Manipulating orbital hybridization of single-atom catalytic sites in metal-organic framework for high-performance lithium-sulfur batteries

Catalytic single-atom catalysts (SACs) have shown superiority in promoting the sluggish redox reaction kinetics and suppressing the shuttle effect of soluble lithium polysulfides (LiPSs) in lithium-sulfur batteries (LSBs) than traditional catalysts, which are however restricted by the limited ratio of single-atom metal sites. Metal-organic frameworks (MOFs) hold great potential as tunable SACs platforms for their talent in regulating catalytic single-atom sites and manipulating the orbital hybridization with guest molecules through modularized design, which however has not been fully designed and understood in LSBs. In this work, a series of porphyrin-based MOF nanosheets (PCN-222(M)-NSs) possessing rational-designed M-N4 (M= Fe3+, Co2+, Ni2+, and Cu2+) single-atom metal sites were synthesized to manipulate the d-p orbital hybridization between M-N4 center and LiPSs. Systematic in-situ/ex-situ electrochemical experiments and theoretical calculations demonstrate that the Cu-N4 center of PCN-222(Cu)-NS exhibits the best effect for promoting LiPSs conversion and suppressing shuttle effect due to the most effective d-p orbital hybridization between Cu-N4 and sulfur species. The assembled LSBs with PCN-222(Cu)-NS/graphene interlayer show remarkably improved discharge capacity and decreased decay rate at low and high area-sulfur loading.