Insights into Ionic Transport and Structural Changes in Magnetite during Multiple‐Electron Transfer Reactions

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Metal oxides, such as Fe3O4, hold promise for future battery applications due to their abundance, low cost, and opportunity for high lithium storage capacity. In order to better understand the mechanisms of multiple‐electron transfer reactions leading to high capacity in Fe3O4, a comprehensive investigation on local ionic transport and ordering is made by probing site occupancies of anions (O2−) and cations (Li+, Fe3+/Fe2+) using multiple synchrotron X‐ray and electron‐beam techniques, in combination with ab‐initio calculations. Results from this study provide the first experimental evidence that the cubic‐close‐packed (ccp) O‐anion array in Fe3O4 is sustained throughout the lithiation and delithiation processes, thereby enabling multiple lithium intercalation and conversion reactions. Cation displacement/reordering occurs within the ccp O‐anion framework, which leads to a series of phase transformations, starting from the inverse spinel phase and turning into intermediate rock‐salt‐like phases (LixFe3O4; 0 < x < 2), then into a cation‐segregated phase (Li2O⋅FeO), and finally converting into metallic Fe and Li2O. Subsequent delithiation and lithiation processes involve interconversion between metallic Fe and FeO‐like phases. These results may offer new insights into the structure‐determined ionic transport and electrochemical reactions in metal oxides, and those of other compounds sharing a ccp anion framework, reminiscent of magnetite.