imide, and Hexafluorophosphate - American Chemical Society

Jan 27, 2017 - imide (FSI) anions is studied under XPS measurements conditions. TFSI and FSI based salts are used extensively, and almost exclusively,...
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X‑ray Photodecomposition of Bis(trifluoromethanesulfonyl)imide, Bis(fluorosulfonyl)imide, and Hexafluorophosphate Ivgeni Shterenberg, Michael Salama, Yosef Gofer, and Doron Aurbach* Department of Chemistry, Bar-Ilan University, Ramat Gan 5290002, Israel ABSTRACT: X-ray photoelectron spectroscopy is one of the workhorses in today’s battery analysis and research. The potential of X-ray radiation damage and degradation during measurements with regularly used salts and organic compounds are often overlooked. In this study, we show that, under common analysis conditions, the exiting X-ray radiation (1468.6 eV) during the XPS analysis does have significant effect on some Mg and Li salts based on TFSI, FSI, and PF6 anions. In all cases, we show that the salts undergo significant photodegradation during the XPS measurements. With XPS, the photodegradation is detected as the solid degradation products remaining on the sample holder, and they are clearly identified by formation of new peaks at lower binding energies for the relevant elements. We were also able to show that in some cases, as expected, some gaseous byproducts evolve during the photodegradation process.

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INTRODUCTION X-ray photoelectron spectroscopy, XPS, is one of the most powerful spectroscopic tools for chemical surface analysis. It is applied in many disciplines where surface chemistry is of interest, like in the study of heterogeneous catalysis, interfacial electrochemistry, electronics, etc. It is also very frequently used in battery research, where anodes, cathodes, and current collector’s surface chemistries are characterized as a function of battery state and history.1−6 Although widely used, the stability of the studied species under X-ray radiation, especially organic materials, is often overlooked. In theory, X-ray radiation’s photons possess energy that is orders of magnitude more than any chemical bond, in particular organic bonds, and therefore many inorganic compounds and all organic compounds are presumed to be susceptible to decomposition during XPS measurements.7−9 Although valence and core electrons are emitted as a consequence of X-ray photon absorption, various rapid relaxation mechanisms act statistically and keep the molecules intact for the measurement time. In practice, thus, most organic compounds, and even biological ones, are sufficiently stable under X-ray radiation for the duration of the measurement. Nevertheless, the literature contains numerous studies where materials had been shown to be unstable under XPS measurements conditions, inflicting inaccurate analysis and, in some cases, damage to the apparatus due to excessive degassing. In this work, the stability of salts based on bis(trifluoromethylsulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI) anions is studied under XPS measurements conditions. TFSI and FSI based salts are used extensively, and almost exclusively, in Li and Mg secondary battery research.10−14 These anions hold many desired properties required for high energy storage devices, the most important of © 2017 American Chemical Society

which is their high resistance toward electrochemical and chemical reduction and oxidation. Mg/Li−TFSI/FSI salts are readily soluble in many aprotic organic solvents, with high dissociation constant even in ethereal solvents and form highly conductive electrolyte solutions.11 Ether based electrolyte solutions are commonly used in cases where a highly stable electrolyte is required in the low potential domain. Li−TFSI/ FSI salts are also extensively used in Li−air systems where a highly reduction medium is necessary in order to inhibit nucleophilic attacks by the superoxides and peroxides formed during the cell’s performance.13 It was established that Mg2+ ions (unlike Li ions) cannot pass through passivation layers.15,16 Thus, in Mg systems, the electrolyte solution must be sufficiently stable with the Mg anode (and also with the cathode) to maintain surface film-free conditions, allowing the transfer of Mg ions to and from the solution. Ethers (mostly THF and glymes) are, so far, the only solvents that conform to this requirement, and thus are important for Mg secondary systems. At the same token, MgTFSI2 is the only readily available “simple” salt that freely dissolves and dissociates in these low polarity solvents.12 MgTFSI2/glyme electrolytic solutions had shown to support highly reversible electrochemical magnesium deposition and dissolution processes albeit with inferior performance compared to the thus far established electrolytes (based on organometallic complex salts).17,18 The most notable deficit in the electrochemical properties of the MgTFSI2/DME solutions is the particularly high overpotential for both the deposition and the dissolution processes.10,11 This developed overpotential for deposition/ Received: November 16, 2016 Revised: January 26, 2017 Published: January 27, 2017 3744

DOI: 10.1021/acs.jpcc.6b11524 J. Phys. Chem. C 2017, 121, 3744−3751

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All measurements were carried out with electron flood gun for charge neutralization. Base pressure was 1 × 10−9 Torr, which increased over the measurement period up to ca. 5−7 × 10−9 Torr. Data analysis was carried out with Kratos Vision 2.19 In most cases Shirley background was subtracted unless the S/N was too bad and then linear background subtraction was utilized. Quantitative data were based on the manufacturer’s software relative sensitivity data, RSF values. X-axis energy was calibrated vs the lowest C 1s peak or fitted peak at 285 eV. Peak fitting was done with the same software. Lorentian: Gaussian, 30:70 line shapes were utilized. Peak heights, widths, and positions were set as free parameters (with upper and lower limits for the peak widths were set within common values). For S 2p, literature value for spin−orbit peak separations, 2.2 eV, and relative peak integration, 2:1, were used. NIST X-ray photoelectron spectroscopy database was used. Minimum number of peaks (or doublets) was selected to be representative of “best” fitting. In cases where smeared features or very badly resolved peaks were found, a very wide peak was fitted as a “good enough” fit. Since the beam size, the total X-ray flux and the flux per area were not constant and changes slightly from sample to sample, only relative changes in the surface chemistry vs time are relevant. Degradation rates are not obtainable for the current study. On the other hand, the qualitative chemical degradation process, namely, the evolved species, features are considered as accurate.

dissolution is often attributed to formation of semipassivating layers. In order to study the chemistry of the hypothesized precipitated layers we, naturally, selected to run a full surface analysis on the anode during the cell performance with the XPS as our main tool. Following a thorough time-dependent study, we have determined that the MgTFSI2 salt, as well as its possible reductive precipitation layers on Mg anodes, is unstable under X-ray radiation and cannot be adequately studied by XPS. In this paper we show that when exposed to Xray, during an XPS measurement of the MgTFSI2 salt, new reduced species are slowly formed on the substrate. Additionally, we will show that the total coverage of the salt is slowly reduced during the measurement, indicating the formation of gaseous byproducts. We have extended our analysis to the LiTFSI, LiFSI, and LiPF6 salts, as those are much more commonly used. In all cases, LiTFSI, LiFSI, and LiPF6 were found to decompose under X-ray in a similar manner to that of MgTFSI2. This study is of importance to researches that use high exiting energy spectroscopic techniques to perform surface analysis in chemical systems containing TFSI/FSI/PF6 salts.

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EXPERIMENTAL SECTION Sample Preparation. Pure powders, MgTFSI2/LiTFSI/ LiFSI, were sprinkled on a Cu-backed, UHV compatible selfadhesive foil. The powder spread with a spatula to homogeneously cover the adhesive side with a very thin, pinhole-free layer. Excess powder was removed by knocking on the back of the foil. Thin layer samples were prepared by dripping a (0.05M) MgTFSI2/LiTFSI/LiFSI solution on the substrate and drying overnight. Solutions were prepared by stirring predetermined amounts of MgTFSI2/LiTFSI/FSI in DME for 6h at 70C0. LiTFSI (99.95%), LiFSI (99.9%), and DME were purchased from Sigma-Aldrich. MgTFSI2 (99.95%) was purchased from Solvionic. Cu conducting tape, code 1181, purchased from “spi supplies”. All samples had been prepared, manipulated and loaded onto the XPS sample holder under strictly pure argon atmosphere. Normal oxygen and humidity levels were below detection level (namely, 1 ppm). The MBraun glovebox had been also equipped with solvent removal absorber unit, and the glovebox atmosphere was not exposed to any other solvent. All samples had been transferred to the XPS spectrometer with especially constructed transfer device without exposure to ambient atmosphere. XPS Analysis. XPS analysis was carried out with Kratos Axis HS spectrometer, with monochromatized Al Kα radiation (1468.6 eV). The spectrometer energy scale is three point calibrated (Au 4f, Ag 3d, and Cu 2p). Survey spectra and high resolution spectra were carried out at 80 and 40 eV pass energy. Most measurements were performed at X-ray power of 150 W, unless otherwise indicated. In all cases, one or two iterative spectra were first acquired for S 2p and F 1s at 75 W in order to obtain spectrum for these atoms at time as close to zero as possible. The rest of the measurements were performed by consecutive measurements of 3−10 spectra (as needed for best S/N ratio) for all the relevant atoms. The X-ray source was held on at all times, including the wait time between measurements. The beam size was roughly 4 × 5 mm, and the actual X-ray flux was not completely constant during the whole measurement and irradiation sequence. Normal time for one sequence of measurement was ca. 60 min. Common time for a full set of time dependent measurement was 7−11 h. All measurements had been carried out under ambient temperature of ca. 25 C.

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RESULTS Decomposition of MgTFSI2. First, a pure MgTFSI2 powder was measured by XPS. Spectra were acquired for C 1s, S 2p, F 1s, O 1s, N 1s, and Mg 2p. Apart from minute intensity changes, no noticeable differences had been observed for the Mg 2p, C 1s, O 1s and N 1s spectra versus irradiation time. On the other hand, significant changes in the shape of the spectra had been observed for the S 2p and F 1s. Also some, but insignificant changes had been noticed as to their peak height. Figure 1 shows the XPS spectra for sulfur, S 2p, and fluorine, F 1s, for three consecutive measurements performed on the MgTFSI2 powder sample at ca. 3 h time intervals. An S 2p spectrum was also acquired at low X-ray source power and short duration. As can be seen, new peaks are emerging at binding energies of approximately 167 and 685 eV, related to new S and F species, respectively. The emergence, and subsequent intensity increase of new peaks at binding energies lower than the initial ones indicates that new, reduced species is/are formed on the surface. The formation of the new peaks is dependent only on exposure to X-ray influx and not on environmental factors, such as ultrahigh vacuum, or aging. To verify this, identical samples were left under UHV for long periods of time, in excess of 56 h, with the X-ray off. These samples have shown the same spectra for all elements, including F and S at time zero. Next, we analyzed in a similar manner greatly thinner samples by spreading MgTFSI2 solution in dimethoxyethane, DME, on a selected substrate, and drying under vacuum. This procedure yields a very thin layer of the MgTFSI2-3DME adduct. Mg forms a very stable complex with DME (MgTFSI-3DME) that is UHV stable.12 We opt to use this sample preparation procedure as it produces a very uniform and nanometer scale thick layer on solid substrates. Also, since MgTFSI2 is very frequently employed as a solution with DME it would make 3745

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Figure 1. XPS spectra of F 1s (a−d) and S 2p (e−h) from MgTFSI2 powder, collected at 3 h intervals.

Figure 2. XPS spectra of F 1s (a−c) and S 2p (d−f) from MgTFSI2/ DME on a Mg substrate, collected at 3 h intervals.

perfect sense to perform the measurements with DME. DME, in any case, has very little effect on the poorly coordinating TFSI− anion.12 The main interaction is between the cation, in this case Mg, and the DME. For the thin MgTFSI2/DME samples we’ve used 2 different substrates, Mg and Au. Mg was chosen since, once again, it is one of the most important anode materials in current battery studies and is often the focal point of surface analysis research. Despite the known reactivity of metallic magnesium, this substrate can be considered, in the current study, as completely inert. Except under very special conditions (not met here) magnesium is covered with a very stable, homogeneous, thin passivating oxide layer.18,20 This passivation layer is not attacked or penetrated by the exceedingly pure and dry MgTFSI2/DME solution when applied under ultrapure, argon atmosphere, glovebox. With these thin MgTFSI2/DME samples we observe similar results (Figure 2) as with the MgTFSI2 pure powder: when exposed to X-ray, over the course of minutes, and then hours, new peaks emerged at 167 and 685 eV for S 2p and F 1s correspondingly. Here the changes in the intensity between the new evolving peaks and the original ones are much more pronounced due to the much smaller sample thickness. Moreover, since the substrate’s XPS peaks are also detected we can obtain a better relative time-dependent quantitative analysis using the substrate as a reference. Table 1 presents the relative atomic concentration of each element during the consecutive XPS measurements vs irradiation time. As the measurement progresses the total concentration of S F and N decreases in favor of Mg. This means that at least part of the TFSI decomposes into gaseous products during X-ray exposure. The evolution of the new, lower BE F 1s and S 2p peaks, in a similar manner to that with pure MgTFSI2 powder, indicates that the photodegradation follows similar pathways. In both cases some of the degradation products change their oxidation

Table 1. Relative Atomic Concentrations (in %) Obtained during X-ray Exposure at 3 h Intervals, for MgTFSI2/DME on Mg F O N C Cl S Mg

at 0 h

after 3 h

after 6 h

7.04 32.24 1.87 34.35 0.57 2.59 21.34

5.14 33.38 1.35 32.48 0.69 2.1 24.86

3.66 33.15 1.24 32.45 0.63 1.91 26.98

state and remain on the substrate. In a separate experiment we verified that none of the surface films evaporate under UHV (under the same conditions, apart from the X-ray irradiation). The atomic concentration of all elements does not change under UHV conditions only (X-ray source off). After establishing the MgTFSI2 photodecomposition using Mg substrates, we performed the same measurements on a more standard substrate, Au, to mitigate any concerns that the MgTFSI2 decomposition is a surface reaction, related to interactions with the Mg. The XPS spectra and the atomic concentration data for the MgTFSI2/DME on Au is presented in Figure 3 and Table 2 respectively. The sample preparation and XPS measurements procedures were performed in a similar manner to that in previous case. For fluorine, a new peak emerges at approximately 685 eV, like the results obtained using the Mg substrate. However, the sulfur spectra show different results; a new peak still emerges at 167 eV, but there is an additional strong peak that emerges at 162 eV. This peak is consistent with sulfur bonded to Au. It can also be related to elemental sulfur and several forms of organic 3746

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measurements with LiTFSI, an important electrolyte in current Li batteries studies.13 The samples preparation, transfer and XPS measurements had been carried in the same way as in the previous section, with MgTFSI2. Figures 4 and 5 show the time

Figure 3. XPS spectra of F 1s (a−c) and S 2p (d−f) from MgTFSI2/ DME on a Au substrate, collected at 3 h intervals.

Table 2. Relative Atomic Concentrations (in %) Obtained during X-ray Exposure at 3 h Intervals, for MgTFSI2/DME on Au F O N C S Au

at 0 h

after 3 h

after 6 h

8.5 12.71 3.87 41.84 5.07 28.11

5.42 10.97 3.75 42.03 3.77 34.05

4.5 9.87 2.47 42.51 3.21 37.43

Figure 4. XPS spectra of F 1s (a−c) and S 2p (d−f) from LiTFSI powder, collected at 3 h intervals.

dependent XPS spectra for sulfur, S 2p, and fluorine, F 1s, for a pure LiTFSI powder and LiTFSI/DME on Au, respectively. The results are very similar to the ones obtained for MgTFSI2, and lead to the same conclusions. The TFSI anion decomposes into reduced species represented by new peaks emerging at lower BE energies for fluorine and sulfur compared with the initial ones. In the LiTFSI/DME thin sample, the relative atomic concentrations for sulfur, nitrogen, and fluorine decrease with irradiation time while the Au atomic concentration increases (Table 3), indicating the formation of gaseous products. At this point we can assert that the decomposition nature of the TFSI anion under X-ray radiation is independent of the cation and would most likely decompose in any salt combination. This is not to say that the reactions are kinetically identical, and there might be difference in the rate of the decomposition. Decomposition of LiFSI. We have extended our study also to the FSI anion which has a similar molecular structure to TFSI and has similar applications, primarily in the Li−air battery research.14 Currently, there is no commercial source for MgFSI2, and its electrochemistry is also not under study as far as we know. All sample preparations, transfer and spectra acquisitions were performed similarly to the previous experiments. As in the previous cases, we focused on the sulfur and fluorine spectra, as they carried all the information regarding the material’s stability. Spectra for sulfur, S 2p, and fluorine, F 1s, of a powdered and a thin layer LiFSI samples are shown in

sulfides and disulfides, but such separated products are highly unlikely to form and stay on the substrate without strong interaction with the gold substrate. It is well established21,22 that Au forms strong bonds with various sulfur compounds, and thus it is logical that whatever the highly reduced sulfur-based compounds are, they are strongly bonded to the gold. In both cases, the F 1s and S 2p new peaks grow on the expense of the original, intact TFSI one. On the basis of these observations, we postulate that the sulfur containing products indicated by the lower BE in the spectrum are bonded to the Au surface. The atomic concentration table shows that during the XPS measurement the total fluorine and sulfur concentration decreases in favor of the Au substrate, indicating again that part of the degradation products are gaseous. We have clearly established that MgTFSI2 is not stable under 1.487 keV X-ray irradiation. This means that XPS measurements might not be suitable for studying chemical systems that contain TFSI anions. Next, we have performed analogous experiments with LiTFSI. Decomposition of LiTFSI. In a similar manner to the study with MgTFSI2, we ran a series of time dependent XPS 3747

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Figure 6. XPS spectra of F 1s (a, b) and S 2p (c, d) from LiFSI powder, collected at 6 h intervals.

Figure 5. XPS spectra of F 1s (a−-c) and S 2p (d−f) from LiTFSI/ DME on a Au substrate, collected at 3 h intervals.

Table 3. Relative Atomic Concentrations (in %) Obtained during X-ray Exposure at 3 h Intervals, for LiTFSI/DME on Au F O N C S Au Li

at 0 h

after 3 h

after 6 h

5.6 11.5 2 35 3.9 36.2 5.8

4 9.5 0.9 35.9 2.2 42.3 5.2

3.12 9.39 0.91 31.98 1.62 47.39 5.59

Figure 7. XPS spectra of F 1s (a, b) and S 2p (c, d) from LiFSI/DME on a Au substrate, collected at 6 h intervals.

Figures 6, 7 correspondingly. In both samples, a new fluorine peak emerges at 685 eV during X-ray exposure, similarly to the TFSI based samples. The results differ from TFSI when observing the S 2p spectra. In the powdered sample, no new peaks are detected even after long periods of X-ray exposure and no significant change in the peak shape is detected. In the thin layer sample, there are only very minor changes for the S 2p spectra as a result of long exposure to X-ray. The features are too small to unequivocally determine that new sulfur species form. The relative atomic concentrations for all elements in the LiFSI samples remained constant during Xray exposure, indicating that no, or only negligible gaseous reaction products form. In any case, from the F 2p spectra, it is obvious that the FSI anion is also affected by X-ray radiation, although its photodegradation mechanism differs from that for TFSI. LiPF6. Finally, we examined the stability of Lithium hexafluorophosphate (LiPF6) under X-ray radiation. LiPF6 is a very important salt and is regularly used in Li secondary

batteries. It contains relatively weak F−P bonds that can be prone to X-ray degradation, especially when considering the results obtained in this paper regarding the formation of reduced fluorine species under X-ray. We carried out several experiments with LiPF6 under the same conditions and protocol as for the other salts. Figure 8 shows the F 1s spectra of a LiPF6/DME sample on an Au substrate. Once again, a new peak forms during the XPS measurement at approx 685.25 eV, corresponding to a fluorine−metal bond. The new peak, for the same time of exposure to X-rays, is not as pronounced as in the previous FSI and TFSI based samples. This may be attributed to two chief causes. First, the kinetics of the photodegradation process in PF6 might be intrinsically much slower than in TFSI and FSI based salts. Second, LiPF6 tends to aggregate when dried from the DME solvent, unlike the TFSI and FSI based salts that form a uniform thin layer. In practice, this results in samples that are much thicker (for LiPF6). As we have observed with the former salts, the relative kinetics of the observed 3748

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containing the studied salts. These chemical systems are most frequently studied with relation to electrodes in lithium, magnesium and sodium based electrochemical cells. The aim of most of these studies is to characterize the surface chemistry of the electrodes that had been in contact with organic solventsbased solutions containing the relevant salts. In many instances, the electrodes examined had been also electrochemically manipulated. The aim of these studies is to characterize precipitated solid reaction products between the solution and the electrodes. In such studies, the chemical identity of the reaction product is sought after, as it is expected to correlate with the experimental conditions, and the electrode response. Thus, it is obvious that the experimental conditions for the current study are of outmost importance with regard to their relevance to common XPS analyses studies. This is the very reason that we carefully chose to carry out all measurements at the same spectrometer parameters (X-ray flux, spectral resolution, time intervals, etc.) with two sample types (ultrathin, and thick, when it was possible). The X-ray energy used for all measurements is the most commonly used, Al anode Kα emission at 1468.xx eV. Moreover, we used only monochromated radiation, which, in addition to better spectral resolution, reduces the source flux on the sample by a factor of 100−1000. Additionally, it does not contribute directly to sample heating, and is not accompanied by high electrons flux as unmonochromated sources do. Ideally, the X-ray flux impinging on the sample, or, better off, its integration with time would have been measured. Unfortunately we do not possess this capability. In any case, the X-ray source settings for the long-term experiments were set to medium value of 150 W (15 kV, 10 mA), very normal in common XPS analyses. Each new sample was first swiftly measured with low flux (15 kV, 5m) for the shortest duration possible, usually 60−90 s. The goal of this measurement was to benchmark the sample’s chemical nature before radiation damage commences. Such conditions are by all means not extraordinary. The changes in the spectra of the irradiated samples have been discernible, in some of the cases, after less than 1 h of irradiation (ca. 15−30 min). Moreover, in some of the cases (e.g., thin MgTFSI2 layers) signs for initial photodegradation had been identified even after the short scan at low source power. Unfortunately, these measurements are plagued with low S/N. Looking at the data, one can observe that the greatest factor for rapid degradation, for each anion, is sample thickness. Depending on the salts and solvents, some samples could have been prepared as a very thin, glassy layer (MgTFSI2 in DME). In other cases we managed to deposit extremely thin, adsorption layers (it is not clear whether it is mono or multilayer) (e.g., washed electrode after immersion in MgTFSI2 in DME,). Very thin samples exhibited much faster photodegradation compared with thick ones. Not once, indications for initial degradation have been observed already in the very first scan, at low source power and short duration. We believe that the photodegradation kinetics is not directly influenced by the sample thickness. It is just that the spectrum background is less affected by the bulk material remaining intact with thick samples. Thus, very thin samples only bring about the spectral features of the degraded salt in a much clearer manner. In normal XPS analysis of electrodes from electrochemical systems, the amount of salt residue and traces of electrodegraded salts are usually of minute quantities. Only seldom these will constitute the major component of the sample’s

Figure 8. XPS spectra of F 1s LiPF6/DME on a Au substrate, collected at 3 h intervals.

photodegradation is, among other parameters, very dependent on the sample’s thickness. As a rule of thumb, the thinner the analyzed samples, the faster and more dramatic the identified degradation features are observed. The distinctive XPS peaks for all other elements showed no discernible changes in their shape. Since the degradation process is less pronounced than in the previous samples, we were not able to determine whether the photodegradation is accompanied by gas evolution. It is reasonable to assume, however, that gaseous products do form since gaseous byproducts such as, POF3, HF and particularly PF3 and PF5 tend to form even under inert environments.

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DISCUSSION One of the key questions vis-à-vis this study is its relevance to “common” XPS analyses done with chemical systems 3749

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accompanied by S−O, S-CF3, and S−N cleavage to form various products such as Mg−N−SO−CFx or, more reasonable Mg−O−SO or MgSO−CFx. The conclusion is that whenever a researcher analyzes samples containing TFSI, FSI, and PF6 residues, or firms containing reaction products of those, (e.g., electrodes from electrochemical cells) he cannot take it for granted that the analyzed species are stable during measurement. It is the responsibility of the researcher to first verify whether the analyte is stable under the analytical conditions. This conclusion is much more important when very thin films are measured, as the contribution of the photodegradation products to the data may become very significant.

surface. This is why we do believe that in many cases the possibility that such traces may undergo photodegradation during XPS analysis, is high. This, in turn, may lead to unreliable and, thus, irrelevant data, leading to incorrect interpretation. We have good reasons to believe that similar photodegradation processes may also influence XPS analyses of partially electrochemically degraded salts (important part of many SEI layers). This, again, may lead to wrong conclusions regarding the electrochemical processes.

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CONCLUSIONS In this work, we have shown that MgTFSI2, LiTFSI, LiFSI, and LiPF6 all decompose under X-ray radiation during XPS measurements. The decomposition of MgTFSI2 and LiTFSI is characterized by the formation of new peaks at lower BE’s for fluorine and sulfur in the XPS spectra, indicating the formation of new, solid, reduced species accumulating as a consequence of the irradiation. Furthermore, by examining the relative elemental concentration during the measurements, we determined that at least some of the photodegradation products are gaseous. We have compared the results of samples loaded onto three different substrates, UHV-compatible glue, Mg and Au. Samples measured on Au substrates showed very different S 2p spectrum than samples that were measured on Mg or as a neat powders. With Au substrates, additional, new sulfur peaks form at lower BEs, compared to Mg substrates. The BE of these S 2p can be attributed to thiols, Au−thiolates, organic sulfides, and disulfides; all are highly reduced forms of sulfur compared with the salts. Of the four possibilities, only Au−thiolates are expected to be stable under UHV conditions, whereas the other are expected to be pumped away. Thus, the difference in the photodegradation residues detected by the XPS on the different substrates, most likely stem from the high affinity of specific sulfur compounds to Au. We do not know whether the decomposition process itself is affected by the presence of Au, or that it only acts as an absorber to volatile, highly reduced sulfur compounds that would otherwise be pumped away in the UHV chamber. On the basis of the results, it is safe to suggest that both MgTFSI2 and LiTFSI follow a similar photodegradation pathway; the TFSI anion decomposition process is independent of the cation. We have discovered that LiFSI also decomposes under X-ray radiation, although its decomposition pathway apparently differs from that for TFSI-based salts, leading to different solid products. We have also detected X-ray photodegradation signs on LiPF6 under the same conditions. The F 1s spectrum in particular showed the formation of a new peak at lower BE. Because of the nature of photoreactions it would be difficult to suggest a detailed reaction pathway without substantial backing evidence elucidating the photoreaction products. We could however suggest some plausible reactions from our limited knowledge on the nature of the photodegradation products, obtained from the XPS measurements. We observed that in all cases (TFSI, FSI, PF6) F goes to a lower oxidation state and is consistent with a metal−fluorine bond. From this observation we can safely assume the cleavage of C−F bonds (for both TFSI and FSI) and the P−F bonds (for PF6). Formation of PF5 and PF3 is highly likely in LiPF6’s case. It is much more difficult to speculate on the reaction products for TFSI and FSI as the bonds breakage are most likely homolitic and result in myriad radical reactions. We can say however that, based on the fact that S goes to a lower oxidation level (for TFSI at least), the C−F cleavage is

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AUTHOR INFORMATION

Corresponding Author

*(D.A.) E-mail: [email protected]. ORCID

Doron Aurbach: 0000-0002-1151-546X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial support for this work was obtained from the Israel Science Foundation (ISF) in the framework of the INREP project.

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REFERENCES

(1) Kim, S. H.; Lee, K. J.; Shim, K. B.; Kim, C. S. Oxidation state of manganese in LiMn2O4 powders and its effect on electrochemical properties. Mater. Sci. Forum 2007, 534−536, 1473. (2) Ensling, D.; Stjerndahl, M.; Nyten, A.; Gustafsson, T.; Thomas, J. O. A comparative XPS surface study of Li2FeSiO4/C cycled with LiTFSI- and LiPF6-based electrolytes. J. Mater. Chem. 2009, 19, 82. (3) Zhang, B.; Chen, G.; Liang, Y.; Xu, P. Structural and electrochemical properties of LiNi0.5Mn0.5−xAlxO2 (x = 0, 0.02, 0.05, 0.08, and 0.1) cathode materials for lithium-ion batteries. Solid State Ionics 2009, 180, 398. (4) Cherkashinin, G.; Nikolowski, K.; Ehrenberg, H.; Jacke, S.; Dimesso, L.; Jaegermann, W. The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte-cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation. Phys. Chem. Chem. Phys. 2012, 14, 12321. (5) Matsuyama, T.; Deguchi, M.; Mitsuhara, K.; Ohta, T.; Mori, T.; Orikasa, Y.; Uchimoto, Y.; Kowada, Y.; Hayashi, A.; Tatsumisago, M. Structure analyses using X-ray photoelectron spectroscopy and X-ray absorption near edge structure for amorphous MS3 (M: Ti, Mo) electrodes in all-solid-state lithium batteries. J. Power Sources 2016, 313, 104. (6) Pearse, A. J.; Gillette, E.; Lee, S. B.; Rubloff, G. W. The reaction current distribution in battery electrode materials revealed by XPSbased state-of-charge mapping. Phys. Chem. Chem. Phys. 2016, 18, 19093. (7) Pan, F.-M.; Lin, Y.-L.; Horng, S.-R. Decomposition of perfluoropolyethers during XPS measurements. Appl. Surf. Sci. 1991, 47, 9. (8) Zubavichus, Y.; Fuchs, O.; Weinhardt, L.; Heske, C.; Umbach, E.; Denlinger, J. D.; Grunze, M. Soft X-ray-induced decomposition of amino acids: an XPS, mass spectrometry, and NEXAFS study. Radiat. Res. 2004, 161, 346. (9) Thomas, J. H. In Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis; Czanderna, A. W., Madey, T. E., Powell, C. J., Eds.; Springer US: Boston, MA, 2002.

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DOI: 10.1021/acs.jpcc.6b11524 J. Phys. Chem. C 2017, 121, 3744−3751

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The Journal of Physical Chemistry C (10) Ha, S.-Y.; Lee, Y.-W.; Woo, S. W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K. T.; Choi, N.-S. Magnesium(II) Bis(trifluoromethane sulfonyl) Imide-Based Electrolytes with Wide Electrochemical Windows for Rechargeable Magnesium Batteries. ACS Appl. Mater. Interfaces 2014, 6, 4063. (11) Shterenberg, I.; Salama, M.; Yoo, H. D.; Gofer, Y.; Park, J.-B.; Sun, Y.-K.; Aurbach, D. Evaluation of (CF3SO2)2N− (TFSI) Based Electrolyte Solutions for Mg Batteries. J. Electrochem. Soc. 2015, 162, A7118. (12) Salama, M.; Shterenberg, I.; Gizbar, H.; Eliaz, N. N.; Kosa, M.; Keinan-Adamsky, K.; Afri, M.; Shimon, L. J. W.; Gottlieb, H. E.; Major, D. T.; Gofer, Y.; Aurbach, D. Unique Behavior of Dimethoxyethane (DME)/Mg(N(SO2CF3)2)2 Solutions. J. Phys. Chem. C 2016, 120, 19586. (13) Kwabi, D.; Ortiz-Vitoriano, N.; Freunberger, S.; Chen, Y.; Imanishi, N.; Bruce, P.; Shao-Horn, Y. Materials challenges in rechargeable lithium-air batteries. MRS Bull. 2014, 39, 443. (14) Han, H.-B.; Zhou, S.-S.; Zhang, D.-J.; Feng, S.-W.; Li, L.-F.; Liu, K.; Feng, W.-F.; Nie, J.; Li, H.; Huang, X.-J. Lithium bis (fluorosulfonyl) imide (LiFSI) as conducting salt for nonaqueous liquid electrolytes for lithium-ion batteries: Physicochemical and electrochemical properties. J. Power Sources 2011, 196, 3623. (15) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype systems for rechargeable magnesium batteries. Nature 2000, 407, 724. (16) Aurbach, D.; Schechter, A.; Moshkovich, M.; Cohen, Y. On the mechanisms of reversible magnesium deposition processes. J. Electrochem. Soc. 2001, 148, A1004. (17) Pour, N.; Gofer, Y.; Major, D. T.; Aurbach, D. Structural Analysis of Electrolyte Solutions for Rechargeable Mg Batteries by Stereoscopic Means and DFT Calculations. J. Am. Chem. Soc. 2011, 133, 6270. (18) Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D. Mg rechargeable batteries: an on-going challenge. Energy Environ. Sci. 2013, 6, 2265. (19) Vinokurov, K.; Bekenstein, Y.; Gutkin, V.; Popov, I.; Millo, O.; Banin, U. Rhodium growth on Cu2S nanocrystals yielding hybrid nanoscale inorganic cages and their synergistic properties. CrystEngComm 2014, 16, 9506. (20) Shterenberg, I.; Salama, M.; Gofer, Y.; Levi, E.; Aurbach, D. The challenge of developing rechargeable magnesium batteries. MRS Bull. 2014, 39, 453. (21) Kacprzak, K. A.; Lopez-Acevedo, O.; Häkkinen, H.; Grönbeck, H. Theoretical Characterization of Cyclic Thiolated Copper, Silver, and Gold Clusters. J. Phys. Chem. C 2010, 114, 13571. (22) Woodruff, D. P. The interface structure of n-alkylthiolate selfassembled monolayers on coinage metal surfaces. Phys. Chem. Chem. Phys. 2008, 10, 7211.

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DOI: 10.1021/acs.jpcc.6b11524 J. Phys. Chem. C 2017, 121, 3744−3751