A wider operational voltage is one of the benefits of using organic solvents for flow battery applications. Aqueous flow batteries are restricted to 1.5 V, but organic solvents such as acetonitrile and propylene carbonate can be used at voltages above 5 V. Most electrolytes, however, have limited solubilities in these organic solvents. Furthermore, as the electrolyte concentration increases, transport properties such as viscosity deteriorate which imposes increased pumping power requirements for flow battery applications and results in slower reaction kinetics. Our prior work with alkyl ammonium counter cations on the redox-active dianionic vanadium complex, vanadiumIV bis-hydroxyiminodiacetate ([VBH]2-), has boosted VBH solubility (0.80 M [N(butyl)4]2[VBH] in acetonitrile). However, observed viscosities increased rapidly as the concentration was raised, reaching 3.63 cP (at 25 °C) for a 0.5 M [N(butyl)4]2[VBH]. To gain a molecular understanding of the apparent concentration dependence of viscosity, we employed full atomistic molecular dynamics (MD) studies on several [alkyl ammonium]2[VBH] electrolyte systems. First, we report the force field bonded parameters for the [VBH]2- anion derived from a PBE-D3BJ/def2-TZVP quantum mechanical data using the JOYCE protocol. Second, we report calculated viscosities following the Green-Kubo formalism and establish an in silico viscosity-concentration dependence for [VBH]2- with different alkyl ammonium cations in acetonitrile benchmarked with experimental viscosity data. Predicted viscosities for [N(butyl)4]2[VBH] showed excellent agreement with experiment for concentrations below 0.3 M. Insights from these simulations could reveal fundamental understanding of the origin of the steep viscosity increase observed experimentally. Further, this current strategy will allow in silico experiments geared at developing electrolyte blends with improved transport properties.

Force field parameterization and viscosity calculations for an anionic nonaqueous redox flow battery active-material: A molecular dynamics study

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