Thermally Stable Polymer Elctrolyte Plasticized with Peg-Borate Ester for Lithium Secondary Battery

We 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 showed good thermal stability and high flash point. Ionic conductivity of the polymer electrolyte increased with increasing amount of the PEG-borate ester and exhibits values greater than 10^-4 S cm^-1 at 30°C and 10^-3 S cm^-1 at 60°C. Furthermore, PEG-borate ester has three EO chains whose length is 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 showed highest ionic conductivity. Furthermore, polymer electrolytes containing PEG-borate esters showed excellent thermal and electrochemical stability. The electrolytes were stable thermally up to 300°C and electrochemically up to 4.5 V vs. Li⁺/Li. The transference number of lithium ions in the polymer electrolyte containing LiCF₃SO₃ or LiClO₄ was found to be higher than that in the electrolyte with LiN(CF₃SO₂)₂. Ab initio calculations were performed to estimate the interactions between the borate ester groups and the anions. The calculated results indicated that the borate atom acts as a Lewis acid center and prefers to interact with "hard" basic anion, CF₃SO₃^- or ClO₄^-. This is in good agreement with the obtained experimental results.