Atomic-scale mapping of dissolution/deposition pathways in MnO₂ polymorphs for aqueous Zn–MnO₂ batteries

Achieving fast and reversible dissolution–deposition in MnO₂ is crucial to advancing aqueous Zn–MnO₂ batteries, yet its microscopic origin and interplay with other storage pathways remain unresolved. Herein, we perform first-principles calculations to map, at the atomic scale, the redox steps of six MnO₂ polymorphs (α, β, γ, δ, λ, and ramsdellite-R) and to establish their polymorph- and surface-dependent mechanistic preferences. Thermodynamic analysis of bulk reactions indicates a clear activation sequence during discharge: H⁺-mediated solid–solid conversion to MnOOH at the highest potentials (1.77–1.88 V), followed by Zn²⁺ intercalation yielding λ-ZnMn₂O₄ (1.30–1.47 V), whereas dissolution/deposition operates at lower potentials (1.23–1.30 V) and exhibits delayed activation. Electronic structure analysis demonstrates superior redox activity at MnO₂ surfaces compared to bulk phases. R-MnO₂ (100) is a dominant dissolution-active facet (ΔG from −1.88 to −0.69 eV), while Mn²⁺ deposition preferentially yields layered δ-MnO₂, in line with its ultralow cleavage/surface energy. Modeling of Mn²⁺ solvation further suggests that OH⁻-rich coordination substantially lowers the desolvation barrier (reduced by 6.34 eV), facilitating O²⁻ acquisition during MnO₂ deposition. These insights deepen the understanding of redox mechanisms across various polymorphs of MnO₂ and provide theoretical guidance for designing high-efficiency dissolution/deposition strategies in MnO₂ cathodes.