Molecular Dynamics Insights into Vanadium-based Electrolyte Systems for Nonaqueous Redox Flow Battery: The Effect of Alkyl Ammonium Cations and Solvent*

Nonaqueous redox flow batteries (NRFB) show great promise as a technology for large-scale energy storage systems.  However, the demand for higher energy density systems, pushing towards higher active-material concentration, has resulted in higher electrolyte viscosity behavior, hindering their practical applications. Our prior work on alkyl ammonium counter cations on the redox-active vanadium^IV bis-hydroxyiminodiacetate ([VBH]^2-) enhanced VBH solubility (0.80 M [N(butyl)₄]₂[VBH] in acetonitrile). But, viscosities sharply increased, 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. Here, we report the force field bonded parameters for the novel [VBH]^2- anion derived from DFT. We also present the predicted viscosities following the Green-Kubo formalism and establish an in silico viscosity-concentration dependence for [VBH]^2- with different alkyl ammonium cations and different solvents (acetonitrile and dimethyl sulfoxide), benchmarked with experimental viscosity data.  Predicted viscosities for [N(butyl)₄]₂[VBH] showed excellent agreement with the experiment for concentrations below 0.3 M. Insights from these simulations could provide a fundamental understanding of the origin of the steep viscosity increase observed experimentally. This current strategy would enable in silico experiments geared at developing electrolyte blends with improved transport properties.