Embedding anion-doped Fe7S8 in N-doped carbon matrix and shell for fast and stable sodium storage

Highlights: • Nitrogen-doped Fe₇S₈ coated by nitrogen-doped carbon shell (N–Fe₇S₈@NC) was designed. • N-doping in the Fe₇S₈ can induce extrinsic defects and modulated band structure. • N–Fe₇S₈@NC shows fast Na⁺ reaction kinetics and good cycling stability. • Capacitive contribution promotes the high rate performance for sodium storage. In order to enhance the electrochemical activity of transition metal chalcogenide anodes for sodium ion batteries (SIBs), anion-doping and carbon modification are demonstrated to be effective methods. In this work, using polydopamine (PDA) coated metal organic framework of MIL-88-Fe (MIL-88-Fe/PDA) as self-template, N-doped Fe₇S₈ embedded in N-doped carbon (N–Fe₇S₈@NC) are successfully fabricated, through a high-temperature sulfurization process. Benefited from the PDA coating, integral peapod-liked nanoarchitecture can be preserved, and N atoms are doped both in the Fe₇S₈ lattice and the carbon matrix and shell. This stable peapod-liked N–Fe₇S₈@NC nanoarchitecture with continuous carbon protection, shows fast Na⁺ insertion/extraction reaction kinetics, demonstrating as fast and stable anode material for SIBs. It displays highly stable capacities of 312.7 mAh g⁻¹ at 5.0 A g⁻¹ after 2000 cycles, giving a capacity retention of 98.1%. Even at ultrahigh current of 10.0 A g⁻¹, after long-term cycling (4000 cycles), a capacity of 238.4 mAh g⁻¹ can be maintained, giving an impressive capacity retention of 81.5%. For the possible kinetics mechanism, the ultrafast pseudocapacitive contribution and higher adsorption energy induced by the anion-doping, may promote its high-rate sodium storage capability.