Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Fluoroethylene Carbonate Induces Ordered Electrolyte Interface on Silicon and Sapphire Surfaces as Revealed by Sum Frequency Generation Vibrational Spectroscopy and X‑ray Reflectivity Yonatan Horowitz,†,§ Hans-Georg Steinrück,‡ Hui-Ling Han,†,§ Chuntian Cao,‡,∥ Iwnetim Iwnetu Abate,‡,∥ Yuchi Tsao,⊥ Michael F. Toney,*,‡ and Gabor A. Somorjai*,†,§ †
Department of Chemistry, Kavli Energy NanoScience Institute, University of California, Berkeley, Berkeley, California 94720, United States ‡ SSRL Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ∥ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ⊥ Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: The cyclability of silicon anodes in lithium ion batteries (LIBs) is affected by the reduction of the electrolyte on the anode surface to produce a coating layer termed the solid electrolyte interphase (SEI). One of the key steps for a major improvement of LIBs is unraveling the SEI’s structure-related diffusion properties as charge and discharge rates of LIBs are diffusion-limited. To this end, we have combined two surface sensitive techniques, sum frequency generation (SFG) vibrational spectroscopy, and X-ray reflectivity (XRR), to explore the first monolayer and to probe the first several layers of electrolyte, respectively, for solutions consisting of 1 M lithium perchlorate (LiClO4) salt dissolved in ethylene carbonate (EC) or fluoroethylene carbonate (FEC) and their mixtures (EC/FEC 7:3 and 1:1 wt %) on silicon and sapphire surfaces. Our results suggest that the addition of FEC to EC solution causes the first monolayer to rearrange itself more perpendicular to the anode surface, while subsequent layers are less affected and tend to maintain their, on average, surface-parallel arrangements. This fundamental understanding of the near-surface orientation of the electrolyte molecules can aid operational strategies for designing highperformance LIBs. KEYWORDS: Lithium ion batteries, electrolyte additives, sum frequency generation vibrational spectroscopy, X-ray reflectivity
T
allowing fast Li-ion diffusion through it, is still obscure.11 For instance, is the SEI’s structure related to the initial arrangement at open circuit potential12 of the electrolyte molecules at the anode surface? In this context, it has recently been shown that fluorinated linear ether electrolyte solution in contact with amorphous silicon has a tighter, up-right interface arrangement leading to an SEI with high lithium ion diffusion rates.13 The beneficial effect of adding the electrolyte additive fluoroethylene carbonate (FEC)14−20 to an ethylene carbonate (EC) solution, e.g., formation of a fluorine-product and a flexible SEI that remains intact after many cycles of expansion and contraction during the silicon anode charge/discharge cycles, can also arise from the rearrangement of the EC molecules by FEC additives at the anode interface.21 To date, it is thought
he appearance of reliable, rechargeable energy storage devices, namely, lithium ion batteries (LIBs), has revolutionized mobility, data accessibility, and communication.1−3 While significant achievements for portable devices are reported yearly, the shift from gasoline powered cars to electric LIB powered vehicles has been less rapid. The first encountered issue is how fast we charge an electric vehicle, as upon charging (“filling up”) a LIB, a redox process occurs on the surface of the anode. This redox process forms an electrically insulating layer that allows lithium ion transport at a reasonable rate while hindering electrolyte decomposition on the anode surface and is termed the solid electrolyte interphase (SEI).4−7 The formation of the SEI affects the overall battery performance as it is closely related to its irreversible capacity loss and cyclability.8 While the formation of the SEI has been addressed extensively,9,10 an understanding of the fundamental process that leads to a formation of the desired SEI with a structure © XXXX American Chemical Society
Received: January 22, 2018 Published: February 16, 2018 A
DOI: 10.1021/acs.nanolett.8b00298 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters that FEC reduction to LiF22 and cross-linked polymeric matrix23 are the reasons why FEC is a desirable additive since they maintain a stable SEI despite silicon’s “breathing” phenomenon as a function of charge/discharge (lithiation/ delithiation) cycles. Recent studies suggest that only when FEC is present a cross-linked polymeric matrix consisting of branched ethylene-oxide-based polymers is formed.24,25 In this Letter, we address the arrangement of an EC electrolyte solution at model electrode interfaces as a function of the electrolyte FEC content by X-ray reflectivity (XRR) and sum frequency generation (SFG) vibrational spectroscopy at two complementary oxide surfaces: native oxide terminated amorphous silicon (a-Si) and sapphire (001). These surfaces are chemically similar, regarding hydroxyl termination and hydrophilicity, as well as their interaction mechanism with different headgroup species of various self-assembled monolayers26−29 and organic liquids.30 We, therefore, expect that the interaction between EC and FEC with silicon oxide is similar to the interaction between EC and FEC with sapphire. Closedring carbonates have been reported to exhibit a preferential adsorption compared to linear carbonates (e.g., dimethyl carbonate) regardless of the electrode material or their bulk solution ratio.31−33 In addition, ethylene carbonate and fluoroethylene carbonate have similar adsorption energies.34 We chose lithium perchlorate (LiClO4) for its stability and to reduce undesired fluorine sources; unlike LiPF6, LiClO4 does not readily dissociate to form hydrofluoric acid that in time can damage the surface.8,35,36 Therefore, by probing this simple electrochemical system (LiClO4 dissolved in EC, FEC, and their mixtures) with surface specific techniques we show the fundamental and common structure of carbonate based electrolyte solutions at the electrode-electrolyte interface. XRR is a surface and interface sensitive technique that can yield Angstrom resolution molecular structural information on the first monolayer and subsequent layers near a surface.37−40 Such high resolution XRR, however, requires well-defined, flat interfaces; hence, our choice of sapphire, which has an atomically smooth surface (root-mean-square (RMS) roughness < 2 Å), is imperative for high-resolution experiments.26,27,41 Previous reflectivity work has been devoted to investigating the intercalation and alloying process of electrode materials as well as the formation and evolution of surface reaction layers such as the SEI. For example, Cao et al. investigated the lithiation and delithiation of crystalline silicon over several cycles and the concomitant evolution of the SEI via in situ XRR with nanometer resolution;42,43 Fister et al. employed a similar approach to investigate the surface structures found during the Li-intercalation in multilayer silicon electrodes.44 Dura and co-workers showed the in situ capability of neutron reflectometry (NR) to investigate the SEI and mechanisms of lithiation/delithiation of active materials in battery systems with several nanometer resolution. For instance, they could show the evolution of the SEI over several cycles and different electrochemical conditions45 as well as the thickness and state of charge of silicon electrodes.46 Several nanometer resolution NR experiments by Veith et al. unraveled the lithiation process of silicon electrodes and focused on the SEI behavior over many cycles,47,48 showing a thickness breathing behavior as a function of state of charge. Furthermore, the group found that the addition of FEC mediates the thickness and composition of the SEI on silicon electrodes.49 Schmidt et al. also performed NR measurements to understand the electrochemistry of amorphous silicon.50,51
In addition to providing insights into the SEI, their results also show mechanistic insights into the lithiation of crystalline silicon, such as suggesting the presence of two zones of lithiation.52 While these previous reflectivity studies investigated the formation of reaction layers at electrode surfaces with nanometer resolution for the XRR studies, and several nanometer resolution for the neutron reflectometry studies, respectively, our present study has a fundamentally different aim, that is, understanding the atomic scale arrangement of individual solvent molecules with Angstrom resolution XRR in combination with SFG vibrational spectroscopy. SFG with its interface only sensitivity53,54 serves as a complemental surface science technique to deliver the chemical composition and structure of the first monolayer. SFG vibrational spectroscopy has characteristics of nondestructive spectroscopy, which allows us to study under working conditions the different molecularsurface adsorption structures and composition of electrolyte solutions.13,32,55−58 In our study, by combining these two techniques we achieve an in situ detailed subnanometerresolution structure (XRR, SFG) and chemical composition analysis (SFG) of the solid−liquid interface and provide insights into the factors governing the formation of the SEI. Qualitatively, SFG and XRR provide similar results and suggest that the addition of FEC realigns the first EC/FEC monolayer at the interface to a more perpendicular alignment, while in layers further away from the surface only minor changes are observed, and a more surface-parallel alignment is preserved. While amorphous silicon and crystalline sapphire both have an oxide termination, SiOx and AlOx, respectively, and their chemical properties are largely similar,26−30 we refrain from quantitative comparisons as SFG, as a vibrational spectroscopy, is more sensitive to the chemical nature of the system. Furthermore, we carried out ex situ X-ray photoelectron spectroscopy (XPS) measurements on both samples presented in the Supporting Information (Figures S3 and S4). The results suggest that no chemical reaction layer is formed on either substrate. Nevertheless, there are some differences between the XPS results, i.e., we find intact FEC molecules remaining on sapphire after the rinsing procedure, which are not evident on silicon. The fact that FEC is harder to wash off on sapphire indicates a somewhat stronger interaction between these molecules and the substrate as compared to silicon. We suggest that this may also be the reason for subtle differences in the structural properties of the surface layer as found via XRR and SFG. The foundational understanding of the orientation of the electrolyte molecules that we obtain can inform strategies that may ultimately lead to faster charging, longer lasting, and safer LIBs. The apparatus and working principle for SFG and XRR were described in detail in previous publications13,42 and therefore will only be briefly explained here. SFG vibrational spectroscopy is achieved when two laser beams, a visible and an infrared one, overlap spatially and temporally on a medium that allows, under the dipole-moment approximation, a symmetry break (e.g., surfaces and interfaces).53,54 In order to probe the adsorption angle of the carbonyl group of both EC and FEC, we polarized the incoming visible (532 nm) and far IR (1700− 2000 cm−1) as well as the reflected SFG beam. We used S and P polarization combinations to probe the orientation of the carbonyl group. Conventionally, we define S as the electric field perpendicular to the incident plane and P as the one parallel to the incident plane. Therefore, an SSP (s-SFG, s-Vis, p-IR) polarization combination probes adsorbates with perpendicular B
DOI: 10.1021/acs.nanolett.8b00298 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
two EC/FEC mixtures, 7:3 and 1:1 wt %, and in FEC at the sapphire interface, all containing 1 M LiClO4 salt. The sapphire was cleaned using piranha acid as described previously.27 A characteristic minimum, qmin, around 0.7 Å−1 is apparent for all four electrolyte solutions. This qmin value corresponds to a surface-normal electron density variation with a length scale of t ≈ π/qmin = 4.5 Å. The molecules’ geometry, in which the elongated axis is ∼7 Å, whereas the shorter axis is ∼5 Å, indicates that the elongated axis of EC and FEC in solution are primarily aligned surface-parallel. We notice a trend in which with increasing concentration of FEC, both the minimum and maximum (in the XRR curves) slightly shift toward lower qz values, implying a slight increase in t. To extract more detailed information on the molecular arrangement of solvent and ion molecules near the interface, we employed a model of the solid-liquid interface with the liquid forming layers that is termed the distorted crystal model (DCM).62,63 This model has been previously used to describe the layering of electrolytes61 and metallic64,65 and ionic liquids41,66−68 at solid surfaces and consists of a series of equally spaced Gaussian functions. Each Gaussian represents one molecular layer, and the loss of order, as a function of separation from the interface, is encoded by their increase in width. Mathematically, the DCM-EDP can be written as
(to the surface) dipole moments; we used an SPS (s-SFG, pVis, s-IR) polarization combination to probe adsorbates with parallel (to the surface) dipole moments. XRR measures the intensity fraction of a monochromatic incoming X-ray beam that is specularly reflected (i.e., exit angle is equal to incoming angle) from a surface. Accordingly, XRR provides information on the surface normal electron density profile (EDP).40,59,60 We develop a physically meaningful EDP model from which the reflectivity is calculated and then compare the experimental reflectivity data to the modeled one, which is varied until the two match. Experiments were carried out at Stanford Synchrotron Radiation Lightsource BL 7−2 using 14 keV Xrays. Details of the experimental procedure can be found in ref 61. All cells were prepared in an argon filled glovebox with oxygen partial pressure below 0.3 ppm. In Figure 1a, we show the Fresnel-normalized reflectivities (R/RF) of 1 M LiClO4 in EC electrolyte solution, along with
⎛ z ⎞⎤ ρsub − ρbulk ⎡ ⎢1 + erf⎜ ⎟⎥ ⎢⎣ 2 ⎝ 2 σsub ⎠⎥⎦
ρ (z ) =
∞
+
∑ n=0
ρbulk d σn 2π
2
e−(z + d0 + nd)
/2σn 2
where ρsub and σsub are the substrate’s electron density and roughness, respectively. ρbulk is the liquid’s bulk electron density, d the spacing between liquid layers, and d0 the distance of the first layer from to the substrate. σn = σ0 2 + σb 2 represents the width of the n-th Gaussian. Here, σ0 is the width of the interface-nearest layer and σb is the broadening from one layer to the next. Accordingly, the DCM model has five free fit parameters. The figure of merit used in the fitting routine is data 2 data ∑i[(Rmodel − Rdata is the measured i i )/Ri ] , in which R model reflectivity, R is the calculated reflected intensity, and each “i” represents one data point. More details about the DCM modeling can be found in our previous report.61 To justify our choice of the DCM, a comparison of model-fits using a more conventional slab approach is presented in the Supporting Information. The failure to produce good fits and the obtained unphysical parameter values within these models shows that the DCM is superior to other models, even if these comprise more free parameters. The corresponding fits are shown in Figure 1a
Figure 1. (a) Measured, Fresnel-normalized, XRR of sapphire in contact with liquid 1 M LiClO4 in EC (gray markers), EC/FEC 7:3 (blue), EC/FEC 1:1 (green), and FEC (red) vs surface-normal scattering vector qz. The solid lines represent the distorted crystal model (DCM) fits discussed in the text. The vertical line highlights the shift in qmin. (b) Same-color fit-derived surface normal electron density profiles ρ(z). We show the confidence interval of the density profiles by the gray shading. All curves are vertically shifted for clarity. The inset shows a sketch of the XRR-derived different orientation of the electrolyte molecules within the surface layer for EC-rich and FEC-rich electrolyte solutions. Due to a reorientation of the solvent molecules to a more surface perpendicular alignment in the presence of FEC, the mean distance d0 between the surface and the center of the molecules along the surface normal increases; this is consistent with our data.
Table 1. XRR Fit-Derived DCM Model Parametersa EC best ρsub (1/Å3) ρbulk (1/Å3) σsub (Å) σb (Å) σ0 (Å) d0 (Å) d (Å) a
1.19 0.41 1.56 1.17 1.33 2.99 4.16
+
0.10 0.13 0.08 0.17 0.20
7:3 −
best
-0.07 -0.13 -0.05 -0.13 -0.17
1.19 0.42 1.73 1.10 1.33 3.19 4.22
1:1
+
0.09 0.05 0.06 0.14 0.14
−
best
-0.03 -0.10 -0.02 -0.05 -0.07
1.19 0.45 1.88 0.98 1.45 3.12 4.27
+
0.04 0.05 0.04 0.07 0.08
FEC −
best
+
−
-0.03 -0.05 -0.02 -0.05 -0.05
1.19 0.46 1.92 1.00 1.52 3.35 4.38
0.02 0.14 0.02 0.04 0.05
-0.12 -0.02 -0.08 -0.19 -0.20
Bold parameters were kept fixed in the model refinement. C
DOI: 10.1021/acs.nanolett.8b00298 Nano Lett. XXXX, XXX, XXX−XXX
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starting from ∼1.2 Å for pure EC to 1.0 for pure FEC (see Figure 2d). These findings suggest that upon adding FEC the layers further away from the interface become slightly better defined. Under the assumption that, due to their molecular geometry, surface-parallel molecules would result in sharper layers, this observation leads us to suggest a tendency of solvent molecules in subsequent layers to maintain their surface-parallel orientation as found in pure EC. Together with the increase in σ0 and d0, the small decrease in σb in combination with a fairly unchanged d indicates that reorientation for higher FEC contents is more prominent for the surface layer compared to layers further into the bulk electrolyte. A sketch of this phenomenon is shown in the inset of Figure 1. While the observed trends are all fairly small relative to the error bars, they collectively support our hypothesis that the addition of FEC molecules results in the solvent molecules at the surface becoming aligned with the FEC/EC plane somewhat more normal to the interface, whereas the subsequent layers are less affected. Our conclusion based on our XRR data is qualitatively supported by our SFG investigations showing a similar trend discussed below. Our SFG results of the first monolayer of the various electrolytes are shown in Figure 3a. It is worth noting that the
(solid lines); the fit-derived EDPs are shown in Figure 1b, all showing interfacial layering; the first layer is denoted surface layer. The fit-derived parameters are tabulated in Table 1; the uncertainties were derived as described elsewhere.27,42,69 The model suggests that a single molecular layer is described by a single Gaussian, despite the fact that the electrolyte solutions consisting of 1 M LiClO4 salt dissolved in either pure EC, mixtures of EC and FEC, or pure FEC are multicomponent liquids. As shown in our previous work,61 this indicates that, on average, the different electrolyte components align within the same z-range, i.e., within a single molecular layer. In Figure 2a−d, we show the fit derived interfacial structural parameters vs the EC content (wt %) in the EC/FEC solutions.
Figure 2. Fit-derived structural parameters as a function of EC (wt %) in the 1 M LiClO4 EC/FEC solution: (a) layering periodicity d; (b) distance d0 of the first layer to the substrate; (c) width of the surface layer σ0; and (d) broadening of the layers into the bulk σb. Figure 3. (a) SFG spectra of pure EC and EC/FEC mixtures at open circuit potential probed at SSP (open circles), sensitive to perpendicular aligned adsorbates, and SPS (open diamonds), sensitive to parallel aligned adsorbates. For clarification, we assign low and high to EC carbonyl group vibrations. The addition of FEC produces two more peaks at ∼1840 cm−1 and ∼1855 cm−1. (b) Top, an illustration showing the CO vector and its angle (θ) from the normal to the surface (Z). Note that θ = 0 when the CO is perpendicular to the X−Y surface. Bottom, the CO bond angle with respect to the surface normal as a function of FEC wt % content.
Figure 2a presents the distance between molecular layers near the surface, d, that shows a slight trend to increase from ∼4.2 to 4.4 Å with increasing FEC concentrations. In Figure 2b, we report the distance of the first interfacial layer to the substrate, d0, which also increases with increasing FEC concentrations, from ∼3.0 to 3.4 Å. Interestingly, d0 seems to increase more than d, 12% vs 5%. This difference is even more apparent when considering that d0 represents, in an idealized case neglecting the substrates surface atom size, only half of the surface layer. The difference indicates different behaviors of the first and subsequent layers upon the addition of FEC. We note that EC and FEC molecules have, from their van der Waals diameters, roughly the same size, and we, therefore, conclude that the increase in d0 may be due to a reorientation toward (on average) a more surface-normal alignment of the molecules. This hypothesis is further supported by the increasing trend by 14% of the initial layer broadening parameter, σ0, with the addition of FEC (Figure 2c); due to the molecular geometry, a reorientation to more perpendicular molecules will result in a broadening of the first layer. The broadening of the layers into the bulk, σb, is perhaps slightly smaller for higher FEC contents,
SFG amplitude is the product of the number of the adsorbed molecules (surface coverage) and their bond angles with respect to the surface normal (for details, refer to Supporting Information).53,54 In Figure 3a, the pure EC solution shows two peaks (labeled as low and high) that are noted in the text as COLOW and COHIGH, respectively. By COLOW, we refer to more tightly packed EC molecules that resemble the strong intermolecular EC interactions in the solid phase. The EC fundamental CO vibration is sensitive to intermolecular interactions (e.g., solid or liquid phase). For example, in the solid phase, strong Raman and IR overtones shift the D
DOI: 10.1021/acs.nanolett.8b00298 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters fundamental vibration frequency to around 1785 cm−1. In the liquid phase, this is shifted to around 1820 cm−1.70 Therefore, we attribute the COLOW peak at 1780 cm−1 to tight packing, strong EC intermolecular interactions. We assign the second peak COHIGH at 1820 cm−1 to loose packing, weak EC intermolecular interactions. When mixing EC and FEC, the SFG spectrum of the interface uniquely changes according to the SFG polarization (SSP or SPS). For SFG at SSP, a perpendicular to the surface sensitive polarization, the addition of FEC to EC yields a new combination of vibrational peaks that, since no redox electrochemical processes take place at open circuit potential, we assume are due to the reorientation of EC by FEC. Specifically, in Figure 3a (open circles), which corresponds to the perpendicular component (SSP) of the EC/FEC mixtures, the COLOW vibration signal is blue-shifted from 1778 cm−1 (pure EC) to 1781 cm−1 at an EC/FEC 7:3 wt % ratio and up to 1792 cm−1 at an EC/FEC with a 1:1 wt % ratio. The C OHIGH vibration maintains its frequency at 1814 cm−1 for pure EC and EC/FEC 7:3 wt % mixture. However, at higher FEC concentrations it too blue-shifts from 1814 to 1823 cm−1 probably due to stronger intermolecular interactions. In contrast to the frequency shifts of EC perpendicular SFG (SSP) components, the parallel to the surface SFG (SPS, Figure 3a, open diamonds) components vanish with increased amounts of FEC. In Figure 3a, the SFG (SPS), which is sensitive to the parallel to the surface dipole alignments of pure EC, distinctly shows two peaks at ∼1780 and 1815 cm−1, the tightly packed (COLOW) and loose structure (COHIGH), respectively. Note that in Figure 3a, as the FEC ratio increases to EC/FEC 7:3 wt % the COLOW amplitude vanishes and the COHIGH drops to the noise level of the background, and at EC/FEC 1:1 wt % there is no feature. The decrease in the amplitude to the noise level of the SPS polarized SFG implies that there is negligible net-dipole moment along the parallel to the surface as FEC is added. We can exclude a scenario of parallel dipole flipping since previous studies21,32,34 show that EC remains adsorbed on the surface through their CO groups as FEC content increases. We assume since both EC and FEC have a rigid planner closed ring structure that wherever the CO dipole, schematically illustrated in Figure 3b, points the five-membered ring skeleton will follow. Therefore, based on our SFG spectra, we suggest that the addition of FEC to EC electrolytes results in a surface layer containing molecules primarily oriented with their CO groups pointing toward the surface. We conclude that as we add more FEC to an EC solution, FEC realigns the EC molecules at the surface to a more up-right (perpendicular to the surface) position. To quantify these observations, we process the amplitude ratio of a specific vibration at two polarizations (SSP and SPS) according to the mathematical model fitting described in the Supporting Information. Based on our fitting, the EC adsorption (CO) angle (Figure 3b), noted as relative to the surface normal, decreases with increasing amounts of FEC; for COLOW, it ranges from 90° (pure EC) to 2° (EC/FEC 1:1 wt %) and for COHIGH, it ranges from 43° (pure EC) to 2° (EC/FEC 1:1 wt %). SFG and XRR show a similar trend in which molecules in the surface layer change their orientation with increasing bulk electrolyte FEC content. The main findings of our XRR data are that, upon increasing the FEC concentration, larger distances of the surface layer to the substrate, d0, and a larger
width of the surface layer, σ0, are observed. Together with our finding that the layering periodicity, d, remains fairly unchanged, and that the broadening parameter, σb, decreases, this suggests a reorientation of the molecules in the surface layer to a somewhat more surface-perpendicular alignment in the presence of FEC, while layers further from the surface are less affected and appear to be arranged surface-parallel independent of the presence of FEC. The reorientation within the surface layer, specifically for ethylene carbonate (EC), is also apparent in our SFG spectra. As we increase the amount of FEC in the electrolyte mixture, we detect an increase of the perpendicular to the surface dipole component (SSP) amplitude, and at the same time, we notice the disappearance of the parallel to surface dipole component (SPS). The correlation between the fitting results of our XRR model and SFG analysis leads us to hypothesize that with increased amounts of FEC a larger portion of EC molecules in the surface layer are oriented surface-perpendicular, as opposed to surfaceparallel. This reorientation occurs more significantly at the surface layer and layers extending further into the bulk electrolyte tend to maintain their surface-parallel orientation. Quantitatively, the EC angle appears different when we compare our XRR and SFG results; while there is no straightforward quantification of the EC angle from XRR, d0 and molecular geometry suggests a larger angle as found from SFG (presented in Figure 3b). Our XPS results discussed above provide some insight into possible explanations for the observed differences. Nevertheless, the combination of SFG and XRR provides new light on the role of FEC as an additive by showing that FEC induces a more up-right EC structure at the interface. This influence is most significant for the surface layer as changes in subsequent solvent layers appear to be minor. In conclusion, by combining sum frequency generation (SFG) vibrational spectroscopy and X-ray reflectivity (XRR), we gain fundamental understanding on the solid−liquid interface structure starting at the surface layer and further into the bulk electrolyte solution of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) at various electrolyte mixtures in contact with two similar surfaces: a native oxide terminated amorphous silicon and sapphire. Our results suggest that the arrangement of the surface layer is slightly different from its adjacent layers extending into the bulk solution. We found that the addition of FEC realigns the EC/FEC surface layer to a more perpendicular alignment, while in layers further away from the surface a more surface-parallel alignment is preserved. We hypothesize that the beneficial properties found in SEIs formed in electrolytes containing FEC may originate in the observed FEC induced reorientation of the surface layer at OCV.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b00298. Explanation of the SFG process, fitting results according to the SFG model and how one calculates the adsorption angle (PDF) E
DOI: 10.1021/acs.nanolett.8b00298 Nano Lett. XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yonatan Horowitz: 0000-0002-7150-9264 Michael F. Toney: 0000-0002-7513-1166 Gabor A. Somorjai: 0000-0002-8478-2761 Author Contributions
Y.H., H.-G.S., and H.-L.H. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Freedom CAR and Vehicle Technologies of the U.S. Department of Energy under contract No. DE-AC02 O5CH1123. The SFG instrumentation was purchased with funding from the Director, Office of Basic Energy Sciences, Materials Science and Engineering Division of the U.S. Department of Energy. This work was partly supported by the Department of Energy, Laboratory Directed Research and Development funding, under contract DE-AC0276SF00515. This work was partly supported by the Joint Center for Energy Storage Research (JCESR). Research carried out at Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract No. DE-AC02-76SF00515.
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