Addressing the controversial mechanism of Na+ reversible storage in TiO2 nanotube arrays: amorphous, anatase and rutile TiO2

Titanium dioxide (TiO2), in its amorphous as well as most common polyphases including anatase, rutile, brookite and various metastable phases, is under intense investigation as anode candidate for advanced sodium-ion electrochemical energy storage. Na-ion batteries (NiB) are attracting the widespread interest of the scientific community because they may offer the most convenient alternative to current leading-edge Li-ion technology (LiB) for large-scale grid energy storage, where size does not matter and cost, safety and reliability are the most stringent requirements. In the recent years, various hypotheses have been proposed on the real mechanism of reversible insertion of sodium ions into the TiO2 structure and literature reports are often controversial in this respect. Interestingly, we experienced peculiar, intrinsically different electrochemical response between amorphous, rutile and anatase TiO2 nanotubular arrays, obtained by simple anodic oxidation, when tested as binder- and conducting additive-free electrodes in lab-scale sodium cells. In particular, after the initial electrochemical activation, anatase TiO2 showed excellent high rate capability and very stable long-term cycling performance at larger specific capacity values, thus definitely outperforming the amorphous and rutile counterparts. To reach deepen insights into the subject, materials were thoroughly characterized by means of scanning electron microscopy and ex-situ X-ray diffraction, and the mechanism of sodium ion insertion in the TiO2 bulk phases was systematically modelled by density functional theory (DFT) calculations. The results we obtained may significantly contribute to get a more systematic selection of proper active material configurations for highly efficient sodium-based energy storage systems.