Excess Li ions in a small graphite cluster

We calculate the optimized geometry and the corresponding electronic structure of Li ions doped in a small graphite cluster with dangling bonds or hydrogen terminations at the edge surrounding the cluster. The calculations imply both covalent and ionic bonds of Li ions to carbon atoms, which may be relevant to explaining the broad signal of the Li-7 NMR Knight shift spectra. Li intercalation, in particular, is possible even at the hydrogen-terminated edges. Because of the finite size effect of the cluster, the ionicity of intercalated Li ions has a large distribution of values, ranging from positive values close to that in graphite intercalation compounds to even slightly negative values, depending on the bonding geometry. We propose that the cluster edge surface plays a special role in accommodating excess Li ions in the disordered graphite system.