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Langmuir 1999, 15, 1470-1479
The Reaction of Lithium with Dimethyl Carbonate and Diethyl Carbonate in Ultrahigh Vacuum Studied by X-ray Photoemission Spectroscopy Guorong Zhuang,† Yufeng Chen,‡ and Philip N. Ross, Jr.*,† Chemical Sciences Division and Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 Received April 21, 1998. In Final Form: October 30, 1998 The reaction of dimethyl carbonate (DMC) and diethyl carbonate (DEC) with clean metallic lithium in ultrahigh vacuum was studied by the use of X-ray photoelectron spectroscopy with the temperatureprogrammed reaction methodology. Both molecules are of interest as solvents in ambient-temperature lithium batteries. The solvent molecules were condensed onto the surface of an evaporated lithium film at 120 K, and spectra were collected as the sample was warmed in ca. 25 to 30-K increments. The reaction of either DMC or DEC with lithium was initiated at 180 K, a temperature much lower than their bulk melting temperatures, producing lithium methyl carbonate, methyllithium and lithium ethyl carbonate, and ethyllithium, respectively. At temperatures greater than 270-300 K, the lithium alkyl carbonates start to decompose with Li2O, elemental carbon, and alkyllithium as products on the surface. Both DMC and DEC are more reactive toward metallic Li than another carbonate solvent, propylene carbonate, which we have studied by the same methodology. Because methyl and ethyllithium are highly soluble in the parent solvent, electrodeposited Li is predicted to have poor stability in an electrolyte composed of either DMC or DEC.
1. Introduction This work is part of a continuing study of the reactions of lithium with nonaqueous solvents used in secondary lithium batteries under well-defined conditions, namely the conditions of an ultrahigh vacuum (UHV) providing truly clean lithium surfaces contacted by truly anhydrous solvents. We have previously published results1 of a study of the reactions of lithium with propylene carbonate (PC), a prototypical alkyl carbonate battery solvent, and tetrahydrofuran (THF), a prototypical ethereal solvent using photoelectron spectroscopy. In the present work, we report on a similar study of the reactions of lithium with two other alkyl carbonate solvents, dimethyl carbonate (DMC) and diethyl carbonate (DEC), and compare their surface chemistry with the previously studied cyclic alkyl carbonate, PC. DMC and DEC are technologically important solvents currently used as cosolvents in commercial Li ion batteries.2 The molecular configurations of DMC and DEC are shown in Figure 1, along with the labeling of carbon and oxygen atoms we use in this paper. DMC is a planar molecule with two inequivalent carbons, denoted C(1) and C(2,2′), and two inequivalent oxygens, denoted O(1) and O(2,2′). DEC has three inequivalent carbons, C(1), C(2,2′), and C(3,3′), and chemically identical oxygens as in DMC. DMC, DEC, and PC are esters of carbonic acid, hence the common name. According to classical organometallic chemistry, one might expect that these molecules, as esters of carbonic acid, would react with lithium to form lithium carbonate and alkyllithium. We show that in fact this is not the case, that all three carbonates react to form an alkyl carbonate, plus alkyllithium compounds in the case of DMC/DEC. * Corresponding author. Fax, 510-486-5530; E-mail, pnross@ csa1.lbl.gov. † Chemical Sciences Division. ‡ Materials Sciences Division. (1) Zhuang, G.; Wang, K.; Ross, P. Surf. Sci. 1997, 387, 199. (2) Guyomard, D.; Tarascon, J-M. J. Power Sources 1995, 54, 92.
In this study we use core-level X-ray photoemission spectroscopy (XPS) to elucidate the pathway of the reaction of DMC and DEC with a multilayer film of clean metallic lithium. The interpretation of the chemical shifts in the C 1s and O 1s binding energies was done with the aid of ab initio Hartree-Fock self-consistent calculations of the binding energies in model compounds/species. 2. Experimental Section All experiments were conducted in a custom-made UHV system described previously.1,3 The excitation photons were created by a dual-anode (Mg/Mg) X-ray source operating at 15 kV and 400 W for XPS. All spectra were acquired at 25-eV pass energy or a spectrometer resolution of about 0.4 eV. Ni(111) (99.995+%) single crystal was obtained from Monocrystal Company (Cleveland, OH) and polished with alumina powder to 0.05 µ, solvent cleaned, and brazed on to a molybdenum plate holder on the UHV manipulator. The crystal was cleaned by combination of Ar+ ion sputtering and thermal annealing to 950 K repeatedly until the contaminants, mainly carbon, sulfur and oxygen, were below the Auger detection limit. It had been demonstrated previously that no lithium diffusion into or alloying with nickel substrate occurs even at elevated temperatures up to 850 K in a vacuum.4 Therefore, Ni(111) is an ideal substrate for lithium deposition. Multilayer Li was vapor deposited on Ni(111) substrate at 140 K. Typically, a lithium film is made so thick that the Ni LMM Auger signal at a kinetic energy of 848 eV is totally attenuated, indicating the film thickness is greater than 30 nm. These thick films were produced by vapor deposition from a compact thermal lithium-beam source consisting of a solid Al-Li alloy with Li atomic concentration of about 9%.5 The source was operated between 820 and 870 K and essentially functioned like an effusion cell. Compared with evaporation from a Li dispenser (SEAS Getters, Colorado Springs, CO) we used previously, the Al-Li source provides large amounts of metallic Li free from oxygen and carbon contamination. The better (3) Wang, K.; Ross, P. Surf. Sci. 1996, 365, 753. (4) Wang, K.; Ross, P. J. Electrochem. Soc. 1995, 142, L95. (5) Schorn, R. P.; Hintz, E.; Musso, S.; Schweer, B. Rev. Sci. Instrum. 1989, 60, 3275; Norton, P. R.; Timsit, R. S. J. Vac. Sci. Technol. A 1994, 12, 3245.
10.1021/la980454y CCC: $18.00 © 1999 American Chemical Society Published on Web 01/07/1999
XPS Study of Li Reaction with DMC and DEC in UHV
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Figure 1. (top) 1-2 denote chemically different C and O atoms in the schematic molecular structure of DMC. (A, middle) calculated binding energies for each carbon atom in DMC by the Hartree-Fock self-consistent field (SCF) method, the bar graph only reflects the binding energy position not intensities; (A, bottom) experimental C 1s spectrum for a thin condensed layer of DMC on Li along with fitted individual component peak areas; (B, middle) calculated binding energies for each oxygen atom in DMC by the HartreeFock SCF method, the bar graph only reflects the binding energy position not intensities; (B, bottom) experimental O 1s spectrum for a thin condensed layer of DMC on Li along with fitted individual component peak areas. stability, reproducibility, and low maintenance of the source was especially suitable for making multilayer Li films in this work. Electron-beam excited Auger electron spectroscopy (AES) was used to characterize the cleanness of the Ni(111) substrate and the lithium film and to estimate thickness of the lithium film. High-purity (99.98%, 350 K) 5. Discussion
Figure 8. The peak area of O 1s (upper panel) and C 1s (lower panel) measured against temperature as the evaporated Li surface exposed to DEC at 130 K was heated. The inset in the lower panel gives the atomic ratio of C to O present on the surface (the area ratios corrected by their relative sensitivity factors).
mately the same binding energy, 288 ( 0.5 eV. The binding energies of the O(2,2′) oxygens are no longer degenerate. The O 1s peak at 533 eV is characteristic of the O(2,2′) bonding with Li, whereas a binding energy near 535 (( 0.5) eV is characteristic of both the O(1) oxygens in both unreacted DEC and the lithium ethyl carbonate. This reaction occurs to a greater extent with increasing temperatures, as shown by the plot of C 1s and O 1s peak intensities as a function of temperature in Figure 7. The intensities of the two C 1s peaks affected by the reaction with lithium, the C(1) peak at 293 eV and the C(2,2′) peak at 290 eV, decreased monotonically, whereas the new states characteristic of ethyllithium and lithium alkyl carbonate at 292 and 288 eV increased concomitantly. The intensities in the O 1s peak for the O(2,2′) oxygens in unreacted DEC decreased monotonically whereas the O 1s peak at 533 eV for the new state of oxygen in lithium ethyl carbonate increased concomitantly. The intensity of the O 1s peak at 535 eV remains approximately constant in this temperature regime because it is the sum of the O(1) oxygen in unreacted DEC and the lithium alkyl carbonate. All the DEC is decomposed by this pathway at about 300 K, as evidenced by the disappearance of the original C(1) peak at 293.5 eV. Above 300 K, both the lithium alkyl carbonate and the ethyllithium begin to decompose, as evidenced by the decrease in the C 1s intensity at 288 eV characteristic of the ethyllithium and the C 1s peak at 292 eV and the O 1s peak at 533 eV characteristic of lithium alkyl carbonate. Like DMC, the total C/O stoichiometries calculated from the total integrated areas of both C 1s and O 1s peaks, remain constant at about 1.6 (( 0.1) up to ca. 350 K, which suggests that
It is of interest to compare the reactions of DMC and DEC with Li to those of PC1 from both Li organometallic surface chemistry and lithium battery perspectives. All three solvents are esters of carbonic acid, and all have a similar first stage of reaction with Li, cleavage of one C-O bond to form a lithium alkyl carbonate. In PC, however, because of its molecular structure, cleavage of one C-O bond does not create an alkyllithium compound as a coproduct, but probably LiH, as represented by the reaction,
CH3CHCH2O2CO + Li f CH2dCHCH2O2COLi + LiH The temperature at which the reaction is initiated differs considerably both in absolute temperature and the temperature to the melting point. In both DMC and DEC, the reaction is initiated at a temperature (ca. 180 K) well below the (bulk) melting points (277 and 229 K, respectively), whereas for PC the reaction is initiated only at its bulk melting temperature and about 40 K higher in absolute temperature. By this measure, DMC and DEC are more reactive solvents than PC. In all three cases, the lithium alkyl carbonate is thermally stable up to about 270-300 K. Above this temperature region the alkyl carbonate undergoes a complex series of decomposition reactions producing hydrocarbon gases such as methane, and ultimately leaving a surface layer composed of elemental carbon and Li2O, the thermodynamically favored products of any CxHyOz compound in the presence of excess Li.1 The implications with respect to Li batteries are limited but interesting. The reactions we observed at 270-300 K in principle would reflect the same chemistry at a freshly deposited Li surface in an ideal electrolyte containing these solvents, i.e., no impurities such as water and a Li salt with an unreactive anion. Lithium alkyl carbonates generally are insoluble in carbonate solvents,12 so that this product of the reaction common to all three solvents would form a protective film, i.e., prevent the corrosion of freshly deposited Li by continuous reaction with solvent. In the case of DEC and DMC, however, the alkyllithium coproducts are highly soluble in most solvents because of their ionicity,13 and because it is 50% of the reaction product, our results would indicate that DEC
XPS Study of Li Reaction with DMC and DEC in UHV
and DMC are (relatively) corrosive solvents for Li. Indeed, Aurbach et al.12 reported that metallic Li is highly soluble in DEC and suggested two pathways (2b and 2c in their notation) for the dissolution,
2b: (CH3CH2)2CO3 + Li f CH3CH2OCO- + CH3CH2OLi 2c:
f CH3CH2- + CH3CH2CO3Li
Our results are fully consistent with reaction path 2c as given by Aurbach et al.12 We predict that the analogous reaction path applies to DMC as well, and that metallic Li is not stable in this solvent. (12) Aurbach, D.; Daroux, M.; Faguy, P.; Yeager, E. J. Electrochem. Soc. 1987, 134, 1611. (13) Streitwieser, A.; Bachrach, S.; Dorigo, A.; Schleyer, P. v. R. In Lithium Chemistry: A Theoretical and Experimental Overview; Sapse, A.-M.; Schleyer, P. v. R., Eds.; John Wiley and Sons: New York, 1995; pp 1-44.
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6. Conclusions The reaction of DMC and DEC with lithium was initiated at 180 K, a temperature much lower than their bulk melting temperatures, producing lithium methyl carbonate, methyllithium and lithium ethyl carbonate, and ethyllithium, respectively. At temperatures above 270-300 K, the lithium alkyl carbonates start to decompose with Li2O, elemental carbon, and alkyllithium as products on the surface. Both DMC and DEC are more reactive toward metallic Li than propylene carbonate, another carbonate solvent we have studied by the same methodology. Because methyl and ethyllithium are highly soluble in the parent solvent, freshly electrodeposited Li is predicted to have poor stability in an electrolyte composed of pure DMC or pure DEC. Acknowledgment. This work was supported by the Office of Energy Research, Basic Energy Sciences, Chemical Sciences Division, of the US Department of Energy under contract no. DE-AC03-76SF00098. LA980454Y
