Facile preparation of surface-modified polypropylene nanofiber separators with enhanced ionic transport and welding performance for lithium-ion batteries

Nanofiber separators are promising for high-performance lithium-ion batteries due to their high porosity and electrolyte uptake. However, most of them have been criticized for the low mechanical strength, particularly polyolefin nanofiber separators, due to the poor welding performance caused by weak interaction among the fibers. The lack of mass-production techniques is another problem for polyolefin nanofiber separators though polyolefin has the competitive cost and stability. In present study, polypropylene nanofiber separators (PPNFS) prepared via nanolayer coextrusion, a new appeared method suitable for large-scale preparation, are surface modified via a simple dip-coating method. The electrostatic self-assembly on the surface of polypropylene nanofibers is achieved. The incorporation of modified polymers significantly enhances the interaction at the nanofiber overlaps, facilitating in-situ welding of nanofibers and resulting in an 800% enhancement in separator mechanical properties. Polybenzimidazole (PBI) is a strange polymer with both electron-donating and electron-accepting sites. Experimental characterization and simulation calculations confirm that PBI-modified PPNFS exhibit a distinctive enhanced ionic transport and promote the formation of a more homogeneous solid electrolyte interphase (SEI) layer. Furthermore, PBI coated PPNFS (PPNFS@PBI) separators demonstrate exceptional thermal stability, superior electrolyte uptake, and enhanced fire resistance. Finally, the effect of molar concentration and molecular weight of PBI on battery performance is evaluated. 15% Molar concentration of PBI with 88,000 molecular weight has been shown to maximize battery performance.