Current Density as a Critical Factor Governing the Cycle Life of Polysulfide-Based Redox Flow Batteries

Abstract Aqueous polysulfide redox flow batteries (RFBs) are promising for large-scale energy storage owing to their low cost and high safety. However, stable operation at high current density remains challenging for polysulfide RFBs (typically cycling below 40 mA cm–2), with the degradation mechanisms remaining poorly understood. Here, we identify current density as a critical factor governing the cycle life and elucidate sulfur deposition-induced degradation. Increasing current density simultaneously accelerates polysulfide crossover and promotes the formation of compact sulfur deposits, which block ion-transport pathways and lead to rapid cell failure. Based on this mechanistic understanding, a surfactant additive was employed to regulate sulfur nucleation and growth, extending the cycle life of polysulfide/ferrocyanide RFBs from 30 cycles to over 500 cycles at 60 mA cm–2. This work elucidates current density and sulfur morphology as critical factors governing polysulfide RFB degradation and provides design principles for durable, high-rate polysulfide-based RFBs.