Composite Membrane for Sodium Polysulfide Hybrid Redox Flow Batteries

Non-aqueous redox flow batteries (NARFBs) using earth-abundant materials, such as sodium and sulfur, are promising long-duration energy storage technologies. NARFBs utilize organic solvents, which enable higher operating voltages and potentially higher energy densities compared with their aqueous counterparts. Despite exciting progress throughout the past decade, the lack of low-cost membranes with adequate ionic conductivity and selectivity remains as one of the major bottlenecks of NARFBs. Here, we developed a composite membrane composed of a thin (<25 µm) Na⁺-Nafion coating on a porous polypropylene scaffold. The composite membrane significantly improves the electrochemical stability of Na⁺-Nafion against sodium metal, exhibiting stable Na symmetric cell performance for over 2300 h, while Na⁺-Nafion shorted by 445 h. Additionally, the composite membrane demonstrates a higher room temperature storage modulus than the porous polypropylene scaffold and Na⁺-Nafion separately while maintaining high Na⁺ conductivity (0.24 mS/cm at 20 °C). Our method shows that a composite membrane utilizing Na⁺-Nafion is a promising approach for sodium-based hybrid redox flow batteries.