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 Fe3C 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 Fe3C accommodates spin‑polarized electrons, enabling decoupled ionic and electronic storage across the interface. Additionally, the lithium-ion hybrid capacitors assembled with the Fe3C 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 Fe3C-based and related magnetic systems.