Effects of Off-Stoichiometry of LiC₆ on the Lithium Diffusion Mechanism and Diffusivity by First Principles Calculations

Lithium diffusion in a stage-1 structure of LiC 6 has been theoretically investigated from first principles on the basis of transition state theory. The calculated chemical diffusion coefficient of lithium atoms under the lithium excess conditions is much larger than that under the lithium deficient conditions, e.g., 2 × 10 −7 cm 2 /s vs 1 × 10 −10 cm 2 /s at room temperature. The calculated activation energies of the chemical diffusion coefficients under the lithium excess and deficient conditions are also different, e.g., 0.30 and 0.49 eV, respectively. The fast diffusion under the lithium excess conditions is attributed to the “interstitialcy” mechanism. These results mean that a small deviation of lithium composition from the exact composition of LiC 6 causes a large difference in the lithium diffusion coefficient. The off-stoichiometry can be the major reason why the experimental diffusion coefficients and activation energies are widely scattered.