Ethan Self

Energy storage systems which meet the requirements 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, our team at Oak Ridge National Laboratory is developing low-cost, high-energy redox flow batteries (RFBs) based on earth-abundant active materials.

This talk will summarize recent work on nonaqueous RFBs containing Na-based catholytes including sodium polysulfides (Na₂S_x) and thiophosphates (Na_xPS_y). Na₂S_x catholytes have outstanding reversibility and cycling stability (e.g., reversible capacities ~200 mAh/g_S with negligible fade over several months of continuous testing). While precipitation of low-order polysulfides (x≤4) during discharge does not 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).

To improve the viability of room temperature Na/Na₂S_x RFBs, our team recently discovered that the addition of P₂S₅ greatly increases the solubility of low-order sodium polysulfides through formation of solvated Na-P-S complexes. This general class of catholytes 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 novel AC impedance methods to identify rate limiting steps in these systems will also be highlighted. Overall, these studies combine electroanalytical measurements with a suite of characterization tools to understand the underlying mechanisms which govern device performance.