Graphene Hybrid Nanosheets: A Highly Stable Anode

Sep 13, 2012 - However, their cycle life remains insufficient for commercial applications. Developing transition-metal oxide anode materials with a lo...
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Ultrathin CoO/Graphene Hybrid Nanosheets: A Highly Stable Anode Material for Lithium-Ion Batteries Yongming Sun, Xianluo Hu,* Wei Luo, and Yunhui Huang* State Key Laboratory of Material Processing and Die & Mold Technology, College of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: Transition metal oxides are promising highcapacity anode materials for next-generation lithium-ion batteries. However, their cycle life remains insufficient for commercial applications. Developing transition-metal oxide anode materials with a long lifespan through a facile route has become an important issue. This work reports the fabrication of ultrathin CoO/graphene hybrid nanosheets consisting of ultrafine CoO nanoparticles (∼5 nm) densely anchored on the graphene nanosheets. They exhibit a high reversible capacity of ∼1018.0 mAh g−1 over 520 discharge/charge cycles, and the Coulombic efficiency remains ∼100% upon cycling, indicating excellent cyclability. The as-obtained CoO/graphene nanocomposite avoids the widespread problem of cracking or pulverization of transition-metal oxide anode materials upon cycling and retains its original morphology and structure even after 520 discharge/charge cycles, benefiting from the synergetic effects of ultrafine CoO nanoparticles and the conductive graphene nanosheets.

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graphene,44 CuO/graphene,45 Mn3O4/graphene,46 and MnO/ graphene47) have been prepared. They exhibit much better cell stability, higher capacities, and better rate capability in comparison to their bare counterparts. Despites these advances, a nanohybrid electrode of the transition-metal oxide and graphene with very long cycle life (e.g., >300 cycles) has seldom been reported. In particular, the particle cracking and pulverization of the electode are usually unavoidable, although this issue is alleviated to some degree by the protection of graphene. This may be because the size of the transition-metal oxides is not small enough, and the combination between graphene and transition-metal oxides is not very satisfying. In contrast, ultrafine particles may not undergo further break during the discharge/charge cycling due to their small sizes. Therefore, to design conversion-type electrode materials with ultrafine nanoparticles on graphene nanosheets may be a promising strategy to achieve long-term structural stability, enabling a long cycle life of lithium-ion batteries. As one of the most promising transition-metal oxide anode materials, CoO attracts extensive interest because of its high lithium-storage capacity.13−20 Recently, CoO quantum dot/ graphene composites were synthesized by a solution process under sonication combination with a thermal treatment at 550 °C for 3 h in N2. Their specific capacity and rate capability were very satisfying.43 However, toxic Co4(CO)12 was used as the cobalt source, and the prepared composites contained a high

INTRODUCTION There is great interest in developing novel high-capacity and long-lasting anode materials for lithium-ion batteries, which is of great importance for critical applications.1−4 Transitionmetal oxides, such as Fe2O3,5,6 Fe3O4,7,8 NiO,9,10 Co3O4,11,12 CoO,13−20 MoO2,21,22 CuO,23−25 Mn3O4,26 and MnO,27−29 have been intensively studied as promising anode materials because of their high theoretical specific capacities. However, the large volume changes and stresses that occur during the lithium insertion/exaction processes cause severe cracking and pulverizing of the electrode, resulting in a significant capacity fade during cycling.30 The transition-metal oxide anode materials often suffer from the major problem of the collapse of the initial structure and a rapid decay in capacity, thus limiting their commercial applications.31 Nanostructuring of electrode materials with decreased sizes or carbon hybrization5,13,26,32−35 has been evidenced as an effective approach to enhance the structural stability of electrode materials. On one hand, decreasing the particle size can not only alleviate the physical strains but also improve the reaction kinectis during the lithium insertion/exaction process.26,32−34,36,37 On the other hand, carbon can buffer the volume effect of transitionmetal oxides and prevent the detachment and agglomeration of pulverized active materials during cycling.5,13,33,35−37 Graphene is the most popular and intriguing two-dimensional carbon material due to its superior electrical conductivity, large surface area, chemical stability, and structural flexibility. To date, many nanostructured transition-metal oxide/graphene composites (e.g., Fe 2 O 3 /graphene, 36 Fe 3 O 4 /graphene, 38 NiO/graphene,39,40 Co3O4/graphene,37,41 CoO/graphene,42,43 MoO2/ © 2012 American Chemical Society

Received: July 16, 2012 Revised: August 30, 2012 Published: September 13, 2012 20794

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Diamond TG/DTA apparatus at a heating rate of 10 °C min−1 in a flowing air to determine the amount of graphene in the samples. Electrochemical Measurements. The working electrodes were prepared by mixing 80 wt % active material (CoO/ graphene hybrid) with 10 wt % acetylene black (Super-P) as a conductive agent and 10 wt % polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone as a binder to form a slurry. The as-prepared slurry was coated onto a copper foil and dried at 80 °C in vacuum for 6 h before pressing. The electrodes were cut into disks (8 mm in diameter) and further dried at 80 °C for 24 h in vacuum. A total of 2032 coin-type cells were finally assembled in an argon-filled glovebox with concentrations of moisture and oxygen below 1.0 ppm, using pure lithium foil as the counter electrode, Celgard 2300 as the separator, and 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v = 1:1) as the electrolyte. A PARSTAT 2273 potentiostat was employed for cyclic voltammetry (CV) measurements at a scanning rate of 0.2 mV s−1 and electrochemical impedance spectrometry tests in a frequency range of 100 kHz to 100 mHz at room temperature. The galvanostatic charge/discharge measurements were performed using a Land Battery Measurement System (Land, China) at various current densities of 200−1600 mA g−1 with a voltage window of 3.00−0.01 V vs Li/Li+ at room temperature.

content of graphene (40 wt %).43 In addition, the Coulombic efficiency was 95%, and the reported cycle number was less than 100,43 which cannot meet the demand for commercial applications. It is highly desirable that further research work should focus on reducing the graphene content in the composites, searching for alternative cobalt sources with low toxicity, improving the Coulombic efficiency, and exploring their electrochemical properties and structural stability over a large number of discharge/charge cycles (e.g., >300 cycles). Herein, we present a simple and easily scaled-up synthetic procedure for fabricating an unprecedented CoO/graphene nanocomposite using Co(OAc)2·4H2O as the cobalt source. Interestingly, the resulting CoO/graphene nanohybrids possess a unique nanoarchitecture comprising ultrafine CoO nanocrystals (∼5 nm) that are densely anchored on the graphene nanosheets. This work provides an ideal model system for studying the function of ultrafine transition-metal oxide nanoparticles on graphene over a large number of discharge/ charge cycles. Encouragingly, the as-formed CoO/graphene nanocomposite with 16 wt % graphene exhibits extended cycling with confinement from the graphene substrate. Even after 520 discharge/charge cycles, the hybrid CoO/graphene electrode still displays a high reversible capacity of ∼1018 mAh g−1 with the Coulombic efficiency of ∼100% at a current density of 200 mA g−1.

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EXPERIMENTAL SECTION Preparation of Graphene Oxide (GO). The GO used in this work was synthesized from natural graphite powder using a modified Hummers method.48,49 The GO product was collected and washed thoroughly using a mixed aqueous solution of 3 wt % H2SO4/0.5 wt % H2O2, and deionized (DI) water until the pH value was 4 according to our previous work.44 Finally, a homogeneous GO suspension (10 mg mL−1) was obtained by dispersing the resultant mixture in DI water under ultrasonication for 5 h. Preparation of the CoO/Graphene Hybrid. An aqueous exfoliated GO suspension (200 mL, approximately 200 mg) was obtained after ultrasonication for 3 h, followed by adding the aqueous Co(CH3COO)2·4H2O (100 mL, 100 mM) solution. Hydrazine hydrate (5 mL, 80 wt %) was then added under stirring. The mixture was further stirred for 24 h. The resulting solid was collected by filtration and then washed thoroughly with DI water. After dry in vacuum at 80 °C, the asprepared Co-precursor/graphene intermediate was further treated at 450 °C (ramp rate: 1 °C min−1) in a 5% N2 atmosphere for 2 h. Finally, the black-colored product of CoO/ graphene hybrid was collected for further characterizations. Materials Characterization. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a X’Pert PRO (PANalytical B.V., Holland) diffractometer with highintensity Cu Kα1 irradiation (λ = 1.5406 Å). The general morphology of the samples was characterized by using fieldemission scanning electron microscopy (FESEM, FEI, Sirion 200) coupled with an energy dispersive X-ray (EDX, Oxford Instrument) spectrometer. Transmission electron microscopy (TEM) images were obtained on a JEOL 2100F microscope. Xray photoelectron spectroscopy (XPS) measurements were carried out on a VG MultiLab 2000 system with a monochromatic Al Kα X-ray source (ThermoVG Scientific). Raman spectra were recorded on a Renishaw Invia spectrometer using Ar+ laser of 514.5 nm at room temperature. TG/DTA analyses were performed with a PerkinElmer

RESULTS AND DISCUSSION As shown in Figure 1, hybrid CoO/graphene nanosheets were synthesized by a simple solution route at room temperature

Figure 1. Schematic illustration of the formation of hybrid CoO/ graphene nanosheets.

combined with a heat-treatment process. In a typical procedure, an ultrathin film of an amorphous Co-based precursor was formed uniformly on the graphene nanosheets in the presence of hydrazine hydrate (Figure S1−S3, see the Supporting Information). Then, the hybrid Co-based precursor/graphene was converted into a CoO/graphene replica through thermal treatment at 450 °C for 2 h in N2. The crystallinity and phase of the product were examined by powder XRD. Figure 2 displays the typical XRD pattern for the product. The diffraction peaks can be indexed to a pure phase of CoO [space group: Fm3m (225)] with a cubic structure (JCPDS no. 43-1004). The significant broadening and reduced intensity of the XRD peaks confirm the nanocrystalline nature 20795

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doublet of core-level Co 2p3/2,1/2, indicating the Co (II) oxidation state.55 The high-resolution C 1s spectrum of the asprepared composite is shown in Figure 3c. The strong C 1s peak (284.5 eV) corresponds to graphitic carbon in graphene, whereas the weaker ones arising from the oxygenated carbons (carbon in C−O at 286.2 eV; carbonyl carbon, CO, 287.9 eV; carboxylate carbon, O−CO, 289.0 eV)56 indicate the deoxygenation process accompanying the reduction of GO. The O 1s peak at 531.8 eV was observed, indicating the existence of residual O2− species bonded with C atoms in graphene (Figure 3d).57 The general morphology of the product was investigated by FESEM. Figure 3a−c shows the representative FESEM images for the CoO/graphene nanocomposite. It is apparent that no significant change in the morphology was observed in comparison with the precursor (Figure S2, see the Supporting Information). A large number of nanosheets with a lateral dimension in micrometer size exist in the product. At a high magnification (Figure 4c), it can be observed that the hybrid

Figure 2. Representative XRD pattern of the resulting CoO/graphene hybrid.

and the ultrafine crystallite size of the products. The characteristic stacking peak of graphene at 22−28° cannot be detected, indicating that the graphene nanosheets are homogeneously dispersed without regular stacks and successfully covered with CoO. The similar phenomenon was also observed in the previous reports on the graphene-based hybrids.50−53 In the Raman spectrum of the CoO/graphene product (Figure S4, see the Supporting Information), two characteristic bands of carbon from graphene were detected at 1350 cm−1 (D-band) for disordered amorphous carbon and 1596 cm−1 (G-band) for graphitic carbon.54 The carbon content in the final CoO/graphene product evaluated by TG analysis is about 16.0 wt % (Figure S5, see the Supporting Information). Important information on the composition and the surface electronic state of the prepared CoO/graphene nanocomposite can be further provided by XPS. The peaks of Co (Co 2s, Co LMM2, Co 2p1/2, Co 2p3/2, Co LMM, Co 3s, Co 3p) and O (O 1s, O KLL) in the survey XPS spectrum for CoO/graphene hybrid (Figure 3a) are expected from CoO. Meanwhile, the peak of C 1s is attributed to graphene nanosheets. The Co 2p peak is further examined by high-resolution XPS (Figure 3b). The Co 2p3/2 and Co 2p1/2 peaks of the CoO are at 780.8 and 796.6 eV with a 15.8 eV peak-to-peak separation, characteristic

Figure 4. (a−c) Representative FESEM images and (d) EDX spectrum of the resulting CoO/graphene hybrid.

CoO/graphene nanosheets are less than 10 nm in thickness, and no obvious particles are on the surface, indicating the very small size of CoO. The corresponding EDX microanalysis (Figure 4d) confirms the existence of Co, O, and C in the CoO/graphene product, which is in good agreement with the XRD and XPS results (Figures 2 and 3). Furthermore, the elemental distribution of Co, O, and C in the CoO/graphene product was explored (Figure S6, see the Supporting Information), suggesting that the ultrafine CoO nanoparticles are distributed uniformly on the graphene sheets. To provide further insights into the morphology and structure on the resulting hybrid CoO/graphene nanosheets, TEM investigations were carried out. Figure 5a, and b show the typical bright-field TEM images of the CoO/graphene product. The TEM images clearly demonstrate flexible and uniform sheet-like structures. The dark/light contrast is clearly observed. The light patches suggest the existence of numerous irregular nanosized pores with diameters less than 3 nm, arising from the decomposition of the amorphous Co-based precursor film on graphene (Figure S3, see the Supporting Information). The gray area can be assigned to the ultrafine CoO nanoparticles. These tiny CoO nanoparticles of ∼5 nm in size are densely

Figure 3. XPS spectra for the CoO/graphene nanocomposite: (a) survey spectrum and high-resolution (b) Co 3d, (c) C 1s, and (d) O 1s spectra. 20796

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Figure 5. (a−b)TEM images, (c) SAED pattern, and (d) HRTEM image of the hybrid CoO/graphene nanosheets.

Figure 6. (a) Discharge and charge curves at a current density of 200 mA g−1 cycled between the voltage of 3−0.01 V vs Li/Li+. (b) CV curves of the CoO/graphene hybrid at a scan rate of 0.2 mV s−1 in the cutoff voltage of 3−0.01 V vs Li/Li+. (c) Cycling performance of the prepared CoO/graphene hybrid electrode at 200 mA g−1. (d) Cycling performance of the prepared CoO/graphene electrode at various current densities of 200, 400, 800, 1200, and 1600 mA g−1.

anchored on the surface of graphene almost without aggregation, forming the hybrid CoO/graphene nanosheets. The low aggregation of CoO nanoparticles indicates that the graphene nanosheets play an essential role in achieving good dispersion of the CoO nanoparticles. Figure 5c shows the corresponding selected-area electron diffraction (SAED) pattern. It exhibits clear diffraction rings, demonstrating the polycrystalline nature. These ring patterns were assigned to the (111), (200), and (222) reflections of the face-centered cubic CoO structure (JCPDS no. 43-1004). High-resolution TEM (HRTEM) images were further carried out. The obvious crystalline lattice with a distance of 0.24 nm (Figure 5d) matches very well with the lattice distance of (111) plane of cubic CoO, consistent with the XRD and SAED results (Figures 2 and 5c). Lithium storage properties of the CoO/graphene nanocomposite were investigated by CV and galvanostatic discharge/charge cycling. Figure 6a shows the galvanostatic discharge/charge voltage profiles for the hybrid CoO/graphene electrode cycled at a current density of 200 mA g−1 over the voltage range of 3.00−0.01 V vs Li/Li+. In the first discharge profile, a broad potential plateau at around 0.7 V reflects the conversion reaction and the formation of the polymer/gel-like film, resulting from the electrochemically driven electrolyte degradation.19,58−62 It is observed that the initial discharge/ charge capacities are 1235.4 and 855.6 mAh g−1, respectively. Hence, there is an irreversible capacity loss of 30.7%, in good agreement with the CV results (Figure 6b). The large irreversible initial discharge capacity could be attributed to the formation of the polymer/gel-like film. From the second cycle onward, the potential plateau at around 0.7 V disappears, which is also illustrated in the CV curves (Figure 6b). Moreover, the reversible formation of Li2O occurs, accompanying the redox of Co nanoparticles and the reversible growth of the polymer/gel-like film in the following discharge/charge cycling.19 As shown in Figure 6c, the reversible capacity gradually increases in the initial cycles and reaches 1097.1 mAh g−1 after 40 discharge/charge cycles. In the subsequent cycles, the capacity remains stable. A high reversible capacity of ∼1018.0 mAh g−1 with 119.0% capacity retention (based on the initial reversible capacity) is achieved even after 520 discharge/ charge cycles, and the Coulombic efficiency remains ∼100% upon cycling, indicating the excellent cyclability (Figure 6c).

The increased capacity in the initial cycles may be assigned to the interfacial Li storage63−65 or the reversible growth of a polymeric gel-like film resulting from the kinetically activated electrolyte degradation.58−62 A similar phenomenon was widely observed in the previously reported metal oxide/graphene composites.37,38,40−42,44,47 Additionally, the synthesized CoO/ graphene nanocomposite shows good rate capability (Figure 6d). At the current density as high as 1600 mA g−1, the CoO/ graphene nanohybrid still exhibits a favorable specific capacity of 531.2 mAh g−1. Importantly, after the high current density measurements, the specific capacities of the as-prepared CoO/ graphene hybrid electrode are able to recover to the initial values, indicating the good reversibility and excellent cyclability. The excellent cycling performance of the CoO/graphene nanocomposite could be attributed to the unique properties of the ultrafine CoO nanocrystals on the graphene nanosheets, which ensures that the electrodes maintain their integrity over many discharge/charge cycles. The small size of the CoO nanoparticles might play a crucial role. The absolute volume change of each single CoO particle is small and thus the small size is hard to envision their further reduction, which is beneficial for preserving electrical contact between the active nanoparticles with electronically conductive graphene as well as the particles’ integrity. Moreover, the interstitial site between the CoO nanoparticles can provide room for expansion during lithium insertion. Additionally, graphene nanosheets serve as the elastic substrate and afford good dispersion of the ultrafine CoO nanoparticles, which can effectively buffer the volume changes and inhibit the aggregation of CoO during the lithiumion insertion/exaction. To confirm the synergistic effect of small particle size and graphene to keep the integrity and stability of the electrodes, the morphology and structure of the CoO/graphene nanocomposite after long-term electrochemical cycles were studied (Figure 7). When discharged/charged at a current density of 200 mA g−1, the nanocomposite featuring ultrafine CoO particles on graphene nanosheets still retain its original morphology even after 520 cycles. This demonstrates the long-term stability of the as-obtained CoO/graphene 20797

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electronically conductive graphene facilitate the lithium-ion and electron transfer and improve the rate capability.

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CONCLUSIONS In summary, we have fabricated the hybrid ultrathin CoO/ graphene nanosheets through a facile approach to successfully address the long-term stability issue of transition-metal oxide. It is demonstrated that the as-formed CoO/graphene nanohybrid can serve as a high-performance anode material with long cycle life (with 119.0% capacity retention over 520 cycles), high reversible specific capacity (1018.0 mAh g−1 over 520 cycles at 200 mA g−1), and excellent rate capability (531.2 mAh g−1 at 1600 mA g−1). The design of ultrafine CoO nanoparticles supported by graphene nanosheets can effectively avoid the particle cracking, pulverization, and aggregation upon cycling. Thus, our successful material design indicates that ultrafine transition-metal oxide nanoparticles on the electrochemical conductive network can achieve long-term structural stability, enabling a long cycle life. Also, the strategy to design conversion electrodes with ultrafine nanoparticles on the electrochemical conductive network can be extended to construct other long-life lithium-storage electrodes.

Figure 7. (a and b) TEM images and (c) SAED pattern of the hybrid CoO/graphene electrode after 520 cycles at a current density of 200 mA g−1 cycled between the voltage of 3−0.01 V vs Li/Li+.

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nanocomposite (Figure 7a,b). It is found that the CoO particles of ∼5 nm did not further break during discharge/charge cycling. It may be because 5 nm is very close the critical breaking size for CoO. Under the protection of graphene, the aggregation of the ultrafine nanoparticles was also avoided. The corresponding SAED result indicates that the electrode material still transforms to CoO during the charge process even after 520 cycles, indicating a highly reversible conversion reaction for the prepared material (Figure 7c). For the electrode made of cobalt oxides, it is reported that the capacity remains constant or even slightly increased, and finally decays after n cycles, where n is dependent on the particle morphology and structure.13,19,20,42,66 Generally, the large volume change and the stresses accompanying phase transitions upon (de)lithiation often cause the partial pulverization and cracking of the electrode. After lithiation, big CoO particles typically break into much smaller ones (e.g., ∼4 nm), forming loose aggregates.19,58 These aggregates are easy to induce into disconnection and peel off from the surface of graphene due to the weak contact (Figure 8a), thus leading to fast decay in capacity upon cycling.

ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, FESEM images, Raman Spectrum, TG/DTA curves, and elemental mapping analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-27-87558241. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant Nos. 51002057, 21271078 and 50825203), the 863 program (Grant No. 2009AA03Z225), the Natural Science Foundation of Hubei Province (Grant No. 2008CDA026), and the PCSIRT (Program for Changjiang Scholars and Innovative Research Team in University). The authors thank Analytical and Testing Center of HUST for the XRD and TG/DTA measurements.

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Figure 8. Schematic illustration of the morphology change before and after many discharge/charge cycles: a big CoO nanoparticle (a) and ultrafine CoO nanoparticles (b) anchored on a graphene nanosheet.

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dx.doi.org/10.1021/jp3070147 | J. Phys. Chem. C 2012, 116, 20794−20799