Preparation Of Silicon-Carbon Composites And Their Anode Performance For Lithium-ion Batteries

The research and development of energy storage technology has always been valued by countries all over the world.Electric energy storage system has been regarded as an important link for safe and economic operation of electric power system.At present,electric energy storage batteries are mainly lithium batteries,and how to increase the capacity of lithium-based energy storage batteries is a hot topic in current research.As one of the most likely lithium battery anode materials that is most likely to be commercialized on a large scale,silicon has the advantages of high specific capacity,high operating voltage,and abundant raw material reserves.It has important research significance in the direction of lithium-based energy storage batteries.However,the volume of silicon-based materials changes greatly during the cycle,and the capacity decays quickly,which seriously restricts its development and application.In this paper,by combining silicon nanomaterials with functional carbon materials,the lithium storage performance of silicon materials is greatly improved.In this paper,silicon particles of 80?100 nm are used as raw materials,and the volume expansion of silicon material during the circulation process is relieved by carbon coating on the surface of the nano silicon particles.This paper innovatively uses polyvinylidene chloride(PVDC)as a carbon source and uses the restricted chemical reaction between silicon material and PVDC to prepare a Si@C composites with a core-shell structure.The core-shell structure can be Significantly improved the electrochemical performance of silicon materials.Specifically,during the mixing process,the polymer-state PVDC is first adsorbed on the surface of the nano-silicon particles to form a polymer film,and the polymer film can form an ultra-thin coated carbon layer on the surface of the nano-silicon after pyrolysis at high temperature.This ultra-thin carbon layer can effectively alleviate the problem of volume expansion of nano-silicon particles during circulation,and also avoid the agglomeration of nano-silicon particles.Further,we assembled the prepared Si@C composites into a coin battery for testing.At a current density of 200 mAg^-1,the specific discharge capacity of the first cycle can reach 1785.9 mAhg^-1,which can be maintained 878.5 mAhg^-1 after 100 cycles.In addition,the button cell electrochemical impedance test shows that the contact resistance of the nano-silicon material coated with carbon layer is significantly reduced compared to pure silicon,thanks to the ultra-thin carbon layer provides an excellent channel for electron transmission,showing good rate performance at different current densities.In order to further improve the electrochemical performance of silicon carbon composites,this paper optimizes the core-shell structure.In the preparation process of Si@C composites,flake graphite was added,and the process was improved,and Si/G@C composites was prepared to further improve the conductivity of the composite material.Specifically,liquid-phase ball milling is used to efficiently mix the nano-silicon particles with flake graphite uniformly.High-temperature pyrolysis PVDC can coat the in-situ carbon of the nano-silicon particles and tightly connect with the flake graphite.This structure can effectively inhibit the agglomeration of nano-silicon particles,and fully play the role of conductive buffer matrix,so that the electrochemical performance of Si/G@C composites is significantly improved compared to Si@C composites.In addition,after adjusting the proportion of silicon content in the Si/G@C composites,it was found that the coin battery assembled with the Si30/G@C(Si accounts for 30 wt.%)composites has a relatively high discharge specific capacity and good cycle stability and rate performance,it can still maintain the discharge specific capacity of 745.6 mAhg^-1 after 100 cycles at a current density of 200 mAg^-1,and the corresponding single-turn capacity attenuation rate is only 0.21%,and it shows good cycle performance under different current densities.