Ion-Replenishing Interlayer and Tailored Electrolyte Jointly Activate Four-Electron Zinc-Iodine Batteries

Activating four-electron iodine chemistry in zinc-iodine (Zn-I₂) batteries promises higher energy density, yet remains challenged by polyiodide shuttling and the instability of high-valence I⁺ species. Here, we demonstrate that a customized NH₄Cl-based aqueous electrolyte, coupled with an ion-replenishing Cl-functionalized covalent organic framework (COF-Cl) interlayer, enables long-lived four-electron Zn-I₂ batteries. The optimized electrolyte promotes I⁺-Cl^- complexation, while the COF-Cl interlayer immobilizes polyiodides and continuously releases Cl^- to stabilize I⁺ against hydrolysis, collectively ensuring reversible I^-/I⁰/I⁺ redox conversion. In situ spectroscopic and theoretical analyses reveal accelerated high-valence redox kinetics and strong I⁺/polyiodide interactions. As a result, the optimized cell delivers high energy density (278 Wh kg^{- 1}), fast kinetics (128 mAh g^{- 1} at 10 A g^{- 1}), and remarkable cycling durability over 45000 cycles at -5°C with an ultralow decay rate of 0.00039% per cycle, with the strategy further validated in pouch cells under low-temperature conditions. This work establishes an effective ion-replenishing interlayer-electrolyte strategy for robust, high-energy aqueous Zn-I₂ batteries.