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.