Effect of high molecular weight on pore formation and various properties of microporous membrane used for lithium-ion battery separator

In this article, iPP cast films with high molecular weight in middle layer were prepared to investigate the effect of high molecular weight component on pore formation and mechanical strength. The results of DSC, SEM and 2D-XRD indicated that higher molecular weight and die draw ratio (DDR) would not alter the thickness of lamellae but enhance the orientation and the content of extend-chain crystal. Then the tensile testing results show that higher DDR would improve the overall strength and the elastic recovery substantially but reduce the elongation at break dramatically. Additionally, the cast film containing higher molecular weight component has longer elongation and yield stress but shows a more gentle decrease of elastic recovery during the repeatedly stretching cycle. Moreover, further detailed characterization of the microporous structure after uniaxial stretching manifest that lower DDR is extremely unfavorable to pore formation, resulting in lowest porosity and poorer pore size distribution. While this inferior microporous structure would be improved by increasing DDR, accompanied by ascending porosity. Moreover, the membrane completely composed of high molecular weight also exhibits more coarse fibrils, which is stemmed from the undivided row-nucleated lamellae. Furthermore, the puncture and tensile strength were reinforced by boosting molecular weight and DDR significantly, which is more beneficial to battery assembly and safety.