Effects of Novel Boric Acid Esters on Ion Transport Properties of Lithium Salts in Nonaqueous Electrolyte Solutions and Polymer Electrolytes

Abstract

Three novel polymerizable anion receptors based on boric acid esters have been synthesized. The addition of these monomers appreciably enhanced the ionic conductivity of certain electrolyte solutions comprised of an aprotic organic solvent of low polarity and a lithium salt of low dissociation ability. Analysis of the viscosity and pulse-field-gradient spin−echo (PGSE) NMR results in association with the ionic conductivity data revealed that the conductivity enhancement originated from the increase in the degree of dissociation, resulting from the addition of these anion receptors. The 11B NMR spectra of dimethoxyethane electrolyte solutions with added boric acid ester monomers substantiated the finding that the ionic dissociation was facilitated by strong interaction between the Lewis-acidic anion receptor and Lewis-basic anions. The polymerizable anion receptor of the catechol borate derivative was cross-linked with a polyether macromonomer containing different lithium salts. The ionic conductivity could be correlated with glass transition temperatures of the polymer electrolytes by the WLF equation. The ionic conduction behavior of the boron polymer electrolytes was compared with that of the reference polymer electrolytes, and the lithium cation transference number was clarified to be higher for the former. The polymer electrolytes showed similar conduction behavior to that of the electrolyte solutions containing the anion receptor monomers.