Spin polarization in Bi/FeS heterostructure enables high-performance sodium ion storage

Bismuth (Bi) has emerged as an appealing anode material in sodium-ion batteries (SIBs), owing to its exceptional volumetric capacity. However, sluggish Na⁺ kinetics remains a critical challenge, which can be mitigated by spin polarization. Yet Bi exhibits strong diamagnetism and lacks viable methods for its spin manipulation. And such manipulation mechanism is difficult to capture using conventional electrochemical characterization methods. Herein, we propose a carbon-coated Bi/FeS heterostructure (Bi/FeS@NSC), where FeS generates metallic Fe⁰ during the conversion reaction, inducing the emergence of spin polarization. Operando magnetometry reveals that the induced-effect contributes to both significant additional capacity and accelerated charge transfer. Furthermore, the complete sodium storage mechanism and capacity derivation of Bi/FeS@NSC electrode are further elucidated through in situ X-ray diffraction (XRD) and theoretical calculations. Benefiting from this unique design, the Bi/FeS@NSC electrode delivers an outstanding capacity (397.5 mAh g⁻¹ at 40.0 A g⁻¹) and stable cycling performance (265.3 mAh g⁻¹ at 20.0 A g⁻¹ after 2200 cycles). This work provides novel perspectives on spin-polarization regulation and corresponding mechanisms in Bi-based anode materials.