LETTER pubs.acs.org/JPCL
Structure of Lithium Peroxide Maria K. Y. Chan,† Eric L. Shirley,|| Naba K. Karan,‡ Mahalingam Balasubramanian,*,‡ Yang Ren,‡ Jeffrey P. Greeley,† and Tim T. Fister*,§ Center for Nanoscale Materials, ‡Advanced Photon Source, and §Chemical Sciences and Engineering, Argonne National Laboratory, Argonne, Illinois 60439, United States Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
)
†
ABSTRACT: The reliable identification of lithium oxide species, especially lithium peroxide (Li2O2), is of vital importance to the study of Li-air batteries. Previous X-ray diffraction studies of Li2O2 resulted in the proposal of two disparate structures by Feher and F€oppl. In this Letter, we assess these competing Li2O2 structures using a combination of the following X-ray and first-principles techniques: (i) high-energy X-ray diffraction (XRD), (ii) comparisons of the measured nonresonant inelastic X-ray scattering (NIXS) spectra with those computed from first principles using the Bethe-Salpeter equation (BSE), and (iii) comparison of thermochemistry data with the formation enthalpies obtained from density functional theory (DFT) calculations using a hybrid functional. All three approaches result in the identification of F€ oppl’s proposal as the more appropriate structure for Li2O2. The measured and computed spectra and data presented in this Letter are useful as benchmarks for future characterization of Li2O2. SECTION: Molecular Structure, Quantum Chemistry, General Theory
F
or competition with the extremely high energy density of fossil fuels, electrochemical cells based on lithium-oxygen (or “lithium-air”) reactions are often touted as the technological heir to lithium ion batteries. In Li-air cells, discharge reactions producing lithium oxide (Li2O), lithium peroxide (Li2O2), and possibly lithium superoxide (LiO2) occur near the surface of the cathode. These discharge products are difficult to characterize because the peroxide and superoxide phases are metastable under ambient conditions. Many traditional spectroscopies for lithium and oxygen, such as X-ray photoelectron spectroscopy, X-ray absorption, and electron energy loss, are hampered by the low core binding energies of lithium and oxygen and typically require ex situ vacuum conditions. X-ray diffraction (XRD) may at times be difficult because of the small scattering cross section of such low-Z compounds and limited crystallinity of discharge products such as Li2O2, Li2O, and possibly LiO2. In the 1950s, two disparate crystal structures were proposed for Li2O21,2 from XRD studies. They have surprisingly different lithium sublattices, as shown in Figure 1. Along the c-lattice direction, Feher’s original structure consists of lithium and oxygen atoms nominally sharing each plane, whereas F€oppl’s revised structure positions the lithium sites between adjacent oxygen planes. Whereas the two structures have similar nearest-neighbor Li O distances (1.91 Å in Feher’s vs 1.98 Å in F€oppl’s), the O O distances in the peroxide anions are drastically different (1.28 and 1.55 Å, respectively). Moreover, whereas there is only one type of Li site in the Feher structure, the F€oppl structure contains two inequivalent Li sites with different nearest-neighbor Li O distances of 1.98 and 2.15 Å. r 2011 American Chemical Society
To distinguish between the two proposed Li2O2 structures, Cota et al.3 symmetrized both structures and used density functional theory (DFT) calculations to determine their relative stabilities and structural changes after relaxations. They found the F€oppl structure to be 0.53 eV per O2 lower in energy than the Feher structure, and they also determined that the latter structure undergoes drastic structural relaxations in DFT, including the lengthening of O O bonds from 1.28 to 1.53 Å. Cota’s DFTrelaxed structure is also shown in Figure 1, showing the lengthening of O O bonds. Cota’s work provided strong evidence of F€oppl’s as the actual structure of Li2O2, and subsequent DFT studies4,5 used the F€oppl structure. We note, however, that the exchange-correlation functional used by Cota et al., the generalized gradient approximation (GGA) of Perdew, Burke, and Erzenhof (PBE),6 is known to have large errors in treating the oxygen molecule7 and oxides.8 Therefore, direct experimental evidence and more accurate first-principles methods are needed to determine the structure more conclusively. In this Letter, we seek to elucidate the structure of Li2O2 using a combination of X-ray and first-principles techniques. Using high-energy X-rays, we obtain the powder XRD pattern and compare the accuracy of the Feher and F€oppl structures using Rietveld refinement. Taking advantage of the coordination sensitivity of nonresonant inelastic X-ray scattering (NIXS), we also measure the lithium and oxygen K-edges of Li2O2. These NIXS Received: August 8, 2011 Accepted: September 12, 2011 Published: September 12, 2011 2483
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spectra are compared with those calculated for the two proposed structures using a Bethe-Salpeter (BSE) treatment, which accurately accounts for electron core hole interactions. Finally, we compare thermochemical data to accurate formation energies calculated from DFT using a hybrid functional. All three approaches result in the identification of the F€oppl structure as the structure of Li2O2. Apart from ascertaining its structure, we believe that the various measured and computed spectra and data of Li2O2 presented here will be useful for future characterization efforts in Li-air battery research. We used a commercial powder Li2O2 sample and took steps to minimize exposure to air. XRD was performed at the Advanced Photon Source (APS) using 114.82 keV X-rays (λ = 0.10798 Å). Rietveld refinement was performed on the powder XRD pattern of Li2O2 using the two proposed structures for Li2O2. On the
Figure 1. Feher’s1 (left), DFT-relaxed Feher’s according to Cota3 (middle), and F€oppl’s2 (right) proposed crystal structures for Li2O2. Red (larger) spheres represent oxygen atoms and blue (smaller) spheres represent lithium atoms. Blue (dashed) horizontal lines indicate Li planes. We can see that Li atoms are roughly in-plane with the O atoms in the Feher structure while Li planes cut through the O O bonds in the F€oppl structure.
Figure 2. Measured powder XRD pattern of Li2O2 (black, lower), and the Rietveld refinement according to the F€oppl (blue, middle) and Feher (red, upper) structures. The residuals (differences between the Rietveld refinement and observed intensities), magnified by a factor of 3, are shown beneath each structure.
basis of the Rietveld refinement, the sample contained ∼12.9% (by weight) impurity phases (mainly LiOH and Li2CO3). Figure 2 summarizes the Rietveld refinement with the experimental data for these two structural models for Li2O2. The space-group (P63/ mmc in both structures) and Wyckoff sites for Li and O in the symmetrized structures obtained from the original Feher and F€oppl structures as reported by Cota et al.3 were used as input for the Rietveld refinements. The cell parameters a and c and variable internal parameters α(Li) (Feher’s structure) and α(O) (both structures) are allowed to vary. Because the DFT-relaxed Feher structure differs from the unrelaxed structure only in a, c, and α, the refinement procedure does not discriminate between the unrelaxed and relaxed Feher structures. The refined structural parameters and the goodness-of-fit parameters are given in Table 1. Comparing the residuals and the goodness-of-fit parameters from Figure 2 and Table 1 for these two structures, we see that the F€oppl structure is a much better fit than the Feher structure. The best fit for the Feher structure shows an O O bond length (1.25 Å) closer to that in the unrelaxed (1.28 Å) than in the DFTrelaxed (1.53 Å) structure of ref 3. The residuals are also particularly pronounced for the Feher structure compared with the F€oppl structure at larger 2θ (2θ > 4°) values. We note that the typical range of 2θ ( 6°), there are also multiple peaks in the measured spectra that are found in the refinement for the F€oppl structure but not in that for the Feher structure. Therefore, whereas it may still be possible to confirm the F€oppl structure by diffraction using conventional lab X-rays, the use of high-energy synchrotron X-ray source is particularly effective in discerning between these two structures. We note that XRD patterns become broadened in samples with reduced crystallinity, such as the case when Li2O2 is processed by ball-milling,9 or when it forms as a discharge product in Li-air batteries.10 Because NIXS is sensitive to short-range order often present in the complicated (i.e., high surface area) morphology in these situations, NIXS should provide a valuable foundation for in situ characterization of Li2O2 in operating batteries. The NIXS spectra of the Li2O2 pellet were measured from the lower energy resolution inelastic X-ray scattering (LERIX) instrument at the APS, simultaneously at momentum transfers q of 0.6 8 Å 1. The measured Li and O K-edges are shown in Figure 3, a and b, respectively. As seen in Figure 3a, the lithium K-edge spectrum evolves substantially with q. These changes arise when 1/q is comparable to the size of the Li 1s orbital, and the matrix element can no longer be approximated by dipole s f p transitions. In contrast, the O K-edge is limited to dipole transitions but is also quite sensitive to changes in its lithium coordination. Theoretical NIXS spectra are obtained by solving the BSE and are also shown in Figure 3. We find that the calculated NIXS spectra are substantially different for the two crystal structures
Table 1. Comparison of Structural Parametersa (a and c lattice parameters, internal variable parameters α(Li) and α(O), and O O bond length), Residuals (unweighted Rp, weighted Rwp), and Goodness-of-fit Parameters (χr2) of Li2O2 Rietveld Refinement Using Two Proposed Structures for Li2O2a
a
R(O O)/Å
α(Li)b
α(O)b
structure
a (Å)
c (Å)
Feher
3.1700(2)
7.7174(5)
1.249(3)
0.156(1)
0.0809(2)
F€ oppl
3.16919(3)
7.71401(8)
1.5638(8)
N/A
0.10136(5)
Rp
Rwp
χr2
0.0463
0.0942
32.28
0.0134
0.0180
1.18
Uncertainties indicated correspond to one standard deviation. b Variable parameters for the 4f Wyckoff sites, as defined in ref 3 tables 3 and 5. 2484
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Figure 3. Measured Li (a) and O (b) K-edges of Li2O2 (top, circles) compared with the spectra computed from the Feher and F€oppl structures with BSE (bottom, solid lines). For the Li K-edge, the momentum (q) dependence is also shown.
Table 2. Formation Enthalpy (ΔH) of Li 2 O 2 Calculated (B) for the Two Structures Using GGA-PBE and HSE06 Compared with Experimental Values (A) (A)
ΔH (eV/O2)
experimental
6.57(9),a 6.557b ΔH (eV/O2)
(B) Feher
F€oppl
PBE
5.69
6.24
HSE06
5.90
6.52
a
Ref 16. b NIST Chemistry WebBook, http://webbook.nist.gov/ chemistry/.
and that the F€oppl structure is in much better agreement with that observed for both the lithium and oxygen edges, and, for Li K-edge, in both the low and high q limits. We note that although the NIXS spectra are computed from crystalline structures, they are applicable for comparison with those measured from samples with reduced crystallinity as well because the NIXS spectra are dependent on local environment instead of long-range order. Bader charge analysis reveals that for the F€oppl structure, the two types of Li atoms have slightly different charges and the oxygen atoms have the same charges, whereas the situation is reversed for the Feher structure. This difference may be responsible for the simple versus complex peaks (F€oppl vs Feher) in the Li spectra for energy loss of 60 to 70 eV, and the reverse in the oxygen spectra for energy loss of 535 to 545 eV. Because the feature at
530 533 eV in the oxygen K-edge is associated with O O bonding, with shorter O O bonds giving rise to the peak at lower energy loss,11 the difference between the F€oppl and Feher structure here likely stems from the difference in O O distances between these two structures. As such, the DFT-relaxed Feher structure by Cota may produce better agreement in the position of this feature. However, the Li O distances are not significantly different between the unrelaxed and relaxed Feher structures, so that the 535 545 eV part of the spectra should not be significantly modified if calculated from the relaxed structure. The sensitivity of the calculated NIXS spectra to local coordination is clearly demonstrated. To supplement the spectroscopic comparisons, we show in Table 2 the formation enthalpies of the two structures as computed using two exchange-correlation functionals: the GGA-PBE7 (as used in ref 3) and the hybrid functional HSE06.12 Whereas the F€oppl structure is lower in energy than the Feher structure in both functionals, the formation enthalpy is consistent with the experimental values only for the F€oppl structure in HSE06. This is a result of the more accurate treatment of the O2 molecule and atomization energies in HSE06 compared with PBE and gives confidence that the total-energy calculations are accurate in HSE06. That this accurate treatment agrees with the NIXSBSE comparison and high-energy XRD allows us to identify conclusively F€oppl’s as the actual structure of Li2O2. To summarize, we have used a multitude of experimental and first-principles techniques to determine that F€oppl’s structure is a better representation than Feher’s of the structure for Li2O2. We demonstrate that the use of high-energy X-ray diffraction or NIXS in conjunction with accurate first-principles methods is a reliable 2485
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The Journal of Physical Chemistry Letters approach to structural determination, with NIXS being particularly useful in situations where the crystallinity is compromised, for example, in situ and nanocrystalline applications.
’ EXPERIMENTAL AND COMPUTATIONAL METHODS For both XRD and NIXS measurements, the samples were prepared by making pellets of commercial Li2O2 powder (Acros Chemicals, 95% pure by weight as per manufacturer’s specification; this information, provided for clarification, does not represent any endorsement or representation of the authors). For XRD, the pellet was enclosed with kapton tape, whereas for NIXS measurements, the pellet was loaded in a hermetically-sealed sample holder. Care was taken to minimize the signal contribution from the aluminized mylar window of the sample holder in both incoming and scattered X-ray paths. The entire process of sample preparation and handling was performed inside a Hefilled glovebox to not expose the sample to air. XRD data were collected at beamline 11-ID-C of the APS (λ = 0.10798 Å) in transmission mode. The NIXS spectra of the Li2O2 pellet were measured from the LERIX instrument at sector 20IDB of the APS. Elastic scattering was at 7.911 keV with a 1.0 eV resolution. Spectra were simultaneously collected at momentum transfers q of 0.6 8 Å 1, and the valence background was subtracted using empirical fits to the Compton scattering. Data were consistent over repeated measurements at two different locations on the sample. Theoretical NIXS spectra were obtained by solving the BSE using the core NIST BSE solver implemented as most recently described by Vinson et al.13 These calculations are based on considering electron core hole excitations from the ground state, including the screened electron core hole interaction, self-energy damping effects, relative core binding-energy shifts for inequivalent atomic sites, and an approximate accounting for vibrational and experimental energy-resolution broadening effects. The BSE results in this work were obtained as part of as a larger group of calculations for several lithium compounds. DFT calculations were performed using the plane wave code VASP,14 with supplied projector augmented wave (PAW)15 potentials for core electrons. We used the hybrid functional HSE0612 to treat exchange-correlation effects as well as GGAPBE7 for comparison. The soft version of the oxygen PAW potential in VASP and kinetic energy cutoffs of 350 eV for the plane wave basis set were used. ’ AUTHOR INFORMATION Corresponding Author
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Sciences, under contract No. DE-AC02-06CH11357. PNC/ XSD (sector 20) facilities at the Advanced Photon Source are additionally supported by a Major Resources Support grant from NSERC, University of Washington, and Simon Fraser University. M.K.Y.C. and J.P.G. gratefully acknowledge use of the Fusion cluster in the Laboratory Computing Resource Center at Argonne National Laboratory.
’ REFERENCES (1) Feher, F.; von Wilucki, I.; Dost, G. Beitr€age zur Kenntnis des € Wasserstoffperoxyds und seiner Derivate, VII. Mitteil.: Uber die Kristallstruktur des Lithiumperoxyds, Li2O2. Chem. Ber. 1953, 86, 1429–1437. (2) F€oppl, H. Die Kristallstrukturen der Alkaliperoxyde. Z. Anorg. Allg. Chem. 1957, 291, 12–50. (3) Cota, L. G.; de la Mora, P. On the Structure of Lithium Peroxide, Li2O2. Acta Crystallogr. 2005, B61, 133–136. (4) Wu, H. Y.; Zhang, H.; Cheng, X. L.; Cai, L. C. The Thermodynamic Properties of Lithium Peroxide, Li2O2. Phys. Lett. A 2006, 360, 352–356. (5) Seriani, N. Ab Initio Thermodynamics of Lithium Oxides: From Bulk Phases to Nanoparticles. Nanotechnol. 2009, 20, 445703(7). (6) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (7) Patton, D. C.; Porezag, D. V.; Pederson, M. R. Simplified Generalized-Gradient Approximation and Anharmonicity: Benchmark Calculations on Molecules. Phys. Rev. B 1997, 55, 7454–7459. (8) Wang, L.; Maxisch, T.; Ceder, G. Oxidation Energies of Transition Metal Oxides within the GGA+U Framework. Phys. Rev. B 2006, 73, 195107(6). (9) Ogasawara, T.; Debart, A.; Holzapfel, M.; Novak, P.; Bruce, P. G. Rechargeable Li2O2 Electrode for Lithium Batteries. J. Am. Chem. Soc. 2006, 128, 1390–1393. (10) Laoire, C. O.; Mukerjee, S; Plichta, E. J.; Hendrickson, M. A.; Abrahama, K. M. Rechargeable Lithium/TEGDME-LiPF6/O2 Battery. J. Electrochem. Soc. 2011, 158, A302–A308. (11) R€uhl, E.; Hitchcock, A. P. Oxygen K-Shell Excitation Spectroscopy of Hydrogen Peroxide. Chem. Phys. 1991, 154, 323–329. (12) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207–8215. (13) Vinson, J.; Rehr, J. J.; Kas, J. J.; Shirley, E. L. Bethe-Salpeter Equation Calculations of Core Excitation Spectra. Phys. Rev. B 2011, 83, 115106(7). (14) Kresse, G.; Furthm€uller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186. (15) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758–1775. (16) Kubaschewski, O.; Alcock, C. B.; Spencer, P. J. Materials Thermochemistry, Pergamom Press: Oxford, U.K., 1993.
*E-mail: fi
[email protected] (T.T.F);
[email protected] (M.B).
’ ACKNOWLEDGMENT Helpful discussion with Brian Toby on the Rietveld refinement of XRD data is gratefully acknowledged. M.K.Y.C., J.P.G., and T.T.F. acknowledge funding from the Center for Electrical Energy Storage: Tailored Interfaces, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES). Work at Argonne National Lab and the Advanced Photon Source is funded DOE-BES under contract DE-AC02-06CH11357. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy 2486
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