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Title Page

Abstract

Contents

Chapter I. Introduction 15

1.1. Motivation 15

1.2. Objective of this work 17

1.3. Thesis outline 17

1.4. References 17

Chapter II. Background and literature review 19

2.1. Overview of conventional Li-ion batteries 19

2.2. Post Li-ion batteries 20

2.3. Rechargeable Li-S battery 20

2.3.1. Basic principles of Li-S battery 20

2.3.2. Components of Li-S battery 21

2.3.3. Challenges of Li-S battery 23

2.3.4. Enhancements of Li-S battery 24

2.4. References 27

Chapter III. Physical and electrochemical characterizations 34

3.1. Physical characterization 34

3.1.1. X-ray diffraction (XRD) 34

3.1.2. Thermal gravity analysis (TGA) 34

3.1.3. Scanning electron microscopy (SEM) 34

3.1.4. Transmission electron microscopy (TEM) 34

3.1.5. Energy dispersive spectroscopy (EDS) 35

3.1.6. X-ray photoelectron spectroscopy (XPS) 35

3.1.7. Ultraviolet-visible (UV-vis) spectroscopy 35

3.1.8. Fourier transform infrared spectroscopy (FT-IR) 35

3.1.9. Brunauer-Emmett-Teller (BET) surface analysis 35

3.1.10. Contact angle (CA) 35

3.2. Electrochemical characterization 36

3.2.1. Cyclic voltammetry (CV) 36

3.2.2. Galvanostatic discharge-charge 36

3.2.3. Electrochemical impedance spectroscopy (EIS) 36

3.2.4. Direct current-internal resistance (DC-IR) 37

3.3. Reference 37

Chapter IV. Revisiting the role of conductivity and polarity of host materials for longlife lithium-sulfur battery 39

4.1. Introduction 39

4.2. Experiment Section 40

4.3. Results 44

4.3.1. Synthesis and characterizations 44

4.3.2. Electrochemical performance 49

4.3.3. Areal and volumetric capacities of pOMS/S80 and pOMC/S80 electrodes 59

4.3.4. Reaction mechanism of sulfur in LSB 63

4.3.5. Interaction of lithium polysulfide with host 66

4.4. Discussion 73

4.5. Conclusion 74

4.6. References 74

Chapter V. A redox-active interlayer for high-energy and long-cycling lithium-sulfur batteries 84

5.1. Introduction 84

5.2. Experimental section 88

5.3. Results and discussion 90

5.3.1. Design rationale of redox-active ILs 90

5.3.2. Structures of ILs and the interaction with polysulifdes 91

5.3.3. Electrochemical performance 103

5.3.4. Characterization after cycling 109

5.4. Conclusion 113

5.5. References 114

Chapter VI. Summary and Future work 122

6.1. Summary 122

6.2. Future work 123

List of publications 124

요약문 127

List of Tables

Table 4-1. Surface properties determined by N₂ isotherms for pOMS, pOMC, and their corresponding sulfur composites. 47

Table 4-2. Comparison of initial capacity, capacity retention, and capacity fading rate in various state-of-the-art... 56

Table 4-3. Comparison of capacity retention and capacity fading rate at high current densities of 1, 2, and 3 C (1... 57

Table 4-4. Electrode resistance values of pOMS/S80 cell obtained from equivalent circuit fitting of... 59

Table 4-5. Comparison of specific and areal capacities in various high active carbon/S electrodes reported... 60

Table 4-6. Comparison of volumetric capacities for various sulfur hosts. 62

Table 5-1. Comparison of the electrochemical performance with current work and previously reported Li-S batteries. 85

Table 5-2. Comparison of the electrochemical performance in current work with Li-S batteries using conductive ILs. 86

Table 5-3. Comparison of the electrochemical performance in current work with Li-S batteries using polar ILs. 87

Table 5-4. Comparison of the surface properties of pOMC, pOMS, and different pOMS/S composites. 100

Table 5-5. Sulfur loading of a IL-free cell and cells with different ILs. 104

Table 5-6. Comparison of the electrochemical performance in this work with Li-S batteries using other various interlayers. 107

Table 5-7. Calculated Re, Rint, and Rct by fitting the EIS data for different cells before and after cycling.[이미지참조] 111

List of Figures

Figure 1-1. Schematic comparison of the theoretical and practical gravimetric energy densities of various... 15

Figure 1-2. Conventional cell configuration of the Li-S cell with four main issues. 16

Figure 2-1. Schematic representation of structure and working of Li‐ion battery. 19

Figure 2-2. (a) Stepwise reduction pathway of octet sulfur (S8) to solid Li₂S₂ and Li₂S products. (b) The...[이미지참조] 21

Figure 4-1. Schematic representations of (a) synthesis process of pOMS/S composite, b) electrochemical reaction... 45

Figure 4-2. TEM images of pOMS at (a) low and b) high magnifications. The pOMS possesses a thin hexagonal... 46

Figure 4-3. (a) XRD patterns of bare sulfur, pOMS, and pOMS/S80. (b) Small‐angle XRD patterns, c) nitrogen... 46

Figure 4-4. (a) nitrogen sorption isotherms, (b) pore size distributions, and (c) cumulative pore volumes of pOMS... 47

Figure 4-5. Characterizations of pOMC and pOMC/S80. (a) Schematic representation of synthesis process of... 48

Figure 4-6. FTIR spectrum of pOMS. 49

Figure 4-7. CV and charge/discharge profiles of (a-c) bare sulfur and (d-f) pOMS/S80 electrodes. The loading... 50

Figure 4-8. (a) CV profiles of pOMS/S80 at a scan rate of 0.1 mV s-1. (b) Rate capability of pOMS/S80 compared...[이미지참조] 51

Figure 4-9. Charge/discharge curves of pOMS/S80 cells at various current densities. 52

Figure 4-10. Charge/discharge profiles and cycle stability of the sulfur-free bare pOMS cell. (a) charge/discharge... 53

Figure 4-11. Voltage profiles of bare sulfur and pOMS/S80 cells in the first cycle and their DCIR and charge... 54

Figure 4-12. The discharging (black) and charging (red) DC-IR plots of pOMS/S80 cell at 50% SOC for 500 cycles. 54

Figure 4-13. Cycling performance of pOMS/S80 with 2 mg cm-2 sulfur loading at current densities of (a) 1, 2,... 55

Figure 4-14. Electrochemical impedance spectroscopy (EIS) profiles of the pOMS/S80 cell with 2 mg cm-2 sulfur...[이미지참조] 58

Figure 4-15. (a) Areal and (b) volumetric capacities along with Columbic efficiency for pOMS/S80 and... 59

Figure 4-16. Cross-sectional morphologies of (a-d) pOMS/S80 and (e) pOMC/S80 electrodes with different sulfur... 61

Figure 4-17. Electrochemical performance at different ratios of electrode components. The weight ratios of active... 63

Figure 4-18. DLS profiles and a modified cell with isolated sulfur trapped between two separators and its LSB... 65

Figure 4-19. SEM images of various (a-c) pOMS/S80 and (d-f) pOMC/S80 electrodes at an initial state (a, d)... 66

Figure 4-20. (a) Digital photograph of bare Li2S6 (DME/DOL) solution (left) and Li2S6 (DME/DOL) solutions...[이미지참조] 67

Figure 4-21. XPS S 2p spectra of (a) Li2S6, (b) pOMC-Li2S6, and (c) pOMS-Li2S6. (black line: original data, open...[이미지참조] 68

Figure 4-22. Morphology of sulfur and pOMS/S80 cathodes. SEM images of sulfur and pOMS/S80 cathodes (a)... 69

Figure 4-23. Characterization of pOMS/S80 composite at fully charged state after 100 cycles at a current density... 70

Figure 4-24. Optimized adsorption structures. (a-c) Top view of Li₂S₄, Li2S6, and Li2S8 cluster adsorption on...[이미지참조] 71

Figure 4-25. Ex-situ XPS S 2p spectra of pOMS/S80 (a) before cycling and (b) in the charged condition after 100... 72

Figure 5-1. Design rationale of redox-active ILs. Illustration of Li-S batteries with (a) conductive, (b) polar, and... 91

Figure 5-2. TGA curves of the bare sulfur powder and pOMC, pOMS, and pOMS/Sx composites under the... 92

Figure 5-3. SEM images of (a) pOMS, (b) pOMS/S30, (c) pOMS/S50, (d) pOMS/S70, and (e) pOMC. 92

Figure 5-4. Structure and computational studies of the ILs. TEM images of (a) pOMC, (b) pOMS, and (c)... 93

Figure 5-5. Contact angle of the electrolyte on the surface for pOMC, pOMS, and pOMS/S50 ILs with electrolyte... 94

Figure 5-6. Optimized structures for Li2Sx adsorption on graphite (001) surface. The green, yellow, and brown...[이미지참조] 95

Figure 5-7. Optimized structures for Li2Sx adsorption on SiO₂ (001) surface. The green, yellow, blue, and red balls...[이미지참조] 96

Figure 5-8. Optimized structures for Li2Sx adsorption on S-containing SiO₂ (001) surface. The green, yellow, blue,...[이미지참조] 97

Figure 5-9. HAADF-STEM image of (a) pOMS/S50, and the corresponding elemental mappings of (b) silicon, (c)...[이미지참조] 98

Figure 5-10. SAXRD patterns of (a) pOMS/Sx composites along with bare pOMS and (b) pOMC. 99

Figure 5-11. WAXRD patterns of pOMS/Sx composites along with bare pOMS and pOMC. 99

Figure 5-12. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distribution curves of the pOMS,... 100

Figure 5-13. Interaction of LiPSs with ILs. (a) Digital photographs of a H-type cell with 0.1 M Li2S6 in...[이미지참조] 101

Figure 5-14. High-resolution S 2p XPS spectra before and after H-type cell test for (a) pOMC, (b) pOMS, and (c)... 102

Figure 5-15. CV curves of (a) IL-free, (b) pOMS IL, and (c) pOMC IL cells for the first 10 cycles at a scan rate... 103

Figure 5-16. Electrochemical performance of Li-S cells with and without ILs. (a) Cycling performances of IL-free... 105

Figure 5-17. Galvanostatic charge-discharge profiles of the (a) IL-free (b) pOMS IL, (c) pOMC IL, (d) pOMS/S30...[이미지참조] 106

Figure 5-18. Self-discharge behaviors of pOMC IL, pOMS IL, and IL-free cells at 0.3 C after rest time of 14 days... 108

Figure 5-19. (a) Photographs of the pristine separator and Li anode. Photographs of separators and Li anodes... 109

Figure 5-20. EIS plots of Li-S batteries employing different IL (a) before and (b) after 100 cycles at 0.5 C in the... 110

Figure 5-21. Characterization after cycling of IL-free and different IL cells. Surface SEM images of (a) fresh Li... 111

Figure 5-22. Cross-section SEM image and EDS mappings of pOMC IL after 500 cycles at 1 C. 112

Figure 5-23. TEM image and elemental mapping of pOMC IL after 100 cycles at 0.5 C. 113

초록보기

각종 환경 문제가 부각되면서 자동차 산업은 화석연료에서 친환경 전기 에너지로 구동되는 전기자동차로 변하고 있으며, 각종 전자제품에서도 장시간 이용이 가능한 고 에너지 저장장치의 수요가 증가하고 있다. 그 중에서 리튬-황 전지는 높은 이론 에너지 밀도를 가지고 있어 장시간 이용이 가능하며, 활성 물질 황은 자연에 풍부하고, 가격이 저렴하여 현재까지 개발되고 있는 이차 전지 중에서 가장 유망한 전지이다. 그러나 활성 물질인 황의 낮은 전기전도도 및 리튬-황 전지의 충전과 방전 과정에서 생성되는 극성 리튬 다황화물이 용해되어 활성 황 물질이 손실 되어 용량 및 수명 감소라는 문제점이 있다. 이러한 단점을 해결하기 위해 황의 손실을 효과적으로 억제하는 황 복합전극 소재 연구가 필요한 실정이다. 이러한 목적으로 인해, 본 박사 학위논문은 우수한 표면 특성을 가지는 극성 실리카 설계 및 합성, 그리고 담지체 및 중간층에 대한 연구를 진행하였다. 진행된 연구는 다음과 같다.

(i) 유황 이용 효율을 향상시키기 위해 표면 특성이 우수한 판 형태의 정령 메조 다공성 실리카를 합성하여 황 담지체로 사용하였다. 본 연구에서는 리튬-황 전지에서 주로 사용되는 다공구조의 전도성 탄소와 극성 다공성 실리카를 황 담지체로 사용하여 세부적인 분석 및 메커니즘을 분석하였다. 극성 다공성 실리카는 비극성 다공성 탄소에 비해 극성 리튬 다황화물과 뛰어난 상호작용을 보여주었으며, 리튬-황 전지의 안전성을 획기적으로 향상됨을 보여주었다.

(ii) 앞서 연구한 극성 다공성 실리카 기반으로, 주기 안전성 및 에너지 밀도를 향상시키기 위한 다기능 중간층을 개발하였다. 이 과정에서 다공구조의 전도성 탄소 중간층과 비교하여 극성 다공성 실리카 중간층으로 사용했을 때 리튬-황 전지의 뛰어난 주기 안전성을 확인 하였다. 추가적으로, 극성 다공성 실리카에 황을 주입함으로써 양극에서뿐만 아니라, 중간층에서도 황을 제공함으로써 리튬-황 전지 황 함유량을 높여 고 에너지밀도 리튬-황 전지를 구현하였다

본 학위논문연구는 우수한 표면 특성을 가지는 극성 다공성 실리카 합성을 통해 개선된 리튬-황 전지 안정성 및 반응 메커니즘의 이해를 연구하였다. 본 연구를 통해 리튬-황 전지 전극 소재 연구에 대한 선택성을 크게 확대 시켜 차세대 전지 개발에 박차를 가할 것으로 기대하며, 더 나아가 향상된 기능을 갖는 고내구성 및 고출력 리튬-황 전지를 개발을 제시한다.

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