Bridging the Gap to Practical Aqueous Zinc–Iodine Batteries: Advanced Hydrogel Electrolytes via Interfacial Chemistry and Architectural Engineering

Aqueous zinc–iodine (Zn–I 2 ) batteries are promising for grid‐scale storage and flexible electronics because of their nonflammable chemistry, low cost, and high theoretical capacity. Their deployment, however, is limited by uneven Zn deposition, water‐driven hydrogen evolution and corrosion, and polyiodide shuttling. Functional hydrogel electrolytes can regulate these coupled processes through solvent confinement, ion‐selective transport, and mechanically persistent electrode contact. This review critically examines recent advances in hydrogel electrolytes for Zn–I 2 batteries from interfacial chemistry to architectural engineering. Distinct from broader hydrogel reviews for aqueous Zn batteries, it focuses on the chemically asymmetric coupling between a water‐reactive Zn anode and a soluble polyiodide cathode and organizes the field through a structure–mechanism–performance framework that links molecular affinity, fixed charge, and water‐state regulation to measurable interfacial parameters, spatially decoupled architectures, and scale‐relevant device metrics. We analyze dual‐interface regulation across solvation and nucleation, polyiodide confinement, tortuosity control, and asymmetric/Janus structures. Broad‐temperature and voltage‐tolerant operation, together with self‐healing, self‐sensing, and biosensing functions, is also evaluated. Finally, we distinguish materials‐level evidence from pouch‐ and Ah‐level validation and propose an application‐specific roadmap centered on high iodine loading, lean electrolyte, limited Zn inventory, water retention, and scalable manufacturing.