Ion storage mechanisms in TiO₂

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.