Interface Investigations of a Commercial Lithium Ion Battery Graphite

Article pubs.acs.org/Langmuir

Interface Investigations of a Commercial Lithium Ion Battery Graphite Anode Material by Sputter Depth Profile X‑ray Photoelectron Spectroscopy Philip Niehoff, Stefano Passerini, and Martin Winter* MEET Battery Research Center, Institute of Physical Chemistry, Westfaelische Wilhelms-Universitaet Muenster, Corrensstr. 46, 48149 Muenster, Germany S Supporting Information *

ABSTRACT: Here we provide a detailed X-ray photoelectron spectroscopy (XPS) study of the electrode/electrolyte interface of a graphite anode from commercial NMC/graphite cells by intense sputter depth profiling using a polyatomic ion gun. The uniqueness of this method lies in the approach using 13step sputter depth profiling (SDP) to obtain a detailed model of the film structure, which forms at the electrode/electrolyte interface often noted as the solid electrolyte interphase (SEI). In addition to the 13-step SDP, several reference experiments of the untreated anode before formation with and without electrolyte were carried out to support the interpretation. Within this work, it is shown that through charging effects during X-ray beam exposure chemical components cannot be determined by the binding energy (BE) values only, and in addition, that quantification by sputter rates is complicated for composite electrodes. A rough estimation of the SEI thickness was carried out by using the LiF and graphite signals as internal references.

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INTRODUCTION Lithium ion batteries (LIBs) have been of increasing interest recently because of their possible use in electrical vehicles (EV), hybrid electrical vehicles (HEV), and plug-in hybrid electrical vehicles (PHEV). Their use in stationary energy storages could be used to compensate for the fluctuating energy production of renewable energies.1 However, LIBs are rather expensive, so increasing their lifetime is of tremendous importance. Although LIBs have been investigated for more than two decades,2,3 their field of application has not specifically demanded long-life capability. Hence, only in recent years were studies of aging mechanisms intensified.4−8 LIBs strongly rely on the so-called solid electrolyte interphase (SEI) forming at the anode and cathode protecting the battery from further degradation as a result of the uncontrolled reaction of lithium with the electrolyte.9,10 It is well known that SEI formation depends on the used active materials and electrolyte composition.11−19 Being able to characterize the SEI structure and composition is therefore of great interest because it helps to reveal major aging mechanisms within the LIB. X-ray photoelectron spectroscopy (XPS) is a rather powerful method of investigating the SEI. Compared to other methods such as impedance spectroscopy,20 spectroscopic ellipsometry,21,22 and transmission electron spectroscopy,23 XPS can obtain very detailed information about the chemical composition of the SEI. However, the use of composite electrodes, as deployed in the battery, leads to drawbacks in the XPS technique. Because of the composition of electrode materials with different conductivities and with porous and rough structures and because of the huge variety of components that may participate in the SEI,24 interpretation is very © 2013 American Chemical Society

complicated. In addition, because of the ultrahigh vacuum conditions in the XPS only nonvolatile SEI species can be investigated and included in the model. In the literature, there are only a few studies that use detailed XPS measurements to characterize the SEI. The first works mainly focused on the determination of decomposition products on lithium metal25,26 and later also on carbon and graphite anodes27 by combining infrared spectroscopy and XPS. Here it was found that mainly the same components form on lithium metal as on graphite. Also, some early models for the SEI structure were derived.28 Later, several studies focused on the cathode side.29−33 Especially informative work was performed by Dedryvere et al.,30,31 who estimated the surface species by valence band analysis and therefore could directly prove major surface species and their signal characteristics. There have been some studies using SDP-XPS to investigate the SEI on lithium and graphite anodes,28,32,34,35 but the ion sources and settings used caused high sputter rates (rate of removal of a certain component) and the depth profiles were thus less detailed than ours. Therefore, only preliminary models could be derived. With possible access to synchrotron sources, nondestructive depth profiling by varying the X-ray wavelength has been carried out.36,37 Although this method probably is the most desirable, the use of standard analysis for aging examinations is not applicable. Received: March 1, 2013 Revised: April 12, 2013 Published: April 15, 2013 5806

dx.doi.org/10.1021/la400764r | Langmuir 2013, 29, 5806−5816

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Figure 1. F 1s, O 1s, and C 1s spectra of the untreated anode. carried out in a glovebox. The electrolyte was used as received from Novolyte. The samples treated with electrolyte were exposed to the electrolyte for about 20 min before the electrolyte was removed under vacuum. The electrodes were transported in a sealed vessel to a glovebox connected to the XPS. The sample was held in the ultrahigh vacuum chamber of the XPS longer than 12 h to remove the electrolyte. XPS (Axis Ultra DLD, Kratos, U.K.) was measured using a monochromatic Al Kα source (hν = 1486.6 eV) at a 10 mA filament current and a 12 kV filament voltage source energy. To compensate for the charging of the sample, we used the charge neutralizer. The measurement was carried out at a 0° angle of emission and a pass energy of 20 eV. The analysis area was approximately 700 μm × 300 μm. Sputter depth profiling was carried out using a polyatomic ion gun, which uses coronene as the ion source. The operating conditions were chosen to be a 12 kV filament voltage and a 4 nA emission current for all measurements. The sputter crater diameter was 1.1 mm. The angle between the surface normal and the ion gun beam was 45°. Measurements were carried out in field of view 2 with a 110 μm aperture and a pass energy of 40 eV. The sputter procedure used was five times for 10 s, five times for 120 s, and two times for 600 s. XPS imaging was carried out using a monochromatic Al Kα source (hν = 1486.6 eV) at a 10 mA filament current and a 12 kV filament voltage source energy. To compensate for the charging of the sample, we used the charge neutralizer. The measurement was carried out at a 0° angle of emission with field of view 3, a high-resolution imaging apparture, and a pass energy of 40 eV. The measurement time was 600 s, and the lateral resolution was 3 μm. The pressure within the analysis chamber was 10−7 Pa. The fitting was carried out with the help of CasaXPS. A standard fit function GL(30) was chosen. The vibrational shake-up of the PVdF was taken into account by the use of a VS(0.35, 0.65, 0.2, 0.1, 0.025)SGL(11) fitting function for the F 1s peak. The C 1s peaks of the PVdF, however, were fitted to the standard GL(30) function. Their BE difference was fixed at 4.15 eV, and their area and fwhm ratio were fixed at 1 because these values were determined by reference measurements of the untreated anode (Results and Discussion section). The position of the poly(ethylene glycol) fit was fixed at +2.2 eV with respect to the graphite peak. The strongly asymmetric peak shape of the graphite signal was fitted using a Donia-Sunjic fit function (DS(0.14, 499)). Calibration of the binding energy (BE) of the measured spectra was performed by using the energy of the C 1s peak (graphite at BE = 284.5 eV) as an internal reference.

In this work, sputter depth profiling (SDP) is used to investigate the SEI structure. With this method, a layered structure of the SEI can be concluded when the relative intensity of the underlying component (substrate) increases. The detection of a slower decrease in one component intensity compared to another is not sufficient because this effect could also be due to different sputter rates. Within this work, the outermost layer of a multilayered structure will be denoted as the overlayer, and the layers between the overlayer and the substrate will be denoted as interlayers. The sputter process is carried out with a polyatomic ion gun, allowing slow sputter rates and low rates of decomposition.38 As decomposition, a nondesired chemical change in a material due to external influences (e.g., UHV, X-rays, or sputter treatment) is denoted. To be sure that the relative intensity changes occur from the removal of different components and not by decomposition, caused by the ion gun beam, of another component, several control experiments have been carried out. Special focus was placed on studying the reported decomposition of LiPF6 and the determination of charging effects. The term charging effects denotes shifts in the binding-energy values. These are caused by the charging of a component due to the equilibrium between emitting photoelectrons and the adsorption of electrons from the charge neutralizer or from the ground. The XPS analysis of battery materials without SDP may lead to a false interpretation. For example, with a multilayered structure of the SEI, certain SEI species may be underestimated or even not identified at all. Therefore, the relative quantification of the composition would be falsified. Further on, SDP may help with the estimation between certain components resulting from their different sputter behavior. For certain sample types, these estimations cannot be done without SDP because of possible charging effects. This will be shown by several examples in the following text.

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EXPERIMENTAL SECTION

The graphite electrodes under investigation were obtained from a lithium nickel cobalt oxide (NMC)/graphite laboratory pouch cell from a commercial supplier. Polyvinylidendifluoride (PVdF) was used as the binder. The electrolyte consists of 1 M lithium hexafluorophosphate in an ethylene carbonate/ethylmethyl carbonate solution. The cell was received after electrochemical formation at the cell maker and opened in a glovebox. The electrodes were not washed with any solvent to avoid the dissolution of SEI components. The reference measurements were carried out using 1 M lithium hexafluorophosphate in EC/DEC (3:7 w/w) electrolyte. The samples’ exposition to the electrolyte and sample preparation on the sample holder were

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RESULTS AND DISCUSSION Initially, the untreated anode (before electrochemical formation and before immersion in the electrolyte) was investigated to support the quantitative interpretation of the anode after electrochemical formation below. For this, more than 40 measurements were carried out. 5807


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In the F 1s spectra shown in Figure 1, a peak can be identified as representing the fluorine of the PVdF. The peak shows a slightly asymmetric peak shape. This asymmetry is due to the partial excitation of stretching modes of the polymer, which results in a loss in kinetic energy of the photoelectron and hence an increase in the estimated BE. This effect is often denoted as a shake-up. To fit this shake-up peak, an asymmetric fitting function was chosen (Experimental Section). All C 1s spectra show three peaks: two symmetrical peaks at 291 and 286 eV representing the CF2 (CF2CH2) and CH2 (CH2CF2) groups of PVdF, respectively, and a strongly asymmetric peak at 284.5 eV related to graphite. The asymmetry was fitted with a Donia−Sunjic fit function as described in the Experimental Section. The fit was chosen to optimize the area ratio of the CF2 and CH2 groups because they should be identical. The drawback of the asymmetric fitting function is the slow fading of the asymmetric tail toward higher binding energies. However, this has only a minor effect on the accuracy of the fit. All of the O 1s spectra show a small, broad signal at 532.7 eV corresponding to the graphite, which usually contains small amounts of oxygen, at the prismatic surfaces. In Table 1, the average binding energies (BEs), the full width half-maximum (fwhm) values, the atomic concentration

This indicates charging effects, which will be further proven below in this work by different experiments. Charging effects could not be compensated for by changing the charge neutralizer parameters. To analyze the structure of the SEI, SDP was deployed. However, SDP using ion sources may cause the decomposition of the sample and therefore produce artifacts.39 Hence, these effects were studied by SDP of the untreated anode first. The untreated anode was sputtered five times for 10 s, five times for 120 s, and then two times for 600 s. In Figure 2, the F 1s intensity is plotted against sputter time. The intensity decreases exponentially. This can be understood

Table 1. Average Binding Energies (BE, eV), Full Width at Half Maximum (fwhm, eV), Atomic Concentration Percentages (%at.), and the Corresponding Maximal Deviations (σmax) Determined from 40 Measurements of the Untreated Anode

Figure 2. Relative F 1s intensity decrease of the untreated anode due to sputter treatment. Triangles, circles, and squares represent results from different measurements.

region

component

BE

σmax

fwhm

σmax

%at.

σmax

F 1s O 1s C 1s C 1s C 1s

PVdF graphite CF2CH2 CH2CF2 graphite

687.53 532.72 290.82 286.35 284.50

0.40 0.50 0.31 0.31 0.00

1.71 2.75 1.28 1.28 0.57

0.21 1.20 0.16 0.16 0.06

15.82 0.87 8.00 8.00 67.31

3.96 0.88 2.26 2.26 4.58

by the structure of the electrode. First, a thin PVdF layer on the surface is removed and then PVdF in the pores of the electrode is removed more slowly, but some PVdF could not be removed at all. The sputter process is highly reproducible (Figure 3); only the amount of PVdF left after the sputter process might vary between 20 and 30% of the initial value. This is possibly

percentages (%at.), and the corresponding maximal absolute deviations (σmax) are given as received from the fitting. The %at. values given in this work reflect the relative intensity ratios of the different components, whose signal intensities where corrected by the relative sensitivity factors. These values do not take structural effects into account, which in this case means that the PVdF intensities are overestimated. The fitting of the anode material is of good quality because of the %at. values of the F 1s peak of the PVdF fits to the C 1s peaks with a deviation of only 1%. The deviation of the PVdF signal intensities of up to 25%at. is probably partially due to inhomogeneities in the electrode. However, organic contamination may also contribute to this variation. Samples of the electrode repeatedly measured several times show deviations of about 10%at.. Therefore, inhomogeneities in the PVdF distribution at the electrode can be estimated to ca. 15%. The deviations of the multiple measurements did not show a definitive tendency. Therefore, it can be concluded that the chosen measurement parameters do not cause the decomposition of certain components of the sample. With respect to the binding energy (BE), maximum shifts of up to 0.4 eV were observed for the F 1s peak, with respect to the graphite peak. If one references the BE of the F 1s peak to the C 1s CH2CF2 peak BE, then the shift is only up to 0.2 eV.

Figure 3. F 1s and C 1s spectra of the SDP of the untreated anode. Spectra shown from top to bottom are from sputter times of 0, 50, 170, 410, 650, and 1850 s. 5808


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Figure 4. F 1s, C 1s, and P 2p spectra of the SDP of the untreated anode in contact with 8.8 (μL/cm2) 1 M LiPF6 in EC/DMC 3/7 electrolyte. Spectra shown from top to bottom are from sputter times of 0, 50, 170, 410, 650, and 1850 s.

Table 2. Average Binding Energies (BE, eV), Full Width at Half Maximum (fwhm, eV), Atomic Concentration Percentages (%at.), and the Corresponding Maximal Deviations (σmax) Determined from Nine Measurements of the Untreated Anode in Contact with 8.8 (μL/cm2) 1 M LiPF6 in EC/DMC 3/7 Electrolyte region

component

BE

σmax

fwhm

σmax

%at.

σmax

F 1s F 1s O 1s C 1s C 1s C 1s C 1s P 2p P 2p

PVdF, LiPFx LiF graphite, contamination CF2CH2 CH2CF2 contamination graphite LiPFx LiPFxOy

687.78 685.48 533.07 290.80 286.37 285.45 284.50 139.12 138.77

0.14 0.20 0.20 0.06 0.13 0.46 0.00 0.27 0.00

2.21 1.34 2.73 1.25 1.25 1.86 0.57 1.86 0.58

0.26 0.20 0.26 0.07 0.07 0.57 0.03 0.08 0.39

23.40 0.82 1.94 5.95 5.95 6.18 52.75 2.95 0.02

2.28 0.25 0.74 0.97 0.98 3.58 2.75 0.97 0.01

shape of the graphite40 and some polymeric species appear at the BE of the shoulder.41,42 Besides these artifacts, it is clearly visible from Figure 2 that about 50% of the PVdF from the surface was removed within the first 50 s. This finding can be used to interpret the SDP of the anode after electrochemical formation because it indicates that some substances are removed faster by sputtering than others. A point often under discussion in the literature is if and how to wash the samples to remove the lithium salt of the electrolyte sticking to the electrode surface. The main reasons for this discussion are on one hand the possible hydrolysis and other reactions of LiPF6 and on the other hand the decomposition of LiPF6 by sputtering. Both processes could alter the SEI and thus influence the result. However, in our case the used XPS instrument has an attached glovebox, so hydrolysis can be excluded. In the following text, we report the effects of the lithium salt under UHV and sputter conditions that were studied. LiPF6 (1 M, 8.8 μL/cm2) in EC/DMC 3/7 electrolyte was placed on the untreated anode. The sample was dried in the pretreatment chamber of the XPS and subsequently measured (Figure 4, Table 2).

due to a different pore structure. Increasing the sputter crater size did not affect the results. The exponential decrease in the sputter rate due to the pore structure complicates quantification by sputter rate and time. Another artifact due to the sputtering is the change in the graphite signal. The graphite peak shows a shoulder, when sputtered, at a slightly higher BE, which was fitted by a standard GL(30) fit. It is probably caused by the amorphization of the graphite. Chemical decomposition of the PVdF to carbon (CC) and hydrocarbon (CH) species can be excluded because the area of the 285.3 eV peak is 2 times larger than that of the initial C 1s PVdF peaks. Furthermore, an additional peak at 287.8 eV occurs with sputtering time and makes it necessary to fit the altered asymmetric peak shape of the graphite with another GL(30) fit function. The fwhm value of the graphite does increase by ca. 0.2 eV. With this fit adoption, it was possible to obtain reasonable correlations of the F 1s and C 1s PVdF intensities, resulting in a maximum deviation of 10%. However, these artifacts make the detailed analysis of the C 1s spectra difficult because carbonate signals appear in the region of the varied asymmetric peak 5809


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Table 3. Atomic Concentration Percentages (%at.) of the Untreated Anode in Contact with 8.8 (μL/cm2) 1 M LiPF6 in EC/ DMC 3/7 Electrolyte after Different Sputter Times (s) region

component

0

10

30

50

170

290

410

530

650

1250

1850

F 1s F 1s C 1s C 1s C 1s C 1s C 1s P 2p

PVdF, LIPFx LIF CF2CH2 shake up CH2CF2 carbon graphite LiPFx

21.42 1.38 6.16 0.01 6.16 5.95 56.69 2.22

18.32 2.83 3.67 1.95 3.67 12.08 55.61 1.87

15.04 4.21 3.21 2.28 3.21 17.08 53.7 1.27

13.81 4.24 2.65 2.58 2.66 19.2 53.85 1.01

11.02 4.5 2 4.83 2 24.52 50.32 0.81

9.9 4.25 0.8 2.63 0.8 22.95 57.89 0.77

8.99 4.21 0.94 4.84 0.94 19.07 60.41 0.6

8.47 3.93 0.52 6.52 0.52 20.01 59.58 0.44

8.14 3.75 0.35 6.89 0.35 24.26 55.88 0.39

6.34 3.17 0.73 2.56 0.73 18.78 67.27 0.43

5.15 3.62 0.41 9.24 0.41 22.65 58.24 0.28

Figure 5. F 1s, O 1s, C 1s, P 2p, and Li 1s spectra of the anode material after electrochemical formation from three samples. 5810


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LiF %at. values for layer thickness estimations the %at. values of the LiPFx values have to be taken into account.

The F 1s spectrum shows two peaks: one at the BE where the PVdF signal has been found in the measurements of the untreated anode and one at a slightly lower BE (684.5 eV). The lower peak corresponds to LiF, as identified by Dedryvere et al.30 by valence band analysis. LiF is present because of the natural equilibrium of LiPF6 ⟨−⟩ LiF + PF5.43 However, the amount of LiF is 0.82%at., a rather low value. The LiPF6 signal in the F 1s spectrum directly overlays the PVdF peak because there is no change in the BE of the PVdF peak and no increase in the fwhm value (Table 2). The interpretation of overlaying LiPF6 and PVdF peaks is also supported by the increased relative atomic concentration of the F 1s PVdF peak found for the anode treated with electrolyte in comparison to that for the untreated anode. It can be derived that the ratio of F to P intensities of the LiPF6 gives an average value of 3.9 (after the subtraction of the PVdF intensity estimated by the C 1s PVdF signals). This indicates that LiPF6 is not stable under UHV conditions on the anode. Therefore, the signal in the literature referred to as LiPF6 should rather be denoted as LiPFx. The take-in time before the measurement was >12, and all decomposition of the LiPF6 was completed as the pressure stabilized. Several measurements at the same sample position showed no further decomposition due to UHV or X-ray influence. The same phenomenon was found by Leroy et al.44 but not discussed. In the O 1s spectrum, the amount of oxygen is slightly increased by about 1%at., and in the C 1s spectrum, another peak is present at a BE of 285.45 eV, probably resulting from some adsorption of electrolyte decomposition products or unidentifiable impurities. The quantification of this peak is related to the high errror due to the antisymmetric peak shape of the graphite signal and the possible differential charging. In the P 2p spectrum at 138.7 eV, a very low intensity decomposition peak identified as LiPFxOy32,45 can be found. The intensity of this peak increases significantly if the sample is exposed to humidity. The peak at 139.1 eV BE is related to LiPFx. Edström et al. report the decomposition of LiPF6 to LiF and PF5 by sputter depth profiling.46 To prove decomposition for our instrumental setup, several samples with the prior reported amount of electrolyte on top were sputtered. It was found (Table 3) that within the first 30 s the %at. value of LiF rises strongly. Afterward, however, no further rise in LiF content can be observed, and the P 2p intensity is also still decaying. A possible explanation of this phenomenon is that the salt decomposes within the first 30 s but PF5 is only slowly removed from the surface. This hypothesis was further proven by sputtering two samples: one treated with 8.8 (μL/cm2) and one treated with only 4.4 (μL/cm2) 1 M LiPF6 in EC/DMC 3/7 electrolyte. There is a direct relationship between the initial %at. value of LiPFx and the %at. value rise of LiF within the first 30 s. Therefore, the amount of LiF produced depends on the amount of LiPFx on the surface. Hence, with our experimental setup the decomposition of LiPFx also takes place. To quantify this decomposition process, eq 1 was used. The percentage of LiPFx that gets decomposed, denoted as x, was estimated to lie between 75 and 100%. This was determined with the %at. values of the LiF component, the LiPFx values from the P 2p signal, and the substrate (sub) %at. values, which combine the %at. values of the graphite, carbon, and shake-up signals. The values were taken from sputter time 0 s (t0) and sputter time tmax, the sputter time where the maximum %at. value of LiF is reached. This results show that when using the

%at.,LiFt max %at.,subt0

x=

%at.,subtmax

− %at.,LiF t0

%at.,LiPFx t0

(1)

An analysis of the SEI formed on the anode after electrochemical formation was carried out. In Figure 5, the F 1s, O 1s, C 1s, P 2p, and Li 1s spectra of an anode after electrochemical formation are shown. Table 4 gives a quantification of the components measured from three different samples. Table 4. Atomic Concentration Percentages (%at.) of the Anode Material after Electrochemical Formation from Three Samples region

component

BE

σmax

fwhm

σmax

%at.

σmax

F 1s

PVDF/ LIPFx LiF RCO3 Li2CO3 Li2O CF2CH2 shake up, carbonates PEO CH2CF2 graphite, CC/CH Li/C species LiPFx LiPFxOy

686.56

0.05

1.64

0.03

15.40

1.47

684.88 533.77 532.07 528.3 290.13 286.86

0.02 0.18 0.12 0.18 0.09 0.13

1.57 1.86 1.90 1.12 1.30 2.80

0.05 0.18 0.00 0.23 0.00 0.00

3.33 4.71 7.67 0.11 3.11 4.47

0.89 0.14 0.28 0.03 0.50 0.99

286.70 285.98 284.50

0.03 0.09 0.03

1.50 1.30 1.29

0.00 0.00 0.02

0.40 3.11 40.26

0.41 0.50 1.85

282.12 136.95 134.41 55.75

0.12 0.03 0.07 0.16

0.47 1.74 1.40 1.89

0.00 0.02 0.09 0.15

0.20 3.31 0.32 12.18

0.06 0.23 0.01 1.95

F 1s O 1s O 1s O 1s C 1s C 1s C 1s C 1s C 1s C 1s P 2p P 2p Li 1s

There are some inhomogeneities revealed from different sample positions (Figure 5, Table 4) leading to maximal differences of up to 26% in %at. values of the LiF species. In the F 1s spectra, the two peaks at a BE of 686.5 and 685 eV can clearly be assigned to preliminary experiments (Figures 1 and 4) and in accordance with literature data32 to LiPFx, PVdF, and LiF. In the O 1s spectrum, three peaks were assigned. There may be more different components hidden within these signals, but the peaks cannot be resolved to distinguish them. The signal at the higher BE of 533.8 eV was assigned to organic carbonates (RCO3), the peak at 532.1 eV was assigned to lithium carbonates,47 and the peak at 528.3 eV was assigned to lithium oxides. These components were found to be major parts of the SEI.30 And this assignment is in good agreement with the SDP experiments reported below in this work. However, oxide signals of polyethylenoxide (PEO) found priorly in the SEI,48 have a binding energy within the Li2CO3 region of the O 1s spectra, and therefore cannot be resolved individually. A distinction between PEO and Li2CO3 with the help of the C 1s spectrum is complicated as explained below. Furthermore, possible lithium alkyl carbonate (RLiCO3) species, which would contribute to the RCO3 and Li2CO3 peak intensities with an intensity ratio of 1:2, cannot be quantified for the same reason. The C 1s spectrum causes significant difficulties in analysis because the graphite shows a strong asymmetric peak shape, 5811


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Figure 6. F 1s, O 1s, C 1s, P 2p, and Li 1s spectra of SDP of the anode material after electrochemical formation. Spectra shown from top to bottom are from sputter times of 0, 50, 170, 410, 650, and 1850 s.

chosen to fit both signals. In doing this, it can be shown how homogeneously the SEI is distributed over the electrode surface. In the C 1s spectrum, a small peak at a BE lower than for the graphite signal can be found. Here, the species was assigned to a lithium/carbon species (lithiated graphite/LiC6 as shown below). This is reasonable because the only species having a negative BE shift with respect to the graphite signal is a metal/carbon species. From Table 4, it can be seen that the difference in BE between CF2H2 and the lithium carbon species is about 8 eV. Therefore, this signal cannot be graphite because from reference measurements it was shown that the shift is

which may be altered as shown by the sputter experiments of the untreated anode. In addition, it is difficult to distinguish between the different species present in the C 1s spectrum because of the number of species present in the C 1s spectrum.24 Only small shifts in the peak positions due to charging effects will cause significant quantification errors. In particular, the carbon/hydrocarbon (CC/CH) signal and the graphite signal, which are separated by only 0.5 eV, cause problems because SEI thickness estimations are based on the graphite signal. Because it is so difficult to distinguish between the graphite and CC/CH signals, only one fitting signal was 5812


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Table 5. Atomic Concentration Percentages (%at.) of the Anode Material after Electrochemical Formation at Different Sputter Times (eV) region

component

0

10

30

50

170

290

410

530

650

1250

1850

F 1s F 1s O 1s O 1s O 1s C 1s C 1s C 1s C 1s C 1s C 1s P 2p P 2p Li 1s

PVdF, LIPF6 LIF RCO3 Li2CO3 Li2O CF2CH2 shake up, carbonates CH2CF2 PEO graphite, CC/CH LiC6 LiPFx LiPFxOy

13.25 4.37 3.84 7.51 0 2.47 2.61 2.46 1.1 44.78 0.18 2.81 0.45 12.94

9.65 7.6 1.73 10.73 0.3 1.88 3.13 1.88 0 37.72 2.84 1.5 0.65 19.41

6.86 10.19 1.05 8.55 0.89 1.04 1.4 1.04 0.02 35.5 6.18 1.03 0.6 25.07

5.98 11.29 0.77 7.24 0.89 0.67 0.42 0.67 0 35.49 8.08 0.87 0.55 26.75

4.39 10.47 0.78 4.85 0.55 0.7 1.22 0.7 0.28 30.46 17.28 0.44 0.24 27.4

3.53 9.99 0.34 4.34 0.35 0.64 2.22 0.64 0.46 31.4 20.01 0.3 0.21 25.45

3.17 9.01 0.34 3.85 0.26 0.59 4.32 0.59 0.14 30.94 23.85 0.21 0.17 22.47

3.14 8.68 0.48 3.46 0.13 0.44 5.32 0.44 0.09 31.39 26.53 0.32 0.16 19.35

2.77 8.09 0.47 3.34 0.15 0.38 4.83 0.38 0.05 31.13 30.1 0.29 0.2 17.73

2.08 6.18 0.23 2.38 0.13 0.72 6.35 0.71 0.01 28.54 37.3 0.1 0.09 15.16

1.65 5.18 0.21 2.06 0.14 0.45 5.04 0.45 0.6 39.91 30.69 0 0 13.6

about 6 eV. The signal assignment at 284.5 eV to graphite and CC/CH is in good accordance with the BE shifts found in the reference data and preliminary experiments. Further proof will be given by SDP analysis below. The P 2p spectrum can be fitted by two components, one at a BE of 137 eV as determined by preliminary experiments according to LiPFx and one at a slightly lower BE according to LiPFxOy. The Li 1s spectrum is fitted by only one signal (at BE 56 eV) because of the small relative sensitivity factor in which the signal intensity is very low. To obtain thickness quantification results, the use of SDP is necessary. Then, it is possible to distinguish between the amount of graphite and the CC/CH signal. In addition, SDP is necessary to obtain structural information. Therefore, SDP was carried out with an anode after electrochemical formation. Spectra and quantification results are shown in Figure 6 and Table 5. The sputter results for the F 1s spectra show, as already found for the untreated anode and the anode treated with electrolyte, that the PVdF/LiPFx signal gets reduced to below 25% of its initial value after 410 s of sputtering. The LiF signal is first increasing and then decreasing extremely slowly again. This finding indicates that LiF is not removed between 30 s and the sputter time where the LiF %at. value reaches its maximum. Therefore, the LiF signal can be used as a substrate signal within this period of sputter time. The increase in the LiF signal is partially due to LiPFx decomposition as concluded by the reported preliminary experiments. However, when the LiPFx signal is included in the LiF signal, this gives an intensity increase of 57%. This indicates the removal of other surface species on top of the LiF. In the O 1s spectrum, the RCO3 signal is decreasing within 50 s to 25% of its initial value. Such a fast decrease is typical for organic species, and thus the interpretation as a mainly organic signal is supported. In contrast, the signal representing inorganic component Li2CO3 initially shows a small increase in intensity and then decreases much slower. It reaches a signal intensity of 25% after about 800 s of sputter time. The decomposition of two entities of RLiCO3 to one entity of Li2CO3 may be occurring. Unfortunately, because of the unavailability of commercial RLiCO3 species, reference experiments were not possible. However, from Table 5 it can be noticed that even if 100% decomposition took place this would not explain the %at. value increase of 3.22 found for Li2CO3

species after 10 s of sputtering because the RCO3 %at. value decreased by only 2.1. Therefore, the increase that is found for Li2CO3 is 3 times higher than the possible increase due to decomposition. At a BE of 528.3 eV, at first no signal can be found. During sputtering, however, a signal occurs that belongs to Li2O. The decomposition of Li2CO3 to Li2O was reported by Edström et al.46 With reference experiments carried out with the sputter setting reported in the Experimental Section at an anode with a thin layer of Li2CO3, only small amounts of decomposition after 170 s were observed. Still, there could be different components in the SEI that serve as the origin of Li2O decomposition products. Therefore, the decomposition to Li2O cannot be excluded. Nevertheless, because the %at. value of the Li2O component never exceeds 1%, the importance of Li2O in the overall discussion is limited. In the C1s spectrum, signal intensities of the PVdF, carbonates, and PEO are decreasing strongly in the beginning of the SDP experiment. The graphite CC/CH peak is decreasing in intensity but the fwhm value is constant, although the CC/CH components should be removed faster by sputtering than graphite. This indicates a full overlay of the signals. Therefore, it is not possible to perform an individual fitting of the components. The intensity of the substrate signal is first decaying but after 50 s it is increasing again. This is more evidence that the graphite peak and the CC/CH peak overlay each other. After 50 s, a majority of the CC/CH SEI species are removed and the graphite signal dominates the signal. In contrast, the lithium/carbon signal is steadily increasing. From electrochemical experiments, it is known that there is only a negligible amount of Li left in a discharged graphite electrode. Lithium carbide species have been reported within the SEI on lithium metal electrodes.39,49 However, this kind of lithium/ carbon species is unlikely in this case because the signal is not decreasing again during the sputter experiment. A more likely interpretation is the formation of LiC6 as an artifact. Thus, this signal has to be taken into account as a substrate signal as well. The LiC6 signal also occurred when Ar sputtering was used, and hence it is no artifact of the ion source. In the P 2p spectrum, the LiPFx signal is removed quickly as found in the reference experiment. The LiPFxOy signal is first slightly increasing and then decreasing more slowly, indicating a more homogeneous spread across the SEI. In reference measurements with higher LiPFx contents, no higher content 5813


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are dominating. The ratio of LiF intensity to Li2CO3 intensity calculated from the maps increases more than 250%. Therefore, it can be concluded that Li2CO3 is building an overlayer on top of LiF. The differences in total intensity are related to the roughness of the electrode. With the obtained SEI model and the help of XPS Multiquant,50 the quantification of the SEI thickness was carried out. Therefore, a two-step procedure was used. In the first step, the organic and fast-sputtered components were quantified by using LiF as an internal standard and applying an effective mean free path for the layer combined from different species. The layer thickness reduction ddiff was determined by (eq 2) the ratio of the LiF %at. value after sputtering, Ia, from the sputter time where the LiF %at. value reaches its maximum, and the %at. value sum of the LiF and LiPFx P 2p component before sputtering, Ib. The tfactor of 0.67 takes the topography of the sample into account.51 Equation 2 is derived from the equation for homogeneous layers (eq 3), which correlates the intensity of the substrate with an exponential decrease with increasing adsorbate layer thickness dads and an exponential increase with increasing mean free path λads of the adsorbate. θe is the angle of emission (Experimental Section). The λ values for PVdF, LiPF6, PEO, and Li2CO3 have been calculated using the TPP equations52,53 within XPS Multiquant. The effective mean free path λads of the layer was calculated using an average value of the single λ (eq 4), resulting in a λads of 25.49 Å for photoelectrons with energies corresponding to the BE of LiF. The second step works as follows: Because of the removal of most of the organic species after 50 s, the combined substrate signal consisting of the shake-up peak, the graphite, CC/CH, and the LiC6 peak can be used as an internal reference. This is valid from the sputter time where the substrate %at. value minimum is reached to the end of the sputtering process. Using the same equations as before and a λads of 32.39 Å calculated from the TPP equations for the combined Li2CO3 and LiF mean free path at BE of the graphite signal, the layer thickness was calculated.

of LiPFxOy formed during sputtering. Therefore, the decomposition of LiPFx to LiPFxOy is not likely. In the Li 1s spectrum, the signal is first increasing and then slowly decreasing, which is in line with the sputter results found for the Li2CO3 and the LiF species. With these results, it can be stated that LiPFx, PVdF, RCO3, PEO, and CC/CH are removed rather fast within the first 50 s of sputtering, resulting in increasing signals of LiF, Li2CO3, LiPFxOy, graphite, and LiC6. However, because the signal for the CC/CH components cannot be resolved from the graphite signal, concrete quantities cannot be given. The described sputter profile proves a layered structure with LiPFx, PVdF, RCO3, PEO, and CC/CH on top of LiF, Li2CO3, LiPFxOy, graphite, and LiC6. The decrease in the Li2CO3 signal is faster (800 s until 25% of the initial %at.) than the decrease in the LiF signal (1850 s until 50%). This may be due to a layered structure and different sputter rates. Unfortunately, no significant increase in the intensity of the LiF signal can exclusively be attributed to the decrease in the Li2CO3 signal. Nevertheless, because the intensity of the Li2CO3 signal is decreasing to values lower than 20%, it can be stated that no larger portion of the Li2CO3 species can be found underneath the LiF layer. The findings derived from the SDP experiments so far can be visualized by two schematic models: a model with LiF and Li2CO3 next to each other (model A) and a model with a layer of Li2CO3 on top of LiF (model B) (Figure 7).

ddiff = ln Figure 7. Model structures of the SEI formed at the graphite anode after electrochemical formation.

To distinguish between these models, XPS imaging was carried out for the LiF and Li2CO3 species after a 50 s sputter time, where most of the organic overlayer is removed, and after 1850 s, where only LiF is left. From Figure 8, it can be seen that after 50 s of sputter time most of the surface is covered with Li2CO3. After 1850 s of sputter time, however, the LiF species

Ia λads cos θetfactor Ib

(2)

I = I0e−dads/ λads cos θe

(3)

∑ λi i

(4)

λads =

The organic overlayer shows a thickness of 8.2 ± 0.9 Å, and the inorganic interlayer, a thickness of 11 ± 1.9 Å. The given deviation results from the measurement of two different samples. However, the systematic error will be higher. On one hand, the average calculated mean free paths might cause a systematic error of up to 7% depending on the actual SEI composition. On the other hand, residual species of the overand interlayer will cause underestimations of the SEI thickness. These underestimations will become stronger with increasing SEI thickness. Also an incomplete decomposition of LiPFx may cause a certain error. Still, with this method it is possible to determine major thickness changes within the SEI as well as differences in structure.

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CONCLUSIONS Within this work, a detailed model of the SEI composition and structure on a graphite in a lithium ion cell could be developed

Figure 8. Images of LiF and Li2CO3 species. (Left) After 50 s of sputter time; (right) after 1850 s. 5814


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(6) Wohlfahrt-Mehrens, M.; Vogler, C.; Garche, J. Aging mechanisms of lithium cathode materials. J. Power Sources 2004, 127, 58−64. (7) Broussely, M.; Biensan, P.; Bonhomme, F.; Blanchard, P.; Herreyre, S.; Nechev, K.; Staniewicz, R. J. Main aging mechanisms in Li ion batteries. J. Power Sources 2005, 146, 90−96. (8) Arora, P. Capacity fade mechanisms and side reactions in lithiumion batteries. J. Electrochem. Soc. 1998, 145, 3647. (9) Peled, E. The electrochemical behavior of alkali and alkaline earth metals in nonaqueous battery systemsthe solid electrolyte interphase model. J. Electrochem. Soc. 1979, 126, 2047. (10) Besenhard, J. O.; Winter, M.; Yang, J.; Biberacher, W. Filming mechanism of lithium-carbon anodes in organic and inorganic electrolytes. J. Power Sources 1995, 54, 228−231. (11) Winter, M.; Imhof, R.; Joho, F.; Novák, P. FTIR and DEMS investigations on the electroreduction of chloroethylene carbonatebased electrolyte solutions for lithium-ion cells. J. Power Sources 1999, 81−82, 818−823. (12) Winter, M.; Novák, P. Chloroethylene carbonate, a solvent for lithium-ion cells, evolving CO2 during reduction. J. Electrochem. Soc. 1998, 145, L27. (13) Krämer, E.; Schmitz, R.; Niehoff, P.; Passerini, S.; Winter, M. SEI-forming mechanism of 1-fluoropropane-2-one in lithium-ion batteries. Electrochim. Acta 2012, 81, 161−165. (14) Wagner, M.; Albering, J.; Möller, K. C.; Besenhard, J.; Winter, M. XRD evidence for the electrochemical formation of Li+(PC)yCn− in PC-based electrolytes. Electrochem. Commun. 2005, 7, 947−952. (15) Wagner, M. R.; Raimann, P. R.; Trifonova, A.; Möller, K. C.; Besenhard, J. O.; Winter, M. Electrolyte decomposition reactions on tin- and graphite-based anodes are different. Electrochem. Solid-State Lett. 2004, 7, A201. (16) Besenhard, J. O.; Winter, M. Advances in battery technology: rechargeable magnesium batteries and novel negative-electrode materials for lithium ion batteries. ChemPhysChem 2002, 3, 155−159. (17) Kohs, W.; Santner, H. J.; Hofer, F.; Schröttner, H.; Doninger, J.; Barsukov, I.; Buqa, H.; Albering, J. H.; Möller, K. C.; Besenhard, J. O.; Winter, M. A study on electrolyte interactions with graphite anodes exhibiting structures with various amounts of rhombohedral phase. J. Power Sources 2003, 119−121, 528−537. (18) Möller, K. C.; Hodal, T.; Appel, W. K.; Winter, M.; Besenhard, J. O. Fluorinated organic solvents in electrolytes for lithium ion cells. J. Power Sources 2001, 97−98, 595−597. (19) Santner, H. J.; Möller, K. C.; Ivančo, J.; Ramsey, M. G.; Netzer, F. P.; Yamaguchi, S.; Besenhard, J. O.; Winter, M. Acrylic acid nitrile, a film-forming electrolyte component for lithium-ion batteries, which belongs to the family of additives containing vinyl groups. J. Power Sources 2003, 119−121, 368−372. (20) Aurbach, D.; Zaban, A. Impedance spectroscope of lithium electrodes. J. Electroanal. Chem. 1994, 367, 15−25. (21) Kwon, K.; Kong, F.; McLarnon, F.; Evans, J. W. Characterization of the SEI on a carbon film electrode by combined EQCM and spectroscopic ellipsometry. J. Electrochem. Soc. 2003, 150, A229. (22) Lei, J.; Li, L.; Kostecki, R.; Muller, R.; McLarnon, F. Characterization of SEI layers on LiMn2O4 cathodes with in situ spectroscopic ellipsometry. J. Electrochem. Soc. 2005, 152, A774. (23) Li, H.; Wang, Z.; Chen, L.; Huang, X. Research on advanced materials for li-ion batteries. Adv. Mater. 2009, 21, 4593−4607. (24) Verma, P.; Maire, P.; Novák, P. A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries. Electrochim. Acta 2010, 55, 6332−6341. (25) Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E. Identification of surface films formed on lithium in propylene carbonate solutions. J. Electrochem. Soc. 1987, 134, 1611. (26) Aurbach, D.; Weissman, I.; Schechter, A.; Cohen, H. X-ray photoelectron spectroscopy studies of lithium surfaces prepared in several important electrolyte solutions. a comparison with previous studies by Fourier transform infrared spectroscopy. Langmuir 1996, 12, 3991−4007. (27) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-Eli, Y. A comparative study of synthetic graphite and Li electrodes in electrolyte

on the basis of XPS analysis combined with sputter depth profiling (SDP). Several of the findings in the reported literature were critically discussed, and alternative and improved data interpretation was provided. In summary, the SDP-XPS investigations led to the following conclusions: (1) Battery electrode materials show charging effects when measured via XPS, and SDP was found to alter these charging effects. Shifts in the binding energy (BE) values of up to 0.4 eV for PVdF and even up to 1 eV for Li2O and LiC6 were found. (2) UHV conditions led to the decomposition of LiPF6 on graphite, which should therefore be denoted as LiPFx. (3) With the used ion gun and ion gun settings, SDP was found to produce the following artifacts: an asymmetric peak shape showing that graphite is altered over the sputter time; LiPFx decomposes to LiF and PFx within a 30 s sputter time; 75−100% of LiPFx does decompose; and by sputtering the SEI on graphite, we have detected the formation of lithiated graphite. (4) A multilayered structure of the SEI could be derived (Figure 7, model B) within a lateral resolution of 3 μm. (5) The organic layer thickness could be estimated to be 8.2 ± 0.9 Å, and the inorganic layer thickness could be estimated to be 11 ± 1.9 Å. (6) The SEI was found to be homogeneous within an analysis area of 110 × 110 μm2. The maximum deviation found for the LiF signal was 25%. The combination of the SDP method with reference measurements is a powerful methodology and will be valuable for the SEI and surface investigations of other electrode materials.

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ASSOCIATED CONTENT

S Supporting Information *

Table of atomic concentration percentages (%at.) of the untreated anode after different sputter times (eV). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Bundesministerium fuer Bildung und Forschung (BMBF) is gratefully acknowledged. REFERENCES

(1) Sauer, D.; Kleimaier, M.; Glaunsinger, W. Relevance of energy storage in future distribution networks with high penetration of renewable energy sources. Proceedings of CIRED: The 20th International Conference on Electricity Distribution, Prague, June 8−11, 2009; The Institution of Engineering and Technology, 2009, paper 0907. (2) Lazzari, M.; Scrosati, B. A cyclable lithium organic electrolyte cell based on two intercalation electrodes. J. Electrochem. Soc. 1980, 127, 773. (3) Besenhard, J. O.; Winter, M. Insertion reactions in advanced electrochemical energy storage. Pure Appl. Chem. 1998, 70, 603−608. (4) Vetter, J.; Novák, P.; Wagner, M. R.; Veit, C.; Möller, K. C.; Besenhard, J. O.; Winter, M.; Wohlfahrt-Mehrens, M.; Vogler, C.; Hammouche, A. Ageing mechanisms in lithium-ion batteries. J. Power Sources 2005, 147, 269−281. (5) Sarre, G.; Blanchard, P.; Broussely, M. Aging of lithium-ion batteries. J. Power Sources 2004, 127, 65−71. 5815


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Article

solutions based on ethylene carbonate-dimethyl carbonate mixtures. J. Electrochem. Soc. 1996, 143, 3809. (28) Kanamura, K.; Tamura, H.; Takehara, Z. XPS analysis of a lithium surface immersed in propylene carbonate solution containing various salts. J. Electroanal. Chem. 1992, 333, 127−142. (29) Edström, K.; Gustafsson, T.; Thomas, J. O. The cathode− electrolyte interface in the Li-ion battery. Electrochim. Acta 2004, 50, 397−403. (30) Dedryvère, R.; Martinez, H.; Leroy, S.; Lemordant, D.; Bonhomme, F.; Biensan, P.; Gonbeau, D. Surface film formation on electrodes in a LiCoO2/graphite cell: a step by step XPS study. J. Power Sources 2007, 174, 462−468. (31) Dedryvère, R.; Laruelle, S.; Grugeon, S.; Gireaud, L.; Tarascon, J. M.; Gonbeau, D. XPS identification of the organic and inorganic components of the electrode/electrolyte interface formed on a metallic cathode. J. Electrochem. Soc. 2005, 152, A689. (32) Andersson, A. M.; Abraham, D. P.; Haasch, R.; MacLaren, S.; Liu, J.; Amine, K. Surface characterization of electrodes from high power lithium-ion batteries. J. Electrochem. Soc. 2002, 149, A1358. (33) Castro, L.; Dedryvère, R.; Ledeuil, J. B.; Breger, J.; Tessier, C.; Gonbeau, D. Aging mechanisms of LiFePO4 // graphite cells studied by XPS: redox reaction and electrode/electrolyte interfaces. J. Electrochem. Soc. 2012, 159, A357−A363. (34) Kanamura, K.; Tamura, H.; Shiraishi, S.; Takehara, Z. Morphology and chemical compositions of surface films of lithium deposited on a Ni substrate in nonaqueous electrolytes. J. Electroanal. Chem. 1995, 394, 49−62. (35) Andersson, A. M.; Herstedt, M.; Bishop, A. G.; Edström, K. The influence of lithium salt on the interfacial reactions controlling the thermal stability of graphite anodes. Electrochim. Acta 2002, 47, 1885− 1898. (36) Herstedt, M.; Andersson, A. M.; Rensmo, H.; Siegbahn, H.; Edström, K. Characterisation of the SEI formed on natural graphite in PC-based electrolytes. Electrochim. Acta 2004, 49, 4939−4947. (37) Herstedt, M.; Stjerndahl, M.; Nytèn, A.; Gustafsson, T.; Rensmo, H.; Siegbahn, H.; Ravet, N.; Armand, M.; Thomas, J. O.; Edström, K. Surface chemistry of carbon-treated LiFePO4 particles for Li-ion battery cathodes studied by PES. Electrochem. Solid-State Lett. 2003, 6, A202. (38) Chang, L.-S.; Lin, Y.-C.; Su, C.-Y.; Wu, H.-C.; Pan, J.-P. Effect of C60 ion sputtering on the compositional depth profiling in XPS for Li(Ni,Co,Mn)O2 electrodes. Appl. Surf. Sci. 2011, 258, 1279−1281. (39) Czanderna, A. W., Madey, T. E., Powell, C. J., Eds.; Beam Effects, Surface Topography, and Depth Profiling in Surface Analysis; Plenum Press: New York, 1998. (40) Zhao, L.; Watanabe, I.; Doi, T.; Okada, S.; Yamaki, J. TG-MS analysis of solid electrolyte interphase (SEI) on graphite negativeelectrode in lithium-ion batteries. J. Power Sources 2006, 161, 1275− 1280. (41) Andersson, A. M.; Edström, K. Chemical composition and morphology of the elevated temperature SEI on graphite. J. Electrochem. Soc. 2001, 148, A1100. (42) Bar-Tow, D.; Peled, E.; Burstein, L. A study of highly oriented pyrolytic graphite as a model for the graphite anode in Li-ion batteries. J. Electrochem. Soc. 1999, 146, 824. (43) Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem. Rev. 2004, 104, 4303−4418. (44) Leroy, S.; Blanchard, F.; Dedryvère, R.; Martinez, H.; Carré, B.; Lemordant, D.; Gonbeau, D. Surface film formation on a graphite electrode in Li-ion batteries: AFM and XPS study. Surf. Interface Anal. 2005, 37, 773−781. (45) Aurbach, D.; Zaban, A. The application of EQCM to the study of the electrochemical behavior of propylene carbonate solutions. J. Electroanal. Chem. 1995, 393, 43−53. (46) Edström, K.; Herstedt, M.; Abraham, D. P. A new look at the solid electrolyte interphase on graphite anodes in Li-ion batteries. J. Power Sources 2006, 153, 380−384. (47) Zhuang, G.; Chen, Y.; Ross, P. N. The reaction of lithium with dimethyl carbonate and diethyl carbonate in ultrahigh vacuum studied

by X-ray photoemission spectroscopy. Langmuir 1999, 15, 1470− 1479. (48) Andersson, A. M.; Henningson, A.; Siegbahn, H.; Jansson, U.; Edström, K. Electrochemically lithiated graphite characterised by photoelectron spectroscopy. J. Power Sources 2003, 119−121, 522− 527. (49) Schmitz, R.; Müller, R.; Krüger, S.; Schmitz, R. W.; Nowak, S.; Passerini, S.; Winter, M.; Schreiner, C. Investigation of lithium carbide contamination in battery grade lithium metal. J. Power Sources 2012, 217, 98−101. (50) Mohai, M. XPS MultiQuant: a step towards expert systems. Surf. Interface Anal. 2006, 38, 640−643. (51) Shard, A. G.; Wang, J.; Spencer, S. J. XPS topofactors: determining overlayer thickness on particles and fibres. Surf. Interface Anal. 2009, 41, 541−548. (52) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculation of electron inelastic mean free paths (IMFPs) VII. Reliability of the TPP-2M IMFP predictive equation. Surf. Interface Anal. 2003, 35, 268−275. (53) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of electron inelastic mean free paths. V. Data for 14 organic compounds over the 50−2000 eV range. Surf. Interface Anal. 1994, 21, 165−176.

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