Excess Li ions in a small graphite cluster

Abstract

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 7 Li 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.

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“…For our calculations we adopted a hexagonal C 54 H 18 cluster in which 18 carbon (C) atoms are terminated by hydrogen atoms at the edge, and the other 36 C atoms are located in the interior region of the cluster, as shown in Fig. 1.…”

“…The edge state which is partially occupied by electrons at the Fermi level plays an important role in accepting electrons. In fact, we calculated the electronic structure of a Li ion at the edge of a hydrogenterminated nanographite in a previous report 18 (see also Ref. 5) and found that the Li ion has an ionicity of +0.6e at the hydrogen-terminated edge and that the donated electron to the nanographite cluster is distributed over the edge carbon sites.…”

“…We previously found 18 that the total charge transferred to the graphite cluster has large variations in value for different Li configurations, and these differences in the charge transfer associated with the interior and edge sites give rise to the two peaks that are observed in the 7 Li NMR experiment. 1,4,6,7 In Sec.…”

“…5) and found that the Li ion has an ionicity of +0.6e at the hydrogen-terminated edge and that the donated electron to the nanographite cluster is distributed over the edge carbon sites. 18 Furthermore, Li-Li bonding on a finite-size cluster can even give negative ionicity by occupying the molecular orbitals corresponding to Li 2s orbitals. 18 However, in the previous paper we did not put as many Li ions on the cluster as is possible.…”

“…18 Furthermore, Li-Li bonding on a finite-size cluster can even give negative ionicity by occupying the molecular orbitals corresponding to Li 2s orbitals. 18 However, in the previous paper we did not put as many Li ions on the cluster as is possible. Thus, the subject of the present paper is the uptake capacity and the stability of Li atoms on a nanographite cluster.…”