Nonaqueous Sodium-Based Catholytes for Redox Flow Batteries

E. Self, M. Lehmann, G. Yang
Oak Ridge National Laboratory,
United States

Keywords: redox flow batteries, AC impedance, sodium polysulfides, sodium thiophosphates, catholytes


Energy storage systems which meet the requirement for long-duration energy storage (LDES) are critical to enable widespread adoption of intermittent renewables (e.g., solar and wind) on the electric grid. DOE’s Long Duration Storage Shot has set an ambitious goal of reducing energy storage costs ≥90% by 2030. To address this challenge, the present work is aimed at developing low-cost, high-energy redox flow batteries (RFBs) based on earth-abundant active materials. This talk will summarize recent work on redox flow batteries containing nonaqueous Na-based catholytes including sodium polysulfides (Na2Sx) and thiophosphates (NaxPSy). Na2Sx catholytes have outstanding reversibility and cycling stability (e.g., reversible capacities ~200 mAh/gS with negligible fade over several months of continuous testing). While precipitation of low-order polysulfides (x≤4) during discharge doesn’t negatively impact the performance of lab-scale prototypes, these ionically/electronically insulating species will present major challenges for system scaleup (e.g., inhibited charge transfer due to current collector passivation). As such, the practical capacity of Na2Sx is only 125 mAh/gS when restricted to cycling soluble species (5≤x≤8). To improve the viability of room temperature Na/Na2Sx RFBs, our team recently discovered that the addition of P2S5 greatly increases the solubility of low-order sodium polysulfides through formation of Na-P-S solvated complexes. This general class of sodium thiophosphates is largely unexplored and can theoretically enable reversible Na capacities exceeding 1,000 mAh/g. This presentation will provide recent findings on key properties (electrochemical reversibility, solubility, and chemical stability) of sodium thiophosphates in nonaqueous solvents. The use of integrated AC impedance measurements to identify rate limiting steps in flow batteries will also be highlighted. Acknowledgements This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy and is sponsored by the U.S. Department of Energy in the Office of Electricity through the Energy Storage Research Program, managed by Dr. Imre Gyuk.