In situ synthesis of CoS2/RGO nanocomposites with enhanced electrode performance for lithium-ion batteries

Article abstract of DOI:10.1016/j.jallcom.2013.05.148

This study reports a novel strategy of preparing CoS₂/reduced graphene oxides (RGO) nanocomposites by employing graphene oxides (GO) as an oxidizing agent and Na₂S₂O₃ as a reducing agent. CoS₂ can be in situ synthesized with GO being reduced. X-ray diffraction (XRD), Raman spectrometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical test are used to characterize the nanocomposite. The CoS₂ particles with the size of 150 nm are dispersed in the networks made from thin RGO nanosheets. The CoS ₂/RGO nanocomposite as an anode material for lithium-ion batteries can deliver excellent reversible capacity retention (640 mA hg^-1) after cycling 50 times when tested at 100 mA g^-1 and rate performance. The enhanced electrochemical properties can be attributed to the nanoscale particles sizes of CoS₂ in addition to the effects of RGO networks in preventing the agglomeration of CoS₂ and absorbing lithium polysulfides during the charge-discharge processes.

Full text of DOI:10.1016/j.jallcom.2013.05.148

Journal of Alloys and Compounds 579 (2013) 372–376
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
In situ synthesis of CoS /RGO nanocomposites with enhanced electrode
2
performance for lithium-ion batteries
a
a
b,
⇑
Bin Qiu , Xiuyun Zhao , Dingguo Xia
a
College of Environmental & Energy Engineering, Beijing University of Technology, Beijing 100124, China
Key Laboratory of Theory and Technology of Advanced Battery Materials, College of Engineering, Peking University, Beijing 100871, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 March 2013
Received in revised form 27 April 2013
Accepted 22 May 2013
Available online 25 June 2013
This study reports a novel strategy of preparing CoS /reduced graphene oxides (RGO) nanocomposites by
employing graphene oxides (GO) as an oxidizing agent and Na S O as a reducing agent. CoS can be
2 2 3 2
in situ synthesized with GO being reduced. X-ray diffraction (XRD), Raman spectrometry, scanning elec-
tron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical test are used to
2
characterize the nanocomposite. The CoS
2
particles with the size of 150 nm are dispersed in the networks
made from thin RGO nanosheets. The CoS
2
/RGO nanocomposite as an anode material for lithium-ion bat-
Keywords:
Lithium ion batteries
Anode
À1
teries can deliver excellent reversible capacity retention (640 mA hg ) after cycling 50 times when
À1
tested at 100 mA g and rate performance. The enhanced electrochemical properties can be attributed
2
to the nanoscale particles sizes of CoS in addition to the effects of RGO networks in preventing the
Reduced graphene oxides
CoS
2
agglomeration of CoS
2
and absorbing lithium polysulfides during the charge-discharge processes.
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
large volume changes, mitigates the aggregation of particles and
increases the electronic conductivity of electrodes [19].
Lithium ion batteries have attracted considerable attentions
with the development of mobile electronics and electric vehicles
1]. Numerous studies are focused on electrode materials, separa-
tors, and electrolytes [2,3]. Particularly, the anode material is re-
garded as one of the key elements in the construction of high
capacity lithium ion batteries. Since Poizot et al. first reported
the mechanism of transition oxides and sulfides [4], metal sulfides
Graphene, as a single two dimensional layer of carbon atoms, is
a good candidate to host active NPs for Li ion batteries because of
its high stability, large surface areas, and abundant functional
groups [20]. Various hybrid nanostructures containing graphene
nanosheet have been extensively studied as electrode materials
in recent years [21–24]. In order to increase its electrical conduc-
tivity as electrode materials for lithium ion batteries, the GO need
reduction treatment in special reducing atmosphere [25,26], which
is usually harsh and tedious.
[
have attracted great attention due to its high theoretical capacity
À1
(
[
much higher than that of graphite ꢀ372 mA hg ) and low cost
5,6]. In particular, cobalt sulfides (CoS, CoS
2
, Co
3
S
4
, Co S
9 8
, etc.)
Herein, we propose a novel method to prepare CoS
by employing graphene oxides (GO) as an oxidizing agent and
Na as a reducing agent. The CoS particles of about 150 nm
were highly uniform dispersed on RGO nanosheets. When used
2
/RGO
are widely investigated and much progress has been made in
Li-ion storage capacities [7–12]. However, according to some
previous reports [13,14], the Li storage process in the conversion
reaction based metal sulfides involves generation of polysulfides
2
2
S O
3
2
as anode material for lithium ion batteries, CoS
2
/RGO composites
Li
2
S
x
(2 < x < 8). The dissolved lithium polysulfides shuttle between
exhibit good cycling stability and rate capability.
the anode and cathode during the charge process, involving side
reduction reactions with lithium anode and reoxidation reactions
at the cathode, which decrease the columbic efficiency and cause
a poor cyclability of LIBs. In order to solve these problems, two
main kinds of strategies are adopted. First, some reports are
focused on designing special structures (hollow spheres, polyhedral
spheres) [15,16]. The special structure of the electrode materials is
believed to facilitate the diffusion of lithium ions [17]. The second
is carbon coating [18], in which carbon acts as a layer to buffer
2. Experimental
2
.1. Synthesis of CoS
2
/RGO composites
In a typical synthesis, 2.5 mmol of CoCl
2
Á6H
2
O was dissolved in deionized water
(
60 mL) at room temperature for 2 h. Then the blue solution was transferred into a
100 mL Teflon-lined stainless steel autoclave followed by the addition of 2.5 mmol
of Na O and 20 mL GO and subsequently agitated it for 2 h. The sealed tank
Á5H
was maintained at 150 °C for 12 h. After reaction, the autoclave was cooled to room
temperature. The black product was harvested by filtration and washed with CS
ethanol, de-ionized water before drying at 80 °C overnight. For comparison, pristine
CoS was prepared following the similar procedures in the absence of GO [27,28].
2 2
S O
3
2
2
,
⇑
Corresponding author. Tel.: +86 10 62767962; fax: +86 10 62768316.
E-mail address: dgxia@pku.edu.cn (D. Xia).
2
0
925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.05.148

B. Qiu et al. / Journal of Alloys and Compounds 579 (2013) 372–376
373
2.2. Characterization
2 2
of GO, CoS , and CoS /RGO, respectively. FTIR spectra verifies the
presence of some oxygen–containing groups in GO, such as CAOH
The obtained products were characterized by X-ray diffraction (XRD, BRUKER
À1
À1
À1
(3390 cm ), CAOAC (1230 cm ), the CAO stretching peak
D8 ADVANCED, Cu K
a
= 1.54 Å) with a scan rate of 0.02° S from 10° to 80°. The
À1
À1
(
1055 cm ) and C@O in carboxylic acid moieties (1730 cm ).
operating voltage and current were kept at 40 kV and 40 mA, respectively. The size
and morphology of the as-synthesized products were examined by using a scanning
electron microscope (SEM-HITACHI S-4800) and transmission electron microscope
JEOL 2010). Raman spectroscopic analysis was performed with a LabRAM HR800
with laser excitation energy of 632.8 nm. Fourier transform infrared spectroscopy
FTIR) was collected by a Thermo Fisher spectrophotometer using the KBr pellet
À1
The peak at 1620 cm is assigned to the contribution from the
skeletal vibrations of the graphitic domains [29]. There is no obvi-
ous peak around 3390 cm and 1730 cm for CoS /RGO, which
2
(
À1
À1
means that GO was reduced, while the strong peak around
(
À1
method. Thermogravimetric analysis (TGA) was carried out on a TG/DTA7300 ther-
1060 cm is attributed to Co@S stretching in CoS
2
[30]
,
another
À1
À1
mal analyzer with a heating rate of 5 °C min in flowing air atmosphere.
weak peak around 1571 cm was assigned to aromatic C@C group
[
31]. In Fig. 2b, CoS
2
/RGO exhibits two typical peaks at 1350 and
À1
2
.3. Cell assembly and testing
1580 cm , which are the characterized peaks of the disorder (D)
and graphite (G) bands of graphene sheets [32]. It is generally ac-
cepted that the I /I ratio reflects the graphitization of carbona-
D G
ceous materials and also the defect density [33]. The intensity
ratio of the D to G band (I /I ) are 1.1 and 1.27 for GO and CoS /
The working electrode was composed of active material (60 wt%), Super P
(
30 wt%) and poly(vinylidene difluoride) (PVDF, 10 %wt). These materials were
mixed in N-methyl pyrrolidone to form homogenous slurry. Then the slurry was
spread onto the nickel foam and dried under vacuum at 120 °C for 12 h. After the
drying process, the foam was pressed under a pressure of 20 MPa. The coin cells
were finally assembled in an argon filled glovebox with the as-prepared materials
as test electrode, metallic lithium as the counter and referenced electrode, 1 M LiPF
in EC:DMC (1:1 in volume)as the electrolyte, and Whatman GF/D borosilicate glass-
fiber sheets as separator, respectively. Coin cells were cycled galvanostatically in
D
G
2
RGO. It is obvious that I
D
/I
G
2
for CoS /RGO increases slightly com-
pared with GO, which may be ascribed to the presence of Na
which can be used as reducing agents to reduce graphene oxides to
graphene [34]. Because Na is environmental friendly, easy to
obtain, and stable in ambient conditions, this strategy to prepare
CoS /RGO in this work is more suitable for large scale synthesis
27].
To investigate the size and morphology of the samples, SEM,
TEM, and HR-TEM were collected for CoS and CoS /RGO in
2 2 3
S O ,
6
2 2 3
S O
À1
the voltage range between 0.02 and 3.00 V at a current density of 100 mA g with
a multichannel battery test system (NEWARE).
2
[
3
. Results and discussion
2
2
Fig. 1a shows the power X-ray diffraction (XRD) patterns of the
(Fig. 3d). The SEM image taken from a typical section of CoS2/
as-synthesized CoS
of both samples correspond to CoS
standard cubic phase CoS (JCPDS Card No. 65-3322) with a space
group of Pa-3 (205). No obvious peaks relevant with Co and S can
be found in the patterns. CoS /RGO is further evaluated from TG
analysis, as shown in Fig. 1b. The main weight loss is between
00 K and 800 K, indicating the complete combustion of RGO. It
can be deduced that the content of RGO in CoS /RGO is about
0% according to TG analysis.
CoS /RGO composite is further characterized by Raman and
FTIR spectroscopy, as shown in Fig. 2. Fig. 2a shows FTIR spectra
2
and CoS
2
/RGO. The dominant diffraction peaks
RGO (Fig. 3a) indicates the size of uniformly dispersed CoS parti-
cles anchored on RGO nanosheets to be about 150 nm. In compar-
2
2
, which can be indexed to the
2
ison with CoS /RGO, CoS particles in the absence of RGO (Fig. 3b)
2
2
show heavily particles aggregation that means RGO nanosheets
play an essential role in achieving good dispersion of the CoS2
NPs. The origin of smaller particles and lower aggregation for
2
4
CoS /RGO can be analyzed in the following. There are many func-
2
2
tional groups such as C@O, OAC@O, and CAO containing negative
2
+
2
charges favor the absorbing of Co on GO [35]. The cobalt ions are
selectively and uniformly anchored onto GO’s surface. In other
words, the functional groups act as nucleation sites for CoS2 NP
2
Fig. 1. (a) XRD pattern of CoS
2
, and CoS
2
/RGO. (b) Thermogravimetric (TG) result of
2 2 2
Fig. 2. (a) FTIR spectra of GO, CoS , CoS /RGO. (b) Raman spectra of CoS /RGO and
CoS /RGO and RGO in air with a heating rate of 5 °C/min.
2
GO.

3
74
B. Qiu et al. / Journal of Alloys and Compounds 579 (2013) 372–376
Fig. 3. (a) SEM images of CoS
2
/RGO. (b) SEM images of CoS
2
. (c) TEM images of CoS
2 2
/ RGO. (d) High-resolution TEM (HRTEM) image of CoS /RGO.
À1
nucleation. The homogeneous nucleation and growth in the bulk
solution are suppressed so that small CoS particles can be ob-
the capacity keeps at the level of 690 mA hg after 20 cycles as
shown in Fig. 4a. Therefore, the enhanced reversible capacity of
2
tained. TEM images of noncomposites are shown in Fig. 3c. it is
clear that the particle size of CoS2 is consistent with SEM images.
2
CoS /RGO composite electrode can be ascribed to the synergic ef-
fects of RGO and CoS2. Furthermore, after 50 cycles under a current
À1
In the high-resolution TEM image of CoS
planes of CoS (200) with a distance spacing of 0.2 nm can be
clearly observed, which is in accordance with XRD pattern.
The electrochemical performance of CoS and CoS /RGO elec-
2
/RGO (Fig. 3d), crystalline
density of 100 mA g , the CoS
2
/RGO nanocomposites still retain a
À1
2
reversible capacity of ꢀ640 mA hg with an average coulombic
efficiency of >97%, whereas the CoS
2
only show a specific capacity
À1
2
2
lower than 210 mA hg (as shown in Fig. 4d).
trode are investigated by galvanostatic discharge-charge tech-
As we have known, transition metal sulfide electrodes suffer a
serious capacity fading because of the drastic volume change of
the electrode during the conversion process [39] and the dissolu-
tion of lithium polysulfides in the electrolyte [2]. Furthermore,
the smaller the particle size is, the less damage the electrodes
nique. Fig. 4a shows the dischargeÀcharge voltage profiles cycled
À1
at a current density of 100 mA g between 0.02 and 3.0 V vs.
+
Li /Li. The plateau at 1.5 V is ascribed to the formation of Li
x
CoS
2
[
36]. While the voltage is further decreased, the plateau at 1.0 V
is followed by a long slope corresponding to the conversion reac-
tion and the formation of SEI film on the surface of the electrode
undergoes during cycling. Therefore, the CoS
150 nm in CoS /RGO nanocomposites shows more stable cyclabil-
ity than pristine CoS with size of 2 m. What’s more, the elastic
2
with size of
2
[
8
15]. The initial charge and discharge capacity of CoS
2
/RGO are
12 mA hg and 1344 mA hg , respectively, based on the total
/RGO nanocomposite with an irreversible capacity
loss of 39.6%. Fig. 4b shows the first charge and discharge capacity
2
l
À1
À1
RGO nanosheets with high surface area inhibit the capacity decay
by buffering volume changes during the lithium insertion and
extraction processes and absorbing the lithium polysulfides [40–
43]. According to previous work [41–43], the reduced graphene
oxide (RGO) is used to envelope micron sized sulfur particles, to
form a highly conductive network around the sulfur particles and
trap the polysulfides through favorable hydrophilic–hydrophilic
interactions.
mass of the CoS
2
À1
À1
of CoS
irreversible capacity loss is about 31%. The large initial discharge
capacity of the CoS /RGO could be attributed to the formation of
SEI during the discharge process and the irreversible decomposi-
tion of Li S [37]. In addition, the graphene networks with high sur-
2
were 823 mA hg and 1197 mA hg , respectively, with an
2
2
face area and many oxygen-containing groups may also contribute
to this issue [38]. Though a large irreversible capacity loss was ob-
Another considerable improvement is the rate capability of the
CoS
in Fig. 5a and b, the CoS
er capacity than the pristine CoS
densities. The reversible capacity of CoS
2
/RGO nanocomposites irrelative to the pristine CoS
/RGO nanocomposites show a much high-
under all investigated current
2
. As shown
served in the first cycle, CoS
specific capacity and better cycling stability in comparison with
the pristine CoS . Indeed, Li intercalation into the RGO was also ob-
served in the Fig. 4. We think that the calculation of capacity based
on total active mass (CoS and RGO) is of more practical signifi-
cance. In order to discern the contribution of RGO and CoS , the
as-prepared pristine CoS and RGO were evaluated as shown in
2
/RGO nanocomposites exhibit a higher
2
2
2
2
/RGO reached
À1
À1
À1
À1
720 mA hg
,
580 mA hg
,
330 mA hg
,
200 mA hg
when
À1
À1
À1
À1
2
tested at 100 mA g
,
200 mA g
,
500 mA g
,
1000 mA g
,
2
respectively. Importantly, after the high-rate measurements, the
À1
2
specific capacities of the CoS
2
/RGO cycled under 100 mA g were
À1
Fig. 4b and c, respectively. The pristine CoS
a reversible capacity of only 250 mA hg and as-prepared RGO
electrode showed a reversible capacity of 150 mA hg . In contrast,
2
electrode remained
able to recover to the 550 mA hg , implying their good reversibil-
À1
ity [44]. In comparison, the pristine CoS
2
only delievered
À1
À1
À1
100 mA hg
when tested at 500 mA g . The good capacity

B. Qiu et al. / Journal of Alloys and Compounds 579 (2013) 372–376
375
Fig. 5. (a) Cycling performance of CoS
2
, CoS
2
/RGO at various current densities. (b)
2
Cycling performance of CoS at various current densities.
CoS
2
/RGO shows a higher specific capacity and batter rate perfor-
. The enhanced electrochemical behaviors
mance than pristine CoS
2
can be attributed to the small particle size and elastic RGO with
high surface area buffering the volume change and absorbing lith-
ium polysulfides in the cycle process. The present work provides a
new method to fabricate CoS
extended to synthesis other metal sulfides (such as NiS, FeS
MoS )/RGO composites to improve the cyclability and rate
2
/RGO nanocomposites, which can be
2
, and
2
performance.
Acknowledgements
The authors acknowledge the financial supports from the major
program of Beijing Municipal Natural Science Foundation (No.
2
1
0110001), National Natural Science Foundation of China (No.
1179001), National High Technology Research and Development
Program (863, No. 2012AA052201).
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