Enhancing the reversibility of SnCoS4 microflower for sodium-ion battery anode material

Article abstract of DOI:10.1016/j.jallcom.2020.154104

As the demand of high-performance energy storage devices rises in many applications, sodium-ion batteries have gained widespread researching attention as an alternative of lithium-ion batteries. Because of the high theoretical capacity, SnS₂ received intensive interests. Yet, the undesirable initial Coulombic efficiency and high hysteresis during charge and discharge reactions restrict their possible applications. To circumvent the poor initial Coulombic efficiency of tin sulfide anode materials and prevent the coarsening of Sn nanograins and reduction of the conversion reaction interface between Na₂S and metallic Sn, SnCoS₄ microflower was successfully designed and synthesized as anode material for sodium-ion batteries. The in situ formed Co nanoclusters restrict the coarsening of the materials and promote the sodiation and desodiation reactions, and therefore highly enhance the reversibility and initial Coulombic efficiency. The SnCoS₄ composite shows a highly reversible capacity of 477.76 mA h/g at 100 mA/g with a lower charge-discharge hysteresis.

Full text of DOI:10.1016/j.jallcom.2020.154104

Journal of Alloys and Compounds 825 (2020) 154104
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Enhancing the reversibility of SnCoS4 microflower for sodium-ion
battery anode material
,
Jiayi Zhong a, Xuezhang Xiao a **, Zunchun Wu a, Nan Zhang a, Ruicheng Jiang a,
,
, b, *
Xiulin Fan a b, Lixin Chen a
a State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
b Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou, 310013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
As the demand of high-performance energy storage devices rises in many applications, sodium-ion
batteries have gained widespread researching attention as an alternative of lithium-ion batteries.
Because of the high theoretical capacity, SnS2 received intensive interests. Yet, the undesirable initial
Coulombic efficiency and high hysteresis during charge and discharge reactions restrict their possible
applications. To circumvent the poor initial Coulombic efficiency of tin sulfide anode materials and
prevent the coarsening of Sn nanograins and reduction of the conversion reaction interface between
Na2S and metallic Sn, SnCoS4 microflower was successfully designed and synthesized as anode material
for sodium-ion batteries. The in situ formed Co nanoclusters restrict the coarsening of the materials and
promote the sodiation and desodiation reactions, and therefore highly enhance the reversibility and
initial Coulombic efficiency. The SnCoS4 composite shows a highly reversible capacity of 477.76 mA h/g at
100 mA/g with a lower charge-discharge hysteresis.
Received 12 December 2019
Received in revised form
27 January 2020
Accepted 28 January 2020
Available online 29 January 2020
Keywords:
Anode materials
Bi-metal sulfides
Solvothermal method
Initial coulombic efficiency
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
large interlayer spacing accelerating the insertion and extraction of
guest ions and accommodating more easily to the volume changes
As more and more countries and areas introduce policies and
laws to stimulate the consumption of new energy vehicles, battery
manufacturing companies and the vehicle companies are working
together to develop high-performance energy storage devices to
meet the growing demand of new energy vehicle market. Although
lithium-ion batteries (LIBs) have commercialized for many years, to
make the price of batteries more acceptable and the source more
sustainable still remains great challenge [1,2]. Driven by the ample
reserve of sodium resources on the earth and cost consideration,
sodium-ion batteries (SIBs) gained widespread researching atten-
tion as an alternative of lithium-ion batteries [3e6]. However, the
obviously larger ion radius of Naþ than that of Liþ gears down the
reaction kinetics and brings about severe volume expansion [7e9].
Two-dimensional (2D) layered metal sulfides, like SnS2 [10,11],
SnS [12,13] and MoS2 [14e16], rely on the layered structure with a
in the host materials during repeated charge and discharge pro-
cesses. Especially, SnS2 undergoes multistep reactions with
sodium-ions to generate Na15Sn4 with a high theoretical capacity of
1136 mA h/g. SnS2 is constituted by a layer of tin atoms sandwiched
between two layers of hexagonally close-packed sulfur atoms. The
large interlayer spacing (0.5899 nm) contributes to the fast and
easy sodium-ion intercalation and diffusion, which could greatly
improve the reaction kinetics, and acts as a buffer to adapt to the
huge volumetric changes [3,10]. Unfortunately, the 2D layered SnS2
tends to present the severe stack of adjacent planes by Van Der
Waals interaction, which leads to the irreversible capacity loss and
short cycle life. More importantly, Sn0 metal and Na2S could just
partly convert back to SnS2 during the desodiation process. The
internal stress of recrystallization causes the migration of the in-
termediate product Sn nanograins, then lead to the aggregation and
coarsening of Sn nanograins and reduction of the conversion re-
action interface between Na2S and metallic Sn0 [17,18]. The ultimate
consequence of coarsening and reduction of conversion reaction
interface is a partially reversible conversion reaction and thus an
unsatisfying initial Coulombic efficiency (ICE) as well as irreversible
capacity loss and structure collapse.
* Corresponding author. State Key Laboratory of Silicon Materials; School of
Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR
China.
** Corresponding author.
E-mail addresses: xzxiao@zju.edu.cn (X. Xiao), lxchen@zju.edu.cn (L. Chen).
https://doi.org/10.1016/j.jallcom.2020.154104
0925-8388/© 2020 Elsevier B.V. All rights reserved.

2
J. Zhong et al. / Journal of Alloys and Compounds 825 (2020) 154104
Many research efforts have been devoted to enhancing the cycle
precipitate was collected and washed 3 times by centrifugation
with de-ionized water and ethanol respectively, after autoclave was
naturally cooled down to ambient temperature. Then obtained
sample was dried at vacuum drying oven overnight at 60 ꢀC. For
contrast, the samples without addition of tin (IV) chloride penta-
hydrate and cobaltous chloride hexahydrate reagent were pre-
pared, respectively.
life of SnS2. For example, Wang et al. reported a freestanding
binder-free hierarchical SnS2 nanoplates decorated on the gra-
phene supported by carbon cloth (SnS2/graphene/CC) as anode
material, in which the graphene acts as the substrate, provides large
specific surface area and supports high mass-loading of SnS2
nanoplates. However, the initial Coulombic efficiency could just
reach 52.5%, far away from practical application [19]. It is thereby
vital to improve the ICE and reversible capacity of SnS2.
2.2. Materials characterization
In 2005, Sony Corporation commercialized a new type of LIB
consisting of SneCoeC ternary alloys system [20]. Then many re-
searchers have demonstrated that the 3d transition metals, like Co
and Fe, could enhance the reversible conversion reaction of SnO2,
attaining high ICE and high reversible capacity of electrode. That is
because the intermediate product Co nanoclusters serve as barriers
to hinder the migration and coarsening of Sn into large grains
during repeated cycling, which greatly boost the reversibility of
conversion reaction and restrain the pulverization of structure
[21e25]. Liang et al. reported a SnO2eCoeC ternary composite
exhibited high ICE of 80.8%. They revealed the notion that Co ad-
ditives dramatically inhibit Sn coarsening in the SnO2eCoeC sys-
tem by ex situ XRD measurements, and the anode material achieve
reversible capacity up to 780 mA h/g at 0.2 A/g after 400 cycles for
LIBs [21]. Compared to monometal sulfide, bi-metal sulfide pos-
sesses higher electronic conductivity and more abundant redox
reactions, which will significantly boost reversible storage of Naþ
[26e29].
Herein, we designed and fabricated a microflower-like SnCoS4
composite as SIB anode material of enhanced ICE and reversible
capacity for the first time. By one-pot solvothermal method, SnCoS4
composite was fabricated with a 3D microflower structure self-
assembled by 2D nanosheets. By designing SneCoeS ternary
composite, we attained the following goals: (1) The in situ formed
Co nanoclusters serve as barriers to hinder the agglomeration and
coarsening of Sn, therefore highly improve the area of conversion
reaction interface of Na2S and metallic Sn0. Thus, the reversible
capacity for the first cycle and initial Coulombic efficiency were
greatly promoted to 637.65 mA h/g and 65.3%, respectively,
compared to SnS2 anode of 228.08 mA h/g and 38.5%. (2) The in situ
formed Co acts as favorable electron conductor uniformly distrib-
uting in the whole microflower and stayed stable since the first
charge process, even without any addition of carbonous materials.
(3) 3D microflower structure supports the 2D nanosheets to pre-
vent the stack of nanosheets during the repeated cycles and gua-
rantees an enlarged surface area. 2D nanosheets could also offer the
benefits of shortened electron and Naþ transport distance and
enhanced reaction kinetics. As a result, the SnCoS4 composite
showed remarkable reversible capacity of 477.76 mA h/g after 60
cycles at 0.1 A/g, high capacity retention of 75% since the second
cycle, and enhanced initial Coulombic efficiency of 65.3% (got a
69.6% enhancement compared to SnS2 anode).
Scanning electron microscopy (Hitachi SU-70), transmission
electron microscopy (Tecnai G2 F30) and high resolution trans-
mission electron microscopy (JEOL JEM-2011) were applied to test
the morphology and micro-structure. X-ray diffraction (XRD) ex-
periments of the samples were performed on an X’Pert Pro X-ray
diffractometer (PANalytical, the Netherlands) with Cu K
The XPS spectra were carried out by VG ESCALAB MARK II with a
monochromated Al K X-ray source.
a radiation.
a
2.3. Electrochemical measurement
The electrochemical tests were performed using CR2025 coin-
type cells, which were assembled in an argon-filled glove box.
The working electrodes were prepared by mixing the as-prepared
active materials, acetylene black and sodium alginate under the
weight ratio of 70: 15: 15 in DI water. Then the slurry was uniformly
coated on the copper foil, last dried in a vacuum oven at 80 ꢀC for
overnight. Electrochemical measurements were carried out using
two-electrode coin cells with sodium metal as counter and refer-
ence electrode. The electrolyte comprised of a solution of 1 M
NaClO4 in ethylene carbonate/diethyl carbonate (1/1; v/v) with
10 wt % fluoroethylene carbonate. Glass microfiber film (GF/D,
Whatman) was used as the separator. The cells were galvanostati-
cally charged and discharged at a LAND CT2001A battery tester in
the potential range of 0.01e3.00 V (vs. Na/Naþ) at different constant
current densities. Cyclic Voltammetry (CV) measurement was
tested by an electrochemical workstation (CHI600E) from 0.01 to
3.00 V (vs. Na/Naþ) at a scan rate of 0.1 mV/s. Electrochemical
impedance spectroscopy measurement was also performed on the
same electrochemical workstation over a frequency range between
100 kHz and 0.1 Hz with an amplitude of 5.0 mV.
3. Results and discussion
The morphology of as-prepared SnCoS4, CoS2 and SnS2 samples
was characterized by scanning electron microscope (SEM). As
shown in Fig. 1a and b, the SnCoS4 composite shows a uniform
flower-like morphology and high monodispersity, with an average
size of about 1.0e1.5 mm. As the time of solvothermal prolonged,
the nanosheets self-assembled into a microflower. This unique
structure could prevent the nanosheet subunit from the agglom-
eration and stack. The nanosheets display a thickness of about
30 nm. The SnS2 sample (Fig. 1c and Fig. S1) displays the
morphology of curved sheets clustered into a littery paper-ball
shape. This evidence illustrate that SnS2 layer structure tends to
agglomerate to reduce its surface area to keep a thermodynamic
stability, which would limit the quantity of electrochemical active
sites and slower the transport and diffusion of Naþ. The
morphology of CoS2 sample (Fig. S2) was granular with an average
size of 30e50 nm.
The crystal structures of obtained samples were determined by
X-ray diffraction (XRD). Fig. 1d shows the XRD pattern of SnCoS4
microflower, SnCoS4 sample exhibits four major diffraction peaks
centered at 17.07ꢀ, 28.74ꢀ, 32.67ꢀ and 50.86ꢀ. The number and the
position of diffraction peaks match well with the previous reported
2. Experimental
2.1. Materials synthesis
All the reactants and solvents were of analytical reagent grade
and were used without further purification. The SnCoS4 micro-
flower composite was prepared through a facile one-pot sol-
vothermal approach. Typically, 3 mmol of cobaltous chloride
hexahydrate, 3 mmol of tin (IV) chloride pentahydrate, and
12 mmol of L-cysteine were added into 70 mL of ethylene glycol
solution under magnetic stirring for 6 h. The obtained solution was
then transferred to a 100 mL Teflon tank and sealed in autoclave,
which was kept in an oven at 180 ꢀC for 12 h. The resulting

J. Zhong et al. / Journal of Alloys and Compounds 825 (2020) 154104
3
Fig. 1. aeb) SEM images of SnCoS4 microflower composite, c) SEM images of SnS2, d) XRD pattern of SnCoS4 and SnS2 composite, e) TEM image of SnCoS4 microflower composite, f)
EDS elemental mapping of SnCoS4 microflower composite: cobalt, tin, sulfur.
work of SnCoS4/graphene composites [30], which demonstrates the
diffraction peaks should be assigned to the pure phase of SnCoS4.
The strong diffraction peaks of the sample without addition of
CoCl2 centered at 28.43ꢀ, 32.30ꢀ, 41.80ꢀ, 50.00ꢀ, 52.37ꢀ, 58.41ꢀ and
60.67ꢀ could be indexed to the specific lattice planes of SnS2 phase
(JCPDS card No. 40e1467). Transmission electron microscopy
(TEM) was applied to further confirm the microstructure of SnCoS4
material. As shown in Fig. 1e, the SnCoS4 displayed a microflower
morphology self-assembled by nanosheet subunits, which is
consistent with the SEM images. Even though the sample went
through ultrasonic treatment when it was prepared, it still exhibits
its original morphology, demonstrating the excellent structure
stability. From the contrast of TEM image, we could find the 2D
layered structure is extremely thin, which could contribute to the
fast and easy electrolyte penetration as well as shorten the diffu-
sion distance of electron/ion. High-resolution transmission electron
microscopy (HRTEM) was applied to investigate the lattice fringe of
SnCoS4 material. As shown in Fig. S3. The HRTEM image shows high
crystallinity of the prepared sample. The parallel atomic planes can
be clearly identified, and the lattice planes with a d-spacing of
0.32 nm and 0.27 nm could be well assigned to SnCoS4 phase [30].
This evidence is clearly verified with the result of XRD pattern.
Energy dispersive spectrometer (EDS) was applied to examine the
elemental distribution of SnCoS4 composite. As shown in Fig. 1f, the
EDS mapping spectra of the SnCoS4 microflower composite illus-
trates that Co, Sn, S elements are homogeneously distributed
within the whole structure.
To investigate the Naþ charge and discharge reactions with the
SnCoS4 microflower electrode, SnS2 electrode and CoS2 electrode, a
cyclic voltammetry (CV) test was performed in the potential range
of 0.01e3.0 V (vs. Naþ/Na) at a scan rate of 0.1 mV/s. CV curves of
the SnCoS4 electrode, SnS2 electrode and CoS2 electrode were dis-
played in Fig. 2(aec), respectively. For SnCoS4 electrode (Fig. 2a), in
the first cathodic scanning, the peaks at 1.3 V and 1.8 V are attrib-
uted to the insertion of sodium-ions into SnCoS4. The broad
reduction peak from 0.5 V to 1.0 V are corresponding to the for-
mation of metallic Sn and Co nanoclusters distributing in Na2S
matrix, and a further sodiation reaction of Sn nanograins on the

4
J. Zhong et al. / Journal of Alloys and Compounds 825 (2020) 154104
Fig. 2. CV curves of the a) SnCoS4 electrode, b) SnS2 electrode and c) CoS2 electrode measured in the potential range of 0.01e3.0 V (vs. Na/Naþ) at a scan rate of 0.1 mV/s.
interface of Na2S/Sn0 [30]. In the first anodic scanning, the peak at
0.07 V and broad peak at 1.2 V are associated with the multi-step
dealloying reaction of NaxSn, the recrystallization of Sn nano-
grains and the reconversion reaction between metallic Sn and Na2S
[31]. In this stage, Co nanoclusters act as excellent electronic con-
ductors and stabilize the Sn nanograins. Overall cathodic and
anodic reactions could be represented as follows:
the first curve and second curve is mainly because the formation of
solid electrolyte interphase (SEI) film on the surface of the CoS2
electrode.
To investigate the function of Co nanoclusters of SnCoS4 elec-
trode generated in the first discharge process, we conducted the
galvanostatic charge-discharge voltage profiles of the SnCoS4, SnS2
and CoS2 electrodes at a current density of 0.1 A/g, in the potential
range of 0.01e3.0 V (vs Na/Naþ). Fig. 3a-c exhibits the galvanostatic
charge-discharge voltage profiles of the SnCoS4, SnS2 and CoS2
electrodes, respectively. The galvanostatic charge-discharge voltage
curves of SnCoS4 electrode showed two evident plateaus (Fig. 3a)
around at 0.5e1.0 and 1.0e1.5 V, which was consistent with the
characteristic voltage position of CV curves. The second and third
curves overlapped very well, which indicated the high reversibility
of SnCoS4 electrode. It delivered a high initial discharge capacity of
975.83 mA h/g and reversible capacity of 637.65 mA h/g, with an ICE
of 65.3%. The irreversible sodium-ion loss was ascribed to the solid
electrolyte interphase (SEI) layer formation in the surface of SnCoS4
electrode. As shown in Fig. 3b, SnS2 electrode delivered the first
discharge and charge capacities of 591.66 and 228.08 mA h/g,
respectively. The ICE value of SnS2 electrode was only 38.5%. The
low ICE of SnS2 electrode should be attributed to the coarsening of
Sn nanograins leading to the irreversibility of conversion reaction.
As displayed in Fig. 3c, the first discharge and charge capacities was
630.24 and 338.27 mA h/g, respectively, with an ICE of merely
54.14% for CoS2 electrode. Compared to SnS2 anode, SnCoS4 elec-
trode shows an enhancement of 69.6% in initial Coulombic effi-
ciency. The charge/discharge potential hystereses of SnCoS4, SnS2
and CoS2 electrodes were demonstrated in Fig. 3d. At 50% state of
charge/discharge, this potential hysteresis of SnCoS4 is only 0.49 V,
which is substantially lower than 0.7 V for SnS2 and 1.24 V for CoS2,
respectively. The ICE of SnCoS4 electrode is 69.6% higher than that
of SnS2 anode. Especially, when it compares to previously reported
SnCoS4 þ (8þ x)Naþ þ (8þ x)e-/ NaxSn þ Co þ 4Na2S
NaxSn þ 4Na2S - (x þ 8)eꢁ 4 Sn þ 4S þ (x þ 8)Naþ
Sn þ 2Na2S 4 SnS2 þ 4Naþ
(1)
(2)
(3)
For SnS2 electrode (Fig. 2b), in the first cathodic curve, a small
peak at 1.7 V was ascribed to the intercalation of sodium-ions into
the (001) plane [32]. The broad peak at around 0.6 V is attributed to
the formation of amorphous Na2S and metallic Sn nanoparticles,
and then further underwent the sodiation process to form Na15Sn4.
During the first anodic sweep, the peaks at 0.07 V and 1.15 V are
associated with the multi-step dealloying reaction of Na15Sn4 and
the reformation of SnS2 [10,32,33]. Compared the areas of the first
two CV curves, we could observe the large irreversible capacity loss
of the initial sodiation/desodiation process. This evidence illus-
trated a poor ICE of SnS2 electrode. For CoS2 electrode (Fig. 2c), in
the discharge process, the peaks at 1.25 V should be assigned to
insertion of sodium-ions into CoS2 to form the intermediate
product NaxCoS2 [34]. In addition, the reduction potentials at 0.57 V
match well with potential positions of the displacement reaction of
Co and Na2S. In the anodic curves, the oxidation broad peaks from
1.8 V to 2.1 V correspond to the reverse reaction and recovery of
CoS2 [34e37]. The intermediate product Co plays a role in an
electron conductor and inactive metal [38]. The difference between

J. Zhong et al. / Journal of Alloys and Compounds 825 (2020) 154104
5
Fig. 3. The galvanostatic charge-discharge voltage profiles of the a) SnCoS4, b) SnS2, c) CoS2 electrodes at a current density of 0.1 A/g, in the potential range of 0.01e3.0 V (vs Na/
Naþ). d) The charge-discharge potential hysteresis of SnCoS4, SnS2 and CoS2 electrodes at a current density of 0.1 A/g.
SnS2/CC anode with a low ICE of 32.8%, the 2D SnS2/CNTs electrode
with corresponding ICE of 46.7%, and 3D SnS2/CNTs electrode with
ICE of 39.5%, SnCoS4 electrode achieves a great enhancement
[33,39]. The generation of Co nanoclusters could greatly reduce the
polarization of electrodes and increase the reversibility of conver-
sion reactions, thus leading to an enhanced initial Coulombic effi-
ciency of anode [40,41]. Galvanostatic intermittent titration
technique (GITT) measurements of SnCoS4 and SnS2 were exhibited
in Figs. S5eS6. The dotted lines represent quasi-equilibrium po-
tentials after fully relaxation for 5 h at open circuit, which are
approximately equal to thermodynamic values. At 50% state of
discharge, the potential hysteresis of SnCoS4 (Fig. S5) is only
0.29 V at the current density of 100 mA/g for 0.5 h followed by a 5 h
rest, which is much lower than that of SnS2 (0.41 V) at the current
density of 100 mA/g.
Fig. 4a reveals the cycling performances of SnCoS4, SnS2 and
CoS2 electrodes at the current density of 100 mA/g. SnCoS4 elec-
trode delivers a superior electrochemical performance retaining a
reversible capacity of 477.76 mA h/g after 60 cycles. Nearly 75%
capacity retention was achieved compared to the second cycle.
After 60 cycles, SnS2 electrode displayed reversible capacities of
224.38 mA h/g, and CoS2 electrode only showed reversible capac-
ities of 83.02 mA h/g, with a poor capacity retention of 24.5%.
Although SnS2 electrode displays a relatively stable cycling per-
formance, its irreversible capacity loss accounts for nearly two
thirds of its first discharge capacity. The great enhancement of
irreversible capacity and cycling performance of SnCoS4 electrode
was mainly attributed to the follows: 1) The unique 3D flower-like
microstructure enlarged the contact area between the electrode
and electrolyte, thus exposed plentiful electrochemical active sites,
contributing to the improvement of sodium-ion storage capacity. 2)
3D microflowers also played a role in supporting the 2D nanosheets
and accommodating more volumetric change during sodiation/
desodiation reactions, which had meaningful contribution to
cycling stability. 3) Due to the generation of Co nanoclusters, the
reversibility of conversion reactions was successfully increased,
providing higher reversible capacity. 4) The synergistic effect of
SneCoeS bimetal system supplied more abundant redox reactions
compared to monometallic sulfide SnS2.
Fig. 4b reveals the rate capabilities of SnCoS4, SnS2 and CoS2
electrodes at various current densities of 0.05, 0.1, 0.2, 0.5 and 1.0 A/
g for 10 cycles each. For SnCoS4 anode, a reversible capacity of
687.69 was achieved at 0.05 A/g. When the current densities
increased to 0.1, 0.2, 0.5 and 1.0 A/g, the reversible capacities of
623.80, 562.30, 453.96, 347.62 mA h/g were delivered, respectively
(Fig. 4b). When the current rate went back to 0.05 A/g, the specific
capacity recovered to 616.66 mA h/g. It equals to a capacity reten-
tion of 89.67% compared with the initial reversible capacity, which
demonstrates an extraordinary rate capability and structural sta-
bility of SnCoS4 anode. SnS2 and CoS2 electrodes were also tested
under 0.05e1.0 A/g conditions, and they manifested inferior rate
capability, delivering specific capacities of 248.41, 229.36, 198.81,
146.82, 104.36 mA h/g and 385.3, 333.33, 272.22, 144.44,
37.76 mA h/g, respectively. Fig. S4 shows the charge/discharge
profile at various current densities within 0.01e3.0 V for SnCoS4.
Electrochemical impedance spectroscopy (EIS) tests were con-
ducted to explore the kinetics performance of SnCoS4 and SnS2
electrodes reacting with sodium-ions. Fig. 4c shows the Nyquist
plots. Every plot consists of a semicircle in the middle frequency
area and an inclined line in the low frequency range. The compo-
nents of the equivalent circuit are exhibited in the inset of Fig. 4c.
Charge transfer resistance (Rct) represents ions and electrons
transport on the interface of electrolyte and electrode materials.
Apparently, owing to the enhanced conductivity of bi-metal sulfide
SnCoS4 electrode, the value of Rct is lower than SnS2 electrode. A
lower Rct of the SnCoS4 electrode can also explain its better rate

6
J. Zhong et al. / Journal of Alloys and Compounds 825 (2020) 154104
Fig. 4. a) Cycling performances of SnCoS4, SnS2 and CoS2 electrodes at the current density of 0.1 A/g. b) Rate capabilities of SnCoS4, SnS2 and CoS2 electrodes at current densities of
0.05, 0.1, 0.2, 0.5, 1.0 A/g, respectively. c) The Nyquist plots of the SnCoS4 and SnS2 electrodes. d) The relationship lines between Z0 vs.
and SnS2 electrodes.
u
ꢁ1/2 in the low frequency range of the SnCoS4
capacity compared to other electrodes. The sharper inclined line in
the low frequency range of SnCoS4 is ascribed to the supported 2D
nanosheets by the 3D flower-like structure and enlarged surface
area, so that the sodium-ion diffusion process could be accelerated.
The advantages of introducing SneCoeS system could be sum-
marized as follows: Tin sulfide provides high specific capacity for
the anode. While the generation of Co nanoclusters serve as bar-
riers to hinder the migration and coarsening of Sn into large grains
during repeated cycling, therefore greatly reduce the polarization
of electrodes and increase the reversibility of conversion reactions,
leading to an enhanced initial Coulombic efficiency compared to
SnS2 anode. For another thing, the in situ formed Co acts as favor-
able electron conductor, so the anode could stay high conductivity
even without addition of carbon-based materials.
Fig. 4d shows the relationship lines between Z’ vs. u
ꢁ1/2 in the low
frequency range of the SnCoS4 and SnS2 electrodes. The sloping line
in low frequency indicates the Warburg impedance [39]. The larger
Warburg diffusion impedance of SnS2 electrode than that of SnCoS4
was attributed to shortened electron/ion transport pathways and
facilitated electrolyte penetration of 3D flower-like structure of
SnCoS4. To figure out the sodium-ion diffusion coefficients (D) of
three electrodes, we used the following equation:
D ¼ R2T2 / 2n4F4A2C2s2
4. Conclusions
In summary, a unique 3D flower-like SnCoS4 anode material was
successfully synthesized by one-pot solvothermal method. Due to
the generation of Co nanoclusters during the first discharge pro-
cess, the reversibility of conversion reactions was greatly improved.
The flower-like SnCoS4 anode material delivers a high sodium-ion
storage reversible capacity of 477.76 mA h/g after 60 cycles with
an initial Coulombic efficiency of 65.3%, in which is increased to
over 69% compared to the SnS2 anode. Employing the synergistic
effect of bi-metal sulfides for effectively enhancing the initial
Coulombic efficiency and reversible capacity of anodes provides
promising solutions of exploration for the next generation high-
performance SIBs.
Where R is the gas constant, T is room temperature, A is the surface
area of the electrode, n is the number of transfer electrons of redox
reaction per molecular, F is Faraday’s constant, C is the concentra-
tion of sodium ions, and
of of SnCoS4 and SnS2 electrodes was calculated, 1122.99 and
1298.59
s is the Warburg factor [42,43]. The value
s
U
/s1/2, respectively. Therefore, diffusion coefficients of
sodium ion in SnCoS4 and SnS2 were 2.57 ꢂ 10ꢁ17 cm [2]/s and
1.92 ꢂ 10ꢁ17 cm2/s, respectively, which illustrates a faster and easier
electrons/ions diffusion on the electrode materials. The kinetic
parameters are displayed in Table 1.
Table 1
Declaration of competing interest
Kinetic parameters of SnCoS4 electrode and SnS2 electrode.
Electrode
Rs(
U
)
Rct
(
U
)
s
(
U
/s1/2
)
D (cm [2]/s)
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
SnCoS4
SnS2
3.329
3.172
63.5
87.7
1122.99
1298.59
2.57 ꢂ 10ꢁ17
1.92 ꢂ 10ꢁ17

J. Zhong et al. / Journal of Alloys and Compounds 825 (2020) 154104
7
[18] J.F. Mao, T.F. Zhou, Y. Zheng, H. Gao, H.K. Liu, Z.P. Guo, Two-dimensional
nanostructures for sodium-ion battery anodes, J. Mater. Chem. A 6 (2018)
3284e3303.
CRediT authorship contribution statement
Jiayi Zhong: Methodology, Data curation, Writing - original
draft. Xuezhang Xiao: Methodology, Data curation, Writing -
original draft, Writing - review & editing. Zunchun Wu: Visuali-
zation, Investigation. Nan Zhang: Software, Validation. Ruicheng
Jiang: Software, Validation. Xiulin Fan: Writing - review & editing.
Lixin Chen: Supervision, Writing - review & editing.
[19] M. Wang, Y. Huang, Y. Zhu, X. Wu, N. Zhang, H. Zhang, Binder-free flower-like
SnS2 nanoplates decorated on the graphene as a flexible anode for high-
performance lithium-ion batteries, J. Alloys Compd. 774 (2019) 601e609.
[20] W. Wang, J. Zhang, B. Li, L. Shi, Electrochemical investigation of Sn-Co alloys as
anode for Na-ion Batteries, J. Alloys Compd. 780 (2019) 565e569.
[21] T. Liang, R. Hu, H. Zhang, H. Zhang, H. Wang, Y. Ouyang, J. Liu, L. Yang, M. Zhu,
A scalable ternary SnO2eCoeC composite as a high initial coulombic effi-
ciency, large capacity and long lifetime anode for lithium ion batteries,
J. Mater. Chem. A 6 (2018) 7206e7220.
Acknowledgements
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The authors gratefully acknowledge the financial supports for
this research from the National Natural Science Foundation of
China (51671173), and the Program for Innovative Research Team in
University of Ministry of Education of China (IRT13037).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.154104.
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