High Efficient Ion Selective Membrane Separator for Room Temperature Sodium Polysulfide Redox Flow Batteries

Redox flow batteries (RFBs) have attracted extensive attention in recent years due to their low cost, safe operation, and design flexibility for large scale energy storage applications. Among all the RFBs, room temperature sodium (Na) polysulfides (PS) RFBs are the most promising because of the abundance and low cost of Na compared to Li. However, both Na-S battery and Na-PS flow batteries suffer from polysulfide diffusion between their two working electrodes which decreases the batteries’ columbic efficiency and shortens their cycle stabilities. This shuttling issue is more detrimental in the case of Na-PS RFBs which doesn’t have carbon matrix to immobilize the solid-state sulfur. Unfortunately, commercial battery separators (e.g. Celgard) cannot be adopted because those membranes are freely permeable to the polysulfide ions. Alternatively, ion selective membranes (e.g. Nafion), which have been employed in aqueous redox flow batteries, have been explored to prevent the polysulfide, however, they also showed poor performances in organic RFBs because of swellings in organic electrolytes and low lithium ion selectivity over polysulfide ions. Here we present a biphenyl backbone-based aromatic hydrocarbon membrane with high sodium conductivity as well as sodium selectivity over polysulfides. The conductivity and sodium-polysulfide (Na2S6) diffusivity of sodiated membrane was measured and compared with Nafion membranes. The Na-PS diffusivity of our IEM (3.17×10-11 cm2/sec) is two order magnitudes lower than that of Nafion (1.02×10-9 cm2/sec), while it showed high Na ion conductivity (3.27×10-5 S/cm) comparable to Nafion (3.37×10-5 S/cm). Thus, Na/PS selectivity of the BP-SA is significantly higher than Nafion. We will discuss the high performance of sodium-polysulfide redox flow battery with the BP-SA membrane and compare its efficiencies with the cell with Celgard2325 and Nafion.