Dynamic Interfacial Complexation Engineering Enables Multielectron Ah-Level Halogen Conversion Batteries

Aqueous zinc-iodine batteries are attractive for safe and cost-effective energy storage because iodine is abundant and supports rich multielectron redox chemistry. However, stabilizing deep iodine conversion in static cells remains challenging because high-valent iodine species are highly susceptible to dissolution, hydrolysis, and shuttle-induced side reactions. Here, we report a dynamic interfacial complexation strategy that enables highly reversible 12-electron iodine conversion with an additional reversible bromine contribution in a static aqueous Zn-halogen battery. By introducing N-butyl-N-methylpiperidinium bromide (BMPBr) as a multifunctional additive, the multistep conversion of a high-loading iodine cathode (10 mg cm^-2) is effectively optimized. At high potentials, BMP⁺ self-assembles with polybromide species to form an oily interphase that encapsulates nascent IO₃^- and stabilizes soluble intermediates while enhancing the Br^-/Br⁰ reversibility. This synergistic chemistry enables a specific capacity of 1937 mAh g_iodine^-1 or 647 mAh g_{iodine+bromine} at 20 A g_iodine^-1, a long and flat discharge plateau above 1.0 V vs SHE, and 99.0% capacity retention over 8300 cycles. Demonstrating practical scalability, a scaled-up 1.2 Ah pouch cell achieves an impressive areal capacity of 19.4 mAh cm^-2. This work establishes a robust paradigm for managing complex multielectron halogen chemistries, offering a viable pathway toward high-energy-density aqueous energy storage.