Low-Concentration and Non-Halogen Aqueous Electrolytes to Achieve Reversible Four-Electron Iodine Conversion reactions

Four-electron aqueous metal-iodine batteries embrace high theoretical capacities, abundant raw materials, and superior safety, making them highly promising for next-generation large-scale energy storage applications with high energy and power densities. However, harnessing this four-electron redox chemistry has traditionally relied on high-concentration, corrosive halogen-containing electrolytes (up to 46 moles) to stabilize hypervalent iodine cations, posing considerable economic and environmental challenges to access their full potential. Here, we proposed a universal electrolyte design, the KA-PA-Nuc standard, which employs kosmotropic anions (KA), polar anions (PA), and nucleophilic species (Nuc) to achieve reversible four-electron aqueous metal-iodine batteries. PA facilitates a water-deficient, anion-enriched interface, while KA disrupts hydrogen bonding between Nuc and their hydration shells, which in turn form stable halogen bonds with hypervalent iodine cations. For the first time, this electrolyte design grounded in anionic chemistry achieves reversible four-electron iodine redox reactions in halogen-free electrolytes at an exceptionally low concentration (2.4 moles). The KA-PA-Nuc standard was validated across diverse Nuc in aqueous Zn-I₂ and Al-I₂ batteries, demonstrating its broad applicability and effectiveness for aqueous metal–I2 batteries. By eliminating the reliance on high-concentration, corrosive halogen-containing electrolytes, this work establishes a new paradigm and provides a new avenue for low-cost and sustainable batteries.