Bislawsone as a robust redox mediator for high-performance aqueous polysulfide redox flow batteries

Aqueous polysulfide-based redox flow batteries (RFBs) offer a low-cost alternative to vanadium-based RFBs for grid storage. However, sluggish S 4 2− -to-S 2 2− kinetics leads to large overpotential and low energy efficiency (EE). Here, we propose bislawsone as a redox mediator that facilitates S–S bond cleavage via an "anchor-drag" effect. The battery exhibits a polarization reduction from 0.47 to 0.08 V at 10 mA cm −2 and achieves a record-high EE of 90.29% among S 4 2− /S 2 2− RFBs. At 60 mA cm −2 , the battery maintains 95.75% capacity utilization, 217 times higher than the bislawsone-free counterpart and outperforming previous solid-catalyst systems. It also exhibits high EE retention (92.43%) and capacity retention (97.88%) over 2,200 cycles. Reduced-state bislawsone alone is highly air-sensitive, and the bislawsone-only RFB with full charge fails after 2-h resting without an inert atmosphere. Conversely, spontaneous electron transfer generates air-tolerant S 2 2− in the bislawsone-polysulfide system, enabling 87.81% capacity retention after 168 h of resting.