Effect of Metal Cation Selection on Phase Properties and Electrochemical Performance of Co-Free High Entropy Spinel Oxide Anodes

Recently, high entropy oxide (HEO) materials received compelling research consideration for lithium-ion batteries (LIBs) owing to their exceptional cycling stability, high specific capacity, and exclusive tailorable properties. This unique tailorable aspect of HEO enables the use of infinite metal cation combinations to develop new electrode materials with exciting and unexplored properties. In this study, we developed a series of Co-free spinel-type HEO (HESO) anodes via a facile hydrothermal method. The effects of various elemental combination on phase purity, morphology, crystallinity, microstructures, valence states, and electrochemical charge-discharge properties of the HESO electrodes are systematically studied. For the first time, we demonstrate that the chemical composition of HESOs is crucial to the phase purity, morphology, and oxygen vacancy concentration. The oxygen vacancy concentration seems to have a substantial impact on the rate capability and reversibility of the HEO electrodes. This study demonstrates that an electrochemically inactive spectator element is not necessary for achieving high cyclability, given that the phase purity of the HEO is wisely controlled. The single-phase (CrNiMnFeCu) 3 O 4 shows a great high-rate capability and no capacity fading after 400 cycles. In addition, the charge/discharge behavior was examined using operando X-ray diffraction. According to the operando XRD analysis, a spinel-to-rock salt phase transition is observed for (CrNiMnFeCu) 3 O 4 before the formation of short-range ordered nano-crystals during the first lithiation. The nano-crystalline nature is preserved upon cycling. The the practical feasibility of HEO anode is evaluated by constructing a 4MCu||LiNi 0.8 Co 0.1 Mn 0.1 O 2 full cell.