Insight to Se-doping effects on Fe7S8/carbon nanotubes composite as anode for sodium-ion batteries

Iron based sulfides are considered as promising candidates for sodium-ion battery anodes. However, the inferior cyclic stability and poor capability hinder their practical application. In this study, a series of Se-doped Fe7S8 modified by carbon nanotubes (Fe7S8-xSex/CNTs) are synthesized through heat treatment and following selenization process. The composition of hybrids can be easily controlled by tuning the proportion of precursors. Electrochemical tests reveal that Fe7S8-xSex/CNTs with optimized nano-structure and composition displays improved sodium storage performances. Specifically, a discharge capacity of 303.4 mAh g−1 can be obtained at a high current of 4 A g−1. In addition, a remarkable capacity of 313.6 mAh g−1 is obtained after 1000 cycles at 2 A g−1, corresponding to a capacity retention of 67.7%. Sodium storage kinetic analysis suggests that Fe7S7Se1/CNTs electrode exhibit an enhanced capacitive-controlled process and improved sodium diffusion coefficient. Density functional theory (DFT) calculation results manifest that Fe7S7Se1 present a stronger binding energy with Na atoms and a decreased Na + diffusion barriers, which are responsible for the superior electrochemical performances. These results demonstrate that Se-doping of iron based sulfides may be a feasible strategy to develop high performance sodium-ion battery anodes.