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MnO nanoparticles anchored on graphene nanosheets via in situ carbothermal reduction as high-performance anode materials for lithium-ion batteries
Materials Letters 84 (2012) 9–12
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
MnO nanoparticles anchored on graphene nanosheets via in situ carbothermal reduction as high-performance anode materials for lithium-ion batteries Danfeng Qiu, Luyao Ma, Mingbo Zheng n, Zixia Lin, Bin Zhao, Zhe Wen, Zibo Hu, Lin Pu, Yi Shi Nanjing National Laboratory of Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
abstract
Article history: Received 5 May 2012 Accepted 11 June 2012 Available online 18 June 2012
A MnO nanoparticle/graphene composite was prepared via in situ carbothermal reduction of Mn3O4 on the surface of graphene nanosheets. The formed MnO nanoparticles with diameters ranging from 20 nm to 250 nm integrated tightly with the graphene nanosheets. As anode material for lithium-ion batteries, the nanocomposite showed a high specific capacity of approximately 700 mA h g 1 at 100 mA g 1, excellent cyclic stability, and good rate capability. During the charge-discharge process, graphene nanosheets served as a three-dimensional conductive network for MnO nanoparticles. Furthermore, the detachment and agglomeration of MnO nanoparticles were effectively prevented due to the tight combination of MnO nanoparticles and graphene. & 2012 Elsevier B.V. All rights reserved.
Keywords: MnO Graphene Nanoparticles Nanocomposites Energy storage and conversion Lithium-ion battery
Introduction Transition metal oxides have long been considered as anode materials for lithium-ion batteries (LIBs) because of their high capacities [1]. However, the large volume change of metal oxides during the Li insertion/extraction process causes mechanical degradation and results in the rapid loss of capacity. Moreover, the low conductivity of metal oxides further hastens the degradation process. To solve these problems, carbonaceous materials with high electrical conductivity and good stability can be used as matrices for metal oxides [2–5]. Among various metal oxides, manganese monoxide (MnO) is extremely attractive because of its relatively low electrochemical motivation force (1.032 V vs. Li/Li þ ), small overpotential, low cost, and environmental benignity [6–8]. Recently, MnO/carbon composite materials, such as coaxial MnO/carbon nanotubes [9], MnO/carbon core–shell nanorods [10], and MnO cubic particles/carbon composite [11], have been reported as anode materials for LIBs. The results showed that the combination of MnO and carbon could improve the electrochemical performance. Graphene is an excellent matrix on which to anchor LIB active materials due to its superior electrical conductivity, excellent mechanical flexibility, and good chemical stability [12,13]. Recently, Zhang et al. reported the synthesis of nitrogen-doped
n
Corresponding author. Tel.: þ86 25 83621220; fax: þ86 25 83621220. E-mail address: [email protected] (M. Zheng).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.045
MnO/graphene hybrid material for LIBs via a hydrothermal method followed by ammonia annealing at 800 1C [14]. In the present work, MnO nanoparticle/graphene composite was produced via in situ carbothermal reduction of Mn3O4 on the surface of graphene nanosheets (GNS). The MnO/GNS nanocomposite showed high performance as an anode material for LIBs.
Experimental GNS was fabricated via the thermal exfoliation method described in our previous work [15]. Briefly, graphite oxide was thermally exfoliated at 300 1C for 3 min in air, and subsequently treated at 900 1C for 3 h in Ar. In a typical synthesis of MnO/GNS nanocomposite, 1.51 g of Mn(NO3)2 aqueous solution (50 wt%) was mixed with 20 mL of enthanol. 100 mg of GNS was added into the solution and then ultrasonically treated for 10 min. The suspension solution was mixed using a magnetic stirrer in a ventilation cabinet; the ethanol in the solution evaporated continuously. Lastly, dried Mn(NO3)2/GNS composite was collected and treated at 700 1C for 5 h in Ar. The final MnO content in MnO/GNS was about 75% by weight. In the control experiments, simplex Mn3O4 sample was prepared by heating Mn(NO3)2 at 700 1C for 5 h in Ar, whereas Mn3O4/GNS nanocomposite was obtained by heating Mn(NO3)2/GNS at 500 1C for 5 h in Ar. The obtained samples were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
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D. Qiu et al. / Materials Letters 84 (2012) 9–12
Electrochemical measurements were performed using 2032type coin cells. 80 wt% active material, 10 wt% carbon black, and 10 wt% polyvinyldifluoride binder were mixed with N-methylpyrrolidone. The obtained slurry was pasted on a copper foil and dried in vacuum. Coin cells were assembled in an argon-filled glove box using an active material as the working electrode,
a Li foil as the counter electrode, 1 M LiPF6 in ethylene carbonate and diethyl carbonate (1:1 vol), and Celgard 2250 as the separator. Charge–discharge measurements were carried out galvanostatically over a voltage range of 0.01–3 V using a battery testing system (LAND, Wuhan Jinnuo Electronics). Furthermore, the tap density of electrode is 1.1 g/cm3.
Results and discussion
Fig 1. XRD patterns of the samples.
The XRD patterns of the samples are shown in Fig. 1. All the diffraction peaks of MnO/GNS can be indexed to the face-centered cubic phase of MnO (JCPDS 07–0230). All the peaks of simplex Mn3O4 are in good agreement with the standard profiles of Mn3O4 (JCPDS 24–0734). In addition, the XRD pattern of Mn3O4/GNS indicates that the sample is mainly composed of Mn3O4 and also consist of a few MnO (peaks indicated by red arrows). The above results showed that the MnO phase was not formed in the absence of graphene. Fig 2a–c present the SEM and TEM images of GNS, which show that GNS has a nanosheet structure. The SEM images also show the presence of abundant macropores within GNS, which facilitate the combination of Mn(NO3)2 and GNS. Fig 2d–f show the SEM images of MnO/GNS under different magnifications. MnO nanoparticles with diameters ranging from
Fig 2. SEM (a and b) and TEM (c) images of GNS; SEM (d–f) and TEM (g and h) images of MnO/GNS; (i) HRTEM image for a single MnO nanoparticle; SEM (j and k) and TEM (l) images of MnO/GNS after 60 cycles.
D. Qiu et al. / Materials Letters 84 (2012) 9–12
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Fig. 3. (a) Discharge and charge curves of MnO/GNS for the first and second cycles at 100 mA g 1; (b) representative charge and discharge curves of MnO/GNS at various current densities; (c) capacity retention of MnO/GNS at various current densities; (d) discharge capacity retention of three samples at 100 mA g 1; Inset of d shows the long-term cyclic performance of MnO/GNS at 1000 mA g 1 (the cell was first cycled at 200 mA g 1 for two cycles). The weight of MnO/GNS (or Mn3O4/GNS) in the working electrode was used to estimate the specific capacity.
20 nm to 250 nm integrated tightly with GNS. Fig 2g and h present the TEM images of the MnO/GNS, revealing the firm attachment of MnO nanoparticles to GNS even after ultrasonication treatment employed to disperse the sample for TEM observation. An HRTEM image of a single MnO nanoparticle is presented in Fig 2i. The lattice fringes with an inter-plane spacing of 0.2205 nm corresponded to the (2 0 0) plane of MnO. During the synthesis, the macropores within GNS were first filled with the Mn(NO3)2 solution. When ethanol evaporated, Mn(NO3)2 nucleated and grew on the GNS surface. Finally, solid Mn(NO3)2 4H2O dispersed uniformly on the GNS surface. During the heat-treatment process, Mn(NO3)2 4H2O was first transformed into Mn3O4. With the increase of the temperature, Mn3O4 reacted with graphene and was reduced into MnO. The mechanism is illustrated as follows: Mn3O4 þC-3MnOþCO. The resulting MnO nanoparticle tightly integrated with GNS due to it was obtained via the in situ carbothermal reduction of Mn3O4. Fig. 3a shows the charge and discharge curves of the MnO/GNS electrode for the first and second cycles at 100 mA g 1. The plateau at about 0.24 V in the first discharge curve was attributed to the reduction of MnO to Mn. In the subsequent cyclic process, the discharge plateau shifted to about 0.55 V, indicating the irreversible formation of crystalline metal nanoparticles and amorphous Li2O matrix [10]. The charge plateau appeared at about 1.2 V, which is much lower than those shown by other metal oxides and favorable for increasing the operation voltage and energy density when the electrode is used as an anode in full batteries [9]. Fig. 3b shows the representative charge and discharge curves of the MnO/GNS anode at various current densities. Fig. 3c shows the cyclic performance of MnO/GNS at various current densities. The cell was first cycled at 100 mA g 1 for 12 cycles, in which a stable specific capacity of about 670 mA h g 1 was observed. Even at 1000 mA g 1, more than 60% of the capacity is retained, indicating that MIO/GNS has good
rate capability. Fig. 3d shows the discharge capacity retentions of the samples at 100 mA g 1. The MnO/GNS had a capacity of 691 mA h g 1 after two cycles, and 782 mA h g 1 after 60 cycles. The electrochemical performance of MnO/GNS is much better than those of Mn3O4/GNS and simplex Mn3O4. The inset in Fig. 3d shows the long-term cyclic performance of MnO/GNS at 1000 mA g 1. The capacity remained at about 460 mA h g 1 after 180 cycles, indicating the excellent cyclic stability of the electrode. Further analysis of the morphology evolution of MnO/GNS after the discharge/charge cycles was performed by SEM and TEM observations. As shown in Fig. 2j–l, MnO nanoparticles remain integrated with GNS after 60 cycles at 100 mA g 1. The high performance of MnO/GNS was attributed to the tight combination of MnO nanoparticles and GNS. The GNS in the composite has a good electrical conductivity and serves as a three-dimensional conductive network for MnO nanoparticles, which decreases the inner resistance of LIBs and is favorable for stabilizing the electronic conductivity. Furthermore, because of the tight combination of MnO and GNS, the GNS not only provides an elastic buffer space to accommodate the volume expansion/contraction of MnO nanoparticles during Li insertion/extraction process but also efficiently prevents the detachment and agglomeration of MnO nanoparticles upon continuous cycling, thus maintaining the structural integrity and avoiding rapid loss of electrode capacity. For Mn3O4/GNS, the Mn3O4 nanoparticle did not tightly integrate with GNS. Thus, the cyclic performance of Mn3O4/GNS is obviously worse than that of MnO/GNS.
Conclusions In summary, a MnO/GNS nanocomposite has been fabricated as LIBs anode material via in situ carbothermal reduction on the
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D. Qiu et al. / Materials Letters 84 (2012) 9–12
GNS surface. MnO nanoparticles integrated tightly with GNS. The flexible GNS provided high electrical conductivity throughout the electrode and served as a mechanically strong framework. As a result, the nanocomposite had a large lithium storage capacity, excellent cyclic stability, and good rate capability.
Acknowledgments This work was supported by China Postdoctoral Science Foundation (No. 20100471296), Postdoctoral Foundation of Jiangsu Province (No. 1001003C), and National Nature Science Foundation of China (No. 60928009 and 61076017). References [1] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM. Nature 2000;407: 496–499. [2] Kang E, Jung YS, Cavanagh AS, Kim GH, George SM, Dillon AC, et al. Adv Funct Mater 2011;21:2430–2438.
[3] Liu J, Li Y, Fan H, Zhu Z, Jiang J, Ding R, et al. Chem Mater 2010;22:212–217. [4] Liu Z, Tay SW. Mater Lett 2012;72:74–77. [5] Zhu Y, Bai YJ, Han FD, Qi YX, Lun N, Yao B, et al. Mater Lett 2011;65: 3157–3159. [6] Yu XQ, He Y, Sun JP, Tang K, Li H, Chen LQ, et al. Electrochem Commun 2009;11:791–794. [7] Zhong K, Xia X, Zhang B, Li H, Wang Z, Chen L. J Power Sources 2010;195:3300–3308. [8] Kokubu T, Oaki Y, Hosono E, Zhou H, Imai H. Adv Funct Mater 2011;21: 3673–3680. [9] Ding YL, Wu CY, Yu HM, Xie J, Cao GS, Zhu TJ, et al. Electrochim Acta 2011;56:5844–5848. [10] Sun B, Chen Z, Kim HS, Ahn H, Wang G. J Power Sources 2011;196: 3346–3349. [11] Liu Y, Zhao X, Li F, Xia D. Electrochim Acta 2011;56:6448–6452. [12] Zhou G, Wang DW, Li F, Zhang L, Li N, Wu ZS, et al. Chem Mater 2010;22:5306–5313. [13] Zhou W, Zhu J, Cheng C, Liu J, Yang H, Cong C, et al. Energy Environ Sci 2011;4:4954–4961. [14] Zhang K, Han P, Gu L, Zhang L, Liu Z, Kong Q, et al. ACS Appl Mater Interfaces 2012;4:658–664. [15] Du QL, Zheng MB, Zhang LF, Wang YW, Chen JH, Xue LP, et al. Electrochim Acta 2010;55:3897–3903.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
MnO nanoparticles anchored on graphene nanosheets via in situ carbothermal reduction as high-performance anode materials for lithium-ion batteries Danfeng Qiu, Luyao Ma, Mingbo Zheng n, Zixia Lin, Bin Zhao, Zhe Wen, Zibo Hu, Lin Pu, Yi Shi Nanjing National Laboratory of Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
abstract
Article history: Received 5 May 2012 Accepted 11 June 2012 Available online 18 June 2012
A MnO nanoparticle/graphene composite was prepared via in situ carbothermal reduction of Mn3O4 on the surface of graphene nanosheets. The formed MnO nanoparticles with diameters ranging from 20 nm to 250 nm integrated tightly with the graphene nanosheets. As anode material for lithium-ion batteries, the nanocomposite showed a high specific capacity of approximately 700 mA h g 1 at 100 mA g 1, excellent cyclic stability, and good rate capability. During the charge-discharge process, graphene nanosheets served as a three-dimensional conductive network for MnO nanoparticles. Furthermore, the detachment and agglomeration of MnO nanoparticles were effectively prevented due to the tight combination of MnO nanoparticles and graphene. & 2012 Elsevier B.V. All rights reserved.
Keywords: MnO Graphene Nanoparticles Nanocomposites Energy storage and conversion Lithium-ion battery
Introduction Transition metal oxides have long been considered as anode materials for lithium-ion batteries (LIBs) because of their high capacities [1]. However, the large volume change of metal oxides during the Li insertion/extraction process causes mechanical degradation and results in the rapid loss of capacity. Moreover, the low conductivity of metal oxides further hastens the degradation process. To solve these problems, carbonaceous materials with high electrical conductivity and good stability can be used as matrices for metal oxides [2–5]. Among various metal oxides, manganese monoxide (MnO) is extremely attractive because of its relatively low electrochemical motivation force (1.032 V vs. Li/Li þ ), small overpotential, low cost, and environmental benignity [6–8]. Recently, MnO/carbon composite materials, such as coaxial MnO/carbon nanotubes [9], MnO/carbon core–shell nanorods [10], and MnO cubic particles/carbon composite [11], have been reported as anode materials for LIBs. The results showed that the combination of MnO and carbon could improve the electrochemical performance. Graphene is an excellent matrix on which to anchor LIB active materials due to its superior electrical conductivity, excellent mechanical flexibility, and good chemical stability [12,13]. Recently, Zhang et al. reported the synthesis of nitrogen-doped
n
Corresponding author. Tel.: þ86 25 83621220; fax: þ86 25 83621220. E-mail address: [email protected] (M. Zheng).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.045
MnO/graphene hybrid material for LIBs via a hydrothermal method followed by ammonia annealing at 800 1C [14]. In the present work, MnO nanoparticle/graphene composite was produced via in situ carbothermal reduction of Mn3O4 on the surface of graphene nanosheets (GNS). The MnO/GNS nanocomposite showed high performance as an anode material for LIBs.
Experimental GNS was fabricated via the thermal exfoliation method described in our previous work [15]. Briefly, graphite oxide was thermally exfoliated at 300 1C for 3 min in air, and subsequently treated at 900 1C for 3 h in Ar. In a typical synthesis of MnO/GNS nanocomposite, 1.51 g of Mn(NO3)2 aqueous solution (50 wt%) was mixed with 20 mL of enthanol. 100 mg of GNS was added into the solution and then ultrasonically treated for 10 min. The suspension solution was mixed using a magnetic stirrer in a ventilation cabinet; the ethanol in the solution evaporated continuously. Lastly, dried Mn(NO3)2/GNS composite was collected and treated at 700 1C for 5 h in Ar. The final MnO content in MnO/GNS was about 75% by weight. In the control experiments, simplex Mn3O4 sample was prepared by heating Mn(NO3)2 at 700 1C for 5 h in Ar, whereas Mn3O4/GNS nanocomposite was obtained by heating Mn(NO3)2/GNS at 500 1C for 5 h in Ar. The obtained samples were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
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D. Qiu et al. / Materials Letters 84 (2012) 9–12
Electrochemical measurements were performed using 2032type coin cells. 80 wt% active material, 10 wt% carbon black, and 10 wt% polyvinyldifluoride binder were mixed with N-methylpyrrolidone. The obtained slurry was pasted on a copper foil and dried in vacuum. Coin cells were assembled in an argon-filled glove box using an active material as the working electrode,
a Li foil as the counter electrode, 1 M LiPF6 in ethylene carbonate and diethyl carbonate (1:1 vol), and Celgard 2250 as the separator. Charge–discharge measurements were carried out galvanostatically over a voltage range of 0.01–3 V using a battery testing system (LAND, Wuhan Jinnuo Electronics). Furthermore, the tap density of electrode is 1.1 g/cm3.
Results and discussion
Fig 1. XRD patterns of the samples.
The XRD patterns of the samples are shown in Fig. 1. All the diffraction peaks of MnO/GNS can be indexed to the face-centered cubic phase of MnO (JCPDS 07–0230). All the peaks of simplex Mn3O4 are in good agreement with the standard profiles of Mn3O4 (JCPDS 24–0734). In addition, the XRD pattern of Mn3O4/GNS indicates that the sample is mainly composed of Mn3O4 and also consist of a few MnO (peaks indicated by red arrows). The above results showed that the MnO phase was not formed in the absence of graphene. Fig 2a–c present the SEM and TEM images of GNS, which show that GNS has a nanosheet structure. The SEM images also show the presence of abundant macropores within GNS, which facilitate the combination of Mn(NO3)2 and GNS. Fig 2d–f show the SEM images of MnO/GNS under different magnifications. MnO nanoparticles with diameters ranging from
Fig 2. SEM (a and b) and TEM (c) images of GNS; SEM (d–f) and TEM (g and h) images of MnO/GNS; (i) HRTEM image for a single MnO nanoparticle; SEM (j and k) and TEM (l) images of MnO/GNS after 60 cycles.
D. Qiu et al. / Materials Letters 84 (2012) 9–12
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Fig. 3. (a) Discharge and charge curves of MnO/GNS for the first and second cycles at 100 mA g 1; (b) representative charge and discharge curves of MnO/GNS at various current densities; (c) capacity retention of MnO/GNS at various current densities; (d) discharge capacity retention of three samples at 100 mA g 1; Inset of d shows the long-term cyclic performance of MnO/GNS at 1000 mA g 1 (the cell was first cycled at 200 mA g 1 for two cycles). The weight of MnO/GNS (or Mn3O4/GNS) in the working electrode was used to estimate the specific capacity.
20 nm to 250 nm integrated tightly with GNS. Fig 2g and h present the TEM images of the MnO/GNS, revealing the firm attachment of MnO nanoparticles to GNS even after ultrasonication treatment employed to disperse the sample for TEM observation. An HRTEM image of a single MnO nanoparticle is presented in Fig 2i. The lattice fringes with an inter-plane spacing of 0.2205 nm corresponded to the (2 0 0) plane of MnO. During the synthesis, the macropores within GNS were first filled with the Mn(NO3)2 solution. When ethanol evaporated, Mn(NO3)2 nucleated and grew on the GNS surface. Finally, solid Mn(NO3)2 4H2O dispersed uniformly on the GNS surface. During the heat-treatment process, Mn(NO3)2 4H2O was first transformed into Mn3O4. With the increase of the temperature, Mn3O4 reacted with graphene and was reduced into MnO. The mechanism is illustrated as follows: Mn3O4 þC-3MnOþCO. The resulting MnO nanoparticle tightly integrated with GNS due to it was obtained via the in situ carbothermal reduction of Mn3O4. Fig. 3a shows the charge and discharge curves of the MnO/GNS electrode for the first and second cycles at 100 mA g 1. The plateau at about 0.24 V in the first discharge curve was attributed to the reduction of MnO to Mn. In the subsequent cyclic process, the discharge plateau shifted to about 0.55 V, indicating the irreversible formation of crystalline metal nanoparticles and amorphous Li2O matrix [10]. The charge plateau appeared at about 1.2 V, which is much lower than those shown by other metal oxides and favorable for increasing the operation voltage and energy density when the electrode is used as an anode in full batteries [9]. Fig. 3b shows the representative charge and discharge curves of the MnO/GNS anode at various current densities. Fig. 3c shows the cyclic performance of MnO/GNS at various current densities. The cell was first cycled at 100 mA g 1 for 12 cycles, in which a stable specific capacity of about 670 mA h g 1 was observed. Even at 1000 mA g 1, more than 60% of the capacity is retained, indicating that MIO/GNS has good
rate capability. Fig. 3d shows the discharge capacity retentions of the samples at 100 mA g 1. The MnO/GNS had a capacity of 691 mA h g 1 after two cycles, and 782 mA h g 1 after 60 cycles. The electrochemical performance of MnO/GNS is much better than those of Mn3O4/GNS and simplex Mn3O4. The inset in Fig. 3d shows the long-term cyclic performance of MnO/GNS at 1000 mA g 1. The capacity remained at about 460 mA h g 1 after 180 cycles, indicating the excellent cyclic stability of the electrode. Further analysis of the morphology evolution of MnO/GNS after the discharge/charge cycles was performed by SEM and TEM observations. As shown in Fig. 2j–l, MnO nanoparticles remain integrated with GNS after 60 cycles at 100 mA g 1. The high performance of MnO/GNS was attributed to the tight combination of MnO nanoparticles and GNS. The GNS in the composite has a good electrical conductivity and serves as a three-dimensional conductive network for MnO nanoparticles, which decreases the inner resistance of LIBs and is favorable for stabilizing the electronic conductivity. Furthermore, because of the tight combination of MnO and GNS, the GNS not only provides an elastic buffer space to accommodate the volume expansion/contraction of MnO nanoparticles during Li insertion/extraction process but also efficiently prevents the detachment and agglomeration of MnO nanoparticles upon continuous cycling, thus maintaining the structural integrity and avoiding rapid loss of electrode capacity. For Mn3O4/GNS, the Mn3O4 nanoparticle did not tightly integrate with GNS. Thus, the cyclic performance of Mn3O4/GNS is obviously worse than that of MnO/GNS.
Conclusions In summary, a MnO/GNS nanocomposite has been fabricated as LIBs anode material via in situ carbothermal reduction on the
12
D. Qiu et al. / Materials Letters 84 (2012) 9–12
GNS surface. MnO nanoparticles integrated tightly with GNS. The flexible GNS provided high electrical conductivity throughout the electrode and served as a mechanically strong framework. As a result, the nanocomposite had a large lithium storage capacity, excellent cyclic stability, and good rate capability.
Acknowledgments This work was supported by China Postdoctoral Science Foundation (No. 20100471296), Postdoctoral Foundation of Jiangsu Province (No. 1001003C), and National Nature Science Foundation of China (No. 60928009 and 61076017). References [1] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM. Nature 2000;407: 496–499. [2] Kang E, Jung YS, Cavanagh AS, Kim GH, George SM, Dillon AC, et al. Adv Funct Mater 2011;21:2430–2438.
[3] Liu J, Li Y, Fan H, Zhu Z, Jiang J, Ding R, et al. Chem Mater 2010;22:212–217. [4] Liu Z, Tay SW. Mater Lett 2012;72:74–77. [5] Zhu Y, Bai YJ, Han FD, Qi YX, Lun N, Yao B, et al. Mater Lett 2011;65: 3157–3159. [6] Yu XQ, He Y, Sun JP, Tang K, Li H, Chen LQ, et al. Electrochem Commun 2009;11:791–794. [7] Zhong K, Xia X, Zhang B, Li H, Wang Z, Chen L. J Power Sources 2010;195:3300–3308. [8] Kokubu T, Oaki Y, Hosono E, Zhou H, Imai H. Adv Funct Mater 2011;21: 3673–3680. [9] Ding YL, Wu CY, Yu HM, Xie J, Cao GS, Zhu TJ, et al. Electrochim Acta 2011;56:5844–5848. [10] Sun B, Chen Z, Kim HS, Ahn H, Wang G. J Power Sources 2011;196: 3346–3349. [11] Liu Y, Zhao X, Li F, Xia D. Electrochim Acta 2011;56:6448–6452. [12] Zhou G, Wang DW, Li F, Zhang L, Li N, Wu ZS, et al. Chem Mater 2010;22:5306–5313. [13] Zhou W, Zhu J, Cheng C, Liu J, Yang H, Cong C, et al. Energy Environ Sci 2011;4:4954–4961. [14] Zhang K, Han P, Gu L, Zhang L, Liu Z, Kong Q, et al. ACS Appl Mater Interfaces 2012;4:658–664. [15] Du QL, Zheng MB, Zhang LF, Wang YW, Chen JH, Xue LP, et al. Electrochim Acta 2010;55:3897–3903.