Bifunctional electrolyte additive with redox mediation and capacity contribution for sulfur cathode in aqueous Zn-S batteries

[Display omitted] •With the TU additive, the Zn-S battery shows excellent cycle performance.•TU− can accelerate the crack of Zn-S bonds to improve the conversion from ZnS to S.•ZnS reacts with Met+ much faster than H2O, inhibiting the formation of SO42−. Aqueous zinc-sulfur (Zn-S) battery has attracted much attention due to the ultrahigh theoretical capacity of S (1675 mA h g−1). Nevertheless, its lifespan is far from satisfactory owing to the sluggish reaction kinetics for ZnS conversion to S, and the formation of irreversible by-product of SO42− during charging. Herein, a novel bifunctional electrolyte additive thiourea (TU) was introduced to solve the above problems. It has been found that TU can undergo reversible redox reactions during cycles, which not only enhances the reversibility between ZnS and S but also contributes to extra capacities. Based on the ex-situ FTIR and XPS characterization, in combination with the electrochemical performance of thiourea and its derivatives, the influential mechanism of TU on the S@KB electrode was proposed. Its negative centers and positive centers of the intermediates can interact with ZnS to weaken the Zn-S bonds to improve the electrochemical dynamics. Simultaneously, the carbonium ions in the intermediate of TU show strong reactivity to ZnS so that the formation of SO42− can be inhibited. As a result, the S@KB electrode shows excellent cyclic performance with 763.7 mA h g−1 capacity after 300 cycles at 5 A/g, corresponding to a low decay rate of only 0.11 % per cycle. This work provides a promising electrolyte additive and an effective strategy to improve the reaction kinetics and cycle stability of Zn-S batteries.