Polysulfide-based redox flow batteries with long life and low levelized cost enabled by charge-reinforced ion-selective membranes

Polysulfide is one of the most promising aqueous redox chemistries for grid storage owing to its inherent safety, high energy and low cost. However, its poor cycle life resulting from polysulfide cross-over has prohibited its successful commercialization. To exploit low-cost and high-capacity polysulfide flow batteries with industrial-relevant cycling stability, we develop a charge-reinforced ion-selective membrane to retain polysulfide/iodide, restrain membrane swelling and prevent water/OH− migration. The polysulfide/polyiodide static cell demonstrates a low capacity decay rate (0.005% per day and 0.0004% per cycle) over 2.9 months (1,200 cycles) at a 100% state of charge. A flow cell containing 4.0 M KI/2.0 M K2S2 demonstrated stable cycling at 17.9 Ah l−1posolyte+negolyte over 3.1 months (500 cycles). Small-angle X-ray scattering and in-situ attenuated total reflectance–Fourier transform infrared/solid-state NMR revealed reduced water cluster size and restrained water movement in the charge-reinforced ion-selective membrane compared to commercial Nafion membrane. Techno-economic analysis shows that the developed polysulfide flow battery promises competitive levelized cost of storage for long-duration energy storage. The development of aqueous redox flow batteries (ARFBs) has been plagued by high material costs and poor operating stability. Here the authors report a membrane design to enable polysulfide-based ARFBs with minimal capacity decay over long cycles and durations as well as competitive cost-effectiveness.