Manipulating stable four-electron zinc-iodine batteries via the introduction of diamine ligand sites

Zinc-iodine batteries (ZIBs) are considered a promising energy storage system, but are still plagued by low energy density and rampant side reactions originating from active H2O molecules in the liquid electrolyte. Realizing the coupled redox reactions within I-/I0/I+ species, i.e., four-electron transfer reactions, is deemed an effective strategy for boosting the energy density of ZIBs, which is mainly blocked by the rapid hydrolysis of nucleophilic I+ ions. To address these issues, urea with diamine ligand sites (-NH2) was introduced into the liquid electrolyte [urea electrolyte (UE)] to achieve durable four-electron ZIBs. As demonstrated by the spectroscopic characterization results, -NH2 groups can bundle the active H2O molecules by reconfiguring the hydrogen bonds, and provide additional electrophilic ligand sites for I+ ions. Based on these advantages, both the side reactions on the Zn anode and the I+ hydrolysis reaction on the I2@AC cathode are remarkably mitigated, and four-electron transfer is realized at low zinc salt concentrations. As a result, the optimized UE electrolyte effectively stabilizes the zinc metal anode, and endows the I2@AC cathode with a high reversible capacity of 187.2 mAh g-1 after 250 cycles at 1 A g-1. The disclosed intermolecular force modulation strategy in this work will offer a comprehensive perspective for the future design of liquid electrolytes for high-energy-density ZIBs.