Non-halogen aqueous electrolytes for four-electron iodine conversion

Abstract Accessing reversible four-electron iodine redox chemistry in aqueous metal–iodine batteries offers a promising route towards high-energy aqueous energy storage, but has conventionally relied on highly concentrated, corrosive halogen-containing electrolytes to stabilize reactive hypervalent iodine cations through interhalogen complexation 1–9 . Here we show a kosmotropic anion–polar anion–nucleophilic species (Nuc) electrolyte framework that harnesses anionic chemistry to coordinate the interfacial microenvironment and molecular interactions, enabling reversible four-electron iodine conversion in halogen-free aqueous electrolytes. Polar anions establish a water-deficient, anion-enriched interfacial microenvironment, whereas kosmotropic anions regulate water–Nuc interactions to increase the availability of Nuc for stabilizing hypervalent iodine cations through Nuc–I + –Nuc halogen bonds. This electrolyte framework is validated across diverse Nuc and in both aqueous Zn–I 2 and Al–I 2 batteries. Moving beyond conventional interhalogen stabilization, these findings establish a distinct electrolyte chemistry framework and represent a mechanistic shift for reversible multi-electron iodine chemistry in aqueous environments.