Mass Spectrometry Investigations on Electrolyte Degradation Products

In the continuing challenge to find new routes to improve the performance of commercial lithium ion batteries cycling in alkyl carbonate-based electro...
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Anal. Chem. 2006, 78, 3688-3698

Mass Spectrometry Investigations on Electrolyte Degradation Products for the Development of Nanocomposite Electrodes in Lithium Ion Batteries Laurent Gireaud,† Sylvie Grugeon,† Serge Pilard,‡ Pierre Guenot,§ Jean-Marie Tarascon,† and Stephane Laruelle*,†

Laboratoire de Re´ activite´ et Chimie des Solides, CNRS (UMR-6007), and Plate-Forme Analytique, Universite´ de Picardie Jules Verne, 33 rue Saint-Leu, 80039, Amiens, France, and Centre Re´ gional de Mesures Physiques de l’Ouest, Universite´ de Rennes 1, 35042, Rennes, France

In the continuing challenge to find new routes to improve the performance of commercial lithium ion batteries cycling in alkyl carbonate-based electrolyte solutions, original designs, and new electrode materials are under active worldwide investigation. Our group has focused on the electrochemical behavior of a new generation of nanocomposite electrodes showing improved capacities (up to 3 times the capacity of conventional electrode materials). However, moving down to “nanometric-scale” active materials leads to a significant increase in electrolyte degradation, compared to that taking place within commercial batteries. Postmortem electrolyte studies on experimental coin cells were conducted to understand the degradation mechanisms. Structural analysis of the organic degradation products were investigated using a combination of complementary high-resolution mass spectrometry techniques: desorption under electron impact, electrospray ionization, and gas chromatography coupled to a mass spectrometer equipped with electron impact and chemical ionization ion sources. Numerous organic degradation products such as ethylene oxide oligomers (with methyl, hydroxyl, phosphate, and methyl carbonate endings) have been characterized. In light of our findings, possible chemical or electrochemical pathways are proposed to account for their formation. A thorough knowledge of these degradation mechanisms will enable us to propose new electrolyte formulations to optimize nanocomposite-based lithium ion battery performance. Rechargeable lithium ion batteries are regarded as the most important advance in energy storage for portable electronic applications (laptop computer, cellular phone, etc.). Their high reversibility is due to the use of (1) Li ion insertion/removal electrode reactions requiring open structure active material, namely, well-known LiCoO2, LiMn2O4, or LiFePO4 as the positive electrode and graphite carbons at the negative electrode; (2) high * Corresponding author: (e-mail) [email protected]; (tel-fax) +33 3 2282 7585. † Laboratoire de Re ´ activite´ et Chimie des Solides, Universite´ de Picardie Jules Verne. ‡ Plate-Forme Analytique, Universite ´ de Picardie Jules Verne. § Universite ´ de Rennes.

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Figure 1. Voltage-composition curve for Li/CoO cell cycled between 0.02 and 3 V at 55 °C in EC/DMC-LiPF6, 1 M. Inset: brightfield TEM image of the CoO active material surface after first lithiation showing the inorganic and organic layer.

ionic conductivity electrolytes (mixtures of a lithium salt and carbonated solvents). Nevertheless, owing to their limited electrode capacity, these batteries fall short from meeting power requirements for transportation applications such as electric or hybrid vehicles. In search of alternatives, numerous worldwide groups are exploring new concepts/designs such as the possibility of using higher specific surface area electrode materials. A few years ago, our group had highlighted that the full reduction of 3d metal oxides (e.g., CoO, NiO) versus lithium led to composite materials consisting of nanometric metallic particles (smaller than 40 Å) dispersed in an amorphous Li2O matrix1 according to the following reaction:

MxOy + 2yLi+ f xMnanometric + yLi2O (with M ) 3d metal oxide) Owing to the nanometric nature of the composite material, this reaction was shown to be highly reversible and to provide improved negative electrode capacities, three times higher than those used in commercial applications. However, this “nanometric-scale” reaction involves a significant electrolyte degradation leading to (1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont L.; Tarascon, J.-M. Nature 2000, 407, 496-499. 10.1021/ac051987w CCC: $33.50

© 2006 American Chemical Society Published on Web 05/05/2006

Figure 2. Cross-sectional view of experimental coin cell device for 3d metal oxides active materials electrochemical testing versus metallic lithium.

the appearance of an inorganic/organic layer surrounding the composite material surface (see Figure 1 and Figure 2). This layer was shown to be thicker than the solid electrolyte interphase (SEI) formed on graphite electrode material. While SEI is known to prevent graphene sheet exfoliation and minimize electrolyte consumption upon cycling in commercial batteries, the thick electronically insulating layer developed on 3d metal oxide surface is rather detrimental to cycling efficiency. With the aim to reduce this phenomenon, studies have been undertaken to determine its chemical composition and to elucidate its formation mechanisms. After the cycling of these nanocomposite electrode materials within the most commonly used electrolyte mixture, namely, ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 weight ratio)-LiPF6 salt (1 M), we first evidenced, by transmission electronic microscopy,1 the presence of a solid layer covered with a thick jellylike film at the surface of the composite material. Carrying out high-temperature (up to 55 °C) cycling experiments, we visualized the growth of the film that could finally embed the electrode separator. As a part of an early analysis on the surface layer, numerous inorganic compounds such as lithium carbonate, lithium methyl carbonate, and lithium fluoride have been clearly identified by means of X-ray photoelectron spectroscopy (XPS)2,3 and infrared spectroscopy;4 these inorganic compounds being similar to those constituting the SEI5 in commercial batteries. Organic compounds were equally spotted using XPS experiments by the appearance of peaks characteristic of C-C and C-O bonds, suggesting that the jellylike film is mainly made of ethylene oxidebased polymers. As these techniques have limited abilities in providing their accurate structures,2-4,6-9 we have embarked on mass spectrometry (MS) analyses of EC/DMC-LiPF6 electrolyte recovered from electrode separators after cycling at 55 °C. Due to the interest of the preliminary results10 and in order to undergo deeper structural investigations, we have pursued this work by (2) Dedryve`re, R.; Laruelle, S.; Grugeon, S.; Gireaud, L.; Gonbeau, D.; Tarascon, J.-M. J. Electrochem. Soc. 2005, 152, 4, A689-A696. (3) Dedryve`re, R.; Gireaud, L.; Grugeon, S.; Laruelle, S.; Tarascon, J.-M.; Gonbeau, D. J. Phys. Chem. B 2005, 109, 15868-15875. (4) Gireaud, L.; Grugeon, S.; Laruelle, S.; Pilard, S.; Tarascon, J.-M. J. Electrochem. Soc. 2005, 152, 5, A850-A857. (5) Peled, E. J. Electrochem. Soc, 1979, 126, 2047-2051. (6) Bar-Tow, D.; Peled, E.; Burstein, L. J. Electrochem. Soc. 1999, 146, 824832. (7) Aurbach, D. J. Power Sources 2000, 89, 206-218. (8) Andersson, A. M.; Edstrom, K. J. Electrochem. Soc. 2001, 148, A1100A1109. (9) Aurbach, D.; Markovsky, B.; Weissman, I.; Levi, E.; Ein-Eli, Y. Electrochim. Acta 1999, 45, 67-86. (10) Laruelle, S.; Pilard, S.; Guenot, P.; Grugeon, S.; Tarascon, J.-M. J. Electrochem. Soc. 2004, 151, 8, A1202-A1209.

developing a global analytical strategy using a combination of complementary high-resolution mass spectrometry techniques (HRMS) such as desorption under electron impact (DEI-HRMS), gas chromatography/mass spectrometry (GC/EI-HRMS and GC/ CI-HRMS), and electrospray ionization (ESI-HRMS). The structures of the degradation products accumulated in the separator during cycling and storage conditions will be presented as analyzed by mass spectrometry. In light of these findings, various chemical or electrochemical pathways will be discussed in order to account for their formation.

EXPERIMENTAL SECTION Cell Preparation. Studies were performed using a coin cell configuration (Figure 2). The negative electrode consisted of a lithium foil (thickness, 380 µm). The positive electrode was made from 2 mg of CoO active material mixed with Ketjen Black carbon (0.3 mg) deposited on a stainless steel current collector. The separator (Whatman-type glass fiber) was soaked with 0.1 mL of electrolyte solution made of anhydrous solvents, namely, EC/ DMC in 1:1 weight ratio containing 1 M LiPF6 salt (commercial name, LP30). Lithium metal was purchased from Sigma-Aldrich and LP30 battery grade from Merck. Cobalt oxide powder was kindly provided by Union Minie`re and Ketjen Black carbon was purchased from Azko Nobel. (1) Cycled Cell. Once assembled in an argon-filled glovebox, coin cells were placed into an oven and cycled at 55 °C between 0.02 and 3 V in galvanostatic mode (0.15 mA/cm2) using a MacPile system. The cells were stopped once an accumulated discharge capacity of ∼250 mA h was reached (after 500 or 600 cycles) and opened in the glovebox. (2) Stored Cell. Coin cells were assembled as mentioned above and then placed in an oven at 55 °C for two months (time to reach 250 mA h) without being subjected to cycling. Mass Spectrometry Analysis. (1) DEI-HRMS Measurements. A small piece of the glass fiber separator recovered from cycled or stored cells was introduced under argon atmosphere to avoid any oxidation of the trapped electrolyte degradation products, in a quartz crucible (1-mm inside diameter). This crucible was placed at the extremity of a direct insertion probe prior to being transferred within a few seconds to the electron impact ionization (EI) source of a high-resolution double sector (BE) mass spectrometer (Varian Mat 311). The probe environment was evacuated to a pressure of 10-7 h Pa, and the quartz crucible was temperature regulated between 25 and 400 °C to ensure complete control of the sample desorption process. Electron energy, emission current, and accelerating voltage of 70 eV, 300 mA, and 3 kV, respectively, were used. The MS scanned over a range of 10-400 Da. Accurate mass measurements of fragment ions were performed using the peak matching method with perfluorokerosene as the internal reference. The remaining part of the separator was extracted using 3 mL of extradry acetonitrile (Sigma-Aldrich) and centrifuged under argon atmosphere. The supernatant was dissolved (1:100) in acetonitrile and analyzed by means of GC/HRMS and ESI-HRMS. (2) GC/EI-HRMS and GC/CI-HRMS Measurements. GC Parameters. Samples were analyzed on an Agilent 6890 gas chromatography system attached to a Waters-Micromass GC-TOF Analytical Chemistry, Vol. 78, No. 11, June 1, 2006

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mass spectrometer. The injection volume was 1 µL for each sample. Helium was used as a carrier gas at a flow rate of 1 mL/min. The GC column was a SGE BPX-70 (30 m, 250-µm i.d., 0.25-µm film thickness). The temperature of the injector was 260 °C, and the transfer line was held at 250 °C throughout the duration of the chromatographic run. The initial oven temperature was 35 °C followed by a temperature gradient of 5 °C/min to 130 °C. Then, a second temperature gradient was applied increasing at 10 °C/ min to 260 °C. Finally, the system was kept at 260 °C for 15 min. MS Parameters. The compounds eluted from the GC column were analyzed using EI and positive chemical ionization (CI). EI. The ion source was operated at 180 °C with an electron energy of 70 eV and a trap current of 200 µA. The MS scanned over a range of 40-500 Da with an acquisition time of 0.45 s and a delay of 0.05 s (2 spectrum/s). CI. The ion source was operated at 100 °C with an electron energy of 70 eV and an emission current of 200 µA. The MS scanned over a range of 40-500 Da with an acquisition time of 0.95 s and a delay of 0.05 s (1 spectrum/s). The reagent gas was methane at a source pressure of 2 × 10-4 mbar. For EI and CI analyses, the instrument was tuned and calibrated for a mass range of 40-500 Da using heptacosaperfluorotributylamine. Chloropentafluorobenzene (M•+, m/z 201.9609 Da) and 2,4,6-tris(trifluoromethyl)-1,3,5-triazine ([M + H]+, m/z 286.0027 Da) were used as lock mass (accurate measurements) for EI and CI, respectively. These reference compounds were infused into the ion source continuously during the chromatographic run. Exact mass chromatograms for the compounds of interest were extracted using a 0.02-Da window. Elemental composition of each compound was obtained using a tolerance of 5 mDa, and only relevant combinations of elements were allowed for the analytes. Data acquisition and processing were performed with MassLynx v 4.0 software. (3) ESI-HRMS Measurements. ESI-HRMS experiments were performed on a Waters-Micromass Q-TOF Ultima Global hybrid instrument equipped with a Z-spray ion source. The prepared solutions were directly introduced (5 µL/min) through an integrated syringe pump in the electrospray source. The source and desolvation temperatures were kept at 80 and 150 °C, respectively. Nitrogen was used as a drying and nebulizing gas at flow rates of 350 and 50 L/h, respectively. The capillary voltage was 2.5 kV, the cone voltage 35 V, and the rf lens1 energy 35 V. Calibration of the instrument was performed using the ions produced by a phosphoric acid solution (0.1% in H2O/CH3CN 50: 50 in volume). For accurate mass measurements, a single internal lock mass correction, using the [M + H]+ ion of leucine enkephalin ([M + H]+, m/z 556.2771 Da) was applied. The mass range was 50-1000 Da, and spectra were recorded at 1 s/scan in the profile mode at a resolution of 10 000 full width at halfmaximum (fwmh). Data acquisition and processing were performed with MassLynx v 4.0 software. For each batch, three different samples were analyzed to verify the reproducibility of the results. Moreover, to avoid false positives due to apparatus or electrolyte contaminations, starting electrolyte (commercial LP30) was analyzed before and after each mass spectrometry experiment. 3690

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Table 1. Phosphate-Containing Fragment Ions Observed during the DEI-HRMS Experiments at a Desorption Temperature of 150 °C for a Cycled Battery Glass Fiber Separator m/z

exp mass

formula

calc mass

109 110 113 125 127 139 151 153 183 197

109.0060 110.0129 113.0010 125.0007 127.0164 139.0152 151.0159 153.0317 183.0423 197.0591

C2H6O3P C2H7O3P CH6O4P C2H6O4P C2H8O4P C3H8O4P C4H8O4P C4H10O4P C5H12O5P C6H13O5P

109.0055 110.0133 113.0004 125.0004 127.0160 139.0160 151.0160 153.0317 183.0422 197.0579

RESULTS After long cycling or storage at 55 °C and cell opening, the glass fiber separators were carefully inspected. Interestingly, cycling induced a separator color change from colorless to yellowish. One part of these separators was analyzed as recovered using, for the first time to our knowledge, DEI-HRMS. The other part of the separator was extracted with acetonitrile, as described in the Experimental Section. The resulting solutions were analyzed by means of GC/HRMS and ESI-HRMS. DEI-HRMS Measurements. For the cycled samples, at desorption temperatures lower than 130 °C, different species have been detected such as H2O (m/z 18), CO (m/z 28), CO2 (m/z 44), and numerous other fragments (m/z 103, 59, 45, etc.) that might come from ethers, aldehydes, esters, or alkyl carbonate salts.11 PF4+ ion (m/z 107) was also observed originating from PF5, which is known to be a major compound of decomposition of the lithium salt LiPF6. At desorption temperatures above 130 °C, these organic species are still desorbing along with other interesting characteristic fragments at m/z 109, 110, 113, 125, 127, 139, 151, 153, 183, and 197, which have been attributed to phosphateending poly(ethylene oxide) (PEO)-type chains by accurate mass measurements experiments (Table 1 and Figure 3a):

The presence of phosphate groups was confirmed by 31P NMR experiments in D2O by the presence of two singlets centered at 3.45 ppm (δPdO). At this point, it is worth pointing out that none of the above phosphate fragment ions was detected for the stored sample (see Figure 3b). Unfortunately, apart from these phosphate-ending PEO-type chains, we failed to trace the origin of the numerous other fragments. Indeed, EI is a “hard” ionization process that makes difficult, as several different molecules simultaneously undergo the desorption process, the fragments’ origin determination. To bypass such a difficulty, GC/HRMS was performed on the acetonitrile supernatant solutions. This technique offers the

Figure 3. DEI-HRMS spectra of the fiber obtained at a desorption temperature of 150 °C (a) after cycling at 55 °C and (b) after 2-months storage at 55 °C.

advantage of separating the volatile electrolyte degradation products12-14 before accurate mass measurement of each ion generated in EI and CI ionization modes. GC/HRMS Measurements. The GC/HRMS chromatogram obtained for the cycled sample is presented on Figure 4a. Note that DMC and EC are detected at retention times of 6.6 and 26.8 min, respectively. For each chromatographic peak, the main fragment ions observed in EI, the elemental composition of the [M + H]+ ions generated in CI and the corresponding formulas, and structures are presented in Table 2. Most of the detected molecules (Rt ) 6.8, 14.4, 23.0, 27.7, 29.1, 30.8, and 33.5 min) have a (CH2-CH2-O)n-based structure presenting various endings such as OCH3 or OCOOCH3 with n values ranging from 1 to 5:

Note the presence of an interesting compound at a retention time of 24.4 min. The interrogation of a spectral database (Wiley Library) enables us to propose the following structure:

As the CI spectrum did not display its [M + H]+ ion (m/z 179) but only an abundant fragment ion at m/z 104 (C4H8O3+•), its exact structure has been confirmed by testing the commercial product (2,5-dioxahexanedioic acid dimethyl, Aldrich). It should be noted, as demonstrated with DEI experiments, that the stored sample GC trace (Figure 4b) reveals no other chromatographic peaks than those corresponding to EC and DMC (Rt ) 6.56 and 27.33 min, respectively). Analytical Chemistry, Vol. 78, No. 11, June 1, 2006

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Figure 4. GC/MS chromatograms of the electrolyte solution (a) after cycling at 55 °C and (b) after 2-months storage at 55 °C. *, Diethyl phthalate (C12H14O4).

Finally, it is worth mentioning that GC/HRMS analyses have failed to detect the oligomers with phosphate endings. This does not come as a surprise since, based on the literature data, monomer molecules such as phosphoric acid 2-hydroxyethyl dimethyl ester (C4H11O5P) and phosphoric acid-methoxy ethyl dimethyl ester (C5H13O5P) exhibit boiling points above 255 and 230 °C, respectively. These temperatures do not permit GC analysis without a preliminary chemical derivatization. The main drawback of the derivatization step is the possible chemical degradation of the compounds that we hope to characterize. To circumvent such a problem, we decided to analyze the nonvolatile and polar electrolyte degradation products, extracted from the glass fiber with acetonitrile, using ESI-MS. ESI-HRMS Measurements. This “soft” ionization technique, enabling the observation of intact molecules in solution, was used in the battery research field for the study of Li ion solvation by carbonated entities in electrolytic mixtures.15 3692

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The ESI-MS spectrum of the cycled sample is reported on Figure 5a. Four sets of peaks (see Table 3) corresponding to ethylene oxide oligomers were clearly identified:

High resolution (see Table 3) was essential to differentiate isobaric ions as illustrated with the separation of the m/z 185.1000 (C7H14O5Li, series 4) and 185.1347 (C8H18O4Li, series 3) ions

Table 2. Retention Times, Fragment Ions Obtained in EI, [M + H]+ Ions Generated in CI, Accurate Mass Measurement of the [M + H]+ Ions, and Proposed Structuresa for the Cycled Electrolyte Solution, Analyzed by Means of GC/HRMS

a

Structure confirmed by the injection of a reference product (Aldrich CAS registry 88754-66-9).

(Figure 6). Among the whole products detected by this MS technique, new polar compounds (series 1 and 2) have been identified while series 3 and 4 were previously spotted by GC/ HRMS analysis. The ESI-MS spectrum obtained for the stored sample (Figure 5b) is mainly composed of an intense peak corresponding to [EC + Li]+ ion (m/z 95). The poly(ethylene oxide)s oligomer molecules attributed to series 1-4 were not observed. As for GC/HRMS, it should be mentioned that the phosphate species were not detected, even through negative ion mode ESIMS experiments, which revealed only PF6- (m/z 145) and [2PF6-, Li+]- (m/z 297) ions. Due to their high affinity toward SiO2, these compounds might remain trapped in the glass fiber separator during the extraction step with acetonitrile. On the other hand, they can easily be desorbed at high temperature under vacuum during DEI-MS analyses. In short, mass spectrometry analyses (DEI-MS, GC/MS, ESIMS) have shown drastic electrolytic decomposition upon cycling (11) Budzikiewicz, H.; Djerassi, C.; Williams, D. H. Holden-Day, Inc. 1967, pp 484-493. (12) Ravdel, B. M.; Abraham, K.; Gitzendanner, R.; DiCarlo, J.; Lucht, B.; Campion, C. J. Power Sources 2003, 119-121, 805-810. (13) Mogi, R.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. Langmuir 2003, 19, 814-821. (14) Arakawa, M.; Yamaki, J. J. Power Sources 1995, 54, 250-254. (15) Mastsuda, Y.; Fukushima, T.; Hashimoto, H.; Arakawa, R. J. Electrochem. Soc. 2002, 149, 8, A1045-A1048.

at 55 °C in nanocomposite-based lithium batteries. Ethylene oxide oligomers with four possible ending groups (OCH3, OCOOCH3, (OCH3)2PO, OH) have been characterized. As the stored sample does not show any degradation product, we can conclude that their formations are initiated by an electrochemically driven process. In light of these findings, various chemical or electrochemical pathways will now be discussed in order to account for their formation. DISCUSSION The proper functioning of a Li-based battery requires electrolytes that are stable over a wide potential range (0-5 V). Nevertheless as we often reach both ends of this potential window, electrolyte mixture degradation unavoidably occurs. At low potential, the passive film generated at the surface of the carbon negative electrode materials enables an excellent cyclability in the presence of carbonated electrolytes. The chemical composition of this layer, known as SEI9 has been widely characterized by XPS2-3,6,7 and FT-IR4,9,16-18 giving direct evidence of inorganic compounds (Li2CO3, LiF, Li2O, ROCO2Li, LiOH). Our study (CoO versus metallic lithium cell) shows that, in the presence of EC/DMC-LiPF6 (LP30) electrolyte, an organic (16) Aurbach, D.; Gofer, Y.; Ben-Zion, M.; Aped, P. J. Electrochem. Soc. 1992, 339, 451-471. (17) Aurbach, D.; Gottlieb, H. Electrochim. Acta 1989, 34, 2, 141-156. (18) Aurbach, D.; Ein-Eli, Y.; Markovsky, B.; Zaban, A. J. Electrochem. Soc. 1995, 9, 142, 2873-2881.

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Figure 5. ESI-MS spectra of the electrolyte solution (a) after cycling at 55 °C and (b) after 2-months storage at 55 °C.

Figure 6. High-resolution effect on isobaric ions separation. (a) Experimental measurement before centroid; (b) experimental measurement after centroid and lock mass correction; (c) calculated mass for C8H18O4Li (series 3); (d) calculated mass for C7H14O5Li (series 4).

jellylike film is generated upon cycling. According to the nature of the species detected by mass spectrometry analyses, it seems clear that ethylene carbonate is consumed, according to a polymerization mechanism. Ethylene Oxide Oligomer Formation. Ethylene carbonate polymerization process mechanisms are described by Vogdanis and al.19,20 In fact, ethylene carbonate ring opening can be chemically initiated at high temperature (150 °C), in the presence 3694

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of a lithium alkoxide catalyst, to form an oxyethyl-oxycarboxyl copolymer:

Table 3. Elemental Compositions of the Main Ionic Species Observed in the ESI-HRMS Spectrum of the Cycled Electrolyte Solution

*Unresolved peaks due to the presence of a mixture of isobaric ions.

This copolymer may transform into PEO chains by a chemical decarboxylation mechanism (e.g., hydrolysis, intra- or intermolecular rearrangements). According to these results, it appeared interesting to test the chemical reactivity at 55 °C of the electrolytic mixture (EC/DMC-LiPF6) with a lithium alkoxide (CH3OLi). Amazingly, after 1-week storage, the same PEO chains as those obtained after extending cycling were spotted by ESI-HRMS measurements (Figure 7). As the same results were also obtained

without LiPF6 salt, we can deduce that alkoxides promote the polymerization process even at temperatures as low as 55 °C. Thus, unlike usual beliefs about Lewis acids’ (HF, PF5) catalytic role in electrolytic degradation,8 we clearly demonstrated that the polymerization process can also be initiated only by lithium alkoxides electrochemically generated during cycling. Thus a legitimate question is, “Where could the lithium alkoxides come from?” In a previous paper, we reported that DMC could be Analytical Chemistry, Vol. 78, No. 11, June 1, 2006

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Figure 7. (a) ESI-HRMS spectrum of LP30 after cycling at 55 °C and (b) ESI-HRMS spectrum of a mixture of lithium methoxide and LP30 after 1-week storage at 55 °C.

electrochemically reduced during cycling at 55 °C in LP30 to give lithium methyl carbonate4 as deduced by means of FT-IR, 13C MAS NMR, and XPS:

Pearson and to the hard-soft acid and bases principle,21 the copolymer structure can be the target of nucleophilic attacks by hydroxide, alkoxide, or carbonate anions (ROCOO-) giving structure 1 as described below:

Furthermore, we have demonstrated an electrochemical reactivity of lithium alkyl carbonate toward lithium leading to the formation of lithium methoxide and a carbon dioxide radical according to the reaction below:

Then, based on early work by Vogdanis and on our previous observations, we can definitely conclude that lithium methoxide can initiate the polymerization process at 55 °C to give the copolymer structure as mentioned above. During cycling, four series of ethylene oxide oligomers having three different terminal functions, OCH3, OCOOCH3, and OH, have been identified by GC/HRMS and ESI-HRMS. According to (19) Vogdanis, L.; Heitz, W. Makromol. Chem., Rapid Commun. 1986, 7, 543547. (20) Vogdanis L.; Martens, B.; Uchtmann, H.; Hensel, F.; Heitz. W. Makromol. Chem. 1990, 191, 465-472. (21) Pearson, R. G.; Songstad, J. J. Am. Chem. Soc. 1967, 89, 1827-1836.

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Then, structure 1 can be involved in a second nucleophilic attack giving structure 2. Thus, these proposed chemical pathways can account for the alkylated chain formation. Note that the hydroxyl ending might come from water contained in the glass fiber separator (∼100 ppm).

Phosphates Formation. Owing to one or both reactions proposed in the literature,12 we think that PF5 reacts directly with methoxide groups to generate phosphates according to these reactions:

For the longer oligomers chains, Morris and Dixon reported the possibility of generating polyether phosphates by reaction between ether-type oligomers and trimethyl phosphate as follows:22

2,5-Dioxahexanedioic Acid Dimethyl Formation. Based on our expertise in the electrochemical reduction of dimethyl carbonate, we can also propose a chemical route to explain the formation of the 2,5-dioxahexanedioic acid dimethyl detected in GC/HRMS at a retention time of 24.4 min (Figure 4a). The first step is the electrochemical reduction of DMC involving a one-electron mechanism to give lithium methyl carbonate:

Then, an anionic attack of the EC structure could take place:

Afterward, a decarboxylation process occurs owing to this mechanism:

The last step consists of the reaction of DMC with the monoglycolate methyl carbonated structure, giving 2,5-dioxahexanedioic acid dimethyl:

However, another pathway involving water12 can also be proposed to account for the phosphate species formation. Experiments are in progress to validate the suggested mechanism.

CONCLUSION Nanocomposite negative electrode-based lithium ion batteries seem very promising for high power requirements. However, due to a high electrolytic decomposition during cycling, they undergo capacity fading. To understand this phenomenon, the degradation of a standard EC/DMC-LiPF6 electrolyte has been studied using high-resolution mass spectrometry. We have clearly shown by means of DEI-HRMS, GC/HRMS, and ESI/HRMS techniques that the organic species generated from 3d metal oxide nanocomposite electrode surface mainly consist of ethylene oxide oligomers (CH2-CH2-O)n (1 < n