Triggering the electronic microenvironment of extraordinary nitrogen-bridged atomic iron coordinated with in-plane nitrogen by manipulating phase-reconfigured 2D vanadium nitride MXenes toward invigorated lithium–sulfur batteries†

Comprehending the electronic configurations of single-atom catalysts (SACs) by fine-tuning the coordination microenvironment for reinforcing electrocatalytic activity for rechargeable lithium–sulfur batteries is of noteworthy significance for boosting sulfur-evolution kinetics, lowering reaction barriers, and alleviating lithium dendrite deterioration. Herein, an extraordinary electronic configuration of isolated Fe coordinated with unsaturated atoms in metallic vacancies derived from atomic arrangement driven by phase-restructured vanadium nitride MXenes was modulated by optimizing the coordination microenvironment during fluoride-free room-temperature organic molten salt in situ etching and using a self-reduced strategy. X-Ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses elucidated that isolated Fe was coordinated with in-plane nitrogen and oxygen atoms, with axial-bridged nitrogen-doped carbon encapsulated on the surface of phase-restructured vanadium nitride. Density functional theory calculations and experimental results comprehensively elucidated axial distortion originating from the bridged nitrogen reordering Fe d-orbital splitting manner to lower the d_z² level, which not only strengthened the adsorption energy to hamper the shuttle effect and decreased the activation energy barrier to boost redox kinetics but also engendered lithiophilicity to lower polarization and homogenize ion flux to suppress lithium dendrite growth. These merits of the Fe_N4-O-NC-VN-modified separator encourage the development of rechargeable lithium–sulfur batteries to promote a dramatic improvement in the reversible capacity on the cathode and a satisfactory cycling lifespan on the anode. This work offers a comprehensive understanding of the electronic configuration of SACs and its modulation by fine-tuning the coordination microenvironment to optimize electrocatalyst activity.