For the cathode materials potentially available for high power capability, reducing their particle size can improve the bulk ionic conductivity due to reduced ion diffusion length, and exploiting new reaction mechanism must be fundamentally advantageous. However, other issues such as synthesis difficulty, poor charge storage stability, and capacity decay can emerge. To simultaneously address these issues, in this work, we first find solid-solid interfacial storage for the ultrafine insertion cathode materials in the space-charge region of a mixed ion/electron conductor through the so-called “job-sharing” mechanism. This mechanism shows that electrons and ions can be stored in the different phases around the interface and transport only inside there, which looks thermodynamically distinct from most of conventional charge storage mechanisms in terms of the relationship between charge storage and cell voltage. The insertion cathodes governed by the “job-sharing” mechanism thus exhibit the outstanding performances with high capacity, fast kinetics, and stable cyclability. Herein, the inverse conceptual compositing between ionic conductor and electronic conductor to harness the size effect offers a potential research direction for not only electrode design in high-power batteries, but also other electrochemical potential applications such as solid-state electrolytes and so on.