Effect of the heat treatment temperature on mechanical and electrochemical properties of polyimide separator for lithium ion batteries

In order to solve the shortcomings of the current traditional commercial polyolefin microporous membranes with worse thermostability at high temperature, polyimide (PI) nanofiber membranes prepared by electrospinning are promising separators for lithium ion batteries that operate at high temperatures. This preparation includes the forming process of poly(amic acid) (PAA) fibers membrane and thermal-imidization process. In this study, we design the experiment of thermal-imidization for PAA fibers to improve the mechanical strength and the electrochemical performance of the obtained PI nanofiber membrane. It is found that the degradation phenomenon and the crosslinking function occur after the imidization during the heat treatment. The mechanical strength of PI nanofiber membrane gets improved with new crosslinking system after being heat-treated at 350 °C. LiCoO2/Li cells based on such PI nanofiber membranes exhibit excellent cycle performance (300 cycles) and rate performance (even at high rates of 6 C), better than those employing polyolefin microporous membranes. During the heat treatment of polyamic acid, a thermal imidization process will occur. At the same time, different heat treatment temperatures will lead to differences in the degree of imidization and the structure of the fiber membrane material. It is important to understand the thermal imidization process. The effect of thermal imidization temperature on the performance of the electrospun polyimide lithium ion battery separator is studied. The experimental results show that although the degree of imidization is decreased by the heat treatment at 350 °C, it has better cycle performance (300 circles) and rate performance (even at high rates of 6 C).