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Cite this: Chem. Commun., 2014, 50, 228 Received 13th September 2013, Accepted 28th October 2013
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In situ grafted carbon on sawtooth-like SiC supported Ni for high-performance supercapacitor electrodes† Song Xie,ab Xiao-Ning Guo,a Guo-Qiang Jin,a Xi-Li Tong,a Ying-Yong Wanga and Xiang-Yun Guo*a
DOI: 10.1039/c3cc47019a www.rsc.org/chemcomm
A novel C–Ni–SiC composite using sawtooth-like SiC as support and carbon as modified material was prepared by hydrothermal synthesis and thermochemical pyrolysis. As a supercapacitor electrode, it exhibits very high specific capacitance (1780 F g 1) and excellent cycling performance (>96% for 2500 cycles).
Supercapacitors, as one of the most promising energy-storage devices, have attracted a great deal of attention because of their higher power density, shorter charging times, longer cycle lifetimes and smaller safety concerns compared to secondary batteries.1,2 Based on the charge and discharge mechanism, there are two kinds of supercapacitors.3 The first comprises electric double layer capacitors (EDLCs), which are based on the charge separation at the electrode–electrolyte interface. Porous carbon, carbon nanotubes, graphene etc. are widely used as the electrode materials (EMs) of EDLCs.4,5 The second comprises pseudocapacitors, which are based on the fast and reversible Faradaic redox reactions at or near the surface of active materials. Their EMs include various kinds of transition-metal oxides (RuO2, NiO, Co3O4, MnO2 etc.) and conductive polymers.6,7 Among these extensively studied EMs, pseudocapacitive nickel oxides or hydroxides have drawn considerable interest due to their high theoretical specific capacitance (B2584 for NiO and 2082 F g 1 for Ni(OH)2 within 0.5 V), abundant resources, better environment compatibility, and lower cost as compared to the state-of-the-art RuO2.8 For reported Ni-based EMs, however, the practical specific capacitance (SC) is much lower than the theoretical value, or the cycle life is less than satisfactory, depending on the ion transmission rate, electronic conductivity, electroactive area and structure stability.9,10 Therefore, it is necessary and meaningful to further develop novel Ni-based EMs with high SC and high cycle stability.
Due to its good thermal stability, mechanical strength, and chemical inertness, SiC has been extensively used to support active metal components in EMs.11 In this communication, we employ sawtooth-like SiC as the supporting material for nickel. The unique physicochemical properties and surface structure of SiC are expected to better disperse active particles and protect them from aggregation during reaction. The grafting of carbon on Ni/SiC is designed to improve the electron and ion transmission ability. The in situ formation of nickel nanoparticles from the Ni(OH)2 film can provide them with a small and uniform size. As a supercapacitor EM, the as-prepared C–Ni–SiC composite exhibits high SC, rapid charge–discharge capacity and excellent cycling stability. Moreover, the SiC used in EM is eco-friendly and can be recycled. Scheme 1 shows a schematic diagram of the preparation of the C–Ni–SiC composite. Generally, it involves hydrothermal growth of Ni(OH)2 on sawtooth-like SiC, forming NiO/SiC through calcination and grafting carbon on NiO/SiC by pyrolysis of methane. The XRD patterns of the as-prepared NiO/SiC and C–Ni–SiC composites are shown in Fig. 1. For NiO/SiC, the peak at 2y = 43.31 corresponds to the NiO (200) plane. The remaining
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Taiyuan 030001, PR China. E-mail: [email protected]; Fax: +86 351 4065282; Tel: +86 351 4050320 b University of Chinese Academy of Sciences, Beijing 100039, PR China † Electronic supplementary information (ESI) available: Detailed experimental procedure, TEM and SEM images, CV and charge–discharge curves, Nyquist plot and cycle stability comparison table. See DOI: 10.1039/c3cc47019a
228 | Chem. Commun., 2014, 50, 228--230
Scheme 1 Schematic diagram of the preparation of the C–Ni–SiC composite.
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Fig. 1
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XRD patterns of C–Ni–SiC and NiO/SiC samples.
Fig. 3
Fig. 2 Low (A, B) and high (C, D) magnification TEM images of NiO/SiC sample.
diffraction peaks are consistent with the planes of b-SiC (JCPDS card 29-1129). For C–Ni–SiC, the diffraction peaks at 2y = 44.51 and 51.81, correspond to the planes (111) and (200) of metallic Ni, respectively. The peak at 2y = 26.31 is associated with the (002) plane of the graphite-like structure of carbon.12 Fig. 2 shows typical TEM images of the NiO/SiC sample. It can be seen from Fig. 2A and B that NiO flakes are homogeneously grown on the SiC support with a tremella-like structure. Fig. 2C further reveals that the NiO flakes are very thin and consist of interlinked nanoparticles. The sizes of these nanoparticles range from 2 to 5 nm (Fig. 2D). These nanoparticles are formed during the dehydration process of Ni(OH)2.13
This journal is © The Royal Society of Chemistry 2014
TEM (A) and HRTEM (B–D) images of C–Ni–SiC sample.
The TEM images of the C–Ni–SiC sample are shown in Fig. 3. From Fig. 3A, it can be seen that the nanoparticles are well dispersed on the sawtooth-like nanorods. The sizes of these nanoparticles are in the range of 5–10 nm, about two times those of previous NiO nanoparticles. From the HRTEM image of the C–Ni–SiC nanostructure shown in Fig. 3B, the lattice spacing of 0.25 nm corresponds to the SiC (111) crystal plane. The figure also presents a nanoparticle with a core–shell structure. The core shows a lattice spacing of 0.20 nm, corresponding to the Ni (111) crystal plane, and the shell shows a lattice spacing of 0.34 nm, corresponding to the carbon (002) crystal plane. Therefore, the increased size of the nanoparticles can be attributed to the coating of carbon. In addition, the aggregation of nanoparticles also contributes to the size increment. Fig. 3C shows an in situ grown carbon nanotube on SiC. Fig. 3D reveals the lattice spacings of 0.20 nm and 0.25 nm, which correspond to the Ni (111) and SiC (111) crystal planes, respectively. These characteristics indicate that the carbon is successfully grafted onto the Ni/SiC nanostructure. Moreover, it can be seen from Fig. 3B and D that the nickel nanoparticles and carbon are fixed well on SiC (more images of the C–Ni–SiC structure can be found in Fig. S3 in the ESI†). The electrochemical measurements were performed using a three-electrode beaker cell, consisting of a C–Ni–SiC working electrode, a platinum sheet counter electrode, and a saturated calomel reference electrode. Cyclic voltammetry and galvanostatic charge–discharge cycling were carried out in 1 M KOH solution at room temperature. Fig. 4A shows the cyclic voltammogram (CV) of C–Ni–SiC at a scan rate of 5 mV s 1. (The CV of NiO/SiC is shown in Fig. S4 of the ESI†.) In the figure, a pair of redox peaks with symmetrical shape can be observed, which are resulted from the reversible process of insertion and extraction of OH anions according to eqn (1).
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Fig. 4 (A) CV of C–Ni–SiC at a scan rate of 5 mV s 1; (B) charge– discharge voltage profiles of C–Ni–SiC at various current densities; (C) calculated capacitance as a function of current density according to the data in (B); (D) the capacitance as a function of cycle number at a constant current density of 52.2 A g 1; inset shows the galvanostatic charge–discharge cyclic curves of the first and last 6 cycles.
Ni(OH)2 + OH 2 NiOOH + H2O + e
(1)
Fig. 4B shows the galvanostatic charge–discharge curves of C–Ni–SiC at various current densities from 0 to 0.5 V. The corresponding SCs are presented in Fig. 4C, which are calculated using the literature method.14 As shown in Fig. 4C, the C–Ni–SiC electrode exhibits a high value of 1780 F g 1 at a charge–discharge current density of 8.7 A g 1. This value has achieved 85% of the theoretical SC of Ni(OH)2. Upon increase of current density, the SC displays a decrease possibly due to the insufficient Faradic redox reaction time under high discharge rates. It is demonstrated that the redox of Ni(II) 2 Ni(III) is a diffusion controlled process.15 Therefore, the reaction under a high discharge rate is restricted by the ion and electron transmission rates. It is noteworthy that a charge–discharge current density of 69.6 A g 1 is larger than most of the reported values.1,2,5,16,17 Moreover at such a high discharge current, the SC of our sample still maintains 46% (812 F g 1) of its original value, better than in the literature.18,19 Fig. 4D shows the cyclic galvanostatic charge–discharge voltage profiles of C–Ni–SiC electrode performed at 52.2 A g 1 (60 mA cm 2). As shown in the inset, the curves for the first 6 and last 6 cycles almost keep the same shape and symmetry, indicating a good reversibility. In the figure, the capacitance presents a decrease for the initial 300 cycles (retaining ca. 89% of its initial value). Because the nickel is coated by carbon or filled in the bottom (top) of carbon nanotubes, the decrease of capacitance is possibly due to the wettability issues, which will lead to the loss of electrical contact between these nanoparticles and current.9,20 The subsequent increase in SC can be related to the improvement of wetting for the active particles
230 | Chem. Commun., 2014, 50, 228--230
during extended cycling.21 After 2500 cycles, the supercapacitor displays an excellent, long cycle life with only 4% deterioration of its initial SC. Moreover, for the last 1000 cycles, the capacitance loss is only 0.4%, demonstrating superior long-term electrochemical stability. The TEM image of C–Ni–SiC (Fig. S5 in the ESI†) after 2500 charge–discharge cycles shows that the active nanoparticles keep good dispersity with slight aggregation, suggesting a good structural stability of the material. In addition, such cycling performance is comparable to those of some other excellent supercapacitors (Table S1 in the ESI†). In summary, a novel C–Ni–SiC composite was prepared using a simple hydrothermal synthesis and thermochemical pyrolysis method. As a supercapacitor EM, it exhibits very high SC (1780 F g 1) and excellent cycling stability (>96%) due to the special dispersion effect of sawtooth-like SiC, suitable size of active particles and excellent conductivity of carbon. Moreover, such a simple and universal fabrication method can also be extended to prepare other catalysts or electrode materials, such as C–Pt/SiC for fuel cells, C–MnO2/SiC for capacitors or Li-ion batteries, and C–ZnO/SiC for sensors. This work was financially supported by SKLCC (2013BWZ006).
Notes and references 1 G. Yu, L. Hu, N. Liu, H. Wang, M. Vosgueritchian, Y. Yang, Y. Cui and Z. Bao, Nano Lett., 2011, 11, 4438–4442. 2 G. Zhang and X. W. D. Lou, Adv. Mater., 2013, 25, 976–979. 3 J. H. Kim, K. Zhu, Y. Yan, C. L. Perkins and A. J. Frank, Nano Lett., 2010, 10, 4099–4104. 4 C. Liu, Z. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett., 2010, 10, 4863–4868. 5 B. Xu, H. Duan, M. Chu, G. Cao and Y. Yang, J. Mater. Chem. A, 2013, 1, 4565–4570. 6 Z. S. Wu, D. W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li and H. M. Cheng, Adv. Funct. Mater., 2010, 20, 3595–3602. 7 K. Zhang, L. L. Zhang, X. S. Zhao and J. Wu, Chem. Mater., 2010, 22, 1392–1401. 8 B. Zhao, J. Song, P. Liu, W. Xu, T. Fang, Z. Jiao, H. Zhang and Y. Jiang, J. Mater. Chem., 2011, 21, 18792–18798. 9 G. Hu, C. Li and H. Gong, J. Power Sources, 2010, 195, 6977–6981. 10 X. Wu, W. Xing, L. Zhang, S. Zhuo, J. Zhou, G. Wang and S. Qiao, Powder Technol., 2012, 224, 162–167. 11 L. Fang, X. P. Huang, F. J. Vidal-Iglesias, Y. P. Liu and X. L. Wang, Electrochem. Commun., 2011, 13, 1309–1312. 12 G. Q. Guo, F. Qin, D. Yang, C. C. Wang, H. L. Xu and S. Yang, Chem. Mater., 2008, 20, 2291–2297. 13 L. P. Zhu, G. H. Liao, Y. Yang, H. M. Xiao, J. F. Wang and S. Y. Fu, Nanoscale Res. Lett., 2009, 4, 550–557. 14 C. Yuan, J. Li, L. Hou, X. Zhang, L. Shen and X. W. D. Lou, Adv. Funct. Mater., 2012, 22, 4592–4597. 15 S. Xie, X. L. Tong, G. Q. Jin, Y. Qin and X. Y. Guo, J. Mater. Chem. A, 2013, 1, 2104–2109. 16 J. H. Kim, S. H. Kang, K. Zhu, J. Y. Kim, N. R. Neale and A. J. Frank, Chem. Commun., 2011, 47, 5214–5216. 17 Z. Gao, J. Wang, Z. Li, W. Yang, B. Wang, M. Hou, Y. He, Q. Liu, T. Mann, P. Yang, M. Zhang and L. Liu, Chem. Mater., 2011, 23, 3509–3516. 18 G. W. Yang, C. L. Xu and H. L. Li, Chem. Commun., 2008, 6537–6539. 19 Z. Lu, Z. Chang, W. Zhu and X. Sun, Chem. Commun., 2011, 47, 9651–9653. 20 J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi and F. Wei, Adv. Funct. Mater., 2012, 22, 2632–2641. 21 V. Ganesh, S. Pitchumani and V. Lakshminarayanan, J. Power Sources, 2006, 158, 1523–1532.
This journal is © The Royal Society of Chemistry 2014
Published on 28 October 2013. Downloaded by Dalhousie University on 14/07/2014 03:51:01.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 228 Received 13th September 2013, Accepted 28th October 2013
View Article Online View Journal | View Issue
In situ grafted carbon on sawtooth-like SiC supported Ni for high-performance supercapacitor electrodes† Song Xie,ab Xiao-Ning Guo,a Guo-Qiang Jin,a Xi-Li Tong,a Ying-Yong Wanga and Xiang-Yun Guo*a
DOI: 10.1039/c3cc47019a www.rsc.org/chemcomm
A novel C–Ni–SiC composite using sawtooth-like SiC as support and carbon as modified material was prepared by hydrothermal synthesis and thermochemical pyrolysis. As a supercapacitor electrode, it exhibits very high specific capacitance (1780 F g 1) and excellent cycling performance (>96% for 2500 cycles).
Supercapacitors, as one of the most promising energy-storage devices, have attracted a great deal of attention because of their higher power density, shorter charging times, longer cycle lifetimes and smaller safety concerns compared to secondary batteries.1,2 Based on the charge and discharge mechanism, there are two kinds of supercapacitors.3 The first comprises electric double layer capacitors (EDLCs), which are based on the charge separation at the electrode–electrolyte interface. Porous carbon, carbon nanotubes, graphene etc. are widely used as the electrode materials (EMs) of EDLCs.4,5 The second comprises pseudocapacitors, which are based on the fast and reversible Faradaic redox reactions at or near the surface of active materials. Their EMs include various kinds of transition-metal oxides (RuO2, NiO, Co3O4, MnO2 etc.) and conductive polymers.6,7 Among these extensively studied EMs, pseudocapacitive nickel oxides or hydroxides have drawn considerable interest due to their high theoretical specific capacitance (B2584 for NiO and 2082 F g 1 for Ni(OH)2 within 0.5 V), abundant resources, better environment compatibility, and lower cost as compared to the state-of-the-art RuO2.8 For reported Ni-based EMs, however, the practical specific capacitance (SC) is much lower than the theoretical value, or the cycle life is less than satisfactory, depending on the ion transmission rate, electronic conductivity, electroactive area and structure stability.9,10 Therefore, it is necessary and meaningful to further develop novel Ni-based EMs with high SC and high cycle stability.
Due to its good thermal stability, mechanical strength, and chemical inertness, SiC has been extensively used to support active metal components in EMs.11 In this communication, we employ sawtooth-like SiC as the supporting material for nickel. The unique physicochemical properties and surface structure of SiC are expected to better disperse active particles and protect them from aggregation during reaction. The grafting of carbon on Ni/SiC is designed to improve the electron and ion transmission ability. The in situ formation of nickel nanoparticles from the Ni(OH)2 film can provide them with a small and uniform size. As a supercapacitor EM, the as-prepared C–Ni–SiC composite exhibits high SC, rapid charge–discharge capacity and excellent cycling stability. Moreover, the SiC used in EM is eco-friendly and can be recycled. Scheme 1 shows a schematic diagram of the preparation of the C–Ni–SiC composite. Generally, it involves hydrothermal growth of Ni(OH)2 on sawtooth-like SiC, forming NiO/SiC through calcination and grafting carbon on NiO/SiC by pyrolysis of methane. The XRD patterns of the as-prepared NiO/SiC and C–Ni–SiC composites are shown in Fig. 1. For NiO/SiC, the peak at 2y = 43.31 corresponds to the NiO (200) plane. The remaining
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Taiyuan 030001, PR China. E-mail: [email protected]; Fax: +86 351 4065282; Tel: +86 351 4050320 b University of Chinese Academy of Sciences, Beijing 100039, PR China † Electronic supplementary information (ESI) available: Detailed experimental procedure, TEM and SEM images, CV and charge–discharge curves, Nyquist plot and cycle stability comparison table. See DOI: 10.1039/c3cc47019a
228 | Chem. Commun., 2014, 50, 228--230
Scheme 1 Schematic diagram of the preparation of the C–Ni–SiC composite.
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Fig. 1
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XRD patterns of C–Ni–SiC and NiO/SiC samples.
Fig. 3
Fig. 2 Low (A, B) and high (C, D) magnification TEM images of NiO/SiC sample.
diffraction peaks are consistent with the planes of b-SiC (JCPDS card 29-1129). For C–Ni–SiC, the diffraction peaks at 2y = 44.51 and 51.81, correspond to the planes (111) and (200) of metallic Ni, respectively. The peak at 2y = 26.31 is associated with the (002) plane of the graphite-like structure of carbon.12 Fig. 2 shows typical TEM images of the NiO/SiC sample. It can be seen from Fig. 2A and B that NiO flakes are homogeneously grown on the SiC support with a tremella-like structure. Fig. 2C further reveals that the NiO flakes are very thin and consist of interlinked nanoparticles. The sizes of these nanoparticles range from 2 to 5 nm (Fig. 2D). These nanoparticles are formed during the dehydration process of Ni(OH)2.13
This journal is © The Royal Society of Chemistry 2014
TEM (A) and HRTEM (B–D) images of C–Ni–SiC sample.
The TEM images of the C–Ni–SiC sample are shown in Fig. 3. From Fig. 3A, it can be seen that the nanoparticles are well dispersed on the sawtooth-like nanorods. The sizes of these nanoparticles are in the range of 5–10 nm, about two times those of previous NiO nanoparticles. From the HRTEM image of the C–Ni–SiC nanostructure shown in Fig. 3B, the lattice spacing of 0.25 nm corresponds to the SiC (111) crystal plane. The figure also presents a nanoparticle with a core–shell structure. The core shows a lattice spacing of 0.20 nm, corresponding to the Ni (111) crystal plane, and the shell shows a lattice spacing of 0.34 nm, corresponding to the carbon (002) crystal plane. Therefore, the increased size of the nanoparticles can be attributed to the coating of carbon. In addition, the aggregation of nanoparticles also contributes to the size increment. Fig. 3C shows an in situ grown carbon nanotube on SiC. Fig. 3D reveals the lattice spacings of 0.20 nm and 0.25 nm, which correspond to the Ni (111) and SiC (111) crystal planes, respectively. These characteristics indicate that the carbon is successfully grafted onto the Ni/SiC nanostructure. Moreover, it can be seen from Fig. 3B and D that the nickel nanoparticles and carbon are fixed well on SiC (more images of the C–Ni–SiC structure can be found in Fig. S3 in the ESI†). The electrochemical measurements were performed using a three-electrode beaker cell, consisting of a C–Ni–SiC working electrode, a platinum sheet counter electrode, and a saturated calomel reference electrode. Cyclic voltammetry and galvanostatic charge–discharge cycling were carried out in 1 M KOH solution at room temperature. Fig. 4A shows the cyclic voltammogram (CV) of C–Ni–SiC at a scan rate of 5 mV s 1. (The CV of NiO/SiC is shown in Fig. S4 of the ESI†.) In the figure, a pair of redox peaks with symmetrical shape can be observed, which are resulted from the reversible process of insertion and extraction of OH anions according to eqn (1).
Chem. Commun., 2014, 50, 228--230 | 229
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Fig. 4 (A) CV of C–Ni–SiC at a scan rate of 5 mV s 1; (B) charge– discharge voltage profiles of C–Ni–SiC at various current densities; (C) calculated capacitance as a function of current density according to the data in (B); (D) the capacitance as a function of cycle number at a constant current density of 52.2 A g 1; inset shows the galvanostatic charge–discharge cyclic curves of the first and last 6 cycles.
Ni(OH)2 + OH 2 NiOOH + H2O + e
(1)
Fig. 4B shows the galvanostatic charge–discharge curves of C–Ni–SiC at various current densities from 0 to 0.5 V. The corresponding SCs are presented in Fig. 4C, which are calculated using the literature method.14 As shown in Fig. 4C, the C–Ni–SiC electrode exhibits a high value of 1780 F g 1 at a charge–discharge current density of 8.7 A g 1. This value has achieved 85% of the theoretical SC of Ni(OH)2. Upon increase of current density, the SC displays a decrease possibly due to the insufficient Faradic redox reaction time under high discharge rates. It is demonstrated that the redox of Ni(II) 2 Ni(III) is a diffusion controlled process.15 Therefore, the reaction under a high discharge rate is restricted by the ion and electron transmission rates. It is noteworthy that a charge–discharge current density of 69.6 A g 1 is larger than most of the reported values.1,2,5,16,17 Moreover at such a high discharge current, the SC of our sample still maintains 46% (812 F g 1) of its original value, better than in the literature.18,19 Fig. 4D shows the cyclic galvanostatic charge–discharge voltage profiles of C–Ni–SiC electrode performed at 52.2 A g 1 (60 mA cm 2). As shown in the inset, the curves for the first 6 and last 6 cycles almost keep the same shape and symmetry, indicating a good reversibility. In the figure, the capacitance presents a decrease for the initial 300 cycles (retaining ca. 89% of its initial value). Because the nickel is coated by carbon or filled in the bottom (top) of carbon nanotubes, the decrease of capacitance is possibly due to the wettability issues, which will lead to the loss of electrical contact between these nanoparticles and current.9,20 The subsequent increase in SC can be related to the improvement of wetting for the active particles
230 | Chem. Commun., 2014, 50, 228--230
during extended cycling.21 After 2500 cycles, the supercapacitor displays an excellent, long cycle life with only 4% deterioration of its initial SC. Moreover, for the last 1000 cycles, the capacitance loss is only 0.4%, demonstrating superior long-term electrochemical stability. The TEM image of C–Ni–SiC (Fig. S5 in the ESI†) after 2500 charge–discharge cycles shows that the active nanoparticles keep good dispersity with slight aggregation, suggesting a good structural stability of the material. In addition, such cycling performance is comparable to those of some other excellent supercapacitors (Table S1 in the ESI†). In summary, a novel C–Ni–SiC composite was prepared using a simple hydrothermal synthesis and thermochemical pyrolysis method. As a supercapacitor EM, it exhibits very high SC (1780 F g 1) and excellent cycling stability (>96%) due to the special dispersion effect of sawtooth-like SiC, suitable size of active particles and excellent conductivity of carbon. Moreover, such a simple and universal fabrication method can also be extended to prepare other catalysts or electrode materials, such as C–Pt/SiC for fuel cells, C–MnO2/SiC for capacitors or Li-ion batteries, and C–ZnO/SiC for sensors. This work was financially supported by SKLCC (2013BWZ006).
Notes and references 1 G. Yu, L. Hu, N. Liu, H. Wang, M. Vosgueritchian, Y. Yang, Y. Cui and Z. Bao, Nano Lett., 2011, 11, 4438–4442. 2 G. Zhang and X. W. D. Lou, Adv. Mater., 2013, 25, 976–979. 3 J. H. Kim, K. Zhu, Y. Yan, C. L. Perkins and A. J. Frank, Nano Lett., 2010, 10, 4099–4104. 4 C. Liu, Z. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett., 2010, 10, 4863–4868. 5 B. Xu, H. Duan, M. Chu, G. Cao and Y. Yang, J. Mater. Chem. A, 2013, 1, 4565–4570. 6 Z. S. Wu, D. W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li and H. M. Cheng, Adv. Funct. Mater., 2010, 20, 3595–3602. 7 K. Zhang, L. L. Zhang, X. S. Zhao and J. Wu, Chem. Mater., 2010, 22, 1392–1401. 8 B. Zhao, J. Song, P. Liu, W. Xu, T. Fang, Z. Jiao, H. Zhang and Y. Jiang, J. Mater. Chem., 2011, 21, 18792–18798. 9 G. Hu, C. Li and H. Gong, J. Power Sources, 2010, 195, 6977–6981. 10 X. Wu, W. Xing, L. Zhang, S. Zhuo, J. Zhou, G. Wang and S. Qiao, Powder Technol., 2012, 224, 162–167. 11 L. Fang, X. P. Huang, F. J. Vidal-Iglesias, Y. P. Liu and X. L. Wang, Electrochem. Commun., 2011, 13, 1309–1312. 12 G. Q. Guo, F. Qin, D. Yang, C. C. Wang, H. L. Xu and S. Yang, Chem. Mater., 2008, 20, 2291–2297. 13 L. P. Zhu, G. H. Liao, Y. Yang, H. M. Xiao, J. F. Wang and S. Y. Fu, Nanoscale Res. Lett., 2009, 4, 550–557. 14 C. Yuan, J. Li, L. Hou, X. Zhang, L. Shen and X. W. D. Lou, Adv. Funct. Mater., 2012, 22, 4592–4597. 15 S. Xie, X. L. Tong, G. Q. Jin, Y. Qin and X. Y. Guo, J. Mater. Chem. A, 2013, 1, 2104–2109. 16 J. H. Kim, S. H. Kang, K. Zhu, J. Y. Kim, N. R. Neale and A. J. Frank, Chem. Commun., 2011, 47, 5214–5216. 17 Z. Gao, J. Wang, Z. Li, W. Yang, B. Wang, M. Hou, Y. He, Q. Liu, T. Mann, P. Yang, M. Zhang and L. Liu, Chem. Mater., 2011, 23, 3509–3516. 18 G. W. Yang, C. L. Xu and H. L. Li, Chem. Commun., 2008, 6537–6539. 19 Z. Lu, Z. Chang, W. Zhu and X. Sun, Chem. Commun., 2011, 47, 9651–9653. 20 J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi and F. Wei, Adv. Funct. Mater., 2012, 22, 2632–2641. 21 V. Ganesh, S. Pitchumani and V. Lakshminarayanan, J. Power Sources, 2006, 158, 1523–1532.
This journal is © The Royal Society of Chemistry 2014
