Molecular orbital calculations on lithium absorption in boron- or nitrogen-substituted disordered carbon

In order to clarify the reason why boron-substituted disordered carbons can store more Li atoms than pristine carbons, we employed several polyaromatic hydrocarbons as model clusters for disordered carbons and investigated the effect of boron and nitrogen substitutions on the stable structures and electronic properties of model clusters, by using a semiempirical molecular orbital method. Boron substitution creates an electron acceptor level in a lower energy region than that for the pristine carbon. This lower acceptor-level accepts electrons from the absorbed Li more easily, so that the Li absorption energy for the boron-substituted cluster is much larger than that for the pristine carbon. On the other hand, for the nitrogen-substituted clusters, the electron acceptor level is almost the same as that for the pristine carbon, so that the Li absorption energy is not enhanced by nitrogen-substitution. These results suggest that the larger Li-storage in boron-substituted disordered carbons is related to the creation of a lower acceptor-level caused by boron substitution.