Interaction between the Lewis acid group of a borate ester and various anion species in a polymer electrolyte containing Mg salt

To clarify the interaction between a Lewis acid and anionic species of the supporting salt incorporated in a polymer electrolyte, we designed a novel solid polymer electrolyte based on Mg salt complexes of poly(ethylene glycol) (PEO) chains cross-linked by a borate ester group as a Lewis acid and examined the ionic conduction mechanism of the electrolyte. Mg(ClO4)(2), Mg(CF3SO3)(2), and Mg[(CF3SO2)(2)N](2) were used as the Mg salt. To change the concentration of the Lewis acid in the polymer electrolyte, two different lengths of PEG chains, which were cross-linked by borate ester group, were used. By estimating the transport number of the Mg2+ cation (t(Mg2+)) of the electrolytes, it was found that the borate ester group interacts with anions with the consequence that t(Mg2+) increases with increasing concentration of borate ester group. By measuring Raman spectra for the electrolyte containing Mg(ClO4)(2) salt, it was also found that the concentration of the free ClO4- anion increased with the increasing concentration of the borate ester group in the polymer electrolyte, which implied that the relative proportion of free Mg2+ carrier ion also increased. The order of the t(Mg2+) value was Mg(ClO4)(2) > Mg(CF3SO3)(2) much greater than Mg[(CF3SO2)(2)N](2). The change in the total energy due to the interaction between the PEG-borate ester and each anion species using ab initio calculation is in good agreement with the results of the t(Mg2+) and Raman spectra. These results indicate that the borate ester group as a Lewis acid interacts with hard anions Of ClO4- or CF3SO3- more strongly than the soft anion of (CF3SO2)(2)N- to enhance the degree of dissociation of the salt and trap the anion in the polymer electrolyte.