Effect of Fumed SiO2 on Pore Formation Mechanism and Various Performances of β-iPP Microporous Membrane Used for Lithium-ion Battery Separator

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In this work, four samples containing different contents of fumed SiO2 were prepared to improve the pore size distribution and various properties of β nucleated isotatic polypropylene (β-iPP) biaxial membrane used for lithium-ion battery separator. The wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) results show that the fumed SiO2 promotes the formation of β-crystal slightly and narrows down the thickness distribution of β-lamellae; meanwhile, evenly distributed SiO2 within β-iPP can be inspected by scanning electron microscopy (SEM). Moreover, further detailed characterization of morphological evolutions during biaxial stretching by tensile testing and SEM manifests that SiO2 can strengthen β-iPP and make the samples deform more homogeneously, resulting in a gradually elaborate and finer oriented microfibril structure after longitudinal stretching, in which more uniform defects distribute between fibrils and restrain the formation of coarse fibrils effectively. Therefore, more superior microporous structure emerges with the addition of SiO2, accompanied by narrower pore size distribution and better connectivity between microvoids, which is confirmed by mercury porosimeter and diminished Gurley value. Moreover, the lower thermal shrinkage, decreased shrinkage rate and suppressed porosity reduction indicate that fumed SiO2 improves thermal and dimensional stability of membrane dramatically. Furthermore, due to the excellent wettability of SiO2 with electrolyte, the microporous membranes doped with SiO2 have higher electrolyte uptake, even after heat treatment at elevated temperature.