Abstract:

Supercapacitors, advanced electrochemical devices, have attracted great interest due to their extraordinary properties, such as high power density, fast charging or discharging rate, and ultra-long cycle life. Currently, great efforts have been devoted to increasing their moderate energy density (typically < 5 Wh·kg^-1). Especially, room temperature ionic liquids (RTILs) have been considered as a promising electrolyte for further improving supercapacitor's performances owing to their large voltage window, high thermal stability, and wide working temperature range. However, RTILs suffer from the high viscosity and poor conductivity stemming from their strong cation–anion interactions. In this work, we investigate the influences of solvent on the capacitive performance within RTIL-based supercapacitors. Activated graphene powders are employed as the electrode active materials, and 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIMBF₄) is chosen as the electrolyte because of the wide applications in electrochemical energy storage. The mole fraction of BMIMBF₄ (ρ_IL) in electrolytes can be regulated with adjusting the ratio of acetonitrile solvents (AN). Electrochemical measurements suggest that the solvent effects on the charge storage capability of supercapacitors depend strongly on the applied scan rate or current density. Specifically, at a lower scan rate of 10 mV·s^-1, solvent exhibits a negligible influence on the electrochemical performance; however, at an elevated scan rate of 200 mV·s^-1, solvent addition could prominently enhance the capacitance by ~2 folds. These results can resolve the controversial solvent effects reported in previous simulation and experimental studies. To interpret the as-obtained results, we further explore the solvent effects on the dynamic properties of electrolytes. It is found that solvent can effectively reduce the strong ion–ion interactions within pristine RTILs, thus decreasing the viscosity by ~29 times. Further electrical impedance spectroscopy tests suggest that the addition of solvent is able to significantly suppress the series resistance (by ~5.5 times) and dielectric relaxation time (by ~6.3 times), which thereby improves the rate capability of supercapacitors. We demonstrate that the maximum specific energy and power density of supercapacitor (ρ_IL = 0.25) are calculated to be 65.2 Wh·kg^-1 at 1 A·g^-1 and 18066.6 W·kg^-1 at 20 A·g^-1, respectively, among the best performances in the state-of-art literatures. More importantly, under an elevated working temperature of 50, its energy density can reach up to 85.5 Wh·kg^-1 at 1 A·g^-1, which is much higher than that of aqueous or organic solution based supercapacitors (< 10 Wh·kg^-1) and lead-acid battery (20–35 Wh·kg^-1), comparable to that of Ni metal hydride (40–100 Wh·kg^-1) and lithium-ion battery (80–150 Wh·kg^-1).

Key words: Supercapacitor, Room temperature ionic liquid, Solvent effect, Electrochemical performance, Activated graphene powder