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electrolyte interface in lithium-ion batteries: solid electrolyte interface formation in trifluoropropylene carbonate solution
Electrochimica Acta 45 (1999) 99±105
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STM study on graphite/electrolyte interface in lithium-ion batteries: solid electrolyte interface formation in tri¯uoropropylene carbonate solution Minoru Inaba*, Yutaka Kawatate, Atsushi Funabiki, Soon-Ki Jeong, Takeshi Abe, Zempachi Ogumi Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 7 February 1999; received in revised form 16 April 1999
Abstract Lithium intercalation within graphite was studied in an electrolyte system, 1 M LiClO4 dissolved in tri¯uoropropylene carbonate (TFPC). Lithium was intercalated within graphite in TFPC. The reversible capacity obtained (275 mAh gÿ1) was smaller than that in ethylene carbonate-based solutions while the irreversible capacity was larger (335 mAh gÿ1). The morphology change of the basal plane of highly oriented pyrolytic graphite (HOPG) was observed by electrochemical scanning tunneling microscopy (STM) to obtain information about passivating ®lm (solid electrolyte interface, SEI) formation in this solvent system. The exfoliation of graphite layers was observed at 1.1 and 1.0 V vs. Li+/Li, and then swelling of graphite layers appeared along step edges at 0.5 V. The feature observed at 0.5 V was considered as SEI itself in this solvent system. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Lithium-ion battery; Graphite; STM; Surface ®lm; Electrolyte solution
1. Introduction In lithium-ion cells, carbonaceous materials are used as negative electrodes [1]. Their charge (lithium intercalation) and discharge (deintercalation) reactions take place at extremely negative potentials close to the redox potential Li+/Li, and thereby nonaqueous electrolyte solutions are used instead of aqueous solutions. Even nonaqueous solvents should not be thermodynamically stable at such negative potentials. It is gener-
* Corresponding author. Tel.: +81-75-753-4933; fax: +8175-753-5889. E-mail address: [email protected] (M. Inaba)
ally recognized that a kind of passivating ®lm, called solid electrolyte interface (SEI) [2], is formed on carbon negative electrode in the initial stage of charging [3,4]. The presence of good SEI prevents further solvent decomposition and improves the safety and cycleability of lithium-ion cells. However, the mechanism of SEI formation as well as its structure and composition has not been fully clari®ed yet. It is widely known that the choice of solvent is very important to obtain good SEI for carbon electrodes in lithium-ion cells. When a well-graphitized carbon electrode is charged in a propylene carbonate PC-based electrolyte solution, the solvent keeps decomposing at about 1.0 V vs. Li+/Li, accompanied by exfoliation of graphite layers [5,6]. This problem has been overcome
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by the use of ethylene carbonate EC-based solutions [7]. On the other hand, such ceaseless solvent decomposition is not observed when disordered carbons heat-treated below 20008C are charged in PC-based solutions. Furthermore, the addition of 12-crown-4, which selectively coordinates lithium ion, to PC-based solutions suppresses solvent decomposition and graphite exfoliation, and enables lithium ions to be intercalated within graphite [3]. Quite recently, it has been reported that lithium can be intercalated within graphite in tri¯uoropropylene carbonate (3-tri¯uoromethyl2,5-dioxa-cyclopentan-1-one, TFPC), which has a structure quite similar to PC but contains ¯uorine atoms [8,9]. TFPC should be less stable against reduction than PC because the former has three ¯uorine atoms that are strongly electron-withdrawing. This fact clearly shows that not only the stability of a solvent against reduction, but also some unknown factors determine the ease of SEI formation on carbon negative electrodes. In previous studies [10±12], we observed topographical changes of the basal plane of highly oriented pyrolytic graphite (HOPG) in 1 M LiClO4/EC+diethyl carbonate (DEC), 1 M LiClO4/EC+dimethoxyethane (DME) and 1 M LiClO4/PC by electrochemical scanning tunneling microscopy (STM) to elucidate the mechanism of surface ®lm formation on graphite. In EC-based solutions, atomically ¯at part raised by ca. 1 nm (hill-like structures) [10,11] and irregular-shaped swelling of the surface (blisters) [12] appeared in the vicinity of step edges on the basal plane surface at about 1 and 0.7 V, respectively. The observed structures, hills and blisters, were attributed to the intercalation of solvated lithium ions [Li(solv)+ n ] between graphite layers and to the accumulation of their decomposition products in the interlayer space, respectively. In contrast to these, only rapid exfoliation and rupture of graphite layers were observed in PC [11]. From these facts, we concluded that SEI formation on graphite is triggered by the intercalation of Li(solv)n and that the stability of the host against solvent cointercalation is one of the key factors for stable SEI formation. In the present work, we applied the electrochemical STM technique to the case in 1 M LiClO4/ TFPC to investigate the mechanism of stable SEI formation in this solvent system. 2. Experimental Highly oriented pyrolytic graphite (HOPG, Advanced Ceramics, STM-1) blocks were used as test electrodes for cyclic voltammetry and STM observation. For charge discharge tests, composite electrodes made of natural graphite powder (Kansai Coke and Chemicals, NG-7, nominal particle size: 7 mm)
Fig. 1. Charge and discharge curves of natural graphite powder (NG-7) in 1 M LiClO4 dissolved in (a) PC, (b) EC+DEC (1:1) and (c) TFPC.
were used. The powder was mixed with a poly(vinilydene di¯uoride) binder, (10% by weight) using 1methyl-2-pyrrolidinone as a solvent to make a viscous slurry. The slurry was spread in a 100-mm-thick layer on a copper foil substrate, and then dried overnight at 1008C. Electrolyte solutions used in the present study were 1 M LiClO4 dissolved in PC (Mitsubishi Chemical Co., Battery Grade), a 1:1 (by volume) mixture of EC and DEC (Mitsubishi Chemical Co., Battery Grade) and
M. Inaba et al. / Electrochimica Acta 45 (1999) 99±105
TFPC (Mitsui Chemicals). The water content in each solution was less than 30 ppm. Charge±discharge tests for graphite powder electrodes were carried out galvanostatically between 0 and 2.0 V using a three-electrode cell and a commercial battery test system (Hokuto Denko, HJ101SM6). Both counter and reference electrodes were lithium metal. The current was 15.5 mA gÿ1-carbon (C/24 rate). An electrochemical STM cell used in this study was described elsewhere [11,12]. Freshly cleaved HOPG was mounted at the bottom of the cell. Only the basal plane (0.20 cm2) was brought into contact with electrolyte solution (1 M LiClO4 dissolved in TFPC). The counter and reference electrodes were platinum wire and lithium metal, respectively. Cyclic voltammograms and electrochemical STM images were obtained with an SPI-3600 system (Seiko Instruments). An apezone wax-coated Pt/Ir tip was used for STM observation. Apiezone wax is stable in carbonate solutions, which are commonly used in lithium-ion batteries, and, in fact, no change was observed after the wax-coated tip was soaked in carbonate solutions for a few days. The scanner was a SEIKO TS20A and its scan range was 20 mm in x- and y- directions and 2 mm in the z-direction. Two kinds of methods were used for STM observation in the present study. (i) Cyclic voltammetry was carried out at a sweep rate of 5 mV sÿ1 between 2.8 and 0 V. Before and after potential cycling, STM images were obtained at a sample potential of 2.8 V. (ii) The potential was lowered stepwise from 3.0 V, and STM images were obtained at various potentials. In both methods, the potential of the STM tip was ®xed at 3.0 V. The tunneling current was 0.5573 nA, and the scan rate of the tip was 1 mm sÿ1. All measurements were carried out at room temperature in an argon-®lled glove box with a dew point < ÿ608C. 3. Results and discussion 3.1. Charge±discharge characteristics Panels (a)±(c) in Fig. 1 show the ®rst charge and discharge curves of natural graphite powder (NG-7) in 1 M LiClO4 dissolved in PC, EC+DEC and TFPC, respectively. In PC, the potential remained nearly constant at ca. 0.9 V up to 2000 mAh gÿ1 upon charging and then dropped suddenly to 0 V. The electrode had no appreciable discharge capacity (not shown). This means that lithium was not intercalated from the PC solution and that the charge was consumed by other processes such as solvent decomposition and rupture of graphite. In EC+DEC, lithium was intercalated within graphite with charge and discharge capacities of 460 and 340
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Fig. 2. Cyclic voltammograms of HOPG basal plane (0.20 cm2) in 1 M LiClO4/TFPC. Scan rate=5 mV sÿ1.
mAh gÿ1, respectively (Fig. 1(b)). The irreversible capacity in the ®rst cycle was 120 mAh gÿ1. During charging, the potential dropped rapidly until it reached about 0.8 V, where a shoulder was observed. This shoulder was observed only upon the ®rst charging process and has been attributed to solvent decomposition and subsequent SEI formation on graphite [3,4]. The charge and discharge curves in Fig. 1(c) con®rmed that lithium can be intercalated within graphite in TFPC as reported by other researchers [8,9]. The reversible (discharge) and irreversible capacities were 275 and 335 mAh gÿ1, respectively. The reversible capacity was smaller than that in EC+DEC while the irreversible capacity was much larger, indicating that TFPC is not as good a solvent for graphite as EC+DEC from the viewpoint of irreversible capacity. Solvent decomposition and SEI formation started at 1.6 V in TFPC, which was higher than in EC+DEC. 3.2. STM observation after cyclic voltammetry in 1 M LiClO4/TFPC Figure 2 shows the ®rst and second cyclic voltammograms of freshly cleaved HOPG basal plane between 2.8 and 0 V in 1 M LiClO4/TFPC. On the ®rst voltammogram, reduction current began to ¯ow at ca. 1.8 V and two major peaks were observed at ca. 1.0 and 0.4 V. These peaks are attributable to solvent decomposition and SEI formation in this solvent system. Both peaks disappeared in the second cycle, which means that the graphite surface was passivated during the ®rst cycle. Redox peaks assigned to lithium intercalation and deintercalation appeared at about 0 and 0.9 V, respectively, but their peak currents were
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Fig. 3. STM images (2 2 mm) and height pro®les of HOPG basal plane surface obtained at 2.8 V (a) before cycling, (b) after one cycle between 2.8 and 0 V and (c) after three cycles in 1 M LiClO4/TFPC. The STM tip potential was 3.0 V.
much smaller than those for SEI formation. This is because only the basal plane was in contact with the solution and the sweep rate was relatively high (5 mV sÿ1) so that surface reactions were emphasized. In Fig. 3 are shown STM images and their height pro®les in 1 M LiClO4/TFPC before and after cyclic voltammetry. Before potential cycling (Fig. 3(a)), a few steps and atomically ¯at terraces, which are typical features of HOPG basal plane, were clearly observed in the image. The pro®le revealed that the height of the largest step running through the center of the image was ca. 30 nm. After one cycle down to 0 V (Fig. 3(b)), the height of the step edge was enhanced by 4 nm at maximum. At some parts in the image, exfoliation of graphite layers was observed, but it was not so pronounced. After three cycles (Fig. 3(c)), swelling along the step edge grew further in height (by 40 nm at maximum). Through analogy from the results by STM observation in 1 M LiClO4/EC+DEC in the previous studies [10±12], it is reasonable to consider that these structures were formed by the intercalation of Li(solv)+ n followed by its decomposition between layers of the host graphite. However, the swelling appeared only in the vicinity of the step edges in TFPC, and did not spread out over the terrace, in contrast to the corresponding structures (blisters) observed in EC+DEC [12]. It should be noted that no feature assigned to socalled surface ®lm was formed on the other part of the basal plane. Nevertheless, the surface was fully passi-
vated after potential cycling as shown in Fig. 2. This fact indicates that the observed swelling along the step edge is SEI itself that works as eective passivating layer.
Fig. 4. Cyclic voltammograms of (a) freshly cleaved HOPG and after the potential was kept at (b) 1.0 and (c) 0.5 V for 2 h in 1 M LiClO4/TFPC. Scan rate=5 mV sÿ1.
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Fig. 5. STM images (2 2 mm) of HOPG basal plane surface obtained at (a) 1.2, (b) 1.0 and (c) 0.5 V in 1 M LiClO4/TFPC.
3.3. STM observation at constant potentials in 1 M LiClO4/TFPC To obtain detailed information about SEI formation in TFPC, the potential of the sample was kept at 1.0 or 0.5 V for 2 h and then cyclic voltammetry was conducted between 2.8 and 0 V. After kept at 1.0 V for 2 h (curve (b) in Fig. 4), the two peaks at 1.0 and 0.4 V were still observed although their peak currents were reduced. Both peaks totally disappeared after the po-
Fig. 6. STM image (500 500 nm) of HOPG basal plane surface obtained at 1.0 V in 1 M LiClO4/TFPC. The image is an expanded view of the square part marked in Fig. 5(b).
tential was kept at 0.5 V for 2 h (curve (c) in Fig. 4). Because the reaction corresponding to the peak at 0.4 V would proceed slowly at 0.5 V, the peak at 0.4 V is assigned to the ®nal step of SEI formation. Panels (a)±(c) in Fig. 5 show in situ STM images obtained at 1.2, 1.0 and 0.5 V, respectively. Each image was obtained a few minutes after the potential was stepped to a given potential. In the ®rst cycle in Fig. 4, reduction current began to ¯ow at ca. 1.8 V. This means that some electrochemical reactions took place at potentials below 1.8 V. However, no change of surface morphology was observed at potentials r1.2 V. The image obtained at 1.2 V (Fig. 5(a)) was exactly the same as that observed at 2.8 V. In Fig. 5(a), a large step of ca. 5 nm in height running diagonally in the image was observed together with several smaller steps. At 1.1 and 1.0 V, graphite layers exfoliated slowly from step edges and the original clear step was fractured gradually during observation as is shown in Fig. 5(b). An expanded view of the square part marked in Fig. 5(b) is shown in Fig. 6. Many small steps of 0.5±1.0 nm in height were newly formed by the exfoliation of graphite layers. Similar exfoliation was observed in 1 M LiClO4/PC in a previous study [11]; however, it was much more vigorous than that in TFPC. When the potential was lowered further, no remarkable change was observed down to 0.6 V. A signi®cant change in morphology was observed at 0.5 V (Fig. 5(c)); irregular-shaped swelling appeared in the image. The image is fairly noisy, but a closer look revealed that swelling was formed along the outlines of newly formed small steps in Fig. 5(b). Height-pro®le analysis revealed that the swellings were of the order of tens of micrometers in height. These structures were quite similar to those observed in Fig. 3(c) and hence they
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are also considered to have been formed by decomposition of Li(solv)+ n at step edges. 3.4. Mechanism of surface ®lm formation in TFPC From STM observation in PC and EC-based solutions in previous studies [10±12], we concluded that SEI formation on graphite is triggered by the intercala+ tion of Li(solv)+ n ; that is, Li(solv)n is ®rst intercalated between graphite layers and then decomposes to leave immobile products, which work as SEI, in the interlayer space. This model was ®rst proposed by Besenhard et al. [13]. When the intercalation of Li(solv)+ n causes a substantial stress between graphite layers [12] or causes partial decomposition of Li(solv)+ n involving gas evolution [3,7], the graphite layers exfoliate and thereby stable surface ®lm is not formed as is the case in PC. The appearance of hill-like structures observed in EC+DEC suggests that lithium ions solvated with EC and/or DEC do not cause such a severe stress and are more stable than Li(PC)+ n , and thereby they can be accommodated securely between graphite layers [10±12]. At lower potentials in EC+DEC, Li(solv)+ decomposes between graphite layers and n leaves decomposition products in the interlayer space. In the case of TFPC, the exfoliation of graphite layers was observed at 1.1 and 1.0 V; nevertheless, stable SEI was formed. The observed exfoliation indicates that the intercalation of TFPC-solvated lithium ions causes a substantial stress between graphite layers or causes partial decomposition involving gas evolution, and thereby cannot be accommodated steadily in the interlayer space. This is reasonable because TFPC has a chemical structure similar to PC. However, the degree of the exfoliation in TFPC is not as intense as that observed in PC [12]; hence, it would not cause fatal deterioration of the host graphite during charging. As shown in Fig. 1(c), the irreversible capacity in TFPC was much larger than that in EC+DEC. This is probably due to the exfoliation of graphite layers in the initial stage. TFPC should be more vulnerable to reduction than EC or PC because it has three ¯uorine atoms that are strongly electron withdrawing. Hence TFPC is expected to decompose more easily than EC or PC. The structures observed in Figs. 3(c) and 5(c) were formed only in quite a narrow area along the step edges on HOPG. These facts indicate that TFPC-solvated lithium ions decomposed as soon as being intercalated. This rapid decomposition left immobile products that worked as stable SEI along step edges. It should be noted that exfoliation of graphite layers was not remarkable after potential cycling in the range 2.8±0 V (Fig. 3(b,c)). This inconsistency was probably brought about because low potentials were forced to the electrode upon the potential cycling at 5 mV sÿ1
and SEI was formed rapidly at step edges before graphite layers began to exfoliate. The above discussion leads us to conclude that at least the following three factors determine whether stable SEI is formed or not on graphite: (i) the ease of Li(solv)+ n intercalation between graphite layers, (ii) the magnitude of the interlayer stress caused by Li(solv)+ n intercalation, in other words, to what extent the graphite host withstands the stress and (iii) the stability of solvent [or Li(solv)+ n ] against reduction. These factors are intricately involved in the SEI formation on graphite, which would be the reason for the complexity and inconsistency in the solvent eects for carbon negative electrodes reported so far. The composition of SEI layer on carbon electrodes has been extensively studied by FT-IR [14±16], electron energy loss spectroscopy (EELS) coupled with transmission electron microscopy (TEM) [17,18], temperature programmed decomposition mass spectroscopy (TPD-MASS) [19], etc. For example, these studies have reported the presence of Li2CO3 and lithium alkylcarbonates (ROCO2Li) in SEI layer formed on carbon electrodes in EC-based solutions. In the present LiClO4/TFPC system, similar compounds may be formed by the decomposition of intercalated Li(TFPC)+ n . Since TFPC contains ¯uorine atoms, the formation of LiF should not be ignored. In addition, our model suggests that part of SEI layer should be embedded within the surface layer of graphite, which enables the SEI layer to be bound tightly on the graphite surface. Recently several researchers have reported that carbon electrodes after cycling are covered with a thick layer of polymer-like substances [20,21]. However, we did not observe such polymer-like substances by STM and were able to observe the basal plane of HOPG even at a low potential of 0.5 V or after potential sweep down to 0 V vs. Li+/Li. Because such polymer layer seems to consist of jelly-like substances containing a large quantities of electrolyte solution as reported by Tatsumi et al. [21], the STM tip would have ploughed its way through polymer-like substances in our observation, indicating that jelly-like polymer layer does not prevent solvent molecules from penetrating the polymer layer. Consequently, it is dicult to consider that such layer of polymer-like substances is the body of SEI on carbon negative electrodes. 4. Conclusions Lithium was electrochemically intercalated within a graphite powder electrode with a reversible capacity of 275 mAh gÿ1 in 1 M LiClO4/TFPC. The irreversible capacity (335 mAh gÿ1) was much larger than that in 1 M LiClO4/EC+DEC (120 mAh gÿ1). From the
M. Inaba et al. / Electrochimica Acta 45 (1999) 99±105
viewpoint of irreversible capacity, TFPC is not as good a solvent as EC+DEC although electrochemical lithium intercalation is possible. Morphology changes of the basal plane of HOPG were observed in 1 M LiClO4/TFPC by electrochemical STM to clarify solvent decomposition and SEI formation processes in this solvent system. The exfoliation of graphite layers was observed at potentials at 1.1 and 1.0 V. The exfoliation is not so fast and did not cause a fatal deterioration of the host. At 0.5 V, swelling of graphite layers along the outlines of step edges was observed. It was considered that these structures were formed by the intercalation of TFPCsolvated lithium ions followed by their decomposition between graphite layers as was concluded for similar structures (blisters) observed in EC+DEC in our previous studies. However, these features appeared in quite a narrow area along step edges. TFPC should be less stable against reduction than EC or PC and thereby TFPC-solvated lithium ions would decompose as soon as intercalated. No feature assigned to socalled surface ®lm was observed in the other part of the basal plane surface; hence, the observed structures were considered as SEI itself formed in TFPC. Acknowledgements This work was partly supported by a Grant-in-Aid for Scienti®c Research (Nos. 09650903 and 10131235) from the Ministry of Education, Science, Sports and Culture, Japan, and by CREST of JST (Japan Science and Technology). References [1] J.R. Dahn, A.K. Sleigh, H. Shi, B.M. Way, W.J. Weydanz, J.N. Reimers, Q. Zhong, U. von Sacken, in: G. Pistoia (Ed.), Lithium Batteries, new Materials and new Prospectives, Elsevier North-Holland, New York, 1993, pp. 1±47.
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[2] E. Peled, J. Electrochem. Soc. 126 (1979) 2047. [3] Z.X. Shu, R.S. McMillan, J.J. Murray, J. Electrochem. Soc. 140 (1993) 922. [4] D. Aurbach, Y. Ein-Eli, O. Chusid,, Y. Carmeli, M. Babai, H. Yamin, J. Electrochem. Soc. 141 (1994) 603. [5] A.N. Dey, B.P. Sullivan, J. Electrochem. Soc. 117 (1970) 222. [6] M. Arakawa, J. Yamaki, J. Electroanal. Chem. 219 (1987) 273. [7] R. Fong, U. von Sacken, J.R. Dahn, J. Electrochem. Soc. 137 (1990) 2009. [8] M. Nagayama, H. Ikuta, T. Uchida, M. Wakihara. Extended Abstracts of the 1995 Fall Meeting of the Electrochemical Society of Japan, Kofu, Japan, 1995, p. 162. [9] J. Arai, H. Katayama, H. Akahoshi. Extended Abstracts of the 49th ISE Meeting, Kitakyusyu, Japan, 1998, p. 642. [10] M. Inaba, Z. Siroma, A. Funabiki, Z. Ogumi, T. Abe, Y. Mizutani, M. Asano, Chem. Lett. (1995) 661. [11] M. Inaba, Z. Siroma, A. Funabiki, Z. Ogumi, T. Abe, Y. Mizutani, M. Asano, Langmuir 12 (1996) 1535. [12] M. Inaba, Z. Siroma, Y. Kawatate, A. Funabiki, Z. Ogumi, J. Power Sources 68 (1997) 221. [13] J.O. Besenhard, M. Winter, J. Yang, W. Biberacher, J. Power Sources 54 (1995) 228. [14] D. Aurbach, Y. Ein-Eli, J. Electrochem. Soc. 142 (1995) 1746. [15] D. Aurbach, Y. Ein-Eli, B. Markovsky, A. Zaben, S. Luski, Y. Carmeli, H. Yamin, J. Electrochem. Soc. 142 (1995) 2882. [16] D. Aurbach, B. Markovsky, A. Shecher, Y. Ein-Eli, J. Electrochem. Soc. 143 (1996) 2809. [17] A. Naji, J. Ghanbaja, B. Humbert, P. Willmann, D. Billaud, J. Power Sources 63 (1996) 33. [18] A. Naji, J. Ghanbaja, B. Humbert, P. Willmann, D. Billaud, J. Power Sources 62 (1996) 141. [19] S. Mori, H. Asahina, H. Suzuki, A. Yonei, K. Yokoto, J. Power Sources 68 (1997) 59. [20] E. Peled. Extended Abstracts of 9th International Meeting on Lithium Batteries, Electrolyte I-4, Edinburgh, UK, 1998. [21] K. Tatsumi, K. Zaghib, S. Koike, T. Sakai, H. Shioyama. Extended Abstracts of the 39th Battery Symposium in Japan, Sendai, 1998, pp. 449±450.
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STM study on graphite/electrolyte interface in lithium-ion batteries: solid electrolyte interface formation in tri¯uoropropylene carbonate solution Minoru Inaba*, Yutaka Kawatate, Atsushi Funabiki, Soon-Ki Jeong, Takeshi Abe, Zempachi Ogumi Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 7 February 1999; received in revised form 16 April 1999
Abstract Lithium intercalation within graphite was studied in an electrolyte system, 1 M LiClO4 dissolved in tri¯uoropropylene carbonate (TFPC). Lithium was intercalated within graphite in TFPC. The reversible capacity obtained (275 mAh gÿ1) was smaller than that in ethylene carbonate-based solutions while the irreversible capacity was larger (335 mAh gÿ1). The morphology change of the basal plane of highly oriented pyrolytic graphite (HOPG) was observed by electrochemical scanning tunneling microscopy (STM) to obtain information about passivating ®lm (solid electrolyte interface, SEI) formation in this solvent system. The exfoliation of graphite layers was observed at 1.1 and 1.0 V vs. Li+/Li, and then swelling of graphite layers appeared along step edges at 0.5 V. The feature observed at 0.5 V was considered as SEI itself in this solvent system. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Lithium-ion battery; Graphite; STM; Surface ®lm; Electrolyte solution
1. Introduction In lithium-ion cells, carbonaceous materials are used as negative electrodes [1]. Their charge (lithium intercalation) and discharge (deintercalation) reactions take place at extremely negative potentials close to the redox potential Li+/Li, and thereby nonaqueous electrolyte solutions are used instead of aqueous solutions. Even nonaqueous solvents should not be thermodynamically stable at such negative potentials. It is gener-
* Corresponding author. Tel.: +81-75-753-4933; fax: +8175-753-5889. E-mail address: [email protected] (M. Inaba)
ally recognized that a kind of passivating ®lm, called solid electrolyte interface (SEI) [2], is formed on carbon negative electrode in the initial stage of charging [3,4]. The presence of good SEI prevents further solvent decomposition and improves the safety and cycleability of lithium-ion cells. However, the mechanism of SEI formation as well as its structure and composition has not been fully clari®ed yet. It is widely known that the choice of solvent is very important to obtain good SEI for carbon electrodes in lithium-ion cells. When a well-graphitized carbon electrode is charged in a propylene carbonate PC-based electrolyte solution, the solvent keeps decomposing at about 1.0 V vs. Li+/Li, accompanied by exfoliation of graphite layers [5,6]. This problem has been overcome
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by the use of ethylene carbonate EC-based solutions [7]. On the other hand, such ceaseless solvent decomposition is not observed when disordered carbons heat-treated below 20008C are charged in PC-based solutions. Furthermore, the addition of 12-crown-4, which selectively coordinates lithium ion, to PC-based solutions suppresses solvent decomposition and graphite exfoliation, and enables lithium ions to be intercalated within graphite [3]. Quite recently, it has been reported that lithium can be intercalated within graphite in tri¯uoropropylene carbonate (3-tri¯uoromethyl2,5-dioxa-cyclopentan-1-one, TFPC), which has a structure quite similar to PC but contains ¯uorine atoms [8,9]. TFPC should be less stable against reduction than PC because the former has three ¯uorine atoms that are strongly electron-withdrawing. This fact clearly shows that not only the stability of a solvent against reduction, but also some unknown factors determine the ease of SEI formation on carbon negative electrodes. In previous studies [10±12], we observed topographical changes of the basal plane of highly oriented pyrolytic graphite (HOPG) in 1 M LiClO4/EC+diethyl carbonate (DEC), 1 M LiClO4/EC+dimethoxyethane (DME) and 1 M LiClO4/PC by electrochemical scanning tunneling microscopy (STM) to elucidate the mechanism of surface ®lm formation on graphite. In EC-based solutions, atomically ¯at part raised by ca. 1 nm (hill-like structures) [10,11] and irregular-shaped swelling of the surface (blisters) [12] appeared in the vicinity of step edges on the basal plane surface at about 1 and 0.7 V, respectively. The observed structures, hills and blisters, were attributed to the intercalation of solvated lithium ions [Li(solv)+ n ] between graphite layers and to the accumulation of their decomposition products in the interlayer space, respectively. In contrast to these, only rapid exfoliation and rupture of graphite layers were observed in PC [11]. From these facts, we concluded that SEI formation on graphite is triggered by the intercalation of Li(solv)n and that the stability of the host against solvent cointercalation is one of the key factors for stable SEI formation. In the present work, we applied the electrochemical STM technique to the case in 1 M LiClO4/ TFPC to investigate the mechanism of stable SEI formation in this solvent system. 2. Experimental Highly oriented pyrolytic graphite (HOPG, Advanced Ceramics, STM-1) blocks were used as test electrodes for cyclic voltammetry and STM observation. For charge discharge tests, composite electrodes made of natural graphite powder (Kansai Coke and Chemicals, NG-7, nominal particle size: 7 mm)
Fig. 1. Charge and discharge curves of natural graphite powder (NG-7) in 1 M LiClO4 dissolved in (a) PC, (b) EC+DEC (1:1) and (c) TFPC.
were used. The powder was mixed with a poly(vinilydene di¯uoride) binder, (10% by weight) using 1methyl-2-pyrrolidinone as a solvent to make a viscous slurry. The slurry was spread in a 100-mm-thick layer on a copper foil substrate, and then dried overnight at 1008C. Electrolyte solutions used in the present study were 1 M LiClO4 dissolved in PC (Mitsubishi Chemical Co., Battery Grade), a 1:1 (by volume) mixture of EC and DEC (Mitsubishi Chemical Co., Battery Grade) and
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TFPC (Mitsui Chemicals). The water content in each solution was less than 30 ppm. Charge±discharge tests for graphite powder electrodes were carried out galvanostatically between 0 and 2.0 V using a three-electrode cell and a commercial battery test system (Hokuto Denko, HJ101SM6). Both counter and reference electrodes were lithium metal. The current was 15.5 mA gÿ1-carbon (C/24 rate). An electrochemical STM cell used in this study was described elsewhere [11,12]. Freshly cleaved HOPG was mounted at the bottom of the cell. Only the basal plane (0.20 cm2) was brought into contact with electrolyte solution (1 M LiClO4 dissolved in TFPC). The counter and reference electrodes were platinum wire and lithium metal, respectively. Cyclic voltammograms and electrochemical STM images were obtained with an SPI-3600 system (Seiko Instruments). An apezone wax-coated Pt/Ir tip was used for STM observation. Apiezone wax is stable in carbonate solutions, which are commonly used in lithium-ion batteries, and, in fact, no change was observed after the wax-coated tip was soaked in carbonate solutions for a few days. The scanner was a SEIKO TS20A and its scan range was 20 mm in x- and y- directions and 2 mm in the z-direction. Two kinds of methods were used for STM observation in the present study. (i) Cyclic voltammetry was carried out at a sweep rate of 5 mV sÿ1 between 2.8 and 0 V. Before and after potential cycling, STM images were obtained at a sample potential of 2.8 V. (ii) The potential was lowered stepwise from 3.0 V, and STM images were obtained at various potentials. In both methods, the potential of the STM tip was ®xed at 3.0 V. The tunneling current was 0.5573 nA, and the scan rate of the tip was 1 mm sÿ1. All measurements were carried out at room temperature in an argon-®lled glove box with a dew point < ÿ608C. 3. Results and discussion 3.1. Charge±discharge characteristics Panels (a)±(c) in Fig. 1 show the ®rst charge and discharge curves of natural graphite powder (NG-7) in 1 M LiClO4 dissolved in PC, EC+DEC and TFPC, respectively. In PC, the potential remained nearly constant at ca. 0.9 V up to 2000 mAh gÿ1 upon charging and then dropped suddenly to 0 V. The electrode had no appreciable discharge capacity (not shown). This means that lithium was not intercalated from the PC solution and that the charge was consumed by other processes such as solvent decomposition and rupture of graphite. In EC+DEC, lithium was intercalated within graphite with charge and discharge capacities of 460 and 340
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Fig. 2. Cyclic voltammograms of HOPG basal plane (0.20 cm2) in 1 M LiClO4/TFPC. Scan rate=5 mV sÿ1.
mAh gÿ1, respectively (Fig. 1(b)). The irreversible capacity in the ®rst cycle was 120 mAh gÿ1. During charging, the potential dropped rapidly until it reached about 0.8 V, where a shoulder was observed. This shoulder was observed only upon the ®rst charging process and has been attributed to solvent decomposition and subsequent SEI formation on graphite [3,4]. The charge and discharge curves in Fig. 1(c) con®rmed that lithium can be intercalated within graphite in TFPC as reported by other researchers [8,9]. The reversible (discharge) and irreversible capacities were 275 and 335 mAh gÿ1, respectively. The reversible capacity was smaller than that in EC+DEC while the irreversible capacity was much larger, indicating that TFPC is not as good a solvent for graphite as EC+DEC from the viewpoint of irreversible capacity. Solvent decomposition and SEI formation started at 1.6 V in TFPC, which was higher than in EC+DEC. 3.2. STM observation after cyclic voltammetry in 1 M LiClO4/TFPC Figure 2 shows the ®rst and second cyclic voltammograms of freshly cleaved HOPG basal plane between 2.8 and 0 V in 1 M LiClO4/TFPC. On the ®rst voltammogram, reduction current began to ¯ow at ca. 1.8 V and two major peaks were observed at ca. 1.0 and 0.4 V. These peaks are attributable to solvent decomposition and SEI formation in this solvent system. Both peaks disappeared in the second cycle, which means that the graphite surface was passivated during the ®rst cycle. Redox peaks assigned to lithium intercalation and deintercalation appeared at about 0 and 0.9 V, respectively, but their peak currents were
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Fig. 3. STM images (2 2 mm) and height pro®les of HOPG basal plane surface obtained at 2.8 V (a) before cycling, (b) after one cycle between 2.8 and 0 V and (c) after three cycles in 1 M LiClO4/TFPC. The STM tip potential was 3.0 V.
much smaller than those for SEI formation. This is because only the basal plane was in contact with the solution and the sweep rate was relatively high (5 mV sÿ1) so that surface reactions were emphasized. In Fig. 3 are shown STM images and their height pro®les in 1 M LiClO4/TFPC before and after cyclic voltammetry. Before potential cycling (Fig. 3(a)), a few steps and atomically ¯at terraces, which are typical features of HOPG basal plane, were clearly observed in the image. The pro®le revealed that the height of the largest step running through the center of the image was ca. 30 nm. After one cycle down to 0 V (Fig. 3(b)), the height of the step edge was enhanced by 4 nm at maximum. At some parts in the image, exfoliation of graphite layers was observed, but it was not so pronounced. After three cycles (Fig. 3(c)), swelling along the step edge grew further in height (by 40 nm at maximum). Through analogy from the results by STM observation in 1 M LiClO4/EC+DEC in the previous studies [10±12], it is reasonable to consider that these structures were formed by the intercalation of Li(solv)+ n followed by its decomposition between layers of the host graphite. However, the swelling appeared only in the vicinity of the step edges in TFPC, and did not spread out over the terrace, in contrast to the corresponding structures (blisters) observed in EC+DEC [12]. It should be noted that no feature assigned to socalled surface ®lm was formed on the other part of the basal plane. Nevertheless, the surface was fully passi-
vated after potential cycling as shown in Fig. 2. This fact indicates that the observed swelling along the step edge is SEI itself that works as eective passivating layer.
Fig. 4. Cyclic voltammograms of (a) freshly cleaved HOPG and after the potential was kept at (b) 1.0 and (c) 0.5 V for 2 h in 1 M LiClO4/TFPC. Scan rate=5 mV sÿ1.
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Fig. 5. STM images (2 2 mm) of HOPG basal plane surface obtained at (a) 1.2, (b) 1.0 and (c) 0.5 V in 1 M LiClO4/TFPC.
3.3. STM observation at constant potentials in 1 M LiClO4/TFPC To obtain detailed information about SEI formation in TFPC, the potential of the sample was kept at 1.0 or 0.5 V for 2 h and then cyclic voltammetry was conducted between 2.8 and 0 V. After kept at 1.0 V for 2 h (curve (b) in Fig. 4), the two peaks at 1.0 and 0.4 V were still observed although their peak currents were reduced. Both peaks totally disappeared after the po-
Fig. 6. STM image (500 500 nm) of HOPG basal plane surface obtained at 1.0 V in 1 M LiClO4/TFPC. The image is an expanded view of the square part marked in Fig. 5(b).
tential was kept at 0.5 V for 2 h (curve (c) in Fig. 4). Because the reaction corresponding to the peak at 0.4 V would proceed slowly at 0.5 V, the peak at 0.4 V is assigned to the ®nal step of SEI formation. Panels (a)±(c) in Fig. 5 show in situ STM images obtained at 1.2, 1.0 and 0.5 V, respectively. Each image was obtained a few minutes after the potential was stepped to a given potential. In the ®rst cycle in Fig. 4, reduction current began to ¯ow at ca. 1.8 V. This means that some electrochemical reactions took place at potentials below 1.8 V. However, no change of surface morphology was observed at potentials r1.2 V. The image obtained at 1.2 V (Fig. 5(a)) was exactly the same as that observed at 2.8 V. In Fig. 5(a), a large step of ca. 5 nm in height running diagonally in the image was observed together with several smaller steps. At 1.1 and 1.0 V, graphite layers exfoliated slowly from step edges and the original clear step was fractured gradually during observation as is shown in Fig. 5(b). An expanded view of the square part marked in Fig. 5(b) is shown in Fig. 6. Many small steps of 0.5±1.0 nm in height were newly formed by the exfoliation of graphite layers. Similar exfoliation was observed in 1 M LiClO4/PC in a previous study [11]; however, it was much more vigorous than that in TFPC. When the potential was lowered further, no remarkable change was observed down to 0.6 V. A signi®cant change in morphology was observed at 0.5 V (Fig. 5(c)); irregular-shaped swelling appeared in the image. The image is fairly noisy, but a closer look revealed that swelling was formed along the outlines of newly formed small steps in Fig. 5(b). Height-pro®le analysis revealed that the swellings were of the order of tens of micrometers in height. These structures were quite similar to those observed in Fig. 3(c) and hence they
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are also considered to have been formed by decomposition of Li(solv)+ n at step edges. 3.4. Mechanism of surface ®lm formation in TFPC From STM observation in PC and EC-based solutions in previous studies [10±12], we concluded that SEI formation on graphite is triggered by the intercala+ tion of Li(solv)+ n ; that is, Li(solv)n is ®rst intercalated between graphite layers and then decomposes to leave immobile products, which work as SEI, in the interlayer space. This model was ®rst proposed by Besenhard et al. [13]. When the intercalation of Li(solv)+ n causes a substantial stress between graphite layers [12] or causes partial decomposition of Li(solv)+ n involving gas evolution [3,7], the graphite layers exfoliate and thereby stable surface ®lm is not formed as is the case in PC. The appearance of hill-like structures observed in EC+DEC suggests that lithium ions solvated with EC and/or DEC do not cause such a severe stress and are more stable than Li(PC)+ n , and thereby they can be accommodated securely between graphite layers [10±12]. At lower potentials in EC+DEC, Li(solv)+ decomposes between graphite layers and n leaves decomposition products in the interlayer space. In the case of TFPC, the exfoliation of graphite layers was observed at 1.1 and 1.0 V; nevertheless, stable SEI was formed. The observed exfoliation indicates that the intercalation of TFPC-solvated lithium ions causes a substantial stress between graphite layers or causes partial decomposition involving gas evolution, and thereby cannot be accommodated steadily in the interlayer space. This is reasonable because TFPC has a chemical structure similar to PC. However, the degree of the exfoliation in TFPC is not as intense as that observed in PC [12]; hence, it would not cause fatal deterioration of the host graphite during charging. As shown in Fig. 1(c), the irreversible capacity in TFPC was much larger than that in EC+DEC. This is probably due to the exfoliation of graphite layers in the initial stage. TFPC should be more vulnerable to reduction than EC or PC because it has three ¯uorine atoms that are strongly electron withdrawing. Hence TFPC is expected to decompose more easily than EC or PC. The structures observed in Figs. 3(c) and 5(c) were formed only in quite a narrow area along the step edges on HOPG. These facts indicate that TFPC-solvated lithium ions decomposed as soon as being intercalated. This rapid decomposition left immobile products that worked as stable SEI along step edges. It should be noted that exfoliation of graphite layers was not remarkable after potential cycling in the range 2.8±0 V (Fig. 3(b,c)). This inconsistency was probably brought about because low potentials were forced to the electrode upon the potential cycling at 5 mV sÿ1
and SEI was formed rapidly at step edges before graphite layers began to exfoliate. The above discussion leads us to conclude that at least the following three factors determine whether stable SEI is formed or not on graphite: (i) the ease of Li(solv)+ n intercalation between graphite layers, (ii) the magnitude of the interlayer stress caused by Li(solv)+ n intercalation, in other words, to what extent the graphite host withstands the stress and (iii) the stability of solvent [or Li(solv)+ n ] against reduction. These factors are intricately involved in the SEI formation on graphite, which would be the reason for the complexity and inconsistency in the solvent eects for carbon negative electrodes reported so far. The composition of SEI layer on carbon electrodes has been extensively studied by FT-IR [14±16], electron energy loss spectroscopy (EELS) coupled with transmission electron microscopy (TEM) [17,18], temperature programmed decomposition mass spectroscopy (TPD-MASS) [19], etc. For example, these studies have reported the presence of Li2CO3 and lithium alkylcarbonates (ROCO2Li) in SEI layer formed on carbon electrodes in EC-based solutions. In the present LiClO4/TFPC system, similar compounds may be formed by the decomposition of intercalated Li(TFPC)+ n . Since TFPC contains ¯uorine atoms, the formation of LiF should not be ignored. In addition, our model suggests that part of SEI layer should be embedded within the surface layer of graphite, which enables the SEI layer to be bound tightly on the graphite surface. Recently several researchers have reported that carbon electrodes after cycling are covered with a thick layer of polymer-like substances [20,21]. However, we did not observe such polymer-like substances by STM and were able to observe the basal plane of HOPG even at a low potential of 0.5 V or after potential sweep down to 0 V vs. Li+/Li. Because such polymer layer seems to consist of jelly-like substances containing a large quantities of electrolyte solution as reported by Tatsumi et al. [21], the STM tip would have ploughed its way through polymer-like substances in our observation, indicating that jelly-like polymer layer does not prevent solvent molecules from penetrating the polymer layer. Consequently, it is dicult to consider that such layer of polymer-like substances is the body of SEI on carbon negative electrodes. 4. Conclusions Lithium was electrochemically intercalated within a graphite powder electrode with a reversible capacity of 275 mAh gÿ1 in 1 M LiClO4/TFPC. The irreversible capacity (335 mAh gÿ1) was much larger than that in 1 M LiClO4/EC+DEC (120 mAh gÿ1). From the
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viewpoint of irreversible capacity, TFPC is not as good a solvent as EC+DEC although electrochemical lithium intercalation is possible. Morphology changes of the basal plane of HOPG were observed in 1 M LiClO4/TFPC by electrochemical STM to clarify solvent decomposition and SEI formation processes in this solvent system. The exfoliation of graphite layers was observed at potentials at 1.1 and 1.0 V. The exfoliation is not so fast and did not cause a fatal deterioration of the host. At 0.5 V, swelling of graphite layers along the outlines of step edges was observed. It was considered that these structures were formed by the intercalation of TFPCsolvated lithium ions followed by their decomposition between graphite layers as was concluded for similar structures (blisters) observed in EC+DEC in our previous studies. However, these features appeared in quite a narrow area along step edges. TFPC should be less stable against reduction than EC or PC and thereby TFPC-solvated lithium ions would decompose as soon as intercalated. No feature assigned to socalled surface ®lm was observed in the other part of the basal plane surface; hence, the observed structures were considered as SEI itself formed in TFPC. Acknowledgements This work was partly supported by a Grant-in-Aid for Scienti®c Research (Nos. 09650903 and 10131235) from the Ministry of Education, Science, Sports and Culture, Japan, and by CREST of JST (Japan Science and Technology). References [1] J.R. Dahn, A.K. Sleigh, H. Shi, B.M. Way, W.J. Weydanz, J.N. Reimers, Q. Zhong, U. von Sacken, in: G. Pistoia (Ed.), Lithium Batteries, new Materials and new Prospectives, Elsevier North-Holland, New York, 1993, pp. 1±47.
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