Applications of graphite intercalation compounds

The properties of graphite intercalation compounds (GIC's) are discussed with respect to possible applications. Five families of intercalates give high electrical conductivity to GIC's: pentafluorides leading to high conductivity, 10⁸ S/m (higher than metallic copper); metal chlorides; fluorine and alkali metals with bismuth giving relatively high conductivity of the order of 10⁷ S/m plus stability in air; and residual halogens showing extremely high stability under severe conditions, though the conductivity is only of the order of 10⁶ S/m. Electrodes of different GIC's have been tried in primary and secondary batteries, where their characteristics are high electrical conductivity and easy diffusion of electrochemically active species between the graphite layers. Primary lithium batteries of a covalent graphite fluoride are now widely used commercially. Secondary batteries using different host graphites and intercalates give interesting results. Large amounts of hydrogen can be stored in the functional space in alkali metal-GIC's. The same GIC's show high coefficients of isotope separation of hydrogen at liquid nitrogen temperature. The structure and texture of the host graphite play a decisive role in the absorption and separation behaviors of GIC's. Exfoliated graphite prepared by rapid heating of GIC's or their residue compounds leads to flexible graphite sheets which have great industrial applications. Some problems connected with the production and use of these sheets are discussed.