Organic iodine electrolyte starting triple I+ storage in In-based metal-organic frameworks for high-capacity aqueous Zn-I2 batteries

Aqueous Zn-I2 batteries show broad prospects for grid-scale energy storage, but face limited capacity and low voltage plateau due to the single conversion of I2/I− coupled with serious dissolution of polyiodide in the electrolyte. Here we report a novel triple I+ storage, started by organic iodine electrolyte for the first time, in In-based metal–organic frameworks (In-MOFs) cathode for high-capacity Zn-I2 batteries. In this electrolyte–electrode cooperation, 1-methyl-3-propylimidazolium iodide (MPII) contained Zn(SO3CF3)2 electrolyte interacts with SO3CF3− to yield I+ for 4 e− I−/I2/I+ conversion. Meanwhile, In-MOFs coordinate with I+ by –C=O and C–OH groups, and alloy with Zn2+ by In centers, to further boost the battery capacity. Such multi-synergy merits deliver an ultrahigh capacity of 481 mAh/g (based on the total mass of In-MOFs from the cathode and I+ ions from the electrolyte) at 1.0 A/g for aqueous Zn-I2 batteries. This study points to a new paradigm for electrolyte–electrode synergistic iodine storage to significantly enhance the capacity of Zn-I2 batteries toward practical applications.