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

Abstract

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 μL mg_S⁻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⁻.