A theoretical study on storage states of Li ions in carbon anodes of Li ion batteries using molecular orbital calculations

Semi-empirical molecular orbital calculations were carried out to clarify storage states of Li ions in amorphous carbon anodes of Li ion batteries. Storage states of Li ions between two graphene sheets were investigated and a favorable structure for a carbon anode to produce large reversible and small irreversible capacities is discussed. A polycyclic hydrocarbon molecule, C54H18, was used as a model of a graphene sheet. Relations between the interlayer distance of two graphene sheets and the storage state of Li ions were investigated, and preferable interlayer distances for specific numbers of Li ions were estimated. In particular, storage states with all U ions on the basal area were treated, because the amount of basal carbons should be larger than that of edge carbons. The charge distribution of Li ions was also investigated. Calculated results suggested that a storage state in which a double Li ion layer was formed was preferable to achieve a larger capacity than the theoretical maximum capacity of graphitic carbons (372 mAh/g) and to reduce hysteresis in the charge-discharge process. Moreover, suitable distance between edges of graphene sheets to prevent the intercalation of electrolyte species was discussed. A recommended structure of carbon anodes suitable for the double Li ion layer storage and prevention of the intercalation of electrolyte species is proposed. (C) 2003 Elsevier Ltd. All rights reserved.