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(ClO 4 ) 2, Mg(CF 3 SO 3 ) 2, and Mg[(CF 3 SO 2 ) 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 Mg 2+ cation ( t Mg 2+ ) of the electrolytes, it was found that the borate ester group interacts with anions with the consequence that t Mg 2+ increases with increasing concentration of borate ester group. By measuring Raman spectra for the electrolyte containing Mg(ClO 4 ) 2 salt, it was also found that the concentration of the free ClO 4 - anion increased with the increasing concentration of the borate ester group in the polymer electrolyte, which implied that the relative proportion of free Mg 2+ carrier ion also increased. The order of the t Mg 2+ value was Mg(ClO 4 ) 2 > Mg(CF 3 SO 3 ) 2 ≫ Mg[(CF 3 SO 2 ) 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 Mg 2+ and Raman spectra. These results indicate that the borate ester group as a Lewis acid interacts with hard anions of ClO 4 - or CF 3 SO 3 - more strongly than the soft anion of (CF 3 SO 2 ) 2 N - to enhance the degree of dissociation of the salt and trap the anion in the polymer electrolyte.