Rational Design of Sandwiched Nanofiber Separators with Controllable Surface Porosity and Thermal Shutdown Functions

The balance between performance and safety in lithium-ion and lithium metal batteries under extreme conditions has become increasingly prominent. In the present study, a sandwich-structured nanofiber separator (PPNFs-PI) with tunable surface porosity and integrated thermal shutdown function is fabricated. It features a polyimide (PI) nanofibrous skeleton (prepared via electrostatic spinning) sandwiched between two polypropylene nanofiber (PPNFs) layers (prepared via multilayer coextrusion). The PI skeleton provides exceptional mechanical strength and thermal stability to suppress dimensional change under harsh conditions, while the PPNFs functional layer substantially enhances surface porosity and ionic conductivity due to its finer fiber diameter. This rational design enables a distinct thermal shutdown function. At temperatures above 170 °C, the PPNFs layers melt to rapidly block ionic transport, preventing combustion during thermal runaway, while the intact PI skeleton maintains a physical barrier against short circuits. Furthermore, the nanofiber-based architecture creates a homogeneous three-dimensional ion transport network, promoting uniform Li⁺ flux and suppressing dendrite growth. The as-prepared PPNFs-PI separator exhibits excellent thermal dimensional stability (no shrinkage at 180 °C), high porosity (66.4%), superior electrolyte uptake (358%), and a high ionic conductivity of 1.18 mS cm^-1. Electrochemical tests verify outstanding interfacial stability, as evidenced by a stable polarization voltage over 1000 h of Li plating/stripping cycling. In NCM811/graphite full cells, it enables remarkable rate performance (145.96 mAh g^-1 at 5C) and cycling stability, retaining 90.8% capacity after 200 cycles at 1C. This work proposes a scalable and cost-effective strategy for designing advanced nanofiber separators for high-performance LIBs and related battery technologies.