PVP pre-intercalation engineering combined with the V⁴⁺/V⁵⁺ dual-valence modulation strategy for energy storage in aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) have become a potential energy storage technology due to their inherent safety, environmental compatibility, and cost-effectiveness. Vanadate compounds have demonstrated considerable potential for AZIB applications among various cathode materials. However, their practical implementation is significantly constrained by intrinsic limitations, including sluggish ion diffusion kinetics, structural instability, and vanadium framework collapse during cycling. To address these challenges, we developed a novel strategy involving polyvinylpyrrolidone (PVP) pre-intercalation into CaV₆O₁₆·3H₂O (CaVO), resulting in a phase transformation to Ca_0.24V₂O₅·H₂O (PVP-CaVO). The embedded PVP acts as a "pillar" between the interlayer spaces, stabilizing the structural stability and thereby enhancing cycling performance. Incorporating PVP introduces additional functional advantages through its amide groups, which possess strong polar characteristics. These groups serve as hydrogen bond acceptors, with nitrogen and oxygen atoms acting as coordination sites. This unique configuration facilitates chemical bond rearrangement and promotes partial reduction of vanadium from higher oxidation states (V⁵⁺) to lower ones (V⁴⁺), establishing a V⁴⁺/V⁵⁺ hybrid valence system. Such electronic structure modification not only enables multi-step redox reactions but also alleviates the strong polarization effect of Zn²⁺ ions. Benefiting from these synergistic effects, the PVP-CaVO cathode demonstrates remarkable electrochemical performance in AZIBs, delivering a specific capacity of 323 mA h g^-1 at 0.5 A g^-1 and maintaining a specific capacity of 169 mA h g^-1 at 10 A g^-1, coupled with excellent cycling stability. Comprehensive <i>ex situ</i> characterization studies further elucidated the energy storage processes, verifying a reversible Zn²⁺/H⁺ co-insertion mechanism. This innovative approach of structural and phase engineering through PVP intercalation provides a valuable approach for optimizing vanadate-based materials.