Thermal-Gated Self-Repairing Polyimide Separator for Dendrite-Suppressed Lithium Metal Batteries

The internal heat generation and the growth of lithium dendrites have raised severe safety issues in lithium metal batteries (LMBs), which significantly hinder their widespread adoption. Therefore, it is critical to develop intelligent separators to improve the security and performance of LMBs. Here, we engineer a self-repairing polyetherimide (PEI)-functionalized polyamide-imide (PAI@PEI) nanofiber separator with a thermal-gated function, in which the thermoplastic PEI core has an automatically thermal shutdown function via intelligent closure of apertures under high temperature, while the thermosetting PAI shell can drive the remodeling of PEI to restore its apertures. The PAI@PEI separator showcases the topmost aperture-closing temperature of 400 °C compared to the cutting-edge separators that typically have an aperture-closing temperature below 200 °C. Morphological characterization confirms that the PAI@PEI separator with a closed aperture can recover its apertures at 350 °C, endowing the PAI@PEI separator with a unique self-repairing function to enhance the longevity and safety of LMBs. Meanwhile, density functional theory calculations reveal that the polar amide and imide groups in PAI@PEI separator, both before and after aperture restoration, can efficiently facilitate Li-ion dissociation and transportation for suppressing lithium dendrite growth. As a result, the aperture-restored PAI@PEI separator (R-PAI@PEI) demonstrates significantly improved overall electrochemical performance. Specifically, the R-PAI@PEI-based Li||Li cell exhibits an exceptional Li-ion transference number of 0.71 and an excellent cycling stability at 1 mA cm^-2 for over 750 h, which significantly outperform commercial and state-of-the-art separator-based LMBs (typically below 0.65 and 500 h, respectively). Importantly, the R-PAI@PEI-based Li||NCM523 battery still exhibits an impressive specific capacity of 99.7 mAh g^-1 at 5C and maintains 90% of its capacity after 100 cycles. These results underscore the feasibility of designing functional separator, opening a new avenue for next-generation highly safe LMBs separators.