Tailoring solvent mixtures to boost electrochemical stability and low-temperature performances of supercapacitors.

The important objective in improving supercapacitor's performance is to increase their operating voltage and expand their working temperature range. Among various organic electrolytes reported so far, the conventional acetonitrile (AN)-based electrolyte features the highest conductivity but suffers from a relatively low operational voltage window (2.5−3.0 V). Our study investigated binary mixtures, combining propionitrile (PN) and AN as the electrolytes designed for high-voltage and high-rate supercapacitors to address this limitation. The physical and electrochemical properties of various binary mixtures are systematically examined. The supercapacitors utilizing these binary electrolyte systems demonstrated remarkable capacitance retention over 10,000 cycles at the upper voltage of 3.4 V, in contrast to the significant capacitance degradation observed in cells using single AN-based electrolytes. Among the various binary electrolytes, carbon-based supercapacitors using a PN−AN with a 20:80 vol ratio achieved an optimal balance among the ionic conductivity (42.2 mS cm−1), long-term stability, working voltage (3.4 V), energy density (32.6 Wh kg−1), and power density (2278 W kg−1). Impressively, it also obtains good rate capability and capacitance retention (>95 %) at a voltage as high as 3.4 V under ultra-low temperature of −40 °C. This approach paves the way to developing high-voltage supercapacitors with optimal performance. [Display omitted] • Binary electrolytes of AN and PN are optimized. • The PA-28 electrolyte represents the optimal compromise. • Supercapacitors based on PA-28 electrolyte can work at 3.4 V and −40 °C. [ABSTRACT FROM AUTHOR]