Spectroscopic Compositional Analysis of Electrolyte during Initial SEI

Article pubs.acs.org/JPCC

Spectroscopic Compositional Analysis of Electrolyte during Initial SEI Layer Formation Gerald Gourdin, John Collins, Dong Zheng, Michelle Foster, and Deyang Qu* Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston, Massachusetts 02135, United States S Supporting Information *

ABSTRACT: The energy density of an electrochemical capacitor can be significantly improved by utilizing a lithiated negative electrode and a high surface area positive electrode. During lithiation of the negative carbon electrode, the electrolyte reacts with the electrode surface and undergoes decomposition to form a solid electrolyte interphase (SEI) layer that passivates the surface of the carbon electrode from further reactions between Li and the electrolyte. The reduction reactions that the solvent undergoes not only form insoluble and gaseous byproducts but also electrolyte-soluble products. In this work, those liquid-phase products generated by reductive decomposition of a carbonate-based electrolyte, 1.2 M LiPF6 in EC/PC/DEC (3:1:4), were analyzed at different stages during the lithiation process of an amorphous carbon electrode. An LCMS analysis of the electrolyte during the initial lithiation process was correlated to a DRIFTS analysis of the carbon electrode surface and the results from the EIS and gas-phase analysis previously reported. It was concluded that the formation reactions of the electrolyte-soluble reduction products are dependent on electrochemical processes but are independent of those reactions that directly lead to the generation of the decomposition gases. This is the first time that the composition of the electrolyte during the initial stages of formation of the SEI layer was systematically determined through the use of an in situ electrochemical-MS analysis. The results presented in this work are focused on the initial analysis, elucidation of the empirical formulas, and determining possible structures for these compounds through a detailed analysis of the spectra from each of these unknowns.

1. INTRODUCTION To address the energy density limitations of electrochemical double-layer capacitors (EDLC),1,2 research has been directed toward the use of pseudocapacitive electrode materials,3 modifying the porous structure of the electrode materials,4,5 and the development of asymmetric capacitors.5,6 In an asymmetric capacitor, the porous carbon material of one of the electrodes is replaced with a material possessing faradaic or pseudocapacitive properties4,7−9 such as lithiated carbons. A Li ion capacitor (LIC) assembled with a prelithiated negative carbon electrode and a high surface area porous carbon positive electrode is one of the more promising asymmetric supercapacitors. During the initial cycling of a lithium ion battery, the lithium ions in the electrolyte intercalate into the structure of the graphitic negative electrode. During this process of lithiating the carbon host material, the electrolyte reacts with the charged electrode surface to form a solid electrolyte interphase (SEI) layer. This layer should effectively passivate the graphite surface and halt the decomposition of the electrolyte. Hence, the development of a stable, conducting layer is crucial for the longterm cyclability of the battery. Unlike graphitic carbons that are used as the Li-ion anode, a high surface area carbon would be used in an asymmetric © 2014 American Chemical Society

supercapacitor. Therefore, the formation of a stable SEI layer is more critical for a prelithiated carbon electrode used in an LIC. Prelithiation of the negative electrode using a sacrificial lithium electrode10 minimizes the consumption of Li ions in the electrolyte during the formation cycles since the SEI layer is formed during that prelithiation process. The formation of the SEI layer is the most significant contributor to the irreversible capacity due to the loss of a significant portion of the Li ions present in the system that are irreversibly consumed during the layer’s formation.11 It has been shown that reduction of the electrolyte begins when the cell potential drops below 1.8 V vs Li.12−14 The initial reduction first results in the formation of a Li-stabilized radical anion, as proposed by Aurbach, Wang, and others.15−21 The radical anion undergoes one of two terminal reduction pathways that result in the generation of the gaseous byproducts, as shown in Schemes 1 and 2.13,22−24 More in depth description of these reactions have been proposed and discussed by Aurbach15,25,26 and others.27−34 Received: April 29, 2014 Revised: July 10, 2014 Published: July 10, 2014 17383

dx.doi.org/10.1021/jp504181b | J. Phys. Chem. C 2014, 118, 17383−17394

The Journal of Physical Chemistry C

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distinct kinetics.15,24,36,41,42 During the lithiation process, there was significant gas generation after the cell potential decreased below 1.0 V, and it was expected to continue to increase with further polarization of the electrode. This turned out not to be the case, and there was an apparent decrease in the generation rate.10,36 In addition, at higher cell potentials there was a significant increase in the passed charge that did not correlate with the generation of decomposition gases. These two phenomena at the higher and lower cell potentials are indicative of other reduction processes that do not directly result in the generation of decomposition gases. An analysis of how the composition of the electrolyte changes during SEI layer formation is an area that has been little explored in the literature, and one of which may provide further insight into these processes. There has been some work that has examined changes in the composition of the electrolyte over a period of many cycles,43 and mechanisms that would result in the formation of oligomeric compounds have also been put forth.44,45 These mechanisms follow as natural extensions of the solvent decomposition mechanisms put forth by Aurbach and others.1,2,15−17,20,46,47 However, while of extreme value, these studies were focused on the long-term changes in the composition of the electrolyte. The focus of this study was to address this gap in the literature and perform a systematic analysis of the changes in the composition of the electrolyte during the initial lithiation of an amorphous carbon electrode. This was accomplished by correlating the results from an LCMS analysis of the electrolyte and DRIFTS analyses of the developing SEI layer to the previously reported gas-phase and EIS analyses during the initial lithiation process. It was determined that there are additional reactions that give rise to unique compounds that would expect to have an effect on the performance of the electrolyte within the cell.

Scheme 1. Detailed Reaction Scheme Illustrating the Probable “2-Electron” Reduction Pathway for EC/PCa

a

A single molecule undergoes two 1-electron reductions, which can then react with other species to form (4) lithium carbonate and ethene/propene or (5) the alkyl carbonate and ethene/propene.

Scheme 1 illustrates the “2-electron” pathway, where an electrolyte component will undergo two 1-electron reductions.12,16,20,27,35 Coordination of the solvent molecule with the Li ion facilitates reduction because it results in a negative free energy change for the first reduction reaction,16,35 which generates the radical anion after overcoming the energy barrier associated with the ring-opening step, as determined by Wang et al.16 Since the electron affinity for the anion is predicted to be much lower than the uncharged molecule, the second reduction is unlikely to immediately follow the first, and since these reactions take place at the carbon interface, the radical anion can be stabilized by the active surface.24 Depending on the concentration of EC (or PC), the molecule can either react with a Li ion (Reaction 4) or with another Li-coordinated solvent molecule (Reaction 5) generating the decomposition gases and either lithium carbonate or the alkyl carbonate, respectively. It has also been shown that under the conditions at lower cell potentials, the reduction of a single EC or PC molecule can proceed through a “1-electron” pathway,15,26,27,29−31,36 which is illustrated in Scheme 2. The

2. EXPERIMENTAL DETAILS 2.1. Materials. The activated carbon powder used in this work was obtained from Calgon. The electrolyte was purchased from Novolyte Technologies and consisted of 1.2 M LiPF6 in a mixed solvent system of ethylene carbonate, propylene carbonate, and diethyl carbonate (EC/PC/DEC) in a 3:1:4 ratio. The moisture content of the electrolyte was determined to be lower than 5 ppm by Karl Fischer titration and was used as received. The Li foil was purchased from Sigma-Aldrich. The separators used in these experiments were trilayer C200 membranes obtained from Celgard. 2.2. Electrochemical Cell and Electrodes. The carbon electrodes were composed of the activated carbon powder and PVDF (binder) in a 7.5% mass ratio dissolved in N-methyl-2pyrrolidone (NMP). A slurry mixture was prepared of the combined components through the addition of acetone, and the mixture was homogenized by a planetary mixer. The mixture was allowed to dry to form a film that was then laminated onto copper mesh that had been painted with a graphite conducting paint. Electrode disks were punched out and dried in a vacuum oven to form the electrodes. The electrode area and the weight of active material were approximately 2.4 cm2 and 44 mg, respectively. The prepared carbon electrodes were employed as the working electrode, and a Li metal electrode was used for the counter/reference electrode. A unique electrochemical cell was manufactured for use in the measurements. A PTFE O-ring spacer was placed between the Li and carbon electrodes to create the necessary spatial arrangement.

Scheme 2. More Detailed Reaction Scheme Illustrating the Probable “1-Electron” Reduction Pathway of EC or PCa

A single (6) molecule is first reduced, which then reacts with another radical anion to form the alkyl carbonate and the corresponding decomposition gas. a

solvent molecule is first reduced and, rather than undergoing a second reduction, reacts with another radical anion to form the alkyl carbonate and decomposition gas directly. In our most recent report, an in situ electrochemical-mass spectroscopic (MS) technique was used to investigate the composition of the gases that are generated during the lithiation of an amorphous carbon electrode used in a LIC.36 In addition, impedance spectroscopy (EIS) was used to correlate the results of that analysis to the computational and experimental studies that have been reported in the literature.12,16,20,27,35 EIS has frequently been used for elucidating how the development of an SEI layer affects the physical and electrical properties of a carbon electrode,37−40 and the results from that analysis showed the development of two semicircular regions. This is evidence that the layer can be subdivided into two phases and that each phase will exhibit 17384



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2.3. Instrument Details. A Kurabo planetary mixer (Mazerustar KK-50S) was used to make the slurry homogeneous for fabrication of the electrode, and a Metrohm 656 coulometer was used for the Karl Fisher titration. The electrochemical measurements, including linear sweep cyclic voltammetry and AC impedance tests, were carried out using an Ecochemie potentiostat/galvanostat (Autolab PGSTAT30) equipped with frequency analyzer (FRA2) module controlled by the Nova software (version 1.7.8, Metrohm Autolab B.V.). All equivalent-circuit fittings of the AC impedance data were performed using ZView software (version 3.3a, Scribner Associates) and Nova software. The gas-phase MS analyses were carried out using a Hiden HPR-20 atmospheric gas analysis system. For the gas analysis, the analyzer source was set to an electron-energy voltage of 70 eV and an emission current of 200 μA for the first experiment. All carbon electrodes were analyzed for surface functional groups using the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique. The analysis was performed using a Thermo Electron Diffuse Reflectance Smart Accessory coupled with a Nicolet, Nexus 670 FTIR spectrometer with a nitrogen-cooled MCT detector. The infrared spectrometer was accessorized with a glovebag that allowed inert atmosphere conditions to be maintained during the anlaysis. Spectra were collected at 300 scans and a resolution of 4 cm−1. All DRIFTS spectra were converted to path-length appropriate Kubelka−Munk units. A single 2 or 3 point baseline correction was employed to each region of interest before final analysis using the Omnic 7.1 software. The LC-MS analysis was carried out using a HP Agilent 1100 HPLC coupled to a Micromass-Waters Quattro LC triple quadrapole mass spectrometer. Five microliters of sample was injected onto a Waters Symmetry C18 column, 4.6 × 150 mm, 3.5 μm particle size, and a 1:1 ACN/H2O mobile phase containing 0.1% trifluoroacetic acid (TFA) at 0.600 mL/min was used for the separation. The mass spectrometer was operated in ESI positive mode set as follows: capillary voltage, 2.50 kV; cone voltage, 25 V; extractor voltage, 0 V; RF lens voltage, 0.1 V; source temperature, 100 °C; desolvation temperature, 200 °C; and nitrogen gas flow at 90 L/h for the nebulizer and 600 L/h for desolvation. A T-fitting was used to split 10% of total LC flow into the inlet of the mass spectrometer. The m/z ratio was monitored from 20 to 400. The analysis of the LC-MS data was carried out using three open source tools. The application MZmine was used to evaluate the TIC chromatographic data and identify the individual peaks.3,48,49 The application Fityk was used to deconvolute the chromatographic data and calculate the peak areas associated with the identified peaks.1,2,4,5,50 Finally, the application mMass was used to isolate individual spectra for processing and interpretation of the spectral data.3,5,6,51,52 2.4. Lithiation Procedure and Analyses. Galvanostatic methods have typically been employed to evaluate the performance of lithium ion batteries.4,5,7−10,53−55 However, in this work, linear scan voltammetry (LSV), a potentiostatic method, was chosen for prelithiation of the carbon electrode. Since the goal of this work was to study the mechanisms involved in SEI layer formation, the use of the LSV technique allows for some separation of the electrochemical processes by controlling the electrode potential. However, both potentiostatic and galvanostatic methods do not alter the electrochemical reactions that occur, and therefore, no discernible difference should be observed in the formation of the SEI layer.

A set of LSV experiments was conducted where the potentiostat was used to decrease the cell potential from its initial OCP value down to a final value of −0.2 V in a controlled manner at a scan rate of 0.8 mV·s−1. The current− potential data from a typical LSV experiment is presented in Figure S13 in the Supporting Information. It is known that PC is susceptible to cointercalation into the interlayers of graphite.10,19,56 The amount intercalation of Li into the graphite paint would be minimal when compared to the amount that inserts into the amorphous carbon. Each experiment was conducted in stages by periodically halting the process at the certain cell potentials so that the analysis could be performed. The hold potentials were 2.6, 1.9, 1.4, 1.2, 1.0, 0.8, 0.6, 0.4, 0.2, 0.0, and −0.2 V, which were chosen to cover the entire range of current changes that is exhibited in a typical LSV current−potential plot. The current flow and cell potential were monitored during all stages of the process. The specific conditions under which each analysis (LC-MS, EIS, DRIFTS, and gas-phase) was conducted were dependent on the particular analysis, and with the exception of the LC-MS analysis, the selected hold potentials were a subset of the larger set. Once the specific hold potential was reached, the cell potential was held for 20 min. During the last minute of the stage, the electrolyte within the cell was well mixed by drawing out and reinjecting a 20-μL sample of electrolyte. This process was performed slowly as to minimize disruption of the cell. After mixing, a 5-μL sample of the electrolyte was taken out and diluted to 100 μL in acetonitrile for analysis by LCMS. Analyses of the unused electrolyte and the electrolyte after being placed into the cell (Li contacted) were also performed. For the electrochemical impedance spectroscopy (EIS) analyses, the cell potential was held until the current flow decreased to a steady-state value, at which time the EIS analysis was performed. For the gas analysis experiments, any generated gaseous byproducts were then drawn into a gas-sampling chamber for analysis and the experiment continued to the next hold potential. The details associated with the gas analysis experiment are provided in an earlier report.5,6,11,36 Lastly, for the electrode analysis experiments, the cell was then taken apart to retrieve the carbon electrode, which was then gently rinsed twice with dry THF and allowed to dry. All DRIFTS samples were diluted with spectrometric grade KBr(Sigma-Aldrich) at 0.5% carbon/KBr ratios. A background electrode sample was also prepared by dipping a fresh electrode into the unused electrolyte for a short time, which was then prepared like the other samples. Each spectrum was standardized by subtracting the spectrum of the background sample in a 3:1 ratio to enhance the changes in the surface functionalities of each IR spectrum.

3. ANALYSIS AND RESULTS 3.1. Supporting Analyses. The processes that result in the development of the SEI layer can be evaluated through the analysis of the carbon surface using DRIFTS and EIS instrumental techniques.4,7−9,12−14,24,40,42,46,57,58 In order to establish a context for the results of the LC-MS analysis with respect to the development of the SEI layer, the results from the DRIFTS analysis, correlated with the previously reported gas analysis and EIS data,10,15−21,36 will first be summarized. Although it is not possible to separately identify all of the components that may compose the SEI layer, much work has been reported in the literature on identifying some of the more 17385



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peaks that correspond with the functional groups associated with compounds noted in Table 1. The intensities of the peaks of interest were determined through a deconvolution of the spectra. The absorptions at 1197 cm−1 (α) and 850 cm−1 (β) are not expected to change significantly over the course of the experiment since they arise from the C−F stretch of PVDF and the P−F stretch of LixPFy, respectively. PVDF is the binder used in the electrode material and so will always be present, and since LiPF6 is a component of the electrolyte, there will always be some residual salt on the carbon surface. In solution, the CO stretch for pure EC is seen at 1796 cm−1 and the overtone of the ring breathing mode at 1771 cm−1. When the molecule becomes adsorbed onto the surface of the carbon, those modes are intensified resulting in an observed blue shift as compared to pure EC (1809 cm−1 (a) and 1789 cm−1 (a1), respectively). Coordination of the molecule on the carbon surface during initial polarization results in a reversal in the intensities of the EC CO band (a) and ring breathing overtone (a1).12,16,20,24,27,35,63 The absorption (shoulder peak) labeled d1 in Figure 1 is an indication of the presence of lithium carbonates. As stated in the Introduction, complete reduction of an EC or PC molecule can follow one of two pathways, referred to as the “2-electron” and “1-electron” pathways. In our previous work, it was predicted that the “2-electron” pathway predominates at the higher cell potentials.15,16,26,27,29−31,35,36 It is through this pathway that two, single electron reductions of an EC molecule can result in the formation of lithium carbonate (see Scheme 1). The presence of lithium carbonates in the SEI layer at cell potentials higher than those associated with the generation of decomposition gases is evidence of the “2-electron” pathway. As the cell potential decreases from 1.2 V to more negative potentials, the absorptions associated with the alkyl carbonates (1680, 1300, 1400, 1090, and 810 cm−1) and ethyl carbonates (1640 and 1330 cm−1) exhibit a developing intensity. This is indicates that the solvent reduction mechanisms are transition-

significant compounds. This has been accomplished through the analysis of pure compounds and through the deposition of those compounds on various substrates. The reduction products from EC have been determined to include Li2CO3,11,13,22−24,26,34,59−61 (CH2OCO2Li)2,12−15,25,26,61 and other more complex alkyl carbonates (ROCO2Li)12,15−21,24,26−34,59−62 with similar products arising from the reduction of PC. In addition, electrolytes that utilize the LiPF6 salt have been shown to produce mixed lithium fluorophosphates (Li x POF y 12,13,16,20,22−24,27,35,60,62 and LixPFy15,16,24−26,34,35,63). Presented in Table 1 is a list of some of the more commonly identified SEI layer components and the particular vibrational modes that give rise to their specific IR bands.12,16,24,26−34,59−63 Table 1. SEI Layer Componentsa component

vibrational mode

approx. position

figure label

EC (coordinated) ROCO2Li

CO stretch CO asym stretch CO sym stretch C−O stretch OCO2− CH2 bend CO asym stretch CO sym stretch CO32− bend C−O stretch P−O stretch P−F stretch C−F stretch

1809/1789 cm−1 1680 cm−1 1300 cm−1 1090 cm−1 810 cm−1 1400 cm−1 1640 cm−1 1330 cm−1 890 cm−1 1490 cm−1 1080 cm−1 850 cm−1 1197 cm−1

a/a1 b b′ b2 b3 b1 c c′ d1 d e β α

(CH2OCO2Li)2 Li2CO3 LixPOFy LixPFy PVDF a

Most common components of the SEI layer and their vibrational modes that produce the specific absorption bands in an IR spectrum.

The DRIFTS spectra taken from a selection of the potential stage analyses are shown in Figure 1. Noted in the figure are the

Figure 1. Selection of DRIFTS spectra at different cell potentials. Labels in figures refer to IR bands that correspond to the vibrational modes from specific functionalities and are listed in Table 1. 17386



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ing to the “1-electron” pathway. In addition, the presence of Licarbonates (d1) becomes increasingly evident in the DRIFTS analysis. Because of the activated carbon’s increasingly catalytic nature, surface oxygen functionalities can be reduced at certain potentials that result in the generation of carbon dioxide, as was detected in the gas analysis experiments at the higher cell potentials.16,36 Carbon dioxide can readily react with lithium ions to form lithium carbonates. These trends also correlate well with the increase in the film resistance determined by the EIS analysis.12,16,20,24,27,35,36 In addition, the increasing intensity of absorption of the IR band associated with LixPOFy (e) is an indication of decomposition of the LiPF6 salt. Salt decomposition products have also been reported to comprise the solid electrolyte interphase layer when the salt used is LiPF6.12,15,26,27,29−31,36−40 3.2. LC-MS Analysis. The baseline electrolyte, after it had been added to the electrochemical cell and exposed to both carbon and lithium electrode, was analyzed. Four large peaks were evident in the chromatogram, which is different from the expected three that would be due to the solvent components. An analysis of the fresh (unexposed) electrolyte produced similar results and is shown in the inset of Figure 2. On the

can be very reactive with the solvent components in the presence of trace amounts of water. A solution of the component solvents was prepared at the same ratios as the baseline electrolyte (3:1:4 EC/PC/DEC), and a portion of this was used to recreate the electrolyte by adding the LiPF6 salt at a concentration of 1.2 M. In addition, solutions of each individual solvent component were prepared with the Li salt, and from these analyses, it was determined that P035 developed from a reaction between the Li salt and propylene carbonate. In addition, the analysis of the in-house prepared electrolyte showed that there was a decrease in the peak areas of all three of the solvent components as compared to the mixed solvent solution. These results were used to estimate the actual concentrations of the components in the baseline electrolyte and the electrolyte prepared from the solvent mixture. Those results are shown in Figure 3, which also shows the percentage decrease in the concentration of each component relative to the in-house prepared solvent (no salt) mixture.

Figure 2. Chromatograms of the electrolyte before (3.0 V) and after the initial lithiation process (−0.2 V). The solvent components were identified and labeled EC, PC, and DEC. The unknown compounds that arise from reaction with the Li electrode or during lithiation of the carbon electrode are labeled P##, where “##” represents the retention time. The inset figure shows the chromatogram of the electrolyte that has not been exposed to lithium metal.

Figure 3. Composition of unused electrolyte. A comparison of the compositions of the Li-contacted and unused electrolyte as compared to a solvent (no salt) mixture at the ratio of 3:1:4 EC/PC/DEC.

basis of the analyses of the individual components, the peaks eluting at 4.8, 5.4, and 10.0 min were identified as ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC), respectively. It is interesting to note that even though the concentration of PC is much lower than that of EC (3:1) it produced a much larger peak compared to that of EC indicating that it is much more ionizable under these analysis conditions. The broad, unidentified peak shows a retention time of approximately 3−3.5 min. The chromatogram for the Li-contacted electrolyte is shown in Figure 2 as a solid, black line. It was determined that the earlier eluting peak was a near coelution of two peaks with retention times at 3.3 and 3.5 min, referred to as P033 and P035, respectively. An LC-MS analysis of a solution of acetonitrile, the diluent for these samples, and the LiPF6 salt showed the development of the P033 peak, but not the P035 peak. It is well known that the PF6 anion is not stable and will dissociate into the PF5 and the fluoride anion, which

The presence of several additional peaks in the chromatogram shown in Figure 2 is evidence that the electrolyte components immediately react with the active Li surface to form a passivation layer on the surface of the Li metal electrode, as described in the literature.15,24,25,36,41,42,58,64 In addition, it has been well reported that, in general, it is the EC and PC molecules that more readily undergo reductive decomposition in the formation of the SEI layer during the lithiation process.10,12,15,16,20,27,29,35,36,47 By examining the peak areas of those two components over the course of the experiment, it was possible to determine their degree of consumption due to the SEI layer formation reactions. Shown in Figure 4 are the percent compositions of EC, PC, and DEC in the electrolyte as the cell potential decreases from a series of experiments. It is evident in Figure 4 that the EC and PC components show a distinct decrease in their concentration over the course of the initial lithiation process. 17387



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to be more easily identified, along with their associated retention times, and an estimation of their peak widths. The unknowns and their retention times, base peaks, and mass peaks identified from the chromatographic data are listed in Table 2. Table 2. Liquid Phase Unknown Compounds

Figure 4. Comparison of electrolyte composition. Change in the % composition of the EC, PC, and DEC components during the initial lithiation process. Dashed lines are just provided to more clearly indicate trends in the data.

peak ID

RT (min)

base peak m/z

mass peak m/z

P035 P036 P040 P044 P065 P076 P113 P127 P145 P158 P162 P176

3.5 3.6 4.0 4.4 6.5 7.6 11.3 12.7 14.5 15.8 16.2 17.6

111.3 89.3 73.5 59.3 73.2 117.4 207.3 295.5 221.3 221.3 309.5 309.6

111.3 89.3 147.4 147.4 231.3 271.4 207.3 295.5 221.3 221.3 309.5 309.6

To understand how these compounds arise during the initial lithiation process, a LSV experiment was conducted where the cell potential was held at different potentials along the LSV curve. The results from this experiment differed slightly from the earlier experiments in that the concentrations of three of the previously identified compounds, labeled P040, P158, and P176 in Figures 2 and 5, and Table 2, were too low to be quantified. The total ion current (TIC) chromatographic data from each analysis was imported into the Fityk application for peak deconvolution. The information obtained from the preliminary analysis using the 2D representation in the application MZmine was used as a guide for the fitting process. Since more concentrated solutions tend to exhibit tailing in liquid chromatographic analyses, the solvent composition peaks and

An examination of the electrolyte chromatogram produced from the analysis of the −0.2 V potential stage (Figure 2) shows that the additional peaks that were just visible in the chromatogram of the Li-contacted baseline electrolyte have significantly increased their concentrations. To obtain a more informative evaluation of the results, the raw LCMS data was imported into the application MZmine for analysis using the “2D Visualizer” representation, which is displayed in Figure 5. Representing the data this way allowed for the individual peaks

Figure 5. 2D visualization of the chromatographic data. A 2D representation of the chromatographic data as m/z of the detected peaks versus the retention. The identified base peaks of the unknowns are circled and labeled with the IDs from Figure 2 and discussed in the text. 17388



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some of the more concentrated unknowns were fitted using a “SplitGaussian” function, while the less concentrated unknowns were fitted using a straight “Gaussian” function. It was assumed that the concentration of that analysis peak (P033) would not significantly change so that it could be used as a pseudointernal standard. Therefore, to account for the variability in the sample concentrations, injection sizes, etc., the integrated peak areas were normalized to the peak area of P033. As stated before, some of the unknowns were detected in the analysis of the electrolyte that was sampled after being added to the cell. To keep the focus of the analysis on the changes in the concentrations of the unknowns during the initial lithiation process, the changes in peak areas relative to the initial starting values were plotted as a function of the cell potential, and these results are presented in Figure 6a, b, and c. As shown in Figure 6, the concentrations of unknowns P035 and P036 show significant decreases as the cell potential drops below 1.4 V, indicating that these compounds are consumed at a rate that is greater than their formation rates. This implies that, although these compounds arise from reactions of the electrolyte with the Li metal, they in turn undergo additional reactions during the electrochemical processes that form the SEI layer and the other unknowns. The concentrations of unknowns P076 and P145 start to show an increase at approximately 1.4 V, higher than what the gas analysis showed for the generation of the decomposition gases (see Figure S.2, Supporting Information) indicating that the energy barriers for these electrochemical processes must be lower than the energy barrier associated with the second 1electron reduction of the radical anion. These results strongly suggest that these compounds must arise from reactions between the radical anions and either of the electrolyte components, P035 or P036. The increase in the concentrations of unknowns P113 and P127 roughly aligns with the increase in the generation of the decomposition gases at approximately 1.2 V, which suggests that the charge-transfer energy barriers associated with these reactions are comparable to the energy barrier associated with the second 1-electron reduction of the radical anion. Finally, the concentrations of unknowns P044 and P162 show an increase at electrode potentials that are more negative than the one associated with the second 1-electron reduction of the radical anion, which indicates that the energy barriers for those reactions must be higher than the energy barrier associated with that second 1-electron reduction. The unknown P044 also exhibits a behavior that is clearly different from the other unknowns. There is an initial increase in its concentration, but at a cell potential of approximately 0.2 V, there is a noticeable decrease, and thereafter, the concentration does not change significantly, which suggests that this unknown is also consumed during SEI layer formation. A less pronounced, but similar, response is seen with P076, which shows a leveling off in its concentration as the cell potential decreases below 0.4 V. This behavior can be explained by conceding that unknown P044 and perhaps P076 may be reactants for one or more of the chemical reactions that result in the formation of the other unknowns or that form the SEI layer. Their formation reactions are electrochemically dependent and have fairly high kinetics, whereas their consumption reactions just appear to be chemically dependent.

Figure 6. Relative change in integrated peak area of unknowns. The change in the integrated peak area of unknowns P035 (a), P036 (a), P044 (b), P076 (b), P145 (b), P162 (b), P113 (c), and P127 (c) as the cell potential changes relative to the initial calculated peak areas. The lines in the figures are only provided to better represent the overall trends in the data points.

undergoes a chemical reaction in the presence of the Li salt. It is also reasonable to expect that the EC molecule may undergo a similar process but that its reaction product may not be detectable under these analysis conditions. When the electrolyte mixture contacts Li metal, the solvent components and the generated unknown(s) undergo electrochemical and chemical reactions that result in the formation of unknowns P036, P044,

4. DISCUSSION As was stated earlier, the unknown P035 was detected in the PC/Li salt mixture, which indicated that the PC molecule 17389



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Table 3. Liquid Phase Unknown Compounds: Empirical Formula, Possible Structures, and Calculated log P Values Using the XLogP,43,65 NC+NHET,44,45,66 ALog98,15−17,20,46,47,67 and KLOP48,49,68 Methods

∼117, m/z values for EC, PC, and DEC. This suggests that these unknown are likely “assembled” from the same or similar components that arise from the decomposition of the solvent components. As stated in the Introduction, Gachot et al.15,24,36,41−45 reported on the identification of electrolyte-soluble oligomers that form from the decomposition of electrolyte components during repeated cycling of a lithium ion battery. In addition, possible mechanisms that could produce these oligomers were proposed. On the basis of that informative work and some additional analysis, it was possible to determine plausible structures for the unknowns reported in this work. The specifics of this process are detailed in section S.1.2 of the Supporting Information, but an overview of the general steps is provided below. On the basis of the mechanisms proposed in those reports,10,36,43,45 the list of possible empirical formulas was compared to the possible oligomer combinations that could result from the decomposition of EC and/or PC in a DECbased LiPF6 electrolyte. For the lower MW unknowns (P035, P036, P040, and P065), structures that could result from combinations of decomposition fragments of the solvent components were created and compared to the identified mass fragment peaks for those unknowns. In the final elimination step, partition coefficient (log P) values for the proposed structures were calculated based on four different methods and compared to establish a possible elution order appropriate in reversed-phase chromatography. The results of this analysis are presented in Table 3. The associated spectrum for each unknown and its identified fragments are presented in section S.2 in the Supporting Information (Figures S1−S12). It was notable that Li was detected in only one of the unknowns. This may seem conspicuous at first, considering that Gachot et al. indicated the presence of lithium in their proposed formula.43,45 However, this lack could be explained by

P076, P113, and P127. Since the interfacial potential across the Li metal and electrolyte interface is negative enough, any energy barriers associated with the charge-transfer process necessary for the formation of these unknowns was easily surmountable. It is also possible that the unknowns P145 and P162 are also formed but that their concentrations are too low to be detectable. In our previous report, the generation rate of the decomposition gases peaked at around 0.6 V and then decreased at lower cell potentials, a phenomenon that would not be expected since that process is electrochemically driven.36−40,43 However, what may be occurring is that the reactions that generate these gases are being dominated by other chemical or electrochemical reactions. The cell potential range at which the initial formation of these unknowns takes place is, in general, independent of the second 1-electron reduction of the radical anion. However, because the cell potential stage at which the initial formation of these unknowns takes place correlates well with the increase in the passed charge, it can be at least concluded that the formation of the unknown compounds is indirectly dependent upon electrochemical processes. An analysis of the spectral data associated with each unknown was performed using the open source applications mMass and MZmine to identify the base peak and the correlating fragment and/or adduct peaks. Additional analysis of the spectral information was used to identify the appropriate m/z that corresponds with the mass of each unknown. It was then possible to determine a list of empirical formulas for each unknown that was considered to be likely candidates. The details of this procedure are provided in section S.1.1 of the Supporting Information. By taking a broader view, some interesting observations can be made here. There are two m/z values that frequently appear associated with significant fragments of several of the compounds: ∼89, ∼103, and 17390



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likely arises from decomposition reactions of EC and PC, providing additional support for that initial reduction step at the higher cell potentials. Lastly, the formation of the P076 compound can be explained through the mechanism that accounts for the decomposition of the LiPF6 salt, as postulated by the Laurelle group.45 This correlates well with the changes in the intensity of the IR band associated with LixPOFy (labeled “e” in Figure 1) and the generation of EC and PC reaction fragments. As the cell potential transitions to the 0.8 V stage, the consumption rates of EC, as well as the compounds P035 and P036, begin to increase. In addition, the concentration of PC in the electrolyte also starts to exhibit a decrease. The DRIFTS analysis indicated a decrease in the coordinated EC concentration, while there was an increase in the lithium carbonate concentrations and in the intensity of several of the bands associated with the alkyl carbonates. These phenomena correlate well with the large increase in the decomposition gases.36 This is further indication that the radical anion has now begun to undergo the second 1-electron reduction, which results in the release of the decomposition gases. The insoluble reaction products, such as the alkyl and lithium carbonates, are building up on the surface of the electrode and the electrolytesoluble compounds, P113, P127, P145, and P162, now also show a noticeable increase in their concentrations. These oligomeric compounds also arise from reactions between the fragments that result from the reduction of the solvent components.45 These are the results that are expected from the “2-electron” reduction pathway, which preferentially results in the formation of lithium carbonate and ethene or propene and is further supported by the impedance analysis, which shows the development of two phases in the SEI layer.36 Concurrently, the insoluble lithium salt decomposition products show a decrease in concentration, mimicking the overall concentration of EC found on the substrate. In contrast, the concentration of the compound P076 exhibits a marked increase in its generation rate. This is an indication that lithium salt decomposition is still occurring, but it is only that it is the formation and/or deposition of the insoluble products that has slowed considerably. As the cell potential decreases to 0.4 V, the DRIFTS analysis shows what seems to be a leveling off in the intensities of most bands, except those of the more easily decomposed LixPFy products. The EIS analysis indicates that the inner phase of the SEI layer is becoming more compact, due to the transition to a more ionic or inorganic character;15,24,36,41,42 therefore, while the resistance attributed to the outer phase stays fairly constant, the resistance of the inner phase has increased, which results in an overall increase in the total resistance of the SEI layer.36 The changes occurring at the electrode surface are accompanied by a substantial increase in the generation of decomposition gases and in the concentrations of the electrolyte-soluble oligomers (P113, P127, P145, and P162), while the EC, PC, and their respective linear isomers P036 and P035 show the most significant decrease in their concentrations. As the electrode potential becomes more negative, more of the solvent molecules undergo the initial 1-electron reduction, and it is this increase in the concentration of the radical anions that initiates the transition from the “1-electron” pathway to the “2electron” reduction pathway, which results in the increased production of alkyl carbonates (ROCO2Li) at the surface of the developing SEI layer and oligomeric compounds in the electrolyte. This is increasingly evident in the results from the

considering that some of the compounds probably exist in two forms: a Li salt form and an acid/alcohol form. The mobile phase used in this analysis was acidified with TFA, and so the likelihood of these unknowns being detected in their protonated form would be substantially increased. It could also easily be assumed that these compounds are just redissolved components of the SEI layer, implying that they are only slightly soluble in the electrolyte. However, the small amount that is soluble would be independent of the amount that had been precipitated onto the electrode surface. As the SEI layer builds up, the concentrations of the soluble decomposition products would remain relatively unchanged since they would be limited by the slight solubility. In contrast, the compounds detected in the LCMS analysis of the electrolyte show an increase in their concentrations that is correlated to increasing polarization of the carbon electrode. Therefore, if these compounds presented in this work are components of the SEI layer, it may be that it is the Li salt form that precipitates out and the protonated form that remains dissolved in the electrolyte and is therefore detectable in the LCMS analysis. Over the potential range from the OCV to 1.4 V, the composition of the electrolyte did not show any substantial changes with the exception EC, which exhibited a steady decrease in its concentration. As described before, the unknowns P035 and P036 showed a very slight decrease in their concentrations. Upon the basis of their structures, it can be seen that P035 and P036 arise from the breakage of the ring in the cyclic carbonates, and so it can be expected that they would exhibit behaviors similar to those of their cyclic isomers under these conditions. Upon contact with the active carbon electrode, solvent molecules can become adsorbed onto its surface. After discharging to 2.6 V, the polarization of the electrode results in the coordination of EC molecules onto the surface, as indicated in the DRIFTS analysis of the electrode from that stage (see Figure 1). At cell potentials above 1.4 V, the double layer has been established (see Figure S13, Supporting Information), and lithium ions, coordinated to the EC and PC molecules, are stabilized at the outer Helmholtz plane. As the cell potential decreases further, the coordinated ions break from the double layer and coordinate with the solvent molecules adsorbed on the carbon surface, where the active carbon substrate adds strains to the C−O bonding of the Li-coordinated, cyclic carbonates. This is marked by the intensification of the absorptions in the region labeled á in Figure 1. When these compounds are strongly adsorbed to the catalytic surface of the carbon’s active edge and surface sites, it facilitates the first 1-electron reduction, which is evident by the large increase in the passed charge not accompanied by the generation of decomposition gases.36 This change is evident in the DRIFTS analysis (Figure 1) by the broadening of the corresponding band’s width as the interface layer creates more bonding modes for the unfolding EC molecules. Below a cell potential of 1.4 V, continued polarization of the carbon electrode ultimately results in breakage of the ring and the formation of the radical anion, which is then stabilized by the coordinated lithium ion and the active carbon surface.15−21 In contrast to P035 and P036, the compounds P044 and P076 start to show a slight increase in their concentration as the cell potential drops below 1.9 V. Although a definitive mechanism that explains the formation of the P044 compound has not yet been finalized, an inspection of the fragmentation pattern (see Figure S6, Supporting Information) indicates that the compound most 17391



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LC-MS and DRIFTS analyses as the cell potential transitions down to −0.2 V. The results from the DRIFTS analysis of the −0.2 V potential stage exhibit the highest, most resolved peaks for all the functionalities of the detected SEI layer components. This potential stage is below the lithium insertion potential, so it would be expected that there would be significant structural changes to the carbon substrate due to the insertion process. The subsequent physical changes results in an electrochemically induced disorder, which thereby increases the number of active surface sites on the carbon substrate. Since the electrochemical system has already transitioned to the “1-electron” pathway, these two factors allow for a substantial increase in the decomposition products and an increased concentration of all detected SEI components. This is, again, supported by the EIS analysis, which showed a substantial increase in the film resistance36 and the LC-MS analysis of the electrolyte. As the cell potential transitioned to the −0.2 V potential stage, EC and, to a certain extent, PC still showed significant decreases in their concentrations and continued increase in the concentrations of the soluble oligomers, an indication of those reductive processes that result in the formation the soluble and insoluble products. However, the compounds P035 and P036 showed a leveling off in their concentrations, as did the concentrations of the compounds P044 and P076. This is an indication that the rates of the reactions that consume P035 and P036 or form P044 and P076 have decreased. This result would be expected if, as was indicated in the Analysis and Results section, that these four compounds are involved in the reactions that form the SEI layer and the other electrolyte-soluble compounds.

Article

ASSOCIATED CONTENT

S Supporting Information *

Process used for first determining the empirical formula for each electrolyte-soluble unknown and then the identification of a reasonable structure using the pertinent references and a comparison of predicted partition coefficients; the mass spectrum and its identified significant fragments for each electrolyte unknown; data from a typical LSV experiment to provide a point of reference; and some of the essential results from our previous report on the analysis of the gaseous decomposition products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 617-287-6036. Fax: 617-287-6185. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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5. CONCLUSIONS The work presented here is a natural extension of the work presented in our previous report, which examined the decomposition gases that are generated during the lithiation of an amorphous carbon electrode.36 This work focuses on determining how the composition of a baseline electrolyte changed during SEI layer formation and to correlate those results to the previously reported results and a DRIFTS analysis of the carbon electrode surface. The results obtained from the DRIFTS analysis showed that the changes in the chemical composition of the SEI layer reflected the changes in its electrical and physical properties. In the LCMS analysis of the composition of the electrolyte, it was determined that there are additional electrochemical and chemical processes that result in the formation of a variety of electrolyte-soluble compounds and to correlate changes in their concentrations with the changes in the composition of the SEI layer and the reactions that produce the decomposition gases. Through a detailed analysis of the mass spectral data obtained for each compound, probable structures for these compounds were developed. On the basis of the electrochemical and chemical reactions occurring during the initial lithiation process, it can be concluded that the higher molecular weight compounds (P113, P127, P145, and P162) are functionally similar to components of the SEI layer and that the lower molecular weight compounds (P035, P036, P044, and P076) likely arise from precursors or intermediates in the reactions that form the SEI layer or may be some of those agents themselves. These results help to lessen some of the complexity associated with the electrochemical processes that occur during the initial development of an SEI layer on a carbon surface. 17392



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Spectroscopic Compositional Analysis of Electrolyte during Initial SEI

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