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Harnessing oxygen redox reactions is an intriguing route to increasing capacity in Li-ion batteries (LIBs). Despite numerous experimental and theoretical attempts to unravel the mechanism of oxygen redox behavior, the electronic origin of oxygen activities in energy storage of Li-rich LIB materials remains under intense debate. In this work, the onset of oxygen activity was examined using a Li-rich material that has been reported to exhibit oxygen redox, namely, Li₅FeO₄. By comparing experimental measurements and first-principles Bethe-Salpeter equation calculations of oxygen K-edge X-ray absorption spectra (XAS), it was found that experimentally-observed changes in XAS originate from the non-bonding oxygen statesmore » in cation-disordered delithiated Li₅FeO₄, and the spectral features of oxygen dimers were also determined. This combined experimental and theoretical study offers an effective approach to disentangle the intertwined signals in XAS and can be further utilized in broader contexts for characterizing other energy storage and conversion materials.« less

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-xIrO₃more » 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.« less

Cuprous oxide (Cu₂O) is a promising catalyst for several important reactions. However, the atomic structures of defective Cu₂O surfaces, which critically affect the catalytic properties both thermodynamically and kinetically, are not unambiguously characterized. High-resolution scanning tunneling microscopy (STM), combined with density functional theory (DFT) calculations and STM simulations, has been used to determine the atomic structure of the (111) surface of a Cu₂O bulk crystal. The single crystal surface, processed by ultrahigh vacuum cleaning and oxygen annealing, shows a (1 × 1) periodicity in the low-energy electron diffraction pattern. The pristine (defect-free) Cu₂O(111) surface exhibits a lattice of protrusions withmore » hexagonal symmetry under STM, which is attributed to the dangling bonds of the coordinatively unsaturated copper (Cu_U) atoms on the surface. Two types of surface atomic defects are also identified, including the Cu_U vacancy and the oxygen-vacancy-induced local surface restructuring. The electronic structure of this surface measured by $dI/dV$ spectroscopy shows an energy band gap of ~1.6–2.1 eV. Consistent with $dI/dV$ measurements, DFT calculations identified surface states within the electronic band gap arising from the Cu ions on the surface. Our results provide a clear picture of the pristine and defective Cu₂O(111) surface structure in addition to the formation mechanism of the reconstructed surface, paving the way toward studying the site-dependent reactivity of this surface.« less

Balancing global energy needs against increasing greenhouse gas emissions requires new methods for efficient CO₂ reduction. While photoreduction of CO₂ is a viable approach for fuel generation, the rational design of photocatalysts hinges on precise characterization of the surface catalytic reactions. Cu₂O is a promising next-generation photocatalyst, but the atomic-scale description of the interaction between CO₂ and the Cu₂O surface is largely unknown, and detailed experimental measurements are lacking. In this study, density-functional-theory (DFT) calculations have been performed to identify the Cu₂O (110) surface stoichiometry that favors CO₂ reduction. To facilitate interpretation of scanning tunneling microscopy (STM) and X-ray absorptionmore » near-edge structures (XANES) measurements, which are useful for characterizing catalytic reactions, we present simulations based on DFT-derived surface morphologies with various adsorbate types. STM and XANES simulations were performed using the Tersoff Hamann approximation and Bethe-Salpeter equation (BSE) approach, respectively. The results provide guidance for observation of CO₂ reduction reaction on, and rational surface engineering of, Cu₂O (110). In conclusion, they also demonstrate the effectiveness of computational image and spectroscopy modeling as a predictive tool for surface catalysis characterization.« less

Anionic redox reactions in cathodes of lithium-ion batteries are allowing opportunities to double or even triple the energy density. However, it is still challenging to develop a cathode, especially with Earth-abundant elements, that enables anionic redox activity for real-world applications, primarily due to limited strategies to intercept the oxygenates from further irreversible oxidation to O₂ gas. Here in this paper, we report simultaneous iron and oxygen redox activity in a Li-rich anti-fluorite Li₅FeO₄ electrode. During the removal of the first two Li ions, the oxidation potential of O^2- is lowered to approximately 3.5 V versus Li⁺/Li⁰, at which potential themore » cationic oxidation occurs concurrently. These anionic and cationic redox reactions show high reversibility without any obvious O₂ gas release. Furthermore, this study provides an insightful guide to designing high-capacity cathodes with reversible oxygen redox activity by simply introducing oxygen ions that are exclusively coordinated by Li⁺.« less

Anionic redox reactions in cathodes of lithium-ion batteries are allowing opportunities to double or even triple the energy density. However, it is still challenging to develop a cathode, especially with Earth-abundant elements, that enables anionic redox activity for real-world applications, primarily due to limited strategies to intercept the oxygenates from further irreversible oxidation to O₂ gas. Here we report simultaneous iron and oxygen redox activity in a Li-rich anti-fluorite Li5FeO4 electrode. During the removal of the first two Li ions, the oxidation potential of O_2- is lowered to approximately 3.5 V versus Li+/Li0, at which potential the cationic oxidation occursmore » concurrently. These anionic and cationic redox reactions show high reversibility without any obvious O₂ gas release. Moreover, this study provides an insightful guide to designing high-capacity cathodes with reversible oxygen redox activity by simply introducing oxygen ions that are exclusively coordinated by Li⁺.« less