Advancing Low-Cost Flow Battery Chemistries
Authors: Saraidaridis James D 
Published: August 2024

Pumped-hydroelectric presently dominates U.S. grid electricity storage capacity resources, but the past decade has brought increasing levels of electrochemical energy storage (EES) installations.1 Policymakers have recognized the need for longer-duration systems as more EES gets added to the grid and sought systems that can provide durations of 8+ hours.2
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3 Although Li-ion presently dominates EES deployments, the average duration of U.S. utility-based-systems are relatively short at 3 hours.4 Redox flow batteries (RFBs) have an advantage over Li-ion systems at long durations due to their system architecture. The marginal cost of another hour of RFB duration asymptotes to the cost of additional active materials and their storage containers, as opposed to the cost of additional Li-ion cells, packs, and modules.
Leading vanadium RFBs have made cost reductions by improving the power density of their energy conversion stacks,5
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6 but eventually are limited by the relatively high cost of their vanadium active materials. Vanadium RFB suppliers are actively seeking to reduce this cost through business innovations. Yet, from a technical perspective, switching to lower cost active materials can also permit economically feasible long duration RFB systems.7

RTX Technology Research Center has previously reported on the low cost active material chemistries including the sulfur/manganese (S-Mn) chemistry and ligand-coordinate metals (LCM, e.g. chromium/iron).8
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9 S/Mn uses active materials based on low-cost sulfur, a waste product of fossil fuel production, and inexpensive manganese, an element presently used commercially in disposable primary cells. LCM chemistries use mass-produced organic ligands and inexpensive metal centers like iron, chromium, and titanium, and can rely on more neutral, and benign, electrolyte pH conditions that offer wider electrochemical stability windows. Along with these benefits, LCM chemistries bring new challenges such as managing pH in an environment where small absolute concentration changes have outsized effects. This abstract will focus on the progress made advancing low-cost chemistries toward industrial relevance via thousands of hours of operation and in full-sized cell/stack form-factors.

Acknowledgements:

The work presented herein was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR000994. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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