Solvation Reconfiguration Enables Two‐Electron Iodine Chemistry Through Chloride Activation for High‐Energy Zn–Iodine Flow Batteries

ABSTRACT

Activating the I ⁰ /I ⁺ redox couple in aqueous Zn–iodine redox flow batteries (ZIRFBs) offers a promising route toward scalable energy storage with high voltage and high energy density. However, the hydrolytic instability of I ⁺ species in water severely limits its practical implementation. Although chloride can stabilize I ⁺ species through ICl ₂ ⁻ formation, substantial side reactions persist in aqueous electrolytes because Cl ⁻ activity remains low even at high chloride concentrations. Here, we report a solvation‐reconfiguration strategy that regulates the chloride microenvironment to enhance Cl ⁻ activity and thereby stabilize I ⁺ species. Guided by solvent descriptors of high donor number and low hydrogen‐bond acidity, N,N‐dimethylformamide (DMF) was identified as the optimal cosolvent. DMF weakens Zn ²⁺ –Cl ⁻ association and disrupts chloride hydration, markedly increasing chloride activity even at low bulk Cl ⁻ concentration. As a result, the ZIRFB delivers stable cycling over 1000 cycles (∼1600 h) with negligible capacity decay, a record‐high Coulombic efficiency of 99.9%, and a high discharge energy density of 72.88 Wh L ⁻¹ based on the catholyte volume. This work establishes chloride‐activity regulation as an effective strategy for stabilizing reactive multielectron redox couples, opening new opportunities for safe, durable, and high‐energy aqueous flow batteries.