Synthesis And Electrochemical Studies On High Capacity Si/C Composite Anodes For Lithium-ion Batteries

Lithium-ion batteries are considered as the most promising powers source in the 21st century for the wide application range from microbatteries for small-size electronic devices to power sources for electrical vehicles. Search for electrode materials with large capacity and long cycle life is a key for developing higher performance lithium-ion batteries. In consideration of the low capacity of graphite-based materials (theoretical capacity: 372mAh/g), which are used extensively in currently commercialized lithium ion batteries, it is urgent to develop new anode materials with larger capacity.In view of small volume change and excellent cyclablity of carbon and large lithium insertion capacity of silicon, they were adopted as matrix and main active material respectively. New types of Si/C composites were prepared by methods of thermal pyrolysis and high energy ball milling and the electrochemical performance was investigated in this thesis.The Si/C composite was obtained by pyrolyzing polyvinyl chloride (PVC), in which fine graphite and silicon powder are homogeneously dispersed. The ductile carbon matrix in the composite can not only buffer the volume change of silicon during lithium insertion but also provide effective electric connection networks surround Si particles. Due to the reasons above, the composite shows large capacity (700tmAh/g) and good cycle stability. After the first cycle, the charge-discharge voltage plateau of the composite shifts toward positive direction for about 0.15V against that of the commercialized CMS anode material. This is favorable for enhancing the charging rate and operating safety of the cell. The crystal Si in the composite would be transformed into amorphous Si after the first lithium insertion-extraction cycle. The amorphous structure can be retained in the following cycles.The introduction of the fine graphite powder (30% content) improves the dispersion uniformity of nanosilicon particles in the composite, reduces the hysteresis between charge and discharge, and enhances the initial charge-discharge efficiency. Studies on the pyrolysis carbon from different organic precursor indicate: the more compact structure and the smaller specific surface area of pyrolysis carbon, the more stable the composite. In addition, coarse composite powder exhibits higher coulombic efficiency and better cycle stability than the fine one. Therefore, the ideal structure concerning Si/C composite is that Si particles are completely enclosed by compact pyrolysis carbon.Furthermore, high energy ball milling technique is combined with thermal pyrolysis for preparing nano-dispersed Si/C composites, in which ball-milled nano-Si mixed with fine graphite is dispersed in pyrolysis carbon uniformly. The composite exhibits excellent cyclability and improved charging rate capability. XRD analysis shows that nano-crystal silicon (partly amorphous) is obtained by high-energy ball milling of raw silicon powders. Two-step ball milling method used in this thesis makes the nano-structured Si and graphite contact intimately and also prevents the formation of electrochemical inactive SiC. By encapsulating the two-step ball milled product with PVC pyrolysis carbon, the specific surface area can be sharply reduced and the irreversible capacity in the first cycle is sequentially decreased.