In-Situ Raman Spectroscopy of Li⁺ and Na⁺ Storage in Anodic Nanotubes

Anodizing is a high-voltage electrochemical conversion process that forms barrier-type oxide layers or self-organized nanoporous/nanotubular structures. So far, the Al 2 O 3 -like nanopores and TiO 2 -like nanotubes could be successfully synthesized on many valve metals and alloys. Recently, it become possible to form nanotubular structures on iron and consequently control the ratio of hematite (Fe 2 O 3 ) to magnetite (Fe 3 O 4 ) by subsequent thermal treatment. 1 Electrochemically active nanotubes, such as self-organized TiO 2 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 an in - situ Raman spectroscopy study under current control for a better fundamental understanding of Li + /Na + storage in nanotubes and correlate the structural fingerprints with the electrochemical data on differential capacity plots of d( Q – Q 0 ) d E –1 . 2 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 in relation to other nanostructures such as nanoparticles 3 will be discussed. 1. L. Fadillah, D. Kowalski, M. Vincent, C. Zhu, S. Kitano, Y. Aoki and H. Habazaki, ACS Applied Materials & Interfaces , 2023, 15, 52563-52570. 2. M. Vincent and D. Kowalski, ACS Applied Nano Materials , 2023, 6, 6528-6537. 3. M. Vincent, S. S. Kumar and D. Kowalski, Electrochimica Acta , 2023, 469, 143161.