Engineering Crystalline–Amorphous Interfaces in Iron Sulfide/Carbon Nanostructures to Boost Sodium Storage Kinetics and Cycling Durability

Iron sulfide (FeS) has emerged as a compelling candidate for sodium-ion storage owing to its abundant availability and intrinsically large theoretical accommodation of Na⁺. However, its real-world utilization remains limited due to inherent issues, such as restricted charge transport, slow Na-ion migration pathways, and pronounced structural breathing upon repeated sodiation-desodiation. Herein, we report a rationally engineered crystalline-amorphous yolk-shell FeS@C heterostructure in which ultrafine crystalline FeS nanocrystals are uniformly encapsulated by an amorphous N-doped carbon shell, forming a sharp and coherent heterointerface. This crystalline-amorphous interface induces interfacial electronic asymmetry, giving rise to a built-in electric field that accelerates directional electron movement and Na⁺ migration. Meanwhile, the conformal N-doped carbon shell effectively buffers the mechanical strain, enhancing structural integrity during long-term cycling. Furthermore, abundant defect sites at the interface introduce pseudocapacitive contributions, promoting rapid charge storage kinetics. Benefiting from these synergistic effects, the FeS@C anode delivers an outstanding reversible capacity of 913.5 mAh g^-1 at 0.1 A g^-1, maintains 462.9 mAh g^-1 at an ultrahigh rate of 20 A g^-1, and exhibits exceptional repeated-cycle performance over 2000 cycles. In a full-cell pairing with a Na₃V₂(PO₄)₃@C positive electrode, the resulting cells demonstrate remarkable energy densities of 261.0 Wh kg^-1 (coin-type) and 232.3 Wh kg^-1 (soft-pack) with over 70% capacity retention upon long-term cycling. This work establishes a universal crystalline-amorphous heterostructure design strategy to modulate interfacial charge dynamics and structural robustness, offering a pathway toward high-performance and durable Fe-based anodes for next-generation sodium-ion batteries.