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–Nx) 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–N2 with asymmetric electron distribution immobilizes LiPSs, accelerates LiPSs conversion, and promotes Li2S deposition/dissociation more effectively than Co–N4 through stronger chemical interactions. As a result, Co–N2 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.