Addressing the Controversial Mechanism of Na+ Reversible Storage in TiO2 Nanotube Arrays: Amorphous versus Anatase TiO2

Among a wide range of applications (e.g., dye-sensitized solar cells, lithium-ion batteries, supercapacitors, etc.), in its amorphous as well as most common polyphases including anatase, rutile, brookite and various metastable phases, TiO2 is under intense investigation as anode candidate for advanced electrochemical energy storage based on the sodium-ion (Na-ion) technology. 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 stringent requirements [1,2]. 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, when tested as binder- and conducting additive-free electrodes in lab-scale sodium cells, we experience intrinsically different and peculiar electrochemical response between amorphous and anatase TiO2 nanotubular arrays obtained by simple anodic oxidation. In particular, after the initial electrochemical activation, anatase TiO2 shows excellent high rate capability and very stable long-term cycling performance at larger specific capacities, thus definitely better response as compared to the amorphous counterpart. To reach deepen insights into the subject, materials are 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 is systematically modelled by density functional theory (DFT) calculations, which may significantly contribute to get a more systematic selection of proper active material configurations for highly efficient sodium-based energy storage systems.