Operando Magnetometry Decodes Space‐Charge Storage in the Solid‐Electrolyte Interphase

The solid‐electrolyte interphase (SEI) is central to ion transport and electrode stability in lithium‐ion batteries (LIBs), yet how charges dynamically distribute and migrate across the SEI/electrode interface during cycling remains elusive. Here, we couple operando magnetometry with an Fe 3 C magnetic probe to track real‑time charge migration across this electrochemical interface. By further integrating operando ambient‐pressure x‐ray photoelectron spectroscopy (AP‐XPS) with multiscale structural and chemical characterizations, we provide converging evidence that supports SEI‐centered space‐charge storage at the electrode interface. This interfacial space‐charge layer delivers an additional ≈236 mAh g −1 within 0.01–1.4 V. The inorganic‐rich SEI forms efficient ionic pathways, whereas Fe 3 C accommodates spin‑polarized electrons, enabling decoupled ionic and electronic storage across the interface. Additionally, the lithium‐ion hybrid capacitors assembled with the Fe 3 C NP@C electrode deliver an energy density of 98.9 Wh kg −1 at a power density of 20,000 W kg −1 together with sustained long‐term stability. These findings expand the functional role of the SEI and show that operando magnetometry can serve as a sensitive real‐time probe of magnetically coupled interfacial processes in the Fe 3 C‐based and related magnetic systems.