Title Page

Contents

Abstract 20

Chapter 1. Introduction 22

1.1. Background of energy storage system 22

1.1.1. Importance of energy storage system 22

1.1.2. History of Li-ion batteries 23

1.2. Next-generation energy storage systems 27

1.2.1. Reasons for developing alternative energy storage systems of Li-ion battery 27

1.2.2. Na/K-ion batteries 30

1.3. Research strategies 34

1.3.1. Layered structure-based metal oxide 34

1.3.2. Redox-inactive metal substitution 35

1.4. The aim of this research 36

Chapter 2. O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂: suppressing irreversible structural changes 41

2.1. Introduction 41

2.2. Experimental 43

2.2.1. Synthesis process of O3-Na₁₋₂ₓ Cr₁-ₓSbₓO₂ 43

2.2.2. Materials characterization 44

2.2.3. Electrochemical characterization 45

2.2.4. Computational details 46

2.3. Results and discussion 47

2.3.1 Crystal structure and morphology of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ 47

2.3.2. Change of electrochemical behavior of O3-NayCr₁-ₓSbₓO₂ as a function of Sb content[이미지참조] 54

2.3.3. Reversible structural change of NaₓCr₀.₈₆Sb₀.₁₄O₂ during charge/discharge 64

2.3.4. Outstanding electrochemical performance and reaction mechanism of Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ 74

2.4. Conclusion 85

Chapter 3. P3-K₀.₇₀Cr₀.₈₆Sb₀.₁₄O₂: suppressing phase transition 86

3.1. Introduction 86

3.2. Experimental 89

3.2.1. Preparation of NaCrO₂ and Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ 89

3.2.2. Na⁺/K⁺ ion-exchange process 90

3.2.3. Materials characterization 91

3.2.4. Electrochemical characterization 92

3.2.5. Computational details 93

3.3. Results and discussion 94

3.4. Conclusion 123

Chapter 4. P3-K₀.₇₁Cr₀.₇₅Ti₀.₂₅O₂: increase in discharge capacity through molar mass reduction 124

4.1. Introduction 124

4.2. Experimental 126

4.2.1. Preparation of NaCrO₂ and Na₀.₇₅[Cr₀.₇₅Ti₀.₂₅]O₂ 126

4.2.2. Na⁺/K⁺ ion‐exchange process for Kₓ[Cr₀.₇₅₊y Ti₀.₂₅₋y]O₂[이미지참조] 127

4.2.3. Materials characterization 128

4.2.4. Electrochemical characterization 129

4.2.5. Computational details 130

4.3. Results and discussion 131

4.4. Conclusion 168

Chapter 5. Summary 169

References 170

논문요약 178

Table 1-1. 6-coordination ionic radius of various ions 39

Table 1-2. The list of ions initially selected and their respective electronegativities. 40

Table 2-1. Structural information for O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ determined from Rietveld refinement. 50

Table 2-2. ICP analyses of atomic ratio of Na, Cr and Sb O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ with standard deviation. 51

Table 2-3. Structural information for O3-NaCrO₂ determined from Rietveld refinement. 59

Table 2-4. Structural information for O3-Na₀.₈₂Cr₀.₉₁Sb₀.₀₉O₂ determined from Rietveld refinement. 60

Table 2-5. ICP analyses of atomic ratio of Na, Cr and Sb in the O3-NaCrO₂ and O3-Na₀.₈₂Cr₀.₉₁Sb₀.₀₉O₂ with standard deviation. 61

Table 2-6. Comparing the available capacities among O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ and other Cr-based layered oxide cathode materials for Na-ion batteries. 82

Table 3-1. The structural information of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂. 98

Table 3-2. The structural information of P3-K₀.₇₀Cr₀.₈₆Sb₀.₁₄O₂. 99

Table 3-3. ICP results of P3-K₀.₇₀Cr₀.₈₆Sb₀.₁₄O₂. 100

Table 3-4. ICP results of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ and after ion-exchange 50 cycles sample. 102

Table 3-5. Comparison of the electrochemical properties between P3-K₀.₇₀[Cr₀.₈₆Sb₀.₁₄]O₂ and other P3-type cathode materials for KIBs. 121

Table 4-1. The structural information of P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂ using Rietveld refinement calculation. 138

Table 4-2. The structural information of O3-Na₀.₇₅[Cr₀.₇₅Ti₀.₂₅]O₂ using Rietveld refinement calculation. 139

Table 4-3. ICP analyses on the atomic ratio of K, Cr, and Ti in the ion-exchanged sample. 142

Table 4-4. Relative formation energies of various P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ configurations. 153

Table 4-5. Relative formation energies of various O3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ configurations. 154

Table 4-6. Comparing the electrochemical properties among P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂ and other cathode materials for KIBs. 160

Figure 1-1. (a) The structure of TiS2 before and after lithium intercalation. (b) Cycling profiles of TiS2 in the voltage range of 1.4 - 3.0 V 25

Figure 1-2. Schematic illustration of the first Li-ion battery (LiCoO₂/Li⁺ electrolyte/graphite). 26

Figure 1-3. Lithium-ion battery demand forecast. 28

Figure 1-4. Virgin raw materials supply, (Ref. United States Geological Survey, Roland Berger) 29

Figure 1-5. A number of publications, related to the sodium for energy storage devices, published in the past three decades. Data was derived from Web of science. The number in 2014 is limited... 32

Figure 1-6. Schematic illustration of Na-ion batteries. 33

Figure 2-1. (a) Rietveld refinement of XRD pattern of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂. (b) TEM-EDS mapping of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂. 49

Figure 2-2. Cr K-edge XANES spectra of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ and Cr₂O₃. 52

Figure 2-3. (a) Crystal structure of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ and (b) 3D BVEL map with all possible positions and diffusion paths of Na⁺ ions. 53

Figure 2-4. Initial charge and discharge profile and coulombic efficiency of (a) O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂, (b) O3-Na₀.₈₂Cr₀.₉₁Sb₀.₀₉O₂, and (c) O3-NaCrO₂ in the voltage range of 1.5-... 56

Figure 2-5. SEM images of (a) O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂, (b) O3-Na₀.₈₂Cr₀.₉₁Sb₀.₀₉O₂ and (c) O3-NaCrO₂. 57

Figure 2-6. (a) Rietveld refinement of XRD pattern of O3-NaCrO₂ (Rₚ: 3.9 %, RI: 8.5 %, χ²: 2.18 %, RF: 3.92 %). (b) Rietveld refinement of XRD pattern of O3-Na₀.₈₂Cr₀.₉₁Sb₀.₀₉O₂ (Rₚ:...[이미지참조] 58

Figure 2-7. XRD patterns of Na₀.₆₄Cr₀.₈₂Sb₀.₁₈O₂. 62

Figure 2-8. (a) Initial charge and discharge profile and coulombic efficiency of Na₀.₆₄Cr₀.₈₂Sb₀.₁₈O₂ (b). Power-capability of Na₀.₆₄Cr₀.₈₂Sb₀.₁₈O₂. (c) Cycle-performance of... 63

Figure 2-9. (a) Formation energy of O3-type and P3-type NaₓCr₀.₈₆Sb₀.₁₄O₂ (0≤x≤1). (b) Cycle profile and predicted theoretical voltage of NaₓCr₀.₈₆Sb₀.₁₄O₂ (0≤x≤0.72) in the voltage range... 66

Figure 2-10. HAADF-STEM analyses of (a) pristine Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ and (b) 1-cycled Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ prepared through charge/discharge in the voltage range of 1.5-4.1 V. 69

Figure 2-11. HAADF-STEM images of (a) pristine O3-NaCrO₂ electrode and (b) 1-cycled NaCrO₂. (c) Ex-situ XRD patterns of pristine NaCrO₂ and 1-cycled NaCrO₂ electrodes. 70

Figure 2-12. HAADF-STEM, BF-STEM and BF-Z contrast of (a) pristine Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂, after cycling at cut-off voltage of (b) 3.6 and (c) 4.1 V Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ electrodes. (d) XRD... 71

Figure 2-13. HAADF-STEM, BF-STEM and BF-Z contrast of (a) pristine NaCrO₂, (b) after cycling at cut-off voltage of (b) 3.6 and (c) 4.1 V NaCrO₂. (d) XRD pattens of these 3 electrodes. 72

Figure 2-14. (a) Crystal structure of tetragonal and octahedral sites where Cr ions can migrate in O3-Na₀CrO₂ and O3-Na₀Cr₀.₈₆Sb₀.₁₄O₂. (b) Comparison of formation energy... 73

Figure 2-15. (a) Charge/discharge profiles of O3-NaₓCr₀.₈₆Sb₀.₁₄O₂ at various current densities. (b) Power capability of O3-NaₓCr₀.₈₆Sb₀.₁₄O₂ at various current densities. 76

Figure 2-16. Cyclic voltammetry profiles of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ repeated 10 times at a scan rate of 0.1 mV s⁻¹ in the voltage range 1.5-4.1 V. 77

Figure 2-17. Charge/discharge profiles of (a) O3-NaₓCrO₂ and (b) O3-NaₓCr₀.₉₁Sb₀.₀₉O₂ at various current density in the voltage range 1.5-4.1 V. 78

Figure 2-18. (a) Predicted Na⁺-ion diffusion motion in O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ and (b) predicted activation barrier energy for Na⁺-ion diffusion in O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂. 79

Figure 2-19. Charge/discharge profiles of O3-NaxCr₀.₈₆Sb₀.₁₄O₂ at various high current densities. 80

Figure 2-20. Charge/discharge capacity and coulombic efficiency of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ over 200 cycles at 2C. 81

Figure 2-21. (a) Integrated spin moments of O3-Na₁Cr₀.₈₆Sb₀.₁₄O₂ (Cr³⁺) and P3-Na₀Cr₀.₈₆Sb₀.₁₄O₂ (Cr⁴⁺). Ex-situ Cr K-edge (b) XANES spectra and (c) EXAFS spectra of NaₓCr₀.₈₆Sb₀.₁₄O₂ (0≤x... 84

Figure 3-1. Rietveld refinement of the XRD pattern of P3-K₀.₇₀Cr₀.₈₆Sb₀.₁₄O₂. 96

Figure 3-2. Rietveld refinement of XRD pattern of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂. 97

Figure 3-3. XRD patterns of samples ion-exchanged during 20, 50, 80, and 100 cycles. 101

Figure 3-4. TEM image and TEM-EDS mapping of the P3-K₀.₇₀Cr₀.₈₆Sb₀.₁₄O₂. 103

Figure 3-5. Cr K-edge of O3-Na₀.₇₂Cr₀.₈₆Sb₀.₁₄O₂ and P3-K₀.₇₀Cr₀.₈₆Sb₀.₁₄O₂ electrodes. 104

Figure 3-6. Convex-hull plot for the formation energy of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ configurations (0≤x≤1) with theoretical voltage. 106

Figure 3-7. Comparison of the calculated redox potential of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ and its experimentally measured cycle curves at 15 mA g⁻¹ during initial charge/discharge process. 107

Figure 3-8. Predicted structural changes of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ as a function of the K content (0.25≤x≤0.75) using first-principles calculations. 108

Figure 3-9. Operando XRD patterns and magnified views of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ (voltage range:1.5 - 4.1 V). 110

Figure 3-10. Change in the c and a lattice parameter as a function of K content in P3-KₓCr₀.₈₆Sb₀.₁₄O₂. 111

Figure 3-11. Change in the volume as a function of K content in P3-KₓCr₀.₈₆Sb₀.₁₄O₂. 112

Figure 3-12. (a) Charge/discharge profiles of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ at various current densities in the voltage range of 1.5 - 4.1 V. (b) Power capability of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ at various current densities. 115

Figure 3-13. Charge/discharge profiles of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ at various current densities in the voltage range of 1.5 - 4.1 V under the KIB system with 1 M of KPF₆ in 1:1 v/v% of EC:DEC. 116

Figure 3-14. Charge/discharge profiles of P3-KₓCrO₂ at various current densities in the voltagerange of 1.5 - 4.1 V. 117

Figure 3-15. dQ/dV profiles of P3-KₓCr₀.₈₆Sb₀.₁₄O₂ at 15 mA g⁻¹. 118

Figure 3-16. dQ/dV profiles of P3-KₓCrO₂ at 15 mA g⁻¹. 119

Figure 3-17. Charge/discharge capacity and coulombic efficiency of (a) P3-KₓCr₀.₈₆Sb₀.₁₄O₂ and (B) P3-KₓCrO₂ over 200 cycles at 150 mA g⁻¹. 120

Figure 3-18. (a) Ex situ Cr K-edge (a) XANES spectra and (b) EXAFS spectra of P3-KₓCr₀.₈₆Sb₀.₁₄O₂. 122

Figure 4-1. Analysis of XRD data of directly synthesized powder for P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂. 133

Figure 4-2. TEM images of the particles on (a) the O3-Na₀.₇₅[Cr₀.₇₅Ti₀.₂₅]O₂ powder, (b) the O3-Na₀.₇₅[Cr₀.₇₅Ti₀.₂₅]O₂ electrode, and (c) the ion-exchanged P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂ electrode. 134

Figure 4-3. TEM-EDS mapping of O3-Na₀.₇₅Cr₀.₇₅Ti₀.₂₅O₂ electrode. 135

Figure 4-4. (a) Rietveld refinement of XRD pattern and (b) corresponding crystal structure of P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂. (c) TEM-EDS mapping of P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂ electrode. 136

Figure 4-5. Rietveld refinement of XRD pattern of O3-Na₀.₇₅[Cr₀.₇₅Ti₀.₂₅]O₂. 137

Figure 4-6. The crystal structure of O3-Na₀.₇₅[Cr₀.₇₅Ti₀.₂₅]O₂ corresponding to the Rietveld refinement results. 140

Figure 4-7. SEM-EDS mapping of P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂ electrode. 141

Figure 4-8. Ex situ analyses of P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂ on (d) Cr K-edge XANES spectra and (e) Ti K-edge XANES spectra. 143

Figure 4-9. Initial charge and discharge profile and Coulombic efficiency of (a) P3-KₓCrO₂ and (b) P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂. 146

Figure 4-10. dQ/dV profiles of P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂ at 16 mA g⁻¹. 147

Figure 4-11. Charge/discharge profiles (a) and power capability (b) of P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ at various current densities. 148

Figure 4-12. Charge/discharge profiles of high-loaded P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ at various current densities. 149

Figure 4-13. Charge/discharge profiles of P3-KₓCrO₂ at various current densities. 150

Figure 4-14. Capacities of charge/discharge and coulombic efficiency of P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ over 300 cycles at 158 mA g⁻¹. 151

Figure 4-15. (a) Convex-hull plot of O3- and P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ configurations (0≤x≤1). (b) Comparison of calculated redox potential of P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ and its experimentally... 152

Figure 4-16. The difference in the c lattice ((a) P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂, (b) P3-KₓCrO₂) and bondinglength of CrO₆ octahedral between x＝0.17 and x＝0.65 ((c) P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂, (d) P3-KₓCrO₂). 157

Figure 4-17. (a) Predicted K⁺ diffusion motion in P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ and (b) predicted activation barrier energy for K⁺ diffusion in the structure of P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂. 158

Figure 4-18. (a) Comparing the electrochemical properties among P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ and other cathode materials for K-ion batteries. (b) Ragone plot of P3-Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ and other cathode... 159

Figure 4-19. (a) Operando XRD pattern of Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂ and (b) expansion of mainpeaks. (c) Changes in c lattice according to the amount to charge and discharge in the... 163

Figure 4-20. Ex situ (a) Cr K-edge and (b) Ti K-edge EXAFS spectra of Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂. Ex-situ (c) Cr K-edge and (d) Ti K-edge XANES spectra of Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂. 164

Figure 4-21. TEM image of (A) the before and (B) after cycled P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂. 165

Figure 4-22. SEM image of (A) the before, after (B) one cycled, and (C) 300 cycled P3-K₀.₇₁[Cr₀.₇₅Ti₀.₂₅]O₂. 166

Figure 4-23. The integrated spin moments of Kₓ[Cr₀.₇₅Ti₀.₂₅]O₂: (E) Cr and (F) Ti. 167