Preparation Of Nanostructured Silicon And Their Application In Lithium-ion Batteries

With the populization of intelligent electronic devices and electric vehicles, the demands for lithium-ion batteries (LIBs) with higher energy density are increasing rapidly. Silicon is a promising new anode material,with a high theoretical capacity 4200 mAh g-1 and low work potential (<0.5V vs. Li/Li+). However,silicon has a volume expansion more than 300% during lithium insertion, which will lead to the electrode pulverization and the rapid capacity loss. The nanostructure of silicon material is an effective method to improve the cycling stability of silicon anodes,because the nano size can reduce the overall mechanical stress and the charge transfer pathway, while the pores or spaces which often exsist in nanostructure can accommodate the volume expansion during lithium insertion. But there are still some problems in the silicon anode research, such as high cost and poor cycling stability.Therefore, in this paper, we mainly focus on the synthesis of silicon nanostructures by chemical vapor deposition (CVD) method and their application in LIBs, as shown in below detailedly.(1) The silicon nanowire with good orientation were grown on silicon substrates by CVD method. Then, we systematically studied the effects of the experimental parameters on the morphology of silicon nanowire arrays, found the growth rate of silicon nanowires was faster at relatively lower growth temperature (455 ?500 ?) or at higher SiH4 flow rate, with higher frequency of silicon nanowires growth with the small diameter. In addition, it was also found that the thickness of Au coating had no significant influence on the length or diameter of the silicon nanowires, but had a positive correlation with the silicon nanowire density, when the thickness of the Au film was not more than 5 nm.(2) The SiC/Si core-shell nanowires on carbon paper were synthesized via a two-step CVD process, directly used as electrode without any additional binder or conductive materials. The composite electrode structures not only have a high specific surface area and a short electron collection pathway, but also improve the quality of the active material per unit area. After 50 cycles,at a constant cycling rate of 0.1 C and 0.5C, the discharge specific capacity of the sample retained 2837 and 1809mAh g-1,respectively. By comparingthe SEM and TEM images of the SiC/Si core-shell nanowires before and after cycling, it is proved that the existence of SiC nanowires can improve the cycling stability of silicon electrode. We also we optimized the time of Si deposition and SiC nanowires growthto further improve the performance of the batteries.(3) Carbon-coated silicon nanotube arrays/carbon cloth were prepared via atwo-step CVD method, by using ZnO nanowire arrays as sacrificial templates, and directly used as the electrode, without any additional binder and conductive materials.Three-dimensional carbon fiber structure provides a high specific surface area and short electronic collection pathway.The hollow structure of silicon nanotubes can accommodate the volume expansion during lithium insertion, and the carbon film not only enhanced the conductivity of the electrode, but also stabilized solid electrolyte interface film. After 100 cycles, the capacity of the carbon-coated silicon nanotube arrays/carbon cloth electrode retained 2198mAhg-1, which also showed a good rate performance. Then, we optimized the synthesis process to further improve the cycle stability by reducing the thickness of the silicon film.The results show that the silicon nanostructures/carbon fiber woven fabriccomposite can improve the cycling stability of silicon anode effectively. Carbon fiber woven fabric is an ideal electrochemical substrate material, because of its high flexibility, high conductivity, high chemical stability and high surface area, so the composite structure has a very good application prospect in the flexible solid LIBs and wearable storage devices.