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 °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 Na2 TiII TiIV O4 solid solution. During the first discharge process, the solid solution converts into a mixture of TiO2 , Na2 TiO3 , 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 Na2 TiII TiIV O4 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.