Branched fiber architectures enable a hierarchical porous nanofiber separator for lithium-ion batteries

Abstract

Poly (p-phenylene benzobisoxazole) (PBO) nanofiber (PNF) holds great promise for high-safety lithium-ion battery separators due to its exceptional mechanical strength and thermal stability, as well as desirable solution processability derived from its rigid-rod molecular structure. However, the strong π-π interactions among PNFs often lead to excessively dense packing and low porosity in sol-gel derived PNF separators, which severely restricts Li⁺ transport. Herein, we design a hierarchical micro-branched fiber architecture as a structural spacer to proactively engineer the pore microstructure. The introduced PBO branched fibers (PBF) effectively disrupt the dense stacking of nanofibers through physical entanglement and steric hindrance, yielding a PNF/branched (PNB) separator with a significantly enhanced porosity of 58%. Benefiting from this well-tailored structure, the PNB separator exhibits remarkable thermal stability with a decomposition temperature above 650 °C, high ionic conductivity of 0.59 mS cm⁻¹, and an elevated Li⁺ transference number of 0.64. In Li||LFP cells, the PNB separator enables excellent rate capability and cycling stability, retaining 95.6% of its initial capacity after 800 cycles at 0.3 C. This work presents a feasible strategy to optimize the porous structure of nanofibers, offering a promising route for the application of PBO nanofibers in LFP-based battery separators.