Single-Atom Catalysts with Unsaturated Co-N₂ Active Sites Based on a C₂N 2D-Organic Framework for Efficient Sulfur Redox Reaction

The lithium-sulfur battery (LSB) is a viable option for the next generation of energy storage systems. However, the shuttle effect of lithium polysulfides (LiPS) and the poor electrical conductivity of sulfur and lithium sulfides limit its deployment. Here, we report on a 2D-organic framework, C2N, with a high loading of low-coordination cobalt single atoms (Co-SAs/C2N) as an effective sulfur host in LSB cathodes. Experimental and computational results reveal that unsaturated Co-N-2 active sites with an asymmetric electron distribution act as effective polysulfide traps, accommodating electrons from polysulfide ions to form strong S-x(2-)-Co-N bonds. Additionally, charge transfer between LiPS and unsaturated Co-N-2 active sites endows immobilized LiPS with low free energy and low electrochemical decomposition energy barriers, thus accelerating the kinetic conversion of LiPS. As a result, S@Co-SAs/C2N-based cathodes exhibit superior rate performance, impressive cycling stability, and good areal capacity at high sulfur loading, 2-fold that of commercial lithium-ion batteries. This work emphasizes the potential capabilities and promising prospects of single-atom catalysts with unsaturated coordination in LSBs.