Lithiation Mechanisms in High Entropy Oxides as Anode Materials for Li-Ion Batteries

Abstract

High entropy oxides based on transition metals, as Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O (TM-HEO), have recently drawn special attention as potential anodes in lithium-ion batteries, due to high specific capacity and cycling reversibility. However, the lithiation/delithiation mechanism of such systems is still controversial and not clearly addressed. Here, we report on an operando XAS investigation on TM-HEO based anodes for lithium-ion cells during the first lithiation/delithiation cycle. This material showed high specific capacity exceeding 600 mAh g-1 at C/10 and Coulombic efficiency very close to unit. The combination of functional and advanced spectroscopic study revealed complex charging mechanisms, developing through an initial Li intercalation reaction during the first charging cycle, followed by the reduction of transition metal (TM) cations, which triggers the conversion reaction below 1.0 V. The conversion is irreversible and not completed, leading to the final collapse of the HEO rock-salt structure. Other redox processes are therefore discussed and called to account the observed cycling behavior of the TM-HEO based anode. Despite the irreversible phenomena, the HEO cubic structure remains intact for ~ 60% of lithiation delivered capacity, so proving the beneficial role of the configuration entropy to enhance the stability of the HEO rock-salt structure during the redox phenomena.