Dynamic Cathode Interlayer for Ultralow Self-Discharge and High Iodide Utilization in Zinc-Iodine Batteries

Aqueous zinc-iodine (Zn-I₂) batteries are highly desirable for grid energy storage but subjected to polyiodide shuttling, which leads to low Coulombic efficiency (less than 98%), severe self-discharge (over 10% after 2 days) and low iodine utilization (below 80%). In this study, we in-situ constructed a dynamic interlayer on the cathode surface using an electrolyte additive that can rapidly reacts with polyiodides. This interlayer effectively prevents polyiodide dissolution and migration in the electrolyte, achieving a high Coulombic efficiency of 99.8% at 0.2 A g^–1 and an ultralow self-discharge rate of 2.9% after 7 days of resting. Remarkably, the interlayer also exhibits good electrochemical activity during cycling. The reacted polyiodides can be reduced to I⁻ ions during discharge, contributing to the cell capacity and improving iodine utilization rate to 89.1% at a high capacity of 2.9 mAh cm^–2. Moreover, the additive greatly enhances zinc plating behavior, resulting in an extended cycle life of over 36,000 without capacity decay at 5.0 A g^–1. At a high mass loading of 15 mg cm^–2 and a low N/P ratio of 1.85, the battery shows 100% capacity retention after 330 cycles with an impressive energy density of 98 Wh kg^–1.