Bromide-Activated Four-Electron Iodine Chemistry Enabled by a Pyridinium-Integrated Hydrogel Electrolyte

Aqueous four-electron zinc-iodine (Zn-I₂) batteries offer high capacity and operating voltage, yet most reported systems rely on Cl^- to activate the I⁰/I⁺ conversion, generating hydrolysis-prone ICl₂ ^- intermediates that cause rapid capacity decay. However, how inner I-X bond polarization (X = F, Cl, and Br) governs the hydrolytic stability of interhalogen intermediates remains unclear. Here, we conduct both experimental and computational analyses, revealing Br^- as the most effective halide. Compared with F^- and Cl^-, Br^- forms less-polarized and more hydrolytically stable IBr₂ ^- intermediates, markedly improving high-voltage iodine reversibility. Building on this Br^--mediated stabilization, we further design a multifunctional PNA-PyBr hydrogel electrolyte by incorporating pyridine hydrobromide into a sulfonate-rich poly(sodium p-styrenesulfonate-co-acrylamide) network. Pyridinium cations immobilize IBr₂ ^- as water-insoluble Py⁺IBr₂ ^- complexes, protecting I⁺ species from hydrolysis, while sulfonate groups regulate the aqueous hydrogen-bond network and establish Zn²⁺-selective transport pathways. The resulting cells deliver a near-theoretical capacity of 422.6 mAh g^-1 and a record lifespan of over 20 000 cycles at 20 C. Moreover, a 500 mAh pouch cell with an areal capacity of 3.5 mAh cm^-2 sustains 600 cycles with negligible capacity decay, demonstrating the practical potential of Br^--activated four-electron Zn-I₂ chemistry.