Structure and ionic conductivity of porous polymer electrolytes based on PVDF-HFP copolymer membranes

The porous poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes were prepared by solvent evaporation method using dibutyl phthalate (DBP), polyvinylpyrrolidone (PVP-k30), polyethylene glycol 200 (PEG200) as additives. The influence of additives on morphology and structure, electrolyte uptake of porous membranes and lithium ionic conductivity of the activated membranes were investigated. The properties of the PVDF-HFP microporous membranes, such as crystallinity, thermal characterization, conductivity and mechanical strength were characterized. The PVDF-HFP gel electrolytes were formed by microporous PVDF-HFP membranes filled and swollen by a liquid electrolyte. The lithium ionic conductivity of polymer electrolytes at 25 °C was measured by ac impendence technique. The membrane took on asymmetric structure and relatively small pore size when DBP and PEG200 were used as additives, whereas nearly symmetric and microporous structure when PVP was used as an additive. Moreover, the average pore size and porosity was about 0.3 μm and 48.7%, respectively. The crystallinity of the membranes was smaller than that of pure PVDF-HFP and decreased in the sequence of PVP, PEG200 and DBP as additives. The maximum electrolyte uptake is 124.6 wt.% and the conductivity is 0.49 mS/cm when PVP as an additive.