Novel Lithium Salt and Polymer Electrolytes for Polymer Lithium Batteries

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NOVEL LITHIUM SALT AND POLYMER ELECTROLYTES

FOR POLYMER LITHIUM BATTERIES

JIAN LIN

Submitted in partial fulfillment of the requirements

For the degree of Doctor of Philosophy

Dissertation Adviser: Dr. Morton H. Litt

Department of Macromolecular Science and Engineering

CASE WESTERN RESERVE UNIVERSITY

August, 2008

CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

Jian Lin candidate for the Ph.D. degree*.

(Signed) Morton H. Litt

(chair of the committee)

Gary Wnek

David Schiraldi

Chung-Chiun Liu

(date) 06/06/2008

*We also certify that written approval has been obtained for any proprietary material contained therein.

Table of contents

Chapter 1 Introduction………………………………………………...…...... 1

1.1. The rocking-chair cell…………………………..………………….……..…….…2

1.2. Polymer electrolytes for secondary lithium batteries…………………….…...... 3

1.2.1. Random copolymers……………………..………………….……………5

1.2.2. Comb polymers and hyperbranched polymers……………………...……7

1.2.3. Block copolymers…………………………………….………………....16

1.2.4. Networks…………………………………………………………..……17

1.2.5. Composite polymer electrolytes and blends………………………...…..18

1.3. Gel electrolytes……………………………………………………….…...…..…20

1.4. Polymer-in-salt electrolytes or ionic rubbers…………………...………....……..24

1.5. Polyelectrolytes……………………….…………… ……….…………..……...27

1.6. Crystalline polymer electrolytes…………………………………………….…...36

1.7. Other cation conductors………………………….…………………...……...…..38

1.8. Ion transport measurement………………………………….……………….…...39

1.9. Temperature dependence of ionic conductivity…………………...………...... …41

1.10. Research goal………………………………………………………..……….…..42

1.11. References……………………………………………..……………..…………..45

Chapter 2. Synthesis and characterization of novel lithium salts with bulky anions ……………………………………………………………………………...58

2.1. Introduction…………………………………………………………...... 58

2.2. Experimental procedures………………………………………………………...65

2.2.1. Materials………………………………………………………………...65

2.2.2. Characterization techniques…………………………………………….65

2.2.2.1. Infrared spectroscopy………………………………………65

2.2.2.2. Matrix-assisted laser desorption/ionization time-of-flight

mass spectroscopy (MALDI-TOF) ……………...………...65

2.2.2.3. Nuclear magnetic resonance Spectroscopy (NMR)……..…66

2.2.2.4. X-ray Photoelectron Spectroscopy (XPS)…... ……………66

2.2.2.5. Thermogravimetric analysis (TGA)………… …………….66

2.2.2.6. Gel Permeation Chromatography (GPC)…………………..67

2.2.3. Synthesis………………………………………………………………...67

2.2.3.1. Attempted synthesis of dilithium pentaerithritol

di(pinacolato)borate……………………………………….67

2.2.3.2. Synthesis of lithium tetrakis(methanoxyato)borate……..…68

2.2.3.3. Synthesis of lithium tetrakis(1, 1, 1, 3, 3, 3-hexafluoro-2-

propoxy)borate………………………………….………….68

2.2.3.4. Synthesis of lithium bis(benzopinacolato(2-)-O,O´)borate..70

2.2.3.5. Synthesis of lithium tetrakis [3,5-

bis(trifluoromethyl)phenyl]borate……………………….....71

2.3. Results and discussion………………………………………….………………..72

2.3.1. Attempted synthesis of dilithium pentaerithritol dipinacolborate………72

2.3.2. Synthesis of lithium tetrakis(1,1,1,3,3,3-hexafluoro-2-propoxy)borate..75

2.3.3. Synthesis of lithium bis[benzopinacolato(2-)-O,O’]borate…………….77

2.3.4. Synthesis of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate…...82

2.4. Conclusions………………………………………………………………………86

2.5. References………………………………………………………………………..87

III

Chapter 3. Synthesis of comb polymers with oligo(ethylene oxide) side chains and their characterization……………………..……………………………………………90

3.1. Introduction ……………………………………………………………………...90

3.2. Experimental procedures………………………………………………………...93

3.2.1. Materials………………………………………………………………..93

3.2.2. Characterization techniques…………………………………………….93

3.2.2.1. Gel permeation chromatography (GPC)….………………..93

3.2.2.2. Infrared spectroscopy………………………………….…...94

3.2.2.3. Nuclear magnetic resonance spectroscopy (NMR)……...…94

3.2.2.4. AC impedance spectroscopy……………………………….94

3.2.2.5. Differential scanning calorimetry (DSC)……….….………95

3.2.2.6. X-ray photoelectron spectroscopy (XPS)…………….……95

3.2.2.7. Elemental analysis…………………………………………96

3.2.3. Synthesis………………………………………………………………..96

3.2.3.1. Attempted preparation of comb polymers from poly(ethylene

oxide-co-epichlorohydrin) by Williamson ether

synthesis……………………………………………………96

3.2.3.2. Preparation of 3,3-bis(bromomethyl)oxetane……………...97

3.2.3.3. Cationic ring-opening polymerization of 3,3-

bis(bromomethyl)oxetane………………………………….97

3.2.3.4. Copolymerization of trimethylene oxide and 3, 3-

bis(bromomethyl)oxetane………………………………….98

3.2.3.5. Preparation of comb copolymer from poly[3,3-

bis(bromomethyl)oxetane]…………………………………98

3.2.3.6. Preparation of comb copolymer from poly[trimethylene

oxide-co-3,3-bis(bromomethyl)oxetane]…………………..99

3.2.3.7. Preparation of polymer/lithium salt complexes…………..100

3.3. Results and discussion………………………………………………………….101

3.3.1. Synthesis……………………………………………………………….101

3.3.1.1. Attempted synthesis of comb polyethers based on

poly(ethylene oxide-co-epichlorohydrin)………………...101

3.3.1.2. Synthesis of comb polyethers with poly(trimethylene oxide)

as backbone and oligo(ethylene oxide) as side

chains……………………………………..……………....105

3.3.2. Solution properties…………………………………………………….113

3.3.3. Thermal properties…………………………………………………….114

3.3.3.1. Thermal stability…………………………….……………114

3.3.3.2. Glass transition temperature……………………………...115

3.3.4. Ionic conductivity……………………………………………………...120

3.3.5. Polarization measurements at low frequencies…….………………….131

3.3.6. Simulation model for electrical behavior…………………….………..135

3.4. Conclusions…………………………………………………………...... 144

3.5. References………………………………………………………………………147

Chapter 4. Synthesis of comb polymers with oligo(trimethylene oxide) side chains and their characterization………………...……..……………………………………151

4.1. Introduction……………………………………………………………….…….151

4.2. Experimental procedures…………………………………………………....….158

4.2.1. Materials……………………………………………………………….158

4.2.2. Characterization techniques………….....…………………………….159

4.2.2.1. Gel permeation chromatography (GPC)…………………..159

4.2.2.2. Infrared spectroscopy……………………………………...159

4.2.2.3. Nuclear magnetic resonance spectroscopy (NMR)………..159

4.2.2.4. AC impedance spectroscopy………………………….…...159

4.2.2.5. Differential scanning calorimetry (DSC)………………….160

4.2.2.6. Matrix-assisted laser desorption/ionization time-of-flight mass

spectroscopy (MALDI-TOF)……………………………...160

4.2.3. Synthesis………………………………………..………………….…..161

4.2.3.1. Synthesis of oligo(trimethylene oxide)monomethylether…161

4.2.3.2. Synthesis of oligo(trimethylene oxide)-disubstituted

oxetane……………………………………………………..162

4.2.3.3. Copolymerization of trimethylene oxide and

oligo(trimethylene oxide)-disubstituted oxetane…………..163

4.3. Results and discussion……………………………….………………………....165

4.3.1. Synthesis ……………………..……………………….……………….165

4.3.1.1. Synthesis of oligo(trimethylene oxide)monomethyl ether...165

4.3.1.2. Synthesis of oxetane monomer with disubstituted

oligo(trimethylene oxide) and its polymerization…………175

4.3.2. Solution properties…………………………………………………….180

4.3.3. Thermal properties…………………………………………………….180

4.3.3.1. Thermal stability…………………………………………..180

4.3.3.2. Glass transition temperature……………………………….182

4.3.4. Ionic conductivity…………………………………………………...... 184

4.3.5. Polarization measurements at low frequencies.…………………….....194

4.4. Conclusions……………………………………………………………………..198

4.5. References………………………………………………………………………200

Chapter 5. Future Work…………………………………………………………….204

References………………………………………………………………………………208

Bibliography…………………………………………………………………………...209

VII

List of tables

Table 3.1. Properties of PEGME, polymer, copolymer and EO comb polymers with

poly(trimethylene oxide) as backbones……….……………………..…..119

Table 3.2. Activation energy (Ea/KJ mol-1) of the ethylene oxide comb polymer

electrolytes using the Arrhenius equation……………………………...130

Table 3.3. Activation energy (Ea/KJ mol-1) of the ethylene oxide comb polymer

electrolytes using the VTF model………….…………………………...130

Table 3.4. Time constants for EO comb copolymer/LiTFSI-[O]/[Li]=50………....142

Table 3.5. Time constants for EO comb copolymer/LiTMPB-[O]/[Li]=50...……..143

Table 3.6. Time constants for EO comb copolymer/LiTMPB-[O]/[Li]=70……....143

Table 4.1. Binding energies (ΔEe, Kcal/mol) of Li+-PEO, Li+-PPO, Li+-PTMO

complexes as a function of coordination number……………………...155

Table 4.2. The calculated activation energies (Ea/KJ mol-1) of the trimethylene oxide

comb polymer electrolytes using the VTF model…………..…………..193

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List of schemes

Scheme 2.1. Attempted synthesis of dilithium pentaerithritol dipinacolborate…….…..73

Scheme 2.2. Two possible routes for the synthesis of lithium bis[1,1,1,3,3,3-hexafluoro-

2-propoxy]borate…………………..……………...…………………..….76

Scheme 2.3. Two attempted synthetic routes to lithium bis[benzopinacolato(2-)-O,O’]

borate……………….…….………………………...…………...…..…….79

Scheme 2.4. Synthesis of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate by two

possible routes……………….………………………...... ………...... ……82

Scheme 3.1. Attempted synthesis of the comb polymer starting from Hydrin C using two

possible routes.………….………………..….…………...………..……102

Scheme 3.2. Synthesis of 3,3-bis(bromomethyl) oxetane……..…………...………….105

Scheme 3.3. Synthesis of ethylene oxide comb polymers by two possible routes..…...109

Scheme 3.4. The proposed three-parallel simulation model …………………...…..…136

Scheme 4.1. Non polymerization of 2-[2-(2-methoxyethoxy)ethoxy]-1, 3-dioxolane 156

Scheme 4.2. The synthetic route to incorporate TMO units into the monomer and anionic

polymerization used to obtain comb polyepoxide ether…………..…..…157

Scheme 4.3. The attempted synthesis of oligo(trimethylene oxide): active chain end

mechanism.……………………………………………...………..… ….169

Scheme 4.4. The synthesis of oligo(trimethylene oxide): activated monomer mechanism

…………………………………………………………………………...170

Scheme 4.5. The synthesis of trimethylene oxide comb polymers by two possible routes …………………………………………………………………………...178

Scheme 5.1. A possible route to the synthesis of oligo(trimethylene oxide) side chains

with controlled length……………………………………………………204

Scheme 5.2. Copolymerization of trimethylene oxide and tetrahydrofuran using the

activated monomer mechanism………………………...………………..205

Scheme 5.3. Preparation of crystalline block from oxetane, bromomethyloxetane and

THF……………………………………………………………………..206

Scheme 5.4. Preparation of triblock copolymer with crystallizable poly(oxazoline)

blocks………………..………………………………………………….207

List of figures

Figure 1.1. Schematic illustration of a lithium rocking chair Battery with graphite and

spinel as intercalation electrode ….……………...…………………………3

Figure 1.2. Preparation of comb polymer using poly(styrene-co-maleic anhydride) as

backbone………………………………………………………………....11

Figure 1.3. Monosubstituted polysiloxane comb polymer…………...…………..….….11

Figure 1.4. Disubstituted polysiloxane comb polymer …………...... ………….………11

Figure 1.5. poly [(4, 7, 10, 13-tetraoxatetradecyl)methylsilane] ………………..……..11

Figure 1.6. Poly[bis(2-(2-methyoxyethoxy)ethoxy)phosphazene] ………….…..……..11

Figure 1.7. Polyphosphazene with 2 and 16 ethylene oxide repeat units as side chain...11

Figure 1.8. Hyperbranched poly[bis(hexaethylene glycol)benzoate]..……………...….16

Figure 1.9. Copolymer of PAN, PMMA and PEGMEM…….………………...... ……..23

Figure 1.10. The mechanism of cooperative ion transport in clusters ……………...….24

Figure 1.11. Poly (diallyldimethylammonium chloride) ……………………...... …..…31

Figure 1.12. Polyphosphazene with charged side chains………………………………31

Figure 1.13. Poly [(oligo(oxyethylene) methacrylate)-co-(alkali-metal methacrylate)]

……………………………………………………………………………..31

Figure 1.14. Polyphosphazene with sulfonate and oligoether side chains………….…..31

Figure 1.15. Poly [lithium-N-(4-sulfophenyl)maleimide-co-methoxyoligo(oxyethylene)

methacrylate]………………………………..…………………………....31

Figure 1.16. Polyacrylate comb polymer with lithium alky sulfonate side group……...32

Figure 1.17. Polyacrylate comb polymer with lithium alky sulfonate side group

separated by siloxane……………...………………………………..……..32

Figure 1.18. Cross-linked polyelectrolyte 1……………………………………….…....33

Figure 1.19. Cross-linked polyelectrolyte 2……………………………………...……..33

Figure 1.20. Ethylene oxide comb polymer with trifluorobutane sulfonate on one side

chain……………………………………………………………………....34

Figure 1.21. Ethylene oxide comb polymer with butane sulfonate on one side chain….34

Figure 1.22. Ethylene oxide comb polymer with carboxylate on one side chain………34

Figure 1.23. Polyelectrolytes incorporating poly(pentaethylene glycol methyl ether

acrylate co-allyloxyethyl acrylate) and lithium

bis(allylmalonato)borate…………………………………………………35

Figure 1.24. The Helmholtz model.……………….…………………………...…….…40

Figure 1.25. The equivalent circuit model.……………….………………...... …..…41

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Figure 2.1. Lithium bis[1,2-benzenediolato(2-)-O,O’]borate…………………………..63

Figure 2.2. Lithium bis[1,2-benzenediolato(2-)-O,O’]borate…..……………...... …..…63

Figure 2.3. Lithium bis[1,2-tetrakis(trifluorom ethyl) ethylendiolate(2-)-O,O’]borate

……………………………………………………………………………...63

Figure 2.4. Lithium bis[1,2-salicylato(2-)-O,O’]borate………………………….……..63

Figure 2.5. Imidazole anion ……………….….……………...... …..……………….64

Figure 2.6. Benzimidazole anion ……………….………………...…...…..………..….64

Figure 2.7. 2-methylbenzimidazole anion ……………….………………...…….…..…64

Figure 2.8. 4-phenylphenyl anion ……………….………………...…….….………….64

Figure 2.9. Mesogenic ester of benzimidazole-5-carboxylic acid anion and its uncharged

analogue……………………………………………………………………64

Figure 2.10. 1H NMR (DMSO-d6) of the dried products from the attempted preparation

of lithium tetrakis(1, 1, 1, 3, 3, 3-hexafluoro-2- propoxy)borate………..69

1 Figure 2.11. H NMR (CDCl3) of the products from the attempted preparation of lithium

bis(benzopinacolato(2-)-O,O´)borate……………...... …..……….………71

1 Figure 2.12. H NMR (D2O) of the attempted synthesized dilithium salt……..…….…74

Figure 2.13. FT-IR spectra of lithium dipinacolborate, boric acid and pinacol.…..…...74

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Figure 2.14. 1H NMR (DMSO-d6) of lithium tetrakis(1,1,1,3,3,3-hexafluoro-2-

propanolato)-borate………………………………………………………76

Figure 2.15. 1H NMR (DMSO-d6) of the lithium bis[benzopinacolato(2-)-

O,O´]borate…………………………………..…………………………..80

Figure 2.16. 11B NMR (DMSO-d6) of lithium bis[benzopinacolato(2-)-O,O´] using

trimethyl borate as a reference.………………...…………………..…….80

Figure 2.17. MALDI-TOF of lithium bis[benzopinacolato(2-)-O,O´]borate……….….81

Figure 2.18. TGA of Lithium bis[benzopinacolato(2-)-O,O´]borate (dried at 95 ºC in

TGA, then equilibrated in glyme at 30 ºC. It picked up 2.5 glyme

molecules per lithium salt.)…………………………………………...… 81

Figure 2.19. TGA of Lithium bis[benzopinacolato(2-)-O,O´]borate with 2.5 coordinated

glyme molecules per lithium salt(saturated by gyme at 30 ºC, and then

heated to 320 ºC)………………………………………………………….82

1 Figure 2.20. H NMR (CDCl3)of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

using route 1……………………………………………………………...83

1 Figure 2.21. H NMR (CDCl3) of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

complexed with 2 glyme molecules using route 2…… ………………….84

XIV

Figure 2.22. TGA of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

equilibrating with glyme vapor at 30ºC. Two glyme molecules were picked

up for each lithium cation………..……………………………………….85

Figure 2.23. TGA of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate with

2 coordinated glyme molecules (equilibrated with glyme vapor at 30 ºC,

and then heated to 400 ºC.)……… ………………………………………85

Figure 3.1. XPS of Hydrin C ……………………………………..…………...…...... 103

Figure 3.2. XPS of the reacted product from Hydrin C…………...….……………….103

1 Figure 3.3. H NMR (CDCl3) of the reaction product from Hydrin C……...………...104

Figure 3.4. IR spectra of polyepichlorohydrin-co-poly(ethylene oxide) (Hydrin C) and the reaction product……………………………………………….….....104

Figure 3.5. 1H NMR (DMSO-d6, 120℃) of poly[3,3-bis(bromomethyl)oxetane]…....110

Figure 3.6. FT-IR spectrum of poly[3,3-bis(bromomethyl)oxetane]………….....……110

1 Figure 3.7. H NMR (CDCl3) of poly(trimethylene oxide)-co-poly[3,3-

bis(bromomethyl)oxetane] ………………………………………….……111

Figure 3.8. FT-IR spectrum of poly(trimethylene oxide)-co-poly[3,3-

bis(bromomethyl)oxetane]…………………………………………….…..111

1 Figure 3.9. H NMR (CDCl3) of the ethylene oxide comb copolymer………………..112

Figure 3.10. FT-IR spectrum of the ethylene oxide comb copolymer………………...112

Figure 3.11. TGA of poly[3,3-bis(bromomethyl)oxetane], poly[3,3-

bis(bromomethyl)oxetane]-co-poly(trimethylene oxide), comb copolymer

and its complexes with LiTFSI or LiTMPB at [O]/[Li]=30…………….115

Figure 3.12. DSC thermograms of PEGME, EO comb homopolymer, EO

combcopolymer and their polymer electrolytes complexed with LiTFSI

and LiTMPB at [O]/[Li]=30, at a heating rate of 10℃/min after quenching

from 120℃……………………………………………………………...118

Figure 3.13. Glass transition temperature (Tg) of EO comb homopolymer and EO comb

copolymer and their polymer electrolytes plotted against salt concentration

([O]/[Li]). (Note: here [O]/[Li]=100 stands for uncomplexed polymer.)

…………………………………………………………………………....120

Figure 3.14. Ionic conductivity vs. the O/Li ratio at 1 MHz of EO comb homopolymers and EO copolymers complexed with LiTFSI and LiTMPB (297K)…………..……………………………………………………....126

Figure 3.15. Ionic conductivity vs. the O/Li ratio at 1 MHz of EO comb homopolymers and EO copolymers complexed with LiTFSI and LiTMPB (366K)……………………………………………………………….….126

Figure 3.16. Arrhenius conductivity plot for EO comb copolymer/LiTFSI and EO comb

copolymer/LiTMPB complexes at [O]/[Li]=50 (σ at 1 MHz)..…….. …....127

Figure 3.17. Arrhenius conductivity plot for EO comb copolymer/LiTFSI and EO comb

copolymer/LiTMPB complexes at [O]/[Li]=30 (σ at 1 MHz)……….…..127

XVI

Figure 3.18. VTF conductivity plot for EO comb copolymer/LiTFSI ([O]/[Li]=50 ,Tg=-

63℃) and EO comb copolymer/ LiTMPB ([O]/[Li]=50, Tg=-58℃)

complexes (σ at 1 MHz)………………………………………………..128

Figure 3.19. VTF conductivity plot for EO comb copolymer/LiTFSI ([O]/[Li]=30,

Tg=-59℃) and EO comb copolymer/ LiTMPB ([O]/[Li]=30, Tg=-52℃)

complexes (σ at 1 MHz)……………………………………..………….128

Figure 3.20. Arrhenius conductivity plot for EO comb homopolymer/LiTFSI, EO comb

homopolymer/LiTMPB, EO comb copolymer/LiTFSI and EO comb

copolymer/LiTMPB at [O]/[Li]=10 (σ at 1 MHz)……………………….129

Figure 3.21. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO comb copolymer/LiTFSI at [O]/[Li]=50 at 297K.……………………..132

Figure 3.22. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO

comb copolymer/LiTFSI at [O]/[Li]=50 and 336K..….…………….….133

Figure 3.23. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO comb copolymer/LiTFSI at [O]/[Li]=50 and 376K..…….………….….133

Figure 3.24. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO

comb copolymer/LiTFSI at [O]/[Li]=30 and 297K……………….…....134

Figure 3.25. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO

comb copolymer/LiTFSI at [O]/[Li]=30 and 336K.……...... …………..134

Figure 3.26. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB

at [O]/[Li]=50, 296K …………………………..……………………….138

Figure 3.27. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB

at [O]/[Li]=50, 336K …………………………………...……..………..138

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Figure 3.28. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB

at [O]/[Li]=50, 376K ………………….…………………….………….139

Figure 3.29. Simulation of resistance and capacitance of EO comb copolymer/LiTFSI at

[O]/[Li]=50, 296K.……………………..…………………….………….139

Figure 3.30. Simulation of resistance and capacitance of EO comb copolymer/LiTFSI at

[O]/[Li]=50, 336K ………………………………..………….………….140

Figure 3.31. Simulation of resistance and capacitance of EO comb copolymer/LiTFSI at

[O]/[Li]=50, 376K ……………………..………………………………..140

Figure 3.32. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB

at [O]/[Li]=70, 296K …………………….………………….………….141

Figure 3.33. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB

at [O]/[Li]=70, 336K…………………..…………………….………….141

Figure 3.34. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB

at [O]/[Li]=70, 366K…………………..…………………….………….142

1 Figure 4.1. H NMR (CDCl3) of the oxetane oligomer initiated by CF3SO3CH3.….....167

Figure 4.2. TGA of oligo(oxtetane) initiated by CF3SO3CH3……...……...…….…….167

Figure 4.3. GPC of oligo(oxetane) initiated by CF3SO3CH3………….……………....168

Figure 4.4. FT-IR spectrum of oligo(oxetane) initiated by CF3SO3CH3……….……..168

1 Figure 4.5. H NMR (CDCl3) of oligo(trimethylene oxide) synthesized via the activated

monomer mechanism……………………………………………………..172

13 Figure 4.6. C NMR (CDCl3) of oligo(trimethylene oxide) synthesized via the activated

monomer mechanism………………………………………….………….172

Figure 4.7. MALDI-TOF of the oligo(trimethylene oxide) synthesized via the activated

XVIII

monomer mechanism ………………………………………….………….173

Figure 4.8. FT-IR spectra of the oligo(trimethylene oxide) synthesized via the active

chain end mechanism and activated monomer mechanism……………....173

Figure 4.9. TGA of oligo(trimethylene oxide) made via the activated monomer

mechanism, and of PEGME purchased from Aldrich……...…………….174

Figure 4.10. GPC (solvent: DMF) of oligo(trimethylene oxide) synthesized via the

activated monomer mechanism……………………….………...……….174

1 Figure 4.11. H NMR (CDCl3) of oligo(trimethylene oxide) disubstituted oxetane.....179

13 Figure 4.12. C NMR (CDCl3) of oligo(trimethylene oxide) disubstituted

oxetane …………………………………………………………………...179

1 Figure 4.13. H NMR (CDCl3) of oxetane-co- (oligo-trimethylene oxide disubstituted

oxetane) ………………………………………………………………....180

Figure 4.14. TGA (at 10℃/min heating rate) of oxetane comb copolymer and its

complexes with LiTFSI or LiTMPB at [O]/[Li]=3…………………...….181

Figure 4.15. DSC thermograms of oligo(oxetane) with 7 repeat units, oligo(oxetane)

with 4 repeat units, oxetane comb copolymer and its polymer electrolytes

complexed with LiTFSI and LiTMPB at [O]/[Li]=30, at a heating rate of

10℃/min after quenching from 120℃……………………….…………..183

Figure 4.16. Glass transition temperature (Tg) of oxetane comb copolymer and its

polymer electrolytes plotted against salt concentration ([O]/[Li]). (Note:

here [O]/[Li]=100 stands for uncomplexed polymer.……..……….……..184

Figure 4.17. Ionic conductivity at 100 KHz vs. the O/Li ratio of oxetane comb

copolymers complexed with LiTFSI and LiTMPB (297K) ……………...187

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Figure 4.18. Ionic conductivity at 100 KHz vs. the O/Li ratio of oxetane comb

copolymers complexed with LiTFSI and LiTMPB (336K and 376K)….187

Figure. 4.19. Conductivity comparison vs. [O]/[Li] ratios of oxetane comb

copolymer/LiTMPB and ethylene oxide comb copolymer/LiTMPB

electrolytes at 297K, 336K and 376K…….………………….………….188

Figure 4.20. Conductivity comparison vs. [O]/[Li] ratios of oxetane comb

copolymer/LiTFSI and ethylene oxide comb copolymer/LiTFSI

electrolytes at 297K, 336K and 376K………………………….……….188

Figure 4.21. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=70 (σ at

100KHz).………………………………………………………………….190

Figure 4.22. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=50 (σ at

100KHz) ………………………………………………………………190

Figure 4.23. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=30 (σ at

100KHz)…...……………..…………………………………………….. 191

Figure 4.24. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=10 (σ at

100KHz)...... ……..191

Figure 4.25. VTF conductivity plot for oxetane comb czopolymer/LiTFSI ([O]/[Li]=70,

Tg=-78℃) and oxetane comb copolymer/ LiTMPB ([O]/[Li]=70, Tg=-77℃)

complexes (σ at 100KHz) ………………..…………………..………….192

Figure 4.26. VTF conductivity plot for oxetane comb copolymer/LiTFSI ([O]/[Li]=50,

Tg=-76℃) and oxetane comb copolymer/ LiTMPB ([O]/[Li]=50, Tg=-74℃)

complexes (σ at 100 KHz)…………………………………...…………...192

Figure 4.27. VTF conductivity plot for oxetane comb copolymer/LiTFSI ([O]/[Li]=10,

Tg=-58℃; [O]/[Li]=30, Tg=-74℃) and oxetane comb copolymer/ LiTMPB

([O]/[Li]=30, Tg=-68℃) complexes (σ at 100KHz)……………………...193

Figure. 4.28. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB at [O]/[Li]=50 and 297K.………….195

Figure. 4.29. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB at [O]/[Li]=50 and 336K.………….195

Figure. 4.30. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB at [O]/[Li]=50 and 376K………...... 196

Figure. 4.31. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB at [O]/[Li]=30 and 297K…………196

Figure. 4.32. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB at [O]/[Li]=30 and 336K ………....197

Figure. 4.33. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and

oxetane comb copolymer/LiTMPB at [O]/[Li]=30 and 376K ………….197

XXI

Novel Lithium Salt and Polymer Electrolytes for Polymer Lithium Batteries

Abstract

JIAN LIN

Synthesis and characterization of a novel lithium salt that operates on the principle of steric occlusion was conducted. Lithium tetrakis[3,5- bis(trifluoromethyl)phenyl]borate (LiTMPB) incorporating a bulky anion with electronic delocalization was synthesized.

Ethylene oxide (EO) comb polymer with poly(trimethylene oxide) backbones and

-O-(CH2CH2O)m-CH3 (m=7) side chains, and trimethylene oxide (TMO) comb polymer with poly(trimethylene oxide) backbones and -O-(CH2CH2CH2O)n-CH3 (n=4) side chains were prepared and complexed with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and LiTMPB salts to form homogeneous electrolytes. All these electrolytes were amorphous and thermally stable up to 150℃. The glass transition temperatures (Tgs) of the pure EO and TMO comb polymers were -68 and -82℃, respectively. The Tgs of the

TMO electrolytes were at least 10℃ lower and rose more slowly with lithium salt concentration than EO counterparts. Their conductivities rose with the increase of salt concentration to a maximum and then dropped. The highest conductivities for the EO comb polymer/LiTFSI and EO comb polymer/LiTMPB electrolytes at ambient temperature and 100 KHz were 3.19*10-5 S/cm at 25:1 [O]/[Li] and 4.65*10-6 S/cm at

XXII

50:1 [O]/[Li], respectively. At 100℃ and 100 KHz, they rose to 9.0*10-4 and 2.0*10-4

S/cm, respectively. For their TMO counterparts, the highest conductivities at ambient temperature and 100 KHz were 4.62*10-5 S/cm at 10:1 [O]/[Li] and 6.34*10-6 S/cm at

30:1 [O]/[Li], respectively. At 100℃ and 100 KHz, they rose to 1.1*10-3 and 2.6*10-4

S/cm, respectively. At given [O]/[Li] ratios from 70/1 to 10/1, TMO comb polymer electrolytes had comparable or even higher conductivities than their EO counterparts. The conductivity/temperature dependence of these electrolytes obeyed the VTF equation instead of the Arrhenius equation, which confirmed their amorphous characteristics.

Conductivity measurements at low frequencies were used as an approximation for cell polarization. It showed that the LiTMPB electrolytes had less cell polarization than their LiTFSI analogues.

An equivalent circuit model composed of three parallel circuits, each with a capacitance and resistance in series, was proposed for electrical behavior simulation and could accurately simulate the ethylene oxide electrolytes’ capacitance and resistance at different frequencies and temperatures.

XXIII

Chapter 1. Introduction

The understanding of the need to use energy more efficiently spread fast from

1980s. A battery with an energy density of at least 120Wh/kg, a stability of more than

1000 charging/discharging cycles, a lifetime of at least 5 years and a reasonable cost became the target. In the US, the lithium-ion battery is considered to be one of the most promising energy storage devices and thus its development is greatly supported.

Lithium is the lightest element that can be safely handled in electrochemical processes and it exhibits the highest oxidation potential of any element, all of which make lithium ideally suited for high energy density batteries. In conventional lithium ion batteries, liquid organic solutions are used as electrolytes and they have severe problems.

First, when the liquid electrolyte is in contact with the lithium-metal electrode, it causes a variety of problems such as the development of passivating layers [1] and dendrite formation [2-3] which in the worst case can lead to fire and explosion. Second, liquid electrolytes may have decomposition reactions because of the high oxidation potential of the metal oxide cathode. Third, the liquid may suffer from volatility, pressure buildup or even explosion, harmful leakage or gassing and flammability. Thus, a wide operating temperature range is impractical. Because of all these shortcomings, the replacement of the liquid electrolyte with a thin polymer membrane would be highly desirable. In that case, both ion conduction and mechanical separation can be realized in a single solid electrolyte membrane. Furthermore, polymer electrolytes expand the options available for battery design. A battery can be designed to conform to the dimensions of any device due to the flexibility of polymer membranes.

1.1. The rocking-chair cell

A simple polymer electrolyte battery would be composed of a thin membrane of a polymer/salt complex sandwiched between an anode and an intercalation cathode.

Because lithium metal can cause severe problems as an anode, it has been replaced by a lithium-ion source such as graphite or other forms of carbon [4-6] and various tin oxide- based glasses [7]. The intercalation cathode materials are typically inorganic insertion compounds. X-ray studies show that these materials typically possess layers or tunnels into which lithium ions may insert. Cobalt oxide and manganese oxide are the most promising due to the yield of large open circuit voltages (up to 4 V) [8-9]. For the electrochemical reactions, during discharging of such batteries, lithium ions would be formed by oxidation at the anode, releasing electrons to the outer circuit, and the formed lithium cations migrate through the polymer membrane to the cathode to form intercalation material. The potential difference between cathode and anode, or the Gibbs free energy, is the driving force for electrons to produce current to power a device. For charging, the reactions would be reversed and lithium ions are “rocked” back to the anode (Fig. 1.1) [9].

Fig. 1.1. Schematic illustration of a lithium rocking chair

Battery with graphite and spinel as intercalation electrode [9]

1.2. Polymer electrolytes for secondary lithium batteries

In 1973, Wright discovered ionic conductivity in alkali metal salt complexes of poly (ethylene oxide) (PEO) [10-12], and this discovery resulted in lots of research of

PEO as a solid electrolyte for secondary lithium batteries [13]. PEO can achieve high molecular weight easily by metallic coordination polymerization of ethylene oxide and is commercially available. Furthermore, the ether oxygen has sufficient electron-donor power to form coordinate bonds with cations and compensate for the lattice energy of the salt. It has also been suggested that PEO has a suitable distance between coordinating

3 sites to form intrapolymer bonds with each cation. Thus, it is a good solvent for lots of salts and facilitates ion-pair separation of dissolved salts. However, PEO has a melting point of 65℃, and is approximately 85% crystalline. Crystalline domains not only block ion paths but also reduce the system flexibility. As a result, the ambient temperature conductivity of PEO/lithium salt complex is very low (<10-8 S/cm) and it is not satisfactory as a polymer solid electrolyte.

Unlike PEO, poly (propylene oxide) (PPO) is completely amorphous, but the conductivities of its lithium salt solutions above 60℃ are much lower than those of PEO and salt solubility in PPO is not comparable to that of PEO. The reason lies in the fact that PPO has a lower dielectric constant and its methyl (CH3) groups limit the segmental motion required to promote conductivity, and hinder the complexation of cations [14-15].

Poly (ethylene imine) (PEI) has been studied as another polymer electrolyte since it has nitrogen on the backbone as a strong electron-donor like ether oxygen in PEO and has a low cohesive energy density. The melting temperature of PEI is 60℃, comparing favorably with that of PEO, 65℃. Ratner’s group [16] complexed PEI with various amounts of sodium triflate and studied their conductivity behavior. They found the complex changed from amorphous to crystalline as the salt concentration increased from ratio of N/Na of 6/1 to 4/1, while the glass transition temperature rose from -5 to +9℃.

Unfortunately, the conductivities were very poor even for amorphous complexes (3.1*

10-7 S/cm at 41℃). To improve the conductivity, branched poly (ethyleneimine) (BPEI), which has primary, secondary and tertiary nitrogen atoms, was studied. Although BPEI has a lower glass transition temperature, its Tg severely increases with the addition of salt, from ~-50 (pure) to ~50℃ (N/Li=4), along with significant decreases in conductivity. For

BEPI complexed with lithium triflate at [N]/[Li]=20, the maximum conductivity was still only 10-6 S/cm at 20℃ [17-18]. Glatzhofer’s group radically cross-linked branched poly

(N-allylethylenimine) to improve PEI’s mechanical strength, using lithium triflate to make a solid polymer electrolyte. However, the highest room temperature conductivity was only 10-8 S/cm which increased to 10-5 S/cm at 80℃ [19].

The limitations of all these polymer hosts have prompted a search for alternatives.

There are several principal requirements for an improved solid polymer electrolyte: (1) the polymer should be amorphous or at least of reduced crystallinity at and below ambient temperature, (2) it should possess solvation and coordination sites for ions which assist ion-pair dissociation, (3) the polymer molecules should be sufficiently mobile to facilitate ion migration. These polymer hosts can be subdivided into random copolymers, comb-branched and hyperbranched polymers, block copolymers, networks and composite and blends. Since there have been a number of reviews published before 2000 [20-32], in this chapter the author will just briefly talk about basic polymer hosts and then focus on the latest developments.

1.2.1. Random copolymers

Crystallinity in PEO can be greatly reduced and it can even become amorphous by copolymerizing ethylene oxide units with small amounts of methylene oxide units, propylene oxide units or epichlorohydrin units. The poly (oxymethylene)-r-poly

(oxyethylene) can achieve high molecular weight of 5*105 Daltons, and when mixed with

-5 LiCF3SO3, the maximum conductivity is 5*10 S/cm at [O]/[Li] =25 which is comparable to values reported for comb polymers containing short poly (oxyethylene)

5 segments. This high conductivity is expected for polymers which do not crystallize at room temperature [33]. This group also synthesized block copolymer polystyrene-b- polyoxyethylene-poly [oxymethylene-oligo(oxyethylene)] to achieve better mechanical properties due to the crystallization of the oxyethylene block. However, this crystallization was detrimental to the low-temperature conductivity of the

-5 copolymer/LiCF3SO3 (σ<10 S/cm) [34]. For PEO-r-PPO (PPO 19 mol%), when mixed with LiAsF6, the room temperature conductivity is almost ten times greater than that in pure PEO. It could also be cross-linked to give better mechanical properties, without damaging its conductivity [35-36].

Ikeda and his coworkers synthesized high molecular weight (MW=106) poly[epichlorohydrin-co-(ethylene oxide)]s [P(EH/EO)s], with high ethylene oxide (EO) content, and used them as matrices for polymer solid electrolytes. They found that, with the increase of EO content, the glass transition temperature decreased and the EO crystalline phase increased. The EO content of the P (EH/EO)s, that gave a maximum conductivity of 2.2*10-5 S/cm at room temperature, was found to be 81 mol%, using

LiClO4 doping at the optimum concentration of [O]/[Li]=20. The optimum lithium salt amount was decided by two opposing factors. The good one is that the increase of salt concentration increases the number of ionic carriers, and decreases the melting point of the complex and the activation energy of diffusion, which contributes to the increase of ionic conductivity. However, the increase of salt concentration enhanced the fraction of complexed ether oxygens. As a result, the mobility of the copolymer matrix segments decreased and Tg increased [37].

1.2.2. Comb polymers and hyperbranched polymers

Comb polymers provide another effective approach to minimize crystallization and enhance ionic conductivity [38]. Normally they are amorphous with low glass transition temperature, high segmental mobility and more free volume, which are important prerequisites for high ionic conductivities. Many different comb polymers with oligo(oxyethylene) side chains have been prepared, using poly (methacrylic acid) [39-41], poly (itaconic acid) [42-43], polyglutamates [44-46], polystyrenes [47], poly (p- phenylstyrene) [48-50], polysiloxanes [51-56], polynorbornene [57] and polyphosphazenes [58-70] as polymer backbones. These systems all show higher room temperature conductivities (~10-5 S/cm), when complexed with alkali metal salts, than the corresponding PEO/salt electrolytes. Since most of these comb polymers are described in previous reviews [20-32], here we just mention several new comb polymers which were published recently or have maintained great interest so far.

Wang’s group fabricated transparent and flexible solid polymer electrolytes from copolymerization of acrylonitrile (AN) and methacrylate ester of PEO macromonomer

(mono substituted) initiated by AIBN [71]. The PAN-PEOs consist of glassy or hard (A) and rubbery or soft (B) segments. The soft PEO segments can dissolve alkali metal while the hard interconnected PAN segments can act as reinforcing filler and hence contribute to the dimensional stability of the polymer electrolytes [72]. Furthermore, since PAN can decrease the crystallinity of PEG and favor the mobility of short chains distributed in the copolymer, the conductivity is greatly improved. The conductivity can be optimized by controlling the PAN content and doping with high molecular weight PEO (Mn=3*106) in the PAN-PEO copolymers. The conductivity reached a maximum of 6.79*10-4 S/cm at

25℃. Another advantage of using PAN, mentioned below in the gel electrolyte part, is that PAN can inhibit dendrite growth during charging of Li batteries [73].

A series of new copolymers using poly(styrene-co-maleic anhydride) as a backbone and poly(ethylene glycol) monomethyl ether (PEGME) (Fig. 1.2) with different molecular weights as side chains were synthesized and their conductivities were studied

[74]. By dynamic mechanical analysis, they found the polymer electrolytes possess two glass transitions, α-transition corresponding to main chain glass transition and β- transition to the side PEO chains. The temperature dependence of the conductivity showed WLF (Williams-Landel-Ferry) behavior. The room temperature conductivities for the copolymer with 12 oxyethylene units as a side chain reached about 10-5 S/cm when complexed with LiBF4. They also found that the segmental motion (evaluated by average relaxation time of the specimen) of the electrolyte also depended on the nature of lithium salt, and its order was LiCF3SO3>LiSCN>LiBF4>LiClO4, which was consistent with the order of conductivity observed for these four salts.

High-molecular-weight (105-106) polyethers, poly[ethylene oxide-co-2-(2- methoxyethoxy)ethyl glycidyl ether] [P(EO/MEEGE)] were prepared by ring-opening copolymerization of ethylene oxide (EO) with 2-(2-methoxyethoxy) ethyl glycidyl ether

(MEEGE) and gave elastic and self-standing polymer electrolyte films without cross- linking [75-77]. With the increase of the MEEGE composition, the conductivity appreciably increased and reached a maximum at MEEGE compositions of 0.2-0.3. There were two possible reasons for the conductivity increase. First, the degree of crystallinity of the copolymers decreased with increasing the MEEGE composition. Second, as mentioned before, the introduction of highly mobile polyether side chains (MEEGE)

8 which coordinated with lithium ions, assisted faster ion transport. A P[EO/MEEGE] containing 27 mol% of MEEGE complexed with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), exhibited high ionic conductivities of

1.4*10-3 S/cm at 60℃ and 3.3*10-4 S/cm even at 30℃, without adding any low molecular weight plasticizers.

Polysiloxanes are valued primarily for their low Tg and outstanding thermo- oxidative stability. The ionic conductivity of comb polysiloxanes based on poly

(methylhydrosiloxane) (PMHS) (Tg=-134.7 ℃ ) substituted with

[methoxyoligo(oxyethylene)ethoxy] side groups was first investigated in 1985 [51, 78-

79]. Following this work, several oligoether-substituted polysiloxane systems have been extensively studied as polymer electrolytes by Smid [53, 55, 80], Shriver [54, 81, 82] and

West [83-85] groups.

Smid’s group synthesized a series of polysiloxane comb polymers with different length of oligo (oxyethylene) side chains and studied their conductivities. These polymers are effective solvent-free media for ion conduction of dissolved lithium salts. The conductivities for poly {[ω-methoxyhepta(oxyethylene)ethoxy]methoxy-siloxane} (with

7 oxythylene repeating units as side chain, Fig. 1.3) complexed with LiClO4 at an

[O]/[Li]=25 was 7*10-5 S/cm at room temperature and 2.5*10-4 S/cm at 70℃. Since these comb polymers are viscous liquids at room temperature, they tried to improve its mechanical strength by either cross-linking or blending with high molecular weight

(MW=4*106) PEO. The conductivity was somewhat lowered but still in the same scale. It is noteworthy that the blends with PEO can be cast into thin, flexible and tough films with good conducting properties [53, 80]. West’s group synthesized a series of double-

9 comb, disubstituted polysiloxane polymers and complexed with LiN(SO2CF3)2. They found their conductivities were superior to monosubstituted analogues. For the polymer host which contains six EO units in each side chain, the maximum ambient conductivity was 4.5*10-4 S/cm, the highest value yet recorded for a lithium-doped comb polymer electrolyte [83-85].

Similarly, polysilane comb polymers, which incorporate oligo(oxyethylene) moieties in the side chains of the polymers, have been prepared and tested as polymer electrolyte hosts [86-88]. The electrolyte

[(CH3CH2OCH2CH2O(CH2)4)Si(CH3)]n/LiCF3SO3 at [O]/[Li]=4 had a room temperature ionic conductivity of 1.25*10-7 S/cm while the copolymer had a higher conductivity of

1.9*10-7 S/cm at [O]/[Li]=8. The moderate conductivity is possibly due to the availability of only two oxygen atoms per side chain, which is insufficient to achieve efficient lithium ion complexation. Another interesting thing is that the fully ether functionalized polymer

[(CH3CH2OCH2CH2O(CH2)4)2Si]n (Fig. 1.4) is the poorest ionic conductor among this series, mostly likely due to the formation of crystalline phases when complexed with

LiCF3SO3 [89]. To improve this series’ conductivities, poly [(4, 7, 10, 13- tetraoxatetradecyl)methylsilane] (Fig. 1.5) was synthesized and studied as a polymer host.

-5 When complexed with LiClO4 (molar ratio [O] / [Li] =8), its conductivity was 2.12*10

S/cm at room temperature and 4.76*10-4 S/cm at 80℃ which was much higher than that of previous polysilanes with only two oxyethylene units [90]. Thus, the conductivities of the polysilane series compare favorably with those of polysiloxanes or polyphosphazenes.

O O

o CH CH CH CH 80 C, 24 h 2 + HO(CH2CH2O)nCH3 m toluene-p-sulfonic acid

O OH O OCH3 60 oC, 24 h CH2 CH CH CH + CH3OH CH2 CH CH CH n (CH ) SO n 3 2 O O(CH2CH2O)nCH3 O O(CH2CH2O)nCH3 (n=7, 12 and 17) Fig.1.2. Preparation of comb polymer using poly(styrene-co-maleic anhydride) as backbone CH 3 Si O n O(CH2CH2O)nCH3

F ig.1.3. Monosubstitued polysiloxane comb polymer

(CH2)4O(CH2)2OCH2CH3 Si O

(CH2)4O(CH2)2OCH2CH3

Fig.1.4. Disubstituted polysiloxane comb polymer

CH3 Si n O O CH 3 3 Fig.1.5. Poly [(4, 7, 10, 13-tetraoxatetradecyl)methylsilane]

OCH CH OCH CH OCH 2 2 2 2 3 N P n OCH2CH2OCH2CH2OCH3

F ig. 1.6. Poly[bis(2-(2-methyoxyethoxy)ethoxy)phosphazene]

OCH2CH2OCH2CH2OCH3 N P n O(CH2CH2O)16CH3

Fig. 1.7. Polyphosphazene with 2 and 16 ethylene oxide repeat units

Polyphosphazene is another inorganic polymer which has aroused tremendous interest as a novel polymer host. It has a very flexible backbone and accordingly very low

Tg. Furthermore, polyphosphazenes are inherently fire retardant and stable to electrochemical oxidation and reduction reactions. Hence, even in the event of damage to an energy storage device, it would be expected that the fire hazard could be minimal in the presence of polyphosphazene. Allcock and Shriver are very active in this field and have developed a series of polyphosphazenes substituted by a variety of different alkyl ether and/or alkoxy side groups and studied their conductivities in a great detail. The poly[bis(2-(2-methyoxyethoxy)ethoxy)phosphazene] (MEEP, Fig. 1.6) showed excellent promise as a solid electrolyte host when complexed with silver triflate (Ag SO3CF3) or

-5 lithium triflate (LiSO3CF3). This system showed a maximum conductivity of 2.7*10

S/cm, which was 2-3 orders of magnitude higher than an analogous complex of poly

(ethylene oxide) with LiCF3SO3 at room temperature. Polarization cell experiments on triflate salt complexes indicated that the transference number for Ag+ was 0.03 or less at

50℃, and for Li+ it was 0.32 under the same conditions [58-59]. The temperature dependence of conductivities obeyed the VTF equation which is typical of amorphous polymer electrolytes and will be discussed in detail later. Again, as the salt concentration increased, the conductivity reached a maximum and then diminished due to the counterbalance of increased ion carriers and greater polymer chain rigidity. When the length of alkyl ether side chain was increased, the maximum conductivity increased slightly and reached a plateau at six to eight oxyethylene groups per side unit (4.8*10-5

S/cm). For side groups of formula, -(CH2CH2O)xCH3, the change from x=1 to x=7 or 8 resulted in an almost doubling of the room temperature (LiSO3CF3) conductivity from

2.6*10-5 to 4.8*10-5 S/cm. This is due to the presence of an increasing number of side chain oxygen atoms, which provide more coordination sites both for ion-pair separation and more free volume (empty sites) to which a cation can be transferred. However, when the side chains exceed roughly eight oxyethylene units, they are now long enough to undergo co-alignment and form microcrystallites. When x=16, the crystalline melting temperature is near 30 ℃. Thus, these polymers begin to develop similar problems to those found with PEO although they still have much higher conductivities and lower crystalline melting temperatures than those of PEO. These results are very similar to

West’s findings for disubstituted polysiloxane/LiN(SO2CF3)2 complexes. In this case, conductivities increasesd with x up to x=6 and then decreased [83-85]. They also synthesized several mixed-substituent oligo(oxyethylene) derivatives and found the crystallinity was reduced since two different types of side groups distributed randomly along the backbone can break the symmetry of the polymer and reduce crystallinity as was expected. For example, a mixed-substituent polymer with side units in which x=2 and x=16 (Fig. 1.7) had a conductivity of 3.9*10-5 S/cm. Unfortunately, although all these polymers with linear side chains have useful conductivities at ambient temperature, they are viscous gums rather than dimensionally stable polymers [60-62].

Another interesting thing they found was that conductivity was dominated by polymer matrix ion coordination ability instead of free volume. By substituting one ion- coordinative oxyethylene side group with alkoxy units of different chain lengths, the conductivity dropped more and more as the chain lengths increased. With

-O(CH2CH2O)2CH3 as one side group and -O(CH2)2CH3 as another side group, the ambient temperature conductivity was only half that of MEEP, and it dropped to 2*10-6

S/cm when -O(CH2)9CH3 was used as a side group. Apparently, alkyloxy groups with different chain lengths provided molecular flexibility and free volume and subsequently lower Tg of the system; however, they had no accessible coordination sites. As a result, the number of coordinative oxygen atoms per unit volume of polymer reduced, which limited ion-pair separation and starved the system of empty coordination sites to which the cations can move [63-64].

In order to obtain dimensionally stable polymers instead of gums while keeping high ionic conductivity, Allcock’s group tried branched alkyl ether side chains. They found that conductivities were similar to those of the linear chain counterparts but they were dimensionally stable without cross-linking. It was reasoned that branched ether side chains may maximize the number of oxygen coordination sites, introduce sufficient conformational disorder to prevent micro-crystallization, while providing more effective side chain entanglement and interdigitation than in the linear side chain analogues and thereby increase the rigidity of the polymers [62, 64-65].

Polyphosphazenes with crown ether side groups and other side groups incorporating sulfur or nitrogen were studied to find out how the coordination strength influences the conductivity. All these polymers showed much poorer conductivities compared to that of MEEP. Thus, it was concluded the cation conductivity in comb polymers depended on the ability of side groups to form weak coordination adducts with the cation sufficient to enhance the dissociation of the ion-paring but insufficient to completely immobilize the cations. Crown ether, sulfur and nitrogen in these side groups apparently binded cations too strongly and restricted the cation motion which also

14 indirectly lowerer the mobility of the anions. As a result, the conductivity dropped tremendously [66, 91-92].

Hyperbranched polymers are similar to comb polymers. For example, they are completely amorphous, highly soluble in common organic solvents, and highly processible because of their highly branched nature. Furthermore, they even have more free chain ends which could greatly contribute to the mobility of lithium ions. Itoh’s group synthesized a series of hyperbranched poly[bis(hexaethylene glycol)benzoate]s with terminal acetyl groups (HBPs, Fig. 1.8) and complexed them with LiCF3SO3 or

LiN(CF3SO2)2. It was found the hyperbranched polymer with a pentaethylene glycol chain when complexed with LiN(CF3SO2)2 ([O]/[Li]=25) exhibited a maximum conductivity of 9*10-5 S/cm at 80℃ [93-94]. Similarly, Hawker’s group [95] synthesized the same hyperbranched polymer but with a terminal hydroxyl group. They reported a maximum conductivity of 7*10-5 S/cm at 60℃ with 5 EO repeating units, which was in the same scale with Itoh’s results.

Tang’s group polymerized AB2 type monomer glycidol using glycerol as an initiator and boron trifluoride diethyl etherate as a catalyst and obtained hyperbranched poly(glycidol) (HPG). They blended HPG with poly(ethylene oxide) polyurethane (PU) which provided good dimensional stability, and complexed the blend with LiClO4. The ionic conductivity of this blend was about 6.6*10-6 and 6.3*10-4 S/cm at 20 and 60℃, respectively [96]. Apparently, the hyperbranched polymer hosts provided lower conductivities than comb polymers.

Fig. 1.8. Hyperbranched poly[bis(hexaethylene glycol)benzoate]

1.2.3. Block copolymers

Amorphous polymer hosts normally have poor mechanical properties. The polymer molecules are so flexible that they flow slowly under light pressure. As a result, they are regarded as a polymer fluid and are not practical in solid polymer electrolytes.

Thus, although they have been proved useful hosts for high-conductivity electrolytes, the poor mechanical properties stimulate people to develop more complex structures in order to optimize both electrical and mechanical properties for practical applications. The use of block copolymers is one way. A long block ABA copolymer based on styrene- butadiene-styrene (SBS) with oligo(oxyethylene) side chains grafted to the B-block was studied. The hard styrene block material associated to form domains which acted as physical cross-linking sites or reinforcement sites controlling the mechanical properties of the material. The PEO, which provided the ion-solvating medium, formed a continuous phase when polymer films were cast from a selective solvent which favors microphase separation. Conductivities, typically 10-5 S/cm at ambient temperatures, were obtained [97].

A variety of di- and triblock phosphazene-ethylene oxide copolymers produced from mono- and diphosphoranimine-terminated poly(ethylene oxide)s were reported recently [98-99]. These polymers were supposed to incorporate the advantages of high

RT conductivity of MEEP and good dimensional stability of poly(ethylene oxide). An ambient conductivity of 3*10-5 S/cm was obtained when complexed with lithium triflate, which was similar to that of MEEP. Of course, as the percentages of PEO in the block copolymers increased, the conductivities decreased due to the formation of crystalline

PEO. However, the mechanical properties were only slightly better than those of MEEP.

1.2.4. Networks

Another method to improve mechanical properties is via cross-linking, and it has been proved to work very nicely. MEEP has been crosslinked chemically to produce a material with increased dimensional stability, with comparable conductivity to the analogous uncrosslinked system when doped with lithium salts [100]. Chemical cross- linking requires the incorporation of a difunctional reagent, such as poly (ethylene glycol), and thus introduces impurities into the system. A better and cleaner method is by 60Co γ- irradiation which involves only side-group coupling reactions. Furthermore, this method allows the cross-linking of the system with the salt already present, producing much greater control over the material properties and the shape of the devices that employ this system [101]. However, its limited accessibility and related expense make this technique less attractive. To solve this problem, inexpensive, readily available ultraviolet radiation was used and produces a pure product [102]. The possible mechanism for cross-linking involves photolytic cleavage of C-H or C-C bonds followed by the resultant carbon radical cross-combination.

1.2.5. Composite polymer electrolytes and blends

Another effective way of improving conductivities and mechanical properties is by the use of composite polymer electrolytes with inert ceramic fillers or blend-based polymer electrolytes. For composite polymer electrolytes, it was initially explored by

Weston and Steele [103], and has been greatly developed [104-106]. For linear PEO,

Croce et al. [107] reported a nanocomposite polymer electrolytes composed of a PEO-

LiClO4 electrolyte and a nanosized titanium oxide (TiO2) or alumina (Al2O3) filler, where addition of the ceramic fillers increased the ionic conductivities of the electrolytes to 10-5

S/cm at 30℃, which reached the scale of those of comb polymer electrolytes. The nanosized fillers kinetically inhibited crystallization of the ionic conductive amorphous phases. The effect was explained by the high surface area of the dispersed fillers. Similar behavior was also reported in other studies for composite polymer electrolytes based on linear PEO [108-110]. For comb polymer polyglycidylether with oligo(oxyethylene) as side chains, fine silica was dispersed within it to enhance the mechanical properties above the melting point. The composite polymer electrolyte had an ionic conductivity of

1.6*10-4 and 1.6*10-3 S/cm at 30 and 90℃, respectively [111]. For hyperbranched polymer electrolytes, Itoh’s group synthesized a hyperbranched polymer, poly[bis(triethylene glycol)benzoate] capped with terminal acetyl groups (HBP) (Fig. 1.8) and complexed it with LiClO4, LiBF4 or LiN(CF3SO2)2 salts. Addition of inert ceramic fillers such as α-LiAlO2 or γ-LiAlO2 produced composite polymer electrolytes. They found the composite electrolytes led to an increase in ionic conductivity and lithium ion transference numbers, and better mechanical performance. For LiN(CF3SO2)2 as a complexing salt ([Li]/[O]=1/9) and 10 wt% α-LiAlO2 as a filler, the conductivity reached

-5 6.6*10 S/cm, while the lithium ion transference number (tLi+) was as high as 0.49 at 80℃

[112].

In addition to the composite polymer electrolytes, a blend-based polymer electrolyte, composed of two conductive components and lithium salts, is another example with favorable electrical properties. In this case, one conductive component acts as a plasticizer to reduce the nonconductive crystalline phase of PEO, leading to an increase in the ionic conductivity at low temperatures. Itoh’s group chose HBP (Fig. 1.8) with 2 EO units as an additive and blended with PEO complexed with LiN(CF3SO2)2.

Addition of 10 wt% of HBP improved ionic conductivity of blend-based polymer electrolytes from 7*10-4 to 8.1*10-4 S/cm and also the cation transference number increased from 0.09 to 0.15 at 80℃ with [O]/[Li]=8. Furthermore, they observed the addition of the hyperbranched polymer was significantly effective for improvement of ionic conductivity in the low temperature region [94].

Finally, Itoh’s group combined the composite polymer electrolyte system with the blend-based polymer electrolyte system and found this complex system produced higher conductivities. In detail, they blended PEO, HBP, ceramic filler (BaTiO3) and lithium salt

[LiN(CF3SO2)2] and found the optimized composite polymer electrolyte had conductivities of 2.6*10-4 S/cm at 30 ℃ and 5.2*10-3 S/cm at 80 ℃ , respectively.

Moreover, it had an electrochemical stability window of 4.0 V and was stable until 312℃ under air [113].

1.3. Gel electrolytes

Gel electrolytes are currently of another great interest, particularly with regard to achieving higher ionic conductivities. They are formed by dissolving a salt in a polar liquid and adding a polymer or polymer mixtures to give the material mechanical stability.

The polymer added is composed of functional groups that are not sufficient to separate ion-pairs. Thus, polar solvents with high dielectric constants such as propylene carbonate

(PC), ethylene carbonate (EC), N, N-dimethylformamide (DMF) and γ-butyrolactone are required to promote sufficient charge separation. In these systems, the major role of the polymer is mostly to maintain a solid and stable matrix, whereas the ion migration within the matrix under an electric field is promoted by the solvents. They can achieve conductivity of about 10-3 S/cm at ambient temperature. However, the mechanical property of such polymer electrolytes is generally very poor because of the characteristics of gels and solvents.

One promising system is based on atactic poly (methyl methacrylate) (PMMA) as a gelation agent [114] and has attracted a lot of interest [115-119]. The research performed by Bohnke’s group [119-120] showed that atactic PMMA formed ionically conductive gels with LiClO4 in propylene carbonate (PC), or mixtures of PC and ethylene carbonate (EC). The addition of PMMA in various proportions to LiClO4-PC electrolyte considerably increased the viscosity to reach a solid rubber-like material. On the other hand, the conductivities at room temperature of these gels decreased very slightly and remained very close to that of the liquid electrolyte. At an about 20 wt% polymer concentration, these gels possessed room temperature conductivities on the order of 10-3

S/cm. This high ion mobility could be explained by the existence of a continuous

20 conduction path available through the solvent. Different characterization techniques showed that PMMA just acted as a matrix and no lithium solvation with PMMA was found. In order to improve their poor mechanical properties at low polymer concentrations, Wunder and his coworkers used stereocomplexed PMMA composed of atactic and isotactic PMMA. Due to association between isotactic PMMA and syndiotactic sequences of atactic PMMA, strong gels were formed with a dynamic elastic modulus one order of magnitude higher than the electrolytes containing only atactic

PMMA while conductivities maintained as high as 4*10-3 S/cm at RT [121].

Gel electrolyte based on poly(vinylidene fluoride) (PVDF) host has been investigated since the early 1980s as another potential system [122-123]. Tsuchida and his coworkers [124-125] in 1983 showed that the physically cross-linked gelled PVDF had a conductivity of 1*10-3 S/cm at 25℃. In 1995, Bell Communications Research Inc. reported that a porous P(VDF-HFP) system had a conductivity of 1*10-3 S/cm at 25℃

[126-127].

Another gel system which has been extensively studied used polyacrylonitrile

(PAN). It has shown higher conductivities than PEO [128] or PPO [129]. Compared to

PEO, the PAN-lithium salt system has many advantages. A typical electrolyte, composed of 38 mol% ethylene carbonate, 33 mol% propylene carbonate, 8 mol% LiClO4 and 21 mol% PAN, had conductivities of 1.7*10-3 S/cm at 20℃ and 1.1*10-3 S/cm at -10℃

[130]. Another electrolyte, composed of 40 mol% ethylene carbonate, 34.75 mol%

-2 propylene carbonate, 4.25 mol% LiAsF6 and 21 mol% PAN, had a conductivity of 10

S/cm at 60℃. Furthermore, PAN proved to be more thermally stable than PEO [73]. In addition to these advantages, the use of PAN-based electrolytes could inhibit most

21 disadvantageous dendrite growth in the charging of lithium batteries [73]. FT-IR and 7Li solid-state NMR techniques showed that the lithium ions interacted with the nitrogen atoms of the AN segments. Thus, PAN improved the dissociation of the lithium salt, while providing mechanical strength.

A comb copolymer was synthesized from poly (ethylene glycol methyl ether)- methacrylate (PEGMEM) and acrylonitrile (AN), and doped with LiClO4 [131]. This copolymer had two Tg’s. One is associated with the amorphous regions of PEGMEM, the other corresponded to PAN segments. Furthermore, both Tgs increased with the concentration of LiClO4, indicating lithium ions coordinated with both ether oxygen of

PEGMEM and nitrogen atoms of the AN segments. However, they found the temperature dependency of the conductivity followed the Arrenius equation, instead of VTF theory, which meant the charge carriers were decoupled from the segmental motion of the polymer chain and transport occured via an activated hopping mechanism. However, the copolymer had a poor conductivity compared to the pure PEMGMEM. The maximum conductivity at 30℃ was only 3*10-6 S/cm.

Similarly, Dong’s group [132] combined PAN, PMMA and comb polymer

(PEGMEM) as a copolymer (Fig. 1.9) doped with LiClO4, and studied its conductivity with added propylene carbonate (PC) and/or silica. The copolymer had stiff chains (PAN and PMMA) as reinforcing backbone moieties for good mechanical strength and flexible side chains (PEGMEM) as the liquid matrix for ion transport. However, the pure

-8 copolymer/LiClO4 was a poor ion conductor (1.51*10 S/cm maximum at RT). When

SiO2 was added, the interaction between lone pair of oxygen and lithium ion was influenced by the nano-SiO2 for the particle effect. The ionic conductivity doubled to

3.75*10-8 S/cm. If propylene carbonate (PC) (50 wt%) was impregnated into the pure polymer, the conductivity jumped to 1.47*10-5 S/cm which was three orders of magnitude higher than “dry” polymer electrolyte. The surprising fact was that when both PC and

-4 SiO2 are added, the conductivity of this gel hybrid film increased even more to 4.16*10

S/cm at ambient temperature. This polymer electrolyte showed VTF behavior, indicating that the lithium ion motion depended on the flexibility of the polymer chains. Unlike the previous materials, the introduction of PMMA segments tremendously reduced the conductivity.

CH3 CH3 a CH2 CH + b CH2 C + c CH2 C CN AIBN O OCH3 O O CH CH O CH 2 2 7 3

CH3 CH3

CH2 CH CH2 C CH2 C a b c CN COO CH2CH2O CH3 O OCH3 7

Fig.1.9. Copolymer of PAN, PMMA and PEGMEM

When N-methylpyrolidone or propylene carbonate was added to the cross-linked

MEEP mentioned before, the conductivities were found to increase one to two orders of magnitude. These additives had little or no effect on the mechanical properties of MEEP at the 30 wt% additive level. Again, the conductivity enhancement was supposed to be due to a high level of solvation of the lithium ions and facile transport of these ion/solvent units through the swollen polymer matrix [133]. Similarly, macrocycle crown ethers and cryptands were used as additives to increase the ion-pair separation [134-136].

1.4. Polymer-in-salt electrolytes or ionic rubbers

The above materials discussed are typically formed by dissolution of salts in poly

(ethylene oxide) (PEO) or other ion-coordinating macromolecules that promote the dissociation of the salt and support ion transport by segment relaxation of polymer chains.

As the salt concentration in the polymer electrolyte is increased, the distance between ions decreases and ion-ion interactions become progressively more significant. The systems with high salt content are called polymer-in-salt electrolytes or ionic rubbers. In this system, the cation motion is decoupled from the polymer matrix and involves a void- to-void jumping or a cooperative bond exchange in large ionic clusters. The hopping between clusters of ions can be shown below (Fig. 1.10). An ion (e.g. Li+) reaches the conducting path at one side of the cluster and another ion of the same type leaves the other side [137]. In solid electrolytes of high salt concentration, separated single clusters come into contact with each other, thus forming an infinite cluster path promoting fast cation transport [138, 139]. The value of the decoupling index reflects the concentration of infinite conducting paths, which increases with an increase of salt concentration. The polymer induces mechanical stability of the conducting composite and suppresses the salt crystallization due to the interaction with the lithium cations. The reason this new material is promising is that it combines the merits of the polymer electrolytes (rubbery properties) and the superionic glasses (high lithium transport number). Such an electrolyte could have conductivities much higher than traditional polymer electrolytes.

Fig. 1.10. The mechanism of cooperative ion transport in clusters [137]

One example is the ionic rubber based on the ternary LiClO4-LiNO3-LiOAc and high molecular weight PEO (MW ~8*106) reported by Angel’s group. They found an ambient conductivity as high as 10-3 S/cm could be obtained from this ionic rubber, with a large rubbery temperature range (~20 ℃ to ~130℃). The current challenge is to find a salt or salt mixtures with wider electrochemical windows (>4.5 V) which is compatible with PEO polymer, or new high molecular weight polymers which are stable in more aggressive melts. Once this problem is settled, the practical application of this new ionic rubber in lithium batteries is very promising [140]. Based on this assumption, they developed a series of lithium sulfonates, which are known to exhibit electrochemical stability. One particular material was formed by complexing the anion of lithium

- chlorosulfonate, ClSO3 , with aluminum chloride to give the chloroaluminate-

- 7 chlorosulfonate anion, [AlCl3-SO3Cl] . Combination of high decoupling index (6*10 )

-3 and a low Tg (-30.4℃) gave an ionic conductivity of 1.6*10 S/cm at room temperature, with a 4.0 V electrochemical window [141].

Recently, polymer-in-salt electrolytes based on polyacrylonitrile (PAN) and acrylonitrile-butadiene copolymers have been extensively studied because the interaction between the lithium ions and nitrile groups is regarded as very suitable for the stabilization of highly conducting amorphous ionic clusters. Wang’s group [142] studied a polymer electrolyte, composed of PAN, propylene carbonate (PC) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), by Raman and AC impedance spectroscopy.

In the polymer electrolyte with relatively low salt content, the cations of the dissolved salt were associated with the polymer and transported by the segmental motion of the polymer, corresponding to the splitting of the 2270 cm-1 component from the 2245 cm-1

25 band of the nitrile stretching. As more salt was added, ionic associates such as

+ - -1 [Lim TFSIn ] (m>n) were formed. In the Raman spectrum, a new peak at 2280 cm

+ - representing the interaction between the polymer and ionic cluster [Lim TFSIn ] (m>n) as well as the 2270 cm-1 peak related to the nitrile-Li+ interaction were observed. When the salt content was extremely high, efficient percolation paths were constructed and contributed the most to the ionic conductivity of the electrolyte. A transition from “salt- in-polymer” to “polymer-in-salt” electrolyte was observed at this moment and conductivities of 10-3~10-4 S/cm were obtained at room temperature, which agreed with

Bushkova’s results that a conductivity of 2.1*10-3 S/cm could be obtained at [CN]/[Li]=2 for PAN copolymer/LiClO4 electrolyte [143]. However, both polymers were hardly soluble in organic solvents, which made the preparation of a solvent-free thin electrolyte film difficult. In order to overcome this problem, Florjanczyk and his coworkers [144] prepared random copolymers of acrylonitrile with butyl acrylate [poly(AN-r-BuA)].

Due to the presence of the BuA monomeric units, these copolymers readily dissolved in acetonitrile, which enabled the preparation of electrolytes by the standard solvent casting technique. They found that as the content of lithium bis(trifluoromethanesulfone)imide increased, the polymer/lithum salt complexes became more and more flexible. By contrast, the PEO systems became less flexible as the salt content increased. The reason for this seemed to be the much stronger donor properties of the ether groups compared to the nitrile groups. However, the conductivities (10-5 S/cm at

RT) were lower than those of pure PAN ionic rubbers. The temperature dependence of conductivity in these systems obeyed the VTF equation, indicating the ion mobility was coupled with the segmental motion of the polymer matrix.

1.5. Polyelectrolytes

By system modeling, Doyle and his coworkers showed that polymer electrolyte systems with a unity transference number for the cation had improved performance over systems with cation transference number of 0.2, even when the conductivity was decreased by an order of magnitude. The reason was that in applications such as electric vehicles, where a high discharge rate was desired, the former cells had larger energy densities and could attain higher peak-power densities while the latter cells would be depleted of electrolyte due to large concentration gradients that develop [145]. As a result, interest is growing in the development of single ion conductors, or polyelectrolytes, which have a positively or negatively charged ion covalently attached to the polymer backbone with only the counterion having any significant mobility. In electrochemical devices, therefore there would not be the same susceptibility to resistance increases owing to the build-up of high or low salt concentration at the interfaces during charging and discharging [32].

Although polyelectrolytes have this obvious advantage over polymer electrolytes, tight ion paring is always likely to occur and concentration and mobility of charge carriers will be largely reduced. Poly (diallyldimethylammonium chloride) (DDAC) (Fig.

1.11) was studied as a polyelectrolyte in early 1980’s [146]. The positive charge was anchored in the polymer backbone, so the conductivity displayed was clearly attributable to the anion chloride. It was hoped that tight ion pairing could be reduced since the positive quaternary nitrogen was surrounded by four alkyl groups, thus segregating chlorine anion from approaching nitrogen. However, the material was too rigid and the conductivity of pure DDAC at 25℃ was still very low (< 10-9 S/cm). To improve the

27 conductivity, poly (ethylene glycol) (MW=300) was used as a plasticizer and an amorphous film with Tg of -37℃ was obtained. The room temperature conductivity now reached 1*10-5 S/cm at 0.67/1 of [Cl]/[OH] molar ratio. A similar anion conductor was developed from polyphosphazene [147]. 80% of side chains were

–OCH2CH2OCH2CH2OCH3 groups while the rest were substituted by

+ - - - –OCH2CH2NMe2R Cl , Br or I groups, where R=CH3, C2H5, and n-C4H9 (Fig. 1.12).

Compared to the previous DDAC, these polyelectrolytes generated amazing conductivities, as high as 2.2*10-5 S/cm at 30℃, with iodine anions as the conducting species. The conductivity increased in the order of Cl-

For cation conductors, poly [(oligo(oxyethylene) methacrylate)-co-(alkali-metal methacrylate)] (Fig. 1.13) was synthesized by free radical copolymerization of oligo(oxyethylene) methacrylate and alkali-metal methacrylates on a Teflon plate. The ionic conductivity was higher than that of DDAC but still only 10-6 S/cm at 80℃ for lithium ions. They pointed out the conductivity was strongly affected by segmental motion of the polymer matrix [148].

Shriver’s group reported the synthesis and conductivity of a class of polyphosphazene with sulfonate and oligoether side chains (Fig. 1.14). The ionic conductivity is only 8*10-7 S/cm at 30℃ and 1.7*10-6 S/cm at 80℃, with a molar ratio of ether oxygen to sodium of 18/1. As a comparison, they prepared a polymer electrolyte formed by MEEP with sodium allylsulfonate [Na(CH2=CHCH2SO3)]. The resulting

28 polymer electrolyte had a comparable ionic conductivity with the polyelectrolyte, but both are two orders of magnitude lower than that of MEEP-NaCF3SO3. All these results again suggest that there is extensive ion-paring formation between sodium cation and sulfonate anion [149-151].

Another novel alternating comb copolymer polyelectrolyte poly [lithium-N-(4- sulfophenyl)maleimide-co-methoxyoligo(oxyethylene)methacrylate] P(LiSMOEn) (Fig.

1.15) with three different oligo(oxyethylene) side chains was synthesized [152]. All the three copolymers showed a glass transition at about -50℃ (Tg1) which was attributed to the oligo(oxyethylene) side chains. In addition, copolymers with n=7 and 12 also exhibited a second glass transition in the temperature range of 30-50℃ (Tg2) which was assigned to the main chain of the copolymer domain. The copolymer with n=16 showed an endothermic peak near room temperature which arised from the melting of the partially crystalline phase formed by the long oligoether side chains. The maximum conductivity at 30℃ was 1.5*10-7 S/cm for n=16. The ionic conduction followed a special dual VTF behavior. Similarly, Kerr’s group developed comb-shaped polyelectrolytes based on polyacrylate ethers and lithium alky sulfonate PAE8-co-

E3SO3Li and PAE8-g-EnSO3Li (n=2, 3) (Fig. 1.16 and 1.17). Again, these polyelectrolytes didn’t improve the ambient conductivities greatly (2.0*10-7 S/cm at RT with [O]/[Li]=40). When 50 wt% of plasticizer, propylene carbonate (PC)/ethyl methyl carbonate (EMC) (1/1, v/v) was added to PAE8-g-E2SO3Li, the ambient conductivity increased three orders of magnitude, due to the increased ion mobility in a micro-liquid environment and an increased concentration of free ions as a result of the much higher dielectric constant of the added liquid [153].

In order to reduce the strong ion-pairing in polyelectrolytes, Armand’s group tried using perfluorocarbon moieties (RF) acting as electron-withdrawing groups adjacent to

- the SO3 group to decrease the basicity of sulfonate anions, hence inducing solubility and dissociation of the ion-pairs. They prepared cross-linked polyelectrolytes (Fig. 1.18 and

1.19) by radical polymerization and ring-opening polymerization, respectively. The best conductivity for the material shown in Fig. 1.18 reached 10-6 S/cm and 10-5 S/cm at 30℃ and 72℃ respectively, at [O]/[Li] =40. They also found the cation mobility increased with lower surface charge Li

85℃. They concluded that although higher conductivities were achieved by the use of far less basic and coordinating perfluorosulfonate anions, strong ion-pairs still existed [154].

Similarly, comb polymer with 1,1,2-trifluorobutane sulfonate group was synthesized and compared to non-fluorinated sulfonate and carboxylate analogues (Fig. 1.20-1.22)

[155].The trifluorobutane sulfonate copolymer (Fig. 1.20) exhibited superior conductivities in all cases, with 7*10-6 S/cm at room temperature, with [O]/[Li] =15. The butane sulfonate copolymer (Fig. 1.21) had the next best conductivity but it was almost a decade lower. The conductivities of the carboxylate system (Fig. 1.22) are very poor (<

10-9 S/cm at room temperature). The latter results are in agreement with previous reports by workers using RCOO-Li+ group as the ion source [156-157]. Apparently it is a consequence of the relatively high ion dissociation energy for this group. It is noteworthy that the polyelectrolyte (Fig. 1.20) has conductivities just 50% lower than those of conventional comb polymer/salt mixtures, which was expected because of the additional

30 contribution from the free anion. Thus they concluded the extra electron-withdrawing capability of the fluorines in the side chain of (Fig. 1.20) facilitated easier release of the

Li+ ion for conduction which is the same from Armand’s result [154].

OCH2CH2OCH2CH2OCH3

n N P n OCH2CH2NMe2R

Cl - N R= CH3,C2H5, n C4H9

Fig.1.11.Poly (diallyldimethylammonium chloride) Fig.1.12. Polyphosphazene with charged side chains

SO3Na

O OR + CH3 COO Li N P N P Cl CH2 C CH2 C OR OR Br C CH3 R=C2H4OC2H4OCH3 I O O(CH2CH2O)nCH3 or (C2H4O)7.22CH3

Fig. 1.13. poly [(oligo(oxyethylene) methacrylate)- Fig. 1.14.Polyphosphazene with sulfonate and oligoether side chains co-(alkali-metal methacrylate)]

CH3 HC CH CH C 2 m O O CH3 O O N o N O O(CH2CH2O)nCH3 CH C 60 C + 2 O O CH CH O CH AIBN 2 2 n 3

SO3Li SO3Li

Fig.1.15. Poly [lithium-N-(4-sulfophenyl)maleimide-co-methoxyoligo(oxyethylene)methacrylate]

Fig. 1.16. Polyacrylate comb polymer with lithium alky sulfonate side group

Fig. 1.17. Polyacrylate comb polymer with lithium alky sulfonate side group

separated Fig. by 1.18.siloxane

Fig.1.18. Cross-linked polyelectrolyte 1

Fig. 1.19. Cross-linked polyelectrolyte 2

O OCH2CH2CHFCF2SO3 C

CH2 CH CH2 CH x y C O O CH CH O CH CH OCH 2 2 n 2 2 3

Fig. 1.20. Ethylene oxide comb polymer with trifluorobutane sulfonate on one side chain

O OCH2CH2CH2CH2SO3 Li C

CH2 CH CH2 CH x y C O O CH CH O CH CH OCH 2 2 n 2 2 3

Fig. 1.21. Ethylene oxide comb polymer with butane sulfonate on one side chain

O OCH2CH2CH2CO2 Li C

CH2 CH CH2 CH x y C O O CH CH O CH CH OCH 2 2 n 2 2 3

Fig. 1.22. Ethylene oxide comb polymer with carboxylate on one side chain

Fig. 1.23. Polyelectrolytes incorporating poly(pentaethylene glycol methyl ether acrylate co-allyloxyethyl acrylate) and lithium bis(allylmalonato)borate (taken from ref. 158)

Lately, Kerr’s group synthesized a new lithium salt, lithium bis(allylmalonato)borate

(LiBAMB) and reacted it with two tetramethyldisiloxane on both allyl sides. The resulting product then reacted with comb poly(pentaethylene glycol methyl ether acrylate co-allyloxyethyl acrylate) by hydrosilation and formed crosslinked single ion conductors

(Fig. 1.23). However, the highest ambient conductivity of the sample was only 2.7*10-8

S/cm, at an [O]/[Li] =40. When it was plasticized with 50 wt% of ethylene carbonate

(EC)/dimethyl carbonate (DMC), the gel polyelectrolyte reached the maximum ambient conductivity of 7.9*10-6 S/cm. They found the gel conductivity was more determined by the dielectric constant than by the viscosity of the solvent. It was encouraging that almost no relaxation and concentration polarization was observed from the preliminary Li/Li cycling profile [158].

1.6. Crystalline polymer electrolytes

Conductivity in polymer electrolytes has long been viewed as confined to the amorphous phase above the glass transition temperature, Tg, where polymer segmental motion creates a dynamic, disordered environment that plays a critical role in facilitating ion transport. Such segmental modes are usually relatively slow, limiting the hopping rate and therefore the maximum conductivity. So far, the maximum conductivity of amorphous polymer electrolytes remains around 10-4 S/cm at room temperature which is too low for many applications. In contrast to this prevailing view, the Tunstall group proposed a new approach to improving ionic conductivity. If the sites to which an ion migrates were already present and aligned in the structure, rather than relying on chain dynamics to generate such sites, then ion hopping could take place as soon as sufficient

36 energy were available for the ion to hop. In such case, there is no need to wait for reorganization of the environment. As a result, the ionic conductivity in the static, ordered environment of the crystalline phase can be greater than that in the equivalent amorphous material above Tg. For detail, they developed a method by which crystal structures of molecular solids may be determined ab initio from powder diffraction data.

By using this method, they determined the structures of three crystalline complexes formed with six ether oxygens per cation: PEO6/LiPF6, PEO6/LiAsF6 and PEO6/LiSbF6.

All three compounds are isostructural: pairs of PEO chains fold to form cyclindrical tunnels within which the lithium cations reside and coordinate with the ether oxygens.

The anions are located outside these tunnels in the interchain space and do not coordinate the cations. Thus lithium cation transport along the tunnels may be possible. The conductivity of the crystalline PEO6/LiAsF6 is more than one order of magnitude higher than the corresponding amorphous phase at lower temperatures. Furthermore, NMR studies of the transference number indicate that whereas the anions in amorphous polymer electrolytes are generally more mobile than the cations, the crystal structure appears to impose selectivity; lithium cations alone carried the current. Although the levels of conductivity achieved so far with crystalline polymer electrolytes are still too low for many applications (10-4 S/cm), their work defines a new direction in the search for higher ionic conductivity in the polymeric state. For example, by analogy with ceramic ionic conductors, introducing more defects such as interstitials or vacancies into these materials should substantially increase the ionic conductivity [159-160].

1.7. Other cation conductors

All the above polymer electrolytes are based on lithium salts and have achieved great success. However, little has been published for materials in which multivalent (di-, tri- and tetravalent) cations are the mobile species [32]. In view of negligible hazards and lower reactivity of magnesium in comparison with lithium, studies on rechargeable magnesium batteries are expected to increase greatly in the future. There has been some research on the polymeric complexes of PEO with magnesium salts [161-163].

Moderately high conductivities, ranging from 10-6 to 10-4 S/cm at 80-100 ℃ were reported for those complexes. However, the transport number of Mg2+ was found to be essentially zero in those polymeric complex systems. Thus, it is supposed that the magnesium-polymer interaction is so strong that such cations are immobile in polyethers

[164-165]. However, Morita’s group got a different result. They prepared a polymer electrolyte consisting of oligo(oxyethylene)-grafted polymethacrylate (PEO-PMA) matrix, doped with poly(ethylene glycol) dimethyl ether (PEGDE, MW=400) and complexed this

-4 with Mg[(CF3SO2)2N]2 or Mg(CF3SO3)2. A high conductivity, 10 S/cm at ambient temperature, was obtained for the polymer electrolyte containing Mg[(CF3SO2)2N]2. The

DC polarization of a Pt/Mg cell using the polymer electrolyte proved that Mg2+ is mobile in the system [166-167].

Furthermore, by using the same polymer host, trivalent rare-earth salts, LnX3

(Ln=La, Ce and Yb) were tried as salts and the complexes obtained had high ionic conductivities in the ambient temperature range [168-171]. About 10-5 S/cm of ionic conductivity was obtained for Ce3+ cation at room temperature, while for La3+ and Yb3+ cations the conductivities were lower. Again, they found both cation and anion mobilities

38 contributed to the ionic conductivity of these systems. In the neat PEO system containing dissolved rare earth salts, no cation transport was detected at ambient temperature [32,

172-173]. Since these cation conductors are out of our scope of lithium conductors, no more detail is discussed here.

1.8. Ion transport measurement

In polymer electrolytes, conductivity occurs by the migration of ions. Because of resistance to ion flow at the electrode-electrolyte interface [174-176], normal measurement of total ionic conductivity does not apply to polymer electrolytes. In general, it is much more difficult to establish a low-resistance interface for ion flow than for electron flow. With the blocking electrodes used for such measurements [177], conductive ions cannot cross through the electrode/electrolyte interface. The conductive ions accumulate near the interface when an electric field is applied. Thus, ion concentration gradient grows and impedes further diffusion of the conductive ions until an equilibrium state is reached. This double layer behavior is explained using the

Helmholtz model (Fig. 1.24). This problem is overcome by the use of ac impedance spectroscopy, which minimizes the effects of cell polarization. Measurements are often made with the electrolyte sandwiched between a pair of electrochemically inert electrodes made of stainless steel, platinum or brass. The detailed methodology of impedance spectroscopy is described in-depth elsewhere [174-177].

Fig.1.24. The Helmholtz Model

During measurement, a sinusoidal potential is applied and the magnitude Im and phase shift (ф) of the current (I) are measured. Thus the sinusoidal dependence of the current I with time (t) is given by

I(t)=Imsin(ωt+ф) (1)

Where ω is the applied frequency. These measurements are repeated at a series of frequencies which can range as low as 10-4 Hz to as high as 10MHz. From these data we can extract the conductivity and dielectric constant of the bulk electrolyte sample. This analysis was originally proposed by Cole and Cole [174] and developed in detail by

Macdonald [175-177]. From the data above we can express the ac current vector (I*) in terms of real (I’) and imaginary (I’’) parts:

I*=I’+ j I’’ j= 1 (2)

A similar expression applies to the ac potential:

V*=V’+ j V’’ (3)

The ac impedance is expressed as

Z*=Z’ + j Z’’, Z*=V*/I* (4)

In an impedance spectrum, also known as Cole-Cole plot, the real part of the impedance (Z’) is plotted against the imaginary part (Z’’) for data collected at various frequencies. The bulk resistance of the electrolyte (Rb) is one of the quantities that can be derived from such a plot. Rb along with geometric factor (g) of the electrolyte sample

(thickness of the sample and electrode area) yields the bulk conductivity: g=thickness/area, σ=g/Rb (5)

For the conductivity interpretation, a single equivalent circuit (Fig. 1.25) is used.

The equivalent circuit consists of a parallel circuit of constant phase element 1 (CPE1) and the bulk electrolyte resistance (Rb) in series with constant phase element 2 (CPE2).

The CPE1 and CPE2 may be associated with the bulk electrolyte and the double layer capacitances, respectively. For detailed interpretation, refer to Sherwood [178] and

Posaldas [179].

CPE1 CPE2

R b Fig. 1.25. The equivalent circuit model

1.9. Temperature dependence of ionic conductivity

The temperature dependence of the conductivity of polymer electrolytes indicates it is an activated process. The conductivity increases with increasing temperature. The

Arrhenius equation, shown in eq. (6),

σ＝A exp (-Ea/RT) (6) often provides a good representation of the data and is typical for conventional solid electrolytes as well as crystalline polymer electrolytes. The activation energy (Ea) and the preexponential factor (A) are obtained from plots of lnσvs 1000/T (in Kelvin). It has been shown that for amorphous polymer complexes the temperature dependence of the conductivity can be fit to the Vogel-Tamman-Fulcher equation, eq. (7),

σ＝Aexp [-B/R(T-T0)] (7) where A is a pre-exponential factor which may be related to ion mobility and ion association, B is an apparent activation energy, which is different from the activation energy Ea, obtained using the Arrenius equation, and T0 is the thermodynamic glass transition temperature of the electrolyte (ideal Tg, at which the free volume is zero, or the mobility becomes zero), the value of which is related to measured Tg and has been proved to be 35-50℃ lower than Tg for many polymer electrolyte systems either theoretically by the Adam-Gibbs analysis or experimentally by many researchers [180-182].

1.10 Research goal

It has been proposed that the main contribution to ion transport may be due to the anion because measured cation transference numbers are often as low as 0.05-0.2 [183].

Furthermore, we know that as the frequency decreases, interfacial polarization of the ionic conductor increases, which results in a continuously increasing dielectric constant and voltage losses; this phenomenon has been predicted by theory [184] and has been reported for inorganic solid electrolytes [185-186]. As a result, good cation transport is desirable for use in high efficiency, reversible lithium batteries. Attempts have been made

42 to reduce the anion mobility and increase the contribution from the cation. A variety of approaches are based on polyelectrolytes which bind the anion covalently to the polymer chain. However, the room temperature conductivities of these materials are very low

(~10-7 S/cm) without plasticizers. Thus, the primary research objective as described in this thesis was to combine the advantage of high ionic conductivity of comb polymers with high cation transference number to make materials that have little or no cell polarization. By the use of bulky anions, it is expected that anion migration in a DC field can be prevented. This was studied with respect to ionic conductivity, especially low- frequency conductivity which is indicative of cell polarization. Along the way, basic structure-property relationships were determined, and new macromolecules were designed and synthesized in an effort to understand the mechanism of conductivity of solid polymer electrolytes complexed with lithium bis(trifluoromethane)sulfonimide

(LiTFSI) or lithium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (LiTMPB), especially their low-frequency conductivities.

More specifically, this thesis focuses on three main research projects:

1. Novel lithium salts with bulky anions which are immobile or hardly mobile in

polymer hosts were designed and synthesized. Their complexing behavior with

diglyme was studied by TGA. LiTMPB was then selected for study of its electrical

properties when complexed with polymers.

2. Comb polymers and copolymers with trimethylene oxide as backbone and

oligo(oxyethylene) (MW=350, 7 repeating units) as side chains were synthesized. The

ionic conductivities, especially low-frequency conductivities of these systems when

complexed with LiTFSI or LiTMPB were studied as well as their thermal and

physical properties.

3. A new synthetic method was used for the synthesis of oligo(trimethylene oxide)

bearing one hydroxyl group. Based on this side chain, novel comb copolymers were

synthesized. The solution properties, thermal properties and ionic conductivities of

these polymers were investigated, and compared with those of previous comb

polymers.

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1977, 124, 1360.

179. M. J. R. Presa, R. I. Tuueri, M. I. Florit, D. Posaldas, J. Electroanal. Chem., 2001,

82, 502.

180. G. Adam, J. H. Gibbs, J. Chem. Phys., 1965, 43, 139.

181. C. A. Angell, Solid State Ionics, 1983, 9/10, 3.

182. M. C. Wintersgill, J. J. Fontanella, in Polymer Electrolyte Reviews, J. R.

MacCallum, C. A. Vincent Eds.; Elesevier Applied Science: London, 1989; Vol.2,

p. 43.

183. H. Cheradame, J. F. Lenest, A. Gandini, M. Leveque, J. Power Sources, 1985, 14,

27.

184. M. E. Lines, Phys. Rev. B: Condens. Matter., 1979, 19,1189-1195.

185. R. D. Armstrong, K. Taylor, J. Electroanal. Chem., 1975, 63, 9-17.

186. I. M. Hodge, M. D. Ingram, A. R. West, J. Am. Ceram. Soc., 1976, 59, 360-366

Chapter 2. Synthesis and characterization of novel lithium salts with bulky anions

2.1. Introduction

The development of commercially viable secondary lithium batteries, based on a lithium metal anode and a lithium ion intercalating cathode material (e.g. Li/LixTiS2) or on two lithium ion intercalating materials (rocking chair cells [1] or lithium ion cells [2-

4], e. g. LixC6/Li1-xMn2O4) not only depends on anode and cathode materials, solvents

(for liquid electrolytes) or polymer matrix (for polymer electrolytes), but also relies on the availability of nontoxic thermally, chemically, and electrochemically stable lithium salts [5]. Further requirements of the lithium salts are sufficiently high solubility (> 1M) in dipolar aprotic solvents (e.g. ethylene carbonate or propylene carbonate), sufficiently high conductivity of the electrolyte solution (> 10-3 S/cm), and compatibility with all cell materials.

Typical electrolytes used in early studies included coordinatively saturated large

- - - - anions such as BF4 , AsF6 , PF6 and ClO4 , which are not easily oxidized or reduced at electrodes and hence were used as the anions of the well-known lithium electrolytes for electrochemical studies. However, some problems associated with these anions have stimulated the search for substitutes [6]. Lithium tetrafluoroborate (LiBF4) is thermally stable [7], however, it often yields poorly conducting solutions, bad cycling efficiencies, and its decomposition product BF3 can initiate polymerization of cyclic ethers. Lithium hexafluoroarsenate (LiAsF6) is also thermally stable [8], but its reduction products are environmentally toxic. Lithium hexaflurophosphate (LiPF6) itself is thermally unstable in the solid state (decomposition temperature, 30℃) and decomposes to yield scarcely soluble LiF and the Lewis acid, PF5. PF5 can initiate the polymerization of solvents and

58 therefore degrade the solution, just like BF3. Lithium perchlorate (LiClO4) solutions are thermally unstable and show explosion risks, especially in ethers [9].

Overall, the high lattice energy of the decomposition product LiF and its low solubility in all solvents is a common reason for stability problems with all lithium salts containing Lewis-acid-based anions [10]. Furthermore, all these anions are thermodynamically unstable with lithium, e.g. LiAsF6 generates about 1600 kJ/mol heat upon reduction [11].

Recently new lithium salts based on the principle of reduced ion-pairing resulting from charge delocalization of the anion have been developed, such as lithium bis(trifluoromethylsulfonyl)imide and lithium tris(trifluoromethylsulfonyl)methide. They show higher chemical stability than the Lewis-acid-based anions, larger radii and hence lower lattice energies, leading to good solubility and low polarizability, especially when perfluorinated, due to the low polarizability of fluorine. Furthermore, they demonstrate low anion/solvent interaction, and generally their large delocalization results in excellent electrochemical stability and a weak anion-lithium cation interaction resulting in higher conductivities. These salts also exhibit good safety characteristics and high thermal stability. For example, the lithium bis(trifluomethylsulfonyl)imide is stable up to 200℃, a very high temperature, when compared with lithium hexafluorophosphate (30℃). Such salts are also useful electrolytes for solid polymer electrolytes [12-14]. In this thesis, lithium bis(trifluoromethylsulfonyl)imide behavior was used to compare with that of other salts.

Another new class of thermally stable lithium electrolytes has recently been discovered with anions generally composed of a chelate complex of boron with aromatic

59 or aliphatic diols (e. g., catechol, Fig. 2.1, tetrafluoro-1, 2-benzenediol, Fig. 2.2, tetrakis(trifloromethyl) ethylenediol, Fig. 2.3) [15-16] or carboxylic acids (e.g., salicylic acid, Fig. 2.4) [17-18]. Typically, these anions have covalent bonds, large radii, and a pronounced charge delocalization due to aromatic substituents or perfluoroaliphatic electron-withdrawing groups, or both. As a consequence of the large anionic radii and charge delocalization, the solubility of the new lithium salts in various dipolar aprotic solvents and some ethers is more than 1 mol/kg. Furthermore, the perfluorinted salts provide stronger charge delocalization of the anions which can suppress ion pair formation even more, and as a result, the conductivity of perfluorinated electrolytes can be greatly improved compared to similar nonfluorinated electrolytes. Gores and his coworkers compared the conductivities of lithium bis[tetrafluoro-1, 2-benzenediolato(2-

)-O, O’]borate with those of the less or nonfluorinated lithium bis[1, 2-benzenediolato(2-

)-O, O’]borates and found solutions of the latter salts in dimethoxyethane (DME) had only about 20 to 40% of the conductivity of the perfluorinated salt at 1M, which confirmed the lower ion-ion interaction from partly fluorinated or perfluorinated borates

[15, 19]. More significantly, Angell and his coworkers [20-21] have shown that the degrees of dissociation for lithium bis(perfluoropinacolato)borate (Fig.2.3) in propylene carbonate (PC) and dimethoxyethane (DME) were larger than those of lithium bis(perfluoromethanesulfonyl)imide (LiTFSI) in the same solvent, reported by Hayamizu et al. [22]. The comparison showed that this borate anion was an even more weakly coordinating anion than TFSI-.

Doyle’s group realized the importance of the lithium ion transference number in lithium/polymer cells. They pointed out the battery’s performance depended not only on

60 the ionic conductivity of the electrolyte but also on a high cation transference number, t+, since this condition could minimize the overpotentials due to the increase of concentration gradients in the vicinity of the electrodes and the depletion of electrolyte inside porous electrodes [23]. However, values for lithium ions commonly found in nonaqueous solutions fall below 0.5 [24-25].

Experimentally, Siddiqui and his coworkers tried phenol and naphthol anions and studied their charge-transfer reactions in the crystalline phase of PEO, but no ionic conductivities were reported [26-27]. Wright’s group studied complexes of PEO with the sodium salts of imidazole (Fig. 2.5), benzimidazole (Fig. 2.6), 2-methylbenzimidazole

(Fig. 2.7), 4-phenylphenyl (Fig. 2.8) and a mesogenic ester of benzimidazole-5- carboxylic acid (Fig. 2.9). The highest conductivities (5*10-4 S/cm at 100℃) were observed with the sodium salt of benzimidazole. In comparison, lithium imidazole salt had the lowest conductivity among all of them due to the low charge delocalization within the anion and subsequent tighter ion pairing. It is noteworthy that the mesogenic complex of the PEO-sodium mesogenic ester of benzimidazole-5-carboxylic acid salt and its uncharged analogue (50/50) system had conductivities of 3.3*10-8 S/cm at 20℃ and

1.8*10-5 S/cm at 80℃. The latter was approximately the same as the other systems containing sodium 2-methylbenzimidazole salt or sodium 4-phenylphenol salt and was surprising high in view of the size of the anion. Supposedly most of the current should be carried by cation but no more experiments were carried out in this paper [28].

Wang’s group prepared three new lithium superacid salts with general formula

CnF2n+1SO3Li (where n is 4, 8 and 10) and used them as salts in polyethylene oxide (PEO) electrolytes with CF3SO3Li and (CF3SO2)2NLi as controls. They found PEO electrolytes

61 containing (CF3SO2)2NLi gave the highest conductivity followed by C4H9SO3Li and

C8H17SO3Li. PEO electrolytes containing C10F21SO3Li and CF3SO3Li showed the lowest room temperature conductivity. However, they neither analyzed why lithium salts with bulky superacid anions had lower conductivity nor studied the electrolytes’ polarization behavior [29].

Kita’s group [30-31] compared the characteristics of some fluoro organic lithium salts and found the fluoro organic lithium salts with SO2 groups showed higher conductivities than those with CO groups (0.1 mol/L lithium salts in ½ v/v PC/DME at

25℃). Methide [(CF3SO2)3CLi] and imide [(RfSO2)2NLi] salts showed higher conductivities than oxide salts RfSO2OLi with only one RfSO2 group. As we know, the conductivities of electrolytes are related to the mobility and concentration of dissociated ions. The mobility of organic anions decreases with the increase of their molecular size. It is interesting that the conductivity of lithium tetrakis[3,5- bis(trifluoromethyl)phenyl]borate (LiTFPB) (2.7 mS/cm) with a molecular weight six times larger than that of CF3SO3Li is higher than that of CF3SO3Li (2.3 mS/cm) (at 25℃ with 0.1 mol/L lithium salt in ½ v/v PC/DME). This indicates that ionic dissociation of

LiTFSB is much higher than that of CF3SO3Li, thus LiTFSB can provide much more free ions than CF3SO3Li and consequently higher conductivities even though LiTFSB has a bulky anion. Furthermore, they also studied the oxidation stability of these salts in propylene carbonate electrolyte in terms of anodic oxidation potential. The oxidation potential of LiTFPB (at 0.5 mA/cm2) was 5.1 V, comparing favorably with that of

(CF3SO2)2NLi (5.2 V), and was 1.3 V higher than that of LiBPh4.

Angell’s group used the pulsed field gradient spin echo method on the NMR resonances of 7Li and 19F in the temperature range of 30-95℃ to study lithium ion self- diffusivity and transport number of lithium bis(perfluoropinacolato)borate (Fig. 2.3).

They found lithium diffusivities were higher than those of the anions in propylene carbonate (PC) or dimethoxyethane (DME). Transport numbers for Li ion were 0.55 for

PC solutions and 0.53 for DME solutions at 50℃. Thus, for the first time, they proved lithium salts with large anions can have cationic transport numbers >0.5 [21].

F F O O F O O F B Li B Li O O F O O F F F

Fig. 2.1. lithium bis[1,2-benzenediol Fig. 2.2. lithium bis[1,2-benzenediol ato(2-)-O,O']borate ato(2-)-O,O']borate

CF CF 3 3 O CF3 O O CF3 O O Li B O O CF Li B CF3 3 CF3 CF3 O O O Fig. 2.3. lithium bis[1,2-tetrakis Fig. 2.4. lithium bis[1,2-salicylato(2-)- (trifluoromethyl) ethylenediolato O,O']borate (2-)-O,O']borate

N N N O N N N

Fig.2.5. Fig.2.6. benzimidazole Fig.2.7. 2-methylbenz- Fig.2.8. 4-phenylphenyl imidazole anion anion imidazole anion anion

NC CN NC CN O O

O(CH2)7CH3 O(CH2)7CH3 N O O N

Fig.2.9. mesogenic ester of benzimidazole-5-carboxylic acid anion and its uncharged analogue

As we have seen above, so far only a few groups have done research on polymer electrolytes with lithium salts bearing big or even bulky anions, and no group has studied their cell polarization behavior in detail. Since bulky anions can hardly move in a polymer matrix, the lithium cation transference number should be near 1 in this type of polymer electrolyte. Consequently, it is intriguing to study the electrical properties of polymer electrolytes with lithium salts bearing bulky anions. In an effort to obtain better electrolytes, our first aim of this project was to synthesize and characterize several lithium salts with bulky anions which could combine the advantages of high lithium mobility with high lithium transport number and low ion-pairing. The interatctions of these salts, when complexed with 1,2-dimethoxyethane (DME, glyme) (a model compound of PEO) was measured by TGA to determine how the interactions between lithium salts and glyme changed with the structures of the different anions.

2.2. Experimental procedures

2.2.1. Materials

3,5-Bis(trifluoromethyl)bromobenzene (Oakwood Products, purity 97%) was dried using 4Å molecular sieves and distilled. 1,1,1,3,3,3-hexafluoro-2-propanol (Aldrich, purity 99+%) was dried over 4Å molecular sieves and distilled. Pinacol (Aldrich, purity

98%), pentaerithritol (Aldrich, purity 99+%), LiOH.H2O (Aldrich, purity 98+%), boric acid (Aldrich, 99+%), lithium borohydride (Aldrich, 2.0 M solution in tetrahydrofuran), lithium wire (Aldrich, purity 99.9%), trimethyl borate (Aldrich, purity 98+%), lithium tetrafluoroborate (Aldrich, purity 98%) and benzopinacol (Aldrich, purity 99%) were used as received.

2.2.2. Characterization techniques

2.2.2.1. Infrared spectroscopy

An ABB Bomem MB104 FTIR spectrophotometer coupled to a computer was used for IR characterization. Solid organic samples (~ 1 mg) for the infrared measurements were ground with KBr (~100 mg) and pressed into pellets. The wavenumber was taken from 4000 cm-1 to 600 cm-1 (resolution 1cm-1) and spectra were calculated after 20 scans.

2.2.2.2. Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF)

The matrix was prepared by mixing ~4 mg of measured sample, 1 mg of sodium iodide, 20 mg of 2-cyano-4-hydroxycinnamic acid or 2-(4-hydroxyphenylazo)benzoic acid and water to 1 mL. A drop of the mixture was dried on the matrix plate and put into the instrument. Ions were formed by laser desorption at 337 nm (N2 laser), accelerated and detected as positive ions. Each experimental trial was performed with a delay setting

65 of 50,000 ns to filter out the low molecular weight signal primary due to the matrix and fragments.

2.2.2.3. Nuclear magnetic resonance spectroscopy (NMR)

1 11 H and B spectra of solutions of different lithium salts in CDCl3 or DMSO-d6 were recorded using a Varian 300 spectrometer. Care was taken to ensure that spectral widths and pulse intervals were sufficient to produce complete relaxation and reliable intensity ratios. Chemical shifts were measured relative to the solvent signal at δ=7.27 ppm (for CDCl3) or δ=2.50 ppm (for DMSO-d6) or δ=0.00 ppm (for trimethyl borate in DMSO-d6)

2.2.2.4. X-ray photoelectron spectroscopy (XPS)

X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a

Kratos AXIS165 system, with a monochromatic Al Kα excitation source. To avoid potential changes to surface composition, no Ar+ surface-etch or other cleaning was performed prior to spectrum acquisition. All spectra were taken with the electron energy analyzer normal to the specimen surface. During all spectra acquisition, a charge neutralization filament was turned on to minimize possible electrical charging effects.

2.2.2.5. Thermogravimetric analysis (TGA)

Thermogravimetric analyses were performed using a 2950 TA instruments TGA analyzer. The weight-pickup experiments were performed under a glyme atmosphere with glyme blown into the heating tubes by dry nitrogen at 30ºC . The temperature was ramped up using a heating rate of 10ºC/min from room temperature to 400ºC under a nitrogen atmosphere.

2.2.2.6. Gel permeation chromatography (GPC)

Analytical GPC was carried out on a Waters 510 GPC, equipped with a Waters

946 UV photodiode array detector and a Waters 410 refractive index detector in sequence.

N, N-dimethylformide (DMF) (HPLC grade) was used as solvent at a flow rate of 1.0 mL/min through two Styragel columns (Waters Styragel HR-4E DMF, HR-5E DMF). A

0.5% concentration was used. Calibration covering the required molecular weight range

(M=500-106) is available from polystyrene standards.

2.2.3. Synthesis

2.2.3.1. Attempted synthesis of dilithium pentaerithritol di(pinacolato)borate

Pinacol (12.06 g, 0.10 mol), LiOH.H2O (4.28 g, 0.10 mol), boric acid (6.18 g,

0.10 mol) and pentaerithritol (6.81 g, 0.05 mol) were transferred to a 1 L round-bottom flask fitted with a reflux condenser. The compounds were purged with argon. 200 mL of water was added under argon. The flask was slowly heated to reflux. More water was added slowly (190 mL water total) until all materials were dissolved when refluxing. A clean pale yellow solution was obtained. The solution was cooled and a lithium salt started to crystallize. The flask was kept in a refrigerator overnight and the salt was filtered. The lithium salt was dried at 150ºC under vacuum (3*10-4 torr) for 2 days, 11.3 g was obtained (70.2%). The product was recrystallized after boiling in water under argon and the salt was filtered and dried at 150ºC under vacuum (3*10-4 torr). 8.36 g pure lithium salt was obtained (yield: 52%). 1H NMR showed the reaction did not

1 proceed as it was expected. The product was lithium pinacolborate [32]. H NMR (D2O):

δ (ppm) 1.058 (s, -CH3, 24H).

2.2.3.2. Synthesis of lithium tetrakis(methanoxyato)borate

It was prepared following the method described by Gores [33]. Lithium wire (6 g,

0.86 mol) was put into 500 mL three-necked flask protected by argon purge. To this flask, cooled externally with ice water, 180 mL of methanol (dried by CaH2 overnight and distilled) was added slowly. Hydrogen came out quickly. Finally, another 180 mL of methanol was added. White solid was obtained. The mixture was heated to reflux and a homogeneous solution is obtained. To this solution, trimethyl borate (91.2 g, 0.86 mol) was slowly dropped while reflux continued, no precipitate was observed. The reaction was stopped and kept in a refrigerator overnight. White needle-like crystal was formed. It was filtered, desiccated under vacuum (3*10-4 torr) for 2 days. Pure white crystal was obtained (112.47 g, yield 92.1%). 1H NMR (DMSO-d6) δ (ppm) 3.165 (s, 12H). 13C

NMR (DMSO-d6): δ (ppm) 49.51. It began to decompose from 50℃.

2.2.3.3. Synthesis of ithium tetrakis(1, 1, 1, 3, 3, 3-hexafluoro-2-propoxy)borate

Route 1. Purified lithium tetrakis(methanoxyato)borate (2.17g, 15 mmol) was dispersed at 47ºC in 20 mL of THF. To this stirred dispersion 1,1,1,3,3,3-hexafluoro-

2-propanol (10.2 g, 0.06 mol) dissolved in 10 mL of THF was slowly added. Upon addition of 8 mL of this solution, all lithium tetrakis(methanoxyato)borate was dissolved and the solution became slightly cloudy. After rotary-evaporating all low boiling points compounds at 45ºC under reduced pressure, the viscous cloudy mixture was dried in an oven at 90ºC under vacuum (3*10-4 torr) for 3 days. White power was obtained and weighed 2.2 g. 1H NMR spectrum of the dried product (Fig. 2.10) is very complicated and apparently it is not the expected product. Some side reactions happened.

100

7.602 5.630 4.475 3.608 3.317 3.181 2.690 2.498 2.167 1.896 1.748 1.143 7.759

DMSO-d6

THF THF

1.00 0.65 0.54 16.27 9.94 0.63 1.62 0.77 0.34 0.16 2.08 3.96 2.08 1.96 0.81 0.66

ppm8.0 (f1) 7.0 6.0 5.0 4.0 3.0 2.0 1.0 1 Fig.2.3.Fig. 2.10. 1H NMR of(DMSO the dried-d6) products. of the dried products from the attempted preparation of lithium tetrakis(1, 1, 1, 3, 3, 3-hexafluoro-2-propoxy)borate

Route 2. 1,1,1,3,3,3-hexafluoro-2-propanol (12.2g, 0.0726 mol) was added to glyme (30 mL) at 0 ºC . lithium borohydride (7.25 mL, 0.0145 mol) was syringed into a dropping funnel and added dropwise to the solution. Gas evolution was seen and the solution remained clear. The reaction was monitored by GPC. After 3 hours, one single peak was seen and the reaction was stopped. Glyme was removed and a white solid was obtained. It was recrystallized from tetrahydrofuran (THF) and 10.3 g of pure compound was obtained (84% yield). 1H NMR (DMSO-d6): δ (ppm) 4.847 (hepta, 4H). when staying under open atmosphere, this salt could easily pick up moisture and hydrolyze to get back to the original 1,1,1,3,3,3-hexafluoro-2-propanol. Thus, it was hydrolytic and chemically unstable and not suitable for lithium battery use.

2.2.3.4. Synthesis of lithium bis(benzopinacolato(2-)-O,O´) borate

Route 1. Benzopinacol (5.55 g, 0.015 mol) was dissolved in N,N-dimethyl acetamide (65 mL) and toluene (5 mL) mixture. Boric acid (0.468 g, 0.0075 mol) was then added and shaken to dissolve. Afterwards LiOH.H2O (0.321 g, 0.0075 mol) was added; it did not dissolve. The mixture was always purged with argon. The solution was heated and the distillate fractionated to collect an azeotropic mixture of water and toluene.

The mixture became pale brown after 5 mins. After 2 h, around 4 mL of azeotropic mixture (~0.5 g water inside) was collected and the top thermometer began to rise to

110ºC which meant that all the water produced had been collected. The reaction was stopped. The cloudy mixture was centrifuged. The supernatant solution was rotary evaporated and 5.8 g of viscous brown liquid was obtained. 1H NMR (Fig. 2.11) showed there were two different types of phenyl groups and GPC showed two peaks. There were at least two compounds in the liquid mixture.

Route 2. Benzopinacol (5.36 g, 0.0145 mol) was added to 1,2- dimethoxyethane (40 mL) and heated to dissolve. Lithium borohydride (3.625 mL,

0.00725 mol) was syringed into a dropping funnel and added dropwise into the benzopinacol solution while refluxing. Gas evolution was observed. The solution color changed from pale blue to dark blue and then translucent. The reaction was monitored by

GPC. After 4 h, one single peak was seen and the reaction was stopped. Glyme was removed and a white solid was obtained. It was recrystallized from toluene and a pure salt was obtained (4.93 g, 90.7%). 1H NMR (300 MHz, DMSO-d6): δ (ppm) 7.386, 7.356

(d, ortho-, 16H); 7.318, 7.294, 7.268 (t, meta-, 16H); 7.217, 7.193, 7.170 (t, para-, 8H).

11 B NMR [B(OCH3)3]: δ (ppm) -16.88 (s, 1B).

350

7.804 7.780 7.776 7.606 7.602 7.578 7.557 7.553 7.494 7.467 7.443 7.383 7.358 7.338 7.314 7.289 7.263 7.242 7.218 7.200 7.165 7.083 7.062 7.056 7.039 7.013 6.987 7.808

300

250 one compound second compound 200

150

100

2.00 1.04 2.10 1.67 2.01 1.06 0.32 -50

ppm (f1) 7.50 7.00 Fig.2.11. 1H NMR (CDCl ) of the products from the attempted preparation of 3 lithium bis(benzopinacolato(2-)-O,O´)borate

2.2.3.5. Synthesis of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

This compound can be prepared using two routes. The first one is ion exchange the commercial sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate to the lithium form.

10 mL wet acid form cation exchange resin (exchange capacity 20.5 mmol) was exchanged to the lithium form resin using a LiCl solution [LiCl (8.7 g, 205 mmol) dissolved in DI water (10 mL)]. The column was then washed with water and methanol.

Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] boronate (1 g, 1.13 mmol) methanol (10 mL) solution was poured into the column which was then washed with methanol (200 mL). The solution was collected, rotary evaporated to remove solvent and dried in a

71 vacuum oven at 80ºC. A pale brown solid was obtained (0.97 g, 98%). 1H NMR (300

MHz, CDCl3): δ (ppm) 7.686 (s, 8H), 7.535 (s, 4H).

Another route is described by Reger et. al [34]. Mg (2.02 g, 83.4 mmol), LiBF4

(1.224 g, 12.8 mmol) and THF (150 mL) were added to a 500 mL 3-necked flask. A solution of 3,5-bis(trifluoromethyl)bromobenzene (21.1 g, 72 mmol) in 30 mL of THF was added dropwise over 1 hour. After all of the 3, 5-bis(trifluoromethyl)bromobenzene had been added, the mixture was heated to reflux for 30 mins. The reaction mixture was allowed to cool and stirred overnight at room temperature. The reaction mixture was dark brown. THF was removed and chloroform (150 mL) was added to dissolve all solids. The chloroform solution was added to LiOH (10g) in water (300 mL), stirred for 30 mins, and filtered. The aqueous layer was extracted with chloroform (50 mL*3), and the combined chloroform solution was treated with activated carbon, and filtered. 50 mL of glyme was added to complex with the lithium salt and the solution was rotary evaporated to remove the solvents. It was then dried in a vacuum oven at 70ºC overnight. 13.5 g of a brown solid was obtained. It was recrystallized from toluene and 9.1 g of pale brown solid was obtained (60.2% yield). 1HNMR showed each lithium salt can coordinate with 2 glyme

1 molecules. H NMR (300 MHz, CDCl3): δ (ppm) 7.698 (s, 8H), 7.549 (s, 4H), 3.541 (s,

8H), 3.356 (s, 12H).

2.3. Results and discussion

2.3.1. Attempted synthesis of dilithium pentaerithritol dipinacolborate

The success in achieving high cationic transference number from lithium bis(perfluoropinacolato)borate made us wonder what would happen if we could make an even bulkier anion. Thus, the synthesis of dilithium pentaerithritol dipinacol borate was

72 attempted (Scheme 2.1). It was expected that pentaerithritol can react with hydroboric acid to form the polyanion. However, the final product’s NMR spectrum (Fig. 2.12) was exactly that of lithium dipinacol borate (δ 1.058 ppm), and infrared spectroscopy also confirmed the formation of this salt (Fig. 2.13). The presence of an absorption band at

966 cm-1 indicates the boron in the lithium dipinacolborate salt is tetracoordinated.

-1 Absorption bands at 1393 and 1363 cm show the presence of the C(CH3)2 group, and

-1 those of 2981 and 2932 cm are from the –CH3 stretch. The absorption bands at 1060 and 1168 cm-1 are due to the C-O group stretch vibrations. The absorption bands at 3450 and1628 cm-1 indicate that the compound has waters of crystallization [32].

All these proved that the reaction was unsuccessful and pentaetrithritol cound not be incorporated into the reaction. Only lithium dipinacol borate was formed. The use of lithium borohydride may work, but it was not tried.

H O 2 O O O O B C(CH2OH)4 + 2 B(OH)3+ 2 + 2 LiOH.H2O Li C B Li O O OH OH O O

Scheme 2.1. Attempted synthesis of dilithium pentaerithritol dipinacolborate

Methyl group of the salt

D O 2

pentaerithritol pinacol

1 Fig.2.12. H NMR (D2O) of the attempted synthesized dilithium salt

Lithium dipinacolborate

Boric acid

pinacol

Fig. 2.13. IR spectra of lithium dipinacolborate, boric acid and pinacol

2.3.2. Synthesis of lithium tetrakis(1,1,1,3,3,3-hexafluoro-2-propoxy)borate

Initially, the most logical synthetic approach for the incorporation of 1,1,1,3,3,3- hexafluoro-2-propanoxy group into a borate anion seemed to be via the replacement of methoxy ligands of lithium tetra(methanolato)borate by the mono ligand 1,1,1,3,3,3- hexafluoro-2-propanol (Scheme 2.2, Route 1). However, the reaction did not give the expected product. The 1H NMR spectrum of the dried product is very complicated and apparently it is not the expected product. It was unclear what the reaction products truly were, except to demonstrate conclusively that the reaction did not proceed as planned and some side reactions happened.

The failure of the above reaction may be due to the high nucleophilicity of the methoxy group compared to that of 1,1,1,3,3,3-hexafluoro-2-propanoxy group. As a result, an alternative approach was taken by using lithium borohydride as an electrophilic agent (Scheme 2.2, Route 2). This approach proved to be successful and a white crystalline product was produced. 1H NMR (Fig. 2.14) showed that the proton chemical shift moved from 5.135 ppm of the starting alcohol to 4.847 ppm for the final product and the proton peaks were separated into a 7 multiple peak due to the two trifluoromethyl groups nearby.

Route 1.

F3C CF3 THF HC O O CH F C CF3 3 B 4 CF3CHCF3 + LiB(OCH3)4 Li + 4 CH3OH F3C HC O O CH CF3 OH F3C CF3

Route 2. F C 3 CF3 HC O O CH CF 4 CF3CHCF3 0 oC F3C 3 + LiBH4 B Li + 4 H2 F3C HC O CF3 OH glyme O CH F3C CF3 Scheme 2.2. Two possible routes for the synthesis of lithium bis[1,1,1,3,3,3-hexafluoro- 2-propoxy]borate

4.916 4.893 4.870 4.847 4.824 4.801 4.780 4.647 4.638 4.637 4.934

F3C CF3 HC O O CH 10.0 Li F3C B CF3 F3C HC O O CH CF3 CF3 CF3

5.0

0.0

0.09 0.37

5.00 4.90 4.80 4.70 4.60 4.50 ppm (f1) Fig. 2.14. 1H NMR (DMSO-d6) of lithium tetrakis(1,1,1,3,3,3-hexafluoro-2- propanolato) borate

It has a high ambient temperature conductivity (0.686 mS/cm) which is comparable to that of LiPF6 (0.692 mS/cm) when both salts were complexed with poly(ethylene glycol)dimethyl ether (MW=500, a fluid polymer that approximates the properties of the solid state variety employed in lithium polymer batteries) at [O]/[Li]=20 and a frequency of 1000 Hz. However, it is very susceptible to hydrolysis and begins to decompose above 50℃. As a result, it was not used in our study.

2.3.3. Synthesis of lithium bis[benzopinacolato(2-)-O,O’]borate

The various problems with the above salts (thermally or chemically unstable) prompted us to synthesize a more stable salt. Since compounds with aromatic rings are normally stable and bulky, we chose benzopinacolborate as an anion. The anion of this salt is very bulky and is expected to be much more thermally and chemically stable. The synthesis was expected to be similar to that of lithium bis[pinacolato(2-)-O,O’]borate, by simply mixing benzopinacol, boric acid and lithium hydroxide monohydrate (Scheme 2.3,

Route 1). However, 1H NMR showed three different types of phenyl groups in the final products and GPC showed three peaks. The reaction may possibly give mono-substituted and di-substituted products. Apparently the nucleophilicity of benzopinacol is less than that of pinacol and this reaction cannot go completely even when the water was removed azeotropically. Therefore, an alternative approach was taken, using lithium borohydride as an electrophilic agent (Scheme 2.3, Route 2), which was similar to that used for the synthesis of lithium tetrakis(1,1,1,3,3,3-hexafluoro-2-propanoxy)borate. Lithium borohydride is reactive enough that this reaction went completely. 1H NMR showed three groups of peaks which were assigned to protons in ortho-,meta- and para- positions

(Fig. 2.15). 11B NMR showed only one boron peak at a chemical shift of -16.88 ppm

77 relative to trimethyl borate as δ=0.0 ppm (Fig. 2.16). Also MALDI-TOF (Fig. 2.17) showed the expected molecular weight of this lithium salt, which proved we made the salt.

In order to determine the stoichiometries of solvate that complexes with glyme, in other words, the ambient temperature composition range, thermogravimetric analysis

(TGA) was applied using two approaches:

1. Dry lithium bis(benzopinacolato)borate (dried at 150℃ in vacuum oven for 2 days until no more weight was lost) was equilibrated in a TGA pan at 30℃ under glyme atmosphere. The glyme uptake (weight increase) was measured (Fig. 2.18).

2. After equilibration, a normal TGA (10℃ /min) was run up to 320℃ and the weight loss was measured (Fig. 2.19).

The data showed that each dried lithium bis(benzopinacolato)borate molecule could pick up 2.5 glyme molecules, presumably solvating lithium ion. Upon heating from room temperature, it immediately began to lose its complexed glyme and at 110℃ all glyme molecules were lost. Then it began to decompose. Apparently this lithium salt did not complex strongly with glyme.

For solubility test, this lithium salt could only be dissolved in a limted variety of solvents, such as water, dimethyl sulfoxide and marginally dissolved in ethyl carbonate.

The reason for the poor solubility of this lithium salt is believed to come from the low dissociation constant of this salt since there are no electron-withdrawing groups on the anion. As a result, the anodic stability is also low (low decomposition temperature verified by TGA) compared to inorganic electrolytes such as LiBF4 or LiPF6. Therefore,

78 the effect of introduction of electron-withdrawing substituents such as trifluoromethyl groups was investigated.

Rou te 1. DMAc/ tolune

O O 2 + B(OH)3 + LiOH H2O B Li +5 H2O O O HO OH

Route 2.

o O O 0 C + 4 H2 2 + LiBH4 B Li glyme O O HO OH

Scheme 2.3. Two attempted synthetic routes to lithium bis[benzopinacolato(2-)- O,O’]borate

400

7.381 7.356 7.329 7.318 7.294 7.290 7.268 7.240 7.217 7.213 7.193 7.170 7.165 7.386 c b a 300 O O B Li O O b a

200

100

2.08 2.22 1.00

7.50 7.40 7.30 7.20 7.10 ppm (f1) Fig. 2.15. 1H NMR (DMSO-d6) of the lithium bis[benzopinacolato(2-)- O,O´]borate

3000

-0.0000000 -16.884 0.474

2500

2000 O O Li B O O 1500

Trimethyl borate 1000 trimethyl borate

500

-500

100 50 0 ppm (f1) Fig.2.16. 11B NMR (DMSO-d6) of lithium bis[benzopinacolato(2-)-O,O´] using trimethyl borate as a reference

Matrix, 2-(4-hydroxyphenylazo)benzoic acid

lithium bis[benzopinacolato(2-)- O,O’]borate salt

Fig. 2.17. MALDI-TOF of lithium bis[benzopinacolato(2-)-O,O´]borate

Each lithium cation can complex with 2.5 glyme molecules.

Fig.2.18. TGA of Lithium bis[benzopinacolato(2-)-O,O´]borate (dried at 95 ºC in TGA, then equilibrated in glyme at 30 ºC.

It lost glyme slowly up to 110 ºC and began to decompose.

Fig. 2.19. TGA of lithium bis[benzopinacolato(2-)-O,O´]borate with 2.5 coordinated

glyme molecules per lithium salt(saturated by gyme at 30 ºC, and then heated to 320ºC)

2.3.4. Synthesis of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

Lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate could be a desired lithium salt for our study since it incorporates a bulky anion and high charge delocalization on the anion. Its synthesis can be either via cation exchange column from the commercially available sodium form to lithium form (Scheme 2.4, Route 1), or via Grignard reaction and then nucleophilic substitution (Scheme 2.4, Route 2). Since the starting material for route 1 is very expensive, we focused on route 2 and a large batch of lithium salt was made. 1H NMR confirmed this lithium salt structure (Figs. 2.20- 2.21) and XPS showed the molar ratio of lithium to boron is 1/1.

Route 1.

F3C CF3 F3C CF3 F3C CF 3 F3C CF3 Na B Li exchange column Li B

F3C CF3 F3C CF3 F C 3 CF3 F C 3 CF3

Route 2. F C 3 CF3 Br MgBr F C CF3 LiBF 3 + M g THF 4 F Li B + Mg F C F C Br F C CF 3 3 3 3 F C 3 CF3

CF F3C 3 Scheme 2.4. Synthesis of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate by two possible routes

7.686 7.535 F3C CF3

150 F3C b CF3 b + a Li B

100 F3C CF3

F3C CF3

1.00 0.48

8.10 8.00 7.90 7.80 7.70 7.60 7.50 7.40 ppm (f1) Fig.2.20. 1H NMR (CDCl )of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate 3 using route 1

4-18-08-litmpb-recrys-from-toluene 3.356

1.0 F3C CF3 c 0.9 CF F3C 3 c 0.8 Li B 2 CH3OCH2CH2OCH3 0.7 d 3.541 F3C 0.6 b CF3 d 0.5 F3C CF3 a

Normalized Intensity Normalized 0.4 b 0.3 a

0.2 7.698 7.549 CDCl3 0.1 7.270

0 8.235 4.037 7.996 11.844

7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 Chemical Shift (ppm) 1 Fig. 2.21. H NMR (CDCl3) of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate complexed with 2 glyme molecules using route 2

In order to determine the Li ion/glyme stoichiometry, the same TGA technique

was applied. The data showed that each dried lithium tetrakis[3,5-

bis(trifluoromethyl)phenyl]borate molecule picks up 2 glyme molecules (Fig. 2.22).

Upon heating, it was stable up to 73ºC and then lost 1 glyme molecule for each lithium

ion between 73ºC and 108ºC. It then plateaued until 200ºC, where it began to decompose

(Fig. 2.23). Apparently this lithium salt is thermally stable and can complex strongly with

glyme molecules. In our later experiments, we chose this salt to add to the comb

polymers we synthesized, and compared their thermal and electrical behaviors with those

of lithium bis(perfluoromethanesulfonyl)imide (LiTFSI) complex.

Each lithium cation can complex with 2 glyme molecules.

Fig. 2.22. TGA of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate equilibrating with glyme vapor at 30ºC. Two glyme molecules were picked up for each lithium cation.

Fig. 2.23. TGA of lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate with 2 coordinated glyme molecules (equilibrated with glyme vapor at 30 ºC, and then heated to 400 ºC.) 85

2.4. Conclusions

Lithium tetrakis(1,1,1,3,3,3-hexafluoro-2-propoxy)borate, lithium bis(benzopinacolato)borate and lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate have been synthesized. Direct reaction of 1,1,1,3,3,3-hexafluoro-2-propanol with lithium tetra(methanolato)borate did not yield the expected lithium salt. It was synthesized using lithium borohydride. Since its anion had a monodentate ligand, this lithium salt was very susceptible to hydrolysis and thermally unstable. As a result, it was not suitable for use in a lithium battery. Lithium bis(benzopinacolato)borate was synthesized by the same method and was shown to complex weakly with glyme, through TGA measurements. Its low solubility might come from the low dissociation constant of this salt since there were no electron-withdrawing groups on the anion. Lithium tetrakis[3,5- bis(trifluoromethyl)phenyl]borate incorporated a bulky anion and enough charge delocalization on the anion and was found to be a reasonable lithium salt for our research.

It was thermally stable and could complex strongly with glyme and had a high solubility in aprotic solvents.

2.5. References

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Electrochemical Technology in Chemistry of Nonaqueous Solutions, G.

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7. V. N. Plakhotnik, V. B. Tul’chinskii, V. K. Steba, Russ. J. Inorg. Chem., 1976, 21,

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11. K. M. Abraham, Electrochim. Acta., 1993, 38, 1233.

12. L. A. Dominey, V. R. Koch, T. J. Blakley, Electrochim. Acta., 1992, 37, 1551.

13. M. armand, W. Gorecki, R. Andreani, Second International Symposium on

Polymer Electrolytes, B. Scrosati, Editor, Elesevier Applied Science, London,

1990, p. 91.

14. L. A. Dominey, T. J. Blakley, V. R. Koch, in Proceedings of 25th Intersociety

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15. J. Barthel, R. Buestrich, E. Carl, H. J. Gores, J. Electrochem. Soc., 1996, 143,

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19. J. Barthel, R. Buestrich, E. Carl, H. J. Gores, Ibid, 1996, 143, 3565.

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Electrochem. Soc., 2003, 150(1), E74-E80.

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22. K. Hayamizu, Y. Aihara, S. Arai, C. G. Martinez, J. Phys. Chem.,1999, 103, 519.

23. M. Doyle, T. F. Fulcher, J. Newman, Electrochim. Acta, 1994, 39, 2073-2081.

24. F. Croce, A. D’Aprano, C. Nanjundiah, V.R.Koch, C. W. Walker, M. Salomon, J.

Electrochem. Soc., 1996, 143, 154-159.

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519-524.

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293-297.

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Chapter 3. Synthesis of comb polymers with oligo(ethylene oxide) side chains and

their characterization

3.1. Introduction

Early studies carried out by Blonsky, Shriver, Austin and Allcock showed that the ionic conductivitiy of salt-complexed polyphosphazenes with linear side-chains increases with increasing side-chain length [1]. Apparently polymers with longer oxyethylene side- chains possess a greater capacity for salt dissolution compared with shorter side-chain polymers, due to larger number of ion coordinating groups per repeat unit for both ion- pair separation and as “empty” sites to which a cation can be transferred [2]. This has been verified by the conductivity difference between polyphosphazenes with one oxyethylene unit (maximum conductivity of 2.6*10-5 S/cm at room temperature) and six oxyethylene units (maximum conductivity of 4.8*10-5 S/cm at room temperature) [3] and was also seen in polysilanes conductivity behavior [4-6]. However, long side chains also increase the opportunities for side group close-packing and hence can induce crystallinity and reduce the ionic conductivity. As we know (chapter 1), ion conduction is via the amorphous region of the polymer matrix and crystallinity can greatly reduce the conductivity. As a result, as the side chain lengths increase, the maximum conductivity increases slightly, reaches a plateau at six to seven oxyethylene groups per side unit and then begins to decrease. This is due to the onset of ordering of the polymeric matrix as low melting crystallites begin to form when the side chains exceed roughly six oxyethylene units. They begin to develop problems similar to those found with PEO. This was also seen for polyphosphazenes with eight oxyethylene units; it had a reduced

90 conductivity of 4.5*10-5 S/cm [3]. Based on the literature results, we decided to use side chains with seven oxyethylene units for our polymers. Initially we tried to use a poly(oxyethylene) backbone via Williamson ether reaction of the side groups with poly(ethylene oxide-co-epichlorohydrin). If this method could work, it is a much easier synthetic route than copolymerization of ethylene oxide and 2-(2-methoxyethoxy)ethyl glycidyl ether by anionic coordination polymerization [7-10]. We also developed a second polymer backbone based on poly(trimethylene oxide) (Tg= -70℃) since it is even more flexible than poly(ethylene oxide) (Tg= -64℃), and consequently is expected to provide better low-temperature electrical performance than a PEO system, based on the criteria for high-conductivity polymer electrolytes introduced in Chapter 1.

Normally, since these electrolytes are binary salt conductors, the salt concentration changes across the membrane with the application of continued direct current during discharges. This is due to the fact that there is no electrode reaction for the anions, which tend to accumulate at the anode unless the salt diffusion coefficient is sufficiently large to allow the concentration gradient to relax. The generation of a salt concentration gradient results in voltage losses due to concentration polarization.

Furthermore, it causes changes in the transport properties that result in poorer performance and even undesirable changes in the electrolyte state such as salt precipitation. A good way to avoid concentration polarization is the use of single ion conductors (polyelectrolytes) in which free movement of anions is prohibited, so the transference number of the lithium cation is unity. However, due to strong ion-pairing, the room temperature conductivities of the polyelectrolytes tested were very poor (<10-7

S/cm) which limits their practical application in lithium batteries.

Thus, the complexing of comb polymers with lithium salts with bulky anions would be interesting. It could combine the advantages of comb polymers and polyelectrolytes. As we know, in any DC operation, as occurs when charging or discharging a battery, polarization becomes very important, and the conductivity under these conditions would be much lower for normal salts than the value found using high frequency AC testing. As a result, normal high frequency conductivity measurements cannot be used when testing for polarization; measurements must be made under conditions where polarization of the system becomes important, possibly by imposing a bias voltage or in a battery. More practically, it can be estimated from measurements at low frequencies. Under this circumstance, the resistance should rise rapidly for the normal electrolytes because of the high mobility of the anions. For LiTMPB electrolytes, the resistance should rise more slowly than with the present materials such as LiTFSI electrolytes.

In this chapter, we studied (a) the physical properties such as glass transition temperature, crystallinity, thermal stability and (b) ionic conduction, especially low frequency conductivity of our ethylenene oxide comb polymer electrolytes using LiTFSI and LiTMPB as salts.

3.2. Experimental procedures

3.2.1. Materials

Poly(ethylene oxide-co-epichlorohydrin) (Hydrin C) (Zeon C2000XL, Mw=7*105) was used as received. Pentaerithritol tribromide (Tokyo Kasei, 98%) was used directly without further purification. Poly(ethylene glycol)monomethyl ether (Aldrich, MW=350) was dried over CaH2 and distilled under high vacuum (1 torr). Oxetane (Alfa Aesar, 97%) was dried over sodium metal and distilled. Sodium hydroxide (Fisher Scientific, 99.2%) was used directly. Boron trifluoride diethyl etherate (Sigma-Aldrich, purified, redistilled) was used directly without further purification.

3.2.2. Characterization techniques

3.2.2.1. Gel permeation chromatography (GPC)

Analytical GPC was carried out on a Waters 510 GPC at room termperature, equipped with UV photodiode array detector Waters 946 and a differential refractometer

Waters 410 in sequence. N, N-dimethylformide (DMF) (HPLC grade) was used as solvent at a flow rate of 1.0 mL/min through two Styragel columns (Waters Styragel HR-

4E DMF, HR-5E DMF). Calibration covering the required molecular weights range

(M=500-106) is available from polystyrene standards (PSS ReadyCal Polystyrene standard kit (SDK-600), Polymer Standard Service). Generally the molecular weights and breadth of distribution relative to polystyrene standards are severely underestimated for comb polymers [11]. As a result, the absolute molecular weights of these comb polymers were obtained using a light scattering detector connected to the GPC with THF as an eluent.

3.2.2.2. Infrared spectroscopy

An ABB Bomem MB104 FTIR spectrophotometer coupled to a computer was used for IR characterization. Oligomers and polymers were smeared on the KBr pellets for measurements. Absorbance was taken from 4000 cm-1 to 600 cm-1 (resolution 1cm-1) and spectra were calculated after 20 scans.

3.2.2.3. Nuclear magnetic resonance spectroscopy (NMR)

1 13 H and C spectra of solutions of monomers and polymers in CDCl3 or DMSO- d6 at various temperatures were recorded using a Varian Gemini 300 MHz spectrometer or a Varian Inova 600 MHz spectrometer. Care was taken to ensure that spectral widths and pulse intervals were sufficient to produce complete relaxation and reliable intensity ratios. Chemical shifts were measured relative to the solvent signal at δ=7.27 ppm (for 1H

1 13 NMR, CDCl3) and δ=2.50 ppm (for H NMR, DMSO-d6) or δ=77.23 ppm (for C NMR,

CDCl3).

3.2.2.4. AC impedance spectroscopy

Ionic conductivity was measured as complex impedance, using a computer- controlled Hewlett-Packard (4284A Model) Impedance Analyser and Capacitance Bridge

Data Acquisition Adaptor (CGA-83 Model). A laboratory-built ceramic cell holder with brass ion-blocking electrodes was used. The viscous polymer electrolyte samples were sandwiched between the brass electrodes. The thickness of the samples ranged from 0.4-

0.7 mm and the area was about 1.5 cm2. The assembly was sealed under dynamic vacuum in a temperature-controlled glass container. Impedance spectra in the frequency range from 20 Hz to 1 MHz (AC amplitude 0.1V) were recorded at various temperatures,

94 during heating (20 to 110℃) and cooling (110 to 20℃) in 10℃ increments. The cell was left for 1/2 h to reach thermal equilibrium before each measurement.

The apparent conductivity was calculated from the real part of the impedance and the sample dimensions. The bulk polymer electrolyte conductivity was calculated from the real part of the frequency-independent plateau observed in the impedance at frequencies above 50 KHz [12].

3.2.2.5. Differential scanning calorimetry (DSC)

DSC measurements were performed using a Dupont Instruments MDSC 2710 under a dry nitrogen atmosphere. A sample of bulk polymer or a polymer electrolyte was loaded in an aluminum pan and hermetically sealed, quenched to approximately -120℃ by liquid nitrogen and then heated to 120℃ at a rate of 10℃/min. It was held there for 5 mins in order to erase any previous thermal history. After that, it was requenched to -120℃ and reheated to120℃ at a rate of 10℃/min. The power and temperature scales were calibrated using pure indium. The glass transition temperature was determined as the mid-point of the transition from the second heating. Melting and crystallization temperatures, when they occurred, were defined as the maxima of the melting endotherms and crystallization exotherms, respectively. Heat of fusion (ΔHm) was measured by the area under the melting endotherm. Crystallinity (χc) was determined from the ratio of the experimentally measured enthalpy to the value of 203 J/g reported for the enthalpy of melting of 100% crystalline PEO [13].

3.2.2.6. X-ray photoelectron spectroscopy (XPS)

X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a

Kratos AXIS165 system with a monochromatic Al Kα excitation source. To avoid

95 potential changes to surface composition, no Ar+ surface-etch or other cleaning was performed prior to spectrum acquisition. All spectra were taken with the electron energy analyzer normal to the specimen surface. During all spectra acquisition, a charge neutralization filament was turned on to minimize possible electrical charging effects.

3.2.2.7. Elemental analysis

Elemental analyses were performed by M.H.W Labs, Phoenix, AZ.

3.2.3. Synthesis

3.2.3.1 Attempted preparation of comb polymers from poly(ethylene oxide-co- epichlorohydrin) by Williamson ether synthesis

10 mL of toluene, 20 mL of diglyme and 17.5 g (50 mmol) of polyethylene glycol monomethyl ether (DP=7, Mw=350) were mixed together and trace of water was removed by azeotropic reflux with toluene using a distilling head (When a thermometer in the distilling head reached 84.1℃ and equilibrated, the azeotrope was taken out.).

Then sodium hydride (0.82 g, 32 mmol) was put into the solution and the mixture was maintained at 100℃ overnight. Hydrogen bubbles were seen coming out gently from the sodium melt surface and a brown solution was obtained. The solution was transferred by a double-tipped needle into a 3-necked flask containing a solution of poly(ethylene oxide- co-epichlorohydrin) (3.18 g, chlorine content: 0.0216 mol, MW= 7*105) in N,N- dimethylacetamide (120 mL). The mixture was reacted at room temperature. After 30 mins, the reaction mixture gelled and the reaction was stopped. The crosslinked polymer

1 partly dissolved in CDCl3. H NMR (300 MHz, CDCl3): δ (ppm) 3.369 (s, -

OCH2CH2OCH3, 3H), 4.283-4.201 (m, -C=CH2, 2H), 4.169-4.056 (m, -CH2C(=CH2)O-,

2H), 3.993-3.954 (t, -OCH2CH2-OC(=CH2)CH2-, 2H), 3.881-3.841 (t, -OCH2CH2-

OC(=CH2)CH2-, 2H), 3.75-3.50 (bm, -CH2O(CH2CH2O)7CH3, -CH2CHO-), 30H). FT-

-1 -1 IR: 2920, 2874 cm (stretching, -CH3, -CH2-), 1664, 1633 cm (stretching, -CH=CH2 double bond), 1113 cm-1 (streching, -C-O-C- ether bond).

3.2.3.2. Preparation of 3,3-bis(bromomethyl)oxetane

Pentaerithritol tribromide (32.5 g, 0.1 mol) and sodium hydroxide (4.44 g, 0.11 mol) were dissolved in a mixture of 40 mL of methanol and 10 mL of water in a 250 mL flask. The solution was refluxed and white solid precipitated in 5 mins. After 1 h, the reaction was stopped and the mixture was filtered leaving a transparent solution. The solvents were removed and the rest mixture was distilled under vacuum (3*10-4 torr) at

1 80℃ to give a colorless oil (21.5 g, 88% yield). H NMR (300 MHz, CDCl3): δ (ppm)

4.429 (s, -OCH2-, 4H), 3.856 (s, -CH2Br, 4H).

3.2.3.3. Cationic ring-opening polymerization of 3,3-bis(bromomethyl)oxetane

A 50 mL 3-necked flask equipped with an internal thermometer, a condenser and a stirring bar was flame dried under vacuum and purged twice with dry argon. The flask was charged with 3,3-bis(bromomethyl)oxetane (6 g, 0.0246 mol) in 10 mL of

-5 chloroform. It was stirred for 15 mins at 0℃ and BF3OEt2 (5 uL, 2.67*10 mol) were injected. The internal temperature, monitored by the thermometer, rose rapidly from 0℃ to 30℃. The reaction mixture became viscous and cloudy almost immediately and polymer separated as the reaction proceeded. The solution turned into a white solid after

5 mins. The solid was washed with 20 mL of chloroform, then with NaHCO3 (0.02g) aqueous solution, and dried under vacuum (3*10-4 torr) at 70℃ for 2 days. A snow-white product was obtained (5.8 g, 97% yield). 1H NMR (300 MHz, DMSO-d6, 120℃): δ (ppm)

3.593 (s, -CH2Br, 4H), 3.491 (s, -OCH2CCH2-, 4H). FT-IR (KBr pellet): 2964, 2905,

-1 -1 -1 2874 cm (stretching, -CH2-), 1114 cm (stretching, -C-O-C- ether bond), 665 cm

(stretching, -CH2Br).

3.2.3.4. Copolymerization of trimethylene oxide and 3,3-bis(bromomethyl)oxetane

A 50 mL 3-necked flask equipped with an addition funnel, a condenser and a stirring bar was flame dried under reduced vacuum and purged twice with dry argon. The

-4 flask was charged with BF3:OEt2 (30 uL, 1.6*10 mol) and chloroform (2 mL). The mixture was stirred for 15 mins at 0℃. A solution of trimethylene oxide (3.42 g, 0.0590 mol) of and 3,3-bis(bromomethyl)oxetane (6 g, 0.0246 mol) in 10 mL of chloroform was added slowly over 1 h. The reaction mixture was neutralized by adding a NaHCO3 (0.02g) aqueous solution. The mixture was precipitated in hexanes twice to produce a gum-like polymer. The polymer was dried under vacuum (3*10-4 torr) at 70℃ for 2 days, yield 7.2

1 g (76.4%). H NMR (300 MHz, CDCl3): δ (ppm) 3,484-3.459 (m, -OCH2CH2CH2-,

13.6H), 3.398 (s, -OCH2CCH2-, 4H), 3.500 (s, -CH2Br, 4H), 1.826-1.792 (m, -

-1 -1 OCH2CH2CH2-, 6.8H). FT-IR: 2948, 2865 cm (stretching, -CH2-), 1108 cm (stretching,

-1 -C-O-C- ether bond), 668 cm (stretching, -CH2Br). Elemental analysis showed that the reacted molar ratio of trimethylene oxide to 3,3-bis(bromomethyl)oxetane was 3.4:1.

(Analytically calculated for 3.4:1 of [trimethylene oxide]/[3,3-bis(bromomethyl)oxetane]:

C, 41.36; H, 6.44; Br, 36.24. Found: C, 40.82; H, 6.49; Br, 36.50.)

3.2.3.5. Preparation of comb copolymer from poly[3,3-bis(bromomethyl)oxetane]

10 mL of toluene, 20 mL of diglyme and 17.5 g (50 mmol) of polyethylene glycol monomethyl ether (DP=7, Mw=350) were mixed and water was removed by azeotrope with tolune using a distilling head. Sodium (0.92 g, 40 mmol) was put into the solution and the mixture was maintained at 100℃ overnight. Hydrogen bubbles were seen coming

98 out gently from the sodium melt surface and a brown solution was obtained. The solution was transferred by a double-tipped needle into a 3-necked flask containing poly[3,3- bis(bromomethyl)oxetane] (3.91 g, 0.016 mol, molecular weight per unit=244) and diglyme (20 mL). The reaction was kept at 120℃ and monitored by titration with 0.01 M

HCl. After 6 h, the reaction was stopped. The mixture was centrifuged and the supernatant solution was rotary evaporated to remove the solvent. 10 mL of cold water was added to the remaining viscous liquid and stirred to form a clear solution. The solution was heated to 90℃. Polymer began to precipitate above 70℃. A viscous liquid polymer (high molecular weight) was obtained (4.84 g, 38.6%) and lower molecular

1 weight polymer was left in water and discarded. H NMR (300 MHz, CDCl3): δ (ppm)

3.381 (s, -(OCH2CH2)7OCH3, 3H), 3.558, 3.549, 3.541 (m,

-CH2(OCH2CH2)6CH2CH2OCH3, 4H), 3.700-3.575 (m, -OCH2C(CH2O-)2CH2-O-,

-1 -1 -O(CH2CH2)7OCH3, 57H). FT-IR: 2922, 2869 cm (stretching, -CH2-, -CH3), 1100 cm

-1 (stretching, -C-O-C- ether bond), 674 cm (stretching, –CH2Br). Elemental analysis showed there was 87.5% substitution of bromine by oligo(ethylene oxide). (Calculated: C,

53.78; H, 8.93; Br, 2.87. Found: C, 53.68; H, 9.43; Br, 2.86.)

3.2.3.6. Preparation of comb copolymer from poly[trimethylene oxide-co-3,3- bis(bromomethyl)oxetane]

10 mL of toluene, 20 mL of diglyme and 17.5 g (50 mmol) of polyethylene glycol monomethyl ether (DP=7, Mw=350) were mixed together and water was removed by azeotrope with toluene using a distilling head. Sodium (0.92 g, 40 mmol) was put into the solution and the mixture was stirred at 100℃ overnight. Hydrogen bubbles were seen coming out gently from the sodium melt surface and a brown solution was obtained. The

99 solution was transferred by a double-tipped needle into a 3-necked flask containing a solution of poly[trimethylene oxide-co-3,3-bis(bromomethyl)oxetane] (7.62 g, 0.016 mol,

([trimethylene oxide]/[3,3-bis(bromomethyl)oxetane]=3.4:1, molecular weight per unit=476) and diglyme (20 mL). The reaction was kept at 120℃ and monitored by titration with 0.01 M HCl. After 5 h, the reaction was stopped. The mixture was centrifuged and the supernatant solution was rotary evaporated to remove the solvent.

Cold DI water (20 mL) was added to the remaining viscous liquid and stirred to form a clear solution. The solution was heated to 90℃. Polymer began to precipitate above 70℃.

A viscous high molecular weight liquid polymer was obtained (5.25 g, 32.3%) and low

1 molecular weight polymer was left in water and discarded. H NMR (300 MHz, CDCl3):

δ (ppm) 3.380 (s, -(OCH2CH2)7OCH3, 3H), 3.612 (s, -OCH2C(CH2O-)2CH2-O-, 8H),

3.656-3.637 (m, -O(CH2CH2)7OCH3, 49H), 3.555-3.547 (m, -OCH2CH2CH2O-, 8H),

3.5-3.45 (m, -OCH2CH2CH2O-, -CH2Br, 8H), 1.85-1.80 (m, -OCH2CH2CH2O-, 8H). FT-

-1 -1 IR: 2914, 2865 cm (stretching, -CH2-, -CH3), 1108 cm (stretching, -C-O-C- ether

-1 bond), 654 cm (stretching, -CH2Br). Elemental analysis showed there was 86.5% substitution of bromine by oligo(ethylene oxide). (Calculated: C, 55.52; H, 9.22; Br, 2.44.

Found: C, 55.35; H, 9.58; Br, 2.43.)

3.2.3.7. Preparation of polymer/lithium salt complexes

The polymer/salt complexes were prepared by dissolving stoichiometric quantities of polymer and salt from the range of oxygen to lithium molar ratio of 10 to 70 in anhydrous methylene chloride. After a homogenous solution was formed, the solvent was removed under vacuum. The complexes were dried under vacuum for 2 days at 70℃ until a constant weight was obtained.

100

3.3. Results and discussion

3.3.1 Synthesis

3.3.1.1. Attempted synthesis of comb polyethers based on poly(ethylene oxide-co- epichlorohydrin)

Initially, the most logical synthetic approach for the incorporation of oligo(oxyethylene) groups into poly(ethylene oxide-co-epichlorohydrin) (Hydrin

C )seemed to be via the nucleophilic displacement reaction of chloride ion by the reaction of polyethylene glycol monomethyl ether sodium salt with the starting polymer. The reaction was run as shown in Scheme 3.1, route 1. However, the reaction did not give the expected 100 percent substituted product. Instead, it cross-linked. The product was characterized by XPS (Figs. 3.1-3.2), 1H NMR (Fig. 3.3) and FT-IR (Fig. 3.4). XPS clearly showed the final product had no chlorine left which made us think the substitution reaction went to 100 percent completion. However, its 1H NMR spectrum indicated there was roughly 10 percent substitution (CH3,δ3.369 ppm) and 90 percent elimination of

HCl with the generation of double bonds (-CH=CH2,δ4.3~4.2 ppm). IR showed peaks at 1664 and 1633 cm-1, characteristic of double bonds, which confirmed the results from the 1H NMR spectrum. Apparently, this PEO sodium alkoxide appeared to be highly basic since it dehydrohalogenated Hydrin C very easily and only 10% of the chloromethyl groups were grafted. Thus, instead of attacking the α-carbon by the SN2 mechanism, this alkoxide abstracted the β-hydrogens of the alkyl bromide by the E2 mechanism. This was unexpected, since primary chloromethyl groups usually react almost 100 percent through nucleophilic substitution by alkoxide. However, the results from Ito’s group corroborated our finding. When they reacted PEO (38 repeat unit) potassium alkoxide with p-(7-bromoheptyl)styrene, they found that the major reaction

101

was elimination (E2) instead of substitution (SN2). All these results suggest that the ether

oxygens in PEO strongly solvate the sodium ions and cause their highly basic terminal

alkoxides to favor the E2 reaction. By contrast, when simple alkoxides without ether

oxygens such as sodium n-butoxide are used, the SN2 reaction becomes almost

quantitative and is of great synthetic value [14].

The strong basicity of this PEO alkoxide prompted us to use a much milder base,

potassium carbonate, for this reaction (Scheme 3.1, route 2). However, its basicity is so

low that the reaction did not go, even under reflux for a long time in the high-polarity

solvent N, N-dimethylacetamide.

R oute 1:

CH3(OCH2CH2) OH + Na CH3(OCH2CH2) ONa + 1/2H2 k k

DMAc CH2CHO CH2CH2O CH2CO NaCl CH2CHO CH2CH2O + CH3(OCH2CH2) ONa + n m k n-p m p CH2 CH2 CH2Cl CH3(OCH2CH2) k O

R oute 2:

K2CO3 CH2CHO CH2CH2O +CH3(OCH2CH2) OH CH2CHO CH2CH2O k m n m DMAc, reflux n-p CH2Cl CH2 O CH3(OCH2CH2) k Scheme 3.1. Attempted synthesis of the comb polymer starting from Hydrin C using two possible routes

102

Fig.3.1. XPS of Hydrin C x104

Cl peak disappears

Fig.3.2. XPS of the reacted product from Hydrin C

103

4.283 4.228 4.201 4.146 4.098 4.056 3.993 3.973 3.954 3.861 3.752 3.637 3.553 3.369

g g eff d c, e, f, CH2CHO CH2CH2OCH2CO CH2CH2O 9 h h CH2 O a CH2CH2OCH3 7.5 c c b g a d b

0.52 2.96 2.12 1.87 2.06 2.60 8.60 1.18 0.49

ppm (f1) 4.00 3.50 . 1 Fig.3.3. H NMR (CDCl3) of the reaction product from Hydrin C

reaction product

Backbone and -HC=CH branched -CH2-,-CH3 2

C-O-C

Hydrin C

Backbone -CH -, -CH- C-O-C 2

Fig. 3.4. IR spectra of polyepichlorohydrin-co-poly(ethylene oxide) (Hydrin C) and the reaction product.

104 3.3.1.2. Synthesis of comb polyethers with poly(trimethylene oxide) as backbones and oligo(ethylene oxide) as side chains

Having recognized the strong basicity of the oligo(oxyethylene) alkoxide, we chose 3,3-bis(bromomethyl)oxetane as a starting monomer since it has a quaternary carbon attached to the CH2Br groups and no elimination reaction can take place.

Furthermore, the bromine atom on the bromomethyl group is a good leaving group and can be easily displaced by a variety of nucleophilic reagents under mild conditions.

Campbell found this enhanced reactivity arises from the altered geometry of the quaternary carbon atom, imposed by the four-membered ring, which forces the halogen atoms into a more available position in space, where they may be displaced more easily

[15]. For its preparation, 3,3-bis(bromomethyl)oxetane can be easily synthesized with very high yield by reacting inexpensive pentaerithritol tribromide with base (sodium hydroxide) in alcohol (Scheme 3.2) [16-17]. A characteristic property of four-membered cyclic ethers is their high reactivity toward cationic ring-opening polymerization due to the great ring strain. For 3,3-bis(bromomethyl)oxetane, since the side-chains are highly ordered and symmetric, upon polymerization initiated by Lewis acid, they pack efficiently to form a crystalline structure [18-24]. It can also copolymerize with other oxetane monomers or THF to produce copolymers [25-30].

Br Br Br methanol + NaOH + NaBr + H2O reflux Br OH Br O

Scheme 3.2. Synthesis of 3,3-bis(bromomethyl)oxetane

105 There are two possible ways of synthesizing comb polymers based on poly[3,3- bis(bromomethyl)oxetane] or its copolymer with trimethylene oxide. One is to synthesize the oligo(oxyethylene)-substituted monomer first, then polymerize or copolymerize this monomer with trimethylene oxide (Scheme 3.3, route 1). However, during this cationic ring-opening polymerization, cationic chain transfer reactions and the resultant cross- linking may be a problem [31].

Because of the complexity of the reaction described above, an alternative approach was taken, to link oligo(oxyethylene) to poly(trimethylene oxide) backbones

(Scheme 3.3, route 2). First, 3,3-bis(bromomethyl)oxetane can homopolymerize upon addition of Lewis acids such as BF3:OEt2. However, an injection of the initiator to the pure monomer 3, 3-bis(bromomethyl)oxetane induces a violent, spontaneous exothermic polymerization and part of the resultant polymer can turn black. Thus, bulk polymerization has problems in controlling the reaction conditions. For precise work, therefore, it was found convenient to adopt a solution polymerization technique; the monomer were dissolved in an inert solvent such as methylene chloride to which the catalyst was added and the polymer separated out as the reaction proceeded. However, the polymer, a white solid, is highly crystalline and is extremely insoluble at room temperature in all the usual organic solvents as well as the more powerful solvents such as dimethyl sulfoxide, formic acid and m-cresol. However, it become soluble in dimethyl sulfoxide, dimethyl formamide and chlorinated hydrocarbons such as o-dichlorobenzene above 150℃. Fig. 3.5 is the 1H NMR spectrum of poly[3,3-bis(bromomehtyl)oxetane] in

DMSO-d6 at 120℃ after being dissolved at 160℃. Since the polymer quickly recrystallized and separated from the solvent, the spectrum is not well shimmed and the

106 integration is not exactly right. However, we can see two single peaks which are assigned to the backbone -CH2O- and side chain -CH2Br respectively. Infrared spectroscopy (Fig.

3.6) helps to confirm the structure of the polymer with -C-O-C- bonds (1114 cm-1) and

-1 -CH2Br groups (665 cm ).

The poor solubility of this homopolymer limits its applications since its chemical modification is fairly difficult. It can only be carried out heterogeneously which proceeds very slowly, or by reaction at very high temperature (>150℃) which may cause partial decomposition. As a result, we decided to copolymerize of 3,3-bis(bromomethyl)oxetane with trimethylene oxide. The copolymer has several advantages over the homopolymer.

First, since the reactivity of 3,3-bis(bromomethyl)oxetane is close to that of trimethylene oxide, the product tends to be a random polymer with a low glass transition temperature and a low melting temperature (if any). Thus, it is soluble in many organic solvents which can lead to easy modification. Second, by changing the molar ratio of 3,3- bis(bromomethyl)oxetane to trimethylene oxide, the distance between two 3,3- bis(bromomethyl)oxetane units separated by trimethylene oxide can be regulated. Upon grafting, the final comb polymer can get the greatest free volume. Thus, an optimum composition can be found [32-34] in the sense of obtaining the best conductivity when the comb polymer is complexed with lithium salts.

The success of the copolymer synthesis was confirmed by 1H NMR and FT-IR.

1H NMR (Fig. 3.7) spectra showed the presence of the poly(trimethylene oxide) backbones (-OCH2CH2CH2-, t, δ 3.484~3.459 ppm) and bromomethyl side groups

(-OCH2C(CH2Br)2CH2-, s, δ 3.500 ppm). Infrared spectroscopy (Fig. 3.8) also confirmed

-1 -1 the presence of -C-O-C- bonds (1108 cm ) and -CH2Br groups (668 cm ). The molar

107 ratio of trimethylene oxide to 3,3-bis(bromomethyl)oxetane is 3.4/1, calculated from the elemental analysis results.

The homopolymer and copolymer were reacted with sodium oligo(oxyethylene oxide) salt in diglyme at high temperature and bromine atoms in these starting polymers were replaced by alkoxy side groups. The process of the reaction can be monitored by sampling the reaction mixture and titration with hydrochloric acid. Fig. 3.9 shows the 1H

NMR spectra of the resultant comb copolymer. From the spectrum we can see that almost all bromine atoms were reacted and there is a substantial side chain terminal methyl group peak (δ 3.380 ppm). FT-IR spectrum (Fig. 3.10) showed the presence of the -C-O-

-1 -1 C- bonds (1109 cm ) and the reduced -CH2Br peak strength (664 cm ) which confirmed that the substitution was successful. Elemental analysis showed that the extent of displacement was around 90% for both the homopolymer and copolymer reaction.

108

Route 1: OBr OO(CH2CH2O)7CH3 diglyme + NaO(CH2CH2O)7CH3 120oC Br O(CH2CH2O)7CH3

OO(CH 2CH2O)7CH3 CH2O(CH2CH2O)7CH3 BF3:OEt2 OCH2CCH2 CH Cl , RT n O(CH2CH2O)7CH3 2 2 CH2O(CH2CH2O)7CH3

OO(CH2CH2O)7CH3 O(CH2CH2O)7CH3 O BF3:OEt2 m + n OCH CH CH OCH CCH 2 2 2 n 2 2 CH Cl , RT m O(CH2CH2O)7CH3 2 2 O(CH2CH2O)7CH3

Route 2:

OBr Br BF3:OEt2 n OCH2CCH2 o CH2Cl2, 0 C n Br Br

OBr Br O BF3:OEt2 n OCH2CH2CH2 OCH2CCH2 + m o CH2Cl2, 0 C m n Br Br

O(CH2CH2O)7CH3 Br diglyme OCH CCH +NaO(CH2CH2O)7CH3 o OCH2CCH2 2 2 n 120 C n Br O(CH2CH2O)7CH3 Br diglyme OCH CH CH OCH CCH +NaO(CH2CH2O)7CH3 o 2 2 2 m 2 2 n 120 C Br

O(CH2CH2O)7CH3 OCH2CH2CH2 OCH2CCH2 m n O(CH2CH2O)7CH3

Scheme 3.3. Synthesis of ethylene oxide comb polymers by two possible routes

109 1-16-polyoxetane-115h-dmso 2.500

1.0

0.9 DMSO-d6

0.8 CH2Br b 0.7 OCH CCH 2 2 n 0.6 CH2Br

0.5 a

0.4 3.593 Normalized Intensity a 3.491

0.3

0.2 b

0.1

4.386 4.000

4.0 3.5 3.0 2.5 Chemical Shift (ppm)

1 Fig.3.5. H NMR (DMSO-d6, 120℃) of poly[3,3-bis(bromomethyl)oxetane]

OCH CCH 2 2 n CH2Br Br

C-O-C bond

Fig.3.6. FT-IR spectrum of poly[3,3-bis(bromomethyl)oxetane]

110 7-17-bbmo-oxetane-copolymer-purified a a CH2Br 3.470 OCH2CH2CH2 OCH2CCH2 1.0 3.4n n b d a CH2Br 0.9 c

0.8 3.480

0.7 3.484 3.500

0.6 3.459 0.5 c b 1.815 3.398 1.805

Normalized Intensity 0.4

1.826 0.3 d

1.795 0.2

0.1

0 262.1944.25 100.00

3.5 3.0 2.5 2.0 Chemical Shift (ppm) 1 Fig. 3.7. H NMR (CDCl3) of poly(trimethylene oxide)-co-poly[3,3- bis(bromomethyl)oxetane]

b Br

OCH2CH2CH2 OCH2CCH2 3.4n n CH Br Br 2

C-O-C bond

Fig. 3.8. FT-IR spectrum of poly(trimethylene oxide)-co-poly[3,3- bis(bromomethyl)oxetane]

111 2-7-new-branched-copolymerat4runat48c 3.645 c c d 1.0 a a a O(CH2CH2O)7CH3 3.648 0.9 OCH2CH2CH2 OCH2CCH2 3.4n n 0.8 c b 0.27(BrH2C) (CH2(OCH2CH2)7OCH3)0.73

0.7 e 3.380

0.6

3.656 d 0.5

Normalized Intensity 0.4 e 0.3

3.637 a b 0.2 3.483 3.555 3.547 3.472 3.612 1.825 1.814 0.1 1.835

0 71.234 6.000 5.092

4.0 3.5 3.0 2.5 2.0 Chemical Shift (ppm) 1 Fig. 3.9. H NMR (CDCl3) of the ethylene oxide comb copolymer

O(CH2CH2O)7CH3 OCH CH CH OCH CCH 2 2 2 3.4n 2 2 n . (BrH C) 0 27 2 (CH2(OCH2CH2)7OCH3)0.73

CH2Br

C-O-C bond

Fig. 3.10. FT-IR spectrum of the ethylene oxide comb copolymer

112 3.3.2. Solution properties

The comb polymers are soluble in most common organic solvents, such as chloroform, methanol, aromatic hydrocarbons and tetrahydrofuran. They are not soluble in aliphatic hydrocarbons. The small hydrodynamic volume of these comb polymers was confirmed by GPC data, showing an abnormally long elution time. As a result, the absolute molecular weights of these polymers were obtained by dynamic light scattering

(Table 3.1). These comb polymers are soluble in cold water, with a cloud point of 70℃.

The existence of a Lower Critical Solution Temperature (LCST) results from the presence of both hydrophilic and hydrophobic centers in these polymers. The hydrophilic centers are the ether oxygen atoms in the side chains and backbones, all of which can form hydrogen bonds with water and favor solubility in water. On the other hand, the

-CH2-CH2-CH2- backbone and -CH2-CH2- side chains are hydrophobic and favor insolubility in water. Below the LCST, hydrogen bonding effects are paramount, and these polymers are soluble. As the temperature rises, the hydrogen bonding decreases, and at the LCST the subtle balance between overall hydrophilicity and hydrophobicity switches over to favor hydrophobicity. As a result, the polymers precipitate. As salt is added to the water, increasing the ionic strength, the LCSTs decrease by 10 to 15℃ as the ionic strength increases from 0 to 1.0 [35-36]. Based on this, the comb polymers can be purified by precipitation after heating the water solution above 70℃ [37] instead of a time-consuming dialysis against water and light alcohols [36, 38].

113 3.3.3. Thermal properties

3.3.3.1. Thermal stability

Thermogravimetric studies were carried out in order to determine the thermal stability of the polymeric systems, in terms of the initial weight loss temperature. The control of this parameter is important because the electrolyte must be thermally inert in the working temperature range (up to 120℃). The thermogravimetric plots (TGA) of poly[3,3-bis(bromomethyl)oxetane], poly[trimethylene oxide-co-3,3- bis(bromomethyl)oxetane], comb copolymer, comb copolymer/LiTFSI ([O]/[Li]=30) and comb copolymer/LiTMPB ([O]/[Li]=30) are shown in Fig.3.11. The starting poly[3,3- bis(bromomethyl)oxetane] is stable up to 300 ℃ . After copolymerization with trimethylene oxide, the copolymer is even more stable, till 330℃. The uncomplexed comb copolymer is also thermally stable up to 300℃. Upon the addition of LiTFSI salt, the thermal stability did not change, while upon the addition of LiTMPB salt, the electrolyte began to decompose above 200 ℃ , which is exactly the decomposition temperature for the LiTMPB salt. Thus, this series of polymer electrolytes are thermally stable up to 200℃ which is high enough for lithium battery applications.

114

100

40 poly(BBMO) Weight/% poly(BBMO-co-oxetane) EO comb copolymer EO comb copolymer- 20 LiTFSI-O/Li=30 EO comb copolymer- LiTMPB-O/Li=30 0

0 100 200 300 400 500 600 Temperature/oC Fig.3.11. TGA of poly[3,3-bis(bromomethyl)oxetane], poly[3,3- bis(bromomethyl)oxetane]-co-poly(trimethylene oxide), comb copolymer and its complexes with LiTFSI or LiTMPB at [O]/[Li]=30

3.3.3.2. Glass transition temperature

The comb homopolymer and comb copolymer are clear, very viscous liquids at room temperature. The thermal transitions of the salt-free and salt-complexed polymers were measured by differential scanning calorimetry (DSC). DSC thermograms were obtained in the second heating cycle at a heating rate of 10℃/min after quenching from

120℃ for the PEGME, starting poly[3,3-bis(bromomethyl)oxetane] and poly[trimethylene oxide-co-3,3-bis(bromomethyl)oxetane], the synthesized comb homopolymer, comb copolymer and their polymer electrolytes complexed with LiTFSI or LiTMPB salt at [O]/[Li]=30. They are shown in Fig. 3.12 and Table 3.1 compiles the values of the experimentally obtained thermal parameters together with the GPC data

115 mentioned above. As was noted before, poly[3,3-bis(bromomethyl)oxetane] is a highly crystalline polymer with a melting point of 220℃. However, after copolymerizing with trimethylene oxide, the copolymer is totally amorphous with a glass transition temperature of -38℃. Poly(ethylene glycol)monomethyl ether (MW=350) shows, in addition to the glass transition temperature of -90℃, a melting region with two peaks, located at -25 and -6℃, respectively. The two synthesized comb polymers showed a single glass transition, thus suggesting that both synthesized polymers behave as homogenous systems. Also, the comb copolymer had slightly lower glass transition temperature (-68℃) than that of comb homopolymer (-64℃). Apparently the randomly distributed poly(trimethylene oxide) in the copolymer backbone separates the oligo(oxyethylene) side chains, increasing the mobility of the side chains, thus lowering the glass transition temperature. Furthermore, it is noteworthy that both comb homopolymer and comb copolymer exhibit a melting endotherm close to that found for the poly(ethylene glycol) monomethyl ether (PEGME, MW=350) used as the side chain.

As a contrast, the comb polymethacrylate PMG-8 (MW=105) with the same pendant oligo(oxyethylene) chain was a viscous melt with a Tg of -58℃ but no melt peak [39-41].

Another interesting thing is that when a liquid nitrogen quenched sample in the DSC was reheated, the DSC scans of both comb polymers clearly showed crystallization exotherms centered at around -40℃. This behavior was found in comb polysiloxane polymers too

[42]. Based on these two facts, we may conclude the poly(trimethylene oxide) backbone has a similar flexibility to that of polysiloxane and can facilitate side-chain crystallization while polymethacrylate has a more rigid backbone that hinders no side chain crystallization. When small amounts of lithium salts were added, side chain

116 crystallization of these comb polymers was prevented, as was shown by the complete elimination of the melting endotherms in these polymer complexes. Obviously lithium ions chelated with the oligo(oxyethylene) side chain and these ion-dipole interactions prevented the side chain crystallization. Thus, all of these polymer/lithium salt complexes are amorphous. Glass transitions were displaced towards higher temperatures relative to the uncomplexed polymers; the magnitude of this displacement depended on the chemical structure of the lithium salt.

Fig.3.13 shows the glass transition temperatures (Tgs) of the polymer electrolytes, determined by DSC, plotted against the LiTFSI or LiTMPB concentrations. In both the homopolymer and copolymer complexes, an increase in Tg with the salt concentration is observed, as has been reported in many polyether-base polymer electrolytes [43-44].

Obviously, higher concentrations of salt increase ionic crosslinking and increase the glass transition temperature. Furthermore, we noticed that for both salts, the copolymer electrolytes had lower Tgs than the homopolymer electrolytes for both salts at the same

[O]/[Li] ratio, which again confirmed our expectations. Another characteristic was that the increase in Tg with salt concentration was much more pronounced in the LiTMPB doped polymer electrolytes than in the LiTFSI doped ones. For LiTFSI salt, it is well known that its anion has two strong electron withdrawing –SO2CF3 groups, which renders charge delocalization and thus promotes dissociation and weakens the interaction with polymer chains [45-46]. Similarly, LiTMPB salt has four aromatic rings with two strong electron-withdrawing -CF3 groups on each ring. As a result, it should possess strong charge delocalization too. Thus the difference is ascribed to the much bulkier

- anion of the LiTMPB salt compared to that of the LiTFSI salt. The bulky B[C6H3(CF3)2]4

117 group takes up more volume and inhibits the mobility of backbone and side chains.

Furthermore, for long-range diffusion, an ion will have to circumnavigate the bulky

boronate group. As a result, the LiTMPB doped electrolytes always have higher glass

transition temperatures than those of LiTFSI doped ones, and as the salt concentration

increases, the Tg rise is more pronounced.

10 PBBMO

PTMO-co-PBBMO 8

PEGME 6

2 uncomplexed EO comb copolymer

0 ℃ -250 -150 -50 50 150 250 -2 uncomplexed EO comb homopolymer EO comb homopolymer-LiTFSI-O/Li=30 -4 EO comb homopolymer-LiTMPB-O/Li=30 EO comb copolymer-LiTFSI- O/Li=30 -6

-8 EO comb copolymer-LiTMPB-O/Li=30

Fig. 3.12. DSC thermograms of PEGME, EO comb homopolymer, EO comb copolymer and their polymer electrolytes complexed with LiTFSI and LiTMPB at [O]/[Li]=30, at a heating rate of 10℃/min after quenching from 120℃.

118 Table 3.1. Properties of PEGME, polymer, copolymer and EO comb polymers with poly(trimethylene oxide) as backbones

b c d d d d Compound Mwt Mw/Mn Tg Tc Tm ΔHm %crystallinity or polymer (℃) (℃) (℃) (J/g)

PEGME 350 1.1 -88 -35, -25, 47 23

-22 -6

PBBMO N/A N/A N/A 173 208, 36 N/A

220

PTMO-co- 2.6*104 1.7 -35 N/A N/A N/A 0

PBBMO

EO comb- 6*104 1.1 -65 -50 -8 39 19.5 homopolymer

EO comb- 7.6*104 1.09 -67 -48 -8 37 18 copolymer

a Determind by 1H NMR spectra and elemental analysis b Absolute weight-average molecular weight determined by Gel Permeation Chromatography (GPC) using a light scattering detector c Polydispersity Index, where Mw and Mn are weight-average and number-average molecular weight d Determined by DSC. Degree of crystallinity was evaluated by using ΔHm and the heat of fusion of 100% crystalline PEO (203 J/g)

119

-10 EOcomb homopolymer-LiTMPB EOcomb copolymer-LiTMPB -20 EO comb homopolymer-LiTFSI EO comb copolymer-LiTFSI -30

C -40 o

/ g T -50

-60

-70

0 20406080100 [O]/[Li] ratio

Fig. 3.13. Glass transition temperature (Tg) of EO comb homopolymer and EO comb copolymer and their polymer electrolytes plotted against salt concentration ([O]/[Li]). (Note: here [O]/[Li]=100 stands for uncomplexed polymer.)

3.3.4. Ionic conductivity

Polymer electrolytes are very viscous liquids at room temperature, and creep

becomes more pronounced with increasing temperature. PTFE ring spacers were used

during the conductivity measurements in order to avoid dimensional changes of the

polymer electrolytes. The temperature dependence of the ionic conductivities of these

polymer/lithium salt complexes with [O] to [Li] molar ratio ranging from 70:1 to10:1

were measured over the temperature range from room temperature to 100℃. As shown in

Fig.3.14, the highest ambient temperature ionic conductivity, which occurred at a 25:1 of

[O]/[Li] for the comb copolymer/LiTFSI complex, was 3.41*10-5 S/cm at a frequency of

1 MHz. This is parallel to the reported values for MEEP/lithium triflate complexes. It

120 rose to 7.5*10-4 S/cm at 93℃ (Fig.3.15). For comb copolymer/LiTMPB electrolyte, maximum conductivity at a frequency of 1 MHz was found at a 50:1 ratio of [O]/[Li]. It was 6.68*10-6 S/cm at ambient temperature and 1.55*10-4 S/cm at 93℃. Although these conductivities are lower than those of their LiTFSI counterparts, they are still much

-8 higher than those of linear PEO/LiClO4 electrolytes (~10 S/cm). Considering the bukly anion of LiTMPB salt, the results imply LiTMPB salt has a larger fraction of dissociated ions than LiTFSI salt, since most or all the current is probably due to lithium cation mobility of LiTMPB.

For all these polymer electrolyte systems, the conductivity rose steeply to a maximum, and then declined as the salt concentration increased. The observation of a maximum in the conductivity of a polymer/salt system, is a feature common to many polymer electrolytes and determines the optimum polymer/salt combination for that system [47-49]. The variation of the conductivity with the amount of salt present in the system can be described by the following equation,

σ=∑nmqmμm (1) where nm, qm and μm are the concentration, charge and mobility of charge carriers of type m. Initially, as salt is added to the pure polymer, more charge carriers are supplied; as a result, the conductivity increases. However, as more and more salt is added, two phenomena tend to counteract this increase. First, for a salt MX dissolved in a polymer host, the formation of ion pairs, [MX]0, increases as more salt is added. Thus, neutral species increase and the fraction of charge carriers drops. Furthermore, larger aggregates

+ - such as triple ions [M2X] or [MX2] have lower mobility due to their size in comparison to free ions, and conductivity will again be adversely affected [43]. Second, the system’s

121 glass transition temperature increases as more salt is added, since the cation complexes with the polymer chains and reduces their motion. As more salt is complexed, transient crosslinks tend to be formed between the chains through ion bridges which restrict chain motion. These effects all tend to reduce the overall conductivity of the system. The conductivity maxima at different lithium salt concentrations for LiTFSI and LiTMPB salts are due to their different chemical structures and sizes. For LiTMPB salt, as we discussed before, the electrolyte’s glass transition temperatures rose steeply with the amount of salt added; and at an [O]/[Li]=50, the disadvantage of salt addition counterbalanced its advantage. As a result, this electrolyte reached its maximum conductivity and began to drop. For LiTFSI salt, due to its much smaller anion size, this effect showed up at a higher salt concentration and reached maximum conductivity at

[O]/[Li]=25.

Another interesting phenomenon we noticed from these two plots is that for a given salt concentration, the comb copolymer complexes always had higher conductivities than their comb homopolymer analogues. Again, this can be rationalized using the difference in matrix free volume and resultant glass transition temperature.

Since the side chains of the comb copolymer have more free volume and lower glass transition temperature, ions inside can move more freely and faster, and subsequently the system has higher conductivity.

The Arrhenius equation has been used to describe the temperature dependence of ionic conductivity:

σ＝σ0 exp (-Ea/RT) (2)

122 where σ is the conductivity, T the absolute temperature, σ0 the conductivity at some reference temperature, Ea the activation energy and R the universal gas constant. This equation is typical for conventional liquid electrolytes as well as crystalline polymer electrolytes. For practical analysis, ln(σ) is often plotted vs. 1000/T. The plot should be linear if the conductivity of the system follows Arrhenius behavior. From the plot the activation energy (Ea) and the preexponential factor, ln(σ0), can be obtained.

The Arrhenius equation usually does not describe amorphous polymer electrolyte systems. Most Arrhenius plots of the conductivity for these electrolytes are curved rather than linear. It has been shown that for amorphous polymer complexes the temperature dependence of the conductivity can be described more accurately by the Vogel-

Tammann-Fulcher equation, eq. (3),

σ＝σ0 exp [-B/R(T-T0)] (3) where B is an apparent activation energy and T0 is the thermodynamic glass transition temperature of the electrolyte. The conductivity σ is now related to a reduced temperature

(T-T0) in which the absolute temperature T is referenced to T0. VTF plots are constructed by plotting ln(σ) vs. 1000/(T-T0). For polymer electrolyte systems, T0 is often assumed to be the measured Tg.

The temperature dependence of ionic conductivities of comb copolymer/LiTMPB complex and comb copolymer/LiTFSI complex at salt concentrations, [O]/[Li]=50 and 30 were studied in detail and Arrhenius plots of the data are shown in Figs.3.16-3.17. Both plots are nonlinear and show curvature, which indicates a non-Arrehenius temperature- dependent behavior. However, the data does fit VTF curves (Figs. 3.18-3.19), producing straight lines when T0 assumed to be the Tg for each system. This indicates that the ionic

123 conductivity behavior for these polymer electrolytes follows the free volume law, the ion transport depends on the segmental motion of the oligoether side chains. However, as the lithium salt concentration increases, segmental mobility is clearly affected by the stiffening of the polymer resulting from ionic crosslinks. This is shown in Fig. 3.20. From the plot we can see that as the LiTMPB salt concentration increases to 10/1 of [O]/[Li], both the comb homopolymer and comb copolymer complexes seem to show Arrhenius behavior within the measured temperature range, while LiTFSI doped electrolytes still show positive curvature. Thus, the LiTMPB doped electrolytes at 10/1 of [O]/[Li] tend to have the characteristics of polymer-in-salt electrolytes (Arrhenius behavior) due to the retardation of the polymer segmental motion caused by the bulky TMPB anion salt and the strong ionic crosslinks between the lithium cation and the segmental side chains, while LiTFSI doped electrolytes at 10/1 of [O]/[Li] still show non-Arrhenius behavior probably due to the much smaller anions and the resultant less stiffening of the polymer.

The activation energy Ea of ionic conduction in the temperature range studied, can be obtained from the slopes of ln(σ) versus 1000/T for all complexes (Arrhenius model- although the plot is curved, a linear equation can be applied to approximately describe it) and ln(σ) versus 1000/(T-T0) for all complexes (VTF model), except for the comb homopolymer/LiTMPB and comb copolymer/LiTMPB complexes at [O]/[Li]=10 (they do not obey the VTF model; they show a curved plot), if T0 is assumed to be the measured glass transition temperature Tg. The results obtained are summarized in Tables

3.2 and 3.3. It is clear from Table 3.2 that the ion transport activation energies measured for comb copolymer complexes are lower than those for comb homopolymer complexes at the same composition, again showing that the comb copolymer has greater flexibility

124 and therefore higher ion mobility than the comb homopolymer has. As the salt concentration increases, the activation energy increases due to the transient ionic crosslinks and consequent lower ion mobility. It is noted that LiTMPB doped electrolytes always have higher activation energies than the LiTFSI analogues, again in accordance with results previously seen from the glass transition temperature measurements.

LiTMPB salt stiffens the polymer segmental chains and reduces the segmental mobility more effectively than LiTFSI salt does due to its higher ion dissociation and much more bulky anion. When the VTF model was used to make the comparison at a constant value for the reduced temperature (T-T0), rather than at the same absolute temperature T, the activation energies (Table 3.3) are much lower than those calculated using the Arrhenius model (Table 3.2). These results are in accordance with results obtained from the comb polysilane electrolytes [50], comb polysiloxane electrolytes [51-52] and crystallizable

PEO polymer electrolytes [53].

125

10-5

-6 10 Conductivity(S/cm) EO comb copolymer/LiTFSI, 297K 10-7 EO comb homopolymer/LiTFSI, 297K EO comb copolymer/LiTMPB, 297K EO comb homopolymer/LiTMPB, 297K 10 20 30 40 50 60 70 [O]/[Li] Fig.3.14. Ionic conductivity vs. the O/Li ratio at 1 MHz of EO comb homopolymers and EO copolymers complexed with LiTFSI and LiTMPB (297K)

10-3

10-4

Conductivity(S/cm) EO comb copolymer/LiTFSI, 366K

-5 EO comb homopolymer/LiTFSI, 366K 10 EO comb copolymer/LiTMPB, 366K EO comb homopolymer/LiTMPB, 366K 10 20 30 40 50 60 70 [O]/[Li] Fig.3.15. Ionic conductivity vs. the O/Li ratio at 1 MHz of EO comb homopolymers and EO copolymers complexed with LiTFSI and LiTMPB (366K)

126

-7.5 -8.0 -8.5 -9.0 -9.5 -10.0

-10.5

-11.0 ln(conductivity) (S/cm) -11.5 EO comb copolymer- LiTFSI-O/Li=50, 1 MHz -12.0 EO comb copolymer- LiTMPB-O/Li=50, 1 MHz -12.5 2.6 2.8 3.0 3.2 3.4 -1 1000/T K Fig.3.16. Arrhenius conductivity plot for EO comb copolymer/LiTFSI and EO comb copolymer/LiTMPB complexes at [O]/[Li]=50 (σ at 1M Hz)

-7.0 -7.5 -8.0 -8.5 -9.0 -9.5 -10.0

-10.5 -11.0 -11.5

ln(conductivity) (S/cm) -12.0 EO comb copolymer- LiTFSI-O/Li=30, 1 MHz -12.5 EO comb copolymer- -13.0 LiTMPB-O/Li=30, 1 MHz -13.5 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 -1 1000/T K Fig.3.17. Arrhenius conductivity plot for EO comb copolymer/LiTFSI and EO comb copolymer/LiTMPB complexes at [O]/[Li]=30 (σ at 1 MHz)

127

-7.5

-8.0

-8.5

) -9.0 S/cm

( -9.5

)

-10.0 -10.5 conductivity

( -11.0 EO comb copolymer- ln LiTFSI-O/Li=50, 1 MHz -11.5 EO comb copolymer- LiTMPB-O/Li=50, 1 MHz -12.0

5 6 7 8 9 10 11 12 13 -1 1000/(T-Tg) K Fig.3.18. VTF conductivity plot for EO comb copolymer/LiTFSI ([O]/[Li]=50 ,Tg=-63℃) and EO comb copolymer/ LiTMPB ([O]/[Li]=50, Tg=-58℃) complexes (σ at 1 MHz)

-7.0 -7.5 -8.0 -8.5 -9.0 -9.5 -10.0

-10.5 -11.0 -11.5 ln(conductivity) (S/cm) ln(conductivity) -12.0 EOcomb copolymer- -12.5 LiTFSI-O/Li=30,1 MHz EO comb copolymer- -13.0 LiTMPB-O/Li=30, 1 MHz -13.5 567891011121314 1000/(T-Tg) K-1 Fig.3.19. VTF conductivity plot for EO comb copolymer/LiTFSI ([O]/[Li]=30, Tg=-59℃) and EO comb copolymer/ LiTMPB ([O]/[Li]=30, Tg=-52℃) complexes (σ at 1 MHz)

128

-7

-8

-9

-10

-11

-12

-13 EO comb copolymer- -14 LiTFSI-O/Li=10 EO comb homopolymer- ln(conductivity) (S/cm) ln(conductivity) -15 LiTFSI-O/Li=10 EO comb copolymer- -16 LiTMPB-O/Li=10 EO comb homopolymer- LiTMPB-O/Li=10 -17

2.6 2.8 3.0 3.2 3.4 -1 1000/T (K ) Fig.3.20. Arrhenius conductivity plot for EO comb homopolymer/LiTFSI, EO comb homopolymer/LiTMPB, EO comb copolymer/LiTFSI and EO comb copolymer/LiTMPB at [O]/[Li]=10 (σ at 1 MHz)

129

Table 3.2. Activation energy (Ea/KJ mol-1) of the ethylene oxide comb polymer electrolytes using the Arrhenius equation [O]/[Li] 70 50 25 10

*CHP-LiTMPB N/A 38.5(±0.7) 48.5(±0.7) 67.5(±0.7)

*CHP-LiTFSI N/A 33.1(±0.3) 41.1(±1.2) 56.0(±1.3)

*CCP-LiTMPB 37.4(±0.7) 38.6(±1) 49.7(±1.3) 66.9(±1.1)

*CCP-LiTFSI N/A 30.9(±0.8) 32.1(±1.1) 46.3(±3)

Table 3.3. Activation energy (Ea/KJ mol-1) of the ethylene oxide comb polymer electrolytes using the VTF model

[O]/[Li] 70 50 25 10

*CHP-LiTMPB N/A 4.6(±0.2) 4.2(±0.2) N/A

*CHP-LiTFSI N/A 4.3(±0.2) 4.2(±0.1) 3.9(±0.2)

*CCP-LiTMPB 4.5(±0.1) 4.7(±0.1) 4.0(±0.2) N/A

*CCP-LiTFSI N/A 4.4(±0.1) 3.9(±0.2) 3.8(±0.1)

* CHP--comb homopolymer, CCP—comb copolymer

130 3.3.5. Polarization measurements at low frequencies

In any DC operation, as is the condition when charging or discharging a battery, polarization becomes very important, and the conductivity under these conditions can be much lower for normal salts (such as LiBF4, LiPF6) than that found using high frequency

AC testing. So far, most of research is focused on secondary lithium batteries’ capacity retention and charging time, with no mention of charging and discharging rates which depends on the lithium batteries’ polarization behavior. As a result, it is very important to study these electrolytes’ polarization behavior. When testing for polarization, normal high frequency conductivity measurements cannot be used; measurements must be made under conditions where polarization of the system becomes important, possibly by imposing a bias voltage or in a battery. For an approximation, it can be tested by running conductivity measurements in the low frequency region. In our experiments, the conductivity measurements were taken from frequencies as low as 20 Hz to 1,000,000 Hz.

For comparison, plots of conductivity vs. frequency for comb copolymer/LiTFSI electrolytes and comb copolymer/LiTMPB electrolytes at 50/1 and 30/1 compositions of

[O]/[Li] at room temperature (297K), middle-range temperature (336K) and high temperature (376K) are shown in Figs. 3.21-3.25. From the plots, we see that at high frequency the conductivity of LiTFSI doped electrolyte is higher than that of LiTMPB doped electrolyte. However, as the frequency decreases, the conductivty of LiTFSI doped electrolytes decreases faster, and at low frequencies even becomes lower than those of the

LiTMPB doped materials in the studied temperature range. At the 30/1 [O]/[Li] composition and 336K, the comb copolymer/LiTMPB electrolyte conductivity at 20 Hz is

10 times higher than that of the comb copolymer/LiTFSI electrolyte. Apparently,

131 LiTMPB doped electrolytes have less cell polarization than LiTFSI doped analogues because the bulky anions of LiTMPB salt are almost immobile. Furthermore, as the temperature rises up, the crossing frequency for the two curves rises from 30 Hz (297K) to 200 Hz (336K) to 1000 Hz (376K), which may indicate the cell polarization for

LiTFSI doped electrolyte becomes more severe than for LiTMPB ones as the temperature goes up and immobility increases. All these results imply that LiTMPB doped electrolytes could have higher DC conductivities than LiTFSI doped analogues. Thus, in lithium batteries, LiTMPB doped electrolytes may have much lower cell polarization and could provide higher power density.

1E-4

1E-5

1E-6

Conductivity (S/cm) EO comb copolymer- LiTMPB-O/Li=50, 297K EO comb copolymer- LiTFSI-O/Li=50, 297K

1E-7 10 100 1000 10000 100000 1000000 Frequency/Hz Fig. 3.21. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO comb copolymer/LiTFSI at [O]/[Li]=50 at 297K

132

1E-4

1E-5

1E-6

Conductivity (S/cm) EO comb copolymer- LiTMPB-O/Li=50, 336K EO comb copolymer -LiTFSI-O/Li=50, 336K

10 100 1000 10000 100000 1000000 Frequency/Hz Fig. 3.22. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO comb copolymer/LiTFSI at [O]/[Li]=50 and 336K

1E-4

1E-5 Conductivity (S/cm) EOcomb copolymer- LiTMPB-O/Li=50, 376K 1E-6 EO comb copolymer- LiTFSI-O/Li=50, 376K

10 100 1000 10000 100000 1000000 Frequency/Hz Fig. 3.23. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO comb copolymer/LiTFSI at [O]/[Li]=50 and 376K

133

1E-5

1E-6

Conductivity (S/cm) EO comb copolymer- LiTMPB-O/Li=30, 297K EO comb copolymer- 1E-7 LiTFSI-O/Li=30, 297K

10 100 1000 10000 100000 1000000 Frequency/Hz Fig. 3.24. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO comb copolymer/LiTFSI at [O]/[Li]=30 and 297K

1E-4

1E-5

Conductivity (S/cm) 1E-6 EO comb copolymer- LiTMPB-O/Li=30,336K EO comb copolymer- LiTFSI-O/Li=30,336K

1E-7 10 100 1000 10000 100000 1000000 Frequency/Hz

Fig. 3.25. Conductivities vs. frequency of EO comb copolymer/LiTMPB and EO comb copolymer/LiTFSI at [O]/[Li]=30 and 336K

134 3.3.6. Simulation model for electrical behavior

There have been some models describing equivalent circuits that can simulate the electrical behavior of polymer electrolytes [54-56]. However, they just focus on the bulk conductivity of polymer electrolytes. So far no models have been able to simulate the electrical behaviors (capacitance and resistance) of polymer electrolytes at all frequency ranges. We tried different models proposed by these authors and no model can do a perfect job. Thus, it was necessary to generate a new model which could simulate the electrical behavior at all different frequencies. After trial and error, we propose a three- parallel model (Scheme 3.4) composed of three parallel circuits each with a resistance and capacitance in series. From this model we can calculate the apparent capacitance and resistance (4-6) and by applying the “Scientist” software [57] non-linear curve fitting (7-

16), we can calculate capacitance and resistance parameters. By plotting the experimental resistance and capacitance compared to calculated ones, we can see how well the model can fit these electrolytes. Furthermore, three relaxation times for each composition and temperature can be obtained, which may possibly be used to compare the different relaxation behaviors for different compositions and temperatures. It is expected that as the temperature rises up or the lithium salt concentration goes down, the relaxation times should become shorter since the electrolyte’s viscosity decreases, which facilitates the ion mobility.

The experimental and simulated resistance and capacitance of the electrolytes of comb copolymer/LiTMPB at [O]/[Li] of 50 and 70 and comb copolymer/LiTFSI at

[O]/[Li] of 50 at 296K, 336K and 376K were chosen since they are representative of these electrolytes at a reasonable concentration, and cover the temperature range. The data are plotted in Figs. 3.26-3.34. From the plots it is clearly seen that this model can fit

135 the experimental data almost perfectly. From the calculation, we get the three relaxation times for each composition at a specific temperature. The data are summarized in Tables

3.4-3.6. Apparently the time constants for each condition do not change a lot with either the lithium salt concentration or the temperature as expected. As a result, although this model can simulate the electrical behaviors of all these electrolytes, it cannot explain the conductivity mechanism. A further perfection of this model needs to be investigated.

R1 C1

R2 C2

R3 C3

Scheme 3.4. The proposed three-parallel simulation model

nn2 IwRCwC1(ii ) i =+∑∑( 22i (4) VRwRCii==11iiiii1(++ ) 1( wRC ) n Ci C ' = ∑ 2 (5) i=1 1(+ wRii C )

n 2 11()⎛⎞wRii C = ∑ ⎜⎟2 (6) RRwRC'1()i=1 iii⎝⎠+

136 Calcuation by Scientist software

// MicroMath Scientist Model File

IndVars: WW

DepVars: R, LNR, C, LNC

Params: R1, R2, R3, C1, C2, C3

T1=C1*R1 (7)

T2=C2*R2 (8)

T3=C3*R3 (9)

B1=1+WW*T1*T1 (10)

B2=1+WW*T2*T2 (11)

B3=1+WW*T3*T3 (12)

LNC=LN(C1/B1+C2/B2+C3/B3) (13)

C=EXP(LNC) (14)

LNR=-LN(WW*T1*T1/(R1*B1)+WW*T2*T2/(R2*B2)+WW*T3*T3/(R3*B3)) (15)

R=EXP(LNR) (16)

***

137

10-6 experimental resistance simulated resistance experimental capacitance 10-7 simulated capacitance

104 10-8

-9 10 Resistance/Ohms Capacitance/Farad 10-10

103 10-11 101 102 103 104 105 106 Frequency/Hz

Fig.3.26. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB at [O]/[Li]=50, 296K

10-6 104 10-7

103 10-8 Resistance/Ohms Capacitance/Farad 10-9 experimental resistance 2 10 simulated resistance experimental capacitance simulated capacitance 10-10 101 102 103 104 105 106 Frequency/Hz Fig.3.27. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB at [O]/[Li]=50, 336K

138

104 10-5

10-6

3 10 -7 10

10-8

102 10-9 Resistance/Ohms Capacitance/Farad experimental resistance simualated resistance -10 experimental capacitance 10 simulated capacitance 101 10-11 101 102 103 104 105 106 Frequency/Hz

Fig.3.28. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB at [O]/[Li]=50, 376K

experimental resistance simulated resistance 10-6 experimental capacitance simulated capacitance

-7 104 10

10-8

10-9 3 Resistance/Ohms Capacitance/Farad

-10 10

10-11 1 2 3 4 5 6 10 10 10 10 10 10 Frequency/Hz Fig.3.29. Simulation of resistance and capacitance of EO comb copolymer/LiTFSI at [O]/[Li]=50, 296K

139

10-5 experimental resistance simulated resistance experimental capacitance -6 4 10 10 simulated capacitance

10-7

3 10 10-8

10-9 Resistance/Ohms Capacitance/Farad

102 -10 10

10-11 101 102 103 104 105 106 Frequency/Hz Fig.3.30. Simulation of resistance and capacitance of EO comb copolymer/LiTFSI at [O]/[Li]=50, 336K

10-5 104 10-6

10-7 103

-8 10

10-9 102 Resistance/Ohms Capacitance/Farad

experimental resistance -10 simulated resistance 10 experimental capacitance simulated capacitance 101 10-11 101 102 103 104 105 106 Frequency/Hz Fig.3.31. Simulation of resistance and capacitance of EO comb copolymer/LiTFSI at [O]/[Li]=50, 376K

140

1.8x104 10-6 experimental resistance 1.6x104 simulated resistance

experimental capacitance -7 10 4 1.4x10 simulated capacitance

1.2x104 10-8

4 10 -9 10

Resistance/Ohms Capacitance/Farad

3 8x10 10-10

10-11 101 102 103 104 105 106 Frequency/Hz Fig.3.32. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB at [O]/[Li]=70, 296K

1E-5 4 10 experimental resistance 3 simulated resistance 8x10 1E-6 experimental capacitance simulated capacitance 6x103 1E-7 3 4x10 1E-8

1E-9

3 Resistance/Ohms 2x10 1E-10 Capacitance/Farad 1E-11

1E-12 101 102 103 104 105 106 Frequency/Hz Fig.3.33. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB at [O]/[Li]=70, 336K

141

104 10-5 experimental resistance

simulated resistance -6 10 experimental capacitance simulated capacitance 10-7

10-8

10-9

3 -10 Resistance/Ohms 10 10 Capacitance/Farad

10-11

10-12 101 102 103 104 105 106 Frequency/Hz Fig.3.34. Simulation of resistance and capacitance of EO comb copolymer/LiTMPB at [O]/[Li]=70, 366K

Table 3.4. Time constants for EO comb copolymer/LiTFSI-[O]/[Li]=50 comb copolymer/ time constant 1 time constant 2 time contant 3

LiTFSI-O/Li=50 (s-1) (s-1) (s-1)

296K 1.09E-7(1±16%) 1.45E-3(1±14%) 1.80E-4(1±15%) 306K 1.27E-7(1±17%) 1.33E-3(1±20%) 1.37E-4(1±12%) 316K 1.60E-7(1±17%) 1.86E-3(1±22%) 1.11E-4(1±9%) 326K 1.70E-6(1±31%) 2.51E-3(1±26%) 1.04E-4(1±7%) 336K 2.37E-5(1±21%) 6.88E-3(1±165) 1.54E-4(1±11%) 346K 3.54E-5(1±47%) 6.03E-3(1±29%) 1.58E-4(1±66%) 356K 2.48E-5(1±12%) 6.70E-3(1±8%) 1.29E-4(1±14%) 366K 2.47E-5(1±10%) 7.65E-3(1±7%) 1.25E-4(1±15%) 376K 2.41E-5(1±8%) 9.00E-3(1±8%) 1.35E-4(1±18%)

142

Table 3.5. Time constants for EO comb copolymer/LiTMPB-[O]/[Li]=50 comb copolymer/ time constant 1 time constant 2 time contant 3

LiTMPB-O/Li=50 (s-1) (s-1) (s-1)

296K 8.82E-8(1±17%) 2.86E-3(1±12%) 2.20E-4(1±22%) 306K 1.23E-7(1±14%) 2.57E-3(1±15%) 1.64E-4(1±18%) 316K 1.32E-7(1±15%) 2.35E-3(1±15%) 1.39E-4(1±11%) 326K 1.58E-7(1±16%) 2.09E-3(1±16%) 1.07E-4(1±10%) 336K 1.87E-7(1±18%) 2.10E-3(1±17%) 7.75E-5(1±10%) 346K 2.48E-7(1±23%) 2.10E-3(1±17%) 5.74E-5(1±10%) 356K 1.40E-6(1±34%) 3.39E-3(1±25%) 6.36E-5(1±11%) 366K 1.68E-5(1±15%) 5.70E-3(1±13%) 1.70E-4(1±13%) 376K 2.81E-5(1±33%) 7.44E-3(1±48%) 2.75E-4(1±55%) 386K 1.42E-5(1±11%) 6.61E-3(1±11%) 1.60E-4(1±11%)

Table 3.6. Time constants for EO comb copolymer/LiTMPB-[O]/[Li]=70 comb copolymer/ time constant 1 time constant 2 time contant 3

LiTMPB-O/Li=70 (s-1) (s-1) (s-1)

296K 6.25E-8(1±14%) 2.47E-2(1±72%) 7.68E-3(1±361%) 306K 4.59E-8(1±20%) 8.89E-3(1±3%) 2.51E-5(1±14%) 316K 1.81E-7(1±29%) 9.61E-3(1±236%) 3.15E-3(1±579%) 326K 9.08E-8(1±10%) 4.57E-3(1±3%) 2.13E-4(1±30%) 336K 9.25E-8(1±11%) 3.60E-3(1±4%) 3.13E-4(1±36%) 346K 9.89E-8(1±11%) 4.60E-3(1±15%) 9.42E-4(1±24%) 356K 1.00E-7(1±11%) 4.65E-3(1±15%) 8.03E-4(1±17%) 366K 1.05E-7(1±12%) 4.76E-3(1±15%) 6.69E-4(1±13%)

143 3.4. Conclusions

The attempt of synthesis of comb polymer from poly(ethylene oxide-co- epichlorohydrin) (Hydrin C) by reacting Hydrin C with oligo(ethylene oxide) monoethyl ether sodium alkoxide failed since this alkoxide was a very strong base and it dehydrohalogenated the primary chloromethyl groups.

Two comb polymers (comb homopolymer and comb copolymer) with oligo(oxyethylene) side chains of the type –O-(CH2CH2O)n-CH3 were made by the cationic ring-opening polymerization of 3,3-bis(bromomethyl)oxetane, or the copolymerization of 3,3-bis(bromomethyl)oxetane with trimethylene oxide, and subsequent Williamson displacement of the bromides by oligo(oxyethylene).

Homogeneous polymer electrolytes were made from these two synthesized comb polymers and two lithium salts having different anion sizes, lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and lithium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

(LiTMPB) using a solvent casting method. Their thermal and electrical, especially the low frequency conductivity properties, were studied as a fuction of temperature. All these polymer electrolytes were amorphous and thermally stable up to 150℃. Their ionic conductivity rose with the increase of salt concentration, reached a maximum, and then dropped. The optimum concentrations for highest conductivites of LiTFSI electrolyte and

LiTMPB electrolyte were 25:1 and 50:1 of [O]/[Li] ratios, respectively. Comb copolymer electrolytes had higher conductivities than those of homopolymer electrolytes due to their higher free volume and lower Tg. Comb copolymer/LiTFSI complex at ambient temperature and 1 MHz had a maximum conductivity of 3.74*10-5 S/cm, which was comparable to that of MEEP/lithium triflate complexes and one of the highest values

144 reported so far. Comb copolymer/LiTMPB complex at ambient temperature and 1 MHz had a maximum conductivity of 6.68*10-6 S/cm. Considering the barely movable bulky anion of LiTMPB salt, most of the current was supposedly to be carried by lithium cation.

This result implied a much higher ion dissociation for the LiTMPB salt compared to that for the LiTFSI salt. For all these electrolytes, ionic conductivity increased with temperature; values of almost 10-3 S/cm were obtained for comb copolymer/LiTFSI complex at higher temperature (110℃). The temperature dependence behavior of these electrolytes obeyed the VTF equation instead of the Arrhenius equation, which confirmed their amorphous characteristics and showed that the ion transport depended on the segmental motion of the oligoether side chains. However, when the salt concentration was high, segmental mobility was clearly affected by the stiffening of the polymer resulting from ionic crosslinks; this was clearly shown by LiTMPB complex at

[O]/[Li]=10. At this concentration, the conductivity behavior obeyed the Arrhenius equation instead of the VTF equation.

Conductivity measurements at low frequency were used as an approximation for cell polarization. Although the LiTMPB doped electrolytes had lower AC conductivity at high frequencies than the LiTFSI doped electrolytes, they had higher conductivity at lower frequencies. This result implied that LiTMPB doped electrolytes may have higher

DC conductivities than LiTFSI doped analogues. Thus, in lithium batteries, LiTMPB doped electrolytes may have much lower cell polarization and could provide higher power density.

An equivalent circuit model, composed of three parallel circuits each with a capacicance and resistance in series, was proposed for conductivity behavior simulation

145 and could accurately simulate these electrolytes’ capacitance and resistance at the different frequencies and temperatures. However, the three relaxation time constants calculated from this model did not decrease regularly with the increasing temperature. As a result, this model still has limitations and needs further investigation.

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150 Chapter 4. Synthesis of comb polymers with oligo(trimethylene oxide) side chains

and their characterization

4.1. Introduction

In the previous chapter we talked about the comb polymers based on oxyethylene oxide units (PEO) as side chains. It is well known that PEO has been the center of focus due to its high lithium salt solubility, commercial availability and ease of use. However, the limited lithium ion transport property of PEO/lithium salt electrolytes is a major obstacle for wide applications [1-2]. Buriez and his coworkers compared the conductivities of oxymethylene-linked PEG 400 (PEMO, amorphous), three different comb polymers: polyvinyl ethers (PVEx), polyepoxide ethers (PEPEx) and polyvinylbenzyl ethers with the side chain lengths chosen to be long enough (DP=7) so that the effect of the backbone architecture on conductivity was minimized. They found that all of the materials had very similar conductivities in the ambient temperature range.

This observation strongly suggests that there is a limit to the conductivity of the polymer electrolytes with ethylene oxide units, even after optimization of the polymer architecture and electrolyte salt. Furthermore, when they replaced the ethylene oxide units with propylene oxide units in a polyacrylate comb polymer, they found a decrease in conductivity of one order of magnitude throughout the temperature range examined (-30℃ to +100℃). It implied that the lithium ion may be too strongly bound to the ether oxygen of propylene oxide for optimum mobility and that a more flexible solvating group would be kinetically more labile, leading to higher conductivities [2]. The apparent limits for the

EO-based polymer electrolytes intrigue people to study other polyethers as possible

151 polymer hosts for lithium battery. Poly(methylene oxide) is a highly crystalline polymer and does not dissolve ionic salts [3]. As described in Chapter 1, so far most of the studies on polymer electrolytes have been guided by the assumption that the spatial disposition of oxygen atoms in PEO is probably the most favorable one for forming salt complexes

[4]. Erker’s group first demonstrated the potential of poly(1,3-dioxolane) in forming polymer electrolytes by both chemical and electrochemical in situ polymerization of 1,3- dioxolane-LiAsF6 (1.0-2.5M) solutions [5]. Goulart’s group reported the preparation of amorphous complexes of poly(1,3-dioxolane) with LiCF3SO3 and LiN(CF3SO2)2 for

[O]/[Li] mole ratios of 8 and 16, possessing conductivitites between 10-6 and 10-5 S/cm at room temperature [6]. Abraham’s group carried out this study further. They found

-6 poly(1,3-dioxolane)/LiClO4 complex has a conductivity of 4.3*10 S/cm at room temperature at an [O]/[Li] mole ratio of 8. This value is significantly higher than that of the corresponding PEO electrolyte and they reasoned that it is due to the presence of the interspersing -CH2O- unit in the polymer which greatly reduces the crystallinity of the polymer electrolyte and increases the amorphous nature and chain flexibility of the polymer hosts [7]. This group also studied poly(tetrahydrofuran) as a possible polymer host for lithium battery [7]. They polymerized THF initiated by benzoyl cation and

-6 complexed the polymer with LiClO4. The complex had a high conductivity of 1.9*10

S/cm at room temperature although it had a melting point of 43℃. Furthermore, the poly(tetrahydrofuran) electrolytes exhibited a lithium transport number of 0.6 which was much higher than that of PEO electrolytes (<0.2). However, the results from low molecular weight poly(tetramethylene glycol) (PTMG) electrolytes seemed to be contradictory. Watanabe’s group [8] and Cameron’s group [9] both found that

152 electrolytes involving PTMG had low conductivities. Farrington’s group carried this research further. They found that PTMG (MW=650)/CoBr2 electrolytes had dramatically lower conductivities (3.6*10-7 S/cm maximum) than those of poly(ethylene glycol) (PEG,

-5 MW=400)/CoBr2 electrolytes (1.45*10 S/cm maximum) at room temperature. This phenomenon was explained on the basis of UV-VIS spectral data which indicated the existence of predominantly neutral species in PTMG. The reason for the difference in the types of species present in PEG and PTMG, as they pointed out, could be rationalized on the basis of the chelate effect. When PEG complexed a cobalt ion with two adjacent ether oxygens, a five-membered chelate rings (-Co-O-C-C-O-) was formed which was regarded by far as the most stable complex. On the other hand, PTMG only formed the far weaker seven-membered chelate rings (-Co-O-C-C-C-C-O-). Thus, PTMG oligomer was unable to displace the anions from the metal ion’s first coordination sphere, and neutral species predominated while PEG was able to compete successfully with the bromide ions for inner coordination positions [10]. Silva’s group investigated polymer electrolytes prepared with LiClO4 and low molecular weight poly(tetramethylene glycol)

(PTMG) homopolymer (Mn=730) and PTMG/poly(ethylene glycol) (PEG) copolymer

(Mn=880, [TMG]/[EG]=2) and found that ionic conductivities at room temperature for the homopolymer (2.2*10-4 S/cm) were lower than those reported for the well-studied

-3 PEG 400/LiClO4, while those for the copolymer were similar (2.2*10 S/cm).

Apparently, the contribution of the OH-end group to the dissociation and conductivity was not negligible [11]. Their results involving low molecular weight polyethers indicated a conduction mechanism typical of a liquid system in which the solvation shell moved with the ionic species [12] and consequently the conductivity results contradicted

153 those found using high molecular weight poly(tetrahydrofuran) electrolytes. As a result, a reappraisal of the suitability of various polyformals and polyethers for forming polymer electrolytes seems warranted. Recently, quantum chemical studies have been carried out to determine the strength of the binding of lithium ions to the oxygens in polyalkoxides

[13] and to determine the height of the energy barriers to the movement of the ions along the chains [14-15]. Table 4.1 lists the results of calculations on the binding of lithium ions to linear polymer chains containing all ethylene oxide units (PEO), all trimethylene oxide (TMO) units and all propylene oxide units. The data shown are binding energies calculated as a function of the coordination number around one lithium ion. The TMO polymer shows stronger binding for coordination numbers up to 5, but then decreases due to steric crowding [13]. In normal salt-in-polymer electrolytes, the coordination number is around 4. If this theoretical calculation is true, it shows that poly(trimethylene oxide) binds more strongly with lithium cation than poly(ethylene oxide) and poly(propylene oxide) analogues, and thus may induce lower conductivity. However, if we observe the data further by substracting the binding energy of 2 oxygen coordination number from that of the 4 oxygen coordination number, we found the difference for Li+-PTMO (35.4 kcal/mol) is less than that of Li+-PEO (37 kcal/mol), which means that Li+-PTMO complex needs less activation energy to remove two coordinating oxygens to allow lithium migration than Li+-PEO complex and this can induce higher conductivity and higher lithium transport number. Furthermore, poly(trimethylene oxide) has a lower glass transition temperature (-74℃) than that of poly(ethylene oxide) (-64℃), which means poly(trimethylene oxide) can provide more segmental mobility to lithium ion and may induce higher conductivity. Thus, it is very interesting to study this polymer as a possible

154 polymer host since it needs less activation energy for lithium to move and has lower glass transition temperature. It may serve as a better polymer host for polymer lithium batteries.

Table 4.1.Binding energies (ΔEe, Kcal/mol) of Li+-PEO, Li+-PPO, Li+-PTMO complexes as a function of coordination number Oxygen coordination Li+-PEO Li+-PPO Li+-PTMO

number

1 39.4 42.9 48.6

2 66.0 68.3 75.0

3 87.1 89.4 97.4

4 103.0 103.0 110.4

5 110.1 110.2 115.6

6 115.4 115.3 112.7

Currently, for polymer hosts besides PEO, most researchers have only focused on preparing linear polyformals or polyethers; comb polymers based on these units are scarcely seen in the literature. Attemptes to polymerize 2-[2-(2-methoxyethoxy)ethoxy]-

1,3-dioxolane by cationic polymerization (Schme 4.1) to form a comb polymer failed, probably due to the electronic and/or steric effects [7]. The electron withdrawing property of the 2-[2-(2-methoxyethoxy)ethoxy] side group probably lowers the electron density on the ring oxygen which in turn inhibits the initiation reaction by the cationic initiator.

Another reason they pointed out is that the bulky side group can inhibit the propagation reaction by preventing the attack of the oxygen lone pair electrons at the carbon in the 2- position. It is even more surprising that they cound not polymerize 2-methoxy-1,3- dioxolane even though it has a much smaller side group. Probably the cation (-O-C+H-O-

155 CH2CH2OCH2CH2OCH3) coming from the protonation of the tertiary carbon through ring opening is very stable compared to the oxonium ion and cannot attack another monomer. Kerr’s group [16] synthesized a new comb polymer with 3 units of trimethylene oxide in the side chains by a complicated step-by-step reaction (Scheme 4.2) and experimentally proved that the TMO containing polymers show better conductivity at very low temperatures than the EO containing polymers. They reasoned that since the

TMO containing polymers had lower binding energy and the resultant higher lability of the lithium ion complexes, they could have higher lithium ion mobility and higher conductivity. Apparently their results agreed with the theoretical calculation. Also, they pointed out the presence of the propylene oxide unit in the backbone of the polymer serves as a coulombic trap for lithium ions, which can reduce conductivity. Thus, there is considerable scope for improvement in lithium ion transport such as elongating trimethylene oxide side chains for better solvation and changing the solvation groups involved, such as avoiding coulombic traps from propylene oxide units.

As a result, the first aim of our work in this chapter was to develop a synthetic methodology for making poly(trimethylene oxide) monomethyl ether with controlled length. If this could work, it would be incorporated into 3, 3-bis(bromomethyl)oxetane via the nucleophilic bromine displacement reactions, and subsequently copolymerized with oxetane. If this novel comb copolymer could be made, its physical properties such as glass transition temperature and crystallinity and then ionic conductivity as polymer electrolytes when complexed with LITFSI or LITMPB salt would be characterized.

BF :OEt or PF OCH2OCH2CH2 O O 3 2 5 n + - OCH2CH2OCH2CH2OCH3 or C6H5CO SbF6 OCH2CH2OCH2CH2OCH3 Scheme 4.1. Non polymerization of 2-[2-(2-methoxyethoxy)ethoxy]-1,3-dioxolane

156

O O

O f O O O OO O O

O O

f) 1M Kt-OBu in THF, 2%; allyl glycidyl ether, 5%.

Scheme 4.2. Synthetic route to incorporate TMO units into the monomer and the anionic polymerization used to obtain comb polyether

157 4.2. Experimental procedures

4.2.1. Materials

Pentaerithritol tribromide (Tokyo Kasei, 98%) was used directly without further purification. Oxetane (Alfa Aesar, 97%) was dried over sodium metal and distilled.

Sodium hydroxide (Fisher Scientific, 99.2%) was used directly. Boron trifluoride diethyl etherate (Sigma-Aldrich, purified, redistilled) was used directly without further purification.

4.2.2. Charaterization techniques

4.2.2.1. Gel permeation chromatography (GPC)

Analytical GPC was carried out on a Waters 510 GPC at room termperature, equipped with UV photodiode array detector Waters 946 and a differential refractometer

Waters 410 in sequence. N, N-dimethylformide (DMF) (HPLC grade) was used as a solvent at a flow rate of 1.0 mL/min through two Styragel columns (Waters Styragel HR-

4E DMF, HR-5E DMF). Calibration covering the required molecular weights range

(M=500-106) is available from polystyrene standards (PSS ReadyCal Polystyrene standard kit (SDK-600), Polymer Standard Service).

4.2.2.2. Infrared spectroscopy

158 4.2.2.3. Nuclear magnetic resonance spectroscopy (NMR)

1 13 NMR, CDCl3) and δ=2.50 ppm (for H NMR, DMSO-d6) or δ=77.23 ppm (for C NMR,

CDCl3).

4.2.2.4. AC impedance spectroscopy

Ionic conductivity was determined by complex impedance measurements using a computer-controlled Novocontrol broadband dielectric converter. A laboratory-built ceramic cell holder with brass ion-blocking electrodes was used. The viscous polymer electrolyte samples were sandwiched between the brass electrodes using a ceramic spacer to prevent samples flow at temperatures higher than 60℃. The thickness of the samples was about 1.3 mm and the area was about 3.0 cm2. The assembly was sealed under dynamic vacuum in a temperature-controlled glass container. Impedance spectra in the frequency range from 0.01 Hz to 100K Hz (AC amplitude 0.1V) were recorded at constant temperature during heating (10 to 100℃) and cooling (100 to 10℃) in ±10℃ increments. The cell was left for 1/2 h to reach thermal equilibrium before each measurement.

Due to the blocking nature of the electrodes, the real part of the impedance fell with frequency to a plateau. The bulk polymer electrolyte conductivity was calculated

159 from the real part of the frequency-independent plateau observed in the impedance at frequencies above 50 KHz [35].

4.2.2.5. Differential scanning calorimetry (DSC)

4.2.2.6. Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy

(MALDI-TOF)

160 4.2.3. Synthesis

4.2.3.1. Synthesis of oligo(trimethylene oxide)monomethylether

Route 1. A 50 mL 3-necked flask equipped with an addition funnel, a condenser and a stirring bar was flame burned under vacuum (3x10-4 torr) and purged with dry argon twice. A dry thermometer was placed in the flask to monitor the internal temperature. The flask was charged with methyl triflate (2.53 g, 0.0154 mol) and chloroform (2mL). The mixture was stirred for 15 mins at -10℃ in a salt/ice water bath before a solution of trimethylene oxide (4.472 g, 0.0771 mol) in chloroform (15 mL) was dropped into the flask over 2 mins using an addition funnel. The internal temperature immediately rose reaching -2℃, and dropped back after 2 mins. The reaction was then immediately terminated by adding sodium hydroxide (0.62 g, 0.0155 mol) aqueous solution. The PH of final mixture was 7, meaning it was neutralized. The mixture was extracted by chloroform (30 mL*2) and the solution was evaporated leaving a colorless oil (4.96 g, 100%). It was further dried in a vacuum oven (1 torr) at 60℃, and only 3.5 g oil remained. NMR, TGA, GPC and FT-IR proved it was not the expected product. It may possibly be the cyclic tetramer.

Route 2. A 50 mL 3-necked flask equipped with an addition funnel, a condenser and a stirring bar was flame burned under vacuum (3x10-4 torr) and purged with dry argon twice. The flask was charged with methanol (0.144 g, 0.0045 mol) and

-5 BF3OEt2 (16 uL, 7.5*10 mol) to give [OH]/[ BF3OEt2] molar ratio of 60/1. The mixture was stirred for 15 mins at 0℃ before a solution of trimethylene oxide (1.826 g, 0.0315 mol) in methylene chloride (5mL) was added at a rate of 0.1 mL/min. After a further 1h of stirring, the reaction was stopped by pouring the reaction mixture into NaHCO3 (0.02 g)

161 aqueous solution. The mixture PH was 7, meaning it was neutralized. The methylene

1 chloride was evaporated to leave a colorless oil (1.96 g, 100%). H NMR (CDCl3): δ

(ppm) 3.725, 3.716, 3.708 (t, 2H, -OCH2CH2CH2OH), 3.579, 3.569, 3.559 (t, 2H, -

OCH2CH2CH2OH), 3.51~3.40 (m, 24H, -(OCH2CH2CH2)6-), 3.294 (s, 3H, -OCH3), 2.45

13 (s, 1H, -OCH2CH2CH2OH), 1.82~1.76 (m, 14H, -(OCH2CH2CH2)7-). C NMR (CDCl3):

δ (ppm) 69.547 (s, CH3OCH2CH2CH2-O-), 68.021 (s, -OCH2CH2CH2OH),

67.657~67.530 (m, -(OCH2CH2CH2)5-), 66.206 (s, CH3OCH2CH2CH2-O-), 61.279 (s,

-OCH2CH2CH2OH), 58.435 (s, CH3OCH2CH2CH2-O-), 31.940 (s, -OCH2CH2CH2OH),

30.281 (s, CH3OCH2CH2CH2-O-), 29.880~29.789 (m, -(OCH2CH2CH2)5-).

For the synthesis of oligo(trimethylene oxide) with average 3.5 repeat units, the procedure was the same, but the amounts of oxetane and initiator were changed.

4.2.3.2. Synthesis of oligo(trimethylene oxide)-disubstituted oxetane

Route 1. Toluene (11 mL), diglyme (3 mL) and oligo(trimethylene oxide)

(DP=7, 2.54 g, 5.80 mmol)) were mixed and traces of water were removed by azeotropic reflux with toluene using a distilling head. (When a thermometer in the distilling head reached 84.1℃ and equilibrated, the azeotrope was taken out.). sodium ((0.13 g, 5.65 mmol, cut quickly and washed with toluene) was put into the solution and the mixture was maintained at 100℃ overnight. Hydrogen bubbles were seen coming out gently from the melted sodium surface. A pink clear solution was obtained, and was transferred by a double-tipped needle into a 3-necked flask containing 3,3-(bisbromomethyl)oxetane

0.5346 g, 2.19 mmol), toluene (15 mL) and azeotropically dried diglyme (3 mL). The mixture was heated at 100℃ and monitored by titrating with 0.01 M HCl titration every

30 mins. After 2 h, the reaction was stopped and centrifuged to remove the precipitated

162 sodium bromide. The supernatant solution was poured into 10x water and heated to 90℃.

0.63 g of a colorless oil separated and was collected (MW=958, yield 30%). 1H NMR

(300 MHz, CDCl3): δ (ppm) 4.454 (s, 4H, cyclic -OCH2-), 3.591 (s, 4H, -

CCH2OCH2CH2CH2O-), 3.56~3.40 (m, 56H, -OCH2CH2CH2-), 3.327 (s, 6H, -CH3),

1.85~1.80 (m, 28H, -OCH2CH2CH2-).

Route 2. Tetrabutylphosphonium bromide (1.87 g, 0.0055 mol), THF (15 mL), 3,3-bis(bromomethyl)oxetane (13 g, 0.053 mol) and oligo(trimethylene oxide)

(DP=4, 25.9 g, 0.11 mol)) were mixed in a 100 mL beaker and stirred to form a homogeneous solution. Then the solution was added to a 200 mL 3-necked flask containing a saturated sodium hydroxide aqueous solution (44 g NaOH, 22 mL H2O) at

64℃. The reaction was stirred at 64℃overnight. The mixture was then poured into 10x water and heated to 90℃. 27.4 g of a colorless oil separated and was collected (MW=552, yield 90%). This batch of disubstituted monomer was used to make comb copolymer by route 2 of the next experiments.

4.2.3.3. Copolymerization of trimethylene oxide and oligo(trimethylene oxide)- disubstituted oxetane

Route 1. A 50 mL 3-necked flask equipped with a condenser and a stirring bar was flame dried under vacuum (3x10-4 torr) and purged with dry argon twice. The flask was charged with trimethylene oxide (1 g, 0.0172 mol), [oligo(trimethylene oxide),

7 repeat units]-disubstituted oxetane (5.5 g, 0.00575 mol) and chloroform (10 mL). The

-4 mixture was stirred for 15 mins at 0℃ and BF3OEt2 (20 uL, 1*10 mol) was injected.

The reaction needed close observation since it could gel without any warning. In one batch we got uncrosslinked polymer. After fractional precipitation, by dissolving the

163 resulting mixture in methanol and precipitating by adding water drop by drop, 2.28 g

1 comb copolymer was obtained (35% yield). H NMR (CDCl3): δ (ppm) 3.55~3.40 (m,

96H, -OCH2CH2CH2-), 3.362 (s, 8H, -C(CH2)4, 3.337 (s, 6H, -OCH3), 1.86~1.79 (m,

48H, -OCH2CH2CH2-). The molar ratio of trimethylene oxide to oligo(trimethylene oxide)-disubstituted oxetane in the final polymer was 10:1 according to the calculation from the 1H NMR integration.

Route 2. A 100 mL 3-necked flask equipped with an addition funnel, a condenser and a stirring bar was flame dried under vacuum (3x10-4 torr) and purged with

-4 dry argon twice. The flask was charged with BF3:OEt2 (60 uL, 3.2*10 mol), methanol

(20 uL, 4.94*10-4 mol) and chloroform (2 mL). The mixture was stirred for 15 mins at room temperature. A solution of (2.07 g, 0.0357 mol) trimethylene oxide and

[oligo(trimethylene oxide), 3.5 repeat units]-disubstituted oxetane (7.6 g, 0.0138 mol) in chloroform (30 mL) was added dropwise at a rate of 5mL/h. After the addition was finished, the reaction mixture was stirred at room temperature for 3 days, and then neutralized by adding NaHCO3 (0.02 g) aqueous solution. The mixture was precipitated in hexanes twice to produce a colorless viscous polymer. The polymer was dried in a vacuum oven at 70℃ for two days and a gum-like polymer was obtained (8.7 g, 90% yield). The 1H NMR spectrum’s peaks and positions were the same to those from the

Route 1 product. The molar ratio of trimethylene oxide to oligo(trimethylene oxide)- disubstituted oxetane in the final polymer was 5:1 according to the calculation from the

1H NMR integration.

164 4.3. Results and discussion

4.3.1. Synthesis

4.3.1.1. Synthesis of oligo(trimethylene oxide)monomethyl ether

Initially, the most logical synthetic approach for poly(trimethylene oxide) monomethyl ether seemed to be via the cationic ring opening polymerization of trimethylene oxide initiated by a variety of catalysts including Lewis acids, such as

+ - BF3OEt2, CF3SO3CH3 (Fig 4.1.), AlCl3, SbCl5, Ph3C PF6 , Et3OBF4, HBF4,

+ - (C2H5)3O PF6 [17-22] and regioselective rhodium-containing catalysts [23], and terminated by water to give a hydroxyl endgroup. Qureshi’s group [24] reported they could obtain poly(trimethylene oxide) (PTMO) with MW> 50000 and polydispersities

(PD) of approximately 2 using BF3OEt2 as a catalyst; however, linear PTMO with controlled molecular weight and low polydispersity (PD<1.5) could not be made by this way.

At first, we tried this conventional method by polymerizing trimethylene oxide initiated by methyl triflate. Fig.4.1 showed the 1H NMR spectrum of the resulting oligo(trimethylene oxide). The degree of polymerization was designed to be 5 for living polymerization. However, from the NMR calculation, the degree of polymerization was

8.2. Furthermore, there was no detectable terminal hydroxyl group peak in the spectrum.

TGA again confirmed it was not a linear polymer with 5 repeating units, since it began to lose weight slightly above ambient temperature (Fig. 4.2). GPC (Fig.4.3) showed there were several peaks, which indicated the molecular weight was uncontrollable. FT-IR

(Fig.4.4) also clearly showed there was almost no detectable hydroxyl peak in the region of 3450 cm-1. It was unclear as to what the reaction products truly were, except to

165 demonstrate conclusively that the reaction did not proceed as planned to give linear polymer with the expected structure. The possible structure of a side product is shown in

Scheme 4.3, a cyclic tetramer. Odian pointed out the cyclic tetramer is the most abundant cyclic oligomer, up to ~50%, with lesser amounts of trimer and others from pentamer to nonamer for oxetane. This possibly explains the found from the NMR, TGA, GPC and

FT-IR spectra. Apparently this conventional cationic ring opening polymerization, later called active chain end mechansim (ACEM), has many chain transfer reactions including chain back-biting and is not useful for the synthesis of the goal product [25].

166 7-26-newbatch-oxetaneoligomer

3.467 1.0 CH (OCH CH CH )nOH 0.9 3 2 2 2

0.8

0.7 9.72+12.08+177.31+1.97=201.08 3.477 3.318 0.6 1.811

3.456 0.5 3/2n=18.33/100.06 1.822 1.801

Normalized Intensity 0.4 n=300/36.66= 8.18

0.3 1.830

0.2 3.438 1.790 3.596 3.507 3.605 3.427 3.743 3.535 1.840 0.1

0 9.72 12.08177.31 18.33 100.06

3.5 3.0 2.5 2.0 Chemical Shift (ppm) 1 Fig.4.1. H NMR (CDCl3) of the oxetane oligomer initiated by CF3SO3CH3

100

40 Weight/%

20 oligo(oxetane) initiated by CF SO CH 3 3 3 0

0 100 200 300 400 500 Temperature/oC

Fig.4.2. TGA of oligo(oxtetane) initiated by CF3SO3CH3

167 dry newbatchoxetaneoligomer.esp 20.85 2.0

21.48

1.5

1.0 23.23

0.5

25.48

0 5 10 15 20 25 30 35

Fig. 4.3. GPC of oligo(oxetane) initiated by CF3SO3CH3

barely detectable hydroxyl group

Fig. 4.4. FT-IR spectrum of oligo(oxetane) initiated by CF3SO3CH3

168

O CH OCH CH CH O CF SO CH 3OS O 2 CF 3 +O CH 3 O CF3SO3 3 2 2 2 3 3

o r O Et OCH CH CH O BF 3 OE t BF3 OEt2 + O Et O BF3OEt 2 2 2

(n +3 ) O Et OCH2 CH 2 CH2 OC H2CH2CH2 OCH2CH2CH2OCH2CH2CH2OCH2CH 2 CH 2 O BF 3OEt n

H2 O Et OCH2 CH 2 CH2 OC H2CH2CH2 OCH2CH2CH2OCH2CH2CH2OCH2CH 2 CH 2 OC H2 CH 2 CH2OH n

Side reaction (50% amount)

Et OCH2CH2CH2 OCH2CH2CH2 OCH2CH2CH2OCH2CH2CH2OCH2CH2CH2 O BF3OEt n

O(CH2)3

Et OCH2CH2CH2 OCH2CH2CH2 O(CH2)3 O(CH2)3 BF3OEt n

(CH2)3O

O Et OCH2CH2CH2 OCH2CH2CH2 OCH2CH2CH2 O BF3 OEt

O(CH2)3

+ O(CH2)3 O(CH2)3

(CH ) O 2 3

Scheme 4.3. The attempted synthesis of oligo(trimethylene oxide): active chain end mechanism

169 Recently, Kubisa and Penczek proposed an activated monomer mechanism

(AMM) for polymerization of cyclic monomers to yield linear polymers with suppressed

cyclic oligomer formation [26-28]. In this process, cyclic ether or cyclic imine is

activated by formation of protonated species in the presence of a protic acid or Lewis

acid in the presence of alcohol. Reaction of the protonated (activated) monomer with

hydroxyl group containing compounds leads to ring opening, reforming the hydroxyl

group. Repetitions of such a reaction constitute a chain process. Apparently, in this

mechanism, alcohol acts as an initiator and a protic acid or Lewis acid is the catalyst. The

monomer is protonated or activated to react with the hydroxyl-ended polymer chains. The

important feature of activated monomer mechanism is that back-biting leading to the

formation of cyclic oligomers can be suppressed due to the absence of charged species at

the growing chain ends (Scheme 4.4).

O CH3OH - CH OCH CH CH OH + HBF4 HO BF4 HBF4 +O HO BF4 3 2 2 2

Slow sequential addition of O O HO BF - CH3OCH2CH2CH2(OCH2CH2CH2)nOCH2CH2CH2OH + HBF4 4

Scheme 4.4. The synthesis of oligo(trimethylene oxide): activated monomer mechanism

170 The 1H NMR and 13C NMR spectra of the synthesized oligo(trimethylene oxide) are shown in Fig.4.5-4.6. From the 1H NMR, we can clearly see the hydroxyl peak around 2.5 ppm; its integration to that of methyl peak (3.294 ppm) is exactly 1:3. The 13C

NMR also confirmed the linear structure with a hydroxyl end group. MALDI-TOF

(Fig.4.7) showed that most of the polymer had 6, 7 and 8 repeating units, with small amounts of material initiated by water to give groups with two hydroxyl ends. The comparison of the FT-IR spectra of the oligo(trimethylene oxide) prepared via different mechanisms is shown in Fig.4.8. The product synthesizd via the activated monomer mechanism has an obvious hydroxyl group peak at about 3450 cm-1, but no detectable peak is seen in that region from the product made via the active chain end mechanism.

TGA (Fig. 4.9) gave another proof. Compared to the high volatility of the product synthesized via the active chain end mechnism, the new product made via the activate monomer mechanism has a much higher boiling point. Instead of beginning to lose weight from room temperature, it begins to lose weight from 150℃, and is comparable to poly(ethylene glycol)monomethyl ether with an average of 7 repeat units. This shows that we have made the oligo(trimethylene oxide) with a linear structure and one hydroxyl end group. However, although the activated monomer mechanism is good for the production of linear polymers, it still competes with the conventional cationic ring-opening polymerization [29]. This was confirmed from the GPC spectrum (Fig.4.10). Instead of a sharp single peak, it still shows a small shoulder at 25 min which is due to the cyclic oligomer.

171 8-16-1h-am-oxetaneoligomer-cdcl3 3.294 a b c b d c e f 1.0 CH3O(CH2CH2CH2O)6CH2CH2CH2OH 3.447 0.9 3.443

3.449 0.8 a b c

0.7

0.6 1.787

3.454 3.437 3.439 1.794 0.5 3.458 1.797 3.433

1.776

Normalized Intensity 0.4

0.3 e d 1.805 3.482 3.414 1.808 3.569 1.767 0.2 3.716 3.492 3.579 3.403 3.725 1.815 3.708 3.712 f 0.1 0 7.11 7.64 80.73 10.00 3.27 47.67

3.5 3.0 2.5 2.0 Chemical Shift (ppm)

1 Fig. 4.5. H NMR (CDCl3) of oligo(trimethylene oxide) synthesized via the activated activated monomer mechanism

8-29-13c-new-am-oxetaneoligomer-ch3oh-cdcl3-25c

67.657 e 1.0 0.9 a g c h e f e i d b CH3OCH2CH2CH2(OCH2CH2CH2)5OCH2CH2CH2OH 0.8

0.7

0.6 f 0.5 g i 29.880

67.639 a 67.530 Normalized Intensity Normalized 0.4 67.553 69.547 68.021 58.435 0.3 d c 29.828

66.206 b

0.2 29.789 h 31.940 30.281 67.519

0.1 61.279

0 0.100.06 0.02 0.06 0.04 0.06 0.38

65 60 55 50 45 40 35 30 Chemical Shift (ppm) 13 Fig. 4.6. C NMR (CDCl3) of oligo(trimethylene oxide) synthesized via the activated monomer mechanism

172

All peaks are due to the oligomer MW+ Na ion

CH3(OCH2CH2CH2)7OH

CH3(OCH2CH2CH2)6OH

15+58*6+17+23=403 CH3(OCH2CH2CH2)8OH

HO(CH2CH2CH2O)8H

HO(CH2CH2CH2O)7H HO(CH2CH2CH2O)6H

Fig. 4.7. MALDI-TOF of the oligo(trimethylene oxide) synthesized via the activated monomer mechanism

-OH end group

Product via Activated Monomer Mechanism

-OH barely seen Product via Active Chain End Mechanism

Fig.4.8. FT-IR spectra of the oligo(trimethylene oxide) synthesized via the active chain end mechanism and activated monomer mechanism

173

100

40 Weight/%

20 oligo(trimethylene oxide) via AM M oligo(ethylene oxide), (PEGME) 0

0 100 200 300 400 500 o Temperature/ C Fig.4.9. TGA of oligo(trimethylene oxide) made via the activated monomer mechanism, and of PEGME purchased from Aldrich

1.4

1.2

1.0

0.8

0.6 peak shoulder: cyclic au oligomer 0.4 oligo(oxetane) 0.2

0.0

-0.2 0 5 10 15 20 25 30 35 Time/min

Fig. 4.10. GPC (solvent: DMF) of oligo(trimethylene oxide) synthesized via the activated monomer mechanism

174 4.3.1.2. Synthesis of oxetane monomer with disubstituted oligo(trimethylene oxide) and its polymerization

There are two possible ways of synthesizing comb polymers with oligo(trimethylene oxide) side chains. One is similar to the way used in Chapter 3, copolymerization of 3, 3-(bisbromomethyl)oxetane with trimethylene oxide first, then using Williamson ether synthesis method to replace bromine groups with oligo(trimethylene) side chains. The other way is to synthesize the side chain substituted monomer first, and then copolymerize this monomer with trimethylene oxide (Scheme

4.5). For the first method, we know from Chapter 3 that it works well. For the second method, if we can find a way to suppress the chain transfer reaction, we could use this method to obtain 100 percent substitution of oligo(trimethylene oxide) side chains in the copolymer. Our work focused on the latter method.

There are two approaches to the synthesis of oligo(trimethylene oxide) disubstituted oxetane monomer. The first one is via typical Williamson ether synthesis, reacting 3,3-bis(bromomethyl)oxetane with oligo(trimethylene oxide) alkoxide, made by reacting the hydroxyl-terminated oligo(trimethylene oxide) with sodium or sodium hydride in an aprotic solvent such as glyme or diglyme. However, the reaction condition is very strict since any moisture needs to be removed completely; otherwise side reactions such as hydrolysis of 3,3-(bromomethyl) oxetane to mono-hydroxyl substituted or di-hydroxyl substituted oxetane dominate. Thus, this procedure gave low yields (~30% conversion).

Another method, developed by Freedman [30], has been used very broadly recently [31-32]. In this process, a mixture of organic solvent (such as THF), 3,3-

175 bis(bromomethyl)oxetane, hydroxyl-terminated oligo(trimethylene oxide), a phase transfer catalyst and a base/water solution such as sodium hydroxide or potassium hydroxide is heated at 60-85℃. The reaction can be finished overnight. Upon completion of the reaction, the product is precipitated by pouring into water. The yield can be raised to 50% since the phase transfer catalyst prefers to bring the oligo(trimethylene oxide) alkoxide into the organic phase. As a result, the competing hydrolysis side reaction is reduced. In this procedure, phase transfer catalysts function by transferring counterions so that they are more soluble in the organic phase. A variety of phase transfer catalysts can be used, such as tetrabutyl ammonium bromide, benzyltrimethyl ammonium bromide, tetramethylammonium chloride, tetramethylammonium iodide, cetyltributylammonium bromide, tetrabutylphosophonium bromide, crown ethers and the like. The preferred catalyst is tetrabutylphosphonium bromide due to its higher stability than ammonium brimides and good solubility in both organic and aqueous mediums. The above reaction can be conducted at temperatures as low as room temperature and as high as above 100℃.

However, at low temperatures, the rate of displacement is extremely low and hydrolysis can dominate. At higher temperatures, the rate of displacement is extremely fast, however, the phase transfer catalyst may be unstable and there will be more side reactions.

Consequently, the preferred reaction temperatures are around 60-85℃. When optimizing the conditions, we found the use of THF as an organic solvent and saturated base water solution at 64℃ instead of 50/50 base/water greatly improves the yield to 90%. The 1H

NMR and 13C NMR are shown in Figs. 4.11 and 4.12. The 1H NMR spectrum shows two

13 single peaks (δ 4.454, 3.591 ppm) due to the cyclic -CH2- and the substituted -CH2-. C

NMR further confirms the structure.

176 The copolymerization of oligo(trimethylene oxide) disubstituted oxetane with trimethylene oxide can be initiated via Lewis acids such as boron trifluoride diethyl etherate or triflic acid and the comb copolymer can be prepared in high yield. However, we discovered that this polymerization is very tricky since it can cross-link very easily due to chain transfer of the oxonium ion. We found the reaction can go to gel without any warning, from several minutes to several hours. It may be better to use an internal thermometer to monitor the reaction. When the temperature stops rising and becomes stable, we can terminate the reaction. To solve this problem, an alcohol initiator and a

Lewis acid cocatalyst were used for polymerization via the activated monomer mechanism. Under these conditions, the final polymer cannot cross-link even at high temperature such as 100℃. The other important character of this copolymerization is that the two bulky side groups sterically hinder the propagation reaction of the growing polymer chain. As a result, the copolymerization rate is much slower than that of the homopolymerization of trimethylene oxide. For the polymerization initiated by Lewis acid, the reaction can take several hours. If it is via the activated monomer mechanism, the reaction can take several days. For longer side chains (7 repeat units), after 4 days 50% disubstituted monomer is still unreacted. For shorter side chains (4 repeat units), the disubstituted monomer conversion can reach 90%. The final product is a gradient copolymer, with decreasing trimethylene oxide units in the chain and increasing disubstituted monomer units as the reaction goes. Fig.4.13 shows the 1H NMR of the final copolymer with 7 repeat units of oligo(trimethylene oxide) side chains. After ring opening, the chemical shift of the substituted –CH2- moves from 3.591 to 3.362 ppm.

Since it has the same chemical environment as the nearby -CH2- in the backbone, they

177 combine to give a single broad peak; its integration relative to -CH3 end group is exactly

8/6. The oxetane comb copolymer with an average of 3.5 repeat units of oligo(trimethylene oxide) side chains was used in later experiments since it can be made in big batch with higher yield than comb copolymer with longer side chains.

Route 1 diglyme HO(CH2CH2O)nCH3 + Na NaO(CH2CH2CH2O)nCH3 120 oC CH2Br OBrBF3OEt2 + OCH2CH2CH2 OCH2CCH2 O m n CH2Cl2 CH2Br Br

CH2Br

OCH2CH2CH2 OCH2CCH2 +Na(OCH2CH2CH2)7CH3 m n CH2Br

CH2O(CH2CH2CH2O)nCH3 diglyme OCH2CH2CH2 OCH2CCH2 120 oC m n CH2O(CH2CH2CH2O)nCH3

Route 2

diglyme HO(CH CH CH O)nCH + Na NaO(CH2CH2CH2O)nCH3 +1/2H2 2 2 2 3 90oC

OBr O O(CH CH CH O)nCH diglyme 2 2 2 3 + NaO(CH2CH2CH2O)nCH3 120 oC Br O(CH2CH2CH2O)nCH3 or

OBr O O(CH CH CH O)nCH THF, saturated NaOH/ H2O 2 2 2 3 + HO(CH2CH2CH2O)nCH3 Bu + Br-, reflux Br 4 O(CH2CH2CH2O)nCH3

O O(CH2CH2CH2O)nCH3 O BF3OEt2 + CH2Cl2 O(CH2CH2CH2O)nCH3

CH2O(CH2CH2CH2O)nCH3

OCH2CH2CH2 OCH2CCH2 m n CH2O(CH2CH2CH2O)nCH3 Scheme 4.5. The synthesis of trimethylene oxide comb polymers by two possible routes

178 1-31-cyclicmonomer-incdcl3 3.327 3.476 a 1.0 e b c b b OO(CH2CH2CH2O)7CH3 0.9 b

0.8 a O(CH2CH2CH2O)7CH3 0.7 d

0.6 3.487 e 3.466 e 3.469 1.822 0.5 a 1.831

4.454

Normalized Intensity 0.4

3.446 c

c 1.812

0.3 3.591

1.839

d 1.841 3.533 b 0.2 d 3.435

1.801 1.849 3.390 0.1

0 3.18 38.93 5.17 0.10 20.00

4.5 4.0 3.5 3.0 2.5 2.0 Chemical Shift (ppm) 1 Fig. 4.11. H NMR (CDCl3) of oligo(trimethylene oxide) disubstituted oxetane

1-31-13c-cyclicproduct.esp a f h g g h g g i e d 30.188 67.846 OOCH2CH2CH2(OCH2CH2CH2)5OCH2CH2CH2OCH3

1.0 67.972 c CDCl3 h 0.9 OCH2CH2CH2(OCH2CH2CH2)5OCH2CH2CH2OCH3 f b

0.8 77.160 77.372 76.950

30.102 0.7 b

0.6

e g 58.752 0.5 69.857 30.051

0.4 Normalized Intensity 72.169 d a 68.554 i 0.3

44.017 0.2 76.485 34.779 69.837 77.466 30.307 0.1 72.007 c

-0.1 75 70 65 60 55 50 45 40 35 30 Chemical Shift (ppm)

13 Fig. 4.12. C NMR (CDCl3) of oligo(trimethylene oxide) disubstituted oxetane

179 3-8-07-newpolymer-nmrrunat45c

3.489 3-8-07-newpolymer-nmrrunat45c 3.489 1.0 1.0 a a a 0.9 a a c CH2O(CH2CH2CH2O)7CH3 0.9 a OCH2CH2CH2 OCH2CCH2 0.8 10 1 0.8 b CH2O(CH2CH2CH2O)7CH3 0.7

3.478 c b d 0.6 0.7 3.500 0.5 1.830

3.478 0.6 3.337 3.500

Normalized Intensity Normalized 0.4

0.3 0.5 d 3.471 3.461

0.2 1.841 3.362 3.450 1.819 3.445

3.337 c Normalized Intensity 0.4 0.1

0 0.3 c 96.26 8.00 6.02 3.471 3.461 3.70 3.65 3.60 3.55 3.50 3.45 3.40 3.35 3.30 3.25 3.20 b 0.2 Chemical Shift (ppm) 1.851 3.362 d 1.809 1.804 3.445 1.793 0.1

0 96.26 6.02 48.33

3.5 3.0 2.5 2.0 Chemical Shift (ppm) 1 Fig. 4.13. H NMR (CDCl3) of oxetane-co- (oligo-trimethylene oxide disubstituted oxetane)

4.3.2. Solution properties

Because the trimethylene oxide structure contains only CH2 and ether group, as does

PEO, the new oxetane comb polymers share many solution properties, as was mentioned

in Chapter 3. Most significantly, they also display a Lower Critical Solution Temperature

(LCST) that results from the presence of both hydrophilic and hydrophobic centers in

these polymers.

4.3.3. Thermal properties

4.3.3.1. Thermal stability

Thermogravimetric studies were carried out in order to determine the thermal

stability of the polymeric systems, in terms of the initial weight loss temperature. The

180 control of this parameter is important because the electrolyte must be thermally inert in the working temperature range (up to 120℃). The thermogravimetric plots (TGA) of oxetane comb copolymer, oxetane comb copolymer/LiTFSI ([O]/[Li]=30) and oxetane comb copolymer/LiTMPB ([O]/[Li]=30) are shown in Fig. 4.14. The uncomplexed oxetane comb copolymer was thermally stable up to 200℃. Upon the addition of LiTFSI salt, the thermal stability was slightly reduced to 180℃, while the LiTMPB electrolyte began to decompose above 165℃. It seems that complexation with metal salts leads to a slight reduction in their thermal stability in an inert atmosphere. This phenomenon is also observed in PEO and PPO electrolytes [33-34]. Since they can stand up to 160℃, these polymer electrolytes are thermally stable enough for lithium battery applications.

100

40 Weight % Weight

oxetane comb copolymer 20 oxetane comb copolymer- LiTFSI-O/Li=30 oxetane comb copolymer- 0 LiTMPB-O/Li=30

0 100 200 300 400 500 Temperature oC Fig.4.14. TGA (at 10℃/min heating rate) of oxetane comb copolymer and its complexes with LiTFSI or LiTMPB at [O]/[Li]=30

181 4.3.3.2. Glass transition temperature

The thermal transitions of the salt-free and salt-complexed polymers were measured using differential scanning calorimetry (DSC). DSC thermograms (Fig. 4.15) were obtained in the second heating cycle at a heating rate of 10℃/min after quenching from 120℃ for the oligo(oxetane) with 7 and 4 repeat units, the oxetane comb copolymer prepared from oligo(oxetane) with 4 repeat units, and its polymer electrolytes complexed with LiTFSI or LiTMPB salt at [O]/[Li]=30. Oligo(trimethylene oxide)with 7 repeat units shows, in addition to the glass transition temperature of -92℃, a melting region with three peaks, located at -21.6, -11.5, and -1℃, respectively, and a recrystallization peak located at -46℃. In contrast, oligo(trimethylene oxide) with 4 repeat unit has no recrystallization peak and melting region. As the chain length increases, poly(trimethylene oxide) tends to crystallize, as was seen for poly(ethylene oxide). The synthesized oxetane comb polymer with oligo(trimethylene oxide) side chain of 4 repeat units is completely amorphous, displaying a single glass transition temperature of -82℃,

10℃ lower than that of the comb polymer with an oligo(ethylene oxide) side chain of 7 repeat units. When lithium salts are added, glass transitions are displaced towards higher temperatures relative to the uncomplexed polymers. Obviously lithium ions chelate with the oligo(trimethylene) side chain and these ion-dipole interactions hinder the segmental mobility of side chains, as was shown for the ethylene oxide comb polymers in Chapter 3.

Since no crystallization was observed in these polymer/lithium salt complexes, their amorhous character should be beneficial for ionic conductivity, as described in Chapter 3.

Fig. 4.16 shows the glass transition temperatures (Tgs) of the polymer electrolytes, determined by DSC, plotted against their LiTFSI or LiTMPB concentrations. An increase

182 in Tg with the salt concentration is observed; higher concentrations of salt increase ionic crosslinking and increase the glass transition temperature. The increase in Tg with salt concentration is much more pronounced in the LiTMPB doped polymer electrolytes than in the LiTFSI doped ones. These results follow those for the ethylene oxide comb polymers and have been discussed fully in Chapter 3. However, we notice one characteristic; the oxetane comb polymer electrolytes have much lower glass transition temperatures (up to 15℃) than ethylene oxide comb polymer electrolytes for both salts.

The low Tgs for our oxetane comb polymer indicate that the barrier to ionic motion is lower for oxetane-based polymers than that of ethylene oxide-based polymers and this agrees with Curtiss’ calculation [13]. As a result, these polymer electrolytes might have higher conductivity.

3.0 oligo(oxetane),n=7 oligo(oxetane),n=4 oxetane comb copolymer 2.5 oxetane comb copolymer- LiTFSI-O/Li=30 2.0 oxetane comb copolymer- LiTMPB-O/Li=30

1.5

1.0 Heat flow(w/g) Heat 0.5

0.0

-0.5 -200 -150 -100 -50 0 50 100 150 200 Temperature/oC Fig. 4.15. DSC thermograms of oligo(oxetane) with 7 repeat units, oligo(oxetane) with 4 repeat units, oxetane comb copolymer and its polymer electrolytes complexed with LiTFSI and LiTMPB at [O]/[Li]=30, at a heating rate of 10℃/min after quenching from 120℃

183

-10 TMO comb copolymer-LiTMPB -20 TMO comb copolymer-LiTFSI -30

-40 C o -50

Tg/

-60

-70

-80

-90 0 20406080100 [O]/[Li]

Fig. 4.16. Glass transition temperature (Tg) of oxetane comb copolymer and its polymer electrolytes plotted against salt concentration ([O]/[Li]). (Note: here [O]/[Li]=100 stands for uncomplexed polymer.)

4.3.4. Ionic conductivity

The temperature dependence of the ionic conductivities of these polymer/lithium salt complexes with [O] to [Li] molar ratio ranging from 70:1 to 10:1 were measured over the temperature range from room temperature to 100℃. As shown in Fig. 4.17, the highest ambient temperature ionic conductivity, which occurred at a 10:1 of [O]/[Li] for the oxetane comb copolymer/LiTFSI complex, was 4.62*10-5 S/cm at a frequency of 100

KHz. This is slightly higher than the ethylene oxide comb copolymer/LiTFSI complex described in Chapter 3 and is one of the highest values reported so far. It rises to 1.1*10-3

S/cm at 103℃ (Fig. 4.18). For oxetane comb copolymer/LiTMPB electrolyte, maximum conductivity at a frequency of 100 KHz was found between the 30:1 and 50:1 [O]/[Li] ratios. It was 6.34*10-6 S/cm at ambient temperature (50:1 ratio of [O]/[Li]) and

184 2.59*10-4 S/cm (30:1 ratio of [O]/[Li]) at 103℃. Again this high conductivity result implies that the LiTMPB salt has a larger fraction of dissociated ions than LiTFSI salt, since most or all the current is probably due to lithium cation mobility of LiTMPB.

Compared to ethylene oxide comb polymer electrolytes in Chapter 3, we find the oxetane comb copolymer electrolytes have higher or comparable ionic conductivities at the same [O]/[Li] ratio. Figs. 4.19 compares oxetane comb copolymer/LiTMPB and ethylene oxide comb copolymer/LiTMPB electrolytes at 297K, 336K and 376K at different [O]/[Li] ratios. From the plot we see at all the compositions measured and from room temperature to high temperature, all the oxetane comb copolymer/LiTMPB electrolytes have higher conductivities than their ethylene oxide counterparts. Fig. 4.20 shows the conductivity comparsion of oxetane comb copolymer/LiTFSI and ethylene oxide comb copolymer/LiTFSI electrolytes. The oxetane comb copolymer/LiTFSI complex shows comparable conductivity to that of ethylene oxide/LiTFSI electrolytes at low lithium salt concentrations (50:1 and 30:1[O]/[Li] ratios) but twice higher conductivites at high lithium salt concentrations (10:1 [O]/[Li] ratio). Apparently low glass transtition temperatures of oxetane comb copolymer electrolytes induce higher polymer chain mobility and this contributes to higher conductivity although ethylene oxide comb polymer electrolytes have higher lithium ion molar concentration. Another characteristic of the oxetane comb polymer electrolytes compared to ethylene oxide comb polymer electrolytes is that the lithium salt concentration at maximum conductivity has changed. Fig 4.17-4.18 show that for LiTFSI salt, the [O]/[Li] ratio for optimum conductivity has moved from 30:1 (for ethylene oxide comb copolymer electroltyte) to

185 10:1; for LiTMPB salt, it moved from 50:1 (for ethylene oxide comb copolymer electroltyte) to 30:1.

186

10-5

-6

Conductivity (S/cm) Conductivity 10 oxetane comb copolymer- LiTFSI, 297K, 100KHz

oxetane comb copolymer- LiTMPB, 297K, 100KHz

0 10203040506070 [O]/[Li] Fig.4.17. Ionic conductivity at 100 KHz vs. the O/Li ratio of oxetane comb copolymers complexed with LiTFSI and LiTMPB (297K)

oxetane comb copolymer- LiTFSI, 376K, 100KHz oxetane comb copolymer- LiTMPB, 376K, 100KHz oxetane comb copolymer- 10-3 LiTFSI, 336K, 100KHz oxetane comb copolymer- LiTMPB, 336K, 100KHz

10 -4

Conductivity (S/cm)

10-5 0 1020304050607080 [O]/[Li] Fig.4.18. Ionic conductivity at 100 KHz vs. the O/Li ratio of oxetane comb copolymers complexed with LiTFSI and LiTMPB (336K and 376K)

187

10-4

10-5

10-6 TMO/LiTMPB, 376K, 100 KHz Conductivity (S/cm) Conductivity EO/LiTMPB, 376K, 100 KHz TMO/LiTMPB, 336K, 100 KHz -7 EO/LiTMPB, 336K, 100 KHz 10 TMO/LiTMPB, 297K, 100 KHz EO/LiTMPB, 297K, 100 KHz

10 20 30 40 50 60 70 [O]/[Li]

Fig. 4.19. Conductivity comparison vs. [O]/[Li] ratios of oxetane comb copolymer/LiTMPB and ethylene oxide comb copolymer/LiTMPB electrolytes at 297K, 336K and 376K

TMO/LiTFSI, 376K, 100 KHz EO/LiTFSI, 376K, 100 KHz TMO/LiTFSI, 336K, 100 KHz EO/LiTFSI, 336K, 100 KHz TMO/LiTFSI, 297K, 100 KHz 10-3 EO/LiTFSI, 297K, 100 KHz

-4

Conductivity (S/cm)

10-5

10 20 30 40 50 [O]/[Li] Fig. 4.20. Conductivity comparison vs. [O]/[Li] ratios of oxetane comb copolymer/LiTFSI and ethylene oxide comb copolymer/LiTFSI electrolytes at 297K, 336K and 376K

188 The temperature dependence of ionic conductivities of oxetane comb copolymer/LiTFSI complex and oxetane comb copolymer/LiTMPB complex at salt concentrations, [O]/[Li]=70, 50, 30 and 10 were studied. Arrhenius plots of the data are shown in Figs. 4.21-4.24. All plots show curvature, which indicates a non-Arrehenius temperature-dependent behavior of the amorphous polymer electrolytes, except oxetane comb copolymer/LiTMPB at 10:1 of [O]/[Li] ratio. However, the data does fit VTF curves (Figs. 4.26-4.27), producing almost straight lines when T0 assumed to be the Tg for each system. This indicates that the ionic conductivity behavior for these polymer electrolytes is like that of the ethylene oxide comb polymer electrolytes described in

Chapter 3; ion transport depends on the segmental motion of the oligoether side chains.

However, as the lithium salt concentration increases, segmental mobility is clearly affected by the stiffening of the polymer resulting from ionic crosslinks. For oxetane comb copolymer/LiTMPB electrolyte at 10:1 of [O]/[Li] ratio, the plot is almost linear and seems to obey the Arrhenius behavior within the measured temperature range, while

LiTFSI doped electrolyte at the same composition still shows positive curvature. Table

4.2 lists the activation energies of the oxetane comb copolymer electrolytes at different compositions from the VTF model. These results are in agreement with results obtained from the ethylene oxide comb polymer electrolytes described in Chapter 3.

189

-8

-9

-10

-11

-12

ln(conductivity) (S/cm) ln(conductivity) oxetane comb copolymer- -13 LiTFSI-O/Li=70, 100KHz oxetane comb copolymer- LiTMPB-O/Li=70, 100KHz -14 2.6 2.8 3.0 3.2 3.4 3.6 1000/T (1/K) Fig.4.21. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=70 (σ at 100KHz)

-8

-9

-10

-11

-12

ln(conductivity) (S/cm) ln(conductivity) oxetane comb copolymer- LiTFSI-O/Li=50,100KHz -13 oxetane comb copolymer- LiTMPB-O/Li=50, 100KHz -14 2.6 2.8 3.0 3.2 3.4 3.6 1000/T (1/K) Fig.4.22. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=50 (σ at 100KHz)

190

-7

-8

-9

-10

-11

-12 ln(conductivity) (S/cm) ln(conductivity) -13 oxtane comb copolymer- LiTFSI-O/Li=30, 100KHz oxtane comb copolymer- -14 LiTMPB-O/Li=30, 100KHz 2.6 2.8 3.0 3.2 3.4 3.6 1000/T (1/K) Fig.4.23. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=30 (σ at 100KHz)

-7

-8

-9 -10 -11

-12

-13

-14

ln(conductivity) (S/cm) ln(conductivity) -15 oxetane comb copolymer- LiTFSI-O/Li=10, 100KHz -16 oxetane comb copolymer- -17 LiTMPB-O/Li=10, 100KHz

2.6 2.8 3.0 3.2 3.4 3.6 1000/T (1/K) Fig.4.24. Arrhenius conductivity plot for oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB complexes at [O]/[Li]=10 (σ at 100KHz)

191

-8

-9

-10

-11

-12

ln(conductivity) (S/cm) oxetane comb copolymer- -13 LiTFSI-O/Li=70, 100KHz oxetane comb copolymer- LiTMPB-O/Li=70, 100KHz -14 5 6 7 8 9 101112 1000/(T-Tg) (1/K) Fig.4.25. VTF conductivity plot for oxetane comb copolymer/LiTFSI ([O]/[Li]=70 ,Tg=-78℃) and oxetane comb copolymer/ LiTMPB ([O]/[Li]=70, Tg=-77℃) complexes (σ at 100KHz)

-8

-9

-10

-11

-12

ln(conductivity) (S/cm) ln(conductivity) oxetane comb copolymer-

-13 LiTFSI-O/Li=50, 100KHz oxetane comb copolymer- LiTMPB-O/Li=50, 100KHz -14 5 6 7 8 9 10 11 12 13 1000/(T-Tg) (1/K) Fig.4.26. VTF conductivity plot for oxetane comb copolymer/LiTFSI ([O]/[Li]=50 ,Tg=-76℃) and oxetane comb copolymer/ LiTMPB ([O]/[Li]=50, Tg=-74℃) complexes (σ at 100KHz)

192

-7 oxetane comb copolymer- LiTFSI-O/Li=10, 100KHz oxetane comb copolymer- -8 LiTFSI-O/Li=30, 100KHz oxetane comb copolymer- -9 LiTMPB-O/Li=30, 100KHz

-10

-11

-12 ln(conductivity) (S/cm) ln(conductivity) -13

-14

6 8 10 12 14 16 1000/(T-Tg) (1/K) Fig.4.27. VTF conductivity plot for oxetane comb copolymer/LiTFSI ([O]/[Li]=10 ,Tg=-58℃; [O]/[Li]=30, Tg=-74℃) and oxetane comb copolymer/ LiTMPB ([O]/[Li]=30, Tg=-68℃) complexes (σ at 100K Hz)

Table 4.2. The calculated activation energies (Ea/KJ mol-1) of the trimethylene oxide comb polymer electrolytes using the VTF model [O]/[Li] 70 50 30 10

*OCP-LiTMPB 6.5 (±0.1) 6.3 (±0.1) 6.5 (±0.1) N/A

*OCP-LiTFSI 5.6 (±0.1) 5.5 (±0.1) 5.2 (±0.1) 4.5 (±0.1)

* OCP—oxetane comb copolymer

193 4.3.5. Polarization measurements at low frequencies

As described in Chapter 3, polarization behavior can be tested by running conductivity measurements in the low frequency region. In our experiments, the conductivity measurements were taken from frequencies as low as 0.01 Hz to 100K Hz.

For comparison, plots of conductivity vs. frequency for oxetane comb copolymer/LiTFSI electrolytes and oxetane comb copolymer/LiTMPB electrolytes at 50/1 and 30/1 [O]/[Li] ratios at room temperature (297K), middle-range temperature (336K) and high temperature (376K) are shown in Figs. 4.28-4.33. From the plots, we see that at high frequency the conductivity of LiTFSI doped electrolyte is higher than that of LiTMPB doped electrolyte. As the frequency decreases, the conductivty of LiTFSI doped electrolytes decreases faster, but is still a bit higher than those of the LiTMPB doped materials in the studied temperature range at low frequencies. At the 30/1 [O]/[Li] composition and 336K, the comb copolymer/LiTMPB electrolyte conductivity at 100K

Hz is one third of that of the comb copolymer/LiTFSI electrolyte, but at 0.01 Hz, it rises to half. Apparently, LiTMPB doped electrolytes have less cell polarization than LiTFSI doped analogues because the bulky anions of LiTMPB salt are almost immobile. However, the cell polarization for these two salts complexed with our oxetane comb copolymer does not differ a lot compared to that for these two salts complexed with ethylene oxide comb polymer electrolytes in Chapter 3 and this needs to be investigated further.

194

1E-5

1E-6

Conductivity(S/cm) 1E-7 oxetane comb copolymer- LiTFSI-O/Li=50, 297K oxetane comb copolymer- LiTMPB-O/Li=50, 297K

1E-3 0.01 0.1 1 10 100 1000 10000 100000 Frequency/Hz Fig. 4.28. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB at [O]/[Li]=50 and 297K

1E-4

1E-5

1E-6

Conductivity(S/cm) oxetane comb copolymer- LiTFSI-O/Li=50, 336K oxetane comb copolymer- LiTMPB-O/Li=50, 336K 1E-7 1E-3 0.01 0.1 1 10 100 1000 10000 100000

Frequency/Hz Fig. 4.29. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB at [O]/[Li]=50 and 336K

195

1E-4

1E-5

Conductivity(S/cm)

oxetane comb copolymer- 1E-6 LiTFSI-O/Li=50, 376K oxetane comb copolymer- LiTMPB-O/Li=50, 376K

1E-3 0.01 0.1 1 10 100 1000 10000 100000 Frequency/Hz Fig. 4.30. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB at [O]/[Li]=50 and 376K

1E-5

1E-6

Conductivity(S/cm) oxetane comb copolymer- LiTFSI-O/Li=30, 297K 1E-7 oxetane comb copolymer- LiTMPB-O/Li=30, 297K

1E-3 0.01 0.1 1 10 100 1000 10000 100000 Frequency/Hz

Fig. 4.31. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB at [O]/[Li]=30 and 297K

196

1E-4

1E-5

Conductivity(S/cm) 1E-6 oxetane comb copolymer- LiTFSI-O/Li=30, 336K oxetane comb copolymer- LiTMPB-O/Li=30, 336K 1E-7 1E-3 0.01 0.1 1 10 100 1000 10000 100000 Frequency/Hz Fig. 4.32. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB at [O]/[Li]=30 and 336K

1E-3

1E-4

1E-5

Conductivity(S/cm) oxetane comb copolymer- LiTFSI-O/Li=30, 376K oxetane comb copolymer- LiTMPB-O/Li=30, 376K

1E-6 1E-3 0.01 0.1 1 10 100 1000 10000 100000 Frequency/Hz

Fig. 4.33. Conductivities vs. frequency of oxetane comb copolymer/LiTFSI and oxetane comb copolymer/LiTMPB at [O]/[Li]=30 and 376K

197 4.4. Conclusions

Oxetane comb copolymer with oligo(trimethylene oxide) side chains of the type –

O-(CH2CH2CH2O)n-CH3 (an average n of 4) was made by the cationic ring-opening copolymerization of oligo(trimethylene oxide) disubstituted oxetane and trimethylene oxide. Homogeneous polymer electrolytes were made from this oxetane comb polymer and two lithium salts having different anion sizes, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium tetrakis[3,5- bis(trifluoromethyl)phenyl]borate (LiTMPB) using a solvent casting method. Their thermal and electrical, especially the low frequency conductivity properties, were studied as a fuction of temperature. All these polymer electrolytes were amorphous and thermally stable up to 150℃. They had much lower glass transition temperatures (~15℃ lower) than ethylene oxide comb polymer electrolytes. As a result, the oxetane comb copolymer could provide more free volume and had less ionic crosslinking at the same composition compared to ethylene oxide comb polymer. Consequently, compared to ethylene oxide comb polymer electrolytes, oxetane comb polymer electrolytes had comparable and even higher conductivities at the same [O]/[Li] ratio. For oxetane comb copolymer/LiTFSI electrolytes, they had slightly lower conductivities at low lithium salt concentrations

(50/1 and 30/1 [O]/[Li]) but twice higher conductivies at high lithium salt concentrations

(10/1 [O]/[Li]); for oxetane comb copolymer/LiTMPB electrolytes, they always had higher conductivities at all the compositions, even 8 times higher at 10/1 [O]/[Li] ratio.

The optimum concentrations for both lithium salts shifted to higher concentrations, for

LiTFSI electrolyte, it shifted to10:1 [O]/[Li], for LiTMPB electrolyte, 30:1 [O]/[Li].

Oxetane comb copolymer/LiTFSI complex at ambient temperature and 100 KHz had a

198 maximum conductivity of 4.62*10-5 S/cm, slightly higher than that of the ethylene oxide comb copolymer electrolytes with oligo(ethylene oxide) side chains of 7 repeat units.

Comb copolymer/LiTMPB complex at ambient temperature and 100 KHz had a maximum conductivity of 6.34*10-6 S/cm, also slightly higher than its ethylene oxide counterpart (4.65*10-6 S/cm). For all these electrolytes, ionic conductivity increased with temperature; values of more than 10-3 S/cm were obtained for oxetane comb copolymer/LiTFSI complex at higher temperature (100℃). Considering that the oxetane comb copolymer we used here has only 4 average repeat units on the side chain, it should have higher conductivity if the side chain length increases to 6 or 7 repeat units; which has been proved by other researchers [36].

The temperature dependence behavior of these electrolytes obeyed the VTF equation instead of the Arrhenius equation, which confirmed their amorphous characteristics and showed that the ion transport depended on the segmental motion of the oligoether side chains. However, when the salt concentration was high, segmental mobility was affected by the stiffening of the polymer resulting from ionic crosslinks; this was clearly shown by LiTMPB complex at [O]/[Li]=10. At this concentration, the conductivity behavior obeyed the Arrhenius equation instead of the VTF equation; the same result was seen with ethylene oxide comb polymer/LiTMPB complex at 10:1

[O]/[Li].

Conductivity measurements at low frequency were used as an approximation for cell polarization. Although LiTMPB doped electrolytes had less cell polarization, the difference was not much and it may due to the molarity difference. At a given [O]/[Li]

199 ratio, the LiTMPB electrolytes have lower concentration of lithium salt, and this means reduced lithium ion numbers and can reduce the conductivity.

4.5. References

1. Y. P. Ma, M. Doyle, T. F. Fuller, M. M. Doeff, L. C. Dejonghe, J. Newman, J.

Electrochem. Soc., 1995, 142(6), 1859-1868.

2. O. Buriez, Y. B. Han, J. Hou, J. B. Kerr, J. Qiao, S. E. Sloop, M. M. Tian, S. G.

Wang, J. Power Sources, 2000, 89(2), 149-155.

3. M. B. Armand, J. M. Chabagno, J. M. Duclot, in Fast Ion Transport in Solids,

Edited by P. Vashista, J. N. Mundy and G. K. Shenoy, Elesevier, New York, 1979,

131.

4. J. R. MacCallum, C. A. Vincent, Polymer Electrolyte Reviews-1, Elsevier, 1987,

Chapter 1.

5. J. S. Foos, S. M. Eeker, J. Electrochem. Soc., 1987, 134, 1724.

6. G. Goulart, S. Sylla, J. Y. Sanchez, M. Armand, Proc. Second Int. Symposium on

Polymer Electrolytes, Italy, Sienna, 1989, p.43.

7. M. Alamgir, R. D. Moulton, K. M. Abraham, Electrochim. Acta., 1991, 36(5/6),

773-782.

8. M. Watanabe, K. Nagaoka, M. Kanba, I. Shinohara, Polymer. J., 1982, 14, 8773.

9. G. Cameron, M. Ingram, K. Sarmouk, Eur. Polym. J., 1990, 26, 1097.

10. M. S. Mendolia, G. C. Farrington, Electrochim. Acta., 1992, 37(9), 1695-1698.

11. S. Chintapalli, R. Frech, Electrochim. Acta., 1995, 40(13/14), 2093.

12. C. A. Furtado, G. G oulart Silva, M. A. Pimenta, J. C. Machado, Electrochim.

Acta., 1998, 43(10-11), 1477-1480.

200 13. P. C. Redfern, L. A. Curtiss, J. Power Sources, 2002, 110(2), 401-405.

14. A. G. Baboul, P. C. Redfern, A. Sutjianto, L. A. Curtiss, J. Am. Chem. Soc., 1999,

121(31), 7220-7227.

15. A. Sutjianto, L. A. Curtiss, J. Phys. Chem. A., 1998, 102(6), 968-974.

16. J. B. Kerr, S. E. Sloop, G. Liu, Y. B. Han, J. Hou, S. Wang, Journal of Power

Sources, 2002, 110, 389-400.

17. J. B. Rose, J. Chem. Soc., 1956, 542, 546.

18. T. Saegusa, Y. Hashimoto, S. Matsumoto, Macromolecules, 1971, 4, 1.

19. G. Pruckmayr, In Cyclic Monomers, K. C. Frisch, ed., Wiley-Interscience, New

York, 1972, p. 1.

20. P. Dreyfuss, M. P. Dreyfuss, Polym. J., 1976, 8, 81.

21. P. E. Black, D. J. Worsfold, Can. J. Chem., 1976, 54, 3225.

22. H. Desi, A. V. Cuncliffe, M. J. Stewart, Polymer, 1993, 34, 642.

23. J. V. Crivello, M. Fan, J. Polym. Sci., Part A, Polym. Chem., 1992, 30, 1.

24. M. Y. Qureshi, M. Ochel, Eur. Polym. J., 1996, 32(6), 691-693.

25. G. Odian, Principles of Polymerization, A John Wiley & Sons, Inc., p. 558.

26. S. Penczek, P. Kubisa, R. Szymanski, Makromol. Chem. Macromol. Symp., 1986,

3, 203.

27. P. Kubisa, S. Penczek, Prog. Polym. Sci., 1999, 24, 1409-1437.

28. P. Kubisa, M. Bednarek, T. Biedron, T. Biela, S. Penczek, Macromol. Symp.,

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645.

201 30. H. H. Freedman, R. A. Dubois, Tetrahedron Letters, 1975, 38, 3251-3254.

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202 Chapter 5. Future Work

The use of oxetane comb copolymers as matrices instead of ethylene oxide comb polymers may be a breakthrough since they have lower glass transition temperatures and less binding energy when complexed with lithium salts. Our experiments have found these polymer/lithium salts electrolytes have higher conductivities than their ethylene oxide counterparts. Since in current stage we can only incorporate 4 trimethylene oxide units as side chains, our first goal in next step is to study how to incorporate longer side chains of oligo(trimethylene oxide) on a polymer backbone to obtain better conductivity.

There may be two ways. The first method is based on cationic ring opening polymerization. We can copolymerize trimethylene oxide with 3,3- bis(bromomethyl)oxetane first, then hydrolyze bromo groups. Once this polymer is obtained, we can use this polymer alcohol as an initiator and grow oligo(trimethylene oxide) with controlled lengths from these hydroxyl groups via the activated monomer mechnism; the detailed procedure is shown in Scheme 5.1.

Another possible route is to choose other monomers which can be radically polymerized instead of tricky cationic ring-opening polymerization, which crosslinks easily. These monomers can be styrene, acrylate etc. Although they may not have flexible backbones like polyethers and can induce higher Tgs, Buriez’s group found that once the side chain lengths were long enough (DP=7), the effect of backbone structures on conductivity was minimized or even negligible [1]. In this case, there are a lot of monomers available and the synthesis procedures can be greatly simplified compared to the strict cationic polymerization conditions and easy crosslinks.

203

CH2Br CH2Br BF3:OEt2 O + O OCH2CH2CH2 OCH2CCH2 m n CH2Br CH2Cl2 CH2Br

CH2OH hydrolyze OCH2CH2CH2 OCH2CCH2 m n CH2OH

CH2OH BF3:OEt2 OCH 2CH2CH2 OCH2CCH2 +Ok m n CH2Cl2 CH2OH

CH2O(CH2CH2CH2O)kOH CH3OSO2OCH3 OCH2CH2CH2 OCH2CCH2 m n CH2O(CH2CH2CH2O)kOH

CH2O(CH2CH2CH2O)kOCH3

OCH2CH2CH2 OCH2CCH2 m n CH2O(CH2CH2CH2O)kOCH3 Scheme 5.1. A possible route to the synthesis of oligo(trimethylene oxide) side chains with controlled length

Once these polymers with longer oligo(trimethylene oxide) side chains can be

made and have demonstrated higher conductivity, we can try to make other

monomethylated polyether–ols with C/O ratios higher than 3. Tetrahydrofuran (THF)

may be a good choice. Poly(tetrahydrofuran) has an even lower glass transition

temperature than that of poly(trimethylene oxide) and this may help to increase

conductivity. Abraham’s group [2] found poly(THF)/LiClO4 electrolyte had a high

conductivity of 1.9*10-6 S/cm at room temperature and a lithium transport number of 0.6

which was much higher than that of its PEO counterpart (<0.2). Furthermore,

204 copolymerization of THF and trimethylene oxide can be easily done (Scheme 5.2) using the activated monomer meachanism. Its molecular weight can be easily controlled by adjusting the monomer/initiator ratio and the comb fragment can have very narrow polydispersity. This new polyether may be more interesting since it has irregular oxygen placement in the polyether with a C/O ratio of 3~4, and solvates lithium ion poorly. Its glass transition temperature may rise more slowly with lithium salt concentration than Tg of poly(trimethylene oxide).

O CH3OH/BF3:OEt2 mn+ O CH3(OCH2CH2CH2)m(OCH2CH2CH2CH2)nOH CH Cl , 0 oC 2 2 Scheme 5.2. Copolymerization of trimethylene oxide and tetrahydrofuran using the activated monomer mechanism

A drawback of the comb polymers for use as solvent-free polymer electrolyte complexes is their poor mechanical strength. At room temperature they are viscous liquids. Their mechanical properties can be improved in several ways. Cross-linking of these polymer electrolytes can yield clear, amorphous, elastomeric materials. The conductivities are not severely damaged by cross-linking which has been proved from

Allcock’s group [3-5].

Another way to make self-supporting membranes is synthesis of block copolymers. The use of block copolymers enables us to attach crystallizable sections at each end of the central block. The one shown in Scheme 5.3 are soluble in many solvents, crystallize well and have high melting points. It has been obtained with almost 100% reaction of the ends in Dr. Litt’s group. The hydroxyl, or the coresponding oxonium ion

205 terminated polymers, can then be reacted with polyethers to make block copolymers. The last step will be reaction of polyether-ol comb segments with the CH2Br group. Thus, a comb block copolymer with high mechanical properties can be made.

Another novel tri- or polyblock polymers with our new polyethers as central block and poly(oxazoline) as crystalline blocks can be made by cationic ring opening polymerization of oxazoline and covalent bond to polyether (Scheme 5.4).

Polymerization of oxazoline has been successfully prepared in Dr. Litt’s group and the polyoxazolines crystallize very well with melting points between 160 and 230℃. This is a living polymerization and the polymers have very narrow molecular weight distributions. Phase separation will produce domains with well defined boundaries.

R SOBr2 NaOH R(CHOH) R(CH2Br)3 O 2 3 CH2Br R=CH , C H 3 2 5 R (CF3SO2)O m + n O + p O O CH2Br

H(OCH 2CH2CH2)m (OCH2CCH2)n (OCH2CH2CH2CH2)p O 2 CH2Br

Scheme 5.3. Preparation of crystalline block from oxetane, bromomethyloxetane and THF

206 R N

- R CH3 NCH2CH2 N O CF3SO3 x O COR A t-butyl-OK CH3 NCH2CH2 O polyether OCH2CH2NCH3 polyether x x COR COR R R

N CF3SO3CH2CH2SO3CF3 O O R NCH2CH2NCH2CH2 N x O COR B t-butyl-OK B + polyether CH2CH2N polyether x O C y R R=pentyl, heptyl, isobutyl, substituted phenyl, etc.

Scheme 5.4. Preparation of triblock copolymer with crystallizable poly(oxazoline)

blocks

The next step after the synthesis of self-supporting membranes would be their characterization, as was done in Chapter 3 and 4. Finally, these polymer electrolytes will be incorporated into a true lithium battery and their performance will be tested.

207 References

1. O. Buriez, Y. B. Han, J. Hou, J. B. Kerr, J. Qiao, S. E. Sloop, M. M. Tian, S. G.

Wang, J. Power Sources, 2000, 89(2), 149-155.

2. M. Alamgir, R. D. Moulton, K. M. Abraham, Electrochim. Acta., 1991, 36(5/6),

773-782.

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