Smart thermal-shutdown separators with fast response for safe Li-metal batteries

Rapid and efficient thermal shutdown separators are essential to enhance Li-metal battery safety by swiftly blocking ionic conduction pathways before thermal runaway. Existing thermal-responsive separators, however, face significant limitations, including high shutdown temperatures, sluggish response times, and inadequate thermal stability, severely restricting their practical deployment. This work proposed a tri-layer smart separator featuring a polybutylene succinate (PBS) thermal-responsive interlayer sandwiched by two robust boron nitride (BN)-enhanced polyvinylidene fluoride (PVDF) outer layers. At approximately 115 ​°C, the PBS interlayer quickly melts to form a pore-free barrier, effectively blocking lithium-ion transport within minutes. Simultaneously, the thermally conductive PVDF-BN layers retain structural integrity and facilitate efficient heat dissipation, ensuring rapid and stable thermal shutdown. This synergistic multi-layer approach achieves significant improvements in battery safety, demonstrated by: (1) an 11-fold reduction in discharge current compared to conventional polyolefin separators after overheated, (2) a delay of thermal-induced internal short circuit onset by up to 293 ​min, and (3) enhanced suppression of lithium dendrite growth, resulting in exceptional cycling stability with 95 ​% capacity retention over 3000 ​h. Additionally, the increased thermal conductivity (1.51 ​W/mK) ensures uniform internal heat distribution, minimizing localized hotspots. Our findings present a practical route for developing advanced separators with integrated thermal management capability.