Assessment methods for identifying suitable charge carriers for non-aqueous redox flow batteries

Redox flow batteries (RFBs) are emerging as a promising battery technology for grid-scale energy storage. The utilization of non-aqueous solvents expands the repertoire of existing electrolytes toward wider electrochemical windows, which is critical for achieving high energy densities. Successful implementation of non-aqueous RFBs on a large scale necessitates identification of suitable charge carriers through the thorough evaluation of key physicochemical properties, such as redox potential, solubility, solution resistance, transport, and electrokinetic properties. These characteristics further inform the performance metrics of the resulting batteries. To date, there is a lack of systematic guidelines and protocols that direct synthetic chemists with consistent procedures to screen electrolytes for practical applications. This is especially true for researchers interested in studying redox-active inorganic molecules as charge carriers for these applications. In this tutorial-review, we discuss the design criteria, testing methods, and H-cell experimental design for inorganic candidates for emergent non-aqueous redox flow battery technologies. We also present a general framework and recommendations on testing procedures that are suitable in different scenarios based on the relevant chemical information that is desired on a given electrolyte. Finally, we conclude the discussion on our envisioned strategies to enable predictive design strategies for next-generation non-aqueous redox flow batteries.