Electrochemical Performance Improvement Of Silicon Monoxide Anode Materials For Lithium Ion Batteries

SiO_x is a promising anode material for lithium-ion batteries.It has been used in Tesla’s automotive batteries,and widely considered by academia and industry as the most promising silicon-based anode material for full-scale commercial use.SiO_x anode materials currently have major problems such as low initial coulombic efficiency,poor cycle performance and high expansion rate,which hinder the commercialization of silicon oxide anode materials.In order to solve these technical problems,this paper carried out the research on the methods of improving electrochemical performance of SiO_x anode materials.To this,three methods of solid phase sintering reaction,chemical liquid phase prelithiation and pyrolytic carbon coating are adopted to improve initial coulombic efficiency and cycle performance.And then,the mechanism and princinple for improving initial coulombic efficiency and cycle performance has also been discussed.The results are as follows:In order to explore lithium compounds to enhance the initial coulombic efficiency of SiO_xanode materials,SiO and Li₂CO₃ which are commonly used in industry were chosed as the research objects.The reaction process and product properties under high temperature conditions has been investigated.At 700℃,SiO and Li₂CO₃ were entirely reacted to form lithium silicate、silicon crystals and silicon dioxide crystals;XPS,XRD and electrochemical performance test results show that both the reversible Si-region and silicon-oxygen region in SiO participated in the sintering reaction with Li₂CO₃,which resulted in a substantial reduction in the reversible capacity of the material(1362.3→245.6m Ah·g^-1).But to a certain extent,the cycle performance of the material were improved by the lithium silicate and other inert products.Meanwhile,after the lithium silicate removed by hydrochloric acid,due to the decrease in the unit mass of the active material,the reversible capacity were slighrly increased.Therefore,the crystalline lithium silicate does not hinder the performance of the reversible capacity.Futher,the chemical liquid state prelithiation were adopted in this paper,the prelithaition effect and principle of Li-4,4’-dimethylbiphenyl/THF composite lithiation reagent on SiO/G composite electrode were studied.The test results show that the initial coulombic efficiency increases from 69.30%to 98.78%after the electrode is immersed in the lithiation reagent for50 minutes.And no significantly decreasing of the reversible capacity and cycle performance,indicating that this method has a good pre-insertion lithium result.However,immersion prelithiation in the liquid phase will result in expansion and deformation of the electrode,which could bring about the active material to fall off.Therefore,the prelithiation level should be controlled.It has been found that the Open Circuit Voltage（OCV）and immersing time could be the key factor to control the degree of prelithiation.To overcome the poor cycle performance of silicon oxide anode materials,this paper also explores the improvement of the cycle performance of silicon oxide materials from the perspective of pyrolytic carbon coating.By comparing with the differences of the microscopic morphology,particle size,specific surface area,and electrochemical properties of the composite materials prepared by the chemical vapor deposition coating method and the pitch coating method,it is concluded that the structure,coating uniformity and carbon content of the carbon layer are the key parameter to improve performance.The results show that the composite material prepared by chemical the vapor deposition method has a uniform and scale-like carbon coating layer with a carbon content of 4.41%,it has better cycle performance and rate performance.Therefore,the chemical vapor deposition coating method will be a promising method to promote the commercialization of silicon oxide anode materials.However the specific manufacturing process conditions need to be explored and optimized for preparing better composite materials.