The need for carbon neutrality and sustainable energy practices is making the development of eco-friendly batteries as substitutes for conventional Li-ion batteries imperative. This is essential for reducing costs, carbon emissions, and energy usage associated with battery manufacturing and disposal processes. Redox flow batteries (RFBs) based on Na-P-S redox active materials are reported as an alternative to Na₂S_x. We have found that incorporating phosphorus into the polysulfide structure improves its solubility in organic solvents, specifically in diglyme, potentially increasing the volumetric capacity of the catholyte solution.
Our work focused on the synthesis and electrochemical characterization of Na-P-S complexes. The synthesis method implemented was based on wet synthesis, which, in contrast with the mechanochemical methods reported in the literature, reduces the amount of energy invested in the synthesis process. After evaluating different ratios of Na:P:S, we decided to evaluate the sodium thiophosphates Na₂P₂S_x, where x = 6, 7, 8, and 9.
Initially, the electrochemical properties of sodium thiophosphate catholytes were compared using CV. In general, Na₂P₂S_x exhibited a larger peak separation (~2 V) and lower peak currents at a given scan rate, potentially indicating a significant activation barrier associated with the rearrangement of the thiophosphate’s polyanionic structure during reduction/oxidation. Despite the substantial voltage hysteresis, Na₂P₂S_x displayed stable cycling performance, with a slight increase in peak current after 100 cycles. This behavior is consistent with measurements on RFBs containing a Na₂P₂S_x catholyte, where an electrode activation process increased the cell’s reversible capacity, likely due to subtle changes in the electrode’s surface chemistry during repeated oxidation/reduction. Finally, preliminary flow cell measurements on Na₂P₂S_x catholyte using a solid/liquid hybrid RFB configuration show reversible voltage profiles after the formation step with deep discharge. Overall, this work demonstrates that Na-P-S catholytes represent a promising material for high-energy, low-cost redox flow batteries.

Ernesto Zuleta Suarez

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