Study On Modification Technologies Of High-rate Graphite Anode Materials For Lithium-ion Batteries

Graphite has become the mainstream anode material for lithium-ion batteries due to its advantages of low voltage platform,good cycle stability,low price,wide source,non-toxic and harmless,but its rate performance is increasingly difficult to meet the current rapid growth of the rapid charge market demand.Because of this,the artificial graphite was improved by hard carbon coating and alumina/hard carbon bilayer coating to prepare high-rate graphite anode materials in this paper.The effects of heating rate,holding time,heat treatment temperature and coating ratio on modified graphite were investigated,and the morphological structure and electrochemical properties of hard carbon coated graphite（AG@HC）and alumina/hard carbon bilayer coated graphite（AG@HC@Al₂O₃）were analyzed in detail.The main research contents and conclusions of this paper are as follows:（1）Using phenol-formaldehyde resin as the carbon source,the artificial graphite was coated with hard carbon by the method of liquid phase coating and high temperature pyrolysis,and the AG@HC anode material was successfully prepared.The results show that hard carbon coating can significantly increase the disorder and specific surface area of the material,thereby effectively improving the rate performance of graphite anode materials.When the ratio of graphite to phenol-formaldehyde resin was 1:0.1,and the heating rate,heat treatment temperature and holding time were 2℃min^-1,800℃and 3 h,respectively,the prepared AG@HC anode material had better electrochemical properties.Its specific capacities at the rate of 0.2 C,0.5 C,1 C,2 C,3 C and 5 C were 353.2 mAh g^-1,328.6 mAh g^-1,285.4 mAh g^-1,218.4 mAh g^-1,128.7 mAh g^-1and 58.9 mAh g^-1,respectively,in which the corresponding reversible specific capacity at 1 C was 54.1 mAh g^-1 higher than that of graphite raw materials.After 150 cycles at 1 C current density,the AG@HC still had a reversible specific capacity of 186.5 mAh g^-1,which was 47.1 mAh g^-1 higher than that of graphite raw materials.（2）On the basis of AG@HC,the aluminum nitrate nonahydrate was used as the coating source,and the alumina coating layer was obtained on the surface of the AG@HC particles by thermal reaction,and the AG@HC@Al₂O₃ anode material was successfully prepared.The research results show that the alumina coating can act as an artificial SEI film and reduce the specific surface area,make up for the loss of the first Coulomb efficiency caused by the hard carbon coating,and more importantly,can improve the surface state of the material and further improve the rate capability of the material.When the alumina coating ratio is controlled to 3%,the initial coulombic efficiency of the material is 2.9%higher than that of AG@HC,and its rate performance is significantly improved.The discharge specific capacities of the AG@HC@Al₂O₃anode material at the rate of 0.2 C,0.5 C,1 C,2 C,3 C and 5 C were 349.4 mAh g^-1,336.9 mAh g^-1,318.5 mAh g^-1,259.8 mAh g^-1,114.2 mAh g^-1 and 69.2 mAh g^-1,respectively,Among them,the corresponding reversible specific capacity at 1 C was increased by 87.2 mAh g^-1 compared with graphite raw materials.AG@HC@Al₂O₃can still maintain a reversible specific capacity of 281.3 mAh g^-1 after 150 cycles at 1C current density,which was 141.9 mAh g^-1 higher than that of graphite material,and the corresponding capacity retention rate was up to 94.4%.