Yolk-Shell Structure and Spin-Polarized Surface Capacitance Enable FeS Stable and Fast Ion Transport in Sodium-Ion Batteries

Iron sulfide (FeS) has been extensively studied as sodium-ion battery anodes due to its high theoretical capacity (609 mAh g⁻¹), but its large volume expansion and low electrical conductivity result in unsatisfactory cycling life and poor rate performance. Moreover, the sodium ion storage mechanism of FeS at a voltage range of 0.01–1 V involving conversion reactions and subsequent ion storage process is unclear yet. Here, the study proposes a vapor-pressure induced synthesis route to fabricate FeS/C yolk-shell structure that ultrathin carbon layers coat on the surface of FeS nanosheets, which can accommodate volume expansion of FeS during sodiation observed via in situ transmission electron microscope and improve its electrical conductivity. Remarkably, an in situ magnetometry reveals that vast spin-polarized electrons can be injected into superparamagnetic Fe nanoparticles (≈3 nm) formed during conversion reaction to induce evolution of electrode magnetization between 0.01 and 1 V, during which spin-polarized surface capacitance effect occurs at Fe/Na₂S interfaces to increase extra ion storage and boost ion transport stably. Consequently, the FeS/C yolk-shell nanosheets deliver a high reversible capacity of 664.9 mAh g⁻¹ at 0.1 A g⁻¹, and 300.4 mAh g⁻¹ after 10 000 cycles at 10 A g⁻¹ with a capacity retention of 81.1%.