A Dual-Anion Synergistic Strategy for Cationic COF Separators toward Enhanced Li–S Batteries

The practical application of lithium–sulfur (Li–S) batteries is severely hampered by the polysulfide shuttle effect and sluggish redox kinetics. Herein, we propose an innovative fixed cationic framework-precise anion regulation strategy to functionalize commercial polypropylene separators for tackling these challenges. A series of cationic covalent organic frameworks (c COFs) with identical skeletons but different coordinated anions (Cl –, NO 3 –, PO 4 3–, and a NO 3 – &PO 4 3– composite) are constructed via a facile ion-exchange process. Systematic investigations reveal that the anion type critically regulates the interfacial properties of the COF-based separator, including electrolyte affinity, polysulfide adsorption, and ion-transport behavior. Notably, the NO 3 – &PO 4 3– composite anion system exhibits synergistic adsorption-transport-catalysis functions: the multidentate PO 4 3– enhances chemical anchoring of polysulfides, the hydrophilic NO 3 – improves electrolyte wettability and Li + mobility, and their combination facilitates efficient sulfur redox conversion. Consequently, the Li–S battery equipped with the NO 3 &PO 4 -TPTG modified separator achieves a high initial discharge capacity of 1090 mA h·g –1 at 0.1 C, excellent rate capability (766 mA h·g –1 at 2 C), and outstanding cycling stability with a retained capacity of 640 mA h·g –1 after 500 cycles at 1 C. Mechanistic studies verify that the composite anion design simultaneously suppresses polysulfide diffusion, reduces interfacial polarization, and accelerates Li 2 S deposition kinetics. This work provides a novel anion-engineering approach for designing multifunctional separators and advances the development of high-energy-density Li–S batteries.