A Study On The Improvment Of Natural Graphite As The Lithium-Ion Battery Anode Materials

Lithium-ion batteries are green and rechargeable secondary power, they are widely used in mobile phones, computers and other equipments, due to high working-voltage, large energy density. The emphases on lithium-ion batteries are further to improve their performances and reduce their costs. Carbon materials are one of the available approaches that enhance the performances and decrease the costs of lithium-ion batteries, since there are a variety of species of carbon materials and their properties improvement would be effective to upgrade the anode. The thesis was concerned on the structures and performances of anode materials, and hope to find ideal materials.In the thesis, the heat-treated natural graphite was analyzed in structures and electrochemical properties through XRD, TEM, SEM, BET, Laser particle diameter and electrochemical test, the electrochemical performances was affected by the factors such as crystal structure, impurity content, particle diameter and BET specific surface area. The compositions and surface characteristics of SEI film were analyzed by FTIR, XRD and SEM. The structures and electrochemical performances were investigated by XRD, TEM and electrochemical test.It is found that heat-treatment could effectively enhance the degree of graphatizing, graphite crystallite size, average graphite diameter of anode materials, and it also decreased BET specific surface area, accordingly improved greatly the charge/discharge performance. With the increase of heat-treatment temperature, electrochemical performance was further improved. The reversible capacity of 800 C-treated graphite was 320 mAh/g, the initial coulomb efficiency was 80 %. Compared with the other graphite, BET specific surface area of 800 癈-treated graphite was the smallest, and the average graphite diameter was the largest, caused to decrease the irreversible capacity. Besides, heat-treatment at 800"C decreased the numbers of active sites of graphite surface, the decrease of the numbers of the active sites reduced greatly the irreversible capacity. Furthermore, the sites, which were originally impossible to intercalate lithium-ion, could intercalate lithium-ion, and increased the reversible capacity consequently.With the FTIR analysis of the SEI film, there were several components, such as (CH2OCO2Li)2, Li2CO3, CH2OCO2Li. With the SEM analysis of the SEI film, it suggested that active lithium reacted with electrode, giving out CH2=CH2, CO2, HF and other gases, reversible capacity decreased with the increase of the current density. In the discharge/charge process, the structure of graphite was changed. In the intercalate/deintercalate process, LiC18, LiC12, LiC6 and other graphite-lithium compounds were produced, and these graphite-lithium compounds were transformed each other in different voltages.Through coating pyrolytic coke on CoSbs, the loss of irreversible capacity was decreased. The initial loss of irreversible capacity decreased from 62 % to 42 %. Compared with CoSbs, the cycling stability of the coke-coated CoSba composite was enhanced. After twenty cycles, the reversible capacity of the coke-coated CoSbs composite enhanced from 160 mAh/g to 50 mAh/g.