A dual-role electrolyte additive for simultaneous polysulfide shuttle inhibition and redox mediation in sulfur batteries

Abstract

In Li–S batteries, the insulating nature of sulfur and Li₂S causes enormous challenges, such as high polarization and low active material utilization. The nucleation of the solid discharge product, Li₂S, during the discharge cycle, and the activation of Li₂S in the subsequent charge cycle, cause a potential challenge that needs to be overcome. Moreover, the shuttling of soluble lithium polysulfide intermediate species results in active material loss and early capacity fade. In this study, we have used thiourea as an electrolyte additive and showed that it serves as both a redox mediator to overcome the Li₂S activation energy barrier and a shuttle inhibitor to mitigate the notorious polysulfide shuttling via the investigation of thiourea redox activity, shuttle current measurements and study of Li₂S activation. The steady-state shuttle current of the Li–S battery shows a 6-fold drop when 0.02 M thiourea is added to the standard electrolyte. Moreover, by adding thiourea, the charge plateau for the first cycle of the Li₂S based cathodes shifts from 3.5 V (standard ether electrolyte) to 2.5 V (with 0.2 M thiourea). Using this additive, the capacity of the Li–S battery stabilizes at ∼839 mA h g⁻¹ after 5 cycles and remains stable over 700 cycles with a low capacity decay rate of 0.025% per cycle, a tremendous improvement compared to the reference battery that retains only ∼350 mA h g⁻¹ after 300 cycles. In the end, to demonstrate the practical and broad applicability of thiourea in overcoming sulfur-battery challenges and in eliminating the need for complex electrode design, we study two additional battery systems – lithium metal-free cells with a graphite anode and Li₂S cathode, and Li–S cells with simple slurry-based cathodes fabricated via blending commercial carbon black/S and a binder. We believe that this study manifests the advantages of redox active electrolyte additives to overcome several bottlenecks in the Li–S battery field.