3334
Langmuir 1999, 15, 3334-3342
X-ray Photoelectron Spectroscopy Study of Surface Films Formed on Li Electrodes Freshly Prepared in Alkyl Carbonate Solutions Alex Schechter and Doron Aurbach* Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
Hagay Cohen Chemical Service Unit of the Weizmann Institute of Science, Rehovoth 76100, Israel Received August 18, 1998. In Final Form: January 12, 1999 Li samples were freshly prepared (shearing) and stored (2 days) in dimethyl carbonate (DMC), and ethyl carbonate-diethyl carbonate (EC-DEC), and dry (20 ppm of H2O) and wet (500 ppm of H2O) EC-DMC solutions of LiAsF6 (1 M), and were then studied by X-ray photoelectron spectroscopy (XPS). The XPS analysis, including depth profiling of these surface films, appears to be reliable on the qualitative level only, because both the X-ray beam and sputtering should be suspected as being partially destructive to the surface films on lithium. These studies basically confirm previous conclusions obtained by Fourier transform infrared spectroscopy spectroscopic studies of Li surfaces. Surface films formed on Li in alkyl carbonate solutions of LiAsF6 are comprised of ROCO2Li, Li2CO3, LiF, LixAsFy, and Li oxides. XPS could also detect surface species with Li-C bonds (e.g., LiCH2CH2OCO2Li). When EC is present, its reduction dominates the surface film formation. The presence of water suppresses both solvent and salt anion reduction and enriches the surface films with Li2CO3 (because of secondary reactions of water with surface species), LiOH, and Li2O. These studies also confirm that the surface films formed on Li have a multilayer structure.
Introduction It is commonly known that the performance of rechargeable Li batteries containing Li metal anodes largely depends on the surface chemistry of the Li electrode in the batteries’ electrolyte solutions.1 From a thermodynamic point of view, Li is highly reactive with any polar aprotic solvent and salt anion relevant to Li battery systems.2 An apparent stability of this active metal in polar electrolyte solutions is obtained through its passivation by surface films.3 These surface films, which are comprised of insoluble Li salts (reduction products of solution species), are usually electronically insulating (when reaching a certain critical thickness), but Li-ion conducting.4 The properties of the surface layers that cover the Li electrodes determine their performance as anodes in rechargeable batteries. For instance, the more uniform their composition and structure, the more uniform the Li deposition processes. When Li deposition-dissolution processes are uniform, the corrosion of the active metal upon cycling can be largely prevented, thus maintaining a sufficiently high Li cycling efficiency, making it suitable for rechargeable battery application. Because of the importance of this issue, extensive efforts have been devoted so far to studying the composition of * To whom correspondence should be addressed. (1) Dominey, L. A. In Lithium Batteries. New Materials, Developments and Perspectives; Pistoia, G., Ed.; Elsevier: Amsterdam, New York, London, Tokyo, 1994; Chapter 4, p 160. (2) Imanishi, N.; Ohashi, S.; Ichicawa, Y.; Takeda, Y.; Yamamoto, O. J. of Power Sources 1992, 39, 185. (3) Yeager, E. B. In Proceedings of the Workshop on Lithium Nonaqueous Battery Electrochemistry; Yeager, E. B., Schumm, B., Jr., Blomgren, G., Blankenship, D. R., Leger, V., Akridge, I., Eds.; The Electrochemical Society, Inc. Softbound Series PV 80-7; The Electrochemical Society: Pennington, NJ, 1980; p 1. (4) Peled, E. In Lithium Batteries; Gabano, J. P., Ed.; Academic Press: London, 1983; Chapter 3, p 43.
surface films formed on lithium electrodes in a large variety of electrolyte solutions.5-10 A review of the relevant literature shows that the most important tools for these studies are Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The former technique enables the identification of functional groups of the various surface species formed,11 while the latter provides an elemental analysis of the surface films, with specific information on the oxidation states of the elements.12 In addition, having sputtering capabilities (usually by Ar+ ions), XPS can provide depth profiling of the sample studied, yielding information on the threedimensional structure of surface films.13 Nevertheless, as already discussed in previous papers,14 XPS alone cannot provide specific enough information that enables a completed chemical analysis of surface species. A simultaneous study of surface films by both FTIR spectroscopy and XPS may be much more conclusive than using each technique (5) Kanamura, K.; Tamura, H.; Shiraishi, S.; Takehara Z.-I. Electrochim. Acta 1995, 40, 913. (6) Kanamura, K.; Tamura, H.; Takehara Z.-I. J. Electroanal. Chem. 1992, 333, 127. (7) Froment, M.; Garreau, M.; Thevenin, J.; Warin, D. J. Micros. Spectrosc. Electron. 1979, 4, 483. (8) Koch, V. R.; Goldman, J. L.; Natwing, D. L. J. Electrochem. Soc. 1982, 129, 1; Koch, V. R.; Young, R. H. J. Electrochem. Soc. 1978, 125, 1371. (9) Aurbach, D.; Zaban, A.; Gofer, Y.; Ein-Eli, Y.; Weissman, I.; Chusid, O.; Abramson, O.; Markovsky, B. J. Power Sources 1995, 54, 78. (10) Aurbach, D.; Zaban, A.; Ein-Eli, Y.; Weissman, I.; Chusid O.; Markovsky, B.; Levi, M. D.; Levi, E.; Schechter, A.; Granot, E. J. Power Sources 1997, 68, 91. (11) Aurbach, D.; Daroux, M. L.; Faguy, P.; Yeager, E. B. J. Electrochem. Soc. 1987, 134, 1611; Aurbach, D.; Daroux, M. L.; Faguy, P.; Yeager, E. B. J. Electrochem. Soc. 1988, 135, 1863. (12) Briggs, D.; Seah, M. P. In Practical Surface Analysis, 2nd ed.; John Wiley & Sons: New York, London, 1990; Vol. 1. (13) Kanamura, K.; Tamura, H.; Shiraishi, S.; Takehara, Z.-I. J. Electroanal. Chem. 1995, 394, 49. (14) Aurbach, D.; Weissman, I.; Schechter, A.; Cohen, H. Langmuir 1996, 12, 3991.
10.1021/la981048h CCC: $18.00 © 1999 American Chemical Society Published on Web 03/31/1999
XPS of Films on Li Electrodes
Langmuir, Vol. 15, No. 9, 1999 3335
Figure 1. An illustration of possible interactions of active metal covered with native films with solutions species. Figure (a) Partial replacement of native surface species by solution reduction products (formation of mosaic-type surface films). (b) Reactions of nucleophilic native surface species (e.g., LiOH) with solvent molecules such as esters and alkyl carbonates, thus forming an additional layer.
Figure 2. An illustration of reactions of fresh active metal exposed to electrolyte solutions. Initially, all solution species react on the active surface at low selectivity. Thus, as the process proceeds, the surface reactions become more selective. This scenario leads to the formation of multilayer surface films.
alone.11,14 Despite the extensive efforts devoted to date to the study of the surface chemistry developed on lithium, because of the complicated structure of surface films formed on Li electrodes, there are still many unanswered questions. For instance, there is a vast difference if the Li is introduced into the solution while covered with native films or if it is exposed fresh to solution species. These two scenarios are illustrated in Figures 1 and 2. In addition, the unavoidable presence of trace water also complicates the situation since water hydrates the surface species (Li salts), diffuses through the surface films, and is reduced within the film. Figure 3 illustrates the dynamics of the Li-solution interphase which has a multilayer structure.14,15 Processes such as continuous reduction of solution species via electron tunneling through the surface films, diffusion of (15) Aurbach, D.; Zaban, A. J. Power Sources 1995, 54, 289.
water and its reduction, further reduction of surface species within the film, and dissolution-reprecipitation of solution species occur simultaneously. This paper reports on the study of Li electrodes, whose surfaces were freshly prepared in alkyl carbonate solutions, by XPS. We chose solutions which are very important for practical Li-battery applications and are largely used in Li-ion batteries.16 These included dimethyl carbonate (DMC), mixtures of DMC and ethylene carbonate (EC), and mixtures of EC and diethyl carbonate (DEC). The major points of interest were the effect of the solvents and solvent mixtures on the Li surface chemistry, as well as the effect of water. We chose LiAsF6 as the electrolyte because it can be purchased in a highly pure state and is much less contaminated by acidic species than salts such (16) Matsumura,Y.; Wong, S.; Mondori, J. J. Electrochem. Soc. 1995, 142, 2914.
3336 Langmuir, Vol. 15, No. 9, 1999
Figure 3. An illustration of the dynamics of surface reactions in surface films formed on Li in alkyl carbonate solutions.
as LiPF6 or LiBF4. On the other hand, the AsF6- anion is reactive on the Li surfaces, and thus, the intensity of its reduction on Li in the various solutions can serve as a good probe for comparison among the solvent reactivities. An important advantage of the present work over similar studies by other groups is that Li surfaces freshly prepared in the same solutions were also studied previously by FTIR spectroscopy.17-19 Thus, the data analysis was made in light of prerequisite knowledge on the complicated surface chemistry of these systems, coming from a different channel of information. Experimental Section We used DMC, EC-DMC (1:1), and EC-DEC (1:1) mixtures containing solvents from Tomiyama Co., and LiAsF6 was obtained from FMC Inc. All the Li samples were freshly prepared in solutions. Figure 4 presents a scheme of the Li surface preparation in solutions. The Li surfaces thus obtained are very flat. After storage for 2 days, the samples were washed (ultrapure 2MeTHF, Tomiyama Co.) and dried. Prior to the sample preparation for the spectroscopic studies, the glovebox atmosphere was circulated through a liquid nitrogen-cooled trap in order to eliminate any interference by atmospheric or volatile organic contaminants. The samples were transferred in sealed vessels to the XPS spectrometer and were introduced into it via a glovebag that was pumped and filled with highly pure argon several times before the sample seals were opened. The samples were loaded onto the system’s manipulator within a glovebag under a stream of highly pure argon (for more details see ref 14). (17) Aurbach, D.; Ein-Eli, Y.; Zaban, A. J. Electrochem. Soc. 1994, 141, L1. (18) Aurbach, D.; Zaban, A.; Schechter, A.; Ein-Eli, Y.; Zinigrad, E.; Markovsky, B. J. Electrochem. Soc. 1995, 142, 2873-2882. (19) Aurbach, D.; Markovsky, B.; Schechter, A.; Ein-Eli, Y.; Cohen, H. J. Electrochem. Soc. 1996, 143, 3809.
Schechter et al.
Figure 4. A scheme of the apparatus for in situ Li surface preparation in solutions: A Li rod is pressed toward two parallel stainless steel wires (0.5 mm thick). The rod is sheared, and thus, three Li bars (4 and 5) with flat surfaces are formed in solution. The middle one (5) was used for the surface analysis (after storage in solution). (1) A stainless steel flange. (2) A polypropylene press. (3) A Li rod. (4) Two Li bars formed with a semicircular cross-section (not used). (5) A rectangular shaped Li bar formed in the middle (whose surface was analyzed by XPS). (6) Two stainless steel wires. Measurements were performed on a Kratos AXIS-HS spectrometer, base pressure ) 10-9 Torr, using a monochromatized Al KR source. The X-ray beam intensity was kept at a relatively low value, 1-3 mA emission current, at 15K eV. The sputtering rate (Ar+ ions at 4000 eV) was estimated as 50 Å/min based on reference experiments with silicon wafers, as well as with standard Ta/Ta2O5 foils, whereas the X-ray beam has a penetration depth of 20-30 Å. Using the “Vision” software, deconvolution of the various peaks that comprise each element spectrum, as well as quantitative analysis of the percentage of each element at each oxidation state on the surface, were performed. Data analysis was based on a Shirley background subtraction and separation of the XPS bands into Gaussian-shaped peaks (particular peaks corresponding to the actual electron pass energies, as independently determined with various reference samples). Surface charging, occurring during the X-ray irradiation, was corrected by an electron flood gun, while final scale determination was done numerically using the F1s line of the LiF species (685 eV) as a reference.
Results and Discussion Typical XPS peaks obtained from the Li samples in their pristine form and after different sputtering periods are shown in Figure 5. F1s, O1s, C1s, Li1s, and As2p XPS peaks obtained from Li surfaces freshly prepared and stored in a EC-DEC/LiAsF6 1 M solution are presented. Figure 6, related to Li samples prepared and stored in a EC-DMC/500 ppm of H2O/LiAsF6 (1 M) solution, presents
XPS of Films on Li Electrodes
Figure 5. Typical F1s, O1s, C1s, Li1s, and As2p XPS peaks obtained from Li surfaces freshly prepared in solutions. Li sample was treated in a EC-DEC (1:1)/LiAsF6 (1 M) solution. The effect of sputtering is also shown, as indicated.
Figure 6. Typical F1s, O1s, C1s, and Li1s XPS bands and their deconvolution to peaks related to the various oxidation states of the elements. Li sample prepared and stored in a ECDMC (1:1)/LiAsF6 (1 M) solution containing 500 ppm of water, as an example.
typical XPS bands (F1s, O1s, C1s, and Li1s) and their deconvolution to peaks (as described above). In general, the carbon 1s spectra (before deconvolution) of unsputtered electrodes show two groups of superimposed peaks around 290 and 286 eV. Upon sputtering, a third peak around 283 eV appears. The oxygen 1s spectra are characterized by two groups of superimposed peaks around 532 and 529 eV. The fluorine 1s spectra are characterized by a major peak around 685 eV, and the lithium 1s spectra are a broad superposition of peaks in the 52-58 eV range. Table 1 provides a partial identification of XPS peaks, based on previous work.5-7,11-14,20-21 A first stage of the present study involved the determination of reliability, accuracy, and reproducibility of the measurements. The stability of the samples under the X-ray radiation was found to be a serious problem. Table 2 provides elemental analysis of Li surfaces freshly prepared and stored in DMC and dry and wet (500 ppm of H2O) EC-DMC (1:1) solutions of LiAsF6 (1 M). The Li sample treated in the DMC solution was measured twice at the same spot, while the lithium samples treated in the EC-DMC solutions were measured once at two different
Langmuir, Vol. 15, No. 9, 1999 3337
locations. These samples were measured in their pristine state with no sputtering. The data in this table demonstrate the role of beam-induced damage: Consecutive measurements at a fixed spot yielded significant variations in the elemental analyses. Moreover, this was also the case for data obtained from the same sample at two different spots. We attribute these discrepancies to the fact that Li substrates are highly reactive and thus, with the energy push of the X-ray beam, some decomposition of surface species can occur in the time scale of the measurements. Hence, the composition of the surface species may change during the measurements. However, as further demonstrated, despite their limitations (which always have to be taken into account), these XPS measurements provide some valuable surface analysis, especially when coupled with other less destructive techniques such as FTIR spectroscopy. Table 3 provides elemental analyses of Li samples freshly prepared and stored in LiAsF6 solutions of DMC, EC-DEC, dry EC-DMC (20 ppm of H2O), and wet ECDMC (500 ppm of H2O). The experiments included Ar+ sputtering followed by XPS measurements. Each spectrum was treated by the deconvolution software to account for the different oxidation states of the elements, yielding a detailed depth profile of the various elements at their different oxidation states. Figure 7 shows the change in the total elemental concentration as a function of sputtering for the lithium surfaces prepared and stored in the four solutions (as indicated). Figures 8-10 compare the percentage of the different chemical shifts resolved for Li, C, and O. In each of these figures, the results obtained from Li surfaces prepared and stored in DMC, EC-DEC, dry EC-DMC (20 ppm of H2O), and wet EC-DMC (500 ppm of H2O) are compared (a-d). It should be emphasized that we estimate the accuracy of the quantitative data in Table 3 and Figures 7-10 as (5% (of the numbers appearing in Table 3). The following general trends characterize the data in Table 3 and Figures 7-10. The surface films formed on Li in the four solutions studied contain LiF (685 eV F peak) and fluorine-arsenic compounds (F1s peak at 687 eV and As2p peaks in the 1324-1332 range) resulting from salt anion reduction. We attributed the latter F and As2p peaks to species of the LixAsFy type.22 The spectra obtained from all the Li samples contain carbon peaks of 282-283, 285, 286-287, and 290-291 eV. The peaks at 285, 286-287, and 290-291 eV correlate well with the information obtained previously from FTIR spectroscopic measurements of Li electrodes treated in similar solutions. This finding suggests that the surface films on Li in alkyl carbonate solutions contain a mixture of ROCO2Li, ROLi, and Li2CO3 species, products of the solvent reduction. Scheme 1 provides the most probable reduction patterns of EC, DMC, DEC, H2O, and AsF6- on lithium surfaces, as obtained from previous FTIR spectroscopic studies.9-11,17-19,22 Thus, the C peak at 285 eV should be attributed to CH3OLi (reduction product of DMC),17,19 or to the methyl carbon in CH3CH2OLi (reduction product of DEC11,18). The C peak at 287 eV should be attributed to the methylene carbon in the major EC reduction product (CH2OCO2Li)2,17-19 and to the methyl carbon in CH3OCO2Li which is the major DMC reduction (20) Aurbach, D.; Daroux, M. L.; McDougal, G.; Yeager, E. B. J. Electroanal. Chem. 1993, 358, 63. (21) Carlson, T. A. In Photoelectron and Auger Spectroscopy; Plenum Press: New York, 1975; Appendix 3. (22) Chusid (Youngman), O.; Aurbach, D. J. Electrochem. Soc. 1993, 140, L1.
3338 Langmuir, Vol. 15, No. 9, 1999
Schechter et al.
Table 1. Assignments for Deconvoluted Peaks in Element Spectra, Obtained from Li Surfaces (Based on refs 7, 11, 12, 14, 20, and 21) carbon 1sa
peak (eV) 282-3 285 285 broad 285 285 285 286-287 286-287 290-291 290-291
peak (eV)
* LiCH2-R * CH3OLi * * LiCH2CH2OCO2Li * * (-CH2CH2-)n * CH3CH2OLi * CH3CH2OCO2Li * CH3OCO2Li * (CH2OCO2Li)2 * Li2CO3b * ROCO2Lib
528 529 529 531 532 533
oxygen 1sa
peak (eV)
* Li2O * LiOH * ROLi * Li2CO3 * ROCO2Li * ROCO2Li
fluorine 1s
peak (eV)
lithium 1s
peak (eV)
arsenic 2p Li3As
685
LiF
53
Li0
1320
687-8
LixAsFy
54
Li-C
1324
55
Li-O
1324-1332
56
LiF
As0 LixAsFy
a The element marked with an asterisk relates to the peak. b Note that the carbonate carbon peak of ROCO Li should appear at a slightly 2 higher eV than that of Li2CO3.
Table 2. Elemental Analysis of a Li Sample Measured Consecutively at the Same Spot (Li Treated in DMC/LiAsF6 Solution) and of Two Li Samples Measured in Two Different Spots (Treated in EC-DMC (1:1)/LiAsF6, Dry and Wet Solutions), No Sputtering atomic concentration % F1s solution DMC/LiAsF6 (1 M) first scan DMC/LiAsF6 (1 M) second scan EC-DMC (1:1)/LiAsF6 (1 M) dry (20 ppm of H2O) dry location A EC-DMC (1:1)/LiAsF6 (1 M) dry (20 ppm of H2O) dry location B EC-DMC (1:1)/LiAsF6 (1 M) wet (500 ppm of H2O) wet location A EC-DMC (1:1)/LiAsF6 (1 M) wet (500 ppm of H2O) wet location B
O1s
685 eV
687 eV
528 eV
531 eV
18.1
11.7
1.2
11.3
21.8
4.2
8.8
5.4
3.4
7.4
1.1
33.9
14.0
0.7
30.5
4.9
4.7
0.9
31.3
2.7
4.3
C1s 533 eV
282-3 eV
1.2
7.0
33.9
product. The 290-291 eV peak is assigned to the carbon bound to the three oxygen atoms in the carbonate species (both Li2CO3 and ROCO2Li).11,14 All the carbon spectra obtained after sputtering contain a 282-283 eV carbon peak. This peak, which reflects carbon at a very low oxidation state, can be attributed only to species containing C-Li bonds (Li carbides). Scheme 1 suggests a few probable paths for the formations of surface species containing Li-C bonds. Of particular interest is the possibility of forming LiCH2CH2OCO2Li species by two-electron EC reduction. A formation of such a species may explain the pronounced 285 eV carbon peaks in the spectra measured from Li surfaces prepared and stored in dry EC-DEC and EC-DMC solutions. Such a 285 eV peak can be a superposition of the carbon peak related to the methylene group bound to Li, and the carbon peak of the other methylene group bound to oxygen. The proximity of these two groups to each other should moderate their oxidation states, thus forming an overlap-
Li1s
285 eV
287 eV
290-1 eV
53 eV
14.4
1.5
2.4
7.8
12.8
0.8
56 eV
F1s
O1s
total C1s Li1s
31.2
29.8
12.5
18.3
39.
0.5
18.5
26.0
26.0
14.2
14.8
44.5
0.5
55 eV
As2p
18.9
11.4
2.3
3.2
17.9
10.8
35.0
30.3
23.4
0.5
13.2
8.1
5.4
7.0
11.9
8.8
14.0
31.3
26.7
27.7
0.3
11.1
14.5
10.4
10.6
16.3
9.6
38.3
24.9
26.9
0.3
4.1
10.0
6.5
26.3
7.0
34.8
25.2
32.8
0.2
ping band centered at 285 eV. It should be emphasized that FTIR spectroscopic studies of Li electrodes are not capable of detecting Li carbide species. Hence, information on the formation of surface RLi species on lithium is a unique contribution of the XPS measurements. Scheme 1 also raises the possibility that the radicals formed are somehow combined and polymerize (path 5). The presence of polyethylene on the Li surface could also be associated with the peak at 285 eV. The possibility that the surface films formed on Li and lithiated carbon electrodes in alkyl carbonate solutions contain polymeric species was raised (23) Genie`s, S.; Yazami, R.; Garden, J.; Frison, J. C. Synth. Met. 1998, 93, 77. (24) Thevenin, J. G.; Muller R. H. J. Electrochem. Soc. 1987, 134, 273. (25) Bar Tow, D.; Peled, E.; Burstein, L. In Proceedings of the Symposium on Batteries for Portable Applications and Electric Vehicles; Holmes, C. F., Landgrebe, A. R., Eds.; The Electrochemical Society, Inc. Softbound Series PV 97-18; The Electrochemical Society Inc.: Pennington, NJ, 1998; p 324.
0 32 93 274 635 1536 2437
0 32 93 274 635 1536
EC-DMC (1:1)/LiAsF6 (1 M) dryb EC-DMC (1:1)/LiAsF6 (1 M) dry EC-DMC (1:1)/LiAsF6 (1 M) dry EC-DMC (1:1)/LiAsF6 (1 M) dry EC-DMC (1:1)/LiAsF6 (1 M) dry EC-DMC (1:1)/LiAsF6 (1 M) dry EC-DMC (1:1)/LiAsF6 (1 M) dry
EC-DMC (1:1)/LiAsF6 (1 M) wetc EC-DMC (1:1)/LiAsF6 (1 M) wet EC-DMC (1:1)/LiAsF6 (1 M) wet EC-DMC (1:1)/LiAsF6 (1 M) wet EC-DMC (1:1)/LiAsF6 (1 M) wet EC-DMC (1:1)/LiAsF6 (1 M) wet 4.9 13.4 9.4 8.6 7.1 5.5
3.4 12.3 11.7 11.4 9.1 7.9 7.0
4.3 5.4 13.8 12.0 8.5 6.9 6.0
13.6
18.1 12.2 10.1
atomic concentration % C1s
4.8
7.4
10.6 7.2
9.3
11.7
4.9 8.4 15.1 17.7 19.6
1.1 4.2 10.1 12.3 13.2 15.6 18.5
1.0 0.4 9.7 13.2 19.4 21.7 20.4
1.2 17.8 17.2 17.5 18.8
31.3 32.9 24.2 19.1 15.4 12.1
28.6 25.4 17.5 16.0 12.6 10.7 10.1
30.5 26.7 19.9 14.1 6.2 5.0 5.1
11.3 5.8 5.5 6.1 5.5
7.0
5.3
1.8
1.6
0.8 1.6 1.4 1.1 2.2 2.3
3.9 3.1 2.4 1.6
1.3 1.5 1.4
1.1
11.2 9.2 6.4 7.0 5.1 4.7
1.0
11.9 8.9 3.5
14.4 0.9
14.5 4.7 4.1 1.6 1.6 1.9
18.9
3.8 7.9 6.6 3.1 2.3 2.1
1.5 1.8 2.3 2.3 1.4
10.4 6.2 4.7 3.9 3.3 2.0
11.4 6.6 3.5 2.3 2.1 2.2 1.3
6.0 9.2 3.1 2.5 0.9 1.2 0.1
2.4 0.4 1.0 0.7 0.1
528 285 286-7 290-1 eV eV eV eV eV 687 533a 282-3 eV eV eV Li2O, ROCO2Li, LiOR, CH3OLi, Li2CO3, As-F LiOH Li2CO3, LiOR ROCO2Li R-R′ ROCO2Li ROCO2Li LiC
531-2a
O1s
2.3 4.0 2.1 7.5 5.1 3.5 7.6
6.0 7.0 5.1 4.0 18.4 10.7 11.3
7.6
7.82
53 eV Li
56 eV LiF
F1s
10.6 15.1 19.0 31.3 48.1 50.1
16.3 9.6 38.3 24.9 22.1 13.4 37.8 10.9 29.9 9.4 32.6 8.7 18.4 8.6 34.2 7.1 5.3 7.1 33.2 6.1 6.9 5.5 31.7 5.7
3.2 17.9 10.8 35.0 30.3 17.7 17.5 12.3 29.7 18.5 29.7 14.1 11.7 27.6 14.3 33.9 8.4 11.4 28.3 10.0 49.6 9.1 25.8 10.2 52.5 7.9 26.3 9.5 48.3 7.0 28.6 8.3
26.9 37.2 48.9 49.6 53.4 57.0
23.4 39.1 45.9 49.8 54.7 56.0 55.9
31.7 34.3 42.9 50.9 59.3 60.6 65.7
39.1 60.9 62.5 62.5 59.2
0.3 0.7 0.4 0.4 0.3 0.1
0.5 0.5 0.4 0.5 0.3 0.3 0.2
0.6 0.4 0.3 0.2 0.1
0.3
0.5 0.3 0.2 0.2 0.4
total C1s Li1s As2p 12.4 18.3 23.6 3.1 22.7 4.6 23.6 4.4 24.2 2.9
O1s
20.7 5.0 14.9 31.5 21.6 17.3 9.9 12.6 27.1 26.0 23.6 14.2 13.8 29.6 13.2 29.1 17.8 12.0 27.2 9.5 31.3 9.7 8.5 25.6 6.3 43.7 6.2 6.9 26.7 5.7 44.8 9.7 6.0 25.5 2.7
31.2 29.7 56.3 4.6 12.2 49.1 13.4 10.1 47.5 15.0 9.3 31.9 19.7 13.6
55 eV OLi
Li1s
Separation among the oxygen peaks of the carbonates was very difficult. They usually appear as a broad O1s peak centered around 531-532 eV. b 20 ppm of H2O. c 500 ppm of H2O.
0 30 60 124 305 666 1567
EC-DEC (1:1))/LiAsF6 (1 M) EC-DEC (1:1)/LiAsF6 (1 M) EC-DEC (1:1)/LiAsF6 (1 M) EC-DEC (1:1)/LiAsF6 (1 M) EC-DEC (1:1)/LiAsF6 (1 M) EC-DEC (1:1)/LiAsF6 (1 M) EC-DEC (1:1)/LiAsF6 (1 M)
a
0 63 244 605 1598
DMC/LiAsF6 (1 M) DMC/LiAsF6 (1 M) DMC/LiAsF6 (1 M) DMC/LiAsF6 (1 M) DMC/LiAsF6 (1 M)
solution
sputtering time (s)
685 eV LiF
F1s
Table 3. Summary of Elemental Analysis of Li Surfaces Treated in the Four Solutions as a Function of Sputtering with Argon Ions (Relevant Functional Groups and Bonds for Each Deconvoluted Peak Are Also Presented [Based on refs 7, 11, 12, 14, 20, and 21)]
XPS of Films on Li Electrodes Langmuir, Vol. 15, No. 9, 1999 3339
3340 Langmuir, Vol. 15, No. 9, 1999
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Figure 7. Summary of the atomic concentration of Li, O, C, F, and As on Li surfaces freshly prepared and stored for 2 days in DMC, EC-DEC, and dry (20 ppm of H2O) and wet (500 ppm of H2O) EC-DMC solutions of LiAsF6 (as indicated) as a function of sputtering time.
Figure 8. The percentage of the various Li peaks (obtained by deconvolution of the spectra) from the total Li content on the Li surfaces prepared and stored in the four solutions as indicated, as a function of sputtering time (and depth). (a) DMC/LiAsF6 (1 M) solution. (b) EC-DEC (1:1)/LiAsF6 (1 M) solution. (c) EC-DMC (1:1)/LiAsF6 (1 M) solution. (d) EC-DMC (1:1)/LiAsF6 (1 M)/500 ppm of water (by volume).
by a number of authors.23-25 However, in our opinion this possibility is questionable because in order to obtain polymerization, a large concentration of radicals has to be formed simultaneously on the surface. In addition, polymerization has to compete with other possible terminations of these radicals, such as Li carbide formation (Scheme 1, path 6), hydrogen abstraction, or double-bond formation (to form ethylene, in the case of EC reduction). As expected, upon sputtering, the overall carbon and fluorine contents decrease while the Li content increases. As the surface species are closer to the Li-film interface, they should be at a lower oxidation state and thus, more lithiated (see Figure 3). It should be stressed that the
sputtering may induce chemical reactions such as the creation of Li carbide species (C1s 282-283 eV peak). The oxygen peaks also change as sputtering proceeds. In general, the O1s 531-532 eV peak decreases and the 528529 eV peak, which relates to species such as Li2O,5,6,13 increases. Interestingly, despite the prolonged sputtering, the C, F, O, and As peaks did not disappear. We estimate the sputtering rate to be close to 50 Å/min ((50%). Hence, on the basis of these experiments, the surface films formed on lithium in the solutions should be at least 2000 Å thick. (After 2000 s of sputtering we still obtained pronounced carbon fluorine and oxygen spectra). This thickness seems too high for surface films formed on
XPS of Films on Li Electrodes
Langmuir, Vol. 15, No. 9, 1999 3341
Figure 9. Same as Figure 8, carbon peaks.
Figure 10. Same as Figures 8 and 9, oxygen peaks.
smooth lithium by chemical reactions with no involvement of electrochemical processes. Hence, these results may indicate low sputtering efficiency. The strong ionic characteristics of the surface species may enhance redeposition of “sputtered” surface species. This further strengthens our conclusion that depth profiling of Li samples by sputtering, coupled by subsequent XPS measurements, is not simple to interpret. Yet, despite the limitation of the above measurements in providing quantitative depth profiling of the surface films on lithium, on the qualitative level XPS is obviously valuable. These studies clearly show that DMC is much less reactive than EC. (See the high percentage of fluorine on Li surfaces prepared in DMC solutions, as compared with Li surfaces prepared in EC-containing solutions (twice as high, Table 3). Hence, it appears that when EC is present,
its reduction process is dominant and, thus, its reduction products are major constituents in the surface films. Since the surface films on Li are formed by competing reactions of solution species, it is clear that EC reduction considerably suppresses the salt anion reduction (compared with the situation in a less reactive, acyclic alkyl carbonate such as DMC). Examining the difference in the F and C atomic concentrations on Li surfaces prepared in EC-DEC and EC-DMC solutions is also important and interesting. The F/C ratio related to EC-DEC is generally considerably higher than that related to a EC-DMC solution (Table 3). It is well-known that Li is not passivated in DEC because DEC reduction products (CH3CH2OLi and CH3CH2OCO2Li)11 are soluble in its solutions. In a EC-DEC solution, Li is passivated because of the formation of the
3342 Langmuir, Vol. 15, No. 9, 1999 Scheme 1. Reduction Patterns of Alkyl Carbonates, AsF6- and H2O on Li (Based on refs 5, 6, 9-11, 13-17-20, 22, and 26 and the Results of This Work)a
a This scheme does not describe possible termination patterns for the radical formed except for Li carbide-RLi formation (path 6) and possible formation of polymers (path 5). b These products are partially soluble in DEC solutions.
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and LiH. The high oxygen mole percentage on the Li surface in this case is in line with paths 7 and 8 (Scheme 1). The high percentage of the 290-291 eV carbon peak is explained by path 10 in Scheme 1. As suggested previously, and on the basis of the intensive studies of these systems by FTIR spectroscopy,11,17,22 an important role of water in these systems relates to its secondary reaction with the ROCO2Li species (formed by alkyl carbonate reduction) to form Li2CO3 (Scheme 1, path 10). Hence, the present results confirm our previous suggestion that one of the expected impacts of the presence of water in an alkyl carbonate solution is the intense formation of Li2CO3 surface species on Li and Li-C electrodes. It should be noted that Li2CO3 is one of the best passivation agents for both Li and lithiated graphite electrodes. Indeed, the presence of water increases the reversibility and cycling efficiency of both Li and Li-C anodes in alkyl carbonate solutions.27 Conclusion
EC reduction product (CH2OCO2Li)2, which is dianionic and, hence, insoluble in EC-DEC. However, these results prove that the role of EC reduction products in the surface film formation on Li in EC-DEC/LiAsF6 solutions is less dominant than in EC-DMC solutions. We assume that this is because (CH2OCO2Li)2 is more soluble in DEC solutions than in DMC solutions. Since it is believed that the most important Li passivating agents in dry EC based solutions are the EC reduction products (mostly (CH2OCO2Li)2), our results indicate that in EC-DMC solutions the passivation of Li is more efficient. This conclusion is especially important because both EC-DEC and ECDMC are the most commonly used solutions in Li ion batteries because of the good passivation of the lithiated graphite anodes in these batteries.26 This passivation, which is essential for obtaining a reversible Li-graphite anode, is achieved by precipitation of the EC reduction products on the lithiated carbon surfaces. The present work suggests that the passivation of active electrodes (Li, Li-C) in EC-based solutions is better when the cosolvent is DMC (compared with DEC). Another important consequence of this study relates to the effect of water. As shown in Table 3 and Figures 7-10, the O/C, O/F, Li/C, and Li/F atomic ratios are considerably higher on the Li surface treated in the wet (500 ppm of H2O) EC-DMC solutions than those treated in the dry solution. In addition, the relative intensity of the 290291 eV carbon peak is much higher in the spectra related to the wet solution than to the dry one. These results demonstrate the importance of the impact of water (when present) on the Li surface chemistry in these solutions. From the increase in the O/F and O/C atomic ratios due to the presence of water, it is clear that water reacts directly with Li, thus suppressing both solvent and salt anion reduction. As shown in Scheme 1 (paths 7, 8, and 9), water reduction may produce LiOH, Li2O, (26) Tarascon, J. M.; Guyomard, D. Abstract 69, p 102 and Abstract 74, p 110. The Electrochemical Society Extended Abstract, Vol. 93-1, Honolulu, HI, May 16-21, 1993. (27) Aurbach, D.; Ein-Eli, Y. J. Electrochem. Soc. 1995, 142, 1746.
The studies of Li surfaces freshly prepared in several LiAsF6 solutions of alkyl carbonates (DMC, dry EC-DEC, and wet EC-DEC) by XPS led to the following conclusions. (1) The quantitative use of XPS for analyzing surface species on Li surfaces is limited because both the X-ray radiation and the sputtering Ar+ beam influence the Li surface chemistry and may induce secondary reactions. (2) Yet, with careful interpretation, XPS is proven to be a valuable qualitative tool for obtaining a more solid understanding of the complicated Li surface chemistry and for confirming conclusions obtained from other spectroscopic studies (e.g., FTIR spectroscopy and energydispersive analysis of X-rays). (3) The present study seems to indicate that sputtering does not necessarily remove completely all surface species (e.g., LiF)20 because of enhanced redeposition of sputtered species on the highly reactive surface. (4) The surface films formed on lithium have a multilayer structure. As close as the surface species are to the Li surfaces, they are more inorganic in nature and have lower oxidation states. (5) EC reduction dominates the Li surface chemistry in EC-DEC and EC-DMC solutions. However, the impact of EC reduction products on the buildup of surface films on Li is more important in EC-DMC than in EC-DEC solutions because solvent reduction products are more soluble in the DEC-based solutions. This suggests that in EC-DMC solutions the passivation of active electrodes such as Li and Li-C is better. (6) Species with a Li-C bond can also be formed on Li in these solutions. This conclusion is unique to the capability of XPS and could not be achieved by other previous spectroscopic studies. (7) When water is present, its reactions suppress solvents and salt anion reduction. The present study confirms the importance of the secondary reaction of trace water with ROCO2Li surface species, which enrich the surface films on Li with Li2CO3. LA981048H
