Monochromatic X-ray Photoelectron Spectroscopy ... - ACS Publications

Subscriber access provided by HACETTEPE UNIVERSITESI KUTUPHANESI

Article

Monochromatic X-ray Photoelectron Spectroscopy Study of Three Different Ionic Liquids in Interaction with Lithium Decorated Copper Surfaces Mark Olschewski, Rene Gustus, Oliver Höfft, Abhishek Lahiri, and Frank Endres J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10139 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Monochromatic X-ray Photoelectron Spectroscopy Study of Three Different Ionic Liquids in Interaction with Lithium Decorated Copper Surfaces Mark Olschewski1, René Gustus2, Oliver Höfft1*, Abhishek Lahiri1 and Frank Endres1*

1 Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str 6, D-38678 Clausthal-Zellerfeld 2 Clausthal Centre of Material Technology (CZM), Clausthal University of Technology, Agricolastrasse 2, D-38678 Clausthal-Zellerfeld

Abstract A comparative study of the chemical reactions of three different ionic liquids prepared by physical vapor deposition on a Li/Cu surface was performed. In this investigation the ionic liquids 1-butyl-1methylpyrrolidinium

bis[fluorosulfonyl]imide

bis[trifluoromethylsulfonyl]imide

([Py1,4]

([Py1,4]FSI), TFSI)

and

1-butyl-1-methylpyrrolidinium 1-octyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([OMIm]TFSI) were characterized by monochromatic X-ray photoelectron spectroscopy after adsorption on Li/Cu. All spectra prove the formation of LiF and Li2O, which verifies partial decomposition of the anions. This is found to be much less intense for [Py1,4]TFSI than for [Py1,4]FSI indicating a more rapid decomposition of the FSI anion in presence of lithium. In addition, changes of the cation structure, as well as changes of the anion/cation ratio, were observed in the spectra. In summary, cations were most likely displaced from the Li/Cu surface and were partially decomposed, especially in case of [OMIm]TFSI, due to the interaction with lithium. The results give an overview on the possible reaction products at the Li surface when using ionic liquids as electrolytes or additives for lithium batteries. Additionally, it gives an insight into the specific role of the cations and anions for the formation of SEI layer in battery systems. Keywords: Ionic Liquids, lithium, X-ray photoelectron spectroscopy, Solid Electrolyte Interphase * To whom all correspondence should be addressed. E-mail: [email protected], [email protected]

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Room temperature ionic liquids (RT-ILs) have been of fundamental interest for a wide field of scientific investigations exhibiting applications in catalysis, electrochemistry or green chemistry, in the past years.1–3 Due to their high temperature stability, wide electrochemical window and good solubility for lithium salts, ionic liquids have been discussed as electrolytes for Li based batteries.4–7 Battery experiments with RT-IL electrolytes show promising results, most notably exhibiting high cycle stability. This seems to be governed by the formation of solid electrolyte interphase (SEI) layers. A couple of investigations focus on SEI layers in battery systems with different ionic liquid electrolytes, as there are nearly unlimited possible combinations of cations and anions covering a wide range of physical and chemical properties.4,6,8–11 To fully understand the interfacial behavior of ionic liquids on electrode surfaces, fundamental experiments have to be performed on a molecular level. Unfortunately interface processes are difficult to access experimentally. Thus a couple of simulation studies give an insight into the mechanisms behind decomposition of the ionic liquid electrolytes for instance in presence of lithium to confirm experimental observations.12–15 In addition, well defined model experiments can be performed to probe electrode surfaces on a fundamental scale. Due to the low vapor pressure of RT-ILs, experiments can be carried out in an ultra-high vacuum (UHV) chamber using surface science techniques like X-ray photoelectron spectroscopy (XPS). Physical vapor deposition (PVD) allows processing of thin films of ionic liquids in UHV and was successfully used to prepare and analyze model interfaces of RT-ILs on crystal metal surfaces like Ag (111), Au (111) and Cu (111).16–21 In addition, first studies were performed aiming the interaction of ionic liquids with lithium and lithium decorated metal surfaces.15,22–24 Howlett et al. used [Py1,4]TFSI as electrolyte to cycle lithium on lithium metal and copper electrodes.24 In a spectroscopic investigation, reaction products of the RTIL and lithium were found on both electrode surfaces. While major amounts of LiF and Li2O indicate decomposition of the anion in presence of lithium, an influence of the cation was also concluded. Similar results were observed by Budi et al. for the interaction of [Py1,3]FSI with lithium.15 After immersion of lithium metal foil in the ionic liquid for 18 days, reaction products like LiF, LiOH and LiO2 were found in XP spectra. The presence of LiOH indicates release of Hydrogen and thus decomposition of the cation, too. In addition quantum-chemical calculations were done, confirming the reaction of the FSI anion with lithium for [Py1,3]FSI adsorbed on a Li (100) surface. When using imidazolium cations a strong reactivity against lithium was observed and traced back to instability of the Carbon in the C2-Position (between the two Nitrogen atoms) of the heterocyclic ring structure.8–10 -2ACS Paragon Plus Environment

Page 2 of 19

Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In a previous work we were able to show the reaction of thin films of [OMIm]TFSI with lithium or lithium decorated copper surfaces.25 To avoid an influence of the SEI layer by ex situ preparation of electrode surfaces this investigation was performed on model interfaces under UHV conditions. Alkali metal and [OMIm]TFSI were deposited on a copper foil using physical vapor deposition (PVD) and afterwards analyzed by non-monochromatic X-ray photoelectron spectroscopy. Next to reaction products of the TFSI anion and lithium, ring break up, leading to formation of Li-N, Li-C and desorption of the residual [OMIm] structure, was observed for the cation. However, the influence of X-ray radiation induced decomposition of the ionic liquids to the reaction with lithium could not be resolved entirely. Buchner et al. observed cation and anion decomposition after codeposition of [Py1,4]TFSI and lithium on a copper surface, too.22 They confirmed an initial reaction even at 80 K and showed changes when increasing the temperature to above 400 K. Structures, which build in the interaction on the copper substrate, were additionally characterized by Scanning Tunneling Microscopy (STM). In the present experimental investigation [Py1,4]FSI, [Py1,4]TFSI and [OMIm]TFSI each were deposited on a Li/Cu surface at room temperature under similar conditions to compare the interaction of each of the ionic liquids with lithium. Reaction products formed were analyzed by monochromatic photoelectron spectroscopy to minimize decomposition of the RT-ILs by beam damage.26

Experiment All experiments were performed in an UHV-XPS/SPM system (SPECS) with a base pressure below 5×10-10 mbar. For each experiment a Cu foil (sigma Aldrich) was used and cleaned by electron beam heating and etching with Ar+ ions accelerated to 2 keV. Purity of the Cu surface before evaporation of Lithium and Ionic Liquids was proved by XPS. For lithium evaporation a lithium-metal source filled in a stainless steel tube (Alvasources AS-3-Li10-C by Alvatec) was used. The dispenser was heated with a current of 10 A. The ionic liquid 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide ([Py1,4]FSI) was purchased from Solvionic, while 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSI) and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([OMIm]TFSI) were purchased in the highest available quality from Io-Li-Tec. They were dried and baked out at 375 K under UHV conditions. To produce a thin layer of the RT-ILs they were evaporated using the TCE-BSC (Kentax GmbH) molecule evaporator immediately after deposition of lithium. Evaporation time and temperature was -3ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

chosen to 3 min at 488 K for [Py1,4]FSI, 3 min at 438 K for [Py1,4]TFSI and 10 min at 413 K for [OMIm]TFSI to ensure similar layer thicknesses. Physical vapor deposition of ionic liquids under UHV conditions in a temperature range of 400 K to 500 K without decomposition has been proven for instance by mass spectroscopy27 or IR measurements28 and already applied in comparable experiments16. Evaporation of intact ion pairs and layer thickness was confirmed by XPS measurements of the RT-ILs deposited on an Au (111) substrate, prior to the experiment. However, due to geometrical reasons (off-axis evaporation of [OMIm]TFSI crucible), the resulting layer thickness of [Py1,4]TFSI and [Py1,4]FSI is in the range of 10 nm, while the [OMIm]TFSI layer is estimated to have a thickness of about 2 nm. The freshly prepared samples were analyzed by XPS measurements after a relaxing time of 15 min, to provide enough time for the surface reactions between lithium and IL before analysis. For Al Kα X-ray generation a XR50M source with a Focus 500 monochromator by SPECS was used in non-focused mode. Electrons emitted from the sample were detected perpendicular to the sample surface by a hemispherical analyzer (PHOIBOS 150 by SPECS) with MediumArea Lens Mode. The pass energy of the analyzer was kept at 10 eV for detail spectra and 30 eV for survey spectra. To minimize the influence of possible X-ray induced decomposition of the ionic liquid (beam damage) the total exposure time did not exceed 20 min. The spectra were displayed as a function of binding energy (BE) with respect to the Fermi level. For all detail spectra reference measurements were provided showing the components in the XP spectra observed when the RT-IL is adsorbed on a pure Cu surface. Since a constant shift in all components assigned to the ionic liquid was observed when adsorbed on Li/Cu, XP spectra have been charge corrected by fixing the F 1s component to the energy in the reference measurement. For quantitative XPS analysis, background-subtraction was employed. Component peak areas were calculated by fitting Voigt profiles optimized by the Levenberg-Marquard algorithm with CasaXPS software. Photoelectric cross-sections calculated by Scofield29, asymmetry factors calculated by Yeh and Lindau30, transmission function of the hemispherical analyzer, as well as an estimation of the mean free path of the electrons in ionic liquids have been taken into account for stoichiometric calculations. In addition, Raman measurements were carried out for neat ionic liquid as well lithium foil immersed in [OMIm]TFSI. The experiments were performed for a period of 4 hours in a sealed glass cuvette. A Raman module RAM III (Nd:YAG laser, 1064 nm) attached to a Bruker Vertex 70 V spectrometer was used for the Raman measurements.

Results -4ACS Paragon Plus Environment

Page 4 of 19

Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The freshly prepared Li/Cu surface has been analyzed by XP spectroscopy to estimate the thickness of the lithium adsorbate. Besides lithium small amounts of fluorine and oxygen were detected in the survey spectrum. This is in good agreement with a rather broad Li 1s detail spectrum (both shown in S.I.) showing contributions of metallic lithium, fluorides and oxides. Based on the decrease in the intensity of the Cu 2p peak before and after Li 1s deposition, the layer thickness is estimated to be 1.8 nm. Impurities of fluorine and oxygen might be present in the vacuum chamber during evaporation of lithium due to heat up and desorption of residual ionic liquid on the stainless steel head of the evaporator. However, those “native” species on the Li/Cu surface have a minor influence on the spectra, as the photoelectrons leaving the surface area were shaded by the adsorbate and the reaction products. [Py1,4]FSI Physical vapor deposition of intact [Py1,4]FSI has been proven by comparing the binding energies and peak areas of the components with spectra measured in bulk ionic liquid. In Figure 2 photoelectron spectra of C 1s and N 1s regions, as well as component peak fits, of [Py1,4]FSI adsorbed on the Li/Cu surface (bottom spectra) are shown in comparison to spectra of the IL on a Cu surface (top spectra). To ensure a good comparability, all spectra were charge corrected by fixing the F 1s component of the anion. Thus, the binding energies reported in this investigation should be considered relative to the F 1s component. The spectra were scaled to equal height of the F 1s species. So the graphs allow an interpretation of stoichiometric changes between single components of the IL in relation to the reference spectrum. In the C 1s region two components C1 at 285.6 eV and C2 at 287.0 eV show contributions of aliphatic and heterocyclic carbon in the [Py1,4] cation.31 Adsorbed on the Li/Cu surface, the peak area of the C2 component in relation to C1 is slightly decreased by approx. 10 % compared to the reference on Cu, while a new component is not observed. This indicates partial decomposition of the cation. In the right graph in Figure 2 the photoelectron spectrum of the N 1s region is shown. Besides two peaks at 402.9 eV and 400.0 eV, which can be assigned to Nitrogen in the [Py1,4] cation and FSI anion, two additional peaks were observed. The first peak at 398.2 eV might be related to LiN322,32, while the second one at 396.3 eV could either be associated with further Li-N bonds or even Cu-N structures. Major changes were observed for the components related to FSI in the F 1s, O 1s and S 2p XP spectra shown in Figure 3. Next to the F-S component of the anion at 688.0 eV BE a prominent peak at 684.7 eV BE can be assigned to LiF33 in the F 1s spectrum. In the middle of Figure 3 the O 1s spectrum with its main component belonging to the sulfonyl-group of the FSI anion at 533.2 eV BE is

-5ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

shown. In addition a shoulder peak OQ1 at 531.4 eV, which might be contributed by LiO/LiOH34,35, next to a peak at 527.9 eV BE, which can be assigned to Li2O35,36, are observed on the Li/Cu surface. Corresponding to the sulfonyl-group of FSI the S 2p spectrum shows a spin doublet structure at 170.1 eV BE (S 2p3/2) and 171.3 eV BE (S 2p1/2). When adsorbed on Li/Cu a couple of new peaks SQ1, SQ2, SQ3 between 164.8 eV and 169.1 eV were detected and confirm anion decomposition, although the origin of all the peaks is not fully understood, so far. Each of the features in SQ1 and SQ2 most likely represents a reduction step of the sulfonyl group. The SQ3 component with its low binding energy may be assigned to LixSOy.24 Structures around 160 eV were reported to represent adsorbed sulfur on a Cu substrate and thus indicate reduction of the sulfuonyl-group.18 [Py1,4]TFSI XP spectra of [Py1,4]TFSI adsorbed on the Li/Cu surface are shown in Figure 4 and Figure 5. In the C 1s region in Figure 4 both (C1 and C2) components of the cation as well as a contribution of C-F3 (TFSI) at 293.1 eV BE were observed. Binding energies and peak areas of the components in C 1s region for [Py1,4]TFSI adsorbed on Li/Cu are in good agreement with the reference measurement on pure Cu. Small differences between both were observed in the N 1s region. The peak area of the [Py1,4] cation component in relation to the TFSI-anion peak at 399.6 eV BE is slightly decreased, although there is no change of the cation components in the C 1s spectrum. In addition a small shoulder peak is observed around 397.3 eV BE and might be related to a decomposition product of the TFSI anion, like observed for [Py1,4]FSI. The F 1s, O 1s and S 2p XP spectra in Figure 5 indicate decomposition of the anion, too. Next to the C-F3 component of the TFSI-anion at 689.0 eV BE in the F 1s spectrum a peak at 685.0 eV BE can be assigned to LiF. In the O 1s spectrum the main component of the sulfonyl-group in TFSI is observed at 532.9 eV BE. On the Li/Cu surface a shoulder peak OQ1 at 531.6 eV BE, which can be assigned to decomposed anion, and a new peak at 528.0 eV BE assigned to Li2O were found. In the S 2p spectrum a shoulder peaks (labeled SQ1) at 167.6 eV BE of the actual TFSI component at 169.2 eV BE (S 2p3/2) confirm partial decomposition of the TFSI anion. [OMIm]TFSI A separation of C1 at 285.0 eV BE and C2 at 286.8 eV BE components, thus between aliphatic and heterocyclic carbon, in the C 1s reference spectrum in Figure 6 is done (comparable with the fit for [Py1,4]). However, due to the rather long octyl-chain the C1 component is much more present compared to the spectrum of the [Py1,4] cation. When adsorbed on the Li/Cu surface both peaks appear strongly decreased and build a broad feature between 284 eV and 287 eV. In addition a new component at 282.3 eV BE is found. Compared to XPS measurements on a lithium surface after immersion in propylene carbonate, this component can be assigned to LixC.37 Additionally the C-F3 component of the TFSI anion component in the C 1s spectrum is broader compared to the reference on Cu. In summary, the relative amount of carbon is massively decreased compared to the other peaks. -6ACS Paragon Plus Environment

Page 6 of 19

Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The cation component in the Nitrogen region, observed at 402.1 eV BE in the Cu reference, vanished completely. Instead, broadening of the TFSI-peak in the N 1s region as well as a new peak at 397.3 eV, which can be assigned to Li-N components, can be observed. Both C 1s and N 1s regions thus indicate desorption of the [OMIm] cation or its decomposition products, while anion related species remain on the surface. In addition, changes concerning the TFSI anion when adsorbed on Li/Cu were observed in the F 1s, O 1s and S 2p XP spectra in Figure 7. In the F 1s spectrum a peak at 685.0 eV BE indicates the presence of LiF, while a new component in the O 1s region shows the presence of Li2O. The TFSI components in the O 1s and S 2p regions are clearly shifted by 0.6 eV to higher BE, now appearing at 533.3 eV and 169.6 eV BE, compared to the reference measurement on pure Cu. The binding energy shift could be explained by a strong interaction of the sulfonyl group with lithium. In addition, both O 1s and S 2p regions show shoulder peaks OQ1 at 532.0 eV BE and SQ1 at 168.0 eV BE, which might be related to decomposed TFSI anions in good agreement to the results shown above.

Discussion The results show reactions of the ionic liquids with lithium, previously deposited on the copper substrate. The substrate might have an influence on the reactions of the ILs with lithium, but no change to the Cu 2p peak was observed (Cu 2p and Li 1s shown in S.I.) for all the experiments. To compare the results for all three ionic liquids Table 1 shows the binding energies and stoichiometric data for the components in the XP spectra shown in Figure 2 - Figure 7. They are separated by their affiliation to the cation structure, anion structure or to decomposition products. Although decomposition of the anion and the formation of LiF and Li2O was observed for all three ionic liquids, the amount of the decomposition products differ a lot. When [Py1,4]FSI is adsorbed on the Li/Cu surface a major amount of LiF can be found, indicating decomposition beginning on the S-F bond of the FSI anion. In addition, different new features were found in the O 1s and S 2p regions, which could be related to residual parts of the decomposed anion. Different components in the S 2p region indicate further reaction to the sulfonyl-group, possibly incorporating reactions with the cation. Reduction of the sulfonyl-group for instance is proved by the presence of Li2O species, which can be figured out in the O 1s spectra. Comparing the stoichiometry between cation and anion in Table 1 the anion with a relative amount of 33% is represented under-stoichiometric on the Li/Cu surface. The products built by reaction of FSI and Lithium altogether account a relative amount of 26%. Strong reactivity of FSI is in good agreement with the results of quantum-chemical calculations by Budi et al., who investigated the exposure of [Py1,3]FSI to a Li(001) metal surface15, and Piper et al. showing decomposition of [Py1,3]FSI and [Py1,3]TFSI on Li13Si4 surfaces6. The calculations showed a rapid break of the S-F bond in FSI by forming LiF and further decomposition of the anion to SO2 and -7ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NSO2, which is reduced by the formation of lithium oxide species. Stepwise reduction of the FSI anion is in good agreement with the multiple peak structure (SQ1, SQ2), which was observed for [Py1,4]FSI on the Li/Cu surface in the XP spectra, shown here. Budi et al. verified their calculations by immersing Li foil for different time periods in [Py1,3]FSI and analyzing the Li surface by XPS.15 The spectra showed good agreement with the predicted interaction mechanism. In the spectra just one S 2p species was observed and assigned to SO2F. The difference to the results, which were shown here, could either be due to the long immersion time or due to the sample preparation of the immersed Lithium foil before analysis. In addition, Budi et al. observed reduction of the cation by loss of Hydrogen, since LiOH species were formed in the interaction. Changes in the N 1s and C 1s spectra in Figure 2 suggest decomposition of the cation, too. Unfortunately those were hard to evaluate as possible decomposition products might not be distinguishable from the original peaks, especially not in the C 1s spectrum. However, the relative increase of C-C like bonds (C1 component) while the amount of C-N like bonds (C2 component) decreases and a new peak in the N 1s region appears, which could be explained by decomposition of [Py1,4] cation, presumably after loss of Hydrogen. When using [Py1,4]TFSI instead, much less amount of decomposition products were detected, like shown in Table 1, although components of reaction products with Lithium (LiF and Li2O) dominate the observed changes in the spectra. In addition, structures NQ1, SQ1 and OQ1 on the low binding energy side of the anion peaks can be observed for [Py1,4]TFSI, in good agreement with [Py1,4]FSI. Comparing the experiments for both ILs adsorbed on Li/Cu indicates enhanced stability of the TFSI anion structure in combination with the same cation. However, the decomposition products observed in the XP spectra are quite similar. Regarding to literature there are two possible mechanisms for the decomposition of TFSI-. Quantum mechanical calculations38 for thermal decomposition of TFSI showed detachment of SO2 and existence of stable CF3-N-SO2CF3-, while in a pulse radiolysis study, break of the N-S bond is found to be the first decomposition step in case of electron impact39. Both studies give a good impression of possible degradation pathways, which might be the initial step in the reaction with lithium. In contrary to [Py1,4]FSI adsorbed on Li/Cu, cation decomposition is not observed in the XP spectra in the case of [Py1,4]TFSI. But when comparing the peak area of the cation components in C 1s and N 1s spectra to the peak area of the anion components in Figure 4 and Table 1 an understoichiometric representation of [Py1,4] cation is observed. This could be explained by desorption of cation molecules, or its fragments, to the vacuum.


Page 8 of 19

Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Possibly Lithium of the Li/Cu surface is solvated as Li+ in a charge transfer process and partially replaces the cation, for instance by forming [Li(TFSI)2]- complexes40,41. Complex formation would be possible for the FSI- anion, too. But following the calculations of Piper et al. a spontaneous break of the S-F bond in the FSI- anion is expected in presence of a lithiated anode surface, whereas TFSI- does not show bond breaking reactions.6 Markevich et al. cycled LiTFSI / [Py1,4]TFSI in a graphite/Li cell setup and analyzed both the gaseous phase, as well as the liquid electrolyte.40 By NMR and FTIR experiments in the electrolyte they could confirm an enrichment of Lithium ions and enhanced formation of a [Li(TFSI)2]- complex on the electrode surface. Interaction of [Py1,4]TFSI with coadsorbed lithium on a Cu (111) surface at different temperatures has been investigated in a XPS, FTIR and STM study by Buchner et al., before.22 Even at a temperature of 80 K decomposition of the anion could be proven by FTIR and XPS measurements. Additionally, decomposition of [Py1,4] cation and formation of LixCHy and Li3N was shown. Carbon-containing species were found to desorb at temperatures ≥ 300K, which could explain the results obtained in the present work. Decomposition and desorption of cation components (mainly Carbon) was observed with a comparable setup using non-monochromatic X-ray radiation, possibly involving beam damage effects, for [OMIm]TFSI adsorbed on a Li/Cu surface in a previous study.25 Moreover new components in N 1s and C 1s spectra were found, which could be assigned to Li-N and Li-C bonds. As this early study is in good agreement with the results presented here, beam damage is not the driving mechanism behind decomposition and desorption of [OMIm]+, when TFSI- is used as anion. Like shown in Table 1 most of [OMIm] cations were decomposed or desorbed in the interaction with lithium. A shift in the components for TFSI in S 2p and O 1s spectra is observed and indicates interaction of the sulfonyl group with Lithium. Thus, the decomposition/desorption of [OMIm] cations could be driven by formation of lithium salts like already suggested for [Py1,4]TFSI. Concluding, charge transfer from the Li/Cu surface to the ionic liquid might be the initial step of the interaction. This is in good agreement with recently published molecular dynamic simulations of Ethylene Carbonate (EC) molecules in interaction with Lithium.42 The authors observed a charge transfer from Lithium to the organic solvent, followed by ring breakup of ethylene carbonate. Differences in Lithium solvation and in reaction stability seem to have a great influence on differences in the SEI layer formation of Lithium ion battery systems using either FSI- or TFSI-. Thus, the interaction of IL and Lithium can’t be discussed independently in terms of anion or cation structure.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Surface reactions of the three ionic liquids after physical vapor deposition on a Li/Cu-surface were discussed, here. Most prominent changes were observed for [OMIm]TFSI. In order to check the stability of bulk IL against Lithium, Raman experiments (spectra shown in S.I.) were performed after the immersion of a piece of lithium foil in [OMIm]TFSI. Decay of the Raman intensity with rising immersion time was observed for the fingerprint region (2750 cm-1–3250 cm-1) of the [OMIm] cation structure as well as for the anion features. For example, the most prominent Raman peak for TFSA occurs at 742 cm-1 which clearly shows a decrease in intensity after 4 hours (fig S4a). Fig S4b shows the changes in the Raman intensity in the anion and cation species from which it is evident that a changes in slope occurs after 1 hour. This implies that in the first 1 hour, there is a significant reaction between Li metal and [OMIm]TFSI which forms the solid-electrolyte interphase (SEI) layer. After 1 hour, it appears that the reaction of Li with IL slows down considerably indicating a stable SEI layer is formed. Thus, reactions at the Li/IL-interface take place for bulk IL, too. The reaction products most probably passivate the Lithium surface, which is in good agreement with saturation of the effects after 1 h as well as absence of new structures or shoulders in the Raman spectra.

Conclusion In a comparative study interactions of [Py1,4]FSI, [Py1,4]TFSI and [OMIm]TFSI with Li/Cu surfaces were investigated. Photoelectron spectra of all the ionic liquids after deposition show contributions of LiF and Li2O, thus prove decomposition of FSI- and TFSI- anion. In addition, new components in S 2p, O 1s and N 1s spectra prove existence of further decomposition products. According to the amount of those species, TFSI- was found to be much more stable than FSI- (in relation to a [Py1,4] cation), although no clear difference in the mechanism of anion decomposition could be observed, here. Decomposition of the cation was proven indirectly by change of the components in photoelectron spectra of [Py1,4]FSI and [OMIm]TFSI. Especially [OMIm]+ is subject to complete decomposition and desorption and exhibits the formation of Li-N and Li-C bonds as remaining species. Evaluating the relative amount of cations and anions showed understoichiometric representation of the cation for [Py1,4]TFSI and [OMIm]TFSI, while in case of [Py1,4]FSI less amount of the FSI anion is observed. This indicates, that the much more stable TFSI anion coordinates with partially dissolved Li or builds stable lithium salts at the surface. Supporting Information Supporting Information with additional spectra was added to the manuscript. The XP survey spectrum of the freshly prepared Lithium/Copper sample is attached as well as Li 1s and Cu 2p spectra before and after physical vapor deposition of the Ionic Liquids. Also Raman spectra of Li in [OMIm]TFSI is shown.

- 10 ACS Paragon Plus Environment

Page 10 of 19

Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acknowledgements The authors like to thank Prof. Dr. Krischok from TU Ilmenau for the valuable discussions on the experimental results. We gratefully acknowledge financial support from the German Research Foundation (DFG: EN370/26-1 and INST 189176-1 FOGG)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 19

(Tables) Table 1: Binding energies, stoichiometric data and group stoichiometry grouped by affiliation to “Cation, Anion, Others” for all three ionic liquids adsorbed on the Li/Cu surface. Binding energies for Cu reference is given in brackets. .……..…. [Py1,4]FSI …...….. Peak

Cation

BE / eV

.……... [Py1,4]TFSI ……..….

at.% SUM %

BE / eV

.……. [OMIm]TFSI ….…….

at.% SUM %

285.6 (285.6)

52%

C 1s (C2)

287.0 (286.9)

39%

N 1s

402.9 (402.8)

9%

402.8 (402.7)

9%

S 2p

170.1 (170.1)

23%

169.2 (169.1)

15%

169.6 (169.0)

10%

293.1 (293.0)

13%

293.0 (293.0)

14%

399.6 (399.4)

7% 25%

C 1s Anion

33%

286.8 (286.8)

43%

43%

284.8 (285.0)

84%

286.6 (286.8)

16%

-

400.0 (399.9)

11%

O 1s

533.2 (533.2)

42%

532.9 (532.8)

F 1s

688.0 (688.0)

24%

689.0 (689.0)

SQ1

169.1

4%

167.6

SQ2

166.7

12%

-

-

SQ3

164.8

3%

-

-

-

282.3

7%

397.3

9%

6%

26%

397.3

50%

40% 20%

6%

7%

399.7 (399.6)

8%

533.3 (532.8)

21%

688.9 (688.9) 168.0

47% 9%

NQ1

398.2

NQ2

396.3

5%

-

OQ1

531.4

30%

531.6

24%

532.0

21%

Li2O

527.9

9%

528.0

10%

528.2

18%

LiF

684.7

31%

685.0

40%

685.0

36%

(Figures)

Figure 1: Cation and anion structures of the ionic liquids [Py1,4]FSI, [Py1,4]TFSI and [OMIm]TFSI


2%

(402.1)

N 1s

LixC Others

-

49%

at.% SUM %

C 1s (C1)

41%

285.5 (285.5)

BE / eV

71%

27%

Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: XPS C 1s and N 1s detail spectra of [Py1,4]FSI adsorbed on Li/Cu and on clean Cu in comparison

Figure 3: XPS F 1s, O 1s and S 2p detail spectra of [Py1,4]FSI adsorbed on Li/Cu and on clean Cu in comparison



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4: XPS C 1s and N 1s detail spectra of [Py1,4]TFSI adsorbed on Li/Cu and on clean Cu in comparison

Figure 5: XPS F 1s, O 1s and S 2p detail spectra of [Py1,4]TFSI adsorbed on Li/Cu and on clean Cu in comparison


Page 14 of 19

Page 15 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: XPS C 1s and N 1s detail spectra of [OMIm]TFSI adsorbed on Li/Cu and on clean Cu in comparison

Figure 7: XPS F 1s, O 1s and S 2p detail spectra of [OMIm]TFSI adsorbed on Li/Cu and on clean Cu in comparison



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

References (1) Binnemans, K. Ionic Liquid Crystals. Chem. Rev. 2005, 105, 4148–4204. (2) Pinilla, C.; Del Pópolo, M. G.; Lynden-Bell, R. M.; Kohanoff, J. Structure and Dynamics of a Confined Ionic Liquid. Topics of Relevance to Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 17922–17927. (3) Endres, F.; Zein El Abedin, S. Air and Water Stable Ionic Liquids in Physical Chemistry. Phys. Chem. Chem. Phys. 2006, 8, 2101–2116. (4) Lewandowski, A.; Świderska-Mocek, A. Ionic liquids as Electrolytes for Li-Ion Batteries—An Overview of Electrochemical Studies. J. Power Sources 2009, 194, 601–609. (5) Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Wei, F.; Zhang, J.-G.; Zhang, Q. A Review of Solid Electrolyte Interphases on Lithium Metal Anode. Adv. Sci. 2016, 3, 1500213. (6) Piper, D. M.; Evans, T.; Leung, K.; Watkins, T.; Olson, J.; Kim, S. C.; Han, S. S.; Bhat, V.; Oh, K. H.; Buttry, D. A.; et al. Stable Silicon-Ionic Liquid Interface for Next-Generation Lithium-Ion Batteries. Nat Comms 2015, 6, 6230. (7) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid Materials for the Electrochemical Challenges of the Future. Nat Mater 2009, 8, 621–629. (8) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Usami, A.; Mita, Y.; Kihira, N.; Watanabe, M.; Terada, N. Lithium Secondary Batteries Using Modified-Imidazolium Room-Temperature Ionic Liquid. J. Phys. Chem. B 2006, 110, 10228–10230. (9) Koch, V. R.; Battista, G.; Scrosati, B. The Interfacial Stability of Li with Two New Solvent-Free Ionic Liquids: 1,2-Dimethyl-3-propylimidazolium Imide and Methide. J. Electrochem. Soc. 1995, 142, L116. (10) Bazito, F. F.; Kawano, Y.; Torresi, R. M. Synthesis and Characterization of Two Ionic Liquids with Emphasis on Their Chemical Stability Towards Metallic Lithium. Electrochim. Acta 2007, 52, 6427–6437. (11) Zhao, L.; Yamaki, J.-i.; Egashira, M. Analysis of SEI Formed with Cyano-Containing Imidazolium-Based Ionic Liquid Electrolyte in Lithium Secondary Batteries. J. Power Sources 2007, 174, 352–358. (12) Dedryvère, R.; Leroy, S.; Martinez, H.; Blanchard, F.; Lemordant, D.; Gonbeau, D. XPS Valence Characterization of Lithium Salts as a Tool to Study Electrode/Electrolyte Interfaces of Li-Ion Batteries. J. Phys. Chem. B 2006, 110, 12986–12992. (13) Valencia, H.; Kohyama, M.; Tanaka, S.; Matsumoto, H. Ab Initio Study of EMIM-BF4 Molecule Adsorption on Li Surfaces as a Model For Ionic Liquid/Li interfaces in Li-ion Batteries. Phys. Rev. B 2008, 78. (14) Valencia, H.; Kohyama, M.; Tanaka, S.; Matsumoto, H. Ab Initio Study of EMIM-BF4 Crystal Interaction With a Li (100) Surface As a Model for Ionic Liquid/Li Interfaces in Li-Ion Batteries. J. Chem. Phys. 2009, 131, 244705. (15) Budi, A.; Basile, A.; Opletal, G.; Hollenkamp, A. F.; Best, A. S.; Rees, R. J.; Bhatt, A. I.; O’Mullane, A. P.; Russo, S. P. Study of the Initial Stage of Solid Electrolyte Interphase Formation upon Chemical Reaction of Lithium Metal and N -Methyl- N -Propyl-PyrrolidiniumBis(Fluorosulfonyl)Imide. J. Phys. Chem. C 2012, 116, 19789–19797. (16) Cremer, T.; Stark, M.; Deyko, A.; Steinrück, H.-P.; Maier, F. Liquid/Solid Interface of Ultrathin Ionic Liquid Films: [C1C1Im][Tf2N] and [C8C1Im][Tf2N] on Au(111). Langmuir 2011, 27, 3662–3671. (17) Cremer, T.; Wibmer, L.; Calderón, S. K.; Deyko, A.; Maier, F.; Steinrück, H.-P. Interfaces of Ionic Liquids and Transition Metal Surfaces-Adsorption, Growth, and Thermal Reactions of Ultrathin [C1C1Im][Tf2N] Films on Metallic and Oxidized Ni(111) Surfaces. Phys. Chem. Chem. Phys. 2012, 14 (15), 5153–5163. - 16 ACS Paragon Plus Environment

Page 16 of 19

Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Uhl, B.; Buchner, F.; Gabler, S.; Bozorgchenani, M.; Jürgen Behm, R. Adsorption and Reaction of Sub-Monolayer Films of an Ionic Liquid on Cu(111). Chem. Commun. (Camb.) 2014, 50, 8601– 8604. (19) Foulston, R.; Gangopadhyay, S.; Chiutu, C.; Moriarty, P.; Jones, R. G. Mono- and Multi-Layer Adsorption of an Ionic Liquid on Au(110). Phys. Chem. Chem. Phys. 2012, 14 (17), 6054–6066. (20) Uhl, B.; Cremer, T.; Roos, M.; Maier, F.; Steinrück, H.-P.; Behm, R. J. At the Ionic liquid | Metal Interface: Structure Formation and Temperature Dependent Behavior of an Ionic Liquid adlayer on Au(111). Phys. Chem. Chem. Phys. 2013, 15, 17295–17302. (21) Uhl, B.; Buchner, F.; Alwast, D.; Wagner, N.; Behm, R. J. Adsorption of the Ionic Liquid [BMP]TFSA on Au(111) and Ag(111): Substrate Effects on the Structure Formation Investigated by STM. Beilstein journal of nanotechnology 2013, 4, 903–918. (22) Buchner, F.; Bozorgchenani, M.; Uhl, B.; Farkhondeh, H.; Bansmann, J.; Behm, R. J. Reactive Interaction of (Sub-)Monolayers and Multilayers of the Ionic Liquid 1-Butyl-1-methylpyrrolidinium Bis(trifluoro-methylsulfonyl)imide with Coadsorbed Lithium on Cu(111). J. Phys. Chem. C 2015, 119 (29), 16649–16659. (23) Uhl, B.; Hekmatfar, M.; Buchner, F.; Behm, R. J. Interaction of the Ionic Liquid [BMP][TFSA] with rutile TiO2 (110) and coadsorbed lithium. Phys. Chem. Chem. Phys. 2016, 18 (9), 6618–6636. (24) Howlett, P. C.; Izgorodina, E. I.; Forsyth, M.; MacFarlane, D. R. Electrochemistry at Negative Potentials in Bis(trifluoromethanesulfonyl)amide Ionic Liquids. Zeitschrift für Physikalische Chemie 2006, 220, 1483–1498. (25) Olschewski, M.; Gustus, R.; Marschewski, M.; Höfft, O.; Endres, F. Spectroscopic Characterization of the Interaction of Lithium With Thin Films of the ionic Liquid 1-Octyl-3methylimidazolium bis(trifluoromethylsulfonyl)amide. Phys. Chem. Chem. Phys. 2014, 16, 25969– 25977. (26) Keppler, A.; Himmerlich, M.; Ikari, T.; Marschewski, M.; Pachomow, E.; Höfft, O.; MausFriedrichs, W.; Endres, F.; Krischok, S. Changes of the Near-Surface Chemical Composition of the 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide Room Temperature Ionic Liquid Under the Influence of Irradiation. Phys. Chem. Chem. Phys. 2010, 13, 1174–1181. (27) Deyko, A.; Lovelock, K. R. J.; Licence, P.; Jones, R. G. The Vapour of Imidazolium-Based Ionic Liquids: A Mass Spectrometry Study. Phys. Chem. Chem. Phys. 2011, 13, 16841–16850. (28) Zaitsau, D. H.; Kabo, G. J.; Strechan, A. A.; Paulechka, Y. U.; Tschersich, A.; Verevkin, S. P.; Heintz, A. Experimental Vapor Pressures of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides and a Correlation Scheme For Estimation of Vaporization Enthalpies of Ionic Liquids. J. Phys. Chem. A 2006, 110, 7303–7306. (29) Scofield, J. H. Hartree-Slater Subshell Photoionization Cross-Sections at 1254 and 1487 eV. J. Electron. Spectrosc. Relat. Phenom. 1976, 8, 129–137. (30) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1 ⩽ Z ⩽ 103. Atomic Data and Nuclear Data Tables 1985, 32, 1–155. (31) Men, S.; Lovelock, K. R. J.; Licence, P. X-Ray Photoelectron Spectroscopy of PyrrolidiniumBased Ionic Liquids: Cation–Anion Interactions and a Comparison to Imidazolium-based analogues. Phys. Chem. Chem. Phys. 2011, 13, 15244–15255. (32) Sharma, J.; Gora, T.; Rimstidt, J. D.; Staley, R. X-ray Photoelectron Spectra of the Alkali Azides. Chemical Physics Letters 1972, 15, 232–235. (33) Poole, R. T.; Jenkin, J. G.; Liesegang, J.; Leckey, R. C. G. Electronic Band Structure of the Alkali Halides. Phys. Rev. B 1975, 11, 5179–5189. (34) Dupin, J.-C.; Gonbeau, D.; Vinatier, P.; Levasseur, A. Systematic XPS Studies of Metal Oxides, Hydroxides and Peroxides. Phys. Chem. Chem. Phys. 2000, 2, 1319–1324. - 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(35) Wu, Q.-H.; Thissen, A.; Jaegermann, W. Photoelectron Spectroscopic Study of Li Oxides on Li Over-Deposited V2O5 Thin Film Surfaces. Appl. Surf. Sci. 2005, 250, 57–62. (36) Carley, A. F.; Jackson, S. D.; O'Shea, J. N.; Roberts, M. W. Oxidation States at Alkali-MetalDoped Ni(110)–O Surfaces. Phys. Chem. Chem. Phys. 2001, 3, 274–281. (37) Kanamura, K. XPS Analysis of Lithium Surfaces Following Immersion in Various Solvents Containing LiBF4. J. Electrochem. Soc. 1995, 142, 340. (38) Kroon, M. C.; Buijs, W.; Peters, C. J.; Witkamp, G.-J. Quantum Chemical Aided Prediction of the Thermal Decomposition Mechanisms and Temperatures of Ionic Liquids. Thermochimica Acta 2007, 465, 40–47. (39) Wishart, J. F.; Shkrob, I. A.; Chemerisov, S. D. The Initial Stages of Radiation Damage in Ionic Liquids and Ionic Liquid-Based Extraction Systems. J. Phys. Chem. B 2007, 111, 11786–11793. (40) Markevich, E.; Sharabi, R.; Borgel, V.; Gottlieb, H.; Salitra, G.; Aurbach, D.; Semrau, G.; Schmidt, M. A. In Situ FTIR Study of the Decomposition of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide Ionic Liquid During Cathodic Polarization of Lithium and Graphite Electrodes. Electrochimica Acta 2010, 55, 2687–2696. (41) Lahiri, A.; Carstens, T.; Atkin, R.; Borisenko, N.; Endres, F. In Situ Atomic Force Microscopic Studies of the Interfacial Multilayer Nanostructure of LiTFSI–[Py1,4]TFSI on Au(111): Influence of Li+ Ion Concentration on the Au(111)/IL Interface. J. Phys. Chem. C 2015, 119, 16734–16742. (42) Ismail, I.; Noda, A.; Nishimoto, A.; Watanabe, M. XPS Study of Lithium Surface after Contact with Lithium-Salt Doped Polymer Electrolytes. Electrochim. Acta 2001, 46, 1595–1603.


Page 18 of 19

Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic


Monochromatic X-ray Photoelectron Spectroscopy ... - ACS Publications

Recommend Documents