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Redox flow batteries are promising candidates for grid storage, and while some large-scale installations exist, current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials costs, and result in less corrosive electrolytes. Examples of quinone derivatives, from benzoquinone to anthraquinone, have been shown to function in at least hundreds of charge/discharge cycles in flow batteries, providing a promising demonstration for the utilization of organics as active materials. Furthermore, if non-aqueous (aprotic) electrolyte solvents were used in place of aqueous electrolytes, a 2- to 3-fold increase in operating voltage could result, yielding a smaller footprint in these enormous stationary batteries. We are interested in developing new organic compounds for both aqueous and nonaqueous electrolytes.

Our research stemmed from our involvement in lithium-ion batteries wherein redox-active organic molecules play an important role. In some cases, oxidation or reduction or organic electrolytes (in many cases dialkylcarbonates) results in polymerization reactions at electrode/electrode interfaces that may lead to stabilizing layers that prevent further electrolyte decomposition and are important to extensive battery operation. In other cases, reversible redox reactions can enable electrolyte additives to serve as redox shuttles that limit the potential of individual cells through an internal mechanism of shuttling charge from one electrode to another, at times when cell potential is higher than its desired maximum value.

Our group evaluated phenothiazine, carbazole, phenoxazine, diphenylamine, and dialkoxybenzene derivatives to determine the structural characteristics that led to improved stability in multiple states of oxidation. This is important, as a redox shuttle functions in two redox states, the radical cation (or radical anion) of which is more susceptible to decomposition. Ultimately, in side-by-side tests, we found that a N-alkylated phenothiazine derivative with trifluoromethyl groups as substituents was just the right combination of redox potential, solubility, and stability to provide the most extensive overcharge protection of cells containing commercial electrodes (LiFePO4 and graphite) ever reported. Since that accomplishment, our work has involved a series of studies, a few of which are summarized here.

One area of interest is the development of stable redox shuttles for higher voltage lithium-ion batteries. This kind of lithium-ion battery would have a different cathode material than LiFePO4 that would allow it to be charged to higher potentials (>4 V/cell). For that kind of lithium-ion battery, a higher oxidation-potential redox shuttle is required, as it is necessary to be able to fully charge a cell prior to redox shuttle activation. Our group has synthesized a few higher-potential phenothiazine derivatives for this purpose, and they do provide overcharge protection, albeit not with the extent to which was afforded by our most promising lower-potential shuttles. A component of our current research involves designing not only new materials with higher redox potentials to test, but also new methods with which to screen these materials both in situ and ex situ.

It turns out that many of the properties required for successful function as a redox shuttle in lithium-ion batteries are consistent with those required for active materials in redox flow batteries. This type of battery works by pumping solutions of electroactive species from tanks to electrochemical reactors. The separation of power and energy results in a modular design that is advantageous for large-scale battery installations, which are needed – in connection to our electrical grid – to increase the amount of intermittent renewable resources that can be deployed onto the grid, ultimately leading to a reduction in the rate of CO2 production by lowering the consumption of fossil fuels. Here our group is making strides in the development of materials for the positive and negative sides of flow cell battery, which you can read about in our recent publications. Feel free to contact me for additional information on ongoing projects and to see if you might be a good fit for our group!