Core-shell nanofiber separators for heat-resistant and stretch-tolerant lithium-ion batteries

In liquid-electrolyte lithium-ion batteries, separators prevent internal short circuits while enabling ion transport. However, commercial polyethylene (PE) separators exhibit severe thermal shrinkage and poor mechanical flexibility, limiting their use in stretch-tolerant batteries. To address these challenges, a coaxial electrospinning approach was employed to fabricate core-shell nanofibers with a polyimide (PI) shell and a PI/thermoplastic polyurethane (TPU) core, yielding a PI@PI/TPU (P@PT) separator that combines thermal stability with elasticity. The P@PT separator exhibited excellent thermal dimensional stability (4.03% shrinkage) compared with PE (96.14% shrinkage) at 150 °C and higher stretchability (66.6% strain at 7.6 MPa) than PI separators. At 25 °C, electrochemical tests showed comparable performance to PE, with higher coulombic efficiency and improved capacity retention at 65–90 °C. Using pre-stretched separators, we clarified the role of separator mechanics, independent of electrode effects, and linked strain history to electrochemical performance, demonstrating viability as a high-temperature stretch-tolerant separator.