Novel microporous poly(vinylidene fluoride) blend electrolytes for lithium-ion batteries

In the present paper, novel microporous poly(vinylidene fluoride)-hexafluoropropylene copolymer (PVDF-HFP) blend electrolyte/electrode films were obtained as the result of phase separation between PVDF-HFP and polyethylene oxide (PEO) oligomer additives, which were cast from a common solvent. Upon solvent evaporation and removal of the additives, an interconnected microporous morphology was formed. The additives can either be removed from the films by vacuum evaporation or methanol extraction. The conductivities of the methanol extracted microporous (pore sizes range from 1 to 5 mu m) films formed from PVDF-HFP/PEO oligomer blends after electrolyte activation are more than 70% higher than those of the methanol extracted nanoporous (pore size in range from 10 to 100 nm) films prepared from PVDF-HFP/dibutyl phthalate blends. Microporous films formed by vacuum evaporation had conductivities similar to solvent extracted nanoporous separators. Battery performance tests were carded out using lithium cobalt dioxide as the cathode and mesocarbon microbeads as the anode. The cells prepared using extracted microporous PVDF-HFP/PEO oligomer blend films as separators show more than 40% higher specific discharge capacity at the C/2 rate, and 70% higher rate capability than those using extracted nanoporous separators. (C) 2000 The Electrochemical Society. S0013-4651(99)12-064-0. All rights reserved.