English

Preparation of High-Strength Polyimide Porous Films with Thermally Closed Pore Property by In Situ Pore Formation Method

• Yuting Bai ^1, ,
• Cenqi Yan ^1, ,
• Zhen Li ^{2, ,} ,
• Jiaqiang Qin ^{1, ,} ,
• Pei Cheng ^{1, ,}

• 1.

  College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China

• 2.

  Science and Technology Research Center of China Customs, Beijing 100026, China

Corresponding author: Email: 729431051@qq.com (Z.L.); Email: jqqin@scu.edu.cn (J.Q.); Email: chengpei@scu.edu.cn (P.C.); Tel.: +86-28-85405401 (P.C.)

• Received Date: 05 June 2023
  Accepted Date: 27 July 2023
  Revised Date: 26 July 2023
  Available Online: 15 September 2024
• Fund Project:

  the National Natural Science Foundation of China 

Abstract: Improving the thermal stability, chemical stability, and mechanical strength of battery separators is crucial to prevent safety incidents like thermal runaway in batteries. This significantly enhances the overall safety performance of batteries. Among various options, polyimide (PI) stands out as an ideal choice due to its outstanding thermal stability, excellent chemical stability, and high mechanical strength. However, existing preparation methods of PI separators, such as non-solvent induced phase separation (NIPS), template method, and electrospinning, often suffer from issues like inadequate mechanical strength. Therefore, this study focused on investigating a novel method to prepare thermoplastic PI porous films with thermally closed pores and enhanced mechanical strength. Several characterization techniques, including scanning electron microscopy (SEM), in situ Fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analyzer (TGA)-FTIR coupling, were employed to understand the pore-forming mechanism of PI porous films. The findings revealed that the temperature range of triethylamine (TEA) removal was consistent with the main stage of the imidization reaction and pore formation. This indicated that the pore structure was formed in situ during the thermal imidization process when TEA was stripped out of the PI film. PI films with varying TEA contents were prepared to investigate the impact on pore structure, showing that pore size could be regulated by TEA content. A more regular reticulated small pore structure on the macroporous pore wall was observed when TEA content was ≥ 100%. SEM analysis showed that the films were thermally self-closed at a heat treatment temperature of 300 ℃. Additionally, TGA indicated that the thermal decomposition temperature of PI porous film reached 580 ℃. The mechanical strength of the PI films before and after pore closure was investigated, demonstrating excellent mechanical strength of approximately 120 MPa. The novel in situ pore formation method for PI porous films through the salt-formation method of poly (amic acid) (PAA) with the organic base TEA, followed by TEA release during thermal imidization, resulted in PI porous films with outstanding thermal stability and high mechanical strength. The self-closure of the PI porous film at high temperatures effectively isolates material and heat transport, providing robust safety assurance for batteries. This advancement has the potential to significantly improve battery safety and performance.

Key words:
• Porous film
•  /  Polyimide
•  /  Thermally closed pore property
•  /  Preparation method
•  /  Mechanical property

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