Influence of Lewis acidic borate ester groups on lithium ionic conduction in polymer electrolytes

Polymer electrolytes having borate ester groups, which are part of the polymer matrix, have been prepared. The transference number of the lithium ions increases with increasing concentration of the borate ester groups, and therefore it is considered that the borate ester groups, having Lewis acidity, interact with Lewis basic anions. Furthermore, the transference numbers of lithium ions in the polymer electrolytes containing LiCF3SO3 or LiClO4 were 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 indicate that the borate ester group prefers to interact with a 'hard' basic anion, CF3SO3- or ClO4-. This is in good agreement with the obtained experimental results.