Correlating the Synthesis and Electrochemical Performance of Complex Multi-Element High Entropy Oxides

High-entropy materials, especially high-entropy alloys and oxides, have sparked immense interest in the past few years due to their novel and curiously complex stoichiometries and structures, thereby allowing for the potential to generate materials with highly tunable and functional properties. Many of the studies on high entropy oxides have focused on five-cation-containing oxides produced via solid-state syntheses. Herein, we report on the practicality of the solution-based synthesis of 9-element high entropy oxide nanoparticles, incorporating Al, Co, Cu, Fe, Mg, Mn, Ni, Ti, and Zn, and have probed reaction conditions that influence the stability of the resulting single-phase spinel crystal structure. The separate but important discrete effects of (a) solvent, (b) surfactant, and (c) heating method, as well as (d) constituent cation composition, respectively, have been systematically explored and correlated with their resulting electrochemical properties and their microstructure, as revealed by complementary HR-TEM analysis. It has been found that nanoparticles characterized by single-phase spinel chemical compositions can be reliably reproduced with a wide variety of cations.