Theoretical Investigation of Migration Pathways for Li Diffusion in h-LiTiS2

Lithium diffusion in highly lithiated hexagonal titanium disulphide (h-LixTiS2, x = 0.88, 1.0) is investigated theoretically with periodic quantum-chemical methods. The calculated lithiation energies confirm that Li preferentially occupies the octahedral site rather than the tetrahedral site. Surprisingly, uncorrected density-functional theory (DFT) methods give better agreement with experiment for the structural parameters than the dispersion-corrected DFT-D approaches. Among the considered point defects, V-Li, V-Ti, and Ti-i, Li point defects are thermodynamically preferred in h-LixTiS2. A moderate relaxation is observed for the atoms surrounding the Li defect or Ti defect site, whereas a pronounced relaxation of the nearest neighboring atoms of a Ti Frenkel defect occurs. Competing pathways for Li diffusion in h-LixTiS2 are investigated using the climbing-image Nudged-Elastic-Band (cNEB) approach. Li+ ions can migrate within the crystallographic ab plane either in a direct pathway through shared edges of neighboring octahedra or via vacant tetrahedral sites. The possibility of three-dimensional Li+ diffusion along the c direction is investigated via inclusion of Ti point defects and Ti Frenkel defects.