One-step hydrothermal fabrication of a novel FeS₂/Fe₃S₄ polycrystalline heterostructure via phase transition engineering for superior sodium storage

Polycrystalline heterostructures are used as anode materials for high-performance sodium-ion batteries (SIBs); however, their use is limited by uncontrolled multi-step growth processes. In this study, we report a facile, safe, and controllable one-step hydrothermal method for synthesizing polycrystalline FeS₂/Fe₃S₄ heterostructure by regulating the sulfur chemical potential and phase-transition process. The heterostructure with a high specific surface area and unique micro‑morphology provides abundant active sites, promoting ions and electrons transport. Ex-situ X-ray diffraction (XRD) and kinetic analyses confirm the reversible phase transitions occurring during cycling and fast reaction kinetics. Consequently, the FeS₂/Fe₃S₄ heterostructure serving as SIBs anodes exhibits outstanding cycling stability (355.3 mAh g⁻¹ after 1000 cycles at 20 A g⁻¹) and remarkable rate capability (497.9 mAh g⁻¹ at 8.0 A g⁻¹). Moreover, the assembled full cell also demonstrates excellent electrochemical performance, confirming the practical potential of the FeS₂/Fe₃S₄ heterostructure. This work presents a novel strategy for the one-step construction of metal sulfide heterojunction electrodes, highlighting their promising prospects for high-performance SIBs.