Hydrothermal Synthesis of Molybdenum Disulfide for Lithium Ion Battery Applications

Chinese Journal of Chemical Engineering, 18(6) 910—913 (2010)

Hydrothermal Synthesis of Molybdenum Disulfide for Lithium Ion Battery Applications*

WANG Shiquan (王石泉)1,2, LI Guohua (李国华)3, DU Guodong (杜国栋)4, JIANG Xueya (江雪娅)2, FENG Chuanqi (冯传启)2, GUO Zaiping (郭再平)4 and KIM Seung- Joo5,**

1 Institue of NT-IT Fusion Technology, Ajou University, Suwon 443-749, Korea

2 Department of Chemistry, Hubei University, Wuhan 430062, China

3 State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technol-

ogy, Hangzhou 310032, China

4 Institute for Superconducting and Electronic Materials, University of Wollongong, Northfield Avenue, Wollon-

gong, NSW 2522, Australia

5 Division of Energy Systems Research, Ajou University, Suwon 443-749, Korea

Abstract Molybdenum disulfide nanoflakes were synthesized by a simple hydrothermal process using sodium molybdate and thiourea as reactants at a relatively low temperature. X-ray diffraction (XRD) and transmission elec-tron microscopy (TEM) indicate that the samples have the structure of 2H-MoS2 and the morphology of nanoflakes with the average thickness around 5-10 nm. The results of electrochemical properties indicate that the morphology and size of MoS2 particles have effects on their capacity when they are used as the anode for lithium ion battery.

The as-prepared MoS2 samples have high reversible discharge capacity up to 994.6 mA·h·g?1 for the MoS2-1 elec-trode and 930.1 mA·h·g?1 for the MoS2-2 electrode and show excellent cycling performances. The MoS2-1 electrode has a better cycling stability than the MoS2-2 electrode due to their difference in the uniformity of the samples.

Keywords molybdenum disulfide, chemical synthesis, electrochemical property, electrode material

1 INTRODUCTION

WS2 and MoS2 materials have shown potential applications in the fields of scanning probe micros-copy [1], solid-state lubrication [2], heterogeneous ca-talysis [3], lithium ion battery [4-7] and electrochemi-cal hydrogen storage [8]. Molybdenum disulfide (MoS2) features a layered structure, in which the at-oms are covalently bonded to form two-dimensional layers that are stacked together through weak van der Waals interactions [9]. This characteristic makes them possibly suitable as electrode materials for lithium ion battery [4-6]. For example, Feng et al. [4] reported that MoS2 nanoflakes prepared by a rheological phase reac-tion had a better lithium intercalation/deintercalation behavior. The as-synthesized MoS2 nanoflakes showed good cycling stability over a wide voltage range. The reversible capacity remains 840 mA·h·g?1 after 20 cycles, which is 84% of the initial reversible capacity. Guo et al. [5] prepared MoS2via hydrothermal method and reported that MoS2 electrode delivered a lithium insertion charge capacity of 801 mA·h·g?1, which is much higher than for WS2 powder [6]. Wang et al. [7] prepared WS2 nanotubes and reported that WS2 nano-tubes electrode delivered a lithium insertion capacity of about 915 mA·h·g?1, which is much higher than that of WS2 powder. The electrochemical performance of transition metal disulfides, including MoS2 showed that the particle size and morphology of materials have a great influence on their electrochemical prop-erties. Nanosized materials with the novel morpholo-gies were believed to have somewhat better perform-ance than bulk materials.

In this work, MoS2 nanoflakes were synthesized through a simple hydrothermal process. The as- synthesized MoS2 nanoflakes electrodes can reversi-bly store lithium with discharge capacities up to 994.6 mA·h·g?1 in a voltage range of 0.01-3.0 V vs. Li/Li+ and show a good cycling stability as electrode materials.

2 EXPERIMENTAL

2.1 Sample preparation

2.1.1Synthesis of MoS2-1

1.21 g (0.005 mol) Na2MoO4·2H2O, 1.56 g (0.02 mol) thiourea (NH2CSNH2) and 0.14 g PEG-1000 were dissolved in 30 ml of distilled water. The result-ing solution was transferred into a 50 ml teflon-lined stainless autoclave. The antoclave was heated at 200 °C for 24 h in a furnace and then cooled to room tempera-ture, by shutting off the furnace. The black precipitates were collected by centrifugation, thoroughly washed with distilled water, and then dried in air.

2.1.2Synthesis of MoS2-2

1.21 g Na2MoO4·2H2O and 0.70 g (0.01 mol) NH2OH·HCl were dissolved in 10 ml of distilled wa-ter, respectively. Then the mixture of Na2MoO4 and NH2OH·HCl solution was added into 100 ml round- bottom flask and heated at 90 °C with stirring. Then a

Received 2010-06-21, accepted 2010-11-20.

* Supported partially by the State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang Univer-sity of Technology and Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0094047).

** To whom correspondence should be addressed. E-mail: sjookim@ajou.ac.kr

Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010911

certain amount of 2 mol·L?1 HCl was added into the above solution to adjust the pH value to about 6. After half an hour, 1.56 g thiourea (NH2CSNH2) was added all at once into the solution with vigorous stirring. The reaction was kept at 90 °C for 10 min. The resulting solution was transferred into a 50 ml teflon-lined stainless autoclave. The autoclave was heated at 180 °C for 12 h in a furnace. Further treatment was similar to that described in MoS2-1.

2.2 Characterization

X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX IIA diffractometer using Cu Kαradiation (λ=0.1541 nm). The morphology of the re-sulting compounds was observed using a transmission electron microscope (TEM). Before the characteriza-tion, the samples were dispersed ultrasonically in etha-nol and dropped onto Cu-grid coated with carbon mem-brane. Electrochemical measurements were carried out using a 2032-type coin cell fabricated in an argon-filled glove box (Mbraun, Germany). The working electrode was fabricated in the mass ratio of 70︰20︰10 active material/carbon black/polyvinylidene difluoride, while lithium foil served as counter and reference electrode. The electrolyte was 1 mol·L?1 LiPF6 dissolved in 1︰1 ethylene carbonate (EC) and dimethyl carbonate (DMC). Constant current charge/discharge cycling was con-ducted on a LAND battery tester (CT2001A) in the voltage range between 3.0 and 0.01 V. Cyclic volt-ammetry (CV) was performed on a CHI660C electro-chemical workstation at a scan rate of 0.1 mV·s?1.

3 RESULTS AND DISCUSSION

Although both samples exhibited a broadened fea-ture in XRD patterns due to low crystallinity (Fig. 1), all diffraction lines were indexed to the hexagonal phase of MoS2 (JCPDS file No. 170744).

Figure 1 XRD patterns of as-prepared MoS2 samples: (a) MoS2-1 and (b) MoS2-2

Figure 2 shows TEM images for the as-repaired MoS2 samples. Figs. 2(a, b) are the TEM images of the MoS2

-1 sample. It is obvious that the product has

(a) MoS2

-1

(b) MoS2

-1

(c) MoS2

-2

(d) MoS2-2

Figure 2 TEM images of the MoS2 samples

Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010

912 the shape of nanoflake with the thickness of about 5-10 nm. Figs. 2 (c, d) are the TEM images of the MoS 2-2 sample. The MoS 2-2 sample also shows a flake-like morphology with the thickness of about 10 nm. The nanoflakes in the MoS 2-2 sample were curled at the edges and more aggregated than the MoS 2-1 sample.

Based on the literature [4, 10] and the experimen-tal conditions, the reaction involves a complex process, which contains three steps: (a) the hydrolysis of NH 2CSNH 2 to form H 2S; (b) the reduction of Mo(VI) to produce Mo (IV); (c) the formation of MoS 2 nan-oflakes. The reaction process for the synthesis of MoS 2-1 could be expressed as follows:

222322CS(NH )2H O 2NH CO H S +??→++ (1)

24222224324Na MoO 15CS(NH )6H O 4MoS Na SO 6NaSCN 24NH 9CO ++??→++++ (2) The formation of the MoS 2-2 sample can be expressed as [11]:

222322CS(NH )2H O 2NH CO H S +??→++ (3) 242422

Na MoO 2NH OH HCl 4HCl MoCl 2NaCl 6H O N +?+??→

+++ (4)

422MoCl 2H S MoS 4HCl +??→+ (5)

The electrochemical properties of MoS 2 samples were measured via coin cell testing. Figs. 3 (a) and 3 (b) show the cyclic voltammograms (CVs) of the

MoS 2-1 and MoS 2-2 electrodes, respectively. In the first scanning cycle [Fig. 3 (a)], there are two reduc-tion peaks (at ~0.4 V and ~1.0 V) and the correspond-ing oxidation peaks (at ~1.8 V and ~2.25 V). During the second cycle, the reduction peak at 0.4 V disap-peared, while a new cathodic peak appears at 1.4 V . From the second cycle, the CV curves remain consis-tent and maintain two reduction peaks (at 1.4 V and 1.8 V) and two oxidation peaks (at ~1.7 V and ~2.25 V). For the MoS 2-2 electrode, in the first cycle, there are two reduction peaks (at ~0.4 V and ~1.5 V) and the corresponding oxidation peaks (at ~1.8 V and ~2.25 V). From the second cycle, the CV curves remain consis-tent and maintain two reduction peaks (at 1.5 V and 1.75 V) and two oxidation peaks (at ~1.8 V and ~2.3 V).

When MoS 2 is cycled between 3.0 and 0.01 V as an anode material, two step reactions occur. First, lithium intercalates into the S slab, and the van der Waals S-S bonds must be broken and replaced by Li-S bonds. MoS 2 then decomposes into Mo nanoparticles embedded in a Li 2S matrix, corresponding to two ca-thodic peaks at about 1.0 V and 0.4 V in the first ca-thodic segment in the cyclic voltammograms (CVs), as shown in Figs. 3 (a) and (b). Both the MoS 2-1 and MoS 2-2 samples show these two peaks. In the follow-ing cycles, an extra peak at about 2.0 V appears. This change can be explained by the formation of a gel-like polymeric layer [12].

Figure 4 shows the charge/discharge profiles of the as-prepared MoS 2 electrodes. For the MoS 2-1 electrode delivered a lithium insertion capacity of 1223.1 mA·h·g ?1 for the first cycle when the discharge current density was 50 mA·g ?1, which was much higher than the reported data of the WS 2 nanotubes [7] and MoS 2 nanopowder [6]. The MoS 2-1 electrode re-tained a reversible capacity of 994.6 mA·h·g ?1. The first discharge curve shows two insertion plateaus at ~1.2 V and ~0.8 V . During the first charge, the charge voltage increases gradually at first, then a short plateau at 2.3 V vs . Li/Li + was observed. The MoS 2-2 electrode delivered a lithium insertion capacity of 1138.4 mA·h·g ?1 for the first cycle and it retained a reversible capacity of 930.1 mA·h·g ?1

, as shown in

(a) MoS 2

-1

(b) MoS 2-2

Figure 3 Cyclic voltammograms (CVs) of MoS 2-1 and MoS 2-2 electrodes at a scanning rate of 0.1 mV·s ?1

1st; 2nd; 3rd; 4th; 5th

Figure 4 First and second charge/discharge curves of MoS 2-1 and MoS 2-2 electrodes

MoS 2-1 1st; MoS 2-1 2nd; MoS 2-2 1st; MoS 2-2 2nd

Chin. J. Chem. Eng., Vol. 18, No. 6, December 2010913

Fig. 4. The first discharge curve shows two insertion plateaus at ~1.8 V and ~0.8 V, which is different from the data of MoS2-1 electrode. In the second cycle, a lithium insertion plateau at about 2.2 V and ~1.5 V as well as a slope starting from 1.5 V down to the cutoff voltage of 0.01 V were observed. From the second cycle, only one plateau (2.3 V for the charge curve and 2.0 V for the discharge curve) connected with a slope can be observed for both charge and discharge curves, similar to that of WS2 nanoflakes electrode [13], indi-cating that the intercalation reaction dominates the electrochemical process after the first cycle.

The cyclic performances within 40 cycles of two MoS2 electrodes are shown in Fig. 5. As mentioned above, we can observe the large initial discharge ca-pacities (1223.1 mA·h·g?1 for the MoS2-1 and 1138.4 mA·h·g?1 for the MoS2-2) in the first cycle. The MoS2-1 and MoS2-2 electrodes exhibit reversible spe-cific capacity as high as 994.6 mA·h·g?1 and 930.1 mA·h·g?1, respectively, at a current density of 50 mA·g?1. After the 20th cycles, the discharge capacities of MoS2-1 and MoS2-2 are 814.8 mA·h g?1 and 843.1 mA·h·g?1. Even in the 40th cycles, MoS2-1 and MoS2-2 electrodes still can sustain high capacities above 705.8 mA·h·g?1 and 483.2 mA·h·g?1, respec-tively. The results shown in Fig. 5 are in good agree-ment with the cyclic voltammograms curves (Fig. 4). The reason why the MoS2 nanoflakes electrodes showed such a high capacities, compared to the re-ported MoS2 electrode [5] is probably that lithium ions were not only intercalated into layers of the MoS2 structure to form Li x MoS2 compounds but also in-serted into defect sites of the clusters composed of the MoS2 nanoflakes [4, 7]. The increase in the number of active sites for Li+ insertion has been also observed in other low dimensional electrode materials such as MoO2 nanowires [14, 15]. The MoS2-1 electrode shows a better cycling stability than the MoS2-2 electrode due to the difference in the uniformity of the particle size distribution. Thus, the morphology and size of MoS2 particles have effects to their capacity when they are used as electrodes [4, 7, 16]. MoS2 electrodes, al-though they have relatively high discharge/charge po-tentials, still can be accepted as anode materials. Con-sidering the high reversible capacity and good cycling stability of the as-prepared two MoS2 nanoflakes elec-trodes, MoS2 nanoflakes could be promising alterna-tive anode materials for lithium ion batteries.

4 CONCLUSIONS

In this work, a hydrothermal method was used to synthesize MoS2 nanoflakes. The as-prepared two MoS2 electrodes exhibit high specific capacities and stable cycling stability. The reversible discharge ca-pacities were 994.6 mA·h·g?1 and 930.1 mA·h·g?1, respectively, for the two types of MoS2 nanoflakes prepared. Although the preparation method is simple, MoS2 nanoflakes so obtained could be promising al-ternative anode materials for lithium ion batteries.

REFERENCES

1Homyonfer, M., Alperson, B., Rosenberg, Y., Sapir, L., Cohen, S.R., Hodes, G., Tenne, R., “Intercalation of inorganic fullerene-like

structures yields photosensitive films and new tips for scanning

probe microscopy”, J. Am. Chem. Soc., 119, 2693-2698 (1997).

2Rapport, L., Bilik, Y., Feldman, Y., Homyonfer, M., Cohen, S.R., Tenne, R., “Hollow nanoparticles of WS2 as potential solid-state lu-

bricants”, Nature, 387, 791-793 (1997).

3Mdleni, M.M., Hyeon,T., Suslick, K.S., “Sonochemical synthesis of nanostructured molybdenum sulfide”, J. Am. Chem. Soc., 120,

6189-6190 (1998).

4Feng, C.Q., Ma, J., Li, H., Zeng, R., Guo, Z.P., Liu, H.K., “Synthesis of molybdenum disulfide (MoS2) for lithium ions battery applica-

tions”, Mater. Res. Bull., 44, 1811-1815 (2009).

5Guo, G.H., Hong, J.H., Cong, C.J., Zhou, X.W., “Molybdenum di-sulfide synthesized by hydrothermal method as anode for lithium

rechargeable batteries”, J. Mater. Sci., 40, 2557-2559 (2005).

6Julien, C.M., “Lithium intercalated compounds: Charge transfer and related properties”, Mater. Sci. Eng., 40, 47-102 (2003).

7Wang, G.X., Bewlay, S., Yao, J., Liu, H.K., Dou, S.X., “Tungsten disulfide nanotubes for lithium storage”, Electrochem. Solid-State

Lett., 7, A321-A323 (2004).

8Chen, J., Kuriyama, N., Yuan, H.T., Takeshita, H.T., Sakai, T., “Electrochemical hydrogen storage in MoS2 nanotubes”, J. Am.

Chem. Soc., 123, 11813-11814 (2001).

9Sibernagel, B.G., “Lithium intercalation complexes of layered transi-tion metal dichalcogenides: An NMR survey of physical properties”,

Solid State Commun., 17, 361-365 (1975).

10Tian, Y., He, Y., Shang, J., Zhu, Y.F., “Hydrothermal synthesis and characterization of lamellar MoS2”, Acta Chimica Sinica, 62,

1807-1810 (2004). (in Chinese)

11Tian, Y.M., Zhao, X., Shen, L.C., Meng, F.Y., Tang, L.Q., Deng, Y.H., Wang, Z.C., “Synthesis of amorphous MoS2 nanospheres by

hydrothermal reaction”, Mater. Lett., 60, 527-529 (2006).

12Wang, Q., Li, J.H., “Facilitated lithium storage in MoS2 overlayers supported on coaxial carbon nanotubes”, J. Phys. Chem. C, 111,

1675-1682 (2007).

13Feng, C.Q., Huang, L.F., Guo, Z.P., Liu, H.K., “Synthesis of tung-sten disulfide (WS2) nanoflakes for lithium ion battery application”,

Electrochem. Commun., 9, 119-122 (2007).

14Yang, L.C., Gao, Q.S., Tang, Y., Wu, Y.P., Holze, R., “MoO2 synthe-sized by reduction of MoO3 with ethanol vapor as an anode material

with good rate capability for the lithium ion battery”, J. Power

Sources, 179, 357-360 (2008).

15Gao, Q., Yang, L., Lu, X., Mao, J., Zhang, Y., Wu, Y., Tang, Y., “Syn-thesis, characterization and lithium-storage performance of MoO2/carbon

hybrid nanowires”, J. Mater. Chem., 20, 2807-2812 (2010).

16Li, H., Li, W., Chen, W.X., Wang, J.M., “Electrochemical lithia-tion/delithiation performances of 3D flowerlike MoS2 powders pre-

pared by ionic liquid assisted hydrothermal route”, J. Alloys Compd.,

471, 442-447 (2009).

Figure 5 Typical cycling performances of two as-prepared MoS2 electrodes

■MoS2-1;●MoS2-2