Study On Rate Performance Of Spherical Graphite Anode Material For Lithium Ion Battery

Natural spherical graphite has been studied as a promising alternative to thecommercial graphite electrode materials. In order to improve the initialcharge-discharge efficiency and the rate performance, micro-expanded andcarbon-coated spherical graphite materials were prepared. Structural andmorphology changes were characterized by using SEM and XRD, respectively;CV, EIS and galvanostatic charge-discharge tests were carried out to evaluate theelectrochemistry performance of the as-prepared graphite electrode materials.Micro-expanded spherical graphite was prepared at60℃by using hydrogenperoxide as the oxidant and formic acid as the intercalation agent. Then theexpandable graphite was heated at800℃for2hours. The influence of reactiontime was investigated. After micro-expansion, no significant changes in thestructure of graphite and d002interlayer distance are found. Micro-expandedgraphite prepared through liquid phase reaction for4hours shows the bestelectrochemical performance. The reversible capacity is325.5mAh/g，235.2mAh/g，109.1mAh/g，38.0mAh/g at the rate of0.1C,0.5C,1C,2C and5C,respectively. And the initial efficiency was80%at the cycle rate of0.1C.Carbon-coated graphite was prepared using pitch as the carbon coatingprecursor via both liquid phase impregnation and solid phase reaction rout es. Theimpacts of preparation routes, coating ratio and carbonization temperature on theelectrochemical properties of carbon-coated graphite were investigatedsystematically. A coating ratio of5mass%carbon and a carbonized temperatureof900℃have been identified as the optimized synthesis condition for bothliquid and solid routes. When the graphite is carbon coated via the liquid route,the reversible capacity can reach354and86.8mAh/g with the cycle rate of0.1and5C, respectively.The electrochemical performances of carbon-coated graphite materialsprepared by two methods were compared, as well. Our result suggests that thecarbon-coated graphite prepared via a liquid phase route possesses a higherreversible capacity at0.1C than the carbon-coated graphite prepared by using solid method. However, no significant difference has been found regarding therate capability and cycle performance.Carbon-coated mild expanded graphite is fabricated using liquidimpregnation method. The result demonstrated that the rate performance can begreatly enhanced by combining the micro-expansion and the carbon coatingprocesses. The reversible capacity of carbon-coated expandable graphite canreach370and114.6mAh/g with the cycle rate of0.1and5C, respectively.