Semi-empirical molecular orbital calculations on the Li ion storage states in heteroatom-substituted carbon materials

Abstract Using semi-empirical molecular orbital calculations, Li ion storage states in heteroatom-substituted carbon materials were studied. Heteroatom-substituted polycyclic hydrocarbon molecules XC 53 H 18 (XB, N, Si, P) were adopted as models of heteroatom-substituted graphene sheets. Electronic structures of the XC 53 H 18 sheets and the interactions between Li ions and the sheets were investigated. Various storage states of Li ions between two XC 53 H 18 sheets were examined. Results indicate that double Li ion layer storage states were stabilized by Si, B, and P substitution, a fact which seems advantageous to achieve large storage capacity. Moreover, we found a single Li ion layer storage state that had a larger capacity than the theoretical maximum for graphitic carbons (372 mAh/g), between two SiC 53 H 18 sheets. For the single layer storage state, the interlayer distance between two SiC 53 H 18 sheets should be small enough to prevent intercalation of electrolyte species. Such storage state is advantageous to reduce irreversible capacity. Mulliken charges of Li ions were almost 1, a fact which is expected to cause hysteresis reduction in charge–discharge cycling. The Si-substituted carbon material seems promising to use as carbon anodes.