Understanding the Behavior and Mechanism of Oxygen-Deficient Anatase TiO₂ toward Sodium Storage

TiO₂ has drawn increasing research attention as negative electrode material in sodium ion battery because of its natural abundance, low cost, nontoxicity, and facile preparation. Despite tremendous studies carried out, the sodium storage mechanism is still under discussion, and the electronic and local structures of TiO₂ during sodiation/desodiation process are not well understood either. Herein, we reported a mechanism study of graphene-supported oxygen-deficient anatase TiO₂ nanotubes (nanowires) as the negative electrode material for sodium ion batteries. Different from the previous reports, the insertion/extraction of Na⁺ ions leads to almost no changes of titanium valence state but there is a charge redistribution of O 2p orbitals which alters the hybridization between O 2p and Ti 3d states, suggested by the combined electrochemical and X-ray spectroscopic study. Both the electronic and local structures of TiO₂ during the reversible sodiation/desodiation process are revealed from the Ti L-edge and O K-edge spectra. This detailed study would shed light on the material design and structural optimization of TiO₂ as energy storage material in different systems.