Unifying Electrochemically‐Driven Multistep Phase Transformations of Rutile TiO2 to Rocksalt Nanograins for Reversible Li+ and Na+ Storage

Abstract Rutile titanium dioxide (TiO 2 (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 TiO 2 (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 TiO 2 (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 TiO 2 (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.