Fabrication Of Polyimide Nonwovens With Cross-Linked Morphology Via Electrospinning For Lithium-Ion Battery Separator Application

Faced with energy shortages and environmental issues in modern times, as a kind of rechargeable battery, lithium ion battery (LIB) has drawn much attention in the new power supply technology due to its outstanding properties such as high working voltage, high specific energy, large capacity, low self-discharge, excellent circulation property, long service life, light quality, and small volume. Separators are used to isolate cathode and anode in lithium ion battery, and also to only allow for ion transport but not for electron when charging and discharging. The separators are playing such an important role that it directly affects the performance and safety of the battery. Traditional separator is mainly polyolefin microporous membrane which presents low porosity, low wettability for electrolyte and inferior thermostability. Hence it can not satisfy the use of high-performance security lithium ion battery, which seriously limits the development of LIBs. That makes it even more significant to develop new separator with higher porosity and thermal stability as well as better electrolyte wettability in fabrication of high performance rechargeable lithium battery.The comprehensive performance of polyimide (PI) materials is very excellent because of its unique aromatic heterocyclic structure. The combination of nanofibers’small size effect and the high heat resistance properties of polyimide make PI nanofiber membrane a potential separator for high volume lithium ion battery. However, since the polyimide nanofiber membrane is usually prepared via electrostatic spinning method for non-woven structure, fibers in the membrane are arranged disorderly and lack of strong interaction between the fibers, which leads to poor mechanical properties, hindering the further development of PI nanofibers membrane.Method of thermal and solvent treatment to polyamide acid (PAA) nonwoven is proposed to enhance the mechanical properties and improve pore structure of PI fiber membrane in this paper. Monomers with flexible units are selected to prepare PAA nonwovens, followed by thermal imidization while micro-melt happens at the same time. After completion of imidization, cross-linked PI nonwovens are successfully fabricated while the morphology varies with the thermal treatment temperature and holding time. As for solvent treatment, we can control crosslinking through adjusting the pretreatment temperature and the soaking time in the solvent of PAA. The characterization analysis indicated that the higher the degree of cross-linking, the lower of the porosity and the electrolyte uptake. It is worth mentioning that the highest tensile strength of the specimens can be improved from 14.76 MPa to 76.10 MPa, and the formation of cross-linked structure does not affect the molecular structure of the PI and thermal dimensional stability, still exhibiting excellent thermal stability. Compared with those of celgard2400 separator, cross-linked PI nonwovens have higher wettability electrolyte, ionic conductivity, excellent discharge specific capacity and cycle performance.