Engineering ion-selective and zincophilic-hydrophilic nanocellulose separator through sulfonation for high-stability zinc-iodine batteries.

Aqueous zinc‑iodine batteries are increasingly recognized as attractive energy storage systems, benefiting from their inherent safety, cost-effectiveness, fast charging, and high energy efficiency. However, their practical application is hindered by polyiodide shuttling effect, unstable Zn deposition, and side reactions. To simultaneously address these issues, a novel interfacial regulation strategy is proposed to modify nanocellulose separator using a urea-sulfamic acid deep eutectic solvent. Theoretical calculations and experimental results confirm that the introduced -SO3- functional group serves a triple mechanism. First, its good zincophilicity modulates Zn2+ ion solvation sheath and enhances Zn2+ ion transport. Second, the ion-dipole interactions between -SO3- and H2O decrease the proportion of strong hydrogen bonds among H2O molecules. Third, the electrostatic repulsion of -SO3- against negatively charged species effectively inhibits the migration of polyiodides and SO42-. These synergistic effects yield alleviated electrochemical polarization, uniformly distributed electric field, lowered desolvation barrier, reduced water activity, and suppressed polyiodide shuttling. Accordingly, the assembled Zn//Zn symmetric cells demonstrate exceptional stability for 1200 h at 10 mA cm-2 and 2 mAh cm-2 and remain stable for 400 h under a remarkable areal capacity of 25 mAh cm-2. Furthermore, the ZnI2 full batteries utilizing the optimized separator deliver outstanding rate performance and long-term stability, with 85.6% capacity retention preserved after 20,000 cycles at 2 A g-1. The assembled pouch cells also offer superior cyclability and excellent mechanical robustness. This work provides a scalable and sustainable approach to separator engineering for high-stability aqueous batteries.