Coordinatively Unsaturated Co Single-Atom Catalysts Enhance the Performance of Lithium–Sulfur Batteries by Triggering Strong d–p Orbital Hybridization

The catalytic activities displayed by single-atom catalysts (SACs) depend on the coordination structure. SACs supported on carbon materials often adopt saturated coordination structures with uneven distributions because they require high-temperature conditions during synthesis. Herein, bisnitrogen-chelated Co SACs that are coordinatively unsaturated are prepared by integrating a Co complex into a conjugated microporous polymer (CMP-CoN₂). Compared with saturated analogues, i.e., tetranitrogen-chelated Co SACs (denoted as CMP-CoN₄), CMP-CoN₂ exhibits higher electrocatalytic activity in polysulfide conversions due to an enhanced hybridization between the 3d orbitals of the Co atoms and the 3p orbitals of the S atoms in the polysulfide. As a result, sulfur cathodes prepared with CoN₂ deliver outstanding performance metrics, including a high specific capacity (1393 mA h g^-1 at 0.1 C), a superior rate capacity (673.2 mA h g^-1 at 6 C), and a low capacity decay rate (of only 0.045% per cycle at 2 C over 1000 cycles). They also outperform sulfur cathodes that contain CMP-CoN₄ or CMPs that are devoid of Co SACs. This work reveals how the catalytic activity displayed by SACs is affected by their coordination structures, and the rules that underpin the structure-activity relationship may be extended to designing electrocatalysts for use in other applications.