Highly enhanced energy density of supercapacitors at extremely low temperatures

Improving the low-temperature tolerance and increasing the energy density of supercapacitors are currently desirable for real-life applications. Commercially available non-aqueous supercapacitors are typically limited in operation to −40 °C or higher, and they usually exhibit poor performance at lower than room temperature. In this work, we investigate symmetric supercapacitors using the 1-butyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid modified with organic solvents as the electrolyte and activated polyaniline-derived carbon (APDC) as the active material. The symmetric coin cells tests are performed in the range of temperatures from −100 °C to 25 °C. Interestingly, with decreasing temperature, the working voltage window of these supercapacitors gradually increases from 2.5 V to 4.0 V, whereas the energy density initially increases and then decreases. The optimal combination of an APDC electrode and the EMIMBF4/acetonitrile/methyl acetate-based electrolyte enables such a supercapacitor to work with a high potential window of 3.5 V and deliver a maximum energy density of 80 Wh kg−1 (based on the total mass of the two-electrode material), a maximum power density of 26 kW kg−1, and a long-term cycle life of 10000 cycles at a low temperature of −50 °C.