A novel quaternary ammonium salt—1,1-dimethylpyrrolidinium tetrafluoroborate (DMPBF4), tetraethylammonium tetrafluoroborate (TEABF4), and spiro-(1,1′)-bipyrrolidinium tetrafluoroborate (SBPBF4) were prepared and dissolved in acetonitrile (ACN) and propylene carbonate (PC) for electrolyte systems of electric double-layer capacitors (EDLC). Physical properties such as thermal stability, solubility, and ionic conductivity were determined and six different electrolyte systems with 1 M concentration were electrochemically evaluated by linear sweep voltammetry, cyclic voltammetry, galvanostatic charge-discharge, and float test. To investigate the origin of the physical and electrochemical properties at the molecular level, quantum calculations for the chemical species in the electrolytes were also carried out. Compared to the standard electrolytes TEABF4 and SBPBF4, the novel DMPBF4 electrolyte exhibits a higher gravimetric capacitance, superior rate capability, and improved cycling stability. The energy and power density of EDLCs using DMPBF4 in the ACN electrolyte reach 40.61 Wh kg−1 and 8.09 kW kg−1, respectively, at the working voltage of 3.0 V. Moreover, EDLCs with DMPBF4 in ACN (PC) retain 87.9% (87.5%) of their initial capacitance after 1000 h of float testing at 3.0 V. The results indicate that DMPBF4 can be a promising electrolyte salt for EDLC application.