- Email: [email protected]
poly(vinylidene fluoride)
Electrochimica Acta 66 (2012) 204–209
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical behaviors of porous SnO2 –Sn/C composites derived from pyrolysis of SnO2 /poly(vinylidene fluoride) Xiaolei Sun, Xinghui Wang, Li Qiao, Duokai Hu, Na Feng, Xiuwan Li, Yingqi Liu, Deyan He ∗ School of Physical Science and Technology, and Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 23 December 2011 Received in revised form 17 January 2012 Accepted 21 January 2012 Available online 1 February 2012 Keywords: Pyrolysis Tin oxide Porous carbon Anode Lithium ion batteries
a b s t r a c t Porous SnO2 –Sn/C composites have been synthesized via directly pyrolysis of mixtures of SnO2 powder and poly(vinylidene fluoride) without any surfactant addition and other treatment. The composites are composed of SnO2 and Sn nanoparticles which are well encapsulated in porous carbon matrix as characterized by transmission electron microscopy, X-ray powder diffraction and micro-Raman spectrometer. The obtained materials were used as anode for lithium ion batteries, a discharge capacity of 1171 mAh g−1 and a charge capacity of 611 mAh g−1 were shown in the first cycle at a current density of 100 mA g−1 , and good cycling performance achieved even at current density as high as 800 mA g−1 . The good electrochemical behaviors could be attributed to the formation of Sn nanoparticles which can increase the reversible capacity and the porous carbon matrix which is of excellent buffering effect and high electronic conductivity. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction As one of energy storage technologies, lithium ion batteries have attracted considerable attention during the past decades [1–4]. Much effort has been devoted to improving the electrochemical properties of lithium ion batteries such as longer cycle life, higher capacity and rate capability [5–10]. One of the main tasks to advance lithium ion batteries is to extend rechargeable capacities of the electrode materials [11–13]. Tin oxide has been intensively studied as an anode material for lithium ion batteries due to its large theoretical reversible capacity of 781 mAh g−1 which is more than twice that of graphitic carbon (372 mAh g−1 ) [14,15]. However, the SnO2 anode materials usually show a rapid loss of the reversible capacity upon cycling due to their large volume expansion-contraction during the charge–discharge process [16,17]. So far, much research has mostly focused on special nanostructures and/or nanocomposites such as nanowires [14,18], nanosheets [17], nanoporous [19], SnO2 –C [20], SnO2 –RuO2 [21], SnO2 –SnS2 [22], SnO2 –CoO [23], Li2 O–CuO–SnO2 –SiO2 [24] for overcoming this issue. In particular, mixing the nanostructures with Sn has been demonstrated to be a relatively effective method to increase reversible capacity [25]. On the other hand, some studies have indicated that carbonaceous matrix can significantly enhance electronic conductivity of electrode materials and maintain their high performances [15,26,27]. Thus, it is very instructive and of
∗ Corresponding author. Tel.: +86 931 8912546; fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.083
great challenge for us to investigate the SnO2 –Sn/C composites as anode material for lithium ion batteries. In this paper, porous SnO2 –Sn/C composites were obtained via directly pyrolyzing SnO2 /poly(vinylidene fluoride) mixtures without any surfactant addition and other treatment. The electrochemical behaviors of the obtained composites as anode material of lithium-ion batteries were evaluated by using conventional galvanostatic and cyclic voltammetry techniques. High reversible capacity and good cycling performance were achieved even at a large current density as high as 800 mA g−1 . 2. Experimental SnO2 powder (50–70 nm, Aladdin, China) and poly(vinylidene fluoride) (PVDF) were used as starting materials in the following procedures. Firstly, SnO2 and PVDF mixtures were prepared by milling 1.4 g SnO2 and 3.6 g PVDF in a 100 mL agate vial at 400 rpm for 2 h. Then, the obtained materials were carbonized at 600 ◦ C in an argon-flowing tube furnace for 3 h. The products were cooled naturally and then thoroughly ground in an agate mortar. The schematic diagram of the apparatus used in our experiment is shown in Fig. 1. The samples were characterized by X-ray powder diffrac˚ tion (XRD, Rigaku RINT2400 with Cu K␣ radiation = 1.5418 A), micro-Raman spectrometer (Jobin-Yvon LabRAM HR800 UV, YGA 532 nm), field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, FEI, Tecnai G2 F30), respectively. The recorded XRD patterns were analyzed by Rietveld refinement method [28] using the software TOPAS 2.1 [29].
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
205
Fig. 1. Schematic diagram of the apparatus used in experiments.
Nitrogen adsorption–desorption isotherm measurements were performed on a micromeritics ASAP 2020 volumetric adsorption analyzer at 77 K. Thermogravimetric analysis (TGA) was carried out in air at a heating rate of 5 ◦ C min−1 using a Perkin Elmer Diamond TG/DTA instrument. Electrochemical experiments were performed with coin cell (CR2032, half cell) using Li foil as counter electrode. The working electrode was prepared by mixing the active material, acetylene black, and PVDF binder at a weight ratio of 85:5:10 in N-methyl2-pyrrolidinone (NMP) solvent. The resultant slurry was then uniformly pasted onto copper foil and dried in a vacuum oven at 120 ◦ C for 24 h. The coating was then pressed under a pressure of approximately 30 kg cm−2 . The cells were assembled in an argonfilled glove box (H2 O < 0.5 ppm, O2 < 0.5 ppm, MBraun, Germany). The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (in a volume ratio of 1:1:1), and a Celgard 2320 microporous polypropylene film was used as separator [30]. The cells were galvanostatically discharged and charged in the voltage range of 0.01–2.0 vs. (Li/Li+ )/V using a battery tester (NEWARE). Cyclic voltammetry (CV) measurements were performed using an electrochemical workstation (CHI 660C) at a scan rate of 0.1 mV s−1 between 0.01 and 2.0 V. 3. Results and discussion Fig. 2a shows Raman spectrum of the typical SnO2 –Sn/C composite synthesized by directly pyrolysis of the SnO2 and PVDF mixture. Characteristic carbon peaks are observed at 1348 and 1597 cm−1 , the former is assigned to disordered carbon and the latter represents the graphitic carbon. The peak at 1597 cm−1 corresponds to an E2g mode of graphite, which is due to the sp2 bonded carbon atoms in a two-dimensional hexagonal graphitic layer [31–33]. The ID /IG ratio for the composites is 0.92, indicating that the PVDF-derived carbon in the SnO2 –Sn/C composites is amorphous carbon [34,35]. As shown in Fig. 2b, Rietveld refinement XRD pattern of the SnO2 –Sn/C composites shows the characteristic lines of SnO2 (JCPDS card no. 41-1445) and Sn (JCPDS card no. 04-0673), respectively. The mole ratio of the two crystalline phases can be estimated to be 85.15:14.85 by a computing analysis (TOPAS 2.1). While the amorphous carbon in the composite might be of insufficient intensity to be detected against the background in the XRD pattern, it was observed by using TEM and TGA measurements. Fig. 3a shows SEM image of the beginning SnO2 powder. It can be seen that the sizes of the particles are around 50–70 nm
Fig. 2. (a) Raman spectrum and (b) Rietveld refinement XRD pattern of the SnO2 –Sn/C composite.
and their surfaces are smooth. Fig. 3b reveals that the pyrolytic SnO2 –Sn/C composite has a porous structure. Such a porous structure is believed to relate to the highly porous PVDF-derived carbon [36]. As shown in Fig. 3c, the SnO2 /Sn nanoparticles (as indicated by ring) are well encapsulated in porous carbon matrix. Fig. 3d shows the high resolution transmission electron microscopy (HRTEM) image of the composite. It is observed that the nanoparticles are well crystallized, and the matrix is a mixture of disordered carbon and graphite which usually shows high electrical conductivity [35]. The lattice fringe with interplanar spacing of about 0.339 nm corresponds to the d-spacing between adjacent (1 1 0) planes in the tetragonal SnO2 structure. The EDS spectrum of the composite is shown in Fig. 3e, the strong peaks are expected for C, O, and Sn elements. Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption analysis was used to investigate the specific surface area and pores size distribution in the SnO2 –Sn/C composite. Fig. 4 shows the typical nitrogen adsorption–desorption isotherm curves of the SnO2 –Sn/C composite, which can be classified as type IV isotherms. It is found that based on the nitrogen adsorption–desorption isotherm measurement, the overall SnO2 –Sn/C composites exhibit a high BET surface area of 442.0 m2 g−1 and a pore volume of 0.148 cm3 g−1 . To further confirm the pore structure of the composite, pore size distribution curves had been plotted by Barrett–Joyner–Halenda (BJH) method from both adsorption and desorption branches. As presented in the inset of Fig. 4, although the peak at around 4.0 nm from the desorption branches is likely an artifact corresponding to capillary
206
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
Fig. 3. SEM images of (a) SnO2 powder, (b) SnO2 –Sn/C composite. (c) A typical TEM image of the SnO2 –Sn/C composite at low magnification. (d) A high resolution TEM image of the SnO2 –Sn/C composite. (e) The EDS pattern of the SnO2 –Sn/C composite.
evaporation at the lower end of the hysteresis loop with a relative pressure of about 0.4–0.5 [37], it can be concluded that the pores are generally smaller than 5 nm from the pore size distribution cures. For determining the contents of Sn, SnO2 , and carbon elements in the SnO2 –Sn/C composites, TGA was carried out in air from 50 to 900 ◦ C with a heating rate of 5 ◦ C min−1 . The recorded TGA curve is shown in Fig. 5. The relatively initial weight increase over the temperature range of 200–390 ◦ C is attributed to the oxidation of tin in the composite [26]. The weight loses rapidly from 400 to 600 ◦ C. Since SnO2 remains stable during the heating process, the weight loss of the composite should correspond to the oxidation of PVDF-derived carbon. Combining the overall weight of final product with the initial mole ratio of SnO2 to Sn from the Rietveld refinement of XRD pattern (Fig. 2b), we can get the weight ratio of Sn:SnO2 :C is about 6.34:46.14:47.52 in SnO2 –Sn/C composite. The reaction mechanism of SnO2 with active carbon is suggested to
be vapor–liquid–solid [38,39]. Based on the experimental results described above, the simple reactions in the present experiment could be the following processes:
PVDF−→C + others
SnO2 + C−→SnO + CO
2SnO−→SnO2 + Sn Electrochemical behavior of the SnO2 –Sn/C composite electrodes was investigated by cyclic voltammetry (CV). Fig. 6a shows the first three CV curves of SnO2 –Sn/C composite electrode in the potential range of 0.01–2.0 V at a slow scan rate of 0.1 mV s−1 . It is
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
207
Fig. 4. Nitrogen adsorption and desorption isotherm curves of the SnO2 –Sn/C composite. The inset is the corresponding pore size distribution calculated by BJH method.
generally accepted that the electrochemical process of SnO2 and/or Sn anodes can be divided into a two-step process [15]: SnO2 + 4Li+ + 4e− → Sn + 2Li2 O
(1)
Sn + xLi+ + xe− ↔ Lix Sn (0 ≤ x ≤ 4.4).
(2)
As shown in Fig. 6a, the cathodic peaks at around 0.57 and 0.68 V in the first cycle could be attributed to the formation of a solid electrolyte interface (SEI) layer, as well as the initial reduction of SnO2 to Sn and the synchronous formation of Li2 O as given by Reaction (1). The cathodic peaks below 0.3 V could be related to the alloying process as described in Reaction (2), the fine in these peaks may be due to the multi-stage lithium ion intercalation of Lix Sn alloy. The anodic peaks from 0.4 to 0.9 V are attributed to the de-alloying process described in Reaction (2). An additional reversible anodic peak is observed at 1.25 V, which demonstrates that Reaction (1) seems to occur reversibly to some extent [20,26]. By comparing with those CV curves of SnO2 or SnO2 /C electrodes reported previously [14,40–42], it is found that the broad background as shown in Fig. 6a might be due to the reaction of PVDF-derived carbon with lithium.
Fig. 5. TGA curve of the SnO2 –Sn/C composite.
Fig. 6. (a) The first three CV curves of the SnO2 –Sn/C composite in a potential range of 0.01–2.0 V at a scan rate of 0.1 mV s−1 . (b) The initial three discharge/charge curves of the SnO2 –Sn/C composite electrode in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at a current density of 100 mA g−1 .
The electrochemical performances of the sample were further evaluated by galvanostatic discharge–charge measurements. Fig. 6b shows the initial three discharge/charge curves of SnO2 –Sn/C composite electrode cycled in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at a current density of 100 mA g−1 . A discharge capacity of 1171 mAh g−1 and a charge capacity of 611 mAh g−1 were achieved in the first cycle, respectively. The irreversible capacity in the first cycle can be calculated to be 560 mAh g−1 and the initial coulombic efficiency is 52.20%. The large capacity loss in the first cycle is mainly attributed to the irreversible reaction as described in Reaction (1) and irreversible trapping of lithium by carbonaceous matrix that might relate to the surface ([H], [O], etc.) reaction and inevitable formation of SEI layer [26,43]. Cycling performance and coulombic efficiency of the SnO2 –Sn/C composite electrode are shown in Fig. 7a. It is observed that the coulombic efficiency is above 95% after 3 cycles at a current density of 100 mA g−1 , a reversible capacity of ∼400 mAh g−1 is remained after 20 cycles with a loss of about 1.15% per cycle. The result is an indicative that the electrode is of an excellent electrochemical reversibility. The SnO2 –Sn/C composite electrode was also tested in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at different current densities of
208
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
surfactant addition and other treatment. The porous SnO2 –Sn/C composites have good electrochemical performances even at high current densities, which could be attributed to the formation of Sn nanoparticles in the porous carbon matrix. The Sn nanoparticles can increase reversible capacity and the porous carbon matrix has excellent buffering effect and high electronic conductivity. The fabrication of porous SnO2 –Sn/C composites according to the pyrolysis of SnO2 and poly(vinylidene fluoride) provides us a feasible way to improve the electrochemical performance of SnO2 –Sn, which should be an elicitation to the researchers in this field. Acknowledgments The authors thank Dr. Yonggang Wang, the College of Chemistry and Molecular Engineering, Peking University for computing analysis on XRD data. The project was financially supported by the National Natural Science Foundation of China with grant (No. 10974073 and No. 11179038). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Fig. 7. (a) Cycling performance and coulombic efficiency of the SnO2 –Sn/C composite electrode in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at a current density of 100 mA g−1 . (b) Cycle performances of the SnO2 –Sn/C composite electrode at different current densities.
800 mA g−1 .
200, 400, and As shown in Fig. 7b, the second-cycle reversible capacities are 607, 524, and 476 mAh g−1 , respectively. Therefore, when the current density increases from 200 to 800 mA g−1 , SnO2 –Sn/C composite electrode retains 78.42% and show relatively exceptional capacity retention upon cycling after the initial capacity loss. To abate the rapid loss of the reversible capacity upon cycling for the tin-based material electrodes, much work has been done on the synthesis of tin-based carbonaceous composites with various methods such as electrospinning [44] hydrothermal method [37,45,46]. However, it is unfortunate that a low first cycle irreversible capacity is obtained for most of the researches due to the high contents of carbon and generated Li2 O in the composites. In the present experiment, the SnO2 –Sn/C composite were obtained by pyrolyzing the SnO2 /PVDF mixtures. Since the Sn:Li2 O ratio is high, the material is beneficial to increase reversible capacity. Hence, the good electrochemical performances of the SnO2 –Sn/C composites could be attributed to the formation of Sn nanoparticles and the porous carbon matrix which has excellent buffering effect and high electronic conductivity. Further work should be done on the dependence of the electrochemical behaviors on the SnO2 :Sn:C ratio with the purpose of the optimum performance. 4. Conclusions In conclusion, porous SnO2 –Sn/C composites have been synthesized by pyrolyzing the SnO2 /PVDF mixtures in argon without any
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
S. Megahed, B. Scrosati, J. Power Sources 51 (1994) 79. C.H. Jiang, E. Hosono, H.S. Zhou, Nano Today 1 (2006) 28. J.M. Tarascon, M. Armand, Nature 414 (2001) 359. K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Science 312 (2006) 885. C.K. Chan, H.L. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31. X. Wang, Z. Yang, X. Sun, X. Li, D. Wang, P. Wang, D. He, J. Mater. Chem. 21 (2011) 9988. X. Wang, X. Li, X. Sun, F. Li, Q. Liu, Q. Wang, D. He, J. Mater. Chem. 21 (2011) 3571. J. Hassoun, S. Panero, P. Simon, P.L. Taberna, B. Scrosati, Adv. Mater. 19 (2007) 1632. V.G. Khomenko, V.Z. Barsukov, Electrochim. Acta 52 (2007) 2829. M. Holzapfel, H. Buqa, L.J. Hardwick, M. Hahn, A. Würsig, W. Scheifele, P. Novák, R. Kötz, C. Veit, F.-M. Petrat, Electrochim. Acta 52 (2006) 973. A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W. van Schalkwijk, Nat. Mater. 4 (2005) 366. J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2010) 587. B. Qu, M. Zhang, D. Lei, Y. Zeng, Y. Chen, L. Chen, Q. Li, Y. Wang, T. Wang, Nanoscale 3 (2011) 3646. M.S. Park, G.X. Wang, Y.M. Kang, D. Wexler, S.X. Dou, H.K. Liu, Angew. Chem. Int. Edit. 46 (2007) 750. N.C. Li, C.R. Martin, J. Electrochem. Soc. 148 (2001) A164. X.W. Lou, Y. Wang, C. Yuan, J.Y. Lee, L.A. Archer, Adv. Mater. 18 (2006) 2325. C. Wang, Y. Zhou, M.Y. Ge, X.B. Xu, Z.L. Zhang, J.Z. Jiang, J. Am. Chem. Soc. 132 (2010) 46. Y.-D. Ko, J.-G. Kang, J.-G. Park, S. Lee, D.-W. Kim, Nanotechnology 20 (2009) 455701. X. Sun, X. Wang, Y. Qin, X. Li, L. Qiao, N. Feng, D. Hu, D. He, Mater. Lett. 66 (2012) 193. L. Noerochim, J.-Z. Wang, S.-L. Chou, H.-J. Li, H.-K. Liu, Electrochim. Acta 56 (2010) 314. Y.S. Yoon, S.H. Choi, J.S. Kim, Electrochim. Acta 50 (2004) 547. K. Chang, W.X. Chen, H. Li, H. Li, Electrochim. Acta 56 (2011) 2856. X.J. Zhu, Z.P. Guo, P. Zhang, G.D. Du, R. Zeng, Z.X. Chen, S. Li, H.K. Liu, J. Mater. Chem. 19 (2009) 8360. A.Y. Shenouda, H.K. Liu, J. Electrochem. Soc. 157 (2010) A1183. A. Sivashanmugam, T.P. Kumar, N.G. Renganathan, S. Gopukumar, M. Wohlfahrt-Mehrens, J. Garche, J. Power Sources 144 (2005) 197. X.W. Lou, J.S. Chen, P. Chen, L.A. Archer, Chem. Mater. 21 (2009) 2868. W.M. Zhang, J.S. Hu, Y.G. Guo, S.F. Zheng, L.S. Zhong, W.G. Song, L.J. Wan, Adv. Mater. 20 (2008) 1160. H. Rietveld, Acta Cryst. 22 (1967) 151. K. TOPAS V2.1. General profile and structure analysis software for powder diffraction data; Bruker AXS, Germany, 2002. S. Ni, X. Wang, G. Zhou, F. Yang, J. Wang, Q. Wang, D. He, Mater. Lett. 63 (2009) 2701. A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. Q. Wang, C.B. Wang, Z. Wang, J.Y. Zhang, D.Y. He, Appl. Phys. Lett. 91 (2007) 141902. J.-Y. Zhou, Z.-Y. Chen, M. Zhou, X.-P. Gao, E.-Q. Xie, Nanoscale Res. Lett. 4 (2009) 814. H. Kim, J. Cho, J. Electrochem. Soc. 154 (2007) A462. L. Su, Z. Zhou, M. Ren, Chem. Commun. 46 (2010) 2590. B. Xu, S. Hou, M. Chu, G. Cao, Y. Yang, Carbon 48 (2010) 2812. X.W. Lou, D. Deng, J.Y. Lee, L.A. Archer, Chem. Mater. 20 (2008) 6562. J.Q. Hu, X.L. Ma, N.G. Shang, Z.Y. Xie, N.B. Wong, C.S. Lee, S.T. Lee, J. Phys. Chem. B 106 (2002) 3823.
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209 [39] B. Wang, Y.H. Yang, C.X. Wang, G.W. Yang, Chem. Phys. Lett. 407 (2005) 347. [40] R. Yang, W. Zhao, J. Zheng, X.Z. Zhang, X.G. Li, J. Phys. Chem. C 114 (2010) 20272. [41] X.M. Yin, C.C. Li, M. Zhang, Q.Y. Hao, S. Liu, L.B. Chen, T.H. Wang, J. Phys. Chem. C 114 (2010) 8084. [42] Z. Yang, G. Du, C. Feng, S. Li, Z. Chen, P. Zhang, Z. Guo, X. Yu, G. Chen, S. Huang, H. Liu, Electrochim. Acta 55 (2010) 5485.
[43] [44] [45] [46]
209
M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv. Mater. 10 (1998) 725. L. Ji, Z. Lin, B. Guo, A.J. Medford, X. Zhang, Chem. Eur. J. 16 (2010) 11543. X. Sun, J. Liu, Y. Li, Chem. Mater. 18 (2006) 3486. S. Ding, D. Luan, F.Y.C. Boey, J.S. Chen, X.W. Lou, Chem. Commun. 47 (2011) 7155.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical behaviors of porous SnO2 –Sn/C composites derived from pyrolysis of SnO2 /poly(vinylidene fluoride) Xiaolei Sun, Xinghui Wang, Li Qiao, Duokai Hu, Na Feng, Xiuwan Li, Yingqi Liu, Deyan He ∗ School of Physical Science and Technology, and Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 23 December 2011 Received in revised form 17 January 2012 Accepted 21 January 2012 Available online 1 February 2012 Keywords: Pyrolysis Tin oxide Porous carbon Anode Lithium ion batteries
a b s t r a c t Porous SnO2 –Sn/C composites have been synthesized via directly pyrolysis of mixtures of SnO2 powder and poly(vinylidene fluoride) without any surfactant addition and other treatment. The composites are composed of SnO2 and Sn nanoparticles which are well encapsulated in porous carbon matrix as characterized by transmission electron microscopy, X-ray powder diffraction and micro-Raman spectrometer. The obtained materials were used as anode for lithium ion batteries, a discharge capacity of 1171 mAh g−1 and a charge capacity of 611 mAh g−1 were shown in the first cycle at a current density of 100 mA g−1 , and good cycling performance achieved even at current density as high as 800 mA g−1 . The good electrochemical behaviors could be attributed to the formation of Sn nanoparticles which can increase the reversible capacity and the porous carbon matrix which is of excellent buffering effect and high electronic conductivity. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction As one of energy storage technologies, lithium ion batteries have attracted considerable attention during the past decades [1–4]. Much effort has been devoted to improving the electrochemical properties of lithium ion batteries such as longer cycle life, higher capacity and rate capability [5–10]. One of the main tasks to advance lithium ion batteries is to extend rechargeable capacities of the electrode materials [11–13]. Tin oxide has been intensively studied as an anode material for lithium ion batteries due to its large theoretical reversible capacity of 781 mAh g−1 which is more than twice that of graphitic carbon (372 mAh g−1 ) [14,15]. However, the SnO2 anode materials usually show a rapid loss of the reversible capacity upon cycling due to their large volume expansion-contraction during the charge–discharge process [16,17]. So far, much research has mostly focused on special nanostructures and/or nanocomposites such as nanowires [14,18], nanosheets [17], nanoporous [19], SnO2 –C [20], SnO2 –RuO2 [21], SnO2 –SnS2 [22], SnO2 –CoO [23], Li2 O–CuO–SnO2 –SiO2 [24] for overcoming this issue. In particular, mixing the nanostructures with Sn has been demonstrated to be a relatively effective method to increase reversible capacity [25]. On the other hand, some studies have indicated that carbonaceous matrix can significantly enhance electronic conductivity of electrode materials and maintain their high performances [15,26,27]. Thus, it is very instructive and of
∗ Corresponding author. Tel.: +86 931 8912546; fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.083
great challenge for us to investigate the SnO2 –Sn/C composites as anode material for lithium ion batteries. In this paper, porous SnO2 –Sn/C composites were obtained via directly pyrolyzing SnO2 /poly(vinylidene fluoride) mixtures without any surfactant addition and other treatment. The electrochemical behaviors of the obtained composites as anode material of lithium-ion batteries were evaluated by using conventional galvanostatic and cyclic voltammetry techniques. High reversible capacity and good cycling performance were achieved even at a large current density as high as 800 mA g−1 . 2. Experimental SnO2 powder (50–70 nm, Aladdin, China) and poly(vinylidene fluoride) (PVDF) were used as starting materials in the following procedures. Firstly, SnO2 and PVDF mixtures were prepared by milling 1.4 g SnO2 and 3.6 g PVDF in a 100 mL agate vial at 400 rpm for 2 h. Then, the obtained materials were carbonized at 600 ◦ C in an argon-flowing tube furnace for 3 h. The products were cooled naturally and then thoroughly ground in an agate mortar. The schematic diagram of the apparatus used in our experiment is shown in Fig. 1. The samples were characterized by X-ray powder diffrac˚ tion (XRD, Rigaku RINT2400 with Cu K␣ radiation = 1.5418 A), micro-Raman spectrometer (Jobin-Yvon LabRAM HR800 UV, YGA 532 nm), field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, FEI, Tecnai G2 F30), respectively. The recorded XRD patterns were analyzed by Rietveld refinement method [28] using the software TOPAS 2.1 [29].
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
205
Fig. 1. Schematic diagram of the apparatus used in experiments.
Nitrogen adsorption–desorption isotherm measurements were performed on a micromeritics ASAP 2020 volumetric adsorption analyzer at 77 K. Thermogravimetric analysis (TGA) was carried out in air at a heating rate of 5 ◦ C min−1 using a Perkin Elmer Diamond TG/DTA instrument. Electrochemical experiments were performed with coin cell (CR2032, half cell) using Li foil as counter electrode. The working electrode was prepared by mixing the active material, acetylene black, and PVDF binder at a weight ratio of 85:5:10 in N-methyl2-pyrrolidinone (NMP) solvent. The resultant slurry was then uniformly pasted onto copper foil and dried in a vacuum oven at 120 ◦ C for 24 h. The coating was then pressed under a pressure of approximately 30 kg cm−2 . The cells were assembled in an argonfilled glove box (H2 O < 0.5 ppm, O2 < 0.5 ppm, MBraun, Germany). The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (in a volume ratio of 1:1:1), and a Celgard 2320 microporous polypropylene film was used as separator [30]. The cells were galvanostatically discharged and charged in the voltage range of 0.01–2.0 vs. (Li/Li+ )/V using a battery tester (NEWARE). Cyclic voltammetry (CV) measurements were performed using an electrochemical workstation (CHI 660C) at a scan rate of 0.1 mV s−1 between 0.01 and 2.0 V. 3. Results and discussion Fig. 2a shows Raman spectrum of the typical SnO2 –Sn/C composite synthesized by directly pyrolysis of the SnO2 and PVDF mixture. Characteristic carbon peaks are observed at 1348 and 1597 cm−1 , the former is assigned to disordered carbon and the latter represents the graphitic carbon. The peak at 1597 cm−1 corresponds to an E2g mode of graphite, which is due to the sp2 bonded carbon atoms in a two-dimensional hexagonal graphitic layer [31–33]. The ID /IG ratio for the composites is 0.92, indicating that the PVDF-derived carbon in the SnO2 –Sn/C composites is amorphous carbon [34,35]. As shown in Fig. 2b, Rietveld refinement XRD pattern of the SnO2 –Sn/C composites shows the characteristic lines of SnO2 (JCPDS card no. 41-1445) and Sn (JCPDS card no. 04-0673), respectively. The mole ratio of the two crystalline phases can be estimated to be 85.15:14.85 by a computing analysis (TOPAS 2.1). While the amorphous carbon in the composite might be of insufficient intensity to be detected against the background in the XRD pattern, it was observed by using TEM and TGA measurements. Fig. 3a shows SEM image of the beginning SnO2 powder. It can be seen that the sizes of the particles are around 50–70 nm
Fig. 2. (a) Raman spectrum and (b) Rietveld refinement XRD pattern of the SnO2 –Sn/C composite.
and their surfaces are smooth. Fig. 3b reveals that the pyrolytic SnO2 –Sn/C composite has a porous structure. Such a porous structure is believed to relate to the highly porous PVDF-derived carbon [36]. As shown in Fig. 3c, the SnO2 /Sn nanoparticles (as indicated by ring) are well encapsulated in porous carbon matrix. Fig. 3d shows the high resolution transmission electron microscopy (HRTEM) image of the composite. It is observed that the nanoparticles are well crystallized, and the matrix is a mixture of disordered carbon and graphite which usually shows high electrical conductivity [35]. The lattice fringe with interplanar spacing of about 0.339 nm corresponds to the d-spacing between adjacent (1 1 0) planes in the tetragonal SnO2 structure. The EDS spectrum of the composite is shown in Fig. 3e, the strong peaks are expected for C, O, and Sn elements. Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption analysis was used to investigate the specific surface area and pores size distribution in the SnO2 –Sn/C composite. Fig. 4 shows the typical nitrogen adsorption–desorption isotherm curves of the SnO2 –Sn/C composite, which can be classified as type IV isotherms. It is found that based on the nitrogen adsorption–desorption isotherm measurement, the overall SnO2 –Sn/C composites exhibit a high BET surface area of 442.0 m2 g−1 and a pore volume of 0.148 cm3 g−1 . To further confirm the pore structure of the composite, pore size distribution curves had been plotted by Barrett–Joyner–Halenda (BJH) method from both adsorption and desorption branches. As presented in the inset of Fig. 4, although the peak at around 4.0 nm from the desorption branches is likely an artifact corresponding to capillary
206
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
Fig. 3. SEM images of (a) SnO2 powder, (b) SnO2 –Sn/C composite. (c) A typical TEM image of the SnO2 –Sn/C composite at low magnification. (d) A high resolution TEM image of the SnO2 –Sn/C composite. (e) The EDS pattern of the SnO2 –Sn/C composite.
evaporation at the lower end of the hysteresis loop with a relative pressure of about 0.4–0.5 [37], it can be concluded that the pores are generally smaller than 5 nm from the pore size distribution cures. For determining the contents of Sn, SnO2 , and carbon elements in the SnO2 –Sn/C composites, TGA was carried out in air from 50 to 900 ◦ C with a heating rate of 5 ◦ C min−1 . The recorded TGA curve is shown in Fig. 5. The relatively initial weight increase over the temperature range of 200–390 ◦ C is attributed to the oxidation of tin in the composite [26]. The weight loses rapidly from 400 to 600 ◦ C. Since SnO2 remains stable during the heating process, the weight loss of the composite should correspond to the oxidation of PVDF-derived carbon. Combining the overall weight of final product with the initial mole ratio of SnO2 to Sn from the Rietveld refinement of XRD pattern (Fig. 2b), we can get the weight ratio of Sn:SnO2 :C is about 6.34:46.14:47.52 in SnO2 –Sn/C composite. The reaction mechanism of SnO2 with active carbon is suggested to
be vapor–liquid–solid [38,39]. Based on the experimental results described above, the simple reactions in the present experiment could be the following processes:
PVDF−→C + others
SnO2 + C−→SnO + CO
2SnO−→SnO2 + Sn Electrochemical behavior of the SnO2 –Sn/C composite electrodes was investigated by cyclic voltammetry (CV). Fig. 6a shows the first three CV curves of SnO2 –Sn/C composite electrode in the potential range of 0.01–2.0 V at a slow scan rate of 0.1 mV s−1 . It is
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
207
Fig. 4. Nitrogen adsorption and desorption isotherm curves of the SnO2 –Sn/C composite. The inset is the corresponding pore size distribution calculated by BJH method.
generally accepted that the electrochemical process of SnO2 and/or Sn anodes can be divided into a two-step process [15]: SnO2 + 4Li+ + 4e− → Sn + 2Li2 O
(1)
Sn + xLi+ + xe− ↔ Lix Sn (0 ≤ x ≤ 4.4).
(2)
As shown in Fig. 6a, the cathodic peaks at around 0.57 and 0.68 V in the first cycle could be attributed to the formation of a solid electrolyte interface (SEI) layer, as well as the initial reduction of SnO2 to Sn and the synchronous formation of Li2 O as given by Reaction (1). The cathodic peaks below 0.3 V could be related to the alloying process as described in Reaction (2), the fine in these peaks may be due to the multi-stage lithium ion intercalation of Lix Sn alloy. The anodic peaks from 0.4 to 0.9 V are attributed to the de-alloying process described in Reaction (2). An additional reversible anodic peak is observed at 1.25 V, which demonstrates that Reaction (1) seems to occur reversibly to some extent [20,26]. By comparing with those CV curves of SnO2 or SnO2 /C electrodes reported previously [14,40–42], it is found that the broad background as shown in Fig. 6a might be due to the reaction of PVDF-derived carbon with lithium.
Fig. 5. TGA curve of the SnO2 –Sn/C composite.
Fig. 6. (a) The first three CV curves of the SnO2 –Sn/C composite in a potential range of 0.01–2.0 V at a scan rate of 0.1 mV s−1 . (b) The initial three discharge/charge curves of the SnO2 –Sn/C composite electrode in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at a current density of 100 mA g−1 .
The electrochemical performances of the sample were further evaluated by galvanostatic discharge–charge measurements. Fig. 6b shows the initial three discharge/charge curves of SnO2 –Sn/C composite electrode cycled in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at a current density of 100 mA g−1 . A discharge capacity of 1171 mAh g−1 and a charge capacity of 611 mAh g−1 were achieved in the first cycle, respectively. The irreversible capacity in the first cycle can be calculated to be 560 mAh g−1 and the initial coulombic efficiency is 52.20%. The large capacity loss in the first cycle is mainly attributed to the irreversible reaction as described in Reaction (1) and irreversible trapping of lithium by carbonaceous matrix that might relate to the surface ([H], [O], etc.) reaction and inevitable formation of SEI layer [26,43]. Cycling performance and coulombic efficiency of the SnO2 –Sn/C composite electrode are shown in Fig. 7a. It is observed that the coulombic efficiency is above 95% after 3 cycles at a current density of 100 mA g−1 , a reversible capacity of ∼400 mAh g−1 is remained after 20 cycles with a loss of about 1.15% per cycle. The result is an indicative that the electrode is of an excellent electrochemical reversibility. The SnO2 –Sn/C composite electrode was also tested in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at different current densities of
208
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209
surfactant addition and other treatment. The porous SnO2 –Sn/C composites have good electrochemical performances even at high current densities, which could be attributed to the formation of Sn nanoparticles in the porous carbon matrix. The Sn nanoparticles can increase reversible capacity and the porous carbon matrix has excellent buffering effect and high electronic conductivity. The fabrication of porous SnO2 –Sn/C composites according to the pyrolysis of SnO2 and poly(vinylidene fluoride) provides us a feasible way to improve the electrochemical performance of SnO2 –Sn, which should be an elicitation to the researchers in this field. Acknowledgments The authors thank Dr. Yonggang Wang, the College of Chemistry and Molecular Engineering, Peking University for computing analysis on XRD data. The project was financially supported by the National Natural Science Foundation of China with grant (No. 10974073 and No. 11179038). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Fig. 7. (a) Cycling performance and coulombic efficiency of the SnO2 –Sn/C composite electrode in a voltage range of 0.01–2.0 vs. (Li/Li+ )/V at a current density of 100 mA g−1 . (b) Cycle performances of the SnO2 –Sn/C composite electrode at different current densities.
800 mA g−1 .
200, 400, and As shown in Fig. 7b, the second-cycle reversible capacities are 607, 524, and 476 mAh g−1 , respectively. Therefore, when the current density increases from 200 to 800 mA g−1 , SnO2 –Sn/C composite electrode retains 78.42% and show relatively exceptional capacity retention upon cycling after the initial capacity loss. To abate the rapid loss of the reversible capacity upon cycling for the tin-based material electrodes, much work has been done on the synthesis of tin-based carbonaceous composites with various methods such as electrospinning [44] hydrothermal method [37,45,46]. However, it is unfortunate that a low first cycle irreversible capacity is obtained for most of the researches due to the high contents of carbon and generated Li2 O in the composites. In the present experiment, the SnO2 –Sn/C composite were obtained by pyrolyzing the SnO2 /PVDF mixtures. Since the Sn:Li2 O ratio is high, the material is beneficial to increase reversible capacity. Hence, the good electrochemical performances of the SnO2 –Sn/C composites could be attributed to the formation of Sn nanoparticles and the porous carbon matrix which has excellent buffering effect and high electronic conductivity. Further work should be done on the dependence of the electrochemical behaviors on the SnO2 :Sn:C ratio with the purpose of the optimum performance. 4. Conclusions In conclusion, porous SnO2 –Sn/C composites have been synthesized by pyrolyzing the SnO2 /PVDF mixtures in argon without any
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
S. Megahed, B. Scrosati, J. Power Sources 51 (1994) 79. C.H. Jiang, E. Hosono, H.S. Zhou, Nano Today 1 (2006) 28. J.M. Tarascon, M. Armand, Nature 414 (2001) 359. K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Science 312 (2006) 885. C.K. Chan, H.L. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31. X. Wang, Z. Yang, X. Sun, X. Li, D. Wang, P. Wang, D. He, J. Mater. Chem. 21 (2011) 9988. X. Wang, X. Li, X. Sun, F. Li, Q. Liu, Q. Wang, D. He, J. Mater. Chem. 21 (2011) 3571. J. Hassoun, S. Panero, P. Simon, P.L. Taberna, B. Scrosati, Adv. Mater. 19 (2007) 1632. V.G. Khomenko, V.Z. Barsukov, Electrochim. Acta 52 (2007) 2829. M. Holzapfel, H. Buqa, L.J. Hardwick, M. Hahn, A. Würsig, W. Scheifele, P. Novák, R. Kötz, C. Veit, F.-M. Petrat, Electrochim. Acta 52 (2006) 973. A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W. van Schalkwijk, Nat. Mater. 4 (2005) 366. J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2010) 587. B. Qu, M. Zhang, D. Lei, Y. Zeng, Y. Chen, L. Chen, Q. Li, Y. Wang, T. Wang, Nanoscale 3 (2011) 3646. M.S. Park, G.X. Wang, Y.M. Kang, D. Wexler, S.X. Dou, H.K. Liu, Angew. Chem. Int. Edit. 46 (2007) 750. N.C. Li, C.R. Martin, J. Electrochem. Soc. 148 (2001) A164. X.W. Lou, Y. Wang, C. Yuan, J.Y. Lee, L.A. Archer, Adv. Mater. 18 (2006) 2325. C. Wang, Y. Zhou, M.Y. Ge, X.B. Xu, Z.L. Zhang, J.Z. Jiang, J. Am. Chem. Soc. 132 (2010) 46. Y.-D. Ko, J.-G. Kang, J.-G. Park, S. Lee, D.-W. Kim, Nanotechnology 20 (2009) 455701. X. Sun, X. Wang, Y. Qin, X. Li, L. Qiao, N. Feng, D. Hu, D. He, Mater. Lett. 66 (2012) 193. L. Noerochim, J.-Z. Wang, S.-L. Chou, H.-J. Li, H.-K. Liu, Electrochim. Acta 56 (2010) 314. Y.S. Yoon, S.H. Choi, J.S. Kim, Electrochim. Acta 50 (2004) 547. K. Chang, W.X. Chen, H. Li, H. Li, Electrochim. Acta 56 (2011) 2856. X.J. Zhu, Z.P. Guo, P. Zhang, G.D. Du, R. Zeng, Z.X. Chen, S. Li, H.K. Liu, J. Mater. Chem. 19 (2009) 8360. A.Y. Shenouda, H.K. Liu, J. Electrochem. Soc. 157 (2010) A1183. A. Sivashanmugam, T.P. Kumar, N.G. Renganathan, S. Gopukumar, M. Wohlfahrt-Mehrens, J. Garche, J. Power Sources 144 (2005) 197. X.W. Lou, J.S. Chen, P. Chen, L.A. Archer, Chem. Mater. 21 (2009) 2868. W.M. Zhang, J.S. Hu, Y.G. Guo, S.F. Zheng, L.S. Zhong, W.G. Song, L.J. Wan, Adv. Mater. 20 (2008) 1160. H. Rietveld, Acta Cryst. 22 (1967) 151. K. TOPAS V2.1. General profile and structure analysis software for powder diffraction data; Bruker AXS, Germany, 2002. S. Ni, X. Wang, G. Zhou, F. Yang, J. Wang, Q. Wang, D. He, Mater. Lett. 63 (2009) 2701. A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. Q. Wang, C.B. Wang, Z. Wang, J.Y. Zhang, D.Y. He, Appl. Phys. Lett. 91 (2007) 141902. J.-Y. Zhou, Z.-Y. Chen, M. Zhou, X.-P. Gao, E.-Q. Xie, Nanoscale Res. Lett. 4 (2009) 814. H. Kim, J. Cho, J. Electrochem. Soc. 154 (2007) A462. L. Su, Z. Zhou, M. Ren, Chem. Commun. 46 (2010) 2590. B. Xu, S. Hou, M. Chu, G. Cao, Y. Yang, Carbon 48 (2010) 2812. X.W. Lou, D. Deng, J.Y. Lee, L.A. Archer, Chem. Mater. 20 (2008) 6562. J.Q. Hu, X.L. Ma, N.G. Shang, Z.Y. Xie, N.B. Wong, C.S. Lee, S.T. Lee, J. Phys. Chem. B 106 (2002) 3823.
X. Sun et al. / Electrochimica Acta 66 (2012) 204–209 [39] B. Wang, Y.H. Yang, C.X. Wang, G.W. Yang, Chem. Phys. Lett. 407 (2005) 347. [40] R. Yang, W. Zhao, J. Zheng, X.Z. Zhang, X.G. Li, J. Phys. Chem. C 114 (2010) 20272. [41] X.M. Yin, C.C. Li, M. Zhang, Q.Y. Hao, S. Liu, L.B. Chen, T.H. Wang, J. Phys. Chem. C 114 (2010) 8084. [42] Z. Yang, G. Du, C. Feng, S. Li, Z. Chen, P. Zhang, Z. Guo, X. Yu, G. Chen, S. Huang, H. Liu, Electrochim. Acta 55 (2010) 5485.
[43] [44] [45] [46]
209
M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv. Mater. 10 (1998) 725. L. Ji, Z. Lin, B. Guo, A.J. Medford, X. Zhang, Chem. Eur. J. 16 (2010) 11543. X. Sun, J. Liu, Y. Li, Chem. Mater. 18 (2006) 3486. S. Ding, D. Luan, F.Y.C. Boey, J.S. Chen, X.W. Lou, Chem. Commun. 47 (2011) 7155.