Lithiated polyimide-reinforced polyimide nanofiber separator for high-performance LiCoO2 batteries

To address the critical challenges of mechanical robustness, lithium-ion transport efficiency, and cycle life in lithium-ion battery separators, we developed a facile surface modification strategy to prepare a high-performance separator (denoted PI-0.5) by poly(amic acid) lithium salt adhesive coating. The PI-0.5 separator exhibits exceptional thermomechanical stability (up to 200 °C), intrinsic flame-retardancy, and a tensile strength 3.5 times higher than that of the pristine PI separators. It achieved an ionic conductivity of 1.05 mS cm −1 and a high Li + transference number of 0.553, which are ∼2.9 and 1.3 times higher than that of the pristine separator, respectively, alongside extended anodic stability up to 4.6 V (that of the pristine PI is 4.28 V). In LiCoO 2 /PI-0.5/Li cells, the PI-0.5 separator enabled excellent rate capability (151.43 mAh·g −1 at 5 C) and outstanding cycling stability, with capacity retentions of 141.91 mAh·g −1 at 2 C after 100 cycles. Symmetric Li/PI-0.5/Li cells further demonstrate stable plating/stripping over 360 cycles with minimal polarization decay. XPS analysis revealed that the PI-0.5 separator suppresses Li 2 CO 3 formation in the solid electrolyte interphase (SEI), inhibiting lithium dendrite growth and improving interfacial stability. This work presents a scalable approach to fabricating multifunctional separators, providing a promising route toward next-generation lithium-ion batteries.