Bulk-Sensitive Characterization of the Discharged Products in Li–O2

Aug 2, 2012 - Understanding the nature of discharged products is critical to identifying suitable electrolyte systems for Li–O2 batteries. We have e...
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Bulk-Sensitive Characterization of the Discharged Products in Li−O2 Batteries by Nonresonant Inelastic X‑ray Scattering Naba K. Karan,† Mahalingam Balasubramanian,*,† Timothy T. Fister,‡ Anthony K. Burrell,‡ and Peng Du‡ †

X-ray Science Division, ‡Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Understanding the nature of discharged products is critical to identifying suitable electrolyte systems for Li−O2 batteries. We have employed nonresonant inelastic X-ray scattering (NIXS), which is a hard X-ray photon-in photon-out technique to monitor low energy core−shell excitations and to obtain bulk sensitive information on the solid discharged products in Li−O2 batteries using various electrolyte solvent/salt combinations. NIXS measurements were performed on cathodes after discharging the Li−O2 cells using low discharge current (∼25 mA/g of carbon). NIXS results reveal that, even in cells containing current state-of-the-art electrolytes, the oxygen in the discharged products is bound predominantly to species other than a peroxide or lithia. This finding shows that electrolyte decomposition is a significant pathway during discharge of Li−O2 batteries using ether and oligoether substituted silane based electrolytes.

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INTRODUCTION Owing to their much higher theoretical energy density over state-of-the-art rechargeable Li-ion batteries, research on rechargeable Li-oxygen (air) batteries have attracted huge interest in the past few years, particularly for electric vehicle applications.1,2 The issue of rechargeability is one of the main obstacles among many others, such as the rate capability, energy efficiency, cycleability etc., that needs to be overcome before rechargeable Li−O2 batteries could become a viable alternative for electrochemical energy storage. The first step in the oxygen reduction reaction in Li−O2 batteries is generally believed to be the formation LiO2, which has been detected spectroscopically using acetonitrile electrolyte.3 Electrolyte (both solvent and salt) instability during Li−O2 battery discharge in presence of highly reactive O2− is now being considered as a major concern. Nonaqueous Li−O2 battery chemistry is complex and strongly depends upon the electrolyte used.4,5 A detailed knowledge of the discharged product(s) formed in a rechargeable Li−O2 battery is important in order to understand the subsequent charging behavior. Understanding of the Li−O2 battery discharge products has been continuously evolving during the last year. Various analytical tools, such as XRD, Raman spectroscopy, DEMS, FTIR, XPS, and 6Li MAS NMR, have been used to investigate the discharged product(s).6−14 These studies have shown that in carbonate based electrolytes discharged products are dominated by electrolyte decomposition (such as lithium alkyl carbonates, lithium carbonates, etc.) arising from the highly active reduced O2 species attack and contain little lithium (per)oxide.6−8,10 On the other hand, discharge products of Li−O2 batteries have been identified to include lithium(per)oxide while using ether based electrolytes, such as DME or TEGDME.9−14 However, © 2012 American Chemical Society

discrepancies exist over the stability of ether-based electrolytes during Li−O2 battery discharge, and a direct correlation between the amount of oxygen consumed and the amount of Li2O2 formed during discharge has not been established.10,11,13 Guided by advanced quantum computations other novel electrolyte systems, such as oligoether substituted silanes (1NM3, for example), are also currently being studied.15,16 Based on theoretical calculations, it has been suggested that introduction of the Si−O group provides added stability for silanes over their carbon analogues.15 Most of the above-mentioned analytical spectroscopic techniques, such as FT-IR and vibrational Raman are well suited to characterize the surface chemistry, which have been successfully used to elucidate the nature of the SEI (solid− electrolyte interphase) layer (typically few nm thick) on the anodes in Li-ion batteries.17,18 As such, FT-IR and Raman are not element specific techniques and structural information is deduced from the presence of specific functional groups with characteristic bands (wave numbers). Element specific spectroscopic techniques such as XPS, XAS using soft X-rays that have been used so far in characterizing the discharged products8,12,16 are again surface sensitive and hence prone to surface contamination. For example, surface contamination is particularly problematic for Li K-XAS and it has been shown that speciation information deduced could be misleading even for standard compounds unless the samples are prepared with exquisite care.19 Equally important, in systems which have distinctly different bulk/surface structures, surface sensitive methods may not provide any useful information relevant to Received: June 26, 2012 Published: August 2, 2012 18132

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electrolyte [1 M LiCF3SO3 in tri(ethylene glycol)-substituted trimethylsilane (1NM3), 1 M LiPF6 in 1NM3, 1 M LiCF3SO3 in tetraethylene glycol dimethyl ether (TEGDME), 1 M LiPF6 in CH3CN]. As received TEGDME (Sigma-Aldrich) was further dried by vacuum distillation and using molecular sieve. 1NM3 was synthesized and purified (water level less than 20 ppm) following the procedure as described in ref 35. Asreceived acetonitrile (Sigma-Aldrich) was further dried using a solvent purifier system (Vacuum Atmospheres Company, part number 103991). As received salts LiCF3SO3 (Sigma-Aldrich) and LiPF6 (Novolyte) were further dried (100 °C) in vacuum oven placed inside an Ar-filled glove box. All of the solvents and salts were stored in an Ar filled glove box. Electrolytes were prepared by dissolving the required amount of salts in the appropriate solvents and were stored in an Ar filled glove box. Li metal foil was used as anode with 1NM3 and TEGDME based electrolytes. Due to the incompatibility of pure Li metal with CH3CN, surface stabilized Li was used as the anode for CH3CN based electrolyte.36 The surface stabilized Li allowed us to obtain a reasonable first discharge capacity. A circular hole (9 mm diameter) in the coin cell base allowed for the oxygen access to the cathode. The assembled coin cells were thereafter placed in a glass chamber. The entire cell assembly and transfer of the coin cells into the glass chamber were performed inside an Ar-filled glove box (O2 < 5 ppm), typically used for Li-ion battery assembly. Thereafter the glass chamber was taken out of the glove box and filled by flowing highly pure oxygen for ∼40 min. Typical OCVs of the Li−O2 cells at this stage were ∼2.9− 3.0 V (vs Li+/Li). The Li−O2 cells were discharged to 2.2 V (Maccor) using a low constant current [∼0.025 mA/cm2 or equivalently ∼25 mA/g(BP carbon)] in order to maximize the amount of discharged products. Representative discharge profiles of Li−O2 cells using different electrolytes are shown in Figure S1 in the Supporting Information (SI). NIXS Measurements. NIXS measurements were performed in the insertion device beamline (ID-20) at the Advanced Photon Source using the LERIX instrument, which allows for collecting simultaneous spectra at various momentum transfer (q) values.37 The energy resolution using typical incident photon energy of ∼8 keV was ∼1.0 eV (measured by fwhm at the elastic energy, ∼7912 eV). The relative uncertainty in energy-loss between different samples is estimated to be within 0.1 eV using the energy value of the elastic scattering. Note that while using incident X-rays of ∼8 keV, the penetration depth in carbon is ∼1 mm and consequently the entire cathode thickness, which includes the coated slurry and the porous carbon paper (gas diffusion layer/current collector), is thoroughly probed. For NIXS measurements, the cathode laminates were recovered from the coin cells after discharging to 2.2 V (full discharge) or partially discharged (for LiPF6 in1NM3 and in CH3CN electrolytes) to 500 mAh/g(BPcarbon), rinsed with acetonitrile and dried in the glovebox antechamber by pulling vacuum for 30 min. Thereafter, several cathode laminates were stacked together and loaded into the sample holder inside an Ar-filled glovebox for NIXS measurements. The sample holder had openings sealed with two layers of aluminized kapton, which allowed for the passage of incident and scattered beams (Figure S2). The sample holder was then placed into the spectrometer with constant flow of He in the sample holder region. Note that the penetration length of ∼8 keV X-ray photons in carbon is ∼1 mm, which makes the NIXS measurements bulk sensitive. NIXS measurements were also performed on some standard compounds by making pellets of

the bulk nature of the system. In addition, formation/ deposition of discharge products in Li−O2 batteries is not limited only to the cathode surface; rather they may be distributed into the micro-pores across the entire cathode thickness.13 In such situations, bulk characterization across the entire cathode thickness would expectedly provide complementary information to those obtained by surface probing techniques. Bulk sensitive information can be obtained from XRD; however, it is limited to probe only products with significant scattering coherence. Bulk sensitive element specific information can also be obtained using NMR. However, performing NMR experiments requires proper choice of isotopes for a particular element of interest. In this regard, nonresonant inelastic X-ray scattering (NIXS) using hard Xrays is a suitable analytical technique than can provide truly bulk sensitive information on Li−O2 battery discharged products (including both crystalline and amorphous components present) with element specificity. NIXS, also known as Xray Raman scattering, can be considered as an alternative approach toward obtaining soft XAS-like information using hard X-rays.20 In a typical NIXS measurement, inelastically scattered photons are measured as a function of energy and momentum transfer. For low momentum transfer regime this is equivalent to XAS, whereas additional information about transitions which are not limited by dipole selection rule can be obtained at high momentum transfer regime.20,21 NIXS has been used to study low energy excitations and local symmetries, in both molecular and condensed matter systems.22−31 In addition, NIXS is particularly suitable for demanding sample environments for high pressure studies.30 Recently, NIXS has also been used to study Li-ion-battery systems.32,33 One disadvantage is that NIXS signal provides an ensemble average over all environments in which the probing element is present. Oftentimes, however, careful comparison to reference standards can help disentangle the various contributions, and thus, key chemical and structural information can be obtained even in complex systems. Finally, advanced ab initio theoretical calculations can also be used to interpret and/or simulate the NIXS spectra.23,26,27,32−34 In the present work we have employed NIXS for bulk characterization of the discharged products in Li−O2 batteries using various state of the art electrolytes.

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EXPERIMENTAL METHODS Li−O2 Battery Electrochemistry. The cathode laminates were prepared using high surface area carbon [Black Pearls 2000 (BP), BET surface area 1467 m2/g] with or without PVDF binder. For laminates with binder, BP carbon and PVDF were mixed in ∼3:1 (weight) ratio. In either case, a slurry was prepared using N-methylpyrrolidone (NMP). The slurry was then hand-coated onto carbon paper (Spectracarb, 2050A, ∼100 μm thick), which was dried overnight in a vacuum oven at ∼75−80 °C. The practice of making cathode laminates by coating a slurry containing some form of carbon on carbon paper is quite common in the literature.9,10 Afterward, circular cathodes were punched out and stored in a desiccator before using in Li−O2 cell assembly. Typical BP carbon loadings in the cathode laminates were ∼0.8−1.2 mg/cm2. Discharge of Li−O2 cells were carried out in coin cell configuration. Li−O2 coin cells were assembled using circular (12.7 mm diameter) cathode laminates (prepared with or without PVDF binder), 2 layers of glass fiber separator (whatman, EPM), and 10−15 drops of nonaqueous liquid 18133

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commercial powders including Li2O2 (Acros Chemicals, 95% pure by weight), Li2O (Sigma Aldrich), LiF (Fisher scientific, >99%), Li2CO3 (Sigma Aldrich, >99%), LiOH (Sigma Aldrich, >98%), and LiOH·H2O (Sigma Aldrich, >98%). The pellets were loaded in a hermetically sealed sample holder. The entire process of sample preparation and handling was performed inside a glovebox.

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RESULTS AND DISCUSSION Before discussing element specific NIXS spectra of the discharged cathodes, it is important to verify the source of the observed signal in the discharged laminates. Measurable O K-NIXS signal could not be detected in the pristine cathode laminates. For example, Figure 1 shows representative single

Figure 1. Representative single (raw) scan O K-NIXS spectra obtained (at q = 5.6 Å−1) from pristine cathode and discharged (in 1 M LiCF3SO3 in 1NM3 electrolyte) Li−O2 cathode laminates. Note that the energy axis is plotted as incident energy. See text for details.

Figure 2. Comparison of the normalized O K-NIXS spectra obtained from Li−O2 battery discharged electrodes using various electrolyte salt/solvents combinations with some reference compounds. FD: full discharge (2.2 V); PD: 500 mAh/g(C); NB: no binder in the laminate; Tf: LiCF3SO3.

(raw) scan O K-NIXS spectra obtained (at q = 5.6 Å−1) from pristine cathode and a discharged (using 1 M LiCF3SO3 in 1NM3 electrolyte) cathode laminates using similar data acquisition parameters. Note that the energy axis in Figure 1 has been plotted as the incident energy. A distinct O K-NIXS signal (vertical line shows the position around which the O Kedge should appear) is evident in the discharged laminate; however, a detectable O K-NIXS signal could not be seen in the pristine laminate. Similar O K-NIXS spectra were also obtained at other momentum transfer values and for other discharged laminates (using different electrolyte) as well. This established that O K-NIXS comes exclusively from Li−O2 discharge activity. Note that oxygen signal from surface adsorbed C−O functionalities can be seen from pristine cathode laminates using surface sensitive spectroscopic techniques, such as XPS.8 Absence of a measurable O K-NIXS signal in the pristine laminates, on the other hand, demonstrates the true bulk nature of the present measurements. Figure 2 shows the O K-edge contribution to the NIXS cross section (averaged over multiple scans) from the discharged electrodes in Li−O2 cells using various electrolytes. The spectra were obtained after washing the discharged electrodes with acetonitrile following the procedures used by others.6,9,11,14 In order to eliminate any possible oxygen contribution from residual oxygen containing electrolytes (e.g., TEGDME and 1NM3), NIXS measurements were also performed on discharged electrodes using 1 M LiPF6 in CH3CN. Each spectrum in Figure 2 has been averaged over the same momentum transfer (q) values and the O K-NIXS is largely dominated by dipole transitions. Effect of possible radiation damage on the observed spectrum was checked by

quickly (∼5 min) measuring the edge spectra and comparing it with subsequent measurements and by taking measurements at various spots on the sample. Within this protocol the effect of beam damage on the discharged cathodes and the inorganic standard samples was minimal. Furthermore, the observed spectra for the standard compounds match well with the theoretically calculated spectra, showing that the spectra do not exhibit large artifacts from radiation damage.33,34 The general appearance of the O K-NIXS spectra in Figure 2 for all electrolytes is somewhat similar with a dominant broad feature between ∼536 and 548 eV. Interestingly, the characteristic strong peak of bulk Li2O2 at ∼532 eV is not obviously clear in any spectrum. Following previous EELS measurements on other peroxides (e.g., hydrogen peroxide, bis(trifluoromethyl) peroxide, and bis(t-butyl) peroxide) and allowing for the differences in absolute energy calibration, this strong peak for Li2O2 can be uniquely assigned to O 1s → σ*(O−O) transition characterizing the very local character of this antibonding orbital in peroxide structures.38 Moreover, there exists a correlation between this excitonic peak position and O−O bond length that has been extensively used to estimate O−O bond lengths for adsorbed peroxide-like moieties over various surfaces. Therefore, presence of various peroxide species, even with a distribution of slightly different O−O bond lengths, is expected to give rise to a strong O1s → σ*(O−O) peak (albeit with some increased broadness). In the absence of such a 18134

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with confidence. In the current study, assigning the broad feature between ∼536 and 548 eV in the O K-NIXS to specific components is difficult as it can potentially arise from several contributions (including for example, LiOH, LiOH·H2O, alkyl carbonates, etc.). NIXS gives elemental specific information averaged over all possible moieties present. Therefore, linear combination analysis without detailed knowledge of various species present will not be unique and was not attempted. Information on various species present in the discharged products cannot be obtained using any one analytical technique and use of multiple techniques has proven to be beneficial in this respect. Recently it has been elegantly demonstrated by chemical methods that superoxide species competitively react with both Li (giving rise to Li2O2) and PVDF binder (giving rise to LiF as side product).14 So, following this suggestion, increased amount of Li2O2 could be expected for laminates that do not contain PVDF binder. However, this cannot be validated by comparing the O K-NIXS data on laminates with or without PVDF binder in 1 M LiCF3SO3/1NM3 electrolyte. Presumably the side reaction involving the PVDF binder is one of many possible reactions involving oxygen and significant electrolyte decomposition products, are also present. The O K-NIXS findings are cross-validated with Li−K NIXS as discussed below. The relative strength of Li K-to O K-NIXS as determined by their edge-steps was small for the discharged cathodes compared to Li2O2 and Li2O. This is illustrated in Figure 3, which shows representative single quick NIXS wide scans obtained (at q = 7.8 Å−1) with coarse energy steps from

feature in the O K-NIXS spectra, it is unlikely that a significant amount of peroxide species containing typical O−O bond distances (∼1.4−1.6 Å) are present in the discharged cathodes. Comparing the normalized intensity at ∼532 eV to that of Li2O2, we estimate that no more than 3−5% of the oxygen in the discharged cathodes can be bound as a peroxide. This observation is in contradiction with previous reports which suggest that the predominant product, during discharge of Li− O2 cells in ether based electrolytes, is Li2O2. However, it should be noted that discrepancies exist over the stability of ether based electrolytes during Li−O2 battery discharge as well. Based on DEMS results McCloskey et al. have postulated that the DME electrolyte is stable during Li−O2 battery discharge and almost exclusively Li2O2 is formed as the discharged product.10 In contrast, Freunberger et al. have shown that, irrespective of the chain length, ether based electrolytes are susceptible to decomposition during the first discharge itself giving rise to various discharged products (such as Li2CO3, poly ethers/esters, etc.) including some Li2O2.11 In subsequent cycles, however, electrolyte decomposition becomes the dominate pathway without any Li2O2 formation. Recently, it has also been shown using surface sensitive XPS and Raman measurements that the Li2O2 is not the sole discharged product in TEGDME based electrolytes; major fraction of the discharge products arises from the salt decomposition.13 Therefore, the present bulk NIXS characterization of the discharged products provides complementary information on the discharged products to that obtained by other surface specific measurements in that Li2O2 is not the sole discharged product using ether based electrolytes. In almost all of the electrodes we examined a feature at ∼534 eV was present. The exception was for LiPF6/NM3 electrolyte, where a feature at slightly lower energy is observed. Comparisons to the spectra of standard compounds suggest that the peak at ∼534 eV may have contributions from Li2CO3 and/or Li2O. However, absence of any appreciable features at ∼540 and 555 eV akin to those seen for Li2O in the discharged cathodes suggests that the feature at ∼534 eV can be attributed to arise from Li2CO3 discounting the presence of a significant amount of Li2O. Comparing the normalized peak intensity of this feature ∼40−55% oxygen bound as Li2CO3 in the discharged cathodes using LiCF3SO3/ 1NM3 electrolyte could be estimated (Figure S3 in SI). Note that this particular feature in Li2CO3 spectra is a characteristic of O 1s → π*(CO) transition for compounds containing carbonyl groups and the position of this peak systematically depends on the relative oxidation of the carbonyl carbon.39 Keeping this in mind, some evidence of the presence of species containing carbonyl group (including Li2CO3) could be deduced from the feature at ∼534 eV for LiCF3SO3/TEGDME electrolyte. Absence of substantial amount of Li2O2 in the discharged products in 1NM3 electrolyte is also not in agreement with previous XPS results (using cathode containing Super P carbon with MnO2 catalyst in LiPF6/1NM3 electrolyte, ∼0.08 mA/ cm2) where it has been claimed that exclusively lithium oxides were formed at the discharged state.16 A broad peak centered at ∼55.7 eV in the Li 1s XPS spectra was assigned to Li2O2 and/ or Li2O in that study. However, note that the Li 1s peak assignment for various standards (particularly their relative energy positions) is not consistent with other studies.8,40 Therefore, we opine that in the absence of O 1s XPS data and spectra from other relevant standard compounds the broad Li 1s XPS spectra alone cannot be used for product identification

Figure 3. Representative quick NIXS wide scans obtained with coarse energy steps (at q = 7.8 Å−1) from pellets of two standards (top panel) and one representative 2.2 V discharged Li−O2 laminate (bottom panel, using 1 M LiCF3SO3 in 1NM3 electrolyte). The insets in the bottom panel shows expanded regions of Li, O K-NIXS (obtained with a finer energy grid) and C K-NIXS (from the wide scan). 18135

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pellets of two standards (top panel) and one representative 2.2 V discharged Li−O2 laminate (bottom panel, using 1 M LiCF3SO3 in 1NM3 electrolyte). The energy axis is plotted as incident energy and the position of various edges is marked with arrows. The broad background (in the wide scans) arises from Compton scattering. The insets in the bottom panel show expanded regions of Li and O K-NIXS, obtained with a finer energy grid and C K-NIXS (from the wide scan). Note the relatively high intensity of the C K-NIXS signal in the discharged cathode, which is dominated by the carbon contribution from the carbon paper due to the bulk sensitivity of NIXS. Clearly, the relative strength of Li to O K-NIXS edge jumps was small for the discharged cathodes compared to Li2O2 and Li2O. Similar features (of Li and O K NIXS) were also seen in wide scans obtained at other q values from discharged cathode laminates using other electrolytes as well. This observation suggests that O K-NIXS is dominated by discharge products poor in lithium and rich in other elements (such as, C and O). The Li K-NIXS exhibits the expected large qdependence, and data for both low (qave = 1.2 Å−1) and high (qave = 7.4 Å−1) q regimes are shown in Figure 4. The presence of a large Compton background in conjunction with the small Li−K edge-step makes the extraction of the Li K-NIXS a bit challenging. Only those cases where the background can be reliably subtracted are shown in Figure 4. In general, the spectral features of the Li K-NIXS of the discharged electrodes were broad and point toward the presence of many Li electronic and structural environments. However, comparison of the spectra of the discharged cathode to standards shows that the Li K-NIXS spectra may contain some Li2CO3 for both 1NM3 and TEGDME electrolytes. Note that even though both Li2O2 and Li2CO3 can potentially contribute to the peaks at ∼67 and 61.5 eV; presence of prominent feature at ∼59.2 eV, particularly at high q-regime, in the Li K-NIXS data in conjunction with the absence of characteristic O K-NIXS feature (Figure 2) indicates that these features mainly arise from Li2CO3 in the discharged cathodes. The absence of any strong feature at ∼64 eV also rules out the presence of substantial amount of Li2O in the discharged cathodes. Also, relatively strong features at ∼62 and 69 eV could be seen in the discharged cathodes using LiPF6 salt in both acetonitrile and 1NM3 electrolytes. On the basis of the LiF standard spectra, particularly the strong peak at ∼69 eV in high q regime, we tentatively assign these features to be arising from LiF while using LiPF6 salt as a result of possible salt decomposition. This is not surprising as LiF is also seen as decomposition product even in cathode laminates of Li-ion cells using LiPF6 salt, subjected to electrochemical cycling.41 As such, Li and O KNIXS data are internally consistent, in that there were no obvious signatures of predominant Li2O2 and/or Li2O in the discharged cathodes and the observed Li and O K-NIXS spectra are dominated by various other components. The positions of distinct peaks observed in the O K and Li K-NIXS spectra of the discharged cathodes using various electrolytes along with the corresponding assigned phases are summarized in Table S1 in SI. It should be noted, however, that such consistency was not seen using soft XAS, where O K-XAS features was assigned to defective Li2O2 but Li K-XAS did not show any Li2O2 signature.12 Also, inspection of the Li K-edge XAS in that study,12 suggests that the reported spectra of the discharged products have significant resemblance to the Li−K excitation spectra (low q-NIXS,32,33 XAS42 and EELS43) of LiF, albeit the discharge was performed using a perchlorate-based electrolyte.

Figure 4. Comparison of the normalized Li K-NIXS spectra obtained from Li−O2 battery discharged electrodes using various electrolytes with several reference compounds for low (panel a) and high (panel b) q-regimes. LiTf: LiCF3SO3; NB: no binder in the laminate.

Finally, the common broad feature between ∼536 and 548 eV in the O K-NIXS spectra of the discharged cathodes for various electrolytes in Figure 2 has similarity to the O K-edge XAS observed for cycled cathode laminates in Li-ion battery (Figure S4 in SI).41 This feature in cycled Li-ion cell cathode surface is generally believed to arise from the presence of various oxygen containing compounds in the SEI layer as a result of electrolyte decomposition. Given the absence of the characteristic O K-NIXS features of Li2O2 and/or Li2O in the discharged Li−O2 cathodes and the prominent presence of the broad peak implies that the solid discharged products mainly consists of species where oxygen is not present in the expected peroxide like forms, and largely exists in products formed by parasitic electrolyte decomposition. We note that the discharge current used in the present study is comparatively small and if the formation of Li2O2/Li2O is rate dependent, then increased amount of Li2O2 could perhaps be deposited in the discharged cathodes at higher rates. However, in that case parasitic side reactions could lead not just to electrolyte usage but also significant loss of Li inventory over repeated cycling. Parasitic 18136

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reactions which “steal” Li are a dominant contributor to diminished long-term cycle life even in well-developed Li-ion technology.44

(5) Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A. J. Phys. Chem. C 2010, 114, 9178−9186. (6) Freunberger, S. A.; Chen, Y.; Peng, Z.; Griffin, J. M.; Hardwick, L. J.; Barde, F.; Novak, P.; Bruce, P. G. J. Am. Chem. Soc. 2011, 133, 8040−8047. (7) Xiao, J.; Hu, J.; Wang, D.; Hu, D.; Xu, W.; Graff, G. L.; Nie, Z.; Liu, J.; Zhang, J. G. J. Power Sources 2011, 196, 5674−5678. (8) Veith, G. M.; Dudney, N. J.; Howe, J.; Nanda, J. J. Phys. Chem. C 2011, 115, 14325−14333. (9) Laoire, C. O.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. J. Electrochem. Soc. 2011, 158, A302−A308. (10) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar., G.; Luntz, A. C. J. Phys. Chem. Lett. 2011, 2, 1161−1166. (11) Freunberger, S. A.; Chen, Y.; Drewett, N. E.; Hardwick, L. J.; Barde, F.; Bruce, P. G. Angew. Chem., Int. Ed. 2011, 50, 8609−8614. (12) Lu, Y. C.; Kwabi, D. G.; Vao, K. P. C.; Harding, J. R.; Zhou, J.; Zuin, L.; Horn, Y. S. Energy Environ. Sci. 2011, 4, 2999−3007. (13) Veith, G.; Nanda, J.; Delmau, L. H.; Dudney, N. J. J. Phys. Chem Lett. 2012, 3, 1242−1247. (14) Black, R.; Oh, S. H.; Lee, J. H.; Yim, T.; Adams, B.; Nazar, L. F. J. Am. Chem. Soc. 2012, 134, 2902−2905. (15) Assary, R. S.; Curtiss, L. A.; Redfern, P. C.; Zhang, Z.; Amine, K. J. Phys. Chem. C 2011, 115, 12216−12223. (16) Zhang, Z.; Lu, J.; Assary, R. S.; Du, P.; Wang, H. H.; Sun, Y. K.; Qin, Y.; Lau, K. C.; Greeley, J.; Redfern, P. C.; Iddir, H.; Curtiss, L. A.; Amine, K. J. Phys. Chem. C 2011, 115, 25535−25542. (17) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-Eli, Y. J. Electrochem. Soc. 1996, 143, 3809−3819. (18) Kong, F.; Kostecki, R.; Nadeau, G.; Song, X.; Zaghib, K.; Kinoshita, K.; McLarnon, F. J. Power Sources 2001, 97−98, 58−66. (19) Ishii, H.; Nakanishi, K.; Watanabe, I.; Ohta, T.; Kojima, K. J. Surf. Sci. Nanotechnol. 2011, 9, 416−421. (20) Schulke, W. Electron dynamics by inelastic x-ray scattering; Oxford University Press: New York, 2007. (21) Mizuno, Y.; Ohmua, Y. J. Phys. Soc. Jpn. 1967, 22, 445−449. (22) Bradley, A J.; Sakko, A.; Seidler, G. T.; Rubio, A.; Hakala, M.; Hamalainen, K.; Cooper, G.; Hitchcock, A. P.; Schlimmer, K.; Nagle, K. P. Phys. Rev. A 2011, 84, 022510−1−022510−8. (23) Pylkkanen, T.; Lehtola, J.; Hakala, M.; Sakko, A.; Monaco, G.; Huotari, S.; Hamalainen, K. J. Phys Chem. B 2010, 114, 13076−13083. (24) Caliebe, W. A.; Soininen, J. A.; Shirley, E. L.; Kao, C. C.; Hamalainen, K. Phys. Rev. Lett. 2000, 84, 3907−3910. (25) Bergmann, U.; Wernet, Ph.; Glatzel, P.; Cavalleri, M.; Pettersson, L. G. M.; Nilsson, A.; Cramer, S. P. Phys. Rev. B 2002, 66, 092107−1−092107−4. (26) Gordon, R. A.; Seidler, G. T.; Fister, T. T.; Haverkort, M. W.; Sawatzky, G. A.; Tanaka, A.; Sham, T. K. Euro. Phys. Lett. 2008, 81, 26004-p1−26004-p6. (27) Conrad, H.; Lehmkuhler, F.; Sternemann, C.; Sakko, A.; Paschek, D.; Simonelli, L.; Huotari, S.; Feroughi, O.; Tolan, M.; Hamalainen, K. Phys. Rev. Lett. 2009, 103, 218301−1−218301−4. (28) Hamalainen, K.; Galambosi, G.; Soininen, J. A.; Shirley, E. L.; Rueff, J. P.; Shukla, A. Phys. Rev. B 2002, 65, 155111−1−155111−5. (29) Huotari, S.; Pylkkanen, T.; Soininen, J. A.; Kas, J. J.; Hamalainen, K.; Monaco, G. J. Synchrotron. Rad. 2012, 19, 106−113. (30) Mao, W. L.; Mao, H. K.; Eng, P. J.; Trainor, T. P.; Newville, M.; Kao, C. C.; Heinz, D. L.; Shu, J.; Meng., Y.; Hemley, R. J. Science 2003, 302, 425−427. (31) Feroughi, O. M.; Sternemann, C.; Sahle, C. J.; Schroer, M. A.; Sternemann, H.; Conrad, H.; Hohl, A.; Seidler, G. T.; Bradley, J.; Fister, T. T.; Balasubramanian, M.; Sakko, A.; Pirkkalainen, K.; Hamalainen, K.; Tolan, M. Appl. Phys. Lett. 2010, 96 (8), 081912−1− 081912−3. (32) Balasubramanian, M.; Johnson, C. S.; Cross, J. O.; Seidler, G. T.; Fister, T. T.; Stern, E. A.; Hamner., C.; Mariager, S. O. Appl. Phys. Lett. 2007, 91, 031904−1−031904−3. (33) Fister, T. T.; Schmidt, M.; Fenter, P.; Johnson, C. S.; Slater, M. D.; Chan, M. Y. K.; Shirley, E. L. J. Chem. Phys. 2011, 135, 224513−1− 224513−5.

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CONCLUSIONS Bulk-sensitive, element-specific NIXS measurements were used to study the nature of discharged products in Li−O2 batteries. Both O− and Li−K NIXS results were internally consistent and show that the discharge reactions in a Li−O2 battery are not dominated by the formation of Li2O2 and/or Li2O while using TEGDME and 1NM3 based electrolytes, at low discharge currents (∼25 mA/g of carbon/∼0.025 mA/cm2). Fundamental to the rechargeable Li-oxygen chemistry is the formation and decomposition of lithium-oxides on repeated discharge−charge cycling, with high efficiency and fidelity. The overarching finding that lithium (per)oxide is not the dominant discharge product(s) reinforces the need for the development of highly stable electrolyte systems which will promote exclusive formation of Li2O2/Li2O during Li−O2 battery discharge.

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ASSOCIATED CONTENT

S Supporting Information *

Discharge characteristics of Li−O2 cells, description of NIXS sample holder, estimation of Li2CO3 in NM3 based electrolyte discharged cathode and comparison to Li-ion cathode laminate SEI spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 630-252-0593. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS PNC/XSD facilities at the Advanced Photon Source, and research at these facilities, are supported by the U.S. Department of Energy - Basic Energy Sciences, a Major Resources Support grant from NSERC, the University of Washington, the Canadian Light Source and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. TTF was supported by the Center for Electrical Energy Storage, an Energy Frontier Research Center funded by DOE/BES. We thank Jack Vaughey and Susi Neuhold for providing surface stabilized Li foil. We gratefully acknowledge the constructive discussions with several Argonne members involved in battery science activities.

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REFERENCES

(1) Bruce, P. G.; Hardwick, L. J.; Abraham, K. M. MRS Bull. 2011, 36, 506−512. (2) Christensen, J.; Albertus, P.; Sanchez-Carrera, R. S.; Lohmann, T.; Kozinsky, B.; Liedtke, R.; Ahmed, J.; Kojic, A. J. Electrochem. Soc. 2012, 159, R1−R30. (3) Peng, Z.; Freunberger, S. A.; Hardwick, L. J.; Chen, Y.; Giordani, V.; Barde, F.; Novak, P.; Graham, D.; Tarascon, J. M.; Bruce, P. G. Angew. Chem., Int. Ed. 2011, 50, 6351−6355. (4) Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A. J. Phys. Chem. C 2009, 113, 20127−20134. 18137

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The Journal of Physical Chemistry C

Article

(34) Chan, M. K. Y.; Shirley, E. L.; Karan, N. K.; Balasubramanian, M.; Ren, Y.; Greeley, J. P.; Fister, T. T. J. Phys. Chem. Lett. 2011, 2, 2483−2486. (35) Dong, J.; Zhang, Z.; Kusachi, Y.; Amine, K. J. Power Sources 2011, 196, 2255−2259. (36) Neuhold, S.; Schroeder, D. J.; Vaughey, J. T. J. Power Sources 2012, 206, 295−300. (37) Fister, T. T.; Seidler, G. T.; Wharton, L.; Battle, A. R.; Ellis, T. B.; Cross, J. O.; Macrander, A. T.; Elam, W. T.; Tyson, T. A.; Qian, Q. Rev. Sci. Instrum. 2006, 77, 063901−1−063901−7. (38) Ruhl, E.; Hitchcock, A. P. Chem. Phys. 1991, 154, 323−329. (39) Lessard, R.; Cuny, J.; Cooper, G.; Hitchcock, A. P. Chem. Phys. 2007, 331, 289−303. (40) Oswald, S.; Mikhailova, D.; Scheiba, F.; Reichel, P.; Fiedler, A.; Ehrenberg, H. Anal. Bioanal. Chem. 2011, 400, 691−696. (41) Balasubramainan, M.; Lee, H. S.; Sun, X.; Yang, X. Q.; Moodenbaugh, A. R.; McBreen, J.; Fischer, D. A.; Fu, Z. Electrochem. Solid State Lett. 2002, 5, A22−A25. (42) Tsuji, J.; Nakamatsu, H.; Mukoyama, T.; Kojima, K.; Ikeda, S.; Taniguchi, K. X-ray Spectrosc. 2002, 31, 319−326. (43) Wang, F.; Graetz, J.; Moreno, M. S.; Ma, C.; Wu, L.; Volkov, V.; Zhu, Y. ACS Nano 2011, 5, 1190−1197. (44) Smith, A. J.; Burns, J. C.; Zhao, X.; Xiong, D.; Dahn, J. R. J. Electrochem. Soc. 2011, 158, A447−A452.

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