High‐Performance Lithium‐Ion Batteries Using Poly‐p‐Phenylene Benzobisoxazole Fiber‐Based Separators With Mesoporous Architecture

ABSTRACT Despite the promise of poly(p‐phenylene benzobisoxazole) (PBO) as a separator material, current fabrication routes remain complex and costly, while nanofiber incorporation introduces a trade‐off between pore size, porosity, and ionic resistance, further aggravated by closed or dead pores, necessitating precise pore structure optimization. In this study, hierarchically structured poly(p‐phenylene benzobisoxazole) (PBO) micro/nanofiber separators with a mesoporous architecture were fabricated via a wet‐laid papermaking process. PBO microfibrillated fibers (PMF) formed a three‐dimensional interconnected network with homogeneously distributed, well‐defined pores, while PBO nanofibers (PNF) were incorporated to reduce the mean pore size, modulate porosity and pore connectivity, and yield a denser, smoother surface. Separators with 0, 5, and 10 wt% PNF loadings exhibited no detectable dimensional shrinkage at 200°C or thermal transitions between 100°C and 250°C. PNF addition progressively reduced porosity (from 51.6% to 38.8%) and electrolyte uptake (from 270% to 200%), while increasing bulk resistance. The 5 wt% PNF separators achieved the optimal balance between pore structure and interfacial resistance, delivering a capacity retention of 77.65% after 100 cycles at 1°C, exceeding that of the commercial polyolefin separator (71.57%). The mesoporous architecture combining PMF backbone and PNF fillers thus offers a scalable design strategy for engineering thermally robust, high‐performance separators for next‐generation LIBs.