Enhanced Li-Ion Diffusion and Cycling Stability of Ni-Free High-Entropy Spinel Oxide Anodes with High-Concentration Oxygen Vacancies.

High-entropy oxide (HEO) is an emerging type of anode material for lithium-ion batteries with excellent properties, where high-concentration oxygen vacancies can effectively enhance the diffusion coefficient of lithium ions. In this study, Ni-free spinel-type HEOs ((FeCoCrMnZn)O and (FeCoCrMnMg)O) were prepared via ball milling, and the effects of zinc and magnesium on the concentration of oxygen vacancy (O), lithium-ion diffusion coefficient (), and electrochemical performance of HEOs were investigated. Ab initio calculations show that the addition of zinc narrows down the band gap and thus improves the electrical conductivity. X-ray photoelectron spectroscopy (XPS) results show that (FeCoCrMnZn)O (42.7%) and (FeCoCrMnMg)O (42.5%) have high O concentration. During charge/discharge, the O concentration of (FeCoCrMnZn)O is higher than that of (FeCoCrMnMg)O. The galvanostatic intermittent titration technique (GITT) results show that the value of (FeCoCrMnZn)O is higher than that of (FeCoCrMnMg)O during charge and discharge. All of that can improve its specific discharge capacity and enhance its cycle stability. (FeCoCrMnZn)O achieved a discharge capacity of 828.6 mAh g at 2.0 A g after 2000 cycles. This work provides a deep understanding of the structure and performance of HEO.