Spinel-Type Sodium Titanium Oxide: A Promising Sodium-Insertion Material of Sodium-Ion Batteries

Spinel-type Li4Ti5O12 (LTO) is expected as a safe and stable negative electrode material of sodium-ion batteries (SIBs). However, its small lattice volume is not sufficient as the Na+ intercalation host. Alternatively, a Na-substituted spinel titanium oxide phase (Na3LiTi5O12, NTO) with a large lattice volume should be promising. In our previous work, we discovered that such a NTO phase is partially generated in each LTO electrode particle during specific SIB cycling. In the present report, we examined the basic properties of this NTO phase in the NTO/LTO composite electrode. The in-situ XRD measurement of the Na-insertion process revealed that smooth Na+ insertion preferentially occurs in the NTO region in the composite electrode because of small lattice expansion in the formation of the Na-inserted NTO phase as well as the smooth Na diffusion in the NTO phase. Thus, the composite electrode containing the NTO phase showed superior rate properties of Na insertion compared to the pristine LTO electrode and showed excellent cycle performance with high efficiency. In addition, the unique stability of the spinel-type NTO phase was examined by the density functional theory calculation, and the effect of the mixed transition-metal (TM)-Li-O network at 16d and 32e sites in lithium-TM oxides was discussed so as to keep the spinel structure for the substitution of 8a-site Li ions by other cationic ions.