Ex situ and operando investigations on (FeCoCrMnZn)3O4 high entropy oxide as an anode for Li-ion batteries

Abstract

(FeCoCrMnZn)₃O₄ high entropy oxide samples were prepared through a solid-state synthesis route involving two major steps: ball milling and annealing. One sample (HEO1) was prepared via 12 hours of ball milling and 5 hours of air annealing at 1000 °C, while the other sample (HEO2) was prepared via 3 hours of ball milling and 12 hours of air annealing at 1000 °C. Structural, morphological and compositional analyses of both samples were performed by XRD, FTIR, HRTEM, SAED, FESEM and EDS measurements. XANES and EXAFS measurements and detailed analyses were performed to establish the possible compositions of both samples, and their relative stabilities were estimated by theoretical calculations, both empirically and using DFT. Lithium-ion half cells were prepared using both samples as anodes, and battery performance was evaluated by galvanostatic charge–discharge (GCD) measurements. It was found that the HEO sample prepared with a shorter ball milling time and longer annealing time (referred to as HEO2) shows better cycle performance as an anode of a Li-ion battery at high current than that prepared with a longer ball milling time and shorter annealing time (HEO1). Finally, operando XANES measurements are carried out during the discharging of HEO anodes to obtain insight into the role of various cations during the lithiation process of the HEO. The detailed structural, morphological and electrochemical insights into the (FeCoCrMnZn)₃O₄ HEO material prepared by solid-state synthesis route, when used as high capacity anodes in Li-ion batteries, as presented in this study, are very useful in the pursuit of new and novel electrode materials for Li-ion batteries. Particularly, this work will help researchers in the future to decide the optimum durations for ball milling and annealing, which are the two important steps for the preparation of a HEO anode material by a solid-state synthesis route.