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Materials Research Bulletin 44 (2009) 1811–1815
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1. Introduction
During the past decade, the properties of molybdenum
disulfide, especially those relating to its uses in such applications
as hydrogen storage [1], catalysts [2], and lubricants [3,4], and its
electrochemical double-layer capacitance [5], have been exten-
sively investigated. Molybdenum disulfide features a layered
structure, in which the atoms are covalently bonded to form two-
dimensional layers that are stacked together throughweak van der
Waals interactions [6]. The weak interlayer interaction allows
foreign ions or molecules to be introduced between the layers
through intercalation. Thus, MoS2 could be developed as an
intercalation host to form a promising electrode material in high
energy density batteries [7–9]. The electrochemical performance
of transition metal disulfides, including MoS2, for Li-ion batteries
has been investigated, and the results showed that the particle size
and morphology of materials have a great influence on their
electrochemical properties. For example, Julien [10] reported on
the electrochemical behavior of crystalline WS2 powders and
found that the lithium insertion capacity was only 0.6 mol Li+ per
mole of crystalline WS2, while Dominko et al. [7] found that 1.7–
3.0 mol of Li+ could be inserted into onemole ofMoS2�xIy nanotube
electrode, depending on the quality of the MoS2�xIy nanotubes.
More recently, work from our group [8] demonstrated that WS2
nanotube electrode could deliver a reversible capacity of above
500 mAh/g, corresponding to 4.7 mol lithium per mole of WS2
nanotubes. We attributed such a high capacity to lithium
intercalation into intratubular and intertubular sites of MS2
(M =W, Mo, etc.) nanotubes, as well as diffusion into the layered
MS2 structure to form LixMS2 intercalation compounds. Since
lithium ions can intercalate into small holes/channels, it may be
imagined thatMS2 nanostructuredmaterials with plenty of defects
can deliver even higher lithium intercalation capacity.
In this work, we report a simple synthesis method (rheological
phase reaction) to synthesize MoS2 nanoflakes [11,12]. The
samples prepared can reversibly store lithium with a capacity of
1175 mAh/g in the voltage range of 0.01–3.0 V vs. Li/Li+,
corresponding to 8 mol lithium per mole of MoS2, which is the
highest capacity reported for MoS2 electrodes so far. Moreover, the
MoS2 exhibited good cycling performance as an electrodematerial.
2. Experimental
The MoS2 material was synthesized by using analytically pure
(NH4)6Mo7O24�4H2O, sulfocarbamide (CS(NH2)2), and oxalic acid
(H2C2O4�2H2O) as starting materials. The Mo/S/H2C2O4 molar ratio
was 1:2:1. These powders were mixed and thoroughly ground in
an agate mortar, and a few drops of water were added to form a
rheological state mixture. The mixture was then put into a 50 ml
sealed Teflon-lined autoclave and maintained at 200 8C for 24 h to
B. Chemical synthesis
B. Electrochemical properties
� 2009 Elsevier Ltd. All rights reserved.
* Corresponding author at: Institute for Superconducting & Electronic Materials,
University of Wollongong, Wollongong, NSW 2522, Australia.
Tel.: +61 2 4221 5225; fax: +61 2 4221 5731.
E-mail address: zguo@uow.edu.au (Z. Guo).
0025-5408/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.05.018
Synthesis of molybdenum disulfide (Mo
Chuanqi Feng a,c, Jun Ma a, Hua Li a, Rong Zeng c, Za
aKey Laboratory for Synthesis and Applications of Organic Functional Molecules, Hube
b School of Mechanical, Materials & Mechatronic Engineering, University of Wollongon
c Institute for Superconducting & Electronic Materials, University of Wollongong, NSW
dARC Centre of Excellence for Electromaterials Science, University of Wollongong, NSW
A R T I C L E I N F O
Article history:
Received 29 October 2008
Received in revised form 29 March 2009
Accepted 29 May 2009
Available online 6 June 2009
Keywords:
A. Inorganic compounds
A. Layered compounds
A. Nanostructures
A B S T R A C T
This paper reports the use
characterization by powd
electronmicroscopy obser
partly form MoS2 nanotub
nanoflake electrode was
higher specific capacity, w
possible reasons for the
discussed. The outstandin
make it possible for MoS2
2) for lithium ion battery applications
ing Guo b,c,d,*, Huakun Liu c,d
iversity, Wuhan 430062, PR China
SW 2522, Australia
2, Australia
2, Australia
a rheological phase reaction method for preparing MoS2 nanoflakes. The
-ray diffraction indicated that MoS2 had been formed. High resolution
on revealed that the as-preparedMoS2 nanoflakes had started to curve and
The lithium intercalation/de-intercalation behavior of as-prepared MoS2
investigated. It was found that the MoS2 nanoflake electrode exhibited
very high cycling stability, compared to MoS2 nanoparticle electrode. The
h electrochemical performance of the nanoflakes electrodes are also
lectrochemical properties of MoS2 nanoflakes obtained by this method
be used as a promising anode material.
arch Bulletin
vier .com/ locate /mat resbu
form a dark gray solid. After that, the solidmixturewas packed into
alumina crucibles and calcined in a tube furnace at 500 8C for 2 h
under a flow of argon gas.
Powder X-ray diffraction (XRD; Rigaku D/max-ra), using Cu Ka
radiation (l = 1.5406 A˚) with a graphite monochromator, was
employed to identify the crystalline phase of the synthesized
materials. The morphology of the resulting compound was
observed using a transmission electron microscope (TEM). The
electrochemical characterizationswere performed using coin cells.
The anode was prepared by dispersing 70 wt% as-prepared MoS2
powder and 20 wt% carbon black in 10 wt% polytetrafluoroethy-
lene (PTFE) solution. TheMoS2 and carbon black powderswere first
added to a solution of PTFE in isopropanol to form a homogeneous
slurry. The slurry was then spread onto copper foil. The coated
electrodes were dried at 125 8C for 24 h in vacuum and then
pressed to enhance the contact between the active materials and
the conductive carbon. Coin test cells were assembled in an argon
filled glove box, where the counter electrode was Li metal and the
electrolyte was 1 mol L�1 LiPF6 dissolved in a 50/50 vol% mixture
of ethylene carbonate (EC) and diethyl carbonate (DEC). These cells
were galvanostatically charged and discharged in the voltage range
of 0.01–3.0 V to measure the electrochemical response at room
temperature.
hydrolysis of CS(NH2)2; (b) the reduction of Mo(VI) and the
Fig. 2. Transmission electron micrographs of MoS2 sample.
Fig. 3. SEM of MoS2 sample.
C. Feng et al. /Materials Research Bulletin 44 (2009) 1811–18151812
3. Results and discussion
3.1. Reaction mechanism and structure characterization
X-ray diffraction (XRD) was performed on the intermediate
product (before the 600 8C calcination) and on the MoS2 final
product (Fig. 1). The results show that the MoS2 phase is nearly
completely formed after hydrothermal treatment in an autoclave
at 200 8C for 24 h (Fig. 1(a)), but the precursor still contains some
impurities resulting from the reactants used during the reaction
process. A lot of small peaks appear in the precursor before
calcinations. These peaks are resulted from impurities (raw
material: H2C2O4, CS(NH2)2), when temperature arrived at
500 8C, these impurities were decomposed, so these small peaks
were disappeared. The hexagonal phase of MoS2 was formed after
sintering the intermediate product at 500 8C for 2 h under a flow of
argon. All diffraction lines can be readily indexed to the hexagonal
phase of MoS2 (JCPDS file No. 170744). The strong and clear peaks
Fig. 1. XRD patterns of as-prepared MoS2 sample: (a) precursor before calcination;
(b) sample formed after calcination at 500 8C.
indicate that the MoS2 was highly crystalline. The crystal cell
parameters were calculated by refining the XRD data:
a = 0.3125 nm, c = 1.2302 nm.
Fig. 2(a) shows a typical TEM image of the as-prepared MoS2
samples. It is obvious that some of the products have taken on the
shapes of nanoflakes. The TEM images (see Fig. 2) of the samples
revealed that the MoS2 nanoflakes had started to partly form
nanotubes which have average diameter of about 240 nm. Fig. 3
contains an SEM image of a MoS2 sample. From the SEM, it can be
seen that at least part of the nanotubes had to have been formed
from nanoflakes. The growth process of the as-prepared MoS2
nanotubes is to some extent similar to themechanism proposed by
Ye et al. for the tube-formation process in materials with layered
structures [13], which involves curving followed by seaming of
molecular layers. This indicates that MoS2 nanotubes could be
formed under proper conditions by this synthesis method. Both
Figs. 2 and 3 show some partially formed MoS2 nanotubes. Based
on the literature [9,14–16] and the experimental conditions that
we used, the formation of MoS2 and then MoS2 nanotubes may
involve a complex process, which contains four steps: (a) the
formation of MoO2; (c) the formation of MoS2; and (d) curving of
nanoflakes to form tubes under heat treatment. The oxalic acid
plays a key role as the reducing reagent, as well as a pH adjustment
agent during the reaction process. Before reaction, the pH of
reactants is about 1, after reaction, the pH of precursor in water is
about 5. The one of roles of water is to react with CS(NH2)2 to
produce H2S (middle product), the other is to speed up diffusion of
ions in reactant. While the sulfocarbamideworks as a sulfurization
reagent. The reaction process for the synthesis of MoS2 could be
expressed as follows:
CSðNH2Þ2þ2H2O ! 2NH3þCO2þ2H2S (1)
ðNH4Þ6Mo7O24þ7H2C2O4 ! 6NH3þ7MoO2þ14CO2þ10H2O
(2)
MoO2þ2H2S ! MoS2þ2H2O (3)
The overall reactions could be expressed as (4):
into the layer sites ofMoS2 nanoflakes to form LixMoS2 compounds.
Once these sites are occupied with Li+ ions, lithium ions will
intercalate into MoS2 defect sites in nanoclusters, intertubular
sites, and intratubal sites (the hollow core) in the MoS2 nanotubes.
During the de-intercalation process, lithium ions are extracted
from the intratubal sites, the defect sites in nanoclusters, and the
layer sites of LixMoS2 in the nanoflakes, successively, which deliver
the reduction/oxidation peaks in the CV curves at lower potentials.
The CV curves become stable after the 5th cycle, in the 5th cycle
and the 8th cycle, two CV curves are nearly overlapped, which
verified that MoS2 material has excellent cycle performances as
anode material. The electrochemical properties of the as-prepared
MoS2 were also measured via coin cell testing. Fig. 5 shows the
charge/discharge profiles of as-prepared MoS2 electrode. In the
first cycle, the MoS2 electrode delivered a lithium insertion
capacity of about 1174.7 mAh/g when the discharge current
density was 60 mA/g, which is much higher than in the reported
data on MoS2 nanopowder [8,10]. The as-prepared MoS2 electrode
retained a reversible capacity of 851.5 mAh/g after 20 cycles. The
first discharge curve shows three insertion plateaus at �1.0 V,
�0.8 V, and �0.4 V. In the second cycle, three lithium insertion
plateaus at �2.1 V, �1.1 V and �0.4 V were observed, as well as a
C. Feng et al. /Materials Research Bulletin 44 (2009) 1811–1815 1813
ðNH4Þ6Mo7O24þ7H2C2O4þ14CSðNH2Þ2þ4H2O
! 7MoS2þ28CO2þ34NH3 (4)
During these reactions, the increased reaction entropy facil-
itates the formation of the expected product (MoS2). This facile
method provides a simple and easily applicable route to synthesize
MoS2 nanoflakes at moderate temperature. It can also be used to
synthesize other transition metal sulfides.
3.2. Electrochemical properties
Cyclic voltammetry (CV) was performed on the MoS2 electrode,
and selected cyclic voltammograms are shown in Fig. 4. During the
first cycle, there are three reduction peaks (at �1.0 V, �0.8 V, and
�0.2 V) and three corresponding oxidation peaks (at �1.5 V,
�1.8 V, and �2.25 V). The reduction peaks at �1.0 V and �0.8 V
overlap with each other, while the two oxidation peaks at the
corresponding potentials (�1.5 V, �1.8 V) are also joined together.
From the second CV cycle, three pairs of reduction and oxidation
peaks were observed. There was no significant change in the
potentials of the oxidation peaks, but the potentials of the
reduction peaks shifted from their original positions (�1.0 V,
�0.8 V, and �0.2 V) to �2.0 V, �1.1 V, and �0.3 V, respectively.
Based on the above situation and our previous research [8], we
suggest that, in the first lithiation process, lithium ions intercalate
Fig. 4. Cyclic voltammograms of theMoS2 electrode in a coin cell (vs. Li) for selected
cycles.
Fig. 5. Typical charge and discharge curves for selected cycles of an as-prepared
MoS2 electrode. (a) For voltage range from 0.01 V to 3.0 V; (b) for a voltage range
from 0.3 V to 3.0 V. Current density was 40 mA/g for both (a) and (b).
slope starting from 0.4 V and running down to the cut-off voltage
of 0.01 V. TheMoS2 nanoflake electrodes were also investigated for
different voltage ranges. When the voltage range was from 0.3 V to
3.0 V, the capacity of the electrode decreased, however, it shows
slightly better cycling performances compared to the electrode
cycled within the voltage range of 0.01–3 V, as shown in Fig. 5(b).
During the first charge, the charge voltage increased gradually
at first, and then a short plateau at about 2.1 V vs. Li/Li+ was
observed. From the second cycle, the discharge plateau is located at
about 2.1 V, while the discharge plateaus at lower potential that
were observed in the first cycle have disappeared. The charge/
discharge behavior of the as-prepared MoS2 electrode is similar to
that of MoS2 nanotube electrodes, which is described in other
reports [7,8]. It should be noted that the irreversible capacity in the
first cycle is high (about 184 mAh/g), which may caused by (1) the
decomposition of electrolyte on the surface of the MoS2 to form a
passivation layer on the electrode; and (2) the fraction of Li+ ions
that were trapped in the nanoclusters or defect sites/intratubal
sites in the MoS2 nanoflakes and nanotubes induced irreversible
capacity. From the second cycle, only one plateau (2.2 V for the
charge curve and 2.1 V for the discharge curve) connected with a
Acknowledgments
Financial support provided by the Australian Research Council
(ARC) through an ARC Discovery project (DP0878611) and by the
Ministry of Education, China, through the Key Laboratory for
Synthesis and Applications of Organic Functional Molecules is
gratefully acknowledged. Moreover, the authors would like to
thank Dr. Tania Silver at the University of Wollongong for critical
reading of the manuscript.
References
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C. Feng et al. /Materials Research Bulletin 44 (2009) 1811–18151814
slope can be observed for both charge and discharge curves,
indicating that the intercalation reaction dominates the electro-
chemical processes after the first cycle. This is in agreement with
the cyclic voltammograms shown in Fig. 4. Although the MoS2
electrode has a relatively high discharge potential, it still can be
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Fig. 6. Typical cycling performance of an as-prepared MoS2 electrode for different
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nanoflake electrodes, MoS2 nanoflakes could be a promising
alternative anode material for lithium ion batteries.
4. Conclusions
In this study, a novel method (the rheological reaction method)
was used to synthesize MoS2 nanoflakes (partly formed nano-
tubes). The synthesized MoS2 shows stable cyclability over a wide
voltage range. (The reversible capacity remains 840 mAh/g after 20
cycles, which is 84% of the initial reversible capacity.) There are
four possibilities for lithium intercalation in the MoS2 nanoflake
electrodes: (1) lithium ions intercalate into nanoflake clusters; (2)
lithium ions intercalate into defect sites in nanoflakes (partly
formed nanotubes); (3) lithium ions intercalate into intratubal
sites (the hollow core) through the open end; and (4) lithium ions
intercalate into the MoS2 layer sites to form LixMoS2. All these four
possibilities contribute to the high lithium insertion capacity of
MoS2 nanoflakes electrodes. Nanoflakes, i.e. partly formed
nanotube materials, may indicate a new possible direction to
further improve the electrochemical performance of electroche-
mically active materials.
Fig. 7. The quantity of Li ions inserted at different working current densities: (a)
40 mA/g and (b) 60 mA/g.
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C. Feng et al. /Materials Research Bulletin 44 (2009) 1811–1815 1815
Synthesis of molybdenum disulfide (MoS2) for lithium ion battery applications
Introduction
Experimental
Results and discussion
Reaction mechanism and structure characterization
Electrochemical properties
Conclusions
Acknowledgments
References
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