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

Abstract

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.