Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries

Abstract

<div>The pre-edges of oxygen-K X-ray absorption spectra have been ubiquitous in transition metal (TM) oxide studies in various fields, especially on the fervent topic of oxygen redox states in battery electrodes. However, critical debates remain on the use of the O-K pre-edge variations upon electrochemical cycling as evidences of oxygen redox reactions, which has been a popular practice in the battery field. This study presents an investigation of the O-K pre-edge of 55 oxides covering all 3d TMs with different elements, structures and electrochemical states through combined experimental and theoretical analyses. It is shown unambiguously that the O-K pre-edge variation in battery cathodes is dominated by changing TM-d states. Furthermore, the pre-edge enables a unique opportunity to project the lowest unoccupied TM-d states onto one common energy window, leading to a summary map of the relative energy positions of the low-lying TM states, with higher TM oxidation states at lower energies, corresponding to higher electrochemical potentials. The results naturally clarify some unusual redox reactions, such as Cr<sup>3+/6+</sup>. This work provides a critical clarification on O-K pre-edge interpretation and more importantly, a benchmark database of O-K pre-edge for characterizing redox reactions in batteries and other energy materials.</div>

Redox Mechanism in Na-Ion Battery Cathodes Probed by Advanced Soft X-Ray Spectroscopy

“…Additionally, some unconventional states that are triggered by electrochemical cycling cannot be sensed through conventional sXAS, as elaborated in this review through several examples of both TMs and O. In particular, we have clarified that sXAS is not conclusive for characterizing the O redox states because the intensity and the lineshape of the O- K sXAS are both dominated by TM characters through strong hybridization effects (Yang and Devereaux, 2018 ; Zhuo et al, 2019 ; Roychoudhury et al, 2020 ). In order to tackle these technical limitations of sXAS in both its FY lineshape distortion and its lack of chemical sensitivity, high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) has been developed.…”

“…To decipher the O redox mechanisms, a direct and reliable spectroscopic probe of the intrinsic O electronic states is essential. While O- K sXAS has been popularly employed in the studies of O redox (Oishi et al, 2015 , 2016 ; Luo et al, 2016a , b ; Ma et al, 2017 ), it has been clarified that the pre-edge feature evolution in both the intensity and lineshape is largely determined by the TM states, which is hard to be distinguished from the oxygen redox states (Yang and Devereaux, 2018 ; Roychoudhury et al, 2020 ). In principle, mRIXS has an additional ability of resolving the fluorescence along the emission energy and thus is capable to disentangle the two features (Qiao et al, 2017 ).…”

Could Irradiation Introduce Oxidized Oxygen Signals in Resonant Inelastic X-ray Scattering of Battery Electrodes?

“…17 This feature gets enhanced at the charged state due to the enhancement of the hybridization at highly oxidized states, corresponding to the O-K XAS pre-edge feature enhancement upon charging. 7,10 Second, at the charged state, a new feature emerges around 530−532 excitation energy and around 523.8 eV emission energy (I in Figure 1c), with another concurrent enhancement of the intensity close to the elastic line at the same excitation energy (III in Figure 1c). Both features, I and III, are signatures of oxidized oxygen states 17 and disappear in the discharged state (Figure 1d), indicating a reversible oxygen redox reaction.…”

“…7,10 Therefore, the key technical challenge for detecting the intrinsic oxidized oxygen state is to separate its signature from the strong TM-O hybridization signals that are largely of TM nature. 7 Fortunately, high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) has been developed and has been shown to be able to solve this technical challenge by resolving the energies of the emitted photons, that is, the emission energy, at each excitation energy across the absorption edge. 11,12 Two groups of mRIXS features are associated with the oxidized oxygen state in oxide cathodes: (i) a relatively sharp feature around the 531 eV excitation energy and 523 to 524 eV emission energy 10,11,13−15 and (ii) an enhanced feature with low energy loss close to the elastic line at the same excitation energy.…”

“…6 Signals from another popular technique, oxygen K-edge (O-K) X-ray absorption spectroscopy (XAS), have been found to be dominated by TM states hybridized with O, in both its intensity and line shape. 7 Therefore, a reliable technique for detecting the intrinsic oxygen oxidation state regardless of its form in TM oxides has become a critical challenge.…”