Synthesis and Characterization of Porous NiSi-Si Composite Anode Materials for Lithium-Ion Batteries

本論文之目的在於開發以矽為主體的鋰離子二次電子負極材料。過去十年當中，與其他負極材料相比，矽因擁有絕佳的理論比電容(~3600 mAh/g)而受到重視。然而由於在充放電過程中伴隨著劇烈的體積膨脹收縮以及矽本身的低導電的特性，導致極板結構上的崩解，使得矽負極材料在商業化的過程中受到阻礙。 以穩定極板結構的觀點為考量，藉由結合高能球磨與蝕刻的製程，成功地製備具有高孔隙度的NiSi-Si複合負極材料，藉由實驗證明，這種預留粒子內孔洞的方法確實能夠緩衝矽在充放電時所造成的體積變化，再者，經由臨場XRD實驗結果發現在充電的過程中有NiSi2的形成，對於極板體積的膨脹有補償的作用。 此外，本研究也嘗試以SEM與XPS的分析，去探討經過充放電於純矽以及矽碳負極材料上所形成的SEI膜的形態與組成 (以LiPF6 為鋰鹽，EC/EMC 為電解液)。結果顯示，於純矽極板上經過一圈的充放電之後就有明顯的表面沉積物，可能是矽表面的矽氧化物表面的親水性所造成電解液的分解，然而經過表面改質鍍碳的矽，能有效的阻絕矽氧化物與電解液的接觸，改變電解液分解的反應，降低氟化合物的生成。The main objective of this research is to explore new materials based on silicon for lithium-ion battery. In the last decade, silicon has attracted much attention because it has the highest specific capacity (~3600 mAh/g) for any of anode materials studied to date. However, Si undergoes a dramatic volume change during cycling and possesses intrinsically poor conductivity, resulting in the mechanical instability and poor cyclability, retard its commercial application. From the view point of stabilizing electrode structure and increasing the conductivity, porous NiSi-Si composite anode material has been synthesized by high-energy ball milling, of mixture of Ni and Si and subsequent dissolution of un-reacted Ni. The preset intra-particle voids have been shown to help to accommodate volume expansion arising form the alloying of Si. Furthermore, Synchrotron XRD indicates that the NiSi component is active toward Li alloying, and Ni2Si is formed during Li alloying. Both preset intra-particles void and presence of Ni2Si help to maintain the integrity of the electrode, resulting in much reduced thickness expansion, as compared with pure Si electrode. In addition, the morphology and composition of SEI layer formation on Si or C-Si anode using LiPF6 in EC/EMC as electrolyte have been carried by means of SEM and XPS analysis. Superficial deposition is vivid even after only one cycle because of the existence of –OH/H2O bounded on SiO2 surface. After coating carbon on Si surface, the formation of Si oxide is reduced.摘要 I Abstract II Table of Contents IV List of Figures VII List of Tables XII Chapter 1 Introduction 1 Chapter 2 Theory and Literature Review 3 2.1 Background and Fundamental Knowledge for Lithium-ion Batteries 3 2.2 Introduction to Silicon and Nickel 8 2.2-1 Silicon 8 2.2-2 Nickel 11 2.3 Investigation on Si and Composite Anode Materials 13 2.3-1 Si Anode 13 2.3-2 Solid Electrolyte Interphase 20 2.3-3 Intermetallic Alloy Anode 28 2.4 Mechanical Alloying and Milling 32 2.4-1 Factors Affecting the MA Process 33 2.5 Chemical Vapor Deposition 39 Chapter 3 Experimental 41 3.1 Chemicals 41 3.2 Synthesis of Anode Materials 42 3.2-3 Synthesis of Nanoporous Structure NiSi/Si/Ni Composite by High-Energy Ball Milling 48 3.3 Analysis and Characterization 51 3.3-1 Phase Identification 51 3.3-2 Microstructure Characterizations 53 3.3-3 Composition Determination 53 3.3-4 X-ray Photoelectron Spectroscopy 54 3.3-5 Pore size & Particle Size Distributions 55 3.4 Electrochemical Characterization 56 3.4-1 Charge and Discharge Strategies 56 3.4-2 Cyclic Voltammetry 56 3.3-3 Electrochemical Impedance Spectroscopy 58 Chapter 4 Results and Discussion 60 4.1 Silicon 60 4.1-1 Physical and Structural Characterization 60 4.1-2 Electrochemical Characterization 61 4.2 Investigation on Solid-Electrolyte-Interphase Film on Si Anode 71 4.2-1 SEI Film on Si Anode 71 4.2-2 the Effects of C-coating by CVD on the SEI Formation 85 4.3 Microstructure Design of Nanoporous NiSi-Si Composite Anode 91 4.3-1 Synthesis and Microstructure Characterization 91 4.3-2 Electrochemical Characterizations and Electrode Morphology of NiSi-Si Composites 98 4.4 Experimental Experience of Mechanical Alloying in Synthesizing NiSi/Si/Ni Composite Materials 109 4.4-1 Size Effect 109 4.4-2 Effect of Mole Ratio of Ni to Si 114 4.4-3 Effet of Weight Ratio of Ball to Powder 117 Chapter 5 Conclusion 119 References 12