- Email: [email protected]
C thin film fabricated by magnetron sputtering
RARE METALS, Vol. 27, No. 5, Oct 2008, p. 507
Lithium intercalation/de-intercalation behavior of a composite Sn/C thin film fabricated by magnetron sputtering ZHAO Lingzhia, b, c, HU Shejuna, c, d, LI Weishana, c, LI Limingb, and HOU Xianhuaa, c a
Department of Chemistry, South China Normal University, Guangzhou 510006, China Faculty of Material and Energy, Guangdong University of Technology, Guangzhou 510006, China c Key Lab of Technology on Electrochemical Energy Storage and Power Generation in Guangdong Universities, Guangzhou 510006, China d Department of Mathematics and Physics, Wuyi University, Jiangmen 529020, China b
Received 28 September 2007; received in revised form 29 October 2007; accepted 30 October 2007
Abstract A tin film of 320 nm in thickness on Cu foil and its composite film with graphite of a50 nm in thickness on it were fabricated by magnetron sputtering. The surface morphology, composition, surface distributions of alloy elements, and lithium intercalation/de-intercalation behaviors of the fabricated films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectrometry (ICP), cyclic voltammetry (CV), and galvanostatic charge/discharge (GC) measurements. It is found that the lithium intercalation/de-intercalation behavior of the Sn film can be significantly improved by its composite with graphite. With cycling, the discharge capacity of the Sn film without composite changes from 570 mAh/g of the 2nd cycle to 270 mAh/g of the 20th cycle, and its efficiency for the discharge and charge is between 90% and 95%. Nevertheless, the discharge capacity of the composite Sn/C film changes from 575 mAh/g of the 2nd cycle to 515 mAh/g of the 20th cycle, and its efficiency for the discharge and charge is between 95% and 100%. The performance improvement of tin by its composite with graphite is ascribed to the retardation of the bulk tin cracking from volume change during lithium intercalation and de-intercalation, which leads to the pulverization of tin. Keywords: lithium-ion battery; anode; magnetron sputtering; composite film; lithium intercalation/de-intercalation
1. Introduction Much attention has been paid to the rechargeable thin film lithium ion batteries in recent years, which have great potential for application in mini-portable electronic devices. And the development of their anode material is considered as one of the breakthroughs to meet the demands for power sources with higher performance. Much work has been done to find a substitute for graphite, used as the anode in a commercial lithium ion battery. As a very promising anode material, tin has a high theoretical specific capacity, which reaches 990 mAh/g. But tin suffers from severe volume variations during lithium intercalation and de-intercalation, leading to quick capacity fade. This drawback limits its practical application as the anode of lithium ion battery. It has been reported that in the case of tin oxide anode, tin is the key to the high discharge capacity and Li2O acts as a matrix that helps to lengthen the cycle life of the electrode [1]. Since then, many efforts have been focused on the seCorresponding author: Li Weishan
E-mail: [email protected]
lection of adequate matrix that can ease the electrode stress, which appears during cycling, and extend effectively the cycle life. Many tin-based alloys and composite, such as Sn-Ni [2], Sn-graphite [3], and SnO2-carbon [4], have been therefore investigated. The cyclic stability of tin can be improved to a great extent but its capacity is reduced with cycles due to the pulverization of tin [5-6]. It has been demonstrated that this drawback can be partly overcome by using different fabrication methods and by reducing the metal particle size. For instance, tin oxide thin films were prepared by the electrostatic spray deposition at 400qC and followed by annealing at 500qC [7]. When the electrode is cycled from 0.05 to 2.5 V (versus Li/Li+), the capacity of about 400 mAh/g is maintained over 100 cycles. Fabricated by electroplating, a Sn film with a rough surface as anode was studied [8]. Its capacity is about 300 mAh/g after 20 cycles when the electrode is cycled from 0.01 to 1.0 V. Obviously, the performance of the tin film electrode needs to be further improved. Recently, it is found in our lab
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that the tin film electrode consisting of fine and compact tin particles (less than 0.5 μm) obtained by electrodeposition onto Cu foil collector has a better cyclic stability than the electrode consisting of large and separated tin particles (about 3 μm) [9]. It is believed that combining tin with graphite can also improve the performance of tin as anode for lithium ion batteries [10]. In this article, magnetic sputtering (MS) is used to fabricate the composite thin film of tin on the Cu foil with graphite on the tin film (Sn/C) and its lithium intercalation/de-intercalation behavior is studied with a comparison of pure Sn thin film. It is expected that the carbon covering of tin can prevent it from becoming electrically disconnected through the conductive graphite layer in the surface and act as a buffer for the alleviation of tin expansion as well as a lithium intercalated material.
2. Experimental Sn and Sn/C thin films were prepared onto Cu-foil substrates in a deposition chamber evacuated to 5 × 103 Pa by multi-target radio frequency magnetic sputtering-4 (RFMS-4) apparatus. The pure tin target (99.9%, I59 mm × 4 mm) and the graphite target (99.95%, I52 mm × 5 mm) were used as starting materials. The Cu-foil substrates were cleaned ultrasonically with acetone and fixed onto a rotatable holder at the position 59 mm above the targets. Pure argon (99.999%) was inputted at 60 mL/min into the chamber till the vacuum demand in the chamber was reached. Direct current (DC) power with 100 W was used to bombard the substrates for 5 min before the fabrication of the thin film. Radio frequency (RF) power with 100 W was used to smelt the target for 5 min to eliminate the impurity of the surface. The Sn thin film was obtained by sputtering tin target with a DC power of 250 W for 20 min. The composite Sn/C film was fabricated by sputtering graphite target with a RF power of 150 W onto the obtained Sn thin film for 10 min. The composition and structure of the fabricated thin films were identified by an X-ray diffractometer (Y-2000 XRD systems, Dandong, China) with Cu K radiation. The morphology of the samples was observed using scanning electron microscopy (SEM, JSM-6380-LA, JEOL). The surface distributions of elements on the surface of the samples were measured by electron probe microanalyzer (EPMA, EPMA-1600, Shimadzu). The ratio of carbon to tin and the electronic state of carbon in the composite thin film were determined using X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Kratos). The content of Sn in the films was determined by inductively coupled plasma atomic emission spectrometry (ICP) and the content of carbon was estimated from the quality margin of the Sn/C and Sn films.
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Cyclic voltammetry (CV) and galvanostatic charge/ discharge (GC) measurements were carried out using an 8-channel testing system (Solartron1480, England) at room temperature with CR2016 button cells. The cells were assembled in an argon-filled glove box (Mikrouna, Sukei1220/ 750). Lithium foil was used as both counter and reference electrodes, and the tin films were used as the working electrodes. The electrolyte was 1 M LiPF6 dissolved in a 1:1:1 (volume ratio) mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Polypropylene membrane (celgard2400) was used as the separator. In the GC test, the current was 0.1 mA/cm2 and the potentials were between 0 and 1.5 V. In the CV test, the potentials were between 0 and 2.0 V and the scan rate was 1 mV/s. The potentials were with respect to Li/Li+. In this article, the discharge and charge processes corresponded to lithium intercalation and lithium de-intercalation, respectively.
3. Results and discussion 3.1. Characterization of thin films Fig. 1 presents the surface morphologies of the fabricated Sn and Sn/C films. It can be seen that both materials are composed of uniform spherical particles. The average particle size on the Sn/C film is larger than that on the Sn film, ~100 nm for the Sn film and ~150 nm for the Sn/C film, in-
Fig. 1. Morphologies of Sn (a) and Sn/C (b) films.
Zhao L.Z. et al., Lithium intercalation/de-intercalation behavior of a composite Sn/C thin film fabricated by…
dicating that the preliminary deposited Sn was covered by carbon of ~50 nm in thickness. The thickness of the Sn film estimated by ICP is 320 nm. Fig. 2 shows the XRD patterns of the Sn and Sn/C films on Cu foil substrates. The diffraction peaks of Sn and Cu appear in the XRD patterns of the Sn and Sn/C films. The Cu diffraction should be ascribed to that of the substrate. No other diffraction peaks can be observed for the Sn film and thus it is pure. The diffraction of graphite in the Sn/C film is weak, indicating that the content of graphite in the film is less. It should be noticed that there is no any diffraction for CuSn alloys. This suggests that during the film fabrication, CuSn alloy cannot form or it may form but its content can be neglected.
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states of carbon. One is at the binding energy of 284.31 eV, corresponding to pure graphite [11], and others are at the binding energy of 287.07 eV, corresponding to carbon in the form of CC and CO [12]. The former makes up ~84 at.% of the total carbon, indicating that carbon in the Sn/C film is mainly in the form of graphite.
Fig. 3. Percentage of graphite in total deposited carbon reflected by the C 1s peak in XPS.
3.2. Voltammetric behaviors
Fig. 2. XRD patterns of Sn and Sn/C films.
To evaluate the average size of crystallites, the Scherrer equation was used with the width of the (200) peak of Sn from XRD data. The average crystallite size of tin in the Sn and Sn/C films has been found to be about 54 nm. To identify the electronic state of carbon in the Sn/C film, XPS analysis in 1-3 nm lengthways deep was carried out. Fig. 3 presents the percentage of graphite in total deposited carbon reflected by the C 1s peak in XPS. There are several
Fig. 4 shows the voltammograms of Sn and Sn/C films for the first three cycles at 0.1 mV/s. The reduction currents at the potentials from 1.7 to 1.1 V and from 1.25 to 0 V in forward scan correspond to the formation of solid electrolyte interphase (SEI) and the lithium intercalation, respectively, and the oxidation current from 0 to 1.25 V corresponds to the lithium de-intercalation [9]. It can be seen from Fig. 4(a) that during the forward scan of the first cycle, the current response for SEI formation is small and the potential of lithium intercalation is more negative. This can be ascribed to the surface state of tin. Tin should be covered with tin oxide formed in the air. SEI cannot be formed on the oxide and lithium intercalation is hard to take place before the oxide is reduced to tin. In the
Fig. 4. Cyclic voltammograms of Sn (a) and Sn/C (b) films.
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following forward cycles, the significant current for SEI formation can be observed, indicating that the fresh tin surface is always available during cycling. This can be ascribed to the crack of bulk tin due to volume change during intercalation and de-intercalation. Different voltammograms for the Sn/C film can be distinguished by comparing Fig. 4(b) with Fig. 4(a). It can be seen from Fig. 4(b) that the current for SEI formation appears during the first cycle and disappears in the following cycle. This suggests that the presence of graphite on Sn can retard the crack of bulk tin. The lithium intercalation capacity of the Sn film, estimated from the current at the potentials for lithium intercalation, decreases with increasing cycle numbers, as shown in Fig. 4(a). However, the lithium intercalation capacity of the Sn/C film is hardly dependent on the cycle number. This suggests that the cyclic stability of the Sn film for lithium intercalation and de-intercalation is poor and can be improved by its composite with graphite.
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mary capacity but improve slightly the reversible capacity. Fig. 6 shows the cyclic stability of the cells with Sn and Sn/C films as anodes. The discharge capacities of the Sn film decrease with an increase in cycle number. In the second cycle, the discharge capacity of the Sn film is 570 mAh/g, and there is 26 % of capacity loss from the 1st cycle to the 2nd cycle. After 20 cycles, the discharge capacity of the Sn film becomes 270 mAh/g, and there is 53% of capacity loss from the 2nd cycle to the 20th cycle. The coulombic efficiency of the Sn film is between 90% and 95%. It is obvious that the cyclic stability of the Sn film is poor.
3.3. Charge and discharge behaviors Fig 5 shows the first charge-discharge curves of the cells with Sn and Sn/C films as the anodes at 0.1 mA/cm2. The discharge (lithium intercalation) and charge (lithium de-intercalation) capacities of the Sn film are 771 and 490 mAh/g, respectively. This loss can be ascribed to the reduction of tin oxide and the decomposition of electrolyte on the newly formed tin without any protection of SEI [13]. The high potentials of the discharge curve of the Sn film at the discharge capacity lower than 200 mAh/g can account for these processes.
Fig. 6. Cycling stability of the cells with Sn and Sn/C films as anodes.
It can be found by comparing the Sn/C film with the Sn film that the cyclic stability of the Sn/C film is much better than that of the Sn film. It can be seen that the discharge capacities of the Sn/C film change little with the increase in cycle number. In the second cycle, the discharge capacity of the Sn/C film is 575 mAh/g, there is 23% of capacity loss from the 1st cycle to the 2nd cycle, less than that of the Sn film. After 20 cycles, the discharge capacity of the Sn/C film becomes 515 mAh/g, there is only 10% of capacity loss from the 2nd cycle to the 20th cycle, far less than that of the Sn film. The coulombic efficiency of Sn/C film is between 95% and 100%, higher than that of the Sn film. It is apparent that the composite of tin with graphite has improved significantly its cyclic stability for lithium intercalation and de-intercalation. 3.4. Morphology and structure change after cycles
Fig. 5. The first charge-discharge curves of the cells with Sn and Sn/C films as anodes.
The discharge and charge capacities of the Sn/C film are 750 and 500 mAh/g, respectively, as shown in Fig. 5. The discharge capacity of the Sn/C film is lower but its charge capacity is slightly higher than that of the Sn film. Apparently, the composite of tin with carbon does reduce its pri-
Figs. 7 and 8 present the surface morphologies and XRD patterns of Sn and Sn/C films after 20 cycles, respectively. It can be easily found from Fig. 7(a) that the Sn film is pulverized after 20 cycles, which is confirmed by the XRD data shown in Fig. 8. No diffraction line of tin in the Sn film can be distinguished. It is well known that there is a volume change of 676% between fully intercalated and de- intercalated tin [10]. The large volume change results in the crack-
Zhao L.Z. et al., Lithium intercalation/de-intercalation behavior of a composite Sn/C thin film fabricated by…
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inner stress resulting from the volume change of tin due to intercalation and de-intercalation, retards the cracking of the film, and keeps the film in integrity. This accounts for the good cyclic stability of the Sn/C film. It is noted from Fig. 6 that from the 15th cycle, there is a decreasing tendency in the discharge and charge efficiency of the Sn/C film. This can be ascribed to the appearance of the crevices of the film shown in Fig. 7(b). More research needs to be done on the composite of tin with graphite to obtain a composite Sn/C film that can be used in practice.
4. Conclusions A composite Sn/C thin film can be fabricated by magnetron sputtering. The composite of tin with graphite can improve its performance as lithium intercalated anode of lithium ion battery. The composite hardly changes the primary discharge and charge capacities for lithium intercalation and de-intercalation of tin, but improves its cyclic stability and efficiency for discharge-charge. This effect is ascribed to the reduction of inner stress resulting from lithium intercalation and de-intercalation in the tin film by the composite graphite, which avoids primary pulverization of tin. Fig. 7. Morphologies of Sn (a) and Sn/C films (b) after 20 cycles.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (Nos. 50771046 and 20373016), the Natural Science Foundation of Guangdong Province (No. 05200534), and the Key Projects of Guangdong Province and Guangzhou City, China (Nos. 2006A10704003 and2006Z3-D2031).
References
Fig. 8. XRD patterns of Sn and Sn/C films after 20 cycles.
ing of the tin film and the formation of SEI on the surface of cracked tin. Thus tin is continuously separated from the bulk due to cycling and leads to pulverization. This accounts for the poor cyclic stability of the Sn film. It can be seen from Fig. 7(b) that after 20 cycles, the Sn/C film keeps its primary film structure though some crevices appear. Fig. 8 demonstrates that tin in the Sn/C film retains its preferable crystal structures. It can be concluded by comparing Fig. 7(b) with Fig. 7(a) that the pulverization of the Sn film can be reduced by its composite with graphite to a great extent. The composite of tin with graphite alleviates
[1] Courteny I. and Dahn J., Electrochemical and in situ X-ray diffraction studies of the reaction of lithium with tin oxide composites, J. Electrochem Soc., 1997, 144: 2045. [2] Mukaibo H., Momma T., and Osaka T., Changes of electro-deposited Sn-Ni alloy thin film for lithium ion battery anodes during charge discharge cycling, J. Power Sources, 2005, 146: 457. [3] Fang T., Hsiao L.Y., Duh J.G., and Sheen S.R., A novel composite negative electrode consist of multiphase Sn compounds and mesophase graphite powders for lithium ion batteries, J. Power Sources, 2006,160: 536. [4] Yuan L., Konstantinov K., Wang G.X., Liu H.K., and Dou S.X., Nano-structured SnO2-carbon composites obtained by in situ spray pyrolysis method as anodes in lithium batteries, J. Power Sources, 2005,146: 180. [5] Li C.M., Zhang R.Y., Li W.S., Zhao L.Z., and Hu S.J., Capacity fade mechanism of metal tin as anode for Li ion battery,
512 Chin. J. Power Sources (in Chinese), 2008, 32 (1): 21. [6] Tan C.L., Lv D.S., Li W.S., Xu M.Q., Zhou D.Y., and Hu S.J., Study of Sn/Ni alloy films electrodeposited as anode materials for Li-ion batteries, Rare Met. Mater. Eng. (in Chinese), 2008, 37: 472. [7] Mohamedi M., Lee S.J., Takahashi D., Nishizawa M., Itoh T., and Uchida I., Amorphous tin oxide films: preparation and characterization as an anode active material for lithium ion batteries, Electrochim. Acta, 2001, 46: 1161. [8] Morimoto H., Tobishima S.I., and Negishi H., Anode behavior of electroplated rough surface Sn thin films for lithium-ion batteries, J. Power Sources, 2005, 146: 469. [9] Li C.M., Huang Q.M., Zhang R.Y., Li W.S., Zhao L.Z., and Hu S.J., A comparison of the performances of two kinds of Sn films as lithium-ion insertion electrodes prepared by electro-
RARE METALS, Vol. 27, No. 5, Oct 2008 deposition, Acta Metall. Sin. (in Chinese), 2007, 43: 515. [10] Wang G.S., Ahn J.H., Lindsay M.J., Sun L. Bradhurst D.H., Dou S.X., and Liu H.K., Graphite-tin composites as anode materials for lithium-ion batteries, J. Power Sources, 2001, 97-98: 211. [11] Evans J.F. and Kuwana T., Introduction of functional groups onto carbon electrodes via treatment with radio-frequency plasmas, Anal. Chem., 1979, 51: 358. [12] Castner D.G., Fisher E.R., and Mackie N.M., Characterization of pulsed-plasma-polymerized aromatic films, Langmuir, 1998, 14: 1227. [13] Zhang X.J., Huang S.T., Wu G.L., Lu S.G., and Cai Z.P., Tin oxide-graphite composite for lithium storage material in lithium-ion batteries, Rare Met., 2003, 22 (3): 226.
Lithium intercalation/de-intercalation behavior of a composite Sn/C thin film fabricated by magnetron sputtering ZHAO Lingzhia, b, c, HU Shejuna, c, d, LI Weishana, c, LI Limingb, and HOU Xianhuaa, c a
Department of Chemistry, South China Normal University, Guangzhou 510006, China Faculty of Material and Energy, Guangdong University of Technology, Guangzhou 510006, China c Key Lab of Technology on Electrochemical Energy Storage and Power Generation in Guangdong Universities, Guangzhou 510006, China d Department of Mathematics and Physics, Wuyi University, Jiangmen 529020, China b
Received 28 September 2007; received in revised form 29 October 2007; accepted 30 October 2007
Abstract A tin film of 320 nm in thickness on Cu foil and its composite film with graphite of a50 nm in thickness on it were fabricated by magnetron sputtering. The surface morphology, composition, surface distributions of alloy elements, and lithium intercalation/de-intercalation behaviors of the fabricated films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectrometry (ICP), cyclic voltammetry (CV), and galvanostatic charge/discharge (GC) measurements. It is found that the lithium intercalation/de-intercalation behavior of the Sn film can be significantly improved by its composite with graphite. With cycling, the discharge capacity of the Sn film without composite changes from 570 mAh/g of the 2nd cycle to 270 mAh/g of the 20th cycle, and its efficiency for the discharge and charge is between 90% and 95%. Nevertheless, the discharge capacity of the composite Sn/C film changes from 575 mAh/g of the 2nd cycle to 515 mAh/g of the 20th cycle, and its efficiency for the discharge and charge is between 95% and 100%. The performance improvement of tin by its composite with graphite is ascribed to the retardation of the bulk tin cracking from volume change during lithium intercalation and de-intercalation, which leads to the pulverization of tin. Keywords: lithium-ion battery; anode; magnetron sputtering; composite film; lithium intercalation/de-intercalation
1. Introduction Much attention has been paid to the rechargeable thin film lithium ion batteries in recent years, which have great potential for application in mini-portable electronic devices. And the development of their anode material is considered as one of the breakthroughs to meet the demands for power sources with higher performance. Much work has been done to find a substitute for graphite, used as the anode in a commercial lithium ion battery. As a very promising anode material, tin has a high theoretical specific capacity, which reaches 990 mAh/g. But tin suffers from severe volume variations during lithium intercalation and de-intercalation, leading to quick capacity fade. This drawback limits its practical application as the anode of lithium ion battery. It has been reported that in the case of tin oxide anode, tin is the key to the high discharge capacity and Li2O acts as a matrix that helps to lengthen the cycle life of the electrode [1]. Since then, many efforts have been focused on the seCorresponding author: Li Weishan
E-mail: [email protected]
lection of adequate matrix that can ease the electrode stress, which appears during cycling, and extend effectively the cycle life. Many tin-based alloys and composite, such as Sn-Ni [2], Sn-graphite [3], and SnO2-carbon [4], have been therefore investigated. The cyclic stability of tin can be improved to a great extent but its capacity is reduced with cycles due to the pulverization of tin [5-6]. It has been demonstrated that this drawback can be partly overcome by using different fabrication methods and by reducing the metal particle size. For instance, tin oxide thin films were prepared by the electrostatic spray deposition at 400qC and followed by annealing at 500qC [7]. When the electrode is cycled from 0.05 to 2.5 V (versus Li/Li+), the capacity of about 400 mAh/g is maintained over 100 cycles. Fabricated by electroplating, a Sn film with a rough surface as anode was studied [8]. Its capacity is about 300 mAh/g after 20 cycles when the electrode is cycled from 0.01 to 1.0 V. Obviously, the performance of the tin film electrode needs to be further improved. Recently, it is found in our lab
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that the tin film electrode consisting of fine and compact tin particles (less than 0.5 μm) obtained by electrodeposition onto Cu foil collector has a better cyclic stability than the electrode consisting of large and separated tin particles (about 3 μm) [9]. It is believed that combining tin with graphite can also improve the performance of tin as anode for lithium ion batteries [10]. In this article, magnetic sputtering (MS) is used to fabricate the composite thin film of tin on the Cu foil with graphite on the tin film (Sn/C) and its lithium intercalation/de-intercalation behavior is studied with a comparison of pure Sn thin film. It is expected that the carbon covering of tin can prevent it from becoming electrically disconnected through the conductive graphite layer in the surface and act as a buffer for the alleviation of tin expansion as well as a lithium intercalated material.
2. Experimental Sn and Sn/C thin films were prepared onto Cu-foil substrates in a deposition chamber evacuated to 5 × 103 Pa by multi-target radio frequency magnetic sputtering-4 (RFMS-4) apparatus. The pure tin target (99.9%, I59 mm × 4 mm) and the graphite target (99.95%, I52 mm × 5 mm) were used as starting materials. The Cu-foil substrates were cleaned ultrasonically with acetone and fixed onto a rotatable holder at the position 59 mm above the targets. Pure argon (99.999%) was inputted at 60 mL/min into the chamber till the vacuum demand in the chamber was reached. Direct current (DC) power with 100 W was used to bombard the substrates for 5 min before the fabrication of the thin film. Radio frequency (RF) power with 100 W was used to smelt the target for 5 min to eliminate the impurity of the surface. The Sn thin film was obtained by sputtering tin target with a DC power of 250 W for 20 min. The composite Sn/C film was fabricated by sputtering graphite target with a RF power of 150 W onto the obtained Sn thin film for 10 min. The composition and structure of the fabricated thin films were identified by an X-ray diffractometer (Y-2000 XRD systems, Dandong, China) with Cu K radiation. The morphology of the samples was observed using scanning electron microscopy (SEM, JSM-6380-LA, JEOL). The surface distributions of elements on the surface of the samples were measured by electron probe microanalyzer (EPMA, EPMA-1600, Shimadzu). The ratio of carbon to tin and the electronic state of carbon in the composite thin film were determined using X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Kratos). The content of Sn in the films was determined by inductively coupled plasma atomic emission spectrometry (ICP) and the content of carbon was estimated from the quality margin of the Sn/C and Sn films.
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Cyclic voltammetry (CV) and galvanostatic charge/ discharge (GC) measurements were carried out using an 8-channel testing system (Solartron1480, England) at room temperature with CR2016 button cells. The cells were assembled in an argon-filled glove box (Mikrouna, Sukei1220/ 750). Lithium foil was used as both counter and reference electrodes, and the tin films were used as the working electrodes. The electrolyte was 1 M LiPF6 dissolved in a 1:1:1 (volume ratio) mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Polypropylene membrane (celgard2400) was used as the separator. In the GC test, the current was 0.1 mA/cm2 and the potentials were between 0 and 1.5 V. In the CV test, the potentials were between 0 and 2.0 V and the scan rate was 1 mV/s. The potentials were with respect to Li/Li+. In this article, the discharge and charge processes corresponded to lithium intercalation and lithium de-intercalation, respectively.
3. Results and discussion 3.1. Characterization of thin films Fig. 1 presents the surface morphologies of the fabricated Sn and Sn/C films. It can be seen that both materials are composed of uniform spherical particles. The average particle size on the Sn/C film is larger than that on the Sn film, ~100 nm for the Sn film and ~150 nm for the Sn/C film, in-
Fig. 1. Morphologies of Sn (a) and Sn/C (b) films.
Zhao L.Z. et al., Lithium intercalation/de-intercalation behavior of a composite Sn/C thin film fabricated by…
dicating that the preliminary deposited Sn was covered by carbon of ~50 nm in thickness. The thickness of the Sn film estimated by ICP is 320 nm. Fig. 2 shows the XRD patterns of the Sn and Sn/C films on Cu foil substrates. The diffraction peaks of Sn and Cu appear in the XRD patterns of the Sn and Sn/C films. The Cu diffraction should be ascribed to that of the substrate. No other diffraction peaks can be observed for the Sn film and thus it is pure. The diffraction of graphite in the Sn/C film is weak, indicating that the content of graphite in the film is less. It should be noticed that there is no any diffraction for CuSn alloys. This suggests that during the film fabrication, CuSn alloy cannot form or it may form but its content can be neglected.
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states of carbon. One is at the binding energy of 284.31 eV, corresponding to pure graphite [11], and others are at the binding energy of 287.07 eV, corresponding to carbon in the form of CC and CO [12]. The former makes up ~84 at.% of the total carbon, indicating that carbon in the Sn/C film is mainly in the form of graphite.
Fig. 3. Percentage of graphite in total deposited carbon reflected by the C 1s peak in XPS.
3.2. Voltammetric behaviors
Fig. 2. XRD patterns of Sn and Sn/C films.
To evaluate the average size of crystallites, the Scherrer equation was used with the width of the (200) peak of Sn from XRD data. The average crystallite size of tin in the Sn and Sn/C films has been found to be about 54 nm. To identify the electronic state of carbon in the Sn/C film, XPS analysis in 1-3 nm lengthways deep was carried out. Fig. 3 presents the percentage of graphite in total deposited carbon reflected by the C 1s peak in XPS. There are several
Fig. 4 shows the voltammograms of Sn and Sn/C films for the first three cycles at 0.1 mV/s. The reduction currents at the potentials from 1.7 to 1.1 V and from 1.25 to 0 V in forward scan correspond to the formation of solid electrolyte interphase (SEI) and the lithium intercalation, respectively, and the oxidation current from 0 to 1.25 V corresponds to the lithium de-intercalation [9]. It can be seen from Fig. 4(a) that during the forward scan of the first cycle, the current response for SEI formation is small and the potential of lithium intercalation is more negative. This can be ascribed to the surface state of tin. Tin should be covered with tin oxide formed in the air. SEI cannot be formed on the oxide and lithium intercalation is hard to take place before the oxide is reduced to tin. In the
Fig. 4. Cyclic voltammograms of Sn (a) and Sn/C (b) films.
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following forward cycles, the significant current for SEI formation can be observed, indicating that the fresh tin surface is always available during cycling. This can be ascribed to the crack of bulk tin due to volume change during intercalation and de-intercalation. Different voltammograms for the Sn/C film can be distinguished by comparing Fig. 4(b) with Fig. 4(a). It can be seen from Fig. 4(b) that the current for SEI formation appears during the first cycle and disappears in the following cycle. This suggests that the presence of graphite on Sn can retard the crack of bulk tin. The lithium intercalation capacity of the Sn film, estimated from the current at the potentials for lithium intercalation, decreases with increasing cycle numbers, as shown in Fig. 4(a). However, the lithium intercalation capacity of the Sn/C film is hardly dependent on the cycle number. This suggests that the cyclic stability of the Sn film for lithium intercalation and de-intercalation is poor and can be improved by its composite with graphite.
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mary capacity but improve slightly the reversible capacity. Fig. 6 shows the cyclic stability of the cells with Sn and Sn/C films as anodes. The discharge capacities of the Sn film decrease with an increase in cycle number. In the second cycle, the discharge capacity of the Sn film is 570 mAh/g, and there is 26 % of capacity loss from the 1st cycle to the 2nd cycle. After 20 cycles, the discharge capacity of the Sn film becomes 270 mAh/g, and there is 53% of capacity loss from the 2nd cycle to the 20th cycle. The coulombic efficiency of the Sn film is between 90% and 95%. It is obvious that the cyclic stability of the Sn film is poor.
3.3. Charge and discharge behaviors Fig 5 shows the first charge-discharge curves of the cells with Sn and Sn/C films as the anodes at 0.1 mA/cm2. The discharge (lithium intercalation) and charge (lithium de-intercalation) capacities of the Sn film are 771 and 490 mAh/g, respectively. This loss can be ascribed to the reduction of tin oxide and the decomposition of electrolyte on the newly formed tin without any protection of SEI [13]. The high potentials of the discharge curve of the Sn film at the discharge capacity lower than 200 mAh/g can account for these processes.
Fig. 6. Cycling stability of the cells with Sn and Sn/C films as anodes.
It can be found by comparing the Sn/C film with the Sn film that the cyclic stability of the Sn/C film is much better than that of the Sn film. It can be seen that the discharge capacities of the Sn/C film change little with the increase in cycle number. In the second cycle, the discharge capacity of the Sn/C film is 575 mAh/g, there is 23% of capacity loss from the 1st cycle to the 2nd cycle, less than that of the Sn film. After 20 cycles, the discharge capacity of the Sn/C film becomes 515 mAh/g, there is only 10% of capacity loss from the 2nd cycle to the 20th cycle, far less than that of the Sn film. The coulombic efficiency of Sn/C film is between 95% and 100%, higher than that of the Sn film. It is apparent that the composite of tin with graphite has improved significantly its cyclic stability for lithium intercalation and de-intercalation. 3.4. Morphology and structure change after cycles
Fig. 5. The first charge-discharge curves of the cells with Sn and Sn/C films as anodes.
The discharge and charge capacities of the Sn/C film are 750 and 500 mAh/g, respectively, as shown in Fig. 5. The discharge capacity of the Sn/C film is lower but its charge capacity is slightly higher than that of the Sn film. Apparently, the composite of tin with carbon does reduce its pri-
Figs. 7 and 8 present the surface morphologies and XRD patterns of Sn and Sn/C films after 20 cycles, respectively. It can be easily found from Fig. 7(a) that the Sn film is pulverized after 20 cycles, which is confirmed by the XRD data shown in Fig. 8. No diffraction line of tin in the Sn film can be distinguished. It is well known that there is a volume change of 676% between fully intercalated and de- intercalated tin [10]. The large volume change results in the crack-
Zhao L.Z. et al., Lithium intercalation/de-intercalation behavior of a composite Sn/C thin film fabricated by…
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inner stress resulting from the volume change of tin due to intercalation and de-intercalation, retards the cracking of the film, and keeps the film in integrity. This accounts for the good cyclic stability of the Sn/C film. It is noted from Fig. 6 that from the 15th cycle, there is a decreasing tendency in the discharge and charge efficiency of the Sn/C film. This can be ascribed to the appearance of the crevices of the film shown in Fig. 7(b). More research needs to be done on the composite of tin with graphite to obtain a composite Sn/C film that can be used in practice.
4. Conclusions A composite Sn/C thin film can be fabricated by magnetron sputtering. The composite of tin with graphite can improve its performance as lithium intercalated anode of lithium ion battery. The composite hardly changes the primary discharge and charge capacities for lithium intercalation and de-intercalation of tin, but improves its cyclic stability and efficiency for discharge-charge. This effect is ascribed to the reduction of inner stress resulting from lithium intercalation and de-intercalation in the tin film by the composite graphite, which avoids primary pulverization of tin. Fig. 7. Morphologies of Sn (a) and Sn/C films (b) after 20 cycles.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (Nos. 50771046 and 20373016), the Natural Science Foundation of Guangdong Province (No. 05200534), and the Key Projects of Guangdong Province and Guangzhou City, China (Nos. 2006A10704003 and2006Z3-D2031).
References
Fig. 8. XRD patterns of Sn and Sn/C films after 20 cycles.
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