The electrochemical properties of 1-ethyl-3-methylimidazolium difluorophosphate ionic liquid have been investigated as an electrolyte for electric double-layer capacitors using activated carbon electrodes. A two-electrode cell test reveals that the capacitance of exhibits a larger voltage dependence than that of a typical ionic liquid electrolyte . At a charging voltage of 2.5 V, the capacitance obtained for is and is larger than obtained for . According to the charge–discharge cycle test, the breaking-up voltage of is lower than that of , which may be caused by the lower anodic stability of than that of . The ionization potential of was calculated by quantum mechanical calculations to estimate its anodic stability and was compared with those of several anions used in typical electrolytes. The results indicate that the anodic stability of is similar to that of and lower than those of and .