Atomic-level modulation of electron density in iron sulfides for enhancing sodium storage kinetics

Iron sulfides (FeS₂) are promising anode materials for sodium ion batteries (SIBs); however, their inferior electronic conductivity, large volume swelling, and sluggish sodium ion diffusion kinetics lead to unsatisfactory rate performance and cycling durability. Heteroatom doping plays a crucial role in modifying the physicochemical properties of FeS₂ anodes to enhance its sodium storage. Herein, ultra-fine Ni-doped FeS₂ nanocrystals derived from a metal-organic framework (MOF) and in-situ anchored on a nitrogen doped carbon skeleton (Ni-FeS₂@NC) are proposed to enhance both structural stability and reaction kinetics. Material characterization, electrochemical performance, and kinetics analysis demonstrate the critical role of Ni doping in sodium storage, particularly in accelerating Na⁺ diffusion efficiency. The N-doped carbon derived from the MOF can buffer the volume expansion and enhance the structural stability of electrode materials during sodiation/desodiation processes. As expected, Ni-FeS₂@NC exhibits a high reversible capacity of 656.6 ± 65.1 mAh g^-1 at 1.0 A g^-1 after 200 cycles, superior rate performance (308.8 ± 6.0 mAh g^-1 at 10.0 A g^-1), and long-term cycling durability over 2000 cycles at 1.0 A g^-1. Overall, this study presents an effective approach for enhancing the sodium storage performance and kinetics of anode materials for high efficiency SIBs.