Sandwich-structured composite separators with an anisotropic pore architecture for highly safe Li-ion batteries

Safety and electrochemical performance are key concerns for porous separators used in Li-ion batteries. We demonstrate the potential application of a sandwich-structured composite separator (SSCS) for highly safe Li-ion batteries, which is comprised of a poly(vinylidene fluoride) (PVDF) porous bulk and a trapped polyethylene terephthalate (PET) nonwoven fabrics layer. The as-prepared SSCS possesses an anisotropic porous structure with large inner pores while small surface ones arising from a nonuniform cooling process along the thickness direction during thermally induced phase separation. The distinctive structure of SSCS gives rise to excellent properties concerning security, such as electrolyte retention, mechanical strength, thermal stability and thermal-shutdown ability. Furthermore, the higher ionic conductivity as well as lower interfacial impedance of SSCS endows the resulting LiFePO4/Li battery with enhanced discharge capacity and cycle stability compared to the cell assembled by the commercialized Celgard 2400 separator. Therefore, the SSCS is expected to be a promising candidate for Li-ion batteries with high cell performances and enhanced safety.