Preparation of PEI-modified PI separators for advanced lithium ion batteries

High-performance polymeric separators are indispensable materials for advanced rechargeable lithium-ion batteries (LIBs). In general, separators must simultaneously possess the following qualifications: flame retardancy, mechanical strength, wettability, and ion conductivity. In this study, polyethylenimide (PEI), which is rich in amino groups, was grafted onto the surface of polyimide fibre membranes by impregnation, resulting in the preparation of PI-PEI composite separator. The grafting of PEI on PI nanofibres resulted in the bonding of fibres, which enhanced the stability of the pore structure and mechanical properties of the nanofibres. Furthermore, the number of polar groups on the surface of the PEI grafted by the PI separator increased, which enhanced the electrolyte affinity of the PEI-PI separator. The influence of PEI concentration on the properties of the separator, electrochemical properties, and cell performances was also discussed, and the optimal PEI concentration (4%PEI) was identified. The PI-4%PEI composite separator exhibited superior wettability, mechanical strength, flame retardancy, and minimal interface impedance. In particular, PI-4%PEI separators display superior capability (127.9 mAh·g⁻¹@5C) in comparison to PP separators (115.4 mAh·g⁻¹@5C). Furthermore, the cell employing the PI-4%PEI separator displays high cycling stability and discharge specific capacity over 100 cycles at 1C. Concurrently, the modified polyimide separator exhibits remarkable stability, ensuring that the lithium-ion battery can operate safely even under high temperature working conditions.