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In situ fabrication of graphene–carbon nanochain webs as anodes for Li-ion batteries† Youlan Zou, Xiangyang Zhou* and Juan Yang*
Received 17th March 2014, Accepted 8th April 2014 DOI: 10.1039/c4cp01137f www.rsc.org/pccp
Carbon nanochain webs-sandwiched graphene has been successfully fabricated via in situ polymerization and subsequent carbonization. Such a novel nano–micro structure not only provides high conductivity, but also improves the cycling stability and rate capability during Li-ion storage. It shows a charge capacity of 1103.2 mA h g 0.05 A g
1
1
at
after 50 cycles.
Recently, intense efforts have been devoted to a search for various advanced anode materials for Li-ion batteries with high reversible capacity, excellent rate capability and cycling stability.1 Polypyrrole (PPy) is most likely chosen to be the candidate because of its ease of synthesis, high conductivity and high capacity.2,3 However rapid expansion and shrinkage of PPy occur during charge–discharge process, which eventually lead to the loss in electrical activity and quick fading of capacity.4 In order to overcome this drawback, PPy has been modified through carbonization.5,6 For example, L. Qie et al., have reported that nitrogen-doped porous carbon nanofiber webs exhibited super high capacity and excellent rate capability when carbonizing and using KOH as an activating agent.5 Another effective way to improve the cycling performance of PPy-based anodes is to make composites with other materials.7–10 Graphene is considered a promising choice when compounding with PPy or other conductive polymers.11,12 It not only serves as a conductive support material for fast electron transport and as a buffer matrix to accommodate the volume change, but also provides large surface for fast Li-ion diffusion and dispersion of PPy. It is interesting to develop a new carbon material from both the fundamental and application points of view.13 Herein, we designed a novel graphene–carbon nanochain web (GCNW) nano–micro structure, in which carbon nanochain webs were inserted between graphene layers to form sandwiched plates. In this system, carbon nanochain webs derived from PPy act as the isolator to prevent the
School of Metallurgy and Environment, Central South University, Changsha, China 410083. E-mail: [email protected], [email protected]; Fax: +86 073188836329; Tel: +86 073188836329 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp01137f
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overlap of graphene layers, which in turn provide high conductivity and large surface for the deposition of carbon nanochain webs. Such a nano–micro structure provides a morphological stability for Li-ion insertion–extraction, and demonstrates high reversible Li-storage capacities and excellent rate performance as an anode for Li-ion batteries. The formation mechanism for GCNW is schematically depicted in Scheme 1. The approach is designed on the basis of the unique negatively charged property of graphene.14,15 When the positively charged ammonium ion of CTAB is combined with graphene dispersion, the surfactant micelles of CTAB would be electrostatically adsorbed on the basal plane of the graphene sheets, forming a graphene-surfactant multilayer structure with the surfactant micelles sandwiched between the graphene sheets. After the Py monomer is added, the monomer will be induced to intercalate into graphene layers by surfactant micelles, forming a kind of weak charge transfer complex. When APS as the oxidant is added, in situ chemical oxidation polymerization of the Py monomer occurs just from the absorbed sites. The strong p–p stacking interaction between the aromatic rings of PPy chains and the sp2-bonded carbon atoms of graphene
Scheme 1 GCNW.
Schematic illustration of the three-step formation process of
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basal planes promotes graphene to coat on the surface of the PPy and form a graphene–PPy nanochain web composite.16,17 Surfactants, certain polymers like PPy and aromatic ‘‘p–p stacking’’ molecules interact with graphene by surface adsorption, micelle formation, and p–p interaction, which electrostatically prevent re-stacking of graphene layers. Sufficient surfactants and high concentration of polymer are beneficial to obtain uniform dispersions with high graphene concentration.18 Finally, the graphene–PPy precursor is placed into a tube furnace to transfer the PPy into carbon and further remove the functional groups of graphene. SEM image (Fig. 1a) shows that GCNW consists of the uniform granules with a diameter of 10 mm from the macroscopic perspective. Under a higher magnification (Fig. 1b), the granule was composed of thin plates within a lot of grids. As revealed by TEM (Fig. 1c), homogeneous morphology of 1D carbon nanochains are dispersed on the surface of graphene films. The thickness of the carbon nanochain is in the range of 7–8 nm, according to the AFM image (Fig. 1d). AFM image further confirms that the oriented carbon spheres organize in series on the base plane of graphene and form the 1D carbon nanochains. 1D carbon nanochains are cross-linked to promote the formation of 3D networks. Compared with the pure carbon nanochain webs (CNW) (Fig. 2a and b), the carbon nanochains
Fig. 1
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of GCNW retain the original cross-linked morphology, indicating that the presence of CTAB facilitates the oriented polymerization of PPy on the basal plane of graphene. From Fig. 2c and d, it could be clearly found that 1D carbon nanochain for GCNW was coated by graphene to form sandwiched plates (G/CNW/G),19 compared with CNW (Fig. S1, ESI†). BET surface areas of graphene (G) and GCNW have been investigated to be 317.02 and 288.17 m2 g 1, respectively. The prime reason for slightly decrease in the surface area in contrast to pure graphene is that the carbon nanochain webs are coated by graphene and assembled to micro-granules. The actual specific surface area of the composite (288.17 m2 g 1) is still considerable, compared with the BET surface area of carbon nanofiber/graphene (315 m2 g 1), as reported by Z. J. Fan et al.20 High surface area has a significant influence on the dispersion and distribution of the carbon nanochain between graphene layers. The nature of nitrogen at the surface of CNW and GCNW is investigated by XPS (Fig 3a and b). Elemental analysis (Table S1, ESI†) reveals that atomic content of N for CNW is 12.08%, much higher than that in other N-doped carbon materials or PPy-based materials used for Li-ion batteries.21 The carbonization of PPy may be a good approach to introduce nitrogen into carbon materials, which is beneficial to improve the electrochemical property of the electrode. As expected, two peaks for CNW located in the interval 400.79 and 398.18 eV are attributed to the pyrrolic-N and hexagonal pyridinic-N, respectively. Although the graphene is added, the N content in GCNW still has 5.11%. But the relative intensity of the peak corresponding to pyrrolic-N becomes weaker than that of CNW, indicating that part of N atoms within the pentagonal ring of PPy are converted to pyridinic-N. The raman spectrum of GCNW (Fig. 3c) displays a strong G-band at 1584.7 cm 1 and a weak D-band at 1367.6 cm 1. The D-band is a measure of disorder originating from defects and the G is representative of sp2-hybridized carbon bonds.22 The G/D intensity ratio (1.04) is a little higher than that of CNW (1.01), but lower than that of pure graphene (1.1). This is due to the antisynergistic effect that the ordered graphene and disordered
(a) and (b) SEM, (c) TEM and (d) AFM images of GCNW.
Fig. 2 (a) SEM, (b) TEM and (c) HETEM images of CNW, (d) HETEM image of GCNW.
10430 | Phys. Chem. Chem. Phys., 2014, 16, 10429--10432
Fig. 3 (a) N1s XPS spectrum of CNW, (b) N1s XPS, (c) Raman and (d) FTIR spectra of GCNW.
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carbon nanochain webs are both introduced to GCNW. The introduction of N heteroatom also leads to the decrease of the intensities of D-band and G-band for GCNW, and the reduction of the order degree for GCNW. The other two peaks for GCNW at about 2700 cm 1 and 2987 cm 1 can be ascribed to the 2D and D+G modes, respectively. The D+G/2D intensity ratio for GCNW decreases significantly in comparison with that of pure graphene, suggesting that the defect concentration of GCNW is much reduced. Additionally, the shape and intensity of the 2D peak are often used to identify the number of layers for graphene. According to A. Gupta et al.,23 the number of the layers of graphene for GCNW is less than that of pure graphene, indicating that carbon nanochain webs serve as the isolators to prevent the overlap of graphene layers. Curve fits of the four bands for the G, GCNW and CNW samples are presented to further investigate the structure of the carbon materials (Fig. S2, ESI†). The full width at half maximum (FWHM) values corresponding to each bond are illustrated in Table S2 (ESI†), which indentifies the combination of carbon nanofibers that enhances the disordered degree of GCNW.24 FTIR spectra of the graphene, CNW and GCNW are shown in Fig. 3d. For the pure graphene, the bands of O–H at around 1300 cm 1, O–H at 3490 cm 1, and CQO at 1760 cm 1 gradually disappear, clarifying that the peaks for oxygen functional groups are removed and the reduced graphene sheets are formed. For the two carbon nanochain-based composites, the bands at 1190 and 1560 cm 1 may be assigned to the stretching vibration of the doping state and vibrations C–N stretching, respectively. Fig. 4a illustrates the CV curves of GCNW in the 1st, 2nd and 50th cycles. It exhibits three apparent peaks at 0.8, 0.5 and 0.01 V in the first reduction process. The peak at around 0.8 V is due to the formation of the solid electrolyte interphase (SEI) films at the electrode–electrolyte interface. A pair of peaks at 0.5 V/0.3 V can be attributed to the reversible Li-ions insertion into/extraction from enclosed spaces that are confined by carbon nanochain webs and graphene.25,26 The peak at 0.01 V corresponds to the reversible Li-ions reaction with carbon materials. No irreversible peak is observed in the second and
fifties cycles, indicating the integrated SEI films have been formed and reserved in the 1st cycle. Fig. 4b demonstrates the first charge–discharge curves of the GCNW anode at various current densities. With the increase of the current density, the charge capacity of the anode decreases. It delivers the first discharge/charge capacities of 1967.8/1221.1 mA h g 1 at 0.05 A g 1. The irreversible capacity loss is mainly owing to the following factors: the high specific surface area resulting in the consumption of Li-ions for the formation of the SEI films; the irreversible Li-ion storage at the defects of the graphene; the irreversible Li-ion storage at the enclosed spaces confined by the carbon nanochain webs and the graphene. Even so, the anode still displays an outstanding cycleability and rate performance, as shown in Fig. 4c and d. The electrode retains a very high reversible capacity of 1103.2 mA h g 1 at 0.05 A g 1 after 50 cycles, which is higher than that of graphene (721.9 mA h g 1), CNW (728 mA h g 1) and almost three times the theoretical capacity of conventional graphite (372 mA h g 1). The coulombic efficiency of the electrode is above 95% besides the first cycle, indicating the irreversible capacity loss decrease significantly (Fig. S3, ESI†). Rate capability is another important factor for the use of Li-ion batteries in power applications. A good electrochemical energy storage device is required to provide its high specific capacities at a high current. When the currents increase to 0.1, 0.5, 1 and 5 A g 1, the anodes maintain the charge capacities of 938, 637, 568.4 and 230 mA h g 1 after 100 cycles at 0.1, 0.5, 1 and 5 A g 1, respectively. The values are higher than those of the graphene/carbon nanofiber electrode reported by Z. J. Fan et al.,20 which highlights the strong synergistic effect between carbon nanochain webs and graphene. The coulombic efficiency of GCNW in the first cycle increases with the increase of the current density. It is due to some side reactions that did not occur at fast ion transportation and the incomplete decomposition of the electrolyte during the formation of SEI film. After the first cycle, the GCNW electrode demonstrates highly reversible behavior at each current density indicating the super cycleability and rate ability of the GCNW electrode (Fig. S4, ESI†). Scheme 2 is the schematic illustration of the electronic transport route and the volume effect of GCNW in the Li-ion insertion–extraction process. The introduction of graphene limits the volume expansion of 1D carbon nanochains. The huge surface
Fig. 4 (a) Cyclic voltammograms, (b) initial charge–discharge, (c) cycling abilities and (d) rate capability plots of GCNW.
Scheme 2 Schematic illustration of the volume change of GCNW during Li-ion insertion–extraction process.
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of graphene leads to sufficient electrode/electrolyte interface to absorb Li-ions and promotes rapid charge-transfer reaction. Huge volume expansion would be effectively alleviated by graphene, when Li+ inserts into the carbon nanochain webs. Cross-linked carbon nanochain webs derived from PPy provide a continuous pathway for electron transport and reduce the transport length of Li-ions.27 N doping in the anode enhances the electrochemical reactivity, which additionally contributes to the exceptional performance. Large numbers of spaces enclosed by graphene and carbon nanochain webs act as storage for Li-ions. Furthermore, cross-linked carbon nanochain webs between the graphene layers facilitate the formation of 3D electronic conductive networks and expand the gap between graphene layers which in turn allow easier migration of Li-ions. Therefore, the unique nano–micro structure ensures the structural stability and provides considerable capacities during Li-ion insertion–extraction process. A simple and practical synthesis route for the fabrication of graphene–carbon nanochain webs is developed via in situ polymerization and carbonization. Carbon nanochain webs are inserted between graphene layers to separate the neighboring graphene. The spaces enclosed by carbon nanochains and graphene are beneficial for ion/electron transfer and the sufficient contact between active materials and electrolyte. Moreover, carbonization of PPy is favorable for efficiently preventing volume expansion/shrinkage. The synergistic effect between carbon nanochain webs and graphene sheets improves the cycling stability and rate capability of the electrode. This is very meaningful both for fundamental study of the anode materials and for industrial application. This work is financially supported by Nature Science Foundation of China. (No: 51204209, 51274240).
References 1 P. G. Bruce, B. Scrosati and J. M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930. 2 T. M. Wu, H. L. Chang and Y. W. Lin, Compos. Sci. Technol., 2009, 69, 639. 3 W. Sun and X. Y. Chen, J. Power Sources, 2009, 193, 924. 4 S. Konwer, R. Boruah and S. K. Dolui, J. Electron. Mater., 2011, 40, 2248. 5 L. Qie, W. M. Chen, Z. H. Wang, Q. G. Shao, X. Li, L. X. Yuan, X. L. Hu, W. X. Zhang and Y. H. Huang, Adv. Mater., 2012, 24, 2047.
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6 C. C. Li, X. M. Yin, L. B. Chen, Q. H. Li and T. H. Wang, J. Phys. Chem. C, 2009, 113, 13438. 7 J. C. Claussen, A. Kumar, D. B. Jaroch, M. H. Khawaja, A. B. Hibbard, D. M. Porterfield and T. S. Fisher, Adv. Funct. Mater., 2012, 22, 3399. 8 F. Han, D. Li, W. C. Li, C. Lei, Q. Sun and A. H. Lu, Adv. Funct. Mater., 2013, 23, 1692. 9 Y. Yang, C. Y. Wang, B. B. Yue, S. Gambhir, C. O. Too and G. G. Wallace, Adv. Energy Mater., 2012, 2, 266. 10 G. P. Xiong, C. Z. Meng, R. G. Reifenberger, P. P. Irazoqui and T. S. Fisher, Adv. Energy Mater., 2014, 4, 1300515. 11 J. T. Zhang, P. Chen, B. H. L. Oh and M. B. Chan-Park, Nanoscale, 2013, 5, 9860. 12 Y. G. Zhang, Y. Zhao, A. Konarov, D. Gosselink, H. G. Soboleski and P. Chen, J. Power Sources, 2013, 241, 517. 13 A. D. Franklin and D. B. Janes, Appl. Phys. Lett., 2008, 92, 013122. 14 L. L. Zhang, S. Y. Zhao, X. N. Tian and X. S. Zhao, Langmuir, 2010, 26, 17624. 15 Y. F. Fan, Y. S. Liu, Q. Cai, Y. Z. Liu and J. M. Zhang, Synth. Met., 2012, 162, 1815. 16 C. Li and G. Shi, Nanoscale, 2012, 4, 5549. 17 J. Zhang and X. S. Zhao, J. Phys. Chem. C, 2012, 116, 5420. 18 D. Parviz, S. Das, H. S. T. Ahmed, F. Irin, S. Bhattacharia and M. J. Green, ACS Nano, 2012, 6, 8857. 19 C. Wu, X. Huang, G. Wang, L. Lv, G. Chen, G. Li and P. Jiang, Adv. Funct. Mater., 2013, 23, 506. 20 Z. J. Fan, J. Yan, T. Wei, G. Q. Ning, L. J. Zhi, J. C. Liu, D. X. Cao, G. L. Wang and F. Wei, ACS Nano, 2011, 5, 2787. 21 L. G. Bulusheva, A. V. Okotrub, A. G. Kurenya, H. K. Zhang, H. J. Zhang, X. H. Chen and H. H. Song, Carbon, 2011, 49, 4013. 22 L. M. Malard, M. A. Pimenta, G. Dresselhaus and M. S. Dresselhaus, Phys. Rep., 2009, 473, 51. 23 A. Gupta, G. Chen, P. Joshi, S. Tadigadapa and P. C. Eklund, Nano Lett., 2006, 6, 2667. 24 A. Sadezky, H. Muckenhuber, H. Grothe, R. Niessner and ¨schl, Carbon, 2005, 43, 1731. U. Po 25 G. X. Wang, X. P. Shen, J. Yao and J. Park, Carbon, 2009, 47, 2049. 26 X. Y. Zhou, J. J. Tang, J. Yang, J. Xie and B. Huang, J. Mater. Chem. A, 2013, 1, 5037. 27 H. R. Byon, B. M. Gallant, S. W. Lee and Y. Shao-Horn, Adv. Funct. Mater., 2012, 23, 1037.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 10429
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In situ fabrication of graphene–carbon nanochain webs as anodes for Li-ion batteries† Youlan Zou, Xiangyang Zhou* and Juan Yang*
Received 17th March 2014, Accepted 8th April 2014 DOI: 10.1039/c4cp01137f www.rsc.org/pccp
Carbon nanochain webs-sandwiched graphene has been successfully fabricated via in situ polymerization and subsequent carbonization. Such a novel nano–micro structure not only provides high conductivity, but also improves the cycling stability and rate capability during Li-ion storage. It shows a charge capacity of 1103.2 mA h g 0.05 A g
1
1
at
after 50 cycles.
Recently, intense efforts have been devoted to a search for various advanced anode materials for Li-ion batteries with high reversible capacity, excellent rate capability and cycling stability.1 Polypyrrole (PPy) is most likely chosen to be the candidate because of its ease of synthesis, high conductivity and high capacity.2,3 However rapid expansion and shrinkage of PPy occur during charge–discharge process, which eventually lead to the loss in electrical activity and quick fading of capacity.4 In order to overcome this drawback, PPy has been modified through carbonization.5,6 For example, L. Qie et al., have reported that nitrogen-doped porous carbon nanofiber webs exhibited super high capacity and excellent rate capability when carbonizing and using KOH as an activating agent.5 Another effective way to improve the cycling performance of PPy-based anodes is to make composites with other materials.7–10 Graphene is considered a promising choice when compounding with PPy or other conductive polymers.11,12 It not only serves as a conductive support material for fast electron transport and as a buffer matrix to accommodate the volume change, but also provides large surface for fast Li-ion diffusion and dispersion of PPy. It is interesting to develop a new carbon material from both the fundamental and application points of view.13 Herein, we designed a novel graphene–carbon nanochain web (GCNW) nano–micro structure, in which carbon nanochain webs were inserted between graphene layers to form sandwiched plates. In this system, carbon nanochain webs derived from PPy act as the isolator to prevent the
School of Metallurgy and Environment, Central South University, Changsha, China 410083. E-mail: [email protected], [email protected]; Fax: +86 073188836329; Tel: +86 073188836329 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp01137f
This journal is © the Owner Societies 2014
overlap of graphene layers, which in turn provide high conductivity and large surface for the deposition of carbon nanochain webs. Such a nano–micro structure provides a morphological stability for Li-ion insertion–extraction, and demonstrates high reversible Li-storage capacities and excellent rate performance as an anode for Li-ion batteries. The formation mechanism for GCNW is schematically depicted in Scheme 1. The approach is designed on the basis of the unique negatively charged property of graphene.14,15 When the positively charged ammonium ion of CTAB is combined with graphene dispersion, the surfactant micelles of CTAB would be electrostatically adsorbed on the basal plane of the graphene sheets, forming a graphene-surfactant multilayer structure with the surfactant micelles sandwiched between the graphene sheets. After the Py monomer is added, the monomer will be induced to intercalate into graphene layers by surfactant micelles, forming a kind of weak charge transfer complex. When APS as the oxidant is added, in situ chemical oxidation polymerization of the Py monomer occurs just from the absorbed sites. The strong p–p stacking interaction between the aromatic rings of PPy chains and the sp2-bonded carbon atoms of graphene
Scheme 1 GCNW.
Schematic illustration of the three-step formation process of
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basal planes promotes graphene to coat on the surface of the PPy and form a graphene–PPy nanochain web composite.16,17 Surfactants, certain polymers like PPy and aromatic ‘‘p–p stacking’’ molecules interact with graphene by surface adsorption, micelle formation, and p–p interaction, which electrostatically prevent re-stacking of graphene layers. Sufficient surfactants and high concentration of polymer are beneficial to obtain uniform dispersions with high graphene concentration.18 Finally, the graphene–PPy precursor is placed into a tube furnace to transfer the PPy into carbon and further remove the functional groups of graphene. SEM image (Fig. 1a) shows that GCNW consists of the uniform granules with a diameter of 10 mm from the macroscopic perspective. Under a higher magnification (Fig. 1b), the granule was composed of thin plates within a lot of grids. As revealed by TEM (Fig. 1c), homogeneous morphology of 1D carbon nanochains are dispersed on the surface of graphene films. The thickness of the carbon nanochain is in the range of 7–8 nm, according to the AFM image (Fig. 1d). AFM image further confirms that the oriented carbon spheres organize in series on the base plane of graphene and form the 1D carbon nanochains. 1D carbon nanochains are cross-linked to promote the formation of 3D networks. Compared with the pure carbon nanochain webs (CNW) (Fig. 2a and b), the carbon nanochains
Fig. 1
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of GCNW retain the original cross-linked morphology, indicating that the presence of CTAB facilitates the oriented polymerization of PPy on the basal plane of graphene. From Fig. 2c and d, it could be clearly found that 1D carbon nanochain for GCNW was coated by graphene to form sandwiched plates (G/CNW/G),19 compared with CNW (Fig. S1, ESI†). BET surface areas of graphene (G) and GCNW have been investigated to be 317.02 and 288.17 m2 g 1, respectively. The prime reason for slightly decrease in the surface area in contrast to pure graphene is that the carbon nanochain webs are coated by graphene and assembled to micro-granules. The actual specific surface area of the composite (288.17 m2 g 1) is still considerable, compared with the BET surface area of carbon nanofiber/graphene (315 m2 g 1), as reported by Z. J. Fan et al.20 High surface area has a significant influence on the dispersion and distribution of the carbon nanochain between graphene layers. The nature of nitrogen at the surface of CNW and GCNW is investigated by XPS (Fig 3a and b). Elemental analysis (Table S1, ESI†) reveals that atomic content of N for CNW is 12.08%, much higher than that in other N-doped carbon materials or PPy-based materials used for Li-ion batteries.21 The carbonization of PPy may be a good approach to introduce nitrogen into carbon materials, which is beneficial to improve the electrochemical property of the electrode. As expected, two peaks for CNW located in the interval 400.79 and 398.18 eV are attributed to the pyrrolic-N and hexagonal pyridinic-N, respectively. Although the graphene is added, the N content in GCNW still has 5.11%. But the relative intensity of the peak corresponding to pyrrolic-N becomes weaker than that of CNW, indicating that part of N atoms within the pentagonal ring of PPy are converted to pyridinic-N. The raman spectrum of GCNW (Fig. 3c) displays a strong G-band at 1584.7 cm 1 and a weak D-band at 1367.6 cm 1. The D-band is a measure of disorder originating from defects and the G is representative of sp2-hybridized carbon bonds.22 The G/D intensity ratio (1.04) is a little higher than that of CNW (1.01), but lower than that of pure graphene (1.1). This is due to the antisynergistic effect that the ordered graphene and disordered
(a) and (b) SEM, (c) TEM and (d) AFM images of GCNW.
Fig. 2 (a) SEM, (b) TEM and (c) HETEM images of CNW, (d) HETEM image of GCNW.
10430 | Phys. Chem. Chem. Phys., 2014, 16, 10429--10432
Fig. 3 (a) N1s XPS spectrum of CNW, (b) N1s XPS, (c) Raman and (d) FTIR spectra of GCNW.
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carbon nanochain webs are both introduced to GCNW. The introduction of N heteroatom also leads to the decrease of the intensities of D-band and G-band for GCNW, and the reduction of the order degree for GCNW. The other two peaks for GCNW at about 2700 cm 1 and 2987 cm 1 can be ascribed to the 2D and D+G modes, respectively. The D+G/2D intensity ratio for GCNW decreases significantly in comparison with that of pure graphene, suggesting that the defect concentration of GCNW is much reduced. Additionally, the shape and intensity of the 2D peak are often used to identify the number of layers for graphene. According to A. Gupta et al.,23 the number of the layers of graphene for GCNW is less than that of pure graphene, indicating that carbon nanochain webs serve as the isolators to prevent the overlap of graphene layers. Curve fits of the four bands for the G, GCNW and CNW samples are presented to further investigate the structure of the carbon materials (Fig. S2, ESI†). The full width at half maximum (FWHM) values corresponding to each bond are illustrated in Table S2 (ESI†), which indentifies the combination of carbon nanofibers that enhances the disordered degree of GCNW.24 FTIR spectra of the graphene, CNW and GCNW are shown in Fig. 3d. For the pure graphene, the bands of O–H at around 1300 cm 1, O–H at 3490 cm 1, and CQO at 1760 cm 1 gradually disappear, clarifying that the peaks for oxygen functional groups are removed and the reduced graphene sheets are formed. For the two carbon nanochain-based composites, the bands at 1190 and 1560 cm 1 may be assigned to the stretching vibration of the doping state and vibrations C–N stretching, respectively. Fig. 4a illustrates the CV curves of GCNW in the 1st, 2nd and 50th cycles. It exhibits three apparent peaks at 0.8, 0.5 and 0.01 V in the first reduction process. The peak at around 0.8 V is due to the formation of the solid electrolyte interphase (SEI) films at the electrode–electrolyte interface. A pair of peaks at 0.5 V/0.3 V can be attributed to the reversible Li-ions insertion into/extraction from enclosed spaces that are confined by carbon nanochain webs and graphene.25,26 The peak at 0.01 V corresponds to the reversible Li-ions reaction with carbon materials. No irreversible peak is observed in the second and
fifties cycles, indicating the integrated SEI films have been formed and reserved in the 1st cycle. Fig. 4b demonstrates the first charge–discharge curves of the GCNW anode at various current densities. With the increase of the current density, the charge capacity of the anode decreases. It delivers the first discharge/charge capacities of 1967.8/1221.1 mA h g 1 at 0.05 A g 1. The irreversible capacity loss is mainly owing to the following factors: the high specific surface area resulting in the consumption of Li-ions for the formation of the SEI films; the irreversible Li-ion storage at the defects of the graphene; the irreversible Li-ion storage at the enclosed spaces confined by the carbon nanochain webs and the graphene. Even so, the anode still displays an outstanding cycleability and rate performance, as shown in Fig. 4c and d. The electrode retains a very high reversible capacity of 1103.2 mA h g 1 at 0.05 A g 1 after 50 cycles, which is higher than that of graphene (721.9 mA h g 1), CNW (728 mA h g 1) and almost three times the theoretical capacity of conventional graphite (372 mA h g 1). The coulombic efficiency of the electrode is above 95% besides the first cycle, indicating the irreversible capacity loss decrease significantly (Fig. S3, ESI†). Rate capability is another important factor for the use of Li-ion batteries in power applications. A good electrochemical energy storage device is required to provide its high specific capacities at a high current. When the currents increase to 0.1, 0.5, 1 and 5 A g 1, the anodes maintain the charge capacities of 938, 637, 568.4 and 230 mA h g 1 after 100 cycles at 0.1, 0.5, 1 and 5 A g 1, respectively. The values are higher than those of the graphene/carbon nanofiber electrode reported by Z. J. Fan et al.,20 which highlights the strong synergistic effect between carbon nanochain webs and graphene. The coulombic efficiency of GCNW in the first cycle increases with the increase of the current density. It is due to some side reactions that did not occur at fast ion transportation and the incomplete decomposition of the electrolyte during the formation of SEI film. After the first cycle, the GCNW electrode demonstrates highly reversible behavior at each current density indicating the super cycleability and rate ability of the GCNW electrode (Fig. S4, ESI†). Scheme 2 is the schematic illustration of the electronic transport route and the volume effect of GCNW in the Li-ion insertion–extraction process. The introduction of graphene limits the volume expansion of 1D carbon nanochains. The huge surface
Fig. 4 (a) Cyclic voltammograms, (b) initial charge–discharge, (c) cycling abilities and (d) rate capability plots of GCNW.
Scheme 2 Schematic illustration of the volume change of GCNW during Li-ion insertion–extraction process.
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Phys. Chem. Chem. Phys., 2014, 16, 10429--10432 | 10431
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of graphene leads to sufficient electrode/electrolyte interface to absorb Li-ions and promotes rapid charge-transfer reaction. Huge volume expansion would be effectively alleviated by graphene, when Li+ inserts into the carbon nanochain webs. Cross-linked carbon nanochain webs derived from PPy provide a continuous pathway for electron transport and reduce the transport length of Li-ions.27 N doping in the anode enhances the electrochemical reactivity, which additionally contributes to the exceptional performance. Large numbers of spaces enclosed by graphene and carbon nanochain webs act as storage for Li-ions. Furthermore, cross-linked carbon nanochain webs between the graphene layers facilitate the formation of 3D electronic conductive networks and expand the gap between graphene layers which in turn allow easier migration of Li-ions. Therefore, the unique nano–micro structure ensures the structural stability and provides considerable capacities during Li-ion insertion–extraction process. A simple and practical synthesis route for the fabrication of graphene–carbon nanochain webs is developed via in situ polymerization and carbonization. Carbon nanochain webs are inserted between graphene layers to separate the neighboring graphene. The spaces enclosed by carbon nanochains and graphene are beneficial for ion/electron transfer and the sufficient contact between active materials and electrolyte. Moreover, carbonization of PPy is favorable for efficiently preventing volume expansion/shrinkage. The synergistic effect between carbon nanochain webs and graphene sheets improves the cycling stability and rate capability of the electrode. This is very meaningful both for fundamental study of the anode materials and for industrial application. This work is financially supported by Nature Science Foundation of China. (No: 51204209, 51274240).
References 1 P. G. Bruce, B. Scrosati and J. M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930. 2 T. M. Wu, H. L. Chang and Y. W. Lin, Compos. Sci. Technol., 2009, 69, 639. 3 W. Sun and X. Y. Chen, J. Power Sources, 2009, 193, 924. 4 S. Konwer, R. Boruah and S. K. Dolui, J. Electron. Mater., 2011, 40, 2248. 5 L. Qie, W. M. Chen, Z. H. Wang, Q. G. Shao, X. Li, L. X. Yuan, X. L. Hu, W. X. Zhang and Y. H. Huang, Adv. Mater., 2012, 24, 2047.
10432 | Phys. Chem. Chem. Phys., 2014, 16, 10429--10432
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6 C. C. Li, X. M. Yin, L. B. Chen, Q. H. Li and T. H. Wang, J. Phys. Chem. C, 2009, 113, 13438. 7 J. C. Claussen, A. Kumar, D. B. Jaroch, M. H. Khawaja, A. B. Hibbard, D. M. Porterfield and T. S. Fisher, Adv. Funct. Mater., 2012, 22, 3399. 8 F. Han, D. Li, W. C. Li, C. Lei, Q. Sun and A. H. Lu, Adv. Funct. Mater., 2013, 23, 1692. 9 Y. Yang, C. Y. Wang, B. B. Yue, S. Gambhir, C. O. Too and G. G. Wallace, Adv. Energy Mater., 2012, 2, 266. 10 G. P. Xiong, C. Z. Meng, R. G. Reifenberger, P. P. Irazoqui and T. S. Fisher, Adv. Energy Mater., 2014, 4, 1300515. 11 J. T. Zhang, P. Chen, B. H. L. Oh and M. B. Chan-Park, Nanoscale, 2013, 5, 9860. 12 Y. G. Zhang, Y. Zhao, A. Konarov, D. Gosselink, H. G. Soboleski and P. Chen, J. Power Sources, 2013, 241, 517. 13 A. D. Franklin and D. B. Janes, Appl. Phys. Lett., 2008, 92, 013122. 14 L. L. Zhang, S. Y. Zhao, X. N. Tian and X. S. Zhao, Langmuir, 2010, 26, 17624. 15 Y. F. Fan, Y. S. Liu, Q. Cai, Y. Z. Liu and J. M. Zhang, Synth. Met., 2012, 162, 1815. 16 C. Li and G. Shi, Nanoscale, 2012, 4, 5549. 17 J. Zhang and X. S. Zhao, J. Phys. Chem. C, 2012, 116, 5420. 18 D. Parviz, S. Das, H. S. T. Ahmed, F. Irin, S. Bhattacharia and M. J. Green, ACS Nano, 2012, 6, 8857. 19 C. Wu, X. Huang, G. Wang, L. Lv, G. Chen, G. Li and P. Jiang, Adv. Funct. Mater., 2013, 23, 506. 20 Z. J. Fan, J. Yan, T. Wei, G. Q. Ning, L. J. Zhi, J. C. Liu, D. X. Cao, G. L. Wang and F. Wei, ACS Nano, 2011, 5, 2787. 21 L. G. Bulusheva, A. V. Okotrub, A. G. Kurenya, H. K. Zhang, H. J. Zhang, X. H. Chen and H. H. Song, Carbon, 2011, 49, 4013. 22 L. M. Malard, M. A. Pimenta, G. Dresselhaus and M. S. Dresselhaus, Phys. Rep., 2009, 473, 51. 23 A. Gupta, G. Chen, P. Joshi, S. Tadigadapa and P. C. Eklund, Nano Lett., 2006, 6, 2667. 24 A. Sadezky, H. Muckenhuber, H. Grothe, R. Niessner and ¨schl, Carbon, 2005, 43, 1731. U. Po 25 G. X. Wang, X. P. Shen, J. Yao and J. Park, Carbon, 2009, 47, 2049. 26 X. Y. Zhou, J. J. Tang, J. Yang, J. Xie and B. Huang, J. Mater. Chem. A, 2013, 1, 5037. 27 H. R. Byon, B. M. Gallant, S. W. Lee and Y. Shao-Horn, Adv. Funct. Mater., 2012, 23, 1037.
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