A dual-mediator for a sulfur cathode approaching theoretical capacity with low overpotential in aqueous Zn-S batteries

Aqueous Zn-S batteries are promising for energy storage. However, the sulfur cathode suffers poor redox kinetics due to the sluggish solid-solid conversion with ZnS. Here, a dual mediator of trimethylphenylammonium iodide (Me3PhN+I-) is introduced. Me3PhN+ functions as a dissolution mediator to generate soluble polysulfide intermediates during discharge, allowing a facile solid-liquid-solid path for S. The ZnS to S oxidation during charge is catalyzed by the I-/I3- redox mediator, and the reduced I3- solubility by Me3PhN+ prevents shuttling while retaining catalyst power. The sulfur cathode exhibits 1659 mA h g-1 capacity with only 0.34 V overpotential at 0.1 A g-1, and 851 mA h g-1 is retained at 5 A g-1. It is superior to 1066 mA h g-1 capacity and 0.75 V overpotential obtained without Me3PhN+I- (0.1 A g-1). With a sulfur loading of 5.1 mg cm-2 and an E/S ratio of 10 mu L mgS-1, the sulfur cathode reaches 1623 mA h g-1/8.3 mA h cm-2 capacity and 0.37 V overpotential with the help of Me3PhN+I-. A dual mediator is introduced in aqueous Zn-S batteries to regulate the reaction paths of S cathodes and enhance their electrochemical performance.

Automated summary

A dual mediator, trimethylphenylammonium iodide (Me3PhN+I-), has been introduced in aqueous Zn-S batteries to regulate the reaction paths of sulfur cathodes and enhance their electrochemical performance. Me3PhN+ functions as a dissolution mediator to generate soluble polysulfide intermediates during discharge, allowing a facile solid-liquid-solid path for sulfur. The I-/I3- redox mediator catalyzes the oxidation of ZnS to sulfur during charge, and Me3PhN+ reduces the solubility of I3-, preventing shuttling while retaining catalyst power.