The Preparation And Electrochemical Performance Of Silicon Anode Material For Lithium Ion Batteries

Silicon(Si) has been considered as one of the most potential anode materials for lithium-ion batteries(LIBs) applications when compared with the typical commercial carbon anodes. Si is abundant on the earth, a wealth of sources, a relatively small cost, and environmentally friendly. Si can host Lithium atom for Li-Si alloy(e.g., Li12Si7, Li13Si4, Li7Si3, and Li22Si5), and provide a specific capacity of 3579 mAh g-1 for room temperature(RT) electrochemical lithiation and low Li-uptake voltage(<0.4 V versus Li/Li+). However, Si exhibits the huge volume change(~300%) in the process of charge/discharge cycles, which can make active materials peeling from the current collector and itself will be pulverized, eventually resulting in rapid capacity fading of electrochemical performance. Furthermore, the intrinsic electric conductivity of silicon is poor(6.7×10-4 S cm-1). In view of the above issues, many approaches in this thesis have been attempted to improve their electrochemical performance including prepared Si/C/MWNTs composite by liquid phase method with heat treatment, Si@PTh composite by chemical oxidative polymerization process, and TiN@Si NRs composite by magnetron sputtering method. Concrete research content is as follows:(1) The Si/C/MWNTs and Si/C composites were prepared by liquid phase method with heat treatment method and using PAN as carbon source. The electrochemical performance of them were further studied and compared with raw Si material. The results show that the cycling stability of the Si/C/MWNTs and Si/C composites are both better than that of raw Si. The addition of MWNTs can further improve the cycling stability and capacity retention of the composite materials. The Si/C/MWNTs composite still remains a relatively high discharge capacity of 618.7 mAh g-1 at 0.5 A g-1 after the 100 cycle. The improved cycling stability is attributed to the carbon matrix as elastic buffer could accommodate the volume change and a small amount of MWNTs added additional electron transport paths.(2) Polythiophene coating on the surface of porous Si core-shell nanospheres(Si@PTh) composite were prepared by a simple chemical oxidative polymerization approach. PTh acts as a flexible layer to buffer the volume expansion of Si during discharge/charge process. In addition, the distinctive porous core and flexible shell structure of Si@PTh composite also can alleviate the volume expansion of Si, increase the contact area of active material and electrolyte, shorten the diffusion path of Li ion, and be advantageous to the diffusion of Li ions and the transport of electrons. Typically, Si@PTh composite electrodes achieve the reversible capacity of 1130.5 mAh g-1 at 1 A g-1 current density after 500 cycles.(3) We have prepared three-dimensional(3D) TiN@Si and TiO2@Si core-shell nanorods arrays structure by the radio frequency(RF) magnetron sputtering and carefully investigated their electrochemical behaviors. TiN with superior mechanical stability and electrical conductivity can provide more effective support and electronic transmission path for Si, at the same time, this unique 3D nanorods array structure could provide enough space to accommodate the volume change of Si. The results show that TiN@Si NRs composite have good cycling stability, which retain a discharge capacity of 3258.8 mAh g-1 with the capacity retention of 94.7% at 1 A g-1 after 200 cycles. TiN@Si NRs composite exhibit a discharge capacity of 2256.6 mAh g-1 at a high current density of 10 A g-1.