Electron Push–Pull Effect Drives Reversible Six-Electron Redox Reaction in Aqueous Zinc–Dual Halogen Batteries

Abstract

Four-electron aqueous zinc–iodine (Zn–I2) batteries with I–/I0/I+ conversion could achieve high energy density, but the development is largely impeded by I+ hydrolysis and sluggish redox kinetics. Here, by introducing 1-ethylpyridine hydrobromide (EB) as an electrolyte additive, we designed aqueous zinc–dual halogen (iodine–bromine) batteries to address these issues, which enable reversible six-electron conversion with high capacity and long-term cycling stability. In situ X-ray absorption spectroscopy, Raman spectroscopy, and theoretical calculations reveal that the donor–acceptor interactions within I–Br interhalogen compounds induce an electron push–pull effect, enabling a kinetically enhanced four-electron pathway (I–/I2Br–/IBr2–) followed by an activated two-electron pathway of bromine (IBr2–/IBr3–) upon charging, while partial IBr3– undergoes an I+/Br0 co-reduction reaction with enhanced bromine utilization during discharge, leading to asymmetric yet overall reversible six-electron conversion. In addition, the cation groups of EB reconstruct the H-bond network of bulk water and electrostatically anchor IBr2– and IBr3– species, thereby reducing the water activity and stabilizing the interhalogen compounds. These synergistic effects enable zinc–dual halogen batteries to achieve a remarkable capacity of 656 mAh g–1 at 0.5 A g–1 and sustained cycling stability with a high initial capacity of 436.8 mAh g–1 over 2200 cycles at 10 A g–1. This work extends the application of interhalogen chemistry for constructing high-energy-density aqueous batteries with multielectron redox reactions.