THE DEPENDENCE OF THE PERFORMANCE OF LI-C INTERCALATION ANODES FOR LI-ION SECONDARY BATTERIES ON THE ELECTROLYTE SOLUTION COMPOSITION

The behavior of synthetic graphite and petroleum coke electrodes was investigated in a variety of Li salt solutions. These solvents included methyl formate, gamma-butyrolactone, tetrahydrofurane, ethylene, dimethyl and diethyl carbonates. The salts included LiAsF6, LiClO4, LiBF4, LiPF6 and LiSO3CF3. The dependence of the stoichiometry and reversibility of Li intercalation of these carbon electrodes in repeated cycling was rigorously investigated. Surface sensitive FTIR spectroscopy was applied in conjunction with electrochemical techniques. It was found that the performance of these Li-C anodes in terms of capacity and cycle life is determined by the passivating ability of the surface films. These are irreversibly formed by the reduction of the solution species on the carbon surface during the first Li-C intercalation step. It was possible to modify this important surface chemistry and thus obtain an improved anode performance by the choice of the appropriate solvent mixture, salt, additives, and the temperature in which the surface films are built up.