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carbon cloth composite electrode materials prepared by magnetron sputtering for Li-ion batteries
Accepted Manuscript Reversible and high-capacity SnO2/carbon cloth composite electrode materials prepared by magnetron sputtering for Li-ion batteries Panpan Xu, Gang Wang, Junfeng Yan, Zhiyong Zhang, Manzhang Xu, Shaobo Cai, Xiongfei Ruan, Zhouhu Deng PII: DOI: Reference:
S0167-577X(16)31957-7 http://dx.doi.org/10.1016/j.matlet.2016.12.071 MLBLUE 21883
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
26 October 2016 16 December 2016 23 December 2016
Please cite this article as: P. Xu, G. Wang, J. Yan, Z. Zhang, M. Xu, S. Cai, X. Ruan, Z. Deng, Reversible and highcapacity SnO2/carbon cloth composite electrode materials prepared by magnetron sputtering for Li-ion batteries, Materials Letters (2016), doi: http://dx.doi.org/10.1016/j.matlet.2016.12.071
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reversible and high-capacity SnO2 /carbon cloth composite electrode materials prepared by magnetron sputtering for Li-ion batteries Panpan Xua, Gang Wangb*, Junfeng Yana*, Zhiyong Zhanga, Manzhang Xua, Shaobo Caic, Xiongfei Ruana, Zhouhu Denga a
School of Information Science and Technology, Northwest University, Xi'an, 710127, P. R. China
b
Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, P. R. China
c
College of Chemistry and Materials Science, Northwest University, Xi’an 710127, P. R. China
*
Corresponding author: Tel.: +86 29 88308280; fax: +86 29 88308330; E-mail: [email protected] (J. F.
Yan), [email protected] (G. Wang) Abstract: The flexible electrode material of SnO2/carbon cloth (SnO2/CC) is fabricated by RF magnetron sputtering method. As an anode material of lithium ion batteries, the SnO2/CC electrode exhibits more excellent cycling stability and rate capability than that of the pure carbon cloth or SnO2. The reversible capacity always maintain about 1.98mAh/cm2 during 50 cycles, which is higher than that of the pure CC (about 1.61 mAh/cm2) and SnO2 (about 0.08 mAh/cm2). After 100 cycles, the capacity of SnO2/CC sample is 1.85 mAh/cm2, still retaining 89.4% of the initial capacity. The good electrochemical properties of SnO2/CC are mainly caused by the high surface area, porous structure and the intrinsic soft characteristics of CC, which can effectively accommodate the volume charge during charge and discharge process. Keywords: SnO2/carbon cloth; flexible electrodes; lithium ion batteries; sputtering; composite materials 1. Introduction Lithium ion batteries (LIBs) have attracted tremendous attention due to their advantages of high
energy density, excellent cycle life and low environmental impact [1, 2]. Graphite has been widely used as anode material for LIBs, however, the low capacity (372 mAh g−1) of graphite can not satisfy the growing requirement for high energy and power density of next generation LIBs [3]. Among the numerous possible alternative anode materials, SnO2 has a high theoretical capacity (781mAh g−1) and low potential of lithium ion intercalation [4]. Therefore, it is considered to be one of the most promising candidates in the future. However, the severe volume change (over 300%) issue in the process of charging and discharging results in pulverization of active material [5] and loss of electrical contact at the anode [6, 7]. Up to now, series of approaches have been adopted to overcome the above-mentioned problems. These methods include (i)reducing the size of particles, or preparing SnO2 material with special morphology [8], (ii)forming tin-base alloy [9], (iii)doping some substance, (iv)synthesizing composite material [10]. However, the traditional electrode materials prepared by aforementioned measures were mainly fabricated by coating method. Videlicet, the mixtures of the active material, conductive material (carbon black) and binder were coated on the current collector. Not only the process is complex, but also the active material can not be fully utilized. Therefore, if SnO2 can be synthesized directly on the flexible CC, which would make it have a good mechanical contact between the active material and the current collector, additionally, the energy density of the anode material has been greatly improved due to avoiding utilizing conductive additive and binder. However, that the SnO2 direct grown on CC by magnetron sputtering method as the anode material of LIBs has not been realized. In this paper, the SnO2 is grown directly on the flexible CC by magnetron sputtering method as the anode material. The electrochemical performance of the sample was obviously improved. 2. Experimental
2.1. Preparation of SnO2/CC The composite SnO2/CC was synthesized by RF magnetron sputtering. Firstly, The CC (WOS 1002, CeTech, the thinkness is 360 µm, Fig. S1 shows the external photo and the low magnification SEM image of pure CC in the supplementary material) was cut into a circle with a diameter of 12 mm, cleaned with with acetone, deionized water and absolute ethyl alcohol for about 30 min using supersonic wave apparatus, respectively. Then the CC was transferred into bake oven at 80 ℃ for 12 h. After that, the CC was introduced into the magnetron sputter apparatus. Subsequently, the sputtering system was evacuated to a pressure of 5.0 × 10-4 Pa, then substrate was heated to 250 ℃ and the Ar/O2 gas (both 99.999% in purity) mixture was adjusted at a flux ratio of 32 : 8. The pre-sputtering was carried out for 20 minutes to ensure a clean SnO2 target surface. Next, the working pressure was adjusted to 0.5 Pa, sputtering power was setted to 100 W. After the CC was sputtered for two hours, the composite SnO2/CC was prepared successfully. 2.2. Characterization The prepared samples were characterized by using X-ray powder diffraction (XRD, SHIMADZU 6100) and scanning electron microscope (SEM, Phenom Prox). The energy dispersive spectrometer (EDS) and elemental mapping studies were performed under the SEM. 2.3. Electrochemical measurements The electrochemical measurements were performed at 25 ℃ using coin cells (CR2025) with pure lithium foil as the counter and reference electrode. The prepared SnO2/CC composite were used directly as the working electrode. Cells were constructed in a glove box in argon atmosphere under a dew point below -65 ℃. The cells were performed by galvanostatic charge/discharge tests. The charge/discharge
tests were carried out on a LAND battery program-control testsystem (CT2001A, Wuhan Jinnuo Electronic Co. Ltd. of China) at a constant current of 200 µA in a cut-off potential window of 0 – 3.0 V. Cyclic voltammetry (CV) measurements were carried out on a CHI660D electrochemical workstation at a scan rate of 0.2 mV s-1 in a potential range of 0 – 3.0 V. 3. Results and discussion 3.1. Microstructure and morphology Fig. 1a demonstrates the structural characteristics of the CC sample before magnetron sputtering investigated by XRD. The sharp peak at around 26° and the weak peak at around 44° are agreed with the carbon material (JCPDS file No. 65-6212) [11]. The XRD patterns of SnO2/CC composite is shown in Fig. 1b, the major diffraction peaks are in accordance with the rutile SnO2 (JCPDS file No. 41-1445), which suggests that high-purity SnO2 products have been synthesized. The typical SEM images of pure CC and the SnO2/CC products are shown in Fig. 2a and 2b, respectively. As shown, every single carbon fiber is closely wrapped by nanometer-sized SnO2. The inset in Fig. 2b shows that there are some fault sections on the surface of CC, which verify that the SnO2 was grown on the CC successfully. Fig. 2c – f show the EDS mapping of the SnO2/CC for the detected elements C, O and Sn, respectively. The C, O and Sn elements are uniformly distributed across the whole SnO2/CC product. In order to further analyze the stoichiometric proportion of the comprised elements in the as-prepared product, the EDS spectrum of SnO2/CC has been carried out, and the results are shown in Fig. S2 in the supplementary material. The values of the atomic ratio among C, O and Sn are 45.9 : 35.5 : 18.6, which means the stoichiometric phase of SnO2 has been obtained.
3.2. Electrochemical performance In order to clarify the electrochemical reactions of the SnO2/CC electrode, CV test in a potential range of 0–3.0 V versus Li/Li+ was implemented. And the result is shown in Fig. 3a. In the first cathodic scan, there is one weak peak at 0.95 V, which can be ascribed to the formation of the solid electrolyte interphase layer on the active materials, and the reduction of SnO2 to Sn and the synchronous formation of Li 2O as described in Eq. (1). The strong peak at about 0.25 V should be related to the formation of LixSn given by Eq. (2), which is attested to be highly reversible during the charge and discharge process [12]. In the first anodic scan, the peaks at 0.65 and 1.85 V can be attributed to Li dealloying from LixSn [13] and partly reversible reaction [14] of the Eq. (1), respectively. Meanwhile, it quite obvious that the CV curves of the second and the third cycles were nearly overlapped, implying that the reactions in equations. (1)-(2) are highly reversible. The major reactions can be summarized as follows: SnO 2 + 4Li + + 4e − → Sn + 2Li 2 O
(1)
Sn + xLi + xe − ↔ Li xSn (0 ≤ x ≤ 4.4)
(2)
The cycling performance of SnO2/CC, pure CC and the SnO2 electrodes were investigated by galvanostatic charge and discharge between 0 and 3.0 V at the constant current of 200 µA, and the results are shown in Fig. 3b. The SnO2/CC electrode displays a high reversible capacity and good cyclic retention. The reversible capacity always maintain at about 1.98 mAh/cm2 during 50 cycles, which is higher than the pure CC (about 1.61 mAh/cm2) and SnO2 (about 0.08 mAh/cm2). After 100 cycles, the capacity of SnO2/CC sample was 1.85 mAh/cm2, still retaining 89.4% of the initial capacity. It is obvious that the SnO2/CC sample demonstrates much better reversible capacity and cycle stability than the pure SnO2. The SnO2/CC sample also shows significantly enhanced rate performance than pure CC and
SnO2. The comparison of the rate results of the three samples was shown in Fig. 3c. As can be seen, after the electrodes cycled at 200 µA for 10 cycles, the applied current densities were increased stepwise to 3200 µA. The average reversible capacities of the SnO2/CC electrode are 1.83, 1.53, 1.22, and 0.86 mAh/cm2 at the constant current of 400, 800, 1600, and 3200 µA. However, the average reversible capacities of the pure CC electrode are 1.65, 1.36, 1.07, 0.78, and the average reversible capacities of the pure SnO2 electrode are 0.59, 0.41, 0.21, 0.11 mAh/cm2 under the same test program respectively. Most notably, it can be seen from Table S1 [15-20] in the supplementary material that the capacity of SnO2/CC composite is much better than most of the previously reported SnO2-based anode materials for LIBs. The improvement of lithium storage properties is mainly caused by synergistic effect of SnO2 and flexible CC. Firstly, compared with the traditional methods (hydrothermal method, molten salt method, etc.), the SnO2 prepared by magnetron sputtering method has better homogeneity and higher purity. Additionally, the SnO2 is grown directly on the flexible CC, the energy density of the anode material has been greatly improved due to avoiding utilizing conductive additive and binder. Secondly, CC with a three-dimensional structure has a good electronic transmission efficiency, which can greatly improve the rate performance of the battery. Thirdly, the high surface area, porous structure and the intrinsic soft characteristics of CC can effectively accommodate the volume change during charge and discharge process [21], which can enhance the cycle performance of the battery.
4. Conclusions The SnO2/CC composite was successfully fabricated by magnetron sputtering method. From the phase and morphology characterization results, it can be seen clearly that the sample has a high purity and uniformity. When used as anode materials for lithium-ion batteries, the SnO2 /CC electrode exhibits better
electrochemical performance than the pure CC and SnO2, which is mainly because the high surface area, porous structure and the intrinsic soft characteristics of CC can effectively accommodate the volume change during charge and discharge process. The results of this work proposed a method for preparing flexible electrode material and enhancing the electric performance of SnO2 as anode materials in lithium-ion batteries.
Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 61306009 and 61405159), the Key Project of Natural Science Foundation of Shaanxi Province (Grants 2014JZ2-003), and the Natural Science Foundation of Shaanxi Province (Grants 2014JM8339 and 2015JM6274).
References [1] M.R. Palacin. Recent advances in rechargeable battery materials: a chemist's perspective. Chem. Soc. Rev. 38 (2009) 2565-2575. [2] X.P. Gao, H.X. Yang. Multi-electron reaction materials for high energy density batteries. Energ Environ Sci. 3 (2010) 174-189. [3] X.Y. Xue, Z.H. Chen, L.L. Xing, S. Yuan, Y.J. Chen. SnO2/α-MoO3 core-shell nanobelts and their extraordinarily high reversible capacity as lithium-ion battery anodes. Chem. Commun. 47 (2011) 5205-5207. [4] L. Luo, W.Z. Xu, Z.K. Xia, Y.Q. Fei, J.D. Zhu, C. Chen, Y. Lu, Q.F. Wei, H. Qiao, X.W. Zhang. Electrospun ZnO–SnO2 composite nanofibers with enhanced electrochemical performance as lithium-ion anodes. Ceram. Int. 42 (2016) 10826-10832.
[5] Y.H. Liu, X. Fang, M.Y. Ge, J.P. Rong, C.F. Shen, A.Y. Zhang, H.A. Enaya, C.W. Zhou. SnO2 coated carbon cloth with surface modification as Na-ion battery anode. Nano Energy. 16 (2015) 399-407. [6] H.X. Zhang, C. Feng, Y.C. Zhai, K.L. Jiang, Q.Q. Li, S.S. Fan. Cross℃stacked carbon nanotube sheets uniformly loaded with SnO2 nanoparticles: A novel binder℃free and high℃capacity anode material for lithium℃ion batteries. Adv. Mater. 21 (2009) 2299-2304. [7] Y. Yu, C.H. Chen, Y. Shi. A tin℃based amorphous oxide composite with a porous, spherical, multideck℃cage morphology as a highly reversible anode material for Lithium℃Ion batteries. Adv. Mater. 19 (2007) 993-997. [8] L.L. Feng, Z.W. Xuan, S.P. Ji, W.R. Min, H.B. Zhao, H. Gao. Preparation of SnO2 nanoparticle and performance as lithium-ion battery anode. Int J Electrochem Sc. 10 (2015) 2370-2376. [9] P. Peng, Z.Y. Wen, Y. Liu, J. Jin. Improvement of lithium storage performance of Sn-alloy anode materials by a polypyrrole protective layer. J. Power Sources. 274 (2015) 1100-1106. [10] N. Wan, X. Lu, Y. Wang, W. Zhang, Y. Bai, Y.S. Hu, S. Dai. Improved Li storage performance in SnO2 nanocrystals by a synergetic doping. Sci Rep. 6 (2016) 18978. [11] Y.D. Zhang, Z.A. Hu, Y.R. Liang, Y.Y. Yang, N. An, Z.M. Li, H.Y. Wu. Growth of 3D SnO2 nanosheets on carbon cloth as a binder-free electrode for supercapacitors. J. Mater. Chem. A. 3 (2015) 15057-15067. [12] Y.P. Sun, C.Y. Zhao, M.Y. Shen, Z.W. Pan, X.G. Liu. SnO2 nanoparticles encapsulated by curved graphite layers as anode materials for Li-ion batteries with high performances. J. Alloys Compd. 683 (2016) 191-197. [13] M.W. Xu, M.S. Zhao, F. Wang, W. Guan, S. Yang, X.P. Song. Facile synthesis and electrochemical
properties of porous SnO2 micro-tubes as anode material for lithium-ion battery. Mater. Lett. 64 (2010) 921-923. [14] P.C. Lian, X.F. Zhu, S.Z. Liang, Z. Li, W.S. Yang, H.H. Wang. High reversible capacity of SnO2/graphene nanocomposite as an anode material for lithium-ion batteries. Electrochim. Acta. 56 (2011) 4532-4539. [15] X.X. Wang, C.H. Zhao, R. Liu, Q. Shen. Hydrothermal synthesis and structural properties of hierarchical flower-like SnO2 nanostructures for lithium ion batteries. J. Nanopart. Res. 16 (2014) 2570. [16] M.V. Reddy, L. Yu Tse, W.K.Z. Bruce, B.V.R. Chowdari. Low temperature molten salt preparation of nano-SnO2 as anode for lithium-ion batteries. Mater. Lett. 138 (2015) 231-234. [17] X.S. Zhu, J.Y. Zhu, Y.N. Yao, Y.M. Zhou, Y.W. Tang, P. Wu. Facile template-directed synthesis of carbon-coated SnO2 nanotubes with enhanced Li-storage capabilities. Mater. Chem. Phys. 163 (2015) 581-586. [18] R. Tian, Y.Y. Zhang, Z.H. Chen, H.N. Duan, B.Y. Xu, Y.P. Guo, H.M. Kang, H. Li, H.Z. Liu. The effect of annealing on a 3D SnO2/graphene foam as an advanced lithium-ion battery anode. Scientific Reports. 6 (2016) 19195. [19] H.K. Zhang, H.H. Song, X.H. Chen, J.S. Zhou, H.J. Zhang. Preparation and electrochemical performance of SnO2@carbon nanotube core–shell structure composites as anode material for lithium-ion batteries. Electrochim. Acta. 59 (2012) 160-167. [20] Z.G. Wu, J.T. Li, Y.J. Zhong, J. Liu, X.D. Guo, L. Huang, B.H. Zhong, S.G. Sun. Synthesis of hierarchical worm-like SnO2@C aggregates and their enhanced lithium storage properties. J. Alloys Compd. 620 (2015) 407-412.
[21] M.Y. Ge, Y.H. Lu, P. Ercius, J.P. Rong, X. Fang, M. Mecklenburg, C.W. Zhou. Large-scale fabrication, 3d tomography, and lithium-ion battery application of porous silicon. Nano Lett. 14 (2013) 261-268.
Figure captions: Fig. 1. XRD patterns of pure CC sample (a) and SnO2/CC composite (b) Fig. 2. SEM images of pure CC sample (a) and SnO2/CC composite (b). The element mapping of C, O and Sn of SnO2/CC (c – f) Fig. 3. The initial three CV curves of SnO2/CC electrode (a) at a scan rate of 0.2 mV/s. The cycling performance (b) and rate performance (c) of SnO2/CC, pure CC and SnO2
Fig. 1
Fig. 2
Fig. 3
The flexible electrode material of SnO2/carbon cloth(SnO2/CC) is successfully fabricated by magnetron sputtering method, the SnO2/CC electrode exhibits more excellent electrochemical performance than the performance of the pure carbon cloth or SnO2.
Graphical abstract
Highlights
The composite SnO2/CC was synthesized by magnetron sputtering method. Electrode material is flexible. Enhanced electrochemical performance are achieved based on synergistic effect of SnO2 and flexible CC.
S0167-577X(16)31957-7 http://dx.doi.org/10.1016/j.matlet.2016.12.071 MLBLUE 21883
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
26 October 2016 16 December 2016 23 December 2016
Please cite this article as: P. Xu, G. Wang, J. Yan, Z. Zhang, M. Xu, S. Cai, X. Ruan, Z. Deng, Reversible and highcapacity SnO2/carbon cloth composite electrode materials prepared by magnetron sputtering for Li-ion batteries, Materials Letters (2016), doi: http://dx.doi.org/10.1016/j.matlet.2016.12.071
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reversible and high-capacity SnO2 /carbon cloth composite electrode materials prepared by magnetron sputtering for Li-ion batteries Panpan Xua, Gang Wangb*, Junfeng Yana*, Zhiyong Zhanga, Manzhang Xua, Shaobo Caic, Xiongfei Ruana, Zhouhu Denga a
School of Information Science and Technology, Northwest University, Xi'an, 710127, P. R. China
b
Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, P. R. China
c
College of Chemistry and Materials Science, Northwest University, Xi’an 710127, P. R. China
*
Corresponding author: Tel.: +86 29 88308280; fax: +86 29 88308330; E-mail: [email protected] (J. F.
Yan), [email protected] (G. Wang) Abstract: The flexible electrode material of SnO2/carbon cloth (SnO2/CC) is fabricated by RF magnetron sputtering method. As an anode material of lithium ion batteries, the SnO2/CC electrode exhibits more excellent cycling stability and rate capability than that of the pure carbon cloth or SnO2. The reversible capacity always maintain about 1.98mAh/cm2 during 50 cycles, which is higher than that of the pure CC (about 1.61 mAh/cm2) and SnO2 (about 0.08 mAh/cm2). After 100 cycles, the capacity of SnO2/CC sample is 1.85 mAh/cm2, still retaining 89.4% of the initial capacity. The good electrochemical properties of SnO2/CC are mainly caused by the high surface area, porous structure and the intrinsic soft characteristics of CC, which can effectively accommodate the volume charge during charge and discharge process. Keywords: SnO2/carbon cloth; flexible electrodes; lithium ion batteries; sputtering; composite materials 1. Introduction Lithium ion batteries (LIBs) have attracted tremendous attention due to their advantages of high
energy density, excellent cycle life and low environmental impact [1, 2]. Graphite has been widely used as anode material for LIBs, however, the low capacity (372 mAh g−1) of graphite can not satisfy the growing requirement for high energy and power density of next generation LIBs [3]. Among the numerous possible alternative anode materials, SnO2 has a high theoretical capacity (781mAh g−1) and low potential of lithium ion intercalation [4]. Therefore, it is considered to be one of the most promising candidates in the future. However, the severe volume change (over 300%) issue in the process of charging and discharging results in pulverization of active material [5] and loss of electrical contact at the anode [6, 7]. Up to now, series of approaches have been adopted to overcome the above-mentioned problems. These methods include (i)reducing the size of particles, or preparing SnO2 material with special morphology [8], (ii)forming tin-base alloy [9], (iii)doping some substance, (iv)synthesizing composite material [10]. However, the traditional electrode materials prepared by aforementioned measures were mainly fabricated by coating method. Videlicet, the mixtures of the active material, conductive material (carbon black) and binder were coated on the current collector. Not only the process is complex, but also the active material can not be fully utilized. Therefore, if SnO2 can be synthesized directly on the flexible CC, which would make it have a good mechanical contact between the active material and the current collector, additionally, the energy density of the anode material has been greatly improved due to avoiding utilizing conductive additive and binder. However, that the SnO2 direct grown on CC by magnetron sputtering method as the anode material of LIBs has not been realized. In this paper, the SnO2 is grown directly on the flexible CC by magnetron sputtering method as the anode material. The electrochemical performance of the sample was obviously improved. 2. Experimental
2.1. Preparation of SnO2/CC The composite SnO2/CC was synthesized by RF magnetron sputtering. Firstly, The CC (WOS 1002, CeTech, the thinkness is 360 µm, Fig. S1 shows the external photo and the low magnification SEM image of pure CC in the supplementary material) was cut into a circle with a diameter of 12 mm, cleaned with with acetone, deionized water and absolute ethyl alcohol for about 30 min using supersonic wave apparatus, respectively. Then the CC was transferred into bake oven at 80 ℃ for 12 h. After that, the CC was introduced into the magnetron sputter apparatus. Subsequently, the sputtering system was evacuated to a pressure of 5.0 × 10-4 Pa, then substrate was heated to 250 ℃ and the Ar/O2 gas (both 99.999% in purity) mixture was adjusted at a flux ratio of 32 : 8. The pre-sputtering was carried out for 20 minutes to ensure a clean SnO2 target surface. Next, the working pressure was adjusted to 0.5 Pa, sputtering power was setted to 100 W. After the CC was sputtered for two hours, the composite SnO2/CC was prepared successfully. 2.2. Characterization The prepared samples were characterized by using X-ray powder diffraction (XRD, SHIMADZU 6100) and scanning electron microscope (SEM, Phenom Prox). The energy dispersive spectrometer (EDS) and elemental mapping studies were performed under the SEM. 2.3. Electrochemical measurements The electrochemical measurements were performed at 25 ℃ using coin cells (CR2025) with pure lithium foil as the counter and reference electrode. The prepared SnO2/CC composite were used directly as the working electrode. Cells were constructed in a glove box in argon atmosphere under a dew point below -65 ℃. The cells were performed by galvanostatic charge/discharge tests. The charge/discharge
tests were carried out on a LAND battery program-control testsystem (CT2001A, Wuhan Jinnuo Electronic Co. Ltd. of China) at a constant current of 200 µA in a cut-off potential window of 0 – 3.0 V. Cyclic voltammetry (CV) measurements were carried out on a CHI660D electrochemical workstation at a scan rate of 0.2 mV s-1 in a potential range of 0 – 3.0 V. 3. Results and discussion 3.1. Microstructure and morphology Fig. 1a demonstrates the structural characteristics of the CC sample before magnetron sputtering investigated by XRD. The sharp peak at around 26° and the weak peak at around 44° are agreed with the carbon material (JCPDS file No. 65-6212) [11]. The XRD patterns of SnO2/CC composite is shown in Fig. 1b, the major diffraction peaks are in accordance with the rutile SnO2 (JCPDS file No. 41-1445), which suggests that high-purity SnO2 products have been synthesized. The typical SEM images of pure CC and the SnO2/CC products are shown in Fig. 2a and 2b, respectively. As shown, every single carbon fiber is closely wrapped by nanometer-sized SnO2. The inset in Fig. 2b shows that there are some fault sections on the surface of CC, which verify that the SnO2 was grown on the CC successfully. Fig. 2c – f show the EDS mapping of the SnO2/CC for the detected elements C, O and Sn, respectively. The C, O and Sn elements are uniformly distributed across the whole SnO2/CC product. In order to further analyze the stoichiometric proportion of the comprised elements in the as-prepared product, the EDS spectrum of SnO2/CC has been carried out, and the results are shown in Fig. S2 in the supplementary material. The values of the atomic ratio among C, O and Sn are 45.9 : 35.5 : 18.6, which means the stoichiometric phase of SnO2 has been obtained.
3.2. Electrochemical performance In order to clarify the electrochemical reactions of the SnO2/CC electrode, CV test in a potential range of 0–3.0 V versus Li/Li+ was implemented. And the result is shown in Fig. 3a. In the first cathodic scan, there is one weak peak at 0.95 V, which can be ascribed to the formation of the solid electrolyte interphase layer on the active materials, and the reduction of SnO2 to Sn and the synchronous formation of Li 2O as described in Eq. (1). The strong peak at about 0.25 V should be related to the formation of LixSn given by Eq. (2), which is attested to be highly reversible during the charge and discharge process [12]. In the first anodic scan, the peaks at 0.65 and 1.85 V can be attributed to Li dealloying from LixSn [13] and partly reversible reaction [14] of the Eq. (1), respectively. Meanwhile, it quite obvious that the CV curves of the second and the third cycles were nearly overlapped, implying that the reactions in equations. (1)-(2) are highly reversible. The major reactions can be summarized as follows: SnO 2 + 4Li + + 4e − → Sn + 2Li 2 O
(1)
Sn + xLi + xe − ↔ Li xSn (0 ≤ x ≤ 4.4)
(2)
The cycling performance of SnO2/CC, pure CC and the SnO2 electrodes were investigated by galvanostatic charge and discharge between 0 and 3.0 V at the constant current of 200 µA, and the results are shown in Fig. 3b. The SnO2/CC electrode displays a high reversible capacity and good cyclic retention. The reversible capacity always maintain at about 1.98 mAh/cm2 during 50 cycles, which is higher than the pure CC (about 1.61 mAh/cm2) and SnO2 (about 0.08 mAh/cm2). After 100 cycles, the capacity of SnO2/CC sample was 1.85 mAh/cm2, still retaining 89.4% of the initial capacity. It is obvious that the SnO2/CC sample demonstrates much better reversible capacity and cycle stability than the pure SnO2. The SnO2/CC sample also shows significantly enhanced rate performance than pure CC and
SnO2. The comparison of the rate results of the three samples was shown in Fig. 3c. As can be seen, after the electrodes cycled at 200 µA for 10 cycles, the applied current densities were increased stepwise to 3200 µA. The average reversible capacities of the SnO2/CC electrode are 1.83, 1.53, 1.22, and 0.86 mAh/cm2 at the constant current of 400, 800, 1600, and 3200 µA. However, the average reversible capacities of the pure CC electrode are 1.65, 1.36, 1.07, 0.78, and the average reversible capacities of the pure SnO2 electrode are 0.59, 0.41, 0.21, 0.11 mAh/cm2 under the same test program respectively. Most notably, it can be seen from Table S1 [15-20] in the supplementary material that the capacity of SnO2/CC composite is much better than most of the previously reported SnO2-based anode materials for LIBs. The improvement of lithium storage properties is mainly caused by synergistic effect of SnO2 and flexible CC. Firstly, compared with the traditional methods (hydrothermal method, molten salt method, etc.), the SnO2 prepared by magnetron sputtering method has better homogeneity and higher purity. Additionally, the SnO2 is grown directly on the flexible CC, the energy density of the anode material has been greatly improved due to avoiding utilizing conductive additive and binder. Secondly, CC with a three-dimensional structure has a good electronic transmission efficiency, which can greatly improve the rate performance of the battery. Thirdly, the high surface area, porous structure and the intrinsic soft characteristics of CC can effectively accommodate the volume change during charge and discharge process [21], which can enhance the cycle performance of the battery.
4. Conclusions The SnO2/CC composite was successfully fabricated by magnetron sputtering method. From the phase and morphology characterization results, it can be seen clearly that the sample has a high purity and uniformity. When used as anode materials for lithium-ion batteries, the SnO2 /CC electrode exhibits better
electrochemical performance than the pure CC and SnO2, which is mainly because the high surface area, porous structure and the intrinsic soft characteristics of CC can effectively accommodate the volume change during charge and discharge process. The results of this work proposed a method for preparing flexible electrode material and enhancing the electric performance of SnO2 as anode materials in lithium-ion batteries.
Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 61306009 and 61405159), the Key Project of Natural Science Foundation of Shaanxi Province (Grants 2014JZ2-003), and the Natural Science Foundation of Shaanxi Province (Grants 2014JM8339 and 2015JM6274).
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Figure captions: Fig. 1. XRD patterns of pure CC sample (a) and SnO2/CC composite (b) Fig. 2. SEM images of pure CC sample (a) and SnO2/CC composite (b). The element mapping of C, O and Sn of SnO2/CC (c – f) Fig. 3. The initial three CV curves of SnO2/CC electrode (a) at a scan rate of 0.2 mV/s. The cycling performance (b) and rate performance (c) of SnO2/CC, pure CC and SnO2
Fig. 1
Fig. 2
Fig. 3
The flexible electrode material of SnO2/carbon cloth(SnO2/CC) is successfully fabricated by magnetron sputtering method, the SnO2/CC electrode exhibits more excellent electrochemical performance than the performance of the pure carbon cloth or SnO2.
Graphical abstract
Highlights
The composite SnO2/CC was synthesized by magnetron sputtering method. Electrode material is flexible. Enhanced electrochemical performance are achieved based on synergistic effect of SnO2 and flexible CC.