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. ff Previous X-ray di raction 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) calcula- tions using a hybrid functional. All three approaches result in the identification € ’ of Foppl s proposal as the more appropriate structure for Li2O2. The measured and computed spectra and data presented in this Letter are useful as bench- marks for future characterization of Li2O2. SECTION: Molecular Structure, Quantum Chemistry, General Theory

or competition with the extremely high energy density of To distinguish between the two proposed Li2O2 structures, Ffossil fuels, electrochemical cells based on lithium-oxygen (or Cota et al.3 symmetrized both structures and used density “lithium-air”) reactions are often touted as the technological heir functional theory (DFT) calculations to determine their relative to lithium ion batteries. In Li-air cells, discharge reactions stabilities and structural changes after relaxations. They found € producing lithium oxide (Li2O), lithium peroxide (Li2O2), and the Foppl structure to be 0.53 eV per O2 lower in energy than the possibly lithium superoxide (LiO2) occur near the surface of the Feher structure, and they also determined that the latter structure cathode. These discharge products are difficult to characterize undergoes drastic structural relaxations in DFT, including the because the peroxide and superoxide phases are metastable under lengthening of OÀO bonds from 1.28 to 1.53 Å. Cota’s DFT- ambient conditions. Many traditional spectroscopies for lithium relaxed structure is also shown in Figure 1, showing the lengthen- and oxygen, such as X-ray photoelectron spectroscopy, X-ray ing of OÀO bonds. Cota’s work provided strong evidence of € ’ absorption, and electron energy loss, are hampered by the low Foppl s as the actual structure of Li2O2, and subsequent DFT core binding energies of lithium and oxygen and typically require studies4,5 used the F€oppl structure. We note, however, that the ex situ vacuum conditions. X-ray diffraction (XRD) may at times exchange-correlation functional used by Cota et al., the generalized be difficult because of the small scattering cross section of such gradient approximation (GGA) of Perdew, Burke, and Erzenhof low-Z compounds and limited crystallinity of discharge products (PBE),6 is known to have large errors in treating the oxygen 7 8 such as Li2O2,Li2O, and possibly LiO2. molecule and oxides. Therefore, direct experimental evidence In the 1950s, two disparate crystal structures were proposed and more accurate first-principles methods are needed to deter- 1,2 ff for Li2O2 from XRD studies. They have surprisingly di erent mine the structure more conclusively. lithium sublattices, as shown in Figure 1. Along the c-lattice direction, In this Letter, we seek to elucidate the structure of Li2O2 using Feher’s original structure consists of lithium and oxygen atoms a combination of X-ray and first-principles techniques. Using nominally sharing each plane, whereas F€oppl’s revised structure high-energy X-rays, we obtain the powder XRD pattern and positions the lithium sites between adjacent oxygen planes. compare the accuracy of the Feher and F€oppl structures using Whereas the two structures have similar nearest-neighbor LiÀO Rietveld refinement. Taking advantage of the coordination sensi- distances (1.91 Å in Feher’s vs 1.98 Å in F€oppl’s), the OÀO tivity of nonresonant inelastic X-ray scattering (NIXS), we also distances in the peroxide anions are drastically different (1.28 and measure the lithium and oxygen K-edges of Li2O2. These NIXS 1.55 Å, respectively). Moreover, whereas there is only one type of Li site in the Feher structure, the F€oppl structure contains Received: August 8, 2011 two inequivalent Li sites with different nearest-neighbor LiÀO Accepted: September 12, 2011 distances of 1.98 and 2.15 Å. Published: September 12, 2011

r 2011 American Chemical Society 2483 dx.doi.org/10.1021/jz201072b | J. Phys. Chem. Lett. 2011, 2, 2483–2486 The Journal of Physical Chemistry Letters LETTER spectra are compared with those calculated for the two proposed basis of the Rietveld refinement, the sample contained ∼12.9% (by structures using a Bethe-Salpeter (BSE) treatment, which accu- weight) impurity phases (mainly LiOH and Li2CO3). Figure 2 rately accounts for electronÀcore hole interactions. Finally, we summarizes the Rietveld refinement with the experimental data compare thermochemical data to accurate formation energies for these two structural models for Li2O2. The space-group (P63/ calculated from DFT using a hybrid functional. All three app- mmc in both structures) and Wyckoff sites for Li and O in the roaches result in the identification of the F€oppl structure as the symmetrized structures obtained from the original Feher and € 3 structure of Li2O2. Apart from ascertaining its structure, we Foppl structures as reported by Cota et al. were used as input for believe that the various measured and computed spectra and data the Rietveld refinements. The cell parameters a and c and variable α ’ α of Li2O2 presented here will be useful for future characterization internal parameters (Li) (Feher s structure) and (O) (both efforts in Li-air battery research. structures) are allowed to vary. Because the DFT-relaxed Feher ff α We used a commercial powder Li2O2 sample and took steps to structure di ers from the unrelaxed structure only in a, c, and , minimize exposure to air. XRD was performed at the Advanced the refinement procedure does not discriminate between the Photon Source (APS) using 114.82 keV X-rays (λ = 0.10798 Å). unrelaxed and relaxed Feher structures. The refined structural Rietveld refinement was performed on the powder XRD pattern parameters and the goodness-of-fit parameters are given in fi of Li2O2 using the two proposed structures for Li2O2.Onthe Table 1. Comparing the residuals and the goodness-of- t 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 OÀO bond length (1.25 Å) closer to that in the unrelaxed (1.28 Å) than in the DFT- relaxed (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 (<80°) for typical Cu Kα and Fe Kα X-ray sources (λ = 1.54 and 1.936 Å, respectively) corresponds to 2θ values of up to approximately 5.16 and 4.11° in the current study. For 2θ < ° € 4.11 , the ratio of residuals (Rp) between the Feher and Foppl structures is reduced from a factor of 3.4 to a factor of 2.0. At very large 2θ values (2θ >6°), there are also multiple peaks in the measured Figure 1. Feher’s1 (left), DFT-relaxed Feher’s according to Cota3 spectra that are found in the refinement for the F€oppl structure but € ’ 2 (middle), and Foppl s (right) proposed crystal structures for Li2O2.Red notinthatfortheFeher structure. Therefore, whereas it may still be (larger) spheres represent oxygen atoms and blue (smaller) spheres possible to confirm the F€oppl structure by diffraction using conven- represent lithium atoms. Blue (dashed) horizontal lines indicate Li planes. tional lab X-rays, the use of high-energy synchrotron X-ray source is We can see that Li atoms are roughly in-plane with the O atoms in the Feher ff structure while Li planes cut through the OÀObondsintheFoppl€ structure. particularly e ective 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 À q of 0.6À8Å 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 Figure 2. Measured powder XRD pattern of Li2O2 (black, lower), and the Rietveld refinement according to the F€oppl (blue, middle) and Feher quite sensitive to changes in its lithium coordination. (red, upper) structures. The residuals (differences between the Rietveld Theoretical NIXS spectra are obtained by solving the BSE and refinement and observed intensities), magnified by a factor of 3, are are also shown in Figure 3. We find that the calculated NIXS shown beneath each structure. spectra are substantially different for the two crystal structures a Table 1. Comparison of Structural Parameters (a and c lattice parameters, internal variable parameters α(Li) and α(O), and OÀO bond length), Residuals (unweighted R , weighted R ), and Goodness-of-fit Parameters (χ 2)ofLi O Rietveld Refinement a p wp r 2 2 Using Two Proposed Structures for Li2O2 À α b α b χ 2 structure a (Å) c (Å) R(O O)/Å (Li) (O) Rp Rwp r

Feher 3.1700(2) 7.7174(5) 1.249(3) 0.156(1) 0.0809(2) 0.0463 0.0942 32.28 Foppl€ 3.16919(3) 7.71401(8) 1.5638(8) N/A 0.10136(5) 0.0134 0.0180 1.18 a 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 dx.doi.org/10.1021/jz201072b |J. Phys. Chem. Lett. 2011, 2, 2483–2486 The Journal of Physical Chemistry Letters LETTER

€ Figure 3. Measured Li (a) and O (b) K-edges of Li2O2 (top, circles) compared with the spectra computed from the Feher and Foppl structures with BSE (bottom, solid lines). For the Li K-edge, the momentum (q) dependence is also shown.

ΔH À À Table 2. Formation Enthalpy ( )ofLi2O2 Calculated 530 533 eV in the oxygen K-edge is associated with O O (B) for the Two Structures Using GGA-PBE and HSE06 bonding, with shorter OÀO bonds giving rise to the peak at Compared with Experimental Values (A) lower energy loss,11 the difference between the F€oppl and Feher structure here likely stems from the difference in OÀO distances Δ (A) H (eV/O2) between these two structures. As such, the DFT-relaxed Feher structure by Cota may produce better agreement in the position À a À b experimental 6.57(9), 6.557 of this feature. However, the LiÀO distances are not significantly different between the unrelaxed and relaxed Feher structures, so (B) ΔH (eV/O ) 2 that the 535À545 eV part of the spectra should not be sig- Feher F€oppl nificantly modified if calculated from the relaxed structure. The sensitivity of the calculated NIXS spectra to local coordination is À À PBE 5.69 6.24 clearly demonstrated. HSE06 À5.90 À6.52 To supplement the spectroscopic comparisons, we show in a Ref 16. b NIST Chemistry WebBook, http://webbook.nist.gov/ Table 2 the formation enthalpies of the two structures as computed chemistry/. using two exchange-correlation functionals: the GGA-PBE7 (as used in ref 3) and the hybrid functional HSE06.12 Whereas and that the F€oppl structure is in much better agreement with the F€oppl structure is lower in energy than the Feher structure in that observed for both the lithium and oxygen edges, and, for Li both functionals, the formation enthalpy is consistent with the K-edge, in both the low and high q limits. We note that although experimental values only for the F€oppl structure in HSE06. This the NIXS spectra are computed from crystalline structures, they is a result of the more accurate treatment of the O2 molecule and are applicable for comparison with those measured from samples atomization energies in HSE06 compared with PBE and gives with reduced crystallinity as well because the NIXS spectra are confidence that the total-energy calculations are accurate in dependent on local environment instead of long-range order. HSE06. That this accurate treatment agrees with the NIXS- Bader charge analysis reveals that for the F€oppl structure, the two BSE comparison and high-energy XRD allows us to identify ff € ’ types of Li atoms have slightly di erent charges and the oxygen conclusively Foppl s as the actual structure of Li2O2. atoms have the same charges, whereas the situation is reversed for To summarize, we have used a multitude of experimental and the Feher structure. This difference may be responsible for the first-principles techniques to determine that F€oppl’s structure is a € ’ simple versus complex peaks (Foppl vs Feher) in the Li spectra betterrepresentationthanFeher softhestructureforLi2O2.We for energy loss of 60 to 70 eV, and the reverse in the oxygen demonstrate that the use of high-energy X-ray diffraction or NIXS spectra for energy loss of 535 to 545 eV. Because the feature at in conjunction with accurate first-principles methods is a reliable

2485 dx.doi.org/10.1021/jz201072b |J. Phys. Chem. Lett. 2011, 2, 2483–2486 The Journal of Physical Chemistry Letters LETTER approach to structural determination, with NIXS being particu- Sciences, under contract No. DE-AC02-06CH11357. PNC/ larly useful in situations where the crystallinity is compromised, XSD (sector 20) facilities at the Advanced Photon Source are for example, in situ and nanocrystalline applications. additionally supported by a Major Resources Support grant from NSERC, University of Washington, and Simon Fraser Univer- ’ EXPERIMENTAL AND COMPUTATIONAL METHODS sity. M.K.Y.C. and J.P.G. gratefully acknowledge use of the Fusion cluster in the Laboratory Computing Resource Center For both XRD and NIXS measurements, the samples were at Argonne National Laboratory. prepared by making pellets of commercial Li2O2 powder (Acros Chemicals, 95% pure by weight as per manufacturer’s specifica- tion; this information, provided for clarification, does not repre- ’ REFERENCES sent any endorsement or representation of the authors). For (1) Feher, F.; von Wilucki, I.; Dost, G. Beitr€age zur Kenntnis des XRD, the pellet was enclosed with kapton tape, whereas for NIXS Wasserstoffperoxyds und seiner Derivate, VII. Mitteil.: Uber€ die Kris- – measurements, the pellet was loaded in a hermetically-sealed tallstruktur des Lithiumperoxyds, Li2O2. Chem. Ber. 1953, 86, 1429 1437. sample holder. Care was taken to minimize the signal contribu- (2) F€oppl, H. Die Kristallstrukturen der Alkaliperoxyde. Z. 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Materials Corresponding Author 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 refine- ment 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 Re- search 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 dx.doi.org/10.1021/jz201072b |J. Phys. Chem. Lett. 2011, 2, 2483–2486