Properties of spinel-type Ti-Li-M composite oxides (M = Li, Na, Cu, and Ag) predicted by density functional theory

Spinel-type titanate is an important material already being used as a stable anode for Li-ion batteries. In addition, spinel titanate shows superconducting properties upon tuning the amount of Li⁺-doping; hence, research on magnetic and superconducting materials has been conducted. However, it is believed that only the tiny Li⁺ monocation can occupy the 8a sites due to its small voids and the charge valence with Ti cations. In recent years, new spinel-type titanium oxides have been discovered, in which 8a sites are occupied by Na⁺ or Ag⁺. Although the application of these new compounds to catalyst and electrode materials has been attempted, the effect of 8a site monocations on the physical properties of spinel-type titanium oxide is unclear. In this study, to systematise the effects of 8a-site monocations on the Ti-O framework, theoretical calculations based on density functional theory (DFT), such as GGA, GGA+U, and hybrid-DFT, were performed for the electronic structures and geometric stabilities of four spinel-titanium oxides: LTO (8a sites occupied by Li⁺), NTO (8a sites occupied by Na⁺), CTO (8a sites occupied by Cu⁺), and ATO (8a sites occupied by Ag⁺). Furthermore, to verify the effect of the partial substitution, Li⁺, Na⁺, Cu⁺, and Ag⁺ doping of LTO, NTO, CTO, and ATO was also investigated. Throughout these calculations, the performance of spinel-type titanates can be categorised by (1) the magnitude of O-displacement and (2) the orbital correlation between the Ti-O framework and the 8a site cations. By appropriately selecting cations, the spinel titanates can be applied to battery materials, catalysts, optical materials, photocatalysts, and precursors to these materials.