Unlocking Surface Sodiation Threshold of Titanium Dioxide via Coupled Electrochemical‐Thermal Activation

ABSTRACT Titanium dioxide anodes for sodium‐ion batteries undergo an electrochemically‐induced phase transformation from crystalline to active amorphous phase in a thin surface layer, resulting in surface‐dependent capacities. The origins of this limited active layer remain unclear. Herein, we reveal that the sluggish movement of the amorphous|crystalline boundary during initial sodiation determines the thickness of the surface‐active layer. To unlock surface sodiation threshold, we propose a coupled electrochemical‐thermal activation protocol to promote the continuous movement of the amorphous|crystalline boundary during the initial cycle, thereby permanently increasing the reversible sodiation capacity. The anatase TiO 2 ‐35 nm anode delivers an enhanced capacity from 116 to 190 mAh g −1 at 0.1 A g −1 after electrochemical‐thermal activation, accompanied by its high‐rate capability and long‐term cyclability. The electrochemical‐thermal activation is also efficient for enhancing the Li + /Na + storage capacities of rutile TiO 2 anodes. This work opens a pathway for enhancing electrochemically‐induced irreversible phase transformations with enhanced charge storage performance.