Next-generation vanadium redox flow batteries: harnessing ionic liquids for enhanced performance

Abstract

Vanadium redox flow batteries (VRFBs) have emerged as a promising contenders in the field of electrochemical energy storage primarily due to their excellent energy storage capacity, scalability, and power density. However, the development of VRFBs is hindered by its limitation to dissolve diverse vanadium salts in the aqueous solution without significantly impacting the viscosity and thereby, the operational efficiency. To address this challenge, a novel aqueous ionic-liquid based electrolyte comprising 1-butyl-3-methylimidazolium chloride (BmimCl) and vanadium chloride (VCl₃) was synthesized to enhance the solubility of the vanadium salt and aid in improving the efficiency. The synthesized novel electrolyte combination showcased a maximum theoretical energy density of approximately 44.24 Wh L⁻¹, a dynamic viscosity of 36.62 mPa s along with a stable potential window of approximately 1.8 V, and an ionic conductivity of 0.201 S cm⁻¹ at room temperature. Furthermore, the aqueous ionic-liquid based VRFB demonstrated an appreciable coulombic efficiency and capacity retention of greater than 85% at a discharge current of 5 mA. The maximum achievable concentration utilizing deionized water was obtained to be 2 M, which can be significantly enhanced by utilizing various component combinations of organic solvents, and ionic liquids to unlock the full potential of VRFBs. This novel electrolyte composition provides a promising pathway for improving the energy density and operational efficiency of VRFBs, paving the way for advanced energy storage solutions.