Pseudocapacitance boosted N-doped carbon coated Fe₇S₈ nano-aggregates as promising anode materials for lithium and sodium storage

Herein, the core-shell structured N-doped carbon coated Fe₇S₈ nano-aggregates (Fe₇S₈@NC) were controllably prepared via a simple three-step synthesis strategy. The appropriate thickness of N-doped carbon layer outside Fe₇S₈ nano-aggregates can not only inhibit the particle pulverization induced by the big volume changes of Fe₇S₈, but can increase the electron transfer efficiency. The hierarchical Fe₇S₈ nano-aggregates composed of some primary nanoparticles can accelerate the lithium or sodium diffusion kinetics. As anode materials for Li-ion batteries (LIBs), the well-designed Fe₇S₈@NC nanocomposites exhibit outstanding lithium storage performance, which is better than that of pure Fe₇S₈, Fe₃O₄@NC and Fe₇S₈@C. Among these nanocomposites, the N-doped carbon coated Fe₇S₈ with carbon content of 26.87 wt.% shows a high reversible specific capacity of 833 mAh·g^-1 after 1,000 cycles at a high current density of 2 A·g^-1. The above electrode also shows excellent high rate sodium storage performance. The experimental and theoretical analyses indicate that the outstanding electrochemical performance could be attributed to the synergistic effect of hierarchical Fe₇S₈ nanostructure and conductive N-doped carbon layer. The quantitative kinetic analysis indicates that the charge storage of Fe₇S₈@NC electrode is a combination of diffusion-controlled battery behavior and surface-induced capacitance behavior.