XPS and SEM-EDX Study of Electrolyte Nature Effect on Li Electrode

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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XPS and SEM-EDX Study of Electrolyte Nature Effect on Li Electrode in Lithium Metal Batteries Rabeb Grissa,*,† Vincent Fernandez,† Neal Fairley,‡ Jonathan Hamon,† Nicolas Stephant,† Julien Rolland,§ Renaud Bouchet,§ Margaud Lecuyer,∥ Marc Deschamps,∥ Dominique Guyomard,† and Philippe Moreau*,†

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Institut des Matériaux Jean Rouxel (IMN), CNRS, Université de Nantes, 2 rue de la Houssinière, BP 32229, Nantes Cedex 3 44322, France ‡ Casa Software Ltd., Bay House, 5 Grosvenor Terrace, Teignmouth, Devon TQ14 8NE, United Kingdom § Institute of Engineering (Grenoble INP), LEPMI, CNRS, Université de Grenoble Alpes, St Martin d’Hères 38402, France ∥ Blue Solutions, Odet, Ergué Gabéric, Quimper Cedex 9 29556, France ABSTRACT: Understanding the solid electrolyte interphase (SEI) in lithium batteries is very important to face the major safety issue of lithium dendritic growth during battery charge. The aim of this work is to study the thickness and the chemical nature of the SEI by XPS, as well as their influence on the electrochemical performance of the battery for different liquid organic electrolytes. XPS imaging is also used in this work to get a chemical mapping of the SEI layer components formed on the metallic lithium electrode surface cycled in different conditions. Data processing based on the principal component analysis (PCA) method has been conducted in order to illustrate the SEI layer heterogeneities. The obtained results are compared with energy-dispersive X-ray spectroscopy (EDX) mapping. Thereby, the benefits and the precision of the XPS imaging technique to identify chemical compounds distribution have been highlighted. These different analyses have led to a better knowledge of the redox processes occurring at the top surface of lithium metal electrodes cycled in different liquid electrolytes. KEYWORDS: lithium batteries, XPS, XPS imaging, DEG, DPG, PEG

1. INTRODUCTION Thanks to the development of innovative solid electrolytes (ceramics and polymers) as well as ionic liquid-based electrolytes,1 a worldwide renewed interest for metal-based lithium batteries has occurred recently, envisaging the effectiveness of suppression of the lithium dendrite formation and growth, therefore promoting a large-scale lithium metal battery technology.2 In this context, the passivation film formed at the lithium/ electrolyte interface (SEI) plays an important role in the lithium deposition process. The understanding of the chemical and morphological properties of this layer is therefore crucial to improve the quality of lithium plating and stripping and select the adequate electrolyte for a given battery system. The ability to understand these interfacial processes requires surface analysis techniques that allow for easy quantification and are capable of providing information on chemical states. Xray photoelectron spectroscopy (XPS) is one of those techniques and is increasingly used in batteries characterization.3 This post-mortem surface analysis technique indeed gives essential information not only on the active material but also on the phenomena occurring during cycling.4 © XXXX American Chemical Society

Nevertheless, for heterogeneous samples, such as electrode surfaces,5 the repartition of the different compounds as well as their concentration in the different zones of the analyzed area, which depends on the topology of the surface are crucial to the understanding of the outermost surface behavior, much more than an average composition. XPS imaging could provide this important information in a lateral dimension, allowing for a correlation of micro- and macroscale materials properties.6 For example, in lithium sulfur (Li−S) batteries, Nandasiri et al.7 have performed in situ XPS imaging on lithium electrodes in order to investigate parasitic reactions of electrolyte and polysulfide species with the Li-anode during cycling. XPS imaging has also recently been deployed to study corrosion phenomena of Ni−Cr−Mo alloys8 and demonstrated to be useful for several industrial applications (such as catalysis processes, biological arrays characterization, etc.).9 It is worth noting that there are two main methods to perform XPS imaging.10 The first one consists of acquiring Received: July 30, 2018 Accepted: September 20, 2018 Published: September 20, 2018 A

DOI: 10.1021/acsaem.8b01256 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Fischer titrations. The BPF porous films are filled with the different electrolytes and sandwiched between two lithium foils. Finally, the cell is thermosealed under vacuum in an aluminized plastic bag. The active surface area of the electrodes is approximately 25 cm2. The cells have been polarized at 300 μA cm−2 at 40 °C under a pressure of 2 bar. During this process, the lithium is stripped from the anode and plated onto the cathode. The theoretical duration to strip the entire 50 μm of lithium anode is about 38 h. 2.2. Scanning Electron Microscopy (SEM) and EnergyDispersive X-ray Spectroscopy (EDX). The morphology of lithium electrodes is investigated with a field-emission gun scanning electron microscope JEOL JSM7600F operating at 2 kV. EDX mappings were performed with a BRUKER SDD energy-dispersive spectrometer mounted on the SEM. Samples were transferred from the glovebox to the SEM using an airtight transfer box, developed in our laboratory.19 2.3. XPS and XPS Imaging Analyses. XPS measurements were performed using a Kratos Axis Nova Instrument, located at the University of Nantes in France, using a monochromated Al K alpha source (1486.6 eV) operating at 300 W. The instrument base pressure was 5 × 10−10 Torr. The sample area analyzed was about 700 μm × 300 μm. A pass energy (PE) of 80 eV, with an all over instrumental resolution of 0.9 eV measured on Fermi edge, was used to acquire wide range survey spectra. A PE of 40 eV was used to acquire narrow spectra of the C 1s, O 1s, Li 1s, N 1s, and S 2p levels with an all over instrumental resolution of 0.55 eV measured on Fermi edge. All data were acquired using charge compensation with low energy electrons gun in order to have homogeneous surface potential. This is necessary to have accessible chemical information. XPS data were analyzed with the CasaXPS software (version 2.3.19) using Gaussian/Lorentzian (30% Lorentzian) line shapes and the Shirley background correction. All spectra were calibrated with hydrocarbon C 1s photoemission set to 285.0 eV binding energy. Parallel imaging across the field of view of 400 μm was performed by acquiring spectrum-images using the imaging XPS mode in the Kratos Axis Nova system, equipped with a spherical mirror analyzer. The maps were acquired in a field of 128 × 128 pixels, which corresponds to a spatial resolution of ∼8 μm. The images were collected under a PE of 160 eV and over the energy range of 700−0 eV with a step of 0.4 eV and a dwell time of 40 s per image. Detailed descriptions of the instrumental part are reported in references.11,20,10 Samples are transferred, under argon atmosphere, from the glovebox using a dedicated transfer chamber system, which is then connected to the XPS apparatus and pumped three times before the samples are introduced in the XPS analysis chamber. All XPS experiments are performed on lithium electrodes without any rinsing or scratching. 2.4. XPS Imaging Data Processing. The XPS imaging data processing was also realized using the CasaXPS software. Given that the raw signal-to-noise in each pixel is too low to meaningfully work with individual spectra at each pixel, images are processed using the following steps to produce spatially resolved spectra of sufficient quality to perform standard XPS quantification.

spectra by scanning an X-ray probe over the sample surface and the second one is known as parallel imaging and consists in acquiring a series of energy-filtered images incremented on energy (by using a spherical mirror analyzer11). This latter mode is preferred because it leads to a better spatial resolution of few micrometers (ideally 3 μm compared to 30−15 for the first mode).6 Nevertheless, although the first commercial XPS involving the imaging mode was available in 1990,12 publications including XPS imaging are much fewer than the ones including standard XPS spectroscopy.10 This could be due to the fact that the main purpose of the instrument manufacturers of including the imaging mode (single energy images) was to provide guidance to select the analysis areas rather than an independent technique.10 This could also be due to the complex structure of the XPS imaging data set obtained from XPS imaging.13 Indeed, the data set stemming from XPS imaging measurements can be considered as a set of images providing us with information, in terms of spectra, at different positions on the sample, i.e., each pixel corresponds to a spectrum. If we want to apply standard XPS data processing, by involving curve fitting for chemical states quantification, we have to perform it to each pixel of the image, which is not possible, given the low signal-to-noise ratio for each pixel. Consequently, the data processing requires sophisticated analysis methods in order to simplify this complex data set structure. In this context, several works11−18 based on multivariate analysis techniques (MVA), and dealing with XPS imaging data processing have been developed. The principal component analysis (PCA) technique has been used11−18 to determine the data set primary components. Thus, the data can be reconstructed using these components, which leads to a considerably higher signal/noise ratio. Other analytical techniques, such as the singular value decomposition (SVD),17 iterative SVD,13 and the nonlinear iterative partial least square method (NIPALS),11,18 have been used, in addition to PCA, to reduce the processing time. In this work, the chemical properties of the passivation film formed on cycled lithium electrodes surfaces will be studied by XPS spectroscopy. The electrodes are cycled in symmetric cells (by lithium stripping/deposition) in different low molecular weight ether-based electrolytes with lithium bis[(trifluoromethyl)sulfonyl]imide) LiTFSI salt. The influence of the electrolyte nature on the passivation film composition and thickness will be therefore investigated. A thorough study will then be conducted using EDX mapping and XPS imaging on the lithium electrode cycled in the selected electrolyte in order to investigate the SEI layer heterogeneities. Mathematical modeling techniques will be applied on our XPS imaging data set in order to identify the main orthogonal vectors (environments), allowing us to reconstruct the sample surface composition.

1 Noise reduction by applying Outlier Filter and PCA noise reduction. 2 Construction of elemental images using quantification regions to sum signal above background for each pixel. 3 Classification of pixels by intensity using appropriate elemental images which group together pixels of similar origin. A small number of false colors compared to the total number of pixels are assigned to pixels with the intention of partitioning images into pixels with common characteristics. 4 Spectra are formed from the XPS image data set by summation of raw signal over each false color used to classify image pixels yielding good quality spectra suitable for constructing peak models.

2. EXPERIMENTAL SECTION 2.1. Electrochemical Analysis. Symmetric pouch cells Li/ electrolyte/Li, using an ∼50 μm-thick lithium electrodes (Blue Solutions) and Bolloré Porous Film (BPF) as a separator (Blue Solutions), have been assembled in a dry room (dew point −48 °C) for three different electrolytes: (i) diethylene glycol dibutyl ether (DEG) (Acros, +99%) + LiN(CF3SO2)2 LiTFSI (1M) (Solvay, battery grade), (ii) dipropylene glycol dimethyl ether (DPG) (Acros, +99%) + LiTFSI (1M), and (iii) polyethylene glycol dimethyl ether 500 g/mol (PEG500) (Aldrich, +99%) + LiTFSI (1 M). All electrolytes contain less than 30 ppm of water according to Karl

Treating the resulting survey spectra as vectors, these false-color spectra21 were manipulated to reveal two spectral forms that were open to physical interpretation, but not available in the raw survey B


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ACS Applied Energy Materials spectra. These two mathematically derived spectral forms were validated by reconstructing each false-color spectrum to confirm these two spectra would account for all significant variations in the spectromicroscopy data set. Each vector revealed distinct chemistry which was interpreted by standard XPS quantification. These two vectors were then used to fit in a linear-least-squares sense the set of raw spectra at pixels for the imaging data set. Spatially resolved chemical state images are extracted based on quantification regions applied to signal partitioned into distinct chemical states. These distinct chemical states are defined by the two vectors (spectral forms) fitted to each spectrum associated with pixel positions within the image data set.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Results. The polarization tests results are displayed in Figure 1. Figure 2. SEM images of (a) a pristine metallic lithium foil and lithium electrodeposited electrodes using different electrolyte formulations (b) DPG+ LiTFSI (1 M), (c) DEG+LiTFSI (1 M), and (d) PEG500+LiTFSI (1M).

for the bare lithium electrode, a smooth surface with some scratches (shiny lines) probably corresponding to the scalpel blade traces. For all electrodeposited electrodes, we observe mossy-like structure of lithium mixed with the electrolyte. This mossy aspect of a cycled lithium electrode surface has been previously observed by high magnification SEM analysis.22 Note that smoother and denser surface is obtained with the PEG500 (Figure 2d) solvent compared to the DEG (Figure 2c) and the DPG (Figure 2b) ones, which is in agreement with their different electrochemical behavior (Figure 1). Indeed, for the DEG solvent (Figure 2c), the lithium surface is composed of spheroid lithium electrodeposits with a quite dense structure whereas the surface mossy character is very pronounced for the DPG solvent (Figure 2b). 3.3. Electrode Composition. XPS spectra obtained for the three lithium electrodes are shown in Figure 3 and all XPS quantitative data are gathered in Table 1. The C 1s core peaks (Figures 3a, f, k) show the presence of the hydrocarbon component at 285 eV, assigned to surface contamination (C−C and C−H bonds), and commonly detected on sample surfaces. This hydrocarbon is also contained in the methyl groups present along the chain in the case of the DPG, in the dibutyl end function in the case of the DEG as well as in reduction products of the three solvents. Note that although the hydrocarbon component involves both surface contaminations and solvent reduction products, the detected proportions (Table 1) are in agreement with the solvents compositions: DEG (25.9%) (Presence of dibutyl end function) compared to the DPG (19.8%) (presence of two methyl groups) and to the PEG500 (9.5%) (no hydrocarbon in the structure). A second component is present at 286.5 eV and corresponds to the ether function C−O−C bond, which is mainly contained in the ether functions of DPG, DEG and PEG500 solvent and also in surface contamination. A third component appearing at 288.5 eV is assigned to a carbon surrounded by two oxygen atoms O−CO, which can be attributed to a surface contamination component or a solvent degradation product.

Figure 1. Galvanostatic polarization tests for Li/electrolyte/Li symmetric cells using three different electrolyte formulations: DPG +LiTFSI (1 M), DEG+ LiTFSI (1 M), and PEG500+LiTFSI (1 M).

We note that the shapes of the curves at the moment of divergence around 25 h are clearly different. In the case of PEG, this divergence is abrupt and could be explained by a complete and clean lithium stripping from the anode. In the case of DEG and DPG, the behavior is more hieratic and could be linked to an inhomogeneous quality of the lithium stripping. A higher polarization plateau is obtained in the case of PEG500 (100 mV) solvent compared to DEG and DPG ones (17 and 25 mV, respectively), which could be linked to the electrolyte ionic conductivity, lower for the PEG500 solvent (1.3 × 10−3 S.cm−1) compared to the DEG and DPG ones (2.2 × 10−3 S.cm−1 and 5.0 × 10−3 S cm−1, respectively). It should also be mentioned that this variable polarization could stem from a more resistive or thicker SEI layer due to changes in the electrode/electrolyte interface morphology (such as thickness) and/or its chemical composition. These results show that the choice of the appropriate solvent is, as expected, crucial in the case of lithium metal electrodes in order to prevent or delay the lithium dendrite formation, but also that a better knowledge of the electrodeposition mechanism is necessary. 3.2. Electrode Morphology. SEM images relative to the pristine as well as to the electrodeposited lithium electrodes in the different solvents (DPG, DEG, and PEG500) are shown in Figure 2. The pristine lithium electrode (Figure 2a) has been analyzed after mechanical erosion with a scalpel blade in order to remove the oxide/hydroxide/carbonate layer formed on the metallic lithium surface even within a very clean argon atmosphere (O2 < 0.5 ppm and H2O < 0.5 pm). We observe, C


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Figure 3. XPS C 1s, F 1s, S 2p, N 1s, and O 1s, respectively, of the lithium electrodeposited samples using(a−e) DPG solvent, (f−j) DEG solvent, and (k−o) PEG500 solvent. Each color in the different spectra is relative to one chemical species or to one chemical compound.

The S 2p core peaks components (Figure 3c, h, m) are split into two subcomponents (S 2p3/2 and S 2p1/2 with an area ratio of 2) because of the spin orbit coupling effect. We note for the DPG and DEG samples the presence of four components assigned to LiTFSI salt (S 2p3/2 at 167.5 eV), reduced LiTFSI (rLiTFSI) Li2NCF3SO2 (S 2p3/2 at 166.5 eV), disulfides S22− (S 2p3/2 at 163.5 eV), and Li2S (S 2p3/2 at 161.5 eV)). For PEG500 sample, only the three first components are observed. The N 1s spectra (Figure 3d, i, n) show one component at 399.9 eV assigned to the LiTFSI lithium salt and one component at slightly lower binding energy (398.5 eV) assigned to the rLiTFSI. A third component at low binding energy (397.5 eV) appears only for the lithium electrode electrodeposited with the DPG solvent (Figure 3d) and is assigned to Li3N. Unlike the C 1s core peaks, the different components constituting the O 1s spectra (Figure 3e, j, o) cannot be easily differentiated, except for the Li2O (lithium oxide present on the initial lithium top surface) component at 528.8 eV, which is well-individualized and observed only for the DPG and DEG solvents (Figure 3e, j). The curve fitting of the O 1s core peak is then performed by deducing each contribution from the

The component observed at 290.2 eV is assigned to a carbon atom surrounded by three oxygen atoms (−CO3), this species can be attributed to lithium carbonate compound, commonly present on the metallic lithium surface. In contrary to the carbonate-based electrolyte where the SEI layer contains also carbonates, ether-based electrolytes and their degradation products contain only C−C and C−O bond types. Therefore, in our case, the carbonate species can be used as a probe of the lithium native layer. The component appearing at higher binding energy (291.5 eV) is assigned to a carbon atom surrounded by three fluorine atoms (−CF3). This species can be contained in the LiTFSI salt or in its degradation products. The F 1s core peaks (Figure 3b, g, l) show the presence of two components. The one at low binding energy (685.0 eV) is assigned to LiF and the one at high binding energy is assigned to the −CF3 species, which can be contained in the LiTFSI salt or in it degradation products. A higher LiF/−CF3 ratio is obtained for the lithium electrode electrodeposited using the DEG solvent compared to the DPG one and a considerably lower LiF/−CF3 ratio is obtained for the PEG500 solvent. D


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Table 1. XPS Quantitative Data of Lithium Electrodes Electrodeposited in DPG+LiTFSI (1 M), DEG+LiTFSI (1 M), and PEG500+LiTFSI (1 M) Electrolytes DPG solvent orbitals C 1s

total C F 1s total F S 2p3/2

S 2p1/2

total S N 1s

total N O 1s

atomic %

B.E. (eV)

atomic %

B.E. (eV)

atomic %

peak assignments

285.0 286.6 288.9 290.1 293.1

19.8 5.5 1.5 4.2 2.2 33.1 7.0 7.3 14.3 0.1 0.2 0.2 0.9 1.2 0.1 0.1 0.1 0.4 0.6 3.7 0.4 0.8 0.9 2.1 1.0

285.0 286.5 289.0 290.2 292.7

285.0 286.8 288.5 290.1 293.1

9.5 5.8 0.8 1.3 7.8 25.2 3.1 25.3 28.4

C−C/C−H C−O O−CO Li2CO3 −CF3

160.5

25.9 3.3 1.5 3.2 1.2 35.1 4.8 4.1 8.9 0.6

162.2 167.8 169.3 161.7

0.2 1.1 0.3 0.3

164.4 167.4 169.3

0.1 1.0 4.4 0,0

163.3 169.0 170.4

0.1 0.5 0.2 3.2

165.5 168.6 170.5

0.1 0.5 2.2 8.2

398.1 399.5

1.2 0.4 1.6 0.8 6.5 9.9 1.5 3.3 2.1 1.9 26.0 1.6 1.6

398.6 399.6

0.7 4.2 4.9

685.2 688.8 160.4 161.8 163.7 167.4 169.2 162.3 163.0 164.9 168.5 170.4 397.41 398.84 399.61 528.4

53.6 54.2 54.7 55.5 55.3 56.0 56.1

total Li total

PEG500 solvent

B.E. (eV)

531.8 532.2 532.3 532.8 533.2 total O Li 1s

DEG solvent

13.4 1.1 2.4 4.1 3.0 25.1 1.6 0.7 1.1 7.7 2.6 7.3 0.9 21.8 100.0

684.8 688.3

528.3 530.6 531.5 531.8 532.2 532.4 533.0 54.7 54.9 55.5 55.9 55.2 56.0 57.0

5.3 9.6 2.2 4.4 0.5 25.2 100.0

other spectra relative to the sample (C 1s, N 1s, and S 2p). We can then deduce, from the C 1s core peak, the presence in the O 1s spectra of C−O (at 533.5 eV), CO (532.5 eV), and Li2CO3 (at 531.7 eV) components, and from the S 2p and N 1s core peaks, the presence of −SO2 groups (at 533.2 eV) (contained in the LiTFSI and the rLiTFSI). The LiTFSI degradation species observed by XPS are in agreement with those already observed in literature.23 For the lithium electrode deposited with the DEG solvent, an additional component appears at 530.8 eV and is assigned to LiOH, which is in agreement with the Li 1s core peak (table1). The presence of lithium hydroxide can be due to water contamination at the electrode surface. The quantification of all the components identified by XPS (after subtraction of the hydrocarbon contamination and

684.9 688.6

531.7 532.2 532.6 533.0 532.5

55.6 56.0 56.1 56.5

2.6 0.8 2.6 11.8 5.0 22.7

2.2 2.5 2.5 3.5 10.7 100.0

LiF 0,0 Li2S LixS S22− Li2NSO2CF3 S(LiTFSI) Li2S LixS S22− Li2NSO2CF3 S(LiTFSI) Li3N Li2NSO2CF3 N(LiTFSI) Li2O LiOH Li2CO3 O−CO Li2NSO2CF3 C-O O(LiTFSI) Li2O Li2S Li3N LiOH Li2CO3 Li2NSO2CF3 LiF Li(LiTFSI)

LiOH contamination) leads to the surface compositions displayed in Figure 4. We note that the Li 2 CO 3 and Li 2 O proportions (representative of the native lithium layer) are higher in the case of the DPG solvent compared to the DEG one and much higher than for the PEG. The Li2O compound is completely covered in this latter case. Two hypotheses can be inferred from this result. The first one, the SEI layer would be homogeneous and thinner than the XPS analysis depth of around 5 nm for all the samples, below which the detection of the electrodes native layers is possible. From the native layer proportions (Li2CO3+Li2O) estimated by XPS, we could conclude that the SEI layer is thicker for the DPG solvent, slightly thinner for the DEG solvent and even thinner for the PEG500 solvent. E


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interface. Furthermore, it seems that the length of the apolar and aprotic alkyl end function plays a lower role than a methyl along the chain which explains the lower electrochemical performance as well as the poor morphological and chemical properties obtained in the case of DPG compared to DEG and to the PEG500. 3.4. Surface Compound Distribution in the PEG500Electrode. To carry out an exhaustive study of the SEI layer chemical composition and of the different chemical compound distribution at the electrode surface, an EDX mapping has been performed on the lithium electrode, cycled with the PEG500 solvent. Figure 5 displays the fluorine−oxygen, sulfur−oxygen, and fluorine−sulfur overlapped maps in a 40 μm × 40 μm zone of the lithium electrode, deposited in PEG500+LiTFSI electrolyte.

Figure 4. Surface chemical composition, determined by XPS, of the lithium samples electrodeposited using DPG, DEG, and PEG500 solvents.

The second hypothesis consists of considering a heterogeneous and discontinuous SEI layer (presenting porosities and cracks), which would mean that in the analyzed area of 300 μm × 700 μm we obtain a denser and more covering SEI layer for the PEG500 solvent, allowing the detection of a lower proportion of Li2CO3 and Li2O native layer compared to the PEG and DEG solvents. Similarly, the SEI layer would be denser for the DEG solvent than for the DPG one. From SEM images, a heterogeneous and mossy structure is formed on the lithium electrode surfaces, which could corroborate the second hypothesis. Nevertheless, the only way to discriminate between the two hypotheses is to obtain spatially dependent analytical information, hence our XPS spectromicroscopy study. This will be developed in the next section. We also observe a better integrity of the LiTFSI lithium salt for the PEG500 solvent: absence of Li2S, Li3N, species and lower rLiTFSI, S22−, and LiF proportions compared to the DEG and DPG solvents. Indeed the (rLiTFSI+ S22−+LiF+Li3N +Li2S)/LiTFSI ratio is estimated to be around 0.2 for the PEG500 solvent, 0.9 for the DEG solvent, and 1.7 for the DPG one. XPS results are therefore in agreement with the electrochemical results and with the morphological aspects of the three electrodes investigated by SEM analyses. Thus, we deduce that for the PEG500 solvent, which leads to better electrochemical performance, we obtain better SEI morphological and chemical properties (less porous morphology with less lithium salt degradation). These findings may originate from the chemical nature of the SEI in the three different cases. Indeed, the ratios between ethers and carbons are very different in the three used solvents. This plays a role on their hydrophilic/hydrophobic properties, i.e., comparing DEG (3O/12C) dibutyl and PEG (5O/10C) dimethyl end functions. In the case of the DPG (3O/8C), a dimethyl end function (like the PEG (5O/10C) is also present, but there are two methyl groups along the chain, which probably makes the whole chain more hydrophobic compared to PEG. These parameters will impact the structure of the ionic complexes especially near the polar electrode/electrolyte

Figure 5. EDX (a) fluorine−oxygen and (b) oxygen−sulfur maps of the lithium electrode electrodeposited in PEG500+LiTFSI (1 M) electrolyte.

The fluorine−oxygen maps (Figure 5a) show two different main environments: the first (in blue) is oxygen-rich and the second (in yellow) is fluorine-rich. This suggests that the sample surface is formed of two nonmiscible oxygen-rich and fluorine-rich environments. Similarly, the sulfur−oxygen mappings (Figure 5b) shows the presence of two main (nonmiscible) sulfur-rich and oxygen-rich environments, matching exactly with those observed in Figure 5a. Fluorine- and sulfur-rich environments could correspond to the LiTFSI salt and its degradation products and the oxygen rich regions could correspond to oxidized and carbonated regions. To get more precise information about the chemical compound distribution in the PEG500-electrode surface, XPS imaging have been conducted on a 400 μm × 400 μm sized region of the same sample. The data-processing detailed in section 2.4 has been performed on the obtained XPS imaging data set and has allowed to map the element chemical states. The XPS maps of components C 1s at 290.0 eV and O 1s at 531.7 eV, both assigned to Li2CO3 compound (Figure 6a and b, respectively) as well as the XPS maps of the components C 1s at 291.4 eV, F 1s at 688.2 eV (Figure 6c and d, respectively), both assigned to LiTFSI (or rLiTFSI) compounds, are displaced in Figure 6. We also observed (Figure 6e) the XPS map of the F 1s component at 685.0 eV, assigned to the LiF compound. These maps confirm the EDX results and give more precision about element chemical states. Indeed, we observe that the Li2CO3 native layer is more or less covered by LiTFSI (or by its reduction products) in the 400 × 400 μm mapped zone. F


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From these vectors, we note a relatively large LiF proportion (60%), compared to XPS spectroscopy measurements (6%). In fact, we have observed −CF3 species degradation to LiF (Figure 8a) under X-rays exposure. This degradation reaches

Figure 6. (a) XPS C 1s (Li2CO3), (b) O 1s (Li2CO3), (c) C 1s (LiTFSI/r-LiTFSI), (d) F 1s (LiTFSI/rLiTFSI), and (e) F 1s (LiF) core peaks components mapping of the lithium electrode electrodeposited in PEG500+LiTFSI (1 M) electrolyte.

Figure 8. Comparison of F 1s XPS variations with X-ray exposure time at (a) ambient temperature and (b) −150 °C.

up to 20% of −CF3 fluorine transformation into LiF after 47 min of XPS measurements at room temperature. Because XPS imaging experiments are very long (around 20 h per sample), the sample is more prone to degradation. It is important to mention that a little proportion of LiTFSI is transformed into LiF after standard XPS spectroscopy measurements. For this reason, only the initial experiments are exploited in this study. These experiments last 15 min for all the samples (leading to the degradation of around 7% of the LiTFSI). However, given that measurement duration is the same for all samples, we assume that the comparative study presented in section 3.3 is quite relevant. To evaluate the LiTFSI salt behavior under long X-ray exposure, the F 1s core peaks relative to the PEG500-sample has been acquired iteratively at room temperature (Figure 8a) and at −150 °C (Figure 8b). We note that the initial LiF/−CF3 ratios are different, which could be due to inhomogeneity in the sample, because both measurements have to be conducted in two different areas. At room temperature, the −CF3 species, characteristic of the LiTFSI salt, is transformed with a significant rate into LiF (around 20% after 47 min of X-rays exposure), compared to only 3% after 109 min of X-rays exposure at −150 °C. This shows that the LiTFSI salt chemistry can be relatively preserved by conducting low temperature measurements. However, this type of experiment is very difficult to setup, especially for long XPS measurements such as imaging. Indeed, liquid N2 has to be supplied every 20 min and it is difficult to maintain the same sample−detector distance all along the experiment because of the sample holder contraction during cooling.

The XPS imaging has not been performed for the DPG ad the DEG-lithium electrodes, but we expect from the XPS spectroscopy results to obtain a less-covered native layer for the DEG-electrode and and even less covered for the DPGone. As described in section 2.4, the PCA allowed determining the two main pure mathematical components (vectors) constituting the sample surface, these vectors are shown in Figure 7 as histograms and the chemical compound

Figure 7. Two orthogonal vectors resulting from the XPS imaging PCA data processing of the lithium electrode electrodeposited in PEG500+LiTFSI (1 M) electrolyte.

■

quantifications are realized after C−C/C−H surface contamination subtraction. We obtain two different compositions: the first one is Li2CO3 and Li2O-rich and the second is LiF, LiTFSI and C−O-rich. This result matches with the obtained elements chemical states mappings and confirms again the EDX chemical maps. Indeed, the electrolyte (LiTFSI and C− O (PEG500)) as well as its degradation products (LiF and rLiTFSI) cover a major part of the Li2CO3 native layer. In some areas, this latter layer is well-detected and only a very small proportion of electrolyte-relative compounds can be observed. This is also in agreement with the observation of a mossy and nonhomogeneous electrode top layer with SEM analyses.

CONCLUSION In this work, we have studied the solvent chemical nature influence on the electrochemical performance of a symmetric Li//Li cell. For this purpose, three solvents, combined with LiTFSI (1 M) lithium salt, were tested: DPG, DEG, and PEG500. Post-mortem SEM and XPS analyses have been conducted on the electrodeposited lithium metal electrodes and we conclude that, depending on the electrolyte composition, the electrodes top surfaces are more or less mossy and dendritic. This shows that the solvent choice is crucial to control the SEI layer as well as the lithium deposition/dissolution properties G


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and then to obtain satisfying electrochemical performance for a lithium metal battery. The PEG500 solvent gave, according to SEM analyses, a smoother and denser electrode surface compared to the DPG and DEG solvents. Besides, according to the XPS analyses, the PEG500 solvent causes less LiTFSI salt degradation and more covering SEI layer compared to the DPG and DEG ones. These results are in agreement with better electrochemical performance obtained for the PEG500 solvent. EDX chemical mapping was performed on the PEG500electrode and has shown the presence of O-rich and F−S-rich different zones on the electrode surface. An XPS imaging analysis has also been conducted on the same electrode and has shown two main environments: a lithium carbonate-rich environment, corresponding to the lithium native layer, and a second one mainly composed of LiTFSI salt with its degradation products. The first environment may correspond to the O-rich zones observed by EDX and the second environments correspond to the S−F-rich zones. Thus, the capability of XPS imaging to map elements chemical states has brought more precision to identify the sample surface homogeneities. A PCA-based data processing method has allowed determining two pure components, from which it is possible to reconstruct all the XPS imaging data set. The first component is Li2CO3-rich and the second one is LiF-rich (and not LiTFSI as expected). This result highlights the LiTFSI salt degradation under long X-ray exposure. We have shown that this degradation could be avoided by conducting low temperature analyses which are, however, very hard to setup in the case of XPS imaging.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rabeb Grissa: 0000-0003-2463-1766 Philippe Moreau: 0000-0002-1691-1592 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was realized with the financial support of BPIFRANCE FINANCEMENT in the context of the ALEPH project (for Accumulateurs au Lithium à Electrolyte Polymère et recyclage Hydrométallurgique), led by Blue Solutions company.

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REFERENCES

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XPS and SEM-EDX Study of Electrolyte Nature Effect on Li Electrode

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