Revitalizing nanoscale solid–solid conversion enables ultrastable aqueous batteries

Aqueous zinc-manganese (Zn-Mn) batteries driven by deposition-dissolution reactions hold significant promise for large-scale grid energy storage. However, their lifetimes are limited by incomplete solid-solid conversion, driven by irreversible pathways forming large-sized solid products-a critical yet overlooked size-dependent challenge. Here, we construct a size-revitalizing layer (SRL) on MnO₂ to regulate the interfacial microenvironment via sustained Mn release and strong interaction with solid products. The SRL governs growth kinetics, stabilizing nanoscale products formation. Such small solid products shorten ion-diffusion paths and reduce concentration polarization, enabling a reversible Mn (Ⅱ, <i>l</i>)-Mn (Ⅳ, <i>s</i>) pathway instead of an irreversible Mn (II, <i>l</i>)-Mn (III, <i>s</i>) route. Systematic screening identifies Bi₂O₃ as the optimal modifier based on its strong p-s (M-Zn) orbital interaction and lattice compatibility, which reduce solid product size from >10 μm to the nanoscale, achieving unparalleled stability and resolving irreversible capacity degradation. With a high-mass-loading cathode (9 mg cm^-2), coin cells achieve >1000 cycles at 2 C and scaled iron-plate cells (16 mg cm^-2, 26.3 mAh) operate stably for 110 days. This size-controlled solid-solid conversion strategy exhibits broad applicability to diverse electrode materials, highlighting its potential for widespread adoption in advanced energy-storage systems.