Probing Electrochemically Induced Structural Evolution and Oxygen Redox Reactions in Layered Lithium Iridate

In order to exploit electrochemical capacity beyond the traditionally utilized transition-metal redox reactions in lithium-metal-oxide cathode materials, it is necessary to understand the role that oxygen ions play in the charge compensation mechanisms, that is, to know the conditions triggering electron transfer on the oxygen ions and whether this transfer is correlated with battery capacity. Theoretical and experimental investigations of a model cathode material, Li-rich layered Li₂IrO₃, have been performed to study the structural and electronic changes induced by electrochemical delithiation in a lithium-ion cell. First-principles density functional theory (DFT) calculations were used to compute the voltage profile of a Li/Li_2-xIrO₃ cell at various states of charge, and the results were in good agreement with electrochemical data. Electron energy loss spectroscopy (EELS), X-ray absorption near-edge spectroscopy (XANES), resonant/nonresonant X-ray emission spectroscopy (XES), and first-principles core-level spectra simulations using the Bethe-Salpeter Equation (BSE) approach were used to probe the change in oxygen electronic states over the x = 0-1.5 range. The correlated Ir M₃-edge XANES and O K-edge XANES data provided evidence that oxygen hole states form during the early stage of delithiation at ∼3.5 V because of the interaction between O p and Ir d states, with Ir-oxidation being the dominant source of electrochemical capacity. At higher potentials, the charge capacity was predominantly attributed to oxidation of the O^2- ions. It is argued that the emergence of oxygen holes alone is not necessarily indicative of electrochemical capacity beyond transition-metal oxidation because oxygen hole states can appear as a result of enhanced mixing of O p and Ir d states. Prevailing mechanisms accounting for the oxygen redox mechanism in Li-rich materials were examined by theoretical and experimental X-ray spectroscopy; however, no unambiguous spectroscopic signatures of oxygen dimer interactions or nonbonding oxygen states were identified.