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

Abstract

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.