Defect and Hydration Synergy Enable Intercalation Pseudocapacitance in TiO ₂ Anodes for High-Performance Aqueous Calcium-Ion Batteries

Aqueous calcium-ion batteries (ACIBs) hold great potential for safe and sustainable energy storage yet are constrained by the scarcity of anodes capable of accommodating Ca 2+ ions with high charge density and large radius while maintaining fast kinetics and long-term stability. Herein, a nanostructured titanium dioxide electrode (HTiO 2 ) is developed to address these limitations, establishing a new performance benchmark for ACIB anodes. The HTiO 2 electrode delivers an exceptional capacity of 198 mAh g –1, a desirable low discharge plateau of −1.25 V vs SCE, and remarkable cycling stability over 20,000 cycles. Theoretical and experimental analyses reveal that the incorporation of oxygen vacancies and lattice water synergistically optimizes local electronic states and lattice configurations. This coupled modification promotes electron/ion transport, mitigating structural strain, and enabling rapid pseudocapacitive Ca 2+ storage within a robust framework. The assembled CuHCF//HTiO 2 full cell achieves a high working voltage of 1.7 V and an impressive energy density of 80.7 Wh kg –1 . This work positions TiO 2 as a superior anode candidate for ACIBs and provides fundamental insights into defect and hydration engineering as an effective paradigm for overcoming the kinetic and structural hurdles in aqueous multivalent-ion batteries.