- Email: [email protected]
C hybrid nanofibers with enhanced lithium storage
Materials Letters 116 (2014) 271–274
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis of 3D networks of C/SnOx/C hybrid nanofibers with enhanced lithium storage Suyuan Li, Hongwei Yue, Qi Wang, Wenhe Xie, Deyan He n School of Physical Science and Technology and Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 September 2013 Accepted 4 November 2013 Available online 9 November 2013
3D networks consisted of C/SnOx/C hybrid nanofibers were prepared by a simple dip-coating process and subsequent CVD growth. The as-prepared material, worked as a binder-free anode electrode, delivered a reversible capacity of 512 mA h g 1 after 200 cycles at a current density of 200 mA g 1. The improved electrochemical performance can be ascribed to the morphological stability and the low resistance of the nanofibers with the CVD carbon skin. Furthermore, the embedded and de-aggregated SnOx nanoparticles in the carbon matrix provide large numbers of reaction sites for lithium ions. The results imply that 3D C/SnOx/C network nanocomposites have potential application in high-performance lithium ion batteries. & 2013 Elsevier B.V. All rights reserved.
Keywords: Composite materials Deposition Semiconductors Energy storage and conversion Networks
1. Introduction Lithium ion batteries (LIBs) have attracted increasing attention in energy storage technologies of portable electronic devices and electric vehicles (EVs) due to their high stability, long cycle life and improved safety [1,2]. As the commercial anode material, graphite cannot meet the rising demands due to its low energy density (theoretical specific capacity is 372 mA h g 1) [3]. SnOx have been considered as one of the potential graphite substitutes due to their higher theoretical specific capacity (792 mA h g 1 for SnO2 and 990 mA h g 1 for metallic Sn), low cost, less toxicity, high abundance and safe working potential [4]. However, the practical application of SnOx as anode materials in LIBs is still hindered by their poor cycle performance, which is caused by the large volume variation during the lithium insertion/extraction processes, giving rise to pulverization of the materials and loss of electrical contact with the current collector [5,6]. To solve these problems, various approaches have been put forward to modify the morphology and structure of SnOx, including fabricating nanosized nanoparticles, designing 3D network structure and incorporating Sn-based materials into active or inactive materials to form composites. However, methods for fabricating these anode materials either suffer from a high-cost or a technologial difficulty in a large area production [7]. In this work, large area 3D networks composed of C/SnOx/C hybrid nanofibers were fabricated by dipping peroxyacetylnitrate
n
Corresponding author. Tel.: þ 86 931 8912546. E-mail address: [email protected] (D. He).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.015
(PAN) carbon fibers (CFs) into SnCl2 2H2O ethanol solution and subsequent CVD growth. The obtained 3D networks, worked as a binder-free anode, exhibited good cycle stability. The improved lithium storage could be ascribed to the de-aggregated SnOx nanoparticles, carbon matrix (carbon core and carbon skin), and the 3D nanostructure of the networks. Moreover, the method is simple and possibly applicable to the other materials. 2. Experimental Fabrications of C/SnOx/C hybrid nanofibers and electrodes: The fabrication of PAN-CFs networks was reported by other authors previously [8]. To obtain CNF/SnO2, SnCl2 2H2O (3.858 g) was dispersed in anhydrous ethanol (60 ml) and magnetic stirred with a water bath refluxing at 70 1C for 1 h to form a homogeneous solution. Then PAN-CF networks were dipped into the solution for 30 s, and vertically drew out at a speed of 60 mm min 1. The networks were dried at 120 1C for 30 min and then calcined in a tube furnace at 400 1C for 1 h. Finally, CNF/SnO2 were placed in a CVD room, and the temperatures were preheated to 650 1C with a heating rate of 5 1C min 1 and maintained for 2 h with a flow rate of 100 sccm (H2:C2H2 ¼20: 80) to fabricate C/SnOx/C networks. The electrode was fabricated by sandwiching the prepared 3D networks between two pieces of nickel foam at a pressure of 20 MPa and the fabrication process of coin cell was similar to our previous report [9]. Structural and morphologic characterization: The morphologies and structures of the materials were characterized by fieldemission scanning electron microscopy (FE-SEM, Hitachi,
272
S. Li et al. / Materials Letters 116 (2014) 271–274
S-4800), transmission electron microscopy (TEM, FEI, Tecnai G2 F30), and X-ray powder diffraction (XRD, Rigaku RINT2400 with Cu Kα radiation λ ¼1.5418 Å). Thermogravimetric analysis (TGA) was carried out in air at a heating rate of 5 1C min 1 using a Perkin Elmer Diamond TG/DTA instrument.
3. Results and discussion Fig. 1 schematizes the preparation process for the 3D networks of C/SnOx/C nanofibers. First, PAN-CF networks are prepared via electrospinning technique and subsequent carbonization. Second, the SnO2 shell is deposited on the PAN-CFs by a simple dip-coating method. Finally, a carbon skin is deposited onto the CNF/SnO2 nanofibers by a CVD growth to embed and de-aggregate the SnO2 nanoparticles. Fig. 2a shows XRD patterns of the CNF/SnO2 and C/SnOx/C composites. The peaks in both of the spectra correspond well to those of rutiletype SnO2 (JCPDS 41-1445). After the deposition of the carbon skin by CVD growth, the XRD pattern shows peaks of metallic Sn (JCPDS 86-2264) with low intensity, which means that partial SnO2 was reduced into Sn during CVD process. TG result shown in Fig. 2b reveals that CNF/SnO2 presents significant weight loss at about 460 1C in air, which can be attributed to the
combustion of the PAN-CFs template. The weight fractions of SnO2 in the composite is 47.5 wt%. SEM and TEM are employed to study the morphologies and microstructures of the composites. As shown in Fig. 3a–f, it is clear that both the samples of CNF/SnO2 and C/SnOx/C still remain the 3D network nanostructures of PAN-CFs, and the surface of C/SnOx/C nanofibers (with a diameter of 100 nm) become rougher than that of PAN-CFs (with a diameter of 80 nm). A carbon skin can be observed on the surface of C/SnOx/C hybrid nanofiber from the TEM image in Fig. 3g, and SnOx nanoparticles are de-aggregated by the carbon skin (Fig. 3h). Selected area electron diffraction (SAED) pattern (Fig. 3i) can be indexed as diffractions from (1 1 0), (1 0 1), (2 1 1) and (3 0 1) crystal plan of rutiletype SnO2. This is caused by the fact that the acetylene penetrates into the SnO2 shell during the CVD process and then, under higher temperature, is split into carbon that can de-aggregate the SnO2 shell into separated SnO2 nanoparticles [10]. Meanwhile, partial SnO2 is reduced into metallic Sn by the generated atomic hydrogen. The de-aggregation of SnOx nanoparticles shorten the diffusion length of lithium ions and SnOx is well embedded in the carbon skin, ensuring good electrical conduction. Cyclic voltammetry was used to investigate the electrochemical processes of C/SnOx/C in the potential window of 0.05 and 2.5 V at
Fig. 1. Schematic illustration of the preparation process for C/SnOx/C hybrid nanofibers and the corresponding transmission electron microscopy images of the resultant samples.
Fig. 2. (a) XRD patterns of CNF/SnO2 and C/SnOx/C hybrid nanofibers; (b) TG curve of CNF/SnO2 nanofibers.
S. Li et al. / Materials Letters 116 (2014) 271–274
273
Fig. 3. ((a), (d)) SEM and TEM images of PAN-CFs, ((b), (e)) SEM and TEM images of CNF/SnO2 and ((c), (f)–(i)) SEM, TEM images and SEAD pattern of C/SnOx/C hybrid nanofibers.
a scan rate of 0.1 mV s 1. The electrochemical reaction between Li þ and SnO2 can be described by the following Eqs. [11]: SnO2 þ 4Li þ þ4e -Sn þ 2Li2 O;
ð1Þ
Sn þ xLi þ þ xe 2Lix Sn ð0 r x r 4:4Þ
ð2Þ
As shown in Fig. 4a, in the first scanning process, three peaks respectively located at 0.64, 0.67 and 0.78 V are observed, which match with those given in the other report on the Sn anode [6]. A broad reduction peak around 0.72 V can be observed, which is attributed to the formation of the solid electrolyte interphase (SEI) and an amorphous lithium oxide as described by Eq. (1). In the subsequent scanning cycles, the peak at 0.72 V shifts to 0.29 and 1.20 V that strongly indicates an irreversible reaction during the initial cycle. As shown in Fig. 4b, CNF/SnO2 electrodes have high initial capacities but show poor capacity retention. The capacity in the 160th cycle is 160 mA h g 1, which is much lower than the capacity of PAN-CFs (344 mA h g 1) at 100 mA h g 1. After CVD growth, C/SnOx/C electrodes show more stable cycle performances (Fig. 4c). The measured 1st, 50th, 100th, 150th and 200th discharge capacities were 1315, 611, 550, 522 and 512 mA h g 1, and the capacities faded slowly with a capacity of 0.66 mA h g 1 per cycle from 50th to 200th cycle (Fig. 4d). From this trend, it is clear that the capacity retention for the C/SnOx/C electrode is much better than that of the CNF/SnO2 electrodes. The enhanced electrochemical performances of the 3D network films can be ascribed to the cooperative contributions of the
grown carbon skin, the de-aggregated SnOx nanoparticles and the 3D network structure: (1) the de-aggregated SnOx nanoparticles are beneficial for providing large numbers of reaction sites for lithium ions, and the SnOx nanoparticles are small enough to shorten the diffusion length during Li-alloying process. (2) The PAN-CFs and the carbon skin with good electrical conductivity can increase the conductivity of the composite. (3) The 3D network nanostructure in the composite helps to alleviate the effect of huge volume change during Li-alloying/de-alloying processes.
4. Conclusions In summary, 3D C/SnOx/C nanofiber networks are fabricated by a simple dip-coating technique and subsequent CVD growth. The material shows good elctrochemical performance. The morphological stability and the high conductivity of C/SnOx/C nanofibers are effectively brought up by the PAN-CF core and deposited carbon skin. The two factors, coupled with the de-aggregated SnOx nanoparticles and the 3D network nanostructures of the material, are responsible for the great improvement in electrochemical properties.
Acknowledgements This project was financially supported by the National Natural Science Foundation of China with grant nos. 11179038 and
274
S. Li et al. / Materials Letters 116 (2014) 271–274
Fig. 4. (a) Initial 3 cyclic CV curves, (b) cycling performances of CNF/SnO2 and PAN-CFs at 100 mA g 1, (c) cycling performances of C/SnOx/C at 200 mA g 1 and (d) discharge-charge profiles of C/SnOx/C at 200 mA g 1.
10974073, and the Specialized Research Fund for the Doctoral Program of Higher Education with grant no. 20120211130005. References [1] Xu YH, Guo JC, Wang CS. J Mater Chem 2012;22:9562–7. [2] Scrosati B, Garche J. J Power Sources 2010;195:2419–30. [3] Tatsumi K, Iwashita N, Sakaebe H, Shioyana H, Higuchi S. J Electrochem Soc 1995;142:716–20.
[4] [5] [6] [7] [8] [9] [10]
Guo ZP, Du GD, Nuli Y, Hassan MF, Liu HK. J Mater Chem 2009;19:3253–7. Zhang L, Zhang GQ, Wu HB, Yu L, Lou XW. Adv Mater 2013;25:2589–93. Ding SJ, Chen JS, Lou XW. Chem Asian J 2011;6:2278–81. Wu P, Du N, Zhang H, Yu J, Qi Y, Yang D. Nanoscale 2011;3:746–50. Qiu Y, Yu J, Tan C, Yin J. Mater Lett 2009;63:200–2. Yang ZB, Wang DS, Li F, Liu DQ, Wang P, Li XW, et al. Mater Lett 2012;90:4–7. Kong JH, Liu ZL, Yang ZC, Tan HR, Xiong SX, Wong SY, et al. Nanoscale 2012;4:525–30. [11] Courtney IA, Dahn JR. J Electrochem Soc 1997;144:2943–8.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis of 3D networks of C/SnOx/C hybrid nanofibers with enhanced lithium storage Suyuan Li, Hongwei Yue, Qi Wang, Wenhe Xie, Deyan He n School of Physical Science and Technology and Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 September 2013 Accepted 4 November 2013 Available online 9 November 2013
3D networks consisted of C/SnOx/C hybrid nanofibers were prepared by a simple dip-coating process and subsequent CVD growth. The as-prepared material, worked as a binder-free anode electrode, delivered a reversible capacity of 512 mA h g 1 after 200 cycles at a current density of 200 mA g 1. The improved electrochemical performance can be ascribed to the morphological stability and the low resistance of the nanofibers with the CVD carbon skin. Furthermore, the embedded and de-aggregated SnOx nanoparticles in the carbon matrix provide large numbers of reaction sites for lithium ions. The results imply that 3D C/SnOx/C network nanocomposites have potential application in high-performance lithium ion batteries. & 2013 Elsevier B.V. All rights reserved.
Keywords: Composite materials Deposition Semiconductors Energy storage and conversion Networks
1. Introduction Lithium ion batteries (LIBs) have attracted increasing attention in energy storage technologies of portable electronic devices and electric vehicles (EVs) due to their high stability, long cycle life and improved safety [1,2]. As the commercial anode material, graphite cannot meet the rising demands due to its low energy density (theoretical specific capacity is 372 mA h g 1) [3]. SnOx have been considered as one of the potential graphite substitutes due to their higher theoretical specific capacity (792 mA h g 1 for SnO2 and 990 mA h g 1 for metallic Sn), low cost, less toxicity, high abundance and safe working potential [4]. However, the practical application of SnOx as anode materials in LIBs is still hindered by their poor cycle performance, which is caused by the large volume variation during the lithium insertion/extraction processes, giving rise to pulverization of the materials and loss of electrical contact with the current collector [5,6]. To solve these problems, various approaches have been put forward to modify the morphology and structure of SnOx, including fabricating nanosized nanoparticles, designing 3D network structure and incorporating Sn-based materials into active or inactive materials to form composites. However, methods for fabricating these anode materials either suffer from a high-cost or a technologial difficulty in a large area production [7]. In this work, large area 3D networks composed of C/SnOx/C hybrid nanofibers were fabricated by dipping peroxyacetylnitrate
n
Corresponding author. Tel.: þ 86 931 8912546. E-mail address: [email protected] (D. He).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.015
(PAN) carbon fibers (CFs) into SnCl2 2H2O ethanol solution and subsequent CVD growth. The obtained 3D networks, worked as a binder-free anode, exhibited good cycle stability. The improved lithium storage could be ascribed to the de-aggregated SnOx nanoparticles, carbon matrix (carbon core and carbon skin), and the 3D nanostructure of the networks. Moreover, the method is simple and possibly applicable to the other materials. 2. Experimental Fabrications of C/SnOx/C hybrid nanofibers and electrodes: The fabrication of PAN-CFs networks was reported by other authors previously [8]. To obtain CNF/SnO2, SnCl2 2H2O (3.858 g) was dispersed in anhydrous ethanol (60 ml) and magnetic stirred with a water bath refluxing at 70 1C for 1 h to form a homogeneous solution. Then PAN-CF networks were dipped into the solution for 30 s, and vertically drew out at a speed of 60 mm min 1. The networks were dried at 120 1C for 30 min and then calcined in a tube furnace at 400 1C for 1 h. Finally, CNF/SnO2 were placed in a CVD room, and the temperatures were preheated to 650 1C with a heating rate of 5 1C min 1 and maintained for 2 h with a flow rate of 100 sccm (H2:C2H2 ¼20: 80) to fabricate C/SnOx/C networks. The electrode was fabricated by sandwiching the prepared 3D networks between two pieces of nickel foam at a pressure of 20 MPa and the fabrication process of coin cell was similar to our previous report [9]. Structural and morphologic characterization: The morphologies and structures of the materials were characterized by fieldemission scanning electron microscopy (FE-SEM, Hitachi,
272
S. Li et al. / Materials Letters 116 (2014) 271–274
S-4800), transmission electron microscopy (TEM, FEI, Tecnai G2 F30), and X-ray powder diffraction (XRD, Rigaku RINT2400 with Cu Kα radiation λ ¼1.5418 Å). Thermogravimetric analysis (TGA) was carried out in air at a heating rate of 5 1C min 1 using a Perkin Elmer Diamond TG/DTA instrument.
3. Results and discussion Fig. 1 schematizes the preparation process for the 3D networks of C/SnOx/C nanofibers. First, PAN-CF networks are prepared via electrospinning technique and subsequent carbonization. Second, the SnO2 shell is deposited on the PAN-CFs by a simple dip-coating method. Finally, a carbon skin is deposited onto the CNF/SnO2 nanofibers by a CVD growth to embed and de-aggregate the SnO2 nanoparticles. Fig. 2a shows XRD patterns of the CNF/SnO2 and C/SnOx/C composites. The peaks in both of the spectra correspond well to those of rutiletype SnO2 (JCPDS 41-1445). After the deposition of the carbon skin by CVD growth, the XRD pattern shows peaks of metallic Sn (JCPDS 86-2264) with low intensity, which means that partial SnO2 was reduced into Sn during CVD process. TG result shown in Fig. 2b reveals that CNF/SnO2 presents significant weight loss at about 460 1C in air, which can be attributed to the
combustion of the PAN-CFs template. The weight fractions of SnO2 in the composite is 47.5 wt%. SEM and TEM are employed to study the morphologies and microstructures of the composites. As shown in Fig. 3a–f, it is clear that both the samples of CNF/SnO2 and C/SnOx/C still remain the 3D network nanostructures of PAN-CFs, and the surface of C/SnOx/C nanofibers (with a diameter of 100 nm) become rougher than that of PAN-CFs (with a diameter of 80 nm). A carbon skin can be observed on the surface of C/SnOx/C hybrid nanofiber from the TEM image in Fig. 3g, and SnOx nanoparticles are de-aggregated by the carbon skin (Fig. 3h). Selected area electron diffraction (SAED) pattern (Fig. 3i) can be indexed as diffractions from (1 1 0), (1 0 1), (2 1 1) and (3 0 1) crystal plan of rutiletype SnO2. This is caused by the fact that the acetylene penetrates into the SnO2 shell during the CVD process and then, under higher temperature, is split into carbon that can de-aggregate the SnO2 shell into separated SnO2 nanoparticles [10]. Meanwhile, partial SnO2 is reduced into metallic Sn by the generated atomic hydrogen. The de-aggregation of SnOx nanoparticles shorten the diffusion length of lithium ions and SnOx is well embedded in the carbon skin, ensuring good electrical conduction. Cyclic voltammetry was used to investigate the electrochemical processes of C/SnOx/C in the potential window of 0.05 and 2.5 V at
Fig. 1. Schematic illustration of the preparation process for C/SnOx/C hybrid nanofibers and the corresponding transmission electron microscopy images of the resultant samples.
Fig. 2. (a) XRD patterns of CNF/SnO2 and C/SnOx/C hybrid nanofibers; (b) TG curve of CNF/SnO2 nanofibers.
S. Li et al. / Materials Letters 116 (2014) 271–274
273
Fig. 3. ((a), (d)) SEM and TEM images of PAN-CFs, ((b), (e)) SEM and TEM images of CNF/SnO2 and ((c), (f)–(i)) SEM, TEM images and SEAD pattern of C/SnOx/C hybrid nanofibers.
a scan rate of 0.1 mV s 1. The electrochemical reaction between Li þ and SnO2 can be described by the following Eqs. [11]: SnO2 þ 4Li þ þ4e -Sn þ 2Li2 O;
ð1Þ
Sn þ xLi þ þ xe 2Lix Sn ð0 r x r 4:4Þ
ð2Þ
As shown in Fig. 4a, in the first scanning process, three peaks respectively located at 0.64, 0.67 and 0.78 V are observed, which match with those given in the other report on the Sn anode [6]. A broad reduction peak around 0.72 V can be observed, which is attributed to the formation of the solid electrolyte interphase (SEI) and an amorphous lithium oxide as described by Eq. (1). In the subsequent scanning cycles, the peak at 0.72 V shifts to 0.29 and 1.20 V that strongly indicates an irreversible reaction during the initial cycle. As shown in Fig. 4b, CNF/SnO2 electrodes have high initial capacities but show poor capacity retention. The capacity in the 160th cycle is 160 mA h g 1, which is much lower than the capacity of PAN-CFs (344 mA h g 1) at 100 mA h g 1. After CVD growth, C/SnOx/C electrodes show more stable cycle performances (Fig. 4c). The measured 1st, 50th, 100th, 150th and 200th discharge capacities were 1315, 611, 550, 522 and 512 mA h g 1, and the capacities faded slowly with a capacity of 0.66 mA h g 1 per cycle from 50th to 200th cycle (Fig. 4d). From this trend, it is clear that the capacity retention for the C/SnOx/C electrode is much better than that of the CNF/SnO2 electrodes. The enhanced electrochemical performances of the 3D network films can be ascribed to the cooperative contributions of the
grown carbon skin, the de-aggregated SnOx nanoparticles and the 3D network structure: (1) the de-aggregated SnOx nanoparticles are beneficial for providing large numbers of reaction sites for lithium ions, and the SnOx nanoparticles are small enough to shorten the diffusion length during Li-alloying process. (2) The PAN-CFs and the carbon skin with good electrical conductivity can increase the conductivity of the composite. (3) The 3D network nanostructure in the composite helps to alleviate the effect of huge volume change during Li-alloying/de-alloying processes.
4. Conclusions In summary, 3D C/SnOx/C nanofiber networks are fabricated by a simple dip-coating technique and subsequent CVD growth. The material shows good elctrochemical performance. The morphological stability and the high conductivity of C/SnOx/C nanofibers are effectively brought up by the PAN-CF core and deposited carbon skin. The two factors, coupled with the de-aggregated SnOx nanoparticles and the 3D network nanostructures of the material, are responsible for the great improvement in electrochemical properties.
Acknowledgements This project was financially supported by the National Natural Science Foundation of China with grant nos. 11179038 and
274
S. Li et al. / Materials Letters 116 (2014) 271–274
Fig. 4. (a) Initial 3 cyclic CV curves, (b) cycling performances of CNF/SnO2 and PAN-CFs at 100 mA g 1, (c) cycling performances of C/SnOx/C at 200 mA g 1 and (d) discharge-charge profiles of C/SnOx/C at 200 mA g 1.
10974073, and the Specialized Research Fund for the Doctoral Program of Higher Education with grant no. 20120211130005. References [1] Xu YH, Guo JC, Wang CS. J Mater Chem 2012;22:9562–7. [2] Scrosati B, Garche J. J Power Sources 2010;195:2419–30. [3] Tatsumi K, Iwashita N, Sakaebe H, Shioyana H, Higuchi S. J Electrochem Soc 1995;142:716–20.
[4] [5] [6] [7] [8] [9] [10]
Guo ZP, Du GD, Nuli Y, Hassan MF, Liu HK. J Mater Chem 2009;19:3253–7. Zhang L, Zhang GQ, Wu HB, Yu L, Lou XW. Adv Mater 2013;25:2589–93. Ding SJ, Chen JS, Lou XW. Chem Asian J 2011;6:2278–81. Wu P, Du N, Zhang H, Yu J, Qi Y, Yang D. Nanoscale 2011;3:746–50. Qiu Y, Yu J, Tan C, Yin J. Mater Lett 2009;63:200–2. Yang ZB, Wang DS, Li F, Liu DQ, Wang P, Li XW, et al. Mater Lett 2012;90:4–7. Kong JH, Liu ZL, Yang ZC, Tan HR, Xiong SX, Wong SY, et al. Nanoscale 2012;4:525–30. [11] Courtney IA, Dahn JR. J Electrochem Soc 1997;144:2943–8.