Theoretical Study on the Correlation between the Nature of Atomic Li Intercalation and Electrochemical Reactivity in TiS2 and TiO2

The electronic structures of LiTiS2 and LiTiO2 (having α-NaFeO2 structure) have been investigated using discrete variational Xα molecular orbital methods. The α-NaFeO2 structure is the equilibrium structure for LiCoO2, which is widely used as a commercial cathode material for lithium secondary batteries. This study especially focused on the charge state of Li ions and the magnitude of covalency around Li ions. When the average voltage of lithium intercalation was calculated using pseudopotential methods, the average intercalation voltage of LiTiO2 (2.076 V) was higher than that of LiTiS2 (1.958 V). This can be explained by the differences in Mulliken charge of lithium and the bond overlap population between the intercalated Li ions and anion in LiTiO2 as well as LiTiS2. The Mulliken charge, which is the ionicity of Li atom, was approximately 0.12 in LiTiS2, and the bond overlap population (BOP) indicating the covalency between Ti and S was about 0.339. When compared with the BOP (0.6) of C−H, which is one of the most famous example of covalent bonding, the intercalated Li ions in LiTiS2 tend to form a quite strong covalent bond with the host material. In contrast, the Mulliken charge of lithium was about 0.79, which means that Li is fully ionized and the BOP, the covalency between Ti and O, was 0.181 in LiTiO2. Because of the high ionicity of Li and the weak covalency between Ti and the nearest anion, LiTiO2 has a higher intercalation voltage than LiTiS2.