Electrochemically in situ formed rocksalt phase in titanium dioxide determines pseudocapacitive sodium-ion storage

Earth-abundant TiO₂ is a promising negative electrode for low-cost sodium-ion batteries (SIBs) owing to its high capacity, rapid (dis)charging capability, safe operation potential and nonflammability. Crystalline anatase TiO₂ is not suitable for reversible Na⁺ (de)intercalation, but it displays pseudocapacitive response after repeated cycles. Herein, we find and demonstrate that ordered rocksalt (RS) NaTiO₂ nanograins are in situ formed by electrochemically cycling with Na⁺ ions in anatase and amorphous TiO₂. The in situ formed RS-NaTiO₂ follows a solid-solution reaction with small volume changes of only 2.0%, that determines the pseudocapacitive "mirror-like" cyclic voltammetry curve with a couple of broad redox peaks at 0.75 V vs. Na⁺/Na, a high capacity of 253 mAh g^-1, high-rate capability and thousands of stable cycles. The multistep crystalline-to-amorphous-to-RS transformations are able to be electrochemically activated during the aging process of assembled full cells. Our finding provides a direction for developing unconventional Ti-based high-performance active materials for SIBs with both high energy and power densities.