Redox-active zinc thiolates for low-cost aqueous rechargeable Zn-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) are promising candidates for large-scale electrical energy storage due to the inexpensive, safe, and non-toxic nature of zinc. One key area that requires further development is electrode materials that store Zn²⁺ ions with high reversibility and fast kinetics. To determine the viability of low-cost organosulfur compounds as OEMs for AZIBs, we investigate how structural modification affects electrochemical performance in Zn-thiolate complexes 1 and 2. Remarkably, modification of one thiolate in 1 to sulfide in 2 reduces the voltage hysteresis from 1.04 V to 0.15 V. While 1 exhibits negligible specific capacity due to the formation of insulating DMcT polymers, 2 delivers a capacity of 107 mA h g⁻¹ with a primary discharge plateau at 1.1 V vs. Zn²⁺/Zn. Spectroscopic studies of 2 suggest a Zn²⁺ and H⁺ co-insertion mechanism with Zn²⁺ as the predominant charge carrier. Capacity fading in Zn-2 cells likely results from the formation of (i) soluble H⁺ insertion products and (ii) non-redox-active side products. Increasing electrolyte concentration and using a Nafion membrane significantly enhances the stability of 2 by suppressing H⁺ insertion. Our findings provide insight into the molecular design strategies to reduce the polarization potential and improve the cycling stability of the thiolate/disulfide redox couple in aqueous battery systems.