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Electrochemical Scanning Tunneling Microscopy Observation of Highly Oriented Pyrolytic Graphite Surface Reactions in an Ethylene Carbonate-Based Electrolyte Solution Minoru Inaba,* Zyun Siroma, Atsushi Funabiki, and Zempachi Ogumi Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
Takeshi Abe, Yasuo Mizutani, and Mitsuru Asano Institute of Atomic Energy, Kyoto University, Uji, Kyoto 611, Japan Received October 9, 1995X In order to elucidate surface reactions on graphite negative electrodes of secondary lithium ion batteries, topographical changes of the basal plane of a highly oriented pyrolytic graphite (HOPG) in 1 M LiClO4/ ethylene carbonate-diethyl carbonate (1:1 by volume) were observed under polarization by electrochemical scanning tunneling microscopy. A step edge on the basal plane of HOPG was treated as a model of the edge plane of HOPG. When the sample potential was stepped to 1.1 V vs Li/Li+, two kinds of hill-like structure of ca. 10 Å height appeared on the HOPG surface. The first hill was formed far apart from a step edge and was almost unchanged with time. The second hill was formed in the vicinity of the step and was spread out with time. The formation of the second hill caused the exfoliation of graphite layers. The observed height of the hills was comparable to the values of the increment of the interlayer spacing for ternary graphite intercalation compounds of alkali metal with solvent molecules prepared by a solution method. It was considered that the intercalation of solvated lithium ion is responsible for the formation of the hills and that this process corresponds to the initial stage of the solvent decomposition and subsequent film formation processes.
1. Introduction Graphite materials have been extensively studied for use as negative electrodes in secondary lithium batteries.1-18 At the graphite electrode, lithium ions are intercalated into graphite upon charging and deintercalated upon discharging. The phase transition mechanism upon electrochemical Li intercalation has been investigated using X-ray diffraction6-9 and Raman spectroscopy.10 Since electrochemical intercalation takes place at the X
Abstract published in Advance ACS Abstracts, March 1, 1996.
(1) Mohri, M.; Yanagisawa, N.; Tajima, Y.; Tanaka, H.; Mitate, T.; Nagajima, S.; Yoshida, M.; Yoshida, Y.; Yoshimoto, Y.; Suzuki, T.; Wada, H. J. Power Sources 1989, 26, 545. (2) Imanishi, N.; Kashiwagi, H.; Ichikawa, T.; Takeda, Y.; Yamamoto, O.; Inagaki, M. J. Electrochem. Soc. 1993, 140, 315. (3) Kanno, R.; Kawamoto, Y.; Takeda, Y.; Ohashi, S.; Imanishi, N.; Yamamoto, O. J. Electrochem. Soc. 1992, 139, 3397. (4) Chusid, O. (Youngman); Ein-Eli, Y.; Aurbach, D.; Babai, M.; Carmeli, Y. J. Power Sources 1993, 43-44, 47. (5) Yazami, R.; Guerard, D. J. Power Sources 1993, 43-44, 39. (6) Dahn, J. R.; Fong, R.; Spoon, J. J. Phys. Rev. 1990, B42, 6424. (7) Dahn, J. R. Phys. Rev. 1991, B44, 9170. (8) Dahn, J. R.; Sleigh, A. K.; Shi, H.; Reimers, J. N.; Zhong, Q.; Way, B. M. Electrochim. Acta 1993, 38, 1179. (9) Ohzuku, T.; Iwakoshi, Y.; Sawai, K. J. Electrochem. Soc. 1993, 140, 2490. (10) Inaba, M.; Yoshida, H.; Ogumi, Z.; Abe, T.; Mizutani, Y.; Asano, M. J. Electrochem. Soc. 1995, 142, 20. (11) Dey, A. N.; Sullivan, B. P. J. Electrochem. Soc. 1970, 117, 220. (12) Arakawa, M.; Yamaki, J. J. Electroanal. Chem. Interfacial Electrochem. 1987, 219, 273. (13) Fong, R.; von Sacken, U.; Dahn, J. R. J. Electrochem. Soc. 1990, 137, 2009. (14) Shu, Z. X.; McMillan, R. S.; Murray, J. J. J. Electrochem. Soc. 1993, 140, L101. (15) Shu, Z. X.; McMillan, R. S.; Murray, J. J. J. Electrochem. Soc. 1993, 140, 922. (16) Ishikawa, M.; Morita, M.; Asano, M.; Matsuda, Y. J. Electrochem. Soc. 1994, 141, 1105. (17) Aurbach, D.; Ein-Eli, Y.; Chusid, O.; Carmeli, Y.; Babai, M.; Yamin, H. J. Electrochem. Soc. 1994, 141, 603. (18) Ein-Eli, Y.; Markovsky, B.; Aurbach, D.; Carmel, Y.; Yamin, H.; Luski, S. Electrochim. Acta 1994, 39, 2559.
interface between graphite and electrolyte solution, the electrolyte solution plays an important role in the intercalation. It is well known that the choice of electrolyte solution is important for minimizing irreversible capacities of graphite electrodes. When a propylene carbonate (PC)based electrolyte is used, the electrolyte keeps decomposing at about 1.0 V, and no lithium is intercalated into graphite.11,12 This ceaseless solvent decomposition has been overcome by use of mixed electrolyte solutions containing ethylene carbonate (EC);13 however, an irreversible capacity is still observed even in EC-based electrolyte solutions during the first charge and discharge at highly graphitized electrodes. The irreversible capacity has been attributed to reductive decomposition of solvent followed by lithium ion conductive surface film (solid electrolyte interface) formation.13-18 The film formed during the first charge is said to suppress further solvent decomposition,13-18 and hence the performance of graphite electrode strongly depends on the properties of the surface film. However, the mechanism of the film formation as well as its composition and morphology has not been fully clarified yet. Electrochemical scanning tunneling microscopy (STM) is a powerful new technique for detailed structural and topographical characterization of electrode/electrolyte interfaces.19 Knowledge of surface structures could be crucial to the understanding of electrochemical processes that are taking place at the electrode surface. Highly oriented pyrolytic graphite is one of the graphite materials prepared by a vapor phase process followed by heat treatment at a high temperature and has a highly anisotoropic structure with a good c-axis orientation. An atomically flat surface of its basal plane can be easily (19) Bard, A. J. In Scanning Tunneling Microscopy and Spectroscopy; Bonnell, D. A., Ed.; VCH: New York, 1993; p 287.
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Figure 1. Electrochemical STM cell used in this study.
obtained by cleaving the surface layers with an adhesive tape, and it has most often been studied by STM.20,21 For electrochemical STM application, HOPG has been used as a substrate of various electrochemical processes such as electrodeposition of Ag,22 Au,23 Pb,24 Li,25etc. In the present study, we observed topographical changes of the HOPG surface in EC-diethyl carbonate (DEC) containing LiClO4 under polarization by electrochemical STM and discuss the surface reaction occurring at the initial stage of charging. Although HOPG has a high crystallinity and an atomically flat basal plane, it still has some defects such as steps and grain boundaries. These defects are considered to be models for highly reactive edge planes of conventional graphite powder. We therefore focused on the reaction at a step edge on a HOPG basal plane. 2. Experimental Section HOPG (Le Carbone-Lorrain, PGGCL) was cleaved with an adhesive tape to obtain a flat basal plane and used as the working electrode. Electrochemical measurements and STM observation were carried out using an electrochemical STM cell shown in Figure 1. The HOPG electrode was mounted in the bottom of the electrochemical STM cell. The geometric area of the electrode was fixed at 0.196 cm2 using an O-ring, and only the basal plane was in contact with the electrolyte solution. The electrolyte solution was a mixture of EC-DEC (1:1 by volume) containing 1 M LiClO4 (Mitsubishi Chemical, Battery Grade). The counter and reference electrodes were platinum wire and lithium metal, respectively. Potential was measured and referred to as volts vs Li/Li+. Electrochemical STM images were obtained with an SPI3600 system (Seiko Instruments) using an apiezone wax-coated Pt/Ir tip in the constant current mode. The potential of the tip electrode was fixed at 3.0 V, and that of the working electrode was varied; hence, the bias voltage for tunneling was the potential difference between the two electrodes. The tunneling current was set at 0.5 nA. The scan rate of the tip was 1 µm s-1. All measurements were carried out at room temperature in an argon-filled glove box. The dew point in the glove box was kept < -60 °C. Prior to electrochemical STM observation, atomic resolution of the system was confirmed without electrolyte solution under argon atmosphere.
3. Results 3.1. Cyclic Voltammetry. Electrochemical behavior of HOPG was first examined by cyclic voltammetry to obtain information about surface reactions on HOPG. The (20) Morita, S.; Tsukada, S.; Mikoshiba, N. J. Vac. Sci. Technol. 1988, A6, 354. (21) Toma´nek, D.; Louie, S. G. Phys. Rev. 1988, B37, 9327. (22) Sonnenfeld, R.; Schardt, B. C. Appl. Phys. Lett. 1986, 49, 1172. (23) Drake, B.; Sonnenfeld, R.; Schneir, J.; Hansma, P. K. Surf. Sci. 1987, 181, 92. (24) Szklarczyk, M.; Bockris, J. O’M J. Electrochem. Soc. 1990, 137, 452. (25) Koura, N.; Tamura, S.; Yoshikawa, M. Denki Kagaku 1995, 63, 623.
Figure 2. Cyclic voltammograms (a) of a freshly cleaved HOPG basal plane and (b) after the potential was kept at 1.1 V vs Li/Li+ for 2 h in 1 M LiClO4/EC-DEC. ν ) 20 mV s-1.
first and second voltammograms of the HOPG basal plane in 1 M LiClO4/EC-DEC are shown in Figure 2a. Three peaks were observed at 733, 558, and 427 mV in the first reduction. Reoxidation peaks corresponding to these reduction peaks were not observed in the following anodic sweep, indicating that these processes are irreversible. These reduction peaks fully disappeared in the second sweep. It was reported that solvent decomposition and surface film formation on graphite electrodes occur at around 0.8 V vs Li/Li+.14,15 The electrochemical processes that take place at 733, 558, and 427 mV are hence closely related to such solvent decomposition and surface film formation. Furthermore, the presence of the three reduction peaks indicates that the surface film formation is not a simple electrochemical process. Since, in general, a basal plane of HOPG is inert, it is reasonable to consider that these processes took place at defects such as steps on the basal plane. Figure 2b shows a cyclic voltammogram measured after the electrode potential was kept at 1.1 V for 2 h, at which potential a reduction process corresponding to the first peak in Figure 2a began and reduction current started to pass. In Figure 2b, the first peak (733 mV) that was observed in Figure 2a disappeared, while the second and third peaks were clearly observed at 636 and 301 mV, respectively. The process corresponding to the first peak in Figure 2a was completed when the electrode potential was kept at 1.1 V, but the following reduction processes corresponding to the second and third peaks in Figure 2a did not proceed. The reaction rate for the first process would have been slow at 1.1 V as shown by a small current at 1.1 V on the cyclic voltammogram (Figure 2a), but this slowness is convenient for us to observe the topographical change because it takes a few minutes to obtain each STM image. Hence, we kept the electrode potential at 1.1 V and observed topographical changes of the HOPG surface with time by electrochemical STM, which is discussed in the following section. 3.2. Electrochemical STM Observation. As mentioned above, it is reasonable to consider that solvent decomposition and surface film formation on HOPG take place at the edge plane rather than at the basal plane. Large scale STM images of a freshly cleaved basal plane of HOPG typically show atomically flat terraces separated by steps. The step edges can be regarded as a kind of active edge plane. We therefore carefully observed an area in the vicinity of a step on the HOPG basal plane. When a freshly cleaved HOPG was immersed in the electrolyte solution, the open circuit potential was about
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Figure 3. STM image and height profile of HOPG basal plane at 2.8 V in 1 M LiClO4/EC-DEC. The frame size of each image is 500 × 500 nm. The potential of the Pt/Ir tip was 3.0 V. The height profile shows the height along the linear line drawn in the image.
3 V. Figure 3 shows an STM image of the basal plane surface of HOPG and the height profile along the linear line drawn in the STM image when the potential of HOPG was kept at 2.8 V. The frame size of the image is 500 × 500 nm. A clear step was observed horizontally in the image. From the height profile, the height of the step was ca. 30 Å. Since the interlayer spacing of pristine graphite is 3.35 Å,26 the height at B corresponds to about nine graphite layers. The image did not change from that observed in air and remained unchanged with time at this potential, indicating that no reaction accompanied by a morphological change or by charge transfer (see Figure 2a) occurred at 2.8 V. The electrode potential was then stepped to potentials lower than 2.8 V; however, no topographical change was observed at potentials higher than 1.1 V, which was in agreement with the results of cyclic voltammetry. When the potential was stepped to 1.1 V, significant changes in morphology were observed. Parts a, b, and c of Figure 4 show the STM images and height profiles at 0.5, 4, and 13 min, respectively, after the potential was stepped to 1.1 V. It took about 2 min to obtain each image; hence, for example, Figure 4a shows an image in the range of 0.5-2.5 min. The tip was scanned horizontally from the left at the bottom of the figure. In Figure 4a, a hill-like structure appeared on the upper part with respect to the step edge. The height profile in Figure 4a revealed that the hill was of 10 Å height at B and had an atomically flat surface. The first hill remained almost unchanged with time as shown in Figure 4b,c. At 4 min, another hill appeared in the vicinity of the step (Figure 4b) in addition to the first one. The height profile revealed that the height of the second hill was 8 Å at B. While the topographical changes of these STM images clearly indicate that the second hill was formed from the step edge and then spread out, it is not clear how the first hill was formed. We (26) Dresselhaus, M. S.; Dresselhaus, G. Adv. Phys. 1981, 30, 139.
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observed the basal plane of an extended area prior to these measurements; however, there was no step except the one in Figures 3 and 4 around this 500 × 500 nm area. The first hill may have been formed through grain boundaries located in the interior of HOPG. In Figure 4c, the second hill in the vicinity of the step edge spread out further, and part of the graphite layers on the second hill exfoliated; the position of the step retreated upward from that in Figure 4b. A part of the original step was separated into two steps near the linear line drawn in Figure 4c. The height of the hill did not change (8 Å at B). The potential was then stepped back to 2.8 V where no electrochemical reaction occurred, and the topographical change was monitored for 40 min; however, no topographical change was observed. Hence, the topographical changes observed at 1.1 V were irreversible. The potential was again stepped to 1.1 V, and STM images were monitored. Figure 5a,b shows the STM images and height profiles at 0.5 and 4 min of the same region as Figure 4, respectively, after the potential was stepped to 1.1 V. The images moved downward by ca. 200 nm compared with that shown in Figure 4c owing to thermal drift. The hill was again observed in Figure 5a,b, and it corresponded to the first (upper) hill in parts a-c of Figure 4. In Figure 5a, two steps, the upper and lower steps, were observed in the lower part of each image, and they retreated from the positions in Figure 4c to the vicinity of the first (upper) hill. It should be noted that the exfoliation of graphite layers occurred on surface regions where no hill-like structure was formed. In Figure 5b, the upper step further retreated upward of the figure from the position in Figure 5a; however, the outline of the first hill remained unchanged. The upper step crossed over the hill in Figure 5b. In the height profile along the linear line drawn in Figure 5b, the region from B to D corresponds the hill of 10 Å height, and the newly formed step edge of 8 Å height was observed at C on the hill. This height profile clearly shows that the hill did not terminate at the step. If the first hill had been a surface structure such as adsorbed molecules formed on the HOPG surface, it would have been lost by the exfoliation of graphite layers. Therefore, the hill was not a surface structure but was an interior structure below the surface. 4. Discussion 27
Lang et al. studied a stage 1 lithium-graphite intercalation compound (GIC) prepared by a liquid phase reaction of HOPG using STM. They observed island-like structures similar to the hills observed in the present study, but of a reduced topographic height of 2-3 nm. These islands had a lateral dimension of 50-200 nm and did not terminate at steps or grain boundaries. They considered these islands to originate from locally missing intercalated lithium; however, they also pointed out a possibility that the observed islands represent surface regions of increased local work function leading to an increased local tunneling barrier height. We also suspected that the hill was formed by lithium intercalation into graphite. However, it is not possible to consider that lithium intercalation into graphite takes place at 1.1 V.6-10 The interlayer spacings of pristine graphite and stage 1 Li-GIC (LiC6) are 3.35 and 3.71 Å, respectively;26 hence, the increase in interlayer spacing by lithium intercalation is 0.35 Å. The observed height of the hill was 8-10 Å, which requires lithium intercalation into over 20 interlayers. This is not unlikely because the (27) Lang, H. P.; Wisendanger, R.; Thommen-Gaise, V.; Gu¨ntherodt, H.-J. Phys. Rev. 1992, B45, 1829.
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Figure 4. STM images and height profiles of HOPG basal plane at (a) 0.5, (b) 4, and (c) 13 min after the potential was stepped to 1.1 V in 1 M LiClO4/EC-DEC. The frame size of each image is 500 × 500 nm. The potential of the Pt/Ir tip was 3.0 V. Each height profile shows the height along the linear line drawn in the corresponding image.
Figure 5. STM images and a height profile of HOPG basal plane at (a) 0.5 and (b) 4 min after the potential was again stepped to 1.1 V in 1 M LiClO4/EC-DEC. The frame size of the image is 500 × 500 nm. The potential of the Pt/Ir tip was 3.0 V. The height profile shows the height along the linear line drawn in the image (b).
height of the original step observed in Figure 3 was ca. 30 Å, which consisted of about nine layers of the pristine graphite. It is also difficult to consider that the change in local work function was responsible for the observed hill formation. We observed the STM image of the sample at 2.8 V after Figure 4c was obtained at 1.1 V. However, no change in morphology including the height of the hill was observed. The bias voltage was the potential difference between the tip (3.0 V) and sample (1.1 or 2.8 V) electrodes in the present study; hence, the hill height was independent of bias voltage. This clearly shows that the observed hill was not an electronic artificiality. In addition, the hill formation was accompanied by the exfoliation of
graphite layers as shown in Figure 4c. Hence, it is reasonable to consider that some species with a much larger volume were intercalated into an interlayer. Table 1 shows the values of the repeat distance (Ic) and increment of interlayer spacing by intercalation (di) of several stage 1 ternary GICs of alkali metal with solvent molecules prepared by a solution method.28 The observed height of the hill, 8-10 Å, is comparable to the increments of interlayer spacing by intercalation of these GICs, in particular, of lithium with tetrahydrofuran. Therefore it is acceptable to assume that the intercalation of solvated (28) Setton, R. In Graphite Intercalation Compounds I. Structure and Dynamics; Zabel, H. S., Solin, A., Eds.; Springer-Verlag: Berlin, 1990; p 320.
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Table 1. Values of Repeat Distance (Ic) and Increment of Carbon Spacing by Intercalation (di) of Several Stage 1 Ternary Graphite Intercalation Compounds of Alkali Metal with Solvent Molecules28 GICsa
Ic (Å)
dib (Å)
Li(THF)1.4C6 Li(THF)2.3C12 Li(THF)3.4C18 Li(DME)2.8C33 Na(THF)3.5C32 Na(DME)2.8C32
12.53 12.44 12.45 7.32, 11.62c 7.24 7.26, 11.27c
9.18 9.09 9.10 3.97, 8.27c 3.89 3.97, 8.42c
a THF: tetrahydrofuran. DME: dimethoxyethane. b d ) I i c 3.35 Å for stage 1 GIC. c Both phases were present simultaneously.
lithium ion was responsible for the formation of the hills. At the present stage, however, it is not clear which solvent, EC or DEC, was intercalated. Besenhard et al.29 studied the crystal expansion of HOPG during electrochemical reduction in 1 M LiClO4/ EC-dimethoxyethane by dilatometry and observed a drastic expansion of the graphite matrix (>150%) at potentials more negative than 1.0 V vs Li/Li+. They attributed this expansion to solvent cointercalation and concluded that the intercalated solvent further decomposes to form an immobile product remaining between the graphite layers and that this reduction product prevents further solvent intercalation and the exfoliation of graphite layers. In the present study, an atomically flat hill-like structure of a height comparable to the increments of solvent cointercalated GICs was formed at 1.1 V vs Li/Li+ and spread out with time. The hill formation was irreversible, and part of the graphite layers on the hill exfoliated. Although it is not clear whether the intercalated solvent molecules decomposed at 1.1 V, the topographical changes observed in the present study are attributed to solvent cointercalation and support the surface film formation mechanism proposed by Besenhard et al.29 which states that the surface film is formed on graphite by the intercalation of solvated lithium ions followed by the decomposition of the intercalated solvent to form an immobile product remaining between the graphite layers. In Figure 4c, the exfoliation of graphite layers seems to have been caused by the hill formation. The intercalation of large solvent molecules induced a sizable strain between the graphite layers adjacent to the intercalates, which led to the exfoliation of the graphite layers above the intercalates. As shown in Figure 5a,b, however, exfoliation took place even on surface regions where no hill was formed. It is reasonable to consider that the exfoliation was also caused by some interlayer strain in this case; that is, the graphite layers exfoliated as soon as solvated lithium ions were intercalated into an interlayer, or even graphite layers on the hill may have been exfoliated by solvated Li+ intercalation into another interlayer in Figure 4c. Whether the hill is formed or not should depend on the stability of solvent molecules intercalated between graphite layers. Since the exfoliation of graphite layers leads to the formation of highly reactive edge planes, the stability of intercalated solvent molecules may be an important factor predominating surface film formation on graphite. On the basis of the above considerations, the observed topographic changes of the HOPG basal plane in the present study are explained by schematic models shown in Figures 6 and 7. At 2.8 V, a step edge of ca. 30 Å height was observed. This step consisted of about nine graphite layers (Figure 6a). When the potential was stepped to 1.1 (29) Besenhard, J. O.; Winter, M.; Yang, J.; Biberacher, W. J. Power Sources 1995, 54, 228.
Figure 6. Schematic models explaining the topographical changes observed in (a) Figure 3, (b) Figure 4b, and (c) Figure 4c.
Figure 7. Schematic models explaining the topographical changes observed in (a) Figure 5a, (b) an intermediate state between parts a and b of Figure 5, and (c) Figure 5b.
V, solvated lithium ions were intercalated between graphite layers in the interior, perhaps from a grain boundary, and the first hill of 10 Å height was formed. At 4 min, solvated lithium ions were intercalated into an interlayer from the step edge on the surface, and the second hill of 8 Å was formed (Figure 6b). While the first hill remained almost unchanged, solvent cointercalation proceeded further in the case of the second hill, and the second hill spread out with time. In addition, part of the graphite layers on the second hill exfoliated (Figure 6c).
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During the second potential step to 1.1 V, the exfoliation took place even on a surface region where no hill was formed (Figure 7a). It was considered that the graphite layers in this region exfoliated as soon as solvated lithium ions were intercalated into an interlayer (Figure 7b). The exfoliation proceeded further, and a new step edge was formed on the first hill (Figure 7c). As shown in Figure 2b, the electrochemical processes corresponding to the first reduction peak at 733 mV in Figure 2a disappeared after the electrode potential was kept at 1.1 V for 2 h. This fact indicates that the electrochemical processes occurring at 1.1 V, the intercalation of solvated lithium ion and the exfoliation of graphite layers, terminated with an elapse of time. As shown in Figure 5b, however, exfoliation did not seem to stop in the time scale of the present STM observation. Consequently, it is not clear how and when solvated Li+ intercalation and the exfoliation of graphite layers terminated. The intercalated solvent molecules are considered to decompose between the graphite layers at more negative potentials as shown by the second and third peaks in Figure 2b. Unfortunately, we have not succeeded yet in obtaining evidence for further decomposition of the intercalated solvent and subsequent surface film formation because clear STM images were not obtained at potentials more negative than 1.1 V under the present experimental conditions. The topographical changes at potentials more negative than 1.1 V and in other electrolyte solutions such as PC-based solutions are currently being investigated.
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5. Conclusions In situ electrochemical STM observation of the basal plane of HOPG in 1 M LiClO4/EC-DEC was performed. When the sample potential was stepped to 1.1 V vs Li/Li+, two kinds of hill-like structure of 8-10 Å height appeared on the basal plane of HOPG. The first hill located far apart from a step edge remained almost unchanged; however, the second hill in the vicinity of the step spread out with time, and graphite layers on the second hill gradually exfoliated. The observed height of the hill was comparable to the values of the increment of the interlayer spacing of ternary alkali metal-solvent moleculegraphite intercalation compounds prepared by a solution method. The topographical changes observed in the present study are attributed to the intercalation of solvated lithium ion, and this supports the surface film formation mechanism proposed by Besenhard et al. which states that the surface film is formed on graphite by the intercalation of solvated lithium ion followed by the decomposition of the intercalated solvent to form an immobile product remaining between the graphite layers. Acknowledgment. This work was partly supported by Grant-in-Aid for Scientific Research (Numbers 07750915 and 05235107) from the Ministry of Education, Science and Culture, Japan. LA950848E
