LITHIUM INSERTION IN BALL-MILLED GRAPHITE

The effects of mechanical milling on the microstructure, morphology an d electrochemical performance of graphite powders with respect to lith ium insertion are studied. After 150 h of ball-milling, the well-graph itized graphite has been pulverized into small particles with a size o f about 50 nm, in which there are a lot of excess vacancies, microcavi ties and metastable carbon interstitial phases with sizes around 13 An gstrom. Due to the large surface energy, the merging of single particl es is favoured and results in the formation of agglomerates with avera ge size about 1 mu m. Voids are formed among the agglomerated particle s. The ball-milled graphite shows reversible specific capacity for lit hium of 700 mA h g(-1) (L1.88C6) with large hysteresis. The large reve rsible capacity is due mainly to Li doping at vacancies, microcavities (or at the edges of the metastable carbon interstitial phase) and voi ds. The bonding change between the interstitial carbon and the carbon in the aromatic plane that is induced by insertion of Li atoms leads t o hysteresis. During charge-discharge cycles, the reversible capacity above I V decreases rapidly, which may be due to some vacancies and mi crocavities being annihilated by moveable and some bound interstitial carbon and to electrolyte penetrating gradually into voids formed by a gglomerated particles during the Li insertion and desertion process. ( C) 1998 Elsevier Science S.A. All rights reserved.