One-step synthesis of nickel cobalt sulphides particles: Tuning the composition for high performance supercapacitors

Article abstract of DOI:10.1039/c6ra10048a

NiS₂-CoS₂ composites with different Ni and Co molar ratios for supercapacitors (SCs) were synthesized by one-step hydrothermal co-deposition method using cheap Na₂S₂O₃·5H₂O as sulfur source. With the increase of Ni content, the composites particle size increases gradually and the hollow sphere structure becomes more obvious. The electrochemical measurements demonstrate that these composites possess a high specific capacitance (C_m) performance, good rate capability and long cycle stability. To be specific, the C_m of Ni/Co/S-1 composite is the largest, up to 954.3 F g^-1 at 1 A g^-1, and as high as 309.5 F g^-1 even at large current density of 20 A g^-1. Furthermore, the Ni/Co/S-1 maintains 99.9% of its initial C_m after 1000 cycles at 5 A g^-1. Moreover, the asymmetric supercapacitors with Ni/Co/S-1 as positive electrode and active carbon as negative electrode are of prominent energy density of 29.3 W h^-1 kg^-1 at the power density of 0.7 kW kg^-1, and superior cycling stability of 99.1% initial value retention after 1000 cycles.

Full text of DOI:10.1039/c6ra10048a

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Oneꢀstep synthesis of nickel cobalt sulphides particles: tuning the composition for high
performance supercapacitors
a,
b
Huaihao Zhang *, Bing Guan, Jiangna Gu , Yu Li, Chi Ma, Jing Zhao, Tianyi Wang and
c,
Changjing Cheng *
a
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR
China
b
AnKao Energy Co., Ltd. No. 29, Huahong Street, East Suhong Road, SuZhou Industrial Park,
SuZhou, 215026, PR China
c
College of Chemistry & Environment Protection Engineering, Southwest University for
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Nationalities, Chengdu, 610041, China)
Abstract: NiS ꢀCoS composites with different Ni and Co molar ratios for supercapacitors (SCs)
2
2
2 2 3 2
were synthesized by oneꢀstep hydrothermal coꢀdeposition method using cheap Na S O ·5H O as
sulfur source. With the increase of Ni content, the composites particle size increases gradually and
the hollow sphere structure becomes more obvious. The electrochemical measurements demonstrate
that these composites possess a high specific capacitance (C
m
) performance, good rate capability
and long cycle stability. To be specific, the C of Ni/Co/Sꢀ1 composite is the largest, up to 954.3
m
ꢀ1
ꢀ1
ꢀ1
ꢀ1
F·g at 1 A·g , and as high as 309.5 F·g even at large current density of 20 A·g . Furthermore,
ꢀ1
the Ni/Co/Sꢀ1 maintains 99.9% of its initial C
m
after 1000 cycles at 5 A·g . Moreover, the
asymmetric supercapacitors with Ni/Co/Sꢀ1 as positive electrode and actived carbon as negative
ꢀ1
ꢀ1
electrode are of prominent energy density of 29.3 Wh·kg at the power density of 0.7 kW·kg , and
superior cycling stability of 99.1% initial value retention after 1000 cycles.
2 2
Keywords: NiS ꢀCoS composites, Hydrothermal coꢀdeposition method, Specific capacitance,
Asymmetric supercapacitors
1. Introduction
Supercapacitors (SCs) have attracted increasing attention due to their prominent properties like
high power density, long lifespan and fast charge and discharge rate, which make them great
1
*
corresponding author. Tel/Fax: +86 51487975244
Eꢀmail address: huaihaozhang@163.com(H.H. Zhang); chengcj@swun.edu.cn(C.J.C)

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potential for applications in next generation energy storage devices. To date, SCs have been applied
in various areas such as largeꢀscale smart grids, hybrid electric vehicles, portable electronic devices
1
ꢀ5
and backꢀup power supplies. Generally, SCs can be typically classified into two categories based
on the energy storage mechanism, electric double layer capacitors (EDLCs) and pseudocapacitors.
EDLCs store energy through electrostatic charge accumulating at the electrode/electrolyte interface.
Pseudocapacitors depending on fast and reversible Faradaic reactions of electrode materials could
6
,7
provide higher specific capacitance. It is widely known that electrode material is one of the most
important factors affecting SCs performance. For example, carbonꢀbased materials are mainly used
in EDLCs, while the materials used in pseudocapacitors are usually transition oxides or conducting
polymers. Overall, according to the previous reports about SCs, improving electroactive materials
capacitance performances (including energy density and power density etc) has always been the
8ꢀ11
research hotspot during the development process.
In recent years, transition metal sulfides with superior electrical, optical, magnetical and
catalytic properties have been widely used in the fields of SCs, Liꢀion batteries, solar cells, sensors
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and catalysts.
The mechanical strength, thermal stability and redox reaction activity of transition
17
metal sulfides used as SCs electrodes are superior to that of the corresponding oxides. Among
them, Co sulphides and Ni sulphides have aroused great interests due to their high redox activity,
1
8ꢀ20
high theoretic C
m
value, abundant raw materials and low toxicity.
The valence states of Ni and
Co are various, which is conductive to redox reactions. Furthermore, the electronegativity of sulfur
is lower than that of oxygen, making Co sulphides and Ni sulphides with more flexible crystal
structure and better ductility. It is well known that crystal structure has great influence on the
capacitance performance of active materials. Better ductility can release the shrinkage and swelling
pressure on materials structure during continuous chargeꢀdischarge process, thus to increase the
21
cycling stability. Meanwhile, the capacitance performance largely depended on the active
2
1
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materials morphology. For example, CoS
2
octahedras, Co
9 8 2
S nanotubes, NiS nanocubes and
2
5
ꢀ1
ꢀ1
Ni S nanosheets as electroactive materials for SCs exhibit high C value of 236.5 F·g (1 A·g ),
3
2
m
2

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1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
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285 F·g (0.5 A·g ), 695 F·g (1.25 A·g ) and 717 F·g (2 A·g ), respectively. However, the
conductivity of single Co sulphide or Ni sulphide is too low to support fast electron transport at
26
high current density, which imposes restrictions on the application in high performance SCs.
However, the physical and chemical properties of Ni and Co are similar, and there exits
various sulfide valence states of Ni and Co ion in bimetallic NiꢀCo sulfides. When the two sulfides
22
are composited, the redox reaction is easy to conduct. Particularly, it is noted that NiꢀCo sulfides
exhibit much higher conductivity and lower optical band gap energy than corresponding NiꢀCo
2
7
oxides. So, NiꢀCo sulfides showed higher C
m
in KOH electrolyte. For instance, porous nanotubes
2
8
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NiCo
2
S
4
,
mesoporous nanoparticles NiCo
2
S
4
and hollow nanoprisms NiCo
2
4
S ,
have
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
considerable high C
m
of 933 F·g (1 A·g ), 1440 F·g (3 A·g ) and 895.2 F·g (1 A·g ),
respectively.
Till now, the above mentioned NiꢀCo sulfides were usually prepared by twoꢀstep reactions.
Typically, the NiꢀCo composite precursors were synthesized and then vulcanized to be transformed
into NiꢀCo sulfides. First, most of precursors were prepared in the presence of heating organic
solvent. Without solvent recycling process, it will lead to raw material consumption. Secondly, the
involvement of CH CSNH , Na S and CS(NH ) as high cost sulfur source can cause environment
3
2
2
2 2
2
7ꢀ29
and health risks.
In this paper, NiꢀCo sulfides have been successfully synthesized through a oneꢀstep hydrother
mal coꢀdeposition method, using cheap Na ·5H O as the sulfur source and H O as the solvent.
The capacitance performance of SCs can be controlled by adjusting the ratios of Ni and Co. Electro
2
S
2
O
3
2
2
ꢀ1
ꢀ1
chemical tests indicated that the C of Ni/Co/Sꢀ1 electrode reached up to 954.3 F·g at 1 A·g and
m
ꢀ
1
m
still maintained 99% of initial C after 1000 cycles at 5 A·g . In order to evaluate the practical app
lication of Ni/Co/Sꢀ1, we prepared asymmetric SCs, using Ni/Co/Sꢀ1 as the positive electrode and a
ctivated carbon as the negative electrode. As the testing results show that the asymmetric SCs has la
ꢀ1
ꢀ1
rge potential of 1.45 V, and high energy density of 29.3 Wh·kg at power density of 0.7 kW·kg , m
anifesting that the NiꢀCo sulfides have great potential for applications in high performance SCs.
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2. Experimental Section
2.1 Synthesis of Ni-Co sulfides
All reagents Ni(NO
The preparation processes of CoS
deionized water, and then 4 mmol Na
3
)
2
·6H
2
O, Co(NO
are as follows: 3 mmol Co(NO
·5H O was added in the above solution under continuous
3
)
2
·6H
2
O and Na
2
S
2
O
3
·5H
2
O are analytical grade purity
2
3 2 2
) ·6H O was dissolved in 30 mL
2
S
2
O
3
2
stirring. After 30 min, the mixed solution was transferred into 50 mL Teflonꢀlined stainless steel
autoclave kept for 16 h at 160 ℃, and then cooled down to room temperature. The black
precipitates were collected by centrifugation, and washed with deionized water and absolute ethanol
repeatedly. The products were dried in vacuum oven at 60 ℃ for 10 h. Subsequently, the NiꢀCo
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sulfides were prepared under such condition: the total mole masses of Ni(NO
Co(NO ·6H O are 3 mmol, with Ni/Co moral ratios of 1:2, 1:1, 2:1 and 1:0, respectively. The
synthetic conditions of NiS /CoS composites and pure NiS are the same as that of CoS mentioned
3 2 2
) ·6H O and
3
)
2
2
2
2
2
2
2
above. The asꢀprepared samples were denoted as Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1, Ni/Co/Sꢀ2 and NiS ,
correspondingly.
2.2 Characterization
The crystal structures and purity of the obtained samples were characterized by Xꢀray
diffraction (XRD, Bruker, D8 Advance, Germany) with Cu K radiation (λ=1.5406 Å). Energy
α
dispersive Xꢀray spectrometer (EDS, OXFORD, INCA300, England) measurements were
performed on the environment scanning electron microscopy (ESEM, Philips, XLꢀ30, Holland) to
confirm the atomic percentages of the samples. The chemical valence states and elemental
compositions were conducted through Xꢀray photoelectron spectroscopy (XPS, Thermo Scientific,
ESCALAB 250Xi, USA). Morphologies, sizes, and parameters of the samples were characterized
by scanning electron microscopy (FESEM, Hitachi, Sꢀ4800 Ⅱ , Japan), transmission electron
1
00
01
1
1
02
03
microscopy (TEM, Philips, TECNAI12, Holland) and high resolution transmission electron
microscope (HRTEM, FEI, TECNAI G2F30 SꢀTWIN, USA). Elemental maps were also confirmed
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by HRTEM. The surface areas and the poreꢀsize distribution were studied by N adsorption analyzer
2
(Micromeritics, TriStar 3000, USA).
2.3 Electrodes preparation and electrochemical measurements
The electrochemical performance of the samples was evaluated using a conventional threeꢀ
electrode system in 3 M KOH electrolyte, with Ag/AgCl electrode as the reference electrode and
platinum plate as the counter electrode. The working electrodes were fabricated according to the
following steps: 85 % active material, 10 % conductive graphite and 5 % polytetrafluoroethylene
(PTFE) were mixed in paste with proper amount of absolute ethanol as solvent. The pastes mixtures
2
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were smeared on Ni foam (1×1 cm ) and then dried at 60 ℃ for 12 h in vacuum oven. Subsequently,
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the electrodes were pressed into sheet under 10 MPa for 30s.
Asymmetric SCs were assembled using the NiꢀCo sulfides as positive electrodes and activated
carbon (AC) as negative electrodes in 3 M KOH electrolyte. The charge storage between the
negative electrode and positive electrode was determined based on the charge balance theory
(Q = Q ). The specific charge storage in each electrode can be determined by the follow equation:
+
−
ꢀ1
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Q = C ×ꢀE×m , where C represents the specific capacitance of single electrode (F·g ), △E is the
m
m
1
19
20
potential range of each electrode (V), and m is the weight of active material of each electrode (g).
-
1
Mass balancing can be calculated by the following equation: (m / m = C ×ꢀE / C × ꢀE+ ). The
+
−
m
m+
−
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fabrication of the negative electrode was the same as that of the positive electrode.
Cyclic voltammetry (CV) and galvanostatic chargeꢀdischarge (GCD) experiments were carried
out on CHI660E electrochemical workstation. Electrochemical impedance spectroscopy (EIS)
5
measurements were performed on AutolabꢀPGSTAT30 in frequency range from 0.01 to 10 Hz, and
alternating current potential amplitude of 5 mV.
3. Results and discussion
3.1 Structure and morphology characterizations
2
Fig. 1A displays SEM images of pure CoS , octahedron structure with edge length around
2
2
180~220 nm, and pure NiS
2
is composed of spherical particles about 435~720 nm (Fig. 1E). Fig.
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1(B~D) reveals that Ni/Co/Sꢀn composites consist of octahedron and spherical particles with
irregular size. The content of octahedral CoS and spherical NiS is associated with the ratios of Co
and Ni. And the surface of Ni/Co/Sꢀn is much rougher than those of pure CoS and NiS
Combining with TEM images, the formation of NiꢀCo sulphides can be ascribed to the growth and
2
2
2
2
.
30
selfꢀaggregation process, followed by Ostwald ripening mechanism. The Ni/Co/Sꢀn samples (Fig.
1(F~J)) are tetragonal and circular projected shapes, consistent with SEM results. However, the
3
1
particles agglomerate together seriously in order to reduce high surface tension and surface energy .
The TEM image (Fig. 1F) shows that the CoS
2
are mainly solid constructions, while the different
are likely to be hollow sphere structures. Rough
brightness in Fig. 1J demonstrates that NiS
2
surfaces and hollow spheres structure not only provide high specific surface areas, but also are
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conductive to improve the wettability of active material for better electrolyte access.
Fig. 2A shows a HRTEM image of Ni/Co/Sꢀ1, octahedral and spherical structure coincided
with SEM image. Fig. 2(B and C) presents the lattice spacing 0.283 and 0.254 nm, corresponding to
(200) and (210) planes of NiS
2
. And the lattice spacing from Fig. 2(D and E) are 0.248 and 0.167
nm, corresponding to (210) and (311) planes of CoS
2
. Meanwhile, EDS mapping of Ni/Co/Sꢀ1 in
Fig. 2F demonstrated the maldistribution of Ni and Co elements in the samples. But in general, the
formation process of Ni/Co/Sꢀ1 belongs to intergrowth process. The distribution of S in the samples
2
indicated that NiS is of hollow sphere structure, matching well with TEM results.
From XRD patterns in Fig. 3A, several diffraction peaks at 2θ values of 27.2°, 31.5°, 35.3°,
38.7°, 45.3°, 53.6°, 56.2°, 58.7° and 61.1° corresponding to (111), (200), (210), (211), (220), (311),
(222), (230) and (321) planes of NiS (JCPDS 11ꢀ0099). With the increase of Ni content, the peaks
2
position have a slight shift toward higher angle and reach to 27.9°, 32.3°, 36.2°, 39.8°, 46.3°, 54.9°,
57.6°, 60.2° and 62.7°, indexed to CoS
2
(JCPDS 41ꢀ1471). A slight shift of peak position can be
attributed to physical and chemical properties similarities of Ni and Co as well as structural
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compatibility. Meanwhile, no peaks of impurities can be detected, demonstrating high purity of
samples. Several intense and sharp diffraction peaks from NiS , indicate that the asꢀprepared NiS is
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of high crystallinity, while the crystallinity of other samples with broad and weak peaks are
relatively low. The EDS analysis in Fig. S1(A~C) demonstrate that the asꢀprepared samples are
mainly composed of Ni, Co and S elements, which agree well with the XRD results. The Ni/Co
atomic ratios of Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1 and Ni/Co/Sꢀ2 are about 1:1.93, 1:1.09 and 1.81:1
respectively, slight different with the initial ratios (1:2, 1:1 and 2:1), which is caused by different
33
reaction activities of Ni and Co ions.
The specific surface area and pore size distribution have important influence on the capacitive
performance in SCs. Fig. 3B shows N adsorptionꢀdesorption isotherms, and pore size distribution
(inset in Fig .3B) calculated from adsorption isotherms. The samples (with the exception of CoS
2
2
)
present type IV isotherms with H3 hysteresis loops, suggesting the existence of mesopores. The
hysteresis loop of Ni/Co/Sꢀ2 was obviously larger than other samples. It is because the Ni/Co/Sꢀ2
possesses plenty of NiS
and produced more hollow spheres, thus leading to more mesopores. The mesoporous structure of
samples is mainly attributed to NiS hollow spheres. However, the CoS displays unconspicuous
hysteresis loop at relative pressure (0.4<P/P <1.0), indicating it is mainly consist of micropores and
2 2
and less amount of CoS , makes Ostwald ripening process happened easier
2
2
0
little amount of small mesopores. This result matches well with pore size distribution curve of CoS2
(Fig. 3B inset).
The BET specific surface areas (SBET) of the CoS
2
, Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1, Ni/Co/Sꢀ2 and
were 11.7, 9.2, 17.3, 19.1 and 18.3 m ·g , respectively. The pore sizes distribution centered at
6.0, 7.1, 4.4 and 5.9 nm for Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1, Ni/Co/Sꢀ2 and NiS , respectively (Fig. 3B
2
ꢀ1
NiS
2
2
inset), while the pore size of CoS was mostly below 5 nm. Generally, the meosopore structure of
2
samples is becoming more obviously with the increase of Ni content. As is known, mesoporous
materials not only provide rich electroactive sites, but also offer more free volume changes during
cycling charge/discharge process, resulting in excellent electrochemical properties of electroactive
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The surface composition and chemical state were further monitored by XPS. Fig. 3C shows the
survey spectrum of the Ni/Co/Sꢀ1 sample and the binding energy ranges from 0~1300 eV. The
peaks at 162.3, 778.5 and 853.5 eV are attributed to S 2p, Co 2p and Ni 2p, respectively, indicating
Ni/Co/Sꢀ1 contains S, Co and Ni elements. The Ni 2p, Co 2p and S 2p spectra were fitted by the
Gaussian fitting method. As illustrated in Fig. 3D, the main peaks of Ni 2p3/2 and Ni 2p1/2 located at
2+ 34
853.5 eV, 870.9 eV can be assigned to Ni . And in Fig. 3E, the two main peaks at 778.5 eV,
2
+ 35,36
793.6 eV for Co 2p3/2 and Co 2p1/2 are the character of Co .
Fig. 3F shows the spectrum of S 2p
region, the two main peaks at 162.3 eV and 163.5 eV correspond to S 2p3/2 and S 2p1/2. The peak at
162.3 eV can be ascribed to the sulphur ion in low coordination on surface, and the peak at 163.5
eV is considered as the metalꢀsulphur bonds. Moreover, the peak at 168.7 eV for O, impurity is due
37ꢀ39
to surface O absorption of the samples exposed to air during testing processing.
3.2 Electrochemical measurements
ꢀ1
Fig. 4A shows the CV curves of NiꢀCo sulphides at scan rate of 5 mV·s in the potential
window from ꢀ0.1~0.5 V. A pair of redox peaks can be observed for Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1,
2 2
Ni/Co/Sꢀ2 and NiS as well as two pairs for CoS , which result from the reversible Faradic reaction
occurring at the electroactive materials/electrolyte interface. The redox reactions for NiꢀCo
2
2,40
sulphides in KOH alkaline electrolyte are as follows
:
−
−
198
199
200
CoS + OH ↔ CoS OH + e
(1)
(2)
(3)
2
2
−
−
CoS + OH ↔ CoS O + H O + e
2
2
2
−
−
NiS + OH ↔ NiS OH + e
2
2
201
202
203
204
205
206
From CV curves, the redox peaks gradually enhanced as Ni content in samples increased. The
CV integrated area of Ni/Co/Sꢀ1 is larger than other samples, suggesting Ni/Co/Sꢀ1 has the best
capacitive performance. As shown in Fig. S2, the redox peaks of Ni/Co/Sꢀ1 are almost symmetric,
demonstrating the excellent reversibility of the oxidation and reduction process. Furthermore, with
the increase of the scan rate, the anodic peaks and cathodic peaks shift to the converse direction
38
because of the polarization at high scan rate.
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Meanwhile, the nonlinear GCD curves of samples in Fig. 4B are typical pseudocapacitive,
much different from that of linear characteristic of EDLCs. Moreover, obvious plateaus in every
m
GCD curves correspond to the redox peaks positions in the CV curves. The C of the samples was
achieved by GCD measurement using the following equation:
2
11
C = I ×ꢀt /
(
m×ꢀV
is the specific capacitance (F·g ), I is the discharge current (A), △t is the discharge time
(s), m is the weight of active materials (g) and △V is the potential range (V). From the discharge
)
(4)
m
ꢀ1
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
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232
where C
m
curves, the C
m
of CoS
2
, Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1, Ni/Co/Sꢀ2 and NiS
2
are 293.9, 682.8, 954.3, 840.8
of NiꢀCo sulphides
ꢀ
1
ꢀ1
and 790.4 F·g at 1 A·g , respectively. Interestingly, it is obvious that the C
m
are increase first and then decrease with the increase of Ni amount because the electrochemical
3
6
activity of the Ni ion is superior to that of the Co ion. Generally, a certain amount of Ni ion is
helpful to enhance capacitive performance of Ni/Co/S composites, but excessive Ni ion will be
2
3
contrary, which agrees with the reported literature. To be specific, Ni/Co/Sꢀ1 possesses the highest
C
m
value, consistent with the CV profiles. From the Fig. S3C, GCD curves of the Ni/Co/Sꢀ1 at
different current densities are all highly symmetrical, revealing that the Ni/Co/Sꢀ1 has excellent
Coulombic efficiency, pseudocapacitive behavior and electrochemical reversibility. Such excellent
capacitive performance of Ni/Co/Sꢀ1 is mainly attributed to the unique structure and synergetic
effects of NiS
2 2
and CoS : 1) Ni/Co/Sꢀ1 with rough surface can utilizes active material and provide
24
3+
rich electroactive sites, beneficial to fast redox reactions ; 2) In the process of redox reaction, Co
2
+
will supply extra holes as pꢀtype doping while Ni will produce extra electrons by nꢀtype doping.
3
+
3+
Thus, coexistence of more Ni and Co will possess higher conductivity and capacitive
41
performance.
According to the GCD curves (Fig. S3), the C
in Fig. 4C. The C gradually decrease as the current density increase, which can be attributed to the
increment of voltage and insufficient active material involved in redox reaction during the high rate
m
of NiꢀCo sulphides can be obtained, as plotted
m
23
ꢀ1
GCD process. The C
m
of Ni/Co/Sꢀ1 is up to 954.3, 886.8, 836.4, 751.8, 578.6 and 309.5 F·g ,
9

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1
233
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corresponding to current densities 1, 2, 3, 5, 10 and 20 A·g , larger than other samples at the same
current densities. Therefore, the electrochemical performance of NiꢀCo sulphides could be
ꢀ1
improved by tuning Ni and Co content. Even at a high current density of 20 A·g , the C
rate of Ni/Co/Sꢀ1 can still be 32.4%. These results indicate that Ni/Co/Sꢀ1 has good rate capability
and the strongest synergistic effect. The C of CoS is relative low, but the capacity retention is
superior to other samples (about 41.5%) due to its good structural stability. Noticeably, at a current
m
retention
m
2
ꢀ1
density of 20 A·g , the capacity retention of Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ2 and NiS
2
is 27.6%, 12.9% and
decreased faster than other samples because the
has a low capacitance retention as well as the C of CoS is low. Cycle performance of
samples under a current density of 5 A·g for 1000 cycles was tested, as shown in Fig. 4D. The C
of CoS , Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1, Ni/Co/Sꢀ2 and NiS retain about 100.9%, 100.4%, 99.9%, 99.6%
ꢀ1
5.9% at 1 A·g , respectively, suggesting their C
m
NiS
2
m
2
ꢀ1
m
2
2
. m
and 103.1% of the original one even after 1000 cycles It is evident the C of samples increase
firstly, dropped slightly and tend to be stable. This phenomenon can be explained as follows: The
electroactivity of materials has been stimulated and enhanced at original GCD phase, while the
active materials could also be degraded to some extent at alkaline condition at following GCD
21
cycling.
Fig. 4E shows the Nyquist plots of samples, including a semicircle in the high frequency
region and an incline line in the low frequency region. The equivalent circuit model and the
enlargement of Nyquist plots are shown in Fig 4E and Fig 4F respectively, where R
equivalent series resistance, W is the Warburg impedance, Rct is the chargeꢀdischarge resistance, C
is the doubleꢀlayer capacitance and C is the Faradaic pseudocapacitor. Here, the R value of
s
is the
d
F
s
samples can be obtained by the semicircle intersect on the real axis. It can be observed that the R
CoS , Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1, Ni/Co/Sꢀ2 and NiS are 0.21, 0.36, 0.18, 0.22 and 0.31 Ω
respectively, indicating that Ni/Co/Sꢀ1 has the lowest intrinsic resistance and contact resistance.
As illustrated in the inset in Fig. 4E, in high frequency `region, the samples show clear
s
of
2
2
40
4
2
semicircles. Moreover, the R obtained by ZSimpWin software for CoS , Ni/Co/Sꢀ0.5, Ni/Co/Sꢀ1,
ct
2
1
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Ni/Co/Sꢀ2 and NiS were 3.76, 1.91, 0.71, 1.35 and 2.2 ꢁ, respectively. To recap, the R of NiꢀCo
2 ct
sulphides are decrease first and then increase with the increase of Ni content, suggesting that a
certain amount of Ni is useful to improve the electrical conductivity of electroactive materials, but
38
excessive Ni goes against electron transfer. It can be known that low Rct means fast electron
36
transport, and also could provide higher C
m
value especially for pseudocapacitor. Therefore,
Ni/Co/Sꢀ1 presents smallest Rct among all the samples, suggesting its superior capacitive
performance. The straight lines in the low frequency region correspond to diffusive resistance of the
43
electrolyte for the redox material, and have not relation with charge storage. As shown in inset of
Fig. 6E, the straight lines of Ni/Co/Sꢀ1 electrodes in the EIS spectra incline at an angle of nearly
44
90° to the Zˊꢀaxis, indicating the capacitive performance are not controlled by diffusion process.
The reason is Ni/Co/Sꢀ1 have more rough surfaces and hollow spheres structures, which could
enhance the wettability of active materials for better electrolyte access.
Fig. 5A shows the CV curves of Ni/Co/Sꢀ1 electrode before and after 1000 GCD cycles. After
1000 cycles, two CV curves are nearly coincident, demonstrating the high cycling stability. The
s
Nyquist plot of Ni/Co/Sꢀ1 electrode before and after 1000 cycles is shown in Fig. 5B. The R of
Ni/Co/Sꢀ1 electrode is only increased from 0.18 ꢁ to 0.49 ꢁ. What’s more, the diameter of the
semicircle has a slight increase and the slope for straight line decreased after 1000 cycles. These
22
results further clarify the Ni/Co/Sꢀ1 exhibits well cycling stability and reversible redox reaction.
Fig. 6 is the SEM images of Ni/Co/Sꢀ1 electrode before (A) and after (B) 1000 cycles. In
comparison Fig. 6A with B, the surface of Ni/Co/Sꢀ1 electrode appeared cracks after 1000 cycles.
This is explained by the fact that continuous redox reaction makes volume of active material
shrinkage and expansion constantly, leading to shell layer fracture of Ni/Co/Sꢀ1 electrode.
In order to evaluate the practical application potential of Ni/Co/Sꢀ1, asymmetric SCs were
fabricated using Ni/Co/Sꢀ1 and AC as the positive and negative electrodes, respectively. Based on
the principle of charge balance, as well as the C
m
of Ni/Co/Sꢀ1 and AC electrode from the CV
ꢀ1
curves at a scanning rate of 5 mV·s (shown in Fig. 7A), the mass ratio of Ni/Co/Sꢀ1 and AC is
1
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determined to be 1:3.07. Due to the different working potential of the Ni/Co/Sꢀ1 and AC, the
voltage of asymmetric SCs can be extended to 0~1.45V. Fig. 7B shows the CV curves of
asymmetric SCs at various scan rates own both electric double layer capacitance and
pseudocapacitance characteristic. Moreover, there is no noticeable distortion of CV curves with
45
increasing scan rate, indicating the fast chargeꢀdischarge quality of the device. Fig. 7C illustrated
a set of GCD curves under various current densities. The C of asymmetric SCs was calculated to
m
ꢀ1
ꢀ1
be 100.4, 88.9, 80.6, 69.2 and 45.9 F·g corresponding to the current density 1, 2, 3, 5 and 10 A·g ,
respectively. The Ragone plot derived from the GCD curves is listed in Fig. 7E. Energy density and
power density of the device were calculated from discharge curves according to the following
equations:
2
2
95
E = C ×
(
ꢀV
)
/ 7.2 (5)
(6)
where E represents energy density (Wh·kg ), C represents the specific capacitance (F·g ), △V
m
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
P = 3.6× E / ꢀt
ꢀ1
ꢀ1
m
ꢀ1
represents voltage interval (V), P represents power density (kW·kg ) and
△
t represents discharge
ꢀ1
time (s). The energy density of asymmetric SCs reaches 29.3 Wh·kg at a power density of 0.7
ꢀ
1
ꢀ1
ꢀ1
kW·kg , and still remains 13.4 Wh·kg at a higher power density of 7.4 kW·kg , indicating their
advantage in the retention of energy density. Moreover, the cycle life of asymmetric SCs was tested
ꢀ
1
and shown in Fig. 7F by repetitive GCD process at a current density of 5 A·g . The device exhibits
good cycle performance, 99.2% of its initial capacitance can be retained after 1000 cycles.
Furthermore, a superior electrochemical reversibility with almost 100% Coulombic efficiency can
be retained (Fig. 7F). All the results demonstrate that the asymmetric SCs are promising in the
application for highꢀperformance energy storage systems.
4. Conclusions
In summary, we have successfully synthesized NiꢀCo sulphides through oneꢀstep hydrothermal
coꢀdeposition, using Na S O ·5H O as the sulfur source. The NiꢀCo sulphides are mainly composed
2
2
3
2
2 2
of octahedral CoS and spherical NiS based on the Ostwald ripening process. Electrochemical
1
2

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measurements results show that the capacitive performance of NiꢀCo sulphides can be controlled
via adjusting the ratios of Ni and Co content. Among them, Ni/Co/Sꢀ1 exhibited the maximum C
m
ꢀ1
ꢀ1
954.3 F·g at a current density 1 A·g as well as desirable cycling stability with capacitance
ꢀ1
retention of 99.9% at 5 A·g after 1000 cycles. Meanwhile, EIS test results show that Ni/Co/Sꢀ1
possesses the lowest Rct (0.71ꢁ), indicating its ideal pseudocapacitance behavior. The remarkable
capacitive performance can be ascribed to the unique structure and strong synergistic effect from Ni
ion and Co ion, which provide short ion diffusion pathway and high electrical conductivity.
Additionally, an asymmetric SCs based on Ni/Co/Sꢀ1 as positive electrode and AC as negative
ꢀ1
ꢀ1
electrode delivers an energy density of 29.3 Wh·kg at a power density of 700 W·kg and a high
ꢀ1
ꢀ1
energy density of 13.4 Wh·kg at a very high power density of 7.4 kW·kg . The NiꢀCo sulphides,
particularly Ni/Co/Sꢀ1, have remarkable capacitive performance, rate capability and cycling
stability, presenting great potential for practical application.
Acknowledgements
This work was financially supported by the Natural Science Foundation of China (NO.
21106124 and NO. 21375116) and Postdoctoral Science Foundation of China (2014M551668). The
related measure and analysis instrument for this work was supported by the Testing Center of
Yangzhou University.
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Figure captions
398
399
400
401
402
403
404
405
406
Figure 1. SEM and TEM images of NiꢀCo sulphides: (A and F) CoS , (B and G) Ni/Co/Sꢀ0.5, (C and H) Ni/Co/Sꢀ
2
1, (D and I) Ni/Co/Sꢀ2, (E and J) NiS
Figure 2. (A~E) HRTEM image of Ni/Co/Sꢀ1, (F) corresponding EDS mapping
Figure 3. (A) XRD patterns of NiꢀCo sulphides, (B) N adsorptionꢀdesorption isotherm and pore size distribution
2
, (insets: the magnified SEM images).
2
curve (the inset) of NiꢀCo sulphides, (C~F) XPS spectrum of Ni/Co/Sꢀ1: (C) survey spectrum, (D) Ni 2p region,
(E) Co 2p region and (F) S 2p region.
Figure 4. Electrochemical performance of CoS
2
2
(a), Ni/Co/Sꢀ0.5 (b), Ni/Co/Sꢀ1 (c), Ni/Co/Sꢀ2 (d) and NiS (e):
ꢀ1
ꢀ1
(A) CV curves at scan rate of 5 mV·s , (B) GCD curves at current density of 1 A·g , (C) C
m
versus current
ꢀ1
density, (D) cycling performance at current density of 5 A·g and (E) Nyquist plots.
407
408
409
410
411
412
Figure 5. (A) CV curves and (B) Nyquist plots of Ni/Co/Sꢀ1 electrode before and after 1000 GCD cycles.
Figure 6. SEM images of Ni/Co/Sꢀ1 electrode: (A) before cycling and (B) after 1000 GCD cycles.
ꢀ1
Figure 7. (A) CV curves of Ni/Co/Sꢀ1 and AC at scan rate of 5 mV·s , electrochemical performance of Ni/Co/Sꢀ
1//AC asymmetric SCs: (B) CV curves at different scan rates, (C) GCD curves at different current density, (D) Cm
versus different current density, (E) Ragone plot and (F) cycling performance and Coulombic efficiency at current
ꢀ1
density of 5 A·g .
1
6

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Figure 1. SEM and TEM images of Ni-Co sulphides: (A and F) CoS , (B and G) Ni/Co/S-0.5, (C
2
and H) Ni/Co/S-1, (D and I) Ni/Co/S-2, (E and J) NiS , (insets: the magnified SEM images).

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Figure 2. (A~E) HRTEM image of Ni/Co/S-1, (F) corresponding EDS mapping

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35
28
21
14
7
0
2
00
0.004
A
B
CoS
2
0
0
0
.003
.002
.001
Ni/Co/S-0.5
Ni/Co/S-1
Ni/Co/S-2
NiS2
Ni/Co/S-2
NiS
2
Ni/Co/S-1
0.000
0
9
18 27
36
Pore size (nm)
Ni/Co/S-0.5
CoS2
2
0
30
40
50
60
70
80
0.0
Ni 2p
0.2
0.4
0.6
0.8
1.0
0
2
? (degree)
Relative pressure (P/P )
C
2 p3/2
D
2
p1/2
sat
sat
1
200 1000 800 600 400 200
0
880
872
864
856
848
Binding energy (eV)
Binding energy (eV)
2
p3/2
2 p3/2
E
F
Co 2p
S 2p
2
p1/2
2
p1/2
sat
sat
sat
8
08
800
792
784
776
176
172
168
164
160
Binding energy (eV)
Binding energy (eV)
Figure 3. (A) XRD patterns of Ni-Co sulphides, (B) N adsorption-desorption isotherm and pore
2
size distribution curve (the inset) of Ni-Co sulphides, (C~F) XPS spectrum of Ni/Co/S-1: (C)
survey spectrum, (D) Ni 2p region, (E) Co 2p region and (F) S 2p region.

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1
2
0.5
B
ꢀ
1
A 8
ꢀ
1
1 A⋅g
e
5
mV⋅s
0.4
d
c
c
b
d
4
0
a
0.3
0.2
0.1
e
a
b
ꢀ
4
ꢀ8
0
.0
ꢀ0.1
0.0
0.1
0.2
Potential (V)
0.3
0.4
0.5
0
200
400
600
800
Time (s)
1
000
800
C
a
c
e
b
d
D
ꢀ
1
5 A⋅g
8
6
4
2
00
00
00
00
0
6
4
2
00
00
00
0
a
d
b
e
c
0
5
10
Current density (A
15
1
20
0
200
400
600
800
1000
ꢀ
⋅
g )
Cycle number
2
0
5
0
5
0
6
F 5
E
a
c
e
b
d
a
c
e
b
d
1
1
4
3
2
1
0
C
d
R
C
F
s
R
ct
W
0
5
10
Z' (Ohm)
15
20
0
1
2
3
4
5
6
Z' (Ohm)
Figure 4. Electrochemical performance of CoS
and NiS (e): (A) CV curves at scan rate of 5 mV·s , (B) GCD curves at current density of 1 A·g ,
C) C versus current density, (D) cycling performance at current density of 5 A·g and (E)
2
(a), Ni/Co/Sꢀ0.5 (b), Ni/Co/Sꢀ1 (c), Ni/Co/Sꢀ2 (d)
ꢀ1
ꢀ1
2
ꢀ1
(
m
Nyquist plots.

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15
10
A 12
B
Before cycling
After cycling
After cycling
Before cycling
8
4
0
5
0
-
4
-8
-
0.1
0.0
0.1
0.2
0.3
0.4
0.5
0
5
10
Z' (Ohm)
15
20
Potential (V)
Figure 5. (A) CV curves and (B) Nyquist plots of Ni/Co/S-1 electrode before and after 1000
GCD cycles.

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DOI: 10.1039/C6RA10048A
Figure 6. SEM images of Ni/Co/S-1 electrode: (A) before cycling and (B) after 1000 GCD cycles.

Page 23 of 24
RSC Advances
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DOI: 10.1039/C6RA10048A
1
2
8
4
0
B 21
A
ꢀ
s
1
ꢀ
1
100 mVꢀ1
⋅
5
mV⋅s
Ni/Co/Sꢀ1
14
50 mV⋅s
ꢀ1
2
0 mVꢀ ⋅s
1
0 mV⋅s
1
AC
7
0
ꢀ1
5
mV⋅s
ꢀ4
ꢀ7
ꢀ8
ꢀ0.9
ꢀ0.6
ꢀ0.3
Potential (V)
0.0
0.3
0.0
0.3
0.6
0.9
1.2
1.5
Potential (V)
C 1.5
ꢀ1
A⋅g
D100
2
1
0
0
0
0
.2
.9
.6
.3
.0
ꢀ
1
3
A
5
1
⋅g
ꢀ1
80
60
ꢀ
1
1 A⋅g
A⋅g
0 A⋅g
ꢀ1
4
2
0
0
0
0
60
120
180
240
300
0
2
4
6
8
10
0
ꢀ
1
Time (s)
Current density (A
⋅
g )
1
00
80
E
F
2
4
0
6
4
2
0
0
0
0
ꢀ1
1
0
1
5 A⋅g
0
60
8
1
0
00
0
.1
120
1
10
0
200
400
600
Cycle nunber
800 1000
ꢀ
1
Power density (kW⋅kg )
ꢀ1
Figure 7. (A) CV curves of Ni/Co/Sꢀ1 and AC at scan rate of 5 mV·s , electrochemical
performance of Ni/Co/Sꢀ1//AC asymmetric SCs: (B) CV curves at different scan rates, (C) GCD
m
curves at different current density, (D) C versus different current density, (E) Ragone plot and (F)
ꢀ1
cycling performance and Coulombic efficiency at current density of 5 A·g .

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NiS
synthesized by one-step hydrothermal co-deposition method. Among them, Ni/Co/S-1 (the Ni:Co
ratio of 1:1) possesses the highest C value at the same current densities.
2 2
-CoS composites with different Ni and Co molar ratios for supercapacitors (SCs) were
m