In Situ TEM Observation of (Cr, Mn, Fe, Co, and Ni)O High-Entropy Spinel Oxide Formation During Calcination at Atomic Scale.

High-entropy oxides (HEOs) are promising anode materials for lithium-ion batteries (LIBs), owing to their stable crystal structure, superionic conductivity, and high capacity. In this study, the (Cr, Mn, Fe, Co, and Ni)O HEO via solid-state reaction is prepared. To improve the synthetic efficiency, it is necessary to understand the formation mechanism. Therefore, a high-resolution transmission electron microscopy (HRTEM) is used to record information during calcination at increasing temperature. The overall formation process included MnO and NiO aggregation at 500 °C, followed by (Mn, and Ni)O combined with CoO at 600 °C to form (Mn, Co, and Ni)O. At higher temperatures, FeO and CrO sequentially combined with (Mn, Co, and Ni)O and formed the (Cr, Mn, Fe, Co, Ni)O at 900 °C. In addition, the valence-state-changing mechanisms and ion arrangements of (Cr, Mn, Fe, Co, and Ni)O are determined using electron energy loss spectroscopy (EELS) and extended X-ray absorption fine structure (EXAFS). This study successfully revealed the formation of HEO at atomic scale. The results provide valuable insights for improving the manufacturing process of (Cr, Mn, Fe, Co, and Ni)O HEOs, which is expected to play a vital role in the development of anode materials for next-generation LIBs.