Cross-Linked Polyimide Nanofiber Menbrane Fabricated By Electrospinning And The Application Research For LIB Separator

Rechargeable lithium ion battery (LIB) with high energy, long cycle life, high working voltage and excellent safety performance has been used for electric vehicles, smart phones and laptops. With the development in the field of new energy, the research and exploitation of the lithium ion battery possessing larger capacity, smaller size, higher temperature resistant property and superior safety performance attracted great attention. The separator in battery plays an important role in electrically isolating cathode and anode to prevent electrical short circuits and at the same time allow rapid transport of ionic charge carriers. Currently, the most of commercially available lithium ion battery separators are polyolefin micro-porous membranes. However, the polyolefin separators can’t meet the requirement of high performance LIB due to its low porosity, low electrolyte uptake, poor wettability and low melting temperature. Therefore, the development of new separators owing high porosity, good thermal stability and high ionic conductivity has become a hot research topic in the field of battery and new energy.Polyimide (PI) nanofiber membranes become the most potential new LIB separator due to combination of the advantages of nanofiber membrane such as high porosity, high specific surface and the specialties of polyimide such as excellent high temperature resistance, chemical stability, self-extinguishing and chemical characteristics of polarity. However, the loose overlap of PI nanofiber and high porosity 80～90% lead to poor mechanical property, which is a major obstacle for practical application.In this paper, Polyimide (PI) nanofiber nonwoven with cross-linked network topographies were prepared for Li-ion battery (LIB) by ammonia pretreatment of the electrospun poly(amic acid) precursor, followed by high-temperature thermal imidization. The ammonia-induced crosslinking degrades the porosity, ionic conductivity and electrolyte uptake by 10～20%, but offers extremely-valuable benefit by converting the loose and weak PI nonwoven to compact and robust fabric, which promotes the tensile strength from 11.4 to 37.4 MPa and shifts the deformation temperature from 328 to 380 ℃. Compared with Celgard separator, the cross-linked PI membrane shows higher porosity, higher electrolyte uptake, better wettability, higher ionic conductivity and better thermal dimensions stability. The cell assembled with the cross-linked PI membranes displays outstanding cycling stability and high capability at 5C, which is 30% higher than that of the cell assembled with Celgard, and could work steadily at 120℃ with no capacity fading. These characteristics make the cross-linked PI membrane advanced separator for high-performance LIBs.