Ambient Temperature Sodium Polysulfide Catholyte for Nonaqueous Redox Flow Batteries

Abstract

Long-duration energy storage (LDES) systems are needed to enable widespread adoption of intermittent energy resources. Redox flow batteries (RFBs) are particularly well-suited for grid-leveling applications because the system’s energy and power densities can be independently scaled. Despite exciting recent progress in both aqueous and nonaqueous systems, a major limitation of RFBs remains their high cost. For LDES applications, developing high energy density electrolytes containing earth abundant active materials is critical to meet targets for installed energy capital costs.Na-based batteries containing sulfur cathodes are attractive for low cost, high energy batteries due to sulfur’s earth abundance and high capacity (e.g., 1,672 mAh/gS for complete reduction to Na2S). Notable examples of S electrodes for reversible Na storage include: (i) molten S (e.g., high temperature Na/S batteries commercialized by NGK Insulators, Ltd. and Tokyo Electric Power Company), (ii) S-based composites containing carbon matrices designed to mitigate dissolution of sodium polysulfides (Na2Sx) in liquid electrolytes, and (iii) RFB electrolytes containing soluble Na2Sx species.This presentation will describe recent work on Na-based biphenyl|polysulfide RFBs and will highlight the effects of soluble vs. insoluble reaction products on reversible capacity and cycling stability. Full cells exhibited stable capacities (up to 200 mAh/gS) with high coulombic efficiency (99.68 ± 0.40%) and negligible capacity fade in both static and flowing cell geometries. For the first time in nonaqueous RFBs, we report the use of 3-electrode galvanostatic AC impedance measurements to identify rate limiting processes in the system. These findings demonstrate that voltage losses are dominated by charge transfer at the cathode, and relevant kinetic parameters (i.e., exchange current density and transfer coefficients) were calculated through a Tafel analysis. In this study, Na+ ß” Al2O3 (BASE) was used as a prototypical membrane, but practical systems require development of low-cost, mechanically robust membranes with high ionic conductivity and minimal crossover. Acknowledgements This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the U.S. Department of Energy through the Energy Storage Program in the Office of Electricity.