Thermoreversible polymer gel electrolytes

Abstract

Thermoreversible polymer gel electrolytes with ionic conductivities in the region of 10 −3 S cm −1 (even at −20°C) have been prepared from a variety of commercially available polymers and organic solvents by gel casting from high-temperature solutions at polymer/solvent ratios down to 10 90 . Lithium trifluoromethanesulfonate has been incorporated as the ionic species necessary for conduction. A typical gel has polymer/solvent in mass ratio 40 60 and has salt incorporated to give an active O/Li ratio of 12 1 . In general, the dynamic modulus ( G′) of these gels is in excess of 10 5 Pa at low strain, but decreases rapidly with increasing strain amplitude despite remaining approximately constant with strain rate. This drop in modulus, which is attributed to breakdown of the gel network, is completely recoverable. In particular, one polymer, poly(vinylidene fluoride), was studied in detail. Gels made from this polymer formed self-supporting transparent films. The incorporation of lithium trifluoromethanesulfonate changed the crystal structure and decreased the solvent evaporation rate, at elevated temperatures, of poly(vinylidene fluoride) gels made with tetraethylene glycol dimethyl ether. Ionic conductivities of liquid electrolytes (dimethylformamide with lithium trifluoromethanesulfonate) and corresponding gels (dimethylformamide, lithium trifluoromethanesulfonate and poly(vinylidene fluoride)) suggest that there is no interaction between salt and polymer in these gels, although this is still under investigation.