Stabilizing Iodine Redox Mediator Enables High-Performance Aqueous Zinc-Sulfur Batteries

Aqueous zinc-sulfur batteries (AZSBs) are regarded as promising candidates for high-energy-density and low-cost energy storage devices. However, sluggish conversion reaction of sulfur-loading cathode and notorious polyiodide shuttle of iodine redox mediator in aqueous electrolytes severely hinder the development of AZSBs. Herein, ammonia-oxidized lignin (AOL) is introduced as electrolyte additive to stabilize the redox mediator function of ZnI₂, which effectively facilitates the reversible sulfur conversion reaction (S₈↔ZnS). As demonstrated, AOL monomer is rich in active hydroxyl/amide moieties, and exhibits strong chemisorption capability for polyiodides as well as remarkable thermodynamic condition for iodine conversion reaction (I₃ ^-↔I^-), which significantly blocks the ZnI₂ mediator loss and I₃ ^-/I₅ ^- shuttle behavior during cycling, thereby maximizing the catalytic effect of ZnI₂ for S₈↔ZnS reaction and high-performance AZSBs. Consequently, the optimized AZSBs deliver high specific capacity of 1532 mAh g^-1 at 0.5 A g^-1, and high reversible capacity of 326.2 mAh g^-1 after 320 cycles at 2 A g^-1. Even if assembled into pouch batteries with high sulfur loading of 10 mg cm^-2, high capacity of 514.5 mAh g^-1 is still maintained after 134 cycles at 0.5 A g^-1. This work provides novel insights to accelerate sulfur conversion reaction kinetics through stabilizing the redox mediators of AZSBs.