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Title Page

Abstract

Contents

Chapter I. (제목없음) 15

1. Lithium ion battery 15

1.1. Basic principle of Li-ion Battery 17

1.2. Materials in Li-ion battery 19

2. Overview Anode material 21

2.1. Carbonaceous materials 23

2.2. Metal alloy Materials 27

2.3. Metal oxide 28

3. Si-based anode materials 30

3.1. The electrochemistry of Si-based materials. 30

3.2. Nano- engineering of Silicon materials 35

3.3. Si-carbon composite structure 42

3.4. Si alloy composite structure 44

4. Reference 48

Chapter II. Critical Thickness of SiO₂ Coating Layer on Core@Shell Bulk@Nanowire Si Anode Materials for Li-ion Batteries 53

1. Introduction 53

2. Experimental section 54

3. Result and discussion 55

4. Conclusion 58

5. Reference 75

Chapter III. Practical Implantation of Si nanoparticles interconnected α-FeSi₂ Anode Materials in Lithium ion Batteries 79

1. Introduction 79

2. Experimental section 80

3. Result and Discussion 81

4. Conclusion 85

5. Reference 97

Chapter IV. Practical investigation of silicon oxide anode material in lithium ion batteries 102

1. Introduction 102

2. Experimental section 104

3. Result and discussion 105

4. Conclusions 110

5. References 126

List of Tables

Chapter I. 18

Table 1. Li-ion batteries configuration and characteristics 18

Table 2. Main required characteristics of application of battery 24

Table 3. The comparison of various graphite 25

Table 4. The characteristics of anode materials for next generation LIB. 29

Table 5. Crystal structure, unit cell volume and volume per Si atom for the Li-Si system 32

Chapter II. 63

Table 1. Discharge/charge capacities, Columbic efficiencies of 1st cycle and capacity retention after...(이미지참조) 63

Table 2. Raman spectral bands assignment of Figure 6. 66

Chapter IV. 125

Table 1. Previous full cell results with Si-based anode 125

List of Figures

Chapter I. 16

Figure 1. (a) Characterization of the main three kinds of EVs; light EV, PHEV and full EV, in terms of... 16

Figure 2. The proportion of material cost in Li-ion battery components 20

Figure 3. Classification of anode materials based on the reversible Li insertion and extraction process 22

Figure 4. (Up) Crystalline structure of artificial graphite depending on the synthetic temperature,... 26

Figure 5. (a) Volumetric capacities calculated at the full lithiated state and (b) gravimetric capacities of... 27

Figure 6. Phase diagrams of Li-Si alloy 30

Figure 7. The structure of various Li-Si alloy composition. 31

Figure 8. Galvanostatic charge-discharge profiles for micro-Si anode. 32

Figure 9. (a) Potential vs specific capacity profiles for the in-situ XRD cell tested under the 0.01C rate... 33

Figure 10. Schematic phase diagram as function of lithiation in crystalline Si anode. 34

Figure 11. Summary of the first lithiation and subsequent cycling of a-Si and c-Si. 34

Figure 12. Si electrode degradation mechanisms 35

Figure 13. TEM images of 10㎚ sized n-Si a) before cycle, b) after 40 cycles, c) voltage profiles of 5,... 36

Figure 14. Theoretical modeling of stress evolution during lithiation in a (a) hollow sphere vs a (b)... 37

Figure 15. (a) The computational modeling of shape changes of a Si tube inside a rigid nanopore during... 38

Figure 16. a) Schematic illustration of the material synthesis for SiNp@CT structure. b) SEM images... 39

Figure 17. a) Schematic of SEI formation on Silicon surface; Designing a mechanical constraining... 40

Figure 18. (a) Schematic of pomegranate-structured silicon describing snug void spaces for volume... 41

Figure 19. Schematic view of the preparation of (a) Si@carbon core-shell nanowires using a SBA-15... 42

Figure 20. a) Schematic of amorphous Si/Graphene composite exhibiting self-compacting behavior... 43

Figure 21. Schematic illustration of pristine STN and nitrated STN electrode during the cycling. 45

Figure 22. Nano-Si/FeSi₂ Ti hetero-structure (a) HR-TEM images of before and (b) after 50 cycles, c)... 46

Chapter II. 59

Figure 1. SEM images of (a) etched Si and (b) cross-sectional image of (a) obtained from focused ion... 59

Figure 2. TEM images of core@shell bulk@ nanowire Si particle with the coating thickness of (a) ~2... 60

Figure 3. TEM images of (a) carbon coated core@shell Si with ~7㎚ thick SiO₂, (b) magnified images... 61

Figure 4. (a) Voltage profiles of samples with SiOx-free, ~2 ㎚, ~7 ㎚, ~10 ㎚, and~15 ㎚ coating... 62

Figure 5. Plots of Coulombic efficiency of core@shell bulk@nanowire Si anodes with ~2㎚, ~7㎚,... 64

Figure 6. Raman spectra of core@shell bulk@nanowire Si particles with ~2㎚, ~ 7㎚, and ~ 15㎚... 65

Figure 7. Electrochemical impedance spectra of the samples with ~ 2, ~ 7, and ~ 15㎚ thick in coin-... 67

Figure 8. TEM image of the sample with ~15㎚ SiO₂ thick at 0.5V charge state after cycling. An inset... 68

Figure 9. SEM images of (a) a pristine sample, and (b, c, and d) samples with ~ 2, ~ 7, and ~ 15 ㎚... 69

Figure 10. Cross sectioned SEM image of the sample with ~ 2㎚ SiOx thick after 1st lithiation.(이미지참조) 70

Figure 11. SEM images of cross-sectioned electrodes of (a, c, and e) pristine electrodes with ~2㎚,... 71

Figure 12. SEM images of core@shell bulk@nanowire Si with ~7㎚ coating thick after 50th cycles.(이미지참조) 72

Figure 13. Electrochemical performance of lithium ion cells made of LiCoO₂/core@shell... 73

Figure 14. 1st voltage profiles of LCO/core@shell bulk@nanowire Si with ~ 2 ㎚ and ~ 7㎚ coating... 74

Chapter III. 86

Figure 1. (a) Schematic illustration of synthesis process of FS and FSC, (b) SEM image of FS (c) HR-... 86

Figure 2. (a) Cross-sectioned TEM images of FS accompanying EDS mapping of FS 87

Figure 3. SEM image of (a) FSC, (b) TEM images of FSC accompanying (c) EDS mapping of FSC 88

Figure 4. (a) SEM images of pristine BM-FS and after thermal C₂H₂ treatment. (b) HR-TEM image... 89

Figure 5. Voltage profiles of (a) FS and FSC during the 1st cycle at 0.2C rate in lithium half cells, (b)...(이미지참조) 90

Figure 6. Half cell cycle performances of (a) FS and FSC at rate of 0.2C charge/discharge. (b) graphite,... 91

Figure 7. Voltage profiles of lithium ion cells made of LiCoO₂// (a) graphite, (b) FS/graphite, (c)... 92

Figure 8. XPS C1, F1s and P 2p spectrum of (a)FS/graphite, (b) FSC/graphite electrodes after 50, 200... 93

Figure 9. (a) Cross sectioned EDS mapping spectrums of Si, Fe, P, F, O, and C after 200 cycles. High... 94

Figure 10. XRD patterns of a) FS, Graphite, FS/graphite electrode before cycle, after 200 cycles (b)... 95

Figure 11. STEM images of (a) FS, (b) FSC after 200 cycles and its EDS mapping images 96

Chapter IV. 112

Figure 1. Possible fading mechanisms of Si-based anode 112

Figure 2. a) Cycle performances of SiOx only and blending half-cells plotted with y-axis of gravimetric...(이미지참조) 113

Figure 3. a) Voltage profiles of SiOx sole half-cell b) Voltage profiles of SG half-cell c) Voltage profiles...(이미지참조) 114

Figure 4. a) SEM image of graphite b) magnified SEM image of graphite c) SEM image of SiMPs d)... 115

Figure 5. a) SEM image of graphite b) magnified SEM image of graphite c) SEM image of SiMPs d)... 116

Figure 6. Surface and core XPS patterns of SiOx powder.(이미지참조) 117

Figure 7. Electrochemical performance of SG electrode. 118

Figure 8. Swelling of SiOx anode. Cross section SEM images of a) Pristine b) after 250 cycles c) after...(이미지참조) 119

Figure 9. Thickness of SEI layers of SiOx blending electrode after full cell a) after 50 cycles b) after...(이미지참조) 120

Figure 10. a) Voltage profiles of MG half-cell b) Cycle performance of MG half-cell c) Formation... 121

Figure 11. Nyquist plot of SiOx full cell a) after formation, b) after 10 cycles, c) after 50 cycles... 122

Figure 12. XPS analysis of SiOx blending electrode a) after 5 cycles, b) after 50 cycles, and c) after... 123

Figure 13. Cross-sectional images of HR-TEM after a) after 50 cycles b) 250 cycles c) after 500 cycles.... 124

초록보기

 Porous silicon materials were made by various synthetic methods and highlighted for anode materials for Lithium ion battery. Most of Porous Silicon has been prepared on the Si wafers but we employed 3D-interconnected pores to silicon at the bulk powder to alleviated the volume expansion during lithium alloy/dealloy. Herein, we applied metal-assisted chemical etching method at bulk silicon powder to produce urchin-like silicon of 1-3 µm in length of wires and 5-8µm sized particles. By controlling etchant concentration, deposed metal concentration, etching time and temperature, we optimized pore depth and core size of the silicon powder for good performance. However, native surface layer of silicon in which SEI layer formation occurs, influence the kinetics of lithiation/delithiation and the interfacial stability during cycling. Accordingly, surface of the sample is modified via functionalized groups (Si-OH, Si-O-Si, Si-H, Si-O-Li) through different chemical treatments, and the characteristic of each samples were confirmed from 29Si-MAS, FT-IR, XPS.

Silicon, one of most promising anode, has been significantly challenged to improve the volumetric energy density related to the both material and electrode expansion, electrical conductivity and long-term cycle performance for the practical application. Herein, we demonstrated a synthesis of Si nanparticles embedded α-FeSi₂ matrix in which α-FeSi₂ acts as a buffer matrix for the expansion of adjacent Si expansion, and inter connected carbon enhance conductivity and reduce the side reaction of electrolyte and structural degradation. Our results reveals that the α-FeSi₂/Si/Carbon (FSC) exhibits excellent electrochemical property in full lithium ion cell compared to α-FeSi₂/Si (FS) and benchmarking sample of FS, FS/CNT, owing to the decreasing formation of SEI layers and good mechanical strength induced by carbon specious. The FSC anode in the full cell shows a significant improved capacity retention of 83% at 0.7C/0.5C charge/discharge rate between 4.4-3V after 200 cycles.

Silicon oxide (SiOx) is one of the silicon (Si)-based candidates for next generation anode materials beyond graphite in lithium ion batteries industry. However the fading mechanisms of Si-based anode have not been researched in detail with full-cells, though the volume expansion of Si is known as foremost issue of capacity decay and half-cells cycling test is not reliable for practical use of full-cells. Here, we report practical investigation of the fading mechanisms of SiOx-graphite mixture (SG) full-cells test. We conclude that the extent of electrode swelling could determine the main fading reason of Si-based anode whether it follows rapid or gradual degradations and that of SG full-cell was estimated as the continuous SEI thickening which causes large over-potential and electrolyte depletion. The 200th cycle retention of our full-cell shows 70% with initial discharge capacity of 3.45 mAh·cm-2, which is the higher area capacity compared to previous Si-based anode full-cell publication. This work will furnish a guide to make practical strategies of Si-based anode.

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