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

## TL;DR

A self-repairing separator for lithium metal batteries is developed to improve safety and performance by suppressing dendrite growth and enhancing thermal stability.

## Contribution

A thermal-gated self-repairing separator is introduced with aperture restoration and high thermal shutdown temperature for safer lithium metal batteries.

## Key findings

- The PAI@PEI separator achieves an aperture-closing temperature of 400°C, much higher than existing separators.
- The R-PAI@PEI separator enables a Li+ transference number of 0.71 and cycling stability for over 750 hours at 1 mA cm−2.
- The separator-based Li||NCM523 battery retains 90% of its capacity after 100 cycles at 5C.

## Abstract

The self-repairing polyetherimide (PEI)-functionalized polyamide-imide (PAI@PEI) nanofiber membrane with a thermal-gated function was designed to enhance the thermal safety properties of lithium metal batteries.The PAI@PEI membrane with the polar amide and imide groups facilitates Li+ dissociation and transport, which is essential for suppressing dendrite growth.The aperture-restored PAI@PEI-based Li||Li cell exhibits an exceptional Li+ transference number of 0.71 and an excellent cycling stability at 1 mA cm−2 for over 750 h.

The self-repairing polyetherimide (PEI)-functionalized polyamide-imide (PAI@PEI) nanofiber membrane with a thermal-gated function was designed to enhance the thermal safety properties of lithium metal batteries.

The PAI@PEI membrane with the polar amide and imide groups facilitates Li+ dissociation and transport, which is essential for suppressing dendrite growth.

The aperture-restored PAI@PEI-based Li||Li cell exhibits an exceptional Li+ transference number of 0.71 and an excellent cycling stability at 1 mA cm−2 for over 750 h.

The online version contains supplementary material available at 10.1007/s40820-025-02050-2.

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.

The online version contains supplementary material available at 10.1007/s40820-025-02050-2.