Unveiling the effect of structural water on Zn-ion storage of polyoxovanadate for high-rate and long-life aqueous zinc ion battery

Aqueous zinc-ion batteries (ZIBs) are promising for grid-scale energy storage because of their low cost, high theoretical energy density, and high safety; however, the rational design of advanced cathode materials with stable internal structures and sufficient Zn²⁺ diffusion channels remains a challenge. To date, most studies have focused on the structural water of electrode materials to improve their electrochemical kinetics without considering its possible negative effects. Herein, we develop the polyoxovanadates of Na₆[V₁₀O₂₈]·nH₂O with controllable structural water (denoted as NVO-n, n = 18, 2.6 and 0) as cathodes for Zn²⁺ storage, which provide model objects to study the effects of water molecules on electrochemical properties. Results reveal that an appropriate amount of structural water improves ion transport by shielding the effective charge of Zn²⁺ ions while reducing the hindrance to the migration of Zn²⁺ ions, enabling fast Zn²⁺ storage. Moreover, the NVO-2.6 possesses multi-electron redox ability and high structural stability, which ensures the highly reversible Zn²⁺ intercalation/de-intercalation at a high working voltage (0.98 V) and small volumetric strain upon cycling. Consequently, the optimized NVO-2.6 delivers a high specific capacity of 228.5 mAh g⁻¹ at 0.1 A g⁻¹ and a long-term cyclability with 89.7% capacity retention after 3000 cycles at 10 A g⁻¹. This study paves a new direction for the design of polyoxovanadate-based cathodes toward high-performance ZIBs.