Real-Space Observation of Li Extraction∕Insertion in Li1.2Mn0.4Fe0.4O2Positive Electrode Material for Li-Ion Batteries

Using a spectrum-imaging scheme based on scanning transmission electron microscopy electron energy-loss spectroscopy, we achieve real-space observation of the extraction and insertion behaviors of Li ions in a high-capacity positive electrode material, Li1.2Mn0.4Fe0.4O2(Li2MnO3–LiFeO2system), consisting of Fe- and Mn-rich nanodomains. The Li ions are first extracted from Fe-rich nanodomains and subsequently extracted from the whole region. After discharge, Li ion recovery occurs. Results indicate the role of nanodomain structures in activating both α‐LiFeO2and Li2MnO3, which are each inactive as pure bulk. We discuss mechanisms of high capacity and degradation associated with oxidation of the oxygen ions.