Cationic\nCovalent Organic Framework-Modified Polypropylene\nSeparator for High-Performance Lithium Metal Batteries

As an important component of lithium\nbatteries, the wettability\nand thermal stability of the separator play a significant role in\ncell performance. Despite the availability of numerous commercial\nseparators, issues such as low ion selectivity and poor thermal stability\ncontinue to limit the efficiency and reliability of the batteries.\nHerein, two cationic covalent organic frameworks (Br-COF and TFSI-COF)\nwith abundant imidazole cationic groups were designed to modify commercial\npolypropylene (PP) separators. The strong lithium-ion affinity of\nthe cationic COF enables the effective dissociation of lithium salt\nion clusters, simplifying the solvent structure of lithium ions to\npromote lithium ions transport. Additionally, solvent anions can be\nanchored to the cationic COF by electrostatic interactions, reducing\nside reactions on the lithium metal anode surface to form a favorable\nSEI layer, which can effectively inhibit the growth of lithium dendrites.\nThe rapid dissociation of anions in lithium salts with some organic\nsolvents and cationic COFs was revealed by a molecular dynamics simulation.\nA LiF-rich SEI layer on the lithium metal anode surface was formed,\nwhich can speed up Li<sup>+</sup> transport at interfaces, leading\nto consistent lithium deposition and outstanding battery performance.\nThe ordered porous structure of the cationic COF provides interconnected\nand continuous channels, improving the wettability between the liquid\nelectrolyte and separators, which is conducive to ion transport. When\npaired with a LiFePO<sub>4</sub> cathode and electrolyte (1.0 M LiTFSI\nin DEC: EC: DMC = 1:1:1), the LiFePO<sub>4</sub>/TFSI-COF@PP/Li cell\ndemonstrates a prominent cycling capacity of 148.0 mAh g<sup>–1</sup> at 0.5 C with a Coulombic efficiency of 98.0% in the first cycle,\nand the capacity retention is 82.0% after 100 cycles, showing good\ncycling stability. Thus, this investigation provides inspiration for\nthe expansion of cationic COF-modified separators for next-generation\nlithium metal batteries.