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.