A strongly coupled CoS2/ reduced graphene oxide nanostructure as an anode material for efficient sodium-ion batteries

Article abstract of DOI:10.1016/j.jallcom.2017.07.339

Sodium-ion batteries (SIBs) are highly attractive electrochemical devices for massive energy storage because of their low cost and abundance of sodium, but insufficient anode performance remains a key challenge for the commercialization of this attractive technology. In this study, a hierarchically porous CoS₂/graphene composite with an architecture of CoS₂ nanoparticles embedded in reduced graphene oxide (rGO) is synthesized through a one-step hydrothermal route allowing the growth of the CoS₂ phase and the reduction of the graphene oxide simultaneously. This composite is applied as an anode material for SIBs, delivering favorable performance. The CoS₂ phase consists of nanoparticles of ～10 nm that are uniformly anchored on the rGO, forming a CoS₂/rGO hybrid with strong phase interaction. As a conversion-type anode for SIBs, the electrochemical testing results show significantly enhanced sodium-storage properties for the CoS₂/rGO composite compared with that of bare CoS₂. Impressively, the CoS₂/rGO nanostructure exhibits a high discharge capacity of approximately 400 mAh g^?1 after 100 cycles at specific current of 100 mA g^?1, corresponding to approximately 80% of the discharge capacity in the second cycle. Such improvement may be due to the two-dimensional conductive network, homogeneous dispersion and immobilization of the CoS₂ nanoparticles, as well as the enhanced wettability of the active material in the electrolyte by introducing rGO. The results suggest that this well-designed conversion-type CoS₂ is a promising anode material for high-performance SIBs.

Full text of DOI:10.1016/j.jallcom.2017.07.339

Journal of Alloys and Compounds 726 (2017) 394e402
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
A strongly coupled CoS2/ reduced graphene oxide nanostructure as an
anode material for efficient sodium-ion batteries
,
, b, c, *
Kongyan Xie a, Li Li b **, Xiang Deng a, Wei Zhou a, Zongping Shao a
a Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engineering, College
of Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, China
b School of Energy Science and Engineering, Nanjing Tech University, Nanjing 210009, China
c Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Sodium-ion batteries (SIBs) are highly attractive electrochemical devices for massive energy storage
because of their low cost and abundance of sodium, but insufficient anode performance remains a key
challenge for the commercialization of this attractive technology. In this study, a hierarchically porous
CoS2/graphene composite with an architecture of CoS2 nanoparticles embedded in reduced graphene
oxide (rGO) is synthesized through a one-step hydrothermal route allowing the growth of the CoS2 phase
and the reduction of the graphene oxide simultaneously. This composite is applied as an anode material
for SIBs, delivering favorable performance. The CoS2 phase consists of nanoparticles of ~10 nm that are
uniformly anchored on the rGO, forming a CoS2/rGO hybrid with strong phase interaction. As a
conversion-type anode for SIBs, the electrochemical testing results show significantly enhanced sodium-
storage properties for the CoS2/rGO composite compared with that of bare CoS2. Impressively, the CoS2/
rGO nanostructure exhibits a high discharge capacity of approximately 400 mAh gꢀ1 after 100 cycles at
specific current of 100 mA gꢀ1, corresponding to approximately 80% of the discharge capacity in the
second cycle. Such improvement may be due to the two-dimensional conductive network, homogeneous
dispersion and immobilization of the CoS2 nanoparticles, as well as the enhanced wettability of the
active material in the electrolyte by introducing rGO. The results suggest that this well-designed con-
version-type CoS2 is a promising anode material for high-performance SIBs.
Received 7 April 2017
Received in revised form
26 July 2017
Accepted 31 July 2017
Available online 3 August 2017
Keywords:
Cobalt sulfide
Graphene
Nanoparticles
Sodium-ion batteries
© 2017 Published by Elsevier B.V.
1. Introduction
for SIBs [1,5e8]. Although some dramatic progress has been made,
further research on anode materials is still urgently needed to
Currently, sodium-ion batteries (SIBs) are known as the most
potential substitute for lithium-ion batteries (LIBs) because of their
natural abundance, low cost, and similar operating principles [1e4].
Unfortunately, the actual applications of SIBs have been blocked, at
least in part, by the lack of proper anode materials to host Naþ,
which has a larger radius than Liþ. Therefore, the foremost issue
that needs to be solved for SIBs is the development of suitable
electrode materials with favorable high specific capacity and
cycling performance. To date, various alternative materials,
including metal chalcogenides, alloy-based materials, layered
metal oxides and carbon, have been exploited as promising anodes
improve the capacity, cycling stability and rate capability of SIBs,
thus making them practically applicable.
Recently, multifarious cobalt sulfides with a certain range of
stoichiometry, such as Co1-xS, Co9S8, CoS, Co3S4 and CoS2, have
drawn a lot of attention because of their remarkable physical,
chemical, magnetic properties [9e12]. In particular, their potential
application in electrochemical energy storage fields, e.g., as elec-
trode materials for supercapacitors, LIBs and SIBs, has received
tremendous attention in recent years [13e16]. As anodes for LIBs
and SIBs, the energy storage in these materials involve
a
conversion-type reaction mechanism with high theoretical capac-
ity. However, the practical application of bulk-phase CoS2 elec-
trodes suffers from a couple of problems. For example, the
dissolution of the polysulfide intermediates in organic solvent will
bring about quick capacity decaying during cycling and the pul-
verization issue coming from the volume expansion during the
* Corresponding author.
** Corresponding author.
E-mail addresses: LILI6023@njtech.edu.cn (L. Li), shaozp@njtech.edu.cn
(Z. Shao).
http://dx.doi.org/10.1016/j.jallcom.2017.07.339
0925-8388/© 2017 Published by Elsevier B.V.

K. Xie et al. / Journal of Alloys and Compounds 726 (2017) 394e402
395
charge-discharge process [17,18]. The formation of a composite
with an elastic material such as carbon may provide buffer space to
adapt the volume change of such conversion-type electrode ma-
terials, thus improving the cycling stability and increasing the
electrode capacity by suppressing the pulverization of the active
materials. For example, superior Li-storage properties of Co9S8 have
been realized through surface coating the active material with
carbon [19]. To date, however, the application of metal sulfides in
SIBs has been less reported, likely due to the lower sodium diffu-
sivity in these materials, which required strict control over the
electrode microstructure and morphology to achieve favorable
electrode performance.
Graphene, as a popular star in materials science with its clearly
two-dimensional honeycomb-like network of carbon atoms, has
aroused wide attention in many fields, including electrochemical
energy storage and conversion, because of its large theoretical
specific surface area, outstanding thermal stability, superior elec-
tronic conductivity, and remarkable structural flexibility [20]. For
example, graphene has been widely used to form composite elec-
trodes for SIBs and LIBs, in which a synergistic effect between
graphene and the main active material may occur, significantly
improving the electrode performance. For example, a brief freeze-
drying and then hydrazine treatment process was used to get
Co3S4 porous nanosheets embedded in graphene sheets, which
delivered an attractive reversible capacity of 450 mAh gꢀ1 at
500 mA gꢀ1 as the anode for SIBs [13]. However, after 50 cycles, a
charge capacity of only 329 mAh gꢀ1 was retained, suggesting the
importance of further cycling stability improvement. The nano-
composite of Co3S4 nanosheets and graphene sheets as reported by
Bao was formed through weak van der Waals interaction. An in-
crease in the interaction between cobalt sulfide and graphene may
further improve the electrode performance by increasing the
charge transfer efficiency and reducing the isolation of cobalt sul-
fide. On the other hand, as the charge carrier could shorten the
diffusion length, 2D nanostructures are very hopeful to supply
more active sites for fast electrochemical reactions [21e23].
Through optimizing the nanostructure and the interaction between
graphene and cobalt sulfide, a further improvement in the perfor-
mance (capacity, cycling stability and rate capacity) of a CoS2
nanoparticles and reduced graphene oxide composite as an anode
for SIBs may be realized.
Ltd), KMnO4 (Shanghai Lingfeng chemical reagent co., Ltd), H2O2
(Shanghai Lingfeng chemical reagent co., Ltd). H2SO4: H3PO4 (180:
20 ml) solution were added to a mixture of graphite flakes (1.5 g)
and KMnO4 (9.0 g) slowly. The reaction was then heated to 50 ꢁC
and stirred for 12 h. The mixture was cooled to room temperature
and poured onto ice with 30% H2O2 and centrifuged several times.
Then, 50 mg of GO powders was diffused in a 53.3 ml of a mixed
solution of ethylene glycol and DI water (1:3 by volume) by ultra-
sonication for 1 h, followed by the addition of 120 mg of L-cysteine
and 0.4 mmol of Co(NO3)2$6H2O. The mixture solution was stirred
for 1 h and shifted to a 100 ml of Teflon-lined autoclave, tighten and
heated in an oven at 160 ꢁC for 8 h. The final product was treated
separately with ethanol and DI water several times, and then dried
in a freeze dryer. Finally, the as-obtained precipitate was treated at
400 ꢁC for 4 h in argon atmosphere to result in the CoS2/rGO
nanostructure. As a control, pristine CoS2 nanoparticles were also
prepared via the similar experimental process but without the
presence of GO in the mixture solution. Pristine graphene sheets
were also prepared by adding GO into the mixture solution, but
without cobalt nitride and the sulfur source.
2.2. Material characterization
The crystallographic information of the samples were evaluated
through RT powder X-ray diffraction (XRD) measurement on a
Bruker D8 Advance diffractometer with Cu K
a source over a 2q
range from 10ꢁ to 80ꢁ. The microstructure images were observed
with field-emission scanning electron microscope (FE-SEM, Hita-
chi, Japan). Transmission electron microscope (TEM) was obtained
using a field emission TEM equipped at 300 kV (Tecnai G2 F30 S-
TWIN, USA). X-ray photoelectron spectroscopy (XPS) analysis was
carried out using a PHI550 system and the as-obtained spectra
were fitted through XPSPEAK41 software. Raman spectra were
collect by an HR800 UV micro-Raman spectrometer. Thermogra-
vimetric (TG) analysis was performed employing a WCT-1 TG
Analyzer under air flow to determine the carbon content of the
product. AutoSorb-iQ3 was employed to character BET made by
Quantachrome. The pore size distribution and specific surface area
of the samples were calculated using the Brunauer-Emmett-Teller
(BET) equation. Besides, we also use DHG-9070A type oven made
by Shanghai Yiheng Science Instrument Co., Ltd., autoclave made by
Zhenghong, SCIENTZ-10Z type freeze dryer was made by Ningbo
Scientz Biotechnology Co., Ltd. DZF-6050 type vacuum oven was
made by Shanghai Boxun Industry Co., Ltd., TG1650-WS type
centrifuge was made by Shanghai Lu Xiangyi Centrifuge Instrument
Co., Ltd.
Herein, we report the synthesis and investigation of a composite
being composed of strongly coupled CoS2 nanoparticles and
reduced graphene oxide (CoS2/rGO) as a potential anode material
for SIBs. The CoS2/rGO nanostructure was synthesized directly from
graphene oxide(GO) and cobalt nitride through hydrothermal
treatment with L-cysteine as the sulfur source. As an anode for SIBs,
good cycling stability (approximately 400 mAh gꢀ1 at 100 mA gꢀ1
after 100 cycles) was achieved. In addition, an outstanding rate
capability with capacity of 247 mAh gꢀ1 at 5000 mA gꢀ1 was
demonstrated. Besides, the electrolyte is critical to the electro-
chemical properties of the SIBs, and different electrolyte will bring
back different electrochemical performance [24e26]. Explanations
of this outstanding performance are discussed.
2.3. Electrochemical measurements
To prepare the working electrode, the active material, conduc-
tive Super P (Shanghai Hersbit chemical) and sodium carbox-
ymethyl cellulose (Na-CMC, Chemical reagents of national
medicine) were mixed in proper weight ratios (80:10:10) by planet
pulp mixer (ARM-30), which were pasted onto copper foil (Hefei Ke
Jing material technology Co., Ltd) current collectors by doctor blade,
and dried at 100 ꢁC for 12 h in vacuum. Each electrode plate con-
tained 1e1.2 mg of active material. The CR2025 coin-type cells were
assembled in a pure-argon filled glove box with 1.0 M NaClO4 in PC:
FEC (98:2 in volume, Cathay Huarong chemical new materials Co.
Ltd) as the electrolyte, metallic sodium (Aladdin chemical reagent)
as the reference and counter electrode and microporous poly-
ethylene film (Celgard 2400) as the separator. RT-galvanostatically
discharge/charge cycling performance was conducted over the
potential range of 3.00e0.01 V on the NEWARE BTS multichannel
battery testing system (5 V, 10 mA). Cyclic voltammetry profiles
2. Experimental section
2.1. Materials synthesis
The hydrothermal synthesis of the CoS2/rGO nanostructure was
conducted as follows. First, graphene oxide (GO) powders were
prepared by an improved version of the graphene preparation
method [27]. To synthesis GO, we employed graphite flakes (sigma-
Aldrich, cat # 33246, ~150
mm falkes), H3PO4 (Sinopharm chemical
reagent co., Ltd), H2SO4 (Shanghai Lingfeng chemical reagent co.,

396
K. Xie et al. / Journal of Alloys and Compounds 726 (2017) 394e402
were acquired from a PARSTAT 273A electrochemical workstation
and the scanning rate was set as 0.1 mV sꢀ1. Electrochemical
impedance spectroscopy (EIS) was tested using a Princeton 2273
electrochemical system at a frequency range of 100 kHz to
100 mHz at open circuit voltage (OCV) over the single cell. The
voltage amplitude is 10 mV, and the applied voltage is 3 V.
attributed to the CoeNH coordination bond in the L-cysteine-Co
[32], and the peak at 786.6 eV can be ascribed to the Co2þ ion of
CoeO bond [33]. The last three peaks corresponded to the Co 2p1/2
orbital. In addition, the peak located at 799.7 eV can be due to the
CoeO bond [13]. Meanwhile, the existence of CoS2/rGO can be
further verified by the S 2p peak at 162.6 (S 2p3/2) and 164.1 eV (S
2p1/2) [34] (Fig. 2d). The presence of oxygen-containing groups
suggests the GO was not fully reduced, which is, however, helpful to
fix CoS2 on the rGO surface.
3. Results and discussion
Fig. 3 presents the N2 adsorption-desorption isotherm and the
pore size distribution diagram of the as-prepared CoS2/rGO com-
posite. The specific surface area of the CoS2/rGO composite was
calculated to be 83.7 m2 gꢀ1, and the main pores were in the range
of 1e10 nm, as shown from the inset of Fig. 3. In comparison, the
specific surface areas of the bare CoS2 and the pristine rGO were
Fig. 1a shows the XRD patterns of the as-prepared CoS2/rGO
composite, bare CoS2, and rGO. Diffraction reflections at 2
q of
approximately 32.4, 36.1, 39.5, 47.2 and 55.2ꢁ were observed from
the XRD patterns of the as-synthesized CoS2/rGO, which can be
indexed based on the cattierite CoS2 phase (JCPDS card 41e1471) at
the (200), (210), (211), (220) and (311) diffraction planes, con-
firming the formation of the CoS2 crystalline phase. Besides,
calculated to be 24.5 and 490.7 m2 ꢀ1, and the main pores of the
g
bare CoS2 and the pristine rGO, respectively, were approximately
3.83 and 3.42 nm, as marked in the inset of Fig. S2. This results
indicated that the surface area of the CoS2/rGO nanocomposite was
relatively enhanced with the addition of GO compared to the bare
CoS2, which would benefit for the diffusion of electrolyte to active
sites and improve the electrochemical performance of CoS2/rGO as
anode in SIBs.
The morphology and microstructure of the CoS2/rGO composite
and the bare CoS2 were examined by SEM and TEM. For the bare
CoS2, as shown in Fig. 4a, they gave rise to large aggregates with
diffraction reflections at 2q
of approximately 26, 43ꢁ were observed
from the XRD patterns of the as-synthesized CoS2/rGO, which can
be due to the presence of rGO phase at the (002) and (100). The
broadened diffraction reflections of both the bare CoS2 and the
CoS2/rGO composite reveal the nanocrystallinity of the CoS2 phase.
Fig. 1b shows the Raman spectra of GO, rGO and CoS2/rGO. All
samples display two bands at approximately 1355 and 1604 cmꢀ1
,
corresponding to the disordered/defected (D) and graphitic (G)
bands of the graphene sheet [17]. The intensity ratios of the D band
to the G band (ID/IG) are 1.004, 0.89, and 1.28 for GO, rGO, and CoS2/
rGO, respectively, indicating a further decreased sp2 carbon domain
[28] when CoS2 nanoparticles formed the composite with rGO, in
good agreement with the disappearance of the characteristic XRD
peaks of graphitized graphene in the CoS2/rGO composite. The
CoS2/rGO composite, as determined by TGA (Fig. S1), was found to
be composed of approximately 50 wt% rGO and 50 wt% CoS2.
In addition, elements in the CoS2/rGO composite are detected by
the XPS survey spectrum from 0 to 1100 eV (Fig. 2a). Specifically,
the C 1s XPS spectra of CoS2/rGO (Fig. 2b) can be fitted into four
peaks which located at 284.7, 286.3, 286.8 and 288.9 eV, corre-
sponding to the carbon atoms in CeC, CeO, C]O, OeC]O groups,
respectively [13,29]. Two peaks related to the Co 2p3/2 and Co 2p1/2
orbitals in the Co 2p XPS spectrum can be deconvolved into six
peaks located at 779.2, 782.9, 786.6 eV, 798.1, 799.7 and 803.7 eV,
respectively (Fig. 2c). The first three peaks can be assigned to the Co
2p3/2 spin-orbit peaks. The peak at 779.2 eV can be ascribed to the
Co2þ ion of CoS2 [30,31], the peak located at 782.9 eV can be
size ranging from 500 nm to 2
mm after calcination. Such large
particle size may inhibit the penetration of the electrolyte,
increasing the diffusion distance for Naþ due to smaller surface area
(24.5 m2
g
ꢀ1) and pore size (~3.83 nm) compared to that of CoS2/
rGO (83.7 m2
g
ꢀ1, ~4.3 nm) as indicated by BET results in Fig. S2a
and Fig. 3. In contrast, as shown in Fig. 4b, the size of CoS2
dramatically decreased to approximately 10 nm when it formed the
composite with rGO, and these CoS2 nanoparticles were homoge-
neously distributed on both sides of the rGO. The intimate inter-
action between the negatively charged graphene oxide and the
positively charged cobalt ions during the early hydrothermal re-
action stage likely played an important part both in inhibiting the
growth of the CoS2 nanoparticles and limiting the aggregation of
the crystalline CoS2 particles to some extent [35], and in allowing
the uniform distribution over the reduced graphite surface. All of
the CoS2 nanoparticles still adhered to the reduced graphene oxide
firmly even after vigorous ultrasonic treatment, further confirming
Fig. 1. (a) XRD patterns of the CoS2/rGO nanocomposite, rGO and bare CoS2, (b) Raman spectra of CoS2/rGO, rGO and GO from 300 to 1800 cmꢀ1
.

K. Xie et al. / Journal of Alloys and Compounds 726 (2017) 394e402
397
Fig. 2. (a) XPS survey spectrum of CoS2/rGO; (b) XPS spectra of C 1s; (c) Co 2p and (d) S 2p.
the pore size of the composite is the largest than that of bare CoS2
and rGO in Fig. S2, and these rich pores in the sample may provide
more effective ionic and electron transport during sodiation/de-
sodiation and buffer space for mediating the volume change dur-
ing the electrochemical conversion reaction. An improved capacity
and cycling stability are then expected.
The TEM images give further evidence that the CoS2 nano-
particles were distributed on the surface of the rGO in the form of
single particles or small particle clusters, and the nature of CoS2
nanoparticles was confirmed by their selected area electron
diffraction (SAED) pattern (inset of Fig. 4c) since the respective
diffraction rings can be well indexed to the (200), (210), (220) and
(311) diffraction planes of CoS2. The HR-TEM image in Fig. 4d shows
crystal lattice fringes with a d-spacing of 0.31 nm of the nano-
particles, matching well with the (210) plane of CoS2, which further
confirms the CoS2 nature of the nanoparticles. The reduced gra-
phene layer in the CoS2/rGO is rather thin, possibly consisting of
single or few-layer sheets, which can be observed from the wrin-
kles and its transparent nature.
As we known, the electrochemical performance of a SIB is
strongly affected by its electrolyte. Four types of electrolyte, i.e., (1)
1.0 M NaClO4 in propylene carbonate and fluoroethylene carbonate
(PC:FEC, 98:2 v/v), (2) 1.0 M NaClO4 in ethylene carbonate and
diethylcarbonate (EC:DEC, 1:1 v/v), (3) 1.0 M NaClO4 in ethylene
carbonate and propylene carbonate (EC:PC, 1:1 v/v), and (4) 1.0 M
NaCF3SO3 in diethyleneglycol dimethylether (DEGDME), were
tested in sodium half cells. The four different organic solvents was
Fig. 3. N2 adsorption-desorption isotherms of the CoS2/rGO nanocomposite.
the presence of strong interactions between the CoS2 nanoparticles
and the rGO in the composite. The natural curvature of the reduced
graphene and the separation of the sheets with the presence of
CoS2 nanoparticle could enrich mesopores inside the CoS2/rGO
composite, which is also consistent with the results in Fig. 3 that

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K. Xie et al. / Journal of Alloys and Compounds 726 (2017) 394e402
Fig. 4. (a) A low-magnification SEM image of bare CoS2; (b) an SEM image of the CoS2/rGO composite; (c) a TEM image of the CoS2/rGO composite. The inset shows the SAED pattern
with indexed diffraction rings, and (d) shows an HR-TEM image of CoS2 nanoparticles in the CoS2/rGO composite.
used to detect the charge-discharge reaction of the CoS2/rGO
composite at an specific current of 100 mA gꢀ1 in the potential
window of 0.01e3.0 V vs Naþ/Na. As shown in Fig. S3, the CoS2/rGO
electrode in 1.0 M NaClO4 in PC: FEC exhibited the best property
with the discharge capacity kept at 400 mAh gꢀ1 after 100 cycles. In
contrast, although the electrode in 1.0 M NaClO4 in EC:PC demon-
strated a high initial discharge capacity of 419 mAh gꢀ1, the
discharge capacity decayed quickly to only approximately 230 mAh
reduction peaks at 0.9 and 0.7 V in the first cycle. According to the
literature, the first reduction peak is associated with the electro-
chemical reactions of CoS2 þ xNaþ þ xeꢀ / NaxCoS2 and
NaxCoS2 þ (4ex)Naþ þ (4ex)eꢀ / Co þ 2Na2S, and we supposed
that the reduction peak at 0.9 V involved with the formation of a
solid-electrolyte interphase (SEI) film on the surface of the elec-
trode materials [41]. Accordingly, there was only one oxidation
peak at 1.4 V, which can be associated with the reactions of Co þ
2Na2S / CoS2 þ 4Naþ þ 4eꢀ. In the second cycle, only one peak at
approximately 0.7 V was observed. This result suggests the reduc-
tion peak at 0.9 V was shifted to 0.7 V, while the initial 0.7 V
reduction peak disappeared because SEI is usually formed during
the first cycle. Such a shift may result from the improved kinetics of
the electrode [42].
Fig. 5c compares the cycling stability of the CoS2/rGO nano-
structure, the bare CoS2 and the pristine rGO at a specific current of
100 mA gꢀ1. The CoS2/rGO electrode exhibited improved cycling
stability compared with the bare CoS2. After 40 cycles, a capacity of
over 400 mAh gꢀ1 was retained for the CoS2/rGO electrode. In
comparison, the capacity of the bare CoS2 electrode decayed
promptly to below 20 mAh gꢀ1 after 40 cycles. Impressively, the
CoS2/rGO nanostructure exhibited a high discharge capacity of
approximately 400 mAh gꢀ1 after 100 cycles, suggesting a capacity
retention of approximately 80% with respect to the discharge ca-
pacity in the second cycle. The excellent cycling ability of the CoS2/
rGO nanostructures was demonstrated by the average 97.3%
coulombic efficiency as an anode for SIBs. To illustrate the superi-
ority of the as-fabricated CoS2/rGO nanostructures for sodium
storage, the rate performance was further investigated to deter-
mine their practical use in SIBs. Fig. 5d show that the CoS2/rGO
electrode delivered stable discharge capacities of 408, 352, 325,
297, and 275 mAh gꢀ1 at specific current of 100, 200, 500, 1000, and
2000 mA gꢀ1. In addition, even though a 50-fold increase in specific
current was tested, a discharge capacity of approximately 247 mAh
g
ꢀ1 after 20 cycles and less than 100 mAh gꢀ1 after 100 cycles. The
CoS2/rGO electrode in 1.0 M NaClO4 in EC: DEC and 1.0 M NaCF3SO3
in DEGDME also presented low capacities and a quick capacity
decay with cycling. In both of electrolytes, a capacity of approxi-
mately 100 mAh gꢀ1 was maintained after 100 cycles. With such
clear differences, the electrolyte clearly has an outstanding effect
on the electrochemical property of the as-prepared CoS2/rGO
electrode. The excellent cycling stability of the CoS2/rGO electrode
in the electrolyte of 1.0 M NaClO4 in PC: FEC suggests that a pro-
pylene carbonate-based electrolyte could likely restrain the reac-
tion between the electrolyte and active material, and this reaction
may cause the fast loss of active materials [36e40].
The electrochemical properties of the CoS2/rGO electrode were
first studied by galvanostatic charge-discharge tests. Fig. 5a show
the 1st, 2nd, 50th and 99th charge-discharge curves of the CoS2/rGO
electrode between 0.01 and 3.0
V at a specific current of
100 mA gꢀ1. Here, the specific capacities of the CoS2/rGO electrode
are calculated based on the whole mass of CoS2 and graphene. As
the capacity of CoS2/rGO electrode, 483 and 744 mAh gꢀ1 was
reached in the first charge and discharge, respectively, corre-
sponding to a coulombic efficiency of 67%. However, the discharge
capacity decreased to 464 mAh gꢀ1 in the second cycle. Fortunately,
capacities of 376 mAh gꢀ1 can be maintained in the 99th cycle.
Cyclic voltammetry (CV) of the CoS2/rGO electrode at a scan rate of
0.1 mV sꢀ1 in the potential range from 0.01 to 3 V was further
measured in Fig. 5b. For the bare CoS2 electrode, there were two

K. Xie et al. / Journal of Alloys and Compounds 726 (2017) 394e402
399
Fig. 5. (a) Charge-discharge curves of CoS2/rGO composite in the voltage range of 0.01e3.0 V at a specific current of 100 mA gꢀ1, (b) CV plots of CoS2/rGO scanned at 0.1 mV sꢀ1 in a
potential window of 0.01e3.0 V. (c) rate performance of the CoS2/rGO electrode at current densities of 100, 200, 500, 1000, 2000, and 5000 mA gꢀ1 in a potential window of
0.01e3.0 V. (d) Cycling performance (left y-axis) and coulombic efficiency (right y-axis) of the bare CoS2, rGO and CoS2/rGO composite at a current density of 100 mA gꢀ1
.
gꢀ1 was still maintained at 5000 mA gꢀ1
outstanding rate capability of the CoS2/rGO electrode. According to
the detailed discharge/charge profiles depicted in Fig. 6a, only a
,
suggesting the
slight increase in the charge plateau appeared when the specific
current was increased from 100 to 5000 mA gꢀ1, illustrating the low
polarization and rapid reaction kinetics of the CoS2/rGO electrode.
Fig. 6. (a) Charge-discharge curves of the CoS2/rGO electrode at various current densities of 0.1 A gꢀ1, 0.2 A gꢀ1, 0.5 A gꢀ1, 1 A gꢀ1, 2 A gꢀ1, 5 A gꢀ1 in a potential window of
0.01e3.0 V. (b) The cycling stability of the CoS2/rGO electrode at three different current densities of 0.1 A gꢀ1, 0.2 A gꢀ1, 0.5 A gꢀ1 in a potential window of 0.01e3.0 V.

400
K. Xie et al. / Journal of Alloys and Compounds 726 (2017) 394e402
This means that the CoS2/rGO electrode could conquer high-rate
cycling without damage the integrity of its structural. Such
outstanding rate performance may be explained by the following
three factors: (i) the highly conductive rGO supplies efficient con-
ducting channels for the CoS2 nanoparticles; (ii) nanoscaled and
well-distributed CoS2 nanoparticles on the surface of graphene are
favorable for rapid Na-ion diffusion; and (iii) the loosely stacked
CoS2/rGO flakes are conductive for better infiltration of the elec-
trolyte and quick Na-ion transport across the electrode/electrolyte
interface. Furthermore, the CoS2/rGO could still achieve a high
specific capacity of 206 mAh gꢀ1 even when the specific current
increased to 500 mA gꢀ1 (Fig. 6b). As a comparison, the capacity of
the bare CoS2 electrode dropped quickly to approximately 20 mAh
gꢀ1 after 100 cycles. Similar poor cycling stability for a bare CoS2
electrode has also been reported by other groups [43e48]. Such
enhanced cycling stability can be due to the incorporation of
reduced graphene oxide in the composite, which not only
restrained the aggregation of the CoS2 nanoparticles but also
buffered the large volume changes during the conversion reactions
by immobilizing them on the rGO. In addition, the highly conduc-
tive rGO would enhance the electrical conductivity of the whole
electrode, and the loosely stacked CoS2/rGO nanocomposite is
conductive for better infiltration of the electrolyte and quick Na-ion
transport across the electrode/electrolyte interface.
Fig. 7a shows the typical EIS in a Nyquist plot, the two partly
semicircles in the high-to-medium frequency zone and a linear tail
in the low frequency zone. Apparently, owing to the highly
conductive rGO, Rct of the CoS2/rGO electrode show a lower value
contrasted with the bare CoS2 electrode [49e51]. Moreover, the
relatively sharper low-frequency region of the CoS2/rGO electrode
suggests the higher sodium-ion diffusivity inside the CoS2/rGO
electrode, ensuring rapid Na-intercalation kinetics [14]. To calculate
the diffusion coefficient of Na-ion through the electrode materials,
we employed the following formula:
low frequency region of the pristine CoS2 and CoS2/rGO composite.
The DNaþ values of the pristine CoS2 and CoS2/rGO composite are
calculated to be 4.10 ꢂ 10ꢀ23 and 2.65 ꢂ 10ꢀ21 cm2 sꢀ1, respectively.
Every character of the electrode materials are conductive, and
furthermore improve the sodium storage properties of the CoS2/
rGO electrode. According to the fitting results, the Rct of the bare
CoS2 electrode is 298.2 U, while it is only 109.8 U for the CoS2/rGO
electrode. We listed the results in Table S1, in which the Rs, Rf and
Rct represented for the ohmic resistance of the electrolyte, the
resistance of Na ion transport through the SEI on the electrode
surface and the resistance of charge transfer reactions inside or on
the surface of active materials, respectively. Meanwhile, the con-
stant phase elements (CPEs) represented the interfacial capacitance
between electrolyte/SEI film/electrode materials and some surface
confined faradic capacitance as well. W is referred to the Warburg
impedance. The restricted particle aggregation is responsible for
the low Rct value for the CoS2/rGO electrode. A lower Rct of the CoS2/
rGO electrode can also explain its better rate capacity compared
with the bare CoS2 electrode. Clearly, the introduction of reduced
graphite sheets to form the composite electrode played a decisive
role in improving the overall capacity and cycling stability of CoS2
even at high specific current.
Finally, in Table S2 [13,53e59], we compare the electrochemical
performance of the CoS2/rGO nanostructure as an anode for SIBs
with other recently reported cobalt sulfide-based electrodes. As can
be seen, electrochemical properties of our CoS2/rGO electrode
outperformed the most of previous reports on cobalt sulfur-based
electrodes in terms of reversible capacity, cycling performance
and initial coulombic efficiency.
4. Conclusions
In summary, a CoS2/rGO hybrid was successfully prepared by
using a hydrothermal method. The CoS2 nanoparticles, with sizes of
1e10 nm, were uniformly embedded on both sides of the rGO,
which provided a structurally stable host for sodium storage
through the conversion reaction. The rGO played an important part
in the formation of the CoS2 nanoparticles. As an anode for SIBs,
good cycle stability (approximately 400 mAh gꢀ1 at 100 mA gꢀ1
after 100 cycles) have been proved by the CoS2/rGO composite. The
improved cycling stability of CoS2/rGO compared to bare CoS2 can
be due to the introduction of the versatile rGO that not only
D ¼ R2T2/2n4F4A2C2s2
In which R is the gas constant, n is the number of electrons
involved in the oxidation/reduction reaction per molecule, T is the
temperature, F is the Faraday constant, C is the concentration of
sodium ions, A is the surface area of the electrode, and
s is the
Warburg factor [52]. Fig. 7b show the Z0 versus ѡꢀ1/2 curves in the
Fig. 7. (a) Electrochemical impedance spectra of the bare CoS2 and CoS2/rGO composite electrodes. (b) Z0 versus ѡꢀ1/2 curves in the low frequency region of the pristine CoS2 and
CoS2/rGO composite.

K. Xie et al. / Journal of Alloys and Compounds 726 (2017) 394e402
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enhanced rate capacity was also realized. Such intriguing electro-
chemical properties of the CoS2/rGO composite make it a potential
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Acknowledgement
This work was supported by the Chinese Academy of Sciences
Innovative and Interdisciplinary Team Award, the Changjiang
Scholars Program (T2011170) and the Youth Project in Nature Sci-
ence Foundation of Jiangsu Province (Grant No.20161006), the
Major Project of Educational Commission of Jiangsu Province of
China under contract No. 13KJA430004, the Priority Academic
Program Development of Jiangsu Higher Education Institutions.
The Six Talent Peaks Project of Jiangsu Province (Grant No. XNY-
CXTD-001).
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