Langmuir 1985,1 , 469-477
469
Reactions of Clean Lithium Surfaces with SO2: Molecular Auger Line-Shape Analysis and Reaction Kinetics? Kenneth W. Nebesny and Neal R. Armstrong* University of Arizona, Department of Chemistry, Tucson, Arizona 85721 Received December 10, 1984 The products of the reaction of SO2with clean metal surfaceshave been examined using Auger electron spectroscopy. The Auger spectra were deconvolved of sample and instrument broadening effects, which allowed for detailed fingerprint spectra of several sulfur oxyanions to be distinguished. At SOz pressures less than 1 mtorr, complete dissociation of the SO2 molecule to form a monolayer of 2:l Li20 and Li2S is observed. At high SO2 pressures, an overlayer of Li2S204/Li2S203 is formed on top of the Li2S/Li20 layer. This growth behavior is apparently similar to the growth of multilayers of oxide on transition-metal surfaces. The rate of reaction of SO2with the clean Li metal (at low surface coverages) was measured for temperatures between 298 and 357 K. An activation energy of 2-5 kcal/mol was determined for the initial reaction to form the Li2S/Li20monolayer. A model for the growth and stability of the layer is presented.
Introduction The surface chemistry and electrochemistry of lithium used in ambient temperature and high-temperature battery environments is of considerable interest.’ It is recognized that the lithium electrode does not exist in a pristine, clean state upon first contact with the electrochemical environment. Lithium also does not remain passive after contact with this environment, building reaction layers at its surface which can vary in excess of microns in thickness.’kJ’ The electrochemical response of these lithium electrodes indicates that polarization of the lithium anode may occur in such environments.’* Previous surface analysis studies have been conducted post-mortem on reactions of lithium with its environment (gas phase or solution) and have concentrated upon layers that form well after the start of the r e a ~ t i o n . ~ ”These layers are therefore much thicker than the sampling depths of the surface analytical technique used to monitor their composition. In those cases where analytical techniques with larger sampling depths have been used, such as infrared spectroscopy, the interface between the lithium metal and the upper reaction layer has still not been probed? Our preliminary surface analysis/electrochemical studies suggested that the initial reaction layer between the bulk lithium metal and its surrounding environment is primarily responsible for its subsequent rea~tivity.~ It is necessary to go back to the clean lithium surface and to build, in a controlled fashion, toward the “real world” environment, by adding reaction products, one molecular layer at a time. We have also determined the energetics (rates) of reactions of small molecules with lithium in its pristine state, as a way of ensuring control of this surface composition in subsequent studies. One of the most popular of the lithium electrochemical systems currently under development is the Li/S02 battery. This system consists of a Li anode in contact with acetonitrile as the solvent, which has been overpressured with up to 4.5 atm of sulfur dioxide.’ The contact of the clean lithium surface with either the solvent or the SO2 should cause complete and immediate reaction. A semipassive film is formed on the lithium surface, however, which protects the lithium from its surrounding environ‘In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
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ment but which may also cause loss of electrochemical energy.l* In this study we removed the Li electrode from this electrochemicalenvironment and studied its reactions, with the strongest oxidizing agent in the system, SOz. Previous studies have demonstrated the viability of producing and maintaining an atomically clean Li surface for several minutes in an ultrahigh vacuum environment and subsequently reacting this Li surface with the small molecule of c h ~ i c e .This ~ ~ ~can be acnomplished by mechanically removing the passive surface layer that normally exists on Li, at the desired moment, to instantaneously form the clean, active surface. Determination of the molecular identity of surface species by Auger spectroscopy has been perfected in several laboratories as a means of complementing other molecular surface spectroscopic techniques.’ The core-hole excitation in an Auger event is localized to a single atom, and the subsequent relaxation and Auger electron-ejection event often involve valence electrons. The shape and energy distribution of an Auger spectrum can therefore provide useful molecular i n f ~ r m a t i o n . ~When the contribution of inelastic electron scattering events to the Auger spectrum are corrected, AES can be used to quantitate the (1) (a) Bro, P; Levy, S. C. Sandia Natl. Lab. [Tech.Rep.] SAND 1982, SAND-82-0785. (b) Watson, T. M. Proc. Power Sources Symp. 1978, 28th, 192. (c) Dey, A. N. Thin Solid Films 1977,43, 131. (d) Chua, D. L.; Merz, W. C. Proc. Power Sources Symp. 1976, 27th, 33. (e) Kilroy, W. P. Proc. Power Sources Symp. 1978,28th, 198. (f) Taylor, H.; Bowden, W.; Barrella, J. Zbid. 1978, Bth, 183. (9) Taylor, H.; McDonald, B. Proc. Power Sources Symp. 1976, 27th, 66. Selim, R.; Bro, P. J. Electrochem. SOC.1974,121, 1457. (i) James, S. D. J. Power Sources, 1983, 10, 105. 6 ) Peled, E. J. Electrochem. SOC.1979,126, 2047. (k) Dey, A. N.; Holmes, R. W. J.Electrochem. SOC.1979, 126, 1637. (1) Dallek, S., James, S. D. and Kilroy, W. P. J.Electrochem. Soc. 1981,128, 508. (m) DiMasi, G. J.; Christopulos,J. A. Roc. Power Sources Symp. 1978,28th, 179. (n) Dey, N.; Holmes, R. W. J. Electrochem. SOC.1980, 127, 1877. (0) Venkatasetty, H. V.; Saathoff, D. J.; Patel, B. K. “Roc.-Electrochem. SOC.1981,81-4. (p) Gardner, C. L.; Fouchard, D. T.; Fawcett, W. R. J. Electrochem. SOC.1981, 128, 2337. (2) David, J.;Froning, M. H.; Wittberg, T. N.; Moddeman, W. E. Appl. Surf. Sci. 1981, 7, 185-218. (3) Anderson, C. R.; Kilroy, W. P. Appl. Surf. Sci. 1972, 9, 1-4, 336. (4) Abraham, K. M.; Pitts, L. J. Electrochem. Soc. 1983, 130, 1618. (5) Nebesng, K. W.; Kaller, R. C.; Armstrong, N. R. J.Electrochem. Soc. 1982, 129, 2861. (6) Nebesny, K. W.; Armstrong, N. R. J. Vac. Sci. Technol.,submitted for publication. (7) (a) Rye, R. R.; Houston, J. E. Acc. Chem. Res. 1984, 17, 41. (b) Jennison, D. R. J. Vac. Sci. Technol. 1982,20,548. (c) Madden, H. H. J. Vac. Sci. Technol. 1981,18, 677; Surf. Sci. 1982, 126, 80. (d) Houston, J. E. In “Methods of Surface Characterization”;Powell, C. J., Madey, T. E., Yates, J. T., Czanderna, A. W., Hercules, D. M., Eds.; Plenum: New York, 1984, Vol. 1. (e) Campbell, C. T.; Rogers, J. W.; Hance, R. L.; White, J. M. Chem. Phys. Lett. 1980, 69,430. (0 Rogers, J. W.; Peebles, H. C.; Rye, R. R.; Houston, J. E.; Binkley, J. S. J. Chem. Phys. 1984,80, 4513.
This article not subject to U S . Copyright. Published 1985 by the American Chemical Society
Nebesny and Armstrong
470 Langmuir, Vol. 1, No. 4, 1985
relative concentration of the surface specie^.^,^ This inelastic background removal process has been computationally tedious in the past.' We have recently accelerated these deconvolution methods through the development of fast Fourier transform (FFT) deconvolution methods, which are used in these studies.1° The reaction of SO2with several different metal surfaces has been recently reviewed.l' The reaction of SOz on surfaces such as platinum and iron can occur dissociatively to either SO plus 0 or complete dissociation to form Sad* plus OadS.l3-l5 In the case of certain metals such as clean silver, SO2chemisorbs weakly as the intact species. Outka and Madix have shown that in the case where oxygen is present first on the surface, SO2 and the surface oxygen can react to form SOaab1l These surface studies provide an interesting contrast to those which we have conducted using the much more reducing, clean Li surface. The reaction of clean Li with sulfur dioxide, at low partial pressures, causes a complete dissociative chemisorption (irreversible) of SO2molecules. This dissociation forms a lithium sulfide/lithium oxide surface mixture which does not completely passivate the surface. The SO2-reactedsurface, along with its lithium oxide counterpart, continues t o react with SO2 at higher partial pressures to form a product layer of sulfur oxyanion salts with sulfur in higher oxidation states. Studies of the energetics of the initial dissociation reaction confirm the expected low activation energy for this dissociation.
Experimental Section The details of the formation and AES characterizationof clean Li surfaces have been described e l ~ e w h e r e . ~A~conventional ~*'~ Auger electron spectrometer (Physical Electronics, thin film analyzer) with a single-pass cylindrical mirror analyzer (CMA) with a resolution (AEIE)of 0.6%, ion pumped to base pressures of 2 X torr, was coupled to another UHV reaction chamber, where the SO2exposures took place. The base pressures in the analysis chamber and the reaction chamber were low enough that transfer of a clean lithium surface could be carried out from one chamber to another or held in one chamber for up to 20 min, with buildup of less than monolayer quantities of oxygen and carbon6 This was more than enough time to complete an experiment. In a typical experiment, a 1cm2,2-3-mm thickness of lithium foil (Alfa) was mechanically scraped to remove its passive layer in an argon glovebox. The residual oxygen and water immediately formed a new, thinner passive layer. This lithium foil was placed on a special transfer stub and mounted in the UHV reaction chambere6After base pressure was obtained, the lithium surface was brought into contact with a mechanical scraper in the reaction chamber which removed this top passive layer. SO2 exposures were initiated at this time by introducing the gas through a precision leak valve and monitoring the SO2partial pressure with a quadrupole mass analyzer (QMA,Anavac, VG-Scientific Inc.). When the rates of reaction with SO2were to be measured, the passive Li surface was placed in the reaction cell and a constant partial pressure of SO2 intrGducedas a flowing stream (constant introduction rate, constant pumping speed). This SO2 was exhausted into the tubomolecular pump (Balzers,TSP-170)which evacuated downstream from the sample in the SO2 flow line. The partial pressure of SO2was digitally recorded as a function of time (8) Burrell, M. C.;Armstrong, N. R Appl. Surf. Sci. 1983, 17, 53. (9) Burrell, M. C.; Armstrong, N. R J. Vac. Sci. Technol., A 1983, 1, 1831. (IO) Nebesny, K. W.; Armstrong, N. R. J . Electron. Spectrosc. Relat. Phenom., submitted. (11) Outka, D.; Madix, R. J.; Surf. Sci. 1984, 137, 242. (12) Brundle, C. R.; Corley, A. F. Faraday Discuss. Chem. SOC.1975, 60,51. (13) Ku, R. C.; Wymblatt, P. Appl. Surf. Sci. 1981, 8, 250. (14) Kohler, U.; Wassmuth, H.-W. Surf. Sci. 1982, 117, 668. (15) Furuyama, M.; Kishi, K.; Ideda, S. J. Electron. Spectrosc. Relat. Phenom. 1978,13,59. (IS) Nebesny, K. W. Ph.D. Dissertation, University of Arizona, Tucson, 1984
by a DEC 11/23 computer (details of the interface are in ref 6). The initiation of the reaction was obtained by a fast mechanical abraision of the passive surface "guillotinetechnique". Following any of these SO2 exposures, the excess reactant was pumped out of the chamber and the sample transferred under UHV conditions to the analysis vessel. Sulfur oxyanion compounds for the standard line shapes were obtained as follows: sodium sulfate (Na2S04),sodium sulfite (Na$03), and sodium thiosulfate (Na2S203)from Fisher Scientific (Fairlawn,NJ) and sodium dithionite (Na2S204)from J. T. Baker Co. (Phillipsburg, NJ). The samples were heated overnight as powders at 100 "C and allowed to cool in a desicator. They were then pressed into pellets at 8 metric tons and mounted directly into the AES analysis chamber. Commercial,group 1sulfides are extremely hydroscopic and no amount of heating and/or pumping was able to fully drive off the adsorbed water. Preliminary standard Auger spectra were obtained on nonhydroscopictransition-metal sulfides obtained from Alfa (Danvers, MA) and prepared as described above. Sodium sulfide (Na2S)was prepared in situ by electron beam reduction of Na2S04with a 30-pA electron beam current for 1 h." The resultant AES line shape produced by this method agreed exactly with the transition-metal sulfide line shapes. As a result, the in situ Na2S line shape was the one used in the spectral comparisons. Auger spectra were obtained with a primary electron excitation energy of 2 KeV, with beam currents of 0.5 pA or less (power density I mtllitorr a
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