Polyimide-Modified PPTA-Coated Separator for Lithium Metal Batteries

The ANF (aramid nanofiber)-coated separator possesses enhanced thermal stability performance when compared to the polyolefin-based separator. The overly dense nanofiber structure reduces the pore size within the separators and increases the impedance. In this study, the polyimide (PI) was introduced to the ODA-PPTA colloidal solution, and a composed PPTA-PI structure was formed. The averaged pore size within the PPTA-PI coating layer (∼367 nm) was larger than that within the PPTA coating layer (∼260 nm) through the thickness. Similar pore sizes were observed on the coating layer, ensuring a uniform lithium ion flux. Meanwhile, the PPTA-PI-coated separator possesses higher tensile strength and puncture strength. The ionic conductivity increases from 0.74 mS/cm (bare PE separator) to 0.76 mS/cm (PPTA-coated separator) and further to 0.82 mS/cm (PPTA-PI coated separator). The Li + transference number of PPTA-PI-coated separator reaches 0.58, which is higher than 0.45 (PPTA-coated separator) or 0.31 from (bare PE separator). Also, the PPTA-PI-coated separator shows improved capacity retention and lithium dendrite suppression compared to the bare PE and PPTA-PE separator after cyclic tests. Compared to other aramid coating methods, the current approach would enable the mass production of PPTA-PI-coated separator at a lower cost.