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

ABSTRACT Aqueous four‐electron zinc–iodine (Zn–I 2 ) batteries offer high capacity and operating voltage, yet most reported systems rely on Cl − to activate the I 0 /I + conversion, generating hydrolysis‐prone ICl 2 − 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 IBr 2 − 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 IBr 2 − as water‐insoluble Py + IBr 2 − complexes, protecting I + species from hydrolysis, while sulfonate groups regulate the aqueous hydrogen‐bond network and establish Zn 2+ ‐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–I 2 chemistry.