Recent Progress on Modulating the Electrochemical Stability Window for Zn Aqueous Batteries

Zn aqueous batteries (ZABs) have gained renewed attention as intrinsically safe, low-cost, and sustainable candidates for stationary energy storage, yet their practical deployment is still constrained by the narrow electrochemical stability window (ESW) of water and its intimate coupling to Zn anode degradation. In this review, we place ESW engineering at the center of the discussion and critically examine recent advances that reshape bulk solvation, regulate proton activity, crowd and confine water, and build protective interphases and hydrophobic interfaces. Concentrated, dual-cation and hydrate-melt electrolytes, water-in-salt and water-in-deep-eutectic formulations, and weakly solvating co-solvents collectively demonstrate that anion-rich, water-poor Zn2+ solvation shells can suppress hydrogen evolution, corrosion, and cathode-side oxygen evolution. Complementary strategies based on pH-buffered or gradient electrolytes, molecularly crowded and "soggy-sand" media, lean-water hydrogels, SEI/interphase architectures, and surfactant or amphiphilic additives further extend the practical ESW by kinetically shielding water at multiple length scales. By systematically comparing these approaches and highlighting their mechanistic commonalities, we aim to provide a coherent toolkit for ESW modulation that can be combined with cathode optimization and cell-level engineering. Finally, we outline remaining challenges and future research directions towards high-voltage, long-life, and application-relevant ZABs that operate close to the intrinsic limits of aqueous electrochemistry worldwide.