An Exfoliation-Evaporation Strategy To Regulate N Coordination Number of Co Single-Atom Catalysts for High-Performance Lithium-Sulfur Batteries

Single-atom catalysts (SACs) with metal-nitrogen (M-N) moiety are effective in boosting the redox kinetics of lithium-sulfur (Li-S) batteries. However, the precise preparation of SACs with controllable M-N coordination number remains challenging and the relationship between M-N coordination number and polysulfide redox kinetics is still unexplored. Herein, a novel exfoliation-evaporation strategy assisted by molten salt is proposed to fabricate Co SACs (named Co-N-x) with different N coordination numbers for Li-S batteries. The key of this strategy is to exfoliate layered Co-based ZIF-L precursors into N-doped graphene with abundant dispersed Co atoms by molten salt and then control Co-N coordination number by selectively introducing Zn evaporation to promote C-N fragments release. Experimental and theoretical calculation results reveal that highly unsaturated Co-N-2 with asymmetric electron distribution immobilizes LiPSs, accelerates LiPSs conversion, and promotes Li2S deposition/dissociation more effectively than Co-N-4 through stronger chemical interactions. As a result, Co-N-2 endows Li-S batteries with long cycle life (0.05% capacity decay per cycle for 700 cycles), excellent rate capability (687 mAh g(-1) at 5 C), and high areal capacity of 8.2 mAh cm(-2) at a high loading of 7.0 mg cm(-2). This work provides an effective strategy to fabricate SACs with controllable N coordination number and establishes the relationship between M-N coordination number and LiPSs redox kinetics, motivating future rational design of SACs for high-performance Li-S batteries.