Exploring Eutectic Mixing of Quinones for Engineering High Energy Density Electrolytes

Eutectic electrolytes can attain high concentrations of redox-active species, offering a path toward 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.