Electrochemical Energy Storage Research Of Silicon Carbide Nanowires

Silicon carbide(SiC) is an attractive material with many outstanding properties such as wide band-gap, high hardness, high thermal conductivity, high breakdown voltage, high electronic mobility, superior oxidation and corrosion stability. It is widely used in aerospace industry, mechanical manufacturing, micro-electronics and other fields.One-dimensional SiC nanomaterials show more excellent performance in terms of mechanics, electrics and optics, etc. In this thesis, silicon carbide materials with different shapes were fabricated by employing modified chemical vapor deposition method(CVD), using different silica sources(monocrystal silicon slice, silicon monoxide powder, silicon dioxide powder, silicon powder). The lithium alloying/de-alloying and electrochemical energy storage performance of as-prepared SiC were studied.SiC is always regarded to be electrochemically inactive in LIBs, which has been established to act as a buffer matrix and/or backbone to enhance the electronic conductivity of composite materials during charge-discharge process. In recent years, due to the rapid development of nanometer materials, a few reports reveal the lithium alloying/de-alloying property of nanometer SiC serving as an anode material for Li-ion batteries(LIBs). In this thesis, silicon carbide materials with different shapes were fabricated by employing modified CVD, using different silica sources. According to the result of material preparation and the requirements of the LIBs, we chose bead-curtain shaped silicon carbide nanowires(NWs)with core-shell structure on graphite paper(GP) using silicon powder as silicon source. After further hydrofluoric acid treatment, bare SiCNWs on GP were obtained for comparison. The as-prepared silicon carbide nanowires core-shell structure and SiCNWs were directly used as working electrodes without addition of binder or electron conductive material, which exhibited high specific capacities and good cycling stabilities. This could be ascribed to the unique nanowire structure, effectively buffering volume changes upon repeated alloying and de-alloying. Comparatively, silicon carbide nanowires core-shell structure presented much better electrochemical properties. Field emission scanning electron microscopy(SEM) and Fourier transform infrared(FTIR) spectra analysis revealed that the SiO2 shell effectively separate the direct contact between active SiC and electrolyte, thus suppressed the rapid growth of solid electrolyte interface(SEI) film in repeated cycling and stabilized the structure of active material.Next, the growth mechanism of SiCNWs and different shapes in the process of nanowires growth impact on electrochemical energy storage were investigated. Firstly, the preparation conditions of material were researched detailly, after many experiments, the most energy-saving preparation conditions in the CVD method were ensured, such as the least time, the least raw material and the lowest preparation temperature. In the preparation conditions, we select 0 h, 0.5 h, 1 h, 3 h, 5 h, 7 h and 10 h. Through the SEM, SiCNWs images showing growth status under different periods were observed. Secondly, the seven samples prepared in the seven periods were applied to the super capacitors respectively to test their electrochemical performances. The results showed that the material prepared with 0.5 h and 1 h presented the highest specific capacity in the first cycle and the charge-discharge capacity of material prepared with 10 h was increasing gradually stabilizing after 200 cycles, showing higher specific capacity 2F cm-2. This may be due to the densely covered silicon carbide balls for the sample prepared with 0.5 h, 1 h contacting directly with graphite paper, which has the largest specific surface area comparing with others, therefore it has the highest specific capacity. However, SiCNWs prepared with 10 h did not directly contact with conductive graphite paper but by thick SiC films to transfer charges, so the material in the initial did not show high specific capacitance. The specific capacity increased after 200 cycles and still remained unchanged after 1000 cycles, showing excellent cycling perdurability.