursor could be easily done by e. g. distillation.

F F POCl3 + 2 LiOAc POCl(OAc )2 + 2 LiCl Eq.14

F F POCl3 + 2 HOAc POCl(OAc )2 + 2 HCl Eq.15

F F POCl(OAc )2 + H2O H[PO2(OAc )2] + HCl Eq.16

F F H[PO2(OAc )2] + LiH Li[PO2(OAc )2] + H2 Eq.17

To assess the frst reaction step thermodynamic calculations have been performed. It can been seen in the Born-Fajans-Haber cycle (Scheme 4) that ∆rG°(g) of reaction between POCl3 and TFA to forming POCl(OAcF)2 is with 50 kJ mol−1 endergonic in the gas phase. And under considerations of Gibbs standard solvation energies (∆solvG°, ε = 89.6) the molecules, the Gibbs free energy of the reaction in solution is more endergonic (∆rG°(solv) = 77 kJ mol−1). Tis is in accordance to the experiments in which no reaction was observed with NMR, and respectively no evolution of HCl occurred. In the case of the reaction between phosphoryl chloride and lithium trifuoroacetate the new lithium salt is formed under elimination of lithium chloride. As can be seen in the Born-Fajans-Haber cycle (Scheme 5) the free reaction enthalpy in gaseous phase is with 275 kJ mol−1 endergonic. Tereby the Gibbs latice energies of the salts (∆latG°) or

Gibbs solvation energies (∆solvG°) are not taken into account. Te applicable Gibbs latice

ΔrG°(g) = 50 F F POCl3(g) + 2 HOAc (g) POCl(OAc )2 (g) + 2 HCl (g)

−4 +40 −18

ΔrG°(solv) = 77 POCl + 2 HOAcF F 3(solv) (solv) POCl(OAc )2 (solv) + 2 HCl (g) Scheme 4!Born-Fajans-Haber cycle for calculation of formation of POCl(OAcF)2 from POCl3 with

TFA (∆solvG° in EC (ε = 89.6) on the vertical arrows; all values calculated at the RI-BP86/def-SV(P) level and in kJ mol−1). 42 Lithium bis(trifluoroacetyl)phosphate

ΔrG°(g) = +275 + F – F + – POCl3(g) + 2 Li (g) + 2 [OAc ] (g) POCl(OAc )2 (g) + 2 Li (g) + 2 Cl (g)

−4 +510 −18 −712

Δ G° = −143 F r (solv) F POCl3(solv) + 2 LiOAc (s) POCl(OAc )2 (solv) + 2 LiCl (s) Scheme 5!Born-Fajans-Haber cycle for calculation of formation of POCl(OAcF)2 from POCl3 F with LiOAc (∆solvG° in EC (ε = 89.6) or ∆latG° on the vertical arrows; all values calculated at the RI-BP86/def-SV(P) level and in kJ mol−1).

F energy ∆latG°[LiOAc ] of +255 kJ mol−1 is over compensated by the Gibbs latice energy of LiCl, which amounts to −356 kJ mol−1. Terefore the formation of LiCl is the driving force of the reaction. Te free reaction enthalpy in solution is −143 kJ mol−1, hence exergonic. Te standard synthesis was performed in solution, so NMR spectroscopy is the most important analysis in the beginning. Due to the absence of chemical shifs in the literature for this class of compounds, like bis(trifuoroacetyl)phosphoryl chloride, values were calculated on the basis of reference structures. Tus it was possible to limit the range of chemical shif in 31PVNMR. As reference for the 31P shif served POCl3 (δ = 2.09 ppm). To compute the isotropic shielding coefcients σ the program MPSHIFT, which computes the NMR properties using the GIAO [147] method, have been used. Te chemical shif of the δsubs can be calculated according to the following equation (Eq.18).

δsubs = δref + σref − σsubs Eq.18

On the basis of the values, shown in the Table 7, it can be seen the tendency of the chemical shif to go up feld upon substitution of chloride with trifuoroacetate groups. Tis is in accordance with the 31PVNMR shifs of trifuoroacetylphosphoric acids[148]

Table 7!Calculated 31P+NMR shifs of (trifluoroacetyl)phosphoryl chlorides (RI-BP86/def-SV(P); reference is POCl3 = 2.09 ppm) and known [145] chemical shifs of several (trifluoroacetyl)phosphoric acids and lithium bis(acetyl)phosphate [144].

Compound exp. / calc. δ [ppm] Solvent POCl2(OAcF) calc. −7.2 — POCl(OAcF)2 calc. −26.0 — PO(OAcF)3 calc. −55.0 — PO(OH)2(OAcF)a exp. −6.13 CDCl3 PO(OH)(OAcF)2a exp. −18.57 CDCl3 PO(OAcF)3a exp. –24.8 CDCl3 Li[PO2(OAc)2]b exp. −19.2 HOAc/Ac2O Syntheses from phosphoryl chloride 43

4.3.2.!Syntheses from POCl3 with LiOAcF in dimethyl carbonate

Te frst atempts to synthesize Li[PO2(OAcF)2] through the conversion of phosphoryl chloride with lithium trifuoroacetate were carried out in dimethyl carbonate. For this, phosphoryl chloride and two equivalent lithium trifuoroacetate were refuxed several hours in dimethyl carbonate. Afer the initial experiments, we atempted to optimize the reaction by activation with trifuoroacetic anhydride (TFAA) and various catalysts, like CaCl2, or AlCl3 (Eq.19). To control the reaction progress NMR samples were taken during the reaction from the reaction mixture, or afer termination of the reaction and afer treatment of the reaction mixture. Despite the meticulous anhydrous working conditions maintained throughout, signals of acidic protons were found in all 1HVNMR spectra.

(activator, catalyst) POCl + 2 LiOAcF POCl(OAcF) + 2 LiCl Eq.19 3 DMC 2

At the beginning POCl3 was refuxed with two equivalents of lithium trifuoroacetate in dimethyl carbonate. In the 31PVNMR spectrum only the signal of POCl3 itself is visible at −4.4 ppm. In the 19FVNMR spectrum the resonance of the −CF3 group from the lithium trifuoroacetate appears at −77.4 ppm. Tus, these are the reactant signals and obviously no reaction occurred. Terefore, in the following catalysts were used in order to lower the activation energy.

0.03 eq CaCl2 POCl + 2 LiOAcF POCl(OAcF) + 2 LiCl Eq.20 3 DMC 2

POCl3

LiOAcF

POCl2OAcF

F −77.5 ppm POCl2OAc TFA

−76 −78 −80 −82 ppm −4 −6 −8 −10 −12 ppm

Figure 11!19F+NMR spectrum of (LM-004) Figure 12!31P+NMR spectrum of reaction from POCl3 with 2 eq. LiOAcF and cat. CaCl2 (LM-004) from POCl3 with 2 eq. LiOAcF and (188.31 MHz in DMC at r.t., toluene-D8 external cat. CaCl2 (81.01 MHz in DMC at r.t., tolu- lock). ene-D8 external lock). 44 Lithium bis(trifluoroacetyl)phosphate

POCl3 LiOAcF

TFA

−77 −78 ppm POCl2OAcF POCl(OAc F )2

0 −5 −10 −15 −20 −25 ppm −50 −100 −150 ppm

Figure 13!31P+NMR spectrum of reaction (LM- Figure 14!19F+NMR spectrum of (LM-005) 005) from POCl3 with 2 eq. LiOAcF and cat. from POCl3 with 2 eq. LiOAcF and cat. AlCl3 AlCl3 (81.01 MHz in DMC at r.t., toluene-D8 ex- (188.31 MHz in DMC at r.t., toluene-D8 external ternal lock). lock). As in the previous experiment no reaction of phosphoryl chloride with lithium trifuoroacetate in dimethyl carbonate to the intermediate compound POCl(OAcF)2 was observed. So it was atempted to add small amount of the Lewis acid CaCl2 (3 mol-% based on POCl3) to catalyse the reaction (Eq.20). In the 31PVNMR spectrum (Figure 12) a main signal at −4.4 ppm and a weak one at −11.8 ppm was observed. Te signal at −11.8 ppm is presumably the monosubstituted phosphorus compound POCl2OAcF, wherein the chlo- rine atoms may have been replaced by oxygen or a hydroxyl group. Te deviation of the calculated shif is 4.6 ppm. By integrating the signals in the 31PVNMR spectrum of a yield of 4.8 % of the monosubstituted product could be determined in solution. Te 19FVNMR spectrum (Figure 11) shows a singlet as the main signal at −77.4 ppm, which is assigned to the −CF3 group of lithium trifuoroacetate. A singlet of low intensity appeared at −76.7 ppm. In addition, a very weak signal at −77.5 ppm was found in the spectrum. Tis signal is the expected doublet of the monosubstituted phosphorus compound POCl2OAcF. Te 4J(19F,31P) coupling constant is 1.3 Hz. Te presence of the 19F,31P-coupling demonstrates that a trifuoroacetyl group is atached to the phosphorus.

0.006 eq AlCl3 POCl + 2 LiOAcF POCl(OAcF) + 2 LiCl Eq.21 3 DMC 2

Te addition of the much more acidic Lewis acid, aluminium trichloride AlCl3 (0.006 eq), to the suspension of phosphoryl chloride and lithium trifuoroacetate in dimethyl carbonate (Eq.21) resulted in a greater conversion. Te 31PVNMR spectrum (Figure 13) showed three resonances. One singlet at −3.6 ppm, which can assigned to the reactant POCl3. Trough integration of the signal intensity the amount of POCl3 in solution was determined as 86.5 %. Further up feld at −11.5 ppm can be seen a weaker broad signal Syntheses from phosphoryl chloride 45

(∆1/2 = 6.2 Hz). It is probably the signal of the monosubstituted phosphorus compound POX2OAcF (X = −Cl, −O−, −OH) Te proportion of the compound in the solution amounts 13.5 %. Te conversion increased to about 9 % through the use of AlCl3. Unfortunately, due to the signal width a coupling to fuorine was not resolved. On closer inspection of the 31PVNMR spectrum a very weak signal can be observed at −22.6 ppm. According to the calculations, the signal is in the range of the expected bis(trifuoroacetyl)phosphoryl chloride. But due to the low intensity and undetectable coupling, there is no certain- ty. In the 19FVNMR spectrum (Figure 14) at −77.5 ppm the resonance of LiOAcF is visible. Tis signal also is greatly broadened (∆1/2 = 6.1 Hz). As a result the expected doublets of the phosphorous compounds are superimposed.

TFAA POCl + 2 LiOAcF POCl(OAcF) + 2 LiCl Eq.22 3 DMC 2

In the next experiment trifuoroacetic anhydride, TFAA, was used as an activator for the acetylation of phosphoryl chloride (Eq.22). For this the stoichiometric mixture of POCl3 and LiOAcF four equivalents of TFAA were added and then refuxed. Afer 4 h refuxing a NMR sample was taken. Te 31PVNMR showed four signals at 3.4, −5.3, −12.5 and −23.2 ppm (Figure 15). Te half-width of the signal at 3.4 ppm is 30 Hz. Tis is the resonance of phosphoric acid, due to the insufcient drying of the trifuoroacetic anhydride batch. Te water content of the trifuoroacetic anhydride batch was 270 ppm. Te singlet at −5.3 ppm is phosphoryl chloride. Te proportion in the solution is according to the integration of the NMR signals 8.4 %. Te main signal at −12.5 ppm is in the region of the mono-substituted compound (49.6 %). Te signal is split by the coupling constant 4J(19F,31P) = 1 Hz to a quartet. Te coupling disappears in a 19F decoupled NMR spectrum. Te signal at −23.2 ppm is in the range of the expected bis(trifuoroacetyl)phosphoryl

H3PO4 POCl2OAcF

4 3 ppm −12.5 ppm

POCl(OAc F )2 POCl3

−5 ppm −23.2 ppm

20 10 0 −10 −20 −30 −40 −50 −60 ppm Figure 15!31P+NMR spectrum of reaction (LM-007) from POCl3 with 2 eq. LiOAcF and 4 eq. TFAA (161.99 MHz in DMC at r.t., toluene-D8 external lock). 46 Lithium bis(trifluoroacetyl)phosphate

F LiOAc POCl2OAcF POCl(OAc F )2

−77.6 −77.7 ppm TFAA

−75.5 −76.0 −76.5 −77.0 −77.5 −78.0 −78.5 ppm Figure 16!19F+NMR spectrum of reaction (LM-007) from POCl3 with 2 eq. LiOAcF and 4 eq. TFAA (376.54 MHz in DMC at r.t., toluene-D8 external lock).

ppm

−25

−20

−15

−10

−5

5 −77.0 −77.5 −78.0 −78.5 ppm Figure 17!19F,31P+COSY+NMR spectrum of reaction (LM-007) from POCl3 with 2 eq. LiOAcF and 4 eq. TFAA (376.54 MHz in DMC at r.t., toluene-D8 external lock). Syntheses from phosphoryl chloride 47 chloride. Te signal shape reveals a spliting due to the 4J(19F,31P) = 1.3 Hz. And it can seen that the multiplet has an odd number of lines, what goes well with an expected septet. Since even here the coupling disappears in a 19F decoupled measurement, it becomes clear that this compound contains two trifuoroacetyl groups bound to the phosphorus. By integrating the signals in the 31PVNMR spectrum, the proportion of the compounds could be determined in the solution as 28.6 %. In addition to the singlet of lithium trifuoroacetate at −76.8 ppm and trifuoroacetic anhydride at −76.9 ppm in the 19FVNMR spectrum (Figure 16) two signals are split into doublets due to a 4J(19F,31P) coupling constants of 1.3 Hz, one at −77.6 ppm and the other at −77.7 ppm. In the 2D-19F,31PVNMR correlation spectrum (Figure 17) the doublet at −77.6 ppm in the 19F axis shows a cross peak to the disubstituted product at −23.2 ppm in the 31P spectrum axis, and the second doublet in the 19FVNMR at −77.7 ppm correlates to the 31PVNMR signal at −12.5 ppm. Otherwise, no further phosphorus compound is shown with fuorine-containing substituent in the 2D-correlation spectrum.

TFAA, cat. AlCl3 POCl + 2 LiOAcF POCl(OAcF) + 2 LiCl Eq.23 3 DMC 2

As could be seen from the previous experiments, the use of a catalyst, the reaction was promoted. Terefore, it made sense to start several atempts (LM-006, LM-009, LM-010, LM-020) to use the catalyst AlCl3 and TFAA together in the synthesis of bis(trifuoroacetyl)phosphoryl chloride (Eq.23). For further discussion only selected NMR spectra are shown, because the diferent experiments showed only marginal diferences. As can be seen in the 31PVNMR spectrum (Figure 18) of the above described reaction there is one sharp signal at −23.3 ppm on a very broad peak in the noise from −17 to −27 ppm.

F F POCl(OAc )2 TFAA POCl(OAc )2

LiOAcF

−21.5 ppm

0 −5 −10 −15 −20 ppm −76.0 −76.5 −77.0 ppm

Figure 18!31P+NMR spectrum of reaction Figure 19!19F+NMR spectrum of (LM-020) (LM-020) from POCl3 with 2 eq. LiOAcF, 4 eq. from POCl3 with 2 eq. LiOAcF, 4 eq. TFAA and TFAA and cat. AlCl3 (161.99 MHz in DMC at r.t., cat. AlCl3 (376.54 MHz in DMC at r.t., tolu- toluene-D8 external lock). ene-D8 external lock). 48 Lithium bis(trifluoroacetyl)phosphate

At closer inspection there is a low intensity resonance a bit down feld at −22.2 ppm. Te main signal is broadened and thus the multiplet is not fully resolved. But it is seen that it has an odd number of lines. It looks that this septet is split by the coupling 4J(19F,31P) = 1.2 Hz. According to this and the chemical shif, the resonance is assigned to the desired disubstituted product, POCl(OAcF)2. Te small signal can be suspected to be the hydrolysed product H[PO2(OAcF)2], because the signal does not occur in the experiment with absolutely dried trifuoroacetic anhydride (LM-020). In the 19FVNMR spectrum (Figure 19) the signals of several −CF3 groups are evident. Te resonance of lithium trifuoroacetate at −76.83 ppm, at −76.78 ppm presumably trifuoroacetic acid and at −77.5 ppm the doublet of POCl(OAcF)2 are visible. Te 4J(19F,31P) coupling is 1.2 Hz. With the use of AlCl3 as catalyst and TFAA as acetylating agent [145] the yield could be increased. Nevertheless, the yield here was less than 10 % (via 19FVNMR integration) and an isolation of bis(trifuoroacetyl)phosphoryl chloride from the starting materials was necessary. All volatile components, like trifuoroacetic anhydride or trifuoroacetic acid and the solvent, could be removed under vacuum or by distillation. Afer removal of all volatile components the remaining solid residue was atempted to be dissolved in dimethyl carbonate, methylene chloride or acetonitrile, but the residue was only moderately soluble in tetrahydrofuran. Heteronuclear NMR measurements of this solution showed signals of the product, but the largest part of the phosphorus compounds were not in solution. It can be assumed that chemical reactions occurred during the work-up. It is most likely, that of condensation reactions of phosphates, due to moisture traces took

Figure 20!ATR-IR spectrum of solid residue of LM-010, LiOAcF powder, dimethyl carbonate and the calculated (RI-BP86/def-SV(P)) IR spectrum of POCl(OAcF)2. Syntheses from phosphoryl chloride 49

Table 8!Comparison of the experimental ATR-IR bands of LM-010 (residue #3.2) with the calculated vibration bands of POCl(OAcF)2 at the RI-BP86/def-SV(P) level. The experimental ATR-IR spectra of LiOAcF and DMC were added but not assigned. w: weak, m: medium, s: strong, sh; shoulder, v: very.

cal. IR band exp. IR band exp. IR band exp. IR band [cm−1] [cm−1] assignment [cm−1] [cm−1] POCl(OAcF)2 residue LM-010 LiOAcF DMC 405 (w) — δ(PO2) — — 488 (vw) 462 (w) δ(CF2) 460 (w) — 526 (w) 522 (w) δ(PClO3) 519 (w) — — — — — — 551 (m) 583 (w) ν(P–Cl) — — — 607 (vw) — 611 (w) — 673 (w), 692 (w) — δ(C–O–P) — — 701 (w) 729 (w) δsymm(C–C–O2) 720 (w) — 722 (m) — ν(O–P–O) — — 801 (w) 802 (m) δ(C–C–O) 803 (m) 792 (w) 815 (m) — δ(C–O–P) — — — 855 (w) ν(C–O–P) 863 (w) 914 (m) — 961 (w) — — 966 (m) 982 (vs) 1018 (w) ν(C–O) — — 1046 (s) 1104 (w) ν(C–O) — — — 1143 (sh) — — — 1127 (s) 1150 (s) ν(C–F) 1151 (vs) — 1182 (vs) 1204 (s) ν(C–F) 1200 (s) 1207 (sh) 1226 (vw) — ν(C–C) — 1261 (vs) 1241 (sh) — ν(C–C) — — 1251 (m) 1321 (m) ν(P=O) — — — 1435 (w) — — 1432 (sh) — 1468 (m) — 1474 (m) 1452 (m) — 1669 (sh) ν(C=O) 1663 (vs) 1605 (vw) — 1682 (vs) ν(C=O) — — — 1704 (sh) ν(C=O) 1712 (sh) — 1763 (m) — ν(C=O) — 1750 (vs) 1803 (s) — ν(C=O) — — — 2971 (vw) ν(C–H) — 2856 (vw) — 3024 (vw) ν(C–H) — 2961 (vw) — 3150 (vw) ν(C–H) — 3009 (vw) 50 Lithium bis(trifluoroacetyl)phosphate

F X2OPO(CH2)4OAcF POCl2OAc POCl(OAc F )2

−12.0 ppm −9.5 ppm −22.5 ppm

POCl3 10.773 16.985 61.401 10.701

0 −5 −10 −15 −20 −25 −30 ppm

Figure 21!31P+NMR spectrum of reaction (LM-017) from POCl3 with 2 eq. LiOAcF and 4 eq. TFAA afer 2.5 h (161.99 MHz in THF at r.t., toluene-D8 external lock)

place. Even atempts to precipitate the sparingly soluble salts or compounds have failed. In any of the atempts, lithium trifuoroacetate could be separated from the disubstituted phosphorus compound. By default, ATR-IR measurements were made of the solid residues. One spectrum is shown below (Figure 20). Te colourless solid was obtained afer concentration, fltration and drying at the vacuum of the reaction mixture (LM-010). Te f gure also includes a calculated vibrational Infrared spectrum of bis(trifuoroacetyl)phosphoryl chloride and a reference spectrum of lithium trifuoroacetate and dimethyl carbonate. Te following Table 8 lists the vibrational bands in detail. As apparent from the measured spectrum the solid sample consists mainly of lithium trifuoroacetate. It is also noteworthy that a substantial amount dimethyl carbonate is contained in spite of the vacuum drying of the solid residue. On closer inspection, it is noticeable that two bands do not ft to the starting materials. Te weak band at 583 cm−1 is assigned to the stretching band of P–Cl unit, which match fairly well to the calculation at 551 cm−1. Further- more the stretching vibration of the C–O single bond of the POCl(OAcF)2 at 1018 cm−1 and 1104 cm−1. Te strong stretching band at 1321 cm−1 is mainly due to the band of the solvent, so the P=O double bond is likely superimposed. It can be concluded from the Infrared spectrum that a phosphate species is in the sample. It is safe to assume that is bis(trifuoroacetyl)phosphoryl chloride. But it is obvious that this is not the main Syntheses from phosphoryl chloride 51 product. Te main components of the residue is lithium chloride and lithium trifuoroacetate with traces of dimethyl carbonate, which are probably coordinating to the lithium cation.

4.3.3.!Syntheses in acetonitrile and tetrahydrofuran

Te experiments of the reaction of phosphoryl chloride with lithium trifuoroacetate, catalytic amounts of aluminium chloride and trifuoroacetic anhydride were carried out in dimethyl carbonate, but dimethyl carbonate has the disadvantage that is not a good solvent for salts, like lithium trifuoroacetate. With a dielectric constant ε = 3.12 dimethyl carbonate is inferior to acetonitrile (ε = 37.5) or tetrahydrofuran (ε = 7.58). For that reason the experiments were repeated in the later solvents (Eq.24). Te reaction in acetonitrile led to the same products as in dimethyl carbonate. Te NMR experiments showed the signals of bis(trifuoroacetyl)phosphoryl chloride and another phosphorus species in the same range of the chemical shif like the product, which could not be identifed, because of the broadened signals in the 31PVNMR and 19FVNMR. Also all at- tempts to isolate the phosphorus compound failed. Even the solubility of the neutral product was similar to the solubility of lithium trifuoroacetate or the other phosphorus impurities. Te signal broadening in acetonitrile is probably due to the constant exchange of trifuoroacetyl groups at the phosphorus atom. On the other hand the results in the NMR experiments with tetrahydrofuran as solvent showed sharp signals, which are described in the following.

TFAA, cat. AlCl3 POCl + 2 LiOAcF POCl(OAcF) + 2 LiCl Eq.24 3 tetrahydrofuran 2

In the 31PVNMR spectrum, from the sample which was taken afer 2.5 h (Figure 21), two main signals at −9.8 ppm and −22.6 ppm can be seen, aside from less intensive signals at −5.0, −12.0, −22.7 and −25.1 ppm. Te same signals were observed in a NMR sample taken from the same batch afer 6 h (Figure 23), difering from this the intensities have changed. It was observed that the intensities of the signal at −5.0, and −12.0 ppm have decreased, meanwhile the resonances at −9.8 and −22.6 ppm have increased. It may well be assumed that the signal at −5.0 ppm belongs to POCl3, because of the range of the chemical shif in the 31PVNMR and the decreasing intensity due to consumption during the reaction. Te resonance at −12.0 ppm was identifed from the further experiments as the monosubstituted product, POCl2OAcF. Also the signal at −22.6 ppm, which is the resonance of the product bis(trifuoroacetyl)phosphoryl chloride. For the frst time the line width is so small that the septet can be seen in the 31PVNMR spectrum. Te 4J(19F,31P) coupling constant is 1.3 Hz. Te amount of product in solution afer 6 h was 54.9 %. Te

O 7 1 2 4 P O 6 CF3 R O R 3 5 O R= Cl, OH, O– Figure 22!Main side product of the reaction (LM-017) from POCl3 with 2 eq. LiOAcF, 4 eq. TFAA and cat. AlCl3. which has been carried out in tetrahydrofuran. The THF ring was cleaved. The R group is not a −OAcF group, phosphate or another aliphatic rest. 52 Lithium bis(trifluoroacetyl)phosphate

X2OPO(CH2)4OAcF POCl(OAc F )2

−9 −10 ppm −22.5 ppm

POCl3 POCl2OAcF 1.033 1.220 2.815 3.894 36.070 54.968

5 0 −5 −10 −15 −20 −25 ppm

Figure 23!31P+NMR spectrum of reaction (LM-017) from POCl3 with 2 eq. LiOAcF and 4 eq. TFAA afer 6 h (81.01 MHz in THF at r.t., toluene-D8 external lock).

ppm

−24

−22

−20

−18

−16

−14

−12

−10

−8

−6

−4

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 ppm

Figure 24!1H,31PVHMBCVNMR spectrum (optimized to 7 Hz) of LM-017 afer 2.5 h (400.17 MHz in THF at r.t., toluene-D8 external lock). Syntheses from phosphoryl chloride 53

ppm

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5 4.5 4.0 3.5 3.0 2.5 2.0 1.5 ppm

Figure 25!1HVTOCSYVNMR spectrum (mixing time: 0.2 s) of LM-017 afer 6 h (400.17 MHz in THF at r.t., toluene-D8 external lock).

ppm 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

4.5 4.0 3.5 3.0 2.5 2.0 1.5 ppm

Figure 26!1HVDQFVCOSYVNMR spectrum of LM-017 afer 2.5 h (400.17 MHz in THF at r.t., toluene-D8 external lock). 54 Lithium bis(trifluoroacetyl)phosphate

ppm 20

100

120

140

160

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 ppm

Figure 27!1H,13CVHMBCVNMR spectrum (optimized to 8 Hz) of LM-017 afer 2.5 h (400.17 MHz in THF at r.t., toluene-D8 external lock).

other impurities at −22.7 and −25.1 ppm located in the range of substituted phosphates or metaphosphates, but could not identifed. Teir share in solution was 3.9 % or 2.8 %. Te other main signal (36 %) at −9.8 ppm has not been observed in previous 31PVNMR spectra. It is split to a triplet, due to a coupling constant of 7.3 Hz, which is rather a J(1H,31P) than a J(19F,31P). A proton decoupled 31PVNMR confrmed that is a proton containing compound, because the coupling disappeared. Te only compound in solution, which contains protons is THF. So the triplet is due to a CH2 group originally from tetrahydrofuran. A 1H,31PVNMR correlation spectrum (Figure 24) showed a cross-peak of the 31PVNMR signal to a 1HxNMR signal at 4.08 ppm, which is in the range of OCH2 groups. Te 1HVNMR signal is split to a doublet of triplets, which shows that another CH2 group is connected to the OCH2 group. In a phosphorus decoupled 1HVNMR spectrum only the triplet is observed. Furthermore another cross-peak from the 31PVNMR signal to an up- feld proton resonance at 1.72 ppm can be seen in the 1H,31PVNMR correlation spectrum. Te 1HVNMR signal belongs to the CH2 group, which is connected to the OCH2 group. Tis all indicates that the ring of tetrahydrofuran was opened during the reaction. In the 1HVTOCSY spectrum (Figure 25) of the reaction can be seen that the proton resonance

Table 9!NMR shifs (in ppm) of side product R2POO(CH2)4OAcF.

R2P(=O)− −OCH2− −CH2− −CH2− −CH2O− −C(=O)CF3 (a) (a) (c) (d) 31P+NMR −9.8 ― ― ― ― ― 1H+NMR ― 4.08 1.72 1.85 4.39 ― 13C+NMR ― 66.2 26.1 24.6 67.5 114.6/156.7 19F+NMR ― ― ― ― ― −76.4 Syntheses from lithium dihydrogenphosphate 55 at −4.08 correlates to further signals of protons at −1.72, 1.85 and 4.39 ppm. Tese are the proton signals of the butyl chain. Te down feld resonance at −4.39 ppm is probably the OCH2 end-group of the chain. Te observed cross-peaks in the 1HVDQFVCOSY spectrum (Figure 26) enabled to determine the sequence of the CH2 groups in the chain. First the PO-CH2 appears at −4.08 ppm (a), second the CH2 group with 1HVNMR shif at −1.72 ppm (b), followed by the group at −1.85 ppm (c) and the OCH2 end-group at −4.39 ppm (d). Te 13CVNMR shifs belonging to the butyl chain were assigned from the 1H,13CVHSQC- and 1H,13CVTOCSYVHSQCVNMR spectra: (a) 66.2 ppm, (b) 26.1 ppm, (c) 24.6 ppm and (d) 67.5 ppm. In the 1H,13C HMBC spectrum a long-range cross-peak from the proton signal at −4.39 ppm (d) to a 13CVNMR resonance at 156.7 ppm can bee seen, which belongs to a carbonyl carbon. Te same 13CVNMR resonance showed a cross-peak in the 19F,13CVHMBCVNMR spectrum to a 19FVNMR signal at −76.4 ppm, which is in the range of the trifuoroacetyl groups. Tis makes it clear that a trifuoroacetyl group is at the end of the chain. From the 19F,13CVHMBCVNMR spectrum (Figure 27) the coupling constants of the trifuoroacetyl group were evident: 1J(19F,13C) = 284 Hz and 2J(19F,13C) = 42 Hz. Te side product from the reaction of lithium trifuoroacetate, phosphoryl chloride and trifuoroacetic anhydride in presence of tetrahydrofuran is R2POO(CH2)4OAcF (Table 9). Te R group is certainly not a trifuoroacetyl group, because there is no correlation signal from a 19F to a 31P in the 19F,31PVCOSYVNMR spectrum. Possible substituents are R = Cl, OH, O−, most likely appears Cl.

4.3.4.!Summary

Te experiments have shown that the phosphoryl chloride reacts with lithium trifuoroacetate under formation of the neutral bis(trifuoroacetyl)phosphoryl chloride (POCl(OAcF)2) and lithium chloride. Supplementary calculations reveals that the formation of LiCl is the driving force in this reaction. Using trifuoroacetic anhydride and catalytic amounts of aluminium chloride the yield of the reaction could optimized to near- ly 70 %. Te studies indicated that the neutral precursor POCl(OAcF)2 is highly sensible against water traces and is a reactive acetylating agent [146]. Even the solvent tetrahy-

F F Li[PO (OAcF) ] + 2 HOAcF LiH2PO4 + 2 Ac-O-Ac 2 2 Eq.29 drofuran was atacked with a ring-opening mechanism and insertion into the P−OAcF bond. Te isolation of the bis(trifuoroacetyl)phosphoryl chloride from the by-products was not successful. Terefore it was not possible to proceed with the synthetic route towards lithium bis(trifuoroacetyl)phosphate.

4.4!Syntheses from lithium dihydrogenphosphate

Tis section describes the formation of lithium bis(trifuoroacetyl)phosphate on the route of esterifcation of lithium dihydrogenphosphate with trifuoroacetic anhydride (Eq.29). Both starting materials are cheap and are available in high purity. In addition excess trifuoroacetic anhydride can easily be removed in the vacuum. Te reaction was carried out in a sealed Schlenk-tube, treated in an ultrasonic bath, in several solvents, like trifuoroacetic acid, trifuoroacetic anhydride, dimethyl carbonate and di- 56 Lithium bis(trifluoroacetyl)phosphate

ethyl ether. Te best results were obtained Li[PO2(OAc F )2] in diethyl ether with the advantage that the lithium dihydrogenphosphate is not soluble in this solvent but the product lithium bis(trifuoroacetyl)phosphate. So a separation of the unreacted starting material can be easily achieved

−23.5 ppm 4.4.1.!NMR characterization

Te 31PVNMR spectrum (Figure 28) of the crude product in THF shows one broad signal at −23.5 ppm, which is the 31P resonance of lithium bis(trifuoroacetyl)phosphate. Due to the signal width of 6.5 Hz the septet is not resolved. In the 19FVNMR spectrum (Figure 29) four signals at −76.3, 0 −10 −20 ppm −76.7, −77.0 and −77.1 ppm can be seen, which Figure 28!31P+NMR spectrum of reaction are all in the area of −CF3 groups. Te sig- from LiH2PO4 with trifluoroacetic anhydride nal at −140.0 ppm is ortho-difuorobenzene, (MS-007) in diethyl ether (161.99 MHz in Et2O an internal standard of 1H and 19F for inte- at r.t., external toluene-D8 lock). gration. Te signal at −77.0 ppm is split to a doublet due to the coupling 4J(19F,31P) = 1.3 Hz. Tere is no doubt that this is the product signal of Li[PO2(OAcF)2]. Te signal at −76.3 ppm is R2POO(CH2)4OAcF (R = OH, O−), which has been characterized in the previous section. It is no impurity from the synthesis,

Li[PO2(OAc F )2] o-DFB o-DFB Et2O

acidic proton LiOAcF

TFA ?

−76 −77 ppm 14 12 10 8 ppm

−80 −100 −120 ppm 12 10 8 6 4 2 ppm Figure 29!19F+NMR spectrum of reaction Figure 30!1H+NMR spectrum of reaction from from LiH2PO4 with trifluoroacetic anhydride LiH2PO4 with trifluoroacetic anhydride (MS- (MS-007) in diethyl ether (376.54 MHz in Et2O 007) in diethyl ether (400.17 MHz in Et2O at r.t., at r.t., external toluene-D8 lock). external toluene-D8 lock). Syntheses from lithium dihydrogenphosphate 57

ppm

110

115

120

125

130

135

140

145

150

155

160

−75 −76 −77 −78 −79 −80 ppm Figure 31!19F,13C+HMBC+NMR spectrum of reaction (MS-007) from LiH2PO4 with trifluoroacetic anhydride (376.54 MHz in Et2O at r.t., toluene-D8 external lock). but it is the product of the decomposition from tetrahydrofuran, which was used as NMR solvent, with the lithium bis(trifuoroacetyl)phosphate. In the two-dimensional 19F,13C HMBC NMR spectrum (Figure 31) one can distinguish between the carbonyl car- bons at 150 ppm and the 13C shifs of the −CF3 groups at 115 ppm. It is safe to assume that all fuorine species are acetate groups. Apart from the product resonance at −77.0 ppm there appeared always the same two unidentifed impurities: down-feld at −76.8 ppm and high feld at −77.2 ppm. It is not possible to identify the impurities by the chemical shifs in the 13CVNMR or 19FVNMR, because the chemical shif highly depends on the concentration. But the 1J(19F,13C) coupling constant of the −CF3 group difer sufciently from each other and are in good agreement with the 1J(19F,13C) of the blind spectrum of lithium trifuoroacetate (1J(19F,13C) = 290.1 Hz) or trifuoroacetic acid (1J(19F,13C) = 285.0 Hz). Te 1J(19F,13C) coupling constant of the 19F signal at −76.8 ppm is 289.3 Hz and correlates to a 13C resonance at 114.9 ppm. Terefore it is lithium trifuoroacetate. Te other signal at −77.2 ppm has a 1J(19F,13C) of 285.1 Hz, which is in accordance with trifuoroacetic acid. In the 1HVNMR (Figure 30) a singlet down feld at 13.84 ppm can be seen, which is likely an acidic proton and belongs to the trifuoroacetic acid. Te existence of trifuoroacetic acid and lithium trifuoroacetate as the main impurities explains the very broad resonances, due to the fast chemical exchange. With the help of the internal NMR standard, ortho-difuorobenzene, it was possible to determine the ratio of lithium bis(trifuoroacetyl)phosphate to lithium trifuoroacetate. Te ratio was 2:1 in the NMR sample.

4.4.2.!IR characterisation

Te colourless powder, which was obtained from experiment MS-002, was investigated by vibrational spectroscopy. Te corresponding spectrum can be seen in (Figure 32). An additional calculated Infrared spectrum of the [PO2(OAcF)2]− anion is superimposed 58 Lithium bis(trifluoroacetyl)phosphate

Table 10!Comparison of the experimental IR bands (Diamond-ATR, corrected) of MS-002 with the calculated vibrational bands of [PO2(OAcF)2]− at the PBE0/def2-TZVPP level. Additionally the experimental ATR-IR band of LiTFA, but the bands are not assigned w: weak, m: medium, s: strong, sh; shoulder, v: very.

cal. IR band exp. IR band exp. IR band [cm−1] [cm−1] assignment [cm−1] [PO2(OAcF)2]− powder MS-002 LiOAcF 465 (w), 473 (w) 501 (m) δ(PO2) 460 (w) 506 (w) 519 (m) δ(CF2) 519 (w) 552 (w) — δ(P–OAcF) — — 590 (w) δ(OAcF) 611 (w) 666 (m), 681 (vw) — ν(O—P–O) — 714 (w), 721 (w) 731 (w) δsymm(C–C–O2) 720 (m) 759 (vw), 766 (vw) 755 (sh), 771 (m), 788 (sh) δ(P–OAcF) — — 802 (m) δ(OAcF) 803 (m) 833 (vw), 839 (vw) 866 (w), 877 (m) δ(OAcF) 863 (m) — 962 (vw) — — — 1014 (w) — — 1107 (s) 1087 (vs) ν(PO2) — 1138 (vs) 1103 (vs) ν(PO2) — — — ν(C–C) 1151 (vs) 1145 (vs), 1169 (w) 1177 (vs) ν(−CF3) — 1187 (s) 1234 (vs) ν(−CF3) 1200 (s) 1338 (w), 1354 (w) 1311 (m), 1338 (m), ν(C–C) 1345 (vw) 1368 (m) 1360 (sh) ν(P=O) — — — — 1474 (m) 1791 (s), 1798 (s) 1790 (sh), 1807 (w) ν(C=O) 1712 (sh)

in the fgure. Te vibrational bands are listed in the Table 10. Te experimental data is in close agreement to the calculation at the PBE0/def2-TZVPP level, but the calculated deformation bands from 900 cm−1 down to 400 cm−1 are slightly red shifed. Te characteristic deformations vibration of the trifuoroacetyl group can be seen in the range of 501-862 cm−1. But in contrast to the bands of lithium trifuoroacetate, the starting material, characteristic band of the POAcF group can be seen at 755 cm−1, 771 cm−1 and 788 cm−1. Te symmetric and asymmetric stretching vibration of the P–O single bonds were calculated at 666 and 681 cm−1, the experimental band of the P–O vibration is presumably one of the bands between 700-800 cm−1. Te vibration of the PO2− group assigned to the strong bands at 1087 and 1103 cm−1, the very weak shoulders of this peak indicates that the vibration is degenerate due to the coordination of lithium at the oxygen atoms. Te P=O double bond was found at 1360 cm−1. Interestingly the carbonyl vibration at 1790 and Syntheses from lithium dihydrogenphosphate 59

MS -002 powder F cal. [P O2(OAc )2] LiOAcF

4000 2000 P–O–AcF O–P=O y

it O–P–O P=O ns 1 te In

666, 68

1500 1000 500 Wavenumber [c m ] Figure 32!ATR-IR spectrum of solid residue of MS-002 and the calculated (PBE0/def2-TZVPP) vibrational spectrum of [PO2(OAcF)2]−.

1807 cm−1 difers signifcantly from the bands of lithium trifuoroacetate, which can be seen at 1712 cm−1. Tis implies that LiOAcF is not the main product in the sample, and on the other side that the C=O bonds are stronger than in lithium trifuoroacetate.

4.4.3.!Purification of the product

As mentioned above in all NMR samples acidic protons were detected, which result from trifuoroacetic acid. So several atempts were made to purify the main product lithium bis(trifuoroacetyl)phosphate from traces of trifuoroacetic acid. At f rst the base lithium hydride or lithium foil was used to deprotonate acidic protons in the solution and to precipitate the trifuoroacetic acid as lithium trifuoroacetate. Te treated solutions were analysed by NMR and a decrease of the TFA signal and an increased signal of lithium trifuoroacetate was observed. But the purifcation was not complete, and remaining acidic proton signals were observed in the 1HVNMR spectra. Another atempt to get rid of the trifuoroacetic acid was to extract the acid from the product solution in non-polar n-butylether. But it failed.

4.4.4.!Stability and purity considerations of the product

Te synthesis of lithium bis(trifuoroacetyl)phosphate from lithium dihydrogenphosphate and trifuoroacetic anhydride is the easiest and most straight forward hitherto investigated. We expected the removal of trifuoroacetic acid as by-product should be easily done under the vacuum. But obviously, it was not. Te question, "Where are the acidic impurities from?" and the even more interesting question, "Why is lithium 60 Lithium bis(trifluoroacetyl)phosphate

Δ G° = +356 + F – r (g) F + F – 3 Li (g) + 3 [PO2(OAc )2] (g) P3O6(OAc )3 (g) + 3 Li (g) + 3 [OAc ] (g)

ΔlatG° = +370 ΔsolvG° = +43 ΔlatG° = -510

Δ G° = -107 F r (s) F F 3 Li[PO2(OAc )2](s) P3O6(OAc )3 (solv) + 3 LiOAc (s)

Scheme 6!Born-Fajan-Haber cycle for the calculation of the thermodynamics of the dismutation of Li[PO2(OAcF)2] (all values calculated at the RI-BP86/def-SV(P) level and in kJ mol−1). trifuoroacetate the main by-product?" requested some answers. First of all it could observed in the experiments that the precipitation of Li[PO2(OAcF)2] from diethyl ether solution started only when the solvent was almost removed. Tis suggested that the product has a high solubility in diethyl ether. But afer complete removal of all volatile compounds at the vacuum, the residue was not as soluble in diethyl ether and an insoluble precipitate always remained. Tis means that something happened during the removal of the solvent. Perhaps the insoluble compound is lithium trifuoroacetate or other unknown compounds? Tere are several ways how lithium trifuoroacetate could be formed. Lithium dihydrogenphosphate is a weak acid (pKa = 7.21 [149]) and the trifuoroacetic acid is a very strong acid (pKa = 0.52 [149]), so it is possible that the LiH2PO4 was protonated to phosphoric acid by the TFA (Eq.25). Tis acid-base equilibrium led to lithium trifuoroacetate and phosphoric acid. In the presence of hygroscopic trifuoroacetic anhydride phosphoric acid could be condensed to diphosphoric acid or metaphosphoric acid, which are both soluble in diethyl ether (Eq.26).

F Li[PO2(OAc )2] Li[PO2(OAc F )2]

unknown phosphate species

−10 −15 −20 −25 −30 ppm −22 −23 −24 ppm Figure 33!31P+NMR spectrum of filtrate (MS- Figure 34!31P+NMR spectrum of 014) afer fractional precipitation in DMC/ Li[PO2(OAcF)2] (MS-002) in diethyl ether CH2Cl2 (161.99 MHz in DMC by r.t., external (81.01 MHz in Et2O by r.t., external toluene-D8 toluene-D8 lock) lock) Syntheses from lithium dihydrogenphosphate 61

F F LiH2PO4 + HOAc H3PO4 + LiOAc Eq.25

6 H3PO4 + 3 TFAA 2 H3P3O9 + 6 TFA Eq.26

Tis condensation reaction would also be possible for the starting material lithium dihydrogenphosphate [143]. Moreover it is possible that trifuoroacetic anhydride could esterify every species of phosphoric acid to trifuoroacetyl phosphoric acid, which is in good agreement to the results of Smyth [145]. Tis sketched reaction cascade leads to a vast range of phosphates! Tis was observed in several 31PVNMR spectra. Figure 33 shows a 31PVNMR spectrum of the MS-014 reaction afer evacuation which supports the presence of three sharp phosphate signals at −22.9, −20.7 and −20.3 ppm, which are on a high and broad peak. All signals are in the region of metaphosphates. Te resonance of Li[PO2(OAcF)2] appears at −22.2 ppm. In contrast to Figure 34 that shows the unique 31PVNMR signal of Li[PO2(OAcF)2] in diethyl ether before evacuation and without the occurrence of other phosphate impurities. All atempts to detect the diferent phosphate species, which were observed in so many experiments, via mass spectrometry (ESI) failed, but the existence of lithium trifuoroacetate was confrmed. Te dismutation of the lithium bis(trifuoroacetyl)phosphate provides another explanation for the presence of lithium trifuoroacetate and phosphate impurities. Tis dismutation of Li[PO2(OAcF)2] is a stepwise intermolecular condensation of the anion with elimination of lithium trifuoroacetate (Eq.27).

F F F 3 Li[PO2(OAc )2] P3O6(OAc )3 + 3 LiOAc Eq.27

Te initial condensation between two lithium bis(trifuoroacetyl)phosphates leads to diphosphate and a lithium trifuoroacetate. Tis diphosphate can condense with another Li[PO2(OAcF)2] to a triphosphate and a further lithium trifuoroacetate. Afer a fnal intramolecular ring-closing condensation step, tris(trifuoroacetyl)metaphosphate and lithium trifuoroacetate are formed. Te driving force for the dismutation is the high latice energy of lithium trifuoroacetate. So the trifuoroacetate salt becomes a good leaving group, which was also observed from Smyth in comparison with non-fuorinated acetate. On the other side the formations of diphosphates due to the elimination of the trifuoroacetate group of phosphoric acid structures has been described by Corby [148]. Te Born-Fajans-Haber cycle of the energetics of this dismutation is Scheme 6. Te Gibbs free energy in the gas phase is highly endergonic (∆rG°(g) = +354 kJ mol−1). Under consideration of the latice energy of both lithium salts (∆lattG°) the Gibbs free energy in the condensed phase is ∆rG°(s) = −107 kJ mol−1. Tis exergonic free reaction enthalpy supports the dismutation of the lithium bis(trifuoroacetyl)phosphate to lithium trifuoroacetate and metaphosphate. Both reactions, the acid-base equilibrium in the reaction mixture (Eq.25) the dehydration of lithium dihydrogenphosphate (or phosphoric acid) leads to the cyclic metaphosphate species (Eq.26) and the dismutation in the condensed phase (Eq.27) are responsible for the side-products lithium trifuoroacetate and 62 Lithium bis(trifluoroacetyl)phosphate

other phosphate impurities. Tis disadvantage of the reaction route cannot be avoided and small amounts of side-products are always formed. In addition, the side-products cannot be separated from the Li[PO2(OAcF)2] by extraction or a fractional precipitation.

4.4.5.!Conclusion

Tis subsection pointed out that the synthesis of lithium bis(trifuoroacetyl)phosphate could easily achieved by the esterifcation of lithium dihydrogenphosphate with trifuoroacetic anhydride in high yields. Drawback of the reaction is: Originating from the acid-base equilibrium of lithium dihydrogenphosphate and trifuoroacetic acid, the main by-product, small amounts of phosphoric acid and lithium trifuoroacetate form. Under the conditions in the reaction vessel, the phosphoric acid can condense to diphosphoric acid or metaphosphoric acid. It is possible that every phosphoric acid species forms various phosphate esters in the presence of trifuoroacetic anhydride. Tese side-products are as soluble in ether as the Li[PO2(OAcF)2] and thus the product is contaminated. Afer isolating the material and dissolving the residue, 31PVNMR spectra indicate several phosphorus containing species, probably diphosphates and metaphosphates. As mentioned above lithium trifuoroacetate is a side-product too. Which brings the same problems with the purity: Lithium trifuoroacetate could not be separated from the lithium bis(trifuoroacetyl)phosphate. Another negative observation has been that the amount of lithium trifuoroacetate increased during work-up. Calculations suggests that lithium bis(trifuoroacetyl)phosphate is stable in solution but dismutates in condensed phase. Tis dismutation is an intramolecular condensation with elimination of lithium trifuoroacetate. Tese disadvantages make it impossible to get lithium bis(trifuoroacetyl)phosphate in batery-grade purity.

4.5!Synthesis from lithium difluorophosphate Te previous sections showed the problems with the use of lithium trifuoroacetate. Due to the incomplete conversion of the starting material during the reaction it is still present in the reaction mixture, and because of the similar solubility like the product, it cannot be removed. One strategy to avoid lithium trifuoroacetate is the reaction of trimethylsilyl trifuoroacetate with lithium difuorophosphate. Tis should lead to the lithium bis(trifuoroacetyl)phosphate and gaseous trimethylsilyl fuoride (Eq.28). Te

Table 11!Product distribution of reaction of Li[PO2F2] with Me3SiOAcF in acetonitrile (MR-071). Percentages from integration of the 31P+NMR spectra signals.

Li[PO2F2] Li[PO2FOAcF] Li[PO2(OAcF)2] 5 h 91.6 % 7.7 % 0.7 % 20 h 90.8 % 9.1 % 0.1 % 35 h 88.2 % 11.2 % 0.7 % Summary 63

Me3SiOAcF should be strong enough to break the P–F bond in favour of formation of the very strong Si–F bond. Te gaseous Me3Si−F could be removed continuously from the equilibrium.

F F Li[PO2F2] + 2 Me3SiOAc Li[PO (OAc ) ] + 2 Me SiF acetonitrile 2 2 3 Eq.28

Te experiment was carried out in acetonitrile and the reaction progress was mon- itored via in-situ NMR afer 5, 20 and 35 h. In the 31PVNMR spectrum (Figure 35) af- ter 35 h three signals from phosphorus species can be seen. Down feld at −17 .4 ppm the main signal from lithium difuorophosphate, which is split into a triplet, due to the coupling 1J(19F,31P) = 932 Hz. At high- Li[PO2F2] er feld at −27.1 ppm a doublet from the monosubstituted fuorophosphate species, Li[PO2FOAcF], appeared. Te coupling constant is 1J(19F,31P) = 914 Hz. Also a very weak resonance of the product lithium bis(trifuoroacetyl)phosphate was observed at −27.6 ppm. Te NMR signals were integrated

F and the product distribution is listed in the Li[PO2(OAc )2] Li[PO2FOAcF ] following table (Table 11). As can be seen only 11.8 % from the starting material was converted afer 35 h. Te amount of product, Li[PO2(OAcF)2], did not increase signifcantly, whereas the amount of mono-fuorinated phosphate grew. Tis suggests that the substitution of the frst fuorine proceeds faster than the second one. It is therefore likely that the reaction will not lead to a complete 0 −10 −20 −30 ppm conversion of the lithium difuorophosphate Figure 35!31P+NMR spectrum of reaction and only a mixture of products will be ob- F (MR-071) from Li[PO2F2] with Me3SiOAc in tained. Previous experiments showed that a acetonitrile afer 35 h (161.99 MHz in MeCN by clean separation of the various lithium salts r.t., toluene-D8 external lock). is not realistic.

4.6!Summary Tis chapter pointed out several ways (Scheme 7) to synthesize lithium bis(trifuoroacetyl)phosphate, Li[PO2(OAcF)2], as a possible new conducting salt in lithium-ion-batteries. Te new lithium salt should be available from cheap starting materials according to economic requirements of the industry. Te used precursors P4O10, LiH2PO4, POCl3, Li[PO2F2], TFAA, and LiOAcF are very cheap to buy in high purity. Te frst detection of the [PO2(OAcF)2]− anion in solution was observed with the reaction of phosphorus pentoxide, lithium trifuoroacetate and trifuoroacetic anhydride. But a yield of more than 12 % could never be realized, so that an excess of unreacted lithium trifuoroacetate was present. Te main difculty on this route remained, that the Li[PO2(OAcF)2] could not be separated from lithium trifuoroacetate. A conducting salt for LIB has to be inherently pure, more importantly for conductivity or cyclovoltammetric measurements. Te 64 Lithium bis(trifluoroacetyl)phosphate

same problem occurred in the reaction of lithium difuorophosphate with trimethylsilyl trifuoroacetate. Te frst fuorine substitution of Li[PO2F2] with the trifuoroacetyl group took place with a yield of only 11 % of the substrate afer 35 hours of refuxing in acetonitrile. Terefore this conversion leads to undesirable mixtures of lithium salts. Te way out of this problem is a synthesis strategy avoiding other lithium salts, especially lithium trifuoroacetate. Tis would be possible with the direct esterifcation of lithium dihydrogenphosphate with trifuoroacetic anhydride or the formation of a non ionic precursor, which can be isolated from other salts. One chosen way to synthesize a neutral precursor was the reaction of phosphoryl chloride with lithium trifuoroacetate. Te resulting product is bis(trifuoroacetyl)phosphoryl chloride, POCl(OAcF)2, obtained in a yield of up to 66 %. Unfortunately the chlorophosphate is highly sensitive to moisture traces and a reactive acetylating agent, and reacts with tetrahydrofuran, which was used as a NMR solvent. All eforts to isolate the compound from the reaction mixture failed. Terefore it was not possible to proceed with the synthesis route towards lithium bis(trifuoroacetyl)phosphate. Te esterifcation as last promising way to get lithium bis(trifuoroacetyl)phosphate has worked, but has showed a drawback of the reaction: Due to an acid-base equilibrium during the synthesis between the starting material lithium dihydrogenphosphate and the by-product trifuoroacetic acid, a small amount of phosphoric acid and lithium trifuoroacetate is formed. Te phosphoric acid leads to a vast range of further polyphosphates, which are similar soluble than the Li[PO2(OAcF)2]. So these impurities can not extracted or separated from the product and remain with the product in the residue. So the lithium salt is contaminated, not only with several polyphosphates but also with lithium trifuoroacetate. Lithium trifuoroacetate is the main impurity, and its content increases afer work-up. Termodynamic calculations support the proposal, that Li[PO2(OAcF)2] is not stable in the solid state and undergoes intermolecular condensation with elimination of lithium trifuoroacetate and formation of a metaphosphate. Even if a characterisation of the dissolved phosphorus species has not be accomplished, the fact that lithium trifuoroacetate is present together with the desired lithium bis(trifuoroacetyl)phosphate, dampen the prospect to get pure lithium bis(trifuoroacetyl)phosphate. Terefore no further endeavours in this direction were undertaken.

F F P4O10 + 4 LiOAc + 2 TFAA H[PO2(OAc )2]

F POCl(OAc )2 F Li[PO2(OAc )2]

F POCl3 + 2 LiOAc

F Li[PO2F2] + 2 Me3Si-OAc LiH2PO4 + 2 TFAA

Scheme 7!Several investigated routes for synthesis of lithium bis(trifluoroacetyl)phosphate. Experimental Section 65

4.7!Experimental Section

4.7.1.!Chemicals

Manufac- Chemical Qality CAS no. Purification before use turer

a) dried at 60 ℃ under counterfow of argon gas for 2 h, afer this at 105 ℃ lithium dihy- Sigma for further 3 h ≥99.999 13453-80-0 drogenphosphate Aldrich b) washed with diethyl ether (5×5 mL; <10 ppm H2O)

trifuoroacetic Alfa Aesar +99 % 407-25-0 refuxed and distilled from P4O10 anhydride phosphorus Alfa Aesar 99.99 % 1314-56-3 — pentoxide trimethylsilyl Sigma ≥97 % 400-53-3 distilled trifuoroacetate Aldrich lithium trifuoro- Alfa Aesar 97 % 2923-17-3 dried under vacuum at 120 ℃ acetate aluminium — +99.9 % 7446-70-0 sublimed chloride phosphoryl Sigma ≥99.9 10025-87-3 refuxed and distilled from CaH2 chloride Aldrich trifuoroacetic for spec- refuxed and distilled from very few VWR/Merck 76-05-1 acid troscopy P4O10

4.7.2.!Syntheses from P4O10

Synthesis of Li[PO2(OAcF)2] in dimethyl carbonate

P4O10 (1.420 g, 5 mmol, 1 eq.) and lithium trifuoroacetate (1.702 g, 20 mmol, 4 eq.) were weighed in a 100 mL fask and suspended with 45 mL dimethyl carbonate. Aferwards an excess of trifuoroacetic anhydride (10 mL, 14.9 g, 71 mmol, 14.2 eq.) was dropped in the suspension. Te reaction mixture was heated under stirring and refuxing for 4 h. Afer that, the mixture was concentrated to one third of the volume and a NMR sample was drawn.

In-situ NMR sample in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −77.24 (br, –CF3), −77.44 (d, 4J(19F,31P) = 1 Hz, –CF3) ppm 31P-NMR (161.99 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −22.07 (∆1/2 = 6 Hz, Li[PO2(OAc )2]) ppm 66 Lithium bis(trifluoroacetyl)phosphate

Synthesis of Li[PO2(OAcF)2] in EC:DMC (1:1)

P4O10 (1.420 g, 5 mmol, 1 eq.) and lithium trifuoroacetate (1.702 g, 20 mmol, 4 eq.) were weighed in a 100 mL fask and suspended with 40 mL ethylene carbonate/dimethyl carbonate (1:1). Aferwards an excess of trifuoroacetic anhydride (10 mL, 14.9 g, 71 mmol, 14.2 eq.) was dropped in the suspension. Te reaction mixture was heated under stirring and refuxing for 5 h. Afer that, the mixture was concentrated to one third of the volume and a NMR sample was taken.

In-situ NMR sample in dimethyl carbonate: 19F-NMR (188.31 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.79 (br, –CF3), −77.34 (d, 4J(19F,31P) = 1.3 Hz, –CF3) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −23.39 (sep, 4J(19F,31P) = 1.3 Hz, Li[PO2(OAcF)2]) ppm

Synthesis of Li[PO2(OAcF)2] in ethyl methyl carbonate

P4O10 (1.420 g, 5 mmol, 1 eq.) and lithium trifuoroacetate (1.702 g, 20 mmol, 4 eq.) were weighed in a 100 mL fask and suspended with 50 mL ethyl methyl carbonate. Aferwards an excess of trifuoroacetic anhydride (10 mL, 14.9 g, 71 mmol, 14.2 eq.) were dropped in the suspension. Te reaction mixture was heated under stirring and refuxing for 5 h. Afer that, the mixture was concentrated to one third of the volume and a NMR sample was taken.

In-situ NMR sample in ethyl methyl carbonate: 19F-NMR (376.54 MHz, EMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.53 (br, –CF3), −77.24 (d, 4J(19F,31P) = 1 Hz, –CF3) ppm 31P-NMR (161.99 MHz, EMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −21.99 (br, ∆1/2 = 5.7 Hz, Li[PO2(OAc )2]) ppm 31P[19F]VNMR (161.99 MHz, EMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −21.99 (s, ∆1/2 = 2.7 Hz, Li[PO2(OAc )2]) ppm Experimental Section 67

4.7.3.!Syntheses from POCl3

LM-003

In a 100 mL round botom fask lithium trifuoroacetate (4.240 g, 35.4 mmol, 2.1 eq.) was suspended in dimethyl carbonate (30 mL). Ten POCl3 (1.55 mL, 2.60 mg, 16.8 mmol, 1 eq.) was added dropwise. Here, a slight warming of the reaction mixture and slight gas evolution was observed. Afer about a minute a gel-like suspension was formed. Te reaction mixture was stirred under refux at 95 ℃ for 4.5 h. From the cooled suspension was taken a NMR (0.2 mL), afer that the reaction mixture was fltered with a glass frit (P4). Te obtained fltrate was a clear liquid, the residue was a colourless fne crystalline powder.

Crude product dissolved in dimethyl carbonate: 19F-NMR (188.31 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.4 (s, –CF3) ppm 31P NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −4.4 (s, POCl3) ppm

Filtrate in dimethyl carbonate: 19F-NMR (188.31 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.4 (s, –CF3) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −4.4 (s, POCl3) ppm 13C-NMR (100.62 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 114.7 (s, 1J(19F,13C) = 285 Hz, LiOCOCF3), 158.1 (s, 2J(19F,13C) = 42 Hz, LiOCOCF3)

Residue from fltrate afer evaporation: ATR-IR ν [cm−1] = 793 (w), 914 (w), 969 (w), 1266 (vs), 1440 (sh), 1454 (m), 1752 (s), 2963 (vw)

Residue: ATR-IR ν [cm−1] = 521 (w), 729 (w), 800 (w), 916 (m), 1014 (m), 1098 (s), 1153 (m) 1214 (m), 1272 (vs), 1435 (w), 1468 (m), 1693 (m), 1732 (m), 2970 (vw) 68 Lithium bis(trifluoroacetyl)phosphate

LM-004 In a round botom fask lithium trifuoroacetate (4.145 g, 34.6 mmol, 2.6 eq.) and calcium chloride (44 mg, 0.4 mmol, 0.011 eq.) were suspended in dimethyl carbonate (60 mL). Ten phosphoryl chloride (1.2 mL, 2.02 mg, 13.2 mmol, 1 eq.) was dropped to the suspension. Afer this the clear mixture was refuxed at 95 ℃ for 4.5 h. Afer warming, the mixture became turbid. Afer the refuxing, the mixture was fltered with a glass frit (P4). Te obtained fltrate was a clear liquid, the residue was a colourless fne crystalline powder. Yield (POCl(OAcF)2) via NMR: 4.8 %.

Filtrate in dimethyl carbonate: 19F-NMR (188.31 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.7 (s, ∆1/2 = 1.0 Hz), −77.4 (s, LiOCOCF3), −77.5 (d, 4J(19F,31P) = 1.3 Hz, POCl2(OCOCF3)) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −4.4 (s, POCl3), −11.8 (q, ∆1/2 = 12.1 Hz, POCl2OAc ) ppm

Residue: ATR-IR ν [cm−1] = 793 (w), 914 (w), 969 (w), 1266 (vs), 1440 (sh), 1454 (m), 1752 (s), 2963 (vw) Experimental Section 69

LM-005 Freshly sublimed aluminium chloride (7 mg, 0.05 mmol, 0.008 eq.) was dissolved in phosphoryl chloride (0.6 mL, 1.01 mg, 6.6 mmol, 1 eq.). Ten the solution was dropped via a syringe into a turbid suspension of lithium trifuoroacetate (2.450 g, 20.4 mmol, 3 eq.) in dimethyl carbonate (25 mL). Tereby the mixture became clear and warm. Afer this it was refuxed for 3.5 h. Afer the refuxing the mixture was fltered with a glass frit (P4). Te obtained fltrate was a clear liquid, the residue (amount not measured) was a colourless fne crystalline powder. Yield via NMR: POCl(OAcF)2 = 4.8 %

Filtrate in dimethyl carbonate: 19F-NMR (188.31 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −77.5 (s, ∆1/2 = 6.1 Hz, LiOCOCF3) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −3.6 (s, POCl3), −11.5 (q, ∆1/2 = 12.1 Hz, POCl2OAc ) ppm LM-006 Freshly sublimed aluminium chloride (9 mg, 0.07 mmol, 0.009 eq.) was dissolved in phosphoryl chloride (0.75 mL, 1.26 mg, 8.2 mmol, 1 eq.). Ten the solution was dropped via a syringe into a turbid and brownish suspension of lithium trifuoroacetate (2.464 g, 20.5 mmol, 2.5 eq.), trifuoroacetic anhydride (4.7 mL, 7.00 mg, 33.6 mmol, 4 eq.) and dimethyl carbonate (35 mL). Tereby the mixture became warm. Afer this it was refuxed for 4 h.

In-situ NMR sample in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.8 (s, ∆1/2 = 1.9 Hz, LiOCOCF3), −76.9 (s, ∆1/2 = 35 Hz, (CF3CO)2O), −77.5 (d, 4J(19F,31P) = 1.2 Hz, POCl(OCOCF3)2) ppm 31P-NMR (161.99 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −22.1 (br, ∆1/2 = 3.4 Hz), −22.2 (sep, ∆1/2 = 6.2 Hz, POCl(OCOCF3)2) ppm 13C-NMR (100.62 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 113.8 (s, 1J(19F,13C) = 228 Hz, LiOCOCF3), 113.8 (s, 1J(19F,13C) = 228 Hz, −CF3), 115.7 (s, 2J(19F,13C) = 45 Hz, POCl(OCOCF3)2), 152.2 (s, 2J(19F,13C) = 44 Hz, POCl2(OCOCF3), 159.6 (s, 2J(19F,13C) = 50 Hz, LiOCOCF3) ppm 70 Lithium bis(trifluoroacetyl)phosphate

LM-007 Lithium trifuoroacetate (2.464 g, 20.5 mmol, 2.5 eq.) and trifuoroacetic anhydride (4.6 mL, 6.85 mg, 32.9 mmol, 4 eq.) were suspended in dimethyl carbonate (25 mL). To this suspension phosphoryl chloride (0.75 mL, 1.26 mg, 8.2 mmol, 1 eq.) were dropped in. A colourless solid precipitated afer two minutes. Te reaction mixture was refuxed for 4 h. Afer fltration a clear fltrate and a fne crystalline powder were obtained.

Filtrate in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = –76.8 (s, LiOCOCF3, ∆1/2 = 0.6 Hz), –76.9 (s, ∆1/2 = 0.6 Hz, (CF3CO)2O), –77.6 (d, 4J(19F,31P) = 1.3 Hz, POCl(OCOCF3)2, –77.7 (d, 4J(19F,31P) = 1.2 Hz, POCl2(OCOCF3)) ppm 31P-NMR (161.99 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 3.4 (br, ∆1/2 = 30 Hz, H3PO4), –5.3 (s, ∆1/2 = 3.4 Hz, POCl3,), –12.5 (q, 4J(31P,19F) = 1.0 Hz, POCl2(OCOCF3)), –23.2 (sept, 4J(31P,19F) = 1.3 Hz, POCl(OCOCF3)2) ppm 13C-NMR (100.62 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 113.2 (s, 1J(19F,13C) = 286 Hz, LiOCOCF3), 113.6 (1J(19F,13C) = 287 Hz, (CF3CO)2O), 114.0 (s, 1J(19F,13C) = 285 Hz, POCl2(OCOCF3)), 114.0 (s, 1J(19F,13C) = 285 Hz, POCl(O- COCF3)2), 149.1 (s, 2J(19F,13C) = 47 Hz, LiOCOCF3), 151.7 (s, 2J(19F,13C) = 44 Hz, POCl2(OCOCF3), 151.8 (s, 2J(19F,13C) = 44 Hz, POCl(OCOCF3)2), 159.6 (s, 2J(19F,13C) = 45 Hz, (CF3CO)2O) ppm LM-009 Freshly sublimed aluminium chloride (14 mg, 0.1 mmol, 0.06 eq.) was dissolved in phosphoryl chloride (1.5 mL, 2.52 mg, 16.4 mmol, 1 eq.). Ten the solution was dropped via a syringe into a turbid suspension of lithium trifuoroacetate (4.929 g, 41.1 mmol, 2.5 eq.) with dimethyl carbonate (70 mL). Afer this it was refuxed for 4.5 h. Ten the solvent was removed mainly by distillation, and the remaining solvent from the stif and brownish residue was totally removed at vacuum.

Crude product dissolved in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = –76.8 (s, LiOCOCF3, ∆1/2 = 2.2 Hz), –76.9 (s, ∆1/2 = 34 Hz, (CF3CO)2O), –77.5 (d, 4J(19F,31P) = 1.2 Hz, POCl(OCOCF3)2) ppm 31P-NMR (161.99 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −22.2 (br, ∆1/2 = 5 Hz), –22.3 (m, ∆1/2 = 6.3 Hz, POCl(OCOCF3)2) ppm 13C-NMR (100.62 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 113.6 (s, 1J(19F,13C) = 289 Hz, LiOCOCF3), 113.8 (s, 1J(19F,13C) = 285 Hz, POCl(O- COCF3)2), 113.9 (s, 1J(19F,13C) = 285 Hz, POCl2(OCOCF3)), 151.8 (s, 2J(19F,13C) = 45 Hz, POCl(OCOCF3)2), 152.2 (s, 2J(19F,13C) = 45 Hz, POCl2(OCOCF3), 159.6 (s, 2J(19F,13C) = 50 Hz, LiOCOCF3) ppm

Crude product: ATR-IR ν [cm−1] = 459 (w), 521 (w), 608 (vw), 730 (m), 802 (m), 860 (w), 960 (w), 1016 (w), 1105 (sh), 1150 (s), 1205 (s), 1332 (m), 1436 (vw), 1469 (m), 1674 (vs), 2972 (vw). Experimental Section 71

LM-010 Freshly sublimed aluminium chloride (14 mg, 0.1 mmol, 0.06 eq.) was dissolved in phosphoryl chloride (1.5 mL, 16.4 mmol, 1 eq.). Ten the solution was dropped via a syringe into a turbid suspension of lithium trifuoroacetate (4.929 g, 41.1 mmol, 2.5 eq.), trifuoroacetic anhydride (12.2 mL, 2.52 mg, 87.7 mmol, 4 eq.) and dimethyl carbonate (70 mL). Afer this it was stirred and refuxed for 3 h. From the reaction mixture 20 mL was separated (#1.0) and the solvent was removed by condensation (condensate #1.2). Te stif and brownish residue was dried at the vacuum and then crushed, afer this a colourless powder was obtained (#1.1). Te remaining reaction mixture was concentrated by distillation until a precipitate was formed (concentrate #2). Te suspension was lef over night. And at the next day a NMR sample was taken from the upper phase (#2.1), then fltered (fltrate #3.1, residue #3.2) and dried at the vacuum.

Crude product dissolved in dimethyl carbonate: 19F-NMR (188.31 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.8 (s, ∆1/2 = 1.2 Hz, LiOCOCF3), −76.9 (s, ∆1/2 = 6.5 Hz), −77.5 (d, 4J(19F,31P) = 1.3 Hz, POCl(OCOCF3)2) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −22.2 (m, ∆1/2 = 2 Hz), –22.3 (m, 4J(19F,31P) = 1.3 Hz, POCl(OCOCF3)2) ppm 13C-NMR (100.62 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 114.3 (s, 1J(19F,13C) = 288 Hz, LiOCOCF3), 114.4 (s, 1J(19F,13C) = 285 Hz, POCl(O- COCF3)2), 152.2 (s, 2J(19F,13C) = 45 Hz, POCl(OCOCF3)2), 159.6 (s, 2J(19F,13C) = 50 Hz, LiOCOCF3) ppm

Powder #1.1 dissolved in tetrahydrofuran: 19F-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −77.09 (d, 4J(19F,31P) = 1.3 Hz, POCl(OCOCF3)2) ppm 31P-NMR (161.99 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −22.1 (m, ∆1/2 = 5.2 Hz), −22.6 (m, ∆1/2 = 6.5 Hz) ppm

Powder #1.1: ATR-IR ν [cm−1] = 521 (w), 585 (vw), 729 (m), 801 (m), 861 (w), 932 (sh), 960 (m), 1015 (m), 1102 (s), 1149 (vs), 1207 (s), 1318 (s), 1435 (w), 1468 (m), 1683 (vs), 1705 (sh), 1807 (w), 2920 (vw), 2971 (vw)

Condensate #1.2 in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.7 (s, ∆1/2 = 0.7 Hz, (CF3CO)2O) ppm

Upper phase #2.1 in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, ∆1/2 = 0.7 Hz, (CF3CO)2O), −77.4 (d, 4J(19F,31P) = 1.3 Hz, POCl(OCOCF3)2) ppm 31P-NMR (161.99 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −21.7 (sept, 4J(19F,31P) = 1.3 Hz, POCl(OCOCF3)2) ppm Filtrate #3.1 in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −77.3 (s, ∆1/2 = 9.1 Hz, LiOCOCF3) ppm 72 Lithium bis(trifluoroacetyl)phosphate

Residue #3.2 in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, ∆1/2 = 1.7 Hz, LiOCOCF3) ppm

Residue #3.2: ATR-IR ν [cm−1] = 462 (w), 522 (w), 583 (w), 607 (vw), 729 (m), 802 (m), 855 (w), 961 (m), 1018 (m), 1104 (w), 1143 (sh), 1150 (s), 1204 (s), 1321 (s), 1435 (w), 1468 (m), 1669 (vs), 1682 (vs), 1704 (sh), 2971 (vw) LM-011 Freshly sublimed aluminium chloride (7 mg, 0.05 mmol, 0.01 eq.) was dissolved in phosphoryl chloride (0.5 mL, 0.84 mg, 5.5 mmol, 1 eq.). Ten the solution was dropped via a syringe into a turbid suspension of lithium trifuoroacetate (1.314 g, 11.0 mmol, 2 eq.), trifuoroacetic anhydride (3.1 mL, 4.62 mg, 21.9 mmol, 4 eq.) and dimethyl carbonate (45 mL). Afer this it was stirred and refuxed for 5 h. Afer 2 h the solution became turbid.

In-situ NMR sample afer 5 h in dimethyl carbonate: 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −76.2 (s, ∆1/2 = 1.7 Hz, LiOCOCF3), −76.3 (s, ∆1/2 = 27 Hz, (CF3CO)2O), −76.9 (d, 4J(19F,31P) = 1.3 Hz, POCl(OCOCF3)2) ppm 31P-NMR (161.99 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −21.7 (sept, ∆1/2 = 5.4 Hz, POCl(OCOCF3)2) ppm LM-013 Freshly sublimed aluminium chloride (10 mg, 0.075 mmol, 0.014 eq.) was dissolved in phosphoryl chloride (0.5 mL, 0.84 mg, 5.5 mmol, 1 eq.). Ten the solution was dropped via a syringe into a suspension of lithium trifuoroacetate (1.253 g, 10.4 mmol, 1.9 eq.), trifuoroacetic anhydride (3.1 mL, 4.62 mg, 21.9 mmol, 4 eq.) and acetonitrile (25 mL). Afer this it was stirred and refuxed for 2.5 h. Afer warming up the mixture was clear, but with time the solution became turbid.

In-situ NMR sample afer 5 h in acetonitrile: 19F-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) F F δ = –76.4 (s, (Ac 2)O, ∆1/2 = 37 Hz), –76.8 (d, POCl(OAc )2, ∆1/2 = 4.9 Hz),

–77.0 (s, ∆1/2 = 1.7 Hz) ppm 31P-NMR (161.99 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) F δ = –22.1 (br, ∆1/2 = 20 Hz), –22.6 (br, POCl(OAc )2, ∆1/2 = 30 Hz) ppm 13C-NMR (100.62 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = 113.9 (s, 1J(13C,19F) = 287 Hz, POCl(OCOCF3)2), 151.6 (s, 2J(13C,19F) = 44 Hz, POCl(OCOCF3)2) ppm Experimental Section 73

LM-014 Freshly sublimed aluminium chloride (18 mg, 0.14 mmol, 0.025 eq.) was dissolved in phosphoryl chloride (0.5 mL, 0.84 mg, 5.5 mmol, 1 eq.). Ten the solution was dropped via a syringe into a suspension of lithium trifuoroacetate (1.253 g, 10.4 mmol, 1.9 eq.), trifuoroacetic anhydride (3.1 mL, 4.62 mg, 21.9 mmol, 4 eq.) and acetonitrile (25 mL). Afer this it was stirred and refuxed for 2.5 h. Afer warming up the mixture was clear, but with time the solution became turbid.

In-situ NMR sample afer 5 h in acetonitrile: 19F-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) F F δ = –76.4 (s, (Ac 2)O, ∆1/2 = 37 Hz), –76.8 (d, ∆1/2 = 1.2 Hz, POCl(OAc )2),

–77.0 (s, ∆1/2 = 1.3 Hz) ppm 31P-NMR (161.99 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) F δ = –22.1 (br, ∆1/2 = 20 Hz), –22.6 (br, POCl(OAc )2, ∆1/2 = 10.2 Hz) ppm 13C-NMR (100.62 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = 113.9 (s, 1J(13C,19F) = 285 Hz, POCl(OCOCF3)2), 151.5 (s, 2J(13C,19F) = 44 Hz, POCl(OCOCF3)2) ppm LM-015 Freshly sublimed aluminium chloride (30 mg, 0.023 mmol, 0.041 eq.) was dissolved in phosphoryl chloride (0.5 mL, 0.84 mg, 5.5 mmol, 1 eq.). Ten the solution was dropped via a syringe into a suspension of lithium trifuoroacetate (1.253 g, 10.4 mmol, 1.9 eq.), trifuoroacetic anhydride (3.1 mL, 4.62 mg, 21.9 mmol, 4 eq.) and acetonitrile (25 mL). Afer this it was stirred and refuxed for 2.5 h. Afer warming up the mixture was clear, but with time the solution became turbid.

In-situ NMR sample afer 5 h in acetonitrile: 19F-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) F F δ = –76.4 (s, (Ac 2)O, ∆1/2 = 19 Hz), –76.8 (d, POCl(OAc )2, ∆1/2 = 4.2 Hz),

–77.0 (s, ∆1/2 = 1.7 Hz) ppm 31P-NMR (161.99 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) F δ = –22.2 (br, ∆1/2 = 20 Hz), –22.6 (br, POCl(OAc )2, ∆1/2 = 16.8 Hz) ppm 13C-NMR (100.62 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = 113.9 (s, 1J(13C,19F) = 287 Hz, POCl(OCOCF3)2), 151.6 (s, 2J(13C,19F) = 45.2 Hz, POCl(OCOCF3)2) ppm 74 Lithium bis(trifluoroacetyl)phosphate

LM-016 In a round botom fask lithium trifuoroacetate (1.972 g, 16.4 mmol, 2 eq.) and trifuoroacetic anhydride (4.6 mL, 6.85 mg, 32.9 mmol, 4 eq.) were dissolved in tetrahydrofuran (25 mL). Ten phosphoryl chloride (0.5 mL, 0.84 mg, 5.5 mmol, 1 eq.) was dropped via a syringe into the solution. Afer this it was stirred and refuxed for 2.5 h. A NMR sample was taken afer this time, the reaction mixture was stirred over night and at the next day it was refuxed again for 3.5 h. Ten a next NMR sample was taken from the clear reaction mixture.

Solution afer 2.5 h in tetrahydrofuran: 19F-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F F δ = –76.50 (s, R2PO(CH2)4OAc , ∆1/2 = 0.7 Hz), –76.51 (s, ∆1/2 = 0.6 Hz), –76.6 (s, LiOAc , F F ∆1/2 = 10 Hz), –76.9 (s, (Ac 2)O, ∆1/2 = 3.8 Hz), –77.2 (d, 4J(19F,31P) = 1.4 Hz, POCl(OAc )2, –77.3 (d, 4J(19F,31P) = 1.3 Hz, POCl2(OAcF) ppm 31P-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = –5.3 (s, POCl3, ∆1/2 = 1.7 Hz), –10.0 (dt, 3J(1H,31P) = 7.3 Hz, R2PO(CH2)4OAc ), –12.2 (q, 4J(31P,19F) = 1.3 Hz POCl2(OAcF)) –22.8 (sept, 4J(31P,19F) = 1.3 Hz, POCl(OAcF)2) ppm

Solution afer 6 h in tetrahydrofuran: 19F-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F F δ = 76.50 (s, R2PO(CH2)4OAc , ∆1/2 = 1.2 Hz), –76.51 (s, ∆1/2 = 0.9 Hz), –76.6 (s, LiOAc , F ∆1/2 = 15 Hz), –76.9 (s, (Ac 2)O, ∆1/2 = 6.4 Hz), –77.2 (d, 4J(19F,31P) = 1.4 Hz, PO- Cl(OAcF)2) ppm 31P-NMR (161.99 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = –5.1 (s, POCl3, ∆1/2 = 1.9 Hz), –9.8 (dt, 3J(1H,31P) = 7.3 Hz, R2PO(CH2)4OAc ), –12.1 (q, 4J(31P,19F) = 1.3 Hz, POCl2(OAcF)), –22.7 (sept, 4J(31P,19F) = 1.3 Hz, POCl(OAcF)2) ppm 13C-NMR (100.62 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 113.8 (s, 1J(13C,19F) = 286 Hz, POCl(OCOCF3)2), 114.5 (s, 1J(13C,19F) = 286 Hz, POCl2(OCOCF3)), 114.6 (s, 1J(13C,19F) = 287 Hz, R2PO(CH2)4(OCOCF3), 115.9 (s, 1J(13C,19F) = 290 Hz, (CF3CO)2O)), 151.8 (s, 2J(13C,19F) = 44.4 Hz, POCl(OCOCF3)2), 152.9 (s, 2J(13C,19F) = 42.1 Hz, POCl2(OCOCF3)), 156.7 (s, 2J(13C,19F) = 41.7 Hz, R2PO(CH2)4(OCOCF3), 159.1 (s, 2J(13C,19F) = 37.9 Hz, (AcF2)O) ppm Experimental Section 75

LM-017 Freshly sublimed aluminium chloride (11 mg, 0.08 mmol, 0.01 eq.) was dissolved in phosphoryl chloride (0.75 mL, 1.26 mg, 8.2 mmol, 1 eq.). Ten the solution was dropped via a syringe into a clear solution of lithium trifuoroacetate (1.972 g, 16.4 mmol, 2 eq.), trifluoroacetic anhydride (4.6 mL, 6.85 mg, 32.9 mmol, 4 eq.) and tetrahydrofuran (25 mL). A NMR sample was taken afer this time, the reaction mixture was stirred over night and at the next day it was refuxed again for 3.5 h. Another NMR sample was taken from the clear reaction mixture.

Solution afer 2.5 h in tetrahydrofuran: 1H-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 1.72 (m, R2POCH2CH2CH2CH2OAcF), 1.85 (m, R2POCH2CH2CH2CH2OAcF), 4.08 (m, R2POCH2CH2CH2CH2OAcF), 4.39 (m, R2POCH2CH2CH2CH2OAcF) ppm 19F-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F F δ = –76.4 (s, R2PO(CH2)4OAc , ∆1/2 = 1.2 Hz), –76.5 (s, ∆1/2 = 0.9 Hz), –76.6 (s, LiOAc , F F ∆1/2 = 14 Hz), –76.8 (s, (Ac 2)O, ∆1/2 = 5.2 Hz), –77.1 (d, 4J(19F,31P) = 1.3 Hz, POCl(OAc )2, –77.2 (d, 4J(19F,31P) = 1.3 Hz, POCl2(OAcF)) ppm 31P-NMR (161.99 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = –5.0 (s, POCl3, ∆1/2 = 2.4 Hz), –9.8 (dt, 3J(1H,31P) =7.2 Hz, PO(CH2)4OAc ), –12.0 (q, 4J(31P,19F) = 1.3 Hz, POCl2(OAcF)), –22.6 (sept, 4J(31P,19F) = 1.3 Hz, F POCl(OAc )2), –22.7 (m, ∆1/2 = 2 Hz), –25.1 (m, ∆1/2 = 7 Hz) ppm 13C-NMR (100.62 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 24.6 (s, R2POCH2CH2CH2CH2OAcF), 26.1 (s, R2POCH2CH2CH2CH2OAcF), 66.2 (s, R2POCH2CH2CH2CH2OAcF), 67.5 (s, R2POCH2CH2CH2CH2OAcF), 114.1 (s, , 1J(13C,19F) = 286 Hz, POCl(OCOCF3)2), 114.2 (s, 1J(13C,19F) = 286 Hz, POCl2(OCOCF3)), 114.6 (s, 1J(13C,19F) = 284 Hz, PO(CH2)4(OCOCF3), 114.8 (s, 1J(13C,19F) = 288 Hz, (CF3CO)2O)), 151.4 (s, 2J(13C,19F) = 43 Hz, POCl2(OCOCF3)), 151.5 (s, 2J(13C,19F) = 44 Hz, POCl(OCOCF3)2), 156.7 (s, 2J(13C,19F) = 42 Hz, PO(CH2)4(OCOCF3) ppm

Solution afer 6 h in tetrahydrofuran: 19F-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F F δ = –76.49 (s, R2PO(CH2)4OAc , ∆1/2 = 1.2 Hz), –76.50 (s, ∆1/2 = 1.1 Hz), –76.6 (s, LiOAc , F ∆1/2 = 14 Hz), –76.8 (s, (Ac 2)O, ∆1/2 = 6.2 Hz), –77.2 (d, 4J(19F,31P) = 1.3 Hz, F F POCl(OAc )2, –77.3 (m, POCl2(OAc ), ∆1/2 = 3.4 Hz) ppm 31P-NMR (161.99 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = –5.1 (s, POCl3, ∆1/2 = 1.6 Hz), –9.8 (dt, 3J(1H,31P) = 7.3 Hz, R2PO(CH2)4OAc ), –12.0 (q, POCl2(OAcF)), –22.6 (sept, 4J(31P,19F) = 1.3 Hz, POCl(OAcF)2), –22.7 (m, ∆1/2 = 2 Hz) ppm 13C-NMR (100.62 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 114.1 (s, 1J(13C,19F) = 286 Hz, R2POCl(OCOCF3)2), 114.5 (s, 1J(13C,19F) = 285 Hz, R2PO(CH2)4(OCOCF3), 151.5 (s, 2J(13C,19F) = 44.6 Hz, POCl(OCOCF3)2), 156.8 (s, 2J(13C,19F) = 41.6 Hz, R2POO(CH2)4(OCOCF3) ppm 76 Lithium bis(trifluoroacetyl)phosphate

LM-018 In a round botom fask lithium trifuoroacetate (1.314 g, 11.0 mmol, 2 eq.) and trifuoroacetic anhydride (3.1 mL, 4.62 mg, 21.9 mmol, 4 eq.) were dissolved in tetrahydrofuran (25 mL). Ten a suspension of phosphoryl chloride (0.5 mL, 0.84 mg, 5.5 mmol, 1 eq.) with calcium chloride (12 mg, 0.02 mmol, 0.004 eq.) was dropped via a syringe into the solution. Afer this it was stirred and refuxed for 6 h. Ten the solvent was removed completely at the vacuum. A colourless powder was obtained.

Crude product dissolved in tetrahydrofuran: 19F-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F F δ = –76.51 (s, R2PO(CH2)4OAc , ∆1/2 = 1.4 Hz), 76.53 (s, ∆1/2 = 1.0 Hz), –76.6 (s, LiOAc , F ∆1/2 = 11 Hz), –76.8 (s, (Ac 2)O, ∆1/2 = 3.4 Hz) –77.2 (d, 4J(19F,31P) = 1.3 Hz, POCl(OAcF)2) ppm 31P-NMR (161.99 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = –9.8 (dt, 3J(1H,31P) = 7.3 Hz, PO(CH2)4OAcF), –22.6 (sept, 4J(31P,19F) = 1.3 Hz, R2PO- F Cl(OAc )2), –22.7 (m, ∆1/2 = 3 Hz) ppm

Powder: ATR-IR ν [cm−1] = 520 (w), 591 (vw), 729 (m), 778 (w), 803 (w), 859 (w), 1046 (m), 1113 (sh), 1150 (vs), 1208 (s), 1299 (br), 1353 (w), 1463 (w), 1688 (s), 1789 (m), 2854 (w), 2923 (w), 2941 (sh) Experimental Section 77

4.7.4.!Syntheses from LiH2PO4

MS-001

Lithium dihydrogenphosphate (0.513 g, 4.936 mmol, 1 eq.), trifuoroacetic anhydride (4.0 mL, 6.0 mg, 28.7 mmol, 5.8 eq.) and dried trifuoroacetic acid (0.4 mL, 0.61 mg, 5.26 mmol, 1.1 eq.) were mixed and treated in an ultrasonic bath for 2 h. Afer this the reaction mixture was fltered, washed three times with diethyl ether (5 mL) and dried under vacuum. A colourless powder was obtained (3.41 g).

A part of the product (0.341 g, 1.15 mmol) was dissolved in tetrahydrofuran (1.5 mL). To this a solution of lithium hydride in THF (1 mL, 0.5 mol L−1) was added under stirring. Afer 20 minutes the mixture was centrifuged. A NMR sample was taken.

Crude product dissolved in tetrahydrofuran: 1H-NMR (200.13 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 13.79 (s, HOAcF) ppm 19F-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F F δ = −77.2 (s, HOAc ), −77.1 (d, 4J(19F,31P) = 1.4 Hz, Li[PO2(OAc )2]), −76.9 (s, ∆1/2 = 10.5 Hz, LiOAcF), −76.4 (s, R2PO(CH2)4OAcF) ppm 31P-NMR (161.99 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −23.0 (s, ∆1/2 = 4.6 Hz, Li[PO2(OAc )2]) ppm 13C-NMR (100.62 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 156.7 (s, 2J(19F,13C) = 40.9 Hz, R2PO(CH2)4OOCCF3), 151.7 (s, 2J(19F,13C) = 40.3 Hz, HOOCCF3), 151.5 (s, 2C, 2J(19F,13C) = 43.9 Hz, Li[PO2(OOCCF3)2] 7Li-NMR (155.52 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −0.1 (s, ∆1/2 = 7.3 Hz, Li[PO2(OAc )2] ppm ATR-IR ν [cm−1] = 499 (w), 591 (vw), 769 (m), 863 (w), 977 (m), 1103 (vs), 1175 (s), 1233 (m), 1291 (m), 1329 (m), 1357 (w), 1799 (s), 2988 (vw)

MS-001 afer treatment with lithium hydride in tetrahydrofuran: 1H-NMR (200.13 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 14.00 (s, HOAcF) ppm 19F-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, HOAcF), −77.1 (d, 4J(19F,31P) = 1.3 Hz, Li[PO2(OOCCF3)2]), F F −76.8 (s, ∆1/2 = 4.0 Hz, LiOAc ), −76.4 (s, R2PO(CH2)4OAc ) ppm 31P-NMR (81.01 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −26.1 (m, unknown compound), −22.9 (s, ∆1/2 = 4.4 Hz, Li[PO2(OAc )2]) ppm 13C-NMR (100.62 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 156.8 (s, 2J(19F,13C) = 41.5 Hz, R2PO(CH2)4OOCCF3)2, 151.9 (s, 2J(19F,13C) = 43.5 Hz, HOOCCF3), 151.5 (s, 2J(19F,13C) = 43.5 Hz, Li[PO2(OOCCF3)2], 115.9 (s, 1J(19F13C) = 290.7 Hz, LiOOCCF3), 114.6 (s, 1J(19F13C) = 286.0 Hz, HOOCCF3), 114.3 (s, 1J(19F,13C) = 286.0 Hz, Li[PO2(OOCCF3)2], 114.0 (s, 1J(19F13C) = 282.0 Hz, R2PO(CH2)4OOCCF3) ppm 7Li-NMR (155.52 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −0.1 (s, ∆1/2 = 7.3 Hz, Li[PO2(OAc )2]) ppm 78 Lithium bis(trifluoroacetyl)phosphate

MS-002 Lithium dihydrogenphosphate (0.50 g, 4.81 mmol, 1 eq.) was suspended in diethyl ether (6 mL). Trifuoroacetic anhydride (4 mL, 6 g, 28.7 mmol, 6 eq.) was added and afer this the reaction mixture was treated with ultrasound for 2 h. Afer this the reaction mixture was fltered, washed three times with diethyl ether (5 mL) and dried under vacuum. A colourless powder was obtained (0.45 g).

Powder dissolved in tetrahydrofuran: 1H-NMR (200.13 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 13.75 (s, HOAcF) ppm 19F-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, HOAcF), −77.1 (d, 4J(19F,31P) = 1.4 Hz, 2J(19F,13C) = 43.4 Hz, 1J(19F,13C) = 285.8 Hz, F F Li[PO2(OOCCF3)2]), −76.8 (s, ∆1/2 = 5.3 Hz, LiOAc ), −76.4 (s, R2PO(CH2)4OAc ) ppm 31P-NMR (81.01 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −26.1 (m, unknown compound), −23.0 (sept, 4J(19F,31P) = 1.3 Hz, Li[PO2(OAcF)2]) ppm

Powder: ATR-IR ν [cm−1] = 501 (vw), 519 (m), 590 (w), 731 (w), 755 (sh), 771 (m), 788 (sh), 802 (m), 877 (w), 962 (sh), 1014 (sh), 1087 (vs), 1103 (vs), 1177 (vs), 1234 (s), 1311 (m), 1338 (m), 1790 (sh), 1807 (s) MS-003 Lithium dihydrogenphosphate (2.042 g, 19.65 mmol, 1 eq.) was suspended in diethyl ether (15 mL). Trifuoroacetic anhydride (13.5 mL, 20.1 mg, 97.06 mmol, 4.94 eq.) was added and afer this the reaction mixture was treated with ultrasound for 2 h. Afer this the reaction mixture was fltered. Te residue was washed four times with diethyl ether (5 mL). Te solvent of the fltrate was removed under vacuum. Te remaining colourless powder was washed with diethyl ether (2 mL) and fltered and then dried under vacuum. A colourless powder was obtained (1.56 g).

Powder dissolved in tetrahydrofuran: 19F-NMR (376.54 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, HOAcF), −77.1 (d, 4J(19F,31P) = 1.3 Hz, Li[PO2(OOCCF3)2]), F F −76.8 (s, ∆1/2 = 11.3 Hz, LiOAc ), −76.4 (s, R2PO(CH2)4OAc ) ppm 31P-NMR (81.01 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −25.3 (m, unknown compound), −23.0 (s, ∆1/2 = 5.9 Hz, Li[PO2(OAc )2]), F −10,3 (s, ∆1/2 = 21.8 Hz, R2PO(CH2)4OAc ) ppm 13C-NMR (100.62 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 156.8 (s, 2J(19F,13C) = 41.9 Hz, R2PO(CH2)4OOCCF3)2), 151.7 (s, 2J(19F,13C) = 44.1 Hz, Li[PO2(OOCCF3)2]), 115.8 (s, 1J(19F,13C) = 283.9 Hz, HOOCCF3), 115.3 (s, 1J(19F13C) = 289.0 Hz, LiOOCCF3), 114.7 (s, 1J(19F13C) = 287.08 Hz, R2PO(CH2)4OOCCF3), 114.2 (s, 1J(19F,13C) = 285.8 Hz, Li[PO2(OOCCF3)2]) ppm 7Li-NMR (155.52 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −0.6 (s, ∆1/2 = 16.0 Hz, Li[PO2(OAc )2]) ppm Experimental Section 79

MS-004 Lithium dihydrogenphosphate (2.01 g, 19.33 mmol, 1 eq.) was suspended in diethyl ether (15 mL). Trifuoroacetic anhydride (13.5 mL, 20.1 mg, 97.06 mmol, 5.02 eq.) was added and afer this the reaction mixture was treated with ultrasound for 2 h. Afer this the reaction mixture was fltered. Te residue was washed four times with diethyl ether (5 mL). Te solvent of the fltrate was removed under vacuum. Te remaining colourless powder was washed with diethyl ether (2 mL) and fltered and then dried under vacuum. A colourless powder was obtained (1.56 g).

Powder dissolved in tetrahydrofuran:: 1H-NMR (200.13 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 13.84 (s, HOAcF) ppm 19F-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, HOAcF), −77.1 (d, 4J(19F,31P) = 1.3 Hz, Li[PO2(OOCCF3)2]), F F −76.9 (s, ∆1/2 = 4.2 Hz, LiOAc ), −76.4 (s, R2PO(CH2)4OAc ) ppm 31P-NMR (81.01 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −25.1 (m, unknown compound), −22.9 (s, ∆1/2 = 4.4 Hz, Li[PO2(OAc )2]), −10.2 (t, 3J(1H,31P) = 6.5 Hz, R2PO(CH2)4OAcF) ppm

Powder: ATR-IR ν [cm−1] = 501 (m), 520 (w), 593 (vw), 732 (w), 771 (w), 878 (vw), 1087 (vs), 1105 (s), 1178 (s), 1234 (m), 1311 (m), 1337 (m), 1807 (s) 80 Lithium bis(trifluoroacetyl)phosphate

MS-005 Lithium dihydrogenphosphate (2.31 g, 22.23 mmol, 1 eq.) was suspended in diethyl ether (6 mL). And trifuoroacetic anhydride (13.5 mL, 20.1 g, 97.06 mmol, 4.37 eq.) was added and afer this the reaction mixture was treated with ultrasound for 2 h. Ten the reaction mixture was fltered, the residue washed four times with diethyl ether (5 mL). Te solvent of the united ether phases was removed under vacuum. A colourless powder was obtained (1.7 g).

Powder dissolved in tetrahydrofuran: 1H-NMR (200.13 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 13.83 (s, HOAcF) ppm 19F-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, HOAcF), −77.1 (d, 4J(19F,31P) = 1.4 Hz, Li[PO2(OOCCF3)2]), F −77.1 (d, 4J(19F,31P) = 1.4 Hz, Li[PO2(OOCCF3)2]), −76.8 (s, ∆1/2 = 5.23 Hz, LiOAc ), −76.4 (s, R2PO2(CH2)4OAcF) ppm 31P-NMR (81.01 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −26.1 (m, unknown compound), −25.2 (m, unknown compound), F F −22.9 (s, ∆1/2 = 7.0 Hz, Li[PO2(OAc )2]), −10.2 (s, R2PO2(CH2)4OAc ) ppm 13C-NMR (100.62 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 156.9 (s, 2J(19F,13C) = 42.9 Hz, R2PO(CH2)4OOCCF3)2), 151.6 (s, 2J(19F,13C) = 43.5 Hz, Li[PO2(OOCCF3)2]), 114.7 (s, 1J(19F13C) = 286.4 Hz, R2PO(CH2)4OOCCF3), 114.3 (s, 1J(19F,13C) = 285.2 Hz, Li[PO2(OOCCF3)2]) ppm 7Li-NMR (155.52 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −0.5 (s, ∆1/2 = 7.1 Hz, Li[PO2(OAc )2]) ppm

Powder: ATR-IR ν [cm−1] = 501 (w), 519 (w), 592 (vw), 731 (w), 771 (w), 878 (vw), 1088 (vs), 1105 (s), 1178 (s), 1234 (m), 1311 (m), 1337 (w), 1808 (s), 2989 (vw)

MS-005 (Purifcation in dimethyl carbonate): Te product mixtures from MS-002, MS-003, MS-004, MS-005 were combined (0.770 g) and recrystallized in dimethyl carbonate (15.5 mL). Te hot solution was fltered and the solvent was removed in vacuum.

Residue dissolved in tetrahydrofuran: 1H-NMR (200.13 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 13.75 (s, HOAcF) ppm 19F-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, HOAcF), −77.1 (d, 4J(19F,31P) = 1.3 Hz, Li[PO2(OOCCF3)2]), F F −76.8 (s, ∆1/2 = 6.9 Hz, LiOAc ), −76.4 (s, R2PO2(CH2)4OAc ) ppm 31P-NMR (81.01 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −22.4 (m, unknown compound), −22.3 (s, ∆1/2 = 7.0 Hz, Li[PO2(OAc )2]) ppm 7Li-NMR (155.52 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = −0.3 (s, Li[PO2(OAcF)2]) ppm Experimental Section 81

MS-006 Lithium dihydrogenphosphate (1.793 g, 17.25 mmol, 1 eq.) was suspended in diethyl ether (13 mL). And trifuoroacetic anhydride (12.0 mL, 17.9 g, 86.27 mmol, 5.0 eq.) was added and afer this the reaction mixture was treated with ultrasound for 3 h. Te ultrasonic bath was cooled with ice. Ten the reaction mixture was fltered, the residue washed four times with diethyl ether (7 mL). Te solvent of the united ether phases was removed under vacuum. A colourless powder was obtained (2.629 g).

A part of the obtained product was separated (0.326 g, 1.1 mmol) and washed with n-butyl ether and then dried in vacuum.

Powder dissolved in diethyl ether: 1H-NMR (200.13 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = 14.00 (s, HOAcF) ppm 19F-NMR (188.31 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (d, ∆1/2 = 4.0 Hz, 1J(19F,13C) = 285.9 Hz, 2J(19F,13C) = 44.8 Hz, Li[PO2(OOCCF3)2]), F −76.4 (s, ∆1/2 = 31.1 Hz, LiOAc ) ppm 31P-NMR (81.01 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) F δ = −23.7 (s, ∆1/2 = 9.4 Hz, Li[PO2(OAc )2]) ppm 7Li-NMR (155.52 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) F δ = −0.5 (s, ∆1/2 = 10.9 Hz, Li[PO2(OAc )2]) ppm

Powder: ATR-IR ν [cm−1] = 500 (w), 592 (vw), 731 (m), 770 (m), 867 (w), 1088 (vs), 1176 (s), 1234 (s), 1312 (m), 1337 (m), 1806 (s), 2990 (vw)

MS-006 in diethyl ether (afer washing with n-butyl ether): 1H-NMR (200.13 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = 13.00 (s, HOAcF) ppm 19F-NMR (188.31 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = −77.3 (s, HOAcF) −77.2 (d, 4J(19F,31P) = 1.4 Hz, Li[PO2(OOCCF3)2]) ppm 7Li-NMR (155.52 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) F δ = −0.6 (s, ∆1/2 = 2.7 Hz, Li[PO2(OAc )2]) ppm 82 Lithium bis(trifluoroacetyl)phosphate

MS-007 Lithium dihydrogenphosphate (6.972 g, 67 mmol, 1 eq.) was suspended in diethyl ether (40 mL). And trifuoroacetic anhydride (48.0 mL, 71.5 g, 345 mmol, 5.1 eq.) was added and afer this the reaction mixture was treated with ultrasound for 3 h. Te ultrasonic bath was cooled with ice. Ten the reaction mixture was fltered, the residue washed fve times with diethyl ether (20 mL). Te solvent of the united ether phases was removed under vacuum. A colourless powder was obtained (3.3 g).

Powder dissolved in diethyl ether: 1H-NMR (400.17 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = 13.01 (s, HOAcF) ppm 19F-NMR (376.54 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = −77.3 (s, HOAcF) −77.2 (d, 4J(19F,31P) = 1.4 Hz, Li[PO2(OOCCF3)2]), F −76.3 (s, ∆1/2 = 18.4 Hz, LiOAc ) ppm 31P-NMR (161.99 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) F δ = −23.7 (s, ∆1/2 = 6.5 Hz, Li[PO2(OAc )2]) ppm 13C-NMR (100.62 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = 157.5 (s, 2J(19F13C) = 40.2 Hz HOOCCF3), 151.8 (s, 2J(19F,13C) = 44.4 Hz, Li[PO2(OOCCF3)2]), 115.0 (s, 1J(19F,13C) = 286.2 Hz, HOOCCF3), 114.3 (s, 1J(19F13C) = 285.3 Hz, Li[PO2(OOCCF3)2]) ppm 7Li-NMR (155.52 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) F δ = −0.6 (s, ∆1/2 = 2.7 Hz, Li[PO2(OAc )2]) ppm

Powder: ATR-IR ν [cm−1] = 502 (w), 522 (w), 593 (vw), 732 (m), 771 (m), 867 (w), 1002 (m), 1103 (vs), 1178 (vs), 1234 (s), 1310 (m), 1801 (s) Experimental Section 83

MS-008 Lithium dihydrogenphosphate (1.50 g, 14.4 mmol, 1 eq.) was suspended in diethyl ether (15 mL). And trifuoroacetic anhydride (10.0 mL, 14.9 g, 71.9 mmol, 5.0 eq.) was added and afer this the reaction mixture was treated with ultrasound for 3 h. Te ultrasonic bath was cooled with ice. Ten the reaction mixture was fltered, the residue washed four times with diethyl ether (5 mL). Te solvent of the united ether phase was concentrated under the vacuum, and washed with n-butyl ether (4 mL) and then dried at the vacuum. A colourless powder was obtained (2.07 g).

A part of the obtained powder (0.996 g) was suspended in diethyl ether (10 mL) and a piece of a lithium wire (0.20 g, 0.03 mmol) was added and treated with ultrasound for 1 h. Ten the solution was fltered and concentrated. Afer several days a yellowish precipitate was observed.

Powder dissolved in diethyl ether: 1H-NMR (200.13 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = 13.08 (s, HOAcF) ppm 19F-NMR (188.31 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (d, 4J(19F,31P) = 1.4 Hz, 2J(19F,13C) = 44.6 Hz, 1J(19F,13C) = 285.3 Hz, Li[PO2(OOCCF3)2]) ppm 31P-NMR (81.01 MHz, Et2O = 1.2 ppm, toluene-D8 external lock, r.t.) F δ = −23.6 (s, ∆1/2 = 6.3 Hz, Li[PO2(OAc )2]) ppm

Powder: ATR-IR ν [cm−1] = 501 (m), 592 (vw), 731 (m), 771 (m), 867 (w), 1002 (m), 1106 (vs), 1173 (vs), 1234 (s), 1310 (m), 1693 (w), 1801 (s), 2967 (vw)

MS-008 dissolved in tetrahydrofuran (afer treatment with lithium foil): 1H-NMR (200.13 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) δ = 13.16 (s, HOAcF) ppm 19F-NMR (188.31 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −77.3 (s, ∆1/2 = 20 Hz, HOAc ), −77.2 (d, 4J(19F,31P) = 1.4 Hz, 2J(19F,13C) = 43.8 Hz, 1J(19F,13C) = 285.3 Hz, Li[PO2(OOCCF3)2]) ppm 31P-NMR (81.01 MHz, THF = 1.72 ppm, toluene-D8 external lock, r.t.) F δ = −23.5 (s, ∆1/2 = 6.0 Hz, Li[PO2(OAc )2]) ppm 84 Lithium bis(trifluoroacetyl)phosphate

MS-010 Lithium dihydrogenphosphate (2.046 g, 19 .7 mmol, 1 eq.), trifuoroacetic anhydride (13.5 mL, 20.1 g, 97.1 mmol, 4.9 eq.), diethyl ether (10 mL) and ethylene carbonate (6.782 g, 77.0 mmol, 3.91 eq.) were mixed together and treated with ultrasound for 3 h. Some diethyl ether (10 mL) was added to the turbid suspension and then fltered. Te residue was washed with diethyl ether (3 mL). Te united fltrates were concentrated under vacuum. A turbid viscous liquid was obtained.

Liquid in ethylene carbonate: 1H-NMR (200.13 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) δ = 12.66 (s, HOAcF) ppm 19F-NMR (188.31 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) F δ = −77.4 (s, ∆1/2 = 7.5 Hz, HOAc ), −77.3 (d, ∆1/2 =6.4 Hz, Li[PO2(OOCCF3)2]), F −76.8 (s, ∆1/2 = 22.6 Hz, LiOAc ) ppm 31P-NMR (81.01 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) F δ = −24.2 (s, unknown compound), −24.1 (s, ∆1/2 = 9.7 Hz, Li[PO2(OAc )2]) ppm 13C-NMR (100.62 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) δ = 157.6 (s, 2J(19F,13C) = 38.9 Hz, HOOCCF3), 151.2 (s, 2J(19F,13C) = 44.7 Hz, Li[PO2(OOCCF3)2]), 114.4 (s, 1J(19F13C) = 285.9 Hz, HOOCCF3), 113.8 (s, 1J(19F,13C) = 284.7 Hz, Li[PO2(OOCCF3)2]) ppm 7Li-NMR (155.52 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) F δ = −0.9 (s, ∆1/2 = 35.9 Hz, Li[PO2(OAc )2]) ppm

Turbid viscous liquid: ATR-IR ν [cm−1] = 691 (vw), 716 (w), 772 (m), 903 (vw), 1066 (vs), 1150 (s), 1391 (m), 1482 (w), 1771 (vs), 1795 (s), 2934 (vw), 2999 (vw) Experimental Section 85

MS-012 Lithium dihydrogenphosphate (2.011 g, 19.35 mmol, 1 eq.), trifuoroacetic anhydride (13.5 mL, 20.1 g, 97.1 mmol, 5.01 eq.), diethyl ether (10 mL) and ethylene carbonate (7.014 g, 79.65 mmol, 4.12 eq.) were mixed together and treated with ultrasound for 3 h. Te warm reaction mixure was fltered, the residue washed with diethyl ether (20 mL) and then all volatile compounds were removed under the vacuum. A colourless solid was obtained.

Solid dissolved in ethylene carbonate: 1H-NMR (400.17 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) δ = 13.09 (s, HOAcF) ppm 19F-NMR (376.54 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) F δ = −77.2 (s, HOAc ), −77.1 (d, ∆1/2 =7.0 Hz, 2J(19F,13C) =43.1 Hz, 1J(19F,13C) =283.7 Hz, F F Li[PO2(OOCCF3)2]), −76.9 (s, ∆1/2 = 118.5 Hz, LiOAc ), −76.4 (s, R2PO2(CH2)4OAc )), −75.4 (m, unknown compound) ppm 31P-NMR (81.01 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) F δ = −23.4 (s, ∆1/2 = 25.7 Hz, Li[PO2(OAc )2]), −22.7 (m, unknown compound) ppm 7Li-NMR (155.52 MHz, EC = 4.2 ppm, toluene-D8 external lock, r.t.) F δ = −0.9 (s, ∆1/2 = 11.5 Hz, Li[PO2(OAc )2]) ppm ATR-IR ν [cm−1] = 715 (w), 771 (w), 893 (vw), 970 (w), 1064 (s), 1152 (m), 1390 (w), 1421 (vw), 1481 (w), 1552 (vw), 1769 (vs), 1794 (s), 1963 (vw), 2932 (vw), 2998 (vw) 86 Lithium bis(trifluoroacetyl)phosphate

MS-013 Lithium dihydrogenphosphate (2.041 g, 19.64 mmol, 1 eq.), dimethyl carbonate (6.5 mL, 7.0 g, 7.7 mmol, 4 eq.) and trifuoroacetic anhydride (13.5 mL, 20.1 g, 97.05 mmol, 4.9 eq.) were mixed together and treated with ultrasound for 3 h. Te ultrasonic bath was cooled with ice. Ten the reaction mixture was fltered. Te residue was washed four times with dimethyl carbonate (5 mL). To the fltrate was added a piece of lithium foil (160 mg, 20.15 mmol, 1.02 eq.). Additional dimethyl carbonate (10 mL) was given to the reaction mixture. And then the reaction mixture was treated with ultrasound until no gas evolution was observed (1 h). Te mixture was fltered, washed with dimethyl carbonate (5 mL) and the solvent was removed under the vacuum. A colourless powder was obtained (2.77 g).

Powder dissolved in dimethyl carbonate: 1H-NMR (400.17 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 13.64 (s, HOAcF) ppm 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −77.2 (s, HOAcF), −77.2 (d, 4J(19F,31P) = 1.3 Hz, 2J(19F,13C) = 43.2 Hz, 1J(19F,13C) =284.2 Hz, F Li[PO2(OOCCF3)2]), −77.1 (s, ∆1/2 = 36 Hz, 1J(19F,13C) = 292.9 Hz, LiOAc ) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −22.9 (m, unknown compound), −22.5 (s, ∆1/2 = 11.5 Hz, Li[PO2(OAc )2]), −22.2 (m, unknown compound) −21.0 (m, unknown compound) ppm 7Li-NMR (155.52 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = 0.01 (s, ∆1/2 = 5.6 Hz, Li[PO2(OAc )2]) ppm

Powder: ATR-IR ν [cm−1] = 520 (vw), 590 (vw), 730 (w), 802 (w), 862 (w), 962 (vw), 1014 (vw), 1106 (m), 1154 (s), 1204 (s), 1322 (w), 1436 (vw), 1470 (w), 1678 (vs), 1810 (w), 2974 (vw) Experimental Section 87

MS-014 Lithium dihydrogenphosphate (2.633 g, 25.33 mmol, 1 eq.), dimethyl carbonate (8.5 mL, 9.1 g, 101 mmol, 4 eq.) and trifuoroacetic anhydride (17.6 mL, 26.2 g, 126.53 mmol, 5.0 eq.) were mixed together and treated with ultrasound for 3 h. Te ultrasonic bath was cooled with ice. Afer one hour lithium hydrid (260 mg, 32.74 mmol, 1.3 eq.) was added and sonicated until no gas evolution was observed. Ten the reaction mixture was fltered and concentrated. Additional lithium hydride (0.024 g, 30.2 mmol, 1.3eq.) and dimethyl carbonate (15 mL) were added and sonicated for 1 h. Afer this the reaction mixture was seperated into two parts. One part (#1) was dried under vacuum. A solid was obtained (4.91 g). To the second part (#2) of the reaction mixture was added dropwise methylene chloride for a fractional precipitation. Afer addition of 30 mL methylene chloride a precipitation was observed. Afer the addition of methylene chloride the mixture was fltered. Te precipitate was dried under the vacuum. Te fltrate was concentrated to a ffh of the volume.

Precipitate #1 dissolved in dimethyl carbonate: 1H-NMR (200.13 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 15.78 (s, HOAcF) ppm 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −82.5 (d, J = 3.4 Hz, unknown compound), −77.5 (s, HOAcF), −77.5 (d, 4J(19F,31P) = 1.3 Hz, Li[PO2(OOCCF3)2]), −77.3 (s, 4J(19F,13C) = 290.9 Hz, LiOAcF), −76.5 (s, unknown compound), −76.3 (s, unknown compound), −75.4 (t, J = 8.3 Hz, unknown compound) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −22.6 (s, unknown compound), −22.2 (s, ∆1/2 = 4.4 Hz, Li[PO2(OAc )2]), −20.7 (m, unknown compound) ppm 7Li-NMR (155.52 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = 0.7 (s, ∆1/2 = 11.6 Hz, Li[PO2(OAc )2]) ppm

Precipitate #1: ATR-IR ν [cm−1] = 520 (vw), 608 (vw), 730 (m), 802 (w), 861 (w), 962 (vw), 1015 (vw), 1106 (m), 1152 (s), 1205 (s), 1320 (m), 1436 (vw), 1469 (w), 1677 (vs), 1807 (vw), 2974 (vw)

Precipitate #1: ESI-MS (3 kV) m/z (%): 112.98 (100) [AcFO−], 226.98 (46) [H(OAcF)2−], 232.98 (49) [Li(OAcF)2−], 354.98 (27) [Li2(OAcF)3−], 472.98 (12) [Li3(OAcF)4−] 88 Lithium bis(trifluoroacetyl)phosphate

Concentrated fltrate #2 in dimethyl carbonate: 1H-NMR (200.13 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 16.05 (s, HOAcF) ppm 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −83.1 (d, J = 3.4 Hz, −CF3 unknown compound), −82.9 (d, J = 3.1 Hz, −CF3 unknown compound), −82.5 (d, J = 3.3 Hz, −CF3 unknown compound), −77.6 (s, −CF3 unknown compound), −77.5 (s, HOAcF), −77.5 (d, 4J(19F,31P) = 1.3 Hz, Li[PO2(OOC- CF3)2]), −77.4 (s, 1J(19F,13C) = 289.9 Hz, LiOAcF), −76.6 (s, −CF3 unknown compound), −76.3 (s, −CF3 unknown compound) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −22.9 (s, unknown compound), −22.2 (s, ∆1/2 = 4.4 Hz, Li[PO2(OAc )2]), −20.7 (m, unknown compound), −20.3 (m, unknown compound) ppm 7Li-NMR (155.52 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = 0.2 (s, ∆1/2 = 11.7 Hz, Li[PO2(OAc )2]) ppm

Precipitate #2 in dimethyl carbonate: H-NMR (200.13 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 14.47 (s, HOAcF) ppm 19F-NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −77.1 (s, HOAcF), −77.1 (d, 4J(19F,31P) = 1.2 Hz, Li[PO2(OOCCF3)2]), −76.7 (s, 4J(19F,13C) = 290.3 Hz, LiOAcF), −76.3 (s, −CF3 unknown compound), −76.1 (s, −CF3 unknown compound), −75.7 (s, −CF3 unknown compound) ppm 31P-NMR (81.01 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) F δ = −22.4 (s, unknown compound), −22.0 (s, ∆1/2 = 10.5 Hz, Li[PO2(OAc )2]), −20.5 (m, unknown compound) ppm Experimental Section 89

4.7.5.!Synthesis from Li[PO2F2]

MR-071

In a round botom fask lithium difuorophosphate (3.00 g, 27.8 mmol, 1 eq.) and trimethylsilyl trifuoroacetate (14.5 mL, 15.53 mg, 83.4 mmol, 3 eq.) were dissolved in acetonitrile (70 mL) and stirred and refuxed for several days. Afer 5 h, 20 h and 35 h NMR samples were taken.

In-situ NMR afer 5 h in acetonitrile: 1H-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = –0.35 (s, (CH3)3–Si–OAcF) ppm 19F-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −71.8 (s, −OAcF), −73.6 (d, 1J(19F,31P) = 957 Hz, Li[PO2FX]), −74.9 (d, 1J(19F,31P) = 916 Hz, F F Li[PO2FOAc ]), −76.8 (d, 4J(19F,31P) = 1.2 Hz, Li[PO2(OAc )2]), −77.1 (s, ∆1/2 = 13.5 Hz, Me3SiOAcF), −85.1 (d, 1J(19F,31P) = 932 Hz, Li[PO2F2]), −157.7 (dec, 3J(19F,29Si) = 7.5 Hz, Me3SiF) ppm 31P-NMR (161.99 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −16.4 (t, 1J(19F,31P) = 932 Hz, Li[PO2F2]), −26.5 (d, 1J(19F,31P) = 916 Hz, Li[PO2FOAcF]), −26.9 (s, Li[PO2(OAcF)2]) ppm 29Si-NMR (79.49 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = 34.3 (s, Me3SiOAcF) ppm

In-situ NMR afer 20 h in acetonitrile: 1H-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = –0.35 (s, (CH3)3–Si–OAcF) ppm 19F-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −71.8 (s, −OAcF), −73.92 (d, 1J(19F,31P) = 960 Hz, Li[PO2FX]), −74.9 (d, 1J(19F,31P) = 915 Hz, Li[PO2FOAcF]), −76.8 (d, 4J(19F,31P) = 1.3 Hz, Li[PO2(OAcF)2]), F −77.1 (s, ∆1/2 = 14.5 Hz, Me3SiOAc ), −85.1 (d, 1J(19F,31P) = 932 Hz, Li[PO2F2]), −157.7 (dec, 3J(19F,29Si) = 7.5 Hz, Me3SiF) ppm 31P-NMR (161.99 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −17.3 (t, 1J(19F,31P) = 933 Hz, Li[PO2F2]), −26.5 (d, 1J(19F,31P) = 915 Hz, Li[PO2FOAcF]), −26.9 (s, Li[PO2(OAcF)2]) ppm 13C-NMR (100.62 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −114.8 (Me3SiOCOCF3), −156.7 (Me3SiOCOCF3) ppm 29Si-NMR (79.49 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = 34.2 (s, Me3SiOAcF) ppm 90 Lithium bis(trifluoroacetyl)phosphate

In-situ NMR afer 5 h in acetonitrile: 1H-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = –0.35 (s, (CH3)3–Si–OAcF) ppm 19F-NMR (376.54 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −71.8 (s, −OAcF), −74.3 (d, 1J(19F,31P) = 914 Hz, Li[PO2FOAcF]), −76.8 (d, F F 4J(19F,31P) = 1.3 Hz, Li[PO2(OAc )2]), −77.1 (s, ∆1/2 = 14.5 Hz, Me3SiOAc ), −85.1 (d, 1J(19F,31P) = 932 Hz, Li[PO2F2]), −157.7 (dec, 3J(19F,29Si) = 7.3 Hz, Me3SiF) ppm 31P-NMR (161.99 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −17.4 (t, 1J(19F,31P) = 932 Hz, Li[PO2F2]), −27.1 (d, 1J(19F,31P) = 914 Hz, Li[PO2FOAcF]), −27.6 (s, Li[PO2(OAcF)2]) ppm 13C-NMR (100.62 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = −114.8 (s, 1J(19F,13C) = 287.2 Hz, Me3SiOCOCF3), −156.7 (s, 2J(19F,13C) = 43.3 Hz, Me3SiOCOCF3) ppm 29Si-NMR (79.49 MHz, MeCN = 1.93 ppm, toluene-D8 external lock, r.t.) δ = 34.3 (s, Me3SiOAcF) ppm References 91

4.8!References

[134]R. Dallenbach, P. Tissot, J. Term. Anal. 1981, 20, 409-417. [135]M. G. Harriss, J. B. Milne, Can. J. Chem. 1971, 49, 3612-3616. [136]P. Sartori, J. Fazekas, J. Schnackers, Journal of Fluorine Chemistry 1972, 1, 463-471. [137]P. V. Radheshwar, R. Dev, G. H. Cady, J. Inorg. Nucl. Chem. 1972, 34, 3913-3915. [138]C. D. Garner, B. Hughes, Inorg. Chem. 1975, 14, 1722-1724. [139]E. Tilo, I. Grunze, Z. Anorg. Allg. Chem. 1957, 290, 223-237. [140]I. Grunze, K. Dostal, E. Tilo, Z. Anorg. Allg. Chem. 1959, 302, 221-229. [141]E. Tilo, I. Grunze, H. Grunze, Monatsber. Dtsch. Akad. Wiss. Berlin 1959, 1, 40-42. [142]I. Grunze, E. Tilo, H. Grunze, Chem. Ber. 1960, 93, 2631-2638. [143]F. Kasparek, Monatsh. Chem. 1961, 92, 1023-1026. [144]J. Neels, H. Grunze, Z. Anorg. Allg. Chem. 1982, 495, 65-72. [145]T. P. Smyth, B. W. Corby, J. Org. Chem. 1998, 63, 8946-8951. [146]F. Efenberger, G. Koenig, H. Klenk, Angew. Chem. 1978, 90, 740-741. [147]R. Ditchfeld, Mol. Phys. 1974, 27, 789-807. [148]N. S. Corby, G. W. Kenner, A. R. Todd, J. Chem. Soc. 1952, 1234-1243. [149]D. R. Lide, in CRC Handbook of Chemistry and Physics, 89th ed., CRC Press/Taylor and Francis, Boca Raton, FL, 2009.

5!Lithium tetrakis(trifluoroethoxy)borate

5.1!Introduction and Overview

Te lithium salt of the symmetric anion tetrakis(2,2,2-trifuoroethoxy)borate (Li[B(OTfe)4], OTfe: –OCH2CF3, Figure 36) is a promising candidate for the use as conducting salt in lithium-ion bateries. Te four 2,2,2-trifuoroethoxy groups, which are connected to the negatively charged central boron atom, have a strong electron

CF3

O CF3 Li O B O

F3C O

CF3

Figure 36!Lewis formula of Li[B(OTfe)4] Figure 37!Electrostatic potential (calulated on RI-BP86/def2-TZVP level) of Li[B(OTfe)4] projected on a isomap of electron density (0.01 e Å−1). withdrawing efect. Tis leads to a delocalization of the negative charge over the large surface of the substituents. Furthermore the fuorinated ethoxy-group is chemically robust, what is necessary to sustain the harsh conditions in bateries. Te sterically demanding substituents protect against nucleophilic atack at the Lewis acidic boron atom. In contrast to more bulky fuorinated alkoxy groups, like 1,1,1,3,3,3-hexafuoro-2- propoxy (–Ohfp), the trifuoroethoxy group is less weighty. Te molecular weight of Li[B(OTfe)4] is 413.88 g mol−1 which is just acceptable for the usage in lithium-ion bateries. Tis is further supported by the fact that the 2,2,2-trifuorethanol is much cheaper than the 1,1,1,3,3,3-hexafuoro-2-propanol (HOhfp) (Sigma Aldrich: 500 g HOTfe = 213 € , HOhfp = 816 €). Te glance on the electrostatic potential map (Figure 37) confrms a rather uniform distribution of the negative charge, with the oxygen atoms being the most negatively charged entities. Tis indicates that tetrakis(trifuoroethoxy)borate is a weakly coordinating anion. Also the calculated molecular orbital energies (Table 12)

Table 12!Comparison of molecular orbital energies of tetrakis(trifluoroethoxy)borate with other weakly coordination anions (RI-BP86/def2-TZVPP).

MO [B(OTfe)4]− [PF6]– [B(Ohfip)4]− [Al(Ohfip)4]− [NTf2]− [BOB]− HOMO −2.7634 eV −3.3598 eV −3.8494 eV –4.1015 eV −2.8531 eV −2.9576 eV LUMO +3.5406 eV +5.5247 eV +2.9813 eV +2.3457 eV +3.1723 eV +0.6044 eV Gap +6.3039 eV +8.8845 eV +6.8307 eV +6.4472 eV +6.0254 eV +3.5620 eV 94 Lithium tetrakis(trifluoroethoxy)borate

give an idea of the stability against reduction and oxidation. Te low HOMO energy of −2.76 eV is similar to the HOMO energy of lithium bis(trifuoromethylsulfonyl)imide (HOMO: Li[NTf2] = −2.85 eV), which suggests a high stability against oxidation on the cathode. Te very high LUMO energy of +3.54 eV presumes a high resistance against reduction. All these facts demonstrate in advance that lithium tetrakis(trifuoroethoxy) borate has potential as a novel conducting salt for lithium-ion bateries. Te frst synthesis, characterisation and electrochemical performance is described in this section. Te synthesis of the lithium salt of the [B(OTfe)4]− anion is not described in the literature yet. But the synthesis of the sodium salt, Na[B(OTfe)4] from sodium borohydride and the alcohol, as product of the disproportion of sodium tris(2,2,2-trifuoroetoxy) borohydride, was described frst by Golden et al. in 1992 [150]. Te related borate anion [B(Ohfp)4]− was frst synthesized and characterized 2011 by the Krossing group [151]. Te lithium salt of the tetrakis(2,2,2-trifuoroethoxy)aluminate (Li[Al(OTfe)4]) is published as conducting salt in lithium-ion bateries [152]. Other symmetric lithium borates with haloacyloxy groups (Li[B(OCORX)4], X = CF3, C2F3, CClF2, CCl3) were published by Yamagughi et al. in 2003 [153]. Besides there are many other lithium borates containing aromatic chelato ligands, such as lithium bis[1,2-benzenediolato-O,O’]borate, lithium bis[3-fuoro-1,2-benzenediolato-O,O’]borate [154], lithium bis[2,2’-biphenyldiolato)-O,O’] borate, lithium bis[salicylato]borate [155] and lithium bis[5-fuoro-2-olato-1-benzenesul- fonato-O,O’]borate [156]. Te Gores group reported in 2000 of the frst lithium spirobo- rate known with a heterocycle: lithium bis[2,3-pyridinediolato-O,O’]borate [157]. Other non-aromatic chelato borate such as lithium bis(malonato)borate [158] and lithium (per- fuoropinacolato)borate [159] are known.

5.2!Synthesis and Characterisation

5.2.1.!Synthesis

Te synthesis of lithium tetrakis(trifuoroethoxy)borate was made from cheap and highly pure starting materials. Te reaction between lithium borohydride and four equivalent 2,2,2-trifuorethanol leads to Li[B(OTfe)4] with formation of hydrogen gas (Eq.30).

Li[B(OCH CF ) ] + 4 H LiBH4 + 4 HOCH2CF3 solvent 2 3 4 2 Eq.30

Tis reaction was carried out in several solvents like toluene and 1,2-dimethoxyethane or mixtures of them. Based on the experiences of the synthesis of Li[B(Ohfp)4] in our working group atention should be paid to the complete conversion [161]. An incomplete conversion leads to the intermediate Li[HB(OTfe)3]. Terefore, the reaction mixture was refuxed for more than four hours with the use of a refux condenser, which was cooled down to −20 ℃ because of the volatile alcohol. Synthesis and Characterisation 95

5.2.2.!NMR characterisation

Te frst synthesis was carried out in toluene and afer one hour refuxing an in-situ NMR sample was taken to control the reaction process. In the 11BQNMR of the sample a doublet at 7.83 ppm was seen. Te doublet is split due to the coupling 1J(1H,11B) = 133 Hz. Tis signal was assigned to Li[HB(OTfe)3] and indicated, that the reaction was not fn- ished. Te reaction was refuxed for further three hours. Aferwards a 11BQNMR signal of the Li[HB(OTfe)3] was not observed in the crude product. But other NMR experiments of the reaction of lithium borohydride with 2,2,2-trifuorethanol in toluene have shown, that the crude product still contained a small amount of the impurity Li[HB(OTfe)3]. In Figure 38 the 11BQNMR of a crude product (MR-058.2) can be seen. Tere are signals at 2.30 ppm and 6.38 ppm. Both signals are in the range of tetracoordi- nated borates. Te resonance of the Li[HB(OTfe)3] at 6.38 ppm is split to a doublet, which disappears in the 1H decoupled 11BQNMR (Figure 39), which verifes the existing of the intermediate Li[HB(OTfe)3]. Te amount of the impurity was around 6 %. Tese signals could not be observed in NMR samples, which were taken in dimethyl carbonate and tetrahydrofuran, because of the mediocre solubility of the product in these solvents. But it is very well soluble in 1,2-dimethoxyethane. For this reason, 1,2-dimethoxyethane was used as solvent for the reaction. Te chelating ether is able to coordinate the lithium of LiBH4, therefore the borohydride anion should be easier accessible for the atack of 2,2,2-trifuorethanol, what should lead to a complete conversion. In the 11BQNMR of the reaction performed in 1,2-dimethoxyethane (Figure 40), one singlet at 2.30 ppm can be seen, the absence of other signals indicated a complete conversion to the desired lithium tetrakis(2,2,2-trifuoroethoxy)borate. Te 1HQNMR spectrum (Figure 41) shows down feld the signal of the −OCH2– unit at 3.86 ppm. Due to the coupling to the fuorine,

[B(OTfe)4]− [B(OTfe)4]−

[H–B(OTfe)3]− [H–B(OTfe)3]−

20 15 10 5 0 ppm 20 15 10 5 0 ppm Figure 38!11B*NMR spectrum of the crude Figure 39!11B{1H}*NMR spectrum of the crude product from the reaction (MR-058.2) of LiBH4 product from the reaction (MR-058.2) of LiBH4 with TFE in toluene (128.39 MHz in DME at r.t., with TFE in toluene (128.39 MHz in DME at r.t., external toluene-D8 lock). external toluene-D8 lock). 96 Lithium tetrakis(trifluoroethoxy)borate

3.9 3.8 ppm

20 10 0 −10 ppm 12 10 8 6 4 2 ppm Figure 40!11B*NMR spectrum of the product Figure 41!1H*NMR spectrum of the product from the reaction (MR-058.4) of LiBH4 with TFE from the reaction (MR-058.4) of LiBH4 with TFE in DME (128.39 MHz in DME by r.t., external tol- in DME (400.17 MHz in DME at r.t., external toluene-D8 lock). uene-D8 lock).

the resonance is split to a quartet with a coupling constant of 3J(1H,19F) = 9.62 Hz. In the 19FQNMR spectrum (Figure 43) the main signal at −76.53 ppm, and two other signals with a very low intensity at −76.30 ppm and −78.20 ppm can be seen. All resonances are in the range of chemically equivalent −CF3 groups. Te main signal is split to a triplet due to the 3J(1H,19F) = 9.62 Hz. Tis indicates that this is the 2,2,2-trifuoroethoxy group bound to the boron atom. Te signal high feld at −78.20 ppm is split to a triplet, despite that it is slightly broadened. By the use of blind samples the signal could be assigned to 2,2,2-trifuorethanol (0.5 %). Tis shows that the product was insufciently dried under the vacuum. Te third signal at −76.30 ppm, down feld to the main signal, is between the 13C satellite and the main signal. Te amount of this unknown impurity in solution is 0.2 %. In the 7LiQNMR spectrum one signal was detected at −1.04 ppm with a half width of 11.54 Hz. Tis may indicate an asymmetric environment of the lithium cation.

As mentioned above the reaction of lithium borohydride with 2,2,2-trifuorethanol was also performed in mixtures of toluene and 1,2-dimethoxyethane, since toluene is cheap and available with low water content (<8 ppm), while 1,2-dimethoxyethane is more ex-

a) F F b) F R R RF R RF

O O O O O O O RF B Li B RF Li B RF O O O O O O O F R RF RF RF

Figure 42!Proposed structures of lithium cation coordination: a) dimeric core of the anion. b) Sol- vation of one DME molecule and one anion to the lithium cation. Synthesis and Characterisation 97

13C satellites pensive and had to dried frst over CaH2. To [B(OTfe)4]− get a homogeneous reaction mixture it was necessary to dissolve the LiBH4 in several equivalents of the chelating 1,2-dimeth- TFE 0.5% oxyethane. For a complete saturation of the lithium cation with the donor solvent only few 1,2-dimethoxyethane was added. In the reaction MR-058.5 four equivalents 1,2-dimethoxyethane were used to dissolve the lithium borohydride. In the further synthesis MR-058.6 only two equivalents were −76 −77 ppm used, but the starting material LiBH4 was not completely soluble in the presence of only two equivalents 1,2-dimethoxyethane. Te reactions were carried out similar to unknown 0.2% the conversion in 1,2-dimethoxyethane. Te NMR experiments showed that a complete −76 −77 ppm conversion to lithium tetrakis(trifuoroethoxy)borate could be achieved. Even using Figure 43!19F*NMR spectrum of the product from the reaction (MR-058.4) of LiBH4 with high concentrations of the crude solid prod- TFE in DME (376.50 MHz in DME at r.t., exter- uct in 1,2-dimethoxyethane (NMR solvent), nal toluene-D8 lock). no signal of Li[HB(OTfe)3] was detected. But at closer look at the highly concentrated solution in the 11BQNMR a small shoulder next to the product signal was visible.

1.7 Hz 3.7 Hz

shoulder

1 0 −1 ppm 1 0 −1 ppm Figure 44!7Li*NMR spectrum of the upper Figure 45!7Li*NMR spectrum of the low- clear phase from a 0.9 mol L−1 solution of er turbid phase from a 0.9 mol L−1 solution of Li[B(OTfe)4] in DME (128.39 MHz in DME at r.t., Li[B(OTfe)4] in DME (128.39 MHz in DME at r.t., external toluene-D8 lock). external toluene-D8 lock). 98 Lithium tetrakis(trifluoroethoxy)borate

Also in the 19FQNMR a slight increase of the previously unknown signal was noticed, downfeld to the main signal. Te intensity of this resonance increases (in the 11BQNMR and 19FQNMR) with the reduction of 1,2-dimethoxyethane during several syntheses. Tis suggests that the resonances comes from the same compound. It is very likely, that the unknown compound is a complex of two borate anions, which chelate one lithium cation (Figure 42 a). Such a dimeric anion would explain the very similar chemical shifs in the 11B and 19FQNMR spectra. Such a structure complies to a known solid state structure which includes such a dimeric anion: Li{[Al(Ohfp)4]2}− with [CPh3]+ or [EMIm]+ cations

[160]. Moreover it is in accordance with the broad 7LiQNMR signals (up to ∆1/2 = 3.7 Hz), which indicate an asymmetric environment. Te 7LiQNMR of two samples are shown in Figure 44 and Figure 45. Te samples were taken from a 0.9 mol L−1 solution of Li[B(OTfe)4] in 1,2-dimethoxyethane (0.9 mol L−1 corresponds to 10.7 DME solvent molecules per one dissolved lithium cation). Te solution was separated into two phases, an upper clear and a lower turbid phase. Te 7LiQNMR spectra of the clear solution (Figure 44) is sharp and symmetric. Te half width of the signal is 1.7 Hz. Tis means that the lithium ion resides in a symmetric environment. In contrast, the 7LiQNMR resonance of the turbid lower phase (Figure 45) is obviously deformed, due to the presence of two lithium signals. Tis implies the lithium is in another coordination environment. Because there are no other impurities, it is likely to assume that the lithium is coordinated from the [B(OTfe)4]−, if there is only litle donor solvent. Otherwise the lithium cation is coordinated by one anion and one DME molecule (Figure 42 b). Such a proposed structure is close to the published solid state structure of Na(DME)[B(Ohfp)4]. Tereby the lithium cation is coordinated by one DME and the oxygen atoms of the borate anion [151].

5.2.3.!Vibrational spectroscopic characterization

Te obtained colourless powder (MR-058.4) synthesized in 1,2-dimethoxyethane, was dried in vacuum (1∙10−3 mbar) to remove all traces of 2,2,2-trifuorethanol or 1,2-dimethoxyethane. Ten an ATR-IR and Raman spectrum was measured. For assignment of the vibrational bands an Infrared and Raman spectrum was simulated from a quantum chemical calculation of [B(OTfe)4]− at the PBE0/def2-TZVPP level and is superimposed in Figure 46. Te bands of the simulated spectra are good accordance to the experimentally measured vibrational bands, except the additional vibration bands at ~2900 cm−1. As it can be seen in the Infrared spectrum there are several bending vibrations of the BO2 moieties 514 cm−1, 568 cm−1 and 602 cm−1. Te corresponding stretching vibrations can be found at 1006 cm−1 and 1053 cm−1. Te strong bands of the C–F vibrations can be observed at 1159 cm−1. All vibrational bands are listed in the Table 13. Furthermore a Raman spectrum was measured and was superimposed with the calculated Raman spectrum. Here again, the calculated bands are in good accordance to the experimental ones measured. In the range from 699 cm−1 down to 470 cm−1 are the bending vibrations of the borate anion. Te intensive breathing vibration of the symmetric anion can be observed at 853 cm−1. An interesting diference to the calculated spectrum in the gas phase to the measured spectrum in solid state can be seen at the C–H stretching vibrations and deformation vibrations of the –OCH2CF3 group. In the gas phase only one δ(CH2CF3) vibration band can be seen at 1242 cm−1, due to the symmetric substituents. But in the solid state the vibration is asymmetric. In the spec- Synthesis and Characterisation 99 trum two bands occur at 1278 cm−1 and 1312 cm−1. Tis means that the four substituents are not degenerate. Tis indicates that the lithium cation is coordinated to the oxygen atoms and infuences the vibrations. Tis is confrmed due to the same behaviour of the corresponding C–H stretching vibration. Te gas phase spectrum predicts only two bands at 2905 cm−1 and 2936 cm−1. Due to the asymmetry, four bands (2757 cm−1, 2819 cm−1, 2906 cm−1, 2966 cm−1) can be found in the measured spectrum.

To verify the assumption that the lithium cation is coordinated to the oxygen atoms further calculations and measurements were done. Terefore a quantum chemical calculation at the simpler RI-BP86/def-SV(P) level was used, to simulate a vibrational spectrum of the anion, which coordinates a lithium cation. For the saturation of the coordination sphere of the lithium cation two formaldehyde molecules were used as additional donor. Tis coordination structure can simulate the efect of the lithium cation on the stretching bands, especially the B–O bands coordination. On the other side, the gas phase calculation of the anion, simulates the ‘naked’ status of the anion. Experi- mentally, the lithium should be detached from the anion in presences of a strong donor solvent, like 1,2-dimethoxyethane. Due to 1,2-dimethoxyethane a ‘naked’ borate anion should be available in solution. Terefore a solution (0.1 mol L−1) of Li[B(OTfe)4] in DME was made to get solvent separated ions: [Li(DME)x]+ and [B(OTfe)4]−. Figure 47 shows the ATR-IR spectra of: solid Li[B(OTfe)4], Li[B(OTfe)4] dissolved in DME, calculated vibrational spectrum of [B(OTfe)4]− and the calculation of the coordinated lithium at the borate anion ([(formaldehyde)2Li][B(OTfe)4]). A comparison of the experimental spectra atract the atention to the range of 1000-1050 cm−1, because of the signifcant deviations. In this range, the stretching vibrations of the B–O bands are observed. And fortunately in this region no vibrations from the solvent DME occur. As mentioned above the

ATR-IR powder of MR-058.4 cal. IR of [B(OTfe)4]− Raman powder of MR-058.4 y t i s cal. Raman [B(OTfe) ]− n 4 e t n I

4000 3000 2000 1000 Wavenumber [cm ]

Figure 46!Experimental ATR-IR and Raman spectrum of the Li[B(OTfe)4] powder (MR-058.4) and the calculated spectra of the [B(OTfe)4]− anion calculated at the PBE0/def2-TZVPP level. 100 Lithium tetrakis(trifluoroethoxy)borate

B–O bands of the solid product were assigned to the bands at 1006 cm−1 and 1053 cm−1. But in the Infrared spectrum of the dissolved Li[B(OTfe)4] these bands disappear, but new bands at 1042 cm−1 and 1032 cm−1 occur! Tis observation is also confrmed by the simulated spectrum. Te B–O stretching vibration of the coordinated anion is visible at

Table 13!Comparison of the experimental ATR-IR and Raman bands of the Li[B(OTfe)4] powder (MR-058.4) with the calculated bands of the [B(OTfe)4]− anion at the PBE0/def2-TZVPP level. w: weak, m: medium, s: strong, sh: shoulder, v: very.

cal. IR band exp. IR band exp. Raman cal. Raman exp. IR band assignment [cm−1] [cm−1] band [cm−1] band [cm−1] TFE [cm−1] — — 470 (vw) 495 (vw) δ(BO2) — — — — 506 (vw) δ(BO2) — 526 (vw) 514 (w) 535 (vw) 523 (vw) δ(BO2) — 551 (vw) 534 (w) 549 (vw) 541 (vw) δ(BO2) 534 (vw) 558 (w) 551 (w) 572 (vw) 561 (w) δ(BO2) 549 (w) 577 (w) 568 (w) 597 (vw) — δ(BO4) — — 602 (w) 633 (w) 605 (vw) δ(BO4) 679 (w) 679 (m) 680 (vw) 655 (vw) δ(B–O–C) 663 (w) 695 (w) 694 (sh) 699 (vw) 670 (vw) δ(B–O–C) — — — 799 (vw) 787 (vw) δbreathing(BO4) — — — 853 (s) 815 (s) δbreathing(B–O–C) — 836 (w) 839 (m) — — δ(C–C) 828 (w) 845 (w) 867 (sh) — — δ(C–C) — 954 (w) 962 (s) — 920 (m) δrocking(C–H) 946 (m) — 973 (sh) 966 (w) 966 (w) δrocking(C–H) — 1007 (s) 1006 (m) 1004 (vw) — ν(B–O) — 1038 (vs) 1053 (vs) — — ν(B–O) — — 1101 (vs) — 1112 (w) ν(C–O) 1080, 1088 (m) 1141 (vs) 1159 (vs) 1164 (w) 1150 (w) ν(C–F) 1141 (vs) 1166 (sh) — 1185 (sh) — δ(C–H) — 1171 (vs) — — — ν(C–O) — 1181 (vs) 1195 (sh) 1210 (vw) — ν(C–C) 1208 (vw) 1282 (s) 1282 (s) 1278 (w) 1243 (m) δ(CH2CF3) 1275 (vs) — 1303 (sh) 1312 (w) — — — — 1379 (vw) 1360, 1369 (vw) δ(C–H) 1374 (w) 1413 (w) 1431 (w) — — δwagging(C–H) 1415 (w) 1467 (vw) 1464 (vw) 1467 (w) 1149 (w) δscissoring(C–H) 1455 (w) — 2825 (vw) 2757 (vw) — ν(C–H) — — 2851 (vw) 2819 (vw) — ν(C–H) — 2942 (m) 2956 (vw) 2906 (m) 2905 (s) νsymm(C–H) 2890 (w) 2979 (w) — 2966 (m) 2936 (s) νasymm(C–H) 2964 (w) — — — — — 3387 (vs) Synthesis and Characterisation 101

1067 cm−1 and 1010 cm−1. In the case of the ‘naked’ anion, one of the B–O bands is markedly shifed to lower energy at 1025 cm−1 and 1012 cm−1. Tese facts demonstrate the signif- icant efect of lithium coordination onto the B–O vibration and also on the corresponding connected –OCH2CF3 group. Te degeneration of the C–H stretching vibrations due to the coordination was also confrmed by the calculation. For completion, at a closer look to the spectrum of Li[B(OTfe)4] in DME a shoulder at 1085 cm−1 is recognizable, this is probably due to the DME, which coordinates the lithium cation. Te coordination of lithium weakens the C–O bond of the ether, which leads to a red shif in the Infrared spectrum. One consequence of the lithium coordination at the anion is the medium solubility: Atempts to make a 1 mol L−1 (or 0.8 mol L−1) solution of Li[B(OTfe)4] in 1,2-dimethoxyethane were overshadowed by the occurrence of phases in the solution. An upper clear phase and a turbid lower phase formed. Even afer stirring, a separation of the phases was observed. Te lower phase was not a solid residue, it was a liquid and equal- ly viscous to the upper clear phase. Terefore every phase was investigated by Infrared spectroscopy. In Figure 48 the ATR-IR spectra are shown and additionally the spectra of 1,2-dimethoxyethane and the ATR-IR spectra of the solid product are superimposed. Te diference between the two phases can be seen in the range between 1000-1050 cm−1. Te spectrum of the turbid phase (red line) resembles the spectrum of the powder (black line). Te B–O stretching vibrations can be found at 1050 cm−1 and 1009 cm−1, which is close to the B–O bands in the solid state (1053 cm−1 and 1006 cm−1)! In contrast, the B–O bands of the clear phase (cyan) can be observed at 1042 cm−1 and 1032 cm−1. So the B–O bands difer signifcantly from the B–O band in the turbid phase and from the powder. Instead these resemble rather the B–O band of the ‘naked’ anion. Tis observa-

BP86/def2-S V(P) [Li(OCH2)2][B(OTfe)4]

BP86/def2-S V(P) [B(OTfe)4] Li[B(OTfe) ]inDME clearphase 4 1067 1025 1012 Li[B(OTfe) ]powderMR-058.4 4 1053 1010 DME 1006 y it ns

te 11042 In

DME 1032

1300 1200 1100 1000 Wavenumber [c m ]

Figure 47!Comparison of the naked borate anion versus the anion which coordinated to a lithium cation. Detailed part of the ATR-IR spectra of Li[B(OTfe)4] powder (black line), DME solution of Li[B(OTfe)4] (blue line), calculated vibrational spectrum of [B(OTfe)4]− (cyan) and a calculated vibrational spectrum of [(formaldehyde)2Li][B(OCH2CF3)4] at the BP86/SV(P) level (gray). 102 Lithium tetrakis(trifluoroethoxy)borate

DME Li[B(OTfe)4] in DME clear phase Li[B(OTfe) ] in DME turbid phase 4 1050 Li[B(OTfe)4] powder 1009 y t i s n e t n I

1042 1032

1500 1400 1300 1200 1100 1000 900 800 Wavenumber [cm ]

Figure 48!ATR-IR spectra of the upper clear and lower turbid phase of a 0.8 mol L−1 solution of Li[B(OTfe)4] in 1,2-dimethoxyethane. Additional the ATR-IR spectra of the solvent DME and the solid lithium tetrakis(2,2,2-trifluoroethoxy)borate (MR-058.4).

tion is similar to the behaviour described above: In the lower turbid phase the lithium appears to be still coordinated to the anion. Tat suggests that the lithium cation, bonding to the borate anion [B(OTfe)4]−, is competitive to the chelating solvent DME. Tis suggest that in the upper clear phase solvent separated ion pairs like [Li(DME)2]+ [B(OTfe)4]− are present, and in the lower phase the ion pairs are not dissociated, e. g. like in [(DME)LiB(OTfe)4].

Conclusion

Lithium tetrakis(trifuoroethoxy)borate could be synthesised with high purity. Te In- frared spectra have shown that the existence of additional B–O and C–H bands in the solid state that indicate a coordination of the lithium cation. For a clean synthesis of Li[B(OTfe)4] respectively a complete conversion of the starting materials, the reaction was carried out in 1,2-dimethoxyethane to inhibit the coordination of the borate anion to the lithium cation, to get solvent separated ion pairs. Te amount of DME used for the synthesis was varied. Even with only two equivalents DME, with respect to lithium borohydride, a clean synthesis could be achieved. Te efect of the lithium coordination on the anion could be determined by vibrational spectroscopy (ATR-IR and Raman) and verifed by quantum chemical calculations. Also the 7LiQNMR spectra underlined this fact. In lower concentrations of the borate in 1,2-dimethoxyethane, the line width of the 7LiQNMR resonance decreases, and at higher concentrations the line width increases or the signal exhibits a shoulder, which indicates an unsymmetrical coordination. It is rea- sonable to assume that [B(OTfe)4]− is coordinating to the lithium cation and competes Ionic Conductivity 103 with DME depending on the concentration. Tis efect could be observed while prepar- ing, for example, a 0.8 mol L−1 solution in DME (corresponds to the ratio 12:1 solvent to salt), thereby a formation of two phases occurred: one upper clear and a lower turbid phase. Both were liquid, but in the clear phase the lithium cation was separated from the solvent and in the lower phase the lithium cation was coordinated at least from one borate anion.

5.3!Ionic Conductivity Te commonly used solvent in liquid non-aqueous electrolytes for lithium-ion bateries are carbonate based solvents, like dimethyl carbonate, ethylene carbonate, propylene carbonate or ethyl methyl carbonate [162]. For lithium-sulfur bateries mainly ethers are used [166]. For that reason the solubility of the new lithium tetrakis(trifuoroethoxy) borate in divers solvents were tested. As in the previous subsection mentioned atempts to dissolve Li[B(OTfe)4] in 1,2-dimethoxyethane were accompanied by formation of two phases. Te same behaviour was observed with other solvents. In Table 14 the results are listed. In the commonly used carbonate mixtures lithium tetrakis(trifuoroethoxy) borate was in concentration range from 1.0-0.2 mol L−1 only soluble with formation of the phases. Solutions in dimethyl carbonate were not even liquid, but rather gel-like or highly viscous. Only in a low concentration (0.1 mol L−1) of the salt in a 1:1 wt-% mixture

Table 14!Solublility behaviour of lithium tetrakis(2,2,2-trifluoroethoxy)borate in serveral solvents.

Relative Concentration Solvent permitivity [163] Solution [mol L−1] [164]

Acetone 21.01 1.0 clear Dichloromethane 8.93 0.5 phase formation Dimethyl carbonate 3.09 1.0-0.1 phase formation Ethylene carbonate 89.78 0.5 phase formation Diethyl ether 4.27 0.5 phase formation Ethyl acetate 6.08 1.0-0.6 phase formation Ethyl acetate 6.08 0.5 clear Propylene carbonate 64.95 0.5 phase formation Tetrahydrofuran 7.52 1.0 clear 1,2-Dimethoxyethane 7.30 1.0-0.6 phase formation 1,2-Dimethoxyethane 7.30 0.5 clear o-Difuorobenzene 13.38 0.5 phase formation Water 80.10 1.0 clear EC:DMC (1:1) — 1.0-0.2 phase formation EC:DMC (1:1) — 0.1 clear EC:EE (1:1) — 0.5 phase formation EC:DMC:DME (1:1:1) — 1.0-0.3 phase formation EC:DMC:DME (1:1:1) — 0.2 clear 104 Lithium tetrakis(trifluoroethoxy)borate

14 DME water 12 13.17 DME:EC:DMC 10 acetone 8 EE ] 6 4.86 THF m c 4 EC:DMC S 3.91 clear m 2 [ y t

i 2.38 v i t 0.5 0.411 clear c

u 0.4 d n

o 0.3 C 0.276 0.2 clear phase 0.1 0.197 0.0 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Concentration [mol L ]

Figure 49!Electric conductivity of lithium tetrakis(trifluoroethoxy)borate in several solvents at 25 ℃.

of ethylene carbonate and dimethyl carbonate it was completely soluble. Te addition of chelating ether, 1,2-dimethoxyethane, to the ethylene carbonate/dimethyl carbonate (1:1) mixture the solubility could be increased. In other solvents like o-difuorobenzene, propylene carbonate, ethylene carbonate, methylene chloride, diethyl ether a 0.5 mol L−1 solution was not clear, but showed phase separation. Good solvents have been ethyl acetate and 1,2-dimethoxyethane. Even beter solubility has been achieved in acetone, tetrahydrofuran and water. In these solvents no phase separation was observed. Fur- thermore the electrical conductivities were measured for some electrolyte solutions. In Figure 49the electrical conductivities for several concentrations are shown and the values of all measurements are listed in the Table 15. Te conductivity of lithium tetrakis(trifuoroethoxy)borate in tetrahydrofuran passes through a maximum at a concentration of 0.9 mol L−1 and is 276 mS cm−1, which is extremely low, considering that the demanding requirements of Li/S bateries are more than 5 mS cm−1. Even in ethyl acetate

Table 15!Electric conductivity of solutions of lithium tetrakis(trifluoroethoxy)borate in several solvents at 25 ℃ (—: not measured).

Solution in Electric conductivity [mS cm−1] [mol L−1] 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Water 11.50 12.82 13.17 13.01 12.52 12.20 11.68 — — — Acetone 4.16 4.51 4.86 4.68 4.79 4.58 4.20 3.82 — — DME 3.36 3.76 3.91 3.63 3.41 — — — — — DME:EC:DMC — — — — — 1.56 2.22 2.38 2.15 1.38 EC:DMC 0.289 0.322 0.341 0.365 0.382 — 0.383 0.406 0.411 0.405 THF 0.271 0.276 0.257 0.234 0.205 — — — — — EE 0.179 0.189 0.197 0.184 0.163 0.100 0.077 0.050 0.022 — DMC 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Thermal stability 105

Table 16!Long-term NMR measurement of Li[B(OTfe)4] (MR-058.4) in D2O at Bruker Avance II+ 400. The D2O shif was calibrated to 4.81 ppm. The chemical reference shifs of 2,2,2-trifluorethanol in D2O are: 1H*NMR: δ = 4.02 ppm, 19F*NMR: δ = –76.80 ppm, 3J(1H,19F) = 9.18 Hz.

1H 3J(1H,19F) 19F 11B ∆ 7Li ∆ Time [h] 1/2 1/2 [ppm] [Hz] [ppm] [ppm] [Hz] [ppm] [Hz] 1 4.02 9.16 –76.69 2.63 135 — — 24 4.02 9.16 –76.69 2.93 139 0.26 1.7 216 4.02 9.16 –76.69 2.99 142 — — 264 4.02 9.16 –76.69 3.12 146 — —

(197 µS cm−1) or ethylene carbonate/dimethyl carbonate (1:1) (411 µS cm−1) a conductivity beyond the 1 mS cm−1 threshold could not be observed. In the gel-like dimethyl carbonate solution no electrical conductivity was measurable. A solution in 1,2-dimethoxyethane had a maximum conductivity of 3.91 mS cm−1 at a concentration of 0.8 mol L−1. And a mixture of ethylene carbonate/dimethyl carbonate/1,2-dimethoxyethane (1:1:1 wt-%) exhibits a maximum conductivity at 0.3 mol L−1 of 2.38 mS cm−1, which is also low. Acceptable conductivities have been observed in acetone and water. But the values in acetone have to be taken with care, because the acetone had a water content of <75 ppm water. Te conductivities in water reached 13.17 mS cm−1 at a concentration of 0.8 mol L−1. Interest- ingly lithium tetrakis(trifuoroethoxy)borate is stable in water at least 11 days. For this a long-term NMR measurement was done (Table 16). Te 11BQNMR shif of Li[B(OTfe)4] is still around 3 ppm, which is in the range of tetra coordinated borates. Te likely decomposition product of a borate in water is boric acid, which was not observed in the NMR, even not afer 11 days. Also the 19FQNMR shif of the −CF3 group difers signifcantly from the blind spectrum of LiOTfe or more likely 2,2,2-trifuorethanol.

Conclusion

Te solubility of lithium tetrakis(trifuoroethoxy)borate is poor in carbonate based solvents and overshadowed by the formation of phases or a gel-like solution. In ethers like tetrahydrofuran the conductivity is less than 1 mS cm−1, which points to the formation of stable ion pairs. Te best result in a non-aqueous solvent was observed in 0.8 mol L−1 solution in acetone (4.86 mS cm−1) and 0.8 mol L−1 solution in 1,2-dimethoxyethane (3.91 mS cm−1). Tese medium conductivities are probably due to the high tendency of the borate oxygen’s to coordinate to the lithium cation. Only chelating solvents or highly polar solvents like water are potent enough to fully solvate the lithium cation. An interesting discovery is the stability of the borate anion in water for at least to 11 days.

5.4!Thermal stability Te thermal stability of the conducting salt is of great importance for the safety of the batery, taking into account that a lithium bateries use organic electrolytes, which have high volatility and fammability. If the batery is exposed to high temperature or other abuse conditions these electrolytes can react with the components resulting a thermal runaway [169] and release of heat and gas. Te main commercial lithium salt LiPF6 is known to begin to decompose at 50 ℃ into LiF and PF5 [170][171][172][173]. PF5 106 Lithium tetrakis(trifluoroethoxy)borate

300 281° C ]

C 250 ° S ample Temperature [ 1 ] e

r Heatflow u t

200 W a r m e [ p w m o

150 l e f t T a e e l 0 p 100 H m a S 50

0 500 1000 1500 2000 2500 3000 Time [cm ] Figure 50!Thermal stability measurement of the lithium tetrakis(trifluoroethoxy)borate (MR-058.4) with diferential scanning calorimetry (DSC) (heat rate: 5 K min−1).

is a strong Lewis acid [174] and can atack the ethylene carbonate with formation of poly(ethylenoxid) and CO2 [175][176]. Furthermore the PF5 can react with water traces with formation of highly toxic hydrofuoric acid, which can also lead to further solvent decomposition and gas generation [177][178][179]. Te DSC measurements of the novel lithium tetrakis(trifuoroethoxy)borate can be seen in Figure 50. Li[B(OTfe)4] begins to absorb heat at 271 ℃, with increasing temperature an endothermic peak appears at 281 ℃, with a heat absorption of 650 kJ mol−1. Tis indicated a decomposition of the lithium borate. Tis means a high thermal stability of the lithium salt, which is similar to the decomposition temperature of lithium bis(oxalato)borate (286 ℃ [180]). Lithium bis(trifuoromethylsulfonyl)imide melts at 234 ℃ [181]. Only lithium tetrafuoroborate decomposes at higher temperatures with 310 ℃ [182][183]. Te new lithium tetrakis(trifuoroethoxy)borate demonstrated a remarkable thermal stability and combined with the long term stability against water, it demonstrates its usefulness in comparison to other conducting salts.

5.5!Electrochemical analysis

5.5.1.!Cyclic voltammetry

To test the electrochemical stability of an electrolyte—conducting salt and solvents—cyclic voltammetry is the method of choice. Tereby the stability of the electrolyte containing the new lithium salt was investigated over a wide potential range of 0 V to 5 V. Half molar solutions of lithium tetrakis(trifuoroethoxy)borate in 1,2-dimethoxyethane and a 0.1 mol L−1 solution in ethylene carbonate/dimethyl carbonate (1:1) have been sur- veyed in two diferent potential ranges. Terefore a platinum working electrode, a lithium metal counter electrode and also a lithium metal reference electrode have been used. Cycling in a potential range of 3 to 5 V against Li/Li+ simulates the charging of a batery cell. Te results of this anodic cycling reveal the oxidation stability of the electrolyte. On the other side, the cathodic cycles, at a range of 3 to 0 V versus Li/Li+ investigates the stability against reduction. In Figure 51 the measurements of the new Electrochemical analysis 107

a) b) 2 70 cycle 1 ] ] 60 cycle 2 Α Α

μ 0 μ cycle 3 [ [ 50 E E 40 W W -2 cycle 1 t t n n cycle 2 30 e e r r r r -4 cycle 3 u

u 20 c c cycle 4 10 -6 cycle 5 0 0 1 2 3 3 4 5 potential WE [V] potential WE [V] c) 2 d) 8 cycle 1 cycle 2 ] ] 6 A A cycle 3 μ μ [ 0 [ cycle 4 E E 4

W cycle 5 cycle 1 W t t n cycle 2 n e e r

r 2 r cycle 3 r u u c -2 cycle 4 c 0 EC decomposition cycle 5 0 1 2 3 3 4 5 potential WE [V] potential WE [V] Figure 51!Cyclic voltamogramms of several solutions of Li[B(OTfe)4]. All potentials against Li/Li+; measured with a scan rate of 0.01 V cm −1. a) cathodic scans of Li[B(OTfe)4] in 1,2-dimethoxyethane (0.5 mol L−1) b) anodic scans of Li[B(OTfe)4] in 1,2-dimethoxyethane (0.5 mol L−1) c) cathodic scans of Li[B(OTfe)4] in ethylene carbonate/dimethyl carbonate (1:1) (0.1 mol L−1) d) anodic scans of Li[B(OT- fe)4] in ethylene carbonate/dimethyl carbonate (1:1) (0.1 mol L−1). lithium salt in several solvents can be seen. Te frst fve scans in carbonate solvents in the anodic scan (d), difer hardly from each other. Te current at the working electrode remains low up to 4.2 V. Te currents at this plateau are low, but indicate an oxidation process, but likely not from the lithium salt, because it is too litle. With higher potential at 4.8 V the current increases to 6 µA. It is likely an irreversible oxidation process of the anion. Te cathodic scan (c) shows a reversible redox reaction at 2.03 V, but the currents are quite low, which suggests a reaction of an impurity. At around 0.5 V the typical ethylene carbonate decomposition was observed. At lower potentials down to 0 V, the currents are low and the Li[B(OTfe)4] salt is probably stable up to 0 V.

Te cyclic voltammetry measurement of the solution of Li[B(OTfe)4] in 1,2-dimethoxyethane difers from the measurement in ethylene carbonate/dimethyl carbonate (1:1). Te same oxidation peak of the impurity occur at 4.2 V. Up to 4.4 V the currents are low in the anodic scan (b), but at higher potentials the currents increases dramatically up to 70 µA. Tis is caused by the oxidation of the solvent 1,2-dimethoxyethane. Te ether is easier to oxidise than the carbonates. In the cathodic scans (a) can be seen a similar oxidation process at 2.05 V of an impurity. Te currents of this oxidation increases with 108 Lithium tetrakis(trifluoroethoxy)borate

1400 100 1200

975 80 ] % ]

1000 [ h y A 837

962 c m n [ 60 800 e i y

t 822 c i i f c f a e

p 600 b a 40 c

discharge capacity m o 400 l

charge capacity u Coulomb efficiency o 20 C 200

0 0 0 10 20 30 40 50 60 cycle no.

Figure 52!Capacity retention of a lithium sulfur test cell during cycling (charge rate: 0.1 C discharge rate: 0.15 C within a voltage range of 1.7 to 2.5 V). The new lithium tetrakis(trifluoroethoxy) borate was used as conducting salt in a 8 wt.-% (0.2 mol L−1) solution of DOL/DME (1:1). The labels are the values from the 5th and 40th cycle.

each cycle, but they are low. Te currents are probably not from the anion, because the currents are too low. Tus, we assume that lithium tetrakis(trifuoroethoxy)borate is stable in the voltammetric range between 0 to 4.8 V.

5.5.2.!Electrochemical cycling

Because of the good solubility of lithium tetrakis(trifuoroethoxy)borate in ether, like 1,2-dimethoxyethane, and electrochemical stability in the potential range of lithium sulfur bateries (~2.2 V) [165] it is a candidate for the use as conducting salt in this bat- tery technology. Terefore the new lithium salt was used in test cells. As test system, pouch bag cells were used with a lithium metal anode. Te cathode consisted of 60 wt.-% elemental sulfur as active material and 35 wt.-% carbon black (Vulcan XC72) as conductive material with polyvinylidene difuoride binder (5 wt.-%). Te electrolyte, a solution of 8 wt.-% lithium salt in a 1,3-dioxolane/1,2-dimethoxyethane mixture with 1:1 weight ratio (0.2 mol L−1), was prepared and cycled in the test cell system. Polypropylene separators were used in all cells. For preparation, the cells were discharged in the frst cycle with a 0.02 C rate. Te cycling tests were performed with a Maccor batery test system at room temperature by using a charge rate of 0.1 C and discharge rate of 0.15 C within the potential range of 1.7 to 2.5 V. Te resulting discharge and charge capacities of each cycle can be seen in Figure 52, also the Coulombic efciency during the cycles. As can be seen, the discharge capacity afer the cycle 40 is 822 mAh. Te capacity of the 5th cycle was set to 100 %. So the capacity lef afer the 40th cycle is 86 %. Furthermore the Coulombic efciency during each cycle is constantly above 97 %. Te discovery was made that the use of the novel lithium borate in the electrolyte, shows a signifcantly improved capacity retention. Because of the main problem of the Li/S batery, the capacity fading during cyclization [165][166][167], it represents a step forward in the Li/S batery research. Tis invention was registered as patent [184][185]. Summary 109

5.6!Summary Tis chapter has demonstrated the successful preparation of lithium tetrakis(trifuoro ethoxy)borate in toluene from the reaction of lithium borohydride with excess of 2,2,2-trifuorethanol. For the complete conversion of the starting material and to avoid the intermediate Li[HB(OTfe)3], it is necessary to saturate the lithium cation at least two equivalents of 1,2-dimethoxyethane. Te preferred route is with use of toluene and four equivalents DME as solvent, this ensures the complete solvation of the Li[BH4] combined with a minimal amount of DME, which has to be rigorously dried frst. Te 1,2-dimethoxyethane chelates the lithium cation and therefore increases the reactivity of the anion. Te donor free Li[B(OTfe)4] could be obtained in multi gram scale and the structure was complete characterised with heteronuclear magnetic resonance and vibrational spectroscopy (Infrared and Raman). It was determined that lithium tetrakis(trifuoroethoxy)borate is only an intermediate weakly coordinating anion, because the oxygen atoms tend to coordinate the lithium cation. Te coordination of the lithium cation infuences the various stretching vibrations of the trifuoroethoxy groups and the corresponding B–O stretching vibrations. In the condensed phase, the lithium is connected to the oxygen atoms. Tis fact has consequences on the solubility, so the lithium coordination is only broken with strong donor solvents. All solutions in the tested organic solvents, with the exception of acetone and tetrahydrofuran, with concentrations >0.5 mol L−1 have shown a separation into two phases. It has been proven for 1,2-dimethoxyethan that a lithium coordination of the anion still exists in the lower turbid phase. In the clear upper phase, solvent separated ion pairs are formed. Nevertheless, 1,2-dimethoxyethane is a suitable solvent. Te maximum ionic conductivity of lithium tetrakis(trifuoroethoxy)borate in 1,2-dimethoxyethane (0.8 mol L−1) is around 4 mS cm−1, which is satisfactory for the use of the electrolyte in lithium sulfur bateries. Due to the formation of gel-like solutions in dimethyl carbonate, an important solvent in lithium-ion bateries, no electrical conductivity could be measured. But the formation of gel-like electrolytes could be an advantage for safety issues of high voltage bateries and for this the lithium borate can be used as an additive to adjust the viscosity of the electrolyte. Additionally, the new lithium salt has shown a remarkable thermal stability. A decomposition of the pure lithium salt (DSC) slowly begins at a temperature above 271 ℃, and ranks similar to other highly stable salts, like Li[BOB] or Li[BF4]. Also the long term stability against water is a great advantage over other fuorine containing conducting salts, like lithium hexafuorophosphate or lithium tetrafuoroborate, which react with water to highly toxic hydrofuoric acid [170][177][178][179]. Te results from the cyclic voltammetry indicate an electrochemical stability in the potential range from 0 V to 4.8 V. Tis meets the requirements for the use in LIB and largely exceeds the demands of lithium sulfur bateries. Te novel lithium salt is a promising candidate for the use in Li/S bateries, because of the solubility in ether and its electrochemical stability. But the most excellent feature of the novel lithium tetrakis(trifuoroethoxy) borate, is the cycling performance in lithium-sulfur bateries. Te use of an electrolyte consisting of a 0.2 mol L−1 solution of Li[B(OTfe)4] in a 1:1 weight ratio mixture of 1,3-dioxolane/1,2-dimethoxyethane revealed a capacity of 86 % afer the 40th cycle. Te decreased capacity retention due to the new lithium salt is an interesting progress in the lithium-sulfur batery technology, and helps to resolve the main problem of this batery type. Te invention of lithium tetrakis(trifuoroethoxy)borate as conducting salt in Li/S bateries was registered as a patent. 110 Lithium tetrakis(trifluoroethoxy)borate

5.7!Experimental Section

5.7.1.!Chemicals

chemical manufacturer quality CAS no. purification before use lithium borohy- for recrystallization from diethyl ether Merck KGaA 16949-15-8 dride synthesis and fnely ground 2,2,2-trifuor- for triple condensing on 3Å molecular Merck KGaA 75-89-8 ethanol synthesis sieve; water content < 43 ppm 1,2-dimethoxy- anhydrous, Sigma Aldrich 110-71-4 distilling from CaH2 ethane 99.5 %

5.7.2.!Preliminary remark

Te clean synthesis of lithium tetrakis(trifuoroethoxy)borate could be achieved by using 1,2-dimethoxyethane as solvent or toluene with at least two equivalents of 1,2-dimethoxyethane. In all cases the complete conversion of the starting material was determined and verifed by NMR spectroscopy. Terefore a sample of the colourless powder, which was obtained afer removal the solvent and surplus alcohol under vacuum, was dissolved in DME and was measured by NMR. Te concentration of the NMR samples were 0.9-0.5 mol L−1. It should be noted that NMR signals with an intensity less then 0.5 % compared to the product signal, are not listed in the following subsections, dealt with the use of DME. Te obtained product is a very fne crystalline powder, and some amounts remained in the glass equipment, while transfer or were lost during evaporation of the solvent. Te values of weight of the obtained product (X g) are referred to the amount of product afer transfer. For those who want to reproduce the synthesis, one has to pay atention to the complete removal of surplus solvent and alcohol, which takes at least three hours, depending on the size of the batch. Experimental Section 111

5.7.3.!Syntheses in toluene

MR-058.1

To an ice-cooled suspension of lithium borohydride (0.309 g, 14.2 mmol, 1 eq.) in toluene (150 mL), 2,2,2-trifuorethanol (5.3 mL, 7.314 g, 0.073 mol, 5 eq.) was dropped into the suspension within 25 minutes. Te ice bath was removed and the mixture was stirred for 1 h at room temperature. Afer this the mixture was refuxed for 3 h. Te refux condenser was cooled to −20 ℃ with a cryostat. An evolution of gas was observed. Te reaction progress was controlled via NMR samples. Afer refuxing, the solvent was removed under vacuum. A colourless solid was obtained (4.8 g).

In-situ NMR in toluene afer 1 h: 1H,NMR (400.17 MHz, toluene = 2.09 ppm, toluene-D8 external lock, r.t.) δ = 3.66 (q, 2H, 3J(1H,19F) = 9.13 Hz, −OCH2CF3), 3.15 (q, 2H, 3J(1H,19F) = 8.80 Hz −OCH2CF3) ppm 19F,NMR (376.54 MHz, toluene = 2.09 ppm, toluene-D8 external lock, r.t.) δ = −76.29 (t, 3F, 3J(1H,19F) = 9.13 Hz, −OCH2CF3), −77.34 (t, 3F, 3J(1H,19F) = 8.80 Hz, −OCH2CF3) ppm 11B,NMR (128.39 MHz, toluene = 2.09 ppm, toluene-D8 external lock, r.t.) δ = 7.83 (d, 1B, 1J(1H,11B) = 132.6 Hz, [H−B(OTfe)3]−) ppm

Solid dissolved in dimethyl carbonate: 1H,NMR (400.17 MHz, DMC = 3.65ppm, toluene-D8 external lock, r.t.) δ = 3.83 (q, 2H, 3J(1H,19F) = 9.53 Hz, [B(OCH2CF3)4]–) ppm 19F,NMR (376.54 MHz, DMC = 3.65ppm, toluene-D8 external lock, r.t.) δ = −77.17 (t, 3F, 3J(1H,19F) = 9.53 Hz, [B(OCH2CF3)4]–) ppm 7Li,NMR (155.52 MHz, DMC = 3.65ppm, toluene-D8 external lock, r.t.) δ = −0.3 (s, ∆1/2 = 1.9 Hz, Li[B(OTfe)4]) ppm 11B,NMR (128.39 MHz, DMC= 3.65 ppm, toluene-D8 external lock, r.t.) δ = 2.25 (s, ∆1/2 = 7.0 Hz, Li[B(OTfe)4]) ppm 112 Lithium tetrakis(trifluoroethoxy)borate

MR-058.3 2,2,2-trifuorethanol (5.0 mL, 6.90 g, 0.069 mol, 5 eq.) was dropped within 25 minutes into a suspension of lithium borohydride (0.30 g, 14.0 mmol, 1 eq.) in toluene (150 mL). A gas evolution was observed. Afer fnishing of the gas evolution the reaction mixture was refuxed for 2 h. Ten the solvent was removed under vacuum a colourless powder (4.2 g, yield: 72.5 %) was obtained.

Powder dissolved in dimethyl carbonate: 1H,NMR (400.17 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 3.59 (q, 2H, 3J(1H,19F) = 9.13 Hz, −OCH2CF3), 3.83 (q, 2H, 3J(1H,19F) = 9.56 Hz, [B(OCH2CF3)4]–) ppm 19F,NMR (376.54 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −75.88 (t, 3F, 3J(1H,19F) = 8.55 Hz, −OCH2CF3), −77.16 (t, 3F, 3J(1H,19F) = 9.56 Hz, [B(OCH2CF3)4]–) ppm 7Li,NMR (155.52 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = −0.3 (s, ∆1/2 = 0.7 Hz, Li[B(OTfe)4]) ppm 11B,NMR (128.39 MHz, DMC = 3.65 ppm, toluene-D8 external lock, r.t.) δ = 2.25 (s, ∆1/2 = 9.14 Hz, Li[B(OTfe)4]) ppm

Powder dissolved in 1,2-dimethoxyethane (0.9 mol L−1) upper clear phase: 1H,NMR (400.17 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 3.82 (q, 2H, 3J(1H,19F) = 9.61 Hz, [B(OCH2CF3)4]−), 4.77 (br, HOCH2CF3) ppm 19F,NMR (376.54 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −76.31 (t, 3F, 3J(1H,19F) = 9.47 Hz, −OCH2CF3), −76.53 (t, 3F, 3J(1H,19F) = 9.70 Hz, [B(OCH2CF3)4]−), −78.21 (br, ∆1/2 =85 Hz, HOCH2CF3) ppm 7Li,NMR (155.52 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −1.07 (s, ∆1/2 = 1.7 Hz, Li[B(OTfe)4]) ppm 11B,NMR (128.39 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 6.4 (d, 1J(1H,11B) = 128 Hz, [H−B(OTfe)3]−), 2.3 (s, ∆1/2 = 10 Hz, [B(OCH2CF3)4]−) ppm

Powder dissolved in 1,2-dimethoxyethane (0.9 mol L−1) lower turbid phase: 1H,NMR (400.17 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 3.81 (q, 2H, 3J(1H,19F) = 9.55 Hz, [B(OCH2CF3)4]−), 3.95 (q, 2H, 3J(1H,19F) = 9.46 Hz, −OCH2CF3) ppm 19F,NMR (376.54 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −76.53 (t, 3F, 3J(1H,19F) = 9.55 Hz, [B(OCH2CF3)4]−), −76.30 (t, 3F, 3J(1H,19F) = 9.70 Hz, −OCH2CF3), −78.2 (br, ∆1/2 =85 Hz, HOCH2CF3) ppm 7Li,NMR (155.52 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −1.03 (s, ∆1/2 = 3.7 Hz, Li[B(OTfe)4]), −1.13 (shoulder) ppm 11B,NMR (128.39 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 6.38 (d, 1B, 1J(1H,11B) = 138.2 Hz, [H−B(OTfe)3]−), 2.30 (s, 1 B, ∆1/2 = 10 Hz, [B(OCH2CF3)4]−) ppm

Powder: ATR-IR ν [cm−1] = 477 (vw), 569 (w), 602 (w), 679 (w), 691 (sh) 840 (w), 963 (m), 973 (sh), 1005 (sh), 1052 (vs), 1162 (vs), 1281 (s), 1430 (w), 1464 (vw), 2248 (vw), 2909 (vw), 2957 (vw) Experimental Section 113

5.7.4.!Synthesis in 1,2-dimethoxyethane

MR-058.4

Lithium borohydride (0.184 g, 8.4 mmol, 1 eq.) was dissolved in 1 ,2-dimethoxyethane (80 mL). Into the solution 2,2,2-trifuorethanol (3.1 mL, 4.23 g, 42.8 mmol, 5 eq.) was dropped within 30 minutes under stirring. A gas evolution was observed. Afer this the reaction mixture was refuxed for 4.5 h. Afer refuxing the solvent was removed under vacuum. A colourless powder was obtained (3.5 g, yield: 100 %) m.p. (DSC) = 281 ℃ (decomposition)

Powder dissolved in 1,2-dimethoxyethane: 1H,NMR (400.17 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 3.86 (q, 2H, 3J(1H,19F) = 9.62 Hz, [B(OCH2CF3)4]−) ppm 19F,NMR (376.54 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −76.53 (t, 3F, 3J(1H,19F) = 9.55 Hz, [B(OCH2CF3)4]−), −76.30 (t, 3F, 3J(1H,19F) = 9.70 Hz, −OCH2CF3), −78.2 (br, ∆1/2 = 85 Hz, HOCH2CF3) ppm 7Li,NMR (155.52 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −1.04 (s, ∆1/2 = 1.7 Hz, Li[B(OTfe)4]) ppm 11B,NMR (128.39 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 2.30 (s, ∆1/2 = 8.9 Hz, [B(OCH2CF3)4]−) ppm

Powder: ATR-IR: ν [cm−1] = 477 (vw), 514 (vw), 534 (vw), 552 (vw), 568 (w), 602 (w), 679 (w), 691 (sh), 839 (m), 962 (s), 973 (sh), 1006 (w), 1053 (vs), 1101 (vs), 1159 (vs), 1279 (s), 1431 (w), 1464 (vw), 2910 (vw), 2956 (vw) Raman: ν [cm−1] = 470 (vw), 536 (vw), 549 (vw), 572 (vw), 597 (vw), 633 (w), 680 (w), 699 (vw), 799 (vw), 853 (s), 966 (w), 1004 (vw), 1164 (m), 1185 (sh), 1210 (vw), 1278 (w), 1312 (w), 1467 (w), 2757 (w), 2819 (w), 2906 (m), 2966 (m)

Powder dissolved in D2O: 1H,NMR (400.17 MHz, D2O = 4.81 ppm, r.t.) δ = 4.02 (q, 2H, 3J(1H,19F) = 9.16 Hz, [B(OCH2CF3)4]−) ppm 19F,NMR (376.54 MHz, D2O = 4.81 ppm, r.t.) δ = −76.69 (t, 3F, 3J(1H,19F) = 9.16 Hz, [B(OCH2CF3)4]−) ppm 7Li,NMR (155.52 MHz, D2O = 4.81 ppm, r.t.) δ = 0.26 (s, ∆1/2 = 1.7 Hz, Li[B(OTfe)4]) ppm 11B,NMR (128.39 MHz, D2O = 4.81 ppm, r.t.) δ = 2.93 (s, 1 B, ∆1/2 = 8.26 Hz, [B(OCH2CF3)4]−) ppm 114 Lithium tetrakis(trifluoroethoxy)borate

5.7.5.!Synthesis in toluene with 4 eq. 1,2-dimethoxyethane

MR-058.5

Lithium borohydride (0.183 g, 8.4 mmol, 1 eq.) were dissolved in 1,2-dimethoxyethane (3.50 mL, 3.05 g, 33.8 mmol, 4 eq.) and then toluene (50 mL) was added. Into this solution 2,2,2-trifuorethanol (3.5 mL, 4.83 g, 42.8 mmol, 5 eq.) was dropped within 30 minutes under stirring. A gas evolution was observed. Afer this the reaction mixture was refuxed for 4.5 h. Afer refuxing the mixture was fltered and than the dried under vacuum. A colourless powder was obtained (2.9 g, yield: 83 %).

Powder dissolved in 1,2-dimethoxyethane: 1H,NMR (400.17 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 3.88 (q, 2H, 3J(1H,19F) = 9.63 Hz, [B(OCH2CF3)4]−) ppm 19F,NMR (376.54 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −76.39 (t, 3F, 3J(1H,19F) = 9.64 Hz, [B(OCH2CF3)4]−), −76.30 (t, 3F, 3J(1H,19F) = 9.70 Hz, −OCH2CF3), −78.2 (br, ∆1/2 = 85 Hz, HOCH2CF3) ppm 7Li,NMR (155.52 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −1.02 (s, ∆1/2 = 1.5 Hz, Li[B(OTfe)4]) ppm 11B,NMR (128.39 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 2.30 (s, ∆1/2 = 8.26 Hz, [B(OCH2CF3)4]−) ppm

Powder: ATR-IR: ν [cm−1] = 471 (vw), 513 (vw), 537 (vw), 554 (vw), 569 (w), 603 (w), 680 (m), 694 (sh), 841 (m), 962 (s), 976 (sh), 1009 (m), 1049 (s), 1100 (vs), 1160 (vs), 1280 (s), 1430 (w), 1465 (vw), 2824 (vw), 2909 (vw), 2950 (vw) Experimental Section 115

5.7.6.!Synthesis in toluene with 2 eq. 1,2-dimethoxyethane

MR-058.6

Lithium borohydride (1.00 g, 46.1 mmol, 1 eq.) was dissolved in 1,2-dimethoxyethane (9.51 mL, 8.27 g, 91.8 mmol, 2 eq.) and then toluene (250 mL) was added. Into this solution 2,2,2-trifuorethanol (16.64 mL, 22.96 g, 230 mmol, 5 eq.) was dropped within 1 h under stirring. A gas evolution was observed. Afer this the reaction mixture was refuxed for 4.5 h. Afer refuxing the mixture was fltered and than dried under vacuum. A colourless powder was obtained (16.7 g, yield: 88 %).

Powder dissolved in 1,2-dimethoxyethane: 1H,NMR (400.17 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 4.74 (br, ∆1/2 = 15 Hz, HOCH2CF3), 3.82 (q, 2H, 3J(1H,19F) = 9.62 Hz, [B(OCH2CF3)4]−) ppm 19F,NMR (376.54 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −76.33 (t, 3F, 3J(1H,19F) = 9.70 Hz, −OCH2CF3), −76.35 (t, 3F, 3J(1H,19F) = 9.70 Hz, −OCH2CF3),−76.48 (t, 3F, 3J(1H,19F) = 9.61 Hz, [B(OCH2CF3)4]−), −78.2 (br, ∆1/2 = 85 Hz, HOCH2CF3) ppm 7Li,NMR (155.52 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = −1.01 (s, ∆1/2 = 2.1 Hz, Li[B(OTfe)4]) ppm 11B,NMR (128.39 MHz, DME = 3.30 ppm, toluene-D8 external lock, r.t.) δ = 2.29 (s, ∆1/2 = 9.45 Hz, [B(OCH2CF3)4]−), 1.92 (s, [B(OR)4]−) ppm

Powder: ATR-IR ν [cm−1] = 472 (vw), 513 (vw), 537 (vw), 554 (vw), 569 (w), 603 (w), 680 (m), 694 (sh), 841 (m), 962 (s), 976 (sh), 1010 (m), 1049 (s), 1101 (vs), 1160 (vs), 1280 (s), 1430 (w), 1465 (vw), 2824 (vw), 2909 (vw), 2951 (vw) 116 Lithium tetrakis(trifluoroethoxy)borate

5.8!References

[150]sJ. H. Golden, C. Schreier, B. Singaram, S. M. Williamson, Inorg. Chem. 1992, 31, 1533-1535. [151]sS. Bulut, P. Klose, I. Krossing, Dalton Trans. 2011, 40, 8114-8124. [152]sS. Tsujioka, B. G. Nolan, H. Takase, B. P. Fauber, S. H. Strauss, J. Electrochem. Soc. 2004, 151, A1418-A1423. [153]sH. Yamaguchi, H. Takahashi, M. Kato, J. Arai, J. Electrochem. Soc. 2003, 150, A312-A315. [154]sJ. Barthel, R. Buestrich, E. Carl, H. J. Gores, J. Electrochem. Soc. 1996, 143, 3565-3571. [155]sJ. Barthel, R. Buestrich, H. J. Gores, M. Schmidt, M. Wuhr, J. Electrochem. Soc. 1997, 144, 3866-3870. [156]sJ. Barthel, M. Schmidt, H. J. Gores, J. Electrochem. Soc. 1998, 145, L17-L20. [157]sJ. Barthel, A. Schmid, H. J. Gores, J. Electrochem. Soc. 2000, 147, 21-24. [158]sW. Xu, L.-M. Wang, R. A. Nieman, C. A. Angell, J. Phys. Chem. B 2003, 107, 11749- 11756. [159]sM. Videa, W. Xu, B. Geil, R. Marzke, C. A. Angell, J. Electrochem. Soc. 2001, 148, A1352-A1356. [160]sI. Krossing, H. Brands, R. Feuerhake, S. Koenig, J. Fluorine Chem. 2001, 112, 83-90. [161]sL. Álvarez Hernández, Dissertation thesis, Albert-Ludwigs-Universität (Freiburg), 2013. [162]s K. Xu, Chem. Rev. 2004, 104, 4303-4418. [163]s D. R. Lide, in CRC Handbook of Chemistry and Physics, 89th ed., CRC Press/Taylor and Francis, Boca Raton, FL, 2009. [164]sC. Daniel, J. O. Besenhard, in Handbook of Batery Materials, 2nd ed., Wiley-VCH, Weinheim, Germany, 2011. [165]sX. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500-506. [166]sD. Bresser, S. Passerini, B. Scrosati, Chem. Commun. (Cambridge, U. K.) 2013, 49, 10545-10562. [167]sY.-X. Yin, S. Xin, Y.-G. Guo, L.-J. Wan, Angew. Chem., Int. Ed. 2013, 52, 13186- 13200. [168]s S. Evers, L. F. Nazar, Accounts of Chemical Research 2012, 46, 1135-1143. [169]sP. G. Balakrishnan, R. Ramesh, T. Prem Kumar, J. Power Sources 2006, 155, 401-414. [170]s S. E. Sloop, J. K. Pugh, S. Wang, J. B. Kerr, K. Kinoshita, Electrochem. Solid-State Let. 2001, 4, A42-A44. [171]sM. Salomon, B. Scrosati, Gazz. Chim. Ital. 1996, 126, 415-427. [172]sB.-T. Yu, W.-H. Qiu, F.-S. Li, L. Cheng, J. Power Sources 2007, 166, 499-502. [173]sK. Amine, J. Liu, S. Kang, I. Belharouak, Y. Hyung, D. Vissers, G. Henriksen, J. Power Sources 2004, 129, 14-19. [174]sK. O. Christe, D. A. Dixon, D. McLemore, W. W. Wilson, J. A. Sheehy, J. A. Boatz, J. of Fluorine Chem. 2000, 101, 151-153. [175]sL. Vogdanis, W. Heitz, Macromol. Chem. Rapid Commun. 1986, 7, 543-547. [176]sL. Vogdanis, B. Martens, H. Uchtmann, F. Henseland, W. Heitz, Makromolekulare Chemie-Macromolecular Chemistry and Physics 1990, 465-472. [177]s U. Heider, R. Oesten, M. Jungnitz, J. Power Sources 1999, 81-82, 119-122. References 117

[178]s D. Aurbach, G. E. Blomgren, L.A. Dominey, I. Galasiu, R. Galasiu, Y. Gofer, S. Komada, X. Liu, T. Osaka, D.A. Sherson, J. Tonstad, I. Weissman, A. Zaban, in Nonaqueous Electrochemistry (Ed.: D. Aurbach), Marcel Dekker Inc., 1999. [179]s K. M. Abraham, D. M. Pasquariello, F. J. Martin, J. Electrochem. Soc. 1986, 133, 661- 666. [180]sE. Zinigrad, L. Larush-Asraf, G. Salitra, M. Sprecher, D. Aurbach, Termochim. Acta 2007, 457, 64-69. [181]sY. Hu, H. Li, X. Huang, L. Chen, Electrochem. Commun. 2004, 6, 28-32. [182]sP. Ping, Q. Wang, J. Sun, H. Xiang, C. Chen, Journal of Te Electrochemical Society 2010, 157, A1170-A1176. [183]sK. S. Gavrichev, G. A. Sharpataya, V. E. Gorbunov, Termochim. Acta 1996, 282/283, 225-238. [184]sPatent pending Pr.: EP 13177155 (19.07.2013), BASF SE – Use of lithium alkoxy- borates and lithium alkoxyaluminates as conducting salts in electrolytes of lithium sulfur bateries. [185]sPatent pending Pr.: EP 13177159 (19.07.2013), BASF SE – Use of lithium alkoxy- borates and lithium alkoxyaluminates as conducting salts in electrolytes of lithium ion bateries.

119

6!Summary

Tis thesis was devoted to the development of novel lithium conducting salts for rechargeable lithium-ion bateries. Terefore, three diferent types of weakly coordinating anions were investigated: boroxinates ([B3O3X4]−, X = F, Ohfp), bis(trifuoroacetyl) phosphate ([PO2(OAcF)2]−) and tetrakis(2,2,2-trifuoroethoxy)borate ([B(OTfe)4]−). Te lithium salts of these anions have been unknown and the results of the fundamental research and the atempts to synthesize the salts are summarized herein and demonstrate their potential for the batery technologies. First a feasibility study was conducted to explore the ways to get access to the boroxinate anion [B3O3X4]−. Terefore, it was atempted to synthesize lithium tetra(hexafuoro-2-propanoxy)boroxinate (Li[B3O3(Ohfp)3F] Figure 53) starting from the neutral boroxine precursor B3O3(Ohfp)3. Te synthesis of B3O3(Ohfp)3 from B2O3 with B(Ohfp)3 was tedious and low in yield. Only very small amounts of precursor could be obtained, which made further steps impossible. Another atempt to obtain the precursor was the substitution of the –OMe groups of trimethoxyboroxine (B3O3(OMe)3), a commercially available boroxine, with the fuorinated –Ohfp groups. It was found that the substitution leads to mixtures of boroxines, with no success in separation. A further interesting boroxinate based anion is tetrafuoroboroxinate ([B3O3F4]− Figure 54) and it was atempted to synthesize it through fuorination of B3O3(OMe)3 with mild fuorination reagents. Tereby an interesting knowledge was gained: Te main products of the fuorination indicated a cleavage of the boroxine ring and a quantitative formation of [BF4]− and [B3O3F6]3−. Te desired singly charged boroxinate was never observed. Tis dimmed the prospect to obtain a lithium boroxinate, so no further endeavours were undertaken.

Figure 53!Lef: Electrostatic potential of [B3O3(Ohfip)3F]− (calculated at the RI-BP86/def2-TZ- VP level) projected on an isomap of electron density (0.01 e Å−1) Right: Ball and stick model of [B3O3(Ohfip)3F]−.

A large part of the synthetic work was dedicated to the synthesis of lithium bis(trifuoroacetyl)phosphate (Li[PO2(OAcF)2] Figure 55), as a possible conducting salt. Four diferent synthetic routes outgoing from P4O10, POCl3, LiH2PO4 and Li[PO2F2] were thoroughly investigated. Both syntheses of Li[PO2(OAcF)2] from P4O10 or Li[PO2F2] demonstrated by NMR spectroscopy, that the new lithium salt was formed, but with a maximum yield of 12 % respectively 0.8 %. But the actual difculty was, that the obtained prod- 120 Summary

Figure 54!Lef: Electrostatic potential of [B3O3F4]− (calculated at the RI-BP86/def2-TZVP level) projected on an isomap of electron density (0.01 e Å−1) Right: Ball and stick model of [B3O3F4]−.

uct mixture was contaminated with other lithium salts, including the reactants, and a separation of Li[PO2(OAcF)2] was not feasible. Terefore another synthesis route was atempted, avoiding other lithium salts: starting with the formation of a non ionic precursor POCl(OAcF)2, which was obtained from the reaction of phosphoryl chloride (POCl3) and lithium trifuoroacetate (LiOAcF). Te reaction was optimized by the use of trifuoroacetic anhydride (TFAA) and aluminium chloride as catalyst. Supplementary calculations revealed that the formation of LiCl is the driving force in this reaction. Unfortunately, POCl(OAcF)2 is highly sensible against water traces and it is a reactive acetylating agent and e.g. reacts with tetrahydrofuran, which was used as NMR solvent. All eforts to isolate the neutral precursor from the reaction mixture failed. Finally the esterifcation of LiH2PO4 with TFAA to lithium bis(trifuoroacetyl)phosphate was investigated. Te synthesis in diethyl ether was successful, and the new lithium salt was characterized in solution with NMR and IR spectroscopy. Tis synthesis comes with one disadvantage: Te main side-product of the reaction is trifuoroacetic acid (TFA), causing a further side reaction. Originating from the acid-base equilibrium of LiH2PO4 and TFA, small amounts of H3PO4 and LiOAcF were formed. Te presence of LiOAcF indicates a major problem, because we found no way to isolate the new conducting salt from contaminating LiOAcF, because of the similar solubility in all used solvents. On the other side the generated H3PO4 could condense under the conditions in the reaction vessel to metaphosphates, which again were esterifcated. Tese parasitic side reactions generated impurities during the reaction, which could not be extracted

Figure 55!Lef: Electrostatic potential of [PO2(OAcF)2]− (calculated at the RI-BP86/def2-TZVP level) projected on an isomap of electron density (0.01 e Å−1). Right: Ball and stick model of [PO2(OAcF)2]−. 121 or separated from the new lithium salt. But the crucial observation was, that during the work up of the reaction mixture, the amount of LiOAcF increased afer evaporation of the solvent. Termodynamic calculations supported the proposal, that Li[PO2(OAcF)2] is not stable in solid state and undergoes a dismutation. Tis dismutation is an intermolecular condensation with elimination of LiOAcF and formation of a metaphosphate. Lithium trifuoroacetate is a good leaving group and its formation is the driving force of the dismutation. Tis crucial point obstructs the access to pure lithium bis(trifuoroacetyl)phosphate in the condensed phase. Hence the use of the salt could not be tested for bateries, despite considerable synthetic efort put into the preparation. A new anion with views to success, was synthesized from lithium borohydride with excess 2,2,2-trifuorethanol in toluene and at least two equivalents of 1,2-dimethoxyethane (DME). Te new Li[B(OTfe)4] (Figure 56) salt was obtained in multi gram scale and without impurities, as long as DME was used during the reaction. Te structure of Li[B(OTfe)4] was completely characterized with heteronuclear magnetic resonance and vibrational spectroscopy (IR and Raman). In the solid state the lithium ion is connected to the oxygen atoms of the anion, which was determined by vibrational spectroscopy. Tis coordination is gradually broken in solution with strong donor solvents, such as DME. For instance, a solution of Li[B(OTfe)4] in DME with a concentration >0.5 mol L−1 showed a phase separation in a clear upper and a turbid lower phase. It could be nicely proven with Infrared measurements that in higher concentrations (>0.5 mol L−1), a lithium coordination of the anion still exists and at lower concentrations solvent separated ion pairs were formed. Te electrical conductivity of the new Li[B(OTfe)4] salt in DME (0.8 mol L−1) is 3.9 mS cm−1 at r.t., which is satisfactory for the use in lithium-sulfur bateries. Solutions in DMC were gel-like and an electrical conductivity was not measurable. Because of this behaviour, the new lithium salt can be used as additive to adjust the viscosity of electrolytes. A remarkable feature of Li[B(OTfe)4] is its high thermal stability, a decomposition slowly begins only above 271 ℃. Furthermore it convinces with the long term stability against water, which is a great advantage over fuorine containing conducting salts, such as Li[BF4] or Li[PF6]. Cyclic voltammetric measurements confrmed the electrochemical stability of Li[B(OTfe)4] in a potential range of 0 to 4.8 V. Tis meets the demands for lithium-ion bateries and largely exceeds the requirements for lithium-sulfur bateries. Terefore the performance of Li[B(OTfe)4] as conducting salt in a 0.2 mol L−1 solution in 1:1 wt.-% DME/DOL was investigated in lithium-sulfur test cells. Te new lithium salt showed an outstanding cycling performance (Figure 57). Afer the

Figure 56!Lef: Electrostatic potential (calculated at the RI-BP86/def2-TZVP level) of Li[B(OTfe)4] projected on a isomap of electron density (0.01 e Å−1). Right: Ball and stick model of the [B(OTfe)4]−. 122 Summary

40th cycle 86 % of the capacity remained, with a Coulombic efciency of around 97 % in each cycle. Tis indicates a signifcant performance improvement of lithium-sulfur batteries. Te favorable features of Li[B(OTfe)4], are the good solubility and conductivity in DME, high thermal and moisture stability. Te salt demonstrated a high electrochemical stability over a wide potential range of 0 to 4.8 V, combined with an excellent cycling performance in test cells. Terefore, this new lithium salt and its use in lithium-ion and lithium-sulfur bateries was registered as patent.

1400 100 1200

975 80 ] % ]

1000 [ h y A 837

962 c m n [ 60 800 e i y

t 822 c i i f c f a e

p 600 b a 40 c

discharge capacity m o 400 l

charge capacity u Coulomb efficiency o 20 C 200

0 0 0 10 20 30 40 50 60 cycle no.

Figure 57!Capacity retention of a lithium sulfur test cell during cycling (charge rate: 0.1 C discharge rate: 0.15 C within a voltage range of 1.7 to 2.5 V). The new lithium tetrakis(trifluoroethoxy) borate was used as conducting salt in a 8 wt.-% (0.2 mol L−1) solution of DOL/DME (1:1). The labels are the values from the 5th and 40th cycle. 123

7!General Experimental and Theoretical Procedures

7.1!Techniques Syntheses were carried out under exclusion of air and moisture. Terefore all operations were done at a vacuum line (1 ⋅ 10−3 mbar, slide vane rotary vacuum pump) with Schlenk techniques or in glove-box (Jacomex GPT4) flled with argon (O2 < 5 ppm, H2O < 2 ppm). All glass devices were cleaned in a KOH/iPrOH bath, washed with phosphoric acid and than neutralized with deionized water. Special reaction vessels were used, which contain a glass frit (P4) and are locked with Produran valves. For the pressure reactions a hydrothermal synthesis reactors were used, which consisted of a stainless steel casing and an inner PTFE vessel with a volume of 25 mL. Te used solvents were dried (H2O < 10 ppm) and stored in Schlenk fasks under argon.

7.2!Analytic methods

7.2.1.!Infrared and Raman measurements

Refective Infrared measurements were performed on a Bruker Alpha P FT-IR spectrometer with a Diamond ATR cell (375-7500 cm−1), which was positioned in the glove-box. All spectra were corrected by the sofware OPUS 6.0 (Diamond ATR correction: angle of refection 45°, index of refraction 1.50) and intensities were normalized. Raman spectra were recorded on a Bruker RAM II Fourier-transform Raman module for the Vertex 70 IR spectrometer, equipped with a germanium detector and Nd:YAG laser. For spectral processing the sofware OPUS 6.0 were used.

7.2.2.!NMR measurements

NMR data were recorded on a Bruker DPX200 or Bruker Avance II+ 400 WB spectrometer at room temperature (r.t. = 25 ℃). Te NMR samples were measured in 3 mm sample tubes, which were implemented in a 5 mm sample tube flled with the external deuterium lock solvent (toluene-D8). Analysis and spectra processing were performed using TOPSPIN 2.1. Te spectra were calibrated to the 1H signals of the sample solvent used in the 3 mm tube (e.g. Et2O = 1.2 ppm, –OCH2CH3) and the feld correction of the 7Li, 11B, 13C, 19F, 29Si, 31P were adjusted to the 1H calibration.

7.2.3.!Mass spectrometry

Mass spectra were performed on a Termo TSQ 700 mass spectrometer (EI:70 eV), the ESI-MS on a Termo LCQ Advantage (5 µL sample direct, 3 kV spray-voltage). 124 General Experimental and Theoretical Procedures

7.2.4.!DSC measurements

Te thermal analysis were performed on a Setaram DSC131 evo calorimeter. Te aluminium crucible had a volume of 30 µL. Te measurement were carried out under argon atmosphere.

7.2.5.!Cyclic Voltammetry

Te Cyclic Voltammetry was performed in an argon flled glove-box by using a Metrohm AutoLab PGSTAT101 potentiostat. A three-electrode electrochemical cell was employed with a platinum working electrode, and a lithium metal counter electrode and lithium metal reference electrode. Te potentiostat was controlled by the NOVA 1.8 sofware.

7.2.6.!Conductivity measurements

Te electrical conductivities were measured by using a Metler Toledo inLab®710 conductivity electrode and a Metler Toledo S30 SevenEasy™ conductometer.

7.2.7.!Qantum chemical calculations

Qantum chemical calculations in this thesis were performed with TURBOMOLE V6.1 to V6.4 [186][187]. Density functionals PBE0 [188][189][190][191][192] and BP86 [188][189] [193][194][195] with RI [199][196][202] approximation were used in combination with the basis sets def-SV(P) [196][197], def2[202]-TZVP [198][199] and def2-TZVPP [200][201]. Te module AOFORCE [203][204][206] was used for the calculation of vibrational frequencies and the zero point energies. Te structures were optimized to a minimum on the energy hyper surface, which was proved by the absence of imaginary frequencies. Ter- mal corrections of the entropy and enthalpies were calculated by using the FREEH module. Raman intensities were calculated with the RAMAN tool. Calculation of the Gibbs solvation energies were calculated using the COSMO [205] module using the default options, otherwise the relative permitivity of the solvent was used (EC = 89.6) Te simulation of NMR shifs were carried out with the MPSHIFT tool [207], which uses the GAIO [208] method. Graphics of electrostatic potentials were created with gOpenMol [209]. Te energies and thermodynamic values obtained from the calculation are listed in Table 17. Analytic methods 125

Calculation of thermodynamics

For the calculation of thermodynamics in the condensed state (Free reaction enthalpy, ∆rG°) Born-Fajans-Haber cycles were used. Te latice energies (∆lattG°) can be obtained from the Gibbs-Helmholtz equation Eq.31 (used values see Table 18). For estimating of the total latice potential energy (Upot) of the Volume-based thermodynamics concept [210] was consulted. Upot of a simple salt MX (1:1) was determined from the formula unit volume Vm (in nm3) and the ionic strength factor I (=1), and with the empirical constants α (=117.3 kJ mol−1) and β (=51.9 kJ mol−1) (Eq.33) [211]. Te entropy was calculated from Eq.34 and Eq.35 where k (=1360 J K−1mol−1) and c (=15 J K−1mol−1) are fting parameters [212][213]. Values of the volume of lithium cation (0.00199 nm3) and the chloride anion (0.047 nm3) were taken from Ref. [210] and the COSMO volumes were used [215] for the other anions.

∆lattG° = ∆lattH° − 298.15 K⋅∆lattS° Eq.31 ∆lattH° = Upot + RT (= 2.48 kJ mol−1) Eq.32 Upot(MX) = 2I(α∛Vm + β) Eq.33 S°(s) = k(Vm/nm3 formular unit−1) + cM[J K−1] Eq.34 ∆lattS° = S°(g) − S°(s) Eq.35

Table 17!Energies and thermodynamic values by using the RI-BP86/SV(P) method and COSMO energies with volumes. Additionally the used Gibbs free solvation enthalpies.

COSMO dielec. SCF energy H FreeH S FreeH ∆solvG° COSMO Compound energy + [Hartree] [kJ mol−1] [kJ mol−1K−1] [kJ mol−1] Volume OC corr. [Hartree]

POCl3 −1796.85752489 39.85 0.32964 −1796.8621455 −4.2 —

TFA −526.43045971 115.80 0.34144 −526.4411787 −20.2 —

HCl −460.69877479 22.83 0.18736 — — —

[OAcF]− −525.89391485 81.97 0.34230 −525.9923603 −250.5 644.65

Cl− −460.12905950 3.72 0.15351 — — —

POCl(OAcF)2 −1928.29982079 209.30 0.59894 −1928.3097027 −18.0 —

[PO2(OAcF)2]− −1543.48630945 212.52 0.58547 −1543.5663072 −202.1 1593.00

P3O6(OAcF)3 −3052.62057780 378.33 0.87774 −3052.6400864 −43.3 —

Table 18!Calculated thermodynamic values.

Compound ∆lattS° ∆lattG° LiCl 0.2126 −711.51 Li[OAcF] 0.3182 −370.37 Li[PO2(OAcF)2] 0.3643 −509.50 126 General Experimental and Theoretical Procedures

7.3!References

[186]MR. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Let. 1989, 162, 165. [187]MO. Treutler, R. Ahlrichs, J. Chem. Phys. 1995, 102, 346. [188]MP. A. M. Dirac, Proc. Royal Soc. A 1929, 123, 714. [189]MJ. C. Slater, Phys. Rev. 1951, 81, 385. [190]MJ. P. Perdew, Y. Wang, Phys. Rev. B 1992, 45, 13244. [191]MJ. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Let. 1996, 77, 3865. [192]MJ. P. Perdew, M. Ernzerhof, K. Burke, J. Chem. Phys. 1996, 105, 9982. [193]MS. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 1980, 58, 1200. [194]MA. D. Becke, Phys. Rev. A 1988, 38, 3098. [195]MJ. P. Perdew, Phys. Rev. B 1986, 33, 8822; 34, 7460 (E). [196]MK. Eichkorn, O. Treutler, H. Öhm, M. Häser, R. Ahlrichs, Chem. Phys. Let. 1995, 242, 652. [197]MA. Schäfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571. [198]MA. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829. [199]MK. Eichkorn, F. Weigend, O. Treutler, R. Ahlrichs, Teor. Chem. Acc. 1997, 97, 119. [200]MF. Weigend, M. Häser, H. Patzelt, R. Ahlrichs, Chem. Phys. Let. 1998, 294,143. [201]MF. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297. [202]MF. Weigend, Phys. Chem. Chem. Phys. 2006, 8, 1057. [203]MP. Deglmann, F. Furche, R. Ahlrichs, Chem. Phys. Let. 2002, 362, 511. [204]MP. Deglmann, F. Furche, J. Chem. Phys. 2002, 117, 9535. [205]MA. Klamt, G. Schüürmann, J. Chem. Soc. Perkin Trans. 2 1993, 799-805. [206]MP. Deglmann, K. May, F. Furche, R. Ahlrichs, Chem. Phys. Let. 2004, 384, 103. [207]MM. Kollwitz, J. Gauss, Chem. Phys. Let. 1996, 260, 639. [208]MR. Ditchfeld, Mol. Phys. 1974, 27, 789-807. [209]ML. Laaksonen, J. Mol. Graph. 1992, 10, 33. [210]ML. Glasser, H. D. B. Jenkins, Chem. Soc. Rev. 2005, 34, 866-874. [211]MH. D. B. Jenkins, H. K. Roobotom, J. Passmore, L. Glasser, Inorg. Chem. 1999, 38, 3609-3620. [212]MH. D. B. Jenkins, L. Glasser, Inorg. Chem. 2004, 42, 8702-8708. [213]MEntropy of: Li+ = 0.13301142 J K−1mol−1, Cl− = 0.15334865 J K−1mol−1, from Sackur-Tetrode equation. [214]MU. P. R. M. Preiss, J. M. Slatery, I. Krossing, Ind Eng. Chem. Res. 2009, 48, 2290- 2296. [215]MTe COSMO volumes were calculated with a correction term: (Vcosmo nm3⋅1.031) + 0.004. see reference [29] 127

8!List of Abbreviations

Ah Ampere hour ATR atenuated total refection BOB bis(oxalato)borate br broad signal C Coulomb calc. calculated COSMO continuum solvation model COSY correlated spectroscopy d doublet (NMR) DEC diethyl carbonate DFT density functional theory DMC dimethyl carbonate DME 1,2-dimethoxyethane DMM dimethoxymethane DOL 1,3-dioxolane DQF double quantum flter DSC diferential scanning calorimetry E° standard voltage EC ethylene carbonate e.g. exempli gratia EI electron impact EMC ethyl methyl carbonate Eq. equation eq. equivalent ESI electrospray ionization eV electron Volt exp. experimental F Faraday constant g gaseous, gram G Gibbs energy h hour 128 List of Abbreviations

H enthalpy HMBC heteronuclear multiple-bond correlation HOMO highest occupied molecular orbital HSQC heteronuclear single quantum correlation Hz Hertz iPr iso-propyl IR Infrared J coupling constant in Hertz K Kelvin lat latice Li/S lithium-sulfur LUMO lowest unoccupied molecular orbital m multiplet (NMR), medium, milli Me methyl MeCN acetonitrile MHz megahertz MS mass spectrometry NMR nuclear magnetic resonance NOESY nuclear Overhauser efect spectroscopy NTf2 N(SO2CF3)2 OAcF OC(=O)CF3 Ohfp OCH(CF3)2 OTfe OCH2CF3 Ox oxalato, C2O4 o-DFB ortho-difuorobenzene PC propylene carbonate ppm parts per million r.t. room temperature s singlet (NMR), strong, solid S entropy SHE standard hydrogen electrode solv solvation TFA trifuoroacetate TFAA trifuoroacetic anhydride 129

TFE 2,2,2-trifuoroethanol THF tetrahydrofuran TOCSY total correlation spectroscopy Upot potential energy v very V Volt vs very strong vs. versus vw very weak w weak Wh Wat hour WCA weakly coordinating anion wt. weight ∆ delta

∆1/2 half width δ chemical shif ε dielectric constant ν wave number σ isotrope shielding coefcient

Abstract

131

New Conducting Salts for Rechargeable Lithium-Ion Bateries

Michael Rohde

February 2014

Abstract

Conducting salts, based on chemically robust and weakly coordinating anions, are par- ticularly important for the improvement of lithium-ion bateries. Tis thesis deals with the development of such lithium conducting salts with three novel anions. Access to the boroxinate anion ([B3O3X4]−, X = F, Ohfp; hfp = CH(CF3)2) was investigated by the synthesis of the neutral precursor B3O3(Ohfp)3. Both the addition reactions of B2O3 with B3O3(Ohfp)3 and the substitution reaction of B3O3(OMe)3 with HOhfp were low in yield and led to undesired mixtures. Fluorination of B3O3(OMe)3 exhibited an exclusive formation of [BF4]− and [B3O3F6]3− anions. Te formation of the novel lithium phosphates (Li[PO2(OAcF)2], OAcF = O(O=)CCF3) were thoroughly examined through four diferent synthetic routes. Te evidence of Li[PO2(OAcF)2] was provided by the synthesis outgoing from P4O10 or Li[PO2F2]. Be- cause of ionic side products and the low yield of 12 % respectively 0.8 %, an isolation the pure salt could not be achieved. An alternative route, along a non-ionic precursor POCl(OAcF)2 from POCl3, has proved difcult, because POCl(OAcF)2 is highly reactive and could not be isolated. Li[PO2(OAcF)2], obtained by the esterifcation of LiH2PO4 and trifuoroacetic anhydride (TFAA), was completely characterized by NMR- and IR spectroscopy. Unavoidable parasitic side reactions caused impurities such as LiOAcF and several metaphosphates, which increased during the work-up. Termodynamic calculations verify the observation, that Li[PO2(OAcF)2] is not stable in condensed phase and tends to eliminate LiOAcF. A promising conducting salt for lithium-ion bateries is considered to be Li[B(OTfe)4] (OTfe = OCH2CF3), which could be synthesized with yield of 100 % from Li[BH4] and TFE (HOCH2CF3) in presence of 1,2-dimethoxyethane (DME). Te structure was completely clarifed with NMR-, IR- and Raman spectroscopy. Te lithium borate captivates with its high decomposition temperature of 271 ℃ and long-term stability in water. Cyclic Voltammetric measurements verify an electrochemical stability in a wide potential range of 0 to 4.8 V. Additionally, it shows a good electrical conductivity of 3.9 mS cm−1 in DME (0.8 mol L−1). Te remarkable performance of the new conducting salt was demonstrated in lithium-sulfur bateries, indicated by a Coulombic efciency of over 97 % per cycle and a remaining capacity of 86 % afer the 40th cycle. Due to the outstanding features of Li[B(OTfe)4] as conducting salt, it was registered as patent for the use in lithium-ion and lithium-sulfur bateries.

Keywords: lithium conducting salt, lithium-ion batery, lithium-sulfur batery, weakly coordinating anion, electrolyte Kurzzusammenfassung 132

New Conducting Salts for rechargeable Lithium-Ion Bateries

Michael Rohde

Februar 2014 Kurzzusammenfassung

Bei der Leistungssteigerung von wiederaufadbaren Lithium-Ionen Baterien kommt Leitsalzen, die auf chemisch robusten und schwach koordinierenden Anionen basie- ren, eine besondere Bedeutung zu. In dieser Arbeit wird auf die Entwicklung solcher Lithium-Leitsalze mit drei neuartigen Anionen eingegangen. Der Zugang zu dem Boroxinatanion ([B3O3X4]−, X = F, Ohfp; hfp = CH(CF3)2) wurde zu- nächst über die Synthese der neutralen Vorstufe B3O3(Ohfp)3 untersucht. Dabei führten sowohl die Additionsreaktionen von B2O3 mit B(Ohfp)3 als auch die Substitutionsre- aktion von B3O3(OMe)3 mit HOhfp zu geringen Ausbeuten und unerwünschten Ge- mischen. Die Fluorierung von B3O3(OMe)3 zum Tetrafuorboroxinat Anion ([B3O3F4]−) zeigte eine ausschließliche Bildung der [BF4]− und [B3O3F6]3− Anionen. Die Darstellung des neuen Lithumphosphats (Li[PO2(OAcF)2], OAcF = O(O=)CCF3) wurde durch vier verschiedene Syntheserouten gründlich untersucht. Bei den Synthesen aus P4O10 oder Li[PO2F2] gelang der Nachweis von Li[PO2(OAcF)2]. Eine Isolierung des reinen Salzes konnte wegen der ionischen Nebenprodukte und der geringen Ausbeute von 12 % bzw. 0,8 % nicht erzielt werden. Eine alternative Route über eine nichtionische Vorver- bindung POCl(OAcF)2 aus POCl3 erwies sich als schwierig, da POCl(OAcF)2 hoch reaktiv ist und nicht isoliert werden konnte. Li[PO2(OAcF)2] konnte aus der Veresterung von LiH2PO4 mit Trifuoressigsäureanhydrid (TFAA) mitels NMR- und IR-Spektroskopie vollständig charakterisiert werden. Unvermeidbare parasitäre Nebenreaktionen führ- ten zu Verunreinigungen, wie LiOAcF und verschiedenen Metaphosphaten, die bei der Aufarbeitung der Reaktionsmischung zunahmen. Termodynamische Rechnungen be- legen die Beobachtung, dass Li[PO2(OAcF)2] in kondensierter Phase nicht stabil ist und zur Abspaltung von LiOAcF neigt. Als vielversprechendes Leitsalz für Lithium-Ionen Baterien kann Li[B(OTfe)4] (OTfe = OCH2CF3) gelten, das aus Li[BH4] und TFE (HOCH2CF3) in Gegenwart von 1,2-Dimethoxyethan (DME) mit einer Ausbeute von 100 % hergestellt werden konnte. Die Struktur wurde vollständig mit NMR-, IR- und Raman-Spektroskopie aufgeklärt. Das Lithiumborat besticht mit seiner hohen Zersetzungstemperatur von 271 ℃ und Langzeitstabilität in Wasser. Cyclovoltammetrische Messungen bestätigen eine elektro- chemische Stabilität in einem großen Potentialbereich von 0 bis 4,8 V. Zusätzlich zeigt es eine gute elektrische Leitfähigkeit von 3,9 mS cm−1 in DME (0,8 mol L−1). Die hervor- ragende Leistungsfähigkeit des neuen Leitsalzes wurde in Lithium-Schwefel-Baterien demonstriert, dabei zeigte es eine Coulomb-Efzienz von über 97 % pro Zyklus und ein verbleibende Kapazität von 86 % nach 40 Zyklen. Wegen der überragenden Eigenschaf- ten von Li[B(OTfe)4] wurde es als Leitsalz in Lithium-Ionen und Lithium Schwefel-Bat- terien patentiert.

Schlagworte: Lithium Leitsalz, Lithium-Ionen Baterie, Lithium Schwefel-Baterie, schwach koordinierendes Anion, Elektrolyt