A Polyanionic Gradient Hydrogel Electrolyte Enabling Anode/Cathode Coregulation for Robust Zn–Iodine Batteries

Aqueous zinc–iodine batteries (AZIBs) are promising energy storage devices owing to their inherent safety, cost-effectiveness, and high specific capacity. However, their practical application is limited by the shuttle effect of polyiodides and the unstable anode–electrolyte interface of Zn. To simultaneously address these problems, a dual-functional gradient polyanionic hydrogel electrolyte (GPAAS) is developed to stabilize the Zn anode and alleviate polyiodide shuttling. This is achieved via an electric field-induced anionic monomer concentration gradient strategy for anionic monomers. The gradient distribution of anionic groups (−SO3–) within the GPAAS facilitates rapid Zn2+ transport and promotes uniform zinc deposition along the (002) crystal plane. Meanwhile, the high concentration of −SO3– reconfigures the solvation structure of Zn2+ and reduces the number of active water molecules at the zinc electrode interface, thereby stabilizing the anode electrode–electrolyte interface. As a result, the Zn||Zn symmetrical cell exhibits exceptional cycling stability, operating for up to 2100 h at 1 mA·cm–2/1 mAh·cm–2. Moreover, the gradient distribution of −SO3– effectively suppresses polyiodide shuttling through electrostatic repulsion, enhancing iodine utilization and reducing capacity decay. Consequently, ZIBs based on GPAAS achieve a high specific capacity of 203 mAh·g–1 at 0.3 A·g–1 and maintain 80% capacity retention after 1000 cycles at 0.5 A·g–1. This work demonstrates that gradient hydrogel electrolytes provide a viable strategy for the development of stable and high-performance ZIBs.