Different solvent mixtures were investigated for non-aqueous vanadium acetylacetonate (V(acac)(3)) redox flow batteries with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The aim of this study was to increase the energy density of the non-aqueous redox flow battery. A mixture of acetonitrile, dimethyl sulfoxide and 1-3-dioxolane nearly doubles the solubility of the active species. The proposed electrolyte system was characterized by Raman and FT-IR spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy and charge-discharge set-up. Spectroscopic methods were applied to understand the interactions between the solvents used and their impact on the solubility. The potential difference between oxidation and reduction of V(acac)(3) measured by cyclic voltammetry was about 2.2 V. Impedance spectroscopy showed an electrolyte resistance of about 2400 Omega cm(2). Experiments in a charge-discharge test cell achieved coulombic and energy efficiencies of similar to 95% and similar to 27% respectively. The highest discharge power density was 0.25 mW cm(-2). (c) 2014 Elsevier B.V. All rights reserved.