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Surface Film Formation on a Graphite Negative Electrode in Lithium-Ion Batteries: Atomic Force Microscopy Study on the Effects of Film-Forming Additives in Propylene Carbonate Solutions Soon-Ki Jeong, Minoru Inaba,* Ryo Mogi, Yasutoshi Iriyama, Takeshi Abe, and Zempachi Ogumi Department of Energy & Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received August 25, 2001. In Final Form: October 17, 2001 In situ electrochemical atomic force microscopy (AFM) observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed during cyclic voltammetry in 1 M LiClO4/propylene carbonate (PC) containing 3 wt % vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfite (ES) in order to clarify the roles of these additives in the formation of a protective surface film on a graphite negative electrode in lithium-ion batteries. Particle-like precipitates appeared on the HOPG surface at the potentials 1.35, 1.15, and 1.05 V versus Li+/Li in PC + VC, PC + FEC, and PC + ES, respectively, and covered the whole surface at lower potentials. No evidence for cointercalation of solvent molecules was observed in the presence of each additive. It was concluded that the layer of the precipitates functions as a protective surface film, which suppresses cointercalation of PC molecules as well as direct solvent decomposition on the surface of the graphite negative electrode.
Introduction In commercially available lithium-ion batteries, lithium ion is electrochemically intercalated into the graphite negative electrode during charging, and deintercalated during discharging.1-3 These reactions are basically reversible; however, the reversibility depends greatly on the kind of electrolyte solution. Early attempts at electrochemical lithium intercalation into graphite in propylene carbonate (PC)-based solutions, which have been used in primary lithium cells for more than two decades, were unsuccessful because of poor compatibility between graphite and PC.4-7 When a graphite electrode is polarized to negative potentials in a PC-based solution, the solvent decomposes ceaselessly at around 1 V versus Li+/Li and thereby lithium ion is not intercalated. Later this problem was overcome by the use of ethylene carbonate (EC) instead of PC,8 and EC-based mixed solvent systems such as EC + dimethyl carbonate (DMC) and EC + diethyl carbonate (DEC) are currently used in commercially available lithium-ion cells employing graphite as a negative electrode. It is widely accepted that these EC-based solutions give a stable surface film through reductive decomposition of the solvents during the first charging process.9,10 The protective surface film is a physicochemical barrier, which is ionically conducting but electronically insulating, and is often called the solid electrolyte inter* To whom correspondence should be addressed. E-mail: inaba@ scl.kyoto-u.ac.jp. (1) Ogumi, Z.; Inaba, M. Bull. Chem. Soc. Jpn. 1998, 71, 521. (2) Winter, M.; Novak, P.; Monnier, A. J. Electrochem. Soc. 1998, 145, 428. (3) Nakajima, T. J. Fluorine Chem. 2000, 105, 229. (4) Dey, A. N.; Sullivan, B. P. J. Electrochem. Soc. 1970, 117, 222. (5) Besenhard, J. O.; Fritz, H. P. J. Electroanal. Chem. 1974, 53, 329. (6) Eichinger, G. J. Electroanal. Chem. 1976, 74, 183. (7) Arakawa, M.; Yamaki, J. J. Electroanal. Chem. 1987, 219, 273. (8) Fong, R.; von Sacken, U.; Dahn, J. R. J. Electrochem. Soc. 1990, 137, 2009. (9) Peled, E. J. Electrochem. Soc. 1979, 126, 2047. (10) Peled, E. Handbook of Battery Materials; Besenhard, J. O., Ed.; Wiley-VCH: Weinheim, 1999; p 419.
face (SEI).9 The chemical and physical properties of the SEI formed in EC-based solutions have been extensively investigated over the past decade, and considerable progress toward understanding of them has been achieved.11-20 We also have investigated SEI formation on a graphite negative electrode in EC-based solutions by scanning tunneling microscopy (STM)13,19,27,28 and atomic force microscopy (AFM).20 We found that SEI formation on a graphite negative electrode is a very complicated (11) Besenhard, J. O.; Winter, M.; Yang, J.; Biberacher, W. J. Power Sources 1995, 54, 228. (12) Aurbach, D.; Ein-Eli, Y.; Markovsky, B.; Zaban, A.; Luski, S.; Carmeli, Y.; Yamin, H. J. Electrochem. Soc. 1995, 142, 2882. (13) Inaba, M.; Siroma, Z.; Funabiki, A.; Ogumi, Z.; Abe, T.; Mizutani, Y.; Asano, M. Langmuir 1996, 12, 1535. (14) Inaba, M.; Siroma, Z.; Kawatate, Y.; Funabiki, A.; Ogumi, Z. J. Power Sources 1997, 68, 221. (15) Naji, A.; Ghanbaja, J.; Humbert, B.; Willmann, P.; Billaud, D. J. Power Sources 1996, 63, 33. (16) Zaghib, K.; Yazami, Y.; Broussely, M. J. Power Sources 1997, 68, 239. (17) Imhof, R.; Novak, P. J. Electrochem. Soc. 1998, 145, 1081. (18) Bar-Tow, D.; Peled, E.; Burstein, L. J. Electrochem. Soc. 1999, 146, 824. (19) Ogumi, Z.; Jeong, S.-K.; Inaba, M.; Abe, T. Macromol. Symp. 2000, 156, 195. (20) Jeong, S.-K.; Inaba, M.; Abe, T.; Ogumi, Z. J. Electrochem. Soc. 2001, 148, A989. (21) Dudley, J. T.; Wilkinson, D. P.; Thomas, G.; LeVae, R.; Woo, S.; Blom, H.; Horvath, C.; Juzkow, M. W.; Denis, B.; Juric, P.; Aghakian, P.; Dahn, J. R. J. Power Sources 1991, 35, 59. (22) Nakamura, H.; Komatsu, H.; Yoshio, M. J. Power Sources 1996, 62, 219. (23) Wrodnigg, G. H.; Besenhard, J. O.; Winter, M. J. Electrochem. Soc. 1999, 146, 470. (24) Fujimoto, H.; Fujimoto, M.; Ikeda, H.; Ohshita, R.; Fujitani, S.; Yonezu, I. J. Power Sources 2001, 93, 224. (25) Biensan, P.; Bodet, J. M.; Perton, F.; Broussely, M.; Jehoulet, C.; Barusseau, S.; Herreyre, S.; Simon, B. Extended Abstracts of The 10th International Meeting on Lithium Batteries, Como, Italy, 2000; The Electrochemical Society: Pennington, NJ, 2000; Abs. No. 286. (26) McMillan, R.; Slegr, H.; Shu, Z. X.; Wang, W. J. Power Sources 1999, 81-82, 20. (27) Inaba, M.; Kawatate, Y.; Funabiki, A.; Jeong, S.-K.; Abe, T.; Ogumi, Z. Electrochim. Acta 1999, 45, 99. (28) Inaba, M.; Kawatate, Y.; Funabiki, A.; Jeong, S.-K.; Abe, T.; Ogumi, Z. Electrochemistry 1999, 67, 1153.
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process that involves solvent cointercalation in the initial stage of charging, and concluded that the SEI layer has two important roles: (i) suppressing cointercalation of solvent molecules from the edge plane and (ii) suppressing direct solvent decomposition on the graphite surface (both the basal and edge planes). The poor compatibility between the graphite electrode and PC results from intensive cointercalation of PC molecules at about 1 V, accompanied by vigorous exfoliation of graphite layers.5,14 The exfoliation regenerates a fresh graphite surface, and thereby a stable SEI is not formed on a graphite electrode in PC-based solutions. Nevertheless, PC-based solutions are attractive as electrolyte solutions in lithium-ion cells because of their superior ionic conductivity at low temperatures.21-24 It has been recently reported that the addition of vinylene carbonate (VC),25 fluoroethylene carbonate (FEC),26 and ethylene sulfite (ES)23 to PC-based solutions greatly suppresses solvent decomposition and graphite exfoliation, and enables lithium ion to be intercalated into graphite. It seems that these additives give stable SEI layers on a graphite surface; however, detailed mechanisms for SEI formation in their presence have not been clarified yet. In the present study, we used highly oriented pyrolytic graphite (HOPG) as a model graphite electrode and observed surface morphology changes in PC solutions containing VC, FEC, and ES as additives by in situ AFM to elucidate their roles in SEI formation on a graphite negative electrode. Experimental Section The base electrolyte solution was 1 mol dm-3 (M) LiClO4 dissolved in PC (Kishida Chemical Co., Lithium Battery Grade). Three kinds of additives, VC (Aldrich), FEC (Kanto Denka Kogyo Co.), and ES (Aldrich), were added (3 wt % each) to the base solution, which was then dried over 4A molecular sieves for weeks. Each solution was used for electrochemical measurements after the water content decreased below 30 ppm. In some experiments, 1 M LiClO4 dissolved in a 1:1 (by volume) mixture of EC and DEC (Kishida Chemical Co., Lithium Battery Grade) was used as an electrolyte for comparison. Natural graphite powder (Kansai Coke and Chemicals Co., NG-7) was used for charge/discharge tests. The graphite powder (90 wt %) was mixed with a poly(vinylidene difluoride) binder (10 wt %) using 1-methyl-2-pyrrolidinone as a solvent to make a viscous slurry. The slurry was coated on copper substrates (10 × 50 × 0.1 mm3) and then dried overnight at 80 °C under vacuum. The thickness of the electrode layer was approximately 100 µm. In some experiments, the test electrode was covered with Ni mesh and used after being pressed moderately. Charge and discharge tests were carried out using conventional threeelectrode cells. The graphite test electrode was immersed in the electrolyte solution (∼20 cm3). The counter and reference electrodes were lithium foil. The test electrode was charged and discharged galvanostatically between 0 and 2.0 V using a battery test system (Hokuto Denko, HJ101SM6). The current was set at 31 mA g-1 (C/12 rate). For AFM observation, HOPG blocks (Advanced Ceramics, ZYH grade, mosaic spread ) 3.5 ( 1.5°) were used. In situ electrochemical AFM observation was carried out with an AFM system (Molecular Imaging, PicoSPM) equipped with a potentiostat (Molecular Imaging, PicoStat) and a laboratory-made electrochemical cell. Freshly cleaved HOPG was mounted at the bottom of the cell. Only the basal plane was brought into contact with the electrolyte solution by using an O-ring. The geometric surface area was 1.2 cm2. The counter and reference electrodes were lithium foil. Pyramidal silicon nitride tips were used for AFM measurements. Cyclic voltammetry (CV) was performed at a slow sweep rate of 0.5 mV s-1 between 2.9 and 0.0 V. AFM images were continuously obtained at an interval of 150 mV during CV measurements. The scan rate of the microcantilever was 5 µm s-1 unless otherwise noted.
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Figure 1. Charge and discharge curves of natural graphite powder (NG-7) in 1 M LiClO4/PC with and without additives. All measurements were carried out at 30 °C in an argon-filled glovebox (Miwa, MDB-1B + MM3-P60S) with a dew point lower than -60 °C. All potentials were referred to as volts versus Li+/ Li.
Results and Discussion Charge and Discharge Characteristics. Figure 1 shows potential profiles during the first charging (lithium intercalation) and discharging (deintercalation) of natural graphite powder (NG-7) in 1 M LiClO4/PC with and without additives. In the absence of additives, the potential dropped rapidly and remained nearly constant at ∼0.9 V, at which solvent decomposition and exfoliation of graphite took place ceaselessly, as mentioned earlier. Lithium ion was not intercalated in the graphite electrode because the intercalation would have taken place at potentials < 0.25 V. In contrast, the addition of 3 wt % VC and FEC greatly suppressed solvent decomposition and exfoliation of graphite. Lithium ion was intercalated and deintercalated over the plateau regions at potentials < 0.25 V. In the presence of 3 wt % ES, a large potential plateau appeared at about 2 V. However, the potential dropped below 0.25 V at about 170 mAh g-1, and then lithium ion was intercalated and deintercalated in a manner similar to those for the other two cases. These results clearly show that all three additives gave effective SEI layers on graphite surfaces in PC solutions. The charge and discharge capacities, and the Coulombic efficiencies during 50 cycles are summarized in Table 1. The results obtained in 1 M LiClO4 dissolved in EC + DEC, which is widely used as an electrolyte solution for graphite negative electrodes, are also shown in Table 1 for comparison. In the first cycle, the charge and discharge capacities and Coulombic efficiencies in PC + VC and PC + FEC are close to those obtained in 1 M LiClO4/EC + DEC. The Coulombic efficiencies of about 80% are due to the presence of the irreversible capacity, which is seen only in the first cycle. It is widely believed that the irreversible capacity is consumed for solvent decomposition and SEI formation.1,2 The Coulombic efficiency was especially lower in PC + ES than in other solutions, though the discharge capacity was comparable. This is due to the presence of the large potential plateau at approximately 2.0 V during charging, as shown in Figure 1.23 The reaction occurring at 2.0 V will be discussed later. Figure 2 shows the variations of discharge capacity with cycle number in the presence of the additives. The addition of VC and ES gave good cycleability, and the capacity retentions were 96 and 85%, respectively, at the 50th cycle. In addition, the Coulombic efficiencies were very high, except for the first cycle, in these electrolyte system, and were comparable to that in EC + DEC, as shown in Table
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Table 1. Charge and Discharge Capacities and Coulombic Efficiencies of Graphite Negative Electrodes (NG-7) during Fifty Cycles in Various Electrolyte Solutionsa charge capacity/mAh g-1 discharge capacity/mAh g-1 (Coulombic efficiency/%) solution PC + 3 wt % VC PC + 3 wt % FEC PC + 3 wt % FECf (Ni mesh-covered) PC + 3 wt % ES EC + DEC (1:1)b a
1st
10th
20th
30th
439 363 (82.7) 436 347 (79.6) 446 353 79.1) 554 356 (64.3) 438 365 (83.3)
366 363 (99.2) c 355 342c (96.3)c 361 351 (97.2) 353 349 (98.9) 371 364 (98.1)
360 359 (99.7) d 461 140d (30.3)d 372 347 (93.3) 347 346 (99.7) 362 361 (99.7)
356 355 (99.7) e 79 70e (88.6)e 348 330 (94.8) 337 336 (99.7) 368 357 (99.7)
Lithium salt: 1 M LiClO4. b By volume. c Fifth cycle.
d
40th 353
50th 353
350
350
(100)
(100)
322 316 (98.1) 328 327 (99.7) 355 354 (99.7)
315 311 (98.7) 320 319 (99.7) 351 351 (100)
Ninth cycle. e 12th cycle. f The electrode was covered with nickel mesh.
Figure 2. Variations of the discharge capacity with cycle number for natural graphite powder (NG-7) in 1 M LiClO4/PC containing 3 wt % VC, FEC, and ES.
1. In contrast, the discharge capacity suddenly dropped at the ninth cycle in PC + FEC. After the cell was disassembled, it was found that part of the electrode layer came off from the copper current collector. The poor cycling characteristics in PC + FEC in Figure 2 were thus considered to be due to an electrical isolation of the electrode during repeated charge and discharge cycles. To prevent electrical isolation, the electrode was covered with nickel mesh and charge/discharge tests were performed. The variations of the charge and discharge capacities of bare and covered electrodes in PC + FEC are shown in parts a and b, respectively, of Figure 3. The cycling characteristics were greatly improved by covering the electrode with nickel mesh, and the capacity retention was 88% at the 50th cycle. However, even the covered electrode gave slightly lower Coulombic efficiencies than those of bare electrodes in the other solutions, as shown in Table 1, indicating lower chemical stability of the SEI layer formed in PC + FEC. It should be noted that the charge capacity was significantly increased just before the discharge capacity dropped, that is, from 345 mAh g-1 at the eighth cycle to 461 mAh g-1 at the ninth cycle, for the bare electrode in Figure 3a. This means that an irreversible capacity was again consumed for SEI regeneration at the ninth cycle. Similar unusual increases in charge capacity are seen at the 7th, 11th, and 13th cycles for the covered electrode in Figure 3b. Although the detailed mechanism for the deterioration is not clear at present, it does not seem that the SEI layer formed in PC + FEC fully suppresses undesirable reactions such as gas evolution that mechanically deteriorate the electrode layer coated on the copper current collector. McMillan et al.26 investigated the charge/discharge characteristics of a graphite negative electrode in 1 M LiPF6 dissolved in a mixed solvent of PC, EC, and FEC (3.5:3.5:1 by volume).
Figure 3. Variations of charge and discharge capacities with cycle number for (a) uncovered and (b) Ni mesh-covered graphite composite electrodes in 1 M LiClO4/PC containing 3 wt % FEC: (2, 9) charge capacities; (4, 0) discharge capacities.
They reported that the graphite electrode exhibited a good capacity retention (more than 88% of the initial value) over 200 cycles. The abnormal behavior in the presence of FEC observed in the present study was not described in their report. We believe that not only FEC but also EC plays a beneficial roll in SEI formation in their solvent system. Morphology Changes of the HOPG Basal Plane during CV. To obtain information about SEI formation, morphology changes of the HOPG basal plane were observed during a slow scan CV using electrochemical AFM. Figure 4 shows cyclic voltammograms between 2.9 and 0.0 V at 0.5 mV s-1 in the presence of the additives. In each solution, a small reduction current began to flow at about 2.3 V, and a large reduction peak appeared in the range 1.0-1.5 V in the first cycle. The peak potentials were 1.3, 1.1, and 1.0 V in PC + VC, PC + FEC, and PC + ES, respectively. These reduction peaks disappeared in the second cycle and hence are attributed to irreversible decomposition of the electrolyte solutions that are closely related to SEI formation. A large reduction current was
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Figure 4. Cyclic voltammograms of the HOPG basal plane in 1 M LiClO4/PC containing 3 wt % VC, FEC, and ES. Sweep rate: 0.5 mV s-1.
observed at potentials lower than 0.6 V in each solution, which may be assigned partly to lithium intercalation because of the presence of the corresponding oxidation peak assigned to lithium deintercalation at around 0.7 V. However, the charge consumed for the reduction current was much larger than that for the oxidation peak in each solution, and hence, a substantial fraction of the reduction current at potentials < 0.6 V was consumed by irreversible processes such as direct decomposition of solvent molecules on the HOPG surface. Figures 5 shows morphology changes of the HOPG basal plane in 1 M LiClO4/PC + VC obtained simultaneously during the first voltammogram shown in Figure 4. Figure 5a shows an AFM image of the HOPG basal plane obtained at 2.9 V before potential cycling. The surface consisted of atomically flat terraces separated by several steps, which
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is a typical structure of the HOPG basal plane. At this potential, the HOPG surface was quite inert and neither deposition nor the intercalation reaction took place. Parts b and c of Figure 5 show AFM images obtained in PC + VC in the potential ranges 1.40-1.25 and 0.95-0.80 V, respectively. The arrows in parentheses denote the direction of raster of the microcantilever; for example, the bottom and top scanning lines of Figure 5b were obtained at 1.40 and 1.25 V, respectively. Morphology changes began at a potential around 1.35 V, and particle-like precipitates appeared on the HOPG surface, as shown in Figure 5b. The number of the precipitates increased with lowering potential down to 0.8 V, and the whole HOPG surface was covered with the precipitates (Figure 5c). Figures 6 and 7 show morphology changes in 1 M LiClO4/ PC + FEC and /PC + ES, respectively. These morphology changes in PC + FEC and PC + ES are very similar to those obtained in PC + VC in Figure 5. In PC + FEC and PC + ES, the precipitates appeared on the HOPG surface at somewhat lower potentials around 1.15 and 1.05 V, respectively, and covered the whole HOPG surface at lower potentials. The potentials at which the precipitates appeared in Figures 5-7 are well correlated with the reduction peaks centered at 1.3, 1.1, and 1.0 V in PC + VC, PC + FEC, and PC + ES, respectively, in the first cycle shown in Figure 4. It should be noted that ceaseless solvent decomposition and exfoliation of graphite take place at about 0.9 V in 1 M LiClO4/PC without additives, as shown in Figure 1. All the additives tested in the present study decomposed at potentials more positive than 1 V, and the resulting precipitate layers effectively worked as a surface film that suppressed the intercalation of solvated lithium ion. In each solution, a minor reduction current began to flow at about 2.3 V in Figure 4; however, the reactions in
Figure 5. AFM images (5 × 5 µm2) of the HOPG basal plane surface obtained (a) before and (b, c) during the first cycle of CV in 1 M LiClO4/PC containing 3 wt % VC. Scan rate of the microcantilever: 5 µm s-1.
Figure 6. AFM images (5 × 5 µm2) of the HOPG basal plane surface obtained (a) before and (b, c) during the first cycle of CV in 1 M LiClO4/PC containing 3 wt % FEC. Scan rate of the microcantilever: 5 µm s-1.
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Figure 7. AFM images (5 × 5 µm2) of the HOPG basal plane surface obtained (a) before and (b, c) during the first cycle of CV in 1 M LiClO4/PC containing 3 wt % ES. Scan rate of the microcantilever: 5 µm s-1.
Figure 8. AFM images of expanded areas (10 × 10 µm2) and height profiles of the HOPG basal plane surface obtained at 2.9 V after the first cycle of CV in 1 M LiClO4/PC containing 3 wt % (a) VC, (b) FEC, and (c) ES.
this potential range brought about no morphology changes, as shown in Figures 5-7. This fact means that the products were highly soluble in the electrolyte solutions. It should be noted that a large irreversible potential plateau was observed at about 2 V in the charge and discharge profile (Figure 1) in PC + ES; nevertheless, any remarkable reduction peaks corresponding to this plateau were not observed in CV in Figure 4. One plausible reason is a difference in the area ratio of the edge plane to the basal plane of the two test electrodes because graphite powder has a much larger edge plane area than HOPG. The reaction therefore may take place predominantly on the edge plane of graphite. Estimation of the Thickness of Precipitate Layers. After the images in Figures 5-7 were obtained, AFM observation was continued during the reverse potential sweep to 2.9 V in each solution. The precipitates formed at lower potentials gradually disappeared during the reverse sweep. This phenomenon is due to surface scratching of the AFM tip because we used the contact mode for AFM observation.29 A similar scratching effect was observed in AFM observation in 1 M LiClO4/EC + DEC in a previous report.20 After the potential was scanned back to 2.9 V, an expanded area of 10 × 10 µm2 including the 5 × 5 µm2 area in Figures 5-7 was observed in each solution. AFM images and height profiles of the 10 × 10 µm2 areas in PC + VC, PC + FEC, and PC + ES are shown in parts a, b, and c of Figure 8, respectively. In each image, the 5 × 5 µm2 area is seen as a rectangular hole, where (29) Frommer, J.; Heckl, W. In Procedures in Scanning Probe Microscopies; Colton, R., Engel, A., Frommer, J., Gaub, H., Gewirth, A., Guckenberger, R., Rabe, J., Heckel, W., Parkinson, B., Eds.; John Wiley & Sons: Chichester, 1998; p 277.
the precipitates were scraped off during repeated scanning. The precipitate layer remained outside the 5 × 5 µm2 area on the surface. From the height profiles, the thicknesses of the precipitate layers formed in PC + VC, PC + FEC, and PC + ES were roughly estimated to be 8, 20, and 30 nm, respectively. The thickness of the precipitate layer is the thinnest in PC + VC. This implies that the precipitate layer formed in PC + VC is dense and solid, and hence the most effective as an SEI, which is in agreement with the superior cycling characteristics in the presence of VC shown in Figure 2. Another important feature seen in Figure 8 is that no evidence for solvent cointercalation was observed inside the rectangular holes in PC + VC, PC + FEC, and PC + ES. In a previous report,20 we investigated SEI formation on a HOPG basal plane in 1 M LiClO4/EC + DEC in a similar manner. In 1 M LiClO4/EC + DEC, atomically flat areas of 1 or 2 nm height (hill-like structures) and large swellings of 15-20 nm height (blisters) appeared on the basal plane of HOPG. We concluded that these features were formed by the intercalation of solvated lithium ions and their decomposition beneath the surface, respectively. Solvent cointercalation in EC + DEC was not so vigorous, and decomposition products of cointercalated solvents between graphite layers play a role in suppressing further solvent cointercalation from EC + DEC. In Figure 8, such features are not seen at all, which confirms that effective SEI layers are formed by decomposition of the additives on graphite surfaces before cointercalation of PC takes place. Effect of Deliberate Elimination of the Precipitate Layer. To confirm the role of the precipitate layer in more detail, AFM observation at a low scan rate was performed
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Figure 9. AFM images of the HOPG basal plane surface obtained at 2.9 V (a) before, (b) during, and (c) after the first cycle of CV at 0.5 mV s-1 in 1 M LiClO4/PC containing 3 wt % ES: (a, b) 2 × 2 µm2; (c) 8 × 8 µm2. The square in part c shows the 2 × 2 µm2 area in parts a and b. Scan rate of the microcantilever: 2 µm s-1.
during CV. It is widely known that contact-mode AFM is very useful to scrape off a thin organic layer deliberately by careful surface scratching.29-33 The lower is the scan rate of the microcantilever, the stronger the adhesive force works between an AFM tip and a sample surface, resulting in vigorous surface scratching.29,30,31 The SEI layer formed at potentials more positive than 1 V thus can be removed by slow scan AFM observation during CV, by which cointercalation of solvent molecules will occur at about 0.9 V even in the presence of the additives. AFM observation of a 2 × 2 µm2 area on a HOPG surface was performed at a scan rate of 2 µm s-1, which was 2.5 times slower than that used in the preceding sections, during CV in 1 M LiClO4/PC + ES. Before CV, a clear step-and-terrace structure of the HOPG basal plane was observed again (Figure 9a). At potentials just below 1 V (Figure 9b), significant exfoliation of graphite layers was observed at a step, as we expected. Vigorous surface scratching at a slow scan rate of 2 µm s-1 scraped off the surface film formed by decomposition of ES, which caused a significant cointercalation of PC molecules at about 0.9 V, which resulted in the observed exfoliation. After the first cycle of CV was completed, the observed area was expanded to 8 × 8 µm2 and then repeated scanning was performed to remove the precipitate layer on the HOPG surface. This enabled us to observe clearly the interior structures, such as the hill-like structures and blisters mentioned in the previous section. The AFM image thus obtained is shown in Figure 9c. It is clear that intensive exfoliation was only limited in the 2 × 2 µm2 area where the surface had been scratched by the tip during CV. This fact confirms that the precipitate layer formed at potentials more positive than 1 V in PC + ES is necessary for suppressing the cointercalation of PC molecules. Another interesting feature in Figure 9c is that many swellings, which are indicated by arrows, are seen on the surface. The presence of these swellings is evidence for cointercalation of solvent molecules, as mentioned in the previous section. In addition, the swellings are only seen on the right-hand side of step S. This fact clearly (30) Overney, R. M.; Meyer, E. MRS Bull. 1993, 18, 26. (31) Overney, R. M.; Takano, H.; Fujihira, M.; Meyer, E.; Guntherodt, H.-J. Thin Solid Films 1994, 240, 105. (32) Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791. (33) Lemoine, P.; Laughlin, J. M. Thin Solid Films 1999, 339, 258.
indicates that solvated lithium ion was intercalated from the exfoliated part of step S in the 2 × 2 µm2 area. Conclusions The charge and discharge characteristics of a graphite negative electrode were tested in 1 M LiClO4/PC containing three kinds of film-forming additives, VC, FEC, and ES. All three additives effectively improved the poor compatibility between the graphite electrode and PC; that is, they greatly suppressed solvent decomposition and exfoliation of graphite layers. In particular, the addition of VC gave the best charge and discharge characteristics with a high reversible capacity (365 mAh g-1) and a good capacity retention (96% at the 50th cycle). In the presence of ES, the Coulombic efficiency in the first cycle was low (64%) because of the presence of a large irreversible capacity. On the other hand, the capacity retention in the presence of FEC was poor, though the initial characteristics were comparable to those in the presence of VC. The poor capacity retention was due to an electrical isolation of the electrode layer upon prolonged charge and discharge cycling. In situ electrochemical AFM observation of the morphology changes of the HOPG basal plane clearly show the role of these additives in PC-based solvent systems. Particle-like precipitates, which are decomposition products of the additives, appeared on the HOPG surface at potentials around 1.35, 1.15, and 1.05 V in PC + VC, PC + FEC, and PC + ES, respectively, and covered the whole surface at lower potentials. No evidence for cointercalation of solvent molecules was observed in the presence of each additive, which showed that the precipitate layer effectively worked as an SEI in suppressing cointercalation of PC molecules as well as direct solvent decomposition on the basal plane. The thickness of the precipitate layers was thinnest (8 nm) in the presence of VC, which may be a primary factor for the superior charge and discharge characteristics. Acknowledgment. The authors thank Kanto Denka Kogyo, Co., Ltd., for donating the FEC sample. This work was supported by CREST of JST (Japan Science and Technology). LA015553H
