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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 Na2TiIITiIVO4 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 Na2TiIITiIVO4 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. [ABSTRACT FROM AUTHOR]

We report TiO2/C as a negative electrode for sodium secondary batteries with an ionic liquid electrolyte. The TiO2/C negative electrode delivers a reversible discharge capacity of 275 mA h g−1 at 10 mA g−1 at 363 K. In sodium secondary batteries, for the first time, the TiO2 negative electrode shows a high reversible discharge capacity of larger than 250 mA h g−1. In addition, the TiO2/C negative electrode also exhibits high rate capability and good cycling performance. [ABSTRACT FROM AUTHOR]