Bimetallic and Bipolar Covalent Organic Framework Cathodes for Polysulfide Catalysis and Trapping for Li-S Batteries

Lithium-sulfur (Li-S) batteries hold great promise for next-generation energy storage due to their high capacity and energy density. However, challenges such as the polysulfide shuttle effect, poor conductivity, etc., during cycling hinder their applications. Here bimetallic (Co/Ni) and bipolar (p-type Ph-NH-Ph/n-type C═O) covalent organic frameworks (COFs) are designed for Li-S battery cathodes, to effectively trap Li polysulfides (LiPSs) and enhance their conversion kinetics. The bimetallic COF composite demonstrated excellent electrochemical performance, delivering a high discharge capacity of 1,420 mAh g^-1 at 0.1 C and maintaining a low capacity decay rate of 0.024% per cycle over 2000 cycles at 1 C. In situ Raman spectroscopy and mass spectroscopy revealed near-complete suppression of soluble polysulfide migration. Ex situ X-ray photoelectron spectroscopy analysis showed catalytic sites and reversible bonding with sulfur intermediates. Computational studies revealed that bimetallic centers synergistically promote strong polysulfide adsorption of Co sites and lowered energy barriers of Ni sites, and bipolar backbone cooperatively adsorb Li⁺ on electron-rich C═O (n-type) sites and polysulfide anions on electron-deficient Ph-NH-Ph (p-type). This synergistic mechanism, coupled with COF's hierarchical porosity, addressed key challenges in Li-S batteries, offering a pathway toward practical high-energy-density energy systems.