Co3O4 Nanocages for High-Performance Anode Material in Lithium

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Co3O4 Nanocages for High-Performance Anode Material in LithiumIon Batteries Nan Yan, Lin Hu, Yan Li, Yu Wang, Hao Zhong, Xianyi Hu, Xiangkai Kong, and Qianwang Chen* Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science & Engineering, University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: Co3O4 nanoparticles have been prepared by a facile strategy, which involves the thermal decomposition of nanoparticles of cobalt-based Prussian blue analogues at different temperatures. The nanoparticles prepared at 450, 550, 650, 750, and 850 °C exhibited a high discharge capacity of 800, 970, 828, 854, and 651 mAhg−1, respectively, after 30 cycles at a current density of 50 mAg−1. The nanocages produced at 550 °C show the highest lithium storage capacity. It is found that the nanocages display nanosize grains, hollow structure, a porous shell, and large specific surface area. At the temperature higher than 650 °C, the samples with larger grains, better crystallinity, and lower specific surface area can be obtained. It is found that the size, crystallinity, and morphology of nanoparticles have different effects on electrochemical performance. Better crystallinity is able to enhance the initial discharge capacity, while porous structure can reduce the irreversible loss. Therefore, the optimal size, crystallinity, and cage morphology are suggested to be responsible for the improved lithium storage capacity of the sample prepared at 550 °C. The as-prepared Co3O4 nanoparticles also have a potential application as anode material for Li-ion batteries due to their simple synthesis method and large capacity.

1. INTRODUCTION In recent years, rechargeable Li-ion batteries (LIBs) have become the power source of choice for modern portable electronic equipment and have been used widely because of their high energy densities.1 For the demands of long cycle life, high capacity, and good rate capability, lots of novel anode electrode materials have been developed such as nanocarbons, alloys, metal oxides, and metal sulfides/nitrides.2−19 As is wellknown, nanomaterials have large specific surface areas and many of active sites, which can be developed to increase the anode capacities. In addition, nanomaterial-based electrodes with stable and solid electrolyte interface (SEI) films and large electrode/electrolyte contact areas can increase the charge− discharge rates and subsequently power densities because of their effective transfer pathways for both Li ions and electrons.2 Co3O4 has attracted much more attention because of its high theoretical capacity (890 mAhg−1) which is due to the conversion reaction mechanism,20 and many Co3O4 nanostructures have been synthesized by various routes.10−16,21,22 However, this material usually suffers from poor capacity retention while cycling and poor rate capability, which still remain major challenges when used in practical cells. To solve this problem, some methods have been used, such as improving the special structure10−12 and coating with graphene.21,22 In the previous reports, Wang et al. summarized that the synergetic effect of small diffusion lengths and sufficient void space in multishelled hollow structures could enhance the cyclic stability and rate capacity.10 Du et al. prepared porous Co3O4 nanotube © 2012 American Chemical Society

structures consisting of the small nanoparticles with the size of 5−10 nm which were responsible for the improved performance.12 Without discussing the influence of crystallinity and others, these previous reports only considered the improved performance caused by the porous structures. However, Lou made a comparison of the electrochemical performance of Co3O4 nanotubes with different crystallinity and proved that the improved crystallinity led to a better cycling performance.26 Although Co3O4 nanoparticles with high capacity have been prepared already, the researchers investigate just one of those factors determining the charge/discharge performance, such as hollow structure or small size. Moreover, Tian et al. synthesized the mesoporous Co3O4 nanotube under different calcination temperatures and indicated the charge−discharge performance could be optimized.15 Poizot et al. suggested that there was an optimum particle size for each metal oxide and hence the best electrochemical performance.20 It can be inferred that the crystallinity, morphology, specific surface area, chemical compositions, and structural stability of nanomaterials all have significant effects on the charge/discharge performance. As we all know, however, the results of some methods to improve the material property are sometimes contradictory. For example, porous structure can provide large specific surface areas and decrease the crystallinity and structural stability at the Received: December 30, 2011 Revised: February 6, 2012 Published: March 5, 2012 7227

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2.2. Characterization. The powder X-ray diffraction (XRD) patterns were collected on a Japan Rigaku D/MAXcA X-ray diffractometer equipped with Cu Ka radiation over the 2θ range of 10−70°. Thermogravimetric analysis (TGA) was carried out using a Shimadzu-50 thermoanalyser under nitrogen gas flow at 10 oC min−1 in the temperature range 30−800 °C. Scanning electron microscopy (SEM) images were performed on a JEOL JSM-6700 M scanning electron microscope. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. Specific surface areas were computed from the results of N2 physisorption at 77 K (Micromeritics ASAP 2020) by using the BET (Brunauer− Emmet−Teller) and BJH (Barrett−Joyner−Halenda). An Xray photoelectron spectrum (XPS) was performed on an ESCALAB 250 X-ray Photoelectron Spectrometer with Al Kα radiation. The FT-IR spectrum was obtained using a Magna-IR 750 spectrometer in the range of 500−4000 cm−1 with a resolution of 4 cm−1. 2.3. Electrochemical Measurements. The electrochemical behavior of the as-prepared porous Co3O4 was examined using CR2032 coin type cells vs Li with 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC:DEC = 1:1, v/v) as the electrolyte. The working electrode was fabricated by compressing a mixture of the active materials, conductive material (acetylene black), and binder (polyvinylidene fluoride) in a weight ratio of Co3O4/carbon/PVDF = 5:3:2 onto a copper foil current collector and then drying at 60 °C for 12 h. For the poor conductivity of Co3O4, the weight ratio of carbon and PVDF should be a little higher than the working electrode of other materials like graphite. A similar high weight ratio has been used in another paper.25 The cells were assembled in an argon-filled glovebox (MBraun Labmaster 130). The electrode capacity was measured by a galvanostatic discharge−charge method in the voltage range between 3.0 and 0.05 V at a current density of 50 mAg−1 on a battery test system (Neware CT-3008W).

same time, which, as a result, leads to a reduction in the capacity after numerous cycles. Li et al. prepared porous Co3O4 nanotubes with high initial discharge capacity but fast fading because of the unstable structure.14 According to the results of the previous studies,10−13 the advantages of porous structure were appreciated, while the disadvantages were ignored. Specifically, Co3O4 nanoparticles formed by decomposition of a precursor material at a relatively low kiln temperature possess porous structure with a poor crystallinity. Therefore, all kinds of influencing factors should be considered in the preparation of Co3O4 nanoparticles to achieve an excellent electrochemical performance, like increasing the crystallinity as much as possible when maintaining the porous structure. Thus, annealing at different temperatures is necessary to get Co3O4 nanoparticles with the above-mentioned optimal structure. Researchers have paid close attention to Prussian blue (PB) and Prussian blue analogues (PBA) for a long time due to their applications in molecular magnetics, CO2 adsorption, heavy metal ion absorption, and so on.23 Using them as reactants to synthesize functional materials has not aroused much attention. As far as we know, besides our team, only Hu et al. thermally converted Prussian blue to mesoporous magnetic iron oxide.24 We recently reported a simple method to prepare monodisperse Co3[Co(CN)6]2·nH2O nanoparticles with a uniform shape,23b and these particles can be thermal converting to Co3O4 without the need of a template or structure-directing agent. Porous Co3O4 nanoparticles with different morphology and structure can be achieved by the structural stability itself and decomposition reaction in the calcination process. The nanoparticles generated at different temperatures are very suitable to study the multifactor combined influences to the lithium storage performance. In this study, a typical PBA, Co3[Co(CN)6]2, has been synthesized by a simple approach and calcined to form porous Co3O4 nanoparticles with different morphology and structure at different temperatures. The electrochemical performances as anode materials for the lithium ion battery and the gap affected by the mentioned factors have been evaluated and analyzed. We also have used XRD, TGA, SEM, TEM, N2 sorption, XPS, and FT-IR to characterize the structure and morphology of Co3[Co(CN)6]2 and Co3O4 nanoparticles. The as-prepared Co3O4 nanoparticles are found to manifest high capacity performance and long cycle life.

3. RESULTS AND DISCUSSION The synthesis includes two steps, formation of Co3[Co(CN)6]2·nH2O as a precursor in solution and then thermal conversion to Co3O4 at different temperatures. Figure 1a shows the XRD pattern of as-prepared Co3[Co(CN)6]2·nH2O, and all of the diffraction peaks in this pattern are in good agreement with the standard Joint Committee on Powder Diffraction Standards (JCPDS) card no. 77-1161 (space group: F43m, lattice constant α = 10.20 Å). The pattern shows very sharp shapes of peaks, and no impurity peaks are observed, which indicates that the sample is a single face-centered cubic (fcc) phase of Co3[Co(CN)6]2 and has good crystallinity. From the thermal-gravimetric (TG) curve which is shown in Figure 1b, the thermal decomposition behavior of the as-prepared sample can be observed. It is clear to find two decomposition steps. The first step occurs at the temperature of 40 °C and shows a weight loss of 26.54% when the platform of the curve reappears at about 180 °C, illustrating the evaporation of adsorbed water. The second step shows an abrupt change of curve with a weight loss of 25.59% near the temperature of 350 °C, which indicates the oxidation of the sample. From the known chemical composition of Co3[Co(CN)6]2·nH2O, the equation of

2. EXPERIMENTAL SECTION 2.1. Synthesis. All chemicals are of analytical grade and used without purification. The typical synthetic experiments were as follows: Solution A: 0.075 mmol (18.7 mg) of Co(CH3COO)2·nH2O was dissolved in a 10 mL H2O system under agitated stirring to get a transparent solution. Solution B: 0.04 mmol of K3[Co(CN)6]2 (16.6 mg) and 0.3 g of polyvinylpyrrolidone (PVP) were dissolved in 10 mL of distilled system. A red turbid solution formed when solution A was added to solution B slowly and regularly using a syringe. The whole reaction process was kept at room temperature with agitated stirring. After 10 min, the reaction was aged at room temperature without any interruption for 4 h. The resulting pink precipitation was collected, washed several times by distilled water, and finally dried in air at 60 °C. The thermal decomposition of the pink precursor to Co3O4 was performed at 450, 550, 650, 750, and 850 °C, respectively, for 1 h in air in the oven with a heating rate of 10 oC min−1. 7228

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Figure 2. XRD patterns of Co3O4 nanoparticles S450, S550, S650, S750, and S850.

dimension of crystallites; λ, the X-ray wavelength; B, the pure diffraction broadening of a peak at half-height; θ, the Bragg angel). The sizes from S450 to S850 are 60.5, 73.9, 98.6, 140.1, and 190.1 nm, respectively (the instrumental error has been deducted). A tendency for minimization of the interfacial surface energy is considered the major factor of crystallite growth during the annealing process.26−28 Different crystalline Co3O4 nanoparticles are obtained by changing the calcination temperature and can impact the electrochemical property, which will be discussed in the following. The morphologies of the precursor Co3[Co(CN)6]2·nH2O and the Co3O4 nanoparticles were examined by scanning electron microscopy (SEM). Figure 3(a) shows the SEM image of Co3[Co(CN)6]2·nH2O, and (b)−(f) show the images of Co3O4 nanoparticles obtained at different sintering temperatures. From Figure 3(a), it can be observed that the

Figure 1. (a) XRD pattern and (b) TG curve under air with a ramp of 10 °C min−1 of the precursor Co3[Co(CN)6]2.

decomposition reaction in the air can be inferred as the following Co3[Co(CN)6 ]2 ·nH2O + O2 → Co3O4 + CO2 + NxOy + H2O

(1)

On the basis of this TG data, the Co3[Co(CN)6]2·nH2O was annealed at 450, 550, 650, 750, and 850 °C for 1 h to obtain Co3O4 nanoparticles with different morphologies and structures, hereinafter designated as S450, S550, S650, S750, and S850, respectively. Further characterizations are carried out in the following. The crystallographic structure of the samples after annealing was analyzed by X-ray powder diffraction (XRD) as is shown in Figure 2. All of the peaks of the five samples correspond well to spinel Co3O4 (JCPDS card no. 42-1467, space group: Fd3m, lattice constant α = 8.08 Å). According to the patterns, no diffraction peaks of Co3[Co(CN)6]2 are observed, which reveal the complete thermal converting of Co3[Co(CN)6]2·nH2O to Co3O4. As expected, with increasing the calcination temperature, a sharpening of the peaks is observed and indicates the improved crystallinity. To understand the influence of annealing temperature on the growth of crystallite size, the mean crystallite sizes are estimated from the (311) peak with the Scherrer formula, D = 0.89λ/(B cos θ) (D, average

Figure 3. SEM images of: (a) the precursor Co3[Co(CN)6]2 and (b)− (f) the Co3O4 nanoparticles S450−S850, respectively. 7229

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Co3[Co(CN)6]2·nH2O particles have uniform size and shape which is called truncated nanocubes, and the mean diameter is about 120 nm based on an average measuring of no less than 50 particles. When the precursor was calcined at 450 °C, the morphology of S450 that displayed in Figure 3(b) does not show much difference with the precursor, in addition to the size of particles decreased and holes that appeared on the surface of nanoparticles. This change means the formation of porous Co3O4 nanoparticles through the thermal decomposition of the precursor. The S550 nanoparticles can also maintain a basic shape of a truncated nanocube which can be seen from Figure 3(c), and more large holes occur on the surface. However, the morphology of the S650 has started to change because the nanocubes consist of primary particles that fall into smaller grains, and the initial structures have collapsed, which is displayed in Figure 3(d).28 This change became clear for the S750, and a new dendritic structure replaced the nanocubes. At 850 °C, the grains gather and recrystallize to a large irregular nanoparticle which can be observed in Figure 3(e) and 3(f). We tend to explain this phenomenon by Ostwald ripening that can be described with the growth of larger crystals from those of smaller size which have a higher solubility. From these SEM images, it can reveal that different morphologies of Co3O4 nanoparticles are obtained by thermal decomposition of the precursor Co3[Co(CN)6]2·nH2O at different temperatures, and the nanoparticles are porous when the temperature is relatively low just like 450 and 550 °C. To observe the detailed morphological and inner structural difference of Co3O4 nanoparticles that exhibit significant change at specific sintering temperature, transmission electron microscopy (TEM) was used to take TEM images. Figure 4 displays the TEM images of Co3[Co(CN)6]2·nH2O as the precursor and the Co3O4 nanoparticles. From Figure 4(a), it can be observed that the inner structure of Co3[Co(CN)6]2·nH2O is a solid nanocube, and the shape is uniform. After annealing, the microstructure of Co3O4 became porous which is revealed in Figure 3(b) and 3(c). This kind of

mechanism of porous structure formation has been interpreted by the Kirkendall effect, which is based on a nonequilibrium interdiffusion process.29 A variety of porous materials were prepared though this effect.29−31 Similarly to the literature, during the thermal decomposition the diffusion rate of Co3[Co(CN)6]2·nH2O was different from that of atmospheric oxygen so that hollows were produced. Moreover, since PVP has been used as surfactant in the synthesis of Co3[Co(CN)6]2·nH2O, it could lead to formation the PVP-protected nanoparticles. Therefore, when annealing at a relatively low temperature such as 450 and 550 °C, the Co 3 [Co(CN)6]2·nH2O could transform to Co3O4 while maintaining the original shape. In the meanwhile, the thermal decomposition took place in the internal nanoparticles and generated gas products like CO2 and N2 which escaped from the inside of Co3[Co(CN)6]2·nH2O nanoparticles. These factors led to the formation of porous Co3O4 nanoparticles with the hollow nanocage structure. With the increasing annealing temperature, the small grains are observed, and the hollow structure became collapsed. At the temperature of 750 and 850 °C, a solid dendritic structure appeared instead of a porous hollow structure and turned to large irregular crystals at last as is shown in Figure 4(d), (e), and (f). Similar to the results observed by SEM, we can conclude that the structure of Co3O4 nanoparticles changes from a hollow porous structure to a solid dendritic structure with the temperature changing. The formation and changing process of Co3O4 nanoparticles could be described as shown in Figure 5. The Co3[Co(CN)6]2·nH2O

Figure 5. Schematic illustration of the preparation of the Co3O4 nanocage and nanoparticle.

nanoparticle turned to the Co3O4 nanocage via thermal decomposition. With the temperature rising, the nanocage became irregular nanoparticles by Ostwald ripening. The proportion of nanocage and nanoparticle depended on the annealing temperature. When calcined at lower temperature such as 350 °C, Co3O4 was also obtained (see the XRD pattern shown in Figure S1, Supporting Information), and the nanocage structure can be seen clearly according to the SEM and TEM images shown in Figure S2 and Figure S3 (Supporting Information). This result also confirmed the fabrication mechanism. According to a schematic illustration of the preparation of Co3O4 nanocages and nanoparticles shown in Figure 5, the morphology and structure of Co 3 O 4 nanoparticles were controlled by the annealing temperature. So, Co3O4 nanocages can be obtained with a different suitable heating rate. N2 sorption was used for characterization of the porous structure of Co3O4 nanoparticles and gathering information about the specific surface area and pore size. The N2 absorption−desorption isotherms at 77 K of each sample are shown in Figure 6 with insets showing the pore size distribution. It can be observed from Figure 6 (a) to (d) that the isotherms are characteristic of a type IV with type H3 hysteresis loop which confirm the mesoporous structure.32 The

Figure 4. TEM images of: (a) the precursor Co3[Co(CN)6]2 and (b)−(f) the Co3O4 nanoparticles S450−S850, respectively. 7230

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Figure 6. N2 adsorption/desorption isotherm (77 K) curve of Co3O4 nanoparticles and porous volume distribution of the pore size (the inset of each curve): (a) S450, (b) S550, (c) S650, (d) S750, and (e) S850.

impact the electrochemical property of Co3O4 nanoparticles due to providing a shorter Li-ion diffusion length29 and facilitating Li-ion insertion into the nanoparticles and deinsertion.33The observations from the N2 sorption are correspond well with the previous microscopy findings in Figure 3 and Figure 4. For further investigation of the chemical composition of the as-prepared Co3O4 nanoparticles at different annealing temperatures, the XPS and FT-IR measurements were carried out. Figure 7(a) shows the spectra of Co 2p of the samples, and two peaks of each sample are similar, centered at 779.8 and 794.8 eV, corresponding to the Co 2p3/2 and Co 2p1/2. The gap

isotherm of S850 which is displayed in Figure 6(e) shows little difference from the others. The BET surface areas of S450 to S850 were calculated to be 45.1, 31.9, 14.1, 9.3, and 3.6 m2 g−1, respectively. Furthermore, the pore size distribution can be studied by the inset. When it comes to the temperature of 450 °C, the pore size distribution has a relatively wide peak of 20 nm. With the annealing temperature increasing to 550 °C, the pore size distribution reveals a bimodal nature with a narrow distribution centered at 3 nm and a wide distribution centered at 9 nm. After that, the remaining three samples obtained at higher temperature do not expose the similar isotherms. This difference of BET surface area and pore size distribution may 7231

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Figure 7. XPS survey spectra of Co3O4 nanoparticles: (a) Co 2p, (b) O 1s, (c) wide scan of S450, and (d) FT-IR spectra of Co3O4 nanoparticles.

the same electrochemical behavior, and consistent with previous reports.10−16,25−28 The initial discharge shows a clear potential plateau at about 1.2 V vs Li+/Li, and the following sloping region may be relative with the formation of a solid electrolyte interphase (SEI) film which can cause the irreversible capacity loss. The first discharge capacities of S450, S550, S650, S750, and S850 are successively 984, 1109, 1150, 1095, and 1025 mAhg−1, all of which are larger than the theoretical value (890 mAhg−1). This is always ascribed to irreversible reactions to form SEI film and possibly interfacial lithium storage.10 The charging curves also show a similar shape, and the capacities are 630, 741, 846, 774, and 624 mAhg−1. Herein, the initial Coulombic efficiencies can be calculated as 64.02%, 66.82%, 73.56%, 70.68%, and 60.88%, respectively. Owing to the good crystallinity, special morphology, and structure, all of these as-prepared samples exhibite excellent cycle performance. The capacity versus cycle number curves at a current density of 50 mAg−1 scanned between 3 and 0.05 V are shown in Figure 8(c). From the profile, it can be seen that the S550 released an initial capacity of 1109 mAhg−1 and maintained a high capacity over 30 cycles. The reversible capacity of S550 after 30 cycles is 970 mAhg−1, which is the best of all the samples. This excellent performance exceeds the theoretical capacity of 890 mAhg−1, and a rough comparison indicates that the reversible capacity is better than those previously reported.10,11,14,16,21 The theoretical capacity of 890 mAhg−1 is predicted by the conversion reaction mechanism20 and calculated by the number of transferred electronics in the reaction. However, this is just an estimated result. Actually, the

between the peaks is about 15 eV (spin orbit splitting), which also corresponds to the standard Co3O4 spectra.34 The major peak of O 1s observed in Figure 7(b) is 530.1 eV, which also corresponds to the standard spectra.34 Besides, a small broadening peak centered at 532 eV can be attributed to the absorbed species at the surface. S450 was selected to carry out a wide scan between 0 and 1100 eV as is shown in Figure 7(c), since this sample was achieved at a low temperature and might contain lots of components. The peaks at 284.8, 406.8, 530.1, 779.8, and 794.8 eV are indexed to the characteristic peaks of C 1s, N 1s, O 1s, Co 2p3/2, and Co 2p1/2. The presence of carbon and nitrogen elements may be related with the incomplete decomposition of surfactant PVP and the CN− group of Co3[Co(CN)6]2·nH2O in the heat treatment. From the FT-IR spectra shown in Figure 7(d), two strong peaks at 670 and 574 cm−1 are related with a cobalt−oxygen bond. The peak at 1381 cm−1 which becomes weak with the increasing temperature proves the existence of nitrate, corresponding to the observations of XPS spectra. The completed decomposition is reasonable to the weakening of the peak at higher temperatures. To study the performance and differentiation as an anode of a Li-ion battery of as-prepared Co3O4 nanoparticles with different morphology and structure, the lithium storage properties were investigated using the standard Co3O4/Li half-battery configuration, where Co3O4 nanoparticles and metal lithium served as positive and negative electrodes, respectively. Figure 8 (a) and (b) shows the discharge and charge capacities versus voltage curves of five samples at the first cycle. The curves are qualitatively very similar, indicating 7232

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Figure 8. (a) and (b) Discharge and charge curves of Co3O4 nanoparticles at first cycle, respectively; (c) discharge capacities versus cycle number of Co3O4 nanoparticles at the same current density of 50 mAg−1 between 3 and 0.05 V; (d) discharge capacity of S550 at different current density between 3 and 0.05 V.

reached the opposite conclusion. Actually, the temperature is just one of the factors affecting the morphology and structure of Co3O4 which really impacts the electrochemical property. Increasing annealing temperature can enhance the material’s crystallinity, decrease the specific surface area, and change the inner structure. Poizot et al. suggested that for each metal oxide there is an optimum particle size and hence the best electrochemical performance.20 They also revealed the electrochemical reactions in Co3O4/Li battery as follows

phenomenon has been reported several times, yet the measured capacity was higher than the theoretical capacity.25−27 Since battery capacity depends on all the materials available to react, the porous material with hollow structure and large surface area would display higher reversible capacities that exceeded theoretical capacity because Li+ ions stored in the interfaces and pores of the porous material could take part in the reaction. Besides, S450, S650, and S750 also show good performance, and the reversible capacities after 30 cycles were 800, 828, and 854 mAhg−1, respectively. Although the S850 exhibits a relative poor cycle performance with the reversible capacity of 651 mAhg−1 after 30 cycles, it is also approximately 2 times higher than the theoretical value of common commercial graphite (372 mAhg−1). It is worth mentioning that all samples have not manifested a capacity fading except the initial irreversible capacity. After the 30th cycle, the irreversible loss of asprepared S450 to S850 can be calculated as 18.7%, 12.5%, 28%, 22%, and 36.5%, respectively, which also are better than previous results.11 For these reasons, the Co3O4 nanoparticles have great potential applications for a Li-ion battery. Some previous reports showed that the calcination temperature can affect the electrochemical lithium storage behaviors of Co3O4.15,26−28,32,33,35,36 For example, Wang et al. reported that the 3D porous Co3O4 prepared at 300 °C had smaller crystallite size and higher capacity compared to 400 °C as well as 500 °C.27 Liao et al. synthesized nanocrystalline Co3O4 thin film by radio frequency (RF) magneton sputtering and found that high-temperature annealing could enhance the capacity and cycle retention obviously.35 It is interesting that the two articles

Cathode: Co3O4 + 8Li+ + 8e− ↔ 3Co + 4Li2O

(2)

Anode: 8Li ↔ 8Li+ + 8e−

(3)

Overall reaction: Co3O4 + 8Li ↔ 3Co + 4Li2O

(4)

According to the equations, the metal cobalt and Li2O formed during the discharge process and then those nanosize metal particles could decompose Li2O in the following charge step. Well-crystalline Co3O4 nanoparticles can maintain the nanosize crystallite and activate the decomposition of Li2O, thus increasing the discharge capacity. Furthermore, the porous structure and the high specific surface area are helpful for the Li-ion insertion/deinsertion into the Co3O4 electrode33 and also provide lower polarization and short Li-ion diffusion lengths. This is conducive to maintaining the stability of the capacity as well. From the above, the material’s property is a result of multifactor determined. It can not be simply concluded that increasing annealing temperature can enhance or reduce the charge−discharge capacity; for instance, the high 7233

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temperature may improve the structural stability with the cost of reducing specific surface area. On the basis of Figure 8 (c) and compared with other samples, S850 with the best crystallinity shows the lowest discharge capacity for the smallest specific surface area and solid irregular nanoparticles. It is found that S550 has a similar porous nanocage structure with a smaller specific surface area than that of S450 but showed a 170 mAhg−1 higher discharge capacity than S450 after 30 cycles, which was caused by the better crystallinity. In addition, the porous nanocage structure of S550 with the pore size distribution centered at 3 and 9 nm provides appropriate Liion diffusion lengths and large contact area with electrolyte. All in all, the S550 shows the best performance as its nanosize grain, hollow structure, porous shell, and large specific surface area and so on. So, an optimum structure of nanoparticles (including crystallinity, morphology, inner structure and chemical composition) rather than an optimum size only is beneficial for improving electrochemical performance. Furthermore, according to the above analysis and experiment results shown in Figure 8, it can be found that better crystallinity is able to enhance the initial discharge capacity, while porous structure can reduce the irreversible loss. Considering the demand for cyclic stability and rate capacity, a porous structure with improved crystallinity is necessary for electrode materials. Because of the best performance in the constant current density test, the rate discharge capacity of S550 was evaluated and shown in Figure 8(d). From the profiles, it can be calculated that the average discharge capacity of 1273, 1117, 824, 487, and 252 mAhg−1 at 30, 50, 200, 1000, and 2000 mAg−1 and then the capacity go back to 811 mAhg−1 at 30 mAg−1. This excellent performance suggests that the pore size distribution centered at 3 and 9 nm, high specific surface area, and porous nanocages may be the optimum Co3O4 nanoparticles for anode material in lithium-ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The SEM and TEM images of Co3O4 nanoparticles obtained at 350 °C were discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (NSFC, 21071137) and the project of National 863 Hi-Tech Plan (2008AA06Z337).

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4. CONCLUSIONS In summary, Co3O4 nanoparticles with different morphology and structure are prepared by simple thermal decomposition of Co3[Co(CN)6]2·nH2O nanoparticles at 450, 550, 650, 750, and 850 °C. With the annealing temperature increasing, the crystallinity of Co3O4 becomes better, while the structure of nanoparticles changed from porous nanocages to solid dendritic grains. The porous Co3O4 nanocage generated at 550 °C shows the best lithium storage capacity (970 mAhg−1) due to its nanosize grain, porous structure (with the pore size distribution centered at 3 and 9 nm) with improved crystallinity, and large specific surface area. By comparing the different morphology and structure of Co3O4 nanoparticles, it is concluded that nanoparticles with an optimum structure (including crystallinity, morphology, inner structure, and chemical composition) rather than an optimum size would possess the improved electrochemical performance. The porous structure of electrode materials needs to be maintained, while the crystallinity increased as much as possible for improving cyclic stability and rate capacity. Simultaneously, the as-prepared Co3O4 nanoparticles have potential application as anode material in lithium-ion batteries due to the simple synthesis method and large capacity. 7234

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