In Situ Atomic-Scale Observation of (Cr,Mn,Fe,Co,Ni)₃O₄ High-Entropy Spinel Oxide Formation During Calcination Process

High-Entropy Oxide (HEO) is a new-type of anode materials for lithium ion batteries (LIBs), owing to their stable crystal structure, superionic conductivity and high capacity. In recent years, High-Entropy Oxide (HEO) nanoparticles were synthesized via surfactant-assisted hydrothermal method [1], which exist problems of complicated synthesis procedure. In this work, we prepared the (Cr,Mn,Fe,Co,Ni)₃O₄ HEO by high-temperature solid state reaction [2]. In order to improve the synthesis efficiency, the formation mechanism is necessary. However, the study on the microstructure changing during calcination is still lacking. Therefore, we recorded detailed information of calcination with the temperature increasing by high-resolution transmission electron microscopy (HRTEM), and the (Cr,Mn,Fe,Co,Ni)₃O₄ HEO was obtained at 900°C. The entire forming process includes the aggregation of precursors at 500°C; when annealing at 600°C, MnO₂ and NiO diffused into Co₃O₄ , and the spinel-structured (Mn,Co,Ni)₃O₄ was formed. With the temperature increasing constantly, Fe₂O₃ and Cr₂O₃combined with (Mn,Co,Ni)₃O₄ sequentially and formed (Cr,Mn,Fe,Co,Ni)₃O₄ spinel-structured HEO at 900°C. Furthermore, from the <i>in-situ</i> TEM observation, the particle growth direction and the structure of intermediate products were identified through the corresponding FFTs. We also unravel the valence change mechanisms and ion arrangements of (Cr,Mn,Fe,Co,Ni)₃O₄ via electron energy loss spectroscopy (EELS), and X-ray absorption near edge spectroscopy (XANES).<br/>This study successfully revealed the formation and growth process of HEO at atomic scale for the first time. The results provide the roadmap on improving the manufacturing process of (Cr,Mn,Fe,Co,Ni)₃O₄ HEO, which are expected to play a vital part in the development of anode materials for next generation LIBs.<br/><br/><b>Keywords</b>: High entropy oxides, TEM, Calcination, Spinel-structured<br/><b>Reference: </b><br/>[1] T.- X. Nguyen, J. Patra, J.-K. Chang, J.-M. Ting, High entropy spinel oxide nanoparticles for superior lithiation–delithiation performance, J. Mater. Chem. A, 8 (2020) 18963<br/>[2] D. Wang, S. Jiang, C. Duan, J. Mao, Y. Dong, K. Dong, Z. Wang, S. Luo, Y. Liu, X. Qi,<br/>Spinel-structured high entropy oxide (FeCoNiCrMn)3O4 as anode towards superior lithium storage performance, J. Alloy. Compd. 844 (2020) 156158