Coupled Role of Zn2+ Reversibility and Reduced Water Activity in Achieving Stable Mild-Aqueous Zn||MnO2 Batteries.

Aqueous Zn||MnO2 batteries are promising for large-scale energy storage but suffer from capacity fading due to irreversible transformations at the MnO2 cathode. Achieving structural reversibility under long-term cycling remains a central challenge. Here, we show that tuning the electrolyte solvation environment can directly stabilize Mn redox chemistry and lattice evolution in mild-aqueous Zn||ε-MnO2 batteries. Using dimethyl sulfoxide (DMSO) as a co-solvent to lower water activity and reorganize the solvation structure, we reveal through operando X-ray diffraction that the modified electrolyte promotes reversible Zn2+ intercalation despite slower transport kinetics. Complementary X-ray photoelectron spectroscopy shows that conventional aqueous electrolytes induce strong Mn-valence gradients between surface and bulk, whereas DMSO suppresses these gradients and maintains uniform redox states. Collectively, these results demonstrate that electrolyte solvation engineering can mitigate parasitic reactions and enable structurally reversible MnO2 redox processes, establishing a holistic pathway to enhance the long-term stability of mild-aqueous Zn-based batteries.