Structurally Coordinated Water Enhances Structural Stability of Tunnel-Type Co(VO₃)₂·2H₂O Cathodes for Aqueous Zinc-Ion Batteries

Tunnel-type cobalt vanadate dihydrate (Co(VO₃)₂·2H₂O, CoVO) is first demonstrated as a robust cathode for aqueous Zn-ion batteries. The orthorhombic framework, built from CoO₆ octahedra and VO₄ tetrahedra, incorporates two structural water molecules directly coordinated to Co²⁺. These structural waters serve critical functions: 1) forming hydrogen-bond networks that buffer local strain, 2) shielding electrostatic repulsion between Zn²⁺ and the polyanionic framework, and 3) creating secondary diffusion channels that accelerate H⁺/Zn²⁺ transport. Owing to this cooperative effect, CoVO delivers 83.6 mAh/g at 0.5C and retains 90.6% capacity after 1000 cycles at 3C. Ex situ XRD and Rietveld refinements confirm a topotactic, zero-strain intercalation mechanism (ΔV = 0.17%) without bulk decomposition, while bond valence sum analysis reveals dual-ion pathways with low migration barriers. The synergy between tunnel topology and structural water coordination underpins the exceptional rate capability, minimal impedance growth, and long-term stability, establishing a general design strategy for advanced multivalent-ion battery cathodes.