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

Abstract

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.