Extra capacity beyond a theoretically predicted value has been widely acknowledged in transition metal-based anodes for application in lithium batteries, but the precise definition of their charge storage behaviors remains elusive owing to the intricate and dynamic interior space. Herein, an urchin-like FeOOH nanostructure is fabricated to reveal the dynamic electrochemical evolution of extra capacity. It is found that electrons continually accumulate at the interface between Fe and the Li₂O/LiOH matrix at a narrow discharge voltage range of 1–0 V, forming a space-charge storage region, thus enabling extra capacity besides the capacity from the reversible conversion reaction. Moreover, using advanced operando magnetometry technology, the significant interfacial space-charge storage behavior could be accurately quantified to be 239.1 mA h g⁻¹. In contrast, the evolution of the partially reversible solid-state electrolyte interface (SEI) functions as a diminished additional source of capacity, significantly lowering the coulombic efficiency and stability of batteries. Further, by fully exploiting the potential of the reversible interfacial space-charge storage mechanism, a well-designed lithium-ion hybrid capacitor is constructed using a reduced FeOOH anode and commercial AC cathode, achieving a high energy density of 140.4 W h kg⁻¹ and a remarkable capacity retention of ∼100% after 1000 cycles at 1 A g⁻¹. This study offers a broad insight into the extended utilization of extra capacity, paving the way for a sustainable strategy in the development of new energy batteries.

The NiO/SnO₂ hybrid interface generates Ni nanoparticles within multilayers, catalyzing Li₂O decomposition and suppressing Sn or Li₂O particle coarsening, thus enhancing reaction kinetics to achieve ultra-high ICE, capacity and long cycling.