Modulating the Proton‐Conducting Lanes in Spinel ZnMn₂O₄ through Off‐Stoichiometry

Abstract The intercalation of protons represents a notable component for energy storage in aqueous zinc‐ion batteries. However, the mechanism of proton transport in metal oxide cathodes, especially related to how the cation distribution modulates the proton‐conducting lanes, remains far from consensus due to the lack of suitable model materials. Here, taking spinel ZnMn₂O₄ cathode as a prototype, it is disclosed that a deficiency of one half of lattice Zn ions can triple its specific capacity at high rates, which is predominantly contributed by proton storage. This promotion can be rationalized by the emergence of facile concerted proton transport in the Zn‐deficient sample, contrasting with the stoichiometric one, where proton intercalation undergoes a slow consecutive process. Furthermore, the restricted Zn motion in spinel phase causes high structural stability during cycling, preventing the recombination of external Zn ions with Zn vacant sites that readily accommodate protons. This work highlights the key role of controlled off‐stoichiometry in optimizing proton transport and storage for aqueous batteries.