A new sodiation-desodiation mechanism of the titania-based negative electrode for sodium-ion batteries

TiO2 is widely investigated as a negative electrode for lithium-ion batteries. In sodium-ion batteries, however, the sodiation-desodiation mechanism of TiO2 is still unclear. Here, we report a new sodiation-desodiation mechanism for an anatase TiO2/C electrode in an ionic liquid electrolyte at 90 degrees C, where it shows a high reversible capacity of 278 mA h g(-1). During the first charge process, TiO2 reacts with Na ions to form a (Na2TiTiO4)-Ti-II-O-IV solid solution. During the first discharge process, the solid solution converts into a mixture of TiO2, Na2TiO3, and TiO, with the former two being X-ray amorphous. In the subsequent cycle, the mixture acts as the active material, reversibly reacting with Na ions to re-form the (Na2TiTiO4)-Ti-II-O-IV solid solution. This mechanism, which has not been reported for Na or Li ion insertion-extraction in anatase TiO2, can help understand this promising electrode material and develop safe sodium-ion batteries with high energy density.