Study of the Decomposition of Propylene Carbonate on Lithium Metal

The compositions of surface films formed on lithium metal in different propylene carbonate (PC)-based solutions and the decomposition processes in the...
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Langmuir 2003, 19, 814-821

Study of the Decomposition of Propylene Carbonate on Lithium Metal Surface by Pyrolysis-Gas Chromatography-Mass Spectroscopy Ryo Mogi,†,‡ Minoru Inaba,*,§ Yasutoshi Iriyama,† Takeshi Abe,† and Zempachi Ogumi† Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan, Shibukawa Laboratory, Kanto Denka Kogyo Co., Ltd., 1497 Shibukawa, Gunma, 377-8513, Japan, and Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan Received July 25, 2002. In Final Form: October 26, 2002 The compositions of surface films formed on lithium metal in different propylene carbonate (PC)-based solutions and the decomposition processes in the surface film formation were investigated by pyrolysisgas chromatography-mass spectroscopy (Py-GC-MS). It was found that the main component in the surface film formed in 1 mol dm-3 (M) LiClO4/PC has a chemical structure of ROCH(CH3)CH2OR′, of which -OR and -OR′ can be -OLi or -OCO2Li. A general scheme for reactions that took place on the lithium electrode in 1 M LiClO4/PC and in the pyrolyzer was elucidated from the results by Py-GC-MS analysis. In 1 M LiClO4/PC containing 5 wt % fluoroethylene carbonate (FEC) as an additive, FEC was dominantly reduced to form an active species, Li+-OCO2•, which further reacted with a PC molecule to form the same main component, ROCH(CH3)CH2OR′. This mechanism suggested that the same active species also might be formed from PC even in the absence of FEC. Another possible route was suggested, in which decomposition of PC is initiated by a nucleophilic attack of an alkoxide. The chemical composition of surface film was greatly affected by the kind of lithium salts. Decomposition products of lithium bis(perfluoroethylsulfonyl)imide (LiBETI) were detected from the surface film formed in 1 M LiBETI/PC. In contrast, the surface film formed in 1 M LiPF6/PC consisted mainly of inorganic compounds with a much smaller amount of organic compounds.

Introduction Lithium metal has a high energy density (3860 mAh g-1) and is expected to be used as a negative electrode in rechargeable lithium batteries. However, a lithium negative electrode usually shows a low cycling efficiency, which is caused by ceaseless reactions with electrolyte solutions and dendritic deposition of lithium metal. The latter dendritic deposition of lithium also causes serious safety issues. It is widely accepted that the surface of lithium metal is covered with a protective film in nonaqueous electrolyte solution, which is often called the solid electrolyte interface (SEI) and that the physical and chemical properties of the film greatly affect the morphology of deposited lithium.1,2 The authors have studied surface film formation on deposited lithium in propylene carbonate (PC) solutions by in situ atomic force microscopy (AFM)3-6 and found that the operation at elevated temperatures of 60 °C or higher greatly improves the cycling characteristics for * To whom correspondence should be addressed. E-mail: [email protected]. † Kyoto University. ‡ Kanto Denka Kogyo Co., Ltd.. § Doshisha University. (1) Peled, E. J. Electrochem. Soc. 1979, 126, 2047. (2) Thevenin, J. G.; Muller, R. H. J. Electrochem. Soc. 1987, 134, 273. (3) Mogi, R.; Inaba, M.; Abe, T.; Ogumi, Z. J. Power Sources 2001, 97/98, 265. (4) Mogi, R.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Electrochem. Soc. 2002, 149 A385. (5) Mogi, R.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Power Sources 2002, 108, 163. (6) Mogi, R.; Inaba, M.; Jeong, S.-K.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Electrochem. Soc. 2002, 149, A1578.

lithium deposition and dissolution.3-5 They also reported that fluoroethylene carbonate (FEC) is an effective additive to improve the cycling efficiency at room temperature.6 The results of AFM observation revealed that under these conditions the decomposition of electrolyte solutions is faster and gives a uniform surface film, which resulted in suppression of the dendritic deposition of lithium metal. It seems that the chemistry of the surface film formation plays a key role in the improvement of the cycling efficiency. The chemical compositions of the surface films formed on lithium metal have been extensively studied using a variety of analytical techniques, such as Fourier transform infrared spectroscopy (FT-IR),7-16 X-ray photoelectron spectroscopy (XPS),8,14,16-24 electrochemical quartz crystal (7) Aurbach, D.; Zaban, A. J. Electroanal. Chem. 1994, 365, 41. (8) Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E. J. Electrochem. Soc. 1987, 134, 1611. (9) Aurbach, D.; Gofer, Y. J. Electrochem. Soc. 1991, 138, 3529. (10) Aurbach, D.; Gofer, Y.; Ben-Zoin, M.; Aped, P. J. Electroanal. Chem. 1992, 339, 451. (11) Aurbach, D.; Ein-Ely, Y.; Zaban, A. J. Electrochem. Soc. 1994, 141, L1. (12) Aurbach, D.; Zaban, A.; Schechter, A.; Ein-Eli, Y.; Zinigrad, E.; Markovsky, B. J. Electrochem. Soc. 1995, 142, 2873. (13) Osaka, T.; Momma, T.; Matsumoto, Y.; Uchida, Y. J. Electrochem. Soc. 1997, 144, 1709. (14) Nazri, G.; Muller, R. H. J. Electrochem. Soc. 1985, 132, 2050. (15) Kominato, A.; Yasukawa, E.; Sato, N.; Ijuuin, T.; Asahina, H.; Mori, S. J. Power Sources 1997, 68, 471. (16) Morigaki, K.; Ohta, A. J. Power Sources 1998, 76, 159. (17) Kanamura, K.; Tamura, H.; Takehara, Z. J. Electroanal. Chem. 1992, 333, 127. (18) Shiraishi, S.; Kanamura, K.; Takehara, Z. J. Electrochem. Soc. 1999, 146, 1633. (19) Kanamura, K.; Shiraishi, S.; Takehara, Z. J. Electrochem. Soc. 1996, 143, 2187.

10.1021/la026299b CCC: $25.00 © 2003 American Chemical Society Published on Web 12/28/2002

Propylene Carbonate on Lithium Metal Surface

microbalance (EQCM),20,25-28 etc. In addition, the mechanisms for reductive decomposition of organic solvents have been recently discussed based on ab initio calculation.29-32 However, the results obtained in these studies are diverse and often controversial, and thus more detailed and comprehensive understanding of solvent decomposition and surface film formation processes on lithium metal is still needed in order to explore solvent systems suitable for lithium metal negative electrode. Our goal is to understand comprehensively the decomposition processes of electrolyte solutions on lithium metal and the compositions of the resulting surface films. In the present work, the authors analyzed the chemical composition of the surface films formed on deposited lithium in PC-based electrolytes by pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS). Py-GC-MS has been recently used for analyzing the compositions of the surface films formed on lithium metal15 and graphite negative electrodes,33-36 because it is a useful tool for identifying the chemical structures of organic compounds. The mechanisms for reductive decomposition of the solvent were discussed in detail with a help of the plentiful results reported so far in the literature. Experimental Section The electrolyte solutions used in the present study were 1 mol dm-3 (M) of lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), and lithium bis(perfluoroethylsulfonyl)imide [LiN(SO2C2F5)2, LiBETI] dissolved in PC. The solutions of 1 M LiClO4/PC and 1 M LiPF6/PC were purchased from Kishida Reagents Chemicals, and 1 M LiBETI/PC was prepared by dissolving LiBETI (3M) in pure PC (Kishida Reagents Chemcals). In some experiments, FEC (Kanto Denka Kogyo) was added as an additive by 5 wt % to 1 M LiClO4/PC. The water content of each solution was less than 30 ppm, which was measured with a Karl Fischer moisture titrator. The working electrode was a nickel plate, and the counter and reference electrodes were lithium foil. Lithium deposition and dissolution were carried out on the nickel electrode at 0.5 mA cm-2 in a three-electrode cell made of poly(tetrafluoroethylene). Lithium was deposited by 0.3 C cm-2 and dissolved to 1.5 V vs Li+/Li. After the fifth dissolution process, the cell was disassembled, and the nickel electrode was dried under vacuum at room temperature to remove the solvent. The sample was set at the sample holder in a pyrolyzer (GP-1028, (20) Naoi, K.; Mori, M.; Naruoka, Y.; Lamanna, W. M.; Atanasoski, R. J. Electrochem. Soc. 1999, 146, 462. (21) Schechter, A.; Aurbach, D. Langmuir 1999, 15, 3334. (22) Kanamura, K.; Shiraishi, S.; Tamura, H.; Takehara, Z. J. Electrochem. Soc. 1994, 141, 2379. (23) Kanamura, K.; Tamura, H.; Shiraishi, S.; Takehara, Z. J. Electroanal. Chem. 1995, 394, 49. (24) Ishikawa, M.; Machino, S.; Morita, M. J. Electroanal. Chem. 1999, 473, 279. (25) Aurbach, D.; Zaban, A. J. Electrochem. Soc. 1995, 142, L108. (26) Aurbach, D.; Moshkovich, M. J. Electrochem. Soc. 1998, 145, 2629. (27) Naoi, K.; Mori, M.; Shinagawa, Y. J. Electrochem. Soc. 1996, 143, 2517. (28) Aurbach, D.; Zaban, A. J. Electroanal. Chem. 1995, 393, 43. (29) Endo, E.; Ata, M.; Tanaka, K.; Sekai, K. J. Electrochem. Soc. 1998, 145, 3757. (30) Endo, E.; Tanaka, K.; Sekai, K. J. Electrochem. Soc. 2000, 147, 4029. (31) Li, T.; Balbuena, P. B. Chem. Phys. Lett. 2000, 317, 421. (32) Wang, Y.; Nakamura, S.; Ue, M.; Balbuena, P. B. J. Am. Chem. Soc. 2001, 123, 11708. (33) Ogumi, Z.; Sano, A.; Inaba, M.; Abe, T. J. Power Sources 2001, 97/98, 156. (34) Sasaki, T.; Inaba, M.; Abe, T.; Ogumi, Z. Extended Abstracts of 2001 Joint International Meeting; The Electrochemical Society: Pennington, NJ, 2001; Abs. No. 271. (35) Mori, S.; Asahina, H.; Suzuki, H.; Yonei, A.; Yokoto, K. J. Power Sources 1997, 68, 59. (36) Ota, H.; Sato, T.; Suzuki, H.; Usami, T. J. Power Sources 2001, 97/98, 107.

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Figure 1. Gas chromatograms of thermal decomposition products of the residual surface film formed on Ni after five cycles of Li deposition and dissolution in 1 M LiClO4/PC. The samples were pyrolyzed at 300 °C (a) without and (b) with exposure to air for 3 min. Yanaco). All of the treatments so far were carried out in an argon glovebox (MDB-1B, Miwa) with a dew point lower than -70 °C. The pyrolyzer was sealed and then transported to the GC-MS apparatus. After the heating zone located at a lower part of the pyrolyzer was heated to 300 °C, the sample was dropped to the heating zone. Gaseous products formed by pyrolysis were analyzed with a gas chromatograph (HP6890, Hewlett-Packard) equipped with a capillary column (PoraPLOT Q, HewlettPackard) and further identified with a mass spectrometer (JMS600W, JEOL). The ionization for MS was carried out by electron impact (EI), in which the voltage and the current for acceleration were set at 70 eV and 100 µA, respectively. A mass spectral search program (Version 1.5) developed by the National Institute of Standards and Technology (NIST), USA, was used for the help to identify the chemical structures of the detected compounds. The reactivity of PC with a nucleophile was investigated as follows: Lithium methoxide solution (ca. 10 wt %) was prepared by dissolving lithium metal in methanol (Wako Pure Chemicals). This solution (0.05 g) was added slowly to pure PC (2 g) with vigorous stirring. A white precipitate was formed immediately in the solution. The precipitate was filtrated and analyzed without drying by Py-GC-MS in a manner similar to that mentioned above.

Results and Discussion Propylene Carbonate. Panel a in Figure 1 shows a gas chromatogram of thermal decomposition products of the residual surface film formed on Ni in 1 M LiClO4/PC. The chemical structures of the decomposition products, which were identified by MS, are specified in Figure 1a. Only PC was detected when a nickel plate was just soaked in the solution; hence, the compounds except for PC in Figure 1a are attributed to the decomposition products of the surface film formed on the lithium metal during the deposition and dissolution cycles. Panel b in Figure 1 shows a gas chromatogram after the sample was exposed to air for 3 min. The peak of propylene glycol became larger, whereas the other peaks disappeared or were greatly suppressed. It is therefore reasonable to consider that the main component gives easily propylene glycol upon hydrolysis. Ota et al.36 also reported that propylene glycol was detected from the surface film formed on graphite negative electrode in PC-based electrolyte by temperature programmed desorption (TPD)-GC-MS. The main component of the surface film thus should have a chemical structure of ROCH(CH3)CH2OR′, of which -OR and -OR′

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groups can be -OLi, -OCO2Li, -O-alkyl, etc., that are stable against strongly reductive lithium metal. Different reductive decomposition mechanisms have been reported for cyclic carbonates by several researchers. Eggert and Heitbaum37 studied the decomposition of PC on a platinum electrode by differential electrochemical mass spectrometry (DEMS) and detected propylene at low potentials. From this fact, they proposed that the following reaction occurred on the electrode:

PC + 2Li+ + 2e- f Li2CO3 + C3H6v

(1)

This reaction, however, does not give the main component, ROCH(CH3)CH2OR′. Yoshida et al.38 analyzed gaseous products generated from graphite/LiCoO2 cells by GC. They detected carbon monoxide from a cell using 1 M LiPF6/ethylene carbonate (EC) as an electrolyte and proposed the formation of a lithium alkoxide, (CH2OLi)2. Though they used a graphite negative electrode, the above reaction also would take place on Li metal. From the similarity of the chemical structure, the formation of an alkoxide, LiOCH(CH3)CH2OLi may be possible in PC [eq 2], though there seems to be no report proposing this mechanism in the case of PC solution in the literature.

glycol formation reported by Rendek Jr. et al.39 as mentioned above, though both groups used similar techniques. The reaction temperature ranges were different between the two groups, which may have resulted in different reaction schemes. Propylene oxide, propanal, and acetone in Figure 1a seem to have the same origin. The formation of these three compounds formally can be regarded as dehydration of propylene glycol; however, it was confirmed by Py-GCMS that dehydration of pure propylene glycol does not occur at 300 °C. It is therefore reasonable to consider that the three compounds were formed by direct pyrolysis of the main component, ROCH(CH3)CH2OR′, rather than dehydration of propylene glycol. The intensities of the peaks for these compounds did not increase after exposure to air as shown in Figure 1b; hence, H2O was not involved in their formation process. Hence, the reactions for the formation of propylene oxide, propanal, and acetone can be described as follows:

PC + 2Li+ + 2e- f LiOCH(CH3)CH2OLi + COv (2) Rendek Jr. et al.39 investigated the decomposition of PC on Li under ultrahigh vacuum (UHV) conditions and confirmed the formation of propylene glycol by infrared reflection absorption spectroscopy (IRAS). Aurbach et al.8,10,12 analyzed a surface film formed on lithium metal in PC electrolyte by FT-IR and proposed a lithium alkyl carbonate, LiOCO2CH(CH3)CH2OCO2Li, which is formed as the main compound in the surface film.

PC + e- + Li+ f •CH(CH3)CH2OCO2Li

(3)

2 •CH(CH3)CH2OCO2Li f LiOCO2CH(CH3)CH2OCO2Li + C3H6v (4) In this reaction scheme, PC is reduced by one electron, and the PC ring is opened by cleavage of the CH(CH3)-O bond to form a Li salt of the resulting radical anion [eq 3]. Two of •CH(CH3)CH2OCO2Li then react to give a lithium alkyl carbonate, LiOCO2CH(CH3)CH2OCO2Li, as in eq 4, which they considered as the main component of the surface film.8,10,40 Furthermore, they examined the stability of LiOCH(CH3)CH2OLi on Li metal and showed that the alkoxide cannot work as a protective film.10 Zhuang et al.41-43 investigated the reactivity of PC with Li metal under ultrahigh vacuum conditions from 130 to 620 K and proposed that the ring opening occurred at the CH3CH-O bond upon reduction of PC, which is in agreement with Aurbach’s scheme. The result reported by Zhuang et al. seemed to be different from ethylene (37) Eggert, G.; Heitbaum, J. Electrochim. Acta 1986, 31, 1443. (38) Yoshida, H.; Fukunaga, T.; Hazama, T.; Terasaki, M.; Mizutani, M.; Yamachi, M. J. Power Sources 1997, 69, 311. (39) Rendek, L. J., Jr.; Chottiner, G. S.; Scherson, D. A. Langmuir 2001, 17, 849. (40) Aurbach, D.; Gottlieb, H. Electrochim. Acta 1989, 34, 141. (41) Zhuang, G.; Ross, P. N., Jr. J. Power Sources 2000, 89, 143. (42) Zhuang, G. R.; Wang, K.; Chen, Y.; Ross, P. N., Jr. J. Vac. Sci. Technol. 1998, A 16, 3041. (43) Zhuang, G.; Wang, K.; Ross, P. N., Jr. Surface Science 1997, 387, 199.

Propylene oxide is formed as in eq 5, which is a ring closure reaction with elimination of R+ and -OR′. Propanal and acetone may be formed through the mechanisms shown in eqs 6 and 7, respectively. A hydride shift is involved in eqs 6 and 7, which is similar to the pinacol rearrangement.44 It should be noted that the relative intensities among propylene oxide, propanal, and acetone changed after exposure to air as shown in Figure 1b. Hence, there should be another route to give these compounds as will be discussed later. Another possible way of elimination without a hydride shift can be described in eq 8. The resulting compound will be hydrolyzed to give allyl alcohol as in eq 9, if a trace amount of water is present in the pyrolyzer. In fact, a trace amount of allyl alcohol was detected in Figure 1a.

ROCH(CH3)CH2OR′ f ROH + CH2dCHCH2OR′ (8) CH2dCHCH2OR′ + H2O f CH2dCHCH2OH + HOR′ (9) 1-Propanol and 2-propanol were detected in Figure 1a, which indicates the ring opening of PC upon reduction. These alcohols are considered to be formed via the reactions in eqs 3 and 10-12.

•CH(CH3)CH2OCO2Li + •H f CH3CH2CH2OCO2Li (10) (44) March, J. Advanced Organic Chemistry, 4th ed.; Wiley: New York, 1992; pp 1058-1074.

Propylene Carbonate on Lithium Metal Surface

2 CH3CH2CH2OCO2Li + H2O f 2CH3CH2CH2OH + Li2CO3 + CO2v (11) PC + e- + Li+ f •CH2CH(CH3)OCO2Li (less stable) f CH3CH(CH3)OCO2Li f CH3CH(CH3)OH (12) PC is first reduced by one electron, and the PC ring is opened. When the CH3CH-O bond of PC is cleaved, a secondary radical species is formed as in eq 3. The resulting radical species traps •H [eq 10], which may be provided from a H2O contaminant or another PC molecule, to form a lithium alkyl carbonate. The lithium alkyl carbonate is hydrolyzed to give 1-propanol [eq 11] with a trace amount of water in the pyrolyzer. On the other hand, the CH2-O bond cleavage of PC gives 2-propanol via the formation of a primary radical species [eq 12]. Because secondary radicals are more stable than primary radicals, 1-propanol should be formed dominantly, which is in good agreement with the results in Figure 1a. Another possible radical termination was elimination of •H as in eqs 13 and 14. Of the resulting three products, CH3CHdCHOCO2Li and CH2dC(CH3)OCO2Li are hydrolyzed to propylene oxide, propanal, and acetone, and CH2dCHCH2OCO2Li is hydrolyzed to allyl alcohol.

•CH(CH3)CH2OCO2Li f CH2dCHCH2OCO2Li and/or CH3CHdCHOCO2Li + •H (13) •CH2CH(CH3)OCO2Li f CH2dC(CH3)OCO2Li + •H (14) The formation of methanol, acetaldehyde and ethanol in Figure 1a indicates the cleavage of the C-C bond of PC upon reduction, though the detailed mechanisms for the formation of these compounds are unclear. In addition, small peaks were detected between 20 and 40 min in Figure 1a; however, their chemical structures could not be identified from their fragmentations of MS. A general scheme for reactions that took place on lithium in 1 M LiClO4/PC and in the pyrolyzer at 300 °C discussed so far is summarized in Figure 2. On lithium metal, the ring of PC is opened by one-electron reduction to form a secondary or a primary radical (routes a and b). Most of the radical reacts with another radical to form ROCH(CH3)CH2OR′ (-OR ) -OR′ ) -OCO2Li). When a twoelectron reduction occurs, ROCH(CH3)CH2OR′ (-OR ) -OR′ ) -OLi) is formed by elimination of CO (route c). These two compounds are assumed as the main components in the surface film. In the pyrolyzer, ROCH(CH3)CH2OR′ is hydrolyzed to propylene glycol, or pyrolyzed to propylene oxide, propanal, and acetone. Allyl alcohol is also formed by pyrolysis in the presence of H2O. Part of the radicals traps •H or eliminates •H to be terminated and the resulting products also precipitate on the lithium metal. They are hydrolyzed to form their respective products (i.e., 1-propanol, allyl alcohol, 2-propanol, propylene oxide, propanal, and acetone) in the pyrolyzer. Route d involves the C-C bond cleavage of PC. In this route, ethanol, acetaldehyde, methanol, etc. are formed through complicated mechanisms. Effects of FEC Addition. The authors previously reported that the cycling efficiency for lithium deposition and dissolution in 1 M LiClO4/PC is greatly improved by the addition of FEC.6 The surface of deposited lithium was observed by in situ AFM, and it was found that the

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surface was uniformly covered with particle-like deposits of 100 nm in diameter. The resistance of the surface film formed in the presence of FEC was lower than that without FEC. It is clear that FEC plays a vital role in the chemistry for the surface film formation. Hence, the surface film formed in 1 M LiClO4/PC + FEC (5 wt %) was analyzed by Py-GC-MS in a similar manner. Figure 3 shows gas chromatograms of thermal decomposition products of the residual surface film formed on Ni in 1 M LiClO4/PC + FEC (5 wt %). Surprisingly, the results shown in Figure 3 were very similar to those in Figure 1. Propylene oxide, propanal, acetone, propylene glycol, and residual PC were detected in Figure 3a, and propylene glycol became the main product after the sample was exposed to air [Figure 3b]. From the similarity, it is obvious that the chemical structure of the main component in the surface film is again assigned as ROCH(CH3)CH2OR′. It should be noted that the compounds directly derived from FEC, e.g., HOCH2CHFOH and HOCH2CHO, which are formed by elimination of HF, were not detected in Figure 3a. The reason will be discussed later. The peaks for alcohols such as methanol, ethanol, 1-propanol, and 2-propanol were greatly suppressed in the presence of FEC in Figure 3a compared with those in Figure 1a without FEC. Propanol is formed by the cleavage of C-O bond and the ring opening of PC upon reduction [eqs 3 and 10-12], whereas the formation of methanol and ethanol involves the cleavage of the C-C bond, as mentioned in the previous section. The number of unidentified small peaks between 20 and 40 min, which are also attributable to PC-derived products formed through other complicated routes, also decreased in Figure 3a. At a glance, all of these facts imply that the addition of FEC suppresses the reductive decomposition and the ring opening of PC. This seems to be in conflict with the fact that a large amount of propylene glycol was detected after exposure to air in Figure 3b. That conflict can be resolved by assuming that FEC decomposes first and the resulting active species reacts with PC to give a PC-derived component of the surface film. In fact, acetaldehyde was detected in Figure 2a, which is evidence for one-electron reduction and the ring opening of FEC, because acetaldehyde can be formed from 1-fluoroethanol, which is analogous to the formation of 1- and 2-propanols from PC as in eqs 3 and 10-12. FEC is reduced by one electron to form two kinds of radical species depending on the direction of the ring opening:

FEC + e- + Li+ f •CHFCH2OCO2Li (less stable) f f (CH2FCH2OH) (15) FEC + e- + Li+ f •CH2CHFOCO2Li (more stable) f f CH3CHFOH f CH3CHO + HF (16) Part of the radicals is terminated by capturing •H to form lithium alkyl carbonates, which are hydrolyzed to form fluoro alcohols. Because the •CHF- radical should be unstable owing to a strong electron negativity of fluorine, the reaction in eq 15 would not proceed, and in fact, CH2FCH2OH was not detected in Figure 3a. In eq 16, CH3CHFOH is unstable, and hence, elimination of HF occurs easily to form acetaldehyde. Acetaldehyde was also detected in the absence of FEC as shown in Figure 1a, and it is formed by decomposition of PC via the C-C bond cleavage. In the presence of FEC, however, acetaldehyde formation from PC was ruled out, because methanol and

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Figure 2. General scheme for the reactions on lithium metal in 1 M LiClO4/PC and in the pyrolyzer heated at 300 °C.

Figure 3. Gas chromatograms of thermal decomposition products of the residual surface film formed on Ni after five cycles of Li deposition and dissolution in 1 M LiClO4/PC + FEC (5 wt. %). The samples were pyrolyzed at 300 °C (a) without and (b) with exposure to air for 3 min.

ethanol, which should be formed simultaneously via the C-C bond cleavage of PC, was not detected in Figure 3a. It should be noted that the formation of propylene oxide, propanal, and acetone was not suppressed so significantly as that of 1- and 2-propanols. This fact indicates that propanol was formed through a mechanism independent of the formation of the former three compounds, as mentioned earlier. Because 1- and 2-propanols are formed from the radical species as in eqs 3 and 10-12, propylene oxide, propanal, and acetone should be formed by mainly pyrolysis of ROCH(CH3)CH2OR′, rather than by hydrolysis of the radical anion species. The energy of the lowest unoccupied molecular orbital (LUMO) of FEC should be lower than that of PC, because the fluorine atom has strong electron negativity. The authors reported that FEC is reduced at 1.1 V on a highly oriented pyrolytic graphite, which is more positive than a potential (about 0.9 V) at which PC decomposes, being accompanied by vigorous exfoliation of the graphite

layers.45 It is therefore reasonable to think that the reduction of FEC is faster than that of PC. The effects of the FEC additive on the decomposition processes are summarized in Figure 4. In the presence of FEC, the reduction and the ring opening of FEC occur dominantly on lithium metal, by which an active species is formed from FEC. The active species reacts with PC to form the main component, ROCH(CH3)CH2OR′, of the surface film. The formation of acetaldehyde in Figure 2a gives evidence for the ring opening of FEC, as mentioned earlier. Because no compounds directly derived from FEC were detected in Figure 3a, the most probable active species is Li+-OCO2•, which is formed by elimination of fluoroethylene from a one-electron reduction product of FEC. The active species would merely react with another FEC molecule, because the solution contained only 5 wt % FEC; hence, most of the active species attacks a PC molecule. The final product is therefore assumed to be LiOCO2CH(CH3)CH2OCO2Li. The formation of the active species, Li+-OCO2•, from PC is also possible through the reductive ring opening followed by elimination of propylene as shown in Figure 4. This route also gives LiOCO2CH(CH3)CH2OCO2Li as the final product and may explain the formation of LiOCO2CH(CH3)CH2OCO2Li from the one-electron reduction products of PC in Figure 2, instead of eq 4. In the presence of FEC, Li+-OCO2• formation is accelerated, because FEC is reduced more rapidly than PC. This rapid decomposition of FEC is one of the reasons for the improvement of the cycling efficiency for lithium deposition and dissolution by addition of FEC, because the surface film is easily selfrepaired when being damaged and thereby the formation of dendritic lithium metal should be suppressed.3-5 Decomposition of PC by a Nucleophile. Aurbach et al.46 reported that a lithium alkyl carbonate was formed by a reaction between EC and lithium tert-butoxide, which is a nucleophile. In their proposal, two consecutive nucleophilic attacks of the alkoxide at both CH2 groups of EC result in the formation of an active species, LiOCO2-, which reacts with another EC to form the lithium alkyl (45) Jeong, S.-K.; Inaba, M.; Mogi, R.; Iriyama, Y.; Abe, T.; Ogumi, Z. Langmuir 2001, 17, 8281. (46) Aurbach, D.; Levi, M. D.; Levi, E.; Schechter, A. J. Phys. Chem. B 1997, 101, 2195.

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Figure 4. General scheme for the reactions on lithium metal in 1 M LiClO4/PC + FEC (5 wt. %).

Figure 5. Gas chromatogram of thermal decomposition products of the white solid formed by the addition of 10% CH3OLi/ CH3OH to pure PC. The sample was pyrolyzed at 300 °C.

carbonate. On the other hand, Yoshida et al.38 reported that trans-esterification occurs on graphite negative electrodes in EC-based mixed solvent systems. For example, they detected ethylene glycol diethyl dicarboxylate (C2H5OCO2CH2CH2OCO2C2H5) in 1 M LiPF6/EC + diethyl carbonate (DEC) (1:1) after storage at the fully charged state. Sasaki et al.47 investigated the mechanism for these trans-esterification reactions and found that addition of nucleophiles such as lithium alkoxides to EC + DEC or EC + dimethyl carbonate (DMC) caused transesterification products. It seems that there is another route for the formation of ROCH(CH3)CH2OR′ that is similar to these trans-esterification reactions upon reduction of PC. The authors thus investigated the products formed by a reaction between PC and lithium methoxide to clarify whether such a route is possible or not. White precipitates were obtained by adding 10 wt % lithium methoxide solution to pure PC. A chromatogram for the thermal decomposition products of the precipitates is shown in Figure 5. Ethylene oxide, propanal, acetone, DMC, and propylene glycol were detected in addition to residual methanol and PC. Some peaks could not be identified with the search program. The decomposition processes of PC in the presence of lithium methoxide are assumed as shown in Figure 6. The nucleophile can attack both the carbonyl carbon (a) and the alkylene carbons (b (47) Sasaki, T.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. Extended Abstracts of the 42nd Battery Symposium in Japan; The Committee of Battery Technology, The Electrochemical Society of Japan, 2001; pp 318-319.

and c) of PC. Yoshino et al. reported in their synthetic studies using EC that a nucleophile formed from polyhydroxy compounds attacked the carbonyl carbon of EC,48 whereas the one formed from substituted phenols attacked either of the alkylene carbons.49 Also, in the synthesis of polycarbonates by the ring-opening polymerization of EC or PC, the resulting copolymers consist of carbonate units (caused by the carbonyl attack) and oxide units (caused by the alkylene attack and subsequent decarboxylation).50-53 In the present case, when a methoxide anion attacks the carbonyl carbon of PC, the ring opening proceeds via route a in Figure 6. The trans-esterification reactions reported by Yoshida et al.38 and Sasaki et al.47 are probably initiated in a similar manner. If the resulting products, A and B, are further attacked by the methoxide anion, DMC and a lithium salt (C) will be formed. Compound C is hydrolyzed to propylene glycol. Compounds A and B are pyrolyzed to propylene oxide, propanal, and acetone, or hydrolyzed to compounds D and E. The two peaks at 26-27 min in Figure 5 are tentatively assigned to compounds D and E from their fragmentation patterns. Because the mass spectra of these compounds are not included in the database of NIST, and their authentic samples are not commercially available, it was not confirmed whether the assignments are correct or not. In contrast to these, when the alkylene carbons are attacked by the methoxide anion, the ring is opened via route b or c to form compounds F or G, respectively. Another methoxide anion attack to either F or G will lead to the formation of compound H. This route was the same as the mechanism proposed for the lithium tert-butoxide nucleophile by Aurbach’s group mentioned earlier.46 Compounds F and G would be hydrolyzed to compounds I and J, respectively, with a trace amount of water. However, compounds H, I, and J were not detected in Figure 5, and thereby decomposition via routes b and c may be minor even if pos(48) Komura, H.; Yoshino, T.; Ishido, Y. Bull. Chem. Soc. Jpn. 1973, 46, 550. (49) Yoshino, T.; Inaba, S.; Ishido, Y. Bull. Chem. Soc. Jpn. 1973, 46, 553. (50) Soga, K.; Hosada, S.; Tazuke, Y.; Ikeda, S. J. Polym. Sci. Polym. Lett. Ed. 1976, 14, 161. (51) Soga, K.; Tazuke, Y.; Hosoda, S.; Ikeda, S. J. Polym. Sci. 1977, 15, 219. (52) Storey, R. F.; Hoffman, D. C. Macromolecules 1992, 25, 5369. (53) Lee, J. C.; Litt, M. H. Macromolecules 2000, 33, 1618.

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Mogi et al.

Figure 6. General scheme for the reactions between a nucleophile, CH3O-, and PC in the solution and in the pyrolyzer heated at 300 °C.

sible. Lithium methoxide was used as a nucleophile in the present study, whereas Aurbach et al. used lithium tertbutoxide in ref 46. The nucleophile, tert-butoxide anion, is bulky and probably cannot attack the carbonyl carbon because of steric hindrance, though the carbonyl attack is kinetically favorable.52,53 It can be concluded that a route via nucleophilic decomposition will be possible, if nucleophiles such as alkoxide anions are formed by reductive decomposition of solvent molecules. However, the compounds detected by Py-GC-MS in Figure 5 were very similar to those formed via the routes in Figure 2, and it is hence difficult to specify the most dominant route. The route via nucleophilic decomposition may play an important role in surface film formation in mixed solvent systems such as PC + DEC and PC + DMC. Influence of Lithium Salts. It is well-known that the composition of surface film on lithium is greatly influenced by the kind of lithium salts.12,17,20,23 Py-GC-MS analysis was carried out in a similar manner using lithium salts such as LiBETI and LiPF6 dissolved in PC. Panel a in Figure 7 shows a gas chromatogram for the pyrolyzed products at 300 °C of a surface film formed in 1 M LiBETI/ PC. The result was generally similar to those obtained for 1 M LiClO4/PC shown in Figure 1a; that is, propylene oxide, propanal, acetone, and propylene glycol were the main detected compounds. They should be pyrolyzed or hydrolyzed products of the surface film component, ROCH(CH3)CH2OR′, as discussed in earlier sections. Propylene glycol was the main product after the sample was exposed to air [Figure 7b]. The peak for 1-propanol was detected, but its intensity was weak. In addition, the peak for 2-propanol was not visible. These facts suggest that the ring opening of PC was suppressed in 1 M LiBETI/PC. The peaks for acetaldehyde and ethanol, which indicate the degree of the C-C bond cleavage, were also suppressed when being compared with those in Figure 1a. Furthermore, the small peaks between 20 and 40 min were diminished. All of these facts indicate that decomposition of PC was suppressed when LiBETI was used as an electrolyte salt. It should be noted that no compounds derived from LiBETI were detected at 300 °C in panels a and b in Figure

Figure 7. Gas chromatograms of thermal decomposition products of the residual surface film formed on Ni after five cycles of Li deposition and dissolution in 1 M LiBETI/PC. The samples were pyrolyzed at (a and b) 300 and (c) 500 °C (a and c) without and (b) with exposure to air for 3 min.

7. To investigate more closely the influence of LiBETI, pyrolysis was carried out at 500 °C, and a chromatogram obtained is shown in Figure 7c. In addition to propylene oxide, propanal, acetone, and propylene glycol, many new peaks were detected. A peak at about 8 min is assigned to SO2, and other peaks designated by black circles showed fragments containing CF units [m/e ) 50 (CF2+•), 69 (CF3+•), 100 (C2F4+•), 119 (C2F5+•), etc.] in their mass spectra. When the solution of 1 M LiBETI/PC was pyrolyzed at 500 °C, PC and its decomposition products, such as propylene oxide, propanal, acetone, etc., were

Propylene Carbonate on Lithium Metal Surface

Figure 8. Gas chromatogram of thermal decomposition products of the residual surface film formed on Ni after five cycles of Li deposition and dissolution in 1 M LiPF6/PC. The sample was pyrolyzed at 300 °C without exposure to air.

mainly detected. A small peak of SO2 was also seen, but the peaks containing CF units were scarcely detected. Hence the peaks designated by black circles are assigned to pyrolyzed products of the surface film, rather than LiBETI itself. The reductive decomposition of LiBETI would have participated in surface film formation. Naoi et al.20 reported that the surface film formed in 1 M LiBETI/ PC contained LiF. They considered that the F- anion of LiF was provided from BETI-, and that residual compounds such as (CF2dCFSO2)2N- and polymers containing CF units may also have existed in the surface film. Figure 8 shows the results for a surface film formed in 1 M LiPF6/PC. Propanal and residual PC were detected, and peaks for other compounds were negligibly small or invisible in Figure 8. Furthermore, propylene glycol was not detected even after the sample was exposed to air (not shown). The residual film was almost dried under vacuum at room temperature, and its color was white after drying, whereas the films formed in the LiClO4 or LiBETI solutions seemed wet and gray. It seems that the composition of the surface film formed in 1 M LiPF6 is quite different from those obtained in the other electrolyte solutions. Kanamura et al.23 have been studied the composition of the surface film formed on lithium metal by XPS and reported that surface films formed in LiPF6/PC and LiPF6/ γ-butyrolactone consisted of inner Li2O and outer LiF layers. They assumed that a trace amount of HF in these solutions reacted with inorganic film components to form the LiF layer. Ishikawa et al.24 also reported the existence of LiF in a surface film formed on lithium in LiPF6/PC + DMC by XPS. Aurbach et al.28 proposed in their FT-IR study that a lithium alkyl carbonate is formed upon immersion of lithium metal in 1 M LiPF6/PC, which further reacts with the HF contaminant to form LiF and LixPOyFz. In contrast to these, Morigaki et al.54 reported that a small (54) Morigaki, K.; Fujii, T.; Ohta, A. Denki Kagaku (presently Electrochemistry) 1998, 66, 824.

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amount of lithium alkyl carbonate was present on Cu electrodes polarized at low potentials in 1 M LiPF6/EC + DMC (1:1) and EC + DEC (1:1). Similar results in 1 M LiPF6/EC + DMC were also reported by Kominato et al.15 Generally speaking, inorganic compounds such as LiF and LixPOyFz have been detected from in surface films formed in LiPF6 solutions by XPS, EQCM, Auger electron spectroscopy, ion chromatography, and atomic absorption spectroscopy, whereas the presence of organic compounds such as lithium alkyl carbonates have been often reported in studies using FT-IR and Py-GC-MS. As shown in Figure 8, smaller amounts of organic compounds were detected in the case of LiPF6/PC. It can be therefore concluded that the solvent decomposition was suppressed in electrolyte solutions containing LiPF6, and inorganic compounds such as LiF and LixPOyFz became predominant compared with organic components. Conclusions Py-GC-MS analysis was used to clarify the compositions of surface films on lithium metal in different PCbased solutions and to elucidate the decomposition processes in surface film formation. It was found that the main component in the surface film formed in 1 M LiClO4/ PC had a chemical structure of ROCH(CH3)CH2OR′, of which -OR and -OR′ can be -OLi or -OCO2Li. A general scheme for reactions that took place on the lithium electrode in 1 M LiClO4/PC and in the pyrolyzer was obtained from the results by Py-GC-MS analysis. In 1 M LiClO4/PC + FEC (5 wt %), FEC was dominantly reduced to form an active species, Li+-OCO2•, which further reacted with a PC molecule to form the same main component, ROCH(CH3)CH2OR′. This mechanism explained the improved cycling efficiency for lithium deposition and dissolution in 1 M LiClO4/PC + FEC, because surface film formation should be rapid in the presence of FEC. This mechanism also suggested that the same active species might be formed from PC even in the absence of FEC, though further investigation is needed. Another possible route initiated by a nucleophilic attack of an alkoxide was suggested. This route is probably minor in the solutions tested in the present study; however, it may play an important role in mixed solvent systems that contain linear carbonates as cosolvents. The chemical composition of surface film was greatly affected by the kind of lithium salts. Many unidentified compounds formed by decomposition of LiBETI were detected from the surface film formed in 1 M LiBETI/PC. In contrast, the surface film formed in 1 M LiPF6/PC consisted mainly of inorganic compounds with a much smaller amount of organic compounds. LA026299B