Crosslinked Polyimide Nanofiber Membrane Prepared via Ammonia Pretreatment and Its Application as a Superior Thermally Stable Separator for Li-Ion Batteries

Polyimide (PI) nanofiber membrane with crosslinked morphology has been prepared by ammonia pretreatment of the electrospun poly(amic acid) precursor followed by an imidization process. The ammonia-induced crosslinking offers an obvious benefit for converting the loose and weak PI nonwoven to compact and robust fabric membrane. This promotes the tensile strength from 10.8 MPa to 37.5 MPa and shifts the onset deformation temperature from 328°C to 380°C. Crosslinking also chemically welds the nanofibers together, which greatly reduces the risk of robust fabric membrane disassembling into the loose nonwovens during long-term battery cycling, and consequently improves the stability of PI nanofiber membrane as separator. The cell assembled with the crosslinked PI nanofiber membrane as separator displays outstanding cycling stability and rate capability with 80% capacity retention up to 5 C (130 mAh/g) and could work steadily at 120°C with no capacity fading. The separator perfectly retains its crosslinked feature without lithium deposition after 100 battery cycles. These characteristics make the crosslinked PI nanofiber membrane advanced separator for high-performance Li-ion batteries.