In Situ Raman Spectroscopy of Li+ and Na+ Storage in Anodic TiO2 Nanotubes: Implications for Battery Design

Anodizing is a powerful method to form electrochemically active materials, among which self-organized TiO2 nanotubes (TiNTs) are of high interest in the battery field due to a unique one-dimensional (1D) geometry offering high volume expansion tolerance and applications without binders and conductive additives. Herein, we report in situ Raman spectroscopy study under current control for a better fundamental understanding of Li+/Na+ storage in TiNTs and correlate the structural fingerprints with the electrochemical data on differential capacity plots of d(Q–Q0) dE–1. Real-time measurements revealed that the nanotubes had undergone two major phase transformations with increasing lithium content, disclosing the sequential steps of a lithium intercalation type of storage. In contrast, sodium-ion insertion induced no significant crystal structure modification but instead a slight crystallinity rupture, signifying a dominant nondiffusion-limited capacitive type of storage. The insight into the charge storage in a 1D material is mandatory for further scale-up of the nanotubes formation and their application as negative electrodes in a lithium-ion battery.