TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Solvent: The electrolyte liquid contains solvent chosen from (gamma)-butyl lactone, propylene carbonate, ethylene carbonate, ethylmethyl carbonate, diethyl carbonate, vinylene carbonate and/or acetonitrile. Preferred Definitions:    R1-R3=at least one is methyl;    a+b+c=1-3;and    A-=tetra-fluoroborate ion.    SPECIFIC COMPOUNDS - 32 quaternary ammonium salts are disclosed, such as N,N-dimethyl-N-ethyl-N-methoxy ethoxyethyl ammonium-bis trifluoro methyl sulfonyl imidate and N,N,N-trimethyl-N-methoxy ethoxyethyl ammonium-bis trifluoro methyl sulfonyl imidate.    EXAMPLE - Dimethyl ethylamine (in g) (5.12), 1-chloro-2-(methoxy ethoxy)ethane (11.6) and acetonitrile (10.2) were mixed and reacted at 80 degrees C. The reaction mixture was then concentrated, and the obtained residue was dried under reduced pressure. Thus, N,N-dimethyl-N-ethyl-N-methoxy ethoxyethyl ammonium chloride (14.8) was obtained. The obtained N,N-dimethyl-N-ethyl-N-methoxy ethoxyethyl ammonium chloride (14.8) was mixed with methanol (29.6) and activated carbon (0.2). The resulting solution was filtered. Then, tetrafluoro sodium borate (7.69) was mixed with filtrate, and stirred at room temperature for 1 hour. The obtained mixture was concentrated and methanol was distilled. Methylene chloride (36.8) was added to the residue, and filtered. Methylene chloride was distilled from the resulting solution, to obtain oily N,N-dimethyl-N-ethyl-N-methoxy ethoxyethyl ammonium tetrafluoroborate (15.3) in yield of 82.2 %. The obtained N,N-dimethyl-N-ethyl-N-methoxy ethoxyethyl ammonium tetrafluoroborate had viscosity of 224.6 mPa.second at 25 degrees C and ionic conductivity of 1.19 mS/cm at 25 degrees C. An electrolyte liquid was formed using the obtained N,N-dimethyl-N-ethyl-N-methoxy ethoxyethyl ammonium tetrafluoroborate and (gamma)-butyl lactone. The obtained electrolyte liquid was excellent in stability even at high temperature. Highly durable electrolytic capacitor and electric-double-layer capacitor were provided by using the electrolyte liquid.