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Polymer gel electrolytes were prepared by soaking poly(vinylidene fluoride) (PVdF) membranes, obtained by casting solutions of PVdF copolymer or homopolymers in a solvent such as acetone, tetrahydrofuran (THF), methyl ethyl ketone (MEK) and N-methylpyrollidone (NMP), into an electrolyte solution of 1 M LiBF4 in propylene carbonate (PC). During the casting process, phase separation occurred leading to non- or low-porous membranes when the phase separation remained in its early stage or to highly porous membranes when it was mostly achieved. The phase separation was enhanced with the increase in the polymer crystallinity and was more significant in the case of the homopolymers. The membranes cast from NMP solutions were non- or low-porous and showed very low uptakes of the electrolyte solution and low conductivity. In this case, the copolymer showed higher uptakes of the electrolyte solution than the homopolymers. It was also found that even for an equivalent uptake of the electrolyte solution the conductivity was higher for the copolymer than the homopolymer, attributable to the lowest crystallinity of the copolymer. On the contrary, the membranes obtained from MEK solutions with slow evaporation rates were highly porous for the homopolymers and absorb much more electrolyte solution than non- or low-porous membranes. The polymer electrolytes from the porous membranes exhibited conductivity of 10−3 S/cm at 30°C. The phase separation process was very important since it allowed a higher uptake of the electrolyte solution and thus a higher conductivity. Nevertheless, increasing porosity affected the mechanical properties of the membranes when it exceeded 50% of the total volume. The principal limitation of the uptake of electrolyte was caused by the loss of mechanical strength. Concerning the potential window and the evolution of the interfacial resistance at the Li/polymer electrolyte interface, no significant difference could be pointed out between the copolymer and the homopolymers. |
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