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 mech-anisms, i.e., 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/non-resonant 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 to 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 due to 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, since 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 interaction or non-bonding oxygen states were identified.