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Suppression of PC decomposition at the surface of graphitic carbon by Cu coating
Electrochemistry Communications 8 (2006) 1726–1730 www.elsevier.com/locate/elecom
Suppression of PC decomposition at the surface of graphitic carbon by Cu coating J. Gao, L.J. Fu, H.P. Zhang, T. Zhang, Y.P. Wu *, H.Q. Wu
*
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Material, Fudan University, Shanghai 200433, China Received 28 June 2006; received in revised form 4 August 2006; accepted 7 August 2006 Available online 6 September 2006
Abstract Cu-coated graphitic carbon was prepared by an electroless plating method, and its physical and electrochemical performance was studied by X-ray diffraction, scanning electronic microscopy, cyclic voltammetry, and measurement of discharge and charge behavior. Copper was uniformly coated on the surface of graphitic carbon. The Cu coating layer prevents the direct contact of electrolyte with the active surface of the graphitic carbon and is probably a part of solid–electrolyte interface (SEI) film. As a result, it suppresses the decomposition of propylene carbonate (PC) and exfoliation of graphite. In a PC-based electrolyte containing 50%(volume) PC, the Cu-coated graphitic carbon markedly shows better electrochemical performance with good cycling as an anode material than original graphitic carbon. This method provides a promising application of lithium ion batteries in low temperature such as 60 °C. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Lithium ion battery; Anode material; Graphitic carbon; Propylene carbonate; Coating
1. Introduction Lithium ion batteries have been widely used in portable electronic devices due to their high voltage and high energy density. Most researchers have concentrated on improving their performance such as capacity, cycling characteristics and low temperature capability [1,2]. As for electrolyte solvent, propylene carbonate (PC) and ethylene carbonate (EC) have been comprehensively studied. EC-based electrolytes are exclusively used in commercial lithium ion batteries because PC decomposes ceaselessly upon lithium intercalation, leading to graphite exfoliation [3–6]. Nevertheless, PC is still attractive as an electrolyte solvent in lithium ion batteries because of its superior ionic conductivity at low temperature, low price and high boiling temperature [7]. One way to hinder the decomposition of PC at the surface of graphite to achieve reversible intercalation and de*
Corresponding authors. Tel./fax: +86 21 55664223 (Y.P.Wu). E-mail address: [email protected] (Y.P. Wu).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.08.011
intercalation of lithium ion is to add some additives into the PC-based electrolytes. The main action is that these additives form an effective SEI (solid–electrolyte interface) layer at potentials higher than 1 V (vs Li+/Li) before PC begins to decompose [8]. For example, vinylene carbonate [8,9], silver hexafluorophosphate [10], tetrachloroethylene[11], triethyl orthoformate [12], and vinyl tris-2-methoxyethoxy silane [13] have been found to be useful. Another way is to modify the graphitic carbon anode materials since it has been found that the main reaction is due to active sites at the surface of graphite [1]. Its purpose is to prevent direct contact of active sites on graphite with PC by coating a layer of other materials on graphite [14,15]. Palladium coating on graphite reduces the initial irreversible capacity of graphite in PC-based electrolyte [16]. Nano-scale copper particles were deposited on the surface of natural graphite to depress the cointercalation of solvated lithium ion [17]. But it mainly studied the electrolyte with the volumetric PC content is 20% and the reported data from cyclic voltammogram still indicates the co-intercalation of solvents. Natural graphite
J. Gao et al. / Electrochemistry Communications 8 (2006) 1726–1730
encapsulated by pyrolytic carbon from polyurea can work stably in a PC-based electrolyte with the volumetric PC content up to 25% [18]. Carbon coated natural graphite has been shown much better electrochemical performance in PC-based electrolyte than bare natural graphite [19,20]. All these results and other surface modification [21] suggest that surface modification of graphitic carbon is a good way to prevent the co-intercalation of PC. To our knowledge, the content of PC in the above reported literature is utmost 30% (by volume). Meanwhile, the above methods do not completely ensure good stability of the formed SEI film since the interfacial resistance will markedly increase at low temperature [22]. In this paper, we found that after coating a layer of copper on the surface of synthetic graphite by an electroless plating method, the decomposition of PC and exfoliation of graphite in PCbased electrolyte with PC content of 50% (by volume) are markedly prevented and good cycling is ensured. By the way, the conductive copper can act as an effective component of the SEI film to ensure good conductivity of the interface and shows promise of the application of this kind of anode material in low temperature.
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Electrochemical performance evaluation was carried out with coin-type half cells which were fabricated as follows. 90 wt.% graphitic carbon or Cu-coated graphitic carbon, the negative electrode material for the Li ion test cells, was mixed with 5 wt.% poly(vinylidene difluoride) (PVDF) binder and 5 wt.% acetylene black in a solvent of Nmethyl-2-pyrrolidone to prepare slurry. The slurry was coated onto a copper foil and dried at 120 °C under vacuum. After drying, the electrode was cut into small pieces and assembled into coin-type model cells in a glove box filled with argon. The pieces were used as working electrodes. Lithium foil was used as the counter and reference electrode. Celgard 2400 separator was placed between graphitic carbon electrode and lithium metal foil. The discharge and charge was galvanostatically measured in 1 M LiClO4 solution of PC–DMC (dimethyl carbonate) (1:1 by volume) by a Land CT2001A tester at about 0.1 mA/ cm2 at the voltage range of 0.0–2.0 V vs Li+/Li. Cyclic voltammograms were measured with the same cells with CHI400 electrochemical analyzer between 2.5 and 0 V at a scan rate of 0.1 mV/s. 3. Results and discussion
2. Experimental X-ray diffraction pattern of Cu-coated graphitic carbon is shown in Fig. 1. Six sharp diffraction peaks are evidently observed at 26.47°, 36.42°, 38.64°, 43.32°, 50.45°, and 74.12°, which correspond to the characteristic peak of graphite (0 0 2), that of cubic Cu2O (1 1 1), that of monoclinic CuO (2 0 0), and those of cubic metallic Cu (1 1 1), (2 0 0) and (2 2 0), respectively. The appearance of Cu2O could be from the incomplete reduction of Cu (II) ion during the electroless plating procedure. CuO could be formed by the oxidation of Cu2O while drying. As to the sensitizing component of Sn and catalyzing component of Ag, they are not observed since the deposition or absorption amounts on the surface of graphitic carbon are very small. The electroless plating of Cu may be a heterogeneous catalyzing reaction. Its detail mechanism remains to be studied in the future. The total equation of Cu depositing is:
500 Cu2O(111)
400 300
CuO
200
Cu(220)
600
Cu(200)
700
Cu(111)
graphite
800
Intensity (a.u.)
Cu-coated graphitic carbon was prepared by an electroless plating method. Before plating, 3 g of graphitic carbon powder (CMS, from heat-treatment at 2800 °C, Shanshan Co. Ltd., Shanghai, China, average diameter 15 lm) was first oxidized by saturated (NH4)2S2O8 solution for 8 h under stirring at 60 °C, and then washed with water until neutral and dried. After oxidation, the graphitic carbon was added under magnetic stirring into a fresh sensitizing solution (60 ml) that was prepared by mixing 1.4 g of SnCl2 Æ 2H2O and 6 ml HCl (37 wt.%) with 50 ml distilled water. After 15 min of stirring, the sensitized graphitic carbon was filtered and washed until neutral with distilled water. Subsequently, it was directly added into a catalyzing solution (60 ml), which was prepared by mixing 1.8 g of AgNO3 and 3 ml of NH3 Æ H2O (25 wt.%) with 50 ml distilled water. After stirring for 5 h, the solid was filtered and washed with water until neutral and dried. To plating Cu, 1.5 g catalyzed graphitic carbon was added to the solution containing A and B under magnetic stirring. Solution A is a mixture of CuSO4 Æ 5H2O (3.5 g), NaKC4H4O6 Æ 4H2O (4.0 g), and Na2EDTA (5.2 g), and solution B is a mixture of NaOH (3.0 g), HCHO (35 wt.%) (11.2 ml). After 10 min of plating, the mixture was immediately filtered and washed with distilled water and dried. Thus Cu-coated graphitic carbon was obtained. X-ray diffraction measurements were performed with the use of Cu Ka target with a Bruker D8 Advanced Spectrometer instrument. Scanning electron microscope (SEM) (Philips XL 30) was used to measure the surface morphology of Cu-coated graphitic carbon.
100 0 0
10
20
30
40
50
60
70
80
90 100
2θ / deg Fig. 1. X-ray diffraction pattern of Cu-coated graphitic carbon.
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J. Gao et al. / Electrochemistry Communications 8 (2006) 1726–1730
Scanning electron micrographs (SEM) of Cu-coated graphitic carbon and the original graphitic carbon are shown in Fig. 2. The comparison shows apparently that the surface of graphitic carbon is uniformly and compactly coated by a layer of Cu particles. These results mean that it is practicable to form a complete coating layer by our above-mentioned electroless plating process. The first discharge–charge profiles of Cu-coated graphitic carbon and the original graphitic carbon in 1 M LiClO4 solution of PC-DMC are shown in Fig. 3. During discharging in the first cycle, the original graphitic carbon presents a very long plateau at about 0.7–0.8 V, and no lithium ion can intercalate into the graphene sheets of graphitic carbon. It is well known that this long plateau is due to PC decomposition and exfoliation of the graphite electrode [3]. As a result, there is no intercalation of lithium in the graphitic carbon. Of course, no charge capacity can be obtained. However, it is not the case at all when the surface of graphitic carbon is coated with a layer of Cu particles. The plateau of about 0.8 V corresponding to PC decomposition disappears almost completely. A large capacity below 0.2 V
Cu-coated CMS original CMS
2.0
+
ð1Þ
Voltage (V vs Li /Li)
Cu2þ þ 2HCHO þ 4OH ¼ Cu þ 2HCOO þ H2 þ 2H2 O
1.5
1.0
0.5
0.0 0
100
200
300
400
Specific capacity (mAh/g) Fig. 3. The first discharge–charge profiles of Cu-coated graphitic carbon (solid line) and the original graphitic carbon (dash dot line) at 0.1 mA/cm2 (0.25 C) in1 M LiClO4 solution of PC–DMC (1:1 by volume).
is obtained corresponding to the intercalation of lithium ion into graphite, and the reversible capacity reaches about 350 mAh/g. These facts indicate that the coating layer of Cu suppressed the decomposition of PC and exfoliation of graphite. The coating layer hides the active structure at the surface of graphite and acts as an obstacle to keep the surface of graphitic carbon from direct contact with PC. As a result, only small size lithium ion can penetrate through the coating layer without restriction. Therefore, lithium ions can intercalate into graphite layer reversibly. This speculation can be proved by the cycling performance shown in Fig. 4. After 10 cycles at 0.1 mA/cm2 (about 0.25 C), the reversible capacity remains very stable because the coating layer of Cu contributes to a stable SEI film. In fact, different thickness of the coating layer will result in different improvement of electrochemical performance, which will be similar with other kinds of coating [14,15,23]. The optimal coating thickness remains further investigation.
Specific capacity (mAh/g)
400
300
200
100
0 0
2
4
6
8
10
Cycle number Fig. 2. Scanning electronic micrographs of (a) Cu-coated graphitic carbon and (b) the original graphitic carbon.
Fig. 4. Cycling performance of Cu-coated graphitic carbon at about 0.1 mA/cm2 (0.25 C) in 1 M LiClO4 solution of PC–DMC (1:1 by volume).
J. Gao et al. / Electrochemistry Communications 8 (2006) 1726–1730
2
Current (mA/cm )
2
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Of course, there are still some things remained to be further solved such as the actions of copper layer on discharge and charge behavior since there is some difference between the discharge and charge profiles of Cu-coated graphitic carbon and those of the other graphitic carbons [25,26].
a
0
-2
4. Conclusion -4
b
-6
-8 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
+
Voltage (V versus Li /Li) Fig. 5. Cyclic voltammograms of (a) Cu-coated graphitic carbon and (b) the original graphitic carbon electrode in 1 M LiClO4 solution of PC– DMC (1:1 by volume) during the first cycle: scan rate: 0.1 mV/s.
Cyclic voltammograms of Cu-coated graphitic carbon and the original graphitic carbon electrode in 1 M LiClO4 solution of PC–DMC for the first cycle at the scan rate of 0.1 mV/s are shown in Fig. 5. The effects of Cu coating can be apparently observed. In the case of the original graphitic carbon, there is a large irreversible peak near 0.45 V vs Li+/Li, which can be definitely ascribed to PC decomposition and graphite exfoliation [19]. In contrast, the cyclic voltammogram of Cu-coated graphitic carbon demonstrates different performance in the same electrolyte. The irreversible peak near 0.45 V vs Li+/Li is barely visible, whereas the reversible peaks at ca. 0.2 V are prominent, which corresponds clearly to the intercalation/deintercalation of lithium ions. The disappearance of the irreversible peak is resulted from the effective suppression of PC decomposition and graphite exfoliation by Cu coating. These results are consistent with the discharge–charge performance in Figs. 3 and 4. Interestingly, there is almost no marked peak corresponding to the decomposition of electrolyte and the formation of SEI film in the case of Cu-coated graphitic carbon. From Fig. 3 it is sure that the SEI is formed during lithium intercalation in the first cycle because of the existence of irreversible capacity in the first cycle. However, it is not so evident in Fig. 5. Perhaps the Cu coating layer acts as a part of SEI film to a certain extent, and the additional formation of SEI film is not a quick process, which corresponds a slow plateau at the range of 0.8–0.2 V in Figs. 3 and 5. It can be predicted that other metals may be used as a coating material according to this method and further study is going on. The content of PC in our used electrolyte is 50% (by volume). Our above method provides very promising application of lithium ion batteries in low temperature since this kind of electrolyte can be easily adjusted to a melting temperature at least below 60 °C [24] and the electronic conductive copper can favor the lithium ion diffusion and electron transference.
Cu-coated graphitic carbon was prepared by an electroless plating method. SEM indicates that the coating is uniform and compact. Measurement of the discharge and charge behavior in the first cycle and cyclic voltammograms shows clearly that the coating copper layer suppresses the decomposition of PC and exfoliation of graphite. It probably acts as a part of SEI film. After coating, the reversible capacity of graphitic carbon is about 350 mAh/g and stable cycling performance has been achieved. This method is very attractive and provides a good way to improve the electrochemical performance of graphitic carbon at low temperature. Acknowledgements Financial supports from Shanghai Science and Technology Committee (04QMX1406) and China Natural Science Foundation (20474010) are greatly appreciated. References [1] L.J. Fu, H. Liu, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Prog. Mater. Sci. 50 (2005) 881. [2] L.J. Fu, H. Liu, H.P. Zhang, C. Li, T. Zhang, Y.P. Wu, R. Holze, H.Q. Wu, Electrochem. Commun. 8 (2006) 1. [3] A.N. Dey, B.P. Sullivan, J. Electrochem. Soc. 117 (1970) 222. [4] J.O. Besenhard, H.P. Fritz, J. Electroanal. Chem. 53 (1974) 329. [5] G. Eichinger, J. Electroanal. Chem. 74 (1976) 183. [6] M. Arakawa, J. Yamaki, J. Electroanal. Chem. 219 (1987) 273. [7] J.T. Dudley, D.P. Wilkinson, G. Thomas, R. LeVae, S. Woo, H. Blom, C. Horvath, M.W. Juzkow, B. Denis, P. Juric, P. Aghakian, J.R. Dahn, J. Power Sources 35 (1991) 59. [8] S.K. Jeong, M. Inaba, R. Mogi, Y. Iriyama, T. Abe, Z. Ogumi, Langmuir 17 (2001) 8281. [9] C. Jehoulet, P. Biesan, J.M. Bodet, M. Broussely, C. Moteau, C. Tessier-Lescouerret, Proc. Electrochem. Soc. 97 (1997) 974. [10] M.S. Wu, J.C. Lin, P.J. Chiang, Electrochem. Solid State Lett. 7 (2004) A206. [11] Y.S. Hu, W.H. Kong, Z.X. Wang, X.J. Huang, L.Q. Chen, Solid State Ionics 176 (2005) 53. [12] L.S. Wang, Y.D. Huang, D.Z. Jia, Electrochim. Acta 51 (2006) 4950. [13] G. Schroeder, B. Gierczyk, D. Waszak, M. Kopczyk, M. Walkowiak, Electrochem. Commun. 8 (2006) 523. [14] Y.P. Wu, E. Rahm, R. Holze, J. Power Sources 114 (2003) 228. [15] L.J. Fu, H. Liu, C. Li, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Solid State Sci. 8 (2006) 113. [16] P. Yu, B.S. Haran, J.A. Ritter, R.E. White, B.N. Popov, J. Power Sources 91 (2000) 107. [17] K. Guo, Q. Pan, L. Wang, S. Fang, J. Appl. Electrochem. 32 (2002) 679. [18] Y.F. Zhou, S. Xie, C.H. Chen, Electrochim. Acta 50 (2005) 4728. [19] M. Yoshio, H.Y. Wang, K. Fukuda, Y. Hara, Y. Adachi, J. Electrochem. Soc. 147 (2000) 1245. [20] H.Y. Wang, M. Yoshio, J. Power Sources 93 (2001) 123. [21] L.J. Fu, H.P. Zhang, Y.P. Wu, H.Q. Wu, R. Holze, Electrochem. Solid State Lett. 8 (2005) A456.
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[22] A.N. Jansen, D.W. Dees, D.P. Abraham, K. Amine, G.L. Henriksen, in: International Meeting on Lithium Batteries, Abstract no. 464, Biarritz, France, June 18–23, 2006. [23] Y.P. Wu, X.B. Dai, J.Q. Ma, Y.J. Cheng, Lithium Ion Batteries – Practice and Applications, Chemical Industry Press, Beijing, 2004.
[24] M.S. Ding, J. Electrochem. Soc. 151 (2004) A731. [25] Y.P. Wu, C.Y. Jiang, C.R. Wan, E. Tsuchida, Electrochem. Commun. 2 (2000) 626. [26] Y.P. Wu, C. Jiang, C. Wan, R. Holze, Carbon 41 (2003) 437.
Suppression of PC decomposition at the surface of graphitic carbon by Cu coating J. Gao, L.J. Fu, H.P. Zhang, T. Zhang, Y.P. Wu *, H.Q. Wu
*
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Material, Fudan University, Shanghai 200433, China Received 28 June 2006; received in revised form 4 August 2006; accepted 7 August 2006 Available online 6 September 2006
Abstract Cu-coated graphitic carbon was prepared by an electroless plating method, and its physical and electrochemical performance was studied by X-ray diffraction, scanning electronic microscopy, cyclic voltammetry, and measurement of discharge and charge behavior. Copper was uniformly coated on the surface of graphitic carbon. The Cu coating layer prevents the direct contact of electrolyte with the active surface of the graphitic carbon and is probably a part of solid–electrolyte interface (SEI) film. As a result, it suppresses the decomposition of propylene carbonate (PC) and exfoliation of graphite. In a PC-based electrolyte containing 50%(volume) PC, the Cu-coated graphitic carbon markedly shows better electrochemical performance with good cycling as an anode material than original graphitic carbon. This method provides a promising application of lithium ion batteries in low temperature such as 60 °C. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Lithium ion battery; Anode material; Graphitic carbon; Propylene carbonate; Coating
1. Introduction Lithium ion batteries have been widely used in portable electronic devices due to their high voltage and high energy density. Most researchers have concentrated on improving their performance such as capacity, cycling characteristics and low temperature capability [1,2]. As for electrolyte solvent, propylene carbonate (PC) and ethylene carbonate (EC) have been comprehensively studied. EC-based electrolytes are exclusively used in commercial lithium ion batteries because PC decomposes ceaselessly upon lithium intercalation, leading to graphite exfoliation [3–6]. Nevertheless, PC is still attractive as an electrolyte solvent in lithium ion batteries because of its superior ionic conductivity at low temperature, low price and high boiling temperature [7]. One way to hinder the decomposition of PC at the surface of graphite to achieve reversible intercalation and de*
Corresponding authors. Tel./fax: +86 21 55664223 (Y.P.Wu). E-mail address: [email protected] (Y.P. Wu).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.08.011
intercalation of lithium ion is to add some additives into the PC-based electrolytes. The main action is that these additives form an effective SEI (solid–electrolyte interface) layer at potentials higher than 1 V (vs Li+/Li) before PC begins to decompose [8]. For example, vinylene carbonate [8,9], silver hexafluorophosphate [10], tetrachloroethylene[11], triethyl orthoformate [12], and vinyl tris-2-methoxyethoxy silane [13] have been found to be useful. Another way is to modify the graphitic carbon anode materials since it has been found that the main reaction is due to active sites at the surface of graphite [1]. Its purpose is to prevent direct contact of active sites on graphite with PC by coating a layer of other materials on graphite [14,15]. Palladium coating on graphite reduces the initial irreversible capacity of graphite in PC-based electrolyte [16]. Nano-scale copper particles were deposited on the surface of natural graphite to depress the cointercalation of solvated lithium ion [17]. But it mainly studied the electrolyte with the volumetric PC content is 20% and the reported data from cyclic voltammogram still indicates the co-intercalation of solvents. Natural graphite
J. Gao et al. / Electrochemistry Communications 8 (2006) 1726–1730
encapsulated by pyrolytic carbon from polyurea can work stably in a PC-based electrolyte with the volumetric PC content up to 25% [18]. Carbon coated natural graphite has been shown much better electrochemical performance in PC-based electrolyte than bare natural graphite [19,20]. All these results and other surface modification [21] suggest that surface modification of graphitic carbon is a good way to prevent the co-intercalation of PC. To our knowledge, the content of PC in the above reported literature is utmost 30% (by volume). Meanwhile, the above methods do not completely ensure good stability of the formed SEI film since the interfacial resistance will markedly increase at low temperature [22]. In this paper, we found that after coating a layer of copper on the surface of synthetic graphite by an electroless plating method, the decomposition of PC and exfoliation of graphite in PCbased electrolyte with PC content of 50% (by volume) are markedly prevented and good cycling is ensured. By the way, the conductive copper can act as an effective component of the SEI film to ensure good conductivity of the interface and shows promise of the application of this kind of anode material in low temperature.
1727
Electrochemical performance evaluation was carried out with coin-type half cells which were fabricated as follows. 90 wt.% graphitic carbon or Cu-coated graphitic carbon, the negative electrode material for the Li ion test cells, was mixed with 5 wt.% poly(vinylidene difluoride) (PVDF) binder and 5 wt.% acetylene black in a solvent of Nmethyl-2-pyrrolidone to prepare slurry. The slurry was coated onto a copper foil and dried at 120 °C under vacuum. After drying, the electrode was cut into small pieces and assembled into coin-type model cells in a glove box filled with argon. The pieces were used as working electrodes. Lithium foil was used as the counter and reference electrode. Celgard 2400 separator was placed between graphitic carbon electrode and lithium metal foil. The discharge and charge was galvanostatically measured in 1 M LiClO4 solution of PC–DMC (dimethyl carbonate) (1:1 by volume) by a Land CT2001A tester at about 0.1 mA/ cm2 at the voltage range of 0.0–2.0 V vs Li+/Li. Cyclic voltammograms were measured with the same cells with CHI400 electrochemical analyzer between 2.5 and 0 V at a scan rate of 0.1 mV/s. 3. Results and discussion
2. Experimental X-ray diffraction pattern of Cu-coated graphitic carbon is shown in Fig. 1. Six sharp diffraction peaks are evidently observed at 26.47°, 36.42°, 38.64°, 43.32°, 50.45°, and 74.12°, which correspond to the characteristic peak of graphite (0 0 2), that of cubic Cu2O (1 1 1), that of monoclinic CuO (2 0 0), and those of cubic metallic Cu (1 1 1), (2 0 0) and (2 2 0), respectively. The appearance of Cu2O could be from the incomplete reduction of Cu (II) ion during the electroless plating procedure. CuO could be formed by the oxidation of Cu2O while drying. As to the sensitizing component of Sn and catalyzing component of Ag, they are not observed since the deposition or absorption amounts on the surface of graphitic carbon are very small. The electroless plating of Cu may be a heterogeneous catalyzing reaction. Its detail mechanism remains to be studied in the future. The total equation of Cu depositing is:
500 Cu2O(111)
400 300
CuO
200
Cu(220)
600
Cu(200)
700
Cu(111)
graphite
800
Intensity (a.u.)
Cu-coated graphitic carbon was prepared by an electroless plating method. Before plating, 3 g of graphitic carbon powder (CMS, from heat-treatment at 2800 °C, Shanshan Co. Ltd., Shanghai, China, average diameter 15 lm) was first oxidized by saturated (NH4)2S2O8 solution for 8 h under stirring at 60 °C, and then washed with water until neutral and dried. After oxidation, the graphitic carbon was added under magnetic stirring into a fresh sensitizing solution (60 ml) that was prepared by mixing 1.4 g of SnCl2 Æ 2H2O and 6 ml HCl (37 wt.%) with 50 ml distilled water. After 15 min of stirring, the sensitized graphitic carbon was filtered and washed until neutral with distilled water. Subsequently, it was directly added into a catalyzing solution (60 ml), which was prepared by mixing 1.8 g of AgNO3 and 3 ml of NH3 Æ H2O (25 wt.%) with 50 ml distilled water. After stirring for 5 h, the solid was filtered and washed with water until neutral and dried. To plating Cu, 1.5 g catalyzed graphitic carbon was added to the solution containing A and B under magnetic stirring. Solution A is a mixture of CuSO4 Æ 5H2O (3.5 g), NaKC4H4O6 Æ 4H2O (4.0 g), and Na2EDTA (5.2 g), and solution B is a mixture of NaOH (3.0 g), HCHO (35 wt.%) (11.2 ml). After 10 min of plating, the mixture was immediately filtered and washed with distilled water and dried. Thus Cu-coated graphitic carbon was obtained. X-ray diffraction measurements were performed with the use of Cu Ka target with a Bruker D8 Advanced Spectrometer instrument. Scanning electron microscope (SEM) (Philips XL 30) was used to measure the surface morphology of Cu-coated graphitic carbon.
100 0 0
10
20
30
40
50
60
70
80
90 100
2θ / deg Fig. 1. X-ray diffraction pattern of Cu-coated graphitic carbon.
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Scanning electron micrographs (SEM) of Cu-coated graphitic carbon and the original graphitic carbon are shown in Fig. 2. The comparison shows apparently that the surface of graphitic carbon is uniformly and compactly coated by a layer of Cu particles. These results mean that it is practicable to form a complete coating layer by our above-mentioned electroless plating process. The first discharge–charge profiles of Cu-coated graphitic carbon and the original graphitic carbon in 1 M LiClO4 solution of PC-DMC are shown in Fig. 3. During discharging in the first cycle, the original graphitic carbon presents a very long plateau at about 0.7–0.8 V, and no lithium ion can intercalate into the graphene sheets of graphitic carbon. It is well known that this long plateau is due to PC decomposition and exfoliation of the graphite electrode [3]. As a result, there is no intercalation of lithium in the graphitic carbon. Of course, no charge capacity can be obtained. However, it is not the case at all when the surface of graphitic carbon is coated with a layer of Cu particles. The plateau of about 0.8 V corresponding to PC decomposition disappears almost completely. A large capacity below 0.2 V
Cu-coated CMS original CMS
2.0
+
ð1Þ
Voltage (V vs Li /Li)
Cu2þ þ 2HCHO þ 4OH ¼ Cu þ 2HCOO þ H2 þ 2H2 O
1.5
1.0
0.5
0.0 0
100
200
300
400
Specific capacity (mAh/g) Fig. 3. The first discharge–charge profiles of Cu-coated graphitic carbon (solid line) and the original graphitic carbon (dash dot line) at 0.1 mA/cm2 (0.25 C) in1 M LiClO4 solution of PC–DMC (1:1 by volume).
is obtained corresponding to the intercalation of lithium ion into graphite, and the reversible capacity reaches about 350 mAh/g. These facts indicate that the coating layer of Cu suppressed the decomposition of PC and exfoliation of graphite. The coating layer hides the active structure at the surface of graphite and acts as an obstacle to keep the surface of graphitic carbon from direct contact with PC. As a result, only small size lithium ion can penetrate through the coating layer without restriction. Therefore, lithium ions can intercalate into graphite layer reversibly. This speculation can be proved by the cycling performance shown in Fig. 4. After 10 cycles at 0.1 mA/cm2 (about 0.25 C), the reversible capacity remains very stable because the coating layer of Cu contributes to a stable SEI film. In fact, different thickness of the coating layer will result in different improvement of electrochemical performance, which will be similar with other kinds of coating [14,15,23]. The optimal coating thickness remains further investigation.
Specific capacity (mAh/g)
400
300
200
100
0 0
2
4
6
8
10
Cycle number Fig. 2. Scanning electronic micrographs of (a) Cu-coated graphitic carbon and (b) the original graphitic carbon.
Fig. 4. Cycling performance of Cu-coated graphitic carbon at about 0.1 mA/cm2 (0.25 C) in 1 M LiClO4 solution of PC–DMC (1:1 by volume).
J. Gao et al. / Electrochemistry Communications 8 (2006) 1726–1730
2
Current (mA/cm )
2
1729
Of course, there are still some things remained to be further solved such as the actions of copper layer on discharge and charge behavior since there is some difference between the discharge and charge profiles of Cu-coated graphitic carbon and those of the other graphitic carbons [25,26].
a
0
-2
4. Conclusion -4
b
-6
-8 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
+
Voltage (V versus Li /Li) Fig. 5. Cyclic voltammograms of (a) Cu-coated graphitic carbon and (b) the original graphitic carbon electrode in 1 M LiClO4 solution of PC– DMC (1:1 by volume) during the first cycle: scan rate: 0.1 mV/s.
Cyclic voltammograms of Cu-coated graphitic carbon and the original graphitic carbon electrode in 1 M LiClO4 solution of PC–DMC for the first cycle at the scan rate of 0.1 mV/s are shown in Fig. 5. The effects of Cu coating can be apparently observed. In the case of the original graphitic carbon, there is a large irreversible peak near 0.45 V vs Li+/Li, which can be definitely ascribed to PC decomposition and graphite exfoliation [19]. In contrast, the cyclic voltammogram of Cu-coated graphitic carbon demonstrates different performance in the same electrolyte. The irreversible peak near 0.45 V vs Li+/Li is barely visible, whereas the reversible peaks at ca. 0.2 V are prominent, which corresponds clearly to the intercalation/deintercalation of lithium ions. The disappearance of the irreversible peak is resulted from the effective suppression of PC decomposition and graphite exfoliation by Cu coating. These results are consistent with the discharge–charge performance in Figs. 3 and 4. Interestingly, there is almost no marked peak corresponding to the decomposition of electrolyte and the formation of SEI film in the case of Cu-coated graphitic carbon. From Fig. 3 it is sure that the SEI is formed during lithium intercalation in the first cycle because of the existence of irreversible capacity in the first cycle. However, it is not so evident in Fig. 5. Perhaps the Cu coating layer acts as a part of SEI film to a certain extent, and the additional formation of SEI film is not a quick process, which corresponds a slow plateau at the range of 0.8–0.2 V in Figs. 3 and 5. It can be predicted that other metals may be used as a coating material according to this method and further study is going on. The content of PC in our used electrolyte is 50% (by volume). Our above method provides very promising application of lithium ion batteries in low temperature since this kind of electrolyte can be easily adjusted to a melting temperature at least below 60 °C [24] and the electronic conductive copper can favor the lithium ion diffusion and electron transference.
Cu-coated graphitic carbon was prepared by an electroless plating method. SEM indicates that the coating is uniform and compact. Measurement of the discharge and charge behavior in the first cycle and cyclic voltammograms shows clearly that the coating copper layer suppresses the decomposition of PC and exfoliation of graphite. It probably acts as a part of SEI film. After coating, the reversible capacity of graphitic carbon is about 350 mAh/g and stable cycling performance has been achieved. This method is very attractive and provides a good way to improve the electrochemical performance of graphitic carbon at low temperature. Acknowledgements Financial supports from Shanghai Science and Technology Committee (04QMX1406) and China Natural Science Foundation (20474010) are greatly appreciated. References [1] L.J. Fu, H. Liu, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Prog. Mater. Sci. 50 (2005) 881. [2] L.J. Fu, H. Liu, H.P. Zhang, C. Li, T. Zhang, Y.P. Wu, R. Holze, H.Q. Wu, Electrochem. Commun. 8 (2006) 1. [3] A.N. Dey, B.P. Sullivan, J. Electrochem. Soc. 117 (1970) 222. [4] J.O. Besenhard, H.P. Fritz, J. Electroanal. Chem. 53 (1974) 329. [5] G. Eichinger, J. Electroanal. Chem. 74 (1976) 183. [6] M. Arakawa, J. Yamaki, J. Electroanal. Chem. 219 (1987) 273. [7] J.T. Dudley, D.P. Wilkinson, G. Thomas, R. LeVae, S. Woo, H. Blom, C. Horvath, M.W. Juzkow, B. Denis, P. Juric, P. Aghakian, J.R. Dahn, J. Power Sources 35 (1991) 59. [8] S.K. Jeong, M. Inaba, R. Mogi, Y. Iriyama, T. Abe, Z. Ogumi, Langmuir 17 (2001) 8281. [9] C. Jehoulet, P. Biesan, J.M. Bodet, M. Broussely, C. Moteau, C. Tessier-Lescouerret, Proc. Electrochem. Soc. 97 (1997) 974. [10] M.S. Wu, J.C. Lin, P.J. Chiang, Electrochem. Solid State Lett. 7 (2004) A206. [11] Y.S. Hu, W.H. Kong, Z.X. Wang, X.J. Huang, L.Q. Chen, Solid State Ionics 176 (2005) 53. [12] L.S. Wang, Y.D. Huang, D.Z. Jia, Electrochim. Acta 51 (2006) 4950. [13] G. Schroeder, B. Gierczyk, D. Waszak, M. Kopczyk, M. Walkowiak, Electrochem. Commun. 8 (2006) 523. [14] Y.P. Wu, E. Rahm, R. Holze, J. Power Sources 114 (2003) 228. [15] L.J. Fu, H. Liu, C. Li, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Solid State Sci. 8 (2006) 113. [16] P. Yu, B.S. Haran, J.A. Ritter, R.E. White, B.N. Popov, J. Power Sources 91 (2000) 107. [17] K. Guo, Q. Pan, L. Wang, S. Fang, J. Appl. Electrochem. 32 (2002) 679. [18] Y.F. Zhou, S. Xie, C.H. Chen, Electrochim. Acta 50 (2005) 4728. [19] M. Yoshio, H.Y. Wang, K. Fukuda, Y. Hara, Y. Adachi, J. Electrochem. Soc. 147 (2000) 1245. [20] H.Y. Wang, M. Yoshio, J. Power Sources 93 (2001) 123. [21] L.J. Fu, H.P. Zhang, Y.P. Wu, H.Q. Wu, R. Holze, Electrochem. Solid State Lett. 8 (2005) A456.
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