Decoding MnO₂ redox chemistry from mechanistic ambiguity to design principles for aqueous Zn-ion batteries

Manganese dioxide (MnO₂) is a leading positive electrode candidate for aqueous zinc-ion batteries, combining safety, high voltage, low cost, and sustainability for grid-scale storage. However, its practical development remains restricted by poor reversibility, rooted in an unresolved mechanistic debate spanning over a decade. Here, we combine operando characterizations, multimodal spectroscopic analyses, and theory to establish a unified picture: proton-primed MnO₂ dissolution and subsequent redeposition as nanocrystalline and disordered MnO_x nanosheets, coexisting with reversible proton intercalation in parent MnO₂ and predominantly in deposited MnO_x, forming a dual redox mechanism. pH-driven insulating byproduct precipitation emerges as a significant kinetic barrier that limits deep dissolution and capacity utilization. Guided by these insights, we introduce surface activation and architectural design strategies toward mitigating kinetic barriers, enabling enhanced capacity and stability in both Swagelok and pouch-type cells. By reconciling mechanistic ambiguity and translating it into actionable design principles, this work demonstrates a framework for developing durable Mn-based positive electrodes for sustainable energy storage.