Unraveling the Rapid Redox Behavior of Li-Excess 3d-Transition Metal Oxides for High Rate Capability

Abstract

Li-excess 3d-transition metal layered oxides are promising candidates in high-energy-density cathode materials for improving the mileage of electric vehicles. However, their low rate capability has hindered their practical application. The lack of understanding about the redox reactions and migration behavior at high C-rates make it difficult to design Li-excess materials with high rate capability. In this study, the characteristics of the atomic behavior that is predominant at fast charge/discharge are investigated by comparing cation-ordered and cation-disordered materials using X-ray absorption spectroscopy (XAS). The difference in the atomic arrangement determines the dominance of the transition metal/oxygen redox reaction and the variations in transition metal-oxygen hybridization. In-depth electrochemical analysis is combined with operando XAS analysis to reveal electronically and structurally preferred atomic behavior when a redox reaction occurs between oxygen and each transition metal under fast charge/discharge conditions. This provides a fundamental insight into the improvement of rate capability. Furthermore, this work provides guidance for identifying high-energy-density materials with complex structural properties.