Systematic selection of solvent mixtures for non-aqueous redox flow batteries - vanadium acetylacetonate as a model system

Employing solvent mixtures in the electrolyte of non-aqueous redox flow batteries can increase energy density and efficiency. In this paper, active species solubility, electrolyte conductivity, and redox reaction rates were examined systematically among a number of binary and ternary mixtures, consisting of acetonitrile and 5 polar aprotic co-solvents to identify mixtures with enhanced active species solubility and redox reaction rates. Although we used vanadium acetylacetonate as a model, the methodologies presented here are applicable when evaluating solvent mixtures for other active species. Our approach is distinctive in that it elucidated the trade-offs in desirable properties that are necessary when solvent mixtures are used for non-aqueous redox flow batteries. We found that in a vanadium acetylacetonate-based non-aqueous redox flow battery, the use of an 84/16 vol% acetonitrile/1,3-dioxolane binary solvent mixture resulted in an increase in the positive and negative sides reaction rates compared to that observed with pure acetonitrile. This binary mixture resulted in an improvement in reaction rate with no decrease in energy density and is a promising solvent system for other active species used for non-aqueous flow batteries.