Ambient Temperature Sodium Polysulfide Catholyte for Nonaqueous Redox Flow Batteries

This study reports a sodium polysulfide catholyte for nonaqueous redox flow batteries (RFBs). We demonstrate reversible capacities up to 200 mAh/gS with negligible fade over 250 cycles at room temperature for sodium polysulfide|biphenyl full cells containing Na+ß″-Al2O3 solid electrolyte (BASE) membranes. Interestingly, formation of insoluble S and Na2S4 phases did not inhibit the catholyte's cycle life which is likely due to the low concentrations used in the lab-scale prototypes. 3-electrode galvanostatic AC impedance measurements demonstrate that voltage losses were dominated by charge transfer at the cathode, and relevant kinetic parameters (i.e., transfer coefficients and exchange current density) were calculated through a Tafel analysis. To the best of our knowledge, this is the first report applying such an impedance approach to nonaqueous RFBs. Overall, the use of low-cost active materials makes sodium polysulfide|biphenyl RFBs promising for long duration energy storage applications. If strategies are developed to increase the solubility of S and/or low order polysulfides (Na2Sx, x ≤ 4), specific energies up to 100 Wh kg-1 (including combined mass of the anolyte and catholyte) can be achieved.