Nanofibrous Covalent Organic Frameworks Based Hierarchical Porous Separators for Fast-Charging and Thermally Stable Lithium Metal Batteries

Engineering multifunctional smart separators are important for the ongoing pursuit of fast-charging and safe batteries. Herein, a novel nanofibrous covalent organic framework (COF) based separator with well-designed hierarchical porous channels is fabricated to effectively regulate mass transport for fast-charging and thermally stable lithium metal batteries (LMBs). Such a hierarchical porous separator consists of electrospun polyacrylonitrile nanofibers with macroporous channels (average diameter of 323 nm) and mesoporous channels (≈7 nm) created between amide-group-bonded COF nanoparticles with intrinsic 1.6 nm lithiophilic microporous channels (PAN/AM-COF). Computational fluid dynamics and density functional theory calculations demonstrate that PAN/AM-COF can simultaneously facilitate high-speed and selective transport of Li⁺, as well as homogeneous deposition of Li, achieving high conductivity (3.33 mS cm⁻¹) and high Li⁺ transference number (0.79). As a result, Li || LFP full cell with PAN/AM-COF displays superior cycling stability at 10 C with an acceptable capacity attenuation (0.037% per cycle) over 1000 cycles. Moreover, when operating under an extreme temperature of 100 °C, the Li || LFP full cell with PAN/AM-COF can still operate stably for 300 cycles at 30 C, highlighting its potential processing scalability for ultrafast-charging energy storage systems. This study gives insights into designing functional separators for fast-charging LMBs.