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Valence Electronic Structure of Li2O2, Li2O, Li2CO3, and LiOH Probed by Soft X-Ray Emission Spectroscopy Aline Leon, Andy Fiedler, Monika Blum, Andreas Benkert, Frank Meyer, Wanli Yang, Marcus Bär, Frieder Scheiba, Helmut Ehrenberg, Lothar Weinhardt, and Clemens Heske J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11119 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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The Journal of Physical Chemistry

Valence Electronic Structure of Li2O2, Li2O, Li2CO3, and LiOH Probed by Soft X-ray Emission Spectroscopy Aline Léon*1, Andy Fiedler2, Monika Blum3, Andreas Benkert1,4, Frank Meyer4, Wanli Yang5, Marcus Bär6,7, Frieder Scheiba2, Helmut Ehrenberg2, Lothar Weinhardt1,3,8, and Clemens Heske1,3,8 1) Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT),

Hermann-von-Helmholtz-Platz

1, 76344

Eggenstein-Leopoldshafen,

Germany 2) Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermannvon-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 3) Department of Chemistry and Biochemistry, University of Nevada, Las Vegas (UNLV), NV 89154-4003, USA

4) Universität Würzburg, Experimentelle Physik VII, Am Hubland, 97074 Würzburg, Germany

5) Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

ACS Paragon Plus Environment

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6) Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB), Hahn-Meitner-Platz 1, 14109 Berlin, Germany 7) Institut für Physik und Chemie, Brandenburgische Technische Universität CottbusSenftenberg, Platz der Deutschen Einheit 1, 03046 Cottbus, Germany 8) Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Engesserstr. 18/20, 76128 Karlsruhe, Germany

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 CO32-, the p-like states of the oxide anion O2-, the p-like states of the peroxide anion O22-, 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.


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1. INTRODUCTION Among several energy storage technologies presently explored, lithium-oxygen (Li-O2) batteries are proposed as an attractive electrochemical energy storage option. With a practical specific energy density of up to 500 Wh/kg,1 the lithium-oxygen battery has the potential to go beyond lithium-ion based systems, which have a specific energy density of 387 Wh/kg.1 However, the application of Li-O2 batteries remains challenging due to a complex interplay between the individual components of the Li-O2 battery cell and the lack of fundamental understanding of the reaction mechanism. The four main challenges2,3 faced by the Li-O2 battery system are (1) a high charge overvoltage leading to low electrical energy efficiency, (2) low current densities that limit the deliverable power, (3) electrolyte and carbon decomposition due to the highly reactive oxygen radical anion, and (4) the lithium reactivity, leading to the formation of dendrites and oxygen crossover. To enable commercial applications, it has thus been pointed out4 that an optimization of each battery component is necessary, which requires a deeper understanding of the chemistry and electronic states involved. The Li-O2 battery is an open cell structure (oxygen is continuously fed into the cell), which generates electricity through a redox reaction between Li and oxygen.1,2,3 During the discharge, the Li metal anode is oxidized, releasing Li+ into the electrolyte and electrons to the external circuit. At the same time, oxygen enters the porous cathode, accepts the electrons from the anode, and is reduced. The dissociated Li ions and oxygen-reduced species migrate across the electrolyte and combine to form lithium oxides. During the charge cycle, the process is reversed, with oxygen evolving at the cathode and Li plating out at the anode. This redox reaction has mostly been studied by applying complementary methods and combining ex-situ/in-situ measurements [x-ray diffraction (XRD),5,6,7,8,9 x-ray photoelectron spectroscopy (XPS),9,10,11


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Raman spectroscopy,6,12,13 hard x-ray non-resonant inelastic scattering (NIXS),14,15 and soft x-ray absorption spectroscopy (XAS)16,17,18] to gain knowledge of the reaction mechanism, the reaction products and by-products due to impurities, and the electrolyte decomposition. XAS has been used to probe and clearly distinguish the unoccupied states of Li2O2, Li2O, and Li2CO3 at the oxygen and lithium K-edge.16 A beam-induced damage study on XAS spectra of these compounds at the Li K-edge16 indicates that Li2CO3 and Li2O2 evolve towards Li2O under soft xray irradiation, while Li2O is the most stable compound. Furthermore, using C and O K-edge XAS, Gallant et al. 17,18 report on chemical and morphological changes of the oxygen electrode (freestanding vertically aligned carbon nanotubes) during cycling in DME (ether solvent; dimethoxyethane). It is shown that crystalline Li2CO3, located at the surface of the carbon nanotubes, most probably results from chemical reactions between carbon and Li2O2 or other discharge intermediates.18 To further improve the Li-ion battery performance,19,20 it is of interest to gain knowledge of the electronic structure of the different compounds taking part in the reaction. This has recently led to efforts to perform (experimentally challenging21) operando measurements using either XAS,20,22,23 a combination of XAS and soft x-ray emission spectroscopy (XES),24 or a combination of soft and hard XAS.25 For Li-O2 batteries, we note that an experimentally derived valence electronic structure of the four major compounds involved, namely Li2O2, Li2O, Li2CO3, and LiOH, is still lacking. Here, we thus use XES to probe the occupied electronic states of these compounds as a first step towards operando experiments. XES26,

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probes the local partial density of states, is an element-specific method, and, as a

photon-in and photon-out technique, generally probes the bulk features of a sample. First, it will be shown that high-brilliance synchrotron radiation causes beam-induced damage of these compounds, and that it is required to derive an approach to minimize this


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damage (e.g., by scanning the sample under the synchrotron beam during data acquisition). Then, we report the (undamaged) local chemical valence structure around the oxygen atoms and compare the experimental results to published partial density of states of oxygen to assign the different emission lines. It will be highlighted that the four compounds have clear distinct spectral features in XES, allowing a spectral interpretation of mixed-species situations.

2. EXPERIMENTAL METHODS Li2O2, Li2O, Li2CO3, and LiOH powders were used as received from Alfa Aesar (purity of 95, 99.998, 99.5, and 98%, respectively). The samples were prepared in an argon glove box (water and oxygen concentrations below 1 ppm) by pressing the powders onto carbon tape mounted on a sample holder. The sample holder was then inserted into a transportable vacuum system and sealed under inert atmosphere in the glove box. Thereafter, the transportable vacuum system was connected to the load lock chamber of the UHV end-station. The load lock and the transportable vacuum system were evacuated before transferring the sample holder to the analysis chamber, thus avoiding any air-exposure of these air-sensitive samples. The pressure during XES acquisition was 5×10-9 mbar or lower.

The experiments were performed at the undulator beamline 8.0.1.2 of the Advanced Light Source, Lawrence Berkeley National Laboratory, using the SALSA endstation28 equipped with a high-transmission soft X-ray spectrometer.29 A fast shutter allows the sample to be exposed to x-rays only for a well-defined irradiation time. The estimated spot size is 150 (h) × 30 (v) µm2. For O K edge XES, we used an excitation energy of 550 eV and recorded two series of


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spectra with the sample fixed in position. In the first series, 300 spectra were recorded with one second beam exposure per spectrum for all four compounds. In the second series, 30 spectra were recorded with 10 seconds beam exposure per spectrum for Li2CO3 and LiOH. Furthermore, for each sample, a spectrum was recorded while continuously scanning the sample with respect to the synchrotron beam (50 µm/s across an area of 1.2 × 1.2 mm2 of the sample, corresponding to an exposure time of approx. 0.6 seconds per spot). Throughout this paper, spectra taken in scanning mode are labeled “scanned”, while all other spectra are labeled by their total exposure time. The pressure during the experiments was 2000 seconds) exposure as well as the spectra taken after 90 s for Li2CO3, 50 s for Li2O, 50 s for Li2O2, and 20s for LiOH seconds. These spectra correspond to the exposure times where the first changes become clearly visible in the XES spectra. The spectral variation over time, shown at the bottom of Figs. 2 – 5 a), is reflected in the difference spectra between the last and the first spectrum of the same series, and between the first spectrum where a change is clearly seen and the first spectrum. The non-resonant 1 s XES spectrum of lithium carbonate, Figure 2, is defined by four main peaks positioned at 519.9, 521.8, 525.5 and 526.8 eV, while the spectrum of lithium oxide, Figure 3, is characterized by two main peaks at 526.5 and 528.9 eV. For lithium peroxide, Figure 4, we denote four main emission lines positioned at 524.3, 525.5, 526.4 and 528.7 eV, and for lithium hydroxide, Figure 5, we find three main peaks, at 521.9, 525.2, and 526.2 eV. It is evident that, in the course of experimental time, the four compounds are subject to gradual radiation damage effects. After 300 seconds, the most prominent change in lithium carbonate is a decrease in intensity of the main peak at 526.8 eV and a broadening of the peak at 525.5 eV, while in lithium oxide, the high-energy shoulder of the main peak and (less pronounced) the peak


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at 528.9 eV decrease in intensity. In lithium peroxide, we observe a decrease in intensity of the peaks at 524.3, 526.4, and 528.7 eV, and an increase of the spectral feature at 525.5 eV. In lithium hydroxide, a decrease of the peak at 521.9 eV, an increase of the peak at 525.2 eV, and a decrease of the main peak at 526.2 eV are found. While the changes after 300 seconds of irradiation are moderate for Li2CO3 and Li2O, they are more pronounced for Li2O2 and LiOH. In the course of time, a redistribution of spectral weight occurs in these four compounds, suggesting a modification of the local chemical environment. After prolonged (> 2000 seconds) exposure, we observe that the emission lines of Li2CO3 are rather stable, with only a small decrease of the main line and an increase of its low-energy shoulder (at 525.5 eV). For Li2O, we note that the spectrum after 2270 seconds becomes similar to the one of LiOH. Further, we observed in additional experiments (not shown) that the handling of Li2O needs special care as it can decompose to LiOH when exposed to air before the measurements. The spectrum of Li2O2 shows a drastic decrease of the intensity of the peak at 528.7 eV (i.e., a reduction in spectral weight of the upper valence states) and a removal of the weak feature at 526.4 eV, but no shifts. For LiOH, we note that after 2610 seconds the intensity of the shoulder at 525.2 eV significantly increases at the expense of the intensity of the peak at 526.2 eV. Not unexpectedly, we thus derive that the four compounds investigated in this study are unstable in the soft x-ray synchrotron beam, making it necessary to scan the beam across the sample during data acquisition to record reliable (i.e., damage-free) XES spectra. Several origins for beam damage in lithium compounds caused by soft x-rays are discussed in Ref.16. As a result of this beam-damage study and the associated optimization of scan parameters, we can now plot representative XES spectra with minimized beam damage effects (if any). These spectra are


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displayed at the top of Figs. 2 – 5 a) and b) and in comparative fashion in Figure 6. As can be seen, the scanned XES spectra of each of these compounds are even better defined than the 1 second spectra due to the even shorter exposure times (0.6 instead of 1 second per sample spot) and higher signal-to-noise ratio, and will be analyzed in the following by comparing with PDOS calculations. 3.3. Assignments of the emission lines using PDOS The oxygen partial density of states (PDOS) of Li2CO3, Li2O, Li2O2, and LiOH is displayed at the bottom of Figures 2 –5 b), respectively. Note that the energy scales of the calculations were shifted to correspond to the experimental scale. For lithium carbonate and lithium oxide, the contributions with p-type symmetry were taken from the theoretical data published by Duan et al.31 (in the case of lithium carbonate separated into contributions from the two types of oxygen atoms [O(I) and O(II)], as described in Section 3.1). For lithium peroxide, we took the p-type PDOS of oxygen constructed from the G0W0-corrected PBE (Perdew-Burke-Ernzerhof) calculations by Garcia-Lastra et al.32. For lithium hydroxide, the calculated data were extracted from the paper by Hermann et al.33 (in this case containing contributions from both s and p symmetry). In order to compare the non-resonant experimental XES spectra with the PDOS, the calculated contribution of different bands were added and convoluted with Gaussian functions with a full width at half maximum (FWHM) of 0.5 eV. The resulting spectra are shown in the center of Figures 2–5 b). Combining theoretical calculations31 with photoelectron spectroscopy,39 Duan et al. concluded that, in lithium carbonate, the first valence band (0 to -2.5 eV relative to the VBM) is mainly formed from the p orbitals of O and the s and p orbitals of Li. The second valence band


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(-4.5 to -8 eV relative to the VBM) is formed from the s and p orbitals of Li, C, and O. The third and fourth valence bands (below -18 eV) are predominantly derived from the s and p orbitals of O and C atoms, with a very small contribution from Li. The convoluted PDOS of lithium carbonate, Figure 2 b), is characterized by five peaks positioned at -7.5, -5.8, -4.7, -1.6, and -0.3 eV relative to the valence band maximum (VBM). By comparing the experimental XES spectrum and the calculated PDOS, one can assign the peaks at 519.9 eV and 521.8 eV to the second valence band, while the peaks at 525.5 eV and 526.8 eV are related to the first valence band. Photoelectron spectroscopy, performed on single-crystal Li2O 40 and compared to density of states obtained from Hartree-Fock calculations,41 indicates that the valence band of Li2O is mainly derived from O 2p-like orbitals with a very small contribution from the s and p orbitals of lithium. The convoluted PDOS, Figure 3b), exhibits two peaks positioned at -2.1 and -0.8 eV relative to the VBM. A comparison of the experimental and calculated data allows assigning the emission line at 526.5 eV to bands derived from the 2p-like states of oxygen. However, the convoluted PDOS does not describe the emission line observed at 528.9 eV. The electronic structure of Li2O2 is dominated by the electronic states of an isolated O22- ion, with almost no contribution from the Li states.32 Based on density functional theory with local density approximation, Wu et al. derived that the lower valence band (below -15 eV) is composed of sharp peaks owing to the contributions of O s- and Li s-states, whereas the upper valence band is composed of bands related to O p- and Li s-states.42 Further, it is shown that the Li contribution is much smaller than the O contribution by a factor of 50 or 70.42 The convoluted PDOS, Figure 4b), is characterized by six peaks positioned at -6.2, -5.6, -5.1, -3.9, -1.7, and -0.6 eV relative to the VBM, which are related to bands derived from the σg (pz), the πu (px, py) and the π*g (px, py)


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orbitals. By comparing the convoluted PDOS and the experimental XES spectrum, we can assign the experimental peaks at 528.7, 525.5, and 524.3 eV to bands derived from π*g, πu, and (σg, πu) orbitals,32 respectively. Note that the (weak) emission line observed in the experimental spectrum at 526.4 eV (-2.9 below VBM in the calculations) and the spectral weight below 522.5 eV (below -7.0 eV in the calculations) are not represented in the convoluted PDOS. We believe that this additional spectral weight is due to an impurity contribution of LiOH. Also, an additional contribution of Li2O cannot be ruled out. The calculated density of states of lithium hydroxide shows that the valence states are dominated by the hydroxide group,33 and that the O-H bond has a strong π-like character.38 The convoluted PDOS, Figure 5b), exhibits two features at -4.6 and -0.9 eV relative to the VBM. One can thus assign the observed emission lines to the orbitals of the hydroxide group. Comparing our experiments with calculated PDOS, we thus derive that the upper valence bands of Li2CO3, Li2O, Li2O2, and LiOH are defined by characteristic emission lines. These emission lines are primarily related to the s and p-like states of the carbonate anion CO32-, the plike states of the oxide anion O2-, the p-like states of the peroxide anion O22-, and the π-like character of the OH- group, respectively.

CONCLUSION

Soft x-ray emission spectroscopy (XES) has been used to study the valence electronic structure of Li2CO3, Li2O, Li2O2, and LiOH. Under high-brilliance soft x-rays, the four compounds degrade, making it necessary to scan the beam across the sample during data


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acquisition to record reliable and representative XES spectra with minimized beam damage effects (if any). Each of the compounds features a specific spectral shape, assigned to the electronic bands of the compound, which allows their differentiation using XES. The availability of such clearly distinguishable spectral shapes will form the basis for studying the chemical processes involving such Li-based compounds in an operando fashion using XES.

AUTHOR INFORMATION Corresponding Author * Aline LEON, Karlsruhe Institute of Technology, Institute for Photon Science and Synchrotron Radiation, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany, Tel: 49 (0) 721 608 23834, Email: [email protected] ACKNOWLEDGMENT We express our gratitude to Y. Duan (National Energy Technology Laboratory, USA) et al. and A. Hermann (The University of Edinburgh, UK) et al. for making their calculated data available. A. Léon gratefully acknowledges O. Fuhr for the crystal structure drawings, as well as M. Haeming and A. Zimina for fruitful discussions. F. Scheiba and A. Fiedler kindly acknowledge the financial support of the German Research Foundation (DFG) in the project SCHE 1714/1-1. M. Bär thanks the Impuls- und Vernetzungsfonds of the Helmholtz Association for funding (VHNG-423). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231.


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24. Asakura, D.; Hosono, E.; Niwa, H.; Kiuchi, H.; Miyawaki, J.; Nanba, Y.; Okubo, M.; Matsuda, H.; Zhou, H.; Oshima, M et al. Operando Soft X-Ray Emission Spectroscopy of LiMn2O4 Thin Film Involving Li-Ion Extraction/Insertion Reaction. Electrochem. Comm. 2015, 50, 93-96. 25. Oishi, M; Fujimoto, T; Takanashi, Y.; Orikasa, Y.; Kawamura, A.; Ina, T.; Yamashige, H., Takamatsu, D.; Sato, K.; Murayama, H. et al. Charge Compensation Mechanisms in Li1.16 Ni0.15Co0.19Mn0.50O2 Positive Electrode Material for Li-Ion Batteries Analyzed by a Combination of Hard and Soft X-Ray Absorption Near Edge Structure. J. Pow. Sourc. 2013, 222, 45-51. 26. Eisebitt, S.; Eberhardt, W. Band Structure Information and Resonant Inelastic Soft X-Ray Scattering in Broad Band Solids. Journal of electron spectroscopy and related phenomena 2000, 110 – 111, 335 – 338. 27. De Groot, F. High-Resolution X-Ray Emission and X-Ray Absorption Spectroscopy. Chem. Rev. 2001, 101 (6), 1779 – 1808. 28. Blum, M.; Weinhardt, L.; Fuchs, O.; Bär, M.; Zhang, Y.; Weigand, M.; Krause, S.; Pookpanratana, S.; Hofmann, T.; Yang, W. et al. Solid and liquid Spectroscopic Analysis (SALSA) – a Soft X-Ray Spectroscopy Endstation with a Novel Flow-Through Liquid Cell. Rev. Sci. Instrum. 2009, 80, 123102, 1-6. 29. Fuchs, O.; Weinhardt, L.; Blum, M.; Weigand, M.; Umbach, E.; Bär, M.; Heske, C.; Denlinger, J.D.; Chuang, Y.D.; McKinney, W. et al. High-Resolution, High-Transmission Soft X-Ray Spectrometer for the Study of Biological Samples. Rev. Sci. Instrum. 2009, 80, 063103, 1-7.


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30. Lusvardi, V.S.; Barteau, M.A.; Chen, J.G.; Eng, Jr., J.; Frühberger, B.; Teplyakov, A. An NEXAFS Investigation of the Reduction and Reoxidation of TiO2(001). Surface Science 1998, 397, 237-250. 31. Duan, Y.; Sorescu, Dan C. Density Functional Theory Studies of the Structural, Electronic, and Phonon Properties of Li2O and Li2CO3: Application to CO2 Capture. Phys. Rev B 2009, 79, 014301, 1-18. 32. Garcia-Lastra, J.M.; Bass, J.D.; Thygesen, K.S. Strong Excitonic and Vibronic Effects Determine the Optical Properties of Li2O2. J. Chem. Phys. 2011, 135, 121101, 1-4. 33. Hermann, A.; Ashcroft, N.W.; Hoffmann, R. Lithium Hydroxide, LiOH, at Elevated Densities. J. Chem. Phys. 2014, 141, 024505, 1-11. 34. Effenberg, H.; Zemann, J. Refining the Crystal Structure of Lithium-Carbonate, Li2CO3. Z. Kristallogr. 1979, 150 (1-4), 133-138. 35. Farley, T.W.D.; Hayes, W.; Hull, S.; Hutchings, M.T.; Vrtis, M. Investigation of Thermally Induced Li+ Ion Disorder in Li2O Using Neutron Diffraction. J. Phys.: Condens. Matter 1991, 3, 4761-4781. 36. Föppl, H. Die Kristallstrukturen der Alkaliperoxide. Z. Anorg. Allg. Chem. 1957, 291, 12– 50. 37. Cota, L.G; de la Mora, P. On the Structure of Lithium Peroxide, Li2O2. Acta Cryst. 2005, B61, 133-136. 38. Mair, S.L. The Electron-Distribution of Hydroxide Ion in Lithium Hydroxide. Acta Crystallogr. Sect. A, 1978, 34, 542-547.


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39. Connor, J.A.; Hillier, I.H.; Saunders, V.R.; Barber, M. On the Bonding of the Ions PO43-, SO42-, ClO4-, ClO3- and CO32- as Studied by X-Ray Spectroscopy and Ab Initio SCF-MO Calculations. Mol. Phys. 1972, 23, 81-90. 40. Liu, L.; Henrich, Victor E.; Ellis, W.P.; Shindo, I. Bulk and Surface Electronic Structure of Li2O. Phys. Rev. B 1996, 54, 3, 2236-2239. 41. Lichanot, A.; Gelize, M.; Larrieu, C.; Pissani, C. Hartree-Fock Ab Initio Study of Relaxation and Electronic Structure of Lithium Oxide Slabs. J. Phys. Chem. Solids 1991, 52, 11551164. 42. Wu, H.; Zhang, H.; Cheng, X.; Cai, L. Ab Initio Calculations of Structural, Elastic and Electronic Properties of Li2O2. Philosophical Magazine 2007, 87, 23, 3373-3383.


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FIGURE CAPTIONS Figure 1. Crystal structure of (a) lithium carbonate, Li2CO3, (b) lithium oxide, Li2O, (c) lithium peroxide, Li2O2, and (d) lithium hydroxide, LiOH. Figure 2. (a) Non-resonant oxygen K XES spectra of Li2CO3 after 1, 10, 90, 100, 300, and 2500 seconds of X-ray exposure. Two difference spectra are also shown, magnified by a factor of 3: between the last and first spectrum of one series (300 – 10 s) and between the first visibly modified and the first spectrum of a second series (90 – 1 s). The scanned XES spectrum of lithium carbonate is shown at the top. (b) Calculated partial density of states of oxygen from Duan et al.31 after convoluting with a Gaussian function. The “scanned” XES spectrum of lithium carbonate is shown at the top. Figure 3. (a) Non-resonant O K XES spectra of Li2O after 1, 50, 100, 300, and 2270 seconds of X-ray exposure. As described in the caption of Figure 2, two difference spectra (300 – 1 s and 50 – 1 s) are also represented (magnified by a factor of 3). At the top, the scanned spectrum is displayed. (b) Calculated partial density of states of oxygen31 and the Gaussian-convoluted spectrum. For comparison, the “scanned” spectrum is displayed at the top. Figure 4. (a) Non-resonant O K XES spectra of Li2O2 (after 1, 50, 100, 300, and 2880 seconds), difference spectra (300 – 1s and 50 – 1 s, magnified by 2), and spectrum recorded in scanning mode. (b) Calculated partial density of states of oxygen extracted from Garcia-Lastra et al.32 and the convoluted PDOS with Gaussian functions. The experimental “scanned” spectrum is displayed at the top. Figure 5. (a) Non-resonant O K XES spectra of LiOH (after 1, 10, 20, 100, 300, and 2610 seconds), difference spectra (300 – 10 s and 20 – 10 s, magnified by 2) and spectrum recorded in


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scanning mode. (b) Calculated (Hermann et al.33) and Gauss-convoluted partial density of states of oxygen. At the top, the “scanned” spectrum is shown. Figure 6. Non-resonant O XES spectra of Li2CO3, Li2O, Li2O2, and LiOH taken in scanning mode.


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(a) Li2CO3

(b) Li2O

(c) Li2O2

(d) LiOH

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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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TOC Graphic


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TOC graphic revised 254x190mm (96 x 96 DPI)



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Crystal structure of (a) lithium carbonate, Li2CO3, (b) lithium oxide, Li2O, (c) lithium peroxide, Li2O2, and (d) lithium hydroxide, LiOH. 1323x991mm (96 x 96 DPI)


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Figure 1 b) 1323x991mm (96 x 96 DPI)



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Figure1c 1323x991mm (96 x 96 DPI)


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Figure1d 1323x991mm (96 x 96 DPI)



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Figure 2. (a) Non-resonant oxygen K XES spectra of Li2CO3 after 1, 10, 90, 100, 300, and 2500 seconds of X-ray exposure. Two difference spectra are also shown, magnified by a factor of 3: between the last and first spectrum of one series (300 – 10 s) and between the first visibly modified and the first spectrum of a second series (90 – 1 s). The scanned XES spectrum of lithium carbonate is shown at the top. (b) Calculated partial density of states of oxygen from Duan et al.31 after convoluting with a Gaussian function. The “scanned” XES spectrum of lithium carbonate is shown at the top. 104x180mm (300 x 300 DPI)


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Figure 3. (a) Non-resonant O K XES spectra of Li2O after 1, 50, 100, 300, and 2270 seconds of X-ray exposure. As described in the caption of Figure 2, two difference spectra (300 – 1 s and 50 – 1 s) are also represented (magnified by a factor of 3). At the top, the scanned spectrum is displayed. (b) Calculated partial density of states of oxygen31 and the Gaussian-convoluted spectrum. For comparison, the “scanned” spectrum is displayed at the top. 104x160mm (300 x 300 DPI)



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Figure 4. (a) Non-resonant O K XES spectra of Li2O2 (after 1, 50, 100, 300, and 2880 seconds), difference spectra (300 – 1s and 50 – 1 s, magnified by 2), and spectrum recorded in scanning mode. (b) Calculated partial density of states of oxygen extracted from Garcia-Lastra et al.32 and the convoluted PDOS with Gaussian functions. The experimental “scanned” spectrum is displayed at the top. 104x160mm (300 x 300 DPI)


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Figure 5. (a) Non-resonant O K XES spectra of LiOH (after 1, 10, 20, 100, 300, and 2610 seconds), difference spectra (300 – 10 s and 20 – 10 s, magnified by 2) and spectrum recorded in scanning mode. (b) Calculated (Hermann et al.33) and Gauss-convoluted partial density of states of oxygen. At the top, the “scanned” spectrum is shown. 104x160mm (300 x 300 DPI)



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Figure 6. Non-resonant O XES spectra of Li2CO3, Li2O, Li2O2, and LiOH taken in scanning mode. 203x279mm (300 x 300 DPI)


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Table of content Image 254x190mm (96 x 96 DPI)


Valence Electronic Structure of Li - American Chemical Society

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