Dynamic Interhalogen Coupling Engineered by Multifunctional Ionic Liquid for High-Energy Aqueous Zn-I2 Batteries

Abstract

Aqueous Zn-I₂ batteries are promising for sustainable energy storage, exhibit excellent safety, low cost, and high energy density. However, their practical application is limited by the two-electron I⁻/I⁰ redox reaction and the severe shuttle effect of polyiodide species. Here, a multifunctional ionic liquid, 1-butyl-3-methylimidazolium bromide (BmimBr), is introduced to establish a dynamic interhalogen coupling between iodine and bromine species, which enables a six-electron transfer pathway involving I⁻/I⁰/I⁺ and Br⁻/Br⁰. In situ characterizations and theoretical calculations show that Br⁻ acts as a dynamic mediator, forming an [IBr₂]⁻ intermediate with iodine. This significantly lowers the energy barrier for oxidizing I⁰ to I⁺, thereby contributing additional capacity and accelerating kinetics. Simultaneously, Bmim⁺ provides dual interfacial functions: confining polyiodides at the cathode to suppress shuttling, and forming a protective layer on the zinc anode, which reduces interfacial water, inhibits dendrite growth and side reactions. Leveraging this synergistic design, the Zn-I₂ battery achieves a high specific capacity of 511 mAh g⁻¹ at 1 A g⁻¹ and retains 90.22% of its capacity after 30 000 cycles at 10 A g⁻¹. The dynamic interhalogen coupling strategy offers a novel route to activate multielectron reactions and stabilize electrode interfaces in Zn-I₂ batteries.