Low-Strain Cathode via Electrochemical Conversion for High-Power and Durable Aqueous Zinc-Ion Batteries

Vanadium-based compounds are promising cathodes for aqueous zinc-ion batteries (AZIBs) due to their multivalent redox chemistry and high theoretical capacity, but their cycling stability is hindered by weak interlayer interactions and strong Zn²⁺-V-O electrostatics, leading to lattice strain (volumetric strain, ∼ 20%) and high ion migration barriers. Herein, we demonstrate a simple, cost-effective, and efficient in situ electrochemical conversion strategy to transform VB₂ into Zn₃(OH)₂V₂O₇·2H₂O (ZnVOH), a vanadium-based cathode featuring low-strain characteristics (volumetric strain of 4.29%). ZnVOH possesses a large open and expanded framework structure, water-mediated shielding layers that weaken Zn²⁺-V-O interactions and lower migration barriers, and a [ZnO₆]-[VO₄] framework that buffers structural stress during Zn²⁺/H⁺ insertion. Meanwhile, soluble borate species (B₄O₇^2- and B(OH)₄^-) generated during conversion stabilize the electrolyte pH and suppress parasitic phase formation, further enhancing durability. To improve the electronic conductivity, a carbon-coated composite (ZnVOH@C) was fabricated. Density functional theory calculations confirm its enhanced electronic transport and Zn²⁺/H⁺ diffusion kinetics. Consequently, ZnVOH@C delivers a high reversible capacity of 422.3 mAh g^-1 at 0.1 A g^-1, along with ultralong cycle life and excellent capacity retention at a high rate (>80% after 8000 cycles at 10 A g^-1). This work provides a design paradigm for developing high-capacity, durable cathodes for next-generation AZIBs.