Nonfluorinated membrane with a decentralized ion-transport network enables efficient and sustainable polysulfide redox flow batteries

Polysulfide-based redox flow batteries are promising for long-duration energy storage, owing to ultralow-cost/earth-abundant active materials and full decoupling of power and energy. However, their practical application has been prevented by poor cycle life resulting from polysulfide crossover and a heavy reliance on costly fluorinated membranes (Nafion 117, USD $800 to $3500 per square meter), along with the environmental concerns. Here, we develop a nonfluorinated sulfonated polyethersulfone (SPES)–based membrane with decentralized ion-transport channels, achieving a 20 times higher ionic selectivity at a markedly reduced cost (USD $12 to $66 per square meter) compared to the commercial Nafion membrane. The low-cost SPES-based membrane enabled stable cycling of polysulfide-ferrocyanide redox flow batteries with a high coulombic efficiency (>99.9%) and energy efficiency (average >75%) for 1600 cycles (>6 months). This strategy demonstrated polysulfide-based redox flow batteries with a record longevity using a low-cost and sustainable membrane, paving the way for their practical commercialization.