High Entropy Oxides with Spinel Structure for Energy Storage Applications

Abstract

To satisfy ever-growing energy demands in modern society, researchers are eager to synthesize functional and tailorable materials. In recent years, a new class material with the potential of exceptional properties, high entropy alloy (HEA), was discovered. Moreover, the concept of HEA has extended to the ceramic system, and a new class of oxide has been proposed, so-called high entropy oxide (HEO). In 2015, Rost et al. reported a equimolar oxide with a single rocksalt phase, (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O, and successfully proved that high configurational entropy is the driven force of stabilizing the structure at high temperature. Further studies regarding functional properties of (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O are also conducted, such as magnetic, optical, catalyst, and electrochemical properties. In 2018, Sarkar et al. have reported that (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O demonstrated outstanding cyclic retention and high capacity as anode material of lithium-ion batteries. Besides, they proposed that high-entropy effect resulted in good cyclic stability, and each cation presents in the bulk tailored electrochemical properties.In this study, a spinel-structured HEO was first synthesized and characterized . Moreover, this spinel-structured HEO was utilized as anode material for lithium-ion battery and demonstrate high capacity (~500 mAh g-1), well cyclic stability with high coulombic efficiency (~100%) over 100 cycles. To investigate the storage mechanism of HEO, in operando X-ray diffraction and in operando X-ray absorption spectroscopy were utilized during charging and discharging. Even though there is no structural change, the valance state of each element changed obviously during lithiation and delithiation. Moreover, in operando transition X-ray microscopy investigation was conducted to observe the volume change during charging and discharging processes. Furthermore, the role of each element in energy storage was also investigated in this study. According to its cyclic stability, high capacity, and tailorable property, this spinel-structured HEO is a promising candidate for the energy storage in the future.