Spatially Decoupled Electron and Lithium Storage at Fe/Solid Electrolyte Interphase Interfaces

Extra lithium storage in transition‐metal nanoparticles remains debated because interfacial reactions are difficult to distinguish. Using Fe/C as a model anode, we combine in situ electron paramagnetic resonance (EPR) and magnetometry to resolve two concurrent storage processes. Magnetometry identifies electron accumulation in Fe 3d orbitals at Fe/Li 2 O rich interfaces, consistent with space charge storage. Furthermore, EPR reveals a reversible, potential‐dependent radical formation within the polymeric solid electrolyte interphase (SEI) after its initial formation. Density functional theory supports Fe‐catalyzed ring opening of carbonate solvents and chain‐radical intermediates that undergo polymerization and reconstruction. These results show that the dynamic organic SEI participates in reversible charge storage alongside inorganic Fe/Li 2 O interfaces. Thus, the traditionally believed inactive Fe‐dominated materials deliver a reversible capacity of 554 mAh g −1 at 2 A g −1 , leading to an energy density of 100 Wh kg −1 at a power density of 100 W kg −1 in an asymmetric lithium ion capacitor with activated carbon as the cathode. These findings extend established interfacial storage concepts to radical‐mediated charge storage within a dynamically reconstructed SEI.