Synergistic Interface‐Bulk Regulation of Biopolymer Electrolytes for Advanced Zn‐Iodine Batteries With Four‐Electron I ⁻ /I ⁰ /I ⁺ Conversion

Aqueous zinc||iodine batteries (AZIBs), which rely on the efficient I − /I 0 /I + conversion chemistry, offer high theoretical energy density. However, their practical application is hindered by the polyiodide (I 3 − and I 5 − ) shuttle effect, I + hydrolysis at the iodine cathode, and poor reversibility at the Zn anode. To address these challenges, we propose a synergistic interface–bulk regulation strategy using oppositely charged biopolymers. A negatively charged sodium alginate (SA) hydrogel serves as the bulk electrolyte matrix, while positively charged quaternized chitosan (qChi) is introduced as a functional additive. In this system, the abundant carboxyl groups along the SA chains not only facilitate uniform Zn 2+ deposition but also electrostatically repel polyiodides. Meanwhile, the amine and quaternary ammonium groups along the qChi chains can strongly bind polyiodides and ICl 2 − species via ionic interactions, thereby effectively suppressing the shuttle effect and stabilizing I + against hydrolysis. Consequently, highly durable two‐electron and four‐electron AZIBs are achieved, delivering remarkable cycling lifespans of 20 000 and 10 000 cycles with ultralow capacity decay rates of 0.00054% per cycle and 0.17% per hundred cycles, respectively. Notably, the AZIBs also maintain excellent performance in a natural seawater‐based electrolyte, highlighting their potential as sustainable and high‐performance energy storage systems.