Eutectic electrolytes can attain high concentrations of redox-active species, offering a path towards high energy density redox flow batteries. Here we introduce a new entropically-driven eutectic mixing approach using organic small molecules. By mixing chemically similar redox-active species, we engineer highly concentrated, low viscosity liquids composed almost entirely of redox active molecules. Using quinones as a model system, we discover a ternary benzoquinone eutectic mixture and a binary naphthoquinone eutectic mixture which have theoretical redox-active electron concentrations of 16.8 and 8.8 M e−, respectively. We investigate compatibility with protic supporting electrolytes and quantify ionic conductivity and viscosity of quinone eutectic electrolytes across multiple states of charge. A binary naphthoquinone eutectic electrolyte with a protic ionic liquid supporting electrolyte (7.1 M e−, theoretical volumetric capacity 188 Ah L−1) achieves a volumetric capacity of 49 Ah L−1 in symmetric static cell cycling. These preliminary results suggest that entropy-driven eutectic mixing is a promising strategy for developing high energy density flow battery electrolytes.
Minor formatting changes and additional data added to the Supplementary Information. Main paper has been updated to reference the new SI figure.