A simple approach for preparation of porous polybenzimidazole membranes as a promising separator for lithium ion batteries

Abstract

A simple and controllable approach has been developed for the preparation of a series of poly(2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole) (OPBI) porous membranes with controlled porosity and pore size. The micropores were formed by simply extracting poly(ethylene glycol) (PEG) from the dry OPBI/PEG blend membranes with water. The effects of PEG molecular weight (M_w = 350–10 000) and PEG weight fraction (OPBI/PEG = 1 : 1–1 : 5) on membrane morphology and properties were investigated. It was found that higher M_w and weight fraction of PEG in polymer blends tended to give higher porosity but lower mechanical strength. The porous membrane, M-4, prepared from PEG10000 (M_w = 10 000) at a weight ratio of OPBI/PEG10000 = 1 : 5 exhibited a high porosity (71%), a high ionic conductivity (1.3 mS cm⁻¹ at room temperature), and reasonably high tensile strength (10 MPa). Furthermore, M-4 showed only ∼5% thermal shrinkage after heating at 200 °C for 1 h and good fire-retardant properties which were much better than those of the commercial separator Celgard 2400. The charge–discharge cycle test and C-rate performance test results revealed that M-4 was a promising candidate as a separator for lithium ion batteries.