Title Page
ABSTRACT
국문 초록
Contents
CHAPTER 1. Introduction 18
CHAPTER 2. Backgrounds 21
2.1. Theoretical background 21
2.1.1. Basic principles of lithium-sulfur battery (LSB) 21
2.1.2. The fundamental challenges of LSBs 25
2.2. The research objective 27
CHAPTER 3. Experimental method 28
3.1. Catalysts synthesis via hydrothermal synthesis method 28
3.1.1. Synthesis of CeFc₃ nanoparticle (CF) 28
3.1.2. Synthesis of hexagoanl CeO₂ nanoplate (HC) and CeO₂ nanorod (CR) 28
3.1.3. Synthesis of nano-RuO₂-decorated HC (NRHC) 28
3.1.4. Preparation of catalyst-modified separators via vacuum filtration 29
3.2. Characterization of LSBs with catalyst-modified separator 30
3.2.1. Material characterization 30
3.2.2. Preparation of the sulfur (S) cathode 30
3.2.3. Electrochemical measurement 31
CHAPTER 4. Results and discussion 32
4.1. Cerium-based polar compounds as efficient polysulfide trapper in lithiumsulfur battery 32
4.1.1. Synthesis of CeF₃ nanoparticle (CF) and hexagonal CeO₂ nanoplate (HC) 32
4.1.2. Efficient chemisorption catalysts for mitigating the LiPSs shuttle effect 43
4.1.3. Optimizing chemisorption catalyst with morphological considerations 51
4.2. An oxygen vacancy-boosted heterostructured RuO₂@CeO₂ as superior catalyst for enhanced LiPS redox kinetics 60
4.2.1. Synthesis of nano-RuO₂-decorated hexagonal CeO₂ (NRHC) 60
4.2.2. Versatile NRHC electrocatalyst for suppressed LiPSs shuttling and enhanced catalytic conversion 78
4.2.3. Electrochemical characterization 94
4.2.4. Applicability of NRHC in LSBs 107
CHAPTER 5. Conclusion 134
References 135
Table 4-1. Comparison of specific capacities of the different full-cells at various scan rates (0.2 C-5 C). 50
Table 4-2. Crystallite sizes of predominant crystal planes of CeO₂ in HC and NRHC calculated from the XRD data. 76
Table 4-3. Contents of SOVs and IOVs in RuO₂, HC, and NRHC, as calculated from the ICP and EPR data. 77
Table 4-4. Electrical resistance of the separators measured using the four-point probe method. 93
Table 4-5. Comparison of specific capacities of the different full-cells at various scan rates (0.2 C-5 C). 130
Table 4-6. EIS fitting results of the bGF and NRHC full-cells before and after 50 cycles at 0.2 C. 131
Table 4-7. Comparison of electrochemical properties of the LSBs with various heterostructured catalysts for modified separators. 132
Figure 2-1. Schematic of the electrochemistry for LSBs. 23
Figure 2-2. A typical charge-discharge voltage profile of LSBs. 24
Figure 2-3. Schematic depiction of the main drawbacks of LSBs. 26
Figure 4-1. Schematic illustration of (a) CF and (b) HC synthesis. 35
Figure 4-2. XRD patterns of (a) CF and (b) HC. 36
Figure 4-3. XPS survey scan of CF and HC. 37
Figure 4-4. TEM image and the corresponding EDX elemental mapping of the (a) CF and (b) HC. 38
Figure 4-5. N₂ adsorption-desorption isotherms of the (a) CF and (b) HC. 39
Figure 4-6. High-resolution (a) Ce 3d and (b) F 1s XPS profiles of the CF. 40
Figure 4-7. High-resolution (a) Ce 3d and (b) O 1s XPS profiles of the HC. 41
Figure 4-8. EPR spectra of the CF and HC. 42
Figure 4-9. (a) Digital photographs of blank Li₂S₆ solution and Li₂S₆ solutions containing CF and HC acquired over 6 h. 45
Figure 4-10. UV-vis spectra of blank Li₂S₆ solution and supernatants of the Li₂S₆ solution with CF and HC acquired after 6 h. 46
Figure 4-11. High-resolution Ce 3d XPS profiles of the HC before and after soaking in a Li₂S₆ solution. 47
Figure 4-12. High-resolution O 1s XPS profiles of the HC before and after soaking in a Li₂S₆ solution. 48
Figure 4-13. Rate performance of the full-cells at different current densities (0.2 C, 0.5 C, 1 C, 2 C, 3 C, and 5 C). 49
Figure 4-14. XRD patterns of CR and HC. 53
Figure 4-15. TEM image and the corresponding EDX elemental mapping of the CF. 54
Figure 4-16. N₂ adsorption-desorption isotherms of the CR. 55
Figure 4-17. EPR spectra of the CR and HC. 56
Figure 4-18. Digital photographs of blank Li₂S₆ solution and Li₂S₆ solutions containing CR and HC acquired over 6 h. 57
Figure 4-19. UV-vis spectra of blank Li₂S₆ solution and supernatants of the Li₂S₆ solution with CR and HC acquired after 6 h. 58
Figure 4-20. Rate performance of the full-cells at different current densities (0.2 C, 0.5 C, 1 C, 2 C, 3 C, and 5 C). 59
Figure 4-21. Schematic illustration of NRHC synthesis. 63
Figure 4-22. XRD patterns of the HC and NRHC. 64
Figure 4-23. DSC/TGA curves of NRHC from room temperature to 1000 °C at N₂ atmosphere. 65
Figure 4-24. XPS survey scans of the HC and NRHC. 66
Figure 4-25. Raman spectra of the HC and NRHC. 67
Figure 4-26. TEM image of the NRHC. 68
Figure 4-27. EDX elemental mapping of the NRHC corresponding to the Figure 4-26. 69
Figure 4-28. ICP-AES measurements of the contents of Ru and Ce in the NRHC. 70
Figure 4-29. N₂ adsorption-desorption isotherms of the NRHC. 71
Figure 4-30. HRTEM images of the NRHC (scale bar: 2nm). 72
Figure 4-31. High-resolution Ce 3d XPS profiles of the HC and NRHC. 73
Figure 4-32. High-resolution O 1s XPS profiles of the HC and NRHC. 74
Figure 4-33. EPR spectra of the NRHC and mixture of HC and RuO₂. 75
Figure 4-34. Digital photographs of blank Li₂S₆ solution and Li₂S₆ solutions containing RuO₂, HC, and NRHC acquired over 6 h. 82
Figure 4-35. UV-vis spectra of blank Li₂S₆ solution and supernatants of the Li₂S₆ solution with RuO₂, HC, and NRHC acquired after 6 h. 83
Figure 4-36. High-resolution Ru 3d XPS profiles of the (a) HC and (b) NRHC before and after soaking in a Li₂S₆ solution. 84
Figure 4-37. High-resolution Ce 3d XPS profiles of the (a) HC and (b) NRHC before and after soaking in a Li₂S₆ solution. 85
Figure 4-38. High-resolution O 1s XPS profiles of the (a) HC and (b) NRHC before and after soaking in a Li₂S₆ solution. 86
Figure 4-39. High-resolution S 2p XPS profiles of the NRHC and HC after soaking in the Li₂S₆ solution. 87
Figure 4-40. Cross-sectional (left) and top-view (middle) SEM images and the corresponding EDX mapping (right) of the NRHC-modified separator. 88
Figure 4-41. Digital images of NRHC-modified separator with (a) pristine CNTs and (b) aCNTs during dynamic mechanical deformation. 89
Figure 4-42. Digital photographs of LiPSs permeation experiments with (a) bGF-, (b) aCNT-, (c) HC-, and (d) NRHC-modified separators. 90
Figure 4-43. (a) Chronoamperometry profiles of the bGF-, aCNT-, HC-, and NRHC- modified separators at a polarization voltage of 10 mV. (b) Nyquist plots before and after... 91
Figure 4-44. (a) Nyquist plots of cells (stainless-steel/separator/stainless-steel) with bGF-, aCNT-, HC-, and NRHC-modified separators and (b) the corresponding calculated ionic conductivities. 92
Figure 4-45. (a) CV curves and (b) Tafel plots of symmetric cells with CCAl, HC, and NRHC electrodes at 1 mV s⁻¹ in an electrolyte with 0.5 M Li₂S₆.[이미지참조] 97
Figure 4-46. Chronoamperometry curves of symmetric cells with the HC and NRHC electrodes. 98
Figure 4-47. Potentiostatic discharge profiles of the (d) HC and (e) NRHC electrodes at a constant voltage of 2.05 V. 99
Figure 4-48. CV curves of LSBs with the bGF-, HC-, and NRHC-modified separator at a scan rate of 0.2 mV s⁻¹. 100
Figure 4-49. CV curves of (a) bGF, (b) HC, and (c) NRHC full-cells at scanning rates from 0.05 to 0.25 mV s⁻¹. 101
Figure 4-50. Fitting line of Ipeak to ν⁰.⁵ for oxidation and reduction peaks measured by CV curves (Figure 4-49) of (a) bGF, (b) HC and (c) NRHC full-cells.[이미지참조] 102
Figure 4-51. Calculated Li⁺ diffusion coefficients of the bGF, HC, and NRHC full-cells. 103
Figure 4-52. (a-c) Tafel plots corresponding to the anodic and cathodic peaks in the CV profiles shown in Figure 4-49, marked as A1, C1, and C2, respectively. 104
Figure 4-53. CV curves of the (a) HC and (b) NRHC full-cell at various temperatures. 105
Figure 4-54. (a) Relation of Li₂S₄ conversion rates with respect to temperatures in the HC and NRHC full-cells. (b) Activation energies (Ea) for Li₂Sn reduction.[이미지참조] 106
Figure 4-55. Rate performance of the full-cells at different current densities (0.2 C, 0.5 C, 1 C, 2 C, 3 C, and 5 C). 113
Figure 4-56. Galvanostatic charge-discharge profiles of the LSBs with different separators at various current densities: (a) 0.2 C, (b) 0.5 C, (c) 1 C, (d) 2 C, (e) 3 C, and (f) 5 C. 114
Figure 4-57. The fifth-cycle galvanostatic charge-discharge profiles of LSBs with different separators at 1 C (inset: magnified nucleation overpotentials). 115
Figure 4-58. Galvanostatic charge-discharge profiles of the LSBs with different separators for the 3rd cycle at current densities of (a) 0.2 C, (b) 0.5 C, (c) 1 C, (d) 2 C, (e) 3 C, and (f) 5 C. 116
Figure 4-59. Capacity contributions of LSBs with different separators at various current densities. 117
Figure 4-60. Shuttle current curves of the bGF, HC, and NRHC full-cells. 118
Figure 4-61. Self-discharge tests of the bGF, HC, and NRHC full-cells conducted after resting for 48 h in the fully charged state (2.8 V) after 10 cycles at 0.5 C. (b) Galvanostatic... 119
Figure 4-62. Cycling performance of the full-cells at 0.2 C. 120
Figure 4-63. Nyquist plots of the bGF and NRHC full-cells before and after 50 cycles at 0.2 C. 121
Figure 4-64. Long-term cycling performance of full-cells at a high current density of 1 C, with the cells activated at 0.2 C for the first three cycles. 122
Figure 4-65. XRD patterns of the fully discharged (1.8 V) and fully charged (2.8 V) NRHC-modified separator after 10 cycles at 1 C. 123
Figure 4-66. (a) SEM images of the fully discharged bGF- and NRHC-modified separator after 10 cycles at 1 C, and the corresponding (b) EDX spectra. 124
Figure 4-67. Comparison of electrochemical properties with the previous studies of heterostructure catalysts for LSBs. The samples with unspecified current rate were all... 125
Figure 4-68. Long-term cycling performance of aCNT full-cell at a current density of 1 C, with the cell activated at 0.2 C for the first three cycles. 126
Figure 4-69. (a) Specific capacities and (b) areal capacities of NRHC full-cell under high sulfur loading at 0.1 C. 127
Figure 4-70. (a) Ex situ SEM images of the Li anode of bGF and NRHC full-cells after 50 cycles at 0.5 C. The corresponding EDX images and spectra of cycled Li anode of cells... 128
Figure 4-71. S 2p XPS profiles of the Li anode of bGF and NRHC full-cells after 50 cycles at 0.5 C. 129