Radical Bearing Solid Electrolyte Interface as an Active Ionic Reservoir for Fast Li + Storage in Coordination Confined Fe@C Anodes

High capacity anodes rely on diffusion limited bulk reactions that induce structural strain, compromising fast and durable Li + storage. Here, carbon‐coated Fe nanocomposites (Fe@C) are constructed through a MOF‐derived coordination confinement strategy that suppresses Fe nanoparticle coalescence and preserves Fe‐centered interfaces for space charge storage, with a protective carbon shell contributing some capacity. This interfacial pathway minimizes strain and shortens charge transport distances, enabling a reversible capacity of 565 mAh g −1 at 2C. With electron accumulation in metal 3d states recognized as the electronic component of space charge storage, the remaining question is how the compensating Li + is accommodated within the solid electrolyte interphase (SEI). In situ electrochemical impedance spectroscopy tracks the reversible formation of the SEI, and ex situ analyses identify organic polymers as the main components for Li + storage. In situ electron paramagnetic resonance with spin trapping captures reversible carbon‐chain radical evolution associated with Li + accommodation. Together, these findings reveal a radical bearing polymeric SEI that undergoes Fe‐catalyzed dynamic reconstruction and serves as the ionic component of space charge storage. This work links coordination confined interface design with radical mediated SEI chemistry, establishing an active ionic reservoir for fast interfacial Li + storage.