Nonaqueous redox flow batteries (NRFBs) are technology for stationary energy storage, which is needed to balance supply and demand of renewables with the electric grid. The use of nonaqueous solvents enables both wide electrochemical windows and a myriad of soluble redox-active organic compounds (redoxmers) that may be tailored for redox behavior, stability, and solubility. High solubility is especially important for achieving high energy densities. An ideal NRFB would operate at 2–4 M concentration. Additionally, the NRFBs fluids require high conductivity and low viscosity for technoeconomic success. Many redoxmers have high solubility in solvent alone or low-concentration electrolytes, but a realistic battery would operate at molar parity, i.e., equal amounts of redoxmer and electrolyte salt. Under this scenario, we previously examined redoxmer systems in both states of charge with a traditional electrolyte salt, lithium bis(triflimide). There is a fluidity-concentration tradeoff where viscosity and conductivity reach a maximum at low molarities (1 M or less) and a gel state is formed at higher concentrations due to increased ion association and network formation. Thus, we asked how to take these fluids to &gt; 2 M <i>and</i> maintain fluidity. We hypothesized that using ionic liquids (ILs) as the electrolytes coupled with small low-melting redoxmers and higher temperatures would solve this predicament. Many ILs are known to exhibit high conductivity and viscosity at &lt;100 °C. We predicted and measured the fluid properties and electrochemical behavior of several small redoxmers/IL combinations and achieved formulations with high concentration and fluidity.

Maximizing energy density and fluidity in concentrated nonaqueous electrolytes

ACS Spring 2022

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