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

Abstract

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.