Solid polymer electrolytes with high ionic conductivities are prepared by using poly(ethylene oxide) (PEO) and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) as polymer matrixes, succinonitrile (SN) as an additive, and lithium bis-tri-fluoromethanesulfonimide (LiTFSI) and lithium bisperfluoroethylsulf'onylimide (LiBETI) as salts. In these systems, the introduction of succinonitrile into the polymer electrolytes increases the material's ionic conductivity and conveys excellent mechanical properties. The described composites, with their beneficial combination of mechanical and electric properties, are expected to have significant potential for lithium batteries.