Abstract: An electrolyte system suitable for use in an energy storage device (such as a supercapacitor), and energy devices which comprising the electrolyte system which is made up of an ionic liquid, such as Li or EMI TFSI and a stabilising amount of a stabilising additive. The stabilising additive preferably contains nitrile and or aromatic (benzene) groups, and may be advantageously benzonitrile, cinnamonitrile or succinonitrile. The stabilising additive stabilises the energy storage device against ESR rise and/or capacitance loss but does not adversely affect other performance characteristics of the ionic liquid.