Langmuir 2001, 17, 849-851
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Reactivity of Lithium toward Propylene Carbonate: Infrared Reflection Absorption Spectroscopy Studies in Ultrahigh Vacuum Louis J. Rendek, Jr.,† Gary S. Chottiner,‡ and Daniel A. Scherson*,† Departments of Chemistry and Physics, Case Western Reserve University, Cleveland, Ohio 44106 Received September 8, 2000 The vibrational properties of vapor-deposited lithium films exposed to propylene carbonate (PC) vapors at room temperature have been examined by infrared reflection-absorption spectroscopy (IRAS) in ultrahigh vacuum (UHV) using a specially designed chamber. The sharply defined spectral features observed were found to be incompatible with those of either, lithium propoxide, lithium alkyl carbonate, lithium carbonate, or mixtures thereof. Instead, the IRAS spectrum of PC/Li/Ni was found to be identical to that obtained by exposing Li to 1,2-propanediol vapors under otherwise identical conditions and may thus be attributed to the corresponding alkoxide derivative.
Introduction Information regarding the reactivity of metallic lithium toward nonaqueous solvents, including polymeric materials, is expected to contribute to the further optimization of high energy density rechargeable lithium batteries. As has been well established, exposure of Li to the atmosphere and electrolyte solutions leads to the spontaneous formation of a passive film on the surface or solid electrolyte interface (SEI). This layer acts as a physical barrier that prevents the metal beneath from coming in contact with the solution and provides a medium for facile Li+ transport1 and, therefore, may be regarded as essential to the operation of the device. This paper examines the reactivity of clean lithium toward propylene carbonate (PC) in ultrahigh vacuum (UHV) using infrared reflection-absorption spectroscopy (IRAS) for the identification of reaction products. In addition to its high degree of specificity, IRAS provides a gentle means of probing the vibrational structure of interfacial films, without compromising their chemical and/or physical integrity. Hence, the interpretation of data obtained with this technique is devoid of complications derived from the use of high energy X-rays or electrons, as with X-ray photoelectron and Auger electron spectroscopies, (XPS and AES), which might change the intrinsic nature of the films. Experimental Section UHV-IRAS measurements were carried out in a customdesigned UHV chamber equipped with XPS, AES, LEED, TPD, and surface preparation capabilities to be described in detail in a forthcoming publication. IRAS spectra in UHV were recorded using a Mattson Cygnus 25 FTIR spectrometer with a focused p-polarized IR beam at an angle of incidence of 69°. More details regarding this specific optical arrangement will also be described elsewhere. Metallic Li films were vapor deposited onto clean polycrystalline Ni substrates by heating resistively a thick piece of a previously degassed Li-Al alloy foil to ca. 845 K.2,3 Freshly deposited Li films were exposed to propylene carbonate (PC) † ‡
Department of Chemistry, Case Western Reserve University. Department of Physics, Case Western Reserve University.
(1) See, for example: Aurbach, D. In Nonaqueous Electrochemistry; Aurbach, D., Ed.; Marcel Dekker: New York, 1999; Chapter 6 and references therein.
vapor introduced into the chamber through a multicapillary array placed directly in front of the Li/Ni surface. Before every experiment, liquid-phase PC was purified by multiple freeze/ pump/thaw cycles using an alcohol/dry ice bath, and the dosing lines were flushed with PC several times until the residual gas composition stabilized. A similar method was employed for the purification and dosing of 1,2-propanediol and n-propanol. The base and working pressures of the UHV chamber were on the order of 5 × 10-8 and 1 × 10-7 Pa, respectively. To avoid possible decomposition, all filaments were turned off during gas dosings.4 As is customary, all spectral data are given in terms of ∆R/R ) (Rref - Rsamp)/Rref vs wavenumber (cm-1), where Rref and Rsam refer to the coadded averaged single beam IRAS spectrum of the Li/Ni surface before (ref) and after (sam) exposure to the gas, respectively.
Results and Discussion The AES spectrum a Li layer ca. 2 nm thick on a Ni(poly) foil (not shown in this work) showed a well-defined peak centered at 52 eV characteristic of metallic Li with no other Li related features at lower energies. Following acquisition of the co-added and averaged IRAS reference spectrum, the specimen was exposed to PC vapors, while keeping the position of the surface strictly fixed, and after a short period of time, the series of sampling spectra were recorded. Since the primary objective of these experiments was to identify the final product of the reaction, the precise exposure was not determined. Instead, the PC dosing was extended until no further changes in the IRAS spectra, involving, in this case, fewer number of scans, could be discerned. The resulting ∆R/R spectrum of Li/Ni exposed to PC vapors shown in curve A, Figure 1, displayed features quite different than those of a rather thick layer of PC condensed at 120 K on bare, clean Ni recorded in separate experiments with the same instrument (curve B in Figure 1). These data indicate that PC vapor reacts with Li at room temperature, and that no intact PC can be found on the surface under these conditions. Further evidence is (2) Schorn, R. P.; Hintz, E.; Musso, S.; Schweer, B. Rev. Sci. Instrum. 1989, 60, 3275. (3) Esposto, F. J.; Griffiths, K.; Norton, P. R. J. Vac. Sci. Technol. A. 1994, 12, 3245. (4) Wang, K.; Chottiner, G. S.; Scherson, D. A. J. Phys. Chem. 1993, 97, 11075.
10.1021/la001287g CCC: $20.00 © 2001 American Chemical Society Published on Web 01/06/2001
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Figure 1. ∆R/R IRAS spectrum: curve A, Li/Ni exposed to PC vapors; curve B, a rather thick layer of PC condensed at 120 K on bare clean Ni; curve C, Ni(poly) exposed to PC; curve D, Li/Ni exposed to n-propanol, curve E, Li/Ni exposed to 1,2propanediol. See text for details.
support of this view is provided by the AES spectra recorded immediately after acquisition of the IRAS data had been completed, for which the signal at 52 eV due to metallic Li (not shown in this work) disappeared. Also prominent in that AES spectrum were peaks associated with carbon and oxygen derived from the reaction products of Li and PC. It must be emphasized that room-temperature exposure of PC to clean Ni yielded an AES spectrum characteristic of clean Ni and a flat featureless ∆R/R IRAS spectra (see curve C, Figure 1), pointing to the inertness of PC toward clean Ni under these conditions. Ross and co-workers have recently reported the results of experiments in which the composition of a rather thick layer of PC condensed on the surface of clean Li at cryogenic temperatures in UHV was monitored by XPS as the temperature of the specimen was raised in stages to 620 K.5 On the basis of their results and quantum calculations, these authors concluded that the features observed for the reacted layer at ca. 300 K correspond to a mixture of lithium propoxide, lithium propyl carbonate, and Li2CO3. The spectral features observed in curve C, Figure 1, however, are not compatible with the presence of any of these species on the surface. Specifically, the following points can be made. i. The IR spectral properties of lithium propoxide were determined by exposing freshly deposited Li film to clean n-propanol, a species which is known to react with Li, at least under more conventional reaction conditions, to generate the desired product. As shown in curve D, Figure 1, the IRAS spectrum of lithium propoxide thus produced exhibits sharp features at energies different than those observed for PC exposed to Li under otherwise identical conditions (curve A, Figure 1). ii. The CO2 symmetric and asymmetric stretching peaks at 1350 and 1650 cm-1 believed to be characteristic of alkyl carbonate-type species6 are prominently absent in the IRAS spectrum of Li/PC (curve C, Figure 1). Furthermore, there was no indication in the XPS spectrum of the PC/Li reacted film recorded after the IRAS measurements had been completed for C(1s) and O(1s) XPS features at high binding energies, ascribed by Ross et al. to alkyl carbonate type of species.5 (5) Zhuang, G.; Wang, K.; Ross, P., Jr. Surf. Sci. 1997, 387, 199. (6) Behrendt, V. W.; Gattow, G.; Drager, M. Z. Anorg. Allg. Chem. 1973, 897, 237.
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iii. The IRAS of Li2CO3 synthesized in UHV by dosing 500 L of CO2 on a vapor-deposited metallic Li film yielded, as expected, features at 875 and 1530 cm-1 in agreement with those reported in the literature for the bulk material.7 These features are clearly absent in the spectrum of Li/Ni exposed to PC, indicating the lack of Li2CO3 in the film produced. Aurbach et al. have suggested, based on experiments performed with liquid solutions at room temperature,8 that the reaction of PC and Li produces primarily a Li alkyl carbonate of formula: CH3CH(CO3Li)CH2CO3Li. Attempts were made to synthesize this specific species by first exposing clean Li to 1,2-propanediol (PD) at room temperature and then to carbon dioxide, while monitoring the surface composition by IRAS. As shown in curve E, Figure 1 (displayed inverted for clarity), the IRAS of PD/ Li was virtually identical to that observed for PC/Li, providing evidence that the reaction product detected by IRAS is the same for both cases and attributed to the corresponding alkoxide, i.e., CH3CH(OLi)-CH2OLi. Further exposure of the reacted film to carbon dioxide, however, failed to reveal any changes in the IRAS, indicating that at room temperature the expected alkyl carbonate is not formed. The same behavior was found upon exposure of Li propoxide to up to 500 L of CO2, which yielded a flat, featureless difference spectrum, using the spectrum of lithium propoxide as a reference. This effect may not be surprising, as previous temperature-programmed desorption experiments performed in our laboratory showed that in the case of n-butanol, the corresponding alkyl carbonate prepared in UHV at cryogenic temperatures undergoes decomposition releasing CO2 at an onset temperature well below 300 K.9 On this basis, it thus seems reasonable to assume that CH3CH(CO3Li)CH2CO3Li would form upon exposure of PC to metallic Li in UHV and decompose immediately at room-temperature, liberating CO2 to yield CH3CH(OLi)-CH2OLi as the final product. It must be stressed at the outset that although species such Li2CO3 have a sufficiently large IR cross section to be identified even at low concentrations, other materials that may be present in the film, such as Li2O, are non-IR active in the region examined and thus would not be detected with this technique. As will be shown in a subsequent publication, the decomposition pathway found for PC appears to be also followed by both symmetric and asymmetric linear alkyl carbonates. In summary, the IRAS experiments reported in this work indicate that the reaction products of Li films exposed to vapor PC in UHV at room temperature are not compatible with the presence of lithium propoxide, a lithium alkyl carbonate or Li2CO3 on the surface, as has been proposed based on XPS for layers of PC first condensed at cryogenic temperatures onto Li films and then brought to room temperature in stages. Instead, the IRAS features of PC/Li are identical to those found upon exposure of clean Li to 1,2-propanediol to form the corresponding lithium alkoxide derivative. It is not clear at this time, whether the markedly different composition of films formed at room temperature and at low temperature via condensation may be ascribed to drastic modifications in reactivity caused by the reaction conditions, or to possible artifacts induced by long-term exposure of condensed films to X-rays, as has been demonstrated in the case of films of tetrahydrofuran exposed to Li by (7) Brooker, M. H.; Bates, J. B. J. Chem. Phys. 1971, 54, 4788. (8) Aurbach, D.; Gottlieb, H. Electrochim. Acta 1989, 34, 141. (9) Zhuang, G.; Chottiner, G. S.; Scherson, D. A. J. Phys. Chem. 1995, 99, 7009.
Reactivity of Lithium toward Propylene Carbonate
temperature-programmed desorption.4 Efforts to resolve these issues are currently in progress and will be reported in due course. Acknowledgment. This work was supported by a grant from the US Department of Energy, Office of Basic Energy Sciences. Additional funding was provided by NEDO, Japan. Enlightening discussions with Prof. Doron
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Aurbach of Bar-Ilan University, Israel, are gratefully acknowledged. The authors may also want to express their appreciation to Mr. George Totir and Mr. Nelson Yee for their help in the design and construction of the Li/Al source and to Mr. Roberto Rioja of Alcoa for providing samples of the alloy. LA001287G
