4.8 Article

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

Journal

CHEMISTRY OF MATERIALS
Volume 31, Issue 12, Pages 4341-4352

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.chemmater.8b04591

Keywords

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Funding

  1. Center for Electrochemical Energy Science, an Energy Frontier Research Center - US Department of Energy, Office of Science, Basic Energy Sciences [DE-AC02-06CH11]
  2. U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
  3. Soft and Hybrid Nano technology Experimental (SHyNE) Resource [NSF ECCS-1542205]
  4. MRSEC program at the Materials Research Center [NSF DMR-1720139]
  5. International Institute for Nanotechnology (IIN)
  6. Keck Foundation
  7. State of Illinois, through the IIN
  8. National Science Foundation [DMR-1809372, ACI-1053575]
  9. Office of Science of the U.S. Department of Energy (DOE) [DE-AC02-05CH11231]

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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/Li2-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 M-3-edge XANES and 0 K-edge XANES data provided evidence that oxygen hole states form during the early stage of delithiation at similar to 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 O2- 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 Xray spectroscopy; however, no unambiguous spectroscopic signatures of oxygen dimer interactions or nonbonding oxygen states were identified.

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