Cationic Covalent Organic Framework-Modified Polypropylene Separator for High-Performance Lithium Metal Batteries

As an important component of lithium batteries, the wettability and thermal stability of the separator play a significant role in cell performance. Despite the availability of numerous commercial separators, issues such as low ion selectivity and poor thermal stability continue to limit the efficiency and reliability of the batteries. Herein, two cationic covalent organic frameworks (Br-COF and TFSI-COF) with abundant imidazole cationic groups were designed to modify commercial polypropylene (PP) separators. The strong lithium-ion affinity of the cationic COF enables the effective dissociation of lithium salt ion clusters, simplifying the solvent structure of lithium ions to promote lithium ions transport. Additionally, solvent anions can be anchored to the cationic COF by electrostatic interactions, reducing side reactions on the lithium metal anode surface to form a favorable SEI layer, which can effectively inhibit the growth of lithium dendrites. The rapid dissociation of anions in lithium salts with some organic solvents and cationic COFs was revealed by a molecular dynamics simulation. A LiF-rich SEI layer on the lithium metal anode surface was formed, which can speed up Li⁺ transport at interfaces, leading to consistent lithium deposition and outstanding battery performance. The ordered porous structure of the cationic COF provides interconnected and continuous channels, improving the wettability between the liquid electrolyte and separators, which is conducive to ion transport. When paired with a LiFePO₄ cathode and electrolyte (1.0 M LiTFSI in DEC: EC: DMC = 1:1:1), the LiFePO₄/TFSI-COF@PP/Li cell demonstrates a prominent cycling capacity of 148.0 mAh g^-1 at 0.5 C with a Coulombic efficiency of 98.0% in the first cycle, and the capacity retention is 82.0% after 100 cycles, showing good cycling stability. Thus, this investigation provides inspiration for the expansion of cationic COF-modified separators for next-generation lithium metal batteries.