Ion transport properties of lithium ionic liquids and their ion gels

Abstract

A new series of lithium ionic liquids were prepared by introducing of two electron-withdrawing trifluoroacetyl groups in borate salts containing two methoxy-oligo(ethylene oxide) groups in the structures. Successive substitution reactions of oligo-ethylene glycol monomethyl ether and trifluroacetic acid from LiBH 4 yielded the lithium salts, which were clear and colorless liquids at room temperature. The fundamental physicochemical properties, such as density, thermal property, viscosity, ionic conductivity, self-diffusion coefficients, and electrochemical stability, were measured. The lithium ionic liquids had self-dissociation ability and conducted ions even in the absence of organic solvents. New polymer electrolytes, named ‘ion gels’, were prepared by radical cross-linking reactions of a poly(ethylene oxide- co-propylene oxide)tri-acrylate macromonomer in the presence the lithium ionic liquid. An increase in the glass transition temperatures ( T g) of the ion gels was very small even with increasing lithium ionic liquid concentration, and the T g's were lower than that of the ionic liquid itself. The ionic conductivity of the ion gels surpassed that of the lithium ionic liquid in the bulk at certain compositions.