Valence Electronic Structure of Li₂O₂, Li₂O, Li₂CO₃, and LiOH Probed by Soft X-ray Emission Spectroscopy

Abstract

The valence electronic structures of Li2CO3, Li2O, Li2O2, and LiOH were determined by soft X-ray emission spectroscopy (XES) at the oxygen K-edge. To ensure the collection of representative high-quality spectra, beam damage effects were characterized, and their influence on the spectral characteristics was minimized by limiting the exposure time (i.e., scanning the sample under the X-ray beam). We find that the spectral shapes of the four spectra are very compound-specific and allow an unambiguous speciation of these compounds. The emission lines are discussed and assigned based on published calculated oxygen-derived partial density of states. It is shown that the oxygen emission of Li2CO3, Li2O, Li2O2, and LiOH in the upper valence band is mainly related to the s- and p-like states of the carbonate anion CO3 2–, the p-like states of the oxide anion O2–, the p-like states of the peroxide anion O2 2–, and the π-like character of the OH– group, respectively. This work thus creates the basis for XES studies of the chemical reaction mechanism in energy storage devices involving these key compounds.

Spectroscopic Signature of Oxidized Oxygen States in Peroxides

“…The outgoing photon’s emission energy is independent of the incident excitation energy, indicating that these are fluorescence-like emission lines from the decay of electrons from valence bands (VBs) to the core holes 13 . Indeed, the energy of this emission line is consistent with the X-ray emission spectra of Li 2 O 19 . The origin of the two regimes of excitation energies could be understood by comparing with the O -K sXAS spectrum (Figure S1).…”

“…Our central goal is to detect and identify the nature of the aforementioned critical O -K mRIXS feature in Lithium peroxide, thus providing benchmarks and guidelines for understanding the O -K mRIXS findings in energy materials. We note that collecting reliable mRIXS data from Li 2 O 2 is a nontrivial issue due to the typical low count rates of RIXS experiments and the radiation sensitivity of the material 14, 19 . These technical challenges have now been solved through our recently commissioned RIXS system with ultra-high detection efficiency 20–21 , and mRIXS results are successfully collected with controlled sample transfer, cooling, and rastering.…”

“…mRIXS of Li 2 O 2 , Li 2 O, and Li 2 CO 3 were collected at the high-efficiency iRIXS endstation of BL8.0.1 of Advanced Light Source 20, 24 . Li 2 O 2 is unstable under air exposure (forming Li 2 CO 3 ), heating (decomposes at 450 °C to Li 2 O), and X-ray excitations (both Li 2 O 2 and Li 2 CO 3 decompose to Li 2 O) 14, 19 . Therefore, despite the high detection efficiency that allows us to collect a full-range mRIXS map in only about 30 minutes, we employed extensive practices on sample transfer 25–26 , liquid N2 cooling, and sample scanning to reduce the radiation effects (see Supplementary Information).…”

Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox

“…Evidence of a new feature appearing at 531.5 eV can also be seen on charging which is consistent with the changes we have seen previously for anion redox materials. On discharge the area of the O K-edge returns to close to that of the pristine material (with the evidence of some carbonate formation on discharge (534 eV) 39 ). This conclusion is further reinforced by the Resonant Inelastic X-Ray Fig 5b, and this indicates that these states are resonant in a narrow energy range (< 1 eV) and hence correspond to newly formed, localised electronic states on oxygen consistent with our previous findings for O-redox materials.…”

Occupied Electronic States of Li in Li, Li2O2, and Li2O Analyzed by Soft X-ray Emission Spectroscopy

“…Léon et al investigated the valence electron structures of Li 2 O and Li 2 O 2 at the O K-edge using soft X-ray emission spectroscopy (SXES) at a synchrotron facility; the resulting spectra describe the valence electron structure of O. 15 We note that Li valence electron structures must be determined experimentally for understanding nature of chemical bonds and identifying oxide phases with Li. High resolution K-edge spectral data can be exploited to visualize chemical state mapping on a microscopic scale, as demonstrated in our previous studies for Be (Z = 4) and B (Z = 5) compounds.…”