Mass Spectrometry As a Suitable Tool for the Li

Dec 14, 2010 - To allow electric vehicles to be powered by Li-ion batteries, scientists must understand further their aging processes in view to exten...
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Anal. Chem. 2011, 83, 478–485

Gas Chromatography/Mass Spectrometry As a Suitable Tool for the Li-Ion Battery Electrolyte Degradation Mechanisms Study Gre´gory Gachot,† Perrine Ribie`re,† David Mathiron,‡ Sylvie Grugeon,† Michel Armand,† Jean-Bernard Leriche,† Serge Pilard,‡ and Ste´phane Laruelle*,† Laboratoire de Re´activite´ et de Chimie des Solides, UMR CNRS 6007, and Plate-Forme Analytique, Universite´ de Picardie Jules Verne, 33 Rue Saint Leu, 80039 Amiens, France To allow electric vehicles to be powered by Li-ion batteries, scientists must understand further their aging processes in view to extend their cycle life and safety. For this purpose, we focused on the development of analytical techniques aiming at identifying organic species resulting from the degradation of carbonate-based electrolytes (ECDMC/LiPF6) at low potential. As ESI-HRMS provided insightful information to the mechanism and chronological formation of ethylene oxide oligomers, we implemented “gas” GC/MS experiments to explore the lower mass range corresponding to highly volatile compounds. With the help of chemical simulation tests, we were able to discriminate their formation pathways (thermal and/or electrochemical) and found that most of the degradation compounds originate from the electrochemically driven linear alkyl carbonate reduction upon cycling and to a lesser extent from a two-step EC reduction. Deduced from these results, we propose an overall electrolyte degradation scheme spanning the entire mass range and the chemical or electrochemical type of processes. Within 2 decades, Li-ion batteries have swept the portable electronic market and are now about to enter the automotive sector. Considered as one of the technical and environmental challenges of the 21st century, this new target is motivating scientists with a demand for increased performances and robustness. Studies are mainly dedicated to the next generation of electrode materials making use of nanomaterials (alloys, conversion materials. . .) or higher voltage insertion materials. Unfortunately, the use of these new materials is limited by the electrolyte electrochemical instability.1,2 The resulting interfacial layer of degradation products (Solid Electrolyte Interphase) which coats both electrodes increases the cell polarization and thus affects the power performances and long-term cyclability. Thus, materials performance enhancement goes necessarily through new elec* To whom correspondence should be addressed. E-mail: stephane.laruelle@ u-picardie.fr. † Laboratoire de Re´activite´ et de Chimie des Solides, UMR CNRS 6007. ‡ Plate-Forme Analytique. (1) Bridel, J.-S.; Grugeon, S.; Laruelle, S.; Hassoun, J.; Reale, P.; Scrosati, B.; Tarascon, J.-M. J. Power Sources 2010, 195, 2036–2043. (2) Dedryve`re, R.; Laruelle, S.; Grugeon, S.; Poizot, P.; Tarascon, J.-M. J. Electrochem. Soc. 2001, 148, A285.

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trolyte compositions and/or additives, encouraging researchers to better understand the decomposition processes. On the other hand, for electric-powered vehicle application, the highly volatile and flammable nature of carbonate solvents is a serious safety concern at masses around 200 kg. This is especially true for rare but inevitable cases of fire or perforation. Even if the packaging is specially designed to eschew such events, numerous efforts are being devoted to reduce toxic fumes3 and/or explosive hazards by exploring, for instance, new classes of electrolyte salts such as the so-called “Hu¨ckel” salts without any fluorine4 and new types of electrolytes such as nonflammable ionic liquids with no vapor pressure.5 For these reasons, either to allow the emergence of new electrode materials or to make batteries safer, mastering the electrode/electrolyte interactions is viewed as essential for further progress in the Li-ion battery development. Hence, researchers attempt to shed light on the electrochemically or thermally driven classical electrolyte decomposition mechanisms. For this purpose, they developed an arsenal of analytical techniques to characterize the inorganic surface layer as X-ray photoelectron spectroscopy (XPS), infrared (IR), atomic force microscopy (AFM), transmission electron microscopy (TEM), pyrolysis gas chromatography/mass spectrometry (Py-GC/MS)6-18 and revealed the now well-known (3) Hammami, A.; Raymond, N.; Armand, M. Nature 2003, 424, 635–636. (4) Niedzicki, L.; Zukowska, G. Z.; Bukowska, M.; Szczecinski, P.; Grugeon, S.; Laruelle, S.; Armand, M.; Panero, S.; Scrosati, B.; Marcinek, M.; Wieczorek, W. Electrochim. Acta 2010, 55, 1450–1454. (5) Galinski, M.; Lewandowski, A.; Stepiak, I. Electrochim. Acta 2006, 51, 5567– 5580. (6) Dedryve`re, R.; Laruelle, S.; Grugeon, S.; Gireaud, L.; Tarascon, J.-M.; Gonbeau, D. J. Electrochem. Soc. 2005, 152 (4), A689–A696. (7) Dedryve`re, R.; Gireaud, L.; Grugeon, S.; Laruelle, S.; Tarascon, J.-M.; Gonbeau, D. J. Phys. Chem. B 2005, 109, 15868–15875. (8) Andersson, A. M.; Edstrom, K. J. Electrochem. Soc. 2001, 148 (10), A1100– A1109. (9) Kamakura, K.; Tamura, H.; Shiraishi, S.; Takehara, Z. J. Electrochem. Soc. 1995, 142, 340. (10) Kamakura, K.; Shiraishi, S.; Takehara, Z. J. Electrochem. Soc. 1996, 143, 2187. (11) El Ouatani, L.; Dedryvere, R.; Siret, C.; Biensan, P.; Gonbeau, D. J. Electrochem. Soc. 2009, 156 (6), A468. (12) Gireaud, L.; Grugeon, S.; Laruelle, S.; Pilard, S.; Tarascon, J.-M. J. Electrochem. Soc. 2005, 152 (5), A850–A857. (13) Aurbach, D.; Ein-Eli, Y. J. Electrochem. Soc. 1995, 142, 1746. (14) Ein-Eli, Y.; Devitt, S. F.; Aurbach, D.; Markovsky, B.; Schecheter, A. J. Electrochem. Soc. 1997, 144, L180. (15) Aurbach, D.; Ein-Eli, Y.; Chusid, O.; Carmeli, Y.; Babai, M.; Yamin, H. J. Electrochem. Soc. 1994, 141, 603. (16) Grugeon, S.; Laruelle, S.; Herrera-Hurbina, R.; Dupont, L.; Poizot, P.; Tarascon, J.-M. J. Electrochem. Soc. 2001, 148 (4), A285–A292. 10.1021/ac101948u  2011 American Chemical Society Published on Web 12/14/2010

lithium derivatives (LiF, Li2CO3, Li alkylcarbonates, phosphates, ethers, and Li alkoxides) in the SEI. To study more specifically the organic molecules, GC/MS experiments have been performed on thermally or electrochemically degraded electrolytes after dilution with CH2Cl2 solvent.19-23 Nevertheless, this “liquid” GC/MS analysis does not allow one to span the whole mass range. Thus, we adopted another strategy consisting in splitting the mass range in two. The highest molar masses (less volatile) were first analyzed by electrospray ionization-high-resolution mass spectrometry (ESI-HRMS) and provided significant results.23,24 Afterward, we pursued our investigation by setting up a “gaseous” GC/ MS experiment to identify the lowest molar masses (most volatile). As reported in this paper, we characterized every volatile product released from a cycled “conversion type”25,26 negative electrode/ Li half-cell electrolyte and then investigated their likely formation mechanisms. EXPERIMENTAL SECTION Cells Assembling. Coin cells (standard 2035-size) were assembled in an argon-filled dry glovebox. The working electrode consisted of a 2 cm2 SUS316L-type stainless steel disk treated at 700 °C under H2/N2 atmosphere during 16 h to favor the growth of a 300-500 nm chromiumIII oxide layer on its surface. It was separated from a lithium foil piece, used as the negative electrode, by a Whatman GF/D borosilicate glass fiber mat imbibed with a 1 M LiPF6 solution in ethylene carbonate (EC) and dimethyl carbonate (DMC) (50/50 w/w) as the electrolyte. Once assembled, the coin cells were kept at 55 °C and cycled between 0.02 and 3 V in galvanostatic mode (0.15 mA cm-2) using a Mac Pile (Biologic, Claix, France) system. The cells were stopped at the end of a charge (3 V) once a cumulated discharge capacity of 115 mAh has been reached (Figure 1). The cell was left to cool and carefully opened in the drybox. The separator was recovered for study and GC/MS analysis. The EC-DMC/LiPF6 (LP30), EC, and DMC selectipur were purchased from Merck, lithium hexafluorophosphate (battery grade) and lithium methoxide from Sigma-Aldrich, and lithium carbonate “puratronic, 99.998 %” from Alfa Aesar. Lithium methyl carbonate was synthesized using a described procedure.12 Sample Preparation for GC/MS Analyses. Samples (electrolyte-impregnated separator or electrolytic solutions) were rapidly placed into an aluminum crucible that was sealed then (17) Kominato, A.; Yasukawa, E.; Sato, N.; Ijuuin, T.; Asahina, H.; Mori, S. J. Power Sources 1997, 68, 471–475. (18) Mogi, R.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Power Sources 2003, 119-121, 597–603. (19) Kumai, K.; Miyaschiro, H.; Kobayashi, Y.; Takei, K.; Ishikawa, R. J. Power Sources 1999, 81-82, 715–719. (20) Sloop, S. E.; Pugh, J. K.; Wang, S.; Kerr, J. B.; Kinoshita, K. Electrochem. Solid-State Lett. 2001, 4 (4), A42–A44. (21) Sloop, S. E.; Kerr, J. B.; Kinoshita, K. J. Power Sources 2003, 119-121, 330–337. (22) Campion, C.; Li, W.; Lucht, B. J. Electrochem. Soc. 2005, 152 (12), A2327– A2334. (23) Gireaud, L.; Grugeon, S.; Pilard, S.; Guenot, P.; Tarascon, J.-M.; Laruelle, S. Anal. Chem. 2006, 78 (11), 3688–3698. (24) Gachot, G.; Grugeon, S.; Armand, M.; Pilard, S.; Guenot, P.; Tarascon, J.-M.; Laruelle, S. J. Power Sources 2008, 178, 409–421. (25) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nature 2000, 407, 496. (26) Grugeon, S.; Laruelle, S.; Dupont, L.; Chevallier, F.; Taberna, P. L.; Simon, P.; Gireaud, L.; Lascaud, S.; Vidal, E.; Yrieix, B.; Tarascon, J.-M. Chem. Mater. 2005, 17, 5041.

Figure 1. Charge/discharge voltage-capacity trace for a SUS316L stainless steel disk treated at 700 °C under H2/N2 for 16 h vs Li° cell. Inset: Charge and discharge capacity retention of the cell as a function of the number of cycles.

pierced prior to being introduced into a laboratory-designed stainless steel furnace cell inside the glovebox. For reproducibility and because of mass spectrometer sensitivity and contamination risk (corrosion by HF followed by solvent adsorption), this furnace cell was carefully washed with acetone then vacuum-dried overnight at 75 °C between each experiment. GC/MS Conditions. All analyses were performed using a trace GC ultra gas chromatograph connected to a ITQ 1100 mass spectrometer (both from Thermo Scientific). The chromatographic separation was performed on a “HP-PLOT/Q” polystyrene-codivinylbenzene-based capillary column (30 m × 0.32 mm i.d., 20 µm) from Agilent J & W Technologies, followed by a postcapillary column “Rtx-1” (15 m × 0.25 mm i.d., 0.25 µm, 100% poly(dimethylsiloxane)) from Restek. Helium was used as the GC carrier gas and maintained at a constant flow rate of 1.3 mL min-1. Each crucible (loaded with samples) was heated up to 200 °C for 3 h in the furnace (Figure 2), then the evolved gases were transferred to a heated, six-port, 2 positions valve (Valco), equipped with an injection loop. The filling time of this loop was adjusted to reach a constant sample volume of 0.5 mL which is then transferred into the split/splitless injector (with a ratio of 10:1) maintained at 200 °C. Note that the separation of the highly volatile sample constituents required the use of a large film stationary phase column (20 µm versus 0.25 µm for a conventional “liquid” GC/MS column) but also of a postcapillary column to preserve the mass spectrometer ion source from the well-known bleeding of this first column as well as from corrosive HF. To achieve the best chromatographic peaks resolution, the programmable temperature gradient was optimized from 40° to 250 °C as follows: the capillary column was ramped from the initial temperature of 40 °C, held for 6 min, increased at 10 °C/min up to 90 °C, increased at 5 °C/min up to 190 °C, held for 5 min, increased at 10 °C/min up to 250 °C, where it was held for 10 min. The total duration of GC analysis was 52 min. The transfer line was maintained at 250 °C. The ion source was set at 200 °C. Tuning of the mass spectrometer was done automatically using the ions resulting from perfluorotributylamine ionization. The mass spectrometer was operated with a filament current of 250 µA and electron energy of 70 eV in the electron ionization (EI) mode. The mass range was 10-300 u and data acquisition processed with the Xcalibur 2.0.7 software. Compounds identificaAnalytical Chemistry, Vol. 83, No. 2, January 15, 2011

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Figure 2. Schematic of the “gas” GC/MS experiment allowing the analysis of volatile products released from battery components (electrolyte, separator, etc.) under heating conditions.

tion and corresponding structural formulas were assigned using the National Institutes of Standards (NIST) library. RESULTS AND DISCUSSION As reported in our previous papers,12,24 we established and generalized a mechanism of inorganic and organic compounds formation coming from the cyclic and linear carbonates-based electrolyte degradation at low-potential upon cycling. The large amounts of products required for the study were generated from a Cr2O3/lithium cell cycled at 55 °C using a classical EC-DMC/ LiPF6 electrolyte. Inorganic compounds contained in the SEI, as lithium methyl carbonate, lithium carbonate, and lithium fluoride were clearly identified by XPS7-11 and IR12-15 while the organic jellylike film observed by transmission electron microscopy was characterized using ESI-HRMS. Ethylene oxide oligomers were identified and differentiated by the nature of their end-groups (carbonate, methoxy and hydroxyl) giving rise to various 1n, 2n, 3n, 4n, 5n, and 6n series as described below. Through chemical simulation, we have demonstrated how lithium methoxide, initially electrochemically generated, could be responsible for the sequential formation of such oligomers 1n, 2n, and then the 3n series. Such chemical reactions imply the precipitation of lithium methylcarbonate only known previously from electrochemical origin. Besides, three other series, 4n, 5n, and 6n with hydroxyl end-groups were evidenced and formed proportionally to water concentration in the cell; trimethyl phosphate and phosphate “P” obviously stem from a reaction with LiPF6. To gain further insight into this degradation issue, we studied lower mass products using “gas” GC/MS. Operating in the 10-300 u mass range, we expected to detect at least the lighter oligomers series and the small molecule DMC. “Liquid” GC/MS analyses were previously attempted on electrolyte solution recovered from a cycled cell, but the resulting chromatogram23 led to the conclusion that such a technique was unsuitable for a low480

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mass compound. Hence, we turned to the “gas” analysis by desorption under electronic impact after separation on a porous layer open tubular (PLOT) column and thus adapted a special device including a tailored stainless steel furnace cell coupled with a specific gas valve at the entrance of the GC/MS apparatus. GC/MS Analysis of the Electrolyte after Cycling. First of all, the electrolyte-soaked separator recovered from the cycled cell was analyzed by GC/MS; the collected chromatogram (Figure 3a) displays numerous and distinct peaks of different intensities retained less than 50 min. The majority of products could be identified; the few remaining apparently are not listed in the NIST library. It is worth noting that the most intense peak assigned to the DMC solvent does not appear until 31.70 min, a longer time than when using the GC/MS in “liquid” mode with samples diluted in acetonitrile (6.60 min) (Figure 3b). Hence, this time extension ensures a much better separation of the products having a lower retention time than DMC. Detailing the analysis, we notice the presence of some of the previously missing monomers 51 (27.16 min) and 31 (36.06 min) that led us to conclude that both ESI-HRMS and gas analysis by GC/MS are complementary techniques providing a global analysis of the degradation compounds over a wide mass range. While proceeding further into the elucidation of the chromatogram, we rapidly met the

Figure 3. (a) “Gas” GC/MS and (b) “liquid” GC/MS (dilution in acetonitrile) chromatograms of the EC-DMC/LiPF6 electrolyte recovered from the cycled stainless steel/Li cell at 55 °C.

multireaction issue, as we could not rule out the assumption that the appearance of identified products could be of electrochemical and/or thermal origin. In order to shed light on the origin of each identified compound, we went through the exercise of discriminating electrochemically and thermally driven reactions by heating first the electrolyte then the components separately, up to 200 °C for 3 h. GC/MS Analysis of the Electrolyte Components after Heating. Identical GC/MS analyses were successively performed on heated compounds enlisting LiPF6, EC-DMC, DMC/LiPF6, EC/LiPF6, and EC-DMC/LiPF6 (LP30) electrolyte. Figure 4 depicts all these chromatograms over the same span, hereafter carefully itemized. Pure LiPF6 chromatogram does not show any degradation peaks, though it is clearly established that this salt decomposes thermally into LiF and PF5 as gaseous phase and would release POF3 and hydrofluoric acid (HF) in the presence of water traces.22,27 Their absence is not surprising as we have to keep in mind that we intentionally added a postcolumn after the separation one in order to withhold fluorinated gas and thus preserve the ion source of the mass spectrometer from corrosion. Note that these gases react with siliceous (Si-O-Si) bonds in the postcolumn explaining that SiMe2F2 is sometimes detected on chromatograms. The mix EC-DMC only reveals DMC solvent and CO2 traces, which indicates decarboxylation of carbonates at such temperature, though to a small extent. Together with the phosphates, EC is not volatile enough to be (27) Campion, C.; Li, W.; Euler, W.; Lucht, B.; Ravdel, B.; DiCarlo, J.; Gitzendanner, R.; Abraham, K.-M. Electrochem. Solid-State Lett. 2004, 7 (7), A194–A197.

detected in these conditions. On the other hand, the presence of LiPF6 in the solvents was found to alter the previous chromatograms, acting as a catalyst for some degradations reactions. In the case of DMC/LiPF6, the CO2 peak increases concomitantly with the formation of CH3F and dimethyl ether. As suggested in the literature,27 the dimethyl ether traces could be explained by further DMC decarboxylation catalyzed by the salt (reaction 1). The CH3F formation is most likely linked to the presence of water traces (reaction 2),22,27 which reacts with PF5 to generate POF3. This oxyfluoride reacts with DMC to yield CH3F, and then the new phosphate obtained after decarboxylation would contribute to increase the CO2 release. With the mix EC/LiPF6, a similar degradation process brings an enhancement of the CO2 peak.

Note that no fluorinated species are released from this mix, in good agreement with the reaction scheme (reaction 3) involving a cyclic solvent.22,27 Regarding the LP30 solution, the chromatogram compares well with that of DMC/LiPF6 solution. However, as the DMC quantity is lower, we see a decrease in the dimethyl ether peak intensity and the disappearance of CH3F. Analytical Chemistry, Vol. 83, No. 2, January 15, 2011

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Figure 4. GC/MS chromatograms stemming from thermal decomposition of (a) LiPF6, (b) EC-DMC, (c) DMC/LiPF6, (d) EC/LiPF6, and (e) EC-DMC/LiPF6 (LP30). The range between 5.5 and 29.5 min is magnified by 8.

In summary, the aforementioned results clearly state that the electrolyte thermal degradation does not account for the large part of the compounds identified in the separator after cycling. Hence, we shifted our focus on the electrochemical degradation by utilizing our latest findings. As we clearly pointed out,24 lithium methoxide (CH3OLi) coming from the DMC reduction triggers the electrolyte degradation, we designed experiments to ascertain whether its presence could lead to additional peaks highlighted in the cycled cell separator chromatogram. Thus, a high concentration of CH3OLi (1 M) was introduced into ECDMC/LiPF6, and this solution was analyzed using the same GC/MS conditions. GC/MS Analysis of LP30 in Presence of CH3OLi (1 M). Figure 5a depicts the GC/MS chromatogram resulting from the EC-DMC/LiPF6 solution with CH3OLi. In comparison with Figure 4e, the presence of CH3OLi noticeably alters the profile, which exhibits a larger number of compounds and in higher concentration. The strong rise of (CH3)2O, CO2, and CH3F peaks proves that the alkoxide also promotes the formation of these gaseous compounds from chemical reaction on LP30. 482

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Indeed, Ravdel et al.28 found that LiPF6 in the presence of MeOLi forms phosphoryl trifluoride POF3, LiF, and CH3F (reaction 4 with R ) CH3). We assumed a nucleophilic attack of CH3OLi on DMC acting as the alkylating agent with the formation of dimethyl ether and lithium methyl carbonate CH3OCO2Li. This carbonate would decarboxylate while regenerating CH3OLi (reaction 5). To confirm this last reaction, a CH3OCO2Li-LP30 solution was subjected to the same GC/ MS experiment. The resulting chromatogram (Figure 5b) was similar to that of the CH3OLi-LP30 mix with smaller peak intensities suggesting that a thermally driven degradation of the lithium methyl carbonate also yields CH3OLi. The occurrence of the other peaks in the chromatogram in Figure 5a comes as a result of the CH3OLi chemical action; from the literature,29 CO2 insertion in dimethyl ether is conceivable in the presence of acid catalyst at high temperature. Apparently, (28) Radvel, B.; Abraham, K.-M.; Gitzendanner, R.; DiCarlo, J.; Lucht, B.; Campion, C. J. Power Sources 2003, 119-121, 805–810.

Figure 5. GC/MS chromatogram of the mixtures of EC-DMC/LiPF6 (LP30) with (a) CH3OLi and (b) CH3OCO2Li.

our experiment (up to 200 °C, in an aluminum crucible with the possible presence of HF) gathers all conditions together to bring about such a carbonylation reaction (reaction 6). The methanol obviously arises from a water reaction on CH3OLi, both being likely to enable the methyl formate formation following the reaction scheme (reaction 7) according to the fact that this product is industrially synthesized through a sodium methoxide catalyzed methanol carbonylation.30

detected compounds are also visible in the first chromatogram resulting from the products found in the separator of cycled cells. Hence, we infer that DMC (or other linear carbonates) electrochemical degradation has a significant influence on the nature of the gases evolved from an aged cell electrolyte.

In the pursuit of a full understanding of each reaction mechanism, we tried to explain the presence of 2-methyl-1,3dioxolane (34.03 min) and 1,4-dioxane (37.44 min). As predicted by our degradation mechanism,24 the reaction of CH3OLi on LP30 yields ethylene oxide oligomers, herein proved again through the 51 and 31 peaks appearance. The nonavoidable traces of water entail OH end-groups in the series (5n, 6n) and specifically the 62 compound (diethylene glycol) that could undergo (reaction 8) a high temperature driven cyclo-dehydration in the presence of hydronium ions (H+)31 to achieve the formation of both cyclic compounds. Ultimately, after scrutinization of this LP30-CH3OLi solution GC/MS chromatogram, it is worth mentioning that all the (29) Cheung, P.; Bhan, A.; Sunley, G.-J.; Iglesia, E. Angew. Chem. 2006, 118, 1647–1650. (30) Chen, L.; Zhang, J.; Ning, P.; Chen, Y.; Wu, W. J. Nat. Gas Chem. 2004, 13 (4), 225–230. (31) Yang, Y.; Duan, P.-G.; Wang, Y.-Y.; Dai, L.-Y. Chem. Eng. Process. 2008, 47, 2402–2407.

Comparison between the Cycled Cell Separator and LP30-CH3OLi GC/MS Analyses. As outlined in the previous section, the CH3OLi deliberately introduced afforded us a proper Analytical Chemistry, Vol. 83, No. 2, January 15, 2011

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Table 1. Retention Times of the Compounds Desorbed from of the EC-DMC/LiPF6 (LP30) Electrolyte Recovered from the Cycled Cell Whose Structural Formula Was Assigned through Matching to the National Institutes of Standards (NIST) Library and Their Origins (Thermal Degradation, DMC, or EC Reduction)

understanding of most of the cycled cell separator (Figure 3a) gas production mechanisms and finally, only five gaseous compounds, namely, EMC (38.65 min), DEC (42.80 min), CH3CH2F (13.30 min), C2H6 (8.03 min), and C2H4 (6.63 min), remain that are assigned neither to the LP30 thermal degradation nor to the lithium alkoxide action. As they are detected after cycling only, we suggested that their formation could originate from electrochemical processes. DMC reduction leads to ionic compounds formation such as lithium alkoxide (reaction 9) or lithium alkyl carbonates (reaction 10) together with radicals such as methylcarboxy or methyl radicals, respectively; these two radicals react together to yield methyl acetate (reaction 11) or ethane (reaction 13). The radical (H•) that arises from HF reduction followed by addition to the carbonate radical yields methyl formate (reaction 12). Note that reactions 11 and 12 and 6 and 7 offer two different paths to methyl acetate and methyl formate. Surprisingly, ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are found in aged EC-DMC-based electrolyte cells. Their appearance would imply a two-step EC reduction as starting reaction in order to reach a radical anion formation (reaction 14). In the presence of H•, this radical forms lithium ethoxide (reaction 15) which could react, as a nucleophile, with DMC yielding EMC (reaction 16) and then DEC from further action of CH3CH2OLi (reaction 17). CH3CH2F is then generated from reaction 4 (with R ) CH3CH2) between PF5 and lithium 484

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ethoxide or from reaction 2, in which DMC is substituted for DEC or EMC. We also see an enhanced C2H4 intensity peak caused by the well-known two electron EC reduction with Li2CO3 precipitation (reaction 18).

To summarize, Table 1 offers a comprehensive survey of all volatile products reported in the cycled cell chromatograms with the corresponding retention times. Even if we acknowledge that further

Figure 6. Global scheme for the EC-DMC/LiPF6 electrolyte low-potential degradation mechanisms resulting from GC/MS and ESI-HRMS analyses.

proofs may be needed, the formation of most of the detected gas could have been logically explained through thermal and/or electrochemical electrolyte solvent degradation pathways. Only the route to acetylene and propylene remains yet unsolved. To go back to their origin, specific GC/MS experiments were performed unveiling that electrolyte thermal degradation releases only a few volatile molecules and with low peak intensities. In fact, the major part of the products arises from the DMC-reduction-initiated chemical degradation upon cycling, which has been simulated by introducing CH3OLi in LP30 electrolyte. It is worth mentioning that half (in number) of these products require the presence of hydrogen radicals or protons to be formed, both emanating from water traces or hydrogen-based groups at the surface of the active materials. Interestingly, these hydrogen-based impurities allowed us to evidence that EC could also undergo an electrochemical reduction (reaction 14) leading to the formation of volatile EMC or DEC and CH3CH2F, compounds detectable by means of GC/MS analyses. The hydrogen radicals or hydronium ions were assumed to come from the presence of the intermediary HF gas; this latter being retained when a postcolumn is present. CONCLUSIONS This study is a continuation in part of our early work aiming to find the best post-mortem analytical tools to provide greater insight into carbonates-based electrolyte degradation processes at low-potential. First, ESI-HRMS analysis proved its outstanding capacity in identifying moderately volatile compounds such as ethylene oxide oligomers with carbonate, alkoxy, and hydroxyl end-groups, enabling us to establish an electrochemically/chemi-

cally driven degradation mechanism. Second, GC/MS experiments were successfully implemented to explore the lower mass range (highly volatile organic compounds) throwing some light on our understanding of the overall electrolyte degradation as schematized in Figure 6, where compounds are sorted out as a function of the MS apparatus, the mass range, and the chemical or electrochemical type of processes. In parallel, chemical simulations were shown to be of great help to discriminate their formation pathways (thermal and/or electrochemical) and stressed that most of the compounds originate from the electrochemically driven DMC reduction upon cycling. To a lesser extent, the EC reduction reaction also gives rise to volatile compounds such as DEC and EMC in the presence of hydrogen radicals or ions. Importantly, it must be mentioned that these mechanisms still hold regardless of the nature of the negative alloying, conversion, and insertion electrode materials as it will be reported in a forthcoming paper. Overall, “gas” GC/MS analyses provided insightful information regarding the electrolyte decomposition products that are essential to comprehend the safety aspects of Li-ion batteries. ACKNOWLEDGMENT The authors acknowledge the financial support of “Re´gion de Picardie” (in part through the BatteryNanoSafe project) and are grateful to Mathieu Morcrette and Jean-Marie Tarascon for helpful comments during the preparation of this manuscript. Received for review July 23, 2010. Accepted November 21, 2010. AC101948U

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