Development of High-Performance Ion Selective Membranes for Lithium Polysulfide Redox Flow Batteries (Li-PS RFBs)

Abstract

There has been a surge of research interest in the lithium polysulfide (Li-PS) redox flow batteries (RFBs) for large scale energy storage applications because of their high theoretical energy density (~2600 Wh/kg and 2199 Wh/L for elemental Li and S) and low material cost. They also can bring additional advantages, such as design flexibility, easy scalability, and safe operation condition. However, the Li-PS battery systems suffer from polysulfide crossover (polysulfide shuttling) between their two working electrodes which decreases the batteries’ columbic efficiency and shortens their cycle lives. This shuttling effect is more detrimental in Li-PS RFBs which doesn’t have carbon matrix to immobilize the solid-state sulfur. In conventional redox flow batteries (e.g. vanadium redox flow battery), the membrane separators has an important role by suppressing the active species crossover between catholyte and anolyte. However, commercial porous battery separators (e.g. Celgard) cannot be adopted due to their low rejection for polysulfide (PSn-) active species and fast capacity decay. Recent few research works revealed the feasibility of using ion exchange membranes (IEMs) as a barrier layer to selectively transport Li+ ions and block PSn- ions. Unfortunately, most commercial IEMs are unstable in the organic polysulfide electrolyte because of its high swelling ratio which can cause the electrolytes crossover. Hence, there is a critical need to develop high performance membrane materials for the Li-PS RFB applications. In this work, we will present our recent progress on developing novel ion exchange membrane materials for the Li-PS RFBs which can greatly suppress the polysulfide shuttling while maintaining high Li+ conductivity and mechanical stability. Our biphenyl polymer membrane (BP-SA) presents excellent stability the DOL/DME organic electrolyte solution, exhibiting potential for Li-PS RFBs application. We studied its selective ion transport properties through diffusion and electrochemical measurement. Diffusion test shows that there is close to 100 % rejection of Li-PS ions by the BP-SA membrane, while considerable amount of Li-PS crossover was found for Celgard and Nafion. Moreover, the BP-SA membrane possess comparable Li conductivity (0.13 mS/cm2) to Nafion (0.29 mS/cm2) in organic solvent. As a result, the Li-PS single cell with the IEM/Celgard composite membrane shows better efficiencies in comparison to the cell with Celgard 2325. Our results unambiguously demonstrate that the BP-SA membrane has greater Li+/PS anion selectivity than Nafion and can be a promising base material to make separators for Li-PS RFBs.