<i>In Situ</i> Transmission Electron Microscopy Observation of the Conversion Mechanism of Fe₂O₃/Graphene Anode during Lithiation–Delithiation Processes

Abstract

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 Fe₂O₃/graphene in LIBs by <i>in situ</i> transmission electron microscopy (TEM). Upon lithiation, the Fe₂O₃ nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li₂O layer. Single-crystalline Fe₂O₃ nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li₂O matrix. Surprisingly, the delithiated product was not Fe₂O₃ 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 Fe₂O₃ 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 Li₂O layer. The macroscopic electrochemical performance of Fe₂O₃/graphene was further correlated with the microcosmic <i>in situ</i> TEM results