Article pubs.acs.org/JPCC
Reduction of Carbonate Electrolytes and the Formation of SolidElectrolyte Interface (SEI) in Lithium-Ion Batteries. 2. Radiolytically Induced Polymerization of Ethylene Carbonate Ilya A. Shkrob,*,† Ye Zhu,† Timothy W. Marin,†,‡ and Daniel Abraham† †
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ Chemistry Department, Benedictine University, 5700 College Road, Lisle, Illinois 60532, United States S Supporting Information *
ABSTRACT: Extensive polymerization of ethylene carbonate (EC) leading to the formation of oligomers with masses up to 1 to 2 kDa in electron beam radiolysis is demonstrated using electrospray ionization mass spectrometry and nuclear magnetic resonance. This polymer has a different structure and morphology than the linear chain copolymer of ethylene oxide and EC that is generated in anionic polymerization of intact EC molecules. This radiolytically generated polymer exhibits chain branching and pendant carbonate groups, and it can form a 3D organic network that is additionally cross-linked through lithium ions. Such a morphology is consistent with the occurrence of anionic and radical polymerization that involve the products of recombination and disproportionation of secondary radicals generated in one-electron reduction of EC. Our examination of this chemistry suggests that the same polymer is likely to occur in electrochemical reduction of EC. The formation of this polymeric network qualitatively accounts for some of unexplained properties of the solid-electrolyte interface (SEI) occurring in electrochemical cells with EC-based electrolyte, including common lithium batteries.
1. INTRODUCTION In lithium batteries, solid-electrolyte interface (SEI) on the graphite anode serves an important role, allowing Li+ ion transport across the solid matrix but blocking the electron flow across this matrix (that causes further breakdown of the electrolyte on the outer surface) and the diffusion of solvent molecules to the anode, where they become decomposed or intercalated between the graphene layers.1−11 Through this selective permeability, SEI prevents runaway electrolyte breakdown and anode degradation. The formation of SEI is a multistep process that is driven by electrochemical reduction of the electrolyte2,12−18 that typically consists of a mixture of ethylene carbonate (EC) and dimethylcarbonate (DMC), containing LiPF6 and additives (like vinylene carbonate, VC) that facilitate the formation of robust SEI (Scheme 1). This interface is a composite containing inorganic salts (such as lithium fluoride and carbonate) and several organic components.2,19 The morphology and chemical composition of the SEI is a subject of ongoing debate. In Part 1 of this series,11 electron paramagnetic resonance (EPR) spectroscopy was used to identify radicals generated in one-electron reduction of organic carbonates and to establish the first chemical steps that lead to the formation of the SEI. Depending on energetics, the electron attachment can be either dissociative or associative, and the former reaction can © XXXX American Chemical Society
Scheme 1. Sketch of Radical Chemistry for EC (to the right) and the Products of Radical Recombination and Disproportionation Occurring Through Secondary Radical Reactions (to the left)
result in either single or double C−O bond fission (Scheme 1). For EC, the resulting QCH2•CH2 radical (where Q is a lithiated carbonate group) either abstracts H from the parent molecule, yielding the EC(−H)• (H loss) radical, or undergoes 1,2migration of hydrogen, yielding a more stable Q•CHMe Received: June 25, 2013 Revised: August 13, 2013
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radical.11 Unlike primary (terminal) radicals, these secondary radicals disproportionate as readily as they recombine. In many of the suggested reaction schemes for SEI formation,2,12,15,17 the QCH2•CH2 radical (generated in a single C−O bond scission) and the carbonate (CO3−•) radicals (generated in double C−O bond scission) were postulated to be the main products of electrolytic reduction. Such primary radicals would overwhelmingly yield recombination products, such as (CH2Q)219,20and (CH2CH2Q)2 dicarbonates2(Scheme 1). Experimentally, only short-chain dicarbonates are observed, which is surprising given that longer-chain dicarbonates should be a side product of the recombination. In Part 1,11we argued that the reaction path to (CH2Q)2, may not directly involve the short-lived QCH2•CH2 radicals but rather might occur through reactions of more stable secondary radicals and related products (Scheme 1). Another realization suggested by our observations was that the involvement of secondary as opposed to primary radicals changes the outcomes of anionic21−23(Scheme 2) and radical
(Scheme 3) polymerizations that are hypothesized to occur during the formation of the SEI. The growing polymer chains can include side carbonate groups (section 4 in Part 1)11 that serve either as sites for subsequent chain branching or as pendant groups “zipped” through shared lithium ions. Because of the occurrence of this Q-branching, as we called it, the resulting polymer matrix becomes a 3D network in which the dendritic polymer is additionally cross-linked through carbonate bridges. This picture is in contrast with the current models visualizing the organic component of the SEI as aggregates of mono- and dicarbonates (or products of CO2 loss from such compounds) or linear oligomers whose growth (by anionic polymerization, Scheme 2) is initiated by these oxide- and Qterminated compounds. According to our results,11 oxidation and reduction of EC yield the same secondary radicals after H abstraction by the primary radicals, and our prediction is that electrochemical reduction yields the same radicals as radiolysis, albeit with different radical yields. This implies that radiolysis yields polymers of the same type as those occurring in SEI (where only reduction takes place). Conversely, the formation of such polymers in radiolysis of EC indicates that the same processes are likely taking place in electrochemical reduction of this electrolyte. Qualitative differences between the radiolysis and electrochemical reduction can still emerge through specific interactions of reaction products with the anode surface and inorganic components. However, as shown below, such interactions may not be required for extensive cross-linking and branching of the polymer that occur even in the solvent bulk. In this study (referred to in the following as Part 2), we test this scenario using electrospray ionization tandem mass spectrometry (ESI MS/MS) and 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. We show that branched polymers with masses up to 1 to 2 kDa are generated in radiolysis of EC solutions. We recall that EC is an inherently unstable molecule that undergoes anionic polymerization (Scheme 2) in which oxideor Q-terminated (macro)molecules attack a solvent molecule and incorporate it as a terminal −CH2CH2Q unit. The latter can lose carbon dioxide,22 and the resulting oxide- or Qterminated species can react again. (The same reaction can be visualized as attack of the base on the carbonyl or alkylene carbons.)21 The growing (C2H4X)n chain (where X is either oxygen or carbonate) consists of ethylene oxide (EO, −C2H4O−) or EX (−C2H4CO3−) units. According to Lee and Litt21 there is general tendency in this polymer toward EC−EO and EC−EO−EO repeat pattern due to the formation of stabilizing macrocyclic intermediates. (See Scheme 2 therein.) The terminal EO or EC group can dehydrate, forming vinyloxide termini that are solvolytically fused to other chains by releasing acetaldehyde.21 There is also intra- and interchain decarboxylation that over time increases the relative fraction of the EO units over the EC units as the polymer ages. Nevertheless, through all of these many transformations the polymer maintains its (EO)x(EC)y composition and the (C2H4X)n chains remain linear. Gireaud et al.24,25 and Gachot et al.26,27used high-resolution ESI MS and gas chromatography−MS to demonstrate the formation of methyl terminated (C2H4X)n oligomers in electrochemical reduction of EC in DMC. It was suggested that reduction of DMC yields MeX anions (where X is O− or Q−) that initiate anionic polymerization. The formation of such
Scheme 2. Anionic Polymerization Occurring in Unirradiated EC (In The Box) and Irradiatiated EC, where it Can Involve Q-Terminated EC(−H)Y Products Shown in Scheme 143
Scheme 3. Radical Polymerization Involving Olefins Generated via Disproportionation of Secondary Radicals Shown in Scheme 1a
a Colored circles indicate different types of carbons that are indicated in Figure 4.
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proton shifts in the same species (Figure 3S in the Supporting Information), so we relied primarily on reference systems (Figure 4S in the Supporting Information) for 1H resonance attributions.
MeX anions (in reduction of DMC) is supported by our EPR results.11 To save space, many of the supporting schemes, tables, figures and the lists of abbreviations have been placed in the Supporting Information. When referenced in the text, these materials have the designator “S”, as in Figure 1S.
3. RESULTS 3.1. Mass Spectrometry Studies. 3.1.1. Before Radiolysis. Because the EO and CO2 have the same nominal mass, the (EO)x(EC)y polymer (Scheme 2) should exhibit mass-44 progressions, with different series of mass peaks corresponding to different terminal groups. (We recall that anionic polymerization can be initiated by more than one impurity or decomposition product.) Such progressions are indeed observed in commercially supplied EC (Figure 5S in the Supporting Information). After 1 week of storage in methanol, well-resolved mass peaks of the oligomer are observed with mass peaks extending to 600 a.m.u. (Figure 6S in the Supporting Information). In the MS2 spectra of these cations (Figure 7S in the Supporting Information), fragments with masses of 44 (EO or CO2), 72 (C2H4CO2), 88 ((EO)2), 132 ((EO)2 and/or (EO)(EC)), and 176 ((EO)3) are observed, suggesting the occurrence of anionic polymerization (Scheme 2). Figure 6S in the Supporting Information demonstrates the decomposition of the MS1 spectrum into several mass-44 series (labeled A−D) with the onsets at 106, 111, 129, and 142 a.m.u.; these four series can also be distinguished through their MS2 spectra that are somewhat different between the series (Figure 7S in the Supporting Information). In particular, D-series cations exhibit additional mass-14 (CH2) and mass-18 (H2O) loss peaks, suggesting methyloxy termination. The same mass-44 progressions were observed in aqueous solutions of synthetic polyethylene glycols (not shown). We also examined MS2 spectra for protonated tetra- and pentaethylene glycol (HO(EO)4−5H, Figure 8S in the Supporting Information) and its methylated derivative, MeO(EO)4Me (Figure 9S in the Supporting Information). The latter cation indicated mass losses of 32 (methanol), 44 (EO), 46 (MeOMe or EtOH), 76 (Me(EO)H), 88 ((EO)2), 90 (MeO(EO)Me), 120 (MeO(EO)2H), and 134 (MeO(EO)2Me). For the glycols, mass losses of 18 (water), 44 (EO), 62 (ethylene glycol), 88 ((EO)2), 106 (HO(EO)3H), and 150 (HO(EO)3H) were observed. We conclude that the EO units are eliminated either as aldehydes or glycols (that for MeO(EO)4Me are methylated) through C−O bond dissociation occurring at different sites in the parent cation. The MS2 spectra shown in Figure 7S in the Supporting Information can be rationalized in the same fashion. Because of the occurrence of this slow anionic polymerization, the samples were analyzed immediately following radiolysis. As the Q-terminated species undergo rapid solvolysis (see ref 20), the presence of such groups can only be established through observation of the corresponding hydroxyl groups, as shown in Scheme 1S in the Supporting Information. 3.1.2. Radiolysis. Figure 1 compares the mass spectra obtained for neat EC before and after 12.1 MGy radiolysis. The MS1 spectrum exhibits a dense comb of mass peaks between 200 and 600 a.m.u. that reveal a mass-14 (CH2) progression and have an average m/z of 430 ± 90 a.m.u. (which would correspond to 4−6 EC units). Figure 2 shows MS2 mass loss spectra for cations with mass peaks shown in Figure 1. It is seen that starting from m/z +350 these MS2 spectra are virtually identical. These MS2 spectra did not change significantly when the collision energy was varied
2. EXPERIMENTAL AND COMPUTATIONAL METHODS Unless otherwise stated, the reagents were obtained from Aldrich and used as supplied without further purification. Perdeutero-substituted ethylene-d4 carbonate (1,3-dioxalan-2on, EC-d4) was obtained from C/D/N isotopes (Pointe-Claire, Quebec, Canada). Battery-grade LiPF 6 and ethylene-h 4 carbonate (Scheme 1) were obtained from Novolyte Technologies (Cleveland, OH). VC was obtained from TCIAmerica and distilled in vacuum. Because solvolysis of Qterminated products was unavoidable during chemical analysis, no effort was exerted in drying the organic solvents used for electrospraying the irradiated material; in fact, in many of experiments, aqueous or mixed solutions have been used. A 5 mm outer diameter borosilicate NMR tube equipped with a low pressure valve was filled with liquid EC or 1 M LiPF6 solution at 30 °C in a glovebox that was purged with dry nitrogen. The valve was closed, and 2.5 cm of the liquid column was irradiated using 2.5 MeV electrons from Argonne’s Van de Graaff accelerator, while the sample tube was immersed in flowing water to maintain the constant temperature at 20 °C during radiolysis. The typical dose rate was 68 kGy/s (1 Gy = 1 J/kg), and the irradiation time varied between 10 and 75 min. Tandem electrospray ionization mass spectra (ESI MSn) were obtained using a Thermo Scientific LCQ Fleet ion trap mass spectrometer operating in positive mode. MS1 corresponds to the first quadrupole and MS2 corresponds to collision-induced dissociation (of mass-selected ions) modes of operation. Liquid samples were injected directly into the ion trap in dilute methanol, acetonitrile, or aqueous (H2O and D2O) solutions. The latter contained dimethylsulfoxide (DMSO) or acetonitrile for better solubility. In the following, MS2 spectra are plotted versus the mass deficit of the fragment ion, so the positions of mass peaks correspond to neutral fragments eliminated in collisional activation of the parent cation. The mass selectivity of this collisional activation was 1 to 2 a.m.u. NMR spectra were obtained in DMSO-d6 using an Avance DMX 500-MHz spectrometer (Bruker); the chemical shifts for 1 H and 13C nuclei are given versus tetramethylsilane (TMS), CFCl3 (for 19F nuclei) and H3PO3 (for 31P nuclei). Twodimensional 1H−1H COSY (Correlation Spectroscopy) and 13 C NMR spectra were acquired over 50 h. Carbon-13 chemical shifts (compiled in Figure 1S in the Supporting Information) were calculated using the NMR option of Gaussian 03,28 using the B3LYP density functional29,30 and 6-31+G(d,p) basis set. Our calculations yielded estimates for these chemical shifts that were close to those for reference systems (where available). Carbon-13 shifts for Qterminated species proved to be very close to the corresponding hydroxyl terminated analogs (Figure 2S in the Supporting Information), which permitted us to take advantage of the existing NMR databases, such as SDBS database from Japan’s National Institute of Advanced Industrial Science and Technology (AIST). In contrast, these density functional theory (DFT) methods generally gave poor estimates for C
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MS2 mass-loss spectra were identical for all mass peaks starting from a mass ∼300 a.m.u. Furthermore, under the same treatment protocol, the MS2 spectra of the irradiated material containing LiPF6 were the same as the ones containing no salt. It appears that the presence of LiPF6 facilitates polymerization but has little effect on the resulting polymer structure. The same is suggested by our NMR studies presented in Section 3.2. When water is added to irradiated EC solution, a precipitate is formed that can be separated by centrifugation. This precipitate was redissolved in a minimal amount of DMSO and then precipitated once again by the addition of water. This procedure was repeated several times to remove all watersoluble components from this residue, and the resulting solid was lyophilized. Figure 15S in the Supporting Information demonstrates the MS1 spectrum of an acetonitrile solution of this residue. The mean mass increases to 830 ± 370 a.m.u. (which is equivalent to 5−14 EC units), suggesting that the residue is composed of macromolecules that can no longer dissolve in pure water. The corresponding MS2 spectra shown in Figure 16S in the Supporting Information are identical to those observed for water-soluble polymers of lower mass (Figure 2). This suggests that starting from ∼300 a.m.u., all radiolytically generated oligomers, regardless of their mass, solubility, and the presence of lithium salt, have the same structural motif. It is also seen, from comparisons to Figures 7S−9S in the Supporting Information, that this motif is different from the one observed for (C2H4X)n polymer that is generated through anionic polymerization of EC by solvolysis. Further inference is possible through the use of EC-d4, as in this compound the isobaric resonance between the EO unit and CO2 is removed. We have repeated the above experiments using EC-d4 (in DMSO/water solutions). Once again, there was no mass variation for MS2 spectra (Figures 17S and 18S in the Supporting Information), but many of mass peaks shifted by either 1 or 2 a.m.u. as light water replaced heavy water (Figure 3), suggesting the occurrence of H/D exchange in the polymer chain (Scheme 1S in the Supporting Information). In Figure 3, we compare MS2 mass loss spectra obtained for EC-h4/H2O and EC-d4/D2O. This corresponded to a situation when the parent cations were either perprotiated or perdeuterated, respectively. All of fragment masses in the ECd4/D2O system can be expressed as sums of the “fundamental” fragments with masses of 20 (water), 32 (formaldehyde and ethylene), 36 (methanol), 44 (CO2), 48 (acetaldehyde (= “EO”)), 52 (propane, ethanol), and 60 (glyoxal); see Table 1S in the Supporting Information. The same “fundamental” masses (after isotope correction) account for the MS spectra in EC-h4/ H2O. In fact, nearly all of the higher mass peaks can be produced by mass-44 (Figure 19S in the Supporting Information) or mass-44/48 (Figure 20S in the Supporting Information) interval shifts from this subset of the low-mass fragments. Because the fragment peaks in Figures 2 and 3 can result from the elimination of more than one molecular fragment, there is ambiguity in the interpretation of these MS2 spectra. To assist interpretation (Figure 21S in the Supporting Information), a genetic algorithm31 program was written that generated isobaric CxDyOz structures corresponding to a given mass peak for EC-d4/D2O. Only structures that can be “decomposed” into the “fundamental” masses (Table 1S in the Supporting Information) were considered. (An example is given in Figure 21S in the Supporting Information.) Initially, all
Figure 1. ESI MS1 spectrum of EC before (ii) and (i) after 2.5 MeV electron beam irradiation to a total dose of 12.1 MGy. The spectrum was obtained by electrospraying the samples in methanol.
Figure 2. ESI MS2 (fragment loss) spectra of irradiated EC (12.1 MGy) electrosprayed in DMSO−water. The m/z (a.m.u.) for the collisionally activated parent cations are indicated in the plot. The upper trace indicates some of the recognizable species that are also observed for solvolytically generated (C2H4X)n polymers.
(Figure 10S in the Supporting Information) or when the mass range of collisional excitation varied from 2.0 to 0.5 a.m.u. (Figure 11S in the Supporting Information) nor did this fragmentation pattern change when the analysis was carried out in different organic solvents (DMSO/water, acetonitrile, and methanol). However, replacement of light water by D2O in the aqueous DMSO solution during electrospraying produced a different MS2 spectrum, suggesting extensive H/D exchange in the oligomers (Figures 12S and 13S in the Supporting Information). When EC containing 1 M LiPF6 was irradiated, even to a lower dose (0.85 MGy), the polymerization proceeded further (Figure 14S in the Supporting Information), producing mass spectra extending over to 1000−2000 a.m.u. Once again, the D
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were irradiated and dissolved in the same batch of DMSO-d6, to exclude the presence of unknown impurities. In 19F and 31P NMR spectra of irradiated LiPF6 solution, two products of anion hydrolysis were observed: PF2O2− and FPO32− (Table 3S in the Supporting Information). No other fluorinated products were found. 13 C NMR spectra of irradiated EC indicated the formation of multiple products (Figure 4 and Figures 23S to 24S in the
Figure 3. Comparisons for ESI MS2 spectra obtained for irradiated EC-h4 and EC-d4 (3.5 MGy) electrosprayed in acetonitrile containing either light or heavy water (to facilitate H/D exchange). In the EC-d4/ D2O trace, mass peaks showing mass-2 satellites upon H2O/D2O exchange are indicated in the plot. See Table 1S in the Supporting Information for a complete list.
of these isobars were given equal weight. These mass peaks were mapped on the EC-h4/H2O spectrum, by giving the isobars a shift of −y mass units. (The isotope effects in fragmentation were neglected.) The resulting trace was normalized and compared with the experimental MS2 spectrum of EC-h4/H2O. The least-squares deviation was minimized using an algorithm that imitates Darwinian evolution on a fitness landscape. This procedure yields the optimum distributions of weights for isobars. As the last step, we eliminated all isobars with relative weights
