Cohesion and structure in stage-1 graphite intercalation compounds

Abstract

We have developed a Thomas-Fermi theory for the structural and elastic properties of the first-stage alkali-metal graphite intercalation compounds. We use a simplified model for the electronic structure of these materials which assumes full charge transfer between the alkali metal and the graphite, no hybridization between metal and carbon states, and a uniform distribution of the donated charge on the graphite planes. We have computed lattice constants, compressibilities, shear moduli, alkali-metal diffusion constants and activation energies, and domain-wall thicknesses; general agreement with available experiments is found. These results indicate that our model of the electronic properties is consistent with known elastic and structural properties. The interplane metal-carbon interaction is mostly determined by a competition between Coulomb attraction and hard-core repulsion. The Li ion is much more compact than that of K, Rb, or Cs, which explains the higher compressibility of the Li graphite intercalation compound and the lower Li ionic mobility. The Na--C bond is found to be very soft, explaining the lack of formation of Na-intercalated graphite. The in-plane alkali-metal--alkali-metal interaction is determined almost entirely by the Coulomb interaction, and is thus relatively independent of the alkali-metal species.