Modification And Surface Treatment Of Carbon Materials Used As Anode Of Lithium Ion Secondary Battery

An artificial graphite (AG), obtained from graphite electrode producing process, was studied as anode of lithium ion secondary battery (LIB) for the first time in the dissertation. The innovative surface chemical deposition coating and chemical film formation on AG treated by mild oxidation were respectively applied to AG surface modification, and the corresponding samples were obtained successfully. Their surface structure and groups, negative performance and lithium ion migration in them were deeply studied by utilizing electrochemical and modern physical methods. Moreover, the mesocarbon microbead (MCMB) was heated with catalyst CoCl2 in low temperature for the first time in order to obtain high capacity and lower irreversible carbon material used as anode of LIB. The mechanism of surface chemical deposition coating with coal-tar pitch is proved as follow: the lamellar molecules, formed in pitch pyrolysis and polymerization, continuously adsorbed on AG surface and turned larger. The first discharge specific capacity of AG with coating treatment is 305.4mAh/g, which is higher than that 255.5mAh/g of AG, and its relevant coulombic efficient increased from 80.8% to 90.2%. And the 50th discharge capacity also increased from 154.1mAh/g to 302.1mAh/g. Meanwhile, AG with pitch-coated treatment could work better than untreated AG with electrolyte dissolved in PC solvent. The charge/discharge curves as a function of the state of charge indicate that irreversible capacities loss was caused by solvent decomposition on AG surface and intercalation into irreversible carbon sites inside the AG. The results indicate that surface chemical deposition coating process could effectively reduce irreversible capacity loss and could reduce inner lattice limitation. Mild oxidation and lithium carbonate treatment with AG successfully adjusted the type and ratio of surface groups and formed chemical SEI film. The results of the first 50th charge and discharge tests proved that the method could reduce the first irreversible capacity and have good re-cycling performance. Impedance spectra measurements show that the impedance spectra were composed of nyquist semi-circle and a line with 45℃ slope, and Li~+ migration in the AG working electrode can be divide into two steps, Li~+ charge-transfer in the film and the diffusion of Li+ between AG layers, the latter is the controlling steps. Finally, the Raman spectrum and XRD analysis showed that catalytic heat treatment for MCMB at low temperature could effectively enlarge carbon micro-crystal dimension. And charge and discharge tests identified that catalytic heated-treatment could let MCMB keeping higher discharge capacity and could enhance its coulombic efficient and cycling performance.