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) 3 O 4 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 2+ and Ni 2+ 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 Cr 3+ cations are reduced to CrO and then Cr 0 during lithiation. A relatively large overpotential is required to activate the Cr reoxidation reaction. The Cr 3+ cations are found after delithiation. These results can guide the material design of HEOs for improving LIB performance.