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 singleatom 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.