Composition and Growth Behavior of the Surface and Electrolyte

Article pubs.acs.org/Langmuir

Composition and Growth Behavior of the Surface and Electrolyte Decomposition Layer of/on a Commercial Lithium Ion Battery LixNi1/3Mn1/3Co1/3O2 Cathode Determined by Sputter Depth Profile X‑ray Photoelectron Spectroscopy Philip Niehoff and Martin Winter* Westfaelische Wilhelms-Universitaet Muenster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, 48149 Muenster, Germany

ABSTRACT: A detailed X-ray photoelectron spectroscopy (XPS) study of the surface and electrolyte decomposition layer of a LixNi1/3Mn1/3Co1/3O2 (NMC) cathode from commercial NMC/graphite cells by intense sputter depth profiling (SDP) using a polyatomic ion gun is provided. Cathodes of a cell after electrochemical formation and a cell at a state of initial capacity (SOIC) of 80%, which was reached after 2500 full cycles at 30 °C, are investigated.

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solid electrolyte interphase (SEI).15,16 The SEI constitutes additional resistance17 and irreversibly consumes lithium,18 which deteriorates cathode performance as a result of the correlating change in the electrode potentials19 and diminishes the overall cell capacity. At the cathode, the function and the influence on the performance of the electrolyte decomposition layers is less clear, and thus the term SEI is not generally used for these layers at the cathode. XPS is a method that is not capable only of determining layer thicknesses but also allows the determination of the chemical composition of these layers. With the help of sputter depth profiling (SDP), it is additionally possible to obtain information about the layer structure. For instance, it can be estimated if a layer consists of multilayers and in what kind of arrangement these layers appear. Several previous XPS studies have been carried out. These were able to determine the layer components and the different compositions of the different electrolytes, additives, salts, active materials, and temperatures used and applied.20,32 Regarding the determination of the layer components, early work by Aurbach et al.20,21 on lithium metal and later on also on carbon and graphite anodes33 by combining infrared spectroscopy and XPS was carried out. In these studies, it could be concluded that mainly the same components form on lithium metal as on graphite. Following this work, several studies focused on the

INTRODUCTION Several countries are enforcing a sustainable energy industry for different reasons. On one hand, carbon dioxide emission reduction is intended to preserve humankind from possible severe impacts due to climate change.1−3 On the other hand, political interest in an independent energy supply is another urge for increasing the amount of renewable energy because fossil energy sources are unequally spread over the world and cannot be regenerated after use. However, most renewable energy forms underlie fluctuations in their feasibility to provide energy. To compensate for this fluctuation, energy storage devices can be used. One promising energy storage system is the lithium ion battery (LIB)4 because it provides a rather high energy efficiency5 and has low self-discharge compared to other batteries6 and the high energy density enables the distribution of renewable energy to mobile applications, for instance, electric vehicles.7,8 Though gradually decreasing, the costs of LIBs are rather high for these applications.9 Therefore, intensive research and development is carried out to reduce the costs by the development of cheaper energy materials and less-expensive manufacturing processes as well as by increasing the lifetime of LIBs,10−14 which reduces the costs for investment. To increase the lifetime, the critical aging effects need to be determined, which requires various analytical methods. Most critically, the function of LIBs is related to and sometimes even dependent on layers of electrolyte decomposition products, which are formed on the electrode surfaces and protect the electrodes and the electrolyte from further reaction with each other. At the anode, this layer is called the © 2013 American Chemical Society

Received: August 23, 2013 Revised: October 20, 2013 Published: December 2, 2013 15813

dx.doi.org/10.1021/la403276p | Langmuir 2013, 29, 15813−15821

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determination of surface species on the cathode side.22−24,28,29 Especially informative work was performed by Dedryvere et al.,23,24 who estimated the surface species by valence band analysis and therefore could directly prove major surface species and their signal characteristics. However, a quantification of the electrolyte decomposition layers has not yet been carried out. In a previous study, we investigated the SEI layer on a commercial graphite surface.34 In this study we examine the electrolyte decomposition layer on a LixNi1/3Mn1/3Co1/3O2 (NMC) cathode used in a commercial lithium ion cell. Apart from the active NMC, the electrode consists of a PVdF binder, a conductive carbon black additive, and an aluminum current collector. For this objective, it is first necessary to determine the layer structure of the decomposition layer, which stems from components of the electrolyte (i.e., organic carbonate solvents35 and LiPF6 as the electrolyte salt19,36). For instance, the presence of a multilayer structure would lead to a overestimation of the outer layers compared to the inner layers because the photoelectrons of the inner layers are partially adsorbed by the outer layers. Hence, the layer structure has to be obtained to correct the intensities with regard to this structure. The layer structure investigation was carried out by a detailed SDP experiment using a 13-step procedure and subsequent analysis of the relative intensity changes. If a multilayered structure is present, then the relative intensities of the lower layer components will increase during the SDP treatment. A faster decrease in one layer component compared to another is not sufficient to determine a multilayered structure. This change can also originate from different rates of removal of the respective component. With the help of a model of the layer structure obtained from the SDP experiment and using the signal intensities of the active material, a quantification can be carried out. Therefore, it is especially important to prove, that the active material signals are not corrupted by any other formed species.

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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. A sputter procedure of five times for 10 s, five times for 120 s, and two times for 600 s was used. The pressure within the analysis chamber was around 1 × 10−6 Pa. The fitting was carried out with the help of CasaXPS. Standard fit function GL(30) was chosen. The vibrational shakeup of the PVdF was taken into account by the use of VS(0.35, 0.65, 0.2, 0.1, 0.025)SGL(11) as the fitting function for the F 1s peak. The C 1s peaks of the PVdF, however, were fitted by the standard GL(30) function. Their binding energy (BE) difference was fixed at 4.15 eV and their area ratio was fixed at 1 because these values were determined by reference measurements of the untreated cathode material (Results and Discussion section). 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 (conductive carbon at BE = 284.5 eV) as an internal reference.

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RESULTS AND DISCUSSION To support the quantitative interpretation of the data of the electrochemically treated samples, the untreated cathode, before electrochemical formation, was investigated first.

Figure 1. F 1s, O 1s, and C 1s spectra of the untreated NMC cathode.

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 9 Measurements of the Untreated NMC Cathode

EXPERIMENTAL SECTION

The LixNi1/3Mn1/3Co1/3O2 (NMC) electrodes under investigation were obtained from an NMC/graphite laboratory pouch cell from a commercial supplier. Polyvinylidene difluoride (PVdF) was used as the binder, and Al foil was used as the current collector. The electrolyte consists of 1 M lithium hexafluorophosphate in a ethylene carbonate/ ethyl-methyl 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 layer components. The electrodes were transported in a sealed vessel to an argon-flooded glovebox connected to the XPS. The cycled cells were cycled at 30 °C until the 1 C capacity between 4.2 and 3.0 V cut-off voltages reached 80% SOIC. The sample take-in time under ultrahigh vacuum was longer than 12 h to remove volatile electrolyte components. XPS (Axis Ultra DLD, Kratos, U.K.) was measured using a monochromatic Al Kα source (hν = 1486.6) at 10 mA filament current and 12 kV filament voltage source energies. To compensate for the charging of the sample, the charge neutralizer was used. 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, being 10 times larger than the analysis area (110 μm). The angle between the surface

region

component

BE

σmax

fwhm

σmax

%at.

σmax

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

PVdF con. carbon NMC CF2−CH2 CH2−CF2 con. carbon

687.63 531.58 529.42 290.87 286.42 284.50

0.10 0.12 0.07 0.12 0.11 0.01

1.61 2.52 1.07 1.19 1.24 0.73

0.03 0.57 0.07 0.12 0.15 0.05

19.13 1.78 1.28 10.49 10.65 56.67

2.10 0.44 0.33 1.24 0.87 3.97

From the F 1s spectra shown in Figure 1, the peak can be identified as relating to the fluorine of the PVdF. The peak shows an asymmetric peak shape due to the partial excitation of stretching modes of the polymer, often referred to as shakeup effects. Therefore, an asymmetric fitting function was chosen (Experimental Section). The O 1s spectra show a small, broad signal at ca. 531.5 eV (Table 1) possibly corresponding to oxidized conductive carbon (“con. carbon”). In addition, a sharp peak at 529.5 eV can be assigned to the cathode material NMC itself. The C 1s spectra of the cathode material shows three peaks: two symmetrical peaks at 291 and 286 eV representing the CF2 (CF2CH2) and CH2 (CH2CF2) groups of PVdF, respectively, 15814


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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 it should be identical for PVdF. In the Co 2p, Mn 2p, Ni 2p, and Li 1s spectra, the signal intensity of the NMC species is too low to allow reliable quantification. In Table 1, the average binding energies (BE), the full width at half-maximum (fwhm), and the atomic concentration percentages (%at.) as well as the corresponding maximal absolute deviations (σmax) are given as received from the fitting data. The %at. values given in this work reflect the relative intensity ratios of the different investigated components, with signal intensities corrected by the relative sensitivity factors. These values do not take structural effects into account. Because of the absorption of photoelectrons by matter, in a multilayer structure the lower-layer intensity will be decreased by the presence of the upper layers. This means in this case that the PVdF intensities are overestimated. Below we introduce the composition percentage value (%comp). This value gives the correct relation of the different components by correcting the decreased intensities of the lower layers. The %at. value of the F 1s peak of the PVdF fits the C 1s PVdF (%at.) values, with a deviation of 10%, reasonably well. A possible explanation for the deviation could be the measurement accuracy as well as small amounts of organic contamination. Furthermore, it can be recognized from Table 1 that in the XPS measurement only a small portion of the signals is attributed to the NMC material. Although the resolution of the XPS with 0.1%at. is sufficient to analyze a surface layer on the NMC material, it has to be realized that only the same %at. value of the NMC material, which was decreased after formation and cycling, can be attributed to a layer on top of the NMC material. If there were more decomposition products forming these belong to a surface layer on top of conductive carbon or PVdF. To estimate the possible decomposition of the samples as a result of the sputtering procedure,34 reference measurements with the untreated cathode (before any contact with electrolyte and before any electrochemical experiment) were carried out. The untreated cathode material was sputtered five times for 10 s, five times for 120 s, and then two times for 600 s.

Figure 2. Relative F 1s intensity decrease of the untreated cathode by sputter treatment.

Figure 3. F 1s, O 1s, and C 1s spectra of the SDP of the untreated cathode. Spectra shown from top to bottom are acquired after sputtering times of 0, 50, 170, 410, 650, and 1850 s.

and one asymmetric peak at 284.5 eV that may carry the XPS data of conducting carbon, used in the cathode to increase the electronic conductivity. The asymmetry was fitted with a

Figure 4. F 1s, O 1s, C 1s, P 2p, and Li 1s spectra of the cathode material after formation (top) and from an 80% SOIC sample (bottom). 15815


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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 Six Measurements of the Cathode after Formation and from an 80% SOIC Sample region

component

BE

σmax

fwhm

σmax

%at.

σmax

BE

σmax

fwhm

σmax

%at.

σmax

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

PVdF/LIPFx LIF RCO3 Li2CO3 NMC CF2−CH2 shake up, RCO3 CH2−CF2 carbon con. carbon LiPFx LiPFxOy

686.87 685.10 534.07 532.37 529.33 290.40 287.28 286.25 285.35 284.50 137.24 135.22 56.08

0.15 0.09 0.23 0.29 0.05 0.04 1.09 0.04 0.21 0.00 0.11 0.79 0.11

1.74 1.60 1.97 2.31 1.07 1.28 3.90 1.28 0.97 0.82 1.99 1.69 1.61

0.03 0.00 0.23 0.19 0.07 0.07 2.20 0.07 0.25 0.06 0.05 0.80 0.68

17.27 3.31 2.73 4.59 0.82 6.25 3.91 6.26 4.32 43.27 1.67 0.24 4.85

0.75 0.83 0.92 1.01 0.17 0.64 3.17 0.65 1.46 3.20 0.62 0.35 2.50

686.78 685.13 533.93 532.34 529.35 290.39 287.49 286.24 285.08 284.50 137.06 135.10 56.09

0.23 0.14 0.17 0.22 0.46 0.09 1.22 0.09 0.23 0.00 0.20 0.24 0.42

1.84 1.60 1.89 1.93 1.07 1.30 3.38 1.30 1.35 0.97 2.00 1.98 1.61

0.13 0.00 0.24 0.35 0.32 0.00 1.33 0.00 0.36 0.09 0.00 0.05 0.44

14.86 3.45 4.54 5.39 0.31 4.63 5.24 4.63 10.57 36.15 2.19 0.60 5.93

3.19 0.34 2.20 1.30 0.25 2.86 4.98 2.87 13.64 15.18 1.23 0.17 2.45

a decomposition product. The amount formed by the used sputtering procedure was about 6% of the initial F 1s PVdF peak. It should be emphasized that these metallic fluorides do not occur in a lithium ion battery. They are artifacts that are formed during the SDP-XPS analysis. This decomposition cannot be excluded when SDP-XPS experiments are performed and should be taken into consideration when interpreting the SDP-XPS data of aged cathode materials. In the O 1s spectra, the NMC %at. value is increasing up to 2.6 over the sputtering time, indicating a relative reduction of the amount of PVdF. In the C 1s spectra, another decomposition artifact can be found. Within the peak representing conductive carbon, a shoulder appears during sputtering, at a slightly higher BE. The chemical decomposition of the PVdF by SDP as the origin of this peak can be disregarded because the intensity of the 285.3 eV peak is about 2 times larger than the initial C 1s PVdF peak’s intensity. In addition, a peak occurs at 287.8 eV due to sputtering. Therefore, the fit of the altered asymmetric peak shape of the conductive carbon needs to be adapted. Furthermore, these decomposition artifacts are not caused by the specific ion source because these artifacts are also present when Ar is used as the ion source. These artifacts do not allow a detailed analysis of the C 1s spectra because carbonate signals37 and some polymeric

Table 3. Composition of the Decomposition Layer (%comp.) of the Cathode Material after Formation and at 80% SOIC region

component

after formation

80% SOIC

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

LIF RCO3 Li2CO3 LiPFx LiPFxOy

0.43 0.12 0.20 0.22 0.03

0.36 0.16 0.19 0.23 0.06

In Figure 2, the F 1s intensity is plotted against the sputter time. It can be noticed that the intensity of the PVdF signal decreases exponentially. This can be understood by the structure of the electrode. First, a thin PVdF layer on the outer electrode surface is quickly removed. Then PVdF in the pores is removed at a much slower pace and some of the PVdF in the pores is not removed at all on the time scale of the experiment. The amount of PVdF left after the sputtering process might vary between 20 and 30% of the initial value, possibly as a result of a different pore structure. Increasing the sputter crater size did not affect the results. This behavior was already found for the investigations of the graphite anode.34 The F 1s spectra in Figure 3 are showing an increasing decomposition peak at 685.6 eV, which increases as a function of sputter time. This peak can be assigned to a metallic fluoride species, which may form by the reaction of PVdF and NMC as

Table 4. Atomic Concentration Percentages (%at.) of the NMC Cathode Material after Formation for Different Sputtering Times (s) region

component

0

10

20

30

40

50

170

290

410

530

650

1250

1850


PVdF/LIPFx LiF RCO3 LiCO3 NMC CF2−CH2 shake up, RCO3 CH2−CF2 carbon con. carbon LiPFx LiPOxFy

16.82 3.83 3.47 3.54 1.04 6.85 7.97 6.83 11.03 31.00 1.05 0.39 5.73

16.47 4.13 3.45 3.29 1.18 7.32 8.11 7.30 10.94 31.25 0.69 0.55 4.80

13.14 3.80 2.38 2.24 2.05 5.31 8.75 5.30 13.79 36.87 0.66 0.31 5.13

11.50 3.95 2.21 2.35 2.24 4.61 7.98 4.60 17.78 38.55 0.58 0.37 3.13

10.69 3.94 1.86 2.48 2.34 4.13 9.66 4.12 20.94 36.68 0.50 0.32 2.18

9.83 3.91 1.68 2.43 2.37 3.83 10.99 3.82 22.24 35.67 0.00 0.47 2.50

8.13 3.78 1.40 2.59 2.52 3.02 9.73 3.01 23.57 37.91 0.38 0.38 3.33

7.40 3.91 1.14 2.30 2.45 2.50 9.28 2.50 24.08 41.33 0.46 0.29 2.28

6.69 3.73 1.17 2.54 2.40 1.91 8.99 1.90 25.34 42.12 0.23 0.00 2.89

6.39 3.59 0.99 2.48 2.45 2.31 7.77 2.30 25.81 45.35 0.00 0.40 0.00

5.68 3.57 0.69 2.58 2.38 1.88 9.20 1.87 27.05 44.62 0.23 0.18 0.00

5.24 3.40 0.82 2.72 2.37 1.47 6.09 1.47 23.72 52.28 0.00 0.33 0.00

4.74 3.11 0.78 2.37 2.21 1.28 5.90 1.28 20.98 56.90 0.00 0.31 0.00

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Table 5. Atomic Concentration Percentages (%at.) of the NMC Cathode Material at 80% SOIC for Different Sputtering Times (s) region

component

0

10

20

30

40

50

170

290

410

530

650

1250

1850


PVdF/LIPFx LiF RCO3 LiCO3 NMC CF2−CH2 shake up, RCO3 CH2−CF2 carbon con. carbon LiPFx LiPOxFy

13.44 3.54 4.92 5.58 0.42 4.11 8.54 4.10 15.42 29.14 1.85 0.46 7.66

11.39 3.87 3.34 3.43 1.16 4.06 8.20 4.05 17.53 34.78 0.85 0.51 6.34

10.56 4.13 2.82 3.14 1.44 3.21 8.88 3.20 20.74 35.48 0.75 0.60 4.58

9.32 4.09 2.53 2.70 1.36 3.40 9.46 3.39 17.85 39.14 0.56 0.38 5.36

8.79 4.03 2.56 2.92 1.44 2.29 10.84 2.29 24.10 35.50 0.41 0.56 3.92

7.96 3.99 2.27 2.87 1.57 3.30 8.46 3.29 19.09 43.11 0.39 0.59 2.85

6.23 3.66 1.64 2.73 1.66 1.58 10.09 1.58 23.49 38.77 0.24 0.26 7.81

5.86 3.62 1.28 2.98 1.67 1.06 9.49 1.05 27.07 42.61 0.35 0.47 2.21

5.36 3.59 1.16 2.82 1.61 0.64 8.20 0.64 29.09 42.83 0.38 0.50 2.87

4.71 3.42 1.21 2.62 1.61 0.68 7.68 0.68 29.53 47.07 0.26 0.38 0.00

4.29 3.04 1.15 2.46 1.56 1.13 8.69 1.13 22.44 50.36 0.29 0.42 2.87

3.88 2.86 1.14 2.25 1.48 1.31 4.59 1.31 18.36 59.71 0.28 0.38 2.31

3.44 2.73 0.91 2.24 1.48 0.92 4.93 0.92 19.73 59.79 0.20 0.15 2.45

the F 1s, O 1s, C 1s, P 2p, and Li 1s spectra of these cathodes are shown. Table 2 gives quantification results of the components, their binding energy (BE), fwhm, %at., and corresponding maximum deviations as received from fitting six different samples each. In the F 1s spectra, two peaks at BEs of 687 and 685 eV, respectively, can be noticed. With the help of preliminary experiments (Table 1, Figure 3), these peaks can be assigned to PVdF and LiF. This result is in accordance with literature data.28 In addition, the LiPF6 signal from the remaining electrolyte salt overlaps the PVdF signal.39 In the O 1s spectra, three peaks were assigned, although there may be more components hidden within these signals. Nevertheless, these peaks cannot be resolved to distinguish the components represented by the peaks. For example, possible lithium alkyl carbonates (RLiCO3) species, which would contribute to the RCO3 and Li2CO3 peak intensities with an intensity ratio of 1:2, cannot be resolved. The signal at the higher BE of 533.5 eV can be assigned to organic carbonates (RCO3), the peak at 531.5 eV, to lithium carbonates, and the peak at 528.3 eV, to lithium oxides and NMC. These components were found to be major parts of the anode SEI and the electrolyte decomposition layer on the cathode.23,24,40

Table 6. Calculated Mean Free Path for Different Decomposition Layer Components Using the TPP Equations component

λi

LiPF6 PEO Li2CO3 LiF

28.39 30.98 27.43 26.85

Table 7. Decomposition Layer Thickness Values Including the Salt and Excluding the Salt (Å) as Well as the Mean Deviations for the Different Samples (%) sample

layer thickness

layer thickness without salt

deviation

after formation 80% SOIC

8.40 26.99

6.31 19.10

16 23

species25,38 would appear in the binding-energy region where these artifacts are present. An analysis of the decomposition layer on the cathode material after electrochemical formation and on a cathode material from a cell at 80% SOIC was carried out. In Figure 4,

Figure 5. Model of the decomposition layer formed at the NMC cathode after formation and at 80% SOIC. The structures of the decomposition layer on the cathode remain uncertain. For comparison, the SEI structure at the graphite anode after formation is shown.34 15817


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on the finding that the decomposition layers can be only partially removed as a result of the porous structure of the electrode. Therefore, if Li2O would be present, then a higher NMC %at. value should be found for the 80% SOIC sample than for the untreated cathode material. By taking a closer look with SDP of the NMC sample after electrochemical formation, it can be noticed that PVdF, LiPF6, and the organic carbonate signals are decreasing as a function of sputtering time, leading to an increase in the conductive carbon, carbon, and NMC signals. However, no increase in the Li2CO3 and LiF signal was found. Hence, the difference in signal change is due to different rates of removal but not due to a multilayer structure. Therefore, it can be concluded that the components of the electrolyte decomposition layer are rather homogeneously distributed. However, it has to be considered that most of the decomposition layer product signals belong to the layer on top of the conductive carbon. Because the conductive carbon is not electrochemically active, the decomposition products are not expected to form on the conductive carbon but rather on the NMC material and subsequently diffuse to the conductive carbon. Hence, the concluded homogeneity of the decomposition layer is evidence only of the layer on the conductive carbon. The structure of the decomposition layer on the NMC material can not be revealed with the help of XPS measurements. The same SDP characteristics can be found for the 80% SOIC sample. Hence, a homogeneous decomposition layer on the conductive carbon can also be concluded for this cathode sample. By considering that the NMC %at. value increases while sputtering, the NMC %at. values of the sample after formation and the sample at 80% SOIC do not reach the NMC %at. values found for the untreated cathode material. This can be explained by the thicker electrolyte decomposition layer of the electrochemically treated samples. This decomposition layer can be reduced only to about 25% of its initial thickness by the sputtering procedure, as was determined for the PVdF layer by the sputtering experiments from the untreated cathode material. Hence, the remaining decomposition layer attenuates the NMC %at. values as a result of the partial adsorption of the photoelectrons. The following calculations will show that the maximum NMC %at. values reached fit very well with the estimated decomposition layer thickness. This finding reinforces the conclusion that no or only minimal amounts of Li2O are present in the decomposition layer. The differences in the %at. values in Tables 2 and 4 and Tables 2 and 5 at 0 s sputtering time can be understood from the difference in present LiPFx on the surface. When the electrolyte is evaporated under vacuum, crystallization of the LiPF6 takes place. This process causes a rather inhomogeneous distribution of LiPF6 on the surface, explaining the differences in the %at. values. For this reason, the decomposition layer thickness with and without the salt will be calculated. Regarding the composition of the decomposition layer on the NMC material only indirect conclusions can be drawn. Most of the decomposition component signals correspond to the layer on the conductive carbon that most likely builds up as a result of the diffusion of decomposition products from the NMC material. The decomposition layer components on the conductive carbon are homogeneously distributed.Therefore, no attenuation of a certain component due to a certain layer structure is present, and the %at. values do not need to be corrected. Therefore, the composition percentage values

The C 1s spectrum was fitted by the use of five fitting functions. Several sigals were estimated from the preliminary experiments and attributed to CF2CH2, shakeup, CH2CF2, and conductive carbon. However, in addition, within the shakeup peak the RCO3 signals are included as well.39 The fitting function at 285.3 eV can be attributed to carbon. The carbon could originate from a hydrocarbon polymer being formed from the decomposition of the organic carbonate electrolyte solvents, but below it will be shown that the origin of the peak can be more likely attributed to the conductive carbon electrode additive. The multitude of different components, which could be present in the decomposition layer and the determined SDP artifacts in the C 1s spectrum, are causing significant quantification errors. The P 2p spectrum shows two components, one at a BE of 139 eV as determined by preliminary experiments according to LiPFx and one at a slightly lower BE according to LiPFxOy.32 The Li 1s spectra are fitted by only one signal (at BE 54 eV) because as a result of the small relative sensitivity factor the signal intensity is very low. By comparison of the cathode after electrochemical formation with the cathode at 80% SOIC, it can be recognized that the intensity of the NMC signal from the 80% SOIC sample is reduced by 62% compared to that of the sample after electrochemical formation. This indicates an increase in the decomposition layer thickness. Li2O and NMC have very similar BEs, and thus the NMC signal could possibly be distorted. However, it will be shown in the sputtering experiments below that Li2O contributes only negligibly to the decomposition layer, if at all. In addtion, it can be noticed from Table 2 that at least 10%at. corresponds to electrolyte decomposition products. As noticed above, only about 0.5%at. belongs to the decomposition layer formed on the NMC material. The remaining 9.5%at. electrolyte decomposition products are located at the conductive carbon or PVdF. Concerning the interpretation of the composition of the electrolyte decomposition layer, it is first necessary to estimate the structure of the decomposition layer because the layer structure affects the quantification of the XPS data as explained above. To analyze the structure of the decomposition layer, SDP was carried out. The results are given in Tables 4 and 5. First, by comparison of the SDP data of the cathode after electrochemical formation and the cathode from the 80% SOIC sample, it can be recognized that with the applied sputtering conditions no notable decomposition of carbonates to Li2O took place. This can be concluded from the finding that the NMC %at. value (in which also any present Li2O component is included) of the 80% SOIC sample never exceeds the value of the sample after formation. Because in the layer on the 80% SOIC sample more carbonates are present, as can be seen from the higher %at. value of Li2CO3 and the lower NMC %at. value indicating a thicker decomposition layer (Table 2), more electrolyte decomposition should have taken place as well. Hence, the relative NMC %at. value should increase to higher values than the NMC %at. value of the cathode after formation. The lower NMC %at. values, during the complete sputtering time, found for the two electrochemically treated samples compared to those for the untreated samples indicate that the NMC %at. value indeed mostly originates from NMC and not from additional Li2O, which might have formed during formation or cycling, respectively. This conclusion is based 15818


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This is due to the fact that the amount of PVdF in the electrode is not changing. To estimate the differences in decomposition layer thickness within one sample, the variation of the thickness was calculated by adjusting the Ib value by the maximum deviation of the NMC %at. value divided by 2. The determined thicker decomposition layer thickness of the 80% SOIC sample correlates with that compared to the untreated NMC cathode material’s lower maximum intensity of the NMC %at. value found during the sputtering process (Table 5). Because the sputtering process is capable of removing surface layers only down to 75%, still 25% of the layer remains and attenuates the substrate signals. In the case of the 80% SOIC sample, this means that a 6.75-Å-thick decomposition layer remains. To estimate the effect on the substrate intensity, eq 3 can be rearranged. Then the calculated %at. value of NMC should be 1.97, which is a first-order approximation in accordance with the measured data. The calculated decomposition layer thickness values also contain the contribution of the electrolyte salt. To obtain the decomposition layer thickness values without the influence of the salt, the contribution of the salt to the decomposition layer, as estimated by eq 1, was determined and subtracted from the total decomposition layer thickness value. The results are given in Table 7. With these results, models of the decomposition layer on the NMC material describing the thickness and composition can be concluded. However, no information on the structure of the decomposition layer could be concluded. These are the models in Figure 5.

(%comp.) can be obtained by calculating the ratios of %at. values correlating to the different components. Therefore, eq 1 was used, with n as the corresponding stoichiometric factor. The %comp. value for the LiF component was calculated from the F 1s LiF signal at 685.1 BE, the RCO3 from the O 1s peak at 534 eV BE with a stoichiometric factor of 3, the Li2CO3 from the O 1s peak at 532.3 eV BE with a stoichiometric factor of 3, the LiPF6 from the P 2p peak at 137.1 eV BE, and the LiPFxOy from the P 2p peak at 135.2 eV BE. Results are given in Table 3. %at., i

%comp., i =

∑

ni %at., i ni

(1)

From these results, it can be seen that the %comp. values of organic carbonates and decomposed salt slightly increased whereas the LiF %comp. values slightly decreased. This indicates that the decomposition processes leading to the formation of LiF are less pronounced after the electrochemical formation process. To quantify the decomposition layer thickness, the change in the O 1s NMC %at. value of the corresponding sample of interest in comparison to the O 1s NMC %at. value of the untreated cathode material is used. The change in this signal is due to the formation of the decomposition layer, which is decreasing the NMC signal intensity. To quantify the thickness of this layer, the inelastic mean free path of this decomposition layer is needed. In the following section, the necessary calculations are discussed in detail. With the help of XPS Multiquant software41 using the TPP equations,42,43 the mean free path λi of different decomposition layer components (Table 6) was calculated to estimate the mean free path of the total decomposition layer λads at the BE of the O 1s NMC signal. For the calculations, the molar weight, density, and number of valence electrons are necessary. Because these values are not known for the RCO3 species, the organic part was taken into account by the addition of the calculated λi of PEO to the λads. Because the different calculated λi values differ only by a maximal 14%, the mean free path λads (eq 2) of the total decomposition layer can be approximated with the average value of the λi values of LiPF6, PEO, Li2CO3, and LiF. With this λads and eq 3, the thickness of the formed decomposition layer can be estimated.

λads =

∑ λi i

ddiff = ln

Ia λads cos θetfactor Ib

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CONCLUSIONS For the first time, the decomposition layer thickness and thickness change due to aging on a commercial NMC material could be determined. The developed method generally allows the investigation of these characteristics with respect to different kinds of cathode materials. The investigations showed that from XPS measurements of battery cathodes containing a conductive agent mainly the electrolyte decomposition products located on the conductive agent are detected. Therefore, the determination of the composition of the decomposition layer on the cathode-active material (i.e., NMC) can be carried out only indirectly. Furthermore, this finding negates the determination of the layer structure on commercial electrodes. The SDP method led to the conclusion that no significant amount of Li2O contributed to the electrolyte decomposition layer on the cathode. Therefore, it was possible to calculate the decomposition layer thickness with the help of the NMC O 1s signal. The decomposition layer on the NMC material after formation has layer thicknesses of 8.4 Å including LiPF6 and 6.3 Å excluding LiPF6. The composition, determined indirectly from the decomposition products located at the conductive carbon surface, is with 43% mainly consisting of LiF. Li2CO3 and RCO3 make 20 and 12% contributions, respectively, to the composition of the decomposition layer. About 3% of the decomposition layer consists of decomposed salt. The decomposition layer on the NMC material at 80% SOIC increased in thickness to 27 Å including LiPF6 and to 19.1 Å excluding LiPF6. This is a decomposition layer thickness increase by a factor of 3. Regarding the composition, the fraction of LiF decreased about 7% whereas the percentage of

(2)

(3)

With eq 2, a value of 28.41 Å was obtained for the mean free path of the photoelectrons corresponding to a BE of 528 eV. With this value and the NMC %at. value of the untreated cathode material for Ib and the NMC %at. value of the corresponding cathode at 80% SOIC for Ia, with a tfactor value, a topographic correction factor for layers on spherical particles of 0.67,44 and a θe of 0°, the obtained ddiff value is 8.59 Å for the decomposition layer thickness of the sample after formation and 26.99 Å for the sample at 80% SOIC. The PVdF binder does not contribute to the calculated thickness values because the attenuating influence of the PVdF layer on the NMC signal is the same for the untreated and treated cathode materials. 15819


Langmuir

Article

(18) Buqa, H.; Blyth, R.; Golob, P.; Evers, B.; Schneider, I.; Alvarez, M.; Hofer, F.; Netzer, F.; Ramsey, M.; Winter, M.; Besenhard, J. Negative electrodes in rechargeable lithium ion batteries - Influence of graphite surface modification on the formation of the solid electrolyte interphase. Ionics 2000, 6, 172−179. (19) Krueger, S.; Kloepsch, R.; Li, J.; Nowak, S.; Passerini, S.; Winter, M. How do reactions at the anode/electrolyte interface determine the cathode performance in lithium-ion batteries? J. Electrochem. Soc. 2013, 160, A542−A548. (20) 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. (21) 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. (22) Edström, K.; Gustafsson, T.; Thomas, J. O. The cathode− electrolyte interface in the Li-ion battery. Electrochim. Acta 2004, 50, 397−403. (23) 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. (24) 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. (25) Andersson, A. M.; Edström, K. Chemical composition and morphology of the elevated temperature SEI on graphite. J. Electrochem. Soc. 2001, 148, A1100. (26) 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. (27) Eriksson, T.; Andersson, A. M.; Bishop, A. G.; Gejke, C.; Gustafsson, T.; Thomas, J. O. Surface analysis of LiMn2O4 electrodes in carbonate-based electrolytes. J. Electrochem. Soc. 2002, 149, A69. (28) 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. (29) 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. (30) 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. (31) 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. (32) 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. (33) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-Eli, Y. A comparative study of synthetic graphite and Li electrodes in electrolyte solutions based on ethylene carbonate-dimethyl carbonate mixtures. J. Electrochem. Soc. 1996, 143, 3809. (34) Niehoff, P.; Passerini, S.; Winter, M. Interface investigations of a commercial lithium ion battery graphite anode material by sputter depth profile X-ray photoelectron spectroscopy. Langmuir 2013, 29, 5806−5816. (35) Tasaki, K.; Goldberg, A.; Winter, M. On the difference in cycling behaviors of lithiumion battery cell between the ethylene carbonate- and propylene carbonate-based electrolytes. Electrochim. Acta 2011, 56, 10424−10435.

RCO3 and decomposed salt increased. This indicates a change in the decomposition processes after formation. (Figure 5)

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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 greatly acknowledged.

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REFERENCES

(1) Arrhenius, S. XXXI. On the influence of carbonic acid in the air upon the temperature of the ground. Philos. Mag. Ser. 5 1896, 41, 237−276. (2) Parry, M. L.; Canziani, O. F.; Palutikof, J. P.; van der Linden, P. J.; Hanson, C. E. Climate Change 2007: Impacts, Adaptation and Vulnerability; Cambridge University Press: Cambridge, U.K., 2007; Fourth Assessment Report. (3) Change, I. P. Climate Change 2007: The Physical Science Basis; Cambridge University Press: Cambridge, U.K., 2007; Fourth Assessment Report. (4) Besenhard, J. O.; Winter, M. Insertion reactions in advanced electrochemical energy storage. Pure Appl. Chem. 1998, 70, 603−608. (5) Tanaka, T.; Ohta, K.; Arai, N. Year 2000 R&D status of largescale lithium ion secondary batteries in the national project of Japan. J. Power Sources 2001, 97−98, 2−6. (6) Winter, M.; Brodd, R. J. What are batteries, fuel cells, and supercapacitors? Chem. Rev. 2004, 104, 4245−4270. (7) Kennedy, B.; Patterson, D.; Camilleri, S. Use of lithium-ion batteries in electric vehicles. J. Power Sources 2000, 90, 156−162. (8) Wagner, R.; Preschitschek, N.; Passerini, S.; Leker, J.; Winter, M.; Münster, U. Current research trends and prospects among the various materials and designs used in lithiumbased batteries. J. Appl. Electrochem. 2013, 43, 481−496. (9) Gerssen-Gondelach, S. J.; Faaij, A. P. Performance of batteries for electric vehicles on short and longer term. J. Power Sources 2012, 212, 111−129. (10) Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W. Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage. Adv. Mater. 2012, 24, 5166−5180. (11) Yi, T.-F.; Li, X.-Y.; Liu, H.; Shu, J.; Zhu, Y.-R.; Zhu, R.-S. Recent developments in the doping and surface modification of LiFePO4 as cathode material for power lithium ion battery. Ionics 2012, 18, 529− 539. (12) Xu, B.; Qian, D.; Wang, Z.; Meng, Y. S. Recent progress in cathode materials research for advanced lithium ion batteries. Mater. Sci. Eng., R 2012, 73, 51−65. (13) Zhang, Y.; Huo, Q.-y.; Du, P.-p.; Wang, L.-z.; Zhang, A.-q.; Song, Y.-h.; Lv, Y.; Li, G.-y. Advances in new cathode material LiFePO4 for lithium-ion batteries. Synth. Met. 2012, 162, 1315−1326. (14) Braun, P. V.; Cho, J.; Pikul, J. H.; King, W. P.; Zhang, H. High power rechargeable batteries. Curr. Opin. Solid State Mater. Sci. 2012, 16, 186−198. (15) 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. (16) Winter, M. The solid electrolyte interphase - the most important and the least understood solid electrolyte in rechargeable Li batteries. Z. Phys. Chem. 2009, 223, 1395−1406. (17) Schranzhofer, H.; Bugajski, J.; Santner, H.; Korepp, C.; Möller, K.; Besenhard, J.; Winter, M.; Sitte, W. Electrochemical impedance spectroscopy study of the SEI formation on graphite and metal electrodes. J. Power Sources 2006, 153, 391−395. 15820


Langmuir

Article

(36) Lux, S.; Lucas, I.; Pollak, E.; Passerini, S.; Winter, M.; Kostecki, R. The mechanism of HF formation in LiPF6 based organic carbonate electrolytes. Electrochem. Commun. 2012, 14, 47−50. (37) Zhao, L.; Watanabe, I.; Doi, T.; Okada, S.; ichi Yamaki, J. TGMS analysis of solid electrolyte interphase (SEI) on graphite negativeelectrode in lithium-ion batteries. J. Power Sources 2006, 161, 1275− 1280. (38) 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. (39) 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. (40) 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. (41) Mohai, M. XPS MultiQuant: a step towards expert systems. Surf. Interface Anal. 2006, 38, 640−643. (42) 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. (43) 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. (44) Shard, A. G.; Wang, J.; Spencer, S. J. XPS topofactors: determining overlayer thickness on particles and fibres. Surf. Interface Anal. 2009, 41, 541−548.

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Composition and Growth Behavior of the Surface and Electrolyte

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