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

Abstract

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 Na₂Ti^IITi^IVO₄ 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 Na₂Ti^IITi^IVO₄ 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.