Diffusion of lithium in highly oriented pyrolytic graphite at low concentrations and high temperatures

Graphite forms intercalates with a variety of elements and molecules because of the absence of chemical bonding between the hexagonal layer planes. The formation of stage-one intercalate ${\mathrm{LiC}}_6$ increases the interlayer distance from 0.335 to 0.371 nm. Although the diffusion of Li into graphite precedes intercalate formation, no data exist for Li transport at low Li concentrations. The release kinetics of Li was measured from highly oriented pyrolytic graphite (HOPG) at Li concentrations between 1 and 10 ppm and temperatures in the range from 1000 to 1300 K, and diffusion coefficients for the isotopes ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ were determined. The Li transport in HOPG is strongly anisotropic, and the contribution to the total release of the diffusion perpendicular to the graphite planes is negligible. The Li diffusion coefficients in the direction of the graphite planes are given by D=(3×${10}^3$ ${\mathrm{cm}}^2$/s) exp(-1.83 eV/kT). The D value at 1070 K is 7.6×${10}^{\mathrm{-}6}$ ${\mathrm{cm}}^2$/s.