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

Using semi-empirical molecular orbital calculations, Li ion storage states in heteroatom-substituted carbon materials were studied. Heteroatom-substituted polycyclic hydrocarbon molecules XC53H18 (XB, N, Si, P) were adopted as models of heteroatom-substituted graphene sheets. Electronic structures of the XC53H18 sheets and the interactions between Li ions and the sheets were investigated. Various storage states of Li ions between two XC53H18 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 SiC53H18 sheets. For the single layer storage state, the interlayer distance between two SiC53H18 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.