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 polyaromati c hydrocarbons as model clusters for disordered carbons and investigated th e effect of boron and nitrogen substitutions on the stable structures and e lectronic 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 cal bon. This lower acceptor -level accepts electrons from the absorbed Li more easily, so that the Li a bsorption energy for the boron-substituted cluster is much larger than that for the pristine carbon. On the other hand, for the nitrogen-substituted c lusters, the electron acceptor level is almost the same as that for the pri stine carbon, so that the Li absorption energy is not enhanced by nitrogen- substitution. These results suggest that the larger Li-storage in boron-sub stituted disordered carbons is related to the creation of a lower acceptor- level caused by boron substitution. (C) 1999 Elsevier Science Ltd. All righ ts reserved.