Advanced Electrolyte System Design of Battery Towards Expanded Applications on Energy Storage

Advanced Electrolyte System Design of Battery towards Expanded Applications on Energy Storage

A Dissertation

Submitted to the Faculty

of the

WORCESTER POLYTECHNIC INSTITUTE

In partial fulfillment

of the requirement for the

Degree of Doctor of Philosophy

Materials Science and Engineering

Jiayu Cao

November 2020

Approved by:

Dr. Yan Wang, Advisor, Professor, Mechanical Engineering

Dr. Brajendra Mishra, Head of the Materials Science and Engineering Program Abstract

The battery technology is approaching a larger share portion of the energy storage demands nowadays.

These sustainable electrochemical-based methods are gradually replacing the traditional fossil resource to power the world with its carry-on capability and eco-friendly nature. Many applications such as, military power supplies, civil transportation, and stationary storage need higher energy density of the rechargeable battery as the development of society. For example, the goal of commercializing next-generation electric vehicles which can satisfy a longer traveling distance is hard to achieve since the lack of lower cost, higher energy density and longer cycling life rechargeable batteries as next-generation energy storage devices. Both lithium sulfur (Li-S) battery and lithium ion battery (LIB) with silicon anode have potential to be the next-generation storage batteries. Because sulfur and silicon are both stored largely on earth, exploited easily and low cost. Therefore, a variety of deep dive and large-scale research have been started about Li-S battery and Si anode applied in

LIB. Over the last few years, Argonne National Laboratory team has developed various ideas and methods to optimize Li-S battery and Si anode used in LIB, such as, introducing appropriate cosolvents and additives, replacing traditional electrolyte with ionic liquid and polymer electrolyte and enhancing Si anode by surface modification. For a comprehensive study, here we discuss four design schemes based on two major problems and one main barrier of Li-S battery and Si anode, respectively.

In terms of Li-S battery, the first problem is the rapid capacity fading during cycling process and the second problem is the lithium polysulfide shuttling. The main barrier of Si anode applied in LIB is the strength and stability of solid electrolyte interphase (SEI) layer formed on the surface of Si anode.

In the first part of this thesis, we report for the first time using hexafluorobenzene (HFB) to tackle the issue of rapid capacity fading during cycling process. When used as an electrolyte additive for Li-S cell, HFB preferably chelates with low-order lithium sulfides Li2S and Li2S2 and subsequently reacts and converts both into active aromatic sulfides: bis(pentafluorophenyl) disulfides and bis(pentafluorophenyl) polysulfides in-situ, thus facilitating the utilization of the electronically insulating Li2S and Li2S2 and improving the cycling stability of Li-S cell. The second part of this thesis is to determine the relationship between various electrochemical properties of Li-S battery and a series of lithium salt concentrations. According to the experimental results, choosing an appropriate salt concentration by balancing different properties the for each electrolyte to maximize their performances is more reasonable than using a certain salt concentration as a standard for all electrolytes and this is also an effective way to constrain the rapid capacity fading during cycling process. In the third part, the impact of the electrolyte solvation structure on the performance of Li-S battery was investigated using an electrolyte system containing various solvent ratios of dioxolane

(DOL) and a nonafluorinated ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether (TPE).

The cell testing results indicate that increasing the ratio of TPE led to both a higher discharge capacity and a higher Coulombic efficiency, because lithium polysulfide shuttling issue is constrained when increasing the ratio of TPE in the electrolyte. In the fourth part, we introduce for the first time using

2,2,2-Trifluoroethyl methyl carbonate (FEMC) to optimize the Si anode. Based on the performance of LIBs, FEMC has potential to be an effective electrolyte co-solvent to protect Si anode due to the rich fluorine groups in its structure. When used as an co-solvent of all-fluorinated electrolyte, FEMC preferably passivates Si anode, helps the formation of a robust and resilient SEI layer, and alleviates the continuous reactions between Si and electrolyte and the delamination of electrode caused by the large volume change of Si during charging and discharging process. Therefore, FEMC co-solvent improves the electrochemical properties of LIB applying Si as anode.

Acknowledgements

I would like to express my deep gratitude to my advisors, Prof. Yan Wang and Dr. Zhengcheng

Zhang for their persistent supports and professional guidance. They discipline themselves with

high standards and show me all the excellent qualities a researcher should have. They will be a role model I am following in my careers.

I am honored to have Professor Richard D. Sisson, Professor Yu Zhong and Professor Qi Wen as committee members. Thanks for their help, encouragement, and guidance during my graduate studies. Special thanks to Professor Richard D. Sisson, who is always there and provides great help to me. His generosity and kindness make MSE department look like a big family for all of us. I want to thank Dr. Rachel Koritala and Dr. Zhenzhen Yang for their continuous help and technical guidance with SEM and XPS equipment. I also want to thank Dr. Quinton James Meisner, who helps me a lot on the organic chemistry and chemical characterization. Many thanks to Rita

Shilansky, who treats me as a sincere friend, gave me generous help and constant care.

I would like to thank all the collaborator and partners in this electrolyte optimization project. Tobias

Glossmann from Mercedes-Benz Research & Development North America, Inc; Andreas Hintennach from

Daimler AG (Mercedes-Benz Cars); Tomas Rojas, Ritu Sahore, Zhenzhen Yang, Paul Redfern, Anh

Ngo, Larry A. Curtiss, from Argonne National Laboratory.

I would like to thank my colleagues in Electrochemical Energy Laboratories of WPI and ANL,

Zhangfeng Zheng, Qiang Wang, Bin Chen, Ruihan Zhang, Sungyool Bong, Jin Liu, Yubin Zhang,

Mengyuan Chen, Xiaotu Ma, Yangtao Liu, Qing Yin, Azhari Luqman, Panawan Vanaphuti, Zifei

Meng, Chao Shen, Dapeng Xu, Yadong Zheng, Jinzhao Fu, Jiajun Chen, Zeyi Yao, Yilun Tang;

Quinton James Meisner, Sisi Jiang, Qian Liu, Zhangxing Shi, Johnson, Noah Mark, Bin Hu,

III

Jingjing Zhang, Jianzhong Yang, Haotian Yang, Mingfu He, Kewei Liu, Yuyue Zhao, Mengxi Yang,

Lily, A Robertson, Lu Zhang, Chen Liao, who have been helping me a lot in the labs.

I thank my parents and friends for their unconditional love and care. Without their support, I won’t achieve anything.

I acknowledge the financial support of the Department of Energy and Mercedes-Benz Research &

Development North America, Inc.

Disclaimer

This report was prepared as an account of work sponsored by an agency of the United States

Government and Department of Energy and Mercedes-Benz Research & Development North

America, Inc. Neither the Department of Energy and Mercedes-Benz Research & Development North

America, Inc, the United States Government, nor any agency thereof, nor any of their employees, makes any warranty, expresses, implies, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States

Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or those of

Department of Energy and Mercedes-Benz Research & Development North America, Inc.

Table of Content

Abstract···································································································································· I

Acknowledgements ···················································································································· III

Table of Content ························································································································ V

Chapter 1 Introduction of Lithium-Sulfur Battery and Si Electrode ······························································· 1

Chapter 2 Tackling the Capacity Fading Issue of Li-S Battery by a Functional Additive – Hexafluorobenzene ·········· 33

Chapter 3 A Systematic Research on the Relationship between Lithium Salt Concentration and Li-S Battery. ·········· 48

Chapter 4 Understanding the Impact of a Nonafluorinated Ether-Based Electrolyte on Li-S Battery ······················ 66

Chapter 5 Optimize the Lithium-Silicon Battery by a New Kind of All Fluorinated Electrolyte ···························· 83

Chapter 6. Summary ··················································································································· 98

Chapter 7. Future Work ·············································································································· 101

Chapter 8. Publications and Presentations ························································································· 103

Chapter 1 Introduction of Lithium-Sulfur Battery and Si Electrode

1.1 Lithium-Sulfur (Li-S) Battery

1.1.1 Basic knowledge of Li-S battery

A Li-S battery is a kind of electrochemical energy storage device, which the electrical energy can be stored in sulfur cathodes. A schematic diagram of lithium-sulfur battery is shown in Figure 1.1 For a traditional Li-S battery, it consists of a sulfur composite cathode (sulfur and carbon black PVDF as binder), a lithium metal anode and organic electrolyte. Carbon black is combined in sulfur cathode to increase the conductivity of sulfur cathode, since the high resistance of pure sulfur material.

Unlike the mechanism of lithium ion battery which is direct lithium insertion/extraction, for Li-S battery, there is a multistep redox reaction during each discharge and charge process.2-4 The reversible redox reaction is shown in Equation (1).5

+ S8 + 16Li + 16e ↔ 8Li2S (1)

This redox reaction will be divided into several steps during cycling and these steps can be explained by the voltage profile of Li-S battery. Figure 2 shows a typical voltage profile of Li-S battery, all intermedia redox reactions can be divided into three regions.2

Region I: Initially, in discharge process the solid-state sulfur is dissolved into electrolyte and forms

−4 liquid-state sulfur, which is further reduced to S2 . In charging process, the soluble long-chain polysulfide species are oxidized to solid sulfur at this region. This reversible reaction is shown in

Equation (2)

2- S8 (s) ↔ S8 (l) + 4e ↔ 2S4 (2)

Each sulfur transfers half-electron charge in this reaction. About 25% of total sulfur capacity is contributed by this process.6 The relationship of voltage and concentration of polysulfide and liquid- state sulfur is shown by the Nernst equation.5 (Equation (3))

[ ( )] = + ln (3) [ 0 ] 𝜃𝜃 𝑅𝑅𝑅𝑅 𝑆𝑆8 𝑙𝑙 2− 2 𝐸𝐸𝑈𝑈 𝐸𝐸𝑈𝑈 𝑛𝑛𝐻𝐻𝐹𝐹 𝑆𝑆4 2- Because the concentration of S4 increases so the voltage decreases in this region. Viscosity of electrolyte also keep increasing in this region since the low solubility of S8 (l) in electrolyte. Once the viscosity rises to a certain level, it is hard to keep the transportation of Li-ion in this electrolyte, so there is a small reverse peak that is circled as point 1 (Figure 2).2

2− In Region II, at discharge process the soluble species S4 are reduced to insoluble lithium disulfide

(Li2S2) or lithium sulfide Li2S. Insoluble Li2S2/Li2S are oxidized to long-chain polysulfide species, at charge process, also there is a reduced polarization caused by the dissolution of solid Li2S2/Li2S is verified by the small peak that is circled as point 2.2, 4 Both reversible reactions is shown in Equations

(4a) and (4b)7.

4− + S + 8Li + 6e ↔ 4Li2S (4a)

4− + S + 4Li + 2e ↔ 2Li2S2 (4b)

2− The long, flat, low voltage plateau appears in this region since the slow rate of soluble S4 being reduced to insoluble and nonconductive Li2S2/Li2S. Therefore, the discharge process stays at this region for a long time.

When cathode is largely covered by both nonconductive solid materials Li2S2/Li2S, the resistance of battery increases a lot causing a huge decrease of voltage, then this reaction is terminated.

In discharge process, there is a solid-to-solid reduction from Li2S2 to Li2S happen in Region III, which is shown in Equation (5).

+ Li2S2 + 2Li + 2e ↔ 2Li2S (5)

Because the diffusion between two solid states is slow, so the conversion of the two sulfides is the

2, 8 most difficult part among all steps and usually Li2S2 cannot fully converts to Li2S.

Figure 1. Schematic diagram of a Li−S cell with its charge/discharge operations.1

Figure 2. A typical initial voltage profile of Li–S battery.2

Lithium-sulfur battery is discovered in 1960s,9 however until 2000, the deep dive and large scale researches of Li-S battery start, since the higher requirements on the energy density of the rechargeable battery in many applications such as, military power supplies, civil transportation, and stationary storage.10-11 Li-S batteries have an overwhelming advantage in energy density as shown in

Figure 3,12 the theoretical value is about 2500 W h kg−1 or 2800 W h L−1, which is greatly larger than that of commercial Lithium-ion batteries (LIBs). The practical energy density of packaged Li-S batteries could be as high as 400 to 600 W h kg−1 (two or three times higher than that of commercial

LIBs), and could satisfy a traveling distance of 500 km for electric vehicles.3, 10 Furthermore, S is one of the most abundant and non-toxicity material exciting on Earth. These advantages make lithium- sulfur battery potential to be the next generation commercial energy storage device.13 Since 2009, 4

Lithium-Sulfur battery has gained even more attention after Nazar et al. reported a Li-S battery which has improved cycling performance.14

Figure 3. The theoretical energy density of three rechargeable battery systems12.

However, the main challenges of Li-S battery industry, either with the materials or with the system are still existence.

First of all, sulfur and Li2S2 and Li2S which are formed during discharge and charge process have poor electronic conductivities, which means cells having very high internal resistance. The high resistance results in a large polarization that reduces the energy efficiency of the battery, furthermore

Li2S2 and Li2S cover on the surface of the S particles during discharge like an insoluble insulation

5 layer, impeding further reduction of S cathode, which finally leads to a poor utilization of active material.1, 15

The second problem lies in the electrochemical process of the Li-S battery. During the discharge

(lithiation), sulfur accepts electron and is reduced stepwise to a series of high-order polysulfide

4, 16 intermediates (Li2Sx, 3

Figure 4. Polysulfide shuttle mechanism in Lithium-Sulfur battery.19

The third problem is related to the large volume change of sulfur cathode upon cycling. In the case of

-3 -3 20-21 the different densities of S8 (2.07 g cm ) and Li2S (1.66 g cm ), it is apparent that sulfur has a

10 huge volume expansion of about 79% from S8 to Li2S. This makes Li2S pulverize and lose their electrical contacts with the conductive material (usually is carbon black) or the current collector. As the large crystalline material of “non-contact S” increases in the battery system, continuous and serious capacity fade starts during cycling. In operando investigation for the traditional Li-S batteries, the volume change of S and the formation of “non-contact S” are inevitable.21 This volume variation problem together with shuttling problem are crucial for practical Li-S batteries, since both of them impact the age and safety and power of Li-S battery significantly.

The Fermi energy of lithium is higher than the low unoccupied molecular orbital of the commonly used ether based liquid electrolytes for Li-S battery, as shown in Figure 5.22 Because of this electrochemical property, there are also several problems when apply lithium as the anode for Li-S battery.

First, electrolytes can be reduced on the surface of the lithium anode forming an interfacial layer between electrolyte and lithium anode, usually this layer is named as solid electrolyte interphase (SEI) layer. This reduction reaction usually happen from first discharge process of Li-S battery, causing a remarkable initial irreversible capacity loss and low coulombic efficiency (CE) of battery23-25, and even worse most of these SEI are not stable since the instability of lithium anode upon cycling, so this consumption of electrolyte and lithium can be continuous resulting in an on-going capacity loss and low CE, finally the lack of electrolyte may causes the difficulty of lithium ion transfer, increasing the over potential then leading to rapid capacity loss.

Second, the growth of Li dendrites, which originates from the inhomogeneous deposition of lithium and can cause safety problems of Li-S batteries.26 The cracks and fissures in the SEI result in a continuous electrolyte and Li consumption by reacting with each other or with dissolved polysulfide during cycling. These reactions finally result in a thick SEI and large lithium dendrites which caused by inhomogeneous deposition of lithium.27 These two structures lead to different drawbacks of Li-S battery, the former one causes high impedance in batteries, the latter one is likely to pierce the separator,27-28 causing a short circuit of the cell.

The third problem of the lithium anode arises from the reaction between lithium and high-order lithium polysulfides. Due to the shuttling effect, soluble high order polysulfides diffusing through the separator and react with Li to form the insoluble sulfides (Li2S2, Li2S). These insoluble sulfides cover on the surface of lithium anode. This gradual growth of insulating sulfides on the lithium anode during cell cycling can seriously raise the impedance and retard the rapid access of Li, then result in low capacity and poor rate capability of Li-S battery.15, 29-30

Last but not the least, due to the electrochemical activity of sulfur in battery system. There are more limitations when design electrolyte system for Li-S battery than the LIB. The common electrolytes for lithium-sulfur battery are ether based electrolytes since they have been reported to be highly active in facilitating the electrochemical reaction between Li and S.10 However, these electrolytes also have very high solubility of polysulfides, causing a serious polysulfide shuttling problem, leading to a low

CE and continuous capacity fading of battery.5, 31 Carbonate electrolytes, which usually be applied in lithium-ion battery, have been tested and shown that it is reactive with the high-order polysulfides.

Therefore carbonate based electrolyte can have an incomplete reduction reaction with high-order polysulfides.32 Other electrolytes, such as ionic liquid electrolytes33 and solid state electrolytes34, are also applied in Li-S batteries. Researchers found that these electrolytes can constrain the shuttling of 8 lithium polysulfide.33, 35 However, ionic liquid and solid state electrolytes also are impeded with the sluggish Li+ diffusion due to their high viscosity or solid-state nature.36

Figure 5. Electrochemistry of the Li-S battery22.

1.1.2 The Improvement and optimization of Li-S battery

As stated above, most of the problems of lithium-Sulfur battery are attributed to the lithium polysulfide shuttling issue and the lack of lithium protection. Many researchers have been put efforts on solving these major problems.

First, start from the sulfur cathode side. In 2003, Lee et al. apply the one-dimensional carbon nanotubes as additives in Li-S battery.37 Lee simply added multi-walled carbon nanotubes (20 wt%) into sulfur cathodes and after adding these nanotubes the capacity retention of Li-S batteries after 50

9 cycles were greatly improved, from 25% to 62%, and there are also an increase in specific capacity,

-1 -1 from about 400 mAhg S (without additive) to 485 mAhg S. This is one of the earliest studies in using carbon nanotubes in Li-S battery However, poor contact between sulfur and carbon nanotubes led to a low discharge capacity and hard to improve.

In order to achieve a better contact between sulfur active material and carbon nanotube, Zhu et al.38 mixed these two materials at 155 °C to functionalized the capillary effect since sulfur are very low viscous at this temperature. As a result, the sulfur cathodes prepared at high temperature showed lower resistance and improved cycling performance compared with the cathodes formed by sulfur and carbon nanotubes mixing at room temperature. Experimental results indicate that, a large specific

-1 discharge capacity of 900 mAhg S was achieved with a 74% capacity retention after 60 cycles at

0.06C.

Compared with previous carbon nanotube, hollow carbon tubes with much larger diameters to encapsulate and confine lithium polysulfides inside the tubes, which are more appropriate applying in lithium-sulfur battery.39 Zheng et al. used hollow carbon tubes as hosts for sulfur and the carbon tube-sulfur cathode having an unique structure that leave sulfur only on the inner surface of the carbon tubes but not the outer surface. Using this unique structure, high specific capacities of about 1400 and

1300 mA h g-1 were achieved at 0.2C and 0.5C, respectively. However, capacity retention of this kind of battery are relatively low in both cases, with about 52% and 48% after 150 cycles, respectively.

Nazar et al. had developed a highly-ordered, mesoporous carbon–sulfur cathode for lithium–sulfur batteries in 2009.14 They used CMK-3 as the mesoporous carbon, which makes by an array of hollow

6.5 nm carbon rods and they have 3 to 4 nm gaps between each other rod as shown in Figure 6. Sulfur was then infiltrated into the rods by a melt-diffusion method at 155 °C.

Using this mesoporous structure to trap lithium polysulfides, Nazar gained a high specific capacity of

1005 mAhg-1 after cycling over 20 cycles at 0.1C.

Figure 6. (a) TEM image and (b) schematic of high-order, mesoporous carbon (black parts) (CMK-

3) in which sulfur (yellow parts) was infiltrated by melt-diffusion at 155 ºC.14

There is also another idea to optimize sulfur cathode by carbon tubes which is using a tube-in-tube structure.40 This structure consists of carbon nanotubes sticking on the inside surrounded by another larger carbon tube on the outside and it combined the advantages of carbon nanotubes and large carbon tubes. This structure was described as the inner carbon nanotubes can improve electronic conductivity of battery; the outer large tube helps to confine the motions of lithium polysulfides in battery. The reported performance of batteries show that batteries delivered high specific capacities

-1 which are 1274 and 787 mAhg S at 0.3C and 1.2C, with 72% and 82% capacity retentions after 50 and 200 cycles, respectively.

Two-dimensional carbon–sulfur structures based on graphene is one of the most popular cathode structures in lithium–sulfur battery so far. Because graphene is potential to be an excellent

11 encapsulation material for sulfur cathodes since its excellent electronic conductivity, ease to be functionalized and trapping lithium polysulfide efficiently.

Dai et al. applied the graphene to encapsulate sulfur particles forming sulfur cathodes at first.41 As a result, the cells applying synthesized graphene-wrapped sulfur cathode showed enhanced capacity and cycling stability compared to the baseline cells. In particular, the capacity and retention of these cells are about 750 mAhg-1 and 70% at the end of 100 cycles at 0.2C. Figure 7 shows the SEM images of graphene-wrapped sulfur composites.

The electrochemical properties of Li-S battery improves by replacing graphene by graphene oxide as a host material for sulfur cathodes.42 The interactions between sulfur and the oxygen functional groups

(epoxide and hydroxyl groups) immobilize the sulfur species, and this sulfur–graphene oxide cathodes showed a high specific capacity of 1000 mAhg-1 with 95% capacity retention after 50 cycles at 0.1C.

Figure 7. (a) Low-magnification and (b) high-magnification SEM images of graphene-wrapped sulfur composites.41

Rong et al. also applied graphene oxide as a host of sulfur and presented relatively better electrochemical properties of Li-S battery43. The idea is coating graphene oxide uniformly on the sulfur particles by utilizing the ionic strength of solution. Main process of this idea is adjusting the ionic strength of solutions so that positive ions are attracted on the surface of graphene oxide in solution, forming the electrostatic repulsion between neighboring sheets. When adding sulfur in this solution, the graphene oxide can precipitate and coat on the sulfur particles forming a core–shell structure. Applying this core–shell structure mixed sulfur–graphene oxide material as a cathode,

Batteries performed enhanced long cycling performances. The specific discharge capacities at the end

-1 of 1000 cycles at 0.6C was 800 mAhg S.

Three-dimensional carbon materials are also appropriate hosts for sulfur material such as porous carbon spheres. Porous carbon spheres can be used to both contain sulfur and trap the intermediate polysulfide species by their pores. Functionalize the carbon surface appropriately and use large porosity and pore volume of porous carbon can further improve the cycling stability and higher areal capacity of sulfur cathode, respectively.

Conducting polymers also can be a promising kind of host materials for sulfur cathodes due to: first, the good electronic conductivity of these kinds of polymers, second, polymers have excellent elastic and flexible nature which can accommodate the volumetric change of sulfur cathode, to some degree, during cycling, third, trap lithium polysulfide efficiently since polymer species have a number of functional groups, so they have strong affinity for lithium polysulfide species.

Lifen Xiao et al. introduced a kind of self-assembled polyaniline (PANI) nanotubes to confine sulfur at the molecular level during cycling.44 During infiltrating sulfur at 280 °C, some of the sulfur would react with PANI forming three-dimensional, cross-linked network with disulfide bonds, which can

13 help to immobilize the sulfur at the molecular level. Due to this unique constraint effect, cells applying sulfur–PANI nanotubes as cathodes displaying a specific capacity of 568 mAhg-1 with 76% capacity retention over 500 cycles at 1C.

Metal-oxide materials are also appropriate host materials for sulfur cathodes, since they have a plenty of oxygen atoms which have strong affinity for lithium polysulfide species.

45 In 2015, Liang et al investigated manganese dioxide (MnO2) as a host material. The authors proposed MnO2 can reacted with Li2S4 forming thiosulfate species, and then further reacted with high- order lithium polysulfides to form catenated polythionate species and low-order polysulfides.

Therefore, MnO2 acted as a polysulfide mediator which can both bind and reduce polysulfides. This

-1 sulfur–MnO2 composite cathode shows a high specific capacity of 1120 mAhg with 92% capacity retention after 200 cycles at 0.2C.

Sun et al. reported that VO2 nanobelts is also one of the most promising host additives for the

46 graphene-based sulfur cathode . Figure 8 shows the UV-vis absorption spectrum of Li2S6 solution with additions. It is obvious that the adsorption signal of Li2S6 fades in the visible region when adding

VO2 as an additive in Li2S6 solution. This experimental result demonstrated that VO2 can attract LiPS strongly. Due to this strong chemical adsorption provided by VO2, the Li-S batteries present impressive cycling and rate performance.

47 Jiang et al. introduced RuO2 nanodot which can also anchor LiPS and participate the surface- mediated reduction. This reaction is likely to contribute to the improvement of reaction kinetics.

Following this work, Shao reported a new idea of optimizing sulfur cathode that embedding RuO2 nanoparticles in the 3D CNT film and combining together with sulfur for Li-S battery48. This creative idea resulted in an outstanding initial capacity of 750 and 1060 mAhg-1at 2 C and 0.5 C, respectively. 14

At the same time, the capacity retention of long term cycling also excellent. The battery applying this composite cathode kept a relatively high capacity of 405 mAhg-1 at 0.5 C after 1000 cycles. The rate of decay during cycling process was only 0.06% per cycle.

Figure 8. UV-vis absorption spectra of the Li2S6 solution without any addition, the Li2S6 solution

46 with rGO and VO2, respectively.

Second, except optimizing the sulfur cathode, modifying the separator is also an effective way to optimize lithium-sulfur battery.

In 2014, Zhou et al. formed a separator - graphene - sulfur sandwich structure that a polymer separator was coated with a thin graphene layer.49 This interlayer served 2 important functions: first, helping to confine lithium polysulfides and second, working as an upper current collector to improve the

15 utilization of sulfur. By applying this graphene-coated separator battery achieved a high specific capacity of 971 mAhg-1 and 70% capacity retention after 300 cycles at 0.9C.

More idea about modifying separator had been found in 2014. Vajo et al. introduced the idea of

50 coating inorganic vanadium (v) oxide (V2O5) onto separators to optimize lithium–sulfur battery.

V2O5 is a kind of solid-state conductor allowing the diffusion of lithium ions, but it is able to constrain the diffusion of lithium polysulfides to the lithium side, since its oxygen-rich structure. By coating only 1 μm layer of V2O5 onto polymer separators through a sol–gel method, a 5mA h lithium–sulfur pouch cell shows a high discharge capacity of 800 mAhg-1 after 250 cycles at 0.07C. This achievement brings us a step closer to commercialize these kinds of high-performance lithium-sulfur batteries.

Third, lithium protection is also a critical issue of Li-S battery. All problems relating to lithium anode can be solved when lithium chip doesn’t contain in the sulfur batteries. Therefore, an attracting idea has been introduced to completely solve this problem, which is replacing lithium with other stable electrodes to be the source of lithium ion and transfer the lithium sulfur battery into lithium-ion sulfur battery.

There are two way to design this lithium-ion sulfur battery. First, apply stable prelithiated electrode replacing lithium chip as anode. Liu et al. 51 used pre-lithiated silicon nanowire (SiNW) as anode for

Li-ion sulfur battery. SiNW was lithiated by directly contact SINW with lithium metal (0.38 mm thick) in electrolyte. Based on this procedure, the authors found that 50% of the full capacity were loaded into the SiNW after 20 minutes. Following this idea, the electrochemical devices have been introduced into this process to increase the efficiency. Shen et al. used the Li–Si half-cell to pre-lithiate the Si anode. First, the cell was assembled with a LiTFSI electrolyte. Then the cell cycled in a voltage window range from 0.01 V to 2 V. After a period of time, the cell was disassembled and the

16 prelithiated Si anode was prepared successfully. foil. Although this pre-lithiation by lithium metal is common and mature in academic studies, the pre-lithiation process still needs to be done in electrochemical devices for example, coin cell and the process need disassemble the coin cells. The destruction of the electrodes also has to be concerned, so it is difficult to be implemented into the commercial battery manufacturing process.

Recently, Li et al.52 have developed a simple and low-cost roll-to-roll mechanical pre-lithiation process. Li reported that, the Sn, Al foils and Si/C anodes were all successfully pre-lithiated by this method. The schematic diagram is shown in Figure 9. In that work, the prelithiation process was completed by thermomechanical pressing, without using any electrolyte. After a series of processes, the side of Sn foil in contact with Li foil has formed a LixSn layer, and the other side is still the Sn foil. The experimental results reported that using prepared LixSn foil as anodes the LixSn//LiFePO4 full cells delivered stable cycling performances even after 200 cycles. Furthermore, LixSn foils also show better stability than pure Li foils when exposed in air. These results indicate that it is possible to apply this pre-lithiated anode in Li-ion sulfur battery and LixAl and LixSi/C anode materials were also formed using the same process.

Figure 9. Schematic diagram of the roll-to-roll pre-lithiation method.52

The second way to design a Li-ion sulfur battery is replacing sulfur with Li2S as cathode which providing lithium ions and Li-metal-free material acts as the anode.

As a product of complete lithiation of elemental S, Li2S has a high theoretical capacity of 1166

-1 mAhg . Therefore, Li2S can provide sufficient Li ions during the charging and discharging process and the Li free anode applied in this Li-ion sulfur battery can avoid a varieties of safety problems

53-54 caused by lithium anode . Second, Li2S has a larger volume state which is much larger than S, therefore the issues caused by volume expansion of sulfur during cycling is hard to happen and Li2S can present a much more stable surface than sulfur cathode55. Last but not least, the melting point of

Li2S is much higher than that of sulfur, which makes applying sulfur battery under high temperature possible and broadens the range of application for sulfur battery19.

However, the poor electronic conductivity, “polysulfide shuttling effect” and high activation barrier voltage are the main obstacles of this Li-ion sulfur battery, many researchers have paid efforts to overcome and solve these problems. 18

56 57 Li2S encapsulation in different kinds of carbon, such as porous carbon , metal–organic frameworks , and free-standing carbon paper58, has become a common solution to address the first two issues, since carbon can both increase the electronic conductivity of sulfur battery and hinder the transfer of polysulfides.

Wang et al. 59 reported a method that carbon-coated Li2S particles can uniformly disperse in a three- dimensional reduced graphene structure. (3D-rGO-Li2S-C) This combined material improves the conductivity and prevents the shuttling of polysulfides of Li-ion sulfur battery, compared with the battery that use Li2S as cathode. The simple schematic diagram of the forming process was shown in

Figure 10, The three-dimensional reduced graphene framework and carbon coating greatly increase the electronic conductivity and the electron transport path of this cathode material. This structure also exerts physical adsorption on polysulfides which is same as the assumption of author.

59 Figure 10. Schematic of 3D-rGO-Li2S-C composite.

The activation barrier (around 4 V) of Li2S in the first charge progress is also a critical drawback of the Li2S cathode. Since a voltage at least around 4 V must be used for the application of normal Li2S cathode in the Li-ion sulfur battery. However, it is well known that the ether based electrolyte for example, DOL/DME based electrolyte which is used as the most common electrolyte in sulfur battery, is likely to oxidation when the voltage is higher than 3.8 V, so, it is necessary to investigate the way to reduce the activation barrier of Li2S in first charge process.

The simple, cheap, effective and universal method to solve this problem is addition of certain additive or solvent that can help electrolyte dissolving Li2S, then convert the solid–solid interface reaction of

20 the activation process into a solid–liquid interface reaction, which is shown in Figure 11. Therefore,

60 its activation barrier of Li2S has significantly reduced.

Figure 11. Schematic illustration of the activation barrier decreasing mechanism by changing the solid–solid interface to solid–liquid interface.60

Compared with the methods relating to the optimization of sulfur cathode, the ideas for improving the electrolytes of Li-S battery are less due to the complex and sensitive electrochemical environment in sulfur battery. Most of them are focus on the protection of lithium anode and the suppression of polysulfide shuttling effect.

In terms of lithium anode protection, there are various methods are protecting the lithium metal surface from attaching dissolved polysulfide species by using electrolyte additives to form a passivating SEI over lithium surface in situ. LiNO3 is an appropriate additive to passivating the lithium anode and improve the Coulombic efficiency (typically 90 to 98%).23, 61-67 Because of this great efficacy, LiNO3 is now used in most of the works on lithium–sulfur battery today as an additive.

However, LiNO3 is a strong oxidizing agent, and can be explosive in nature under high concentrations

4 and temperatures. Therefore, a safer additive is needed to replace LiNO3.

Xiong et al. introduced another additive lithium bis(oxalato) borate (LiBOB) suggesting a lithium

68 surface film richer in organic components rather than the inorganic NOx-species would be more persistent and safer. Some beneficial effects on the initial discharge capacity and the surface morphology were observed, but with increasing cycling, the cells without additives showed the same performance as with LiBOB. Phosphorus pentasulfide, biphenyl-4,4-dithiol and lithium iodide are also reported to optimize Li-S batteries by passivating the surface of lithium anode.69-71 Lithium polysulfide (LiPS) themselves could also be used as additives to improve the compatibility of the electrolyte with the lithium metal anode.72-75

Instead of using additives forming a passivation layer to protect lithium anode in situ, another approach is to protect the lithium ex situ by a physical film. For example, a thin, porous layer of

-2 aluminum oxide (Al2O3) nanoparticles (0.58 mg cm ) was coated on a lithium metal surface by spin-

76 coating method. The thickness of Al2O3 is limited, thinner layer could not protect the lithium metal against lithium polysulfides effectively and thicker layer blocked the diffusion of lithium ions. As a result, A high discharge capacity of 1215 mA h g-1 was demonstrated, with capacity retention of 70% after 50 cycles at 0.1C.

Similar like sulfur cathode and separator, electrolyte can also be optimized to suppress the solubility of lithium polysulfide and become the redox mediators that facilitate electrochemical processes for lithium sulfur battery.

Redox mediators are molecules with reversible redox couples that undergo electrochemical reduction or oxidation at the electrode and diffuse to the active material. They allow the redox of insoluble

22 species to be electrochemically coupled to the electrode surface, even when the active material is not in direct electronic contact. Their electrochemical potential must be matched to that of the process that they are mediating. They can control precipitation of Li2S on the cathode host on discharge, and decrease the overpotential by their oxidation on charge.77

Aurbach et al. were the first to prove that metallocenes are highly effective as a redox mediator for

78 lithium sulfur battery. The metallocene oxidizes the Li2S to polysulfides, then it can diffuse to the electrode surface to be re-oxidized for another cycle.

Benzo[ghi]peryleneimide (BPI) was introduced as a mediator on the lower volt age plateau where

79 reduction of Li2S4 to Li2S occurs. Since the reduction potential of BPI is slightly less than the plateau voltage, so it is reduced at the electrode surface and diffuses away to reduce polysulfides in solution,

Therefore, the formation of large insulating intractable films of Li2S covering over the surface of sulfur cathode and then increasing the resistance of cathode largely is avoided. After the addition of

BPI, Li2S is preferentially deposited on pre-existing sulfide nuclei, favoring localized three- dimensional sulfide deposition, and doubling the capacity.

The dissolution of LiPSs mainly relies on the solvation of Li+ ions. Electrolyte solutions with lower electron donor ability are less able to solvate Li+ ions and thus can suppress the solubility of LiPS.

Amide-based room-temperature ionic liquids (RTILs) with lithium salt (mostly LiTFSI) is an appropriate choice to replace traditional electrolyte suppressing the dissolution of LiPSs. Lacking any free solvent molecules, the donor ability of RTILs is solely dependent on the anionic component, which is related to the LiPS solubility. LiPSs have much lower solubility in an ionic liquid with a low

Gutmann donor number (DN).80

Polymer and solid-state electrolytes are also the potential choices to replace the traditional liquid electrolytes as the second-generation electrolyte for Li-S battery due to their capability of reducing the solubility of polysulfides and blocking the shuttle of polysulfides in sulfur batteries1. In addition, they could protect the lithium metal anode and minimize dendrite formation, which is beneficial for improving the safety and cycle life of Li−S batteries. For example, the long-chain PEO polymers have been widely used in Li−S batteries as one of the major components in the designed polymer electrolyte.

81 Hassoun et al. reported a solid-state Li−S battery applying a Li2S−C composite cathode and PEO- based gel-type polymer membrane containing a nanosized zirconia dispersed PEO/ LiCF3SO3 polymer matrix. Liang et al.147 developed a PEO18/Li(CF3SO2)2N polymer electrolyte. 10 wt % SiO2 and a sulfur−mesoporous carbon sphere composite were also included in this design for Li−S batteries.

Collected data shows the discharge capacity of Li-S batteries were more than 800 mAhg−1 after 25 cycles at 70 °C.82

Highly conductive glass-ceramic electrolytes have also been evaluated in Li−S batteries. Hayashi et

83 al. have introduced an all-solid-state Li−S battery employing Li2S−P2S5 electrolytes. This all-solid- state Li−S battery was able to retain a capacity of over 650 mAhg−1 for 20 cycles at room temperature.

Lin et al.148 developed a novel class of sulfur-rich cathode materials, lithium poly(sulfidophosphate)s, which has a high Li ion conductivity of 10−5-10−6 S cm−1. The Li-S battery using this cathode material displayed a reversible capacity of over 1200 mAhg−1 at C/10 over 300 cycles at 60 °C when applying the solid-state Li3PS4 electrolyte.

1.2 Silicon Electrode

1.2.1 The main barriers for the commercialization of Si electrode

Si has been reported as one of the most promising active materials of anodes for lithium-ion battery

(LIB), because its gravimetric capacity is ten times higher than that of conventional graphite (3579 mAhg-1 for Si and 372 mAhg-1 for graphite).84 Its abundance and low working voltage lead to it becoming a potential active material for the next-generation anode.85

However, its direct utilization is hampered by several fundamental drawbacks. First is the Si particle volume change and pulverization. Silicon is an alloy anode and each silicon atom can host four lithium atoms. This mechanism of lithiation is associated with massive volume changes due to their intake of large amounts of lithium. The relative volume increase of electrode per Si atom in this phase can be expressed by the relation:86

VSi(atom) = Vcell /NSi (6) where, Vcell and NSi is the unit cell volume and number of Si atoms in the unit cell, respectively. The

Si has a diamond cubic structure with 8 atoms per unit cell, and Li4.4Si has a more complex structure.

When transforming from Si to Li4.4Si, the volume expansion is about 420%. This large volume expansion/contraction during lithium insertion/extraction induces large stresses, which can cause cracking and pulverization of Si, leading to loss of electrical contact and capacity fading.87

Due to the large volume expansion and shrink during each cycle, after a few cycles, the structure of the Si begins to crumble away and can no longer hold Li+ ions effectively, so bulk Si breaks and drastically fades capacity.88 As this process happening, the binder lose efficacy that it can’t hold the silicon particles anymore, then the Si particles stack together on the electrode and is likely to delaminate from the metal foil current collector, which results in the beginnings of the device capacity fade. As the process continues, the Si particles peel-off from the surface of the current collector as the

25 cycles continue and the morphology of electrode surface is changed, which eventually leads to drastic capacity fade. Last, in a common electrode situation, this then means that the remaining connected Si particles will experience a much greater current flow, which places further strain on the remaining Si particles leading to further electrode degradation.

Apart from pulverization and morphology change of Si. Large volume change during electrochemical cycling also results in another severe issue: unstable solid electrolyte interphase (SEI) layer.89 When the potential of the anode is below ≈1 V versus Li/Li+, SEI is formed on the anode surface, due to the reductive decomposition of the organic electrolyte. In order to prevent continuous side chemical reactions between electrolyte and Si anode, this layer should be dense, stable, robust and electronically insulating but ionically conducting. However, the SEI cracks due to the volume change of Si anodes during cycling. The exposed fresh electrode surface keeps reacting with electrolyte leading to the continuous formulation of SEI and consumption of lithium. Finally, very thick SEI layer is formed, resulting in low Coulombic efficiency, low electronic conductivity and higher resistance to ionic transport of the whole electrode. Therefore, it is a big challenge to maintain a stable

SEI layer during cycling.

1.2.2 The Optimization of Si electrode

Based on the barriers of the commercialization of Si anode we need to solve, there are different aspects to optimize the Si anode.

90 First aspect is focusing on the composition of Si anode. Ruijun et al. reported the Mg2Si anode materials synthesized by hydrogen-driven chemical reaction delivered an initial capacity of about

1095 mAhg-1 with an initial coulombic efficiency of 92%. They also found that the dissociation and irreversible lithiation of Mg was the main factor for capacity fading of the batteries.

Because stability of the electrodes formed by Si-based alloys could be improved by compositing with

91 92 carbon. Xiao et al. synthesized and introduced Mg2Si/C composites. This material formed with an uniform carbon coating layer by in situ decomposition of C2H2 gas onto the surface of the pre- synthesized Mg2Si. This alloy composite with a carbon content of 1.19% showed a large first discharge capacity of 1405 mAhg-1. The batteries applying this composite electrode maintained a capacity of 320 mAhg-1 after 100 cycles and keep a stable coulombic efficiency over 95%. This electrochemical performance was better than that of employing bare Mg2Si anode.

Besides the binary alloys, tertiary alloys were also investigated by different research groups. Jo et al. developed a core-shell structure of nano-Si/FeSi2Ti with a traditional scalable melt-spinning technique. The nano-Si/FeSi2Ti heterostructure delivered a discharge capacity of 780 mAhg-1 at 0.1

C. It maintained about 70% of its initial capacity at 0.2 C after cycling at 5 C.

Micro and nanoscale Si powder is also a way to solve the problems of utilizing Si anode. Because the small dimensions of macro- or nanoscale SI powders are able to reduce the volumetric change-based stresses to acceptable levels.93

Li et al. reported that an electrode constructed from 78 nm diameter Si particle powder presented better capacity retention than bulk Si powder.94 Mazouzi et al.95 reported that the most stable reversible capacity of 960 mAhg−1 after 700 cycles can be achieved in Si particle-based anodes utilizing 100 nm diameter Si nanoparticles when the state of charge was limited to minimize volumetric expansion.

Applying composite Si nanoparticle-based instead of bare Si nanoparticles as the active material of anodes is an effective way. They can also constrain the significant expansion of Si and maintaining the structural integrity of the anode and enhance their stability by reducing Si aggregation or electrochemical sintering.96

Lee et al.97 developed a nano-Si dispersed oxide/graphite mixture composite material, which exhibited a stable capacity of over 600 mAhg−1 after 40 cycles, and reduced irreversible capacity.

In addition, carbon coating of Si electrodes modified the SEI morphology and showed a smoother surface with limited pore structure than the visibly cracked and porous morphology presented in bare nanostructures Si electrodes.

Si nanowires (SiNWs) is also good choices for anode materials. Because, there was strong electrical contact between the current collector and every SiNW, so that SiNWs have more active materials contributing to the capacity than normal Si particles. Second, no binders or conducting carbon were needed, which lower the total weight and increase the energy density of battery. Doping the nanowires is an effect method to increase their electrical conductivity and optimize the capacity retention of battery at high charge/discharge rates.

Ge et al.98 developed doped SiNWs electrodes for LIBs. The doped SiNWs was synthesized by direct etching of boron-doped Si wafers and alginate as binder. The doped SiNWs electrode showed a discharge capacity of larger than 1000 mAhg −1 at 1 C current rate after 2000 cycles. This improved electrochemical performance was contributed by porous silicon with a large pore size and high porosity and maintaining its structure after lithium ion intercalation. Therefore this SiNWs kept low stress during cycling which was beneficial for the battery that having high capacity and long cycle life.

Thin Si films are also able to release stresses due to volume expansion and contraction during cycling and maintain their mechanical integrity. Takamura et al.99 synthesis a vacuum deposited Si film with a thickness of 50 nm as an anode for LIBs. The LIB applying this ultrathin Si film as anode present a discharge capacity over 2000 mAhg−1 at 30 C current rate after 3000 cycles. The thin Si films with

50 nm thickness exhibited robust characteristics without pulverization and improved capacity

28 retention when patterned on roughened current collectors. However, some degree of capacity fading still happened due to the limited space for release the facile strain expansion and decreased accessibility of electrolyte to SEI between Si nanofilm and electrolyte.

Spray drying is a method for manufacturing dry powder from a liquid solution by rapidly drying with a heated gas. The spray-drying process rapidly produces a dried product from a liquid solution in a single step, and can also be carried out continuously, which is advantageous for facile synthesis and mass production.100

In LIB applications, Li et al.101 synthesis the core-shell structure graphite/Si-porous carbon composite prepared by spray drying with various carbonaceous additives, including polyvinylpyrrolidone (PVP), pitch, and citric acid, to have a core– shell structure. The mass ratio of graphite to Si to carbonaceous additives in the raw materials was designed to be 72:10:18, and the homogeneous suspension, containing the raw materials, was spray dried at inlet and outlet temperatures of 120 and 808 °C, respectively. The primary precursor was further spray dried with a pitch-based carbon solution. The obtained powder was then calcinated at 900 °C for 3 h to make the core–shell porous Si/C composite.

The composite had a dense graphite core and a porous amorphous carbon shell with a thickness of 1

– 3 mm. In addition, Si nanoparticles were embedded in the porous carbon shell, which not only constrain the volume change of Si during the charging and discharging process but also impregnates the graphite core with liquid electrolyte to ensure a large electroactive surface area for the graphite.

As a result, the Si/C composites exhibited a relatively high initial capacity of 723.8 mAhg-1 and a good cycle retention of 81.8% after 100 cycles.

Recently, Xu et al.102 developed and introduced a watermelon-inspired Si/C microsphere prepared by spray drying with flake graphite and carbon-coated nano-Si. The watermelon-inspired Si/C microsphere is made as a secondary structure and made up of the C-coated nano-Si and the ball-milled

29 flake graphite. Such a novel Si/C design shows following advantages. Firstly, the graphite framework acts as a network and suppresses the aggregation of nano-Si. Secondly, the carbon coating on the nano-Si constrain the volume change of the Si during continuous charging and discharging process and enhances the ionic/electric conductivity between the graphite and Si. Thirdly, numerous internal pores in the composite formed from the spray drying contributes additional space for the accommodation of the volume change. Therefore, the Si/C microspheres showed an high initial CE of 89.2% with a capacity of 620 mAhg-1 and achieved improved capacity retention (75.2%) over 500 cycles at a high current rate.

Second aspect is optimizing binder. In traditional anodes, active materials are fixed to the current collector by additive binders. However, in terms of Si, the weak heterogeneous connection makes Si particles easy to peel off from the current collector during the continuous volume expansion and shrink process then, lose the electrical contact with the electrode. This process is dominant for the electrochemical performance of LIB, which may lead to increase in internal resistance and fast capacity fading. Hence, it is essential to develop effective approaches to optimize the electrical contact between nano-Si and the current collector. Recently, conductive polymer binders have shown great effect in improving the electrochemical performance of Si anode. Conductive polymers, which are stable, ionically or electronically conductive and structural flexible, have been applied as binders for

Si anodes as they may stabilize the interface of Si anode and electrolyte and improve charge transfer at the interface and among Si nanoparticles.

Wu et al.103 modified silicon nanoparticles with conductive hydrogel coating layer through an in-situ polymerization method. The hydrogel coating not only serves as the continuous conductive network but also acts as the soft cushion for the integrity of the structure. This optimized electrode delivered a capacity of 550 mAhg−1 at 6 Ag−1 after 5000 cycles.

Higgins et al.104 reported to use a conductive polymer poly(3,4-ethylenedioxythiophene)/poly-

(styrene-4-sulfonate) (PEDOT: PSS) to bond silicon nanoparticles tightly to the current collector. The electrode presented an initial specific capacity of 3685 mAhg-1 at 1 Ag−1 and a capacity of 1950 mAhg−1 after 100 cycles with a capacity retention of 78%.

Choi et al.105 reported that by integrating 5wt% polyrotaxane into conventional polyacrylic acid binder, the composite polymer framework showed perfect elasticity that could keep pulverized Si particles together without crumbling and improved the cycle life of LIBs.

Third aspect is improving the electrochemical properties of electrolyte. It was reported that VC enhanced the electrochemical performance, including cycle life and thermal stability of the lithiated

Si anode.106 Upon addition to the electrolyte, VC undergoes electrochemically induced polymerization, leading to the formation of poly(VC) on the Si electrode surface, thus making SEI denser and more stretchable.107

Kennedy et al.108 reported the effect of 3 wt% VC as an electrolyte additive on Si nanowire electrode.

VC outperformed other additives, such as FEC, lithium bis(oxalato) borate (LiBOB), and VEC, with the same composition.

The most investigated additive for Si-based anodes is FEC and it is widely treated as a key electrolyte additive for Si and Si-based composite anodes. The FEC-derived SEI protects against not only the decomposition of the electrolyte constituents, but also against oxidation of the Si electrode.109

Molecular additives containing nitrogen have attracted huge interest as enabling additives, mainly in graphite and to a limited extent of Si anode for LIBs. These functional additives are believed to undergo electrochemical reductive polymerization at higher potential and thereby suppress further electrolyte degradation on Si anode.110

Silane is another class of molecular additives used to directly modify the surface of Si particles by forming a surface protective layer, a siloxane network. This result in the formation of a robust and stable SEI layer over the Si surfaces. Improvement in the electrochemical performance of the LIBs with the incorporation of silanes is contributed by the stabilization of the SEI by the Si-O-Si linkage.

Ryu et al.111 reported a series of alkoxysilanes as potential additives for the Si anode. Surface passivation is realized through the reaction between silane-based additives and interfacial silanol groups, by converting them into more stable siloxane or silicate bonds.

Li et al.112 learned the effect of SA on the performance of the Si-C composite nanofiber anodes. The addition of 3 wt% SA into 1 M LiPF6 EC + DEC + DMC (1:1:1, volume ratio) increased the capacity retention of Si-C anode material. The discharge capacity of the SA-containing electrolyte was reported to be 34% higher than that of the base electrolyte after 50 cycles. The capacity retention achieved to 82% after 150 cycles, thus demonstrating the potential of SA as an SEI-inspired additive.

Last but not the least, silicon oxide (SiOx, x ~ 1), a Si derivative, has received much attention for its smaller volume change relative to Si, good cycling stability and low cost.113 The volume change of

SiOx charging and discharging process could be effectively confined due to the formation of Li2O and

Li4SiO4. Therefore, this mechanism can effectively help battery accommodating the volume variation of Si and stabilize the SEI layer between the anode and the electrolyte.114 During the Li uptake process,

SiOx only shows a volume change of up to 160% while maintaining a high theoretical specific capacity of 3170 mAhg−1, making it a potential anode candidate for LIBs.115

Chapter 2 Tackling the Capacity Fading Issue of Li-S Battery by a Functional

Additive – Hexafluorobenzene

Abstract

Due to the strong electron-withdrawing fluorine groups, the aromatic ring of the hexafluorobenzene

(HFB) becomes more electron-deficient and strongly interacts with the lone-pair electrons of the media. When used as an electrolyte additive for Li-S cell, HFB preferably coordinates with low-order lithium sulfides Li2S and Li2S2 and subsequently reacts and converts both into active aromatic sulfides

- bis(pentafluorophenyl) disulfides and bis(pentafluorophenyl) polysulfides in-situ, thus facilitating the utilization of the electronically insulating Li2S and Li2S2 and improving the cycling stability of Li-

S cell. This research opens a new avenue to tackle the inherent issues associated with the Li-S chemistry.

1. Introduction

Li-ion battery (LIB) has been one of the most competitive energy storage systems due to its high theoretical energy density and long calendar life.116 Nevertheless, the practical energy density of most commercial LIB is around 200-300 Wh/kg117 which has already been a limitation when applying them in pure electric vehicles. Therefore, to achieve the requirement of higher energy density, lithium sulfur

(Li-S) battery is considered a promising chemistry to replace the traditional LIB as next generation energy storage118-119 due to its high theoretical capacity (1675 mAh/g), which is an order of magnitude higher than that of the traditional insertion-type oxide cathodes (e.g., LiCoO2, LiNi0.5Mn0.3Co0.2O2,

118 and LiFePO4), and the fact that sulfur is currently the cheapest solid state cathode material.

However, there are several technical issues facing Li-S chemistry such as low sulfur utilization, redox shuttling effect, severe self-discharge, poor cycle life, and low specific energy due to the excess

33 amount of electrolyte. Under normal conditions, sulfur atoms form cyclic octatomic crowns with a chemical formula S8. During the discharge (lithiation), sulfur accepts electron and reduced stepwise to a series of high-order polysulfide intermediates (Li2Sx, 3

4 and Li2S. The former, which are soluble in the ether-based electrolyte, diffuses to the lithium anode side, accepts electron and gets reduced to lower-order polysulfides Li2Sx (2

Tremendous research efforts are focused on constraining the dissolution of the high-order polysulfides by new material design using mesoporous carbon,121 strong absorption polymers,122 carbon interlayers,123 and metal oxide hosts.124

LiNO3 is an indispensable electrolyte component for Li-S cells. It could effectively eliminate the shuttling effect via passivating the lithium anode and improve the Coulombic efficiency.23, 61, 66

However, the capacity of Li-S cell still fades rapidly despite nearly 100% Coulombic efficiency.71, 125

Xiong et al. introduced another additive lithium bis(oxalato) borate (LiBOB) suggesting a lithium

68 surface film richer in organic components rather than the inorganic NOx-species would be more persistent and safer. Some beneficial effects on the initial discharge capacity and the surface morphology were observed, but with increasing cycling, the cells without additives showed the same performance as with LiBOB. Phosphorus pentasulfide, biphenyl-4,4-dithiol and lithium iodide are also reported to optimize Li-S batteries by passivating the surface of lithium anode.69, 71 Lithium polysulfide (LiPS) themselves could also be used as additives to improve the compatibility of the

34 electrolyte with the lithium metal anode.126 Recently, a number of papers127-131 reported the organic disulfide such as dimethyl disulfide (DMDS) as a co-solvent can enhance cell capacity and capacity retention of Li-S cells due to the electrochemically active nature of the S−S bond in these organic disulfides. However, the theoretical capacity of Li-S cell cannot be accurately determined since the

S-S bond of the disulfide contributes to the overall capacity. This leads to an artificially inflated specific capacity of the cell. So far, no additives are reported to tackle the issues associated with the crystallization/precipitation of Li2S/Li2S2 thus prevent active material loss which is believed to be the key to the stable cyclability of Li-S cells.

In this paper, we report for the first time such an additive hexafluorobenzene (HFB) (Figure 1a) to tackle the above issue. The designed idea is based on the fact that HFB has an electron deficient

2- (Lewis acidic) core due to the six fluorine atoms which strongly interacts with polysulfides Sx (Lewis basic), especially the stronger Lewis base Li2S/Li2S2 forming a new complex which could be recharged back to the system thus enabling the reutilization of isolated active materials. Our experimental results support our initial hypothesis that HFB could curtail the loss of the active material and improve the performance of Li-S battery. DFT calculations shed light on the energetics of the HFB-Sx2- solvation and possible reaction pathways.

2. Materials & Methods

2.1 Materials

1,3-Dioxolane (DOL), 1,2-dimethoxyethane (DME), hexafluorobenzene (HFB, 99%), lithium nitrate

(LiNO3, 99.99%), lithium sulfide (Li2S, 99.9%), and lithium bis(trifluorosulfonyl)imide (LiTFSI,

99.95%) were purchased from Sigma-Aldrich. Elemental sulfur (Alfa Aesar), pentafluorothiophenol

(Oakwood Chemical), and anhydrous diethyl ether (HPLC grade, Fischer Scientific, 99.9%) were purchased from their respective suppliers, respectively. HFB was distilled under reduced pressure 35 before drying. Water content was measured by Karl-Fischer titration method and controlled <20 ppm for all electrolyte solvents and additives. The baseline electrolyte is 1.0 M LiTFSI DOL/DME (1:1 volume ratio) + 0.1 M LiNO3. A fluorinated ether-based electrolyte – 1.0 M LiTFSI DOL/TTE (1:1 volume ratio) – containing 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE, Synquest

Laboratories) as co-solvent was used to verify the impact of HFB additive.

2.2 Li-S cell testing

2032-Coin cells were assembled in an argon-filled glovebox with O2 level <5 ppm. Lithium chips were used as the anode (15.6 mm in diameter, MTI), Celgard 2325 (16.0 mm in diameter) as separator with various electrolyte to sulfur (E/S) ratios. Sulfur loading is 1.4-1.8 mg/cm2. After cell assembly, the cells were transferred to a battery cycler (Maccor Test Unit 4100) and cycled at a C/10 current

(1C = 1675 mA/mg) with a cutoff voltage 3.0-1.6 V.

2.3 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was measured by Solartron Cell Test® 1470E with

ZView software. A frequency range from 1 MHz to 0.1 Hz and an amplitude of 10 mV was applied for performing the measurements.

2.4 Post-test Analysis

Cycled lithium anodes were rinsed with 2 mL of dry DME twice, then airtight transferred to the column of the SEM instrument for surface morphology analysis. FT-IR spectra were collected on a

Nicolet iS5 FT-IR Spectrometer (Thermo Fisher Scientific) using an attenuated total reflectance (ATR) attachment placed in an argon-filled glovebox. Cycled electrolyte was recovered from cells after disassembly by soaking the cell components in anhydrous diethyl ether (1 mL) overnight to both

36 dissolve analytes and desalt the electrolyte materials. The resulting supernatant was then passed through a 0.2 μm filter and then used for GC-MS analysis without further dilution.

3. Results and Discussion

Li2S Li2S (2.5 mM in DME) (2.5 mM in DME + HFB)

F rest F F

Li2Sx (x = 8) Li2Sx (x = 8) (2.5 mM in DME) (2.5 mM in DME + HFB) F F rest F

(a) (b) (c)

Figure 1. (a) Chemical structure of hexafluorobenzene – HFB, (b) coordination and reactivity of HFB

st with lithium sulfide Li2S and high-order Li2S8, and (c) 1 cycle charge and discharge voltage profiles for the Li-S cell using 1 M LiTFSI DOL/DME and 1 M LiTFSI DOL/DME + 10% HFB.

HFB was selected as a neutral molecule receptor which is capable of binding lone pair bearing molecules including anions through multiple pair-π interactions. HFB is susceptible to nucleophilic attack, and this type of reaction has been widely studied with a variety of nucleophiles. Sulfur-

2- containing nucleophiles of the type of Sx have been somewhat neglected. Since a series of polysulfides is generated during the discharge and charge process of Li-S battery and the low-order ones are known to be easily isolated and electrochemically inactive, we were curious about the impact of the HFB-Sx2- coordination/reaction on the cell performance. Figure 1b illustrates the strong pair-

2- π interactions between HFB and Sx by a solubility test. Apparently, when HFB was added, the suspension of Li2S in DME turned to a dark solution from the opaque suspension indicating a /reaction occurs between HFB and S2- anion; similarly, HFB also coordinates and reacts with the high-order polysulfides exemplified by Li2S8 solution in DME which changed from the initial dark-red solution 37 to a colorless one when HFB was added. This strong pair-π interaction was also observed electrochemically, i.e. the addition of HFB to the 1.0 M LiTFSI DOL/DME electrolyte significantly increases the dissolution of the lithium polysulfides causing the severe Li2Sx redox shuttling (flat, long charging plateau) as shown in Figure 1c.

3.1 Cell performance improvement

As shown in Figure 1c, the HFB cell suffers from severe shuttling effects due to the strong interaction of HFB with the discharged polysulfide products. To examine the positive effect of HFB on the cell performance, LiNO3 was added to the electrolyte to minimize the shuttling mechanism so that the regular cycling of HFB cell could be achieved as shown in Figure 2a. Surprisingly, during the charging process, an extra small “plateau” was shown on the voltage profile for all cells containing

HFB, indicating a new species formed likely from the complex of HFB with the lithium polysulfides.

This “plateau” remains persistent even after 50th cycle as shown in Figure 2b.

Compared with the baseline electrolyte, the delivered capacity is more stabilized for the 3% HFB cell and maintains high retention after 20 cycles as shown in Figure 2c. It is worth noting that all the sulfur electrodes used in this test were made by simply mixing commercial sulfur powder, carbon black and PVDF binder, and not optimized or specially designed.

However, further increase in HFB concentration (from 3% to 20%) deteriorates the cell performance in both capacity retention and the Coulombic efficiency, indicating the additional HFB participates in the parasitic reaction and causes deterioration of the cell performance (Figure 2d). Possible reactions of HFB with the polysulfide anions were proposed in Figure 3. HFB preferably interacts with Li2S,

2- a stronger nucleophile compared with polysulfide anions Sx , via a nucleophilic aromatic substitution

(SNAr) reaction generating lithium pentafluorophenyl sulfide (1) and LiF (Route a). Compound (1) can be reversibly oxidized (or charged) to bis(pentafluorophenyl) disulfide (3) and

38 bis(pentafluorophenyl) polysulfide (4) and reduced (or discharged) back to (1) in the cell following

Route d. The use of dimethyldisulfides (DMDS) and alkyl polysulfides to activate the dead lithium sulfides has been previously reported by other groups127-130 without clear understanding of the mechanism.

In an attempt to potentially identify these species, compound (1) and compound (3) was synthesized ex-situ. The synthesized compound (1) and (3) was then added to the baseline electrolyte and tested in the cell. The charge voltage profiles of HFB, compound (1) and compound (3) cells have exactly the same shoulder shown in the voltage range from 2.63 to 2.85 V during the charging process, as

2- shown in Figure 4, confirming compound (1) and (3) as the interaction products of HFB and Sx .

Further evidence for the formation of the above proposed species was provided by the post-test analysis of the cycled electrolyte.

However, when additional HFB are present, it interacts with high-order polysulfides as shown in

Route b generating lithium pentafluorophenyl polysulfide (2). Compound (2) can be further reduced to give (1) and Li2S, resulting in active material loss and the capacity fade.

3.2 3.0 0.1 M LiNO3 0.1 M LiNO3 0.1 M LiNO3+3wt% HFB 3.0 0.1 M LiNO3+3wt% HFB 0.1 M LiNO3+5wt% HFB 0.1 M LiNO3+5wt% HFB 2.8 0.1 M LiNO +20wt% HFB 0.1 M LiNO3+20% HFB 2.8 3 2.6 2.6 2.4 2.4 2.2 2.2 Voltage (V) Voltage Voltage (V) Voltage 2.0 2.0 1.8 1.8 1.6 (a) (b) 1.6 1.4 0 200 400 600 800 1000 1200 1400 0 100 200 300 400 500 600 700 800 Capacity (mAh/g) Capacity (mAh/g)

1200 100 0.1 M LiNO3 0.1 M LiNO3+ 3wt% HFB 0.1 M LiNO + 5wt% HFB 1000 3 90 0.1 M LiNO3+20wt% HFB 800 80 600 70

400 0.1 M LiNO3 0.1 M LiNO3+3wt% HFB Capacity (mAh/g) Capacity 60 0.1 M LiNO3+5wt% HFB 200 0.1 M LiNO3+20wt% HFB Coulombic Efficiency (%) Coulombic Efficiency (c) (d) 0 50 0 20 40 60 80 100 0 20 40 60 80 100 Cycle Number Cycle Number

Figure 2. (a) 1st cycle and (b) 50th cycle charge and discharge voltage profile of the Li-S cell using baseline electrolyte 1.0 M LiTFSI DOL/DME + 0.1 M LiNO3, baseline + 3% HFB, baseline + 5%

HFB, baseline + 20% HFB, respectively; (c) capacity retention with cycling number, and (d)

Coulombic efficiency with cycling number.

F F F F Li F F + S + LiF (a) Li F S + F F Li F F (1) + F F Li+ Li S S S F F + F F S S S S Li + S S S + LiF (b) F S S S F F F F (2) F F F F + F F Li [reduction] (c) + Li Li S S S S F S S S F S F F Li+ (1) F F F F F F F F [oxidation] F F F F F (d) S F or F S F S S [reduction] F F S S F F + Li F F F n F F (3) (4)

Figure 3. (a) Proposed nucleophilic aromatic attack of Li2S on HFB to produce lithium pentafluorophenyl sulfide (1), (b) lithium polysulfide Li2S6 with HFB generating lithium pentafluorophenyl polysulfide (2), (c) lithium pentafluorophenyl polysulfide reduced to (1) and Li2S, 40 and (d) reversible activation of low order Li2S back to the electrochemically active species (3) and

(4).

3.0 2.8 2.85 V 2.63 V 2.6 2.4 2.2 Voltage [V] 2.0

0.1 M LiNO3 0.1 M LiNO +10wt% HFB 1.8 3 0.1 M LiNO3+5wt% LiC6F5S 1.6 0.1 M LiNO3+5wt% C12F10S2 0 200 400 600 800 1000 1200 1400 Capacity (mAh/g)

Figure 4. 1st cycle voltage profiles of Li-S with baseline electrolyte 1.0 M LiTFSI DOL/DME + 0.1 M LiNO3 (black lines), baseline + 5wt% HFB (red lines), baseline + 5wt% compound (1) LiC6F5S (blue lines), and baseline + 5wt% compound (3) C12F10S2 (magenta line).

35 40 0.1 M LiNO3 0.1 M LiNO3 (b) (a) 0.1 M LiNO3+3wt% HFB 30 0.1 M LiNO3+3wt% HFB 35 0.1 M LiNO3+20wt% HFB 0.1 M LiNO3+20wt% HFB 30 25 SOC=0% SOC=0% 25 20 (Ohm) (Ohm) 20 img

15 img Z -Z 15 - 10 10 5 5 0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 40 Z (Ohm) Zreal (Ohm) real

Figure 5. (a) Nyquist plots of the Li-S cells with baseline electrolyte and HFB electrolytes (inset:

R|C equivalent circuit model), and (b) Nyquist plots of the same cells after one hundred charge- discharge cycles. All impedance measurements were performed at fully discharged state.

Figure 5 presents the Nyquist plots of the cells before cycling and after one hundred discharge-charge cycles with various concentrations of HFB. All cells showed similar R1 (Ohmic resistance of the cell) that ranges from 2.5 to 4.0 Ohm before cycling. After cycling, both R1 and Rint became smaller for the low HFB content cells compared with both baseline and 20% HFB cell, indicating the reutilization of the low-order Li2S and Li2S2 in the sulfur electrode. The biggest difference is the charge transfer resistance Rct shown in Figure 5b caused by the high concentration of HFB. Rct for the 20 wt% HFB cell increases dramatically after 100 cycles. This result is well correlated to the cell testing data that excess amount of HFB leads to the deterioration of cell performance and the possible mechanism discussed in Figure 3.

900 100 800 700 80 600 60 500 400 Baseline 40 Baseline 300 10wt%HFB Baseline + 10wt% HFB 15wt%HFB Capacity (mAh/g) 200 20wt%HFB 20 Baseline + 15wt% HFB

100 (%) Efficiency Coulombic Baseline + 20wt% HFB (a) (b) 0 0 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Cycle Number Cycle Number

Figure 6. (a) Capacity retention and (b) Coulombic efficiency of the Li-S cells with fluorinated electrolyte 1.0 M LiTFSI DOL/TTE baseline electrolyte, baseline + 10wt%, 15wt% and 20wt% HFB, respectively.

The strong interaction of HFB-Li2Sx was confirmed by the solubility data (Figure 1b) and the electrochemical test data (Figure 1c and Figures 2a-2d). However, due to the increased solubility of the sulfide species in the electrolyte, LiNO3 additive is mandatory to be present in the base electrolyte in order to evaluate its impact on the cell performance. Apparently, side reactions of HFB with LiNO3

42 thus altering the SEI chemical composition and morphology on Li become problematic. With increasing HFB concentration from 1% to 20%, the chemical composition and morphologies of the

SEI on cycled Li anodes dramatically changes. (FT-IR data shown in Figure 7 and SEM/EDS data shown in Figure 8 in the Supporting Information) with 20% HFB cell resembling that of the SEI formed in the LiNO3-free baseline electrolyte (Figure 8h).

-1 -1 -1 1455 cm 1281 cm 1074 cm -1 -1 1367 cm 1200 cm -1 DOL:DME 1515 cm

DOL:DME+0.1 M LiNO3

DOL:DME+0.1 M LiNO3+5wt% HFB

DOL:DME+0.1 M LiNO3+10wt% HFB

DOL:DME+0.1 M LiNO3+15wt% HFB

DOL:DME+0.1 M LiNO3+20wt% HFB

3000 2500 1500 1000 Wavenumbers (cm-1)

Figure 7. FT-IR spectra of Li anode harvested after 100 cycles from Li-S cells with 1.0 M LiTFSI

DOL/DME, baseline electrolyte DOL/DME + 0.1 M LiNO3, and baseline + 3wt%, 10wt%, and 20wt%

HFB, respectively.

Figure 8. SEM images of Li anode harvested from Li-S cells after 100 cycles with different electrolytes (a) 1.0 M LiTFSI DOL/DME, (b) baseline electrolyte 1.0 M LiTFSI DOL/DME + 0.1 M

LiNO3, and (c-h) baseline electrolyte + 1%, 3%, 5%, 10%, 15%, 20% HFB, respectively.

Figures 8c-8e show the surface morphology of the cycled anode in different electrolytes. Clearly, the addition of HFB and its different concentrations affect the SEI formation on the surface of lithium anode. The smooth surface indicates the fewer precipitation of Li2S and Li2S2. However, when the

HFB concentration increases, the surface of Li anode resembles the DOM/DME/LiTFSI cycled

44 electrode where large agglomeration of lithium sulfides due to the side reaction of HFB with LiNO3

(Figures 8f-8h).

To verify the impact of HFB additive in LiNO3-free electrolyte, a fluorinated ether-based electrolyte

1.0 M LiTFSI DOL/1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was used as a new baseline electrolyte. TTE as an electrolyte co-solvent has been thoroughly studied by our group132-134 and other groups135-137 and significant improvement in Li-S battery performance has been reported due to its low lithium polysulfide solubility and excellent lithium surface passivation capability.

Therefore, no LiNO3 additive is required for this electrolyte to eliminate the redox shuttling effect caused by the activation of low-order lithium polysulfides and dissolution of HFB-Li2Sx species resulting from the strong solubilizing effects of HFB. Compared with the DOL/DME and DOL/TTE cell, the DOL/TTE + HFB cells showed much improved capacity and capacity retention especially for the low HFB concentration as evidenced in Figure 6a. Counterintuitively, the higher HFB concentration actually slightly lowered both the capacity retention and the Coulombic efficiency. It is speculated that higher HFB content increases the solubility of the active sulfides, which detracts from the function of TTE co-solvent to suppress the solubility of sulfide species.

Figure 9. DFT optimized structures of encounter complex, transition state, and product from the reaction of HFB and Li2S showing a small barrier for the reaction (energy unit is in eVs).

Table 1. DFT calculated free energies ∆G relative to encounter complex for different lithium polysulfides (Li2Sx, x=1, 2, 3, 4, 6, and 8) with HFB additive.

Polysulfide Structure ∆G, eV (298 K) Encounter complex with HFB 0.0 Transition state 0.52 Li2S Product with HFB -3.08 Encounter complex with HFB 0.0 Transition state 0.75 Li2S2 Product with HFB -2.08 Encounter complex with HFB 0.0 Transition state 0.91 Li2S3 Product with HFB -1.61 Encounter complex with HFB 0.0 Transition state 1.12 Li2S4 Product with HFB -1.58 Encounter complex with HFB 0.0 Transition state 1.20 Li2S6 Product with HFB -1.17 Encounter complex with HFB 0.0 Li2S8 Transition state 1.19 Product with HFB -1.13

Density functional theory (DFT) calculations were then carried out to determine the mechanism by which the HFB might coordinate with low-order lithium sulfide (Li2S) and lithium disulfide (Li2S2) and subsequently react and convert both into active aromatic sulfides, thus facilitate the sulfur utilization and the cell performance. Calculations of the potential energy surfaces were performed with Gaussian 09, Revision E.01[44] with the B3LYP/6–311++G** method.[45] Zero-point energies were scaled by 0.9877.[46] The polarizable continuum solvation method was used for implicit solvation calculations of solvation energies.[47] The calculation results show that the nucleophilic aromatic attack of Li2S with HFB was found to have a barrier of 0.52 eV for conversion from an encounter complex C6F6...Li2S to a pentafluorophenyl sulfide-dilithium fluoride structure C6F6S-Li2F via a 46 transition state. The free energy and the predicted structure are illustrated in Figure 9. Compared with Li2S, the reaction of HFB with Li2S2 was found to have a slightly high barrier of 0.75 eV for conversion from an encounter complex C6F6...Li2S2 to a pentafluorophenyl disulfide-dilithium fluoride structure C6F6S2-Li2F. Same calculations were also performed for high-order polysulfide

2- anions (Li2Sx , x=3, 4, 6, 8) and the results are summarized in Table 1. The higher polysulfides had even larger energy barriers and were much less favorable compared with the low-order ones. This result is consistent with the mechanism by which HFB acts as an additive to preferably coordinate and activate the low-order polysulfides thus enhance the active material utilization and capacity retention of the Li-S cells.

4. Conclusions

A new functional electrolyte additive hexafluorobenzene (HFB) was reported for the first time for lithium-sulfur batteries. The impact of HFB was verified experimentally both in 1.0 M LiTFSI

DOL/DME + 0.1 M LiNO3 and 1.0 M LiTFSI DOL/TTE electrolyte cells. Although high concentration (>20wt%) additive causes detrimental effect on the former cell performance probably due to the interference with LiNO3, low concentration improved the capacity decay especially at the later stage of the cycling. DFT calculation results indicate that HFB is preferably coordinate with low- order polysulfides: Li2S and Li2S2 over higher-order Li2Sx (x>2) and further react forming redox- active species, thus reactivating the electronically isolated active materials back to the system and reduces the gradual capacity loss during later cycles. The reaction mechanism was proposed, and possible species was identified. This research opens a new approach to tacking the fundamental issue of the Li-S chemistry.

Chapter 3 A Systematic Research on the Relationship between Lithium Salt

Concentration and Li-S Battery.

Abstract

Salt concentration is a basic factor of electrolyte that is usually unconsidered during the design of electrolyte. Based on our previous experience, reducing the lithium salt concentration as an effective method to optimize 1,3-dioxolane (DOL)/ 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether

(TTE) electrolyte system is firstly reported in this paper. Decreasing the lithium salt concentration in

DOL/TTE electrolyte helps electrolyte releasing free DOL solvent molecules dissolving sulfur.

Therefore, the solvation of sulfur is easier, and electrolyte dissolve more sulfur than baseline electrolyte. The relationship between various electrochemical properties of lithium sulfur battery and a series of lithium salt concentrations is presented in this paper. According to the experimental results, choosing an appropriate salt concentration by balancing these properties in an ideal way for each electrolyte is more reasonable than using a certain salt concentration as a standard for all electrolytes.

This research introduces a new and easy approach to tacking the issue of Li-S chemistry.

1. Introduction

The requirement for energy increases continuously with time due to the growth of population and economy.1 To reduce the dependence on fossil fuel, it is essential to unearth alternative and renewable energy sources, such as solar and wind energies. However, since these renewable energy sources are intermittent, therefore, it is critical to develop efficient and economical storage of electricity.138

Lithium sulfur (Li-S) batteries have been considered to be promising replacement of lithium-ion batteries (LIBs) as the next generation energy storage due to their energy density are two to three folds higher than those of LIBs and the abundant source of raw material.10, 139

However, the large-scale commercialization of Li-S batteries have not been fulfilled over decades due to many problematic issues. The problems include poor cycle life, low coulombic efficiency and sulfur utilization and a high self-discharge rate.39, 139-141 All these problems are related to polysulfide shuttling effect, insulating properties of elemental sulfur 142 and loss of active material during the charging and discharging process.

Under normal conditions, sulfur atoms form cyclic octatonic crowns with a chemical formula S8.

During the discharge (lithiation), sulfur accepts electron and reduced stepwise to a series of high-

4 order polysulfide intermediates (Li2Sx, 3

In order to address these problems, tremendous efforts have been made by many research groups. Part of them have focused on optimizing sulfur electrodes by incorporating sulfur into various host materials, for example, carbonaceous materials.123, 144-146 However, the high cost and complex formation process of these electrodes are main barriers. Some of research groups apply appropriate additives to electrolyte to protect lithium anode or increase the utilization rate of active material.

23, 61, 66 LiNO3 has been utilized widely to passivate lithium anode. Phosphorus pentasulfide and lithium iodide are also reported to protect lithium anode.69, 71 Recently, organic disulfide, such as dimethyl disulfide, and hexafluorobenzene are applied as additives to reutilize the isolated lithium sulfide which has been deposited in cell and can’t be charged back.129, 147

Coordinating the lithium salt concentration applied in the electrolyte is also a simple and effective way to optimize the properties of lithium sulfur batteries. Minjeong Shin,148 Azusa Nakanishi,136

Marzieh Barghamadi,149 Hai Lu,150 Syed Abdul Ahad,151 Ryota Tamate,152 Yokoyama Yuko 153 and

Ka-Cheong Lau,154 reported that the usage of high lithium salt concentration in electrolyte help optimizing the properties of Li-S batteries, with other researchers. In addition, applying low salt concentration in electrolyte to optimize Li-S batteries have also been reported.155-156 However, there are few research group focusing on the relationship between a series of lithium salt concentrations and the electrochemical properties of Li-S battery especially using 1,1,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether (TTE) as one of the co-solvent. Numerous research groups set one molar as a standard lithium salt concentration rather than applying the appropriate lithium salt concentrations when designing different electrolytes for Li-S battery. We believe that there is an appropriate lithium salt concentration for every electrolyte that used in Li-S battery and several experiments and test are schemed out to verify our assumption.

We have reported the application of a low lithium salt concentration in another electrolyte system which contains a partially fluorinated ether like TTE as one of its co-solvents.157 It is possible to attempt adjusting the lithium salt concentration of electrolyte containing TTE.

In this paper, we studied how the a series of concentrations of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane (DOL)/TTE and DOL/dimethyl ether

(DME) based electrolytes impact on the electrochemical properties of lithium sulfur batteries, with the aim of carrying out further studies on the importance of applying appropriate lithium salt concentration into various electrolytes. We firstly report the impacts of a low lithium salt concentration applied in DOL/TTE electrolyte on the electrochemical properties of Li-S battery. We reported the electrochemical performances of Li-S battery and the harvested batteries and electrodes 50 were studied systematically by several characterized and analytical techniques such as air-free scanning electronic microscopy (SEM) and electrochemical impedance spectroscopy (EIS) attempting to explain the experimental results and learning the mechanisms. Our experimental results indicate that, in term of DOL/TTE electrolyte system, using an appropriate low concentration of

LiTFSI (0.5 M) optimizes the specific capacity and capacity retention but decreases the coulombic efficiency (CE) of lithium sulfur batteries. However, some reverse results are concluded from the batteries applying DOL/DME electrolytes that high concentration of LiTFSI (2 M) amplify the specific capacity and capacity retention but lessen the CE in first 50 cycles. These inverse results in different electrolyte systems confirm to our assumption that it is essential to choose appropriate concentrations of lithium salt for various electrolytes with the purpose of enhancing the properties of

Li-S batteries. The conclusion analyzed by the results of lithium symmetric cells and other electrochemical tests is that adjusting lithium salt concentration improve specific properties of battery but weaken others. Therefore, it is necessary to balance all properties that the salt concentration impacts before choosing an certain concentration of salt.

2. Materials & Methods

2.1 Materials

1,3-Dioxolane (DOL), lithium bis(trifluorosulfonyl)imide (LiTFSI, 99.95%) and polyvinylidene fluoride (PVDF) were purchased from Sigma-Aldrich. Elemental sulfur was purchased from Alfa

Aesar. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was purchased from Synquest

Laboratories. Carbon black (Super P) was purchased from Timcal. Water contents were measured by

Karl-Fischer titration method and controlled <20 ppm for all electrolyte solvents and additives. The baseline electrolytes are 1.0 M LiTFSI DOL/TTE and DOL/DME (1:1 volume ratio) + 0.2 M LiNO3,

51 and 0.1, 0.5, 2 M LiTFSI DOL/TTE and DOL/DME (1:1 volume ratio) + 0.2 M LiNO3 are testing electrolytes.

2.2 Bulk ionic conductivity

The bulk ionic conductivity of the electrolyte solutions was determined using stainless-steel coin cell assembly with a PTFE washer filled with electrolyte. The thickness of the

2 2 PTFE washer (l) was 0.177 cm and internal area (πr ) was 0.502 cm . The Ohm resistance (RΩ) was measured by electrochemical impedance spectroscopy at the highest frequency Z’ intercept from the

Nyquist plots, and the ionic conductivity (σ) was calculated using Eq. 1, where r is the internal radius of the washer and all other variables as previously defined.

= 1000 (1) 𝑙𝑙 2 𝜎𝜎 𝜋𝜋𝑟𝑟 ∙𝑅𝑅Ω ∙ 2.2 Li-S cell testing

Electrodes were fabricated by coating a slurry of carbon-sulfur composite, carbon black (Super P), and polyvinylidene fluoride binder in a weight ratio of 80:12:8 onto an aluminum foil. After drying at 50°C for 3 h, Sulfur loading is 1.4 - 1.5 mg/cm2. 2032-Coin cells were assembled in an argon-filled glovebox with O2 level <5 ppm. Lithium chips were used as the anode (15.6 mm in diameter, MTI),

Celgard 2325 (16.0 mm in diameter) as separator. Electrolyte to sulfur (E/S) ratio equal to 10. After cell assembly, the cells were transferred to a battery cycler (Maccor Test Unit 4100) and cycled at a

C/10 current (1C = 1675 mA/mg) with a cutoff voltage 2.6 V - 1.6 V.

2.3 Li-Li symmetric cell test

Li/Li symmetric cells were constructed with two identical 15.6 mm diameter Li metal electrodes and a glass fiber separator and 170 µL electrolyte. After assembly, the Li/Li symmetric cells were

52 transferred to the battery cycler and cycled galvanostatically at +/- 0.27 mA for 100 hours or until cell voltage reached +/-1 V. After the electrochemical protocol was completed, the cells were transferred back to an argon-atmosphere glovebox and disassembled for SEM characterization.

2.4 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) and lithium transference number were measured by

Solartron Cell Test® 1470E with ZView software. A frequency ranges from 1 MHz to 0.1 Hz at open- circuit voltage (OCV) and an amplitude of 10 mV was applied for performing the measurements.

2.5 Scanning Electron Microscope

Cycled lithium anodes were rinsed with 2 mL of dry DME twice, then airtight transferred to the column of the SEM instrument for surface morphology analysis.

3. Results and Discussion

To research and understand the impacts of a series of lithium salt concentrations on the Li-S battery completely, the ionic conductivity of each electrolyte was tested and shown in Table Ⅰ. The ionic conductivity of each DOL/TTE electrolyte is much smaller than the DOL/DME electrolyte when applying same concentration of LiTFSI. These results are consistent with a couple of paper132, 134, 157-

161 which have pointed out that although applying TTE as a solvent will passivate lithium anode and constrain the polysulfide shuttling, TTE has a low ionic conductivity and solubility of sulfur.

Therefore modifying the salt concentration applied in DOL/TTE electrolyte is an attempt to balance these properties and find an appropriate salt concentration to optimize Li-S battery that performing better than the one using one molar as a standard lithium salt concentration.

Table Ⅰ. Bulk Ionic Conductivity Data of Selected Electrolyte

[LiTFSI] (M) 0.1 0.25 0.5 0.75 1 2

DOL/TTE 0.084 0.23 0.72 1.39 1.79 2.01

DOL/DME + LiNO3 1.42 4.52 8.83 9.89 10.27 11.53

The passivation layer between lithium anode and electrolyte has been characterized and analyzed by

Li-Li symmetric cell. The voltage curves of batteries applying various concentration of LiTFSI have considerable differences between each other in DOL/TTE batteries as shown in Figure 1a. The 0.1

M batteries have the largest voltage in each cycle which is 0.2 V at beginning and 0.8 V after 100 hours. The large, unstable, and sharp peaks in these curves indicate that the passivation layer over lithium anode is unstable and formed continuously, so the layer should be thick and have a rough and irregular morphology. This inference has been approved by the large overpotential of the batteries and the SEM images of the lithium anodes harvested from these symmetric cells as shown in Figure

3e and 2k, respectively. The curves of other three groups are stable and similar in first 40 cycles, then the voltage of 0.5 M and 1 M batteries start increasing in different rates. The voltage of 1 M batteries increases from 0.02 V to 0.06 V in a slow rate and the voltage of 0.5 M batteries increases from 0.03

V to 0.2 V in a relatively high rate, which means the passivation layer of 2 M battery is the most stable and durable and the one in 0.5 M battery is the least among three groups. These results reveal that, when electrolyte solvent is DOL/TTE, LiTFSI plays an important role in passivating lithium anode and also the concentration of LiTFSI closely relate to the passivation of lithium anode. These conclusions are consistent with the characterizations concluded from the SEM images of lithium

54 anodes harvested from the symmetric cells as shown in Figure 2. The morphologies of passivation layers in 2 M and 1 M batteries are similar and the passivation layers are both smooth and integrated.

However, there are numerous fragments present on the passivation layer of 0.5 M batteries by observing at high resolution displaying that the passivation layer becomes unstable and more fragmentized than 2 M and 1 M batteries. Last, the morphology of passivation layers in 0.1 M batteries are rough and various cracks and shreds present on the surface of lithium anodes. It is hard to find an integrated passivation layer on the surfaces of lithium anodes. A large amount of lithium dendrites appear and overspread on the surface of lithium anode when observe the harvested lithium anodes at a very high resolution. These images point out the fact that the passivation layer is hardly formed and interphase between lithium and electrolyte in 0.1 M battery is unstable. Combine all these results, it is obvious that lithium salt (LiTFSI) plays an important role during forming passivation layer on lithium anode in the DOL/TTE battery and the stability and quality of the passivation layer would reduce if decrease the lithium salt concentration.

On the contrary, in Figure 1b, the voltage curves of batteries containing a series of concentration of

LiTFSI in DOL/DME batteries are stable and similar with each other. All curves are in the same voltage range from 0.02 V to 0.06 V. These results imply that, in a sulfur free environment, LiNO3 is the dominant factor of forming passivation layer in DOL/DME battery and it is likely that lithium salt impact slightly on the formation of passivation layer.

1.0 1 M LiTFSI 0.10 1 M LiTFSI 0.8 0.5 M LiTFSI 0.08 0.5 M LiTFSI 0.1 M LiTFSI 0.1 M LiTFSI 0.6 2 M LiTFSI 0.06 2 M LiTFSI 0.4 0.04 0.2 0.02 0.0 0.00 Voltage (V) Voltage Voltage (V) Voltage -0.2 -0.02 -0.4 -0.04 -0.6 -0.06 -0.8 a -0.08 b -1.0 -0.10 0 20 40 60 80 100 0 20 40 60 80 100 Time (Hour) Time (Hour)

Figure 1. (a) Voltage/Time curves of Li-S batteries containing various DOL/TTE electrolyte 1 M

LiTFSI, 0.1 M LiTFSI, 0.5 M LiTFSI, 2 M LiTFSI, respectively; (b) Voltage/Time curves of Li-S batteries containing various DOL/DME electrolyte 1 M LiTFSI, 0.1 M LiTFSI, 0.5 M LiTFSI, 2 M

LiTFSI, respectively.

Figure 2. (a-c) the SEM images of morphology of lithium anode harvested from li symmetric cell using 2 M LiTFSI DOL/TTE in different resolution 250 times, 1000 times, 4500 times, respectively;

(d-f) the SEM images of morphology of lithium anode harvested from li symmetric cell using 1 M

LiTFSI DOL/TTE in different resolution 250 times, 1000 times, 4500 times, respectively; (g-i) the

SEM images of morphology of lithium anode harvested from li symmetric cell using 0.5 M LiTFSI

DOL/TTE in different resolution 250 times, 1000 times, 4500 times, respectively; (j-l) the SEM

57 images of morphology of lithium anode harvested from li symmetric cell using 0.1 M LiTFSI

DOL/TTE in different resolution 250 times, 1000 times, 4500 times, respectively.

3.1 Electrochemical performance

Figure 3a, 3b and 3c, 3d show the cycling performances and CE of Li-S batteries containing

DOL/TTE and DOL/DME electrolytes with different lithium salt concentrations, respectively. 0.5 M batteries show the best specific capacities and capacity retentions among four DOL/TTE battery groups as shown in Figure 3a. Different from the DOL/TTE batteries, 2 M batteries present the best cycling performance among four DOL/DME batteries groups. Since the cycling performances of both baseline electrolyte batteries are not the best, therefore, our assumption has been confirmed that it is essential to choose appropriate lithium salt concentrations for different electrolyte systems to enhance the properties of Li-S batteries. On the other hand, the large difference between two ideal salt concentrations indicate that the mechanisms of choosing appropriate salt concentration are inconsistent.

Because TTE has a very low solubility of sulfur and there are possibly not enough DOL molecules dissolving sulfur if adding one molar LiTFSI into electrolyte. Therefore, for the DOL/TTE electrolyte system, it is probably that a low lithium salt concentration would release more DOL molecules as the free solvent molecules to dissolve more sulfur active materials during cycling. This inference is consistent with the results analyzed from the voltage profiles of DOL/TTE batteries. Figure 3e and

3f present the voltage profiles of the Li-S batteries using a variety of DOL/TTE electrolytes.

Comparing the red curves (0.5 M) with other three curves at both first and last cycle, the first discharge stage, which sulfur is dissolved into electrolyte as S8, of 0.5 M battery processes at a higher voltage and last for a longer time. Both characterizations suggest that batteries using 0.5 M LiTFSI dissolve more sulfur and do this more easily than other batteries at first discharge stage. Therefore, in Li-S 58 battery applying DOL/TTE electrolyte, lower the salt concentration increase the solubility and the utilization ratio of sulfur, resulting in a better cycling performance. However, it is worth noting that decreasing the concentration of LiTFSI also causes a more serious shuttling problem due to a higher solubility of polysulfide and forms an unstable passivation layer with worse quality than the one in baseline cell since there are less amount of salt participating in the passivation. These worse properties result in low CE, large overpotential, unstable, thick interface and a increasingly growth of lithium dendrites. Therefore, modifying the lithium salt concentration used in DOL/TTE electrolyte is trying to balance all the electrochemical properties that the salt impacts on and discover an acceptable lithium salt concentration to optimize the cycling performances of Li-S battery without sacrificing other properties hardly.

Compared with DOL/TTE electrolyte, the idea of optimizing lithium salt concentration applied in

DOL/DME electrolyte are dissimilar. DME has a much higher solubility of sulfur than TTE, so the goal for optimizing DOL/DME batteries should be increasing the lithium salt concentration to improve the ionic conductivity of DOL/DME electrolyte. Batteries applying 2 M LiTFSI in electrolyte show the highest specific capacity, capacity retention, ionic conductivity and least overpotential as shown in Figure 3c, Table Ⅰ, Figure 3g and 3h, respectively. These experimental results confirm the feasibility of the goal. Figure 3h shows more sulfur being utilized in 2 M batteries convincing the assumption that high ionic conductivity of electrolyte might helping to restrain the deposition of large crystalized Li2S which cannot be recharged in the battery and slow down the rate of capacity fading.

Likewise, it is necessary to balance the increasement of salt concentration and other properties.

Because as the growth of salt concentration, the raise of ionic conductivity of electrolyte becoming smaller and the solubility of sulfur and viscosity of the electrolyte come to be two important factors that need to be considered and balanced. Therefore, it is probably that the electrochemical

59 performances of Li-S battery hardly improve when applying an extremely large salt concentration in

DOL/DME electrolyte.

1 M LiTFSI 1000 1 M LiTFSI 0.5 M LiTFSI 0.5 M LiTFSI 0.1 M LiTFSI 900 0.1 M LiTFSI 100 2 M LiTFSI 2 M LiTFSI 800

700 95 600

500 90 Capacity (mAh/g) Capacity 400

300 (%) Efficiency Coulombic a b 200 85 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Cycle Number Cycle Number

1100 1 M LiTFSI 103 1 M LiTFSI 0.5 M LiTFSI 0.5 M LiTFSI 1000 0.1 M LiTFSI 102 0.1 M LiTFSI 2 M LiTFSI 2 M LiTFSI 900 101 100 800 99 700 98

Capacity (mAh/g) Capacity 600 97

Coulombic Efficiency (%) Efficiency Coulombic 96 500 c d 95 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Cycle Number Cycle Number

st 1 Cycle 90th Cycle 2.6 2.6

2.4 2.4

1 M LiTFSI 1 M LiTFSI 2.2 2.2 0.5 M LiTFSI 0.5 M LiTFSI 0.1 M LiTFSI 0.1 M LiTFSI 2 M LiTFSI 2.0 2 M LiTFSI Voltage [V] Voltage 2.0 Voltage [V] Voltage

1.8 1.8 e f 1.6 1.6 0 200 400 600 800 1000 0 100 200 300 400 500 600 Capacity (mAh/g) Capacity (mAh/g)

th 2.6 1st Cycle 2.6 90 Cycle

2.4 2.4

2.2 2.2

2.0

2.0 [V] Voltage Voltage [V] Voltage 1 M LiTFSI 1 M LiTFSI 0.5 M LiTFSI 0.5 M LiTFSI 0.1 M LiTFSI 0.1 M LiTFSI 1.8 1.8 2 M LiTFSI 2 M LiTFSI g h 1.6 1.6 0 200 400 600 800 1000 0 100 200 300 400 500 600 700 Capacity (mAh/g) Capacity (mAh/g)

Figure 3. (a, c) capacity retention with cycling number, and (b, d) Coulombic efficiency with cycling number of the Li-S cell using various DOL/TTE and DOL/DME electrolytes, respectively; (e, g) 1st cycle and (f, h) 90 cycle charge and discharge voltage profile of the Li-S cell using various DOL/TTE and DOL/DME electrolytes, respectively.

SEM characterization of lithium anodes harvested from the Li-S cells containing various DOL/DME electrolytes are shown in Figure 4. The morphology of lithium anode in 2 M batteries are the best among four battery groups which show the most smooth and integrated passivation layers. In other three battery groups, there are a large amount of grains, fragmentized particles and cracks founded on the passivation layers of lithium anodes. These differences on the morphology of passivation layers reveal that lithium salt also take part in the formation of passivation layer together with LiNO3 and

DOL/DME solvents in Li-S battery, and a higher salt concentration helps the battery forming a more stable and sturdy passivation layer. This conclusion are identical with some papers162-164 that lithium salt is likely to react with electrolyte containing LiNO3 and lithium anode forming passivation layer.

Figure 4. (a-c) the SEM images of morphology of lithium anode harvested from li symmetric cell using 0.1 M LiTFSI DOL/DME + 0.2 M LiNO3 in different resolution 240 times, 500 times, 1000 times, respectively; (d-f) the SEM images of morphology of lithium anode harvested from li symmetric cell using 0.5 M LiTFSI DOL/DME + 0.2 M LiNO3 in different resolution 240 times, 500 times, 1000 times, respectively; (g-i) the SEM images of morphology of lithium anode harvested from li symmetric cell using 1 M LiTFSI DOL/DME + 0.2 M LiNO3 in different resolution 240 times, 500 times, 1000 times, respectively; (j-l) the SEM images of morphology of lithium anode harvested from

62 li symmetric cell using 2 M LiTFSI DOL/DME + 0.2 M LiNO3 in different resolution 240 times, 500 times, 1000 times, respectively.

Figure 5a and 5b presents the Nyquist plots of the batteries after ninety discharge-charge cycles with various concentrations of LiTFSI in DOL/TTE and DOL/DME electrolytes, respectively. In Figure

5a, 0.1 M batteries show the largest R1 (Ohmic resistance of the cell) among four battery groups, 0.5

M group is the second large and other two battery groups have similar R1. These results are consistent with the ones collected from bulk ionic conductivity tests. On the contrary, in Figure 5b, all cells showed similar R1 that ranges from 7.5 to 9 Ohm which indicates that bulk electrolytes which have high enough ionic conductivities have similar R1. In Figure 5a, only the interface in 0.1 M batteries are thick enough so that Nyquist plot can identify the Rct and Rint. 0.1 M batteries have the largest R

(R= Rct + Rint), 0.5 M battery shows the second large of R and 2 M battery shows the smallest R.

These results indicate that due to the thicker, rougher, and unstable interphase in battery applying low salt concentration also cause a high resistance of battery. Therefore, the resistance of battery is another property that need to be considered during modifying the salt concentration for DOL/TTE electrolyte.

The identical results concluded from Figure 5b, in DOL/DME batteries, a smooth and sturdy interphase in battery also results in a relatively small resistance R.

700 1 M LiTFSI 0.5 M LiTFSI 600 0.1 M LiTFSI 2 M LiTFSI 500 SOC=0% 400 LiTFSI DOL/TTE

300 Z'' (Imaginary) Z'' Z'' (Imaginary) Z'' 200

100 a

0 0 100 200 300 400 500 600 700 Z' (Real) (Ohm)

Figure 5. (a, b) Nyquist plots of the cells using various DOL/TTE and DOL/DME electrolytes after ninety charge-discharge cycles, respectively. (inset: R|C equivalent circuit model) All impedance measurements were performed at fully discharged state.

4. Conclusion

The relationships between a series of lithium salt concentration and the electrochemical properties of

Li-S battery applying two electrolyte DOL/TTE and DOL/DME was reported. In both electrolyte groups the performances of baseline batteries were not the best, which convince our assumption that there is an appropriate concentration of lithium salt for each electrolyte system that used in lithium sulfur battery. The idea of introducing an appropriate low lithium salt concentration into DOL/TTE electrolyte system to optimize the cycling performances of lithium sulfur battery was reported for the first time. Apply a low lithium salt concentration in DOL/TTE electrolyte would release more DOL molecules as the free solvent molecules which could dissolve more sulfur and the dissolving process are also easier than baseline electrolyte. Therefore, the decreasing of lithium salt concentration is an effective way to increase the specific capacity and capacity retention. However, it was worth noting that reducing the salt concentration would sacrifice several electrochemical properties of Li-S battery to a certain degree, so it is necessary to balance all properties influenced by salt concentration in an

64 ideal way to maximize the optimization without sacrificing other properties hardly and finally choosing an appropriate lithium salt concentration for Li-S battery. This research a new and easy approach to tacking the issue of Li-S chemistry.

Chapter 4 Understanding the Impact of a Nonafluorinated Ether-Based

Electrolyte on Li-S Battery

Abstract

The impact of the electrolyte solvation structure on the performance of Li-S battery was investigated using an electrolyte system containing various solvent ratios of DOL and a nonafluorinated ether:

1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether (TPE). The cell testing results indicate that increasing the TPE ratio led to both a higher discharge capacity and a higher Coulombic efficiency.

The Li electrodes from the Li/Li symmetric cells showed that the one in higher TPE-content electrolyte has a smoother and more continuous SEI layer. AIMD simulation revealed the solvation structure of DOL/TPE – lithium polysulfide changes when the TPE concentration increases, which dictates the lithium polysulfide solubility in such electrolyte. These results shed light on the underpinned mechanism of the improved Coulombic efficiency and cycling performance of the Li-S cell.

1. Introduction

The consumption of limited fossil fuel resources is continuously rising during the past two decades,132,

165 and atmospheric pollution and consequent climate changes caused by using fossil fuel166 becomes more serious. These problems have prompted interest in renewable energies, such as water, wind, solar energies, to replace traditional fossil fuel.167 Among the various kinds of electrical energy storage systems, the Li-ion battery (LIB) has been one of the most competitive candidates for energy storage due to their high theoretical energy density and long calendar life.132, 165-170 Nevertheless, the energy density of most commercial LIBs are about 200−300 Wh/kg117, 132 which has already been a limitation when applying them in pure electric vehicles. The limited energy density of LIBs results in a short driving range (i.e., <300 km) for pure electric vehicles.15, 117, 171 In order to achieve the

66 requirement of a higher energy density, the lithium sulfur (Li-S) battery is a promising candidate to replace traditional LIB as a next generation energy storage device.1, 12 The theoretical charge-storage capacity of sulfur (1675 mAh/g) is an order of magnitude higher than that of the traditional insertion-

1 compound cathodes (e.g., LiCoO2, LiNi0.5Mn0.3Co0.2O2, and LiFePO4), and also sulfur is the cheapest solid-state cathode material for energy storage devices.

The capacity of Li-S batteries comes from the reversible redox reaction S + 2Li ↔Li2S. The final insoluble product Li2S is generated during the last redox reaction. Before the Li2S is produced, sulfur is reduced to various long-chain polysulfide intermediates, which are soluble in the organic electrolyte.4, 172 The polysulfides can have a negative impact on the Coulombic efficiency (CE) and capacity of the Li-S battery:120, 173-174 during charging and discharging, various kinds of soluble higher-order lithium polysulfides Li2Sx (4

Polysulfide redox shuttling causes extremely low CE because it shortens the discharge step and extends charge step of a voltage-limited battery, and it also contributes to capacity fading of the battery during cycling due to the loss of the active material from the deposition of final discharged product, Li2S, on lithium anode.

The instability of the Li anode is also a main obstacle for the commercialization of Li-S batteries.27,

167 The volume change causes cracks and fissures in the solid-electrolyte-interface (SEI) that results in continuous electrolyte and Li consumption by reacting with each other or with dissolved polysulfide during cycling. This finally results in a thick SEI and inhomogeneous deposition of Li 67 dendrites,28 the former causes high impedance in batteries, the latter is likely to pierce the separator,27-

28 causing a short circuit of the cell. The polysulfide redox shuttling phenomenon and stabilizing the

Li anode/electrolyte interface can be alleviated by forming a resilient and robust passivation SEI layer, which can help overcome the poor cycle life and low sulfur utilization of Li-S batteries.133

177 porous carbon,121, 145 carbon nanotube,146, 178 and graphene179-180), conducting polymers,122 metal oxides,124 and metal or covalent organic frameworks.181 These host materials can promote electron transfer, trap the polysulfide to constrain the redox shuttling process and relieve the volume expansion of Li-anode. However, the high cost and complex formation process of these electrodes are main barriers. In another direction, some of research groups add additives to the electrolyte helping to form a protective passivation SEI layer over the Li-anode. For example, LiNO3 has been utilized by many

5 researchers. Mikhaylik et al. found that LiNO3 can suppress polysulfide shuttling, this result was explained by Aurbach et al. by LiNO3 decomposing at the lithium surface, preventing reactions

23 between lithium and polysulfide. However, as an additive, LiNO3 has several drawbacks. Firstly,

66 LiNO3 is consumed gradually during cycles and also decomposes at the cathode below 1.8 V.

Furthermore, LiNO3 is easily to be oxidized, in pouch cells to produce extreme gassing that causes serious safety issues.182 Taking into consideration the issues discussed above, new electrolytes are considered to alternative methods for the development of reliable Li-S batteries are desirable. New electrolyte design and materials have been attempted to solve these issues.183 One of the prominent strategies to mitigate the lithium polysulfide crossover is highly concentrated electrolytes, also described by different groups as sparingly solvating electrolytes, salt-in-solvent electrolytes, solvate

68 ionic liquids or localized high-concentration electrolytes for lithium-ion battery, to drastically

184-185 minimize Li2Sx dissolution. Examples for Li-S battery application include

186 187 (CH3CN)2LiTFSI/TTE, and (glyme)/LiTFSI (triglyme or tetraglyme).

Previously, a great improvement on the electrochemical performance of lithium-sulfur cells has been reported by using a fluorinated ether -1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether -TTE

Figure 1a as cosolvent blended with 1,3-dioxolane (DOL)132-134, 169, 188

In this paper, we studied a new nonafluorinated ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3,3- pentafluoropropyl ether (TPE) (Figure 1b), to a Li-S electrolyte system. The TPE has one more F substituting H compared with previously reported TTE, therefore TPE has a higher fluorination degree compared with TTE. We report the electrochemical performance of the Li-S battery, which composed of a sulfur composite cathode, a DOL/TPE blended electrolyte and a lithium-metal anode.

In order to understand the mechanism in the new Li-S system, the electrode and electrolyte were studied systematically by electrochemical methods, analytical technique such as air-free scanning electronic microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and ab-initio molecular dynamics simulation.

Overall, a comparison of cells containing different solvent ratios of DOL/TPE show that increasing the TPE concentration leads to both an improved discharge capacity and higher CE due to the solvation structure change and its effect on suppressing the dissolution of Li2Sx and redox shuttling behavior.

F F F F F F F F F O F O F F F F F F F

(a) (b)

Figure 1. (a) Chemical structure of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) and

(b) 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether (TPE).

2. Experimental

Materials. Nonafluorinated ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether (TPE) was purchased from Manchester Organics (97%) and dried with 4Å molecular sieves before use. 1,3- dioxolane (DOL) was purchased from Sigma-Aldrich, vacuum distilled, dried over 4Å molecular sieves for 24 h, and filtered before use. Lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) was purchased from Sigma-Aldrich (99.95%) and dried at 150°C under vacuum overnight. 0.5 M LiTFSI solutions were formulated with 4:1, 2:1, or 1:2 DOL: TPE solvent ratios by volume. The synthesis and electrochemical performance of porous carbon used in this study has been reported before.189

Carbon-sulfur composites were prepared via melt-infiltration method. Typically, carbon and sulfur powders in a weight ratio of 1:4 were mixed in a mortar and pestle to disperse the sulfur uniformly into the carbon. The mixture was then sealed in a stainless-steel pressure reactor and heated at 160°C for 18 h. Electrodes were fabricated by coating a slurry of carbon-sulfur composite, carbon black

(Super P), and polyvinylidene fluoride binder in a weight ratio of 80:12:8 onto an aluminum foil.

After drying at 60°C for 12 h, the sulfur loading was determined to be 1.2 mg/cm2. Lithium metal electrodes (MTI) were 15.6 mm in diameter. Polyolefin separators (Celgard 2325) punched to 16 mm diameter were used with Li-S cells, and glass fiber (Fisher Scientific) separators were used with the

Li/Li symmetric cells.

Li-S battery cycling test. 2032 coin cells were used for all experiments, and all coin cell assembly was conducted in an argon-atmosphere glovebox. Sulfur electrodes were punched to 14.2 mm disks and dried at 50°C in an Ar-filled glovebox before use. The coin cells were assembled using 20 µL electrolyte per milligram of sulfur, with half of the electrolyte added on the Li anode and the other half on the separator. After assembly, the cells were transferred to a battery tester (Maccor 4100) and cycled at a C/10 rate from 2.6 to 1.0 V, with C-rate corresponding to the theoretical capacity of the sulfur content of the electrode (1675 mA/g of sulfur).

Li/Li symmetric cell test. Li/Li symmetric cells were constructed with two identical 15.6 mm diameter

Li metal electrodes and a glass fiber separator and 170 µL electrolyte. After assembly, the Li/Li symmetric cells were transferred to the battery cycler and cycled galvanostatically at +/- 0.27 mA until 1.5 mAh charge had passed, or until cell voltage reached +/-1 V, for 50 cycles. After the electrochemical protocol was completed, the cells were transferred back to an argon-atmosphere glovebox and disassembled for FT-IR and SEM characterization.

SEM characterization. Scanning electron microscopy (SEM) and energy dispersive spectroscopy

(EDS) were performed with a JEOL JSM-6610LV SEM and Oxford Instruments XMax detector with

AZtecEnergy software, respectively. Lithium anodes harvested from the Li/Li symmetric cells were rinsed in 2 mL of dry DOL twice and dried via evaporation. The Li samples were mounted inside an air-tight transfer holder and placed into the SEM column.

FT-IR measurement. FT-IR spectra were collected on a Thermo Scientific Nicolet iS5 FT-IR equipped with a Thermo Scientific iD5 ATR attachment placed in an argon filled glovebox. Pieces of the anode were gently pressed unto the ATR attachment and the spectrum collected for 1000 scans at 0.482 cm-

1 resolution with background subtraction.

Simulations. Ab-initio molecular dynamics (AIMD) calculations was used within the generalized

71 gradient approximation in the form of the exchange-correlation functional developed by Perdew,

Burke, and Ernzerhof (PBE)190 as implemented in the Vienna ab initio Software Package (VASP).191-

194 The energy cutoff for the plane waves was set at 600 eV, and the Brillouin zone was sampled using the Г point. The electrolyte systems are created in a 15×15×15 Å supercells with ~300 atoms in all cases and considering periodic boundary conditions in all directions. Volume ratios of 4:1, 1:1 and

1:2 for DOL to TPE are considered for the electrolyte; these molecules are initially assigned random positions and orientations inside the cell. The number of molecules of each compound is estimated to be consistent with volume ratios and experimental densities. However, we are still limited to a relatively small supercell size due to the computational complexity of the system; this means that adding one molecule of Li2S6 in this volume represents a concentration close to 0.5 M, which is large in comparison with the experimental values. Nevertheless, the locality of the solvation as well as the comparison among the volume ratio continues to be valid and complementary to the experimental results. Each electrolyte was equilibrated at 300 K for 14 ps using a time step of 1 fs; the temperature was maintained using a Nose-Hoover thermostat.

3. Results and Discussion

3.1 Electrochemical performance

The cycling metrics of three DOL/TPE/LiTFSI electrolytes in the Li-S batteries are shown in Figure

2. In Figure 2a, the initial discharge capacity ranges from 1100-1170 mAh/g for these three kinds of electrolytes, with final gravimetric capacities of 630, 660, 770 mAh/g for the 4:1, 1:1, and 1:2 ratios, respectively. Figure 2b shows the CE of the three electrolytes surveyed, with stark differences in the measured CE. It should be noted that as the electrochemical protocol begins with a discharge step followed by a charge step, the CE is calculated from the (n+1th discharge)/(nth charge), otherwise CE

72 would be artificially inflated and could be higher than 100% for some cells. In Figure 2b, it can be seen that the 4:1 ratio leads to exceptionally low initial CE of ~50%, although capacity loss from one cycle to another is much lower than the CE indicates. Throughout the cycling protocol, the CE of the

4:1 ratio and 1:1 ratio increases, while the 1:2 ratio stays roughly constant at 99%.

Figures 2a-2b indicate some clear insight into the stabilities of both electrode-electrolyte interfaces with the various electrolytes. First, the reasonably stable cycling (55% retention after 50 cycles) shows that with the 4:1 ratio, the low CE and large capacity of irreversible reactions can be attributed to shuttling reactions. These shuttling reactions therefore preclude the existence of a properly passivated

Li electrode, as a functional SEI would prevent the reduction of the shuttling component. The high

CE and stable cycling of the 1:2 ratio therefore indicates either the presence of a functional SEI on the lithium or significantly lower solubility of the shuttling component. Both of these qualities of an electrolyte could be tied to the relative composition and lithium salt concentration used.

1200 100

1000

800 80

600

400 60 4:1, 0.5 M LiTFSI 4:1, 0.5 M LiTFSI 200 1:1, 0.5 M LiTFSI (%) Efficiency Coulombic 1:1, 0.5 M LiTFSI Discharge Capacity (mAh/g) Capacity Discharge (a) (b) 1:2, 0.5 M LiTFSI 1:2, 0.5 M LiTFSI 0 40 0 10 20 30 40 50 0 10 20 30 40 50 Cycle Number Cycle Number

Figure 2. (a) Li-S cell capacity with 4:1, 1:1, and 1:2 DOL/TPE/LiTFSI electrolyte and (b) corresponding Coulombic efficiency. (Average sulfur loading was 1.2 mg/cm2, E/S = 20 (mL/g), current density was 2.01 mA/cm2).

In order for an electrolyte to be ionically conductive, there must be a lithium salt that then dissociates to form mobile Li ions. In traditional Li-S batteries with organic electrolytes composed of DOL and

1,2-dimethoxyethane (DME), both solvents can solvate the lithium salt. If these electrolytes are not saturated with the lithium salt, free, uncoordinated solvent molecules can then solvate the intermediate discharge products, polysulfides of the form Li2Sx. Lithium polysulfides have been shown to participate in shuttling reactions in the literature42, 162, 195-197. Furthermore, these undercoordinated solvents can also participate in the dissolution of SEI components on the lithium electrode, which are intrinsically lithium salts as well.

For this system, using the TPE co-solvent the sole coordinating solvent component is DOL. As the solvent ratio of DOL:TPE decreases (with a constant lithium salt), the concentration of undercoordinated DOL is decreasing as well. As was shown in previous work with a fluorinated ether cosolvent, lithium polysulfide solubility decreases, and CE increases substantially with a non- solvating co-solvent.

Figures 3a shows the voltage profiles of cells contain three testing electrolytes at the first cycle. For the discharge curves, while the 4:1 ratio shows a high (~2.1 V) plateau, the lower DOL ratio electrolytes show a depressed plateau, with the 1:2 showing a plateau of ~1.7 V. At the end of the discharge plateaus, the 4:1 ratio shows a sharper polarization (more vertical line), while the 1:2 ratio shows a more sloped profile. The differences in plateau voltages for the three electrolytes on discharge are not mirrored for the subsequent charge, as the charge voltage plateaus all occur at approximately the same voltage.

Figures 3b-3c summarize the voltage profiles at cycles 1, 10, 20, 30, 40, and 50 with various electrolytes. First, looking at the 4:1 electrolyte, the initial discharge has both the largest capacity and

74 highest plateau of all subsequent cycles. The overpotential on both discharge and charge cycles grow monotonically throughout the cycling protocol, with a commensurate decrease in cycling capacities, while the general shape is preserved. The 1:1 ratio shows a slightly different trend, in that the first discharge shows a depressed voltage profile, which may be due to electrode wetting. The first charge profile and subsequent discharge profiles (10th and 20th cycle) do not show a significantly higher overpotential than the 4:1, so this increased overpotential appears to be limited to the initial charge.

However, in contrast to 4:1 ratio, both discharge and charge overpotential of 1:1 ratio group increase steadily and largely over the rest of the cycling protocol, suggesting a rise of interfacial impedance.

The 1:2 ratio shows a large overpotential on all discharge voltage profiles, with an initial plateau of roughly 1.9 V.

2.6 st 1 Cycle 4:1 DOL/TPE 0.5M LiTFSI 2.4 2.6 2.4 2.2 2.2 2.0 2.0 1.8 1.8 Voltage Voltage (V) Voltage 1 1.6 1.6 10 20 1.4 4:1 0.5 M 1.4 30 1:1 0.5 M 1.2 1.2 1:2 0.5 M 40 (a) 50 1.0 (b) 1.0 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600 Capacity (mAh/g) Capacity (mAh/g) 2.6 1:1 DOL/TPE 0.5M LiTFSI 1:2 DOL/TPE 0.5M LiTFSI 2.6 2.4 2.4 2.2 2.2 1 2.0 1 10 10 2.0 20 1.8 20 30

Voltage 30 1.8 1.6 Voltage 40 40 1.6 50 1.4 50 1.4 1.2 1.2 1.0 (c) 1.0 (d) 0 200 400 600 800 1000 1200 Capacity (mAh/g) 0 200 400 600 800 1000 1200 Capacity (mAh/g)

Figure 3. (a) 1st cycle voltage profiles of Li-S cells contain three testing electrolytes (4:1, 1:1, and 1:2 ratios of DOL:TPE), and voltage profiles of the Li-S cell with (b) 4:1 ratio DOL:TPE electrolyte, (c)

1:1 ratio DOL: TPE electrolyte, and (d) 1:2 ratio DOL:TPE electrolyte. (Average sulfur loading was

1.2 mg/cm2, E/S = 20 (mL/g), current density was 2.01 mA/cm2).

3.2 Post-test analysis

Solvation structure plays a significant role in the cell performance. Furthermore, the electrolytes also showed different passivation capability on the surface of lithium anode when different electrolytes were used as indicated by the Coulombic efficiency data shown in Figure 2b. To understand if the lithium anode passivation also impacts the cell performance, Li/Li symmetrical cells were assembled and cycled repeated with different electrolytes. Figure 4a showed the voltage profiles of Li/Li cell cycled with three electrolytes. These Li/Li symmetric cells showed minimum overpotential growth after 220 hours deposition and stripping, indicating that TPE-based electrolytes are able to passivate the lithium anode. However, the solid-electrolyte-interface (SEI) formed by the electrolytes differs in terms of morphology and chemical composition. The harvested lithium anode was then subject to

SEM and FT-IR analysis. Figure 4b showed the FT-IR spectra of the lithium anode harvested from the Li/Li symmetric cells. Characteristic broad peaks associated with the decomposition/polymerization of the DOL were observed from the 4:1 DOL:TPE cycled anode, different vibrational peaks that originated from the fluorinated species were observed from 1300 cm-

1 to 1000 cm-1 for the both 1:1 and 1:2 DOL:TPE electrolytes. These peaks did not simply arise from a component of an initial electrolyte component as evidenced by the FT-IR spectra collected for the individual solvent DOL and TPE.161 Both SEM and the FT-IR data indicate that higher TPE concentration electrolytes dominates the SEI process on the surface of lithium anode rather than the

DOL forming the protection layer and preventing the redox reaction of dissolved Li2Sx in the electrolyte.

(b) 100 DOL/TPE 4/1 (a) DOL:TPE 1:2 DOL/TPE 1/1 DOL/TPE 1/2 50

DOL:TPE 1:1

0 Voltage (mV) Voltage DOL:TPE 4:1 -50

-100 0 40 80 120 160 200 240 Time [h] 3000 2500 1500 1000 Wavenumbers (cm-1) Figure 4. (a) Voltage profiles of Li-Li Symmetric cells contain three testing electrolytes (4:1, 1:1, and 1:2 ratios of DOL: TPE), (b) FT-IR spectra of the Li anode harvested from the cycled Li/Li symmetric cells.

Figures 5 are the SEM images of cycled lithium anode from Li/Li symmetric cells. Figure 5a shows the lithium morphology cycled in 4:1 DOL:TPE electrolyte. The surface was heavily deposited by the decomposition products of the electrolyte with large isolated agglomeration caused by the polymerization of DOL. When the TPE concentration is increased in the electrolyte, the lithium morphology changes. Figures 5b-5c show the SEM images of lithium cycled with the higher TPE concentration electrolytes (1:1 and 1:2 DOL:TPE). The surface of the lithium was transitioned to a more smooth surface with fine distributed particles (Figure 5b) and more integrated and homogeneous without large particles and no cracking was observed (Figure 5c) for the anode cycled with the highest TPE concentration (1:2 DOL:TPE), which significantly contrasts with that shown in

Figure 4a. Figures 5d-5f are the magnified SEM images of Figures 5a-5c. Electrochemical data and

77 post-test analysis all indicate that lithium anode surface passivation and the electrolyte solvation power of Li2Sx contribute simultaneously to the improved Li-S cell electrochemical performance.

Figure 5. (a-c) SEM images of lithium anode harvested from Li/Li symmetric cells containing three electrolytes (4:1, 1:1, and 1:2 ratios of DOL:TPE) and (d-f) magnified SEM images of (a-c).

3.3 AIMD Simulation

To deeply understand the improvement in Li-S cell with TPE based electrolytes, the solvation structure was calculated by AIMD simulation for the electrolyte, the isolated Li2S6 molecule, and electrolyte in the presence of a Li2S6 molecule. The solvation energy is defined as = E

elec+Li2S6 E + E , where E , E and E represent the free energy of Δthe𝐸𝐸 last AIMD step−

elec Li2S6 elec+Li2S6 elec Li2S6 for� electrolyte in� the presence of the Li2S6, the electrolyte and the isolated Li2S6, respectively. Under this definition, the most negative solvation energy represents the strongest solvation energy of the lithium polysulfide in the electrolyte. We also found negligible charge transfer between the electrolyte and polysulfide by performing Bader analysis using the code developed by Henkelman et al. 198

Figure 6 showed the Li2S6 solvation structures in the DOL/TPE electrolytes, along with the solvation energies from the last step of the calculation. The most negative solvation energy is represented by the 4:1 volume ratio at -1.61 eV, followed by the 1:1 volume ratio at -0.92 eV and 1:2 volume ratio at -0.41 eV. From these results, the magnitude of the solvation energy of the Li2S6 depends on the concentration of TPE concentration in the electrolyte.

The radial pair distribution function was calculated using the average trajectories of the molecules over the last picosecond of the calculation. As shown in Figures 7a-7c, the first solvation shell of the

Li2S6 (located around 3Å) is composed by DOL molecules for all three volume ratios considered, while the TPE molecules are located in outer shells. The location of the closest TPE molecules also shows proportionality with the concentration, for 4:1 volume ratio electrolyte, the TPE appear at around 7 Å from the Li2S6, for 1:1 volume ratio at around 6 Å and finally at 5 Å for 1:2 volume ratio.

The coordination number of the Li2S6 was also calculated and the data were shown in Figures 7d-7f.

These results reveal that as the TPE molecules reach closer to the LiPS, the solvation with DOL molecules presents a coordination number in the first shell of ~1.8 at the 4:1 volume ratio, and ~1.5 and 0.5 for 1:1 and 1:2 volume ratio.

Based on these calculations, as the concentration of TPE increases, the number of DOL molecules in the immediate vicinity of the LiPS decreases, thus lowering the solvation energy and consequently decreasing the solubility of the LiPS in the electrolyte. This result correlates well with the Coulombic efficiency and cycling performance.

Figure 6. Solvation structures and energies of Li2S6 in 4:1, 1:1 and 1:2 DOL:TPE electrolytes.

0.12 Li2S6-TPE 0.12 Li2S6-TPE 0.12 Li2S6-TPE (c) (a) (b) Li S -DOL Li S -DOL Li S -DOL 2 6 0.10 2 6 0.10 2 6 0.10 0.08 0.08 0.08

g(r) DOL:TPE 1:1 g(r) g(r) DOL:TPE 1:2 0.06 0.06 DOL:TPE 4:1 0.06 0.04 0.04 0.04 0.02 0.02 0.02 0.00 0.00 0.00 2 4 6 8 10 2 4 6 8 10 2 4 6 8 10 r(A) r(A) r(A)

10 10 Li S -TPE Li S -TPE 10 2 6 (d) 2 6 (e) Li2S6-TPE (f) Li S -DOL Li S -DOL 8 2 6 8 2 6 8 Li2S6-DOL

6 DOL:TPE 4:1 6 DOL:TPE 1:1 6 DOL:TPE 1:1 4 4 4 2 2 2 Coordination Number Coordination Coordination Number Coordination Coordination Number Coordination 0 0 0 2 4 6 8 10 2 4 6 8 10 0 2 4 6 8 10 r(A) r(A) r(A)

Figure 7. (a-c) Pair distribution function g(r) for the DOL and TPE molecules with respect to the

Li2S6, and (d-f) coordination number for the DOL and TPE molecules with respect to the Li2S6. For all cases the large peak at ~3Å in the g(r) represents the first solvation shell of the lithium polysulfide, which is carried out exclusively by the DOL molecules.

Figure 8. (a) Top and side view of the coordinates of TPE with Li (001) surface in the lowest reaction energy configuration before performing electronic optimization, and (b) top and side view of the TPE-

Li interaction after electronic relaxation. (Green, brown, gray and pink represent Li, C, F, and H atom, respectively).

We also simulated the interaction of TPE with the lithium. TPE molecules are adsorbed on the Li

(100) surface with a binding energy of -0.28 eV with O atom in TPE as close as possible to the Li surface. It has been reported that the absorption of a Li atom with TPE in exchange for an H atom is also energetically favorable when it is considered that the H atom is adsorbed by the Li surface.161

To complement the study of that reaction, we calculated the adsorption of the TPE configuration with the lowest reaction energy over the Li slab with an H atom on the surface. This calculation is illustrated in Figure 8. After the electronic relaxation, the Li atom that was initially attached to the molecule returns to the surface, on top of that, a fluorine atom migrates from the TPE towards the surface, which suggests the formation of LiF as part of SEI and the unsaturated fluorinated ether. The binding energy of this configuration, with respect to the optimized TPE configuration and Li surface with the H atom, is estimated to be -2.25 eV, which suggest that the reaction is energetically favorable.

4. Conclusions

The impact of a nonafluorinated ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether

(TPE), was explored as an electrolyte co-solvent for Li-S battery. The high TPE concentration electrolyte showed significantly improved Coulombic efficiency and capacity retention. the unique passivation reaction of TPE with lithium anode are the underpinned mechanism. AIMD simulation revealed the solvation energy and solvation structure of DOL/TPE electrolyte changes with varying

TPE concentrations. Furthermore, the reaction of TPE with lithium anode is calculated to be thermodynamically favorable forming LiF and unsaturated fluorinated species as the dominate SEI components. These simulation results correlate well with the electrochemical data such as Coulombic efficiency and capacity retention.

Chapter 5 Optimize the Lithium-Silicon Battery by a New Kind of All

Fluorinated Electrolyte

Abstract

2,2,2-Trifluoroethyl methyl carbonate (FEMC) has potential to be an effective electrolyte co-solvent to protect Si anode due to the rich fluorine groups in its structure. When used as an co-solvent of all- fluorinated electrolyte, FEMC preferably passivates Si anode, helps the formation of a robust and resilient SEI layer and alleviates the continuous reactions between Si and electrolyte and the delamination of electrode caused by the large volume change of Si during charging and discharging process. Therefore, FEMC co-solvent improves the electrochemical properties of Li ion battery applying Si as anode. This research introduces a new material FEMC to solve the critical problems of applying Si anode in lithium ion battery.

1. Introduction

Li-ion battery (LIB) has been a popular energy storage system and applied in various area such as consumer electronics, precise instrument and transportation system.117, 199-203 However, the practical energy density of most commercial LIB, used graphite as anode, is around 200-300 Wh/kg,117, 204-205 which cannot meet the energetic requirement of the pure electric vehicles. Therefore, materials having high theoretical capacity should be applied in the next generation of LIB as electrodes to achieve this requirement of higher energy density. Silicon is an appropriate choice for anode since its theoretical capacity is 4200 mAh/g,206-207 which is over 10 times higher than that of the graphite.208

However, the large volume changes (over 300% times) of Si electrode during lithium insertion and extraction processes,208-210 which leading to a number of fading mechanisms such as, the pulverization and delamination of Si particles, continuous reaction between Si and electrolyte and the formation of thick, unstable, inhomogeneous solid-electrolyte interphase (SEI). All of them are the challenges of 83 utilizing Si anode.208, 211-212 The intrinsic electrical conductivity of Si is another problem which causes low capacity retention and large initial irreversible capacity loss.212

To address these problems and commercialize the Si anode, a variety of ideas have been proposed and studied. The ideas can be roughly divided into two kinds. One is trying to optimize the Si electrode, including structural designs of Si material and formation of Si alloy/alloy composite anode and the other is designing or optimizing the formulation of electrolyte system to form a robust and stable SEI layer on the surface of Si anode. Si nanowires is an idea to optimize the structure of Si,

Cui213, Lee214, Zhi215 and many other research groups216-218 put an effort on this area. Si nanotubes is also an optional idea to optimize the structure of Si.219-221 Three-dimensional porous Si particles has drew considerable attention from researchers222-225 since it has larger mass loading than Si nanowires and nanotubes. Si alloys are widely learned and developed by many research groups, including

226 227 228 229 Mg2Si , SiNi , Fe3O4-SiO2 and Ti4Ni4Si7 .

Design and optimize the formulation of electrolyte system is another way to solve the problems caused by Si anode. The addition of additives into electrolyte is a popular method and has attract extensive attention from many research groups230-237, since it is low cost and easy to be commercialized.

Fluoroethylene carbonate (FEC), which is used for graphite at beginning238, now is widely used as additive to stabilize the SEI layer and protect Si anode. A plenty of investigations of FEC have been done239-249, results show that since SEI of Si anode contains more inorganic compounds, for example

LiF, and less organic compounds when apply FEC as additive, so SEI is more stable, integrated and homogenous. The impedance of cell decreases due to the thinner and smother SEI. FEC also suppressed the degradation of lithium salt in the electrolyte.241, 245, 250-252 However the improvement of electrochemical performances of cells can’t meet the requirements of next-generation rechargeable battery if only introduce fluorinated materials as additives. All-fluorinated electrolyte has been 84 developed to achieve the higher requirements of batteries,253-255 since the SEI of cells have more inorganic compounds and likely to be more resilient and robust. More importantly, the ignition points of all-fluorinated electrolytes are much higher than the common organic electrolyte. In consideration of safety issue, all-fluorinated electrolytes have greater possibility to be commercialized.

2,2,2-Trifluoroethyl methyl carbonate (FEMC) (Figure 1) is usually applied in high voltage lithium ion batteries as a kind of co-solvent since it can increase the oxidation stability of electrolyte and stabilize the batteries under high voltage condition.256-259 However, there isn’t any paper report its functionalities to pure Si electrode.

In this paper, we report for the first time that FEMC tackle the issues of pure Si anode as a co-solvent of a new formulation of all-fluorinated electrolyte. The designed idea is based on the fact that FEMC has an electron deficient (Lewis acidic) core due to the three fluorine atoms which can form more lithium fluoride (LiF) during the degradation process resulting in the SEI layer formed by more inorganic compounds meaning that this SEI layer should become more stable and help improving the electrochemical performances of the battery. Our experimental results support this inference and shows that FEMC is necessary and contributes a lot in this all-fluorinated electrolyte.

2. Materials & Methods

2.1 Materials

A Gen 2 electrolyte (1.2 m LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with weight ratio 3:7) was provided by Tomiyama Pure Chemical Industries. Fluoroethylene carbonate

(FEC) was purchased from Solvay, dried, and distilled before use. FEMC was synthesized by the

Materials Engineering Research Facility of Argonne National Laboratory (Argonne), dried, distilled under reduced pressure before use. Lithium hexafluorophosphate (LiPF6) was purchased from Stella

Chemif, dried before use. Silicon nanoparticles (70–130 nm) were purchased from Paraclete Energy.

Conductive carbon particles (C45, 50–60 nm) were purchased from Timcal. Poly(acrylic acid) (PAA) was purchased from Sigma-Aldrich. The baseline electrolyte is Gen 2 + 10 wt% FEC (GF electrolyte).

NCM 622 positive electrodes, which contain 90 wt% LiNi0.6Co0.2Mn0.2O2, 5 wt% C45, and 5 wt%

PVDF binder with a loading of 10 mg cm−2, were produced by the Cell Analysis, Modeling, and

Prototyping facility of Argonne National Laboratory (Argonne). Pure Si electrodes, which contain 70 wt% Si, 10 wt% C45, and 20 wt% PAA binder with a loading of 0.75 mg cm−2, were produced in lab by physically mixing Si, C45, 10 wt% PAA aqueous solution and appropriate amount of deionized water, then coating the slurry on copper foil (10 μm). Heat the electrodes at 80 °C under vacuum overnight, cut into pieces and heat again at 130 °C under vacuum 5 hours before use. The baseline electrolyte is also GF electrolyte. The all fluorinated electrolyte is 1 M LiPF6 FEC/FEMC (1:1 volume ratio).

2.2 Li-S cell testing

2032-Coin cells were assembled in an argon-filled glovebox with O2 level <5 ppm. Lithium chips

(15.6 mm in diameter, MTI) were used as the anode, Celgard 2325 (16.0 mm in diameter) as separator,

Si electrode (14 mm) as cathode to assemble lithium-ion half cells with silicon anodes (Li-Si cells).

Preformed Si electrode (15 mm) were used as anode, Celgard 2325 (16.0 mm in diameter) as separator,

NMC 622 as cathode to assemble NMC-Si full cell. After assembly, the cells were transferred to a battery cycler (Maccor Test Unit 4100) and cycled at a C/20 current (1C = 3600 mA/mg) with a cutoff voltage 1.5 – 0.01 V 3 cycles as formation cycles and then finished 100 cycles at a C/3 current (the capacity that 1C charging one hour equal to the charge capacity of the third formation cycle) with a same cutoff voltage 1.5 – 0.01 V.

Cells are lithiated at C/10 and delithiated at different C-rate (C/10, C/5, C/2, 1C, C/10) and each C- rate run 10 cycles with a cutoff voltage 0.01 V – 1.5 V to test the rate capability. Three formation cycles are finished as the preformation of Si anodes before test.

2.3 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was measured by Solartron Cell Test® 1470E with

ZView software. A frequency range from 1 MHz to 0.1 Hz and an amplitude of 10 mV was applied for performing the measurements.

2.4 Post-test Analysis

Scanning Electron Microscopy Analysis

Cycled silicon anodes were rinsed with 2 mL of dry DMC twice then airtight transferred to the column of the SEM instrument for surface morphology analysis. SEM data were acquired using Hitachi S-

4700-II system.

X-ray Photoelectron Spectroscopy Analysis

The Si anodes after cycling was washed in DMC solution and dried before moving to the XPS chamber. XPS data were acquired using PHI 5000 VersaProbe II system (Physical Electronics) that attached to an argon-atmosphere glovebox to avoid any contamination of moisture and air. The base pressure in the XPS chamber was 1 × 10−8 torr. The X-ray source was operating at 25 W equipped with monochromatic Al Kα radiation∼ (hυ = 1486.6 eV). The spot size for X-ray beam was set to 100

μm. The Shirly background data were subjected from all spectra. The spectra were fitted to multiple

Gaussian-Lorentz peaks by using the software package (Multipack) that Physical Electronics provided.

Figure 1. Chemical structure of 2,2,2-Trifluoroethyl methyl carbonate – FEMC.

3. Results and Discussion

3.1 Cell performance improvement

As shown in Figure 2b, in terms of lithium-ion half cells that using silicon as anodes (Li-Si cells), the all-fluorinated electrolyte cells have better cycling performance than baseline cells. The capacity retention of all-fluorinated electrolyte cell is 96% in first 100 cycles, and 65% after 200 cycles.

Baseline cells keep 62% capacity retention at the first 100 cycles, but the capacity fades rapidly after

80 cycles and is close to nothing at 125th cycle. Compared with the baseline electrolyte, the delivered capacity is more stabilized for the all-fluorinated cells and maintains high retention after 50 cycles.

The coulombic efficiency (CE) of both cells are similar at first 80 cycles, then the CE of baseline cells start decreasing which is similar with the fading of its capacity. The cycling performances of NMC-

Si full cells show analogous tendency, shown in Figure 2c, that all-fluorinated cells still store 70% capacity after finished 100 cycles which is 20% more than baseline cells.

The functionalities of FEMC should be supported by experimental data. The cycling performances of cells shown in Figure 2d presents the functionalities of FEMC in the all-fluorinated electrolyte. In

Figure 2d, the capacity retentions of cells drop from 96% to 82% and 72% when replace FEMC to the EMC and the solvents of Gen 2 electrolyte respectively. These results indicate that FEMC gives a large contribution in the all fluorinated electrolyte cells.

Compared with baseline cells, all-fluorinated electrolyte cells have more robust and resilient SEI layers on Si anodes. This optimization results in the huge improvement on capacity retention of all fluorinated cells. We also propose that FEMC plays an important role in the formation of SEI layer, forming various compositions which helps increasing the capacity retention of cells. This assumption is supported by the differential capacity curves of various Li-Si cells. The differential capacity curves of first discharge process were blown up and shown in Figure 2a, from top to bottom representing the cell using Gen 2 (Pink) , Gen 2 + 10wt% FEC (Baseline, black), Gen 2 + 10wt% FEMC (Green),

Gen 2 + 10wt% FEMC + 10wt% FEC (Blue), 1 M LiPF6 FEC/FEMC (1:1 volume ratio) (Red), respectively. The range of these amplified curves is -0.2 V to 0.2 V which represents the formation of SEI at first formation cycle. The peaks in curves are located at different positions which means the compositions of SEI layers are various in different cells. There are several information of peaks need to be mentioned in this plot. First, it is distinct that only the curves of cells having FEMC in electrolyte present an extra peak around 1.5 V, which means FEMC helps forming SEI of Li-Si cells and this result supports our assumption. Second, the appearances of the FEMC peaks are earlier than other peaks, which suggests that FEMC is more active than FEC and other non–fluorinated solvent in electrolyte. Third, the peaks of curves appear earlier when increasing the ratio of fluorinated solvent as electrolyte, so the formation process of SEI layer in the all-fluorinated electrolyte cell is the quickest among all chosen fluorinated electrolyte cells. Therefore, all-fluorinated electrolyte can reduce the time of Si interact with electrolyte and saving more electrolyte and active material.

3000 100 a b 2500 95 2000

0.67 V 1500 90 1.48 V 0.73 V

0.85 V 1000 0.95 V 1.53 V 85 Gen2 + 10wt% FEC 0.85 V 500 1 M LiPF6 FEC/FEMC (1:1 volume ratio)

1.56 V (%) Efficiency Coulombic

0.98 V

Differential Capacity [mAh/V] 0.89 V Discharging Capacity (mAh/g) 0 80 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0 50 100 150 200 Voltage [V] Cycle Number

3000 100 180 100 d 98 160 98 2500 c 96 96 140 94 94 2000 120 92 92 100 1500 90 90 80 88 88 60 1000 86 86 1 M LiPF6 FEC/FEMC (1:1 volume ratio) 40 Gen2 + 10wt% FEC 1 M LiPF (EC/EMC 3: 7 weight ratio)/FEC (1:1 volume ratio) 84 84 6 1 M LiPF FEC/FEMC (1:1 volume ratio) 500 6 1 M LiPF6 FEC/EMC (1:1 volume ratio) Coulombic Efficiency (%) Efficiency Coulombic Coulombic Efficiency (%) Efficiency Coulombic 20 82 82 Discharging Capacity (mAh/g) Discharging Capacity (mAh/g) 0 80 0 80 0 20 40 60 80 100 0 20 40 60 80 100 Cycle Number Cycle Number

Figure 2. (a) Part of the differential capacity curves of cells at first formation cycle (range from -0.2

V to 0.2 V), using electrolyte Gen 2 (Pink) , GF (Baseline, black), Gen 2 + 10wt% FEMC (Green),

Gen 2 + 10wt% FEMC + 10wt% FEC (Blue), 1 M LiPF6 FEC/FEMC (1:1 volume ratio) (Red), respectively; Cycling performance of (b) Li-Si cells; (c) NMC-Si full cells using baseline electrolyte

GF (black), 1 M LiPF6 FEC/FEMC (1:1 volume ratio) (red) respectively; (d) cycling performance of

Li-Si cells using three electrolytes 1 M LiPF6 FEC/FEMC (1:1 volume ratio) (black), 1 M LiPF6

(EC/EMC 3:7 weight ratio)/FEC (1:1 volume ratio) (red), 1 M LiPF6 FEC/EMC (1:1 volume ratio)

(blue), respectively.

Figure 3 shows the rate capability of the all-fluorinated electrolyte cells. Cells are lithiated at C/10 and delithiated at different C-rate (C/10, C/5, C/2, 1C, C/10) and each C-rate run 10 cycles. In Figure

3, cells have excellent specific capacity, good capacity retention and large recover ratio of capacity at

90 last 10 cycles with a C-rate of C/10. These results indicate that the all-fluorinated electrolyte cell has a good rate capability which, as our proposal, is mainly due to a stable, thinner, more conductive and homogeneous SEI layer. The cycling performance of the all-fluorinated electrolyte cells cycling with a high C-rate are shown in supporting information. In Figure S1, the all-fluorinated electrolyte cells have large specific capacity and capacity retention after 200 cycles. These perfect results also point out the all-fluorinated electrolyte cell has a good rate capability.

4000 100

3500 C/10 C/5 C/2 1C C/10 3000 95

2500

2000 90

1500

1000 1 M LiPF FEC/FEMC (1:1 volume ratio) 85 6 Coulombic Efficiency (%) Efficiency Coulombic 500

Discharging Capacity (mAh/g) 0 80 0 10 20 30 40 50 Cycle Number

Figure 3. Rate capability of the all-fluorinated cells.

3.2 Electrochemical impedance spectroscopy

All improved properties of SEI layer formed by the all-fluorinated electrolyte need to be supported by the results of post-test analysis.

Figure 4a and 4b present the Nyquist plots of the Li-Si cells before and after one hundred C/3 cycles using baseline electrolyte GF (black) and 1 M LiPF6 FEC/FEMC (1:1 volume ratio) (red) respectively.

The impedance of all-fluorinated cell is about 85% of the baseline cell’s impedance at the start of cycling and is only 50% after one hundred cycles. Figure 4c shows the results of ASI tested by Hybrid

Pulse Power Characterization (HPPC) test for NMC-Si full cells after one and nighty seven cycles, using baseline electrolyte GF (black) and 1 M LiPF6 FEC/FEMC (1:1 volume ratio) (red) respectively. 91

The results of ASI at 20% SOC were selected for comparison and analysis. First, ASI of cells having all-fluorinated electrolyte are smaller than baseline cells at both start and end of cycling process.

Second, in terms of the increasing rate of ASI from start to the end of cycling process, all-fluorinated cells are much smaller than the baseline cells which is similar with the impedance results of Si-Li cells. Combine all these analysis results of Li-Si cells and NMC-Si full cells, we propose that first the lower impedance of the all-fluorinated cells can help increasing capacity retention and alleviate the issue of overpotential when cycle at high C-rate, which supports the rate capability test. Second, the conductivity of SEI layer formed in the all-fluorinated cells are superior to the ones in baseline cells since all-fluorinated cells having smaller impedance at beginning of cycling process. Third, because the all-fluorinated cells have tinier increasing rate of impedance than the baseline cells, therefore, all- fluorinated cells have more stable and thinner SEI layers than the baseline cells.

80 180 Gen2 + 10wt% FEC Gen2 + 10wt% FEC 1 M LiPF FEC/FEMC (1:1 volume ratio) 160 1 M LiPF6 FEC/FEMC (1:1 volume ratio) 70 6 140 60 SOC=0% SOC=0% 120 50 100 40 80

30 (Imaginary) -Z'' -Z'' (Imaginary) -Z'' 60 20 40 10 a 20 b 0 0 0 10 20 30 40 50 60 70 80 0 20 40 60 80 100 120 140 160 180 Z' (Real) (Ohm) Z' (Real) (Ohm)

180 Baseline at the start of cycles Baseline at the end of cycles 160 All fluorniated electrolyte cell at the start of cycles All fluorniated electrolyte cell at the end of cycles ) 2 140 cm

• 120

100

ASI (ohm ASI 80 60 c 40 0 10 20 30 40 50 60 70 80 State of Charge (%)

Figure 4. Nyquist plots of (a) Li-Si cells before 100 C/3 cycles; (b) Li-Si cells after 100 C/3 cycles, and (c) ASI plots of NMC-Si full cells’ tested by HPPC protocol at start and end of cycles using baseline electrolyte GF (black) and 1 M LiPF6 FEC/FEMC (1:1 volume ratio) (red) respectively.

3.3 SEM characterization

Figure 5a, 5b, 5c, 5d, 5e and 5f show the morphology and cross-section of Si electrodes harvested from baseline cells after one hundred and one hundred and twenty-five cycles, respectively. Figure

5g, 5h, 5i, 5j, 5k and 5l show the morphology and cross-section of Si electrodes harvested from all- fluorinated cell after one hundred and two hundred cycles, respectively. SEI layer in baseline cell starts cracking after one hundred cycles, plenty of large cracks appear in the SEM images, and this

SEI becomes worse after another 25 cycles. Cracks cover all SEI layer and huge cracks show in the

SEM images. The shred SEI in baseline cell should be one of the main reasons causes the rapid fading of capacity after 70th cycle. The cross-section view of Si electrodes harvested from baseline cells present the delamination of Si electrode appeared during first one hundred cycles and became incredibly severe after one hundred and twenty-five cycles. Figure 5c and 5f indicate that the SEI layer formed in baseline cell cannot avoid the delamination of Si electrode and it is likely that this

SEI is inflexible and hard to expand.

Differ from the baseline cell. The all-fluorinated electrolyte cell has a perfect SEI layer after on hundred cycles, fewer cracks appear on SEI, no large cracks even under 1000 magnification time.

However, the breaking of SEI happen more frequent after one hundred cycles. The extremely stable and resilient SEI layer at first one hundred cycles of all-fluorinated cell is the primary reason resulting in the high capacity retention, since this SEI layer protect Si anode from interacting with electrolyte and also constrain the volume expansion of Si anode during cycling process then alleviate the extent 93 of the delamination. In Figure 5i and 5l, the electrodes harvested from all-fluorinated electrolyte cells have favorable cross-section view which present the delamination of Si electrode, even after two hundred cycles, didn’t arise, and the thickness of Si electrode and SEI layer in all-fluorinated cell is smaller than the one in baseline cell, revealing there are less side reactions and delamination of Si electrode proceeding in cell, which support the deduction that SEI formed in all-fluorinated cell is more stable and resilient than SEI formed in baseline cell.

Figure 5. The SEM images of Si electrodes harvested from baseline cells (a) and (b) after one hundred cycles, (d) and (e) one hundred and twenty-five cycles; cross-section images of Si electrodes (c) after one hundred cycles, (f) one hundred and twenty-five cycles; two hundred cycles (d); SEM images of

Si electrodes harvested from all-fluorinated cells (g) and (h) after one hundred cycles, (j) and (k) two hundred cycles; cross-section images of Si electrodes (i) after one hundred cycles, (l) two hundred cycles.

3.4 X-ray photoelectron spectroscopy

F1s, O1s, C1s XPS spectura of the Si electrodes from harvested baseline and all-fluorinated cells are shown in Figure 6 and devided into three groups based on the different elements. The scale of intensity (Y axis) are same in each group. F1s XPS spectura are shown in Figure 6a and Figure 6b.

The intensity of Li-F bond in the spectura of all-fluorinated cell is almost twice over the one of baseline cell, which means the SEI of all fluorniated cell containing a larger amount of LiF than the

SEI in the baseline cell. The intensity of O-P-F bond in the spectura of all-fluorinated cell is noticeably small compared with the one of baseline cell illustrating that the degradation of LiPF6 are allivated in the all fluorinated electrolyte. C1s XPS spectura are shown in Figure 6c and Figure 6d.

It is obvious that the intensity of all peaks in the spectura of the all-fluorinated cell is less than the one of baseline cell, revealing that SEI layer of the all-fluorinated cell includes less organic compounds than the SEI layer of baseline cell. O1s XPS spectura are shown in Figure 6e and Figure

6f. The O1s XPS spectura display similar result as what the C1s XPS spectura presenting.

LiF has already been learned by a great quantity of research groups and been treated as a benefical compound in the SEI layer of Si electrode240-241, 243, 245-246, 249, 251 which decrease the ratio of organic compounds of SEI layer and facilitates SEI layer becoming more robust and resilient. Based on the results of XPS sepctura, previous test results and research of LiF by other research groups, we propose, first the all fluorinated electrolyte passivating Si electrode and forming a large amount of

LiF as the compound of SEI optimizing the SEI in cell supported the previous analysis and inference. Second, the all fluorinated electrolyte helps protecting the LiPF6 from degradation stablizing the salt concentration and the transfer rate of lithium ion. Third, the new co-solvent

FEMC, is as important as the FEC that not only contributing more LiF to form SEI but also assisting electrolyte passivate Si electrode quickly and effectively due to the results shown in differential

95 capacity plot that its degradation peak appear in this plot earlier than all other solvents in the test.

This high reactivity of FEMC might be the key factor about constrain the degradition of LiPF6, since fast passivating cause LiPF6 don’t have enough time to react and degradate.

F F1s 1s Li-F

Li-F Intensity (cps) Intensity Intensity (cps) Intensity O-P-F O-P-F a b

690 688 686 684 682 690 688 686 684 682 Binding Energy (eV) Binding Energy (eV)

C=O O1s O1s

C=O C-O C-O Intensity (cps) Intensity Intensity (cps) Intensity

c d

536 535 534 533 532 531 530 529 536 535 534 533 532 531 530 529 Binding Energy (eV) Binding Energy (eV)

C C-C-H 1s C1s

C-C-H C-O

C-O Intensity (cps) Intensity Intensity (cps) Intensity C-O-C(=O)O O-C=O O-C=O C-O-C(=O)O e f

292 290 288 286 284 282 292 290 288 286 284 282 Binding Energy (eV) Binding Energy (eV)

Figure 6. F1s, O1s, C1s XPS spectra of Si electrode harvested from baseline cell (GF electrolyte) after one hundred cycles respectively (a), (c), (e); F1s, O1s, C1s XPS spectra of Si electrode harvested from all-fluorinated cell (Electrolyte is 1 M LiPF6 FEC/FEMC (1:1 volume ratio)) after one hundred cycles respectively (b), (d), (f).

4. Conclusion

A new co-solvent FEMC for all fluorinated electrolyte was reported for the first time. The all- fluorinated electrolyte is verified to protect Si electrode by optimizing the properties of SEI layer in lithium ion battery and FEMC contributes a lot that be a suitable co-solvent of this all-fluorinated electrolyte and passivate the Si electrode quicker than other solvents protecting the Si effectively, and last but not the least, FEMC also offer a large amount of F ions forming LiF and other inorganic materials as the compounds of SEI layer which is helpful to optimize its properties. The experimental results indicate that first SEI layer formed by the all-fluorinated electrolyte contains more inorganic compounds, especially LiF, which makes SEI more resilient and robust and finally optimize the electrochemical performance of batteries. Second, all-fluorinated electrolyte suppressed the degradation of lithium salt in electrolyte, which help keep the transportation of lithium ion stable and increase the rate capability and capacity retention of cell. This research introduces a new material to tackle the critical issue of the silicon electrode.

Chapter 6. Summary

Lithium-ion batteries (LIBs) are occupied in our daily life, from consumer electronics, energy storage systems to electric vehicles. The recent prosperous market in electric transportation induces enormous interests and this high demand of LIBs will accelerate the progress of developing the next generation secondary battery. The next generation secondary battery should have a large improvement on the energy density and battery life. Sulfur and silicon are potential to be the active material of anode for the next generation secondary battery. However, there are several fundamental issues need to be solved before the commercialization of these two anodes. Prof. Wang’s group at Worcester

Polytechnic Institute has developed various advanced NMC cathode material to increase the energy density of lithium ion battery and match the new anodes, and Dr. Zhang’s group at Argonne National

Laboratory has developed a variety of methods to optimize these anode materials by applying appropriate electrolyte solvents and additives. During my Ph.D. studies, I was first making enhanced lithium-manganese-rich cathode for Li-ion batteries, including the formation of precursors and screening test of the formulation of doping materials, then I started working in Dr. Zhang’s lab as a visiting student and focusing on employing new kinds of solvent and additives to optimize lithium sulfur battery and protect the silicon anode in lithium ion battery. The following parts are a brief summary of the research.

Firstly, the impact of a nonafluorinated ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether (TPE), was explored as an electrolyte co-solvent for Li-S battery. The high TPE concentration electrolyte showed significantly improved Coulombic efficiency and capacity retention. The unique passivation reaction of TPE with lithium anode are the underpinned mechanism. AIMD simulation revealed the solvation energy and solvation structure of DOL/TPE electrolyte changes with varying

TPE concentrations. Furthermore, the reaction of TPE with lithium anode is calculated to be

98 thermodynamically favorable forming LiF and unsaturated fluorinated species as the dominatnt SEI components. These simulation results correlate well with the electrochemical data such as Coulombic efficiency and capacity retention.

Secondly, a new functional electrolyte additive hexafluorobenzene (HFB) was reported for the first time for lithium-sulfur batteries. The impact of HFB was verified experimentally both in 1.0 M

LiTFSI DOL/DME + 0.1 M LiNO3 and 1.0 M LiTFSI DOL/TTE electrolyte cells. Although high concentration (>20wt%) additive causes detrimental effect on the former cell performance probably due to the interference with LiNO3, low concentration improved the capacity decay especially at the later stage of the cycling. DFT calculation results indicate that HFB is preferably coordinate with low- order polysulfides: Li2S and Li2S2 over higher-order Li2Sx (x>2) and further react forming redox- active species, thus reactivating the electronically isolated active materials back to the system and reduces the gradual capacity loss during later cycles. The reaction mechanism was proposed, and possible species was identified. This research opens a new approach to tacking the fundamental issue of the Li-S chemistry.

Thirdly, the relationships between a series of lithium salt concentration and the electrochemical properties of Li-S battery applying two electrolyte DOL/TTE and DOL/DME was reported. In both electrolyte groups the performances of baseline batteries were not the best, which convince our assumption that there is an appropriate concentration of lithium salt for each electrolyte system that used in lithium sulfur battery. The idea of introducing an appropriate low lithium salt concentration into DOL/TTE electrolyte system to optimize the cycling performances of lithium sulfur battery was reported for the first time. Apply a low lithium salt concentration in DOL/TTE electrolyte would release more DOL molecules as the free solvent molecules which could dissolve more sulfur and the dissolving process are also easier than baseline electrolyte. Therefore, the decreasing of lithium salt

99 concentration is an effective way to increase the specific capacity and capacity retention. However, it was worth noting that reducing the salt concentration would sacrifice several electrochemical properties of Li-S battery to a certain degree, so it is necessary to balance all properties influenced by salt concentration in an ideal way to maximize the optimization without sacrificing other properties hardly and finally choosing an appropriate lithium salt concentration for Li-S battery. This research a new and easy approach to tacking the issue of Li-S chemistry.

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Chapter 7. Future Work

Overall, with further understanding and optimization of electrode design, I envision an acceleration in the development of commercial lithium–sulfur batteries and Si anode in the near future.

As for the Li-S battery, the sluggish sulfur redox reaction kinetics is a major cause for the shuttle effect of LiPSs, which severely hinders the improvement of electrochemical performance. To further enhance the sulfur reaction kinetics and promote the overall performance, there is a lot of room for investigations. These can be clarified as the following field.

Material Optimization: The fast conversion between the adsorbed LiPSs and the insoluble deposits

(Li2S/Li2S2) is the key to suppress the dissolution of LiPSs.

Fundamental Theory: Although the development of useful additives or solvents to promote the conversion of sulfur to Li2S/Li2S2 has achieved substantive progress, it is important to trace the full chemical reaction by appropriate characterization techniques (e.g., in situ Raman/X-ray absorption spectroscopy/TEM etc.) to gain the reliable conversion process and establish the correct theory about the redox of sulfur.

Safety Guarantee: Lithium anode suffers severe lithium dendrite problems during reduction, so all- solid-state electrolyte may be an alternate to address the safe threat induced by the liquid electrolyte.

Cost Consideration: For the commercial application, low-cost and large-scale production of catalysts such as nitrides, metal oxides, and sulfides are highly recommended.

As for the commercialization of Si anode in LIBs, the addition of additives has great potential to be the approach to realize this goal. In this regard, the following points need to be taken into consideration when design new electrolyte system in future.

Systematic design and molecular-level high-throughput screening, as well as characterization of electrolyte additives and polymeric binders, need to be research properly.

101

A fundamental understanding of the effect of electrolyte additives and binders on Si anode interfacial chemistry is required. The use of both in situ experimental and theoretical calculations could be of very important.

The future large-scale deployment of large-format Si anode-based LIBs requires the systematic evaluation and cautious safety evaluation.

Advanced electrolyte system must be validated on a larger scale to fully assess their applicability.

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Chapter 8. Publications and Presentations 1. Publications  Cao, J., Tornheim, A., Glossmann, T., Hintennach, A., et al. (2019). Understanding the Impact of a Nonafluorinated Ether-Based Electrolyte on Li-S Battery. Journal of the Electrochemical Society, 166(15), A3653-A3659.

 Meisner, Q. J., Rojas, T., Rago, N. L. D., Cao, J., et al. (2019). Lithium-Sulfur Battery with Partially Fluorinated Ether Electrolytes: Interplay between Capacity, Coulombic Efficiency and Li Anode Protection. Journal of Power Sources, 438, 226939.

 Vanaphuti, P., Chen, J., Cao, J., Bigham, K., et al. (2019). Enhanced Electrochemical Performance of the Lithium-Manganese-Rich Cathode for Li-Ion Batteries with Na and F CoDoping. ACS Applied Materials & Interfaces, 11(41), 37842-37849.

 Meisner, Q. J., Rojas, T., Glossmann, T., Cao, J., et al. (2020). Impact of Co-Solvent and LiTFSI Concentration on Ionic Liquid-Based Electrolytes for Li-S Battery. Journal of The Electrochemical Society, 167(7), 0528.

 Cao, J., Meisner, Q. J., Glossmann, T., Hintennach, A., et al. (2020). Tackling the Capacity Fading Issue of Li-S Battery by a Functional Additive – Hexafluorobenzene. ACS Applied Energy Materials, 3(4), 3198- 3204.

2. Presentations

2020 MRS Spring Meeting & Exhibit (Poster)

Title: Solving the Capacity Fading Issue of Li-S Battery by a Fluorinated Additive – Hexafluorobenzene

103

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