Liquid Catholyte Molecules for Non-Aqueous Redox Flow Batteries

Non-aquoes redox flow battery is a unique energy storage technology. Unlike traditional battery systems, the active energy storage material is in mobile phase and could basically be stored anywhere separately. This gives a lot of flexibility in terms of capacity, power and safety control, making it an idea candidate for large scale stationary energy storage application.

Material development for this technology is crucial for possible implementation. Molecular engineer of organic redox active molecules could lead to tunable physical and electrochemical properties, including solubility in organic solvents, molecular mobility, redox potential, and electrochemical reversibility, which are key factors to energy storage applications. A novel series of catholyte molecules, ANL-8, ANL-9, and ANL-10, has been developed by incorporation of PEO chains with different lengths on one or both sides of the dimethoxy-di-tert-butyl-benzene based redox structure. All of these molecules can afford electrochemically reversible behavior with similar redox potentials around 4.0 V vs Li/Li+. Due to the asymmetrical incorporation of PEO groups, ANL-8 and ANL-9 are found to be liquid at room temperature. The results from the diffusion, viscosity, DFT calculation, overcharge evaluation, and cell cycling indicate that ANL-8 is likely to be the best candidate among the redox active molecules in this study and does have promise to be a novel catholyte material for NRFBs. To our knowledge, it is the first time that the liquid catholyte molecules have been discovered for the application of NRFBs. The liquid nature of these catholyte molecules may open a new door to utilization of these molecules as the solo or co- solvents for high energy NRFB applications.