Electrochemical Reaction Mechanism of High-Entropy Oxides in Li-Ion Batteries

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Recently, high-entropy oxides (HEOs) have emerged as an alternative conversion electrode material for next-generation Li-ion batteries (LIBs). The concept of entropy stabilization of numerous transition metal oxides (TMOs) in a single disordered solid solution phase opens new boundaries of materials design, providing the possibility of overcoming individual TMOs limitations by tailorable properties. Although conversion anode TMOs deliver high capacity due to multi-electron redox processes, they still suffer from large volume variation and severe structural changes, leading to electrode pulverization and resulting in drastic capacity fading. The individual contribution of each transition metal (TM) in the HEO structure to the electrochemical reaction mechanism and its structural stability is not yet well understood and prevents the rational design of HEOs with different electrochemically active sites that can be used to improve the Li-storage performance and battery cyclability. In this work, (MgCoNiCuZn)O nanoparticle anode materials were synthesized by solid state reaction and subsequent high energy ball milling. The system consists of five component bivalent ions sharing the same transition metal (TM) site in a rock-salt structure. Extended x-ray absorption fine structure (EXAFS) was used to investigate the change in the local structure around each TM in the pristine and galvanostatically cycled electrodes. The 1st, 2nd and 100th cycles in lithiated and delithiated states were studied, showing an initial capacity of ~550 mAh/g at a current density of 50 mA/g for the first couple of cycles, and ~650 mAh/g at 100 mA/g after 100 cycles. Our preliminary data reveal significant changes from the initially octahedral coordination of each TM, giving us insights about the electrochemical and structural role of each component. In the lithiated state, Co, Cu and Ni are reduced to a metallic state while Zn remains oxidized. Subsequent delithiation shows that Co and Ni are re-oxidized to some degree while Cu and Zn remain unchanged. In this presentation we will discuss the structural analysis of the local environment of each TM with a focus on their individual roles in the electrochemical performance of the HEO anode.