Microstructure of poly(vinylidene fluoride)-based polymer electrolyte and its effect on transport properties

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF/HFP)-based membranes have been prepared using tri-o-cresyl phosphate, dibutyl phthalate, propylene carbonate, or triethyl phosphate as plasticizers. Scanning electron microscopy and the gas adsorption/desorption method were used to reveal the microporous structure of the membranes. Results indicate that the boiling point and viscosity of the plasticizers are important factors during the membrane-forming process. The membrane prepared with a plasticizer of higher boiling point can achieve higher porosity, electrolyte uptake, and thus higher ionic conductivity. The pore sizes of the PVDF/HFP membranes obtained in this study are mostly below 40 nm while the average pore sizes are smaller than 20 nm. The conductivities at room temperature are higher than 10(-3) S cm(-1). The transference numbers measured using the dc polarization method coupled with ac impedance spectra are found to be above 0.7 when 1 M LiPF6 in ethylene carbonate/gamma-butyrolactone (1: 1 v/v) was used as the liquid electrolyte counterpart. (C) 2002 The Electrochemical Society.