NOVELTY - Electrolyte (15) (I) comprises a cationic moiety comprising silicon compound (II), where the electrolyte further comprises an anion such that the cationic moiety is present in a salt and the anion is bis(trifluoromethanesulfone)imide anion, bis(oxalato)borate anion or tetrafluoroborate anion.    USE - (I) is useful in an electrochemical double-layer capacitor (claimed). (I) is useful in electric and hybrid electric vehicles (preferably automotive starters and hybrid automotive vehicles), batteries and fuel cells.    ADVANTAGE - (I): enables the capacitor to operate at high voltages (preferably 5 volts or above) for extended periods; has high room temperature ionic conductivity (hence providing low series resistance), high thermal and electrochemical stability, and low volatility, toxicity and flammability; is relatively easy to make, and can be synthesized in high yields from available and relatively inexpensive starting materials; and is stable in storage and processing.    DETAILED DESCRIPTION - Electrolyte (15) (I) comprises a cationic moiety comprising silicon compound of formula ((R5)(R6)(R7)Si-R4-P(R2)(R3)(R1)) (II), where the electrolyte further comprises an anion such that the cationic moiety is present in a salt and the anion is bis(trifluoromethanesulfone)imide anion (preferred), bis(oxalato)borate anion or tetrafluoroborate anion.    R1-R3 = alkyl moieties of less than ten carbons or hydrogen, where at least one of R1-R3 are alkyl moieties;    R4 = alkyl moieties of less than ten carbons or alkoxy groups of less than ten carbons;    R5-R7 = alkyl moieties of less than ten carbons, H or -O-SiR8R9R10; and    R8-R10 = alkyl moieties with less than ten carbons or H.    An INDEPENDENT CLAIM is included for a quaternary phosphonium salt of formula (((R5)(R6)(R7)Si-R4-P(R1)(R2)(R3))+.A-) (II).    A = anion, preferably bis(trifluoromethanesulfone)imide anion, bis(pentafluoroethanesulfonyl)imide anion, bis(oxalato)borate anion, tetrafluoroborate anion or cyanamido anion.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of an electrochemical double-layer capacitor.    Electrochemical double-layer capacitor (10)    Electrode (11)    Separator (12)    Current collector (13)    Electrolyte (15)