Tri-monomer polyamide acid nanofiber separator for safe and high performance lithium-ion batteries

The separators play a crucial role in ensuring the safety of lithium batteries and greatly affect the electrochemical performance of the battery. However, the weak thermal stability and non-polar surface of commercial polyolefin separators severely restrict the development of the high-performance lithium batteries. In this work, we initially proposed polyamide acid (PAA) nanofiber separator with three monomers through a two-step synthesis method, which was prepared by electrospinning. By controlling different processing temperatures, PAA separators containing different amount of carboxyl groups were obtained. PAA separator had better wettability and thermal stability than PE separator. Meanwhile, the electrospinning method brings ultra-high porosity (95.3%) to the membrane, resulting a much higher electrolyte absorption and retention rate of PAA (initially 3214%, 240% at 50 min) compared to PE separator. In particular, carboxyl groups serve as functional groups to provide an electronegative atmosphere, promoting the transport of lithium ion between the anode and cathode, and playing an important role in improving ionic conductivity. Consequently, batteries with PAA-SOH-50 separator showed the best rate charging/discharging performance, maintaining 87.7% and 73.6% of its initial capacity at 2.0 C. Moreover, cells with PAA separator exhibited much higher initial discharge capacity (164.3 mAh g-1) than that with PE separator (126.8 mAh g-1). After 100 cycles at 0.5 C, the PAA nanofiber separator achieved an ultralow attenuation rate of only 0.04% per cycle, which was less than half of the commercial PE separator (0.10% per cycle). This work provides a new vision for the development of separator materials, revealing the great influence of functional groups, such as carboxyl groups, on the performance of separators, which provides a strong basis for the further research of high-performance separators.