Impact of Synthetic Parameters on Structure and Electrochemistry of High-Entropy Layered Oxide LiNi0.2Co0.2Mn0.2Al0.2Fe0.2O2

Abstract

This study explored a high-entropy layered oxide (HELO), LiNi_0.2Co_0.2Mn_0.2Al_0.2Fe_0.2O₂, prepared by coprecipitation followed by heat treatment. Coprecipitation yielded a kinetically favored product, and the subsequent heat treatment under various temperatures and times allowed tuning the material toward more thermodynamically favored structures. Refinement of X-ray powder diffraction (XRD) revealed that after 450 °C treatment, ∼12% of the Ni²⁺ in the structure was located within the lithium cation layer. After heat treatment at 600, 700, and 800 °C, there was a continued decrease of Ni²⁺ in the lithium cation layer to 9.3, 6.3, and 3.7%, respectively. Longer heat treatment times at 800 °C decreased the level to 2.5%. Electrochemical behavior was evaluated by cyclic voltammetry, galvanostatic cycling, rate capability testing, and electrochemical impedance spectroscopy, where increased loaded voltage, functional capacity, rate capability, and decreased impedance were observed for samples treated at higher temperatures with lower cation mixing. X-ray absorption near edge spectroscopy (XANES) data indicated that the redox activity of Ni and Co was dominant in the electrochemistry, while the participation of Mn or Fe was minimal. The level of Ni oxidation state change between charge and discharge related to the heat treatment temperature where the samples with high cation mixing showed lower capacity consistent with some Ni³⁺ in the transition metal layer inaccessible for electron transfer. Longer heat treatment times at 800 °C did not continue to provide a benefit in electrochemical function even with some additional reduction in cation mixing.