Ethyl Methyl Sulfone-Based Electrolytes for Lithium Ion Battery Applications

energies

Article Ethyl Methyl Sulfone-Based Electrolytes for Lithium Ion Battery Applications

Peter Hilbig 1, Lukas Ibing 2, Ralf Wagner 2, Martin Winter 1,2 and Isidora Cekic-Laskovic 1,2,* 1 Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstrasse 46, 48149 Münster, Germany; [email protected] (P.H.); [email protected] (M.W.) 2 MEET Battery Research Center/Institute of Physical Chemistry, University of Münster, Corrensstrasse 46, 48149 Münster, Germany; [email protected] (L.I.); [email protected] (R.W.) * Correspondence: [email protected]; Tel.: +49-251-83-36805

Received: 20 July 2017; Accepted: 28 August 2017; Published: 1 September 2017

Abstract: Sulfone-based electrolytes, known for their higher oxidative stability compared to the typically used organic carbonate-based electrolytes, are considered promising electrolytes for high voltage cathode materials towards the objective of obtaining increased energy density in lithium ion batteries. Nevertheless, sulfones suffer from high viscosity as well as incompatibility with highly graphitic anode materials, which limit their application. In this paper, the effect of ﬂuoroethylene carbonate (FEC) as an electrolyte additive for the application of ethyl methyl sulfone (EMS) electrolytes containing LiPF6 as conducting salt, is studied in graphite-based cells by means of selected electrochemical and spectroscopic methods. In addition, inﬂuence of ethylene acetate (EA) as co-solvent on the electrolyte viscosity and conductivity of the EMS-based electrolytes is discussed, revealing improved overall nickel cobalt manganese oxide (NMC)/graphite cell performance. X-ray photoelectron spectroscopy (XPS) measurements provide information about the surface chemistry of the graphite electrodes after galvanostatic cycling. The concept of EA as co-solvent is found to be applicable for other sulfones such as isopropyl methyl sulfone (MeiPrSO2) and ethyl isopropyl sulfone (EtiPrSO2).

Keywords: single solvent-based nonaqueous electrolyte; ethyl methyl sulfone; graphite; solid electrolyte interphase; ﬂuoroethylene carbonate

1. Introduction Nowadays, lithium-ion batteries (LIBs) find application in a very broad field of industrial applications [1–3] as they are, for instance, used in small portable devices, such as mobile phones or laptop computers, as well as traction batteries in the automotive industry [4]. Unfortunately, the driving range for electric cars (before being recharged) is still too low [5]. For this reason, it is of high importance to develop high voltage and high capacity materials delivering higher energy density compared to the commonly used materials. Numerous efforts towards designing and developing high voltage cathode materials have been made and are still in focus of exhaustive research [6,7]. One of the main obstacles related to application of high voltage cathode materials in LIBs with graphite as anode material, refers to selection of an appropriate electrolyte formulation that fulfills the mandatory minimum requirements. Among them, the electrolyte has to maintain electrochemical stability, which is not the case for the commercially used organic carbonate-based electrolytes containing lithium hexafluorophosphate (LiPF6) as conductive salt [8], as they decompose at a potential of 4.5 V vs. Li/Li+ [9]. On the other hand, the desired electrolyte has to enable formation, like the organic carbonate based electrolytes [10–12], of a kinetically stable solid electrolyte interphase (SEI) on the graphite electrode [13–16]. Sulfones represent a class of solvents reported to be stable at high potentials [17–21]. Furthermore, sulfones, especially ethyl methyl sulfone (EMS), are environmentally

Energies 2017, 10, 1312; doi:10.3390/en10091312 www.mdpi.com/journal/energies Energies 2017, 10, 1312 2 of 14 friendly and have high flashpoint, thus increasing the safety of the electrolyte. The major barrier to commercialization of sulfone-based electrolytes is their poor compatibility with graphite anodes [22]. One of the possibilities is to replace graphite with lithium titanate (LTO), however this anode material delivers lower energy density [23]. To overcome this challenge, SEI-forming electrolyte additives can be added to the sulfone-based electrolyte, thus leading to the formation of an effective SEI on the graphite surface [13]. A wide variety of SEI additives have been reported in the literature so far [24–28]. In particular, fluoroethylene carbonate (FEC) shows promising results on both graphite anode and high voltage cathode materials [29]. Another challenge arising with the use of sulfone-based electrolytes is related to their low wettability of separator and electrodes as well as their poor electrolyte conductivity when used as electrolyte solvents [22]. Different co-solvents are reported to decrease the viscosity and to increase the conductivity of sulfone-based electrolytes [30]. These co-solvents are for instance dimethyl carbonate (DMC) or ethyl methyl carbonate (EMC) [23,31]. In this paper, the influence of FEC as SEI-forming additive on sulfone-based electrolytes was investigated. Furthermore, by using ethylene acetate (EA) as co-solvent in EMS-based electrolytes, the challenges of poor wettability and poor conductivity were addressed. The electrolyte formulations were investigated in lithium NMC/graphite cells. To analyze the main decomposition products and to show the beneficial behavior of FEC in the electrolyte, X-ray photoelectron spectroscopy (XPS) and XPS sputter depth profiling measurements were performed.

2. Experimental All electrolytes were prepared in an argon-filled glovebox with water and oxygen content below 0.1 ppm. EMS (99%, ABCR, Karlsruhe, Germany) was heated up to 60 ◦C prior to electrolyte formulation. LiPF6 (battery grade, BASF, Ludwigshafen, Germany), FEC (battery grade, BASF) and EA (99.8% anhydrous, Sigma Aldrich, St. Louis, MO, USA) were used as received. All investigated electrolytes were formulated by volume percent (vol. %). For reasons of comparison, 1 M LiPF6 in EC/DEC (3:7 wt. %) (LP47, BASF, battery grade) was used as received. T44 graphite was used as model material for the electrochemical investigations, as its high BET surface area leads to increased electrolyte reduction [32], thus revealing additional minor electrolyte decomposition reactions. Graphite electrodes were composed of 87 wt. % graphite (Imerys, Paris, France) 8 wt. % polyvinylidene difluoride (PVdF, Arkema, Colombes, France) and 5 wt. % conductive additive Super C65 (Imerys). The composition of the lithium manganese oxide spinel, (LMO)-based electrodes was 80 wt. % LMO (Toda, Hiroshima, Japan), 10 wt. % PVdF (Arkema), and 10 wt. % carbon black C65 (Imerys). The preparation procedure of aforementioned electrodes has been described in detail elsewhere [29]. Balanced NMC111 and graphite electrodes (both 12 mm diameter, Litarion, Kamenz, Germany) were used. The electrochemical measurements were performed in a three-electrode T-cell setup (Swagelok®, Solon, OH, USA) as well as in CR2032 coin cells (Hohsen Corp. Osaka, Japan). Depending on the measurement setup, lithium foil (Albemarle, Charlotte, NC, USA) was used for counter electrode (CE) and reference electrode (RE). Constant current constant potential (CCCP) measurements were performed at 20 ◦C using a battery cycler (MACCOR Series 4000, Tulsa, OK, USA). The graphite/Li cells were cycled in the potential range between 0.02 V–1.50 V vs. Li/Li+ starting with three formation cycles at charge/discharge rate of 37.2 mA·g−1 (corresponding to C/10, when considering a practical specific capacity of 372 mAh·g−1). After the formation sequence, cells were cycled at charge/discharge rate of 1C (372 mA·g−1). Each intercalation step (1C) was followed by a constant potential step with 1 h duration at a potential of 0.025 V vs. Li/Li+. The cathode capacity-limited NMC/graphite cells (15% oversized anode) were cycled with five formation cycles at 0.1C. After the formation, cells were cycled at 1C over 100 charge/discharge cycles in the voltage range from 3 V to 4.3 V. AC impedance measurements were conducted to determine the conductivity of the electrolyte. All measurements were carried out on a Solartron 1260A (AMETEK, Berwyn, PA, USA) impedance Energies 2017, 10, 1312 3 of 14 gain phase analyzer, connected to a Solartron 1287A (AMETEK, Berwyn, PA, USA) potentiostat using a customizedEnergies 2017, 10 cell, 1312 having two stainless steel disk-electrodes. A frequency range from 1 kHz3 of to14 1 MHz − ◦ usinga customized an AC amplitude cell having oftwo 20 stainless mV was steel applied disk-electrodes. to the cell A infrequency the temperature range from range1 kHz to from 1 MHz40 C to ◦ 60usingC, regulated an AC amplitude via a climate of 20 mV chamber was applied (Binder, to th Tuttlingen,e cell in the Germany).temperature Differences range from –40 in the°C to determined 60 conductivity°C, regulated values via a areclimate in-between chamber the(Binder, error Tuttlingen, margin of Germany). the system. Differences in the determined conductivityThe contact values angle are measurementsin-between the error were margin performed of the system. on a Drop Shape Analyzer DSA 100 (Krüss, Hamburg,The contact Germany) angle placed measurements in a dry were room performed (dew point: on a −Drop65 ◦ ShapeC). For Analyzer each measurement, DSA 100 (Krüss, a drop of electrolyteHamburg, was Germany) dispersed placed on ain surface a dry room of the (dew Celgard point: 2500 –65 °C). (Celgard, For each Charlotte, measurement, NC, USA) a drop separator. of electrolyteFor the was X-ray dispersed photoelectron on a surface spectroscopy of the Celgard (XPS) 2500 investigations, (Celgard, Charlotte, an AXIS NC, USA) Ultra separator. DLD (Shimadzu Corporation,For the Kyoto,X-ray photoelectron Japan) was spectroscopy used. An area (XPS) of investigations, 300 µm × 700 anµ AXISm was Ultra irradiated, DLD (Shimadzu while using a Corporation, Kyoto, Japan) was used. An area of 300 µm × 700 µm was irradiated, while using a filament voltage of 12 kV, an emission current of 10 mA and a pass energy of 20 eV. The obtained filament voltage of 12 kV, an emission current of 10 mA and a pass energy of 20 eV. The obtained spectraspectra were were calibrated calibrated against against the the carbon carbon signal signal at 284.5 at 284.5 eV. For eV. the For XPS the sputter XPS sputter depth profiling depth profiling measurements,measurements, a sputter a sputter crater crater diameter diameter of of 1.1 1.1 mm, mm an, an emission emission current current ofof 88 mAmA andand aafilament filament voltage ofvoltage 0.5 kV of as 0.5 well kV asas well a pass as a energy pass energy of 40 of eV 40 andeV and a 110 a 110µ mµm aperture aperture werewere applied. applied. The The fitting fitting of of the resultedthe resulted spectra spectra was was performed performed by by means means of of CasaXPS. CasaXPS.

3. Results3. Results and and Discussion Discussion AsAs shown shown inin FigureFigure 11a,a, EMS-basedEMS-based electrolyte electrolyte is isnot not capable capable of forming of forming an effective an effective SEI onSEI on graphite.graphite. The The decompositiondecomposition of of EMS EMS starts starts at ata apotential potential of 1.2 of 1.2V vs. V vs.Li/Li Li/Li+ (Figure+ (Figure 1a). For1a). the For the applicationapplication of of EMS-basedEMS-based electrolytes, electrolytes, the theuse useof SEI of-forming SEI-forming additives additives is inevitable. is inevitable. With this in With line, this in different concentrations of FEC were considered as SEI-forming additive for the application of EMS- line, different concentrations of FEC were considered as SEI-forming additive for the application based electrolytes in graphite/Li cells. The potential profiles of afore described electrolyte offormulations EMS-based are electrolytes depicted in in Figure graphite/Li 1. cells. The potential proﬁles of afore described electrolyte formulations are depicted in Figure1.

3.0 3.0 3.0

2.5 a) 2.5 b)2.5 c)

2.0 2.0 2.0 (V) (V) (V) + + +

1.5 1.5 1.5

1.0 1.0 1.0 Potential vs. Li/Li Potential vs. Li/Li Potential vs. Li/Li 0.5 0.5 0.5

0.0 0.0 0.0

0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 Specific capacity (mAh/g) Specific capacity (mAh/g) capacity [mAh/g]

3.0 3.0

2.5 d) 2.5 e)

2.0 2.0 (V) (V) + +

1.5 1.5

1.0 1.0 Potential vs. Li/Li Potential vs. Li/Li 0.5 0.5

0.0 0.0

0 100 200 300 400 500 0 100 200 300 400 500 Specific capacity (mAh/g) Specific capacity (mAh/g)

FigureFigure 1. 1. First cycle cycle potential potential profiles proﬁles of graphite/Li of graphite/Li cells containing cells containing 1 M LiPF6 in 1 EMS M LiPF with;6 in(a) EMS0% with; (a)FEC 0% ( FECb) 1% (b FEC,) 1% (c FEC) 2%,( FEC;c) 2% (d) FEC;3% FEC (d )and 3% (e FEC) 4% FEC, and in (e )the 4% potential FEC, in range the from potential 0.02 to range 1.5 V vs. from 0.02 toLi/Li 1.5 V+. vs. Li/Li+.

As seen in Figure 1b, the presence of 1% FEC does not prevent the decomposition of EMS, as As seen in Figure1b, the presence of 1% FEC does not prevent the decomposition of EMS, decomposition of the electrolyte on the electrode deteriorates the performance of the cell in terms of as decomposition of the electrolyte on the electrode deteriorates the performance of the cell in terms of low Coulombic efficiency and reversible capacity (Table 1). However, when 2% FEC was added to low Coulombic efficiency and reversible capacity (Table1). However, when 2% FEC was added to the 1 M LiPF6 in the EMS-based electrolyte (Figure 1c), decomposition of EMS is inhibited, at least in thethe 1 Mfirst LiPF cycle,6 in thus the EMS-basedenabling reversible electrolyte lithium (Figure intercalation1c), decomposition and deintercalation of EMS isinto/from inhibited, the at least ingraphite the first structure, cycle, thus as indicated enabling by reversible the formation lithium of the intercalation respective potential and deintercalation plateaus at potential into/from the graphitebelow 0.2 structure, V vs. Li/Li as+ indicated[33]. In addition, by the the formation irreversible of capacity the respective in the first potential cycle and plateaus the potential at potential + below 0.2 V vs. Li/Li [33]. In addition, the irreversible capacity in the first cycle and the potential Energies 2017, 10, 1312 4 of 14

differenceEnergies due 2017 to, 10, IR 1312 drop and overpotential (=“potential-drop”) are lower compared to the4 of values 14 obtained for 1% FEC-containing electrolyte (Figure1a), which results in higher energy efficiencies [ 34]. Whendifference increasing due the to amountIR drop ofand FEC overpotential to 3% or 4%,(=“potential-drop”) the Coulombic are efficiency lower compared increases, to the whereas values the potential-dropobtained decreases.for 1% FEC-containing Addition of electrolyte more than (Fig 4%ure of FEC1a), towhich the electrolyteresults in formationhigher energy has no efficiencies [34]. When increasing the amount of FEC to 3% or 4%, the Coulombic efficiency increases, further influence on the electrochemical performance of the graphite/Li cell. For this reason, higher whereas the potential-drop decreases. Addition of more than 4% of FEC to the electrolyte formation concentrationshas no further of FEC influence are not on shown. the electrochemical Table1 summarizes performance the Coulombic of the graphite/L efficiencyi cell. andFor this the reason, measured potentialhigher drop concentrations values of the of firstFEC cycleare not for shown. the aforementioned Table 1 summarizes electrolyte the Coulombic formulations. efficiency and the measured potential drop values of the first cycle for the aforementioned electrolyte formulations. Table 1. Coulombic efficiency values and measured potential drop between the charge and the dischargeTable curve 1. Coulombic of the first efficiency cycle of values the graphite/Li and measured cells potential depicted drop in Figure between1. the charge and the discharge curve of the first cycle of the graphite/Li cells depicted in Figure 1. Electrolyte Formulation Coulombic Efficiency (%) Potential Drop (V) Electrolyte Formulation Coulombic Efficiency (%) Potential Drop (V)

1 1M M LiPF LiPF6 in6 EMSin EMS 30.2 30.2 0.300 0.300 11 M M LiPF LiPF6 in6 in EMS EMS with with 1% 1%FEC FEC 53.7 53.7 0.170 0.170 11 M M LiPF LiPF6 in6 in EMS EMS with with 2% 2%FEC FEC 68.6 68.6 0.020 0.020 11 M M LiPF LiPF6 in6 in EMS EMS with with 3% 3%FEC FEC 70.8 70.8 0.010 0.010 11 M M LiPF LiPF6 in6 in EMS EMS with with 4% 4%FEC FEC 80.7 80.7 0.009 0.009

Pronounced decomposition of EMS on the graphite electrode in the first cycle is prevented, when Pronounced decomposition of EMS on the graphite electrode in the ﬁrst cycle is prevented, when more than 1% FEC is added to the electrolyte formulation. To further investigate the minimum more than 1% FEC is added to the electrolyte formulation. To further investigate the minimum amount of FEC required for achieving stable cycling behavior, long-term cycling measurements were amountperformed of FEC required(Figure 2). for achieving stable cycling behavior, long-term cycling measurements were performed (Figure2).

Figure 2. Cycling profiles of graphite/Li cells containing 1 M LiPF6 in EMS with (a) 1% FEC; (b) 2% Figure 2. Cycling profiles of graphite/Li cells containing 1 M LiPF6 in EMS with (a) 1% FEC; (b) 2% FEC; (c) 3% FEC and (d) 4% FEC, in the potential range from 0.02 to 1.5 V vs. Li/Li+. FEC; (c) 3% FEC and (d) 4% FEC, in the potential range from 0.02 to 1.5 V vs. Li/Li+. The cell containing 1% FEC (Figure 2a) does not reach the theoretical capacity value of graphite The(372 cell mAh·g containing−1) [35], due 1% to FEC continuous (Figure2 reductiona) does not of EMS reach on the graphite theoretical electrode capacity (compare value Figure of graphite 1b). (372 mAhIn the·g first−1)[ cycle,35], due the specific to continuous discharg reductione capacity reaches of EMS a onvalue graphite of 50 mAh·g electrode−1. Moreover, (compare the Figurecycling 1b). In thebehavior first cycle, over the 100 specific charge/discharge discharge capacitycycles is not reaches stable. a In value case of the 50 mAhelectrolyte·g−1. containing Moreover, 2% the FEC, cycling −1 behaviora specific over 100discharge charge/discharge capacity of 280 cycles mAh·g is not is reached stable. in In the case formation of the electrolyte cycles (Figure containing 2b). After 2% the FEC, a specific discharge capacity of 280 mAh·g−1 is reached in the formation cycles (Figure2b). After the Energies 2017, 10, 1312 5 of 14

Energies 2017, 10, 1312 5 of 14 formation step, the specific capacity decreases and the Coulombic efficiency values indicate neither a stable cyclingformation performance step, the specific nor capacity complete decreases prevention and the of theCoulombic EMS decomposition efficiency values after indicate the firstneither cycle. Therefore,a stable the cycling amount performance of FEC was nor increased complete to prevention 3% (Figure of2 thec). TheEMS cell decomposition cycled with after 3% the FEC-containing first cycle. electrolyteTherefore, shows the more amount stable of cycling FEC was behavior increased compared to 3% to(Figure the cells 2c). containingThe cell cycled 1% and with 2% 3% FEC. FEC- In the initial cycles,containing a specific electrolyte discharge shows more capacity stable of cycling 240 mAh behavior·g−1 is compared achieved. to Increasing the cells containing the amount 1% and of FEC 2% FEC. In the initial cycles, a specific discharge capacity of 240 mAh·g−1 is achieved. Increasing the in the electrolyte to 4%, decomposition of EMS-based electrolyte is even more suppressed, as depicted amount of FEC in the electrolyte to 4%, decomposition of EMS-based electrolyte is even more in Figure2d. In the first, formation cycles, a specific discharge capacity of 325 mAh ·g−1 is reached. suppressed, as depicted in Figure 2d. In the first, formation cycles, a specific discharge capacity of Over 100325 charge/dischargemAh·g−1 is reached. cycles,Over 100 a stablecharge/discharge cycling behavior cycles, a withoutstable cycling any fadingbehavior can without be observed. any The capacityfading retentioncan be observed. (discharge The capacitycapacity retention (cycle 100)/discharge (discharge capacity capacity (cycle (cycle 100)/discharge 7)) amounts capacity to 98.9%. The CCCP(cycle measurements 7)) amounts to showed 98.9%. The that CCCP addition measurements of 4% FEC showed to the 1 Mthat LiPF6 addition in EMS of 4% electrolyte FEC to the results 1 M in the bestLiPF6 long-term in EMS cycling electrolyte behavior results inin graphite/Lithe best long-term cells comparedcycling behavior to lower in graphite/Li amounts ofcells FEC. compared Increased amountto oflower FEC (>4%)amounts does ofnot FEC. improve Increased the overallamount electrochemical of FEC (>4%) performance,does not improve including the long-termoverall cycling.electrochemical Due to the high performance, viscosity including of EMS, long-term leading tocycling. a low Due conductivity, to the high viscosity an EMS-based of EMS, electrolyteleading wouldto never a low compete conductivity, with the an carbonate-basedEMS-based electrolyt electrolyte.e would Tonever overcome compete this with challenge the carbonate-based the EMS-based electrolyte. To overcome this challenge the EMS-based electrolyte had to be modified by adding a electrolyte had to be modified by adding a dilution agent to the formulation. Ethylene carbonate (EA) dilution agent to the formulation. Ethylene carbonate (EA) is a well-known co-solvent with excellent is a well-known co-solvent with excellent properties regarding its viscosity and conductivity. Having in properties regarding its viscosity and conductivity. Having in mind, that the performance of the mind, thatelectrolyte the performance is strongly of related the electrolyte to its conductivity. is strongly related Further to itsadvantages conductivity. of EA Further are its advantages good of EA areenvironmental its good environmental friendliness and friendliness the ability to and solvate the the ability lithium to solvate salt as well the as lithium the high salt wettability as well asof the high wettabilitythe separator. of theHence, separator. EA was Hence, added EAto the was high added viscose to theEMS-based high viscose electrolyte, EMS-based resulting electrolyte, in an resultingelectrolyte in an electrolyte with good conductivity, with good compatible conductivity, with compatible the standard with carbonate-based the standard electrolyte carbonate-based (Figure electrolyte3). Decreasing (Figure3). the Decreasing amount of the EA amount leads to of a EAlowe leadsr conductivity to a lower of conductivity the electrolyte of theformulation. electrolyte formulation.Increasing Increasing the amount the amountof EA would of EA enhance would the enhance flammability. the flammability. Therefore, it Therefore, desirable to it desirablekeep the to keep theamount amount of EMS of EMS as high as highas possible. as possible. Nevertheless, EMS-based electrolytes are known for much lower ionic conductivity, originating Nevertheless, EMS-based electrolytes are known for much lower ionic conductivity, originating from their high viscosity, compared to the state-of-the-art organic carbonate-based electrolytes. from their high viscosity, compared to the state-of-the-art organic carbonate-based electrolytes. Therefore, ethylene acetate (EA) as low viscosity co-solvent, known for enabling high ionic Therefore, ethylene acetate (EA) as low viscosity co-solvent, known for enabling high ionic conductivities in the resultant electrolyte, was added to the 1 M LiPF6 in EMS electrolyte formulation. conductivitiesThe obtained in the conductivity resultant electrolyte, values of sele wascted added electrolytes to the are 1 Mshown LiPF in6 in Figure EMS 3. electrolyte formulation. The obtained conductivity values of selected electrolytes are shown in Figure3.

1M LiPF6 in EA+ 4% FEC

1M LiPF6 in EMS/EA (1:1) + 4% FEC 1M LiPF in EC/DEC (3:7) 10 6 1M LiPF6 in EMS + 4% FEC ) -1 8

2 Conducitvity (mS cm Conducitvity 0

-2 -40-200 204060 Temperature (°C)

FigureFigure 3. Conductivity 3. Conductivity measurements measurements of 1of M 1 M LiPF LiPF6 in6 in EMS EMS + + 4%4% FEC,FEC, 1 1 M M LiPF LiPF6 6inin EA EA + 4% + 4% FEC FEC as as well aswell 1 M as LiPF 1 M6 LiPFin EMS:EA6 in EMS:EA (1:1) (1:1) + 4% + 4% FEC FEC and and 1 M1 M LiPF LiPF6 6in in EC/DECEC/DEC (3:7), (3:7), in in the the temperature temperature range range from −from40 ◦ C−40 to °C 60 to◦C. 60 °C.

The determined conductivities of 1 M LiPF6 in EMS + 4% FEC are much lower compared to ◦ ◦ the values obtained for 1 M LiPF6 in EC/DEC (3:7) in the temperature range from −40 C to 60 C. Energies 2017, 10, 1312 6 of 14

Because of the limited liquid range, pure EMS-based electrolytes are not conductive in the temperature range fromEnergies−20 2017◦C, 10 to, 1312 0 ◦C. At 20 ◦C, the conductivity amounts to 1.73 mS·cm−1 and increases6 of 14 further to 4.81 mS·cm−1 at 60 ◦C. In contrast to this, the conductivity of the carbonate-based electrolyte The determined conductivities of 1 M LiPF6 in EMS + 4% FEC are much lower compared to the −1 ◦ −1 ◦ −1 reaches avalues value obtained of 3.98 for mS 1 M·cm LiPF6 atin EC/DEC 0 C, increasing (3:7) in the temperature to 5.58 mS ·rangecm fromat 20 −40 C°C andto 60 9.85°C. Because mS·cm at ◦ 60 C. Comparedof the limited to this,liquid a range, high pure conductivity EMS-based values electrolytes can beare achieved,not conductive in 1 in M the LiPF temperature6 in EMS range electrolyte containingfrom 4% −20 FEC °C asto 0 additive. °C. At 20 °C, At the−20 conductivity◦C, a conductivity amounts to value 1.73 mS of 5.65cm−1 mSand· increasescm−1 is reached,further to increasing4.81 −1 to 8.08 mSmS·cm·cm−1 atat 60 0 ◦°C.C. In At contrast 20 ◦C, to the this, conductivity the conductivi ofty the ofelectrolyte the carbonate-based reaches aelectrolyte value of reaches 9.58 mS a ·cm−1. value of 3.98 mS cm−1 at 0 °C, increasing to 5.58 mS cm−1 at 20 °C and 9.85 mS cm−1 at 60 °C. Compared Further increase in temperature results in a conductivity of 11.21 mS·cm−1 at 60 ◦C. After addition of to this, a high conductivity values can be achieved, in 1 M LiPF6 in EMS electrolyte containing 4% EA to theFEC EMS-based as additive. electrolyte At −20 °C, a in conductivity a 1:1 (EMS/EA) value of ratio, 5.65 mS the cm conductivity−1 is reached, increasing of the 1 M to LiPF8.08 mS6 in cm EMS/EA−1 −1 (1:1) + 4%at FEC0 °C. electrolyteAt 20 °C, the is conductivity comparable of tothe the electrolyte value of reaches the carbonate-based a value of 9.58 mS electrolyte cm−1. Further (5.23 increase mS· cm ). The additionin temperature of EA to theresults EMS-based in a conductivity electrolyte of 11.21 increases mS cm−1 not at 60 only °C. theAfter conductivity, addition of EA but to the also EMS- influences the wettabilitybased electrolyte of the electrolyte in a 1:1 (EMS/EA) formulation, ratio, the conductivity enabling its of applicabilitythe 1 M LiPF6 in on EMS/EA different (1:1) + separators. 4% FEC In −1 the following,electrolyte the is values comparable obtained to the fromvalue of the the contact carbonate-based angle measurement electrolyte (5.23 are mS depicted. cm ). The addition In the related of EA to the EMS-based electrolyte increases not only the conductivity, but also influences the measurement,wettability a very of the low electrolyte contact formulation, angle (Θ) indicatesenabling its a applicability high wettability. on different For theseparators. 1 M LiPF In the6 in EMS electrolyte,following, a contact the anglevalues of obtained (103 ± from3), was the determinedcontact angle as measurement an average are value depicted. from fourIn the measurements related performedmeasurement, on a Celgard a very 2500 low separator. contact angle Compared (Θ) indicates to a this high very wettability. high value, For the the 1 M contact LiPF6 in angle EMS of the commerciallyelectrolyte, used a contact 1 M LiPF angle6 in of EC:DEC(103 ± 3), was (3:7) determ amountsined as to an (57 average± 5), value thus from ensuring four measurements better wettability. Additionperformed of EA to theon a EMSCelgard electrolyte 2500 separator. (1:1 ratio), Compared results to inthis the very contact high value, angle the value contact of (60angle± of3), the which is commercially used 1 M LiPF6 in EC:DEC (3:7) amounts to (57 ± 5), thus ensuring better wettability. similar to the one obtained for the reference electrolyte. Addition of EA to the EMS electrolyte (1:1 ratio), results in the contact angle value of (60 ± 3), which Withis both,similar conductivity to the one obtained as well for the as contactreferenceangle electrolyte. measurements it is shown, that the addition of EA to the EMS-basedWith both, electrolyte conductivity has as awell significant as contact influence angle measurements on improved it is shown, performance. that the addition In the following, of the EA/EMS-basedEA to the EMS-based electrolytes electrolyte were characterized has a significan byt meansinfluence of on selected improved electrochemical performance. In methods the on both anodefollowing, and cathode the EA/EMS-based materials, aselectrolytes well as in were NMC/graphite characterized by cells. means To investigateof selected electrochemical the electrochemical methods on both anode and cathode materials, as well as in NMC/graphite cells. To investigate the stability of 1 M LiPF6 in EMS:EA (1:1) with 4% FEC electrolyte and in particular the influence of EA on electrochemical stability of 1 M LiPF6 in EMS:EA (1:1) with 4% FEC electrolyte and in particular the the stability against graphite electrodes, a potential vs. specific capacity diagram of the first cycle of a influence of EA on the stability against graphite electrodes, a potential vs. specific capacity diagram graphite/Liof the cells first iscycle shown of a graphite/Li in Figure 4cells. is shown in Figure 4.

Figure 4. First cycle potential proﬁle of graphite/Li cell containing 1 M LiPF6 in EMS:EA (1:1) + 4% Figure 4. First cycle potential profile of graphite/Li cell containing 1 M LiPF6 in EMS:EA (1:1) + 4% + FEC electrolyte,FEC electrolyte, in the potentialin the potential range range from from 0.02 0.02 to to 1.5 1.5 V V vs.vs. Li/Li +. .

+ The firstThe electrolyte first electrolyte decomposition decomposition starts starts at at approximately approximately 1.5 1.5 V vs. V vs.Li/Li Li/Li, which+, whichis attributed is attributed to a reaction of the SEI-forming additive FEC. While decreasing the potential to 0.02 V, the typical to a reaction of the SEI-forming additive FEC. While decreasing the potential to 0.02 V, the typical intercalation plateaus of lithium ions into graphite [36,37] are discernible. A small potential-drop intercalation(0.003 plateausV) indicates of lithiuma low-restive ions SEI, into similar graphite to the [36 SEI,37 ]formed are discernible. with 1 M LiPF A small6 in EMS-based potential-drop (0.003 V)electrolyte indicates containing a low-restive 4% FEC. SEI, By similar increasing to the the SEI potential formed to with 1.5 V 1 vs. M Li/Li LiPF+,6 thein EMS-baseddeintercalation electrolyte of containing 4% FEC. By increasing the potential to 1.5 V vs. Li/Li+, the deintercalation of lithium ions takes place. The Coulombic efficiency amounts to 76.9%. The addition of FEC to the EMS/EA solvent mixture is still required, as, similarly to EMS, EA is not able to form an effective SEI on graphite during constant current cycling. To prove this assumption, XPS sputter depth profiling measurements Energies 2017, 10, 1312 7 of 14

Energies 2017, 10, 1312 7 of 14 were carried out on graphite electrodes after constant current cycling in graphite/Li cells containing investigatedlithium ions electrolyte takes place. formulations. The Coulombic efficiency amounts to 76.9%. The addition of FEC to the EMS/EA solvent mixture is still required, as, similarly to EMS, EA is not able to form an effective SEI Figure5a depicts the XPS S 2p core spectrum of a graphite electrode cycled with the 1 M LiPF 6 in EMSon graphite electrolyte. during The constant signal atcurrent 167.59 cycling. eV is To attributed prove this to assumption, EMS, which XPS is decomposedsputter depth profiling on the graphite measurements were carried out on graphite electrodes after constant current cycling in graphite/Li surface. During sputtering, this signal decreased slightly indicating a very thick decomposition layer cells containing investigated electrolyte formulations. on the electrode. Compared to the aforementioned XPS S 2p core spectra, the core spectra of graphite Figure 5a depicts the XPS S 2p core spectrum of a graphite electrode cycled with the 1 M LiPF6 cycledin EMS with electrolyte. 1 M LiPF The6 in signal EMS:EA at 167.59 (1:1) eV electrolyte is attributed (Figure to EMS,5b) which look is very decomposed similar, thuson the showing graphite that EA is notsurface. able During to protect sputtering, EMS againstthis signal decomposition. decreased slightly In indicating both cases, a very the sulfurthick decomposition signal cannot layer stem from impuritieson the electrode. present Compared on the pristine to the aforementioned graphite surface, XPS since S 2p core the referencespectra, the measurement core spectra of ongraphite a non-cycled graphitecycled with electrode 1 M LiPF does6 in not EMS:EA show (1:1) any electrolyte sulfur signal (Figure (Figure 5b) look5e). very When similar, adding thus 1% showing FEC to that the EA EMS/EA electrolyte,is not able the to protect sulfur signalEMS against in the decomposition. XPS S 2p core spectrumIn both cases, decreases the sulfur (Figure signal5c) cannot compared stem tofrom the signal obtainedimpurities without present FEC. on the Increasing pristine graphite the amount surfac ofe, since FEC tothe 4% reference (Figure measurement5d) shows nearly on a non-cycled no sulfur signal ingraphite the XPS electrode S 2p core does spectrum not show of any the sulfur cycled signal graphite (Figure electrode 5e). When anymore. adding 1% A FEC quantitative to the EMS/EA analysis of electrolyte, the sulfur signal in the XPS S 2p core spectrum decreases (Figure 5c) compared to the the XPS data shows a strong decrease of atomic sulfur concentration within the formed SEI. Figure6 signal obtained without FEC. Increasing the amount of FEC to 4% (Figure 5d) shows nearly no sulfur depicts relative composition of the organic and inorganic part of the SEI, as well as the sulfur content. signal in the XPS S 2p core spectrum of the cycled graphite electrode anymore. A quantitative analysis Whenof the using XPS data 1 M shows LiPF a6 strongin EMS decrease as electrolyte, of atomic sulfur sulfur concentration content amounts within the to 12%formed due SEI. to Figure the massive decomposition6 depicts relative of thecomposition EMS solvent. of the Decreasing organic and the inorganic amount part of EMSof the in SEI, the as electrolyte well as the formulation sulfur to 50%,content. by adding When using EA, lowers 1 M LiPF the6 amountin EMS as of electrolyte, sulfur, from sulfur 12% content relative amounts composition to 12% todue 6%. to Duethe to the ongoingmassive decomposition decomposition of EMSthe EMS and solvent. EA, relative Decreasing composition the amount of inorganic of EMS in as the well electrolyte as organic part rises.formulation Overall, to EA 50%, does by notadding inhibit EA, decompositionlowers the amount of EMSof sulfur, onthe from graphite 12% relative surface. composition By adding to FEC to the6%. electrolyte Due to the formulation, ongoing decomposition decomposition of EMS of and EMS EA, and relative with composition this the relative of inorganic composition as well as of sulfur inorganic the SEI part decreases rises. Overall, from 6% EA to does 2%, not thus inhibit indicating decomposition formation of ofEMS a passivation on the graphite layer. surface. The proportionBy adding FEC to the electrolyte formulation, decomposition of EMS and with this the relative regarding the inorganic and organic part is not as expected. The decomposition of FEC on the graphite composition of sulfur in the SEI decreases from 6% to 2%, thus indicating formation of a passivation surface, results in formation of hydroﬂuoric acid (HF) and vinylene carbonate (VC). HF reacts on the layer. The proportion regarding the inorganic and organic part is not as expected. The decomposition graphiteof FEC on surface the graphite to lithium surface, ﬂuoride results (LiF) in formation whereas of VC hydrofluoric polymerizes acid on (HF) the and electrode vinylene surface. carbonate We would expect(VC). anHFincrease reacts on of the the graphite relative surface composition to lithium of fluoride the inorganic (LiF) whereas part and VC apolymerizes decrease ofon the the relative compositionelectrode surface. of the We organic would part. expect The an increase obtained of resultsthe relative for thecomposition electrolyte of the formulation inorganic part containing and 1% FECa decrease may be of correlated the relative to composition the mean errorof the deviation organic part. of theThe system.obtained Increasedresults for amountthe electrolyte of FEC (4%) reducesformulation the atomic containing sulfur 1% concentration FEC may be withincorrelated in theto the SEI mean to 1%. error Nearly deviation no EMS of the decomposition system. is observedIncreased on amount the electrode of FEC (4%) surface reduces indicating the atomic formation sulfur concentration of an effective within SEI. in Whenthe SEI usingto 1%. Nearly 4% FEC in the electrolyteno EMS decomposition formulation, is the observed inorganic on the part electrode of the surface SEI increases, indicating whereas formation the of organic an effective part SEI. of the SEI When using 4% FEC in the electrolyte formulation, the inorganic part of the SEI increases, whereas decreases, compared to the results obtained for the cell without FEC. This can be attributed to the the organic part of the SEI decreases, compared to the results obtained for the cell without FEC. This previously described decomposition of FEC. The obtained XPS results are in good agreement with can be attributed to the previously described decomposition of FEC. The obtained XPS results are in electrochemicalgood agreement measurements with electrochemical (Figures measurements1 and2). (Figures 1 and 2).

S 2p a) S 2p b) S 2p c)S 2p d) S 2p e)

0 s 0 s 0 s 0 s 0 s

60 s 60 s 60 s 60 s 60 s

120 s 120 s 120 s 120 s 120 s

600 s 600 s 600 s 600 s 600 s

172 168 164 160 171 168 165 162 171 168 165 162 171 168 165 162 171 168 165 162 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) Figure 5. XPS S 2p core spectra of graphite electrodes, after five charge/discharge cycles at 0.1C in Figure 5. XPS S 2p core spectra of graphite electrodes, after ﬁve charge/discharge cycles at 0.1C in graphite/Li cells with different amounts of FEC in the electrolyte (a) 1 M LiPF6 in EMS; (b) 1 M LiPF6 graphite/Li cells with different amounts of FEC in the electrolyte (a) 1 M LiPF6 in EMS; (b) 1 M LiPF6 in EMS:EA (1:1); (c) 1 M LiPF6 in EMS:EA (1:1) with 1% FEC and (d) 1 M LiPF6 in EMS:EA (1:1) with in EMS:EA (1:1); (c) 1 M LiPF in EMS:EA (1:1) with 1% FEC and (d) 1 M LiPF in EMS:EA (1:1) with 4% FEC; (e) pristine electrode. 6The electrodes where sputtered for 60 s, 120 s and 6006 s, respectively. 4% FEC; (e) pristine electrode. The electrodes where sputtered for 60 s, 120 s and 600 s, respectively.

EnergiesEnergies 2017 2017, 10, 101312, 1312 8 of8 14 of 14 Energies 2017, 10, 1312 8 of 14

140140 Sulfur Sulfur 120120 Inorganic Inorganic Organic Organic 100 1 100 6 6 2 2 1 12 12 80 80

60 60 83 83 85 85 90 90 82 82 40 40 Relative Composition / % / Composition Relative 20 % / Composition Relative 20 15 6 10 10 15 9 9 0 0 6 1 M LiPF 1 M LiPF6 1 M LiPF6 1 M LiPF6 1 M LiPF6 1 M LiPF6 1 M LiPF6 1 M LiPF6 6 in in in in in in in in EMS:EA+ 4% FEC EMSEMS EMS:EAEMS:EA EMS:EAEMS:EA + 1% + FEC1% FEC EMS:EA+ 4% FEC FigureFigureFigure 6. 6. Relative 6.Relative Relative composition composition composition of of sulfurof sulfur sulfur content, content, content, organic organic organic and and and inorganic inorganic inorganic part part part of of theof the the SEI. SEI. SEI.

Having this in mind, as well as the results obtained from the CCCP measurement on graphite HavingHaving this this in in mind, mind, as as well well as as the the results results obtained obtained from from the the CCCP CCCP measurement measurement on on graphite graphite with EMS and different FEC amounts (Figure 2) in the following measurements 4% FEC were added withwith EMS EMS and and different different FEC FEC amounts amounts (Figure (Figure2) in 2) the in the following following measurements measurements 4% FEC4% FEC were were added added to to the investigated electrolyte formulation. In order to get a deeper insight on the influence of EA on theto investigatedthe investigated electrolyte electrolyte formulation. formulation. In order In order to get to a get deeper a deeper insight insight on the on inﬂuence the influence of EA of on EA the on the cycling performance, long-term cycling behavior was investigated in graphite/Li cells (Figure 7). cyclingthe cycling performance, performance, long-term long-term cycling cycling behavior behavior was investigated was investigated in graphite/Li in graphite/Li cells (Figurecells (Figure7). 7).

500500 105105

450450 100100 400400 95 350350 95

300300 90 90 250250 85 85 200200 150 150 80 80 Coulombic efficiency(%) Specific capacity (mAh/g) 100 Coulombic efficiency(%) Specific capacity (mAh/g) 100 Discharge capacity Discharge capacity 75 Charge capacity 75 50 50 Charge capacity Efficiency Efficiency 0 0 70 70 00 102030405060708090100 102030405060708090100 Cycle number Cycle number Figure 7. Cycling proﬁle of graphite/Li cell containing 1 M LiPF in EMS:EA (1:1) + 4% FEC electrolyte, FigureFigure 7. Cycling 7. Cycling profile profile of graphite of graphite/Li /Licell cell containing containing 1 M 1 LiPFM LiPF6 in6 6 EMS:EA in EMS:EA (1:1) (1:1) + 4% + 4% FEC FEC electrolyte, electrolyte, + inin thein the the potential potential potential range range range from from from 0.02 0.02 0.02 to to 1.5to 1.5 1.5V V vs. V vs. vs.Li/Li Li/Li Li/Li+. +..

TheTheThe CCCP CCCP CCCP measurement measurement measurement of of ofgraphite/Li graphite/Li graphite/Li cells cells cells indica indicates indicatestes a astable a stable stable cycling cycling cycling performance, performance, performance, excluding excluding excluding capacitycapacitycapacity fading fading fading overover over 100100 100 charge/discharge charge charge/discharge/discharge cycles. cycles. cycles. In theIn Inthe formation the formation formation cycles, cycles, acycles, specific a specifica dischargespecific discharge discharge capacity capacityofcapacity 344 mAh of of344·g −344 1mAhis mAh achieved. g− 1 gis−1 achieved.is After achieved. the After formation After the the formation cycles, formation the cycles, specific cycles, the capacity the specific specific increases capacity capacity to 353increases increases mAh· gto− 1to −1 −1 353in353 themAh mAh 5th g cycle. gin thein Thisthe 5th 5th valuecycle. cycle.is This comparable This value value is comparableis to comparable the one received to theto the one in one thereceived received CCCP in measurement thein the CCCP CCCP measurement ofmeasurement 1 M LiPF 6 ofin of1 EMS M1 MLiPF without LiPF6 in6 inEMS EA EMS as without co-solvent. without EA EA Overas asco-solvent. 100co-solvent. charge/discharge Over Over 100 100 charge/discharge charge/discharge cycles, the Coulombic cycles, cycles, efficiencythe the Coulombic Coulombic values efficiencyindicateefficiency avalues stable values indicate cycling indicate performance.a stable a stable cycling cycling To perfor show perfor themance.mance. anodic To To stabilityshow show the ofthe anodic EMS-based anodic stability stability electrolytes, of ofEMS-based EMS-based linear electrolytes,sweepelectrolytes, voltammetry linear linear sweep sweep (LSV) voltammetry measurementsvoltammetry (LSV) (LSV) on meas LMO measurements areurements depicted on onLMO in LMO Figure are are depicted8. depicted in Figurein Figure 8. 8.

Energies 2017, 10, 1312 9 of 14

0.4

1M LiPF6 in EMS

1M LiPF6 in EMS + 4% FEC

) 1M LiPF in EMS:EA (1:1) + 4% FEC 2 0.3 6

0.2

0.1 Current density (mA/cm Current density

0.0

3.50 3.75 4.00 4.25 4.50 4.75 5.00 Potential vs. Li/Li+ (V)

Figure 8. Electrochemical stability window of 1 M LiPF6 in EMS, 1 M LiPF6 in EMS + 4% FEC and 1 M LiPFFigure6 in 8. EMS:EA Electrochemical + 4% FEC, stability respectively window on LMO of 1 M (WE) LiPF using6 in EMS, lithium 1 M as LiPF both6 in CE EMS and + RE, 4% at FEC a scan and rate 1 M −1 ofLiPF 1006 µinV ·EMS:EAs . + 4% FEC, respectively on LMO (WE) using lithium as both CE and RE, at a scan rate of 100 µV s−1.

The electrochemical stability window show, that all three investigated electrolytes (1 M LiPF6 The electrochemical stability window show, that all three investigated electrolytes (1 M LiPF6 in in EMS, 1 M LiPF6 in EMS + 4% FEC and 1 M LiPF6 in EMS:EA + 4% FEC) are stable on LMO upEMS, to a1 potentialM LiPF6 in of EMS 5 V vs.+ 4% Li/Li FEC +and. The 1 M peak LiPF maxima6 in EMS:EA of the + 4% deintercalation FEC) are stable peaks on LMO of LMO up areto a + positionedpotential of at 5 4.05 V vs. V Li/Li vs. Li/Li. The+ peakand 4.16maxima V vs. of Li/Lithe deintercalation+, a further proof peaks that of theLMO addition are positioned of EA as at + + co-solvent4.05 V vs. toLi/Li the EMS-basedand 4.16 V electrolytevs. Li/Li , hasa further no negative proof that effect the on addition the electrochemical of EA as co-solvent stability ofto the the electrolyte.EMS-based As electrolyte known from has the no literature,negative aluminumeffect on th dissolutione electrochemical depends stability on various of the factors, electrolyte. including As aluminumknown from surface the literature, structure, aluminum electrochemical dissolution operation depends conditions on various as well factor as thes, including nature and aluminum type of thesurface electrolyte. structure, EMS-based electrochemical electrolytes operation have been cond intensivelyitions as studied well as in respectthe nature to aluminum and type current of the collectorelectrolyte. dissolution EMS-based by Meisterelectrolytes et al. have [38]. Inbeen this in contributiontensively studied constant in currentrespect measurementsto aluminum current using aluminumcollector dissolution current collector by Meister as WE, et al. were [38]. conducted In this contribution and the electrode constant polarizedcurrent measurements to 5 V vs. Li/Li using+ + foraluminum 24 h. Thereafter, current collector SEM studies as WE, were were performed conducted to and quantify the electrode aluminum polarized dissolution. to 5 V Thevs. Li/Li authors for 24 h. Thereafter, SEM studies were performed to quantify aluminum dissolution. The authors pointed pointed out that an electrolyte based on 1 M LiPF6 in EMS shows no pit formation (areas where aluminumout that an dissolution electrolyte took based place) on on1 M the LiPF aluminum6 in EMS current shows collector no pit formation whereas 1 (areas M LITFSI where in EMSaluminum leads todissolution extensive pittook formation. place) on Due the toaluminum the addition current of LITFSI, collector no stablewhereas passivation 1 M LITFSI layer in can EMS be formedleads to onextensive the aluminum pit formation. foil, triggered Due to the by theaddition high dielectricof LITFSI, constantno stable of passivation EMS. The layer absence can of be aluminum formed on dissolutionthe aluminum spots foil, provides triggered an indicationby the high of thedielectric formation constant of a sufficientlyof EMS. The stable absence passivation of aluminum layer, dissolution spots provides an indication of the formation of a sufficiently stable passivation layer, formed by the conducting salt LiPF6, which inhibits the anodic dissolution of aluminum. formedWith by this the inconducting line, electrochemical salt LiPF6, which performance inhibits the in aanodic cell setup, dissolution where of reactions aluminum. at the anode With this in line, electrochemical performance in a cell setup, where reactions at the anode and and cathode influence each other [39,40], 1 M LiPF6 in EMS:EA (1:1) with 4% FEC electrolyte, was investigated.cathode influence Decreasing each theother amount [39,40], of EA 1 leadsM LiPF to6 a in lower EMS:EA conductivity (1:1) with of the 4% electrolyte FEC electrolyte, formulation. was Increasinginvestigated. the amountDecreasing of EA the would amount enhance of EA the flammability.leads to a lower The cycling conductivity profile ofof NMC/graphitethe electrolyte cellformulation. containing Increasing the aforementioned the amount electrolyte of EA would composition enhance is the shown flammability. in Figure9 .The cycling profile of NMC/graphite cell containing the aforementioned electrolyte composition is shown in Figure 9.

Energies 2017, 10, 1312 10 of 14 EnergiesEnergies 20172017, 10, 10, 1312, 1312 1010 of of 14 14

225 105

225 105 200 100 200 175 100 95 175 150 95 150 125 90

125 90 100 85

100 75 85 80

75 Coulombic eff iciency(% ) Specific capacity (mAh/g) capacity Specific 50 80 Cha rge capaci ty Coulombic eff iciency(% ) Specific capacity (mAh/g) capacity Specific 50 75 25 Disch arge capa ci ty Efficiency Cha rge capaci ty 75 25 Disch arge capa ci ty 0 Efficiency 70 0 102030405060708090100 0 70 0 102030405060708090100Cycle number Cycle number Figure 9. Cycling profile of NMC/graphite cell containing 1 M LiPF6 in EMS:EA (1:1) + 4% FEC, in the

voltageFigureFigure range9.9. CyclingCycling from profile 3.0 profile and of of4.3 NMC/graphite NMC/graphite V. cell cell containing containing 1 1M M LiPF LiPF6 in6 in EMS:EA EMS:EA (1:1) (1:1) + +4% 4% FEC, FEC, in in the the voltagevoltage range range from from 3.0 3.0 and and 4.3 4.3 V. V. Formation of an effective SEI on graphite in the above mentioned cell set-up, as achieved by the first fiveFormationFormation cycles were of of an an performed effective effective SEI SEIat onaon rate graphite graphite of 0.1C in in andthe the aboveresulted above mentioned mentioned in a specific cell cell set-up,capacity set-up, as asof achieved achieved173 mAh by by g the−1 the. Thefirstfirst Coulombic five five cycles cycles efficiencywere were performed performed reaches at ata avalue arate rate ofof of 0.1C99.3%. 0.1C and and After resulted resulted the formation in in a a specific specific cycles, capacity capacity the cellof of 173 173was mAh mAh cycled ·gg−−1 . 1. withTheThe 1C,Coulombic Coulombic resulting efficiency efficiency in a stable reaches long-term aa valuevalue cycling. ofof 99.3%. 99.3%. The Aftercapacity After the the retention formation formation amounts cycles, cycles, the to the cell99.7% cell was inwas cycled the cycled 20th with cyclewith1C, to resulting1C, 99.3% resulting in in the a stablein 60th a stable long-termcycle long-term and cycling.to 98.9% cycling. The in the capacity The 100th capacity retentioncycle. retention amounts amounts to 99.7% to 99.7% in the 20thin the cycle 20th to cycle99.3%A to further in99.3% the proof 60thin the cycle for 60th comparable and cycle to and 98.9% electrochemicalto in98.9% the 100thin the cycle. 100th performance cycle. of the EMS/EA-based electrolyte to the carbonate-basedAA further further proof proof electrolytefor for comparable comparable is provided electrochemical electrochemical by the charge/discharge pe performancerformance of of therate the EMS/EA-based EMS/EA-basedmeasurement, electrolyteas electrolyte depicted to to inthe theFigure carbonate-based carbonate-based 10. After the electrolyte electrolytecharge/discharge is is provided provided test byup by theto the 20C,charge/discharge charge/discharge cells were cycled rate rate measurement,with measurement, 1C for 30 asadditional as depicted depicted chargeinin Figure Figure discharge 10. 10 .After After cycles, the the charge/dischargeas charge/discharge shown in Figure test 10, test up depicting up to to 20C, 20C, stablecells cells were cycling were cycled cycled performance with with 1C 1C forand for 30 comparable 30 additional additional specificchargecharge dischargecapacity discharge values. cycles, cycles, as as shown shown in in Figure Figure 10, 10 depicting, depicting stable stable cycling cycling performance performance and and comparable comparable specificspecific capacity capacity values. values.

100 100 200 200 98 98 100 0.1C 100 175 0.2C 175 0.2C 200 0.1C a)96 200 b) 96 1C 98 98 1C 0.1C1C 1C 1C 150175 0.2C1C 94 150175 0.2C 94 0.1C a)96 b) 96 1C 2C 125 1C 92 125 1C 1C 1C 92 150 1C 2C 94 150 94 90 90 100 100 2C 125 2C 92 125 92 88 88 90 90 75100 75100 86 86 88 88 Coulombic efficiency (%) efficiency Coulombic (%) efficiency Coulombic Specific capacity (mAh/g) capacity Specific 5075 (mAh/g) capacity Specific 5075 5C 5C 84 84 Charge capacity 86 Charge capacity 86 Discharge capacity 25 (%) efficiency Coulombic 25 (%) efficiency Coulombic

Specific capacity (mAh/g) capacity Specific (mAh/g) capacity Specific Discharge capacity 50 10C 82 50 5C10C 82 5C Efficiency Efficiency 15C 20C 84 84 Charge capacity 15C 20C Charge capacity 0 Discharge capacity 80 0 80 25 10C 82 25 10C Discharge capacity 82 0 1020304050607080 Efficiency 0 1020304050607080 Efficiency 15C 20C 15C 20C 0 Cycle number 80 0 Cycle number 80 0 1020304050607080 0 1020304050607080 Cycle number Cycle number Figure 10. Cycling proﬁles including charge/discharge rate test of NCM/graphite cells containing Figure 10. Cycling profiles including charge/discharge rate test of NCM/graphite cells containing (a) (a) 1 M LiPF6 in EMS:EA (1:1) with 4% FEC and (b) 1 M LiPF6 in EC:DEC (3:7) electrolytes in the 1 M LiPF6 in EMS:EA (1:1) with 4% FEC and (b) 1 M LiPF6 in EC:DEC (3:7) electrolytes in the voltage Figurevoltage 10. range Cycling between profiles 3.0 including and 4.3 V charge/discharge vs. Li/Li+. rate test of NCM/graphite cells containing (a) + range1 M LiPF between6 in EMS:EA 3.0 and (1:1)4.3 V with vs. Li/Li 4% FEC. and (b) 1 M LiPF6 in EC:DEC (3:7) electrolytes in the voltage range between 3.0 and 4.3 V vs. Li/Li+. TheThe concept concept of ofusing using EA EA as asco-solvent co-solvent and and FEC FEC as asSEI-forming SEI-forming additive additive was was extended extended to toother other sulfones, namely isopropyl methyl sulfone (MeiPrSO ) and ethyl isopropyl sulfone (EtiPrSO ). For this sulfones,The namelyconcept isopropylof using EA methyl as co-solvent sulfone (MeiPrSOand FEC 2as2) andSEI-forming ethyl isopropyl additive sulfone was extended (EtiPrSO2 to2). other For reason, constant current constant voltage (CCCV) measurements of NMC/graphite cells with the thissulfones, reason, namely constant isopropyl current constantmethyl sulfone voltage (MeiPrSO (CCCV) measurements2) and ethyl isopropyl of NMC/ sulfonegraphite (EtiPrSO cells with2). theFor aforementionedthisaforementioned reason, constant sulfones sulfones current are are constantdiscussed discussed voltage in inthe the (follCCCV) following.owing. measurements Similar Similar to tothe of the cyclingNMC/ cyclinggraphite performance performance cells with of of1 theM 1 M LiPFaforementionedLiPF6 in6 inEMS:EA EMS:EA (1:1)sulfones (1:1) with with are 4% 4%discussed FEC FEC electrolyte electrolyte in the onfoll on NMCowing. NMC also alsoSimilar the the CCCV CCCVto the measurement cycling measurement performance of of 1 M 1 M LiPF LiPFof 16 in6M in EtiPrSO :EA (1:1) with 4% FEC on graphite exhibits a good cycling stability, as shown in Figure 11. EtiPrSOLiPF6 in2 :EAEMS:EA2 (1:1) with(1:1) with4% FEC 4% onFEC graphite electrolyte exhibits on NMC a good also cycling the CCCV stability, measurement as shown ofin 1Figure M LiPF 11.6 in EtiPrSO2:EA (1:1) with 4% FEC on graphite exhibits a good cycling stability, as shown in Figure 11.

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225 105 225 105

200 200 a) 100 b) 100 175 175 95 95 150 150

125 90 125 90

100 85 100 85

75 75 80 80 Coulombic efficiency (%) Coulombic efficiency (%) Specific capacity(mAh/g) Specific capacity (mAh/g) 50 50 Charge capacity Charge capacity 75 75 25 Discharge capacity 25 Discharge capacity Efficiency Efficiency

0 70 0 70 0 102030405060708090100 0 102030405060708090100 Cycle number Cycle number

Figure 11. Cycling proﬁles of NMC/graphite cells containing (a) 1 M LiPF6 in EtiPrSO2:EA (1:1) + 4% Figure 11. Cycling profiles of NMC/graphite cells containing (a) 1 M LiPF6 in EtiPrSO2:EA (1:1) + 4% FEC and (b) 1 M LiPF6 in MeiPrSO2:EA (1:1) + 4% FEC, in the voltage range from 3.0 to 4.3 V. FEC and (b) 1 M LiPF6 in MeiPrSO2:EA (1:1) + 4% FEC, in the voltage range from 3.0 to 4.3 V.

TheThe capacity capacity retention retention amounts amounts to to96.9% 96.9% in inthe the 100th 100th cycle. cycle. In Incontrast contrast to tothe the other other sulfone-based sulfone-based electrolytes,electrolytes, the the cycling cycling profiles proﬁles of of1 M 1 M LiPF LiPF6 in6 inMeiPrSO MeiPrSO2:EA2:EA (1:1) (1:1) with with 4% 4% FEC FEC indicate indicate a strong a strong capacitycapacity fading. fading. The The capacity capacity retention retention amounts amounts to to93 93.5%.5% in inthe the 100th 100th cycle. cycle. It may It may be beconcluded, concluded, that that EMSEMS shows shows a better a better long long term term cycling cycling behavior behavior comp comparedared to tothe the other other two two investigated investigated sulfone-based sulfone-based electrolytes.electrolytes. Furthermore, Furthermore, the the concep conceptt of of using using EA EA as as co-solvent co-solvent in in su sulfone-basedlfone-based electrolytes as wellwell as as thethe presence presence of FECof FEC as SEI as additiveSEI additive represent represent a successful a successful approach approach towards alternativetowards alternative nonaqueous nonaqueouselectrolyte electrolyte formulations formulations for application for application in lithium ionin lithium batteries. ion batteries.

4. 4.Conclusions Conclusions In Inthis this contribution, contribution, the the concept concept of ofFEC FEC as asSEI-fo SEI-formingrming electrolyte electrolyte additive additive and and EA EA as aselectrolyte electrolyte co-solventco-solvent was was introduced introduced for for sulfone-based sulfone-based electr electrolytes.olytes. The The optimal optimal amount amount of of FEC FEC in in the the 1 M 1 M LiPF LiPF6 6 in inEMS EMS electrolyte, electrolyte, usedused to to achieve achieve stable stable cycling cycling performance performance on graphite on graphite electrodes electrodes was determined. was determined.It was shown It was that shown the addition that the of 4%addition FEC to of 1 M4% LiPF FEC6 into an1 M EMS-based LiPF6 in electrolytean EMS-based results electrolyte in a stable resultslong-term in a cyclingstable long-term performance cycling of graphite/Li performance cells. of graphite/Li Further tailoring cells. ofFurther this electrolyte, tailoring of in this terms electrolyte,of higher in conductivity terms of higher and conductivity wettability, was and attainedwettability, by usingwas attained EA as co-solvent.by using EA The as co-solvent. conductivity ◦ · −1 Thevalue conductivity of 1 M LiPF value6 in of EMS:EA 1 M LiPF (1:1)6 in EMS:EA + 4% FEC (1:1) at + 20 4% CFE (5.23C at 20 mS °Ccm (5.23) ismS·cm comparable−1) is comparable to the one −1 to ofthe the one commercially of the commercially available carbon-basedavailable carbon-based electrolyte (5.58electrolyte mS·cm (5.58), whereasmS·cm−1), the whereas conductivity the · −1 conductivityvalue (1.73 mSvaluecm (1.73) of puremS·cm 1− M1) LiPFof pure6 in EMS1 M is LiPF much6 in lower. EMS Contact is much angle lower. measurements Contact angle further measurementsconﬁrmed similar further behavior confirmed of thesimilar 1 M behavior LiPF6 in EMS:EAof the 1 M (1:1) LiPF with6 in EMS:EA 4% FEC to(1:1) the with carbonate-based 4% FEC to ◦ theelectrolyte, carbonate-based resulting electrolyte, in a contact resulting angle of in≈ a60 contact. On the angle other of hand, ≈ 60°. the On contact the other angle hand, of the the EMS-based contact ◦ angleelectrolyte of the EMS-based without EA electrolyte shows a much without higher EA valueshows (103 a much± 3) higherleading value to a (103 lower ± 3)° wettability leading to of a the lowerused wettability Celgard 2500 of the separator. used Celgard CCCP 2500 measurements separator. ofCCCP graphite measurements using a 1 Mof LiPFgraphite6 in EMS:EA using a 1 based M LiPFelectrolyte6 in EMS:EA indicating based noelectrolyte negative indicating effect of EA no on negati the cyclingve effect performance of EA on the of cycling the cell. performance Results obtained of theby cell. XPS Results measurements obtained revealed by XPS decompositionmeasurements ofrevealed EMS as decomposition well as EA on graphite,of EMS as resulting well as inEA a thickon graphite,decomposition resulting layer in a thick on the decomposition graphite surface. layer Theon the decomposition graphite surface. of EMS The decomposition (sulfur signal at of168 EMS eV) (sulfurwas notsignal observed at 168 when eV) was adding not 4% observ FECed to thewhen 1 M adding LiPF6 in4% EMS:EA FEC to electrolyte the 1 M LiPF formulation.6 in EMS:EA CCCP electrolytemeasurements formulation. of graphite/Li CCCP measurements cells, using an of EMS-based graphite/Li electrolyte cells, using with an andEMS-based without electrolyte EA, showed withthat and in both without cases EA, that showed 4% FEC that are requiredin both tocase preventings that 4% the FEC decomposition are required of to the preventing electrolyte the over decomposition100 charge/discharge of the electrolyte cycles. In theover NMC/graphite 100 charge/discharge cell conﬁguration, cycles. In a stablethe NMC/graphite cycling performance cell configuration,was achieved a with stable the cycling 1M LiPF performance6 in EMS:EA was + 4% achieved FEC electrolyte. with the In 1M addition, LiPF6 in the EMS:EA concept + of 4% using FEC EA electrolyte.and FEC inIn combinationaddition, the withconcept EMS-based of using EA electrolyte and FEC is in found combination applicable with for EMS-based other sulfones, electrolyte such as is EtiPrSOfound applicable2 and MeiPrSO for other2. sulfones, such as EtiPrSO2 and MeiPrSO2.

Acknowledgments: Financial support by the German Federal Ministry for Education and Research (BMBF) Acknowledgementswithin the project Electrolyte: Financial Lab,support 4E (project by the referenceGerman Federal 03X4632) Mi isnistry gratefully for Education acknowledged. and Research The authors (BMBF) would withinlike tothe thank project B. StreipertElectrolyte for Lab, his support4E (project in analyzingreference 03X4632) the XPS data.is gratefully acknowledged. The authors would like to thank B. Streipert for his support in analyzing the XPS data.

Energies 2017, 10, 1312 12 of 14

Author Contributions: Peter Hilbig, Isidora Cekic-Laskovic and Martin Winter conceived and designed the experiments; Peter Hilbig and Lukas Ibing performed the experiments; Peter Hilbig, Lukas Ibing, Ralf Wagner, Isidora Cekic-Laskovic and Martin Winter analyzed the data; Peter Hilbig and Isidora Cekic-Laskovic wrote the paper.” Conﬂicts of Interest: The authors declare no conﬂict of interest.

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