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Langmuir 2002, 18, 6554-6558
Reactivity of Linear Alkyl Carbonates toward Metallic Lithium: Infrared Reflection-Absorption Spectroscopic Studies in Ultrahigh Vacuum Louis J. Rendek, Jr.,† Gary S. Chottiner,‡ and Daniel A. Scherson*,† Departments of Chemistry and Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106 Received January 7, 2002 The reactivity of symmetric and asymmetric alkyl linear carbonates of the type ROCO2R′, where R and R′ represent methyl or ethyl groups, toward metallic Li was studied by infrared reflection-absorption spectroscopy (IRAS) in ultrahigh vacuum. Comparison of the IRAS spectra of clean Li surfaces exposed to dimethyl, diethyl (DEC), and ethyl-methyl carbonates (EMC) with those obtained in very similar experiments involving judiciously selected alcohols provided unambiguous evidence that the products of such reactions are primarily Li alkoxides (ROLi and R′OLi). Spectral evidence was also found for the presence of Li ethyl carbonate for experiments involving DEC and EMC. No changes in the IRAS spectra could be discerned upon exposure of RCO2R′-reacted Li surfaces to large doses of CO2 at ca. 300 K. Furthermore, predosing Li surfaces with CO2 at 300 K to form small amounts of Li2CO3 prior to exposure to ROCO2R′ did not alter the nature of the reaction products. The conclusions emerging from this study support by and large the mechanism proposed by Aurbach et al. for reactions between Li and condensed phase linear and cyclic alkyl carbonates.
Introduction Linear alkyl carbonates, either as mixtures or in combination with their cyclic alkyl analogues, particularly ethylene carbonate, have emerged as a very promising class of solvents for lithium battery applications. Key to the operation of both metallic Li and Li+-intercalated carbon electrodes in such devices is the so-called solid electrolyte interface (SEI).1 This term refers to the layer formed by the spontaneous reaction of these highly reactive electrodes with atmospheric gases present as impurities in the solution and/or the solvent and electrolyte salts, which isolates both physically and electronically the electrode beneath from the solution but allows facile migration of Li+. Not surprisingly, much effort has been devoted to the study of the structure and properties of these layers, including, in addition to electrochemical techniques, a growing number of in situ and ex situ spectroscopic probes.1 Our group has been mainly concerned with the development and implementation of electron- and photon-based methods for the identification of reaction products of Li films vapor-deposited on clean, nominally inert supports and Ar+ sputter-cleaned Li foils exposed to vapor phase nonaqueous solvents, including selected alcohols, in ultrahigh vacuum (UHV).2-4 This paper examines the reactivity of clean Li thick foils and thin Li films supported on Ni(poly) toward dimethyl, diethyl, and ethyl-methyl carbonates by infrared reflection-absorption spectroscopy (IRAS) in UHV, using procedures and instrumentation developed in our research group,3,4 and complements an analogous investigation reported recently involving propylene carbonate (PC).2 † ‡
Department of Chemistry. Department of Physics.
(1) Aurbach, D. In Nonaqueous Electrochemistry; Aurbach, D., Ed.; Marcel Dekker: New York, 1999; Chapter 6 and references therein. (2) Rendek, L. J.; Chottiner, G. S.; Scherson, D. A. Langmuir 2001, 17, 849. (3) Zhuang, G. R.; Chottiner, G.; Scherson, D. A. J. Phys. Chem. 1995, 99, 7009. (4) Wang, K.; Chottiner, G. S.; Scherson, D. A. J. Phys. Chem. 1993, 97, 11075.
Figure 1. Schematic diagram of the UHV chamber for IRAS measurements.
On the basis of the results obtained, a rather general reaction mechanism for the Li metal induced decomposition of both linear and cyclic carbonates has been deduced, which may be of relevance to SEIs formed in electrochemical environments. Experimental Section IRAS measurements were performed in a custom-designed UHV chamber shown schematically in Figure 1. This system is equipped with a hemispherical electron energy analyzer (VG Scientific CLAM100) for X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), employing a twin (Mg and Al) anode X-ray source (VG Scientific model MK II) and a Varian (model 981-2145) electron gun, respectively. The unit also has low-energy electron diffraction (LEED) optics (Varian model 981-2148) for single-crystal work, a quadrupole mass spectrometer (Ametek-Dycor M106M) for the analysis of residual gases and temperature-programmed desorption (TPD) studies,
10.1021/la0200212 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/26/2002
Reactivity of Alkyl Carbonates toward Metallic Li and an Ar+ sputtering gun (Varian, model 981-2046). As indicated in Figure 1, a collimated p-polarized IR beam emerging through one of the side ports of a Mattson Cygnus 25 FTIR spectrometer is focused onto the specimen at an angle of incidence of 69° through a flat flange-mounted ZnSe window, using a coated, plano-convex KBr lens (Spectral Systems 945-3862, focal length of 10 cm). An identical ZnSe/KBr window/lens assembly is used to direct the beam, after reflection from the sample, onto an off-axis parabolic mirror (Janostech A8037-262), which refocuses the beam onto the active element of a HgCdTe (MCT) IR detector (EG&G Judson model J15D). The detector housing and spectrometer bench are enclosed and purged with dry nitrogen gas to remove spectral background contributions due to atmospheric gases, mostly water vapor and carbon dioxide. Two types of Li specimens were utilized in these studies: (a) A coin-shaped disk (ca. 1 mm thick) obtained by cutting a half-inch diameter Li rod (99.95% pure, Alfa Aesar) normal to its main axis under liquid n-hexane in a high-quality glovebox (Vacuum Atmospheres), following the procedure described by David et al.5 This sample was then pressed between two glass slides to create an optically smooth surface. While still under the inert atmosphere of the glovebox, the specimen was mounted on a transferable UHV holder attached to a portable UHV transfer arm equipped with a magnetically coupled linear manipulator and a gate valve to isolate the specimen from the atmosphere. This sample transfer arm was then removed from the glovebox and mounted to a sample introduction chamber. After the introduction chamber was turbo-pumped to high vacuum, the isolating gate valve was opened and.the sample was transferred to the main UHV chamber XYZ manipulator. The surface impurities (mostly oxygen and carbon) were removed through extensive Ar+ sputtering (ca.100 h), and the sample cleanliness was inspected with Auger electron spectroscopy. Once this procedure was completed, only a few hours were required to restore the clean surface after gas exposures. These Li foils were transferred to UHV after the system had been baked so as not to subject the Li foil to main chamber bake temperatures of ca. 200 °C. (b) Metal Li films vapor-deposited on clean polycrystalline Ni foil substrates by resistive heating of a 2 mm thick Li-Al alloy foil (Alcoa 2090, 2.6 wt % Li) to a temperature of about 845 K. Prior to the actual evaporation, the Li-Al alloy was outgassed at ca. 760 K, yielding only H2 as the major species detected by the mass spectrometer, and the Ni foils were cleaned by a series of Ar+ sputtering/thermal annealing cycles, followed by characterization with AES. The performance of this Li source was found to be superior to that of commercial SAES Getters devices used in earlier studies in terms of long-term stability, low outgassing, and durability.6,7 The thickness of the Li films was estimated from AES based on the homogeneous attenuation model.8 A few milliliters of each of the compounds examined, that is, diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC) (EM Industries, Selectipur grade, 99+%;
