Ultrasmall Hollow Covalent Triazine Framework Nanosphere/Aramid Nanofiber Composite Separator for High‐Energy Lithium Metal Batteries

Sluggish and uneven mass transport in separators significantly accelerates lithium metal batteries (LMBs) degradation. Here, via scaffold-coating synergistic strategy, composite separator integrating heat-resistant, polar aramid nanofiber (ANF) porous scaffold with ultrasmall hollow covalent triazine framework nanosphere (SCTF) coating is fabricated to boost the cycling performance of LMBs across diverse operating conditions. The highly porous ANF layer serves as robust electrolyte reservoir, while the SCTF layer functions as efficient ion redistributor leveraging its extensive interconnected pathways present within staggered layers of intrinsic nanopores with intra- and interparticle open spaces. To highlight the importance of mesoscale structure of coatings, the ion transport uniformity and kinetics of SCTF/ANF separator are compared with those of nanosheet-like CTF and large hollow CTF nanosphere coated separators by time-of-flight secondary ion mass spectrometry and molecular dynamics simulations. Owing to its exceptional ionic conductivity (1.41 mS cm^-1) and Li⁺ transference number (0.79), the SCTF/ANF separator endows the Li//LiFePO₄ cell with capacity retention of 72% after 3000 cycles at 5 C. The practical viability of this separator is demonstrated by its stable cycling performances in high-rate, high-voltage, and high-temperature LMBs, along with pouch cells. This work highlights the scaffold-coating synergy and mesoscale coating design of separators for high-performance LMBs.