Insights into ionic transport and structural changes in magnetite during multiple-electron transfer reactions

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 (O^2-) 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 (Li_xFe₃O₄; 0 < x < 2), then into a cation-segregated phase (Li₂OFeO), 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. In order to better understand the mechanisms of multiple-electron transfer reactions leading to high capacity in Fe₃O₄, a comprehensive experimental investigation, assisted with ab initio calculations, is made on local ionic transport and ordering in the material. It is revealed that the cubic-close-packed O-anion array is sustained in Fe₃O₄ during delithiation, thereby enabling multi-Li intercalation and conversion reactions.