graphene composite as anode materials for lithium-ion batteries

graphene composite as anode materials for lithium-ion batteries

Electrochimica Acta 56 (2011) 2306–2311 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elec...

882KB Sizes 2 Downloads 129 Views

Electrochimica Acta 56 (2011) 2306–2311

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

CuO/graphene composite as anode materials for lithium-ion batteries Y.J. Mai, X.L. Wang, J.Y. Xiang, Y.Q. Qiao, D. Zhang, C.D. Gu, J.P. Tu ∗ State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 27 August 2010 Received in revised form 7 November 2010 Accepted 11 November 2010 Available online 19 November 2010 Keywords: Graphene Cupric oxide Lithium-ion battery Coulombic efficiency

a b s t r a c t CuO/graphene composite is synthesized from CuO and graphene oxide sheets following reduced by hydrazine vapor. As the electrode material for lithium-ion batteries, CuO nanoparticles with sizes of about 30 nm homogeneously locate on graphene sheets, and act as spacers to effectively prevent the agglomeration of graphene sheets, keeping their high active surface. In turn, the graphene sheets with good electrical conductivity server as a conducting network for fast electron transfer between the active materials and charge collector, as well as buffered spaces to accommodate the volume expansion/contraction during discharge/charge process. The synergetic effect is beneficial for the electrochemical performances of CuO/graphene composite, such as improved initial coulombic efficiency (68.7%) and reversible capacity of 583.5 mAh g−1 with 75.5% retention of the reversible capacity after 50 cycles. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Cupric oxide (CuO), well-known material of p-type semiconductor, has been employed for gas sensors [1], CO oxidation catalysts [2], solar energy conversion [3], and lithium-ion battery anode materials [4], because of their low band-gap energy and high catalytic activity, as well as their nontoxic nature and affordable price. However, just like other metal oxides, CuO suffers from poor electronic conductivity and large volume variation during charge/discharge process, leading to severe mechanical strains and rapid capacity decay. Extensive attempts have been made to solve the above problems, including carbon-coating, electronically conductive additives or hierarchical nanostructures with various morphologies [5–8]. Recently, our group is quite successful in synthesizing CuO with various morphologies by simply self-assemble. The morphology effect on the electrochemical properties of CuO and Cu2 O has been investigated [9–11], showing that the large specific surface area of these hierarchical nanostructures afford sufficient contact area for CuO/electrolyte and short the diffusion length of Li+ , but so far proved not to be very successful in enhancing the cyclability of CuO based electrode as anode for lithium ion batteries. Graphene, the name given to a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, exhibits many unusual and intriguing properties such as the thinnest known material but the strongest and record stiffness and thermal conductivity ever measured [12,13]. Its charge

∗ Corresponding author. Tel.: +86 571 87952856; fax: +86 571 87952573. E-mail address: [email protected] (J.P. Tu). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.11.036

carriers exhibit giant intrinsic mobility, 200,000 cm2 V−1 s−1 , and can travel for micrometers without scattering at room temperature if extrinsic disorder is eliminated [14,15]. In particular, its superior electrical conductivities, high surface areas and chemical tolerance intrigue great interest in energy storage technologies, such as lithium-ion batteries [16–18] and supercapacitor [19,20]. For example, the supercapacitor devices based on chemically converted grapheme and polyaniline nanofibers showed large electrochemical capacitance (210 F g−1 ) at a discharge rate of 0.3 A g−1 ; pseudocapacitor based on single-crystalline Ni(OH)2 hexagonal nanoplates grown on GS delivered a high energy density of 37 Wh kg−1 and a high power density of 10 kW kg−1 [21,22]. Yoo et al. [23] pointed out the specific capacity of graphene nanosheets increased form 540 mAh g−1 to 784 mAh g−1 by the incorporation of macromolecules of CNT and C60 . Pan et al. [24] argued that disordered graphene nanosheets could find promising applications in high-capacity Li-ion batteries with exceptionally high reversible capacities (794–1054 mAh g−1 ) because of additional reversible storage sites such as edges and other defects. Although metal oxide (Mn3 O4 , TiO2 , SnO2 , Co3 O4 , Cu2 O)/graphene composites have been synthesized [20,25–28], to the best of our knowledge, there are few reports devoted to the synthesis of CuO/graphene composite [29–31]. To circumvent the poor electronic conductivity and large volume variation during the charge/discharge process of CuO electrode, herein, we have synthesized CuO/graphene composite by an in-situ chemical synthesis approach. The key strategy in this work is to develop a synergetic effect between CuO and graphene. The initial coulombic efficiency and reversible capacity, especially the improved cycling performances of the CuO/graphene composite in comparison with the ordinary CuO were evaluated.

Y.J. Mai et al. / Electrochimica Acta 56 (2011) 2306–2311

2307

2. Experimental Graphene oxide (GO) sheets were synthesized by chemical exfoliation of flake graphite powder by a modified Hummers method as originally presented by Kovtyukhova et al. [32]. CuO/GO composite was synthesized by Wang’s method [22] with a little modification. In a typical process, 2 ml (1.2 mg ml−1 ) of aqueous dispersion GO were dispersed in 12 ml of N,N-dimethylformamide (DMF). The resulting suspension was heated to 90 ◦ C in a round bottom flask with magnetic stirring. After the suspension reached 90 ◦ C, 1.2 ml of 0.2 M cupric acetate aqueous solution was injected in the resulting solution. The mixture was kept at 90 ◦ C with stirring for 1 h. After that, the CuO/GO was collected by centrifugation and washed with water. The CuO/GO was then dispersed in 30 ml of water, and sealed in Teflon lined stainless steel autoclaves for hydrothermal reaction at 180 ◦ C for 10 h. We found that hydrothermal reaction at 180 ◦ C for 10 h was important to obtain CuO in CuO/graphene composite with well crystallinity and good electrochemical performance as anode materials. The final product was collected by centrifuge, water-washing and dried. For comparison, the CuO were synthesized with the same process except the addition of the aqueous dispersion GO. The GO sheets in the composite were reduced by hydrazine vapor [33]. In brief, the CuO/GO composite was placed in a perfectly cleaned glass Petri dish inside a larger glass dish which also contained 1 mL of hydrazine monohydrate (wt.% = 85%). The larger dish was covered with a glass lid, sealed with Parafilm tape, and placed over a plate at 50 ◦ C for 12 h, after which the dish was opened and dried at 90 ◦ C in vacuum. The structure and morphology of the CuO and CuO/graphene composite were analyzed by X-ray diffraction (XRD, Philips PC-APD with Cu K␣ radiation), field emission scanning electron microscopy (FESEM, S-4800), transmission electron microscopy (TEM, JEOL JEM-2010F), atomic force microscopy (AFM, SEIKO SPA400) and X-ray photoelectron spectroscopy (XPS, PHI 5700). TG analysis of the CuO/graphene was measured on a Netzsch-STA 449C apparatus from 30 to 800 ◦ C at a heating rate of 10 ◦ C min−1 in air. Electrochemical performances of the CuO and CuO/graphene composite were investigated with two-electrode coin-type cells (CR 2025). The working electrodes were prepared by a slurry coating procedure. The slurry consisted of 85 wt.% as-synthesized powder, 10 wt.% acetylene black and 5 wt.% polyvinylidene fluorides (PVDF) dissolved in N-methyl pyrrolidinone (NMP) were incorporated on nickel foam with 12 mm in diameter. After dried at 90 ◦ C for 24 h in vacuum, the foam was pressed under a pressure of 20 MPa. Test cells were assembled in an argon-filled glove box with the metallic lithium foil as both the reference and counter electrodes, 1 M LiPF6 in ethylene carbonate (EC)–dimethyl carbonate (DME) (1:1 in volume) as the electrolyte, and a polypropylene (PP) micro-porous film (Cellgard 2300) as the separator. The galvanostatic charge–discharge tests were conducted on LAND battery program-control test system at a rate of 67 mA g−1 in the voltage range of 0.02–3.0 V (versus Li/Li+ ) at room temperature (25 ± 1 ◦ C). Cyclic voltammetry (CV) was performed on CHI660C electrochemical workstation at a scan rate of 0.1 mV s−1 from 0 to 3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI660c potentiostat over a frequency range from 100 kHz to 10 mHz. A three-electrode cell was used for EIS test, where lithium foil act as both the counter and reference electrodes.

3. Results and discussion Fig. 1 shows the XRD patterns of CuO and CuO/graphene composite. The CuO/graphene composite shows broader peaks but with lower intensity than the bare CuO, suggesting that the

Fig. 1. Typical XRD patterns: (a) CuO, (b) CuO/graphene composite, (c) graphene films obtained by filtrating aqueous dispersion of GO and then reducing by hydrazine vapor.

CuO/graphene composite is composed of small CuO particles with relatively low crystallization, owing to the high density of oxygen functional groups, including carboxylic, hydroxyl, and epoxy groups, on GO surface to hinder diffusion, crystallization and growth of CuO grains [34]. What is more, some small peaks of Cu2 O are observed for both materials, because DMF, which used as solvent during the synthesis process, is a weak reducing agent [1]. Yu et al. demonstrated that Cu2 O nanospheres with hollow interiors can be fabricated by dissolving Cu(NO3 )2 in DMF [3]. Finally, an additional small and low broad (002) diffraction peak appears at about 2 = 25◦ , showing the disorderedly stacked graphene sheets, but this broad peak is weaker than that of graphene film, which obtained by filtrating aqueous dispersion of GO and then reducing by hydrazine vapor (shown in the inset of Fig. 1), suggesting less agglomeration and more disordered stacking for graphene sheets in the composite. Fig. 2a shows the morphology of CuO grown in free solution, which is composed of granules and slabs with sizes of 50–200 nm. In the CuO/graphene composite, granules of CuO with uniform size of about 30 nm are selectively and directly grown on the GO sheets (Fig. 2b). It is believed that both chemisorptions and van der Waals interactions between CuO and GO exist at oxygen-containing defect sites and pristine regions of the GO, respectively. In addition, oxygen functional groups located at the surface of GO effectively hinder diffusion, recrystallization and grown of CuO grains. The following reduction of GO by hydrazine vapor had a little effect on the original composite state between CuO and GO sheets. Fig. 3a shows the bright-field image of GO, whose central part normally appears on the TEM image as homogeneous and featureless region, whereas edges tend to scroll. The selected area electron diffraction (SAED) pattern of the flake in Fig. 3a shows the typical six-fold symmetry expected for graphite/graphene. Fig. 3b shows an AFM image of the exfoliated GO. The obtained GO has a lateral dimension of several micrometers and a thickness of 1.08 nm which is slightly larger than the pure graphene sheet [13,35]. It can be attributed to the functional groups of GO as well as the tapping AFM mold, but this height value is within the range of previously reported GO thicknesses and is characteristic of a fully exfoliated GO [36–38]. Fig. 3c shows the TEM images of the CuO/graphene composite. It can be seen that the CuO nanopaticles with a size of about 30 nm are uniformly distributed on 2D graphene sheets. It is note that the morphology of graphene sheets has little change after reducing by hydrazine vapor. The CuO nanoparticles on the surface of graphene sheets acting as spacers ensure to efficiently prevent the closely restacking of sheets, avoiding the loss of their high active surface [39,40].

2308

Y.J. Mai et al. / Electrochimica Acta 56 (2011) 2306–2311

Fig. 2. SEM images of (a) CuO and (b) CuO/graphene composite.

Fig. 4a shows a high-resolution C1 s XPS spectrum of GO. Five types of carbon which correspond to carbon atoms in different functional groups appear clearly. The C 1s peak of graphite is observed at 284.6 eV, C 1s of C–OH at 285.7 eV, C 1s of epoxy at 286.7 eV, C 1s of C O at 288.0 eV, and C 1s of C(O)O at 289.1 eV, respectively. Fig. 4b shows XPS data of C 1s of CuO/graphene hybrid, and the inset is wide-scan XPS spectrum of CuO/graphene composite. The graphene in composite reduced by hydrazine vapor exhibits the same oxygen-containing functionalities, but their intensities are much smaller than GO, confirming that most of the epoxide, hydroxyl, and carboxyl functional groups are successfully removed. Furthermore, there is an additional component at 285.6 eV corresponding to C–N bond, because the reductive is hydrazine vapor. Becerril et al. [33] showed that the graphene reduced by hydrazine vapor had conductivities of about 20 S cm−1 , more than six orders of magnitude of GO. Fig. 5 shows CV curves of CuO/graphene composite and CuO electrodes at a scan rate of 0.1 mV s−1 . In the first cycle, three cathodic peaks of the CuO/graphene electrode locate at 1.59, 0.97, and 0.74 V (vs. Li+ /Li), corresponding to the reductive reaction from CuO to a solid-solution phase with a CuO-type structure, following transforming into a Cu2 O phase; further decomposition of Cu2 O into copper nanograins embedded into a Li2 O matrix; and growth of an organic-type coatings, respectively. During the following charge, three anodic peaks locate near 1.34, 2.46 and 2.86 V, which corresponds to decomposition of the organic layer; Li2 O, acting as oxygen reservoir, releases oxygen to partially oxidize copper particles to the Cu(I) and Cu(II), respectively [41,42]. In the second cycle, decreases of individual peak intensity and integral area suggest reversible capacity losses. Three cathodic peaks are observed with shifts to 2.19, 1.14, and 0.78 V, but with little shift of anodic peaks. However, compared to ordinary CuO electrode, there are three differences: (1) the areas of all CV peaks of the CuO/graphene

Fig. 3. (a) Bright-field TEM image of GO, the inset is electron diffraction pattern (SAED), (b) AFM image of exfoliated GO sheets with height profile, (c) bright-field TEM image of CuO/graphene composite.

Y.J. Mai et al. / Electrochimica Acta 56 (2011) 2306–2311

Intensity / a.u.

C-OH (285.7eV)

a

C(epoxy) (286.7eV)

0.2 CuO/graphene CuO

0.1

Specific current / A g-1

a

C in graphite (284.6eV) C=O (288.0eV) C(O)O (289.1eV)

2309

0.0 -0.1 -0.2 -0.3 -0.4 -0.5

280

284

288

Intensity / a.u.

Specific current / A g-1

b

Binding Energy / eV

1.0

1.5

2.0

2.5

3.0

2.5

3.0

+

b

Intensity / a.u.

0.5

Potential vs. (Li / Li ) / V

Binding Energy / eV

C-N (285.6eV)

0.0

292 0.2

CuO/graphene 2nd CuO/graphene 3rd CuO 2nd CuO 3rd

0.1 0.0 -0.1 -0.2 -0.3 0.0

0.5

1.0

1.5

2.0 +

280

284

288

292

Binding Energy / eV Fig. 4. C 1s XPS spectra of (a) GO and (b) graphene in composite reduced by hydrazine vapor, the insert is wide-scan XPS spectrum of CuO/graphene composite.

composite are larger than pure CuO, indicating that more reactions toward Li+ occur in the composite. It is attributed to the graphene conducting network which facilitates Li+ transfer on active material/electrolyte interfaces, ensuring the improved electrical contact of CuO/CuO and CuO/current collector. The improved electrical conductivity of CuO/graphene composite can enhance the kinetics and the extent of electrode reactions. (2) The CuO electrode owns faster decrease rate of individual peak intensity and integral area, indicating more reversible capacity losses. (3) The interval between the cathodic and anodic peaks is broader for the CuO, exhibiting a bigger polarization than the CuO/graphene electrode. The electrochemical performance of the CuO/graphene composite, CuO and graphene were evaluated by galvanostatic charge/discharge cycling at a current density of 67 mA g−1 . As shown in Fig. 6a and b, the first discharge/charge voltage profiles as well as the stabilized ones for the CuO/graphene and CuO electrodes are very similar. There are three sloping potential ranges (2.0–1.6, 1.25–1.0, and 1.0–0.02 V), which are consistent with three cathodic peaks in the CVs for the lithium reaction with the CuO/graphene composite during the first discharge, but no obvious voltage plateau is observed for grapheme (Fig. 6c). The initial discharge capacity is about 817, 752.4 and 950 mAh g−1 , for the CuO/graphene, CuO and graphene electrode, respectively, which are larger than their theoretical capacities (670 mAh g−1 for CuO and 754 mAh g−1 for graphene). The extra capacity compared with the theoretic capacity usually is ascribed to formation of SEI during the first discharge process, the reduction of the adsorbed impurities on active material surfaces, the initial formation of lithium oxide as well as possibly interfacial lithium storage [43]. However, the CuO/graphene composite shows more extra capacity compared with CuO electrode which can be attributed to large

Potential vs. (Li / Li ) / V Fig. 5. Cyclic voltammogram curves of CuO/graphene composite and bare CuO at a scan rate of 0.1 mV s−1 from 0 to 3.0 V: (a) the first cycle, (b) the second and third cycles.

electrochemical active surface area of well-dispersed graphene and more grain boundary areas of the CuO nanoparticles in the composite. But, interestingly, the CuO/graphene composite has higher initial coulombic efficiency than the CuO (68.7% vs. 60.9%). The introducing of graphene shows a strong synergistic effect in the composite for improving the reversible capacity, considering the initial coulombic efficiency of graphene in the range of 42–62% [24,39,44,45], including ours (42.2%). In addition, the enhanced reversible capacity of CuO/graphene composite is clearly shown with neglected decay of charge capacity in the first three cycles (Fig. 6a). Another advantage of the CuO/graphene composite is significantly improved cycling performance compared with the CuO. Fig. 6d shows a decrease in the capacity of CuO from 752.4 to 359.3 mAh g−1 up to 5 cycles and then to only 226 mAh g−1 , 49.3% retention of the reversible capacity up to 50 cycles. Compared with the CuO we synthesized before with such morphologies as flower-like, caddice clew-like, bird nest-like and needle-like, nanotube-like [9,10,46], the CuO here with granules and slabslike morphology, without hierarchical structure, shows poor cyclic performance. In turn, it demonstrates again that the hierarchical nanostructured CuO with various morphologies enhances cyclic performance by affording larger specific surface area, leading to sufficient contact area for CuO/electrolyte, shorting the diffusion length of Li+ , and enhancing reactivity of electrode reaction during cycling. On the other hand, the CuO/graphene composite exhibits a reversible capacity of 561.4 mAh g−1 . After 50 cycles the discharge capacity still remains 423 mAh g−1 , which is about 75.3% retention of the reversible capacity. This result was inferior to the leaf-like CuO/MWCNT composite [47], sisal-like hierarchical microstructures CuO/C films [48], but it can compare with CuO/CNTs nanomicrospheres [8], and still better than that of bird

2310

Y.J. Mai et al. / Electrochimica Acta 56 (2011) 2306–2311

a

c +

2

1

0

200

400

0

Discharge Capacity / mA h g -1

+

Potential vs. (Li / Li ) / V

1

1 st

2 nd

400

600

Specific capacity / mA h g

Utilization of CuO / mAh g

600

800

1000

-1

CuO CuO/graphene graphene

600

400

200

0 0

10

-1

20

30

40

50

Number of cycles

1000 maximum utilized capacity of ordinary CuO utilization of CuO in composite

-1

e

1 st

2 nd

800

800

100

800

Weight(%)

200

400

Specific capacity / mA h g

2

0

200

-1

d

3 rd

3 rd

800

3

0

1

0

600

Specific capacity / mA h g

2

1 st

2 nd

3 rd

0

b

3

Potential vs. (Li / Li ) / V

+

Potential vs. (Li / Li ) / V

3

-1

(462.4mAhg )

95 90 85

loss 12%

600 0

200

400

600 o

Temperature / C

400 0

10

20

30

40

50

Number of cycles Fig. 6. Discharge–charge curves for (a) CuO/grapheme; (b) CuO; (c) grapheme; (d) cycling performances of bare CuO, graphene and CuO/graphene electrode; (e) utilization of CuO component in CuO/graphene composite when assuming the maximum utilization of graphene component, the inset is TG curve of CuO/graphene.

nest-like [9], needle-like [10], hollow sphere and urchin-like CuO [4]. In order to estimate the synergetic effect between the conducting graphene sheets and CuO, we use the parameter of the capacity utilization of one component by Wu’s method [39] with a little modification as follow: composite

Cuo = csp

csp

max − cgraphene × wt%graphene

wt%CuO

(1)

Cuo is the utilized capacity of CuO in composite, and where csp max cgraphene , the maximum utilized capacity of ordinary graphene, is equal to the first charge capacity considering that the initial discharge capacity is significantly affected by the formation of SEI film. composite It is supposing that the total reversible capacity csp of composite is the sum of the above two parts, and wt.%CuO , wt.%graphene are estimated to be ∼88 wt.% and ∼12 wt.%, respectively, based

on the TG curve of CuO/graphene composite (inset of Fig. 6e). According to Eq. (1), the utilized capacity of CuO in composite exceeds the possible maximum reversible capacity of the bare CuO max = 458.5 mAh g−1 ) in the first 23 cycles. After the 50th cycle, (cCuO it still remains 425.4 mAh g−1 , which can be compared with the maximum reversible capacity of ordinary CuO (Fig. 6e). It means that with the help of graphene, the CuO in the composite almost performs with a maximum reversible capacity of bare CuO even up to 50 cycles. Based on the above results, it is concluded that the synergetic effect between the conducting graphene sheets and CuO is responsible for the improved electrochemical performance of CuO/graphene such as highly reversible capacity, good cyclic performance and high coulombic efficiency. It can be explained as follows: CuO nanoparticles located on the surface of graphene effectively prevent the agglomeration of graphene sheets, keeping their high active surface, excellent electronic conductivity and

Y.J. Mai et al. / Electrochimica Acta 56 (2011) 2306–2311

References

350 CuO CuO/graphene

[1] [2] [3] [4] [5] [6] [7] [8]

210

"

-Z / Ω

280

[9]

140 [10] [11]

70

0 0

2311

[12] [13]

70

140

210

280

350

'

Z/Ω

[14] [15] [16] [17]

Fig. 7. Nyquist plots of the CuO and CuO/graphene electrodes. [18]

high carrier mobility. This can be understood from the EIS study. Fig. 7 shows Nyquist plots of the CuO and CuO/graphene electrodes. It is believed that the semicircle at middle frequency region is ascribed to the charge-transfer process of lithium ion at the CuO/electrolyte interface, while the line in the lower frequency region is corresponded to the limited diffusion of lithium ions into CuO. The CuO/graphene electrode exhibits smaller semicircles compared with the bare CuO, indicating a lower charge transfer resistance of the CuO/graphene composite. What is more, during the synthesis process, GO sheets, rather than graphene, with high density of oxygen functional groups were selected as precursor which effectively hindered the growth of CuO grains. Small CuO grains show stronger chemisorptions and van der Waals interactions between the CuO and GO at the oxygen-containing defect sites and pristine regions of the GO. Furthermore, the small CuO grains provide sufficient contact area for CuO/electrolyte and short the diffusion length of Li+ . Finally, the reduced graphene sheets with good electronic conductivity server as a conducting network for fast electron transfer between the active materials and the collector, and the flexible graphene can also be used as buffered spaces to accommodate the volume expansion/contraction during the discharge/charge process. 4. Conclusions In summary, the CuO/graphene composite was obtained by an in-situ chemical synthesis approach. The presence of CuO nanoparticles between the graphene sheets effectively keeps the neighboring graphene sheets separated. In turn, GO with high density of oxygen functional groups effectively hindered the growth of CuO grains. As electrode materials for lithium-ion batteries, the CuO/graphene composite ensures fast electron transfer between the active materials and collector, and accommodates the volume expansion/contraction during discharge/charge process. The CuO/graphene composite is expected to account for the improved electrochemical performances, in comparison with the ordinary CuO, such as highly reversible capacity, high initial coulombic efficiency and good cyclic performance.

[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48]

J.T. Zhang, J.F. Liu, Q. Peng, X. Wang, Y.D. Li, Chem. Mater. 18 (2006) 867. M.F. Luo, Y.J. Zhong, X.X. Yuan, X.M. Zheng, Appl. Catal. A: Gen. 162 (1997) 121. Y. Chang, J.J. Teo, H.C. Zeng, Langmuir 21 (2004) 1074. J.C. Park, J. Kim, H. Kwon, H. Song, Adv. Mater. 21 (2009) 803. X.P. Gao, J.L. Bao, G.L. Pan, H.Y. Zhu, P.X. Huang, F. Wu, D.Y. Song, J. Phys. Chem. B 108 (2004) 5547. S.Q. Wang, J.Y. Zhang, C.H. Chen, Scripta Mater. 57 (2007) 337. L.B. Chen, N. Lu, C.M. Xu, H.C. Yu, T.H. Wang, Electrochim. Acta 54 (2009) 4198. S.F. Zheng, J.S. Hu, L.S. Zhong, W.G. Song, L.J. Wan, Y.G. Guo, Chem. Mater. 20 (2008) 3617. J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, Electrochim. Acta 55 (2010) 1820. J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, J. Power Sources 195 (2010) 313. J.Y. Xiang, X.L. Wang, X.H. Xia, L. Zhang, Y. Zhou, S.J. Shi, J.P. Tu, Electrochim. Acta 55 (2010) 4921. C. Lee, X. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385. A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Nano Lett. 8 (2008) 902. X. Du, I. Skachko, A. Barker, E.Y. Andrei, Nat. Nano 3 (2008) 491. S.V. Morozov, K.S. Novoselov, M.I. Katsnelson, F. Schedin, D.C. Elias, J.A. Jaszczak, A.K. Geim, Phys. Rev. Lett. 100 (2008) 016602. P. Guo, H.H. Song, X.H. Chen, Electrochem. Commun. 11 (2009) 1320. Z.F. Du, X.M. Yin, M. Zhang, Q.Y. Hao, Y.G. Wang, T.H. Wang, Mater. Lett. 64 (2010) 2076. N. Zhu, W. Liu, M.Q. Xue, Z. Xie, D. Zhao, M.N. Zhang, J.T. Chen, T.B. Cao, Electrochim. Acta 55 (2010) 5813. T. Lu, Y.P. Zhang, H.B. Li, L.K. Pan, Y.L. Li, Z. Sun, Electrochim. Acta 55 (2010) 4170. B. Wang, J. Park, C.Y. Wang, H. Ahn, G.X. Wang, Electrochim. Acta 55 (2010) 6812. Q. Wu, Y.X. Xu, Z.Y. Yao, A.R. Liu, G.Q. Shi, ACS Nano 4 (2010) 1963. H.L. Wang, H.S. Casalongue, Y.L. Liang, H.J. Dai, J. Am. Chem. Soc. 132 (2010) 7472. E. Yoo, J. Kim, E. Hosono, H.S. Zhou, T. Kudo, I. Honma, Nano Lett. 8 (2008) 2277. D.Y. Pan, S. Wang, B. Zhao, M.H. Wu, H.J. Zhang, Y. Wang, Z. Jiao, Chem. Mater. 21 (2009) 3136. J. Yao, X.P. Shen, B. Wang, H.K. Liu, G.X. Wang, Electrochem. Commun. 11 (2009) 1849. S.M. Paek, E. Yoo, I. Honma, Nano Lett. 9 (2008) 72. D.H. Wang, D.W. Choi, J. Li, Z.G. Yang, Z.M. Nie, R. Kou, D.H. Hu, C.M. Wang, L.V. Saraf, J.G. Zhang, I.A. Aksay, J. Liu, ACS Nano 3 (2009) 907. C. Xu, X. Wang, L.C. Yang, Y.P. Wu, J. Solid State Chem. 182 (2009) 2486. J.W. Zhu, G.Y. Zeng, F.D. Nie, X.M. Xu, S. Chen, Q.F. Han, X. Wang, Nanoscale 2 (2010) 988. D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.L. Taberna, P. Simon, Nat. Nano 5 (2010) 651. N. Li, Z.Y. Wang, K.K. Zhao, Z.J. Shi, S.K. Xu, Z.N. Gu, J. Nanosci. Nanotechnol. 10 (2010) 6690. N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V. Buzaneva, A.D. Gorchinskiy, Chem. Mater. 11 (1999) 771. H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, ACS Nano 2 (2008) 463. H.L. Wang, J.T. Robinson, G. Diankov, H.J. Dai, J. Am. Chem. Soc. 132 (2010) 3270. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nano 3 (2008) 101. I.V. Lightcap, T.H. Kosel, P.V. Kamat, Nano Lett. 10 (2010) 577. Y. Si, E.T. Samulski, Nano Lett. 8 (2008) 1679. Z.S. Wu, W.C. Ren, L. Wen, L.B. Gao, J.P. Zhao, Z.P. Chen, G.M. Zhou, F. Li, H.M. Cheng, ACS Nano 4 (2010) 3187. J. Yan, T. Wei, B. Shao, F.Q. Ma, Z.J. Fan, M. Zhang, C. Zheng, Y.C. Shang, W.Z. Qian, F. Wei, Carbon 48 (2010) 1731. S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Poizot, J.M. Tarascon, J. Electrochem. Soc. 148 (2001) A285. A. Debart, L. Dupont, P. Poizot, J.B. Leriche, J.M. Tarascon, J. Electrochem. Soc. 148 (2001) A1266. J. Maier, Nat. Mater. 4 (2005) 805. G.X. Wang, X.P. Shen, J. Yao, J. Park, Carbon 47 (2009) 2049. P.C. Lian, X.F. Zhu, S.Z. Liang, Z. Li, W.S. Yang, H.H. Wang, Electrochim. Acta 55 (2010) 3909. J.Y. Xiang, J.P. Tu, X.H. Huang, Y.Z. Yang, J. Solid State Electrochem. 12 (2008) 941. J.Y. Xiang, J.P. Tu, J. Zhang, J. Zhong, D. Zhang, J.P. Cheng, Electrochem. Commun. 12 (2010) 1103. H.B. Wang, Q.M. Pan, J.W. Zhao, W.T. Chen, J. Alloy Compd. 476 (2009) 408.