Tailoring Alkyl Chain Length to Customize Dual‐Electrode Interface for Reversible Four‐Electron Iodine Chemistry in Zn–I 2 Pouch Batteries

ABSTRACT Aqueous zinc–iodine (Zn–I 2 ) batteries show great promise for large‐scale energy storage, yet realizing four‐electron transfer for high energy density requires cooperation between zinc interface regulation and high valence iodine anchoring. Herein, the dual‐electrode interface microenvironment is customized via a series of bromide quaternary ammonium salts with varied alkyl chain length. The moderately‐sized tetraethylammonium bromide (TEAB) constructs a well‐ordered electric double layer that suppresses water penetration and hydrogen evolution while promoting uniform Zn 2+ deposition. Simultaneously, TEAB stabilizes I + through a synergistic coordination‐hydrophobic anchoring mechanism, preventing iodine hydrolysis and shuttling. This synergistic customization of the anode–cathode dual‐interface microenvironment successfully activates the reversible four‐electron I − /I + redox chemistry. With this design, the symmetric Zn cell operates stably for over 50 days at 5 mA cm −2 and 5 mAh cm −2 as well as shows wide‐temperature adaptability from −20°C to 50°C, the full cell retains 84.5% capacity after 8000 cycles at 1 A g −1 , and an Ah‐scale pouch cell delivers stable 428 cycles with a high‐capacity retention of 98.74% even under a heavy iodine loading of 20 mg cm −2 . This work establishes a molecular‐level design paradigm for simultaneous anode and cathode interface engineering, advancing practical high‐energy aqueous zinc–iodine batteries.