The Fabrication Of Si-C Composite Anode Materials And Their Electrochemical Performances

Silicon is considered as a promising anode material for rechargeable Li-ion batteries, owing to its high theoretical capacity and high-security. However, large volume changes during the Li+insertion/extraction process and intrinsic low electric conductivity limits the development of silicon-based anode materials. In this paper, we hope to solve the application bottlenecks of silicon through adding carbon as buffer substance. We have also revealled the structure-function relationship by designing different morphology structures of Si-C composites. Using abundant and cheap silica as raw material, we create the yolk-shell structured porous Si-C microspheres, at the same time, systematically study the influence of the void volume on electrochemical performance. Taking advantages of easy crosslinking when sodium alginate meets the divalent metal ions, three-dimensional porous Si-C composite microspheres were fabricated using the commercial Si nanoparticles as silicon sources. These two designed structures improved cycle performance and rate capability effectively and the morphology of Si-C microspheres can be controlled well through above two experiments.The main works are as follow:(1) The yolk-shell structured porous Si-C microspheres have been created by using SiO2spheres as templet through magnesiothermic reduction. A carbon shell was first coated on the SiO2, followed by selective etching to create spaces required for expansion. Finally the SiO2cores were in situ magnesiothermic reduced to porous Si inside the hollow carbon spheres. The hollow design between core and shell ensures the structural stability. Thus after100cycles, the specific capacity still stabilized at530mAh·g-1and the capacity retention was67%. The microspheres also exhibit good rate performance because of the interconnected carbon shells and porous silicon cores. With the current density ranging from200mA·g-1to2000mA·g-1, the capacity retention was still as high as54%.(2) Using sodium alginate aqueous solution with Si nanoparticles as aqueous phase, isooctane as oil phase, three-dimensional structured Si-C microspheres have been prepared by emulsification-gelation and freeze drying methods. Si nanoparticles were well-distributed in the interconnected3D network of carbon membranes, which ensures the good cycling and rate performances of microspheres. The initial discharge capacity reached2280mAh·g-1, and after200cycles the reversible capacity stabilized at970mAh·g-1. The capacity remained650mAh·g-1at2000mA·g-1, which recoverd to1400mAh·g-1when the current density back to100mA·g-1.