Ionic conductivity of microporous PVDF-HFP/PS polymer blends

Separators made from blends of poly(vinylidene fluoride-co-hexafluoropropyl ene) (PVDF-HFP) and polystyrene (PS) were microporous when cast from solven t/nonsolvent mixtures of acetone/dimethylcarbonate(DMC)/butanol(BuOH), wher e BuOH was the nonsolvent for both polymers. The ionic conductivities of th ese separators reached 2 mScm(-1) and could be increased to 4 mScm(-1) with addition of fumed silica. The morphology of the separators was characteriz ed by scanning electron microscopy, nonsolvent (BuOH) and solvent (propylen e carbonate, PC) uptake, and differential scanning calorimetry, and correla ted with ionic conductivity measurements using 1 M LIPF6 in ethylene carbon ate/DMC/diethyl carbonate, in the ratio 4:5:1 by weight. The solvent filled the microporous network and caused the pore wails to swell. The greater th e former contribution, which increased with increasing PS content, the high er the ionic conductivity. Passing two identical microporous films through a double-roll laminator at a temperature above the glass transition tempera ture, T-g, of PS (T-g = 95 degreesC) and below the melt temperature, T-m, o f PVDF-HFP (T-m = 133 degreesC) preserved the microporous structure, with a conductivity decrease of only 20% compared with unlaminated Alms. (C) 2001 The Electrochemical Society. All rights reserved.