Sodium Metal Hybrid Redox Flow Batteries Containing Sulfide and Thiophosphate Catholytes

Abstract

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 proposed work is aimed at developing low-cost, high-energy redox flow batteries (RFBs) based on earth-abundant active materials. Prior work on room temperature Na2Sx catholytes demonstrated outstanding reversibility and cycling stability in batch and flow cell configurations (e.g., reversible capacities ~200 mAh/gS with negligible fade over 250 cycles and 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 cycling window of Na2Sx is restricted to soluble species (5≤x≤8) with a sulfur utilization of 125 mAh/gS. 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 previously unknown Na-P-S solvated complexes. This general class of sodium thiophosphates has tremendous, untapped potential for next-generation nonaqueous catholytes and can potentially enable reversible Na capacities exceeding 1,000 mAh/g. This presentation will describe ongoing efforts aimed at: (i) assessing key properties (electrochemical reversibility, solubility, chemical stability) of sodium thiophosphates in nonaqueous electrolytes, (ii) benchmarking the performance of lab-scale RFB prototypes containing Na-P-S catholytes and ceramic membranes, and (iii) quantifying voltage losses on electrodes polarized far from equilibrium using 3 electrode AC impedance measurements. 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.