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A study on the deformation and fracture of anodes in Li ion batteries : 리튬 이온 전지 음극 물질에서의 변형 및 파괴 거동 관찰

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dc.contributor.advisor오규환-
dc.contributor.author최용석-
dc.date.accessioned2017-07-13T05:52:42Z-
dc.date.available2017-07-13T05:52:42Z-
dc.date.issued2016-08-
dc.identifier.other000000136485-
dc.identifier.urihttps://hdl.handle.net/10371/118090-
dc.description학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 8. 오규환.-
dc.description.abstractSilicon and germanium are considered as one of the best candidates of anode material for the Li-ion batteries due to its enormous capacity. However, it has been known that the Li ion insertion and extraction during electrochemical cycling results in a huge volume expansion and shrinkage, which can lead to fracture of the anode. This mechanical degradation can lead to the fading of the capacity of the battery by isolating active materials, and by creating new surface area for solid electrolyte interface (SEI) growth. Thus, understanding of how electrodes are able to sustain electrochemical reaction without mechanical degradation is essential for the development of high-capacity Li-ion batteries. This thesis explores the deformation and fracture of anode materials during electrochemical cycling to provide a guideline for the practical design of high-capacity lithium ion batteries to avoid fracture.
First, we report observations of microstructural changes in {100} and {110} oriented silicon wafers during initial lithiation under relatively high current densities. Evolution of the microstructure during lithiation was found to depend on the crystallographic orientation of the silicon wafers. In {110} silicon wafers, the phase boundary between silicon and LixSi remained flat and parallel to the surface. In contrast, lithiation of the {100} oriented substrate resulted in a complex vein-like microstructure of LixSi in a crystalline silicon matrix. A simple calculation demonstrates that the formation of such structures is energetically unfavorable in the absence of defects due to the large hydrostatic stresses that develop. However, TEM observations revealed micro-cracks in the {100} silicon wafer, which can create fast diffusion paths for lithium and contribute to the formation of a complex vein-like LixSi network. This defect-induced microstructure can significantly affect the subsequent delithiation and following cycles, resulting in degradation of the electrode.
Second, we have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration using a simple bending test. Silicon thin-films on copper substrates were lithiated to various states of charge. Then, bending tests were performed by deforming the substrate to a pre-defined shape, allowing for a variation in the curvature along the length of the electrode. From the bending test, the critical strains at which cracks initiate in the lithiated silicon were determined. Using the substrate curvature technique, we also measured the elastic modulus and stresses that develop in the electrodes during electrochemical lithiation. From these measurements, the fracture energy was calculated as a function of lithium concentration using a finite element simulation of fracture of an elastic film on an elastic-plastic substrate. The fracture energy was determined to be 12.0 ± 3.0 J m-2 for amorphous silicon and 10.0 ± 3.6 J m-2 for Li3.28Si, with little variation in the fracture energy for intermediate Li concentrations.
Third, we measure stresses that develop in sputter-deposited amorphous Ge thin films during electrochemical lithiation and delithiation. Amorphous LixGe electrodes are found to deform plastically at stresses that are significantly smaller than those of their amorphous LixSi counterparts. The stress measurements allow for quantification of the elastic modulus of amorphous LixGe as a function of lithium concentration, indicating a much-reduced stiffness compared to pure Ge. Additionally, we observe that thinner films of Ge survive a cycle of lithiation and delithiation, whereas thicker films fracture. By monitoring the critical conditions for crack formation, the fracture energy is calculated using an analysis from fracture mechanics. The fracture energies are determined to be 8.0 J m-2 for a-Li0.3Ge and 5.6 J m-2 for a-Li1.6Ge. These values are similar to the fracture energy of pure Ge and are typical for brittle fracture. Despite being brittle, the ability of amorphous LixGe to deform at relatively small stresses during lithiation results in an enhanced ability of Ge electrodes to endure electrochemical cycling without fracture.
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dc.description.tableofcontentsChapter 1. Introduction 1
1.1 Introduction to Li ion batteries 1
1.2 Basics of Li ion batteries 3
1.3 Various electrode materials for Li ion batteries 6
1.4 Characteristics of Si and Ge (Group IV elements) as an anode material 10
1.5 References 26

Chapter 2. Microstructural evolution induced by micro-cracking of silicon during fast lithiation 30
2.1 Introduction 30
2.2 Experimental procedures 32
2.3 Results and discussion 34
2.4 Conclusion 51
2.5 References 52

Chapter 3. The fracture energy of lithiated thin-film silicon electrodes at various lithium concentrations 58
3.1 Introduction 58
3.2 Experimental procedures 60
3.3 Results and discussion 65
3.4 Conclusion 90
3.5 References 91

Chapter 4. Measurements of stress and fracture in germanium electrodes during electrochemical cycles 95
4.1 Introduction 95
4.2 Experimental procedures 98
4.3 Results and discussion 104
4.4 Conclusion 120
4.5 References 121

Chapter 5. Conclusion 126
Appendix I. Interface-enhanced Li ion conduction in LiBH4-SiO2 solid electrolyte 129
6.1 Introduction 129
6.2 Experimental procedures 132
6.3 Results and discussion 135
6.4 Conclusion 162
6.5 References 163
Appendix II. In-situ microstructure evolution of oxide dispersion strengthened ferritic steel under uniaxial deformation 169
7.1 Introduction 169
7.2 Experimental procedures 171
7.3 Results and discussion 175
7.4 Conclusion 187
7.5 References 188
Appendix III. Dynamic recrystallization in high-purity aluminum single crystal under frictionless deformation mode at room temperature 191
8.1 Introduction 191
8.2 Experimental procedures 193
8.3 Results and discussion 197
8.4 Conclusion 209
8.5 References 210

요약 (국문 초록) 213
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dc.formatapplication/pdf-
dc.format.extent5189509 bytes-
dc.format.mediumapplication/pdf-
dc.language.isoen-
dc.publisher서울대학교 대학원-
dc.subjectLI ion batteries-
dc.subjectAnode material-
dc.subjectSilicon-
dc.subjectGermanium-
dc.subjectMicrostructure-
dc.subjectNon-uniform lithiation-
dc.subjectMicro-crack-
dc.subjectElastic modulus-
dc.subjectDeformation-
dc.subjectFracture energy-
dc.subjectBending test-
dc.subject.ddc620-
dc.titleA study on the deformation and fracture of anodes in Li ion batteries-
dc.title.alternative리튬 이온 전지 음극 물질에서의 변형 및 파괴 거동 관찰-
dc.typeThesis-
dc.description.degreeDoctor-
dc.citation.pagesXXIII, 221-
dc.contributor.affiliation공과대학 재료공학부-
dc.date.awarded2016-08-
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