Nafion Inhibits Polysulfide Crossover in Hybrid Nonaqueous Redox Flow Batteries

We report on the effectiveness of sodium-exchanged Nafion membranes for inhibiting polysulfide crossover during redox flow battery operation. The solubility of polysulfides allows them to be used as a high-capacity catholyte for nonaqueous redox flow batteries (NARFB). The NARFB cathode capacity is controlled by the total tank catholyte volume and polysulfide concentration and, as such, is independent of polysulfide adsorption sites on carbon fibers or nanotubes as found within traditional Na or Li-S batteries. However, one of the major barriers to the realization of polysulfide NARFBs is associated with developing a robust membrane that has minimal polysulfide crossover under the operating conditions. We found that sodium-exchanged Nafion membrane separators effectively inhibit polysulfide crossover within a Na-sulfur NARFB. These membranes significantly improve both capacity retention (70%) and Coulombic efficiencies (99%) after 50 cycles. Commercial porous membranes showed a large crossover as detected by UV-vis spectroscopy and resulted in low capacity retention (20%) and Coulombic efficiency (74%) after 45 cycles. Electrochemical impedance spectroscopy (EIS) measurements highlighted the trade-off between innate reactivity and ionic conduction of the membranes. The results show that dense, single ion conducting Nafion enables a long cycle life; however, it reacts with Na metal to form a resistive passivation layer and increases the cell resistance. On the contrary, the open pore structure of Celgard allows for higher current charge/discharge, and its chemical nature is compatible with Na; however, it has a high degree of polysulfide crossover.