THERMALLY STABLE POLYMER ELCTROLYTE PLASTICIZED WITH PEG-BORATE ESTER FOR LITHIUM SECONDARY BATTERY

摘要

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 LiCF_3SO_3 or LiClO_4 was found to be higher than that in the electrolyte with LiN(CF_3SO_2)_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, CF_3SO_3~- or C1O_4~-.This is in good agreement with the obtained experimental results.