Deformation and fracture behaviors of microporous polymer separators for lithium ion batteries

The functionality and reliability of the separator are crucial to the abuse tolerance of a battery since the separator serves as the physical barrier to prevent any contact (short circuit) between the positive and negative electrodes. Therefore, understanding the mechanical behavior, especially the deformation and fracture behaviors of the separator are of great importance for battery design and manufacturing. Here we report the deformation behaviors of five commercially available microporous polymer separators investigated by conventional tensile testing coupled with in situ tensile testing under an atomic force microscope. Morphological models were developed to elucidate the tensile deformation mechanisms. For anisotropic separators (Celgard 2325 and 2400) made by the dry process, material direction dictates the significant diversity in overall mechanical integrity of the separator: they have limited mechanical properties when stretched in the transverse direction (TD), whereas they are rather robust when pulled in the machine direction (MD). The anisotropy of these separators is a result of the distinct deformation mechanism of the stacked lamellae in the separator. Separators manufactured by the wet technique (Toray V20CFD and V20EHD, Teijin Lielsort) behaved more biaxially - all mechanical properties were nearly identical in both MD and TD. Moreover, in order to evaluate the fracture properties of these separators, the essential work of fracture (EWF) approach was adopted. The EWF results show that the fracture properties for the dry processed separators also present orientation dependence. When stretching in the MD, the MD-oriented slit-like pores serve as crack tip blunters to inhibit the propagation of cracks whereas the TD-oriented pores exactly facilitate the crack propagation by linking up the pores with the crack tip when stretching in the TD. The same toughening mechanism (tip blunting) was also found in the case of wet processed separators.