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Preparation of Silicon Oxide Coated KS-6 Graphite Composite Anode Materials by Sol-gel Method in Lithium Ion Batteries
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 61 (2014) 1428 – 1433
The 6th International Conference on Applied Energy – ICAE2014
Preparation of silicon oxide coated KS-6 graphite composite anode materials by sol-gel method in lithium ion batteries 1,2
1
2
Chun-Chen Yang, 1,2 Jeng-Ywan Shih, 1,2 Min-Yen Wu
Department of Chemical Engineering, Ming Chi University of Technology, New Taipei City 243, Taiwan, R.O.C.
Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei City 243, Taiwan, R.O.C. (1Corresponding author, E-mail: [email protected])
Abstract
This work reported a sol-gel preparation method for the SiO2 coated KS-6 composite anode material by using KS-6 graphite, silicon oxide, and glucose (C6H12O6). Silicon oxide was used as a surface modification dopant and C6H12O6 was used as a carbon source. The composite anode material was sintered at 600-700oC in an Ar atmosphere for 5h. The characteristic properties of the composite anode materials were examined by micro-Raman spectroscopy, XRD, SEM, EA and AC impedance method. The coin cell was used to investigate the electrochemical performance at various rates. It was found that the specific discharge capacities of 16wt.%SiO2 coated KS-6 composite anode materials were 404, 422. 323, 192, 67, 29 and 13 mAh g-1 at 0.1, 0.2, 0.5, 1, 3, 5 and 10C rate, respectively. However, they all showed the current efficiency of ca. 96-99%. Apparently, the as-synthesized SiO2 coated KS-6 composite anode can be a good candidate for high power Li-ion battery applications. KeywordsǺAnode, KS-6 graphite, Silicon oxide (SiO2), Sol-gel method, surface modification
1.
Introduction
Lithium-ion batteries, due to their relatively high specific capacity, are considered for electric vehicle (EV), cell phones, laptop computers, digital cameras, renewable energy storage, and smart grid applications [1]. At present, graphite has been the most popular anode material for Li-ion batteries, mainly used to 3C small size devices that need high energy and a medium charge/discharge rate. However, the graphite cannot carry out at a high C-rate; it is due to easily form a dendritic on the surface of the graphite anode. The Li-ion battery (LIB) will degrade severely in performance, causing overheating and thermal runaway. Moreover, the graphite anode also suffers the particle expansion and shrinkage during the lithium intercalation/de-intercalation process. The safety problem of LIB is now more critical and challenge. There are two models have been proposed to describe the formation of the SEI film. The first
model is that the SEI film id established through the co- intercalation of solvent molecules from the decomposition electrolyte along with lithium ions insert into the graphite layers. The second model describes that the SEI film is formed by the decomposed electrolyte species on the graphite surface [2-4]. Nevertheless, the thickness of SEI film on the graphite surface is very critical and very important, because it affects the electrochemical performance of the carbon electrode. The graphite has two main problems, which are high irreversible capacity loss and poor cycling stability. It was well-known that the SEI film on the graphite surface can lead to the decomposition of electrolyte molecules at elevated temperature and cell potential and result in the high irreversible capacity loss and cycling performance deterioration. The surface modification of the graphite anodes has thus received a lot of attention. Recent studied showed that the surface modification is an effective way to improve its electrochemical
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.140
Chun-Chen Yang et al. / Energy Procedia 61 (2014) 1428 – 1433
performance [4-7]. The SiO2 surface coating on graphite can enhance the capacity and also prevent the direct contact electrolyte, which can deduce the crack or volume expansion/shrinkage problem during the charge/discharge cycling [5-9]. In this work, we prepare the SiO2-coated KS-6 graphite material by a sol-gel method, using KS6 graphite, silicon oxide, and glucose (C6H12O6) as the raw material. Silicon oxide was used as a surface modification dopant and C6H12O6 was used as a carbon source. The composite anode material was sintered at 600-700oC in an Ar atmosphere for 5h. The characteristic properties of the composite anode materials were examined by micro-Raman spectroscopy, XRD, SEM, EA and AC impedance method. The coin cell was used to investigate the electrochemical performance at various rates. The electrochemical performances of the SiO2-coated KS6 graphite composite samples were also examined by an automatic galvanostatic charge/discharge unit and a cyclic voltammetry method in detail. 2.
Experimental
2.1 Preparation of SiO2-coated KS-6 graphite materials A suitable amount of KS-6 graphite (Timcal Co. Ltd., Switzerland) and 16wt.%nano-SiO2 (Aldrich) was mixed with 5%glucose (Aldrich) in ethanol at 50oC for 10h under stirring. The mixed precursor was then dried at 80oC in a vacuum oven for 12h. The dry mixed powder was sintered at a tube furnace at 400oC for 4 h, then 600oC for 5 h in an Ar atmosphere. 2.2 Physical Property Characterization The crystal structure of SiO2-coated KS-6 graphite samples was examined by an X-ray diffraction (XRD) spectrometer (Philip, X’pert Pro System). The surface morphology was conducted by a scanning electron microscope (SEM, Hitachi). The Micro-Raman spectra were recorded on a confocal micro-Renishaw with a 632 nm He-Ne laser excitation. The residual carbon content in the sample was analyzed using Elemental Analyzer (Perkin Elmer 2400). The electron conductivity of the composite samples was measured by AC impedance method. 2.3 Electrochemical measurements The electrochemical performances of KS-6 graphite and 16wt.%SiO2-coated KS-6 graphite composite battery were measured by using a two-
electrode system (CR 2032 coin cell assembled in an argon-filled glove box). The SiO2-coated KS-6 graphite composite electrodes were prepared by mixing active KS-6 graphite or 16wt.%SiO2-coated KS-6 graphite LiFePO4/C materials, Super P, and poly(vinyl fluoride) (PVDF) binder in a weight ratio of 80:10:10, pasted on an copper foil (Aldrich), and then dried in a vacuum oven at 120oC for 12 h. The lithium foil (Aldrich) was used as the counter and reference electrode. A micro-porous PE film was used as the separator. The electrolyte was 1 M LiPF6 in a mixture of EC and DEC (1:1 in v/v, Merck). The KS-6 graphite or SiO2-coated KS-6 graphite half cells were charged by a constant current profile (CC) and discharged by a constant current profile, over a potential range of 0.02 – 1.50 V (vs. Li/Li+) at varied C rates with an Autolab PGSTAT302N potentiostat. The cyclic voltammetry (CV) was conducted by using an Autolab instrument at a scanning rate of 0.1 mV s-1 between 0.02 and 1.50 V. 3. Results and discussion The XRD patterns of KS-6 graphite and the SiO2-coated KS-6 graphite prepared by the sol-gel method are shown in Fig. 1, respectively. The XRD diffraction patterns revealed that both the K-S 6 graphite and SiO2-coated KS-6 graphite have similar pattern, including (002), (100), (101), and (004) diffraction peaks. However, the peak intensities of the peaks are different. It was found that the peak intensity of SiO2-coated KS-6 graphite is lower than that of KS6 graphite. It may be due to some of amorphous soft carbon deposited on graphite. There is no obvious SiO2 XRD peak occurred. It is because the particle size of SiO2 powder is small. Their (002) peaks are very narrow and sharp, which indicates graphite crystallites are perfect and the structure defects are very few. It was found that that the XRD patterns of SiO2-coated KS-6 graphite and KS-6 graphite are very close to the standard graphite (JCPDS card number 41-1487). The SiO2 surface coating and glucose carbon precursor have no observable influence on the structure of graphite materials.
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Fig. 2 SEM images of (a).KS-6 graphite; and (b). 16%SiO2-KS-6 graphite. 2000 1500
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Fig. 1 XRD patterns of (a). KS-6 graphite; and (b). SiO2-KS-6 graphite composite. The SEM images of KS-6 graphite and the SiO2-coated KS-6 graphite samples are shown in Fig. 2(a) and (b), respectively. As can be seen from the figures, KS-6 graphite has flake morphology; however, has irregular granular nano-SiO2 covered KS-6 graphite morphology and shape. It shows clearly that the morphology of KS-6 graphite is totally different from the SiO2-coated KS-6 graphite. The electrochemical properties of graphite samples are strongly related with the degree of graphitization and the total carbon content, and the value of ID/IG ratio. The carbon content of the KS-6 graphite and SiO2coated KS-6 graphite samples are approximately 99.30% and 82.93%, respectively.
25
According to the EA analysis result, we can find that the composition of 16%SiO2-coated KS-6 graphite is composed of 16wt.%SiO2, 82.93% graphite and 1.07% amorphous soft carbon, which is the residual carbon after sintering glucose. The cyclic voltammetries of the KS-6 graphite and the SiO2coated KS-6 graphite in the second cycle at a scan rate of 0.1 mV s-1 are shown in Fig. 3. The reduction (Liintercalation) and oxidation (Li-deintercalation) peaks of the KS-6 graphite sample are at approximately 0.16 and 0.265V, respectively, and the potential difference between the two peaks is 0.10V. In contrast, two reduction (Li-intercalation) peaks and one oxidation (Li-deintercalation) peak of the SiO2-coated KS-6 graphite sample are approximately at 0.175V, 0.06V, and 0.260V, respectively. It was found that the discharge and charge capacity of the SiO2-coated KS6 graphite is much larger than that of the KS-6 graphite. It was demonstrated that the discharge capacity of the SiO2-coated KS-6 graphite is larger than that of pristine KS-6 graphite. The SiO2 material is also active anode material; it can also deliver extra discharge capacity through the conversion reaction.
(a) KS-6 graphite (b) 16%SiO 2 coated KS-6 graphite 0.0020 0.0015 (b) 0.0010
Current/ A
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0.0005
(a)
0.0000 -0.0005 -0.0010 -0.0015 -0.0020 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 +
Potential/ V (vs. Li /Li)
Fig. 3 The CV curve of KS6 graphite and SiO2-graphite composite samples at 0.1 mV s-1.
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(a)
(b)
Voltage/ mV
1200 1000 800 600 400 200 0 0
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Capacity/ mAh g -1
Fig. 4 The initial charge-discharge curve for KS-6 graphite and the SiO2 coated graphite composite at 0.1C rate. Fig. 5 shows the rate capability of KS-6 graphite and the 16%SiO2-coated KS-6 graphite at various rates (from 0.2C to 10C). The KS-6 graphite delivers the specific capacities of 341, 221, 112, 16, 10, and 5 mAh g-1 at rates of 0.2, 0.5, 1, 3, 5, and 10C, respectively. It shows very poor high rate capability. However, the 16%SiO2-coated KS-6 graphite delivers the specific capacities of 420, 287, 164, 61, 27, and 13 mAh g-1 at rates of 0.2, 0.5, 1, 3, 5, and 10C, respectively. It shows that the the high rate capability is improved, as compared with pristine KS-6 graphite. very poor high rate capability. the LiFePO4/C sample without both PSS template and Nb doping delivers the specific capacities of 133, 131, 124, 106, 93, and 75 mAh g-1 at 0.2, 0.5, 1, 3, 5 and 10C rates, respectively.
(a) KS-6 graphite (b) 16% SiO 2 coated KS-6 graphite
(b)
400 (a)
Capacity/ mAh g
-1
350 300 250 200 150 100 50 0
5C 10C 1C 3C 0.2C 0.5C 1-3cycle 1-3cycle 1-3cycle 1-3cycle 1-3cycle 1-3cycle
Fig. 5 The rate capability curve of KS6 graphite and the SiO2 coated KS-6 graphite composite samples at varied rates. Moreover, the long-term cycling performance is also very important. iThe t was revealed that the discharge capacity of the KS-6 graphite at 0.1 C rate is 320 mAh g-1 and a coulomb efficiency was ca. 98%. However, the discharge capacity and the coulomb efficiency of the 16%SiO2coated KS-6 graphite composite during 30 cycles test at 0.1C/0.1C rate are 362 mAh g-1 and 98%, respectively, as shown in Fig. 6. Two advantages for SiO2 modification on graphite, one increase discharge capacity due to the conversion reaction; the other prevent direct contact between electrolyte and graphite, which can avoid the crack or expansion of graphite during the long-term cycling test. The dissolution of cathode materials, such as Mn4+ and Fe2+ from LiMn2O4 or LiFePO4 cathode materials is one of the main reasons for capacity fading mechanism during high temperature cycling. The Mn4+ and Fe2+ cations will deposit on graphite surface, it results in increasing the thickness of SEI film. It was well-known that the HF generated from LiPF6 electrolyte was responsible for the dissolution of Fe during cycling. The un-coated graphite material in LiPF6 electrolyte will also react with HF and thus leads to a gradually capacity loss. The SiO2 surface coating can prevent graphite from direct contacting with electrolyte and greatly decreasing the capacity loss [1-4].
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Capacity/ mAh g
Fig. 4 shows the initial charge-discharge profiles of KS-6 graphite and the SiO2-coated KS-6 graphite at 0.1C/0.1C charge/discharge rate at 25oC. It was found that the discharge and charge capacities of KS-6 graphite are 357 and 340 mAh g-1, and the irreversible capacity loss and the current efficiency are 17 mAh g-1 and 95%, respectively. However, it was found that the discharge and charge capacities of the 16%SiO2-coated KS-6 graphite are 413 and 404 mAh g-1, and the irreversible capacity loss and the current efficiency are 9 mAh g-1 and 98%, respectively. It is clearly that the charge capacity performance of the 16%SiO2-coated KS-6 graphite is superior to that of pristine KS-6 graphite without SiO2 coating. It may be due to the synergistic effect of nano-SiO2 and the soft carbon coating.
250 50
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0 5
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Chun-Chen Yang et al. / Energy Procedia 61 (2014) 1428 – 1433
Fig. 6 The cycling performance of KS-6 graphite and the SiO2 coated KS-6 graphite composite samples at 0.1C rate.
( a ) K S - 6 g r a p h it e ( b ) 1 6 % S iO 2 c o a te d K S - 6 g r a p h i te
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ω − 1 /2
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The AC impedance spectroscopy is used to study the interface properties of the KS-6 and 16%SiO2-coated KS-6 graphite samples. The AC spectra of the the KS-6 and 16%SiO2-coated KS-6 graphite samples at open circuit potential are shown in Fig. 7(a). Each AC plot consisted of one semicircle at higher frequency followed by a linear portion at lower frequency. The lower frequency region of the straight line is considered as Warburg impedance. It is for long-range lithium ion diffusion in bulk phase. The Rb indicates the bulk resistance at the electrolyte; Rct is attributed to the charge transfer resistance at the active material interface; CPE represents the double layer capacitance and some surface film capacitance. The lithium chemical diffusion coefficients (Di) of the electrode were calculated based on the equation (1):
(a )
120 100 80 60
(b )
40
Di =
1 RT 2 ( ) 2 AF 2σC
0
1
Where σ is Warburg impedance coefficient (obtained a slope from plot of Zre vs.ϖ-0.5, as seen in Fig. 7(b); Di is the lithium diffusion coefficient; R is the gas constant; T is the absolute temperature; F is Faraday’s constant; A is the area of the electrode; C is the molar concentration of Li+ ions (CLi+=1.0×10-3 mol cm-3). Table 1 summaries the calculated Rb, Rct, Di, jo parameters for two samples. It was found that the Rb values of KS-6 graphite sample without any surface modification and the 16%SiO2-coated KS-6 graphite are around 4.32 and 2.60 ohm, respectively. The values of Rct and Di for two graphite samples are greatly varied. After SiO2 surface coating on graphite, the charge resistance (Rct) values and the lithium diffusion coefficients (DLi) are effectively improved, which is good for the enhancement of rate capability of graphite materials. By comparison, we found that KS-6 graphite shows the Rct value of 56.04 ohm and the lithium diffusion coefficient (Di) of 3.14×10-13 cm2 s-1. In contrast, it was observed that the 16%SiO2coated KS-6 graphite shows the lower Rct value of 45.97 ohm and the higher lithium diffusion coefficient (Di) of 8.42×10-11 cm2 s-1. Table 2 lists these AC impedance parameters in detail. As a result, the surface modification of graphite can indeed improve both the electrochemical performance and rate capability.
2 ω
(1)
3
4
− 1 /2
Fig. 7 The Nyquist plot and Z’ vs w-1/2 plot for K6 graphite and the SiO2-KS-6 graphite.
Table 1 The carbon content of KS-6 graphite and the SiO2 coated graphite samples Sample# 1 weight/
Sample# 2 weight/
Samples
mg
mg
KS-6 graphite
0.51
16%SiO2 coated KS-6 graphite
0.42
Data
C/ %
C/ %
Aver age C%
0.59
100.2 4
98.36
99.3 0
0.34
83.70
82.15
82.9 3
Table 2 AC parameters for KS-6 graphite and the SiO2 coated KS-6 graphite samples
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Data
4.
Conclusions
This work reported a sol-gel preparation method for the SiO2 coated KS-6 composite anode material by using KS-6 graphite, silicon oxide, and glucose (C6H12O6). Silicon oxide was used as a surface modification dopant and C6H12O6 was used as a carbon source. The composite anode material was sintered at 600-700oC in an Ar atmosphere for 5h. The characteristic properties of the composite anode materials were examined by micro-Raman spectroscopy, XRD, SEM, EA and AC impedance method. The coin cell was used to investigate the electrochemical performance at various rates. It was found that the specific discharge capacities of 16wt.%SiO2 coated KS-6 composite anode materials were 404, 422. 323, 192, 67, 29 and 13 mAh g-1 at 0.1, 0.2, 0.5, 1, 3, 5 and 10C rate, respectively. However, they all showed the current efficiency of ca. 96-99%. Two advantages for SiO2 modification on graphite, one increase discharge capacity due to the conversion reaction; the other prevent direct contact between electrolyte and graphite, which can avoid the crack or expansion of graphite during the long-term cycling test. The un-coated graphite material in LiPF6 electrolyte will also react with HF and thus leads to a gradually capacity loss. The SiO2 surface coating can prevent graphite from direct contacting with electrolyte and greatly decreasing the capacity loss Apparently, the as-synthesized SiO2 coated KS-6 composite anode can be a good candidate for high power Li-ion battery applications. 5.
Acknowledgements Financial support from the National Science Council, Taiwan (Project No: NSC 101-2221-E131 -037) is gratefully acknowledged.
6.
References
[1] M. Broussely, S. herreyre, P. Biensan, P. Kasztejan, K. Nechev, R.J. Staniewicz, J. Power Surces 97-98 (2001) 12-21. [2] Y.P. Wu, C. Jiang, C. Wang, R. Holze, Solid State Ionics 156 (2003) 283-290. [3] Y.P. Wu, E. Rahm, R. Holze, J. Power Sources 114 (2003) 228-236. [4] J.P. Olivier, M. Winter, J. Power Sources 151-
Rb/ ohm
Rct/ ohm
D/ cm2 s-1
4.32
56.04
3.14 10-13
Sample KS-6 graphite
16%SiO2 coated KS2.60 6 graphite 155 (2001) 151-155.
45.97
8.42 10-11
[5] H. Buqu, P. Golob, M. Winter, J.O. Besenhard, J. Power Sources 97-98 (2001) 122. [6] M. Lu, Y. Yan, X. Zheng, J. Gao, B. Huang, J. Power Sources 219 (2012) 188-192. [7] G. Zhao, Z. Wei, N. Zhang, K. Sun, Mater. Lett. 89 (2012) 243-246. [8] X. Guo, H.F. Xiang, T.P. Zhou, W.H. Li, X.W. Wang, J.X. Zhou, Y. Yu, Electrochim. Acta 109 (2013) 33-38. [9] J. Lai, H.J. Guo, X.Q. Li, Z.X. Wang, X.H. Li, X.P. Zhang, S.L. Huang, L. Gan, Transactions of Nonferrous Metals Society of China 23 (2013) 1413-1420.
ScienceDirect Energy Procedia 61 (2014) 1428 – 1433
The 6th International Conference on Applied Energy – ICAE2014
Preparation of silicon oxide coated KS-6 graphite composite anode materials by sol-gel method in lithium ion batteries 1,2
1
2
Chun-Chen Yang, 1,2 Jeng-Ywan Shih, 1,2 Min-Yen Wu
Department of Chemical Engineering, Ming Chi University of Technology, New Taipei City 243, Taiwan, R.O.C.
Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei City 243, Taiwan, R.O.C. (1Corresponding author, E-mail: [email protected])
Abstract
This work reported a sol-gel preparation method for the SiO2 coated KS-6 composite anode material by using KS-6 graphite, silicon oxide, and glucose (C6H12O6). Silicon oxide was used as a surface modification dopant and C6H12O6 was used as a carbon source. The composite anode material was sintered at 600-700oC in an Ar atmosphere for 5h. The characteristic properties of the composite anode materials were examined by micro-Raman spectroscopy, XRD, SEM, EA and AC impedance method. The coin cell was used to investigate the electrochemical performance at various rates. It was found that the specific discharge capacities of 16wt.%SiO2 coated KS-6 composite anode materials were 404, 422. 323, 192, 67, 29 and 13 mAh g-1 at 0.1, 0.2, 0.5, 1, 3, 5 and 10C rate, respectively. However, they all showed the current efficiency of ca. 96-99%. Apparently, the as-synthesized SiO2 coated KS-6 composite anode can be a good candidate for high power Li-ion battery applications. KeywordsǺAnode, KS-6 graphite, Silicon oxide (SiO2), Sol-gel method, surface modification
1.
Introduction
Lithium-ion batteries, due to their relatively high specific capacity, are considered for electric vehicle (EV), cell phones, laptop computers, digital cameras, renewable energy storage, and smart grid applications [1]. At present, graphite has been the most popular anode material for Li-ion batteries, mainly used to 3C small size devices that need high energy and a medium charge/discharge rate. However, the graphite cannot carry out at a high C-rate; it is due to easily form a dendritic on the surface of the graphite anode. The Li-ion battery (LIB) will degrade severely in performance, causing overheating and thermal runaway. Moreover, the graphite anode also suffers the particle expansion and shrinkage during the lithium intercalation/de-intercalation process. The safety problem of LIB is now more critical and challenge. There are two models have been proposed to describe the formation of the SEI film. The first
model is that the SEI film id established through the co- intercalation of solvent molecules from the decomposition electrolyte along with lithium ions insert into the graphite layers. The second model describes that the SEI film is formed by the decomposed electrolyte species on the graphite surface [2-4]. Nevertheless, the thickness of SEI film on the graphite surface is very critical and very important, because it affects the electrochemical performance of the carbon electrode. The graphite has two main problems, which are high irreversible capacity loss and poor cycling stability. It was well-known that the SEI film on the graphite surface can lead to the decomposition of electrolyte molecules at elevated temperature and cell potential and result in the high irreversible capacity loss and cycling performance deterioration. The surface modification of the graphite anodes has thus received a lot of attention. Recent studied showed that the surface modification is an effective way to improve its electrochemical
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.140
Chun-Chen Yang et al. / Energy Procedia 61 (2014) 1428 – 1433
performance [4-7]. The SiO2 surface coating on graphite can enhance the capacity and also prevent the direct contact electrolyte, which can deduce the crack or volume expansion/shrinkage problem during the charge/discharge cycling [5-9]. In this work, we prepare the SiO2-coated KS-6 graphite material by a sol-gel method, using KS6 graphite, silicon oxide, and glucose (C6H12O6) as the raw material. Silicon oxide was used as a surface modification dopant and C6H12O6 was used as a carbon source. The composite anode material was sintered at 600-700oC in an Ar atmosphere for 5h. The characteristic properties of the composite anode materials were examined by micro-Raman spectroscopy, XRD, SEM, EA and AC impedance method. The coin cell was used to investigate the electrochemical performance at various rates. The electrochemical performances of the SiO2-coated KS6 graphite composite samples were also examined by an automatic galvanostatic charge/discharge unit and a cyclic voltammetry method in detail. 2.
Experimental
2.1 Preparation of SiO2-coated KS-6 graphite materials A suitable amount of KS-6 graphite (Timcal Co. Ltd., Switzerland) and 16wt.%nano-SiO2 (Aldrich) was mixed with 5%glucose (Aldrich) in ethanol at 50oC for 10h under stirring. The mixed precursor was then dried at 80oC in a vacuum oven for 12h. The dry mixed powder was sintered at a tube furnace at 400oC for 4 h, then 600oC for 5 h in an Ar atmosphere. 2.2 Physical Property Characterization The crystal structure of SiO2-coated KS-6 graphite samples was examined by an X-ray diffraction (XRD) spectrometer (Philip, X’pert Pro System). The surface morphology was conducted by a scanning electron microscope (SEM, Hitachi). The Micro-Raman spectra were recorded on a confocal micro-Renishaw with a 632 nm He-Ne laser excitation. The residual carbon content in the sample was analyzed using Elemental Analyzer (Perkin Elmer 2400). The electron conductivity of the composite samples was measured by AC impedance method. 2.3 Electrochemical measurements The electrochemical performances of KS-6 graphite and 16wt.%SiO2-coated KS-6 graphite composite battery were measured by using a two-
electrode system (CR 2032 coin cell assembled in an argon-filled glove box). The SiO2-coated KS-6 graphite composite electrodes were prepared by mixing active KS-6 graphite or 16wt.%SiO2-coated KS-6 graphite LiFePO4/C materials, Super P, and poly(vinyl fluoride) (PVDF) binder in a weight ratio of 80:10:10, pasted on an copper foil (Aldrich), and then dried in a vacuum oven at 120oC for 12 h. The lithium foil (Aldrich) was used as the counter and reference electrode. A micro-porous PE film was used as the separator. The electrolyte was 1 M LiPF6 in a mixture of EC and DEC (1:1 in v/v, Merck). The KS-6 graphite or SiO2-coated KS-6 graphite half cells were charged by a constant current profile (CC) and discharged by a constant current profile, over a potential range of 0.02 – 1.50 V (vs. Li/Li+) at varied C rates with an Autolab PGSTAT302N potentiostat. The cyclic voltammetry (CV) was conducted by using an Autolab instrument at a scanning rate of 0.1 mV s-1 between 0.02 and 1.50 V. 3. Results and discussion The XRD patterns of KS-6 graphite and the SiO2-coated KS-6 graphite prepared by the sol-gel method are shown in Fig. 1, respectively. The XRD diffraction patterns revealed that both the K-S 6 graphite and SiO2-coated KS-6 graphite have similar pattern, including (002), (100), (101), and (004) diffraction peaks. However, the peak intensities of the peaks are different. It was found that the peak intensity of SiO2-coated KS-6 graphite is lower than that of KS6 graphite. It may be due to some of amorphous soft carbon deposited on graphite. There is no obvious SiO2 XRD peak occurred. It is because the particle size of SiO2 powder is small. Their (002) peaks are very narrow and sharp, which indicates graphite crystallites are perfect and the structure defects are very few. It was found that that the XRD patterns of SiO2-coated KS-6 graphite and KS-6 graphite are very close to the standard graphite (JCPDS card number 41-1487). The SiO2 surface coating and glucose carbon precursor have no observable influence on the structure of graphite materials.
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Fig. 2 SEM images of (a).KS-6 graphite; and (b). 16%SiO2-KS-6 graphite. 2000 1500
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70
2 T h e a ta
Fig. 1 XRD patterns of (a). KS-6 graphite; and (b). SiO2-KS-6 graphite composite. The SEM images of KS-6 graphite and the SiO2-coated KS-6 graphite samples are shown in Fig. 2(a) and (b), respectively. As can be seen from the figures, KS-6 graphite has flake morphology; however, has irregular granular nano-SiO2 covered KS-6 graphite morphology and shape. It shows clearly that the morphology of KS-6 graphite is totally different from the SiO2-coated KS-6 graphite. The electrochemical properties of graphite samples are strongly related with the degree of graphitization and the total carbon content, and the value of ID/IG ratio. The carbon content of the KS-6 graphite and SiO2coated KS-6 graphite samples are approximately 99.30% and 82.93%, respectively.
25
According to the EA analysis result, we can find that the composition of 16%SiO2-coated KS-6 graphite is composed of 16wt.%SiO2, 82.93% graphite and 1.07% amorphous soft carbon, which is the residual carbon after sintering glucose. The cyclic voltammetries of the KS-6 graphite and the SiO2coated KS-6 graphite in the second cycle at a scan rate of 0.1 mV s-1 are shown in Fig. 3. The reduction (Liintercalation) and oxidation (Li-deintercalation) peaks of the KS-6 graphite sample are at approximately 0.16 and 0.265V, respectively, and the potential difference between the two peaks is 0.10V. In contrast, two reduction (Li-intercalation) peaks and one oxidation (Li-deintercalation) peak of the SiO2-coated KS-6 graphite sample are approximately at 0.175V, 0.06V, and 0.260V, respectively. It was found that the discharge and charge capacity of the SiO2-coated KS6 graphite is much larger than that of the KS-6 graphite. It was demonstrated that the discharge capacity of the SiO2-coated KS-6 graphite is larger than that of pristine KS-6 graphite. The SiO2 material is also active anode material; it can also deliver extra discharge capacity through the conversion reaction.
(a) KS-6 graphite (b) 16%SiO 2 coated KS-6 graphite 0.0020 0.0015 (b) 0.0010
Current/ A
60000
0.0005
(a)
0.0000 -0.0005 -0.0010 -0.0015 -0.0020 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 +
Potential/ V (vs. Li /Li)
Fig. 3 The CV curve of KS6 graphite and SiO2-graphite composite samples at 0.1 mV s-1.
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(a)
(b)
Voltage/ mV
1200 1000 800 600 400 200 0 0
50
100
150
200
250
300
350
400
450
Capacity/ mAh g -1
Fig. 4 The initial charge-discharge curve for KS-6 graphite and the SiO2 coated graphite composite at 0.1C rate. Fig. 5 shows the rate capability of KS-6 graphite and the 16%SiO2-coated KS-6 graphite at various rates (from 0.2C to 10C). The KS-6 graphite delivers the specific capacities of 341, 221, 112, 16, 10, and 5 mAh g-1 at rates of 0.2, 0.5, 1, 3, 5, and 10C, respectively. It shows very poor high rate capability. However, the 16%SiO2-coated KS-6 graphite delivers the specific capacities of 420, 287, 164, 61, 27, and 13 mAh g-1 at rates of 0.2, 0.5, 1, 3, 5, and 10C, respectively. It shows that the the high rate capability is improved, as compared with pristine KS-6 graphite. very poor high rate capability. the LiFePO4/C sample without both PSS template and Nb doping delivers the specific capacities of 133, 131, 124, 106, 93, and 75 mAh g-1 at 0.2, 0.5, 1, 3, 5 and 10C rates, respectively.
(a) KS-6 graphite (b) 16% SiO 2 coated KS-6 graphite
(b)
400 (a)
Capacity/ mAh g
-1
350 300 250 200 150 100 50 0
5C 10C 1C 3C 0.2C 0.5C 1-3cycle 1-3cycle 1-3cycle 1-3cycle 1-3cycle 1-3cycle
Fig. 5 The rate capability curve of KS6 graphite and the SiO2 coated KS-6 graphite composite samples at varied rates. Moreover, the long-term cycling performance is also very important. iThe t was revealed that the discharge capacity of the KS-6 graphite at 0.1 C rate is 320 mAh g-1 and a coulomb efficiency was ca. 98%. However, the discharge capacity and the coulomb efficiency of the 16%SiO2coated KS-6 graphite composite during 30 cycles test at 0.1C/0.1C rate are 362 mAh g-1 and 98%, respectively, as shown in Fig. 6. Two advantages for SiO2 modification on graphite, one increase discharge capacity due to the conversion reaction; the other prevent direct contact between electrolyte and graphite, which can avoid the crack or expansion of graphite during the long-term cycling test. The dissolution of cathode materials, such as Mn4+ and Fe2+ from LiMn2O4 or LiFePO4 cathode materials is one of the main reasons for capacity fading mechanism during high temperature cycling. The Mn4+ and Fe2+ cations will deposit on graphite surface, it results in increasing the thickness of SEI film. It was well-known that the HF generated from LiPF6 electrolyte was responsible for the dissolution of Fe during cycling. The un-coated graphite material in LiPF6 electrolyte will also react with HF and thus leads to a gradually capacity loss. The SiO2 surface coating can prevent graphite from direct contacting with electrolyte and greatly decreasing the capacity loss [1-4].
500 450
CE% (c)
100
400
(d) (b)
350 (a)
75
300 capacity
CE%
(a) KS-6 (b) 16%SiO 2 coated KS-6
1400
450
-1
1600
500
Capacity/ mAh g
Fig. 4 shows the initial charge-discharge profiles of KS-6 graphite and the SiO2-coated KS-6 graphite at 0.1C/0.1C charge/discharge rate at 25oC. It was found that the discharge and charge capacities of KS-6 graphite are 357 and 340 mAh g-1, and the irreversible capacity loss and the current efficiency are 17 mAh g-1 and 95%, respectively. However, it was found that the discharge and charge capacities of the 16%SiO2-coated KS-6 graphite are 413 and 404 mAh g-1, and the irreversible capacity loss and the current efficiency are 9 mAh g-1 and 98%, respectively. It is clearly that the charge capacity performance of the 16%SiO2-coated KS-6 graphite is superior to that of pristine KS-6 graphite without SiO2 coating. It may be due to the synergistic effect of nano-SiO2 and the soft carbon coating.
250 50
200 150 (a) KS-6 (Capacity) (b) SiO2 coated KS-6 (Capacity)
100
25
(c) KS-6 (CE%) (d) SiO2 coated KS-6 graphite (CE%)
50 0
0 5
10
15
20
Number of cycles
25
30
35
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Chun-Chen Yang et al. / Energy Procedia 61 (2014) 1428 – 1433
Fig. 6 The cycling performance of KS-6 graphite and the SiO2 coated KS-6 graphite composite samples at 0.1C rate.
( a ) K S - 6 g r a p h it e ( b ) 1 6 % S iO 2 c o a te d K S - 6 g r a p h i te
200
(a) 180 160
Z'/ ohm
140 120 100 80 (b )
60 40 0
1
2
3
4
ω − 1 /2
( a ) K S - 6 g r a p h ite ( b ) 1 6 % S i O 2 c o a t e d K S - 6 g r a p h it e
200 180 160 140 Z'/ ohm
The AC impedance spectroscopy is used to study the interface properties of the KS-6 and 16%SiO2-coated KS-6 graphite samples. The AC spectra of the the KS-6 and 16%SiO2-coated KS-6 graphite samples at open circuit potential are shown in Fig. 7(a). Each AC plot consisted of one semicircle at higher frequency followed by a linear portion at lower frequency. The lower frequency region of the straight line is considered as Warburg impedance. It is for long-range lithium ion diffusion in bulk phase. The Rb indicates the bulk resistance at the electrolyte; Rct is attributed to the charge transfer resistance at the active material interface; CPE represents the double layer capacitance and some surface film capacitance. The lithium chemical diffusion coefficients (Di) of the electrode were calculated based on the equation (1):
(a )
120 100 80 60
(b )
40
Di =
1 RT 2 ( ) 2 AF 2σC
0
1
Where σ is Warburg impedance coefficient (obtained a slope from plot of Zre vs.ϖ-0.5, as seen in Fig. 7(b); Di is the lithium diffusion coefficient; R is the gas constant; T is the absolute temperature; F is Faraday’s constant; A is the area of the electrode; C is the molar concentration of Li+ ions (CLi+=1.0×10-3 mol cm-3). Table 1 summaries the calculated Rb, Rct, Di, jo parameters for two samples. It was found that the Rb values of KS-6 graphite sample without any surface modification and the 16%SiO2-coated KS-6 graphite are around 4.32 and 2.60 ohm, respectively. The values of Rct and Di for two graphite samples are greatly varied. After SiO2 surface coating on graphite, the charge resistance (Rct) values and the lithium diffusion coefficients (DLi) are effectively improved, which is good for the enhancement of rate capability of graphite materials. By comparison, we found that KS-6 graphite shows the Rct value of 56.04 ohm and the lithium diffusion coefficient (Di) of 3.14×10-13 cm2 s-1. In contrast, it was observed that the 16%SiO2coated KS-6 graphite shows the lower Rct value of 45.97 ohm and the higher lithium diffusion coefficient (Di) of 8.42×10-11 cm2 s-1. Table 2 lists these AC impedance parameters in detail. As a result, the surface modification of graphite can indeed improve both the electrochemical performance and rate capability.
2 ω
(1)
3
4
− 1 /2
Fig. 7 The Nyquist plot and Z’ vs w-1/2 plot for K6 graphite and the SiO2-KS-6 graphite.
Table 1 The carbon content of KS-6 graphite and the SiO2 coated graphite samples Sample# 1 weight/
Sample# 2 weight/
Samples
mg
mg
KS-6 graphite
0.51
16%SiO2 coated KS-6 graphite
0.42
Data
C/ %
C/ %
Aver age C%
0.59
100.2 4
98.36
99.3 0
0.34
83.70
82.15
82.9 3
Table 2 AC parameters for KS-6 graphite and the SiO2 coated KS-6 graphite samples
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Chun-Chen Yang et al. / Energy Procedia 61 (2014) 1428 – 1433
Data
4.
Conclusions
This work reported a sol-gel preparation method for the SiO2 coated KS-6 composite anode material by using KS-6 graphite, silicon oxide, and glucose (C6H12O6). Silicon oxide was used as a surface modification dopant and C6H12O6 was used as a carbon source. The composite anode material was sintered at 600-700oC in an Ar atmosphere for 5h. The characteristic properties of the composite anode materials were examined by micro-Raman spectroscopy, XRD, SEM, EA and AC impedance method. The coin cell was used to investigate the electrochemical performance at various rates. It was found that the specific discharge capacities of 16wt.%SiO2 coated KS-6 composite anode materials were 404, 422. 323, 192, 67, 29 and 13 mAh g-1 at 0.1, 0.2, 0.5, 1, 3, 5 and 10C rate, respectively. However, they all showed the current efficiency of ca. 96-99%. Two advantages for SiO2 modification on graphite, one increase discharge capacity due to the conversion reaction; the other prevent direct contact between electrolyte and graphite, which can avoid the crack or expansion of graphite during the long-term cycling test. The un-coated graphite material in LiPF6 electrolyte will also react with HF and thus leads to a gradually capacity loss. The SiO2 surface coating can prevent graphite from direct contacting with electrolyte and greatly decreasing the capacity loss Apparently, the as-synthesized SiO2 coated KS-6 composite anode can be a good candidate for high power Li-ion battery applications. 5.
Acknowledgements Financial support from the National Science Council, Taiwan (Project No: NSC 101-2221-E131 -037) is gratefully acknowledged.
6.
References
[1] M. Broussely, S. herreyre, P. Biensan, P. Kasztejan, K. Nechev, R.J. Staniewicz, J. Power Surces 97-98 (2001) 12-21. [2] Y.P. Wu, C. Jiang, C. Wang, R. Holze, Solid State Ionics 156 (2003) 283-290. [3] Y.P. Wu, E. Rahm, R. Holze, J. Power Sources 114 (2003) 228-236. [4] J.P. Olivier, M. Winter, J. Power Sources 151-
Rb/ ohm
Rct/ ohm
D/ cm2 s-1
4.32
56.04
3.14 10-13
Sample KS-6 graphite
16%SiO2 coated KS2.60 6 graphite 155 (2001) 151-155.
45.97
8.42 10-11
[5] H. Buqu, P. Golob, M. Winter, J.O. Besenhard, J. Power Sources 97-98 (2001) 122. [6] M. Lu, Y. Yan, X. Zheng, J. Gao, B. Huang, J. Power Sources 219 (2012) 188-192. [7] G. Zhao, Z. Wei, N. Zhang, K. Sun, Mater. Lett. 89 (2012) 243-246. [8] X. Guo, H.F. Xiang, T.P. Zhou, W.H. Li, X.W. Wang, J.X. Zhou, Y. Yu, Electrochim. Acta 109 (2013) 33-38. [9] J. Lai, H.J. Guo, X.Q. Li, Z.X. Wang, X.H. Li, X.P. Zhang, S.L. Huang, L. Gan, Transactions of Nonferrous Metals Society of China 23 (2013) 1413-1420.