The ionic conductivity of a solid polymer electrolyte based on copper(II) trifluoromethanesulfonate dissolved in poly(ethylene oxide) (PEO) was significantly improved by addition of a non-volatile solvent sulfolane (tetramethylene sulfone, TMS). The bulk conductivity of (PEO)(15)Cu(CF3SO3)(2) thin-film electrolyte can be increased from similar to 10(-5) Omega(-1) cm(-1) at ambient temperature to almost 5 x 10(-4) Omega(-1) cm(-1) for (PEO)(15)Cu(CF3SO3)(2)TMS20wt.% composite electrolyte. The charge transfer resistance of a Cu\electrolyte\Cu symmetrical cell decreases after addition of the solvent from similar to 2.9 k Omega cm(2) (at ambient temperature) to similar to 0.2 k Omega cm(2). The activation energy of ionic conductivity obtained from In sigma =f(T-1) plot was 98.9 kJ mol(-1) for the (PEO)(15)Cu(CF3SO3)(2) and 79.5 kJ mol(-1) for the (PEO)(15)Cu(CF3SO3)(2)TMS20wt.% electrolyte. (C)