Preparation And Electrochemical Properties Study Of Si-Based Alloy Anode Materials

Currently, commercialized lithium-ion battery anode material was mainly graphite. Althought having excellent cycle performance, it can not meet the market requirements for high-capacity lithium-ion battery because of it poor theoretical capacity of 372 mAh/g. Silicon-based materials have attracted much attention because of the highest theoretical specific capacity （4200 mAh/g）. However, the huge volume change generated in the process of insertion/extraction will cause pulverization of materials and low conductivity for electrode, therefore restricts the application of silicon-based anode materials. In this thesis, two methods were applied to improve the electrochemical performance of silicon-based materials.1. Si-Sb-ZnO_x composite anode materials were synthesized by chemical reduction-mechanical alloying method. The addition of ZnO can improve the electrochemical performance of Si-Sb alloy efficiently. Since the lithiation products can relieve the volume effect by making the Li_xSi, Li_xSb and Li_xZn buffering matrix for each other, they can improve the cycle proformance of materials. Ratio of active substance, testing temperatures and cutoff voltage has significant influences on the specific capacity and cycle performance of electrode materials. Systematical analysis at the open circuit voltage and electrochemical impedance spectroscopy are used to study the lithiation mechanism of the addition of ZnO. Results showed that the Si-Sb-Zno.3 has better cycle performance, with the second discharge specific capacity of 859 mAh/g, and the capacity remains at 690.6 mAh/g after 200 cycle,2. Si/Ni₁7Si₃/C nanofiber composite anode materials were prepared by electrospinning method. XRD results showed that the material was composed of elemetal Si and Ni₁7Si₃ alloy. Nano-silicon powders can easily aggregate due to the large specific surface. Electro spinning method dispersed the powder evenly in material substrate by electrostatic effect. And the products exhibited one-dimentional nanofiber structure, which was convenient for the ion diffusion and charge transfer. Due to the good ductility, inert metal Ni is commonly used in the preparation of Si-based alloy anode materials. It was a good buffering matrix for the volume expansion in the process of insertion/extraction. The Si/Ni₁7Si₃/C showed high reversible capacity of 839 mAh/g for the 2nd discharge, when the ratio of Si and C6H6O4Ni·4H2O is 1:2, the retention of capacities was 1132.4 mAh/g after 200th discharge-charge cycle. This increase of capacity with cycle number could be attributed to a reversible growth of the polymeric gel-like film resulting from kinetically activated electrolyte degradation. And after 200 cycles, the fiber structure of the electrode material was still maintained-which showed a superior stability.