Cross-Linking Synergistic Effects to Enhance the Comprehensive Properties of Electrospun Polyimide Nanofiber Membranes for Advanced Lithium-Ion Batteries

Electrospun polyimide (PI) nanofiber membranes have served as promising separators for safe lithium-ion batteries (LIBs) due to their excellent thermal stability. However, electrospun PI nanofiber membranes have excessively large pores and low mechanical strength, which have been shown to be unsuitable for direct utilization as LIB separators. To solve these issues, we proposed a novel electrospun PI nanofiber membrane obtained using a triple cross-linking strategy, including precalendering, microsolvent dissolution, and chemical imidization. The precalendering process, as a prerequisite, densified the poly(amic acid) (PAA) nanofiber membrane and followed by microsolvent dissolution and chemical imidization, which provided synergistic cross-linking effects. This resulted in a mutual anchoring effect of the ultralong molecular chains at the cross-linking interface, forming a robust cross-linking structure and efficient imidization transition. As a result, the mechanical properties of the samples significantly improved, while the tensile strength increased from 9.9 to 95.5 MPa. Furthermore, as the pressing degree of the membranes increased, the electrolyte uptake, ionic conductivity, and capacity decreased; however, the ability to inhibit lithium dendrite growth and uniformly produce solid electrolyte interfaces (SEIs) increased. Compared to the noncross-linked PI membrane, the PI membrane obtained using the triple cross-linking strategy had a stable interface with the lithium metal electrodes and a high-capacity retention rate. This robust PI nanofiber membrane could serve as a promising replacement for traditional polyolefin separators in high-performance LIBs.