Sodium insertion and de-insertion mechanism of spinel-type sodium titanium oxide studied by in situ XRD

Abstract The reaction mechanism of a spinel-type sodium titanium oxide (Na3LiTi5O12) was investigated using in situ XRD during the Na insertion/extraction process. The XRD profiles gradually shifted to lower and higher angles during the sodiation and de-sodiation processes, respectively, while maintaining a spinel-type structure and no new peaks were observed, indicating a solid-solution reaction according to the following reaction; (Na) 3 8 a (Li1Ti5)16dO 12 192 i + 3Na+ + 3e− ↔ (Na) 6 16 c (Li1Ti5)16dO 12 192 i . The evolution of the XRD profile was confirmed to be highly reversible, which explains the high cycling stability of the NTO electrode. There are two possible sites for Na to occupy: 8a and 16c. The Na ion initially occupies only the 8a site, as confirmed by the 220 peak. The intensity of the 220 peak gradually decreased with increasing SOC and was hardly detected at an SOC of 50%, indicating that all the Na ions at the 8a sites moved to the 16c sites. Topological analysis combined with XRD revealed that the Na ion at the 8a site is stable with one other Na ion at an adjacent 16c site despite the interatomic distance being too short.