First-Principles Study of Spinel MgTiS2 as a Cathode Material

Spinel intercalation hosts are well known to to facilitate high rate capability and high voltage Li-ion batteries. A recent experimental study has shown that Mg can reversibly intercalate in spinel TiS2, demonstrating the viability of Li intercalation host crystal structures for Mg-ion batteries [ Sun, X. ; Energy Environ. Sci. 2016, 9, 2273−2277 ]. We report on a first-principles statistical mechanics study of Mg insertion into spinel TiS2, accounting for occupancy on both octahedrally and tetrahedrally coordinated interstitial sites. In contrast to Li-containing spinels, we predict mixed octahedral and tetrahedral site occupancy at nondilute Mg concentrations consistent with the recent experimental study of Sun et al. The onset of mixed occupancy is correlated with an increase in the spinel volume upon Mg insertion, which is more pronounced in Mg x TiS2 than in its Li counterpart. The results in this study suggest that the degree of mixed occupancy could be controlled through the volume of the host with addition of electrochemically inactive species.