While conversion-type metal selenides are highly promising for fast-charging batteries, the microscopic origin of their ultrafast interfacial kinetics has remained largely unclear. Here, we developed a spatially confined CoSe@N-C architecture as a well-defined model platform. Using advanced operando magnetometry, we achieved quantitative decoupling of the space-charge storage contribution from conventional diffusion-controlled processes, providing direct insight into the interfacial charge storage mechanism of conversion-type metal selenides. The rigid porous carbon framework not only mitigates volume expansion but, more importantly, enforces intimate, atomic-scale contact between the in situ generated electronic (Co) and ionic (Li2Se/Na2Se) phases, maximizing the density of space-charge interfaces. Driven by this enhanced spin-electronic and ionic coupling, the spatially confined CoSe@N-C anodes deliver exceptional fast-charging capabilities and stable wide-temperature stability (from −20 to 50 °C) in both Li- and Na-ion batteries. In addition to demonstrating a high-performance electrode, this work offers quantitative insights into the space-charge mechanism, establishing a crucial physical perspective for designing next-generation ultrafast energy storage materials.