Title Page

Contents

Abstract 17

Chapter 1. Introduction 18

1.1. Background of energy storage system 18

1.1.1. Importance of energy storage system 18

1.1.2. History of lithium-ion battery system 19

1.2. Need for development of conversion cathode 24

1.2.1. Limit of Intercalation-based cathode 24

1.2.2. Characters of conversion-based cathode 26

1.2.3. Challenges of conversion-based cathode 29

1.3. The aim of this research 32

1.3.1. Strategy 1: Increase redox potential 32

1.3.2. Strategy 2: Enhance sluggish kinetics 36

1.3.3. Novel conversion cathode materials for Alkali-ion batteries 39

Chapter 2. Development of Novel Cathode with Large Lithium Storage Mechanism Based on Pyrophosphate‐Based Conversion Reaction for Lithium-ion Batteries 41

2.1. Introduction 41

2.2. Experimental 44

2.2.1. Materials preparation 44

2.2.2. Electrochemical characterization 44

2.2.3. Materials characterization 45

2.2.4. Computational details 45

2.3. Results and discussion 47

2.3.1. Theoretical average operation voltage of Cu₂P₂O₇ 47

2.3.2. Preparation of nano-C-CPO 49

2.3.3. Electrochemistry of nano-C-CPO 65

2.3.4. Conversion reaction mechanism of Cu₂P₂O₇ 75

2.4. Conclusion 82

Chapter 3. High-energy Conversion-based Cathode Activated by Amorpholization for Li-ion Batteries 83

3.1. Introduction 83

3.2. Experimental 86

3.2.1. Materials preparation 86

3.2.2. Electrochemical characterization 86

3.2.3. Materials characterization 87

3.2.4. Computational details 88

3.3. Results and discussion 89

3.3.1. Material investigating for A-CPO/C 89

3.3.2. Electrochemical properties of A-CPO/C in LIB system 100

3.3.3. Demonstration for conversion reaction of Cu(PO₃)₂ 115

3.4. Conclusion 124

Chapter 4. The conversion chemistry for high-energy cathodes of sodium/potassium-ion batteries 126

4.1. Introduction 126

4.2. Experimental 129

4.2.1. Materials preparation 129

4.2.2. Electrochemical characterization 129

4.2.3. Materials characterization 130

4.2.4. Computational details 130

4.3. Results and discussion 132

4.3.1. Conversion-based cathode material with high operation voltage via inductive effect 132

4.3.2. Morphology and crystal structure of N-CSO/C 139

4.3.3. Electrochemistry of CuSO₄ in Na/K cell 148

4.3.4. Conversion reaction mechanism of N-CSO/C in Na/K cell system 157

4.4. Conclusion 165

Chapter 5. Conclusion 166

References 168

논문요약 178

Chapter 1. Introduction 15

Table 1.1. Standard thermodynamic values, theoretical redox potential, and band-gap of various compounds. 35

Chapter 2. Development of Novel Cathode with Large Lithium Storage Mechanism Based on Pyrophosphate‐Based Conversion Reaction for Lithium-ion Batteries 15

Table 2.1. Formation energies of CuO, Li₂O, Cu₂P₂O₇, Li₄P₂O₇, Cu and Li obtained by first principles calculation 48

Table 2.2. Detailed structural information of nano-C-CPO obtained by Rietveld refinement 58

Table 2.3. Comparing electrochemical performances of cathode materials for LIBs 74

Chapter 3. High-energy Conversion-based Cathode Activated by Amorpholization for Li-ion Batteries 15

Table 3.1. ICP analyses on the atomic ratio of Cu, P and O of Cu(PO₃)₂. 96

Table 3.2. Formation energies of Cu(PO₃)₂, Li(PO₃), Cu and Li and predicted theoretical redox potentials of (a) Cu(PO₃)₂, (b) CuO using first principles calculation 101

Chapter 4. The conversion chemistry for high-energy cathodes of sodium/potassium-ion batteries 15

Table 4.1. Predicted theoretical redox potentials of CuSO₄ and CuO using first principles calculation in NIB system 136

Table 4.2. Predicted theoretical redox potentials of CuSO₄ and CuO using first principles calculation in KIB system 137

Table 4.3. Comparison of theoretical voltage of CuSO₄ corresponding to counter metal of Na, Mg and K 138

Chapter I. Introduction 9

Figure 1.1. Crystal structure of LiₓTiS₂ during Li⁺ intercalation. Reproduced with permission. 21

Figure 1.2. A schematic presentation of the discharging (a) and charging (b) processes in lithium-ion batteries. Reproduced with permission. 22

Figure 1.3. Crystal structure of olivine, layered and spinel-type. Reproduced with permission. 23

Figure 1.4. Theoretical gravimetric and volumetric capacities and theoretical potential of selected conversion cathode materials: (a, b) chalcogens and chalcogenides; (c, d) halogens and metal halides.... 25

Figure 1.5. Two types of conversion reactions using lithiation of S and FeF₂ as examples: (a) true conversion with the formation of two new phases; (b) chemical transformation with a single new phase... 28

Figure 1.6. Scheme of challenges of conversion-based cathode. Reproduced with permission. 31

Figure 1.7. Scheme of higher redox potential of Cu-P bond than that of Cu-O bond arising from inductive effect by sulfur with high electronegativity. Reproduced with permission. 34

Figure 1.8. Scheme of merits of carbon coating and nano-sizing. 38

Chapter 2. Development of Novel Cathode with Large Lithium Storage Mechanism Based on Pyrophosphate‐Based Conversion Reaction for Lithium-ion Batteries 9

Figure 2.1. Scheme of nano-sizing and carbon mixing of nano-C-CPO particles using high-energy ball milling 50

Figure 2.2. XRD patterns of nano-C-CPO composites which coated with different carbon content 51

Figure 2.3. Cycle-performances of nano-C-CPO composites which coated with different carbon content 52

Figure 2.4. Comparing XRD intensity between nano-C-CPO and pristine Cu₂P₂O₇ using XRD patterns which measured for 1 h on each sample and crystal structure of nano-C-CPO composite 54

Figure 2.5. Comparing XRD patterns of the as-prepared nano-C-CPO sample and the other nano-C-CPO sample which exposed in air for one week 55

Figure 2.6. Refined XRD pattern and crystallite size using Scherrer equation for selected h k l reflections of pristine Cu₂P₂O₇ (Rₚ＝3.11%, RI＝3.84%, RF＝4.03%, and χ²＝7.95%) XRD pattern[이미지참조] 56

Figure 2.7. Refined XRD pattern and crystallite size using Scherrer equation for selected h k l reflections of nano-C-CPO composite (Rₚ＝2.51%, RI＝4.45%, RF＝3.49%, and χ²＝2.80%) XRD pattern[이미지참조] 57

Figure 2.8. SEM image of (a) nano-C-CPO (b) pristine Cu₂P₂O₇ 60

Figure 2.9. Bright-field TEM image of nano-C-CPO composite 61

Figure 2.10. EDS elemental mapping (Cu: blue, P: purple, O: yellow, C: green) of fresh nano-C-CPO composite 62

Figure 2.11. Thermogravimetric (TGA) spectra profiles of pristine Cu₂P₂O₇ and nano-C-CPO composite 63

Figure 2.12. Electrochemical impedance spectra profiles of pristine Cu₂P₂O₇ and nano-C-CPO composite 64

Figure 2.13. (a) Power-capability of nano-C-CPO at various current rates. (b) Rate performance of nano-C-CPO 66

Figure 2.14. Power capability of pristine Cu₂P₂O₇ at various current density 67

Figure 2.15. CV curve for (a) nano-C-CPO, (b) pristine Cu₂P₂O₇ electrode measured at 5 μVs⁻¹ in the voltage range of 1.0-4.0 V 68

Figure 2.16. Cyclic performance and coulombic efficiency of nano-C-CPO over 400 cycles at 360 mA g⁻¹ after 1 cycle at 72 mA g⁻¹ 70

Figure 2.17. (a) Charge/discharge curves of nano-C-CPO at 1, 10, 50, 100, 200, 300 and 400th over 400 cycles at 360 mA g⁻¹ after 1 cycle at 120 mA g⁻¹. (b) Cyclic performance and coulombic efficiency of...[이미지참조] 71

Figure 2.18. Comparing (a) XRD patterns, (b) SEM images, (c) SAED patterns of nano-C-CPO before cycle and after 400 cycles 72

Figure 2.19. Cyclic performances of nano-C-CPO and pristine Cu₂P₂O₇ over 400 cycles at 360 mA g⁻¹ after 1cycle at 120 mA g⁻¹ 73

Figure 2.20. Operando XRD patterns of nano-C-CPO electrode during the initial cycle 76

Figure 2.21. Ex-situ XRD patterns of nano-C-CPO electrode during the initial cycle 77

Figure 2.22. Cu K-edge (a) XANES and (b) EXAFS spectra of nano-C-CPO 79

Figure 2.23. HRTEM image and SAED pattern of (a) fresh, (b) discharged, and (c) charged nano-C-CPO composite 80

Figure 2.24. HRTEM image and SAED pattern of (a) fresh, (b) discharged, and (c) charged nano-C-CPO composite 81

Chapter 3. High-energy Conversion-based Cathode Activated by Amorpholization for Li-ion Batteries 11

Figure 3.1. Scheme of fabrication process from bare Cu(PO₃)₂ to LC-CPO/C and A-CPO/C with approximate material structure images 90

Figure 3.2. Comparing XRD patterns and maximum intensities of bare Cu(PO₃)₂, LC-CPO/C and A-CPO/C 92

Figure 3.3. TEM SAED patterns of (a) A-CPO/C, (b) LC-CPO/C 93

Figure 3.4. EDS elemental mapping (Cu: yellow, P: green, O: blue) of A-CPO/C 94

Figure 3.5. Thermogravimetric (TGA) spectra profiles of bare Cu(PO₃)₂ and A-CPO/C 95

Figure 3.6. Comparing SEM images and average particle sizes of (a) A-CPO/C, (b) LC-CPO/C, (c) bare Cu(PO₃)₂. 98

Figure 3.7. XPS spectra of A-CPO/C: (a) XPS full spectrum survey, (b) Cu 2p, (c) O 1s, (d) P 2p 99

Figure 3.8. Power capability of (a) A-CPO/C and (c) LC-CPO/C at various current densities in the voltage range of 2.0-4.3 V. Rate capability of (b) A-CPO/C and (d) LC-CPO/C. 104

Figure 3.9. Comparing charge/discharge curves for pre-cycle at 12 mA g⁻¹ of A-CPO/C and LC-CPO/C. 105

Figure 3.10. Comparison charge/discharge hysteresis of A-CPO/C and LC-CPO/C electrodes measured at 12 mA g⁻¹. 106

Figure 3.11. (a) XRD pattern, (b) SEM image and average particle size of A-CPO/C_1. (c) Comparing power capability performances of samples for (c) A-CPO/C, (d) A-CPO/C_1.... 107

Figure 3.12. Comparing CV curves of (a) A-CPO/C, (b) LC-CPO/C electrode measured at 5 mV s⁻¹ in the voltage range of 2.0-4.3 V (vs. Li⁺/Li) 108

Figure 3.13. EIS measurements and values of charge transfer resistance of bare Cu(PO₃)₂, LC-CPO/C and A-CPO/C 109

Figure 3.14. (a) Power capability at various current density, (b) cycle performance for 100 cycles at 480 mA g⁻¹ of bare Cu(PO₃)₂. 110

Figure 3.15. Cycle performances of A-CPO/C and LC-CPO/C for 300 cycles at current density of 480 mA g⁻¹ after initial cycle at 30 mA g⁻¹. 112

Figure 3.16. Comparing (a) XRD patterns, (b) SEM image, (c) TEM image and SAED pattern of both of as-prepared/after 300cycles for A-CPO/C electrode 113

Figure 3.17. (a) Charge/discharge curves of A-CPO/C half-cell (vs. Li metal), full-cell (vs. graphite) and graphite half-cell (vs. Li metal) at 12 mA g⁻¹, (b) Cycle performance of A-CPO/C... 114

Figure 3.18. (a) Cut-off voltage points of ex-situ samples on charge/discharge curve of A-CPO/C. (b) Ex-situ XRD patterns of A-CPO/C. 116

Figure 3.19. Ex-situ XRD patterns of discharged A-CPO/C electrodes at 2.4, 2.6 and 2.8 V 117

Figure 3.20. HRTEM images with SAED patterns at (a) as-prepared, (b) discharged, (c) charged A-CPO/C 118

Figure 3.21. Ex-situ measurements of A-CPO/C electrodes for XANES spectra 120

Figure 3.22. Ex-situ measurements of A-CPO/C electrodes for EXAFS spectra 121

Figure 3.23. Ex-situ measurements of A-CPO/C electrodes for (a) XPS Cu 2p spectra, (b) XPS O 1s spectra 122

Figure 3.24. Ex-situ measurements of as-prepared, discharged and charged A-CPO/C electrodes for XPS P 2p spectra. 123

Figure 3.25. Comparison of energy densities (based on weights of active cathode materials) A-CPO/C and the other conventional cathodes in full-cell system 125

Chapter 4. The conversion chemistry for high-energy cathodes of sodium/potassium-ion batteries 13

Figure 4.1. Scheme of overall conversion reaction mechanism of CuSO₄ under the NIB and KIB system 134

Figure 4.2. Scheme of higher redox potential of CuSO₄ than that of CuO arising from inductive effect by sulfur with high electronegativity 135

Figure 4.3. Scheme of preparation of N-CSO/C using high-energy ball milling 141

Figure 4.4. SEM image of (a) pristine CuSO₄ and (b) N-CSO/C 142

Figure 4.5. Intensities and FWHM of XRD peaks of N-CSO/C composite and pristine CuSO₄ 143

Figure 4.6. Crystallite size of (a) pristine CuSO₄ and (b) N-CSO/C composite determined using Scherrer equation for selected h k l reflections of the XRD patterns 144

Figure 4.7. (a) Rietveld refinement of XRD pattern of N-CSO/C (Rₚ＝3.64%, RI＝1.98%, and RF＝2.12%) and detailed structural information of N-CSO/C composite[이미지참조] 145

Figure 4.8. EDS elemental mapping of N-CSO/C composite (Cu: red, S: green, O: blue, and C: yellow) 146

Figure 4.9. Thermogravimetric spectra profiles of pristine CuSO₄ and N-CSO/C composite 147

Figure 4.10. Power capability of N-CSO/C at various current density in Na cell 149

Figure 4.11. (a) Power capability (b) rate performance of N-CSO/C at various current density in K cell 150

Figure 4.12. Cyclic performance and Coulombic efficiency of N-CSO/C over 300 cycles at 720 mA g⁻¹ after 1cycle at 120 mA g⁻¹ in Na cell 151

Figure 4.13. Cyclic performance and coulombic efficiency of N-CSO/C over 200 cycles at 360 mA g⁻¹ after 1 cycle at 120 mA g⁻¹. 152

Figure 4.14. The comparison of SEM images of N-CSO/C electrodes measured (a) before cycle and (b) after 300 cycles. 153

Figure 4.15. Comparing (a) SEM images, (b) SAED patterns of N-CSO/C before cycle and after 200 cycles. 154

Figure 4.16. (a) Charge/discharge profiles at 1, 5, 10, 30 and 50th, (b) cyclic performance and Coulombic efficiency over 50 cycles at 360 mA g⁻¹ of N-CSO/C full cell in Na cell[이미지참조] 155

Figure 4.17. (a) Charge/discharge profiles of N-CSO/C|Hard-carbon full-cell at 1st, 5th, 10th, 50th, 100th, 150th, 200th, 250th and 300th cycles in K cell. (b) Cyclic performance and coulombic...[이미지참조] 156

Figure 4.18. (a) Operando XRD patterns of N-CSO/C electrode during the first cycle in Na cell (b) Ex-situ XRD patterns of N-CSO/C electrode during initial and second cycles at various... 158

Figure 4.19. (a) Operando XRD patterns of N-CSO/C electrode during the first cycle in K cell. (b) Ex-situ XRD patterns of N-CSO/C electrodes at various voltages in K cell. 159

Figure 4.20. Cu K-edge of (a) XANES (b) EXAFS spectra of N-CSO/C samples in Na cell 160

Figure 4.21. Cu K-edge of (a) XANES (b) EXAFS spectra of N-CSO/C samples in K cell 161

Figure 4.22. ToF-SIMS graphs of nano-CuSO₄/C samples during charge and discharge in Na cell 163

Figure 4.23. ToF-SIMS graphs of (a) CuSO₃⁺ (b) KSO₃⁻ (c) Cu⁺ during charge and discharge in K cell 164