Mixed Solvents to Optimize the Energy Density of Nonaqueous Vanadium Redox Flow Batteries
Authors: Smith Kirk P. , Petit Luke F , Monroe Charles W. 
Published: May 2020

Nonaqueous redox flow batteries have the potential to be more energy-dense than their aqueous counterparts. The wider electrochemical stability windows of organic solvents enable higher-voltage redox reactions, which have already been demonstrated to improve molar energy density (Wh/molactives). Exceeding the theoretical maximum volumetric energy density (Wh/L) of state-of-the-art commercial active liquids (37.5 Wh/L for aqueous vanadium at 1.4 V and 2.0 M, a standard concentration [1]) requires increasing the active-species concentration past a threshold that is inversely proportional to the nonaqueous cell’s potential. Vanadium acetylacetonate (V(acac)3) is a potentially useful active species for nonaqueous flow batteries because its disproportionation delivers 2.2 V [2].
As well as containing an electrochemically active species, flow-battery electrolytes must also contain a supporting salt, whose ions maintain electroneutrality during cell operation by travelling across the separator membrane. The minimum ionic strength needed in the electrolyte to maintain this charge balance up to the maximum theoretical state of charge depends on the number of charges received by the active species across all of its oxidation states. This minimum support concentration may be different from the optimal concentration of salt required to achieve sufficient conductivity.
For a given nonaqueous electrolyte, optimization for a given active-species chemistry to achieve maximum volumetric energy density requires careful selection of solvent systems and supporting electrolyte, subject to the constraints of solubility limits (at all oxidation states of the active species), electroneutrality, and conductivity [3]–[5].
In this work we use the well-established disproportionation chemistry of the V(acac)3 active species, with tetraethylammonium tetrafluoroborate (TEABF4) as supporting salt. The solubility limit of V(acac)3 in several solvents is determined densitometrically [4]. Promising binary mixtures of mutually miscible solvents are explored further, with the aim of choosing one solvent that preferentially solvates V(acac)3 and the other that solvates TEABF4. Optimization of the binary electrolyte identifies a solvent system of acetonitrile and dioxolane which offers a maximum V(acac)3 solubility of 0.95 M and a theoretical energy density of 29.3 Wh/L. The reversible electrochemistry of this mixture is confirmed with voltammetry, and stable cyclability is demonstrated in a benchtop flow cell [6]. Key performance metrics will be reported, including energy, voltaic, coulombic, and utilization efficiencies, electrolyte conductivity, and peak power density.
While the results for the V(acac)3/ TEABF4 system are promising, this work provides a logical workflow and strategy for maximizing energy density that can be applied to other nonaqueous chemistries to achieve their true potential.

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