Charge-Discharge Mechanism of High-Entropy Co-Free Spinel Oxide Toward Li Storage Examined Using Operando Quick-Scanning X-Ray Absorption Spectroscopy.

Transition metal high-entropy oxides (HEOs) are an attractive class of anode materials for high-performance lithium-ion batteries (LIBs). However, owing to the multiple electroactive centers of HEOs, the Li storage mechanism is complex and debated in the literature. In this work, operando quick-scanning X-ray absorption spectroscopy (XAS) is used to study the lithiation/delithiation mechanism of the Cobalt-free spinel (CrMnFeNiCu) O HEO. A monochromator oscillation frequency of 2 Hz is used and 240 spectra are integrated to achieve a 2 min time resolution. High-photon-flux synchrotron radiation is employed to increase the XAS sensitivity. The results indicate that the Cu and Ni cations are reduced to their metallic states during lithiation but their oxidation reactions are less favorable compared to the other elements upon delithiation. The Mn and Fe cations undergo two-step conversion reactions to form metallic phases, with MnO and FeO as the intermediate species, respectively. During delithiation, the oxidation of Mn occurs prior to that of Fe. The Cr cations are reduced to CrO and then Cr during lithiation. A relatively large overpotential is required to activate the Cr reoxidation reaction. The Cr cations are found after delithiation. These results can guide the material design of HEOs for improving LIB performance.