Unraveling the Origins of the "unreactive Core" in Conversion Electrodes to Trigger High Sodium-Ion Electrochemistry

Electrochemical storage via conversion reactions in crystalline electrode materials critically rests upon the sizes of the guest ions. Here we report an unusual behavior that renders CuO inactive in the process of sodium-ion insertion with a synergistic combination of a variety of synchrotron X-ray microscopic, spectroscopic, and structural probes. We reveal that the "unreactive core" formation is closely associated with the microstructural integrity of battery active materials. In light of these findings, we also demonstrate that this undesirable process can be inhibited by the materials' microstructural design to trigger the potential of high electrochemical properties.