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

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) a 8(Li1Ti5)16dO192'+ 3Na+ + 3e- <-> (Na)16c(Li1Ti5)16dO' 3 12 6 12 192 . 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. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The reaction mechanism of a spinel-type sodium titanium oxide was investigated using in situ XRD, showing a solid-solution reaction during Na insertion/extraction process. The XRD profile evolution was found to be highly reversible, leading to high cycling stability of the NTO electrode. The movement of Na ions between 8a and 16c sites was confirmed to contribute to the changes in XRD profiles.