Solvation Reconfiguration Enables Two-Electron Iodine Chemistry Through Chloride Activation for High-Energy Zn-Iodine...

Activating the I⁰/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 ICl₂ ^- 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 Zn²⁺-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.