Nonfluorinated Membrane with a Decentralized Ion-Transport Network Enables Efficient and Sustainable Polysulfide Redox Flow Batteries

Aqueous redox flow batteries are one of the most promising electrochemical energy storage technologies for long-duration energy storage. Polysulfide-based redox flow batteries are particularly attractive owing to ultra-low-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 (Nafion117, USD $800 to $3,500 per square meter), along with the environmental concerns associated with the fluorine-processing industries. Common fluorinated membranes such as Nafion are not only expensive but also inadequate to prevent polysulfide-crossover due to oversized ion-transport channels resulting from aggregated microphase separation. Here, we develop non-fluorinated sulfonated polyethersulfone (SPES)-based membrane with decentralized ion-transport channels to create high-quantity, smaller ion-transport channels. The reduced channel size helps to mitigate polysulfide-crossover while the high-quantity channels help to maintain high ionic conductivity. The developed SPES membrane showed a 20 times higher ionic selectivity at a drastically 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 full polysulfide-ferrocyanide redox flow batteries with a high coulombic efficiency (>99.9%) and energy efficiency (average > 75%) for 1,600 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 practical commercialization of polysulfide redox flow batteries. Acknowledgement: The work was supported by grants from the Research Grant Council (RGC) of the Hong Kong Special Administrative Region, China (project nos. RFS2223-4S03, CUHK 14302823, CUHK 14310124, and C1017-22G) and City University of Hong Kong Projects 7006111 and 7020112.