Amorphous hydrated vanadium oxide with enlarged interlayer spacing for aqueous zinc-ion batteries

Aqueous zinc-ion batteries (aqZIBs) are low cost and highly safe; however, the development of optimal cathode materials for them is challenging. Although layered vanadium oxides with high specific capacity have been extensively used as an aqZIB cathode material, the applicability of layered VO₂·0.5H₂O (VOH) as an aqZIB cathode material has not been investigated. In this light, herein, the electrochemical properties of a VOH cathode, prepared by a facile hydrothermal process, were examined and the applicability of the VOH cathode in aqZIBs was verified. VOH was electrochemically oxidized at high voltage during pre-charging; anodic oxidation caused a change in the valence state of VOH and induced an unexpected crystalline-to-amorphous phase transformation. Furthermore, the insertion of water in the pyramidal VO₅ framework of oxidized VOH (ox-VOH) facilitated the highly reversible V³⁺/V⁵⁺ redox reaction with Zn ions. Moreover, ox-VOH was found to possess a highly stable amorphous phase and achieved high diffusion-controlled contribution at a low current density (50 mA g⁻¹), affording superior, and stable long-term cyclability with ~88% retention after 240 cycles. These achievements signify a new milestone in developing suitable cathodes for high-performance aqZIBs.