Coulombic-hinderance regulation on pyrovanadates for practicable calcium-ion batteries: a solid-solution strategy

Pyrovanadates are considered a promising host material for the reversible intercalation of highly charged Ca²⁺ ions due to their favorable layered structure and the presence of rich interstitial confined species. However, in calcium-ion battery (CIB) systems, the diffusion kinetics of the Ca²⁺ ions are slower, and the electrostatic interactions are stronger (compared to Li⁺), which limits the effectiveness of pyrovanadate's structural advantages. In this study, we employ an allelic reconfiguration strategy to develop novel solid-solution phase pyrovanadate materials, specifically Zn_3-xCu _х (OH)₂V₂O₇·2H₂O (x = 0, 1, 1.5). By incorporating 'twin' isotopic Cu elements from the adjacent ds-block, we activate redox reactions at non-vanadium metal sites through the modulation of electronic properties. As a result, a pronounced plateau zone during the discharge/charge process is observed. Using theoretical simulations and X-ray absorption spectroscopy, we have clarified the mechanism by which the solid solution enhances the interlayered confinement of species such as lattice water and hydroxide radicals, improving structural stability and facilitating the diffusion of highly charged Ca²⁺ ions. This approach effectively addresses the issue of layer shrinkage, which typically arises from the intense Coulombic interaction between the carrier and the host. When assembled with an active carbon anode, coin-cell CIB devices can operate steadily at a charge rate of 100 mA g^-1 for over 1000 reversible cycles. This demonstrates the potential of innovative solid-solution design strategies to create Coulombic-force-resistant host materials for future multivalent metal-ion battery technologies, including CIB systems.