Rutile titanium dioxide (TiO₂(R)) lacks octahedral vacancies, which is not suitable for Li⁺ and Na⁺ intercalation via reversible two-phase transformations, but it displays promising electrochemical properties. The origins of these electrochemical performances remain largely unclear. Herein, the Li⁺ and Na⁺ storage mechanisms of TiO₂(R) with grain sizes ranging from 10 to 100 nm are systematically investigated. Through revealing the electrochemically-driven atom rearrangements, nanosize effect and kinetics analysis of TiO₂(R) nanograins during repeated cycling with Li⁺ or Na⁺, a unified mechanism of electrochemically-driven multistep rutile-to-rocksalt phase transformations is demonstrated. Importantly, the electrochemically in situ formed rocksalt phase has open diffusion channels for rapid Li⁺ or Na⁺ (de)intercalation through a solid-solution mechanism, which determines the pseudocapacitive, "mirror-like" cyclic voltammetry curves and excellent rate capabilities. Whereas, the nanosize effect determines the different Li⁺ and Na⁺ storage capacities because of their distinct reaction depths. Remarkably, the TiO₂(R)-10 nm anode in situ turns into rocksalt nanograins after 30 cycles with Na⁺, which delivers a reversible capacity of ≈200 mAh g^-1, high-rate capability of 97 mAh g^-1 at 10 A g^-1 and long-term cycling stability over 3000 cycles. The findings provide deep insights into the in situ phase evolutions with boosted electrochemical Li⁺ or Na⁺ storage performance.

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