Tracking the Effect of Sodium Insertion/Extraction in Amorphous and Anatase TiO2 Nanotubes

We report a mechanistic investigation of the electrochemical behavior of TiO2 nanotubes (NTs) in amorphous and anatase phases during sodiation/desodiation. The local structure variations of these two host structures upon Na+ uptake/release are comparatively examined by X-ray absorption near edge structure (XANES) at the Ti K and L, O K, and Na K edges. Upon Na insertion/extraction, the surface/near surface XANES analysis at the Ti K edge suggests that the surface/near surface of TiO2 NTs in both phases is partially reduced to metallic Ti from the unstable sodium titanate formed at the electrolyte/electrode interface, and the sodiation of amorphous NTs is more effective than anatase ones. More importantly, it further reveals that irreversible phase transformations from pure amorphous and anatase TiO2 to amorphous sodium titanate take place on the NT surface/near surface after the first cycle of discharge/charge. The bulk XANES analysis at the Ti L edge demonstrates that irreversible phase transformation also proceeds in the bulk of these two phase structures, where, however, the formation stable amorphous sodium titanate is observed. All the findings are corroborated by energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), and XANES analysis at the O K and Na K edges.