Identifying the Decomposition of Diethyl Carbonate ... - ACS Publications

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Identifying the Decomposition of Diethyl Carbonate in Binary Electrolyte Solution in Contact with Silicon Anodes - A Sum Frequency Generation Vibrational Spectroscopy Study. Yonatan Horowitz, Hui-Ling Han, and Gabor A. Somorjai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03774 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Industrial & Engineering Chemistry Research

Identifying the Decomposition of Diethyl Carbonate in Binary Electrolyte Solutions in Contact with Silicon Anodes - A Sum Frequency Generation Vibrational Spectroscopy Study Yonatan Horowitz*,§,†,‡ Hui-Ling Han,§,† and Gabor A. Somorjaj†,‡ †Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ‡Department of Chemistry, University of California, Berkeley, California 94720, United States

ABSTRACT

The key factor in the use of lithium ion batteries is the formation of an electrically insulating solid layer that allows lithium ion transport but stops further electrolyte redox reactions on the electrode surface, therefore termed the solid electrolyte interphase (SEI). We have studied the preceding stages to the SEI formation, specifically the reduction of diethyl carbonate (DEC) mixed with ethylene carbonate (EC) with dissolved 1.0 M LiPF6; a common binary electrolyte. We used a p-doped crystalline silicon (100)-hydrogen terminated wafer as the anode in a lithium

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(Li) half-cell system. We employed in situ sum frequency generation (SFG) vibrational spectroscopy with interface sensitivity to probe the molecular composition of the SEI surface species under various applied potentials where only diethyl carbonate reduction to form a Siethoxy (Si-OCH2CH3) surface is expected. We found that even at open circuit potential (OCP) DEC decomposes on the silicon (100)-hydrogen surface to form Si-ethoxy bonds. These findings shed new light on the interfacial Si anode-binary electrolyte solutions chemistry at stages preceding the formation of the SEI on Si anodes.

INTRODUCTION Lithium ion battery systems are electrochemical devices comprised of an anode, porous separator, cathode, and an electrolyte that solvates a lithium salt. During discharge, electrons leave the anode structure, pass through the current collector to a specific circuit (i.e., an electrical device) before entering the cathode. To maintain the neutrality of the surface charge, Li+ ions are released into the electrolyte to carry positive charge to the cathode. During charging, a voltage potential is applied to release electrons from the cathode while Li ions are released into the electrolyte from the cathode to accumulate in the anode. The electrode/electrolyte interface is the only location where reactions take place, and the ion and electron exchange at the interface dictates the energy release or accumulation rate in rechargeable systems.1 Presently, vehicular applications implement graphitic carbon anodes and metal oxide cathodes with a combination of ethylene carbonate (EC) and diethyl carbonate (DEC) or dimethyl carbonate (DMC) and LiPF6 as salt. Additives are also added to improve stability and performance at low-temperatures.2-4 With the growing interest to employ lithium-ion batteries for electric vehicle applications, anode materials that can deliver higher theoretical gravimetric


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capacity than carbon based anodes (372 mA h g−1) have attracted much attention in recent years.5 One of the top contenders is silicon (Si) due to its higher theoretical gravimetric capacity of 4200 mAh g−1. Unlike carbon based anodes, Si allows lower working potentials of 0.2 – 0.3 V vs. Li/Li+ and so avoids the risk of lithium plating. In addition, Si presents high volumetric capacity (≈ 9786 mAh cm−3), and is naturally abundant on Earth.6 However, the commercial use of Si is still facing some considerable challenges: extreme volume expansion due to the lithium alloying/dealloying process,7 low intrinsic electronic and ionic conductivity, and unstable solidelectrolyte interphase (SEI).8-9 Ideally, upon charging the Li-ion battery, the anhydrous organic electrolyte10 in contact with the anode material is reduced to form an insoluble electrical insulating layer. This layer allows lithium ion transport at a reasonable rate while hindering further electrolyte consumption on the anode surface, therefore termed the solid electrolyte interphase (SEI).11 However, for Si anode materials this is not the case and the SEI formed has soluble products that diffuse into the bulk electrolyte solution exposing fresh Si surface.12-13 Consequently, the electrolyte is gradually consumed over the charge/discharge cycles lowering the discharge capacity and coulomb efficiency until rendering the battery useless. It is imperative to understand the chemical nature of each electrolyte compound in contact with the anode through the charge and discharge cycles, and at preceding stages to its reduction potential, as this knowledge can modify operational strategies for designing lithium ion batteries for vehicular applications. To specifically probe under reaction conditions the surface chemistry a surface specific, nondetrimental technique is required. Sum frequency generation (SFG) vibrational spectroscopy14 is a non-destructive, in situ nonlinear optical method that yields vibrational spectra of adsorbates that are molecules found solely at surfaces (and interfaces).15 Accordingly, SFG vibrational


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spectroscopy is used to identify interfacial (adsorbed) species on a surface and probe their dynamics.16-17 As SFG vibrational spectroscopy utilizes laser beams, by polarizing the laser beams, it can provide the interfacial species orientation in respect to the surface plane.18 SFG vibrational spectroscopy was successfully applied in Li-ion battery studies on either anode19 or cathode20-21 electrode materials in contact with various electrolyte solutions, as well as under open circuit potential (OCP), and applied potential (e.g., reaction conditions).22-24 In situ SFG vibrational spectroscopy having an interface sensitivity often leads to surprising findings reinforcing the growing notion that the electrode-electrolyte interface has different properties than that of the bulk solution electrolyte. For example, it was found that cyclic carbonates such as ethylene carbonate (EC) have a preferential adsorption on a LiCoO2 cathode surface compared to the linear carbonates such as diethylene carbonate (DEC).21 Consequently, for a mixed solution of EC : DEC with a 1:1 bulk volume ratio the molecular ratio at the interface is roughly 9:1. Here we show by SFG vibrational spectroscopy performed while applying a potential that in a binary electrolyte solution comprised of EC and DEC, the linear carbonate (DEC) is first to dissociate to form Si-ethoxy when in contact with a Si(100) hydrogen terminated anode starting at open circuit potential. At a constant potential of 1.0 V, there is a shift in the vibration frequency of the ethoxy group (-OCH2-) that we suggest to originate from further DEC decomposition leading to higher surface coverage and stronger intermolecular interactions. When we reduce the applied potential to 0.8 V vs. Li/Li+, the vibrational modes have further redshifts. Subsequent cycling in the potential between 0.8 V to 1.1 V does not induce further frequency shifts. Therefore, we suggest that the constant Si-ethoxy surface coverage is achieved once we lower the potential to 0.8 V and is maintained over the cycling. By comparing the


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applied field at the electrode-electrolyte interface to reported systems we estimate that a Stark effect induces a ~ 1.5 cm-1 shift as well. EXPERIMENTAL SECTION Sum Frequency Generation Vibrational Spectroscopy. The picosecond laser system consisted of a 1064 nm Nd:YAG pump laser (PL2230, Ekspla) with a repetition rate of 50 Hz and an average peak power of 25 mJ. A LaserVision optical parametric generator and amplifier system converted the 1064 nm to a visible 532 nm beam and a mid-IR beam ranging between 2200 and 4000 cm−1. The visible and infrared beams overlap spatially and temporally on a medium. The beam angles were 34° and 48°, respectively, regarding the perpendicular plane to the sample surface (reference plane). We collected the SFG beam (in the UV range) by a Hamamatsu photomultiplier tube. We added several band-pass filters to minimize stray 532 nm light. Unless specified differently, we used an SSP (SFG, vis, IR) polarization combination because it is sensitive to the adsorbate dipole moment perpendicular to the interphase. Optical electrochemical half-cell (“eCell”). The optical electrochemical half-cell (ECC-OPTO-STD, EL-CELL) hence, “eCell” serves to monitor the optical properties of an electrode material in the course of galvanostatic measurements (electrochemical charging and discharging). For this purpose, the working electrode (Si anode) is placed right below an optical window. To reduce IR absorption in the mid-IR (2800 cm-1 – 3400 cm-1) we used CaF2 windows and a 400 nm thick copper thin film is deposited on the window in the shape of an open ring (refer to S1) to ensure a good electric contact with the current collector. We chose copper as it is stable and does not lithiate (e.g., form


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a LixCuy alloy) at anodic potential range applied in this study.25 A 9 mm in diameter lithium disc performs as both counter electrode and a reference electrode. The Si anode is pressed from below by the Li disc onto the CaF2 window to minimize IR absorption due to the electrolyte. A Celgard 2400 separator placed in between the Li disc and Si anode ensures electrical insulation. In short, the IR and visible lights shine from above through the optical window and electrolyte layer onto the topside of the Si anode, and the reflected SFG signal is collected with the photon detector. To minimize the IR absorption of the electrolyte solution, we diluted the electrolyte with deuterated tetrahydrofuran (d-THF) to about 2% in v/v. THF is known to be stable in the potential range that we chose.26-28 Prior to any experiment, the disassembled test cell was dried over night at 80 C° in a vacuum oven. We assembled (and disassembled) the test cell in an inert (ultra-pure argon) atmosphere with 0.1-ppm concentration of water and oxygen (LabStar, MBruan). Materials. The electrolyte was a battery-grade 1.0 M lithium hexafluorophosphate (LiPF6) solvated in an ethylene carbonate (EC) and diethyl carbonate (DEC) solution having a 1:1 v/v ratio (SigmaAldrich) and was used without further purification. Lithium foil was purchased from Alfa-Aeser. All electrochemical cells were assembled in an argon filled glovebox (Labstar, MBraun) where water and oxygen levels were < 0.1 ppm. A commercial p-type Si(100) wafer, the anode, was purchased from MTI corporation. The Si(100) was boron doped, having a 5-10 Ω/cm resistivity and was polished on both sides. The Si(100) wafer was cut to ~ 7 x 7 mm2 pieces so it would fit into the 10 mm in diameter circular piston design of the eCell. The Si squares were cleaned by submerging them in a hot piranha for 10 minutes. After which, they were thoroughly rinsed with TDW. Aqueous hydrofluoric acid (50% by weight) was obtained from Sigma Aldrich. The removal of the oxide layer was carried out by dropping the Si squares in hydrofluoric acid for 5


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minutes. Immediately, after they were thoroughly rinsed with TDW, and transferred into a glovebox. All H2O is deionized with a resistivity of 18.2 MΩ × cm. CAUTION: Fluoride-containing solutions such as 27 M (48% by weight) HF pose a severe contact hazard. Hydrofluoric acid is highly toxic and corrosive and may cause serious burns, which may not be immediately painful or Visible. Fluoride ions readily penetrate the skin and can cause the destruction of deep tissue and bone. Electrochemical Measurements. We chose a two-electrode configuration for simplicity. All potentials reported herein are referred to the Li/Li+ redox couple. We applied various constant potentials using a VersStat3 (Ametek Scientific Instruments) potentiostat. We chose a common electrolyte solution: 1.0 M LiPF6 salt dissolved in ethylene carbonate (EC) and diethyl carbonate mixture (DEC) in a volumetric ratio 1:1, purchased from Sigma-Aldrich and used as is. We have diluted all electrolyte solution with deuterated tetrahydrofuran (d-THF, Sigma-Aldrich) to about 2% in v/v. THF is known to be stable in the potential range that we chose.26-28 The anode material was a p-doped Si(100) wafer (MTI corporation) that we cut into 7 mm by 7 mm pieces. A lithium piece of 1 mm thick and 9 mm in diameter was used as a reference electrode. We used Celgard 2400 as the separator. RESULTS AND DISCUSSION To study the preferential reduction of diethyl carbonate (DEC) on Si(100)-H electrodes, we have carried out SFG measurements at open circuit potential (OCP), and at known DEC reduction conditions23 for prolonged times (chronoamperometry, CA) at 1-hour intervals. To avoid an abrupt polarization of the Si(100)-H terminated anode, we applied a linear scan


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voltammetry from OCP to the desired reduction potential. In this paper, we also compare the obtained SFG spectra after CA to ones taken after 5 sweeps at the same potential range (1.1 V to 0.8 V) with a sweep rate of 0.1 mV/second. In Figure 1a, we show the current drop from ~ 0 ampere (A) at OCP to ~ -89 µA/cm2 over 30 minutes during a linear sweep from OCP to the reduction potential of 1.0 V with 1 mV/sec sweep rate. Once the potential reaches 1.0 V, the current rises for 180 minutes until reaching a steady state current of about -4 µA/cm2. In Figure 1b, we present the CA plot at 0.8 V constant for just over 24 hours. At first, the current plunges to ~ -58 µA/cm2, afterwards it monotonically increases to around ~ -2.5 µA/cm2 within 180 minutes and stabilizes around -1.5 µA/cm2 after 360 minutes. In Figure 1c, we present the current vs. time of 5 consecutive cycles in which the potential was swept from 0.8 V to 1.1 V. The 5 sweeping cycles were taken after the silicon anode was held at 0.8 V for 24 hours. Each sweep took about 50 minutes to complete. The initial current starts ~ -6 µA/cm2 and monotonically increases to about 3 µA/cm2 within 50 minutes. Similar current behavior is observed for the later cycles. The increase of the current when the potential is at 0.8 V leads us to the conclusion that a subtle reduction process takes place which we associate with the sole reduction of DEC.23


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Figure 1. The electrochemical measurements (chronoamperometry and sweeping potential) of Si(100)-H anode in contact with an electrolyte (1.0 M LiPF6 in EC:DEC, 1:1 v/v). Chronoamperometry: (a) from OCP to 1.0 V, and (b) from 1.0 V to 0.8 V. (c) The current vs time of 5 consecutive cycles run between 0.8 V to 1.1 V with a scan rate of 0.1 mV/second. All potentials reported herewith are vs. Li/Li+. For example, we note the slow decrease of the maximal positive current values (at 1.1 V) that decrease from 3 µA/cm2 to 1.5 µA/cm2. This trend leads us to the conclusion that DEC decomposes on the Si(100)-H surface as no SEI formation is expected at these potentials23, 29-30 since EC is well above its reduction potential.23, 31 In our former study, we have concluded that as a single solvent (component) electrolyte, DEC is reduced at 0.8 V, and EC is reduced bellow 0.5 V when in contact with a Si-H terminated anode.23 An open question remained whether the reduction of DEC in a binary (two) component


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electrolyte solution is the same.32 We are addressing this question, after conducting in situ SFG measurements under constant potentials, in the following section. In Figure 2 we compare between SFG spectra of a freshly assembled eCell taken at open circuit potential (black line, OCP fresh) to ones taken after 24 hours (red line, OCP overnight), and as a function of the applied potentials. We have chosen an SSP polarization (s-SFG, s-vis, and p-IR) that is more sensitive to adsorbed species with a dipole moment perpendicular to the surface plane.14-15 We performed our SFG scans between 2800 to 3100 cm-1 since we did not detect visible features bellow 2900 cm-1 as expected since EC is supposed to dominate the Si anode surface,21, 33 and DEC methylene peak intensity vanishes near the Si surface.34 Therefore, we present our SFG spectra from 2900 cm-1 to 3015 cm-1. The amplitude at a given frequency can also be associated with the ordering of the molecules at the interface. The more dipole moments are aligned normal to the anode surface the stronger SFG intensity.


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Figure 2. SFG spectra at open circuit potential and under applied potentials. The SFG spectra of the Si(100)-H / electrolyte interface taken at OCP from a fresh sample (black), and after 24 hours at OCP (red). Followed by SFG spectra of the same sample after 4 hours at 1.0 V (green), 24 hours at 0.8 V (blue), and after sweeping the potential between 0.8 V ➝ 1.1 V for 5 cycles (purple). The Si anode was in contact with 1 M LiPF6 : EC: DEC (1:1, v/v). The spectra are vertically shifted for clarity. Open symbols represent the data while solid lines are the model according to Eq. S2. Inset: illustrations of diethyl carbonate (DEC, top) and ethylene carbonate (EC, bottom) and our assigned -OCH2 groups β and γ, respectively. In Figure 2, we assign four major vibrations as follows: (α) asymmetric CH3 stretching (asCH3) at 2955 cm-1, (β) asymmetric OCH2 stretching (as-OCH2) at 2972.5 cm-1, (γ) asymmetric OCH2 stretching of ethylene carbonate (as-OCH2 EC) at ~2990 cm-1, and (δ) asymmetric OCH2 stretching (as-OCH2) at ~3006 cm-1.23,

28, 35-36

We assign the γ vibration at ~2990 cm-1 to the

ethylene carbonate -OCH2 group since this vibration was reported as both Raman and IR active, thus possibly SF active.15, 37 For further vibrational properties (e.g., amplitude and peak width) refer to Figure S2. We notice that except for as-CH3 (α group) all other modes shift from their original vibrations at OCP by up to 2.5 cm-1 as a function of the applied potential. However, after the anode was cycled, we did not detect further shifts. The methyl (R–CH3) is a terminal group in either DEC or its reduction product Si-OCH2-CH3, and the negligible shift of as-CH3 indicates that the α group frequency is less likely affected by the surface field and also the increase in amplitude suggests that more R-CH3 species are present on the anode surface. The wavenumbers of the β and γ groups (both assigned to as-OCH2) are affected by the applied potential as can be seen in Figure S3. These two features have red-shift by ~1.5 cm-1 at 0.8 V. Interestingly, we also


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detect a minuscule red-shift in wavenumbers of the β and γ groups after 24 hours in contact with Si(100)-H anode under OCP condition. We associate this observation with spontaneous electrolyte decomposition on an active anode surface at open circuit potential.38 A red-shift in frequency often reflects weaker intermolecular interactions of neighboring molecules, e.g., a loose structure as in the case of Si-methoxy (Si-OCH3).39 When we consider that Si-OCH3 group has a high steric repulsion,40 it is reasonable to assume that a larger group such as Si-ethoxy will have an even stronger steric repulsion, consequently having fewer neighboring molecules. Hence, our suggestion that the red-shift of the -OCH2 groups is due to the partial dissociation of DEC to form the ethoxy anion. Bsaed on previous studies we assume that linear carbonates decompose via a linear alkyl anion (in our case the ethoxy anion CH3CH2O− featured in Scheme 1).29 The ethoxy anion readily reacts with acidic surface Si sites, Si(100)−H, and substitutes the proton with an ethoxy group to produce a Si-ethoxy bond (Si−OCH2CH3).23 Scheme 1. Proposed Diethylene Carbonate (DEC) Reduction Pathway

At the potential window studied in this paper (OCP to 0.8 V) DEC decomposes to form the CH3CH2O− anion. We suggest that this anion replaces the hydrogen terminated Si with an ethoxy group. The γ–group which we assigned as the as-OCH2 mode of the ethylene carbonate was found to have a significant red-shift when we applied 0.8 V. This shift can originate from a vibrational Stark effect,41 i.e, the vibration frequency is affected by the interfacial electric field. We therefore elaborate and examine this effect on the -OCH2 groups in the following text. Let us assume two vibrational states and a given applied electric field (E). The applied electric field (E)


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will produce a shift in the vibrational transition energy (∆v) that is proportionally linear to the difference of the two vibrational states dipole moments (∆µ), to give: ∆v = − ∆µ · E. We can also correlate between the anode-electrolyte interface under applied potential to other interfacial systems and estimate the vibrational Stark effect at the interface as the vibration’s difference dipole is directly related to the vibrational frequency’s sensitivity to the external electric field, e.g., the C-H stretch has small a transition dipole due to the small difference in electronegativity between carbon and hydrogen. Recently, Lian et al., measured an 8 cm-1 vibrational Stark effect shift for bound C≡N of 1,4-phenylene diisocyanide (PDI) when they applied potentials from 0.1 to -0.4 V vs. Ag/AgCl.42 The measured value of |∆µ| for the PDI C≡N stretch was reported as 1.8 cm-1/(MV/cm). Bearing in mind that the C-H stretch has a smaller transition dipole than an acetonitrile group (C≡N) and that the |∆µ| of acetonitrile is 0.44 cm-1/(MV/cm) a reasonable estimation for of hydrocarbons’ C-H stretch |∆µ| is

Identifying the Decomposition of Diethyl Carbonate ... - ACS Publications

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