• unlimited access with print and download
    $ 37 00
  • read full document, no print or download, expires after 72 hours
    $ 4 99
More info
Unlimited access including download and printing, plus availability for reading and annotating in your in your Udini library.
  • Access to this article in your Udini library for 72 hours from purchase.
  • The article will not be available for download or print.
  • Upgrade to the full version of this document at a reduced price.
  • Your trial access payment is credited when purchasing the full version.
Buy
Continue searching

Lithium bis(oxalato)borate-based electrolyte for lithium-ion cells

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Fadhel A Kh A Azeez
Abstract:
Compact, light weight rechargeable batteries offering high-energy densities have become necessary in the 21st century especially for applications such as portable electronics devices, hybrid electric vehicles, and load leveling in electric power generation/distribution. Among rechargeable batteries, lithium-based systems seem able to fill these needs. The state-of-art electrolyte for Li-ion batteries of LiPF6 dissolved in organic-carbonate solvents has disadvantages in low- and high-temperature environments. At high temperature, the thermal instability of LiPF6 is believed to be the main cause for poor performance of lithium-ion batteries. At low temperature, the high viscosity of ethylene carbonate, which is a major component in the solvent mixture, restricts use to above -20 °C. These factors limit the operation of lithium-ion batteries to be between -20 and 60 °C. In an attempt to improve the performance, enhance the safety, and lower the cost of lithium-ion cells, we use a stable salt at high temperature, lithium bis(oxalato)borate (LiBOB), and dissolve it in mixtures of γ-butyrolactone (GBL), ethyl acetate (EA), and ethylene carbonate (EC), with and without fumed silica (FS) nano particulates as a gelling agent. Conductivity, cycling studies of cathode half-cells, rheology, and FTIR measurements are performed for LiBOB in such mixtures as a function of salt concentration, solvent composition, temperature, and FS content and type. We find that LiBOB in a mixture of GBL:EA:EC yields a technologically acceptable conductivity, and LiBOB in GBL:EA:EC is a potential candidate for Li-ion cells. For example, LiBOB based-electrolyte with a salt concentration of 0.7M LiBOB in a GBL: EA: EC (wt) composition of 1:1:0 has a conductivity ∼ 6.0 and 11.1 mS/cm at -3 at 25 °C, respectively, and at 1 M LiBOB in solvent composition of 1:1:0.1, the conductivity is ∼10.8 and 20.0 mS/cm at 25 and 60 °C, respectively. These conductivities are higher than that of the state-of-art electrolyte, which is 9.5 mS/cm at 25 °C. The product of conductivity with viscosity, which an indication for ion disassociation is essentially independent of temperature. Altough LiBOB in GBL:EA:EC (1:1:1) has the highest product value, it's conductivity is the lowest. This indicates that our system is viscosity dominated. Adding FS to a LiBOB-based electrolyte yields a mixture with an elastic modulus independent of frequency and larger than the viscous modulus in a dynamic rheology experiment, which indicates formation of a 3-D gel structure. Fumed silica enhanced the mechanical properties of the electrolyte without sacrificing its conductivity. The surface chemistry of FS (native silanol vs octyl-modified) has no effect on conductivity but a significant effect on rheological properties of the mixture. Using a gel electrolyte is anticipated to enhance the safety of lithium-ion batteries by eliminating leakage problems associated with a liquid electrolyte. Cathode half-cells using a LiBOB-based electrolyte give good performance, and in the case of LiMn2 O4 half-cells, the performance is better than that using state-of-art electrolyte. It is expected that LiMn 2 O4 cathodes will lower the cost of lithium-ion batteries based on material cost. The performance of LiFePO4 and LiCoO 2 half-cells using the gel electrolyte is comparable to half-cells using state-of-art electrolyte. In addition, a Celgard(TM) separator can be eliminated by using gel electrolyte, which should lower the cost of lithium-ion batteries. Our results show that using 1M LiBOB in GBL:EA:EC + 20% R805 can prevent contact between the cathode and the anode without Celgard and give a better performance than cells using the separator. Results obtained in this dissertation support further study of LiBOB-based gel electrolyte as a potential replacement for the state-of-art electrolyte for use in lithium-ion batteries.

TABLE OF CONTENTS

LIST OF TABLES ........................................................................................... viii LIST OF FIGURES ............................................................................................. x

CHAPTER 1: INTRODUCTION ........................................................................ 1 1.1. Motivation for this research .......................................................................... 1 1.2. Objectives of this study.................................................................................. 3 1.2.1. Liquid electrolyte ................................................................................. 3 1.2.2. Gel electrolyte ...................................................................................... 4 1.3. Our approach .................................................................................................. 5 1.4. Outline of thesis ............................................................................................. 8 1.5. References ...................................................................................................... 9

CHAPTER 2: AN OVERVIEW ON LITHIUM-ION BATTERIES ............... 12 2.1. Introduction .................................................................................................. 12 2.2. What is a battery? ........................................................................................ 13 2.2.1. Primary battery ................................................................................... 14 2.2.2. Secondary battery ............................................................................... 14 2.2.3. How does a battery work? ................................................................. 14 2.2.4. Galvanic cells vs. batteries ................................................................. 16 2.3. Battery performance .................................................................................... 16 2.3.1. Voltage ............................................................................................... 18 2.3.2. Capacity ............................................................................................. 19 2.3.3. Self discharge ..................................................................................... 20 2.3.4. Energy content ................................................................................... 20 2.3.5. Specific Energy and Energy Density ................................................. 21 2.4. History of lithium-based batteries ............................................................... 22 2.4.1. Problems with metallic lithium anode ............................................... 25 2.5. Lithium-ion batteries .................................................................................... 26 2.6. Features and benefits of lithium-ion cells .................................................... 29 2.7. Future trend of Li-ion batteries .................................................................... 31 2.8. Design principles ......................................................................................... 33 2.9. Lithium-ion battery components and their development ............................. 36

v

2.9.1. Anode hosts ........................................................................................ 36 2.9.2. Cathode materials ............................................................................... 41 2.9.3. Electrolyte .......................................................................................... 43 2.9.4. Separator materials ............................................................................. 46 2.10. Overcharge/ over discharge ....................................................................... 51 2.10.1. Discharge characteristics ................................................................. 52 2.10.2. Charging characteristics ................................................................... 52 2.10.3. Protection circuit .............................................................................. 53 2.11. Summary .................................................................................................... 55 2.12. References .................................................................................................. 56

CHAPTER 3: ELECTROLYTES FOR LITHIUM-ION BATTERIES .......... 60 3.1. Introduction .................................................................................................. 60 3.1.1. Solvents .............................................................................................. 61 3.1.2. Salts .................................................................................................... 63 3.2. State-of-art electrolyte ................................................................................. 65 3.2.1. LiPF 6 .................................................................................................. 66 3.3. LiBOB .......................................................................................................... 68 3.4. Limitation of the state-of-art electrolyte at low temperature ....................... 69 3.5. Fumed silica ................................................................................................. 71 3.5.1. Advantages of fumed silica ................................................................ 73 3.6. References .................................................................................................... 77

CHAPTER 4: EXPERIMENTAL AND METHODS ....................................... 85 4.1. Material preparation ..................................................................................... 85 4.1.1. Composite electrolyte preparation ..................................................... 85 4.1.2. Cathode preparation ........................................................................... 87 4.1.3. Preparation of coin cells .................................................................... 89 4.2. Electrochemical measurements ................................................................... 91 4.2.1. Electrolyte resistance ......................................................................... 91 4.2.2. Conductivity measurements ............................................................... 92 4.2.3. Cell cycling ........................................................................................ 96 4.3 Rheological measurements ........................................................................... 97 4.3.1 Dynamic measurements ...................................................................... 97

vi

4.4. FTIR-ATR measurements ........................................................................... 98 4.5. References ..................................................................................................101

CHAPTER 5: CONDUCTIVITY OF LIBOB-BASED ELECTROLYTE FOR LITHIUM-ION BATTERIES .........................................................................102 Abstract .............................................................................................................103 5.1. Introduction ................................................................................................104 5.2. Experimental ..............................................................................................106 5.2.1. Conductivity measurements .............................................................107 5.2.2. Viscosity measurement ....................................................................109 5.3. Result and Discussion ................................................................................109 5.3.1. Salt concentration effect on conductivity and viscosity ..................109 5.3.2. EC content effect on conductivity and viscosity .............................112 5.3.3. Temperature effect on conductivity .................................................114 5.4. Summary ....................................................................................................114 5.5. References ..................................................................................................123

CHAPTER 6: RHEOLOGICAL PROPERTIES OF LIBOB-BASED GEL ELECTROLYTE FOR LITHIUM-ION BATTERIES ..................................126 Abstract .............................................................................................................127 6.1. Introduction ................................................................................................128 6.2. Experimental ..............................................................................................132 6.2.1. Materials...........................................................................................132 6.2.2. Rheological measurement ................................................................132 6.2.3. FTIR-ATR measurements ................................................................134 6.3. Result and Discussion ................................................................................134 6.3.1. Effect of fumed silica content on rheological properties .................134 6.3.2. Effect of solvent composition on rheological properties .................135 6.3.3. Effect of salt concentration on rheological properties .....................137 6.4. Summary ....................................................................................................140 6.5. References ..................................................................................................148

CHAPTER 7: ELECTROCHEMICAL PROPERTIES OF LIBOB-BASED GEL ELECTROLYTE FOR LITHIUM-ION BATTERIES .........................151 Abstract .............................................................................................................152

vii

7.1. Introduction ................................................................................................153 7.2. Experimental ..............................................................................................156 7.2.1. Materials...........................................................................................156 7.2.2. Rheological measurement ................................................................157 7.2.3. Conductivity measurements .............................................................157 7.2.4. Cathode half-cell preparation and cell cycling ................................158 7.3. Result and Discussion ................................................................................160 7.3.1. Fumed silica effects on conductivity and rheology .........................160 7.3.2. Salt concentration effect on conductivity ........................................162 7.3.3. EC content effect on conductivity ...................................................163 7.3.4. Cathodes half-cells using LiBOB- and LiPF 6 - based electrolyte ....163 7.4. Summary ....................................................................................................166 7.5. References ..................................................................................................176

CHAPTER 8: LiBOB-BASED GEL ELECTROLYTE AS A SEPARATOR FOR Li-ION BATTERIES ..............................................................................180 Abstract .............................................................................................................181 8.1. Introduction ................................................................................................182 8.2. Experimental ..............................................................................................183 8.2.1. Conductivity measurements .............................................................183 8.2.2. Rheological measurements ..............................................................183 8.2.3. Cathode half-cell preparation and cell cycling ................................183 8.3. Results and Discussion ..............................................................................185 8.4. Conclusion .................................................................................................187 8.5. References ..................................................................................................192

CHAPTER 9: CONCLUSIONS AND RECOMENDATIONS ......................194 9.1. Conclusions ................................................................................................194 9.2. Recommendations ......................................................................................197 9.2.1. Li-ion batteries .................................................................................197 9.2.2. Lithium batteries ..............................................................................202 9.3. References ..................................................................................................203

viii

LIST OF TABLES

Table 2.1. The electromotive series for some battery components .................... 17

Table 2.2. Technical comparison for some rechargeable batteries ................... 21

Table 2.3. Sequence development of components and systems for rechargeable lithium batteries (LE for liquid electrolytes and PE for polymer electrolytes) . 24

Table 2.4. General performance characteristics of lithium-ion batteries .......... 31

Table 2.5. Structure and properties of some solvents used for lithium battery electrolytes ......................................................................................................... 45

Table 2.6. Separators used in secondary lithium batteries ................................ 47

Table 2.7. Major manufacturers of lithium-ion battery separators along with their typical products ......................................................................................... 48

Table 2.8. Manufacturing process of typical microporous film ........................ 48

Table 2.9. Typical properties of some commercial microporous membranes .. 49

Table 2.10. TMA data for typical Celgard separators ....................................... 49

Table 2.11. Safety and performance tests for lithium-ion batteries and the corresponding important separator property and its effect on cell performance and/or safety ....................................................................................................... 50

Table 2.12. Properties of prototypes for HEV application, Exxon standard grades, and some commercial Products ............................................................. 51

Table 3.1. Organic carbonates and esters as electrolyte solvents ...................... 62

Table 3.2. Organic ethers as electrolyte solvents .............................................. 63

ix

Table 3.3. Typical salts used in research and industry for lithium-ion cells ...... 64

Table 3.4. Novel lithium salts and their major properties ................................. 67

Table 4.1. Liquid electrolytes used in gel electrolyte preparation. .................... 86

Table 4.2. Typical composition for the cathodes in this study. .......................... 88

Table 4.3. Electrolyte compositions that are used in cycling study. .................. 89

Table 4.4. Gel electrolyte compositions. ............................................................ 94

Table 4.5. Cutoff voltages for cathodes in this study. ........................................ 96

Table 4.6. Gel electrolyte compositions used in dynamic measurements. ......... 98

Table 4.7. Samples used in FTIR-measurements. ............................................100

x

LIST OF FIGURES

Figure 2.1. Comparison of the gravimetric and volumetric energy densities of rechargeable lithium batteries with those of other systems ................................ 13

Figure 2.2. Galvanic cell during discharge process ............................................ 15

Figure 2.3. Cell polarization as a function of operating current ........................ 18

Figure 2.4. Comparison of cells of high- and low-rate services ........................ 19

Figure 2.5. Lithium-ion battery operation ......................................................... 27

Figure 2.6. Electrochemical potential ranges of some lithium insertion compounds in reference to metallic lithium ...................................................... 29

Figure 2.7. Li-ion batteries market .................................................................... 32

Figure 2.8. Schematic energy diagram of a cell at open circuit ........................ 35

Figure 2.9. Three types of carbon used in lithium-ion batteries ........................ 37

Figure 2.10. Model of lithium-ion intercalation into carbon material ................ 38

Figure 2.11.Discharge characteristics of lithium-ion with coke and graphite electrodes ............................................................................................................ 39

Figure 2.12. Lithium-ion battery charge ............................................................ 53

Figure 3.1. Schematic of fumed silica synthesis ............................................... 72

Figure 3.2. Overview of the types of fumed silica surface groups ..................... 73

Figure 3.3. Elastic modulus and conductivity of PEGdm composites with R805 ............................................................................................................................. 75

xi

Figure 3.4. Cycling behavior for Li/V 6 O 13 cells w/o F.S (1.8-3 V, C/15, RT) .. 75

Figure 3.5. Steady-shear behavior of R805 composites ..................................... 76

Figure 4.1. Coin cell for cycling studies (not to scale) ....................................... 90

Figure 4.2. Two-electrode cell for conductivity measurement ........................... 93

Figure 4.3. Equivalent circuit used for electrolyte resistance calculation, where R E is bulk electrolyte resistance, R ct is charge transfer resistance, and Q is constant phase element. ...................................................................................... 95

Figure 5.1. Conductivity cell ............................................................................116

Figure 5.2. Equivalent circuit used for electrolyte resistance calculation, where R E is bulk electrolyte resistance, R ct is charge transfer resistance, and Q is a constant phase element. ....................................................................................116

Figure 5.3. Concentration dependence of conductivity for electrolytes containing LiBOB in GBL:EA:EC of a) 1:1:0, b) 1:1:0.1, c) 1:1:0.5, and d) 1:1:1 (wt) composition at 0 and 60 °C ..............................................................117

Figure 5.4. Concentration dependence of molar conductivity for electrolytes containing LiBOB in GBL:EA:EC of a) 1:1:0, b) 1:1:0.1, c) 1:1:0.5, and d) 1:1:1 (wt) compositions at 0 and 60 °C ............................................................118

Figure 5.5. Concentration dependence of viscosity for electrolytes containing LiBOB in GBL:EA:EC composition of 1:1:0.1 (wt) at 5 and 25°C .................119

Figure 5.6. Product of viscosity and conductivity for 0.7 M LiBOB dissolved in GBL:EA:EC as a function of temperature for various solvent compositions ..120

Figure 5.7. Dependence of conductivity on EC content for 0.2 and 1.2 M LiBOB dissolved in GBL:EA:EC at 0 and 60 °C .............................................120

xii

Figure 5.8. Dependence of viscosity on EC content in 0.7 M LiBOB in GBL:EA:EC at 10 and 25 °C ............................................................................121

Figure 5.9. Temperature dependence of conductivity for a) 0.2, b) 0.5, c) 0.7, d) 1.0, and e) 1.2 M LiBOB in a GBL:EA:EC solvent mixture of varying EC content ...............................................................................................................122

Figure 6.1. Components of studied gel electrolytes .........................................131

Figure 6.2. Elastic and viscous moduli for 0.2 M LiBOB in GBL:EA:EC (1:1:0) + various content of A200 ................................................................................142

Figure 6.3. Elastic modulus for 0.2 M LiBOB in GBL:EA:EC (1:1:0) + various content of R805 .................................................................................................142

Figure 6.4. Elastic modulus for 0.2 M LiBOB in GBL:EA:EC (1:1:1) + A200 ...........................................................................................................................143

Figure 6.5. Elastic modulus for 1.0 M LiBOB dissolved in various composition of GBL:EA:EC + 10% fumed silica (FS) .......................................................143

Figure 6.6. Elastic modulus for various LiBOB concentrations in GBL:EA:EC (1:1:0) + 10 % FS ..............................................................................................144

Figure 6.7. Elastic modulus for various LiTFSI concentration in GBL:EA:EC (1:1:0) + 10% + A200 .......................................................................................145

Figure 6.8. Spectra for acetonitrile (AN) with and without fumed silca and LiBOB between wavenumber of 1700-1910 cm -1 ............................................146

Figure 6.9. Elastic modulus for 0.2 M LiBOB in GBL:EA:EC (1:1:0) + 10 % fumed silica .......................................................................................................147

Figure 6.10. Elastic modulus for 1.0 M LiBOB in GBL:EA:EC (1:1:1) + fumed silica ..................................................................................................................147

xiii

Figure 7.1. Coin cell for cycling studies (not to scale) .....................................168

Figure 7.2. Temperature dependence of conductivity for 0.7 M LiBOB in mixture of GBL:EA:EC of 1:1:0 composition for liquid electrolyte with and without 10% fumed silica (R805 or A200). .....................................................168

Figure 7.3. Elastic and viscous moduli for 1.0 M LiBOB in GBL:EA:EC (1:1:1) + various content of R805 .................................................................................169

Figure 7.4. Elastic modulus (G') for 0.7 M LiBOB in mixture of GBL:EA:EC of 1:1:0 composition with 10% fumed silica (R805 or A200) .............................169

Figure 7.5. Conductivity for 0.7 M LiBOB in mixture of GBL+EA+EC of 1:1:1 composition with various fumed silica content (R805) ....................................170

Figure 7.6. Conductivity of LiBOB in GBL:EA:EC +10% R805 at 0 and 60°C ...........................................................................................................................170

Figure 7.7. Conductivity of 1 M LiBOB in GBL:EA:EC +10% A200 at 0, 25, and 60°C ............................................................................................................171

Figure 7.8. Discharge capacity for Li/LiFePO 4 half cells using LiBOB-based electrolyte ..........................................................................................................172

Figure 7.9. Discharge capacity of Li/LiMn 2 O 4 half cells using LiBOB-based liquid and gel electrolytes .................................................................................173

Figure 7.10. Discharge capacity of Li/LiFePO 4 half cells using 1 M LiBOB/GBL:EA:EC (1:1:0) + fumed silica.Figure 7.10. Discharge capacity of Li/LiFePO 4 half cells using 1 M LiBOB/GBL:EA:EC (1:1:0) + fumed silica 174

Figure 7.11. Discharge capacity of Li/LiCoO 2 half cells using LiBOB/GBL:EA:EC +10% A200 ....................................................................175

xiv

Figure 8.1. Conductivity of 0.7 M LiBOB in mixture of GBL:EA:EC of 1:1:1 composition (wt) with and without 20% fumed silica (R805) .........................188

Figure 8.2. Elastic (G') and viscous (G") moduli of 1.0 M LiBOB in GBL:EA:EC (1:1:1) + 20% R805 ....................................................................188

Figure 8.3. Discharge capacity of LiCoO 2 half-cells using 1 M LiBOB in mixture of GBL:EA:EC of 1:1:1 composition (wt) with and without 20% fumed silica (R805). Performance with base-liquid electrolyte using Celgard separator is also shown .....................................................................................189

Figure 8.4. Discharge capacity of LiMn 2 O 4 half-cells using 1 M LiBOB in mixture of GBL:EA:EC of 1:1:1 composition (wt) with and without 20% fumed silica (R805). Performance with base-liquid electrolyte using Celgard separator is also shown .....................................................................................190

Figure 8.5. Discharge capacity of LiFePO 4 half-cells using 1 M LiBOB in mixture of GBL:EA:EC of 1:1:0 composition (wt) with and without 20% fumed silica (R805). Performance with base-liquid electrolyte using Celgard separator is also shown .....................................................................................................191

Figure 9.1. Concentration of Mn dissolved from stabilized lithium manganese oxide spinel (SLMOS) and LiMn 2 O 4 powders stored in LiPF 6 /EC: DEC (1:1) electrolyte at 55 °C for 4 weeks ........................................................................198

Figure 9.2. Concentration of Mn dissolved from stabilized lithium manganese oxide spinel (SLMOS) and LiMn 2 O 4 powders stored in 1 M LiBOB/EC:DEC (1:1) electrolyte at 55 °C after 4 weeks ............................................................199

Figure 9.3. Amount of Fe 2+ ions dissolved from C-LiFePO 4 powder that has been aged in 1.2 M LiPF 6 /EC:PC:DMC (1:1:3) and 0.7 M LiBOB/EC:PC:DMC (1:1:3) for one week at 55 °C ...........................................................................199

Figure 9.4. Discharge capacity for MCMB/Li cell using 1 M LiBOB-based electrolyte and state-of-art electrolyte ..............................................................200

xv

Figure 9.5. Current density obtained on an Al electrode at various potentials vs. Li in electrolyte containing 1.0 M LiBOB in an EC/EMC (1:1) mixture. Inset: Dependence of steady-state current density on applied potential on an Al electrode in electrolyte containing various Li-salts ..........................................202

1

CHAPTER 1: INTRODUCTION 1. 1. Motivation for this research The growth in portable electronics devices such as cellular phones and laptop computers during the past two decades has created great interest in compact, light-weight batteries offering high energy densities that show good rechargeability and reliability. In addition, strengthened environmental regulation and a more rational use of available energy resources prompt the development of advanced batteries for electric vehicles. Currently, the state-of-art electrolyte for the lithium-ion battery, which is the best battery presently available for a variety of applications, is composed of lithium hexafluorophosphate (LiPF 6 ) dissolved in a mixture of ethylene carbonate (EC) and linear esters such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Lithium hexafluorophosphate is the common salt in commercial lithium- ion batteries because it has many advantages such as good conductivity in solution, good mobility, and the ability to passivate the positive electrode’s aluminum (Al) current collector. Ethylene carbonate is a major solvent in commercial lithium-ion batteries because of its high- dielectric constant and ability to form a good solid electrolyte interface (SEI) on the graphitic anode in lithium-ion batteries. However, current lithium-ion batteries operate only up to 60 o C due to the low thermal stability of LiPF 6 1-5 and its susceptibility to hydrolysis. In addition, the low-temperature performance of current lithium-ion batteries is limited to -20 o C and is affected by high melting point of EC 6-8 . Improving the high- and low-temperature performance of lithium-ion batteries electrolyte will have a significant impact on batteries

2

and their applications. In order to improve the high- and low-temperature performances of lithium-ion batteries, researchers have been trying to replace LiPF 6 and reduce or eliminate EC from the electrolyte system. To improve the possibility of using lithium-ion batteries in hybrid electric vehicle (HEV), issues like high cost and safety of currently used lithium-ion batteries need to be addressed. The high cost of current lithium-ion batteries stems, in part, from using expensive cathodes such as LiCoO 2 . Cheap cathodes such as LiFePO 4 and LiMn 2 O 4 are not being used with LiPF 6 because of detrimental effects of HF, which is a side product from the reaction between H 2 O and LiPF 6 , on these cathodes. By not using LiPF 6 as the salt, we can eliminate these effects and use other cathodes to lower the cost of lithium-ion batteries. The second issue of improving the lithium-ion battery performance, which is related to safety, can be enhanced by eliminating the leakage problem of liquid electrolyte by adding fumed silica nano particulates to the mixture and convert the liquid to a gel electrolyte. Efforts have been devoted to develop alternative lithium salts to replace LiPF 6 , including lithium perchlorate (LiClO 4 ), lithium arsenate (LiAsF 6 ), lithium tetrafluoroborate (LiBF 4 ) , lithium triflate (LiCF 3 SO 3 ), and lithium bis(trifluoromethane sulfonyl)imide (LiN(SO 2 CF 3 ) 2 ). However, each salt has its own challenges that prevent it from being used in commercial Li-ion batteries. 9-14

Recently, lithium bis(oxalato)borate (LiBOB), which was independently disclosed by Lischka et al. in Germany and Angell et al. in the USA 15, 16 , has attracted attention as a promising candidate for Li-ion batteries 17-20 . The LiBOB salt has many advantages such as high thermal stability (up to 302 o C), ability to passivate aluminum, and ability to form a

3

solid electrolyte interface (SEI) on graphite even in the absence of EC 21, 22 , which is a major solvent component due to its ability to form the SEI. Almost all the studies done with LiBOB in the past six years use mainly carbonate solvents. Although LiBOB has many advantages, it also has disadvantages when used with linear carbonate solvents, which are used in state-of- art electrolytes as co-solvents to lower the viscosity of the electrolyte. The problems of using LiBOB with linear carbonate solvents comes from LiBOB essentially being insoluble in these solvents 23 and, consequently, the formulation of the solvents is restricted to be EC- or PC-rich. However, high-EC or -PC content in an electrolyte formulation tends to increase its viscosity, which renders the electrolyte with poor low-temperature performance and rate capability. For low-temperature application, one needs to find a proper solvent mixture that is tailored for LiBOB to give high conductivity and good salt solubility. Since it is reported that esters (such as ethyl acetate (EA)…) can improve the low-temperature performance of Li-ion batteries 24-25 , we decided to use esters instead of linear carbonate solvents in our system. Our hypothesis is that LiBOB in GBL:EA:EC has the potential to improve the performance (by eliminating problems caused by HF), lower the cost (by using cheaper salt and cathodes), and enhance the safety of lithium-ion batteries (by eliminating leakage problem associated with liquid electrolyte). 1.2. Objectives of this study 1.2.1. Liquid electrolyte The first objective of this study is to determine if LiBOB-based electrolytes can be used in practical lithium-ion batteries, as determined by cycling performance of cells using

4

these electrolytes. In this work, we study the cycling performance of cells using LiBOB- based electrolyte as a function of LiBOB salt concentration and solvent composition. For these studies, half cells are made using three types of metal-oxide cathode: lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate. The cycling performance of cathode half-cells using all three cathode materials and anode half-cells using graphite with LiBOB in GBL+EA+EC are evaluated and compared with cells using state-of-art electrolyte (LiPF 6 in EC:EMC). 1.2.2. Gel electrolyte The second objective of this study is to evaluate the cycling performance of cells using LiBOB-based gel electrolyte. A first step in our efforts to achieve this objective is to examine the ability of a LiBOB-containing liquid electrolyte to form a gel electrolyte. Two types of fumed silica are chosen: R805 and A200 (Evonik 26-27 ). The R805 fumed silica contains octyl surface group at 48% coverage and silanol surface group at 52% coverage, and the A200 fumed silica contains only native silanol on the surface. These two silica types are chosen to examine the effect of surface chemistry of fumed silica on conductivity and rheology of the gel electrolytes. In this study, conductivity and rheological properties of the gel electrolytes as a function of salt concentration, solvent composition, fumed-silica content, fumed-silica type and temperature are evaluated and compared to those of liquid electrolyte. From these results, an understanding for the effects of solvent composition, salt concentration, temperature, fumed silica content and type is gained. That understanding can enable us to determine the best solvent composition, salt concentration and fumed silica

5

percentage in the electrolyte to produce a mixture that yields good cycleability, mechanical strength, and conductivity. In order to determine if the gel electrolyte can be used in lithium-ion cells, the performance of cathode and anode half-cells using the gel electrolyte is evaluated. Three types of cathodes: lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate are used in this study. The cycleability of all these cells is be evaluated and compared with those of LiBOB-based liquid electrolyte. 1.3. Our approach To enable an electrolyte capable of working in a wide temperature range, our strategy was to choose a salt (other than LiPF 6 ) that satisfies the requirements given in section 3.1.2 and gives better high-temperature performance than the currently used LiPF 6 . Suitable solvents must be used with the chosen salt to improve the low-temperature performance of lithium-ion batteries. We have chosen LiBOB as the salt for this study, and mixtures of - butyrolactone (GBL), ethyl acetate (EA) and ethylene carbonate (EC) as the solvents. The solvent -butyrolactone (GBL) is chosen because it has a reasonably high- dielectric constant (~39), a relatively moderate viscosity (~1.73 cP at 25 o C), a similar structure to EC, and good solubility for LiBOB 28-30 . In addition, it was reported that GBL can improve the low-temperature performance of a LiBOB based electrolyte system 31-32 . The solvent EA is chosen because it has a low-melting point (~ -84 o C), which increases the liquid range of the electrolyte. In addition, the low viscosity (~ 0.45 cP at 25 o C) of ethyl acetate (EA) improves the conductivity of the electrolyte. Ethyl acetate (EA) also

6

has the ability to improve the low-temperature performance for lithium-ion cells by improving solution transport properties 33-34 . To achieve our objectives, electrochemical impedance spectroscopy (EIS) is used to measure the conductivity of the LiBOB-based electrolyte as a function of salt concentration, solvent composition, fumed-silica content, fumed-silica type and temperature. A rheometer is used to study the rheological properties of our gel electrolytes as a function of salt concentration, solvent composition, fumed-silica type and fumed-silica content. An Arbin battery cycler controlled by Arbin ABTS software is used to measure cycleability for the cathode and anode half-cells. In these cells, three types of cathodes are used: lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate. Finally, IR spectroscopy is used to study the effect of surface chemistry of fumed silica on rheological properties. The full plan to achieve our goals is schematically illustrated in scheme 1.1, which shows our approach to formulate and characterize LiBOB-based liquid and gel electrolytes for use in lithium-ion and lithium cells.

7

Scheme 1.1. Work plan.

Conductivity and viscosity measurements were done to check if candidate electrolyte can be used for lithium - ion batteries. The effect of salt concentration and solve nt composition on conductivity and viscosity of liquid electrolyte is investigated. Best solvent composition and salt concentration is c hosen according to the results

Examine the ability of fumed - silica filler to make a gel electrolyte and study silica s urface - chemistry effect on both conductivity and rheology

Examine the performance of liquid electrolyte by studying the cycling performance of cathode and anode half - cells

Study effect s

of salt concentration, solvent composition, fumed silica content an d temperature on conductivity and rheology of gel electrolyte

Study performance of gel electrolyte using anode and cathode half - cells with different cathode materials (lithium cobalt oxides, lithium manganese oxide, and lithium iron phosphate)

Gel elect rolyte

Liquid electrolyte

8

1.4. Outline of thesis

An overview about lithium-ion batteries and electrolytes for Li-ion batteries are presented in Chapters 2 and 3, followed by an experimental section detailing the material preparation and characterization in Chapter 4. Submitted (or prepared for submission) manuscripts to peer-reviewed journals are presented in the following four chapters. Chapter 5 reports the effect of salt concentration, solvent composition, and temperature on the conductivity and viscosity of the LiBOB-based liquid electrolyte. Chapter 6 reports the effect of salt type and concentration, solvent composition, fumed silica type and content on rheological properties of LiBOB-based electrolyte. Chapter 7 reports the effect of salt concentration, solvent composition, fumed silica type and content, temperature, and cathode type on conductivity of LiBOB-based gel electrolyte and on cycling performance of cells using LiBOB-based electrolyte and state of art electrolyte. Chapter 8 reports performance of half cells using LiBOB-based electrolyte without using Celgard separator and compare its performance with a cell using Celgard separator. In Chapter 9, conclusions from the experimental results and recommendations for future work are presented. Experimental results are presented in Appendix A, which illustrates the effect of salt concentration, solvent compositions, fumed-silica content, fumed-silica type, temperature, and cathode type on conductivity, rheology, and cell performance of LiBOB-based electrolyte for lithium-ion batteries.

Full document contains 224 pages
Abstract: Compact, light weight rechargeable batteries offering high-energy densities have become necessary in the 21st century especially for applications such as portable electronics devices, hybrid electric vehicles, and load leveling in electric power generation/distribution. Among rechargeable batteries, lithium-based systems seem able to fill these needs. The state-of-art electrolyte for Li-ion batteries of LiPF6 dissolved in organic-carbonate solvents has disadvantages in low- and high-temperature environments. At high temperature, the thermal instability of LiPF6 is believed to be the main cause for poor performance of lithium-ion batteries. At low temperature, the high viscosity of ethylene carbonate, which is a major component in the solvent mixture, restricts use to above -20 °C. These factors limit the operation of lithium-ion batteries to be between -20 and 60 °C. In an attempt to improve the performance, enhance the safety, and lower the cost of lithium-ion cells, we use a stable salt at high temperature, lithium bis(oxalato)borate (LiBOB), and dissolve it in mixtures of γ-butyrolactone (GBL), ethyl acetate (EA), and ethylene carbonate (EC), with and without fumed silica (FS) nano particulates as a gelling agent. Conductivity, cycling studies of cathode half-cells, rheology, and FTIR measurements are performed for LiBOB in such mixtures as a function of salt concentration, solvent composition, temperature, and FS content and type. We find that LiBOB in a mixture of GBL:EA:EC yields a technologically acceptable conductivity, and LiBOB in GBL:EA:EC is a potential candidate for Li-ion cells. For example, LiBOB based-electrolyte with a salt concentration of 0.7M LiBOB in a GBL: EA: EC (wt) composition of 1:1:0 has a conductivity ∼ 6.0 and 11.1 mS/cm at -3 at 25 °C, respectively, and at 1 M LiBOB in solvent composition of 1:1:0.1, the conductivity is ∼10.8 and 20.0 mS/cm at 25 and 60 °C, respectively. These conductivities are higher than that of the state-of-art electrolyte, which is 9.5 mS/cm at 25 °C. The product of conductivity with viscosity, which an indication for ion disassociation is essentially independent of temperature. Altough LiBOB in GBL:EA:EC (1:1:1) has the highest product value, it's conductivity is the lowest. This indicates that our system is viscosity dominated. Adding FS to a LiBOB-based electrolyte yields a mixture with an elastic modulus independent of frequency and larger than the viscous modulus in a dynamic rheology experiment, which indicates formation of a 3-D gel structure. Fumed silica enhanced the mechanical properties of the electrolyte without sacrificing its conductivity. The surface chemistry of FS (native silanol vs octyl-modified) has no effect on conductivity but a significant effect on rheological properties of the mixture. Using a gel electrolyte is anticipated to enhance the safety of lithium-ion batteries by eliminating leakage problems associated with a liquid electrolyte. Cathode half-cells using a LiBOB-based electrolyte give good performance, and in the case of LiMn2 O4 half-cells, the performance is better than that using state-of-art electrolyte. It is expected that LiMn 2 O4 cathodes will lower the cost of lithium-ion batteries based on material cost. The performance of LiFePO4 and LiCoO 2 half-cells using the gel electrolyte is comparable to half-cells using state-of-art electrolyte. In addition, a Celgard(TM) separator can be eliminated by using gel electrolyte, which should lower the cost of lithium-ion batteries. Our results show that using 1M LiBOB in GBL:EA:EC + 20% R805 can prevent contact between the cathode and the anode without Celgard and give a better performance than cells using the separator. Results obtained in this dissertation support further study of LiBOB-based gel electrolyte as a potential replacement for the state-of-art electrolyte for use in lithium-ion batteries.