Preparation Of SiO_x/C Nanocomposites And Their Electrochemical Performance As Anodes For Li-ion Batteries

Si is considered to be the most promising anode material for lithium ion batteries due to its highest theoretical capacity,abundant resources and moderate lithiation/delithiation potential.However,the large volume change and low conductivity experienced during the cycling process seriously affect the cycling stability and rate performance of the electrode.As an alternative to silicon,silicon oxide?SiO_x,0<x?2?has attracted the attention of researchers due to its advantages of smaller volume expansion and easy synthesis in liquid phase.In this work,a series of SiO_x/C nanocomposites with different morphology and microstructure were synthesized by using siloxane with different molecular structure as silicon source,and the electrochemical performance of the composites as lithium ion battery anode were studied.The following two research contents are carried out specifically:Firstly,the effects of siloxane with different molecular structure as silicon source on the electrochemical properties of SiO_x/C nanocomposites were studied.Using methyl vinyldiethoxy silane,vinyl trimethoxy silane,dimethyl diethoxy silane,3-mercaptopropyl trimethoxy silane,phenyltriethoxy silane and diethoxymethyl phenylsiloxane as the silicon source,respectively,resorcinol and formaldehyde as polymerization monomer of extra carbon source precursor,SiO_x/C nanocomposites with different morphology and structure were synthesized through coassembly,sol-gel method and high temperature heat treatment.The results showed that the molecular structure of siloxane had important influence on the micromorphology,structure and composition of composite materials.The number of siloxy groups in siloxane affected the x value of SiO_x in the product,and the number of hydrocarbon groups affected the ratio of SiO_x to C in the product,thus showing differences in capacity,cycling performance and rate performance.The electrochemical test showed that vinyltrimethoxysilane was the optimal siloxane for the synthesis of SiO_x/C nanocomposites.Secondly,the optimized synthesis and electrochemical properties of SiO_x/C nanocomposites prepared with vinyl trimethoxysilane as silicon source were investigated.The effects of the ratio of alcohol to water,surfactant dosage and extra carbon content on the electrochemical performance of products were studied.The results showed that when the ratio of alcohol to water was 2:5,the dosage of surfactant CTAB was 0.2 g,and the dosage of resorcinol was 0.2 g,the as-prepared SiO_x/C nanocomposite presented trollflower-like aggregated by interconnected small spheroid and it has the best electrochemical performance,and it had the optimized electrochemical performance.And the trollflower-like SiO_x/C nanocomposite exhibited excellent cyclic stability and rate performance.At a current density of 800mA g^-1,the capacity retention of the electrode was 82.5%verus the second cycle after397 cycles,and the coulombic efficiency rapidly increased to more than 99%after few cycles except for an initial coulombic efficiency of 72.9%.The excellent electrochemical performance of the trollflower-like SiO_x/C nanocomposite was attributed to the highly homogeneous distribution of carbon and SiO_x,which not only accelerated the electron transfer rate,but also alleviated the volume expansion of Si during cycling.