Probing the Participation of Oxygen Redox in the Electrochemical Cycling of Li-Rich Li2IrO3
Eric L. Shirley, John T. Vinson, Liang Li, Maria K. Chan, Joong S. Park, Eungje Lee, John W. Freeland, Zhenpeng Yao, Fernando Castro, Timothy T. Fister, Christopher M. Wolverton, Michael Thackeray
To exploit extra capacity beyond the traditionally postulated transition metal redox in Li-ion batteries, it is imperative to understand the exact role of oxygen in the charge compensation, i.e., what triggers the electron occupation change in oxygen and whether such a change is correlated with battery capacity. We perform theoretical and experimental investigations to study the electrochemical-delithiation-induced structural and electronic evolution of Li2IrO3, a model Li-rich layered cathode material. A first-principles scheme is proposed to compute the structures and voltage curve of Li2IrO3 at various states of charge, and excellent agreement with electrochemical experiments is obtained. Specifically focusing on the electronic response of oxygen, we employed Electron Energy Loss Spectroscopy (EELS), X-ray Absorption Near-Edge Spectroscopy (XANES), as well as first-principles core-level spectra simulation, to probe the change in oxygen electronic states. Correlating Ir and O edges in XANES demonstrate that oxygen hole states are formed in the early stage of delithiation at 3.5 V due to the interaction between O p and Ir d states, and this Ir oxidation is the dominant source of capacity; at higher voltages, the capacity is largely due to oxygen contribution. We further show that the emergence of oxygen holes cannot be regarded as the sole indicator of extra capacity beyond transition metal oxidation, as it sometimes is a result of enhanced mixing of O p and Ir d states.