Ion storage mechanisms in TiO ₂

Abstract Titanium dioxide (TiO2) is a non-toxic, stable, naturally abundant, and easily synthesized material that has been extensively investigated in various energy fields, including rechargeable ion batteries, solar cells and photocatalysis. Recently, TiO2 has garnered significant attention as a promising negative electrode material for sodium-ion batteries. However, the ion storage mechanism of TiO2 remains poorly understood, with existing studies often presenting inconsistent or contradictory results. This review critically summarizes recent researches on advances in understanding the ion storage mechanisms of TiO2 electrode materials in both lithium-ion and sodium-ion batteries, with a particular focus on the latest findings related to in-situ electrochemically induced crystalline-to-amorphous-to-rock-salt phase transformations. This emerging concept challenges the traditional perspective that irreversible phase evolution is inherently detrimental. Instead, the in-situ formation of new rock-salt phases with enhanced electrochemical storage capabilities for Li+ or Na+ offers a promising alternative strategy for the design of next-generation TiO2–based negative electrodes. Furthermore, we highlight a rich yet underexplored research area with significant potential to deepen the understanding of electrochemical phase transitions in TiO2 and other metal oxide electrodes within energy storage systems.