Dynamic Reconstruction of Ion Transport Pathways by a Shape Memory Separator for Safer Lithium Metal Batteries

ABSTRACT Thermal safety is a key challenge for high‐energy lithium metal batteries. Conventional separators deform under overheating while sustaining ion transport, accelerating thermal runaway. Herein, we report an aramid‐supported shape memory separator that converts temperature rise into reconstruction of ion transport pathways. A covalently crosslinked PETMP‐TMPTA‐IBOA network is photopolymerized within a porous aramid scaffold. At normal operating temperatures, the interconnected pore structure affords an ionic conductivity of 0.29 mS cm −1 and enables stable Li plating/stripping for over 3000 h. At 140°C, the shape memory network undergoes thermomechanical rearrangement, constricting the percolating pores and increasing the ionic resistance by one order of magnitude. Meanwhile, the aramid scaffold preserves dimensional integrity up to 175°C and maintains electrode isolation after ionic shutdown. The separator is compatible with high‐energy NCM811 and Li‐rich Mn‐based oxide cathodes. In particular, a Li‐rich cathode with high mass loading of 16 mg cm −2 cycles stably for 100 cycles at 0.5 C, delivers nearly 250 mAh g −1 at 60°C, and remains operable up to 125°C. This work establishes thermomechanical reconstruction of ion transport pathways as a general design principle for intrinsically self‐protective lithium metal batteries that reconcile high energy operation with autonomous overheating protection.