Ionic conductivity of microporous PVDF-HFP/PS polymer blends

Separators made from blends of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and polystyrene (PS) were microporous when cast from solvent/nonsolvent mixtures of acetone/dimethylcarbonate(DMC)/butanol(BuOH), where BuOH was the nonsolvent for both polymers. The ionic conductivities of these separators reached 2 mScm(-1) and could be increased to 4 mScm(-1) with addition of fumed silica. The morphology of the separators was characterized by scanning electron microscopy, nonsolvent (BuOH) and solvent (propylene carbonate, PC) uptake, and differential scanning calorimetry, and correlated with ionic conductivity measurements using 1 M LIPF6 in ethylene carbonate/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 the former contribution, which increased with increasing PS content, the higher the ionic conductivity. Passing two identical microporous films through a double-roll laminator at a temperature above the glass transition temperature, T-g, of PS (T-g = 95 degreesC) and below the melt temperature, T-m, of 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.