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)₃O₄ 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 Cu²⁺ and Ni²⁺ 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^2+/3+ and Fe^2+/3+ 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 Cr³⁺ cations are reduced to CrO and then Cr⁰ during lithiation. A relatively large overpotential is required to activate the Cr reoxidation reaction. The Cr³⁺ cations are found after delithiation. These results can guide the material design of HEOs for improving LIB performance.