A Radical-Cationic Covalent Organic Framework to Accelerate Polysulfide Conversion for Long-Durable Lithium–Sulfur Batteries

Covalent organic frameworks (COFs) have emerged as promising metal-free sulfur hosts to facilitate the conversion kinetics and suppress the shuttling effect of lithium polysulfides (LiPSs) in lithium–sulfur (Li–S) batteries. However, constructing COFs with stable and high electrocatalytic functionality for LiPS conversion remains unexplored. Herein, we develop a radical-cationic COF (R-TTF^•+-COF) with superior electrical conductivity of 3.9 S m^–1 at room temperature, which features both nucleophilic and electrophilic sites for effective LiPS chemisorption and conversion. With this novel radical-based catalyst, the Li–S battery achieves superior longevity of 1500 cycles with a capacity fading of 0.027% per cycle at a current density of 0.5 C. The capacity retention of the Li–S battery based on R-TTF^•+-COF at the current density of 2.0 C is nearly twice as high compared to a COF without radicals. The crucial role of radical cations in catalyzing LiPS conversion has been systematically elucidated through solid-state nuclear magnetic resonance spectroscopy, electron paramagnetic resonance spectroscopy, and theoretical simulations, which verify the reversible interactions between LiPSs and [TTF]₂^•+ moieties. This intriguing radical-assisted mechanism opens a new avenue for designing efficient catalytic sulfur hosts using organic molecules, offering a significant step toward the practical application of Li–S batteries.