Eutectic Mixing of Quinones—A Strategy for Engineering Highly-Energy Dense Liquid Electrolytes

To address the low energy density of redox flow batteries, eutectic electrolytes have emerged as an approach to achieving high concentrations of redox-active species. Here we introduce an entropically-driven eutectic mixing approach which, through eutectic mixing of chemically similar redox active species, can be used to engineer highly concentrated liquids composed of nearly all redox active molecules. Demonstrated using quinones, we investigate a ternary benzoquinone eutectic and a binary naphthoquinone eutectic which,if the total charge could be accessed, have a theoretical volumetric capacities of 16.8 and 8.8 M e-, respectively. We quantify melting point, ionic conductivity, and viscosity across multiple states of charge. We also explore application to redox flow batteries using a binary naphthoquinone eutectic electrolyte with a protic ionic liquid supporting salt, achieving a volumetric capacity of 37 Ah/L in symmetric static cell cycling. Though reactivity of the redox active quinones and capacity fade during cycling present challenges for the electrolytes in this study, the entropically-driven eutectic approach presents a general framework which could be used to develop highly concentrated, low viscosity electrolytes using redox-active organic small molecules.