Phosphonate-based supercapacitor electrolyte integrating the advantages of flame retardancy, extreme temperature adaptability, and anti-supergravity

Abstract

Traditional acetonitrile-based organic electrolytes continue to dominate in carbon-based supercapacitors due to their high performance-to-price ratio. However, its flammability, low extreme temperature applicability, and unknown anti-supergravity limit its application in some special scenarios (e.g., space exploration). For the first time, an electrolyte system based on potassium hexafluorophosphate (KPF6) and diethyl ethylphosphonate (DEEP) as the electrolyte salt and solvent, respectively, was proposed. The KPF6/DEEP system combines excellent flame retardancy, high capacitance performance at high (60 °C) and low (-70 °C) temperatures, anti-supergravity (60 G), and an operating voltage of up to 3.0 V, resulting in a stable and high power energy output of up to 16 Wh kg−1 for carbon-based supercapacitors at −60 °C. These benefits are primarily attributed to the distinct physical properties of DEEP (e.g., flame retardancy, low freezing point, high boiling point, and high compatibility with KPF6). Such outstanding overall performance, which incorporates excellent flame retardancy, extreme temperature adaptability, anti-supergravity, and cycle stability (capacity retention > 95 % after 14,000 cycles), outperforms the traditional acetonitrile-based system, opening up a new option for power devices for some applications with extreme environmental demands.