CoS2 Hollow Spheres - American Chemical Society

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CoS2 Hollow Spheres: Fabrication and Their Application in Lithium-Ion Batteries Qinghong Wang, Lifang Jiao,* Yan Han, Hongmei Du, Wenxiu Peng, Qingna Huan, Dawei Song, Yuchang Si, Yijing Wang, and Huatang Yuan Institute of New Energy Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), MOE (IRT-0927), Nankai University, Tianjin 300071, P. R. China ABSTRACT: Uniform hollow spheres of CoS2 have been successfully synthesized via a facile solvothermal method and electrochemically investigated as anode material for lithium-ion batteries. The key strategy is that sulfur powder is used as the sulfur source, while absolute ethanol (EtOH) serves as the solvent and reducing agent simultaneously. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) patterns demonstrate the high purity of the product. SEM images display that the hollow spheres are about 2
3 μm in diameter and 300 nm in shell thickness. The mechanism for the formation of the final hollow structure is discussed. Temperature and reaction concentrations are found to be the key factors in controlling the morphologies. Meanwhile, electrochemical measurements reveal that the as-prepared CoS2 delivers high discharge capacity (1210 mAh g
1) and good cycle stability, indicating that it might find possible application as anode material for lithium-ion batteries in the long term.

’ INTRODUCTION Cobalt sulfides of different stoichiometric composition such as CoS, CoS2, and Co9S8 have attracted great attention due to their potential application in catalysis,1 semiconductor,2 magnetic materials,3
5 lithium-ion batteries,6
10 and other fields.11,12 Many approaches have been explored to synthesize metal sulfides, including the high temperature solid phase process,9,13 the hydrothermal and solvothermal method,14
19 arc-discharge method,20 low-temperature procedures,21 chemical vapor decomposition,22 and so on.23,24 Among the above methods, hydrothermal and solvothermal methods are considered to be a facile method because it is easy to control the reaction condition and it provides relatively abundant sulfur sources. In recent years, considerable efforts have been made to the preparation of CoS2 for its specific properties. It is known that CoS2 possesses higher electronic conductivity and thermal stability compared to other metal sulfides (such as FeS2). Therefore, it has attracted particular interest in the field of secondary lithium-ion batteries.13,25 Yan et al.13 prepared CoS2 by heating Co and S powders in Ar atmosphere and used it as the anode material of a lithium-ion battery. Poizot et al.26 reported that the reaction mechanism of sulfides with Liþ can be described as follows: Mnþ ðXÞ þ ne
þ nLiþ T M0 þ nLiðXÞ X ¼ ðS, OÞ Recent research has shown that hollow and porous structure is an ideal host for reversible Liþ intercalation with a high stability r 2011 American Chemical Society

for this structural character.27,28 However, to the best of our knowledge, there are no reports about the solvothermal preparation and electrochemical property of CoS2 hollow spheres. Herein, we report a facile and efficient solvothermal route to synthesize uniform hollow spheres of CoS2. Absolute ethanol (EtOH) in the reaction acted not only as a solvent but also as a weak reducing agent. Sulfur powder is used as a sulfur source. Moreover, the electrochemical property of this material as the anode has been investigated in lithium-ion batteries.

’ EXPERIMENTAL METHODS In a typical synthesis, 2 mmol of CoCl2 3 6H2O was dissolved in absolute ethanol. Then the blue solution was transferred into a 50 mL Teflon-lined stainless steel autoclave followed by the addition of 4 mmol of sulfur powder and subsequent agitation. The sealed tank was maintained at 240 °C for 24 h. After reaction, the autoclave was cooled to room temperature naturally. The resulting precipitates were collected by centrifugation and then washed with deionized water and absolute ethanol three times, respectively. Finally the CoS2 precipitated was dried at 60 °C for 10 h, and the CoS2 spheres with hollow structures were obtained. For comparison, the synthetic parameters including temperature, concentration, and the reaction time of the precursors were also Received: December 7, 2010 Revised: February 21, 2011 Published: April 05, 2011 8300

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Figure 1. XRD patterns and refinement results of the observed (red fork), calculated (green line), and difference (pink line) XRD profiles of the CoS2 hollow spheres. (insert) EDS spectrum of the CoS2 hollow spheres.

examined. The molar ratio of CoCl2 3 6H2O to sulfur powder was 0.5 for each sample. The crystalline structures of the as-prepared samples were analyzed by X-ray diffraction (XRD). The XRD data were recorded by a Rigaku D/MAX-2500 powder diffractometer with a graphite monochromatic and Cu KR radiation (λ = 0.154 18 nm) in the 2θ range of 3
80°. Rietveld analysis was performed using GSAS (General Structure Analysis System) software.29,30 The elemental composition of the product was detected by energy dispersive spectroscopy (EDS-7421, Oxford Instrument). The morphology and microstructure of the as-synthesized samples were characterized with a scanning electron microscopy (SEM, Hitachi X-650). The as-prepared CoS2 was used as the anode material for rechargeable lithium-ion batteries. The anode was fabricated by mixing the active material, carbon black, and polytetrafluoroethylene (PTFE) binder in a weight ratio of 80:10:10. The testing cells were assembled with the anodes as-fabricated, metallic lithium cathode, Celgard 2300 film separator and 1 M LiPF6 in 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte. The assembly of the testing cells was carried out in an argon filled homemade glovebox, where water and oxygen concentration was kept less than 5 ppm. The discharge
charge cycle tests were run at a current density of 100 mA g
1 between 3.00 and 0.01 V (vs Li/Liþ), respectively. Cyclic voltammetry (scan rate, 0.1 mV s
1; the potential interval, 0.01 to 3.00 V) was conducted by a Zahner IM6e electrochemical workstation. All the tests were performed at room temperature.

’ RESULTS AND DISCUSSION XRD pattern and Rietveld refinements of the as-synthesized CoS2 hollow spheres are shown in Figure 1. As shown in Figure 1, the refinement of the XRD patterns fit the experimental data points very well (wRp = 17.45%, Rp = 9.49%). All the diffraction peaks can be indexed to the standard diffraction data of the corresponding cubic phase CoS2 with a space group of Pa3(205). The lattice constants are calculated to be a = 5.533 Å, which are in good agreement with the reported data for the pure phase of cubic CoS2 structure (JCPDS no. 89-1492, a = 5.538 Å). Thus, the obtained CoS2 is of high purity and in good crystallinity. Moreover, EDS was performed to further determine the chemical composition. The results inserted in Figure 1 reveal that no other element was observed except Co and S. Furthermore, the

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Figure 2. SEM (a, b) and TEM (c, d) images of the CoS2 hollow spheres prepared at 240 °C for 24 h. Scale bars: (a) 10 μm; (b) 2 μm; (c) 50 nm; (d) 10 nm.

Figure 3. SEM images of the CoS2 microspheres synthesized from (a, b) 0.01 M and (c, d) 0.1 M CoCl2 3 6H2O solution, respectively. Scale bars: (a) 2 μm; (b) 500 nm; (c) 3 μm; (d) 1 μm.

quantification of the peaks shows that the molar ratio of S and Co is 1.98, which is quite close to the refinement results and stoichiometric CoS2. SEM image shown in Figure 2a illustrates that the sample prepared at 240 °C for 24 h is composed of uniform hollow sphere structures. From the typical SEM image, it is observed that the as-prepared CoS2 hollow spheres are about 2 to 3 μm in diameter and the shells are about 300 nm in thickness. Parts c and d of Figure 2 are magnified TEM images from the edge of a microsphere, revealing that the CoS2 spherical shell is comprised of nanoparticles with the diameter of about 20 nm. Various microstructures of CoS2 have been controllably synthesized by adjusting experimental parameters. It is found that concentration of the raw materials and temperature play an important role in controlling the sizes and morphologies of the CoS2 microcrystals. As shown in Figure 3, when the concentration of CoCl2 3 6H2O was 0.01 M, CoS2 hollow spheres with a diameter of about 1 μm were produced (Figure 3a,b). The shell with the thickness of about 200 nm packs loosely. As the concentration of CoCl2 3 6H2O was 0.1 M, CoS2 core
shell microspheres with a diameter of 3 μm were obtained. The average size of the cores are about 2 μm, while the thickness of the shell is only about 200 nm. Interestingly, multishell CoS2 hollow spheres can be observed (Figure 3c). Obviously, it tends 8301

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Figure 4. SEM images of the CoS2 microspheres synthesized at (a, b) 210 °C and (c, d) 180 °C for 24 h, respectively. Scale bars: (a) 5 μm; (b) 500 nm; (c) 2 μm; (d) 500 nm.

to form hollow structures with a low reaction concentration and form core
shell structures with high concentration. Figure 4 shows the influence of temperature on the morphologies of the CoS2 materials. At the reaction temperature of 210 °C, the sample is composed of uniform hollow spheres about 1 μm in diameter and 300 nm in shell thickness. There are flakes which are not obvious on surface of the microspheres. At the reaction temperature of 180 °C, uniform microspheres were obtained and no hollow spheres were observed. A higher magnification image displays that many obvious nanoflakes grow on the surface of the microspheres. To investigate the formation and evolution of the CoS2 hollow structures and core
shell structures, time-dependent experiments were carried out. XRD patterns of the samples obtained at 240 °C for different hours were taken. As shown in Figure 5a, at the reaction time of 4 h, CoS2 is formed and no other intermediate are produced during the process. Since no strong reducing agents were added in our system. It is thought that EtOH may serve as the reducing agent, moreover many studies have reported that ethanol, EG, and DMF can act as reducing agents at high temperatures.31,32 The essence of this solvothermal synthesis is the reduction of sulfur powder but not cobalt ions by ethanol solvent (reduction potential: Eθ = 0.14 V for S þ 2Hþ þ 2e f H2S; Eθ =
0.28 V for Co2þ þ 2e = Co). Therefore, S can be reduced to S2
in these solvents, form S22
with the superfluous S, and then react with Co2þ to form CoS2.33
36 The shape evaluation are recorded by SEM. As shown in Figure 5b, at the early reaction stage (4 h), the product is composed of coherent spheres, which are about 1
1.5 μm in diameter. SEM image of the sample after grinding (Figure 5b, insert) illustrates that the primary nanoparticles tend to aggregate together to form solid microspheres. When reacted for 8 h, the solid spheres start to evacuate and the immediate surface layer forms, resulting in a homogeneous core
shell structure (Figure 5c). When the reaction was prolonged to 16 h, singlewall hollow spheres and core
shell structures about 2
2.5 μm in diameter coexist (Figure 5d). Finally, when the dwell time was processed for 24 h, uniform hollow spheres with the diameter of 2
3 μm are observed. Therefore, the formation process of the CoS2 hollow spheres can be schematically illustrated in Figure 5e. As can be seen, stage (i) belongs to a synthesis of CoS2 nanoparticles, stage (ii) is an aggregation of CoS2 nanoparticles

Figure 5. (a) XRD patterns, SEM images of the samples synthesized at 240 °C for (b) 4 h, (c) 8 h, (d) 16 h, and (e) schematic illustration of the formation of the CoS2 hollow spheres. Scale bars: (b,c) 2 μm; (d) 3 μm.

Figure 6. Charge
discharge curves of the CoS2 hollow spheres prepared at 240 °C for 24 h.

to solid spheres driven by the minimization of the total energy of the system, and stage (iii) belongs to the evacuation of the solid core and the generation interior space. Furthermore, stage (iv) is the further growth for the formation of the core
shell structures and hollow structures. On the basis of the above analysis, it is believed that Ostwald ripening should be the main formation mechanism for the hollow spheres, which has been investigated in detail.37
40 Electrochemical performance of the as-prepared CoS2 as the anode material of lithium-ion battery was evaluated. It is reported that13 the charge
discharge voltage is the key factor to affect the cyclic performance. The intercalation reaction xLiþ þ xe
þ CoS2 T LixCoS2 occurs on the CoS2 electrode when the discharge voltage was set at 1.6 V. Also, the structure of material is nearly not destroyed in this reversible topotactic reaction process. When the discharge voltage was 0.02 V, the mechanism is CoS2 þ 4Liþ þ4e
T Co þ 2Li2S and the reaction results in the collapse of the CoS2 structure. Figure 6 illustrates the charge
discharge curves of the sample in the first and 10th cycles in the voltage range of 0.01
3.0 V at a 8302

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Figure 7. Cyclic voltammograms for the CoS2 hollow spheres with scan rate 0.1 mV s
1. The scan potential ranges 0.01
3.0 V.

Figure 8. Cycle performance of the CoS2 hollow spheres prepared at 240 °C for 24 h.

current density of 100 mA g
1. The anode in the first charge
discharge process displays a charge capacity of 900 mAh g
1 and a high discharge capacity of about 1210 mAh g
1, corresponding to 4.13 Li and 5.55 Li per CoS2, respectively. The large irreversible capacity in the first cycle may be attributed to the formation of solid-electrolyte interphase (SEI) film onto the surface of the electrode materials.41 The first discharge curve has three plateaus at 1.6, 1.3, and 0.8 V. The reduction potential of the material at about 1.6 V may be attributed to the decomposition reaction, which is irreversible. The two plateaus at 1.3 and 0.8 V shift to 1.8 and 1.4 V, respectively, in the following cycles, and the two plateau potentials of the charge curves were 2.0 and 2.3 V. The plateau at 1.8 V in the discharging process represents the insertion of a small amount of lithium. Also, the flat voltage observed at 1.4 V corresponds to the displacement reaction of Li2S þ Co8. The slope from 1.2 to 0.01 V might be attributed to the formation of a surface polymeric layer, which in general contributes to additional capacity.42 Figure 7 shows the cyclic voltammograms for the as-prepared CoS2 electrode at a scan rate 0.1 mV s
1 in the potential range of 0.01
3.0 V. In the first scanning cycle, two reduction peaks at 0.6 and 1.0 V and two oxidation peaks at 2.2 and 2.6 V are observed. However, the reduction peaks shift to 1.1 and 1.6 V, while the

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oxidation peaks slightly shift negatively in the subsequent cycles, which correspond well to the discharge
charge plateaus. The peak areas of the electrode decrease as the cycle number increases, indicating the decreases of discharge capacity. The cycle performances of the as-prepared CoS2 electrodes between 0.01
3.0 V and 1.0
3.0 V are shown in Figure 8. In the range of 0.01
3.0 V, the obvious capacity decay in the first five cycles may be caused by the complicated side-reactions and irreversible structure transformation.43 Then, the discharge capacity retains about 320 mAh g
1 after 40 cycles, which is equal to that of CoS2 with 30 wt % acetylene black at the current density of 50 mA g
1 reported by Yan et al.,13 exhibiting a favorable cycle performance. In the range of 1.0
3.0 V, the electrode displays the highest discharge capacity of only 746 mAh g
1 and the capacity is lower; however, it also displays good cycle stability. With respect to the facile preparation and high purity of the product, the present CoS2 shows a potential application in lithium ion batteries.

’ CONCLUSIONS In summary, various hollow sphere structured CoS2 have been synthesized by a simple and facile solvothermal method, which can also be used for the preparation of other metal sulfides, such as MnS, FeS, CuS, and so on. The as-prepared product is composed of a large number of CoS2 hollow spheres with the average diameter of about 2
3 μm and with the shell thickness of 300 nm. It is concluded that the Ostwald ripening mechanism is responsible for the formation of the hollow structures. The assynthesized CoS2 hollow spheres display low charge and discharge potential plateaus, high discharge capacity, and good cycle performance in lithium ion storage and retrieval, illustrating that it may be a promising anode material candidate for lithium ion batteries. Furthermore, the CoS2 sample may also find potential applications in other fields, such as catalysis, semiconductor, alkaline rechargeable batteries, and so on. A great deal of work is still underway. ’ AUTHOR INFORMATION Corresponding Author

*Address: Institute of New Energy Material Chemistry, Nankai University, Tianjin, 300071, P. R. China. Fax: 86-022-23502604. Phone: 86-022-23504527. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by NSFC (Grants 20801059, 21073100), 973 program (Grant 2010CB631303), TSTC (Grant 10JCYBJC08000), and MOE Innovation Team (Grant IRT0927). ’ REFERENCES (1) Hoodless, R. C.; Moyes, R. B.; Wells, P. B. Catal. Today 2006, 114, 377–382. (2) Sadjadi, M. S.; Pourahmad, A.; Sohrabnezhad, S.; Zare, K. Mater. Lett. 2007, 61, 2923–2926. (3) Sun, X. C.; Parvin, K.; Ly, J.; Nikles, D. E. IEEE Trans. Magn. 2003, 39, 2678–2680. (4) Otero-Leal, M.; Rivadulla, F.; Rivas, J. IEEE Trans. Magn. 2008, 44, 4503–4505. (5) Pasquariello, D. M.; Kershaw, R.; Passaretti, J. D.; Dwight, K.; Wold, A. Inorg. Chem. 1984, 23, 872–874. 8303

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