Energetic aqueous zinc-sulfur battery achieved via co-solvent and redox mediator synergistic regulation

Abstract

Aqueous Zn–S batteries (AZSBs) have received significant attention due to the outstanding energy density, low cost, and high safety. However, the slow kinetics and poor reversibility of the sulfur conversion reaction, coupled with the formation of dendrites in zinc anode and hydrogen evolution reaction (HER), hinder the practical applications of AZSBs. In this paper, we developed a novel co-solvent electrolyte that utilizes N-methylpyridine (NMP) and KI as additives to regulate the cathode reversibility and anode stability synergistically. Firstly, the electrophilic group of NMP (–CO–NR2-) effectively activates the oxidation of I− to form I3−. The resulting I3−/I− redox mediator participates in the sulfur conversion reaction and lowers its energy barrier, thus enhancing the reaction kinetics of ZnS ↔ S and inhibiting the formation of irreversible by-products. Secondly, NMP alters the solvation structure of Zn(H2O)62+, achieving uniform deposition of Zn2+ and inhibiting the growth of zinc dendrites. Thirdly, NMP forms a dense solid-electrolyte-interphase layer between the zinc metal and H2O during cycling, inhibiting HER. Under the synergistic effect from solvent and redox mediator, AZSBs exhibit an impressive capacity of 775 mAh g−1 at 5 A g−1 (10 C) with 77.5 % capacity retention after 300 cycles.