<i>In Situ</i> Transmission Electron Microscopy Observation of the Conversion Mechanism of Fe<sub>2</sub>O<sub>3</sub>/Graphene Anode during Lithiation–Delithiation Processes

Transition metal oxides have attracted tremendous attention as anode materials for lithium ion batteries (LIBs) recently. However, their electrochemical processes and fundamental mechanisms remain unclear. Here we report the direct observation of the dynamic behaviors and the conversion mechanism of Fe<sub>2</sub>O<sub>3</sub>/graphene in LIBs by <i>in situ</i> transmission electron microscopy (TEM). Upon lithiation, the Fe<sub>2</sub>O<sub>3</sub> nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li<sub>2</sub>O layer. Single-crystalline Fe<sub>2</sub>O<sub>3</sub> nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li<sub>2</sub>O matrix. Surprisingly, the delithiated product was not Fe<sub>2</sub>O<sub>3</sub> but FeO, accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. The charge–discharge processes of Fe<sub>2</sub>O<sub>3</sub> in LIBs are different from previously recognized mechanism, and are found to be a fully reversible electrochemical phase conversion between Fe and FeO nanograins accompanying the formation and disappearance of the Li<sub>2</sub>O layer. The macroscopic electrochemical performance of Fe<sub>2</sub>O<sub>3</sub>/graphene was further correlated with the microcosmic <i>in situ</i> TEM results.