Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A

Article pubs.acs.org/JPCC

Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A Postmortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES) Niloofar Ghanbari, Thomas Waldmann,* Michael Kasper, Peter Axmann, and Margret Wohlfahrt-Mehrens

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ZSW, Zentrum für Sonnenenergie und Wasserstoff, Forschung Baden-Württemberg, Helmholtzstrasse 8, D-89081 Ulm, Germany ABSTRACT: Durability and performance of Li-ion cells are impaired by undesirable side reactions, observed as capacity decreases and resistance increases during their usage. This degradation is caused by aging mechanisms on the material level including surface film formation, especially in the case of graphite-based anodes. The present study evaluates the applicability of glow discharge optical emission spectroscopy (GD-OES) as a powerful tool to study aging-induced film formation on graphite anodes of Li-ion cells, including deposition of metallic Li. The technique provides depthresolved information on the elemental distribution in the samples from the anode surface to the current collector (through-plane resolution). Additionally, conducting GD-OES depth profiling at different positions of an aged graphite anode reveals differences in surface film growth across the anode plane (in-plane resolution). After verification of the GD-OES method by well-established analytical techniques, aged anodes from commercial state-of-the-art Li-ion cells are analyzed. The results show through-plane and in-plane inhomogeneity in surface film growth: local island-like Li deposition is revealed for 16Ah pouch cells cycled at 45 °C and high charging current density while a more homogeneous Li plating gradient is found for cycling 26650-type cells at −20 °C.

1. INTRODUCTION The human-induced climate change due to increasing CO2 emission1−3 can be counteracted by moving toward zero emission transport systems.4,5 High gravimetric and volumetric energy density of Li-ion cells has made them a promising candidate for zero/low emission means of transport.4,5 However, unwanted side reactions occurring during the use of Li-ion cells limit their lifetime.6,7 Understanding the root causes of aging mechanisms is the key to improve lifetime and sustainability of Li-ion cells while complying with the demands of the automotive industry. State-of-the-art Li-ion cells utilize anodes composed of graphite particles, which are prone to chemical reactions with electrolyte leading to growth of the solid electrolyte interphase (SEI) during usage.6−12 Another less known aging mechanism is deposition of metallic Li as a parallel reaction to Li intercalation into graphite. Due to the comparably high reactivity of metallic Li with electrolyte, Li deposition leads to rapid capacity loss9,13−18 and can impair cell safety.19,20 Li deposition is so far often referred to as “Li plating”; however, the term “metal plating” is only correct in the case of a homogeneously deposited metal film. For example, Jalkanen et al. and Bauer et al. have recently shown that Li deposition can also be more inhomogeneous and localized.21,22 Li deposition can be detected by negative anode potentials vs Li/Li+ in three-electrode full cells with an additional reference electrode.9,17,18,20,23 However, localized and/or small amounts of Li deposition are likely not to be detected in the © 2016 American Chemical Society

electrochemical data. Li deposition conditions are reported to be fulfilled when charging at low temperatures in combination with too high C-rates (charging current densities).24,22,23,9,14,13,18,20 The temperature dependency of aging mechanisms9,17 in combination with temperature gradients inside Li-ion cells25−29 or with local inhomogeneities12,21,30 suggest nonuniform aging inside Li-ion cells, which is not well understood. Since the aging phenomena are taking place on the material level, the method of choice to investigate them is disassembly of Li-ion cells and subsequent analysis of the cell components by physicochemical methods. This process is known as postmortem analysis.7−9,13,30−33 Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) are wellknown methods for morphological and compositional characterization of electrode surfaces in postmortem analysis.9,12,30 However, EDX is not sensitive to light elements such as Li and H, which are important in the analysis of aged Li-ion cells. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is sensitive to Li but provides an average elemental concentration in a few cm2 scraped off an electrode; hence, no surface information is obtained. X-ray photoelectron spectroscopy (XPS) in combination with Ar+ sputtering has the Received: July 21, 2016 Revised: September 9, 2016 Published: September 9, 2016 22225

DOI: 10.1021/acs.jpcc.6b07117 J. Phys. Chem. C 2016, 120, 22225−22234

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The Journal of Physical Chemistry C drawback of very slow removal of material from the sample, resulting in depth profiling in the sub micrometer range. In contrast, glow discharge optical emission spectroscopy (GD-OES) provides fast (∼1 μm/min) depth-resolved analysis of metal samples.34 Depth profiling in GD-OES is achieved through sample sputtering using plasma (usually Ar+ ions) and the simultaneous detection of the removed elements by an optical emission spectrometer. Applying the GD-OES technique on porous electrodes from Li-ion cells, recently Saito and Rahman35 and Takahara et al.36 measured depth profiles across the whole depth from the sample surface to the current collector (through-plane resolution). Further research by Takahara et al.10,37 and in our group8 focused on quantification of the Li distribution in graphite anodes with SEI growth. First results on graphite anodes with additional Li plating in our lab showed that it is possible to detect metallic Li on graphite anodes by GD-OES,33 which is usually not possible by other methods. The present study focuses on Li quantification in aged graphite anodes to differentiate between SEI growth and Li deposition/plating. In addition to the depth-resolved data acquisition, in-plane aging inhomogeneity in graphite anodes of large-format cells are studied through conducting GD-OES measurements at multiple positions of the same aged anodes. Such investigations have, to the best of our knowledge, not been reported before. In order to yield a sound basis for further experiments, the present study first validates the GD-OES method by comparison with other physicochemical analysis methods and by exclusion of measurement artifacts. Second, we define the terms “anode surface” and “anode bulk”. Third, applying the GD-OES method to postmortem analysis of commercial Li-ion cells, we show that different aging phenomena including SEI growth and Li deposition/plating can be present inside such cells.

Figure 1. Linear correlation between results of the calibrated GD-OES method and ICP-OES for Li. The bars show the error ranges of multiple GD-OES measurements. The calibration has a relative error of 18%.

The GD-OES results were further complemented by SEM/ EDX (LEO 1530VP Gemini) and laser-scanning microscopy (LSM) (Keyence VK-X200). We note that due to our experimental setup, samples were in contact with air for these methods. For GD-OES, we found in this study that air traces within the pores of anode samples can lead to invalid measurements due to changes of the sputtering rate. Therefore, all GD-OES were kept under Ar atmosphere (MBraun, O2 < 0.1 ppm, H2O < 0.1 ppm) and transferred to GD-OES using an airtight sample holder. 2.2. Cell Aging Tests and Sample Preparation. Graphite anodes were harvested from commercial 16Ah Liion pouch cells with stacked Li(NixMnyCoz)O2 (NMC) cathodes and graphite anodes. The anode dimensions were 202 mm × 102 mm. The cells had a voltage range of 2.7−4.2 V and a specified maximum charging rate of 3 C. Samples are taken from different in-plane positions of the electrode (inplane resolution, see Figure 2a). Each sample is measured from the surface to the current collector (through-plane resolution, see Figure 2a). The sample selection is illustrated in Figure 2b,c. A fresh cell of this type (cell 1) was opened as received from the manufacturer to provide a baseline (pouch-A) for comparison with the aged cell. Samples pouch-B and pouch-C were chosen from an aged cell (cell 2) of the same type with 89.4% capacity retention. Cell 2 was subject to cycling aging at 45 °C with a charging rate of 3 C and a discharging rate of 1 C using standard constant current−constant voltage (CC−CV) charging and a constant current (CC) discharging protocols. The detailed cycling behavior of cell 2 will be reported in a future paper. Additionally, commercial 26650-type cells with a nominal capacity of 2.5Ah and a voltage range of 2.0−3.6 V were tested. Anode and cathode consist of graphite and LFP, respectively. For the experiments, we used fresh cells (cell3) and cells cycled 700 times at −20 °C (cell4). For measurements of temperature gradients, a 26650-type cell was equipped with an internal temperature sensor (type K). The procedure was described in detail for similar cells in our earlier papers.26,28,32 For cell disassembly in an Ar-filled glovebox, all tested cells were discharged to their end-of-discharge voltage. After separation of cell components, anodes were rinsed with dimethyl carbonate (DMC, ≥99%, Sigma-Aldrich) three times for 60 s. In order to be sure about the results, all GDOES experiments were reproduced at least once.

2. MATERIALS AND METHODS 2.1. Analysis Methods. The GD-OES analysis was carried out using a GDA750 device (Spectruma). A method was developed in pulsed radio frequency (rf) mode with 550 V and 2hPa gas pressure. A mixture of 1% H2 with Ar was chosen as discharge gas, which was reported to provide better measurement sensitivity and depth resolution for analyzing porous carbon-based coatings.38,39 For comparison to our previous studies and to secure reproducibility,8,33 also pure Ar was used. The analyzed sample area had a diameter of 2.5 mm. The resulting crater depth was measured mechanically using a profilometer (Taylor Hobson, Form Talysurf 50). Roughness values were extracted from the width of Gaussian curves fitted to the profilometer data as implemented in the software provided by the manufacturer. GD-OES calibration was performed using a set of reference coatings prepared by systematic variation of LiH2PO4 in waterbased graphite slurries (0−10 mass %). The reference coatings exhibit a similar porosity and chemical composition like the aged graphite anodes, which is necessary for a matrix specific calibration. The calibrated method was evaluated through correlation of the quantified GD-OES results with ICP-OES analysis (Arcos, Spectro) of the samples with known composition and is shown in Figure 1. The linearity with a regression factor of R2 = 0.963 verifies the calibration. The observed elements and the emission lines used for this study were H (121 nm), O (130 nm), C (156 nm), and Li (670 nm). 22226


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The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION 3.1. Analysis of Sputtered GD-OES Crater. Conducting GD-OES depth profiling leaves a crater on the analyzed sample area. Figure 3a shows a profile of the crater of sample pouch-C. The crater bottom is flat (not concave or convex) and has a similar roughness as the sample surface before sputtering, which eliminates doubts of major layer interference during the sputtering process. Figure 3b shows an SEM image of the crater bottom. The structure of the graphite particles on the bottom of the sputtered crater is preserved, indicating that (i) there was no disturbing redeposition due to sputtering and (ii) there was no significant binder melting as reported after sputtering NMC cathodes by Saito et al.35 and Takahara et al.36 This is in agreement with the profilometer measurements in Figure 3a, which show a similar roughness inside the crater compared to the anode surface. Redeposition of sputtered particles would interfere with the accuracy of the GD-OES measurement and could be an issue when analyzing graphite coatings with pure Ar plasma.36,38 Table 1 summarizes the roughness values extracted from the profilometer measurement shown in Figure 3a. It can be seen Table 1. Overview on Roughness of Graphite Anode Surfaces and Inside the Crater after GD-OES Sputtering sample pouch-A (fresh cell) pouch-B (dark areas) pouch-C (bright areas)

Figure 2. (a) Illustration of a typical graphite anode for GD-OES analysis, showing through-plane (z direction) and in-plane (x/y plane) resolution. (b) Photographic image of the fresh graphite anode (pouch-A) and the marked area where sample was taken from (top view on x/y plane); (c) photographic image of the aged anode and the marked areas of where the samples were taken from (pouch-B and pouch-C).

roughness on anode surface [μm]

roughness inside sputtered crater [μm]

2.81

1.92

3.74

4.08

4.16

3.04

that the surface roughness is smallest for the fresh anode surface (pouch-A). For the aged samples, pouch-B and pouchC, the surface roughness values are higher by 33% and 48%, respectively. This implies a rough aging-induced film formation on these samples. Figure 3c shows the SEM image of the entire crater produced by the GD-OES experiment. From this Figure, a clearly round

Figure 3. Sputtered crater of sample pouch-C: (a) profilometer scan of the sputtering crater, (b) SEM image of the particles at the bottom the crater, (c) SEM image of the crater, (d) EDX mapping of the anode surface next to the sputtering crater, and (e) EDX mapping of the border of the crater. 22227


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reasonable depth profiling resolution yielding a sound basis for further GD-OES experiments. 3.1.1. Exclusion of GD-OES Surface Artifacts. Film formation on the surface of graphite anodes obtained from postmortem analysis of Li-ion cells is commonly observed as concentration peaks of the aging-relevant elements (e.g., Li) at the beginning of GD-OES measurements.8,33,36 In order to verify that these peaks are no measurement artifacts due to the GD-OES plasma ignition, we conducted a stepwise sputtering analysis of a graphite anode from a fresh cell (after initial SEI formation). In this experiment, which has to the best of our knowledge not been conducted before, sample sputtering is stopped every 160 s (∼1.0 μm) and then continued with the same sample staying inside the GD-OES device. Therefore, the measurement area does not change. Figure 5a shows that the elemental peaks appear only during the first sputtering step (step 1 in Figure 5a), which corresponds to removal of the topmost layers of the sample surface. In contrast, there are no such peaks in further sputtering steps, which correspond to deeper regions of the sample (steps 2−4 in Figure 5a). This observation specifies that the surface peaks represent a layer different from the bulk, covering the anode surface due to the formation of the initial SEI during the first cycles of the cell.40 It can also be concluded that the onset of aging-induced film formation occurs within the outer anode surface, which is consistent with earlier reports.8,33,36 Figure 5b shows the GD-OES depth profile of a graphite anode before being built into a cell (SEI free). As expected, no foreign elements such as Li or P were present, which indicates that possible elemental contamination of previous measured samples in the GD-OES device is not influencing the measurement. Carbon is distributed homogeneously on its average value, and there is only a minor oxygen peak on the anode surface originating most likely from carboxylic, carbonyl, and phenol surface groups.41,42 We note that in this GD-OES experiment, depth profiling was conducted across the entire coating thickness; however, only the first 5 μm from the anode surface are shown in Figure 5b since the elemental distribution remained constant afterward. To summarize, the GD-OES experiments in this section showed that the elemental peaks (Li, P) for anode samples are no artifact. The differences of elemental concentrations on anode surfaces expressed in the elemental peaks of GD-OES measurements and the absence of such changes in deeper layers

shaped crater with a diameter of 2.5 mm is observed. A disturbed measurement would have resulted in a smaller and/or deformed crater area. Figure 3d,e shows EDX mappings of the sample surface next to the bottom of the sputtered crater and at the border of the crater, respectively. The bottom of the crater is mainly composed of Cu from the current collector, which is covered by carbon from remaining particles of active material and binder. This is consistent with the SEM images. It can be seen from the EDX mappings in Figure 3d,e that the anode surface of pouch-C next to the crater contains oxygen. This is reasonable since the corresponding sample is later proved to contain Li deposition, which easily reacts with air to form oxides. In contrast to the anode surface, the bottom of the sputtered crater contains only a very small amount of oxygen, indicating an effective removal of the surface film by the GDOES sputtering process. The LSM image in Figure 4 depicts a rough film formation on the surface of sample pouch-C. Roughness data extracted

Figure 4. Aging induced surface film on the surface of sample pouch-C disclosed by LSM. The surface exhibits a standard deviation of 5.0 ± 0.1 μm.

from this LSM image (5 μm) is consistent with the profilometer method. As can be seen in Table 1, the roughness values for the crater bottoms for samples pouch-A, -B, and -C measured with the profilometer method are similar to those on the anode surfaces, indicating a reasonable depth profiling resolution in the GD-OES measurement. To summarize this part, detailed investigation of the sputtered GD-OES crater by complementary methods reveals

Figure 5. Verification of GD-OES surface peaks: (a) interrupted sputtering with 160s-long steps and (b) depth profile of the SEI free anode. 22228


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Figure 6. GD-OES depth profiles of (a) pouch-A (from fresh cell), (c) pouch-B (SEI growth), and (e) pouch-C (Li deposition) and respective SEM micrographs (b,d,f).

of the anodes call for a definition of “anode bulk” and “anode surface”. This will be evaluated in the next section. 3.1.2. Defining “Anode Surface” and “Anode Bulk”. Definitions of surface and bulk have been given for nonporous solid bodies.43 Depending on the sensitivity of the applied method, the surface ranges from one atomic layer to 103 atomic layers.43 Based on the lattice constant c/2 of graphite, the latter case corresponds to 0.33 μm, which is thinner than a typical Li surface peak in a GD-OES measurement of an anode from a Liion cell. In order to give a comparable definition of “anode bulk” and “anode surface” for samples from Li-ion cells, the decrease of Li from the sample surface to the bulk is used. This decrease of Li is sharper closer to the anode surface but tends to get more flat when going deeper into the anode bulk. The point at which the decrease in Li content reaches 10%/μm or less is defined as the start of the anode bulk in this article. The defined surface-bulk border is applied to the aged samples discussed in the following sections together with the respective depth profiles. We would like to note that the resolution and therefore the depths of surface and bulk are influenced by the surface roughness (see Table 1). In the case of a higher surface roughness (e.g., an anode composed of micrometer-sized graphite particles or with uneven film formation), the surface will be thicker than in case of lower surface roughness (e.g., in the case of a graphite single crystal (HOPG)). 3.2. Inhomogeneous Degradation of Graphite Anodes of Aged Li-Ion Cells. 3.2.1. Local Li Deposition on Anodes of 16Ah Pouch Cells Cycled at High Temperature and High Charging C-Rate. In this section, graphite anodes from commercial 16Ah pouch cells are analyzed. As can be seen from Figure 2b, a typical anode from the fresh 16Ah cell (cell1) shows no obvious differences by visual inspection. Figure 6a shows the GD-OES depth profile analysis of the anode taken from a fresh cell (pouch-A). The observed elements (Li, P) are typical for the initial SEI of state-of-the-art cells and consistent with literature.8,40,44−46 Applying the previously defined surface-bulk border based on the Li profile, a surface film of ∼0.1 μm thickness has formed on pouch-A, corresponding to the initial SEI created during the first cycles by the

manufacturer. Depending on the electrolyte components and aging conditions, typical SEI thicknesses are reported to be between 0.01 and 0.1 μm.40,47 Using TEM, Nie et al. detected a 0.05 μm thick SEI on the delithiated graphite anode harvested after formation of a half-cell with EC/EMC/LiPF6 electrolyte.47 The thickness of 0.1 μm for the initial SEI in Figure 6a is consistent with literature, supporting the capability of GD-OES in detecting thin porous deposited layers, despite the unavoidable roughness of the anode surface. Figure 6b shows an SEM image of pouch-A. The shape of the graphite particles is recognizable, which indicates a relatively thin initial SEI covering the particles. From previous studies on different types of Li-ion cells, it can be expected that the elevated operating temperature of 45 °C accelerates the decomposition of electrolyte on the graphite anode6,8,9,21 but counteracts with the deposition of metallic Li on the anodes.9,18 However, the comparably high charging rate of 3 C increases the possibility of Li deposition.17,18 Visual inspection of an aged anode from cell 2 (Figure 2c) shows nonuniform aging across the anode area. Figure 6d,f shows representative SEM images of the dark (pouch-B) and bright (pouch-C) regions of the harvested graphite anode. While graphite particles are still visible for pouch-B, pouch C is covered with a thick dendritic film. Since metallic Li is wellknown for its dendritic growth,48−50 it is likely that the bright regions are local “island-like” Li deposition. The decrease in carbon intensity recorded by EDX analysis indirectly indicates a thicker surface film in pouch-C compared with pouch-B and pouch-A. This decrease stems from the coverage of the graphite anode with aging-induced film growth. Figure 6c,e shows GD-OES depth profiles of the samples pouch-B and pouch-C, respectively. The measurements were carried out using Ar and Ar−H2 as analyzing gas. Using Ar−H2 as the sputtering gas had the advantage of increased sputtering rate38 but caused difficulties in analyzing pouch-C, possibly due to the comparably high Li content. It can clearly be seen that the amount of Li on the anode surface is significantly higher in the case of pouch-C (bright regions of anode) and covers a wider range of the anode thickness. 22229


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the detected Li (Lidetected) and LiSEI gives the amount of metallic Li (Limetallic) on the sample:

Due to the relatively large size of the anodes, it was possible to analyze the regions represented by pouch-B and -C separately using ICP-OES. ICP-OES measurements resulted in 1.42 ± 0.6 mass % Li and 2.03 ± 0.45 mass % Li for samples pouch-B and pouch-C, respectively. For comparison, GD-OES concentration profiles cLi(z) were integrated according to zi 1 c Li = c Li(z) dz zi 0 (1)

Li metallic = Lidetected − LiSEI

(2)

This assumption serves a straightforward basis for extracting the minimum contribution of deposited metallic Li in the overall detected Li content of the samples. In this approach it is assumed that the entire detected oxygen represents Li2O, which demands a certain amount of Li mass % (LiSEI). Since LiSEI represents the highest amount of Li in the structure of the SEI, excess detected Li must originate from deposition of metallic Li. This approach gives the minimum amount of metallic Li, Limetallic > 0 means there is metallic Li present in the sample, while Limetallic < 0 means there is a very low probability that metallic Li is present. The more negative Limetallic, the lower the Li deposition probability. This estimation was applied to every data point of the recorded GD-OES depth profiles of the samples to extract the estimated distribution of metallic Li over the anode coating thickness. Figure 8a shows the estimation applied to samples pouch-B and pouch-C. The calculated metallic Li depth profile of pouchC shows an average of 5.8 ± 1.1 mass % (integrated profile, see eq 1) metallic Li spreading to a depth of ∼13 μm from the anode surface. This result is well in agreement with the SEM image of the FIB cross-section of this sample shown in Figure 8b. The growth of Li deposition to a depth of ∼10 μm from the anode surface is clearly observed in the FIB-SEM imaging. Figure 9 compares the EDX mapping for O and F on the cross sections of the samples pouch-B and pouch-C. EDX of pouch-B shows both O and F signals, whereas F is distributed through the whole anode and O is mostly located on the anode surface. For pouch-C, EDX shows a thicker film on the anode surface, which is rich in O but not F. While F cannot be detected with GD-OES using Ar, the O detected by EDX is consistent with the GD-OES measurements. It can be concluded from the concentration gradient of metallic Li that the particles of the graphite anode surface are more susceptible to Li deposition. This observation is in agreement with recent simulations by Hein and Latz, who reported the condition for Li plating is first fulfilled on the anode surface and then begins to spread into the anode bulk.51 Pouch-C exhibits a higher amount of Li plating spreading into deeper layers compared with pouch-B, which suggests an inhomogeneous charge distribution and degradation of the graphite anode from cell2, possibly as a result of severe electrolyte decomposition during fast charging at high temperature. A similar analysis is applied to the graphite anodes from 26650 cylindrical cells in the following section. 3.2.3. Li Plating Gradient on Graphite Anodes of 26650 Cells Cycled at Low Temperature. Li deposition can also be expected for cycling at low temperatures.9,15−19,23,33,51 In contrast to the results from high temperature/high C-rate cycling, where the Li deposition was localized, a more homogeneous Li deposition was recently reported for 26650 cells14 and pouch cells33 cycled at low temperature. This more homogeneous Li deposition is known as “Li plating”. Petzl et al. reported hints on a gradient of Li plating in a cylindrical 26650-type cell with a wound jelly roll consisting of a LiFePO4 (LFP) cathode and graphite anode.14 The authors found differences in the mass area of the anode inside and

∫

where c Li is the amount of Li until a depth zi was reached. The ICP-OES results are consistent with these integrated GD-OES results on the dark- (1.4 ± 0.3 mass % Li) and light-colored regions (2.4 ± 0.5 mass % Li). A similar nonuniform aging of anodes from cycled Li-ion cells across the x/y plane was recently reported by Jalkanen et al.21 However, they did not detect Li deposition chemically, but got an indirect indication of it by visual inspection and by reaction of the deposition with water.21 We would like to note that the reaction with water is not a proof of metallic Li since Li intercalated into graphite might show a similar behavior. Jalkanen et al. suggested that the elevated temperature during cycling leads to dried areas and gas bubbles, causing higher local current densities and therefore local Li deposition.21 It is likely that this is also the case for the cells tested in the present study. Since the focus of the present study is on the investigation by GD-OES, the reason for this behavior will be tested in more detail in a future paper. Following the surface-bulk border defined in section 3.1.2, a film thickness of ∼1.0 and ∼5.0 μm for pouch-B and pouch-C is measured, respectively. A more detailed analysis of the depth profiles, including estimation for the possible metallic Li in the samples, is given below. 3.2.2. Estimation of Metallic Li. The detected Li mass % in the GD-OES depth profiles of the aged graphite anodes is the summation of the SEI growth and the Li deposition contribution. In order to extract the amount of metallic Li present on the studied aged graphite anodes, a singlecomponent SEI model is assumed. Figure 7 summarizes the

Figure 7. Li content in the commonly reported SEI species.

Li mass fraction in different commonly reported SEI species. According to Figure 7, Li2O introduces the highest amount of Li to the aged graphite anode engaged in the structure of the SEI. Therefore, assuming the SEI solely consists of Li2O, the contribution of the SEI on the overall Li content of the aged anode is maximized and the probability of the presence of metallic Li is minimized. Hence, a positive difference between 22230


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Figure 8. (a) Estimated minimum amount of metallic Li for samples pouch-A and pouch-B. (b) SEM images of sample pouch-C (FIB cross section), showing the penetration of Li deposition into a depth of 10 μm from the electrode top surface.

Figure 9. EDX mappings of O and F of the cross sections of samples pouch-B (SEI growth) and pouch-C (Li deposition).

outside the jelly roll.14 However, they did not prove Li plating or the differences therein using a physicochemical method.14 In the present study, we performed the cell aging under similar cycling conditions for the same cell type to further investigate the inhomogeneous anode degradation by GD-OES and SEM. For comparison, a fresh cell was disassembled and analyzed as well. Figure 10 shows the positions of the unwound graphite anode where the samples were taken together with the corresponding SEM images. Based on their positions, the analyzed samples are called 26650-outside (near the cell housing) and 26650-inside (near the cylindrical core of the cell). For the anodes from the fresh cell, the SEM images inside and outside the jelly roll both show graphite particles and are very similar. In contrast, the SEM images show thicker films on the anode from the cell cycled at −20 °C. On the outside of the jelly roll, a film is covering the anode; however, the shapes of the graphite particles are still visible through this film. On the inside of the jelly roll this film is significantly thicker, burying the graphite particles. We note that the microstructure of this film is not dendritic as in Figure 6f. The reason might be deformation of the metallic Li into a denser film caused by the higher pressure in the 26650 cell compared to the pouch cell. Figure 11a,b shows the GD-OES depth profiles of the graphite anode inside and outside of the jelly roll, respectively. The measurements were carried out and reproduced using Ar

Figure 10. Position where samples were taken from the unwound jelly roll of the cylindrical 26650-type cells and corresponding SEM micrographs for the fresh and cycled cells.

as analyzing gas since the Ar−H2 mixture caused measurement interruptions, possibly due to the high Li content of the samples. For inside and outside of the jelly roll, there is a clear difference in the Li peak shape and intensity. For the outside of the jelly roll, the maximum of the Li peak starts at the very anode surface, whereas it located at a depth of ∼0.6 μm for the sample from the outside of the jelly roll. Applying the previously defined surface-bulk border, film thicknesses of ∼0.8 and ∼2.0 μm corresponding to the anodes from the inside and outside of the jelly roll are found, respectively. This is significantly thicker compared to the anode surface of a sample containing only SEI. Figure 11c shows the extracted metallic Li for 26650-inside and 26650-outside based on the previously introduced estimation (see section 3.2.2). While 26650-inside exhibits an average of 3.6 ± 0.7 mass % covering spreading to ∼2.5 μm below the top surface, 26650-outside shows an average of 6.4 ± 1.2 mass % metallic Li spreading to about 7 μm from the anode surface. 22231


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Figure 11. GD-OES measurements of the graphite anode from a cylindrical 26650-type cell cycled at −20 °C (a) inside the jelly roll and (b) outside the jelly roll. The vertical dashed line corresponds to the defined borderline between anode surface and bulk. (c) Calculated metallic Li based on the recorded Li and O mass fractions in the GD-OES depth profiles of 26650-inside and 26650-outside.

design and cycling temperature. However, it seems that low temperature Li plating causes a more concentrated Li-rich anode surface, while high temperature could spread the Li depositions across wider anode surface layers. This observation is in agreement with our previous study, where the graphite anode was cycled in pouch cells at 5 °C.33 Figure 12b illustrates local Li deposition (high temperature), and Figure 12c, Li plating (low temperature) based on the GD-OES depth profiling results. The detected Li plating gradient on the two ends of the jelly roll is well in agreement with the different film thicknesses observed by SEM. Furthermore, our results are in agreement with the differences of the mass per area of the anode of the same cell type reported by Petzl et al.14 The observed differences in the amount of Li plating inside and outside of the jelly roll might be a result of the temperature gradient inside the cell25,26,28 in combination with the temperature dependency of the anode potential.9,18 Lower local temperatures lead to lower anode potentials, and if the anode potential gets more negative vs Li/Li+, this leads to more Li deposition. This nonuniformity reflects a nonuniform utilization and degradation of the anode, which is greatly influenced by the cell design. To further investigate this, a 26650-type cell was equipped with a type-K temperature sensor inside the jelly roll as well as on the outer cell surface. Operando measurements with this cell at −20 °C showed that a maximum temperature difference of 1.5 °C is building up during charging. From earlier measurements,9 a change of the anode potential vs Li/Li+ of 15−22 mV/°C can be estimated. Together with the maximum temperature difference this translates into a maximum difference of 21−31 mV vs Li/Li+ between inside and outside of the jelly roll. Therefore, the temperature difference over 700 cycles is most likely the cause for the different thickness of the Li plating at low temperature operation of 26650-type cells.

The analysis of metallic Li is summarized in Figure 12a. For pouch-A and pouch-B, no metallic Li is found. In contrast,

Figure 12. (a) Integrated metallic Li mass % in the first 2 μm of the studied samples. (b) Illustration of local Li deposition observed in the graphite anode of the studied pouch cell. (c) Illustration of the Li plating gradient in the graphite anode from the studied 26650 cell.

4. CONCLUSIONS The present study proves GD-OES as a powerful tool for studying aging-induced changes to graphite anodes from disassembled Li-ion cells, including Li deposition. Verification of the method was carried out through: • Calibration by lab-coated samples with known Li content/verified by ICP-OES. • Detailed investigation of the sputtered crater after the GD-OES measurement by SEM and EDX-mapping, comparison of graphite anode surface roughness by profilometer data, and laser-scanning microscopy.

pouch-C shows a significantly positive value, indicating metallic Li deposition. According to Figure 12a, metallic Li is detected on both ends of the jelly roll of the 26650 cell, but almost twice as much on the outer side of the jelly roll, which makes it likely to be more responsible for the capacity loss of the cell. This illustration discloses the tendency of Li plating to grow on the outer electrode surface, forming a Li-rich layer on the surface particles of the graphite anode. 26650-inside and 26650-outside exhibit a significantly higher amount of surface Li spread across a thinner anode surface compared with pouch-C. This difference could be due to cell 22232


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The Journal of Physical Chemistry C • Exclusion of measurement artifacts for GD-OES surface peaks by interruption of sputtering and by comparison with graphite anodes without any electrode surface layers. Therefore, the observed elemental peaks on the anode surfaces are no artifacts of the GD-OES measurements. • The point at which the decrease in Li content reaches 10%/μm or less is defined as the start of the anode bulk. Analyzing graphite anodes revealed: • Discriminating between the aging mechanisms of SEI growth and Li deposition using quantified depth profiling analysis. Such quantitative discrimination has, to the best of our knowledge, not been reported elsewhere. • Applying GD-OES at different planar positions provided information on nonuniform aging of graphite anodes (inplane resolution): SEI growth and Li deposition were detected on the same anode and were distinguished by GD-OES. • Depth-resolved elemental distribution across the complete graphite anode coating revealed the progress of Li from the anode surface to the anode bulk (through-plane resolution). This is in agreement with FIB-SEM experiments. • Deposition of metallic Li on graphite anodes can occur during long-term cycling at 45 °C with a high charging C-rate (3 C). • Li deposition can be localized (“island-like”, see Figure 12b) and can coexist with the different aging mechanism of SEI growth on the same graphite anodes. This deposition is visible as bright areas. We stress that this appearance of metallic Li should not be referred to as “Li plating” since it is not a homogeneous film. It has likely resulted from nonuniform planar utilization of the electrode due to intense electrolyte drying. • Differences of more homogeneous Li plating were found for the inside and outside of the anode from the unrolled jelly roll for cylindrical 26650-type cells by GD-OES. Higher amounts of Li plating were found near the cell housing (Figure 12c), which is in agreement with SEM. The aging differences in the 26650 cells are likely to originate from the temperature distribution in the 26650 cell and the resulting differences in the anode potential vs Li/Li+. The GD-OES measurements in the present article have broadened the knowledge on Li deposition/Li plating on graphite anodes of Li-ion cells. Further extensive investigations on this topic are under investigation in our lab.

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analysis, as well as Dr. A. Pfeifer (Carl Zeiss AG) for the FIBSEM analysis and CIC Energigune (Spain) for providing the pouch cell aged at 45°C.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49(0)731-9530212. Fax: +49(0)731-9530-666. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has been performed within the MAT4BAT project (http://www.mat4bat.eu) and received funding from the European Community’s Seventh Framework Program (FP7/2007-2013) under grant agreement no. 608931. We thank G. Arnold (ZSW) for conducting the ICP-OES measurements and Dr. C. Pfeifer (ZSW) for SEM/EDX 22233


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Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A

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