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^-, NO₃^-, PO₄^3-, and a NO₃^-&PO₄^3- 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 NO₃^-&PO₄^3- composite anion system exhibits synergistic adsorption-transport-catalysis functions: the multidentate PO₄^3- enhances chemical anchoring of polysulfides, the hydrophilic NO₃^- improves electrolyte wettability and Li⁺ mobility, and their combination facilitates efficient sulfur redox conversion. Consequently, the Li-S battery equipped with the NO₃&PO₄-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 Li₂S 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.