Safety assessment of polyolefin and nonwoven separators used in lithium-ion batteries

This paper features a comprehensive safety assessment of lithium nickel manganese cobalt oxide (NMC)/graphite pouch cells incorporating several different types of polyolefin and nonwoven separators, including polyethylene (PE), polypropylene (PP), tri-layer PP/PE/PP, Al2O3-coated polyethylene terephthalate (hereinafter called CPET) and Al2O3-filled polyethylene terephthalate (hereinafter called FPET). We investigate the cell safety by means of thermal ramp, overcharge, internal short circuit, and external short circuit tests; at the same time, we characterize the structure and properties of the separator materials used in the cell samples. The results disclose that thermal dimensional stability, melt or melt rupture temperature and shutdown function of the separators greatly impact the lithium-ion battery safety. The nonwoven separators demonstrate exceptional thermal resistance and dimensional stability and greatly enhance the safety of lithium cells under certain conditions of abuse; however, the non-uniform distribution of the ceramic particles filled in the nonwoven separators could cause uneven current-density distribution leading to cell failures. •Physical properties of polyolefin and nonwoven separators compared.•Safety performance of the Li-ion battery cells using 5 different separators assessed.•The strength and a potential problem of nonwoven separators shown.•The key properties of a separator to battery safety identified.