Structural stability of Na-inserted spinel-type sodium titanium oxide

Highlights: • A Na-inserted structure of spinel-type sodium titanium oxide (NTO) was examined. • The XRD Rietveld refinement analysis of Na-inserted NTO electrode was performed. • Na position was changed from 8a-site to 16c-site in spinel lattice by Na-insertion. • Na site changing provoked relaxation of Ti–O and Na–O atomic distance of NTO. • The crystal stability of NTO lattice should be enhanced by Na-insertion reaction. -- Abstract: Spinel-type sodium titanium oxide (Na₃LiTi₅O₁₂, NTO), which has an analogous structure to Li₄Ti₅O₁₂ (LTO), is prepared as a single-phase material for the negative electrode of Na-ion batteries. A superior Na insertion and extraction cycle performance is achievable based on its similarity with the LTO reaction mechanism. However, the detailed structure of the NTO material in the Na insertion state remains obscure. Consequently, the crystallographic features of the Na insertion mechanism have not been sufficiently elucidated. In this study, the structural analyses of NTO and Na-inserted NTO (Na-NTO) electrodes were performed via X-ray diffraction and Rietveld refinement. The Na occupation site of the NTO spinel lattice was altered from the oxygen tetrahedral to the octahedral site by the electrochemical Na insertion reaction. The lattice constants of NTO and Na-NTO were refined for a_NTO = 8.73 Å and a_Na-NTO = 8.84 Å; approximately 1% lattice expansion, which was caused by the local compressions of the Ti–O atomic distance existing in the NTO and their relaxation in the Na-NTO, was confirmed by Na insertion. Notably, Ti–O atomic distances in NTO are quite heterogeneous with values of 1.8 Å and 2.2 Å, while they are homogenized to 2.0 Å, 2.1 Å, and 2.2 Å in the Na-NTO lattice. Furthermore, the oxygen position in the NTO lattice is modified to enhance lattice symmetry by Na insertion, similar to the Li insertion reaction of the LTO lattice. Thus, the Na-NTO lattice structure is considerably more stable than the NTO lattice, which promises stable Na insertion and extraction cycle performances for NTO materials toward Na-ion battery utilization.