TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation (Disclosed): A secondary sulfide and an alkyl halide are mixed and heated to obtain tertiary sulfonium halide salt. The obtained salt is dissolved in aqueous medium and subjected to anion exchange reaction with reagent such as hexafluorophosphate to obtain the sulfonium salt. Preferred Composition: The concentration of the sulfonium salt in the electrolyte solution is 0.5-3M. The non-aqueous organic solvent contains dimethyl carbonate or propylene carbonate, preferably ethylene carbonate, propylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, gamma -butyrolactone and/or acetonitrile. Preferred Definitions:    R1-R3=1-4C organic group;    R4=1-3C linear alkyl;    n=4;and    Y'-=N(SO2F)2-, N(SO2CF3)2-, N(SO2C2F5)2-, BF4-, PF6-, CF3SO3- or CF3CO2-, preferably BF4-.    SPECIFIC COMPOUNDS - The sulfonium salt is chosen from 7 disclosed compounds such as n-butyl dimethyl sulfonium bis(trifluoromethyl sulfonyl) imide of formula: S+(-CH3)2-C4H9.N(CF3SO2)2- (A), 1-methyl-tetrahydro-thiophenium bis(trifluoromethyl sulfonyl) imide, dimethyl ethyl sulfonium bis(trifluoromethyl sulfonyl) imide, diethyl methyl sulfonium bis(trifluoromethyl sulfonyl) imide and ethyl methyl sulfonium bis(trifluoromethyl sulfonyl) imide.    EXAMPLE - Acetonitrile (in g) (20) and n-butyl methyl sulfide (10.22) were stirred in a flask. Iodomethane (14.2) was further added and the contents were reacted at 25 degrees C for 2 weeks. The precipitated crystals were separated, washed with diethyl ether and vacuum dried. The dried crystals (1.99) were dissolved in pure water (50 ml) and added to lithium bis(trifluoromethyl sulfonyl) imide (2.82). Methylene chloride (50 ml) was added and the organic phase was extracted, purified and vacuum dried to obtain n-butyl dimethyl sulfonium bis(trifluoromethyl sulfonyl) imide (1) with melting point of -2 degrees C, ionic conductivity of 5.2 mS/cm at 25 degrees C and viscosity of 63 mPa.second at 25 degrees C.