3D Li Diffusion in c-Li_xTiS₂ (x = 0.69 and 0.75): A Theoretical Study

Lithium diffusion in the spinel type cubic titanium disulphide (c-Li_xTiS₂, x = 0.69 and 0.75) is investigated theoretically with periodic density-functional theory (DFT) method. The calculated unit cell length and bond distances for a series of Li_xTiS₂ (0 < x < 1) compounds are in agreement with the experimental data, with the maximum deviation of +0.06% for the lattice parameter and -1.2% for the bond length. In agreement with Vegard's law, the unit cell length a is an almost linear function of x. The calculated average intercalation potentials for the series Li_xTiS₂ (0 < x < 1) range between 1.6 to 1.9 V which is in the range of experimental findings. Competing pathways for Li diffusion in c-Li_xTiS₂ (x = 0.69 and 0.75) are investigated using the climbing-image Nudged-Elastic-Band (cNEB) approach. Li⁺ ions can migrate along the '100' and '110' directions suggesting that Li⁺ diffusion in c-Li_xTiS₂ is three-dimensional (3D). The calculated activation energy values for the considered migration pathways show that Li⁺ diffusion along the '100' directions is more preferable that that along the '110' directions.