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

Zinc-iodine batteries (ZIBs) are considered as a promising energy storage system, but are still plagued by the low energy density and rampant side reactions originating from active H₂O molecules in liquid electrolyte. Realizing the coupled redox reactions within I⁻/I0/I⁺ species, i.e., four-electron transfer reactions, is deemed as effective strategy to 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 (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. On basis of these advantages, both the side reactions on Zn anode and I⁺ hydrolysis reaction on 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 I₂@AC cathode with 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 comprehensive perspective for the future design of liquid electrolyte for high-energy density ZIBs.