Reaction Mechanism of Electrodeposited ε-MnO₂: A Proton-Centered Pathway in Aqueous Zn-Ion Systems

Aqueous Zn/MnO₂ batteries have garnered significant interests owing to their abundance, high theoretical specific capacity, safety, and low cost. However, large-scale application of these systems is limited by the incomplete understanding of the MnO₂ reaction chemistry. The different crystal lattice structures among MnO₂ polymorphs contribute to the variations in reported reaction mechanisms. Among them, ε-MnO₂ polymorph, the dominant phase in electrolytic manganese dioxide (EMD), is notably observed during the charge cycles of aqueous Zn/MnO₂ batteries. In this work, we investigate the electrochemical behavior of an ε-MnO₂ cathode synthesized via electrodeposition from a ZnSO₄ and MnSO₄ electrolyte_, onto a 3-dimensional carbon cloth substrate. Proton intercalation emerges as the dominant charge storage mechanism, critically enabling the reversibility of ε-MnO₂ during cycling, as revealed by operando synchrotron X-ray diffraction and X-ray absorption spectroscopy. Additionally, a proton-coupled dissolution/redeposition pathway operates alongside minor Zn²⁺ intercalation, as quantified by Rietveld refinement. Morphological and chemical heterogeneities are studied by transmission X-ray microscopy further validates this reaction mechanism. These mechanistic insights provide the foundation for rationally designing Zn/MnO₂ batteries with optimized proton dynamics and charge transfer, advancing these systems as a viable solution for safe, cost-effective grid-scale energy storage.