Pulse Microwave Deposition of Cobalt Oxide Nanoparticles on

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Pulse Microwave Deposition of Cobalt Oxide Nanoparticles on Graphene Nanosheets as Anode Materials for Lithium Ion Batteries Chien-Te Hsieh,*,† Jiun-Sheng Lin,† Yu-Fu Chen,† and Hsisheng Teng‡ †

Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan 32003, Taiwan Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

‡

ABSTRACT: We report the high capacity, superior rate capability, and excellent cyclic stability of graphene nanosheets (GNs) decorated with Co3O4 nanoparticles as anodes in Li-ion batteries. A pulse microwave-assisted (MA) polyol method (total period: 30 min) is adopted to deposit cobalt oxides onto the GNs in water and ethylene glycol (EG), forming two types of Co3O4@GN hybrids. The selection of solvent in the MA process plays a central role in affecting the crystallinity, dispersion, and particle size of cobalt oxide particles. The resulting Co3O4@GN hybrid, prepared by the MA method in EG, shows a homogeneous dispersion of Co3O4 nanocrystals with an average size of 10 nm. The Co3O4@GN hybrid displays advantages of high reversible capacity, excellent cycleability, and high rate capability. This improved cyclic performance can be attributed to the formation of a three-dimensional GN framework decorated with Co3O4 nanocrystals, leading to fast diffusion of Li ions (diffusion coefficient: 5.82 × 10−12 cm2 s−1) and low internal resistance (equivalent series resistance: 92.2 Ω) determined by electrochemical impedance spectroscopy. With its ease of MA fabrication and good performance, the Co3O4@GN hybrid will hold promise in practical Li-ion batteries.

1. INTRODUCTION Lithium-ion batteries have been receiving considerable attention for a variety of applications in consumer electronics and electric vehicles.1−3 The development of Li-ion batteries with higher specific energy, higher power density, and longer cycle stability is a challenge in the field of energy-storage devices.4 The energy and the power density of the Li-ion battery strongly depend on the properties of cathode and anode materials. Therefore, the nanostructuring of electrode materials recently has shown a great potential because of short transport length of both electron and lithium ions, high electrode/ electrolyte contact area, and good accommodation of strain during charge/discharge cycling.4,5 As for anodes, graphite is the commercial anode material for Li-ion batteries with a theoretical specific capacity of 372 mAh g−1. An increasing effort has been made to seek alternative anode materials with higher energy and power densities, such as transition-metal oxides (Fe2O3, NiO, MnO2, SnO2, and Co3O4). Among them, Co3O4 anode offers a high theoretical capacity of 890 mAh g−1, which is 2 times higher than that of graphite anode. Pioneering studies have pointed out various types of Co3O4 anodes including Co3O4 nanopowders by citrate-gel method,4 Co3O4 microspheres by hydrothermal synthesis,6,7 Co3O4 nanowires by template-free method,8 and Co3O4 particles by thermal decomposition.9 Despite its high specific capacity, as-prepared Co3O4 anodes still suffer from two drawbacks. First, a large volume expansion/contraction and severe particle aggregation associated with Li+ insertion/extraction induce the anode pulverization and loss of interparticle contact, thus resulting in a large irreversible capacity and poor cycling stability.3,10 Second, © 2012 American Chemical Society

the formation of Li2O during the alloying process would cause the electrical isolation of anode materials.11 Accordingly, commercializing Co3O4 anodes with better cycle stability and higher energy density is still a challenge. Recently, graphene nanosheet (GN) with a flat one atomthick single layer or a few layers is expected to be a promising anode material because of its high surface area, good electric conductivity, and strong mechanical stability. Such superior properties originate from its unique nanoarchitecture that consists of carbon atoms tightly packed into a two-dimensional (2D) honeycomb sp2 carbon lattice.12 Theoretically, a singlelayer GN offers a Li-storage capacity of 744 mAh g−1 if Li ions are attached to both sides of the graphene sheets. A single or a few layers of GNs can be stabilized in solvent; however, the GNs tend to easily restack after drying because of the van der Waals interaction. The heavy aggregation would significantly result in the loss of extraordinary properties, and even the graphite may be subsequently formed. Accordingly, it has been demonstrated that GN-based anode materials exhibit large initial discharge capacity and reversible capacity, but low initial Coulombic efficiency and fast capacity fading.13,14 To resolve the problems, one strategy has been proposed to insert nanoparticles as spacers that can efficiently reduce the degree of restacking of GNs and keep their high active surface area. In fact, a number of research achievements have been obtained by anchoring Co3O4 nanoparticles into 2D GNs by chemical Received: April 30, 2012 Revised: June 28, 2012 Published: July 6, 2012 15251

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Figure 1. Schematic illustration of the Co3O4@GN hybrid, starting from (i) exfoliation of natural graphite, (ii) anchoring of Co oxide nanoparticles by the pulse MA method in different solvents, and (iii) thermal treatment.

impregnation followed by calcination,3 hydrothermal synthesis,5 microwave-assisted synthesis,12 chemical deposition,15 and in situ reduction process.16 Such strategies are directed toward the introduction of Co3O4 nanoparticles to create preferred sites for Li storage, thus leading to an improved capacity. In this Article, we report an alternative strategy for the anchoring of Co3O4 nanoparticles on GNs. Our approach is based on the work previously reported by our group on an MA polyol synthesis and analogous systems on carbon nanotubes.17,18 In this Article, a pulse MA synthesis of Co3O4 nanoparticles was carried out in two kinds of solvents (i.e., ethylene glycol (EG) and distilled water) under atmospheric pressure. The pulse MA synthesis only takes one-half an hour in a household microwave oven, inducing a homogeneous dispersion of Co3O4 nanoparticles over GNs. When compared to the conventional heating, the pulse MA synthesis takes advantages of energy efficiency, speed, uniformity, and simplicity in execution, showing a commercial feasibility. The Co3O4@GN hybrids in H2O (CGH) and in EG (CGE) deliver different cyclic performances, characterized by charge− discharge cycling and electrochemical impedance spectroscopy (EIS). The merit of the present work confirms one facile approach to fabricate Co3O4@GN hybrid with superior electrochemical performance for Li-ion batteries.

were prepared at ambient temperature. Next, 200 mL cobalt nitrate solutions were carefully poured into the GO slurries. The recipe of C/Co ratio was chosen according to our preliminary study. The pH value of the mixture was adjusted to 11 by adding a sufficient amount of 0.04 M KOH. The mixture was then uniformly dispersed by sonication for 1 h. After that, the beaker was placed inside a household microwave oven (Tatung Co., 900 W, 2.45 GHz) and heated under pulsed microwave irradiation for 30 min. The microwave power and the pulse frequency (i.e., ton:toff = 3 s:2 s) were set at 700 W and 1.5, respectively. The pulse MA synthesis was carried out at ca. 85 and 160 °C for water- and EG-based solutions, respectively. The treated GO samples were then separated from the solution and dried at 105 °C in a vacuum oven overnight. In step (iii), the GO powders were placed in a horizontal quartz tube and heated to 400 °C under N2 atmosphere. Thermal reduction process was performed and kept at that temperature for 1 h, giving two types of Co3O4@GN hybrids (i.e., CGH and CGE). The crystalline structures of obtained GN composites were characterized using X-ray diffraction (XRD, Shimadzu Labx XRD-6000), equipped with Cu Kα radiation emitter. The particle size and dispersion of oxide nanoparticles onto the GNs were observed using a field-emission scanning electron spectroscope (FE-SEM, JEOL JSM-5600) and a high-resolution transmission electron microscope (HR-TEM, JEOL, JEM2100). The Co3O4@GN hybrids were prepared by ultrasonically dispersing the composite in ethanol. A drop of the suspension was applied onto a copper grid and was dried in air. A thermogravimetric analyzer (TGA, Perkin-Elmer TA7) was used to analyze the amount of Co3O4 deposited on graphene sheets. The TGA analysis was conducted under an oxygen atmosphere with a heating rate of 10 °C min−1, ramping between 30 and 900 °C. The Co3O4@GN hybrids were mixed with a binder (poly vinylidenefluoride) and two conducting media (Super-P and KS-6) in a weight ratio of 82:10:4:4 in N-methyl pyrrolidinone (NMP) solvent to form the electrode slurry. The mixture was blended by a three-dimensional (3-D) mixer using Zr balls for 3

2. EXPERIMENTAL SECTION The scheme for fabricating Co3O4@GN hybrid consists of three steps: (i) exfoliation of natural graphite by the modified Hummers’ method, (ii) anchoring of Co3O4 nanoparticles by the pulse MA method in different solvents, and (iii) thermal treatment, as illustrated in Figure 1. Step (i) is to prepare the graphite oxide (GO) sheets using a modified Hummers’ method, as reported previously.4,19 The as-synthesized GO sheets (ca. 0.5 g) were placed in 300 mL of distilled water and EG and then put in an ultrasonic bath for 10 min, forming the types of GO slurries. In step (ii), two kinds of Co-containing solutions, 0.1 M Co(NO3)2·6H2O in distilled water and EG, 15252

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irradiation with the polar bond in the complex. When a wet body is exposed to microwave radiation, the section that has the highest moisture content adsorbs the microwave most strongly and therefore becomes the hottest part of the body. Accordingly, the rate of evaporation is the greatest from the wettest region.20 Thus, the interaction between polar solvent and surface oxides onto GO sheets is an important factor in determining the uniformity of deposits. It is known that the interaction potential is different in each solvent,21 and water has a dipole moment, showing stronger polarity than EG. Therefore, the nucleation rate (or crystal growth rate) in water is thus faster than that in EG under microwave irradiation. As observed from the inset of Figure 2a, the size of cobalt oxide cubes is approximately 300−500 nm, which is larger than the nanoparticles. In comparison, the CGE sample possesses a homogeneous dispersion of cobalt oxide nanoparticles with a size ranging from 10 to 15 nm (see the inset of Figure 2c). As a result, the effect of solvents in the MA synthesis acts as a crucial role in affecting the dispersion of cobalt oxide nanoparticles over GNs. Typical TGA analysis has been adopted to analyze the weight loading of Co3O4 in the GN hybrids, prepared from the pulse MA synthesis in H2O and EG. The residual weight corresponds to the loading of cobalt oxide deposits onto the GNs. The weight percentages of cobalt oxides for CGH and CGE samples are 35.2 and 34.7 wt %, respectively. This result reveals that both hybrids exhibit an almost identical loading of cobalt oxides onto the GNs. Figure 3 shows typical XRD patterns of pristine GN and Co3O4@GN hybrids. There is a characteristic peak of graphene

h to prepare uniform slurry. The resultant slurry then was uniformly pasted on a copper foil substrate with a doctor blade, followed by evaporating the solvent, NMP, with a blow dryer. The prepared anode sheets were dried at 135 °C in a vacuum oven for 12 h and pressed under a pressure of approximately 200 kg cm−2. The electrode layers were adjusted to control a thickness of ∼100 μm. CR2032-type coin cells were assembled in a glovebox for the electrochemical characterization. In the test cells, the Li foil and the porous polypropylene film served as the counter electrode and the separator, respectively. The electrolyte solution was 1.0 M LiPF6 in a mixture of ethylene carbonate, polycarbonate, and dimethyl carbonate with a weight ratio of 3:2:5. The half cells were charged using a conventional protocol of constant current−constant voltage (i.e., different C rates to 0.01 V with a 0.017 mA cutoff current). The cycling test at different C rates (from 0.1 to 5C) was performed within the voltage region of 0.01−3.0 V vs Li/Li+ at ambient temperature. The EIS measurement was carried out in the frequency range of 100 kHz to 0.01 Hz at open circuit potential with an alternating current perturbation of 5 mV. The EIS analysis of Co3O4@GN hybrid anodes was performed using an impedance analyzer (CH Instrument, Inc., CHI 608), and the equivalent circuit was analyzed using a computer software (Zview).

3. RESULTS AND DISCUSSION Figure 2a,b and c,d shows the FE-SEM images of CGH and CGE hybrids with low and high magnifications. Generally, GNs

Figure 2. FE-SEM images of (a,b) CGH and (c,d) CGE composites with different magnifications. The insets of (a) and (b) show highmagnification FE-SEM images of CGH and CGE samples, respectively.

Figure 3. Typical XRD patterns of pristine GN, CGH, and CGE powders.

are not perfectly flat but look like a crumpled structure with few carbon layers. In the images, cobalt oxide particles appear as white dots on a curved substrate corresponding to the planar graphene sheets. A large number of Co3O4 were decorated to the graphene sheets, forming Co3O4@GN hybrids. There is an obvious different dispersion between CGH and CGE hybrids, where there are two types of cobalt oxide particles (i.e., nanoparticles and cubes) attached to the CGH sample, inducing a random dispersion over the surface of GNs. Under microwave irradiation, energy is transferred to inside the complex, and heat is produced from the interaction of

at ca. 24.8° for pristine GN samples, indicating that the interplanar spacing of d002 is approximately 0.358 nm. The characteristic (111), (220), (311), (511), and (440) peaks are well observed, reflecting the presence of Co3O4 phase in the hybrids. The XRD results of the as-prepared nanohybrids confirm the formation of face-centered cubic (fcc) Co3O4 phase (JCPDS78-1970) in both CGH and CGE samples. It is worth noting that no reflections for impurities are discerned for the CGE sample, whereas weak CoO crystalline reflections appear in the CGH sample. The trace amount of CoO crystals displays the rock-salt structure, possibly originating from a thermal 15253

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decomposition: Co(OH)2 → CoO + H2O. The pulse MA synthesis in the presence of H2O tends to form Co(OH)2 attached to GO sheets, and the Co(OH)2 crystals are partially annealed to convert them into CoO@GN hybrid.5 Additionally, after the deposition of Co oxide in the hybrid, the (002) peaks are shifted to ca. 23.7° for GNs, and the corresponding interspacing distance is 0.375 nm. This is presumably because of the insertion of partial Co3O4 nanoparticles into the GNs, thereby increasing the interplanar distance. The microstructural observation of Co3O4@GN hybrids was characterized using HR-TEM and selected-area diffraction (SAD), as shown in Figure 4. The images clearly illustrate

nm, confirming the presence of the (111) plane of the Co3O4 fcc crystals. Figure 5 presents cyclic voltammogram (CV) curves of both Co3O4@GN hybrid electrodes at 0.1 mV s−1. In the first cycle,

Figure 4. HR-TEM images of (a,b) CGH and (d,e) CGE composites with low and high magnifications. HR-TEM images focusing on cobalt oxide nanoparticles in (c) CGH and (f) CGE composites, showing the lattice fingers. The insets of (b) and (d) show SAD patterns of CGH and CGE samples, respectively.

Figure 5. Typical CV curves of (a) CGH and (b) CGE anodes at 0.1 mV s−1.

two cathodic peaks were observed at ca. 1.1 and 0.05 V vs Li/ Li+ for the CGH anode, assigning to a multiple electrochemical reaction: reduction of Co3O4 or CoO and lithiation of GNs with Li ions. As for the CGE anode, the cathodic peaks take place at ca. 0.8 and 0.05 V vs Li/Li+, corresponding to the reduction of Co3O4 and GNs with Li ions. For both anodes, the observed anodic lump within the potential region of 1.8−2.2 V vs Li/Li+ is attributed to the oxidation (delithiation) reaction of Co3O4@GN hybrids. In the second cycle, the main reduction peaks are shifted to 0.8 and 1.0 V vs Li/Li+ for CGH and CGE anodes, respectively. The oxidation peaks for both anodes remin at the same potential (ca. 2.1 V), indicating that Co oxide is electroactive for Li storage. The reversible reaction on the Co3O4@GN hybrid can be proposed as follows:15,16

planar views of GNs covered with cobalt oxide nanoparticles. As expected, not only nanoparticles but also submicrometer cubes over the CGH composite can be viewed. As for the CGE sample, nanosized Co3O4 are uniformly dispersed upon the basal plane of GNs. The cube-like Co3O4 nanoparticles are found to have an average size of 12 nm. The homogeneous dispersion can be ascribed to a possible linkage between surface oxide groups (e.g., carboxyl and hydroxyl) of GO sheets and metallic ions,22 favoring the nucleation of Co3O4 under microwave irradiation. The SAD patterns from the area focusing on the as-prepared cobalt oxide nanoparticles are illustrated in the inset of Figure 4b and e, reflecting the presence of bright diffraction spots along with diffraction rings. The presence of such rings suggests that the Co3O4 grain consists of nanocrystallites. Figure 4c and f shows the atomic resolution HR-TEM images of the CGH and CGE samples, focusing on the cobalt oxide nanoparticles in both composites. For the CGH composite, the lattice fingers with a spacing of 0.46 nm are apparently visible in the nanograins, in which the (111) plane of the CoO rock-salt crystals can be viewed. As for the CGE sample, there is an obvious interlayer distance of 0.24

Co3O4 @GN + 8Li+ + 8e− ↔ 3Co@GN + 4Li 2O

(R1)

GN + x Li+ + x e− ↔ LixC

(R2)

Following the above parallel reactions, Co3O4 nanocrystals are capable of offering active sites for reversible reactions, and the GNs also provide a number of Li-storage sites from basal plane 15254

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and edge of graphene.16 In addition, a reversible reaction may occur in the CGH anode because of the presence of CoO crystals:5 CoO@GN + 2Li+ + 2e− ↔ Co@GN + Li 2O

Table 1. Components and Related Parameters of the Equivalent Circuit for Both GN Anodesa

(R3)

As compared to the first and second CV cycles, it has shown the irreversible capacity for both anodes, which can be attributable to the formation of a solid electrolyte interface (SEI) layer on the surface of GNs or Li-ion reaction with unreduced functional groups on the GNs.3 The characteristic EIS curves (Nyquist plots) of Co3O4@GN hybrid anodes are shown in Figure 6a and b. The Nyquist plots

electrode

RE (Ω)

RSEI (Ω)

RCT (Ω)

RES (Ω)

DLi (cm2 s−1)

CGH CGE

6.9 6.9

116.5 64.9

28.0 20.4

151.4 92.2

9.99 × 10−13 5.82 × 10−12

a The deviation for each component is CGH (280 mAh g−1) > pristine GN (220 mAh g−1). Again, this superior rate capability can be supported by one reason that the deposition of Co3O4 nanoparticles into the GNs promotes the fast diffusion of ions, electrons, and electrolytes in the stereo framework (i.e., the DLi value in CGE ≫ the DLi value in CGH), and their electrical conductivity also is improved (i.e., the RES value in CGH > the RES value in CGE), which has been well demonstrated by the EIS analysis. More importantly, the Co3O4@GN hybrids exhibit a stable capacity at 1C, as shown in Figure 9b. Among these anodes, the CGE anode displays not only the best cyclic stability but also the highest reversible capacity of ca. 650 mAh g−1 after 50 cycles. In contrast, the CGH and GN anodes decrease from 520 to 450 mAh g−1 and from 401 to 300 mAh g−1, respectively. It is worth noting that an inherent poor electrical/ionic conductivity of Co3O4/Co/Li2O matrix appears during the charge/discharge process, inducing capacity fading.8 Moreover, Co3O4 is converted to Co and Li2O, causing a volume expansion during Li uptake. Such a dramatic volume expansion leads to lattice destruction and thus provides unwanted mechanical stress and strain of pure Co3O4 anode. In our case, because the Co3O4 acts not only as a redox site but also as a nanospacer, the Co3O4@GN hybrid creates a 3D architecture that effectively accommodates the stress and strain of volume change, that is, a buffering effect.28 This finding confirms that there is a strong synergistic effect in the Co3O4@GN hybrid, in which homogeneous dispersion of Co3O4 plays as the key factor in the excellent cyclic performance and superior rate capability.

Figure 8. Typical charge−discharge curves of (a) CGH and (b) CGE anodes charged and discharged at various C rates.

theoretical capacity of GNs (i.e., 744 mAh g−1). The initial charge capacities are 861 and 934 mAh g−1 at 0.1C for the CGH and CGE anodes, respectively. The first discharge capacities of CGH and CGE anodes are approximately 1127 and 1129 mAh g−1, respectively. Such high specific capacities are found to be higher than that of fresh GN anodes (ca. 600− 1000 mAh g−1)12,14 and pure Co3O4 (890 mAh g−1)15 reported previously. This improved capacity can be attributed to a synergetic effect between GNs and the Co3O4 in the 3D framework, where Li ions are capable of adsorbing on the abundant nanocavities or defects of GNs and interspacing between neighboring Co3O4 nanocrystals. In other words, the framework provides a large amount of void spaces, imparting more available sites for Li storage. Therefore, the introduction of Co3O4 crystals into the 2D graphene network acts in a positive role to facilitate Li storage capacity, that is, the x number in (R2). As observed from Figure 8, the first discharge curves for both anodes exhibit a well-defined voltage plateau at ca. 0.90−1.1 V, which is followed by a decrease to the cutoff voltage of 0.01 V. The observation of voltage plateau is in agreement with the CV analysis. Additionally, similar phenomena have been viewed for Co3O46,7 and Co3O4@GN hybrid anodes.15 The CGE anode displays a Coulombic efficiency at the first cycle better than the CGH anodes, that is, 76.4% (CGH) and 82.9% (CGE). The presence of irreversible capacity can be ascribed to the 15256

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 886-3-4638800 ext. 2577. Fax: 886-3-4559373. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. Hsieh is grateful for the financial support from the National Science Council of Taiwan under contracts NSC 1012628-E-155-001-MY3 and NSC 99-2632-E-155-001-MY3. Prof. H. Teng acknowledges the support from the National Science Council of Taiwan (NSC 101-3113-E-006-011) and the Indus trial Technology Res earc h Institute South (B327HK3310), and the Bureau of Energy, Ministry of Economic Affairs, Taiwan (101-D0204-2).

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REFERENCES

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Figure 9. (a) The capacity as a function of C rate and (b) the capacity at 1C as a function of cycle number for pristine GN, CGH, and CGE anodes.

4. CONCLUSIONS In summary, we developed an efficient MA deposition approach to fabricate Co3O4@GN hybrids as anodes for Liion batteries. The selection of solvent in the pulse MA process played a central role in affecting the crystallinity, dispersion, and particle size of cobalt oxide particles. The Co3O4 nanoparticles, prepared by the MA method in EG, were 10 nm in size and homogenously anchor on GNs not only as spacer to keep the neighboring GNs separated but also as active sites for Li storage. By embedding Co3O4 nanoparticles in the GN network, such a confinement of Co3O4 nanoparticles by the surrounding GNs buffers the volume change upon Li insertion/ extraction. The resulting Co3O4@GN hybrid anode thus exhibited superior Li-ion performance with high reversible capacity, excellent cycleability, and good rate capability. This improved performance could be attributed to the formation of 3D GN framework decorated with well-dispersed Co3O4 nanocrystals, thus inducing fast diffusion of Li ions and low internal resistance. With the results as the basis, the present work delivers a fast MA synthesis of Co3O4@GN anode materials for high-performance Li-ion batteries. 15257

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