Cubic MnS-FeS Composites Derived from a Prussian Blue Analogue as Anode Materials for Sodium-Ion Batteries with Long-Term Cycle Stability.

Cubic N,S codoped carbon coating MnS-FeS composites (MnS-FeS@NSC) with a hollow structure were prepared and used as anode materials for sodium-ion batteries. MnS-FeS@NSC exhibits excellent cycle performance and high rate capability and delivered a reversible capacity of 501.0 mAh g after 800 cycles at a current density of 0.1 A g with a capacity retention of 81%. More importantly, the MnS-FeS@NSC anode holds long-term cycle stability; the capacity can remain 134.0 mAh g after 14 500 cycles at 4 A g. Kinetic analysis demonstrated that Na storage follows a pseudocapacitive dominating process, which is ascribed to the origin of the outstanding rate performance of the MnS-FeS@NSC material. The enhancement of electrochemical performance is attributed to the hollow structure and the N,S codoped carbon coating structure, which can reduce the diffusion distance for sodium ions and electrons, alleviate volume expansion during sodium-ion insertion/extraction, and retain the structural integrity effectively. Furthermore, a two-step sodiation processes with FeS sodiation prior to MnS was demonstrated by X-ray diffraction (XRD), and the electrochemical impedance spectroscopy (EIS) spectra might indicate that the accumulation of the metallic elements in the preconversion reaction can accelerate the transfer of electrons and ions in the further conversion process.