We have focused on the PEG-borate ester as a new type of plasticizer for solid polymer electrolyte composed of poly(ethyleneglycol) methacrylate (PEGMA) and lithium bis-trifluoromethanesulfonimide (LiTFSI). The PEG-borate ester shows good thermal stability and high flash point. Ionic conductivity of the polymer electrolyte increases with increasing amount of the PEG-borate ester and exhibits values greater than 10(-4) S cm(-1) at 30 degreesC and 10(-3) S cm(-1) at 60 degreesC. Furthermore, PEG-borate ester has three EO chains whose lengths are variable, and various ionic conductivities are expected to depend on EO chain length. As a result, polymer electrolyte containing the PEG-borate ester whose EO chain length is n = 3 shows highest ionic conductivity. Furthermore, polymer electrolytes containing PEG-borate esters show excellent thermal and electrochemical stability. The electrolytes are thermally stable up to 300 degreesC and electrochemically up to 4.5 V vs. Li+/Li.