Thermal Modification And Electrochemical Performances Of SiO_x Anode For Li-ion Batteries

Silicon monoxide（SiO_x）has the advantages of high theoretical specific capacity（2600 mAh/g）and good cycle stability,which is considered to be one of the most promising new anode materials for commercial application,however,still has poor conductivity and low initial coulombic efficiency.Disproportionation reaction or compounding with carbon is an effective means to improve the performance of SiO_xanode.These means inevitably involve heat treatment steps.At present,the effect of heat treatment on the microstructure changes of SiO_x and the mechanism of its effect on electrochemical performance are still ambiguous.This paper focuses on the internal relationship between heat treatment and SiO_x microstructure changes as well as its influence on the lithium storage performance and mechanism of SiO_x.The main research contents and main conclusions are as follows:（1）By studying the effect of different heat treatment temperatures on the changes of SiO_x microstructure at Ar atmosphere,it is found that:（1）Heat treatment can make the oxygen inside SiO_x diffuse to the surface,and at the same time,Si nanocrystals and oxygen-rich regions surrounding Si nanocrystals are gradually formed inside;（2）The formation and growth of crystalline Si inside SiO_x are mainly controlled by the heat treatment temperature,and the oxygen-rich area formed by the diffusion and aggregation of oxygen mainly changes with the heat treatment time.（2）The HF acid etching treatment of SiO_x after heat treatment could effectively wash off the oxygen-rich area and surface SiO₂ layer formed by heat treatment,reduced the oxygen content in SiO_x,increased the number of pores in SiO_x,and increased the specific surface area of SiO_x,so as to improve the ICE and cyclic stability.Compared with the raw material,the material prepared by HF etching after heating at 1050°C for3 h,the ICE could be increased from 60.08%to 67.8%,with a reversible specific capacity of 1285.2 mAh/g after 500 cycles.（The SiO_x lost its lithium storage performance after 500 cycles）.（3）After heat treatment,SiO_x was rapidly cooled with liquid nitrogen,and its microstructure and electrochemical properties were studied.It was found that:（1）Compared with the natural cooling method,rapid cooling would form more Si nanocrystals with smaller particle size inside SiO_x,thereby effectively reducing the relative content of amorphous substances in the material;（2）The quenching in liquid nitrogen could prevent the formation of a complete SiO₂ coating layer on the surface of SiO_x,promoting the generation of Si and SiO_x regions with low oxygen content,which are beneficial to improve the lithium-ion transport performance and improve the electrochemical activity of the material.The modified material after heat treatment and quenching was activated at a current density of 0.1 A/g,showing a specific discharge capacity of 1635.3mAh/g and an ICE of 62.7%,and the capacity retention rate after 300cycles at a current density of 0.5 A/g was 70.6%.The same heat treatment-quenching modification was performed on SiO_x@C prepared by CVD method,and it was found that its ICE and specific capacity were also improved.The ICE was 71.96%,and the specific capacity was 978.3mAh/g after 300 cycles at a current density of 0.5 A/g.