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

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 TiO₂-35 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 TiO₂ anodes. This work opens a pathway for enhancing electrochemically-induced irreversible phase transformations with enhanced charge storage performance.