In Situ Studies of Solid Electrolyte Interphase (SEI) Formation on

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In situ Studies of Solid Electrolyte Interphase (SEI) Formation on Crystalline Carbon Surfaces by Neutron Reflectometry and Atomic Force Microscopy Miriam Steinhauer, Michael Stich, Mario Kurniawan, Beatrix-Kamelia Seidlhofer, Marcus Trapp, Andreas Bund, Norbert Wagner, and K. Andreas Friedrich ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09181 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 21, 2017

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ACS Applied Materials & Interfaces

In situ Studies of Solid Electrolyte Interphase (SEI) Formation on Crystalline Carbon Surfaces by Neutron Reflectometry and Atomic Force Microscopy Miriam Steinhauer1,§, Michael Stich2,§,*, Mario Kurniawan2, Beatrix-Kamelia Seidlhofer3, Marcus Trapp3, Andreas Bund2, Norbert Wagner1,*, K. Andreas Friedrich1,4

1German

Aerospace Center (DLR), Institute of Engineering Thermodynamics, Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany

2Technische

Universität Ilmenau, Electrochemistry and Electroplating Group,

Gustav-Kirchhoff-Straße 6, D-98693 Ilmenau, Germany 3Helmholtz-Zentrum

Berlin für Materialien und Energie, Institute for Soft Matter and

Functional Materials, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany 4University

of Stuttgart, Institute for Energy Storage,

Pfaffenwaldring 31, D-70569 Stuttgart, Germany

1

ACS Paragon Plus Environment


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Keywords: Solid electrolyte interphase, atomic force microscopy, neutron reflectometry, in situ, lithium-ion battery, X-ray reflectometry, formation, carbon

Abstract:

The solid electrolyte interphase (SEI) is a complex and fragile passivation layer with crucial importance for the functionality of lithium-ion batteries. Due to its fragility and reactivity, the use of in situ techniques is preferable for the determination of the SEI’s true structure and morphology during its formation. In this study, we use in situ neutron reflectometry (NR) and in situ atomic force microscopy (AFM) to investigate the SEI formation on a carbon surface. It was found that a lithium rich adsorption layer is already present at the open circuit voltage on the carbon sample surface and that the first decomposition products start to deposit close to this potential. During the negative potential sweep, the growth of the SEI can be observed in detail by AFM and NR. This allows precise monitoring of the morphology evolution and the resulting heterogeneities of individual SEI features. NR measurements show a maximum SEI thickness of 192 Å at the lower cut-off potential (0.02 V vs. Li/Li+) which slightly decreases during the positive potential scan. The scattering length density (SLD) obtained by NR provides additional information on the SEI’s chemical nature and structural evolution.

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Introduction The “solid electrolyte interphase” (SEI) on carbon anodes has been studied extensively in the past, due to its crucial importance for the operation of lithium-ion batteries (LIBs). This interphase layer is initially formed by the reduction of the battery’s organic electrolyte on the anode surface during its first negative potential scan. It passivates the surface, avoiding further decomposition of the electrolyte while maintaining a Li+ conductivity. A vast range of different surface sensitive characterization techniques, like XPS,1–5 SEM,2,4,5 NMR,3 XRD,2,5 FTIR,2,3 AFM,6–11 STM,12 etc. has been used to better understand the role and composition of the SEI. Some of these techniques have to be performed ex situ and require a sample treatment before the analysis. Often samples have to be dried before the experiment, leading to solidification of substances otherwise dissolved in the electrolyte. On the other hand rinsing the surface with solvents can wash out certain SEI components.13 Reactions with solvent impurities (especially water, reacting to LiF14,15) can change the chemical composition and morphology.16,17 For some experiments the sample transfer has to be done in air (i.e. exposing it to relatively high levels of humidity) or vacuum, which further alters the SEI. Therefore, in order to observe the true structure of the SEI and its evolution inside an electrochemical cell, the use of in situ techniques is necessary. In this work, in situ neutron reflectometry (in situ NR) and in situ atomic force microscopy, (in situ AFM) as well as ex situ Xray reflectometry were used to study the SEI formation during the first potential scans. These techniques are surface sensitive and complement each other in revealing the morphological and compositional changes of the SEI at very small lateral scales (< 1 µm, AFM) and at larger lateral scales (averaged over the whole lateral scale, > 1 cm, NR) with a resolution in the sub-nanometer scale perpendicular to the surface. The ex situ X-ray reflectometry measurements were performed as preparatory measurements in order to determine the feasibility of reflectometry

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measurements for determining the SEI thickness and serve as a reference to the results obtained by the in situ techniques. Industrial carbon anodes for LIBs are usually produced by coating and drying a slurry of active material, binder, conducting agent and solvent on a copper foil. This production process results in a complex electrode surface, varying in thickness, morphology, and composition, making it difficult to observe the unobstructed SEI formation steps on carbon in detail. In order to reduce the complexity inherent to the SEI growth on an industrial carbon anode, we chose a model electrode with a smooth, sputtered carbon surface for the investigation of the basic processes of the SEI evolution.

Experimental

Model Electrodes For the preparatory X-ray reflectometry (XR) experiments a thin layer of amorphous carbon was vapor-deposited onto 1 mm thick silicon wafers with a size of 20 × 20 mm² (Sil’tronix ST, Archamps, France). The XR experiments showed an inhomogeneous distribution of the carbon layer thickness. Therefore the model electrodes for NR and AFM measurements were prepared by sputtering a thin layer of carbon (thickness ≈ 30 nm, root mean square surface roughness, Rq ≈ 1 nm) from a graphitic target onto highly n-doped, single-side polished silicon crystals with roughnesses < 0.6 nm (resistance < 0.005 Ω m, orientation ‹100›, Sil’tronix ST, Archamps, France). The model electrodes were subsequently annealed at 800°C. The carbon layer thus contains a crystalline structure, as confirmed by Raman measurements. The sputtering process was performed by the Nanoelectronic Materials Laboratory (NamLab gGmbH, Dresden, Germany). For the AFM measurements the wafers had a thickness of 0.5 mm and a diameter of

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20 mm; for the NR measurements, the silicon wafers had a thickness of 10 mm and a diameter of 60 mm.

Ex situ X-ray Reflectometry To test whether reflectometry measurements are able to measure the SEI thickness, preparatory XR measurements were performed. Therefore the model electrodes were measured outside the electrochemical cell cycling on a home-built reflectometer at the Helmholtz-Zentrum Berlin (HZB) using a Cu Kα line with a wavelength of 1.541 Å. The incident beam was defined for the line focus of the X-ray tube and a diaphragm of 0.1 mm. The instrument resolution was set to δQz = 0.003 Å-1. The reflected beam was monochromatized and detected by a scintillation detector. For low angles, a 0.1 mm nickel absorber was inserted into the reflected beam to prevent saturation of the detector. For all X-ray measurements, the sample was kept under an inert argon atmosphere in a home-built container with a 0.025 mm thick Kapton HN window (DuPont, Hamm, Germany). Details on the home-built X-ray instrument and its operational modes can be found elsewhere.18

In situ Neutron Reflectometry NR measurements were performed at the time-of-flight (TOF) reflectometer BioRef (V18) at the Helmholtz-Zentrum Berlin (HZB).19,20 The chopper frequency was 25 Hz. At the open circuit voltage (OCV) three different angular positions θ (θ = 0.5, 1.5 and 2.6, respectively) were measured to cover a scattering vector Q-range from 0.008 Å -1 to 0.44 Å-1, where Q = 4π sin (θ)/λ with λ corresponding to the wavelength band defined by the choppers and the scattering angle θ. The total measuring duration at OCV was 7.5 h. For all other measurements, two angular positions θ (θ = 0.5 and 1.5, respectively) were measured to cover a scattering vector range from 0.008 Å-1 to 0.23 Å-1. This corresponds to a measuring time of 4 h. The resolution was set to a 5



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constant value of 5% in ∆λ/λ. The scattered neutrons were detected by a position sensitive area detector (PSD, DENEX, Lüneburg, Germany) consisting of a 3He-filled wire chamber with an active area of 300 × 300 mm².

Data Reduction and Analysis of the Neutron Reflectometry Data Neutron data reduction and normalization were performed using an in-house-developed software suite available at BioRef. The raw data were normalized to the direct beam and binned with a constant step width of 2%. The experimental reflectivity curves were fitted using the MOTOFIT package implemented in the IGOR Software.21 Here, the Abeles matrix method is used to calculate the reflectivity of n layers with thickness di, scattering length density ρi and roughness σi, i+1, with i = 0 (fronting) – n (backing).

In situ Atomic Force Microscopy The in situ atomic force microscope measurements were conducted with a Dimension Icon AFM (Bruker Co., Billerica, USA) inside an argon-filled glovebox (M. Braun Inertgas-Systeme GmbH, Garching, Germany) with H2O and O2 concentrations below 10 ppm. The probes used in the experiment were ScanAsyst-Fluid probes (Bruker Co., Billerica, USA) made out of silicon nitride with a denoted tip radius of 20 nm. The AFM data was obtained in the PeakForce Tapping mode. All displayed AFM images, as well as the data for the surface roughness calculations, underwent a 3rd order polynomial flattening procedure using the NanoScope Analysis software (Bruker Co., Billerica, USA) to remove tilt and curvature. The fast scan direction in all images is horizontal. An artificial shading filter was added to the AFM images to better visualize their three-dimensionality. All in situ AFM images obtained during the

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experimental procedure were merged into a movie clip, which can be found in the supporting information, Video S1.

Electrochemical Procedures The electrochemical experiments were performed in a three-electrode cell for the AFM measurements and a two electrode cell for the NR and X-ray reflectometry (XR). The model electrodes were cycled against a lithium foil (99.9% purity, Sigma-Aldrich, St. Louis, USA) as a counter and (if applicable) reference electrode. As electrolyte 1 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v, Sigma-Aldrich, St. Louis, USA) with H2O and HF concentrations below 15 ppm and 50 ppm, respectively was used. The electrochemical cells were designed for lithium-ion battery materials and were sealed off during the measurement to avoid evaporation of the electrolyte. During the experiment the anode potential was negatively changed at a constant scan rate of 1 mV s-1, starting from the open circuit potential to the following plateaus, where the voltage was held constant (Figure 1), allowing for AFM and NR data to be gathered, before moving to the next plateau: 1.5 V, 1.25 V, 1.0 V, 0.8 V, 0.6 V, 0.5 V, 0.4 V, 0.3 V, 0.2 V, 0.1 V, 0.02 V. The used devices for the electrochemical procedures were a BaSyTec Cell Test System (BaSyTec GmbH, Asselfingen, Germany) for NR and X-Ray reflectometry and a SP-300 potentiostat/galvanostat (Bio-Logic Science Instruments SAS, Seyssinet-Pariset, France) for the AFM measurement.

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Figure 1. Negative potential sweep procedure during the in situ NR and AFM measurements. Potential scan between voltage plateaus was performed at a constant voltage rate of 1 mV s-1. During voltage plateaus (a)-(l) the measurement data was gathered. The duration of the plateaus (dashed lines) varied between several minutes for AFM and several hours for NR and is not to scale in the graph. After completing the negative sweep, the anode potential for the NR measurements was also positively changed to the voltage plateaus of 1.0 V, 1.5 V and 3.0 V with a scan rate of 1 mV s-1. For the ex situ X-ray reflectometry measurements the cell was disassembled in an argon-filled glovebox (GS Glovebox Systemtechnik GmbH, Malch, Germany) after the cycling and the electrodes were washed with 50 µL DEC to remove the remaining electrolyte.

Results and discussion

Preparatory X-ray reflectometry results X-ray reflectometry measurements were performed to measure the SEI thickness ex situ. Figure 2a-d show the measured and fitted reflectivity curves of two identically produced 8


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samples, uncycled (black) and cycled (red) curves. The best fit of the XR data was achieved by applying a multibox model to the data. This allows modeling of a continuous SLD profile. Figure 2e and Figure 2f show the electron density profiles as a function of the distance to the electrode surface as a result of the fit.

Figure 2. (a) and (b) measured (black symbols) and fitted (line) reflectivity curves of the two uncycled wafers (error bars are presented in blue color), (c) and (d) reflectivity curves of the two wafers after cycling; (error bars are presented in blue color), (e) and (f) scattering length density profiles as a function of the distance to the electrode surface before (black line) and

Two layers can be distinguished on the silicon substrate: a native SiO2 layer present at the silicon surface and the sputtered carbon layer. For the fit, the scattering length densities of silicon and SiO2 were set parameters while roughness and thickness of the SiO2 and carbon layer as well as the electron density of the carbon layer were variable parameters. First fits of the uncycled wafers revealed that the carbon layer could not be simulated as a single layer. A second box was necessary to fit the carbon layer correctly. This was probably caused by an inhomogeneous carbon layer due to the depositing process. For the cycled wafer, the model was extended with two further layers for the SEI. The SEI is reported in the literature to consist of a dense, inorganic inner layer and a less dense, organic outer layer.17,22 The detailed fitting results are given in Tables S1-S4 in the supporting information. The fits revealed a total thickness of the 9



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carbon layer of 632 Å for sample #1 and 544 Å for sample #2. The scattering length densities of the modeled carbon layers were between 11.28 × 10-6 Å-2 and 15.80 × 10-6 Å-2. These values are smaller than the electron density of graphite (19.22 × 10-6 Å-2)23 calculated for a graphite density of 2.26 g cm-3. The calculated scattering length densities give a physical density of 1.33 g cm-3 to 1.86 g cm-3 of the carbon layer which resembles the physical densities of amorphous carbon that were described in the literature.24 After the cycling, the carbon thickness on sample #1 has decreased by approximately 80 Å as shown in Figure 2e. This might be an effect of the rather inhomogeneous distribution of the carbon layer. In both samples, the scattering length densities of the carbon layers decreased after cycling. This can be caused by moieties of the electrolyte and lithium species that remain in the carbon layer. For the inner SEI layer, the fit gave layer thicknesses of 200 Å and 205 Å with corresponding scattering length densities of 13.82 × 10-6 Å-2 and 13.01 × 10-6 Å-2. The outer, less dense layer was 74 Å and 80 Å thick with scattering length densities of 8.30 × 10-6 Å-2 and 7.92 × 10-6 Å-2. The observation that the SEI consists of an inorganic inner layer and an organic outer layer is frequently described in literature.25,26 Consistent with the literature, the scattering length densities of the inner layer also point to an inorganic composition with species such as Li2CO3, Li2O or LiF whereas those of the outer layer point to an organic composition. However, it should be mentioned that the scattering length densities are also influenced by the porosity of the SEI layers and a porosity gradient could also explain the SLD curve progression of the SEI layers. The porosity of the SEI also leads to the observed lower SLD values compared to bulk materials.

SEI growth monitored by in situ AFM and NR The X-Ray reflectometry measurements were preparatory measurements for in situ neutron reflectometry measurements. For the NR measurements model electrodes with a more 10


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homogeneous carbon coating were chosen, as described in more detail in the experimental part. Figure 3a and Figure 3b show the reflectivity curves during negative and positive potential sweeps. In Figure 3c and Figure 3d, the corresponding scattering length density profiles are shown as results of the fits. While the SLD curves displayed in Figure 2e and f and in Figure 3c and d show the qualitative SLD changes between individual layers very well, a fitting procedure (as described above) is required to obtain quantitative SLD and thickness values for the different layers. The fitting procedure takes the roughness, the layer thickness of the various layers and their SLDs into account as is demonstrated for clarification in Figure S1 in the supplementary information. Since the roughness influences the SLD course strongly, thickness and SLD values cannot directly be read from the diagram but have to be extracted from the fitting procedure. Detailed fitting results are given in Tables S5-S6 in the supporting information.

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Figure 3. (a) Measured (symbols) and fitted (line) neutron reflectivity curves during the negative potential sweep. (b) Measured (symbols) and fitted (line) reflectivity curves during the positive

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potential sweep. (c) SLD profiles during the negative potential sweep and (d) positive potential sweep.

At OCV a rough adsorption layer is present between the carbon surface and the electrolyte with a scattering length density of 0.42 × 10-6 Å-2 and a thickness of 73 Å. At voltages between 1.5 V and 1.0 V the scattering length density further decreases and reaches slightly negative values between -0.10 × 10-6 Å-2 and -0.03 × 10-6 Å-2. This indicates that the surface layer gets richer in lithium (SLD of pure lithium: -0.88 × 10-6 Å-2). Between 1.0 V and 0.5 V, the scattering length density increases to 0.70 × 10-6 Å-2 whereas the thickness stays constant at a value of approx. 55 Å. While in the ex situ XR-experiments two SEI layers, one inorganic and one organic, could be identified, this distinction could not be made in the in situ NR measurements. This limitation of the in situ measurement is caused by the SEI being soaked with electrolyte in the electrochemical cell and therefore exhibiting SLD values which “blur” the transition between different SEI layers so that inorganic and organic SEI layers cannot properly be distinguished. In the voltage region between 1.0 V and 0.5 V, the scattering length density of the carbon layer decreases from 3.45 × 10-6 Å-2 at 1.0 V to 2.95 × 10-6 Å-2 at 0.5 V which shows that the carbon starts to get lithiated. Between 0.4 V and the lower cut-off voltage of 0.02 V, the layer thickness constantly increases until it reaches a maximum of 192 Å. In this voltage area, the scattering length density of the SEI stays rather constant at a level between 0.80 and 0.84 × 10-6 Å-2. During charging, the SEI thickness decreases slightly to 140 Å and at the same time the scattering length density increases to 0.94 × 10-6 Å-2. At 1.5 V and 3.0 V upon charging the SEI has a thickness of 123 Å and 124 Å respectively with corresponding scattering length densities of 1.01 × 10-6 Å-2 and 1.15 × 10-6 Å-2.

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Figure 4. (a) Thickness of the carbon and SEI layer during negative potential sweep (left) and positive potential sweep (right), (b) SLD of the carbon and SEI layer during negative sweep (left) and positive sweep (right). Figure 4a shows the evolution of the SEI and carbon thickness upon negative potential sweep (discharge) and positive potential sweep (charge), Figure 4b shows the corresponding development of the scattering length density. The development of the thickness and SLD shows that the SEI starts growing below 0.8 V and increases its thickness as well as changes its chemical composition distinctly from 0.3 V. The behavior during charging shows that the SEI thickness is not constant after its initial formation but rather a function of the state of charge. A similar behavior was already observed and described for silicon anodes.27

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Figure 5. (a)-(l) Surface morphology of the carbon model electrode during the first negative potential sweep as depicted by in situ AFM at the voltages indicated in the individual images. The blue asterisk marks the position of the same surface feature and its evolution during the experiment. All displayed voltages were measured vs. a Li/Li+ reference 15



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electrode. Figure 5 displays the surface morphology of the carbon model electrode during the first negative potential sweep. The in situ AFM image at OCV (Figure 5a) confirms that the surface is initially very smooth with an Rq of 0.76 nm. It also can be seen in Figure S2 in the supplementary information that the initial formation of small round features on the surface already starts at OCV with increasing contact time with the electrolyte. At 1.5 V (Figure 5b) the surface is still smooth (Rq = 1.49 nm), but already shows a bigger cluster of round surface features. At 1.25 V (Figure 5c) the cluster of round structures has drifted downwards due to uncompensated sample drift, revealing that those features only cover parts of the surface. Nevertheless, the overall roughness Rq of the surface increases to 2.47 nm due to the growth of the individual features with diameters in the range of 50 nm. At this stage the round features seem to be loosely attached to the surface and have a high mobility, resulting in quick changes of the surface appearance (this can be seen in more detail in the supporting information Video S1). Until 1.0 V (Figure 5d) the round features continue to grow slowly and the surface roughness increases to 3.44 nm. At 0.8 V (Figure 5e) the previously smooth parts of the surface begin to get rougher, the round features continue to grow bigger. The most significant change in the surface morphology, however, is seen at 0.6 V (Figure 5f). The round surface features have grown a lot larger and now cover the whole image, resulting in a surface roughness Rq of 8.77 nm. Small nodules on the sides and between the round features begin to form. The surface features now seem to be attached stronger to the surface at this voltage, making them immobile. Below 0.6 V the surface morphology is relatively stable and changes slowly, which is at first surprising, because the NR data shows, that the SEI continues to grow fast at these potentials. Pinson and Bazant simulated SEI formation under the assumption that solvent diffusion is the rate-determining transport mechanism. They reported that once an initial SEI layer is formed growth of fresh SEI takes place underneath the existing SEI, which is one possible explanation why the surface 16


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morphology does not change significantly once the initial SEI has formed even though the SEI continues to grow below 0.5 V.28 The voltage region around 0.6 V could indicate the transition from the initial SEI growth at the interphase SEI/electrolyte to the grow at the interphase electrode/SEI once the SEI thickness has reached values which no longer allow for an electron transport to the SEI/electrolyte interphase. Between 0.5 V and 0.2 V (Figure 5g-j) the round features remain similar in shape and size, but the nodules are continuing to grow, resulting in a slight increase in the roughness Rq from 9.12 nm at 0.5 V to 9.37 nm at 0.2 V. At 0.1 V the round features still only change slightly, but the nodules are growing strongly, as seen by the roughness increase of the surface to 10.1 nm. At 0.02 V (Figure 5l) some round features have grown further and the nodules have overall decreased in size. At the top and bottom of the image smoother areas are visible. During the in situ AFM measurement, the sample drift was compensated by continuously repositioning the AFM tip. As a consequence, the growth and evolution of the SEI could be observed at one local spot throughout the first negative potential scan. In the AFM images of Figure 5, an easily distinguishable surface feature is highlighted by an asterisk. Note that this feature at its early development is already visible at 1.5 V. Its size evolution during further polarization is displayed as a horizontal cross-section cut along its center in Figure 6a. The areas of the cross-sections were evaluated and are displayed together with the overall roughness evolution in Figure 6b. Below 1.5 V this surface feature first grows slowly and merges with an adjacent feature at 1.0 V resulting in a sudden increase in size. Another major size increase of this feature is seen between 0.8 and 0.6 V, corresponding, according to literature to the electrolyte decomposition29. At lower voltages, the feature grows steadily until 0.2 V and experiences a larger growth until 0.1 V, where its size reaches a maximum. The small size reduction at 0.02 V can be partially attributed to the decrease of the nodules at the side of the feature. 17



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The roughness of the surface derived from the AFM images exhibits the same development as the cross-section area increase of the previously discussed feature (Figure 6). It is clearly visible that the roughness of the surface already starts to grow from voltages close to the OCV and that the biggest change in roughness occurs between 0.8 and 0.6 V. At lower voltages the roughness only increases slightly.

Figure 6. (a) Development of the size of the feature highlighted by an asterisk in Figure 5 during the first negative polarization. (Horizontal cross section cut through the middle of the feature), (b) Roughness evolution of the carbon model electrode surface (black points) and size development of the feature highlighted by an asterisk in Figure 5 (blue triangles) during the first negative sweep monitored by in situ AFM. According to literature, the onset potential for the SEI formation is typically in the range of 0.8 V vs. Li/Li+ for the used electrolyte.16,30,31 Usually the first decomposition products found on the carbon surface are considered to be organic species, formed by the electrolyte reduction,32,33 which are being partially transformed to inorganic components at lower potentials4 and during further cycling.33 This results in a two-layered structure with inorganic components close to the carbon electrode and organic species near the electrolyte.17,25,26 Also the in situ AFM results show that the main growth of the SEI is seen in the potential range between 0.8 and 0.6 V. While at 18


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higher potentials smooth areas could be seen in the in situ AFM images, which can be attributed to the carbon substrate, those areas are beginning to get overcast at 0.8 V and at 0.6 V no smooth areas can be detected any longer. This points towards a film formation of the abovementioned organic species on the surface. However, the initial roughness increase starts already at higher voltages and is already present at the first investigated plateau at 1.5 V. Also other in situ AFM experiments11,32 show a deposition at this voltage, using an HOPG working electrode and a similar electrolyte (1.5 M LiTFSI/EC or 1 M LiPF6 in EC:DMC (1:1), respectively). On the other hand, the initially seen surface structures are mobile and occasionally move, merge and disappear. This behavior rather points towards a precipitation and crystallite growth on the surface.33 Furthermore Anderson and Edström found that LiF, the major component of the SEI, can also be found on the uncycled electrode forming separate crystallites on the surface at elevated temperatures.5 LiF can be a product of the decomposition of the conducting salt LiPF6, especially in the presence of H2O and HF contamination12,15 and could therefore also be forming at higher voltages. This finding could explain the roughness increase at voltages above 0.8 V and is supported by Lu et al. who observed the first deposition products from 1 M LiPF6 in EC/DMC (1:1) at higher voltages (0.8 V, Cu vs. Li) being nanometer size crystallites of LiF.4 The surface changes seen mainly between 0.1 and 0.02 V (partial smoothening of surface structures, decrease of nodules size) could be the result of the transformation of some organic species to inorganic compounds at low voltages.4

Conclusions In situ neutron reflectometry measurements show that at the open circuit voltage a lithium rich adsorption layer forms at the electrode surface. The morphology and growth of this layer can be observed by in situ atomic force microscopy (AFM) during the negative potential scan and 19



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shows the formation and growth of surface structures with a high mobility and a tendency to merge. It is important to remark that morphological heterogeneities in the fully developed solid electrolyte interphase (SEI) can be traced back to small surface aberrations, which are already starting to form at higher voltages (above 1.5 V vs. Li/Li+). While slow changes of the surface morphology can be seen by in situ AFM throughout the negative potential scan, the most rapid change of the SEI happens below the potential of 0.8 V vs. Li/Li+, which is in good agreement with the onset of SEI thickness increase obtained by in situ neutron reflectometry (NR). The rapid morphology change is, agreeing with literature, considered to be due to organic decomposition products forming from the electrolyte’s reduction. The roughness of the surface and the size of the observed surface feature also show their strongest change between 0.8 and 0.6 V vs. Li/Li+. At lower voltages the speed of SEI growth further increases according to in situ NR while the morphology of the surface observed by in situ AFM changes at a lower rate. This could be explained by a transition of the SEI growth process. While the SEI is initially growing on the interphase SEI/electrolyte it might continue to grow underneath the existing SEI once it has reached a thickness that no longer allows for an electron transport to the SEI/electrolyte interphase. The SEI thickness reaches a maximum of 192 Å at the lower cut-off voltage of 0.02 V. During positive polarization the thickness of the SEI layer slightly decreases while the scattering length density stays at a constant level.

Associated Content Video clip of SEI formation as monitored by in situ AFM. Tables of the fitting results for XR and in situ NR measurements. Images of simulated SLD behavior and AFM images of the model electrode at OCV.

Author Information Corresponding Authors 20


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* N. Wagner: [email protected]; Tel.: +49 711 6862 631; Fax: +49 711 6862 747 * M. Stich: [email protected]; Tel.: +49 3677 69 3148; Fax: +49 3677 69 3104 ORCID M. Stich: 0000-0002-3655-6487 A. Bund: 0000-0001-9837-2408 M. Kurniawan: 0000-0003-3171-6573 Author Contributions §M.

Steinhauer and M. Stich contributed equally to this work.

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the German Federal Ministry of Education and Research (BMBF) as part of the project “Li-EcoSafe – Entwicklung kostengünstiger und sicherer Lithium-IonenBatterien” (FKZ: 03X4636B). M. Stich and M. Kurniawan are grateful for the support by the “Thüringer Graduiertenförderung” and M. Stich acknowledges funding by the “Promotionsabschluss-Stipendium der TU Ilmenau”. We gratefully acknowledge financial support for the in situ atomic force microscope (Forschungsgroßgerät "InertgasRastersondenmikroskop") by the German Research Foundation (DFG), the Free State of Thuringia and the European Regional Development Fund (EFRE) (DFG-Gz: INST 273/56-1 FUGG). We thank HZB for the allocation of neutron beam time and Magali Camargo for the helpful discussions.

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Graphical Abstract

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In Situ Studies of Solid Electrolyte Interphase (SEI) Formation on

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