High-temperature resistant semi-crystalline poly(ether ether ketone) separator

Lithium metal batteries (LMBs) are considered promising energy storage devices, but electrolyte-induced swelling and thermal shrinkage of separators significantly limit battery performance and safety, especially for extreme-condition applications. Here, semi-crystalline poly(ether ether ketone) (PEEK) separator is developed to effectively boost cycling performance and safety of LMBs at temperatures above 80°C via crystallinity engineering strategy involving reversible chemical modification and clamping-annealing processes. The abundant polar groups of PEEK enable electrolyte wetting while promoting Li⁺ desolvation for fast ion transport. The ionic conductivity and Li⁺ transport number are 0.61 mS cm⁻¹ and 0.71, respectively. Its recovered crystalline structure provides excellent Young's modulus (1.0 GPa), thermal stability (300°C) and durability against electrolyte swelling. Leveraging the synergistic interplay between molecular structure and polymer chain alignment, semi-crystalline PEEK separator effectively suppresses lithium dendrite growth while enabling consistent and rapid ion transport. A high capacity retention of 94% after 100 cycles of pouch cell with industry-level mass loading of active materials confirms its practical applicability. At 100°C, the Li||LiFePO₄ cells with semi-crystalline PEEK separator achieve ultralong cycle life exceeding 500 cycles, 16 times that of cells with low-crystallinity PEEK separator, whereas Celgard 2400 separator completely fails to operate. This work demonstrates the feasibility of limiting electrolyte-induced swelling of separator through crystallization and establishes the relationship between separator swelling and battery performance. This crystallization approach provides a promising avenue to highly durable separators for long-term cycling LMBs, especially at task-specific high-temperature conditions.