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j.matchemphys.2018.08.048

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Accepted Manuscript
Hydrothermal synthesis of Co-doped-MoS2/reduced graphene oxide hybrids with
enhanced electrochemical lithium storage performances
Shurui Xu, Qing Zhu, Tao Chen, Weixiang Chen, Jianbo Ye, Jianguo Huang
PII:
S0254-0584(18)30712-0
DOI:
10.1016/j.matchemphys.2018.08.048
Reference:
MAC 20891
To appear in:
Materials Chemistry and Physics
Received Date:
22 January 2018
Accepted Date:
19 August 2018
Please cite this article as: Shurui Xu, Qing Zhu, Tao Chen, Weixiang Chen, Jianbo Ye, Jianguo
Huang, Hydrothermal synthesis of Co-doped-MoS2/reduced graphene oxide hybrids with enhanced
electrochemical lithium storage performances, Materials Chemistry and Physics (2018), doi:
10.1016/j.matchemphys.2018.08.048
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ACCEPTED MANUSCRIPT
Graphical Abstract (Figure)
ACCEPTED MANUSCRIPT
Hydrothermal synthesis of Co-doped-MoS2/reduced graphene oxide
hybrids with enhanced electrochemical lithium storage performances
Shurui Xu, Qing Zhu, Tao Chen, Weixiang Chen*, Jianbo Ye, Jianguo Huang
Department of Chemistry, Zhejiang University, Hangzhou 310027, P.R. China
Abstract
This work reports a facile one-pot hydrothermal route to fabricate Co-dopedMoS2/reduced graphene oxides (RGO) hybrids. The effects of Co-doping on the
morphology, microstructure and electrochemical lithium storage performance of these
hybrids are investigated. It is demonstrated that the rational Co-doping can change the
morphologies and microstructures of the hybrids. Especially, the Co-dopedMoS2/RGO-2 prepared with 1:4 mole ratio of CoCl2 to Na2MoO4 in the hydrothermal
solution shows that the numerous Co-doped MoS2 layers with shorter crystal fringes
and more defect sites are well anchored on the surface of RGO, resulting in the
significantly enhanced electrochemical performance for reversible lithium storage. In
comparison with the MoS2/RGO, the Co-doped-MoS2/RGO-2 can not only deliver
much larger reversible capacity of 1236 mAh g-1 with good cyclic stability at the
current density of 100 mA g-1, but also exhibit significantly enhanced high-rate
capability of 895 mAh g-1 at a high current density of 1000 mA g-1. The Co-dopedMoS2/RGO-2 also shows much higher Coulombic efficiency of 89.2% at the first
cycle than MoS2/RGO (67.2%). The greatly enhanced electrochemical performance is
ascribed to its robust heterostructure by hybridizing of Co-doped MoS2 layers and
RGO sheets.
Keywords? Molybdenum disulfide; cobalt doping; reduced graphene oxide sheets;
hydrothermal route; lithium ion battery.
-------------------
*Corresponding author: Fax: 86-571-87951895; Tel: 86-571-87951352.
E-mail address: weixiangchen@zju.edu.cn (W. X. Chen)
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1. Introduction
As one typical example of advanced electrochemical energy storage systems,
lithium ion batteries (LIBs) with high energy density, low self-discharge and long
service life have become the dominant power sources for portable electronic products
and electric vehicles [1-3]. However, the performance of current LIBs cannot meet
ever-growing demands for superior energy density and power density. In principle,
the performance of LIBs largely depends on electrode materials [4, 5]. Even if
graphite is currently used as anode material for most commercial LIBs because of its
excellent cycling stability, its low specific capacity (theoretical capacity of 372 mAh
g-1) and poor rate-capability limit the development of next-generation LIBs.
Therefore, it is essential to devote more efforts to design and create novel anode
materials with improved lithium storage performances to satisfy the ever-increasing
performance demands. [6, 7]
Recently, nanostructured MoS2 materials have attracted a great deal of attention
on account of their unique two-dimension (2D) morphology, fascinating properties
and various applications [8]. MoS2 comprises of a molybdenum layer located between
two sulfur layers features a layered structure, in which S-Mo-S atoms are covalently
bonded to form closed packed hexagonal structures that are stacked together through
van der waals interactions. The interlayer spacing of MoS2 (0.63 nm) is remarkably
greater than that of graphite (0.33 nm), indicating that MoS2 is a kind of suitable host
among the numerous candidates for LIBs [10, 11]. For instance, the single- or fewlayer MoS2 sheets have been revealed to exhibit high reversible specific capacity of
900-1000 mAh g-1 for electrochemical lithium storage [12, 13]. Nevertheless, MoS2
sheets have their own shortcomings as electrode materials of LIBs. The van der waals
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interaction between basic plane will result in aggregation of MoS2 layers, which will
reduce the stability of 2D nanostructure, leading to the poor cyclic performance [14].
Additionally, as a typical semiconductor [15], the low conductivity of MoS2 will
greatly limits its practical application as electrode materials. What?s more, during
intercalation/extraction, the volume changes give birth to concomitant mechanical
stress, which leads to a conspicuous degradation and loss of contact between active
material and current collector. These are the major problems faced by practical
applications of MoS2-based materials in LIBs [12, 13].
As one of the effective methods to settle these problems, hybridization with
conductive substrates (such as graphene, carbon nanotubes and conducting polymers)
has drawn considerable attention [3]. Owing to its superior electrical conductivity,
high charge mobility, large specific surface area and intrinsic flexibility, graphene is
believed as one of the most promising carbon matrices [16]. It is well verified that the
horizontal growth of 2D materials on graphene can intensify interaction between 2D
nanosheets and graphene, which can increase the electrical conductivity and
avoid
the phenomenon of aggregation and pulverization, resulting in the enhanced
performances for electrochemical energy storage [17]. Besides, 2D MoS2 nanosheets
are similar to graphene in the microstructure and morphology. Naturally, graphene
can be employed as a suitable matrix to support MoS2 layers to form a sheet-on-sheet
heterostructural hybrid, which can greatly improve its conductivity and enhance
electron transfer rate, resulting in significantly improved electrochemical performance
for reversible lithium storage [18]. The robust heterostructure of MoS2/RGO hybrids
not only ameliorates lithium ion accommodation, but also shortens the transport
pathway of ions, leading to high specific capacity and enhanced rate capability [1922]. In addition, the intrinsic flexibility of graphene can dramatically mitigate the
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volume change during long-term charge/discharge cycling [23]. Therefore, this kind
of heterostructural MoS2/graphene hybrids could exhibits significantly enhanced
electrochemical performance for LIBs. In our previous work [18], the layered
MoS2/graphene composite prepared by hydrothermal route and annealing in H2/N2
manifested a high specific capacity of about 1100 mAh g-1 at a current of 100 mA g-1.
Few-layered MoS2/graphene composite synthesized with the assistance of a watersolvable supramolecule (N-methylimidazole pillar[5]arene) delivered a high
reversible specific capacity of 1100-1200 mAh g-1 at a current density of 100 mA g-1
with stable cyclic performance and enhanced rate capability for electrochemical
lithium storage [24].
On the other hand, heteroatom doping has been demonstrated one general
strategy to change the microstructure and increase the electrical conductivity of metal
sulfides and oxides [25]. It has been reported that rational cation-doping (such as Ni2+,
Co2+) could adjust the structure and morphology of MoS2 nanomaterials, resulting in
improved electrochemical properties [26, 27]. Change in crystal engineering leads to
cobalt or nickel atoms being located exclusively at the sulfur edges; hence achieving a
high density of active sites [28-30]. Density functional theory (DFT) calculation and
electrochemical measurement have revealed that Co-doping could greatly increase the
activity of MoS2 for hydrogen evolution reaction (HER) [28]. Dai et al have
demonstrated that the optimized Co-doped MoS2 catalyst fabricated by a depositionprecipitation method showed superior electrochemical activity and excellent stability
for HER [31]. Zhang et al have also reported that the 3D defect-rich MoS2
nanomesh/RGO foam (5 wt% RGO to MoS2 and 2 mol% Co to Mo) achieved by a
one-pot hydrothermal reaction exhibited prominent activity for HER [26]. In our
previous work [32], Co-doped MoS2/graphene hybrids synthesized through a one-pot
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hydrothermal method showed a remarkable electrocatalytic activity toward HER. Codoped MoS2 is mostly applied for HER. We believe that hybridizing of Co-doped
MoS2 layers with RGO sheets can significantly improve the electrochemical
performance for reversible lithium storage.
Herein, we present a facile one-pot hydrothermal route to synthesize Co-doped MoS2/RGO hybrids with different mole ratios (1:9, 1:4, 3:7, 1:1) of Co:Mo as anode
materials for LIBs. The morphologies and microstructures of these hybrids were
examined using X-ray diffraction (XRD) patterns, scanning electron microscope
(SEM) images, high-resolution transmission electron microscopy (HRTEM) and Xray photoelectron spectroscopy (XPS). The effects of Co-doping on the
microstructure and the electrochemical performance of MoS2/RGO for lithium ion
storage were investigated. The results indicate that rational Co-doping can change
microstructures and morphologies of Co-doped-MoS2/RGO hybrids, leading to
significant improvement in their electrochemical performance. Especially, the Codoped-MoS2/RGO-2 hybrid with 1:4 mole ratio of Co:Mo shows a high reversible
specific capacity of 1236 mAh g-1 with stable cyclic performance and enhanced highrate capability. The Co-doped-MoS2/RGO-2 hybrid with excellent electrochemical
lithium storage property will hold high potential as host electrode material in LIB.
2. Experimental section
2.1 Synthesis of Co-doped-MoS2/RGO hybrids
The natural graphite power (Shanghai Colloid Chemical Plant, China) was used
for preparing graphene oxide sheets (GOS) by the modified Hummers method, the
details of which were described elsewhere [18, 33], Co-doped-MoS2/RGO hybrids
were synthesized by one-pot hydrothermal reaction. 20 mL of the solution containing
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CoCl2�2O (0.15, 0.30, 0.45 or 0.75 mmol) was dropped in 20 mL suspension of
GOS (3.0 mmoL) along with vigorous stirring. Due to a large amount of oxygencontaining functional groups (such as ?COO, C=O, ?OH), Co2+ ions could be well
absorbed on the surface of GOS by coordination with these functional groups after
stirring of 12 h at room temperature. Then, the mixed solution of Na2MoO4�2O
(1.35, 1.20, 1.05 or 0.75 mmol) and L-cysteine (7.50 mmol) in 35 mL deionized water
was added into the above suspension. The final mixed suspension was stirred for
another 2.0 h. After that, the mixture was transferred into 100 ml Teflon-lined
stainless steel autoclave, which was heated to 240 癈 and kept this temperature for 24
h. The black edimentation was collected by the centrifugation and washed for several
times with water, and freeze-dried for 48 h. Finally, four hybrid samples were
obtained, which are denoted as Co-doped-MoS2/RGO-1, Co-doped-MoS2/RGO-2,
Co-doped-MoS2/RGO-3 and
Co-doped-MoS2/RGO-4, respectively, prepared with
different mole ratios (1:9, 1:4, 3:7, 1:1) of CoCl2:Na2MoO4 in hydrothermal reaction
solution. For the control experiment, MoS2/RGO hybrid was prepared by the same
hydrothermal route except for adding CoCl2�2O.
2.2 Characterizations
The crystal structures of samples were characterized by XRD on a Thermo
X?TRA X-ray diffractometer with Cu ?? radiation (?=0.154056 nm). The
morphologies were observed by using a SIRION-100 field emission scanning electron
microscop (FE-SEM). Thermogravimetric analysis (TGA) was carried out with a
NETZSCH STA 409 PC apparatus at a heating rate of 5 oC min-1 in flowing air. High
resolution transmission electron microscope (HRTEM) characterization was
performed by using a JEOL JFL-2010 TEM operating at 200 kV. In order to HRTEM
observation, sample was well dispersed in ethanol and drop-casted onto a 200 mesh
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copper grid coated with holey carbon. X-ray photoelectron spectrum (XPS) analysis
was performed on a PHI 5000 Versaprobe system using monochromatic Al K?
radiation (1486.6 eV). All binding energies were referenced to the C 1s peak at 284.6
eV.
2.3 Electrochemical measurements
The electrochemical measurements were carried out by using CR2032 buttontype cells with lithium foil served as both counter electrode and reference electrode.
The working electrode was prepared by a slurry coating procedure. The slurry
consisted of 80 wt% active material (MoS2/RGO or Co-doped-MoS2/RGO hybrid), 10
wt% acetylene black and 10 wt% polyvinylidene fluorides dispersed in N-methyl-2pyrrolidinone. The obtained slurry was spread on a copper foil, dried at 120 癈 for 12
h under vacuum, and then pressed to form the working electrode. The test cells were
assembled in an argon-glove box, in which both the moisture and oxygen contents
were below 1.0 ppm. A polypropylene film (Celgard-2300) was used as the separator.
1.0 mol L-1 LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate
(EC/DMC, 1:1 in volume) was employed as electrolyte. Cyclic voltammetry was
implemented on an electrochemical workstation (CHI 660E) at a scanning rate of 0.5
mV s-1 in a potential windrow of 0.005-3.00 V. Galvanostatic charging/discharging
cycles were carried out on a LAND 2001A Battery Tester in a voltage range of 0.0053.00 V at various current densities. Electrochemical impendence spectroscopy (EIS)
was performed on the electrochemical workstation (CHI 660E) by applying a sine
wave with an amplitude of 5.0 mV over the frequency range from 200 kHz to 0.01 Hz
3. Results and discussions
Fig. 1
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Fig. 1 shows the XRD patterns of the MoS2/RGO and Co-doped-MoS2/RGO
samples prepared by one-pot hydrothermal route. As illustrated in Fig. 1(a), even if
three broad and poor peaks at 32.3o, 43.1o and 57.0o can be observed, respectively
corresponding to (100), (006) and (110) of 2H-MoS2 (JCPDS No. 37-1492), the peak
at 2?=14.3� attributed to (002) plane of 2H-MoS2 cannot be found. The fact indicates
the extremely low crystallinity of MoS2 in as-prepared sample and poor-stacking of
MoS2 along c-axis. Fig. 1(b, c) shows that both Co-doped-MoS2/RGO-1 and Codoped-MoS2/RGO-2 almost display the same XRD patterns in comparison with
MoS2/RGO. The reflections of CoS2 hardly appear form the Co-doped-MoS2/RGO-1
and Co-doped-MoS2/RGO-2 hybrids. This means that that when the hydrothermal
solution contains 0.15 mol or 0.30 mmol CoCl2 with CoCl2:Na2MoO4 of 1:9 or 1:4 in
mole ratio, Co2+ ions can substitute for Mo form the Co-doped MoS2 layers by the
simultaneous hydrothermal reaction of Co2+ and MoO42- with L-cysteine. From the
XRD pattern of Co-doped-MoS2/RGO-3 and Co-doped-MoS2/RGO-4, the sharp
peaks at 27.8 o, 32.1o, 36.2, 39.9o, 46.6o, 55.0o, 60.4o and 63.0 o can be found, which
are respectively induced to (111), (200), (210), (211), (220), (311) (023) and (321)
planes of CoS2 (JCPDS No. 65-3322). The fact indicates that there are CoS2 phase in
both Co-doped-MoS2/RGO-3 and Co-doped-MoS2/RGO-4. It is due to that the
excessive Co2+ ions cannot be doped into MoS2 lattices and are transferred to CoS2
particles during hydrothermal reaction. It is worth notice that all samples display a
pronounced peak at around 9.0o (marked by #1) and a poor peak at 17.2o (marked by
#2). By using Bragg's equation, the d-spacing of peak #1 is about 0.98 nm, it may be
regarded as the distance of adjacent MoS2 layers. The interlayer expansion can be
attributed to the incorporation of RGO and trapping of foreign species, such as NH4+
ions that are generated from the L-cysteine
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under hydrothermal condition
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(HSCH2CHNH2COOH+H2O?CH3COCOOH+NH4++S2-) [25, 34]. The interlayer
spacing of the reflection #2 at 2?=17.2o can be calculated to be about 0.52 nm, which
can be ascribed to the spacing between MoS2 layer and RGO sheet as described in Fig
1(f). In addition, it can be found from Fig. 1 that the intensity of peak #2 at 2?=17.2o
becomes more and more poor with increasing of Co2+ content in the hydrothermal
solution. In particular, the peak #2 is too weak to be observed from the Co-dopedMoS2/RGO-4. It is due to that the excessive Co2+ ions preferentially react with Lcysteine to form CoS2 particles under hydrothermal condition, disturbing the wellgrowth of MoS2 layers on the surface of RGO. Finally, the peak at 2?=26.2�, induced
to the restacked graphene (JCPDS No. 75-1621), does not appear for all the samples,
which means that the RGO is synchronously prevented from restacking due to the
hybridization with MoS2 or Co-doped MoS2 layers.
Fig. 2
The general morphological features of as-prepared samples are characterized by
SEM as shown in Fig 2. It is observed from Fig. 2(a) that the MoS2/RGO hybrid
displays a graphene-like architecture consisted of the curved ultrathin flakes, from
which the individual MoS2 flakes or particles can not be found, indicating that MoS2
layers uniformly growth on the surface of RGO to form sheet-on-sheet
heterostructure. As shown in Fig 2(b, c), Co-doped-MoS2/RGO-1 and Co-dopedMoS2/RGO-2 hybrids display different morphological features to some extent
compared to MoS2/RGO. It can be found from Fig. 2(b, c) that the curled MoS2 layers
are well anchored on the surface of RGO. Especially, as shown in Fig. 2(c), numerous
curled MoS2 layers can be clearly observed on the surface of RGO from the Codoped-MoS2/RGO-2 sample. Of course, these MoS2 layers should be Co-doped MoS2
for these two samples. The fact demonstrates that the rational Co-doping alters the
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morphology of MoS2/RGO hybrid. When the hydrothermal solution contains 0.45 mol
of CoCl2 and 1.05 mmol of Na2MoO4 (Co:Mo=3:7 in mole), a small amount of CoS2
particles are dispersed in the Co-doped-MoS2/RGO-3 sample as shown in Fig. 2(d).
Especially, when the hydrothermal solution contains 0.75 mol of CoCl2 and 0.75
mmol of Na2MoO4 (Co:Mo=1:1 in mole), the as-prepared Co-doped-MoS2/RGO-4
clearly shows that quite a few of CoS2 particles with the sizes of 40-90 nm disperse on
the curled graphene-like flakes as shown in Fig. 2(e), which agrees with its XRD
analysis as shown in Fig.1 (e).
Fig. 3
HRTEM characterization is carried out to obtain more structural details of the
samples. Fig. 3(a, b) exhibits that interlaced MoS2 layers are scattered on RGO, which
displays a few-layered structure with d-spacing of 0.97 nm. HRTEM images of Fig.
3(c-h) shows that the microstructures of Co-doped-MoS2/RGO-1, Co-dopedMoS2/RGO-2 and Co-doped-MoS2/RGO-3 are on the whole similar to that of
MoS2/RGO. However, the three Co-doped-MoS2/RGO hybrids show that the MoS2
layers display shorter crystal fringes with less layer number and more exposed edges,
particularly for Co-doped-MoS2/RGO-2 as shown in Fig. 3(e, f). In addition, the
interlayer spacing of 0.96, 0.97 nm and 0.94 nm in Fig. 3(d, f, h) are consistent with
their XRD analysis. No crystal fringes of CoS2 can be found from both Co-dopedMoS2/RGO-1 and Co-doped-MoS2/RGO-2, indicating the well-doping of Co2+ ions into
MoS2 lattices. Even if HRTEM image of Co-doped-MoS2/RGO-3 hardly presents the
crystal fringes of CoS2 as shown Fig.3 (g, h), its XRD pattern shows the peaks of CoS2
phase as shown in Fig. 1 (d). For the Co-doped-MoS2/RGO-4, quite a few CoS2
particles can be clearly viewed from Fig. 3(i, j). The lattice spacing of 0.27 nm is
assigned to the (200) planes of CoS2 (JCPDS No. 65-3322).
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Fig. 4.
In order to investigate the specific capacity contribution of MoS2 (or Co-doped
MoS2) and RGO, TGA was employed to determine the content of RGO in the different
hybrids as shown in Fig. 4. Fig. 4(a) shows the MoS2/RGO hybrid displays three mass
losses in the TGA curve. The first mass loss from room temperature to 90-100 oC
should be due to the volatilization of the trace moisture. The second appeared at about
230 oC is attributed to the removal of oxygen-containing groups of RGO. The third is a
large continuous weight loss in the range of 350-470 oC, which is caused by the
decomposition of RGO and the oxidation of MoS2 to MoO3 in the flowing air. As
shown in Fig. 4(b-e), the Co-doped-MoS2/RGO hybrids exhibit the similar TGA curves
to that of MoS2/RGO, except for a small mass loss in the range of 530-600 oC. The
RGO would be completely oxidized to CO2 under 700 oC in air [24]. The finally
remained substance should be MoO3 for MoS2/RGO sample or Co3O4 and MoO3 for
Co-doped-MoS2/RGO hybrids. Therefore, the contents of RGO can be calculated to be
respectively about 18.7wt% for MoS2/RGO, 22.1wt% for Co-doped-MoS2/rGO-1,
22.7wt% for Co-doped-MoS2/rGO-2, 23.8wt% for Co-doped-MoS2/rGO-3 and
24.6wt% for Co-doped-MoS2/rGO-4.
Fig. 5
XPS was used to further analyze the elemental composition and chemical state of
the typical sample, Co-doped-MoS2/RGO-2 hybrid. As shown in Fig. 5(a), the survey
spectroscopy reveals that the Co-doped-MoS2/RGO-2 sample consists of C, O, S, Mo
and Co elements. The high-resolution scan of C 1s in Fig. 5(b) exhibits four peaks
located at 284.6 eV, 285.4 eV, 286.4 eV and 288.6 eV, which are ascribed to the
carbon-functional groups C=C, C?O, C?O?C and C=O, respectively [35]. Fig. 5(c)
states that the S 2p peak can be deconvoluted to four peaks. The two peaks at 161.2
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eV and 162.4 eV manifest the S 2p3/2 and S 2p1/2 for S2-, while the other two peaks at
163.4 eV and 164.1 eV can be attributed to the S 2p3/2 and S 2p1/2 for apical S2ligands or/and bridging disulfides S22- [36-38]. The high-resolution XPS of the hybrid
in the Mo 3d region can be divided into six peaks. One peak at 225.7 eV actually is
assigned to the S 2s of MoS2. Two main peaks arising from 228.4 eV and 231.7 eV
correspond to Mo (IV) 3d5/2 and Mo (IV) 3d3/2 binding energy, respectively. The two
relatively small peaks at 232.7.0 eV (Mo 3d5/2) and 235.6 eV (Mo 3d3/2) suggest that
Mo (IV) is partially oxidized to Mo (VI) in the air [39-41]. Moreover, a shoulder peak
at 229.4 eV manifests that Mo (VI) was partially reduced to Mo (V) [40-42]. The Co
2p XPS spectra in Fig. 4(e) can be well fitted with two spin?orbit doublets, which are
characteristic of Co2+ and Co3+, and two shake-up satellites (identified as ??Sat.??).
The two prominent peaks at 779.5 eV and 798.3 eV, attributed to Co 2p3/2 and Co
2p1/2 of Co2+ species, are very near to the binding energy of the Co species in CoS2
[43-45]. The slight difference is due to the Co doping into the MoS2 lattice. Even if
CoS2 phase in the Co-doped-MoS2/RGO-2 hybrid can not be detected by XRD, SEM
and HRTEM, a XPS peak for Co2+ can be observed, indicating that Co2+ ions can be
well doped into the MoS2 lattice. The quantitative analysis of the XPS peak states that
the atomic ratio of Mo:Co:S is about 0.80:0.23:2.5, which generally agrees with the
mole ratios of CoCl2 and Na2MoO4 in the hydrothermal solution and also is
approaching to the stoichiometry of Co0.2Mo0.8S2.
Fig. 6.
To clearly reveal the redox reaction of the MoS2/RGO and Co-doped-MoS2/RGO
hybrids for reversible lithium storage, the cycle voltammetry (CV) was performed in
the potential windrow of 0.005-3.0 V at a scan rate of 0.5 mV s-1. Fig. 6(a) shows that
two conspicuous peaks appear in the 1st cathodic scanning. The peak at 1.40 V is
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ascribed to the intercalation of lithium ion into the MoS2 lattices (MoS2+xLi+ +xe?LixMoS2) [10, 46]. Lithium intercalates into the S slab, and the van der Waals S?S
bonds are broken to be replaced by Li?S bonds, resulting in a phase change from 2H
to 1T. The peak at 0.40 V is attributed to the conversion reaction of LixMoS2 into
metallic Mo and Li2S matrix (LixMoS2+(4-x)Li++(4-x)e ? Mo+2Li2S) [47, 48]. These
metallic Mo clusters are highly uniformly embedded in the Li2S matrix in atomic
level. There are abundant interface phases between Mo clusters and Li2S matrix. In
the subsequent anodic sweep, two peaks can be distinguished at about 1.92 V and
2.28 V. The peak at 1.92 V can be attributed to the removal of lithium associated with
Mo or partial oxidation of Mo [46, 47]. The peak at 2.28 V is assigned to the
delithation of Li2S (Li2S ? S+2Li++2e-). At the end of the 1st cycle, most of the
MoS2 should be converted to Mo and S (MoS2+4Li++4e 2Li2S+Mo and
Li2S?S+2Li++2e) [10]. In the 2nd and 3rd cathodic sweep, the two peaks at 1.4 V and
0.40 V do not appear, while two new peaks at about 2.0 V and 1.37 V arise. The peak
at 2.0 V pairs with anodic peak at 2.28 V is the reduction of S forming Li2S
(S+2Li++2e-? Li2S). The peak at 1.37 V is attributed to the association of Li+ ions
with molybdenum [46]. The increasing cathodic current below 0.3 V is due to the
lithium storage in the defect sites and the interfaces between Mo clusters and Li2S. As
shown in Fig. 6(b, c, d), Co-doped-MoS2/RGO-1, Co-doped-MoS2/RGO-2 and Codoped-MoS2/RGO-3 electrodes exhibit the similar CVs feature compared to
MoS2/RGO, except for that at the 1st cathodic sweep, the peaks of Li+ insertion
slightly move from 1.40 V to lower potential (1.12 V, 1.08 V or 0.90 V). For Codoped-MoS2/RGO-4 hybrid electrode, Fig. 6(e) shows the three reduced peaks at 1.30
V, 0.95 V and 0.55 V at the 1st cathodic sweep. Since XRD, SEM and HRTEM
characterizations have demonstrated that there are quite a few CoS2 particles in Co13
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doped-MoS2/RGO-4 hybrid, the three reduced peaks should be attributed to the first
lithium storage process of CoS2 and Co-doped-MoS2/RGO, including intercalation of
lithium ions into CoS2 and Co-doped MoS2 lattices (MS2+xLi++xe?LixMS2, M=Co,
Mo) [48] and the successive conversion reaction into metallic Co and Mo embedded
in a Li2S matrixes (LixMS2+(4-x)Li++(4-x)e ?M+2Li2S, M=Co, Mo) [49, 50].
Fig. 7
Fig. 7 shows the first three charge/discharge voltage curves of MoS2/RGO and
Co-doped-MoS2/RGO hybrid electrodes at the current density of 100 mA g-1 between
0.005 V and 3.00 V. As shown in Fig. 7(a), in the 1st discharge (lithiation process),
the plateau at 1.5 V represents the insertion of lithium ions into MoS2 lattice to form
LixMoS2 with the change of phase from 2H to 1T and the variation of plateau is due to
the poor crystallinity of MoS2. Another plateau at around 0.65 V is indicative of the
conversion reaction of LixMoS2 to generate metallic Mo highly embedded in Li2S
matrixes. The sloping potential curve below 0.4 V is attributed to lithium storage in
the defect sites of active material and the interfaces between metallic Mo and Li2S
matrix. During the 2nd and 3rd discharge, there are two plateaus located at 2.1 V and
1.5 V, which are ascribed to the lithation of S forming Li2S and association of lithium
ions with molybdenum. In the charge process (delithiation), MoS2/RGO electrode
displays two plateaus at 1.85 V and 2.20 V, which well agree with its CVs. As
pictured in Fig. 7(b, c, d, e), the four Co-coped-MoS2/RGO hybrids almost exhibit the
sample voltage plateau features in their charge/discharge curves compared to the
MoS2/RGO electrode, expect for that the 1st discharge curve of the Co-copedMoS2/RGO-4 is different form those of other samples due to the existence of quite a
few of CoS2 particles. Fig. 7 also shows that the Co-coped-MoS2/RGO-2 hybrid
exhibits higher specific capacity for electrochemical lithium storage than other
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samples. As shown Fig. 7(a), at the first cycle, MoS2/RGO delivers an initial
discharge capacity of 1382.6 mAh g-1 and a reversible charge capacity of 929.3 mAh
g-1 with a Coulombic efficiency of 67.2%. The reversible specific capacity delivered
by MoS2/RGO is much higher the theoretical value of MoS2 (670 mAh/g), which is
due to the defects or disorder structures of MoS2 layers dispersed on RGO and the
synergism between MoS2 layers and RGO. The irreversible capacity loss in the first
cycle is mainly caused by the formation of a solid electrolyte interface (SEI) film and
other the incomplete conversion reaction such as reduction of oxygen-containing
groups [47, 50]. Besides, during the charge/discharge process, a small amount of
lithium ions are trapped in the defect sites and hardly delithated, leading to
irreversible capacity. Fig. 7(c) shows that the Co-coped-MoS2/RGO-2 can deliver an
initial discharge capacity of 1385.3 mAh g-1 and a reversible capacity of 1235.2 mAh
g-1 with a Coulombic efficiency of 89.2%. The corresponding values are respectively
1292.5 mAh g-1 and 1120.6 mAh g-1 with a Coulombic efficiency of 86.7% for Cocoped-MoS2/RGO-1, 1171.6 mAh g-1 and 1020.5 mAh g-1 with a Coulombic
efficiency of 87.1% for Co-coped-MoS2/RGO-3, 1173.5 mAh g-1 and 905.6 mAh g-1
with a Coulombic efficiency of 77.2% for Co-coped-MoS2/RGO-4. It is concluded
that rational Co-doping not only increases the reversible specific capacity, but also
greatly improves the Coulombic efficiency at the first cycle, which is one of important
factors for the practical application in LIB. Especially, the first Coulombic efficiency
of Co-doped-MoS2/RGO-2 is up to 89.2%, which is much higher than that (67.2%) of
MoS2/RGO in this work and also higher than other MoS2/RGO composites reported
elsewhere [27, 53, 54]. In comparison with MoS2/rGO, the rational Co-doping can
alter the microstructure and morphology, which facilitates the diffusion of ions during
electrode reaction and the accessibility of electrolyte into active materials. In addition,
15
ACCEPTED MANUSCRIPT
it was reported that the Co-doping greatly enhanced the conductivity of MoS2 [51],
which would further reduce charge-transfer resistance of Co-doped-MoS2/RGO-2
electrode for reversible lithium storage. Therefore, Co-doped-MoS2/RGO-2 not only
delivers higher reversible specific capacity, but also exhibits higher Coulombic
efficiency at the 1st cycle than MoS2/RGO. At the 2nd and 3rd cycle, the Coulombic
efficiencies increase up to above 96% for all electrodes as shown in Fig. 7.
Fig. 8.
Fig. 8(a) illustrates the cyclic performance of MoS2/RGO and Co-dopedMoS2/RGO electrodes for reversible lithium storage at a current density of 100 mA g1.
It has been reported that the MoS2 nanostructures prepared by different methods
including hydrothermal routes could deliver a specific capacity of 800-1000 mAh g-1
[12, 13]. But the pristine MoS2 exhibits a poor cyclic stability due to its agglomeration
and pulverization during repeat lithiation/delithation process, leading to the
destruction of electrode and the loss of capacity [12, 13].
The hybridization with
graphene (or RGO) nanosheets has been demonstrated an effective strategy to
improved the cyclic durability. Thus, it is reasonable that all electrodes exhibit good
cyclic stability as shown Fig. 8(a). It can also be observed in Fig. 8(a) that after 5-10
cycles, Co-doped-MoS2/RGO-2 electrode exhibits a reversible capacity of 1236 mAh
g-1, which is much larger that of MoS2/RGO (898 mAh g-1), and also larger than those
of Co-doped-MoS2/RGO-1 (1085 mAh g-1), Co-doped-MoS2/RGO-3 (1019 mAh g-1)
and Co-doped-MoS2/RGO-4
(902 mAh g-1). The specific capacity of RGO prepared
by L-cysteine assisted hydrothermal method is about 860 mAh g-1 [52]. According to
the contents of RGO in the hybrids, the specific capacity contribution of MoS2 or Codoped-MoS2 can be calculated to be 907, 1149, 1341, 1068 and 917 mAh g-1,
respectively, for MoS2/RGO, Co-doped-MoS2/RGO-1, Co-doped-MoS2/RGO-2, Co16
ACCEPTED MANUSCRIPT
doped-MoS2/RGO-3 and Co-doped-MoS2/RGO-4. It was reported that the specific
capacity contribution of the exfoliated MoS2 sheets in MoS2/PEO composite was
1131 mAh g-1, due to that the more defects or disorder structures of the exfoliated
MoS2
greatly enhanced the lithium accommodate capability. One can be found that
the specific capacity contributions of Co-doped MoS2 in Co-doped-MoS2/RGO-1 and
Co-doped-MoS2/RGO-2 are very closed to or higher than that (1131 mAh/g) of the
exfoliated MoS2 in MoS2/PEO. The higher specific capacity contribution of Codoped-MoS2 in Co-doped-MoS2/RGO-2 should be attributed to that Co-doped-MoS2
layers exhibit more defects or disorder structures than the exfoliated MoS2 sheets.
Another reason for such high specific capacity of the Co-doped-MoS2/RGO-2 is due
to the synergistic effects between Co-doped MoS2 layers and RGO, as well as the
enhanced conductivity of Co-doped MoS2.
At the 100th cycle, Co-doped-
MoS2/RGO-2 still retains a reversible specific capacity of 1223 mAh g-1, indicating its
excellent cyclic stability.
High-rate capability is an important aspect to obtain high power density in LIB.
Fig. 8(b) shows the rate cycling behavior of the MoS2/RGO and Co-dopedMoS2/RGO electrodes at different current densities. It can be seen from Fig. 8(b) that
at a high current density of 1000 mA g-1, Co-doped-MoS2/RGO-2 can delivers a
reversible capacity of 894 mAh g-1, which is larger that of MoS2/RGO (692 mAh g-1),
Co-doped-MoS2/RGO-1 (795 mAh g-1) and Co-doped-MoS2/RGO-3 (750 mAh g-1).
The fact indicates that rational Co-doping can significantly improve the rate capability
of MoS2/RGO based electrode materials. In addition, Fig. 8(b) also shows that when
the current density return to 100 mA g-1 from 1000 mA g-1, all electrodes can recover
the specific capacity to their original values, indicating that they still can keep good
cyclic durability after the cycling at the different current densities.
17
ACCEPTED MANUSCRIPT
The results of electrochemical measurements reveal that the rational Co-doping
can further improve the electrochemical performance of MoS2/RGO hybrid, because
the rational Co-doping changes the microstructure and morphology of the hybrid.
These changes not only provide more sites for the lithium accommodate, but more
facilitates the diffusion of ions and the access of electrolyte. Among of all samples in
this work, the Co-doped-MoS2/RGO-2 hybrid displays best electrochemical property.
It not only delivers a high specific capacity of 1236 mAh g-1 with stable cyclic
performance, but also exhibits significantly enhanced high-rate capability.
In order to well understand the remarkable electrochemical performance of Codoped-MoS2/RGO-2 hybrid in comparison with MoS2/RGO and gain insight into their
kinetics characteristics of the electrochemical lithium storage process, EIS of
MoS2/RGO and Co-doped-MoS2/RGO-2 electrodes are analyzed after 10 cycles as
shown in Fig. 8(c). Fig. 8(d) is the corresponding equivalent circuits for EIS fitting by
using the Z-view software. Fig. 8(c) shows that the Nyquist plots of MoS2/RGO and
Co-doped-MoS2/RGO-2 electrodes consist of an incline line in the low-frequency
region and two depressed semicircles in the medium- and high-frequency region. The
incline line in the low-frequency region corresponds to Warburg impedance (Zw),
reflecting the diffusion process of lithium ions from electrolyte into the bulk of the
electrode material. The semicircle in the high-frequency region is related to the
interface resistance (Rf) and capacitance (CPE1) of electrode active particles, while
the semicircle in the medium-frequency region reflects the charge-transfer resistance
(Rct) of the electrode reaction and the electrochemical double layer capacitance
(CPE2) between electrode materials and electrolyte [55, 56]. The parameter Re
corresponds to internal Ohmic resistance of test cell, which contains the resistance of
current collector, electrolyte and contact resistance. As shown in Fig. 8(b), the EIS
18
ACCEPTED MANUSCRIPT
fitting results well agree with the experimental date. The values of Re, Rf and Rct are
respectively 28.8 ?, 16.3 ? and 8.1 ? for Co-doped-MoS2/RGO-2 electrode, and 29.9
?, 21.2 ?, 12.9 ? for MoS2/RGO electrode. It can be clearly found that the Codoped-MoS2/RGO-2 electrode exhibits lower Rct than MoS2/RGO, manifesting that
Co-doping effectively improve charge-transfer process for reversible lithium storage.
As well known, the incorporation of graphene (or RGO) can greatly promote the
charge transfer process of electrode reaction due to its superior conductivity and high
charge mobility, resulting in the great reduce of charge-transfer resistance of
MoS2/RGO electrode. Thus, in this work, MoS2/RGO electrode also shows a low Rct
of 12.9 ?, which is much lower than that (71.5 ?) of the pristine MoS2 electrode
prepared through the similar hydrothermal route in our previous work [24]. In
comparison with MoS2/RGO, the further improvement in the electrode kinetics of Codoped-MoS2/RGO-2 should be attributed to the rational Co-doping. It has been
reported that the Co-doping could greatly improve the conductivity of MoS2 [55]. At
room temperature, only 3% Co-doping can increased the conductivity of MoS2 layers
by one order of magnitude [55]. Therefore, the rational Co-doping can further reduces
charge-transfer resistance of MoS2/RGO-based electrode for reversible lithium
storage. Additionally, the current work also demonstrates that the rational Co-doping
can change the microstructure and morphology of MoS2/RGO hybrids, facilitating the
diffusion of ions during electrode reaction and the accessibility of electrolyte into
active materials. Therefore, the electrochemical lithium storage performance of Codoped-MoS2/RGO-2 is significantly improved in comparison with MoS2/RGO.
4. Conclusions
In summary, this work has presented a facial one-pot hydrothermal route to
19
ACCEPTED MANUSCRIPT
fabricate Co-doped-MoS2/RGO hybrids by simultaneous hydrothermal reaction of
Co2+ ions and MoO42- with L-cysteine in the presence of GOS. It is demonstrated that
the rational Co-doping can change the morphology and microstructure of the Codoped-MoS2/RGO hybrids, resulting in significantly enhanced electrochemical
lithium storage property. When the mole ratio of CoCl2:Na2MoO4 in the hydrothermal
solution is 1:9 or 1:4, Co2+ ions can effectively dope into MoS2 lattices. When the
mole ratio of CoCl2:Na2MoO4 in the hydrothermal solution is 3:7 or 1:1, the excessive
Co2+ ions would be transferred into CoS2 particles dispersed in the hybrids.
Electrochemical measurements show that the Co-doped-MoS2/RGO-2 hybrid with the
Co:Mo of 1:4 in mole exhibits a high reversible capacity of 1236 mAh g-1 at the
current density of 100 mA g-1 with excellent cyclic stability and enhanced high-rate
capability of 895 mAh g-1 at the current density of 1000 mA g-1. In addition, the Codoped-MoS2/RGO-2 exhibits much higher Coulombic efficiency of 89.2% at the first
cycle than that of MoS2/RGO (67.2%). The superior electrochemical performance of
Co-doped-MoS2/RGO-2 is attributed to the more defects or disorder structures of Codoped MoS2 layers dispersed on the surface RGO, which greatly enhance lithium
accommodation capability. In addition, due to the synergistic effects between Codoped-MoS2 layers and RGO, Co-doped-MoS2/RGO-2 exhibits significantly
enhanced high-rate capability and electrode reaction kinetics, including low chargetransfer resistance and improved CE in the 1st cycle. The present results turn out that
the Co-doped-MoS2/RGO-2 hybrid can be used as a promising electrode material for
high-performance LIBs.
Acknowledgements
This work is financially supported by the Natural Science Foundation of China
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ACCEPTED MANUSCRIPT
(21473156), the Science and Technology Project of Zhejiang Province of China
(2015C01001), the International Science and Technology Cooperation Program of
China (2012DFG42100) and the Fundamental Research Funds for the Central
Universities (2017XZZX008-06).
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Graphical Abstract (text)
Co-doped-MoS2/reduced graphene oxides (RGO) hybrids are synthesized by facile
one-pot hydrothermal method. The Co-doped-MoS2/RGO hybrid prepared with 1:4
mole ratio of CoCl2 to Na2MoO4 in the hydrothermal solution delivers a reversible
capacity as high as 1236 mAh g-1 with the excellent cyclic stability and enhanced
high-rate capability for electrochemical lithium storage.
ACCEPTED MANUSCRIPT
023
321
311
111
Intensity/a.u.
??
220
210
211
200
??
e
10
20
30 40
2?/deg
Fig. 1.
110
006
100
d
c
50
60
b
a
70
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Fig. 2.
ACCEPTED MANUSCRIPT
Fig. 3.
ACCEPTED MANUSCRIPT
100
Weight loss /%
90
80
(a) (b)
(c)
70
(a) MoS2/RGO
60
(c) Co-doped-MoS2/RGO-2
(b) Co-doped-MoS2/RGO-1
(d) Co-doped-MoS2/RGO-3
50
40
(e) Co-doped-MoS2/RGO-4
0
(d)
(e)
100 200 300 400 500 600 700
o
Temperature / C
Fig. 4
ACCEPTED MANUSCRIPT
(a)
O1s
Intensity (a.u.)
C1s
Mo3d 5/2
Mo3d 5/2
S2s
Co2p
Mo3p 3/2
N1s
S2p O2s
Mo4p
Mo4s
Mo3s
800
(b)
600
400
200
Binding energy (eV)
(c)
C=C
0
2-
S 2P
2-
3/2
S 2P
Intensity (a.u.)
Intensity (a.u.)
1/2
C-O
C=O
C-O-C
Mo(VI)3d3/2
Mo(V)3d5/2
S2s
240
236
232
228
Binding energy (eV)
1/2
164
162
160
Binding energy (eV)
Fig. 5
Co
2+
Co
sat.
810
224
158
3+
(e)
Mo(IV)3d5/2
Mo(VI)3d5/2
S2 2P
Intensity (a.u.)
Intensity (a.u.)
Mo(IV)3d3/2
3/2
2-
166
294 292 290 288 286 284 282 280
Binding energy (eV)
(d)
2-
S2 2P
2+
Co
2p1/2
2p3/2
3+
Co
sat.
800
790
780
Binding energy (eV)
770
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-1
0.0
-0.5
-1.0
1st
2nd
3rd
-1.5
2.0
Current/(mA mg )
1.5
0.0
0.5 1.0 1.5 2.0 2.5
Potential/(V vs Li+/Li)
0.0
-0.5
-1.0
0.0
-0.5
-1.0
1st
2nd
3rd
-1.5
0.5 1.0 1.5 2.0 2.5
Potential/(V vs. Li+/Li)
0.0
0.5 1.0 1.5 2.0 2.5
+
Potential/(V vs. Li /Li)
(d)
0.5
0.0
-0.5
-1.0
1st
2nd
3rd
-1.5
0.0
0.5 1.0 1.5 2.0 2.5
Potential/(V vs. Li+/Li)
2.0
-1
Current/(mA mg )
1.5 (e)
1.0
0.5
0.0
-0.5
-1.0
1st
2nd
3rd
-1.5
-2.0
0.0
0.5 1.0 1.5 2.0+ 2.5
Potential/(V vs. Li /Li)
Fig. 6.
3.0
1.0
-2.0
3.0
1st
2nd
3rd
-1.5
1.5
0.5
0.0
0.5
2.0
(c)
(b)
1.0
-2.0
3.0
1.0
-2.0
Current/(mA mg )
1.5
0.5
-2.0
-1
2.0
(a)
1.0
-1
Current/(mA mg )
1.5
Current/(mA mg-1)
2.0
3.0
3.0
ACCEPTED MANUSCRIPT
3.5
2.5
Potential /V
2.0
1.5
3rd
1.0
0.0
1st
disc
harg
e
2nd
0
3.5
300
Potential /V
2.0
1.5
1.0
0.0
2nd
1st
0
1.5
2nd
3rd
disch
arge
1st
1.0
0
3rd
dis
cha
rge
300
3.5
(e)
3.0
1.0
3rd
2nd
1st
0
300
2nd
1st
ge
ar
ch
1.5
3rd
0.5
0.0
1st
dis
cha
rge
2nd
0
300
600
900 1200
Capacity /mAh g-1
Fig. 7?
disc
harg
e
600
900 -1 1200
Capacity /(mAh g )
2.5
1.0
2nd
1st
1.5
3rd
2.0
1500
ge
ar
ch
2.0
0.0
1500
3rd
2.5
0.5
300
600
900 1200
-1
Capacity /(mAh g )
600
900 -1 1200
Capacity /(mAh g )
(d)
3.0
1st
0.5
e
arg
ch
2.0
3.5
e
arg
ch
2nd
1st
2.5
0.0
1500
3rd
2nd
2.5
3rd
0.5
600
900 1200
-1
Capacity/(mAh g )
(c)
3.0
(b)
3.0
ge
ar
ch
Potential /V
Potential/V
2nd
3rd
1st
0.5
Potential /V
3.5
(a)
3.0
1500
1500
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1200
1000
800
(1) MoS2/RGO
600
(2) Co-doped-MoS2/RGO-1
400
(4) Co-doped-MoS2/RGO-3
200
160
(3) Co-doped-MoS2/RGO-2
(5) Co-doped-MoS2/RGO-4
0
20
40
60
80
Cycle number
MoS2/RGO(experiment date)
140
MoS2/RGO(fitted date)
120
Co-doped-MoS2/RGO-2 (fitted date)
(3)
(2)
(4)
(1)
(5)
1400
(c)
Co-doped-MoS2/RGO-2(experiment date)
-Zimg /?
80
60
40
20
0
20
40
60
80 100 120 140 160
Zreal /?
Fig. 8
100 mA g
-1
1000
800
600
400
200
100
100
0
1200
(b)
0m
A g -1
20
0
5 0 mA - 1
g
0
1 0 mA 00
g 1
mA
g -1
-1
charge
discharge
10
1400
Capacity /(mAh g )
1600
(a)
Capacity /(mAh g-1)
1600
MoS2/RGO
Co-doped-MoS2/RGO-1
Co-doped-MoS2/RGO-2
Co-doped-MoS2/RGO-3
0 10 20 30 40 50 60 70 80 90 100
Cycle number
ACCEPTED MANUSCRIPT
Figure Captions
Fig. 1. XRD patterns of the (a) MoS2/RGO, (b) Co-doped-MoS2/RGO-1, (c) Codoped-MoS2/RGO-2, (d) Co-doped-MoS2/RGO-3 and (e) Co-doped-MoS2/RGO-4
hybrids
prepared
by
hydrothermal
method;
(f)
Schematic
illustration
of
microstructures of MoS2/RGO.
Fig. 2. SEM images of (a) the MoS2/RGO, (b) Co-doped-MoS2/RGO-1 (c) Co-dopedMoS2/RGO-2, (d) Co-doped-MoS2/RGO-3 and (e) Co-doped-MoS2/RGO-4 hybrids
prepared by hydrothermal method.
Fig. 3. TEM/HRTEM images of the (a, b) MoS2/RGO, (c, d) Co-doped-MoS2/RGO-1,
(e, f) Co-doped-MoS2/RGO-3, (g, h) Co-doped-MoS2/RGO-3 and (i, j) Co-dopedMoS2/RGO-4 hybrids prepared by hydrothermal method.
Fig. 4. TGA curves of the MoS2/RGO and Co-doped-MoS2/RGO hybrids.
Fig. 5.
XPS high-resolution scans of (a) survey, (b) C 1s, (c) S 2p, (d) Mo 3d and (e)
Co 2p of the Co-doped-MoS2/RGO-2 hybrid.
Fig. 6. Cyclic voltammograms of (a) the MoS2/RGO, (b) Co-doped-MoS2/RGO-1, (c)
Co-doped-MoS2/RGO-2, (d) Co-doped-MoS2/RGO-3 and (e) Co-doped-MoS2/RGO-4
hybrid electrodes at a scan rate of 0.5 mV s-1.
Fig. 7?Galvanostatic discharge/charge voltage profiles of the (a) MoS2/RGO, (b)
Co-doped-MoS2/RGO-1, (c) Co-doped-MoS2/RGO-2, (d) Co-doped-MoS2/RGO-3
and (e) Co-doped-MoS2/RGO-4 electrodes for the first three cycles at a current
density of 100 mA g-1.
Fig. 8. (a) Cycling performances of MoS2/RGO and Co-doped-MoS2/RGO electrodes
at current density of 100 mA g-1; (b) Rate capabilities of MoS2/RGO and Co-dopedMoS2/RGO electrodes at different current densities; (c) Nyquist plots of the
MoS2/RGO and Co-doped-MoS2/RGO-2 electrodes obtained by applying a sine wave
with amplitude of 5.0 mV over the frequency range from 200 kHz to 0.01 Hz and (c)
equivalent circuit model of the studied system (CPE represents the constant phase
element).
ACCEPTED MANUSCRIPT
Highlights
1. Co-doped-MoS2/RGO hybrids are synthesized by one-pot hydrothermal method.
2. Co-doping changes the morphology and microstructure of the hybrids.
3. Co-doped-MoS2/RGO can deliver a reversible capacity as high as 1236 mAh g-1.
4. The hybrid exhibits significantly enhanced high-rate capability.
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