In redox flow batteries, electricity is stored in the form of chemical energy in a liquid electrolyte. This results in a relatively low energy density. Limiting is the limited solubility of the vanadium ions and the narrow electrochemical window of aqueous electrolytes. With electrolytes based on ionic liquids, TSE aims to overcome these two limitations. In the case of aqueous electrolytes, TSE is working with partners to improve the functional materials (membrane and graphite felt electrodes) and to develop a state-of-charge sensor.