In Situ AFM Study of Surface Film Formation on the Edge Plane of

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In Situ AFM Study of Surface Film Formation on the Edge Plane of HOPG for Lithium-Ion Batteries Yasuhiro Domi,† Manabu Ochida,† Sigetaka Tsubouchi,† Hiroe Nakagawa,† Toshiro Yamanaka,† Takayuki Doi,*,† Takeshi Abe,‡ and Zempachi Ogumi† † ‡

Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan ABSTRACT: Changes in the surface morphology of the edge planes of graphite during a potential sweep were studied using highly oriented pyrolytic graphite (HOPG) in an ethylene carbonate (EC) + diethyl carbonate (DEC)-based electrolyte solution by in situ atomic force microscopy (AFM). The effects of the microscopic structures of graphite, i.e., edge and basal planes, on surface film formation are discussed. The formation of fine particles and precipitates was observed depending on the electrode potential between 1.0 and 0 V. These were considered to be remnants of blisters that could be observed at the basal plane and decomposition products of the electrolyte solution. The surface films were 56 and 66 nm thick after the first and second cycles, respectively. The precipitate layer formed on the edge plane was thinner than that observed on the basal plane after the second cycle. These results enabled us to elucidate the difference in the formation of surface films on the edge and basal planes of HOPG.

1. INTRODUCTION Graphite is widely used as a negative electrode in lithium-ion batteries due to its excellent properties, such as a high reversible capacity and a negative electrochemical potential close to that of lithium metal (
3.045 V vs a standard hydrogen electrode).1
3 The reversibility of a graphite negative-electrode differs depending on the kind of electrolyte solution used. The electrochemical intercalation of lithium ion into graphite did not proceed in propylene carbonate (PC)-based solutions containing ca. 1 mol dm
3 of lithium salts due to the poor compatibility between graphite and PC;4 when the graphite electrode is polarized to negative potentials in a PC-based solution, solvent decomposition and graphite exfoliation occur continuously on the surface at around 0.9 V vs. Li+/Li, and no lithium ion is intercalated. Recently, however, it has been reported that the addition of certain kinds of organic molecules (additives), such as vinylene carbonate, ethylene sulfite, fluoroethylene carbonate, and so on, to a PC-based solution suppresses solvent decomposition and graphite exfoliation, and enables lithium ion to be intercalated into graphite.5
9 On the other hand, the intercalation of lithium ion proceeds in ethylene carbonate (EC)-based solutions even without additives.10 It has been widely recognized that the electrochemical intercalation of lithium ion into graphite in an EC-based solution proceeds due to the formation of a stable passivating surface film. The surface film is formed on a graphite negative-electrode through the reductive decomposition of electrolyte solution in the initial stage of charging.11,12 This passivating film, called a solid electrolyte interphase (SEI),11 is conductive for lithium ion but electronically insulating, and hence it suppresses further decomposition of the electrolyte solution and enables lithium ion r 2011 American Chemical Society

to be intercalated within graphite. The SEI also plays a major role in improving the safety and cyclability of lithium-ion batteries that use a graphite negative-electrode. Over the past two decades, many researchers have sought to understand the nature of the SEI. For instance, the chemical composition of the SEI has been studied by means of Fourier transform infrared spectroscopy (FT-IR),13
17 X-ray photoelectron spectroscopy (XPS),18
20 gas chromatography (GC),21 transmission electron microscopy (TEM),8b,22,23 and so on. However, this problem still remains to be solved, along with SEI formation and deterioration under prolonged charge and discharge cycles. If the ideal chemical composition and thickness of SEI can be clarified, the performance of lithium-ion batteries should be improved remarkably. In situ scanning probe microscopy (SPM) is a very powerful tool for obtaining morphological information on interfacial reactions in various electrolyte solutions under potential control.24
32 We have previously investigated changes in the morphology of the basal plane of highly oriented pyrolytic graphite (HOPG) in several kinds of electrolyte solution using in situ scanning tunneling microscopy (STM).24
26 In an EC + diethyl carbonate (DEC)-based electrolyte solution, atomically flat parts raised by about 1 or 2 nm (hill-like structures) were observed at around 1 V versus Li+/Li. In addition, much larger irregular-shaped swellings of the surface (blisters) 15
20 nm high appeared in the vicinity of step edges on the basal plane surface at potentials close to 0.8 V versus Li+/Li. These hills and blisters were considered to be formed by the intercalation of solvated lithium ion between Received: July 8, 2011 Revised: November 2, 2011 Published: December 02, 2011 25484

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The Journal of Physical Chemistry C graphite layers and the accumulation of their decomposition products in interlayer spaces, respectively. These morphological changes are consistent with the solvent cointercalation model for SEI formation proposed by Besenhard et al.33 We have also investigated the thickness of the precipitate layer on the HOPG basal plane by means of in situ atomic force microscopy (AFM).29
31 The precipitate layer was formed by the direct decomposition of solvents on the surface below 0.65 V versus Li+/Li, and its thickness increased with potential cycling. We reported that the thickness varied depending on the kinds of additives in the electrolyte solution30 and the operating temperature.31 On the basis of our previous findings and those of other research groups, it has been recognized that the edge planes of graphite predominantly participate in the intercalation/deintercalation reactions of lithium ion and surface film formation. However, there have been no detailed reports on the mechanism of the formation of SEI on the edge plane of graphite. In this paper, we used the HOPG edge plane as a model electrode, and investigated both the changes in its surface morphology during a potential sweep and the thickness of the precipitate layer by means of in situ AFM. On the basis of these observations, we discuss here the differences in the formation of a surface film between the edge and basal planes of HOPG.

2. EXPERIMENTAL SECTION The electrochemical properties of the edge plane of HOPG (Momentive, ZYH, Mosaic Spread: 3.5 ( 1.5°) were studied by cyclic voltammetry (SP-300, Biologic) at a slow sweep rate of 0.5 mV s
1 between 3.0 and 0 V using laboratory-made threeelectrode cells. The HOPG edge plane was pretreated carefully and used as a working electrode. In addition, the HOPG edge plane was characterized by XPS. In some experiments, a freshly cleaved HOPG basal plane was used for comparison. Only the edge or basal plane was brought into contact with the electrolyte solution. The effective electrode surface area was limited to 0.07 cm2 by an O-ring. The reference and counter electrodes consisted of lithium foil. All potentials in the text reflect V versus Li+/Li. The electrolyte solution was 1 M LiClO4 dissolved in a 1:1 (by volume) mixture of EC and DEC (Kishida Chemical). The water content in the solution was less than 20 ppm. In situ AFM observations coupled with cyclic voltammetry were performed in a conventional contact mode using a 5500 AFM/SPM Microscope (Agilent) with laboratory-made electrochemical cells and pyramidal silicon nitride tips (spring constant; 0.02 N m
1, Olympus). The geometric surface areas of the electrode were fixed at 0.25 cm2 through the use of some square silicon rubber. All AFM observations were carried out at a room temperature of ca. 25 °C in an argon-filled glovebox with a dew point of less than
80 °C. 3. RESULTS AND DISCUSSION 3.1. SEI Formation during the First Cycle. In C1s spectrum of a pretreated HOPG edge plane, a minor broad peak at around 286.1 eV, in addition to a sharp peak at about 284.4 eV, was observed. In addition, a single peak was seen at about 532.3 eV in the O1s spectrum. These peaks were also observed before pretreatment of the HOPG edge plane. These results indicate surface functional group attached on the graphite edge, such as alcohol and ether groups.34 The surface of the pretreated HOPG

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edge plane was flat enough to be observed within a range of 100 μm as described hereinafter. Figure 1 shows the first and second cyclic voltammograms of the HOPG edge plane in 1 M LiClO4 dissolved in EC/DEC (1:1, v/v%) and the roughness factor calculated from Figure 2 (discussed later). In the first cycle, cathodic current began to flow at about 1.0 V, and a discernible shoulder appeared at about 0.6 V (inset in Figure 1). This cathodic shoulder, including a small current at around 1.0 V, disappeared in the second cycle. In the initial cyclic voltammogram of the HOPG basal plane, three cathodic peaks were observed at around 1.0, 0.8, and 0.5 V.29 On the basis of the literature, these cathodic peaks are assigned to (1) the intercalation of solvated lithium ion beneath the surface through the step edge of HOPG at about 1.0 V, (2) its decomposition between graphite layers at around 0.8 V, and (3) direct decomposition of solvents on the surface to form a precipitate layer at potentials below 0.65 V, respectively. Among them, only (1) and (2) should be observed on the HOPG edge plane, and the corresponding cathodic current appeared as a shoulder peak at about 0.6 V in Figure 1. Since the number of intercalation sites for Li+ is much greater for the edge plane than the basal plane, the cathodic current did not separate into two peaks, but formed a broad peak. Thus, a passivating film should be formed by irreversible decomposition reactions of the electrolyte solution in the initial cycle.24,25 In addition, direct decomposition of solvents also occurs on the HOPG edge plane, as is the case in for the basal plane, at potentials below 0.6 V, which is discussed later. A large increase in cathodic current observed at about 0 V could be assigned primarily to the intercalation of lithium ion, and a corresponding anodic peak due to deintercalation appeared at potentials close to 1.2 V. However, the cathodic charge consumed at about 0 V was greater than the anodic charge. Therefore, a substantial fraction of the cathodic current would be consumed by irreversible processes such as solvent decomposition. Furthermore, this cathodic current peak was more than 10 times greater than that obtained for the HOPG basal plane.29 This difference may be due to the fact that the HOPG edge plane has many more active sites for the intercalation/deintercalation of lithium ion, i.e., graphene layers on the surface, than the basal plane. Figures 2 and 3 show changes in the morphology of the HOPG edge plane obtained during the first cathodic scan at 0.5 mV s
1 in 1 M LiClO4/EC+DEC (1:1, v/v%) and the cross-sectional profiles, respectively. From the cross-sectional profiles, the roughness factor (root-mean-square roughness, Rq) was calculated according to the following equation.35 The calculated values were plotted against the potential shown in Figure 1. v" ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi #ffi u u 1 L Rq ¼ t ðhi
pÞ2 L i¼1

∑

L, hi, and p denote the number of data points, the height of each point, and the average surface height, respectively. Figure 2a shows an AFM image obtained at an open circuit potential (OCV) of 2.9 V before CV. The morphology of the edge plane did not change over several hours; the HOPG edge plane is quite inert and neither deposition nor intercalation takes place at this potential. No topographical change was observed between 2.9 and 1.0 V (Figure 2a,b and Figure 3). In addition, the Rq values were almost constant in this potential range. A small current flowed below 2.0 V in the first cycle (inset in Figure 1), whereas 25485

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Figure 1. Cyclic voltammograms of the HOPG edge plane in 1 M LiClO4/EC+DEC (1:1, v/v%). Sweep rate: 0.5 mV s
1. The solid and dashed lines represent the first and second cycle, respectively. The solid circles denote the root-mean-square roughness with the scan of negative direction. The inset shows the expansion of the first cycle in the negative direction between 3.0 and 0.55 V.

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Figure 3. Cross-sectional profile of the HOPG edge plane obtained at each potentials during the first cycle of 0.5 mV s
1 in 1 M LiClO4/EC +DEC (1:1, v/v%).

Figure 4. AFM image (45 μm  45 μm) of the HOPG edge plane obtained at 3 V at the (a) first, (b) fifth, (c) 10th, and (d) 20th AFM scans after first cycle in 1 M LiClO4/EC+DEC (1:1, v/v%). Figure 2. AFM images (10 μm  10 μm) of the HOPG edge plane obtained at the potentials of (a) 3 V (pristine, before CV), (b) 1.51
1.04 V, (c) 1.01
0.54 V, (d) 0.53
0.06 V during the first cycle of 0.5 mV s
1 in 1 M LiClO4/EC+DEC (1:1, v/v%).

no noticeable change in surface morphology was observed; very similar behavior was observed at the HOPG basal plane.29 When the potential was scanned in a more negative direction, a shoulder peak of a cathodic current appeared at around 0.6 V (inset in Figure 1) and fine particles smaller than 0.1 μm were observed at potentials from 1.0 to 0.6 V (Figures 2c and 3). This fine change in morphology influenced the surface roughness; the value of Rq gradually increased. At potentials below 0.6 V, particle-like precipitates were observed on the surface, and they ranged in size from 0.3 to 0.6 μm, as shown in Figure 2c,d. The size of the particles, as well as their height, which could be estimated from the cross-sectional profiles shown in Figure 3, reached a maximum at about 0.6 V, and then gradually became smaller with a decrease in the potential down to 0 V, while the number of precipitates increased. These results also indicate that the current rise at potentials close to 0 V in Figure 1 can be attributed not only to the intercalation of lithium ion, but also to the decomposition of electrolyte on the edge plane. We previously reported that SEI formation on the basal planes of HOPG should consist of the following three steps:29,31 (1) the

intercalation of solvated lithium ion beneath the surface at about 1.0 V, (2) its decomposition between graphite layers at around 0.8 V, and (3) the direct decomposition of solvents on the basal plane to form a precipitate layer at potentials below 0.65 V. The swellings on the surface arising from the intercalation of solvated lithium ion and its decomposition are called hills and blisters, respectively. Such hills and blisters formed beneath the surface cannot be observed on the HOPG edge plane because the scanning plane is horizontal to the graphene layers. However, the potential-dependency of the morphological changes observed on the HOPG edge plane is quite similar to that for the HOPG basal plane, as described above. Therefore, the fine morphological changes on the HOPG edge plane from 1.0 to 0.6 V should be attributed to the remnant of the blisters formed between graphene sheets. In addition, the formation of particlelike precipitates at potentials below 0.6 V was attributed to the direct decomposition of solvents on the edge plane. In an AFM image (10 μm  10 μm) of the HOPG edge plane obtained at 3 V after the first cycle (not shown), the number of precipitates and their sizes were almost the same as those obtained from 0.53 to 0.06 V (Figure 2d). In a wide scan (89 μm  89 μm), on the other hand, some larger precipitates of ca. 3 μm were observed. Thus, particle-like precipitates ranging from 0.3 to 3 μm were formed in the first cycle. 25486

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Figure 5. AFM image (89 μm  89 μm) and a cross-sectional profile of the HOPG edge plane surface obtained at the potential of 3 V after the first cycle. The dotted square in the center shows the area scraped off by AFM cantilever.

Table 1. Thicknesses of Precipitate Layers on the HOPG Edge and Basal Planes after the First and Second Cycles edge plane

basal plane

first cycle

56 ( 8 nm

47 ( 4 nm

second cycle

66 ( 3 nm

77 ( 5 nm

3.2. Thickness of the Precipitate Layer. In our previous study, the precipitate layers on the HOPG basal plane, which were formed by the direct decomposition of solvents at potentials below 0.65 V, could be scraped off by scanning the surface repeatedly in contact-mode AFM.29,31 The thickness of the precipitate layers was then estimated from the difference in height between scraped and unscraped parts. In this study, this measuring technique was applied to the precipitate layers formed on the HOPG edge plane. Figure 4 shows AFM images (45 μm  45 μm) of the HOPG edge plane obtained at the (a) first, (b) fifth, (c) 10th, and (d) 20th AFM scans at 3 V after the first potential cycle in 1 M LiClO4/EC+DEC (1:1, v/v%). Many particles of various sizes were observed at the first and fifth scans, as shown in Figure 4a,b, respectively. Particles larger than ca. 1 μm almost disappeared in the 10th scan (Figure 4c), and only fine particles smaller than 0.1 μm remained in the 20th scan (Figure 4d). No change in the AFM image was observed in further scans. Fine particles of the same size were observed in Figure 2c, and hence they should be identified as remnants of the blisters between graphite layers. Figure 5 shows an AFM image (89 μm  89 μm) including the 45 μm  45 μm area shown in Figure 4, and a cross-sectional profile. Many precipitates are clearly seen on the surface outside the 45 μm  45 μm area, while such precipitates were not observed within this area. The thickness of the precipitate layer in Figure 5 was roughly estimated to be 56 nm from the cross-sectional profile (see Table 1); the thickness was estimated at over 15 places in the same way, and then the averaged values and standard deviation were evaluated. After Figure 5 was obtained, the second cycle of CV was performed in the same potential range. Figure 6 shows the AFM image obtained at 3 V after the second cycle. Precipitate layers were again deposited in the lower and upper right of the area surrounded by the dotted line, indicating that further decomposition of the electrolyte occurred in these areas. On the other hand, no precipitate

Figure 6. AFM images (89 μm  89 μm) of the HOPG edge plane obtained at 3 V after the second cycle in 1 M LiClO4/EC+DEC (1:1, v/v%).

layers were deposited in the upper left of the same area. These observations indicate that very thin surface films should remain even after scraping by repeated scans in contact AFM, and could still act as an effective passivating film during the second cycle. These results suggest that the surface films formed on the HOPG edge plane in an EC+DEC-based electrolyte should not be uniform. After the second cycle, the upper right area (25 μm  25 μm), indicated by the dashed square in Figure 7a, was again scraped off by repeated scans of AFM. The 89 μm  89 μm area was then again observed by AFM. The obtained image is shown in Figure 7a together with a cross-sectional profile. The precipitate layers remained in the lower part in the area surrounded by the dotted line, while those inside the dashed square were scraped away. On the basis of the cross-sectional profile, the heights of the upper left area in the dotted square and the inside of the dashed square were almost the same. Furthermore, the precipitate layers outside the dashed square increased from 56 (Figure 5) to 66 nm during the second cycle (see Table 1). These facts indicate that the left area outside the dotted square was not completely passivated after the first cycle, and hence the precipitate layer grew in this area during the second cycle. Therefore, the surface film formed on this area was very similar to those in the lower and upper right in the area surrounded by the dotted line in terms of the passivation effect. Figure 7b shows an AFM image of the basal plane after the same procedures as in Figure 7a. For the basal plane, the precipitate layers were estimated to be 47 and 77 nm thick after the first and second cycles, respectively. These values were consistent with the values in our previous report.29 Table 1 summarizes the thicknesses of precipitate layers formed on the HOPG edge and basal planes after the first and second cycles. The thicknesses on the edge plane were greater than those on the basal plane after the first cycle, whereas the increase in thickness from the first to second cycle on the basal plane was greater than that on the edge plane. This fact revealed that the surface film formed on the edge plane in the first cycle could serve as an effective passivating film, compared to that on the basal plane, and suppressed further direct reductive decomposition of the solvents in the second cycle or later. Based on reports in the literature, the precipitates formed on the basal plane contain organic compounds including 25487

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precipitate layer on the HOPG edge plane was 56 nm, and this increased to 66 nm after the second cycle. On the other hand, for the basal plane, these values were 47 and 77 nm after the first and second cycles, respectively. Therefore, the increase in thickness from the first to second cycle on the basal plane was greater than that on the edge plane. On the basis of these results, the edge plane was found to be effectively covered with a passivating surface film in the first cycle compared to the basal plane. This difference should come from reaction activity for electrolyte decomposition; that is, the edge plane of graphite is highly active to decompose an electrolyte and to form effective passivating films, compared to the basal plane. However, the surface film formed on the edge plane in EC+DEC-based electrolyte solution was not uniform; the electrode surface could not be passivated completely and homogeneously. Some additives in the electrolyte solution should be useful for the formation of a uniform passivating surface film.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: +81-774-38-4977; fax: +81-774-38-4996.

’ ACKNOWLEDGMENT This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under contract from the Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING). ’ REFERENCES

Figure 7. AFM image and a cross-sectional profile of the (a) HOPG edge plane (89 μm  89 μm) and (b) HOPG basal plane (89 μm  89 μm) obtained at a potential of 3 V after the second cycle. The dotted and dashed square in the center shows the area scraped off by AFM cantilever after the first and second cycle, respectively.

lithium alkoxides, lithium alkyl carbonate, and their polymerized compounds.13a,16,19,29,36 The composition ratio in of precipitates formed on the edge plane may be different from that of the basal plane taking into account the degree of passivation. In addition, the precipitate layers formed on the edge plane in the first cycle were somewhat thicker than those on the basal plane, while they effectively served as a passivating surface film. This would likely be due to the high activity of the edge plane compared to the basal plane.

4. CONCLUSIONS We investigated the changes in the surface morphology of the HOPG edge plane during a potential sweep and the thickness of precipitate layers in EC+DEC-based electrolyte solution by in situ AFM. In the potential range of 1.0
0.6 V, the formation of fine particles smaller than 0.1 μm was observed, which should represent remnants of blisters formed between graphite layers. These fine particles remained even after scraping by repeated scans in contact-mode AFM, and should serve as effective SEI. At potentials below 0.6 V, particle-like precipitates appeared on the surface of the edge plane. After the first cycle, the thickness of the

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