SnS2 has been investigated as anode material for lithium ion batteries. To obtain anode material having high
capacity and long-term lifespan, in the present work, a nanostructured composite comprising SnS2 nanosheets and
CMK-3 ordered mesoporous carbon has been designed. The CMK-3/SnS2 composite is fabricated via incipient wetness
impregnation, followed by chemical reduction and chemical conversion in an inert gas at high temperature. The
obtained composite exhibits boosted lithium storage behaviors involving high specific capacity, fast rate response and
stable cyclability. At a discharge-charge rate of 100 mA g-1, the CMK-3/SnS2 electrode delivers a specific capacity of
985.2 mA h g-1 with the utilization efficiency of SnS2 present in the composite of 90.5 %. Even, after 500 cycles testing
at higher discharge-charge rates of 0.5 and 1 A g-1, CMK-3/SnS2 still can maintain specific capacities of 556.2 and
402.9 mA h g-1, respectively, much higher than the theoretical specific capacity of commercialized graphite anode
material. This work demonstrates considerable application potential of the CMK-3/SnS2 anode in Li-ion batteries.
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Cite this paper: Vietnam J. Chem., 2020, 58(5), 622-629 Article
DOI: 10.1002/vjch.202000050
622 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH
Fabrication and lithium storage performances of a composite of
tin disulfide and ordered mesoporous carbon
Le Thi Thu Hang
*
, Hoang Thi Bich Thuy, Dang Viet Anh Dung, Dang Trung Dung
School of Chemical Engineering, Hanoi University of Science and Technology,
1 Dai Co Viet, Hai Ba Trung district, Hanoi 10000, Viet Nam
Submitted March 31, 2020; Accepted April 28, 2020
Abstract
SnS2 has been investigated as anode material for lithium ion batteries. To obtain anode material having high
capacity and long-term lifespan, in the present work, a nanostructured composite comprising SnS2 nanosheets and
CMK-3 ordered mesoporous carbon has been designed. The CMK-3/SnS2 composite is fabricated via incipient wetness
impregnation, followed by chemical reduction and chemical conversion in an inert gas at high temperature. The
obtained composite exhibits boosted lithium storage behaviors involving high specific capacity, fast rate response and
stable cyclability. At a discharge-charge rate of 100 mA g
-1
, the CMK-3/SnS2 electrode delivers a specific capacity of
985.2 mA h g
-1
with the utilization efficiency of SnS2 present in the composite of 90.5 %. Even, after 500 cycles testing
at higher discharge-charge rates of 0.5 and 1 A g
-1
, CMK-3/SnS2 still can maintain specific capacities of 556.2 and
402.9 mA h g
-1
,
respectively, much higher than the theoretical specific capacity of commercialized graphite anode
material. This work demonstrates considerable application potential of the CMK-3/SnS2 anode in Li-ion batteries.
Keywords. Tin disulfide, lithium storage, composite, cyclability.
1. INTRODUCTION
Since the first commercialization in 1991, lithium
ion batteries (LIBs) have been regarded as the most
popular storage technology for mobile and portal
applications including mobile phones, laptops,
digital cameras because of their low self-discharge,
high energy density, nearly zero-memory effect, and
long lifespan.
[1]
Graphite is established as a main
anode active material for commercial LIBs since it
possesses some advantages such as low working
potential, low cost, and high stability. Nevertheless,
it also exhibits some disadvantages such as high
lithium ion diffusion resistance, harmful lithium
dendrite growth on the graphite surface, and low
theoretical specific capacity (372 mA h g
-1
).
[2]
To
meet the growing demand on high power density
and high energy density, the development of new
electrode active materials with outstanding
electrochemical properties is indispensable.
Much effort has been dedicated to developing
alternatives to graphite anode for LIBs. In this
context, conversion-alloying anode materials have
recently received great attention, especially, SnS2, a
tin based anode material since it possesses
outstanding properties such as high specific capacity
(1230 mA h g
-1
), low cost, low working potential,
environmental friendliness, and ready availability.
[3]
Unfortunately, this material also suffers from large
stress caused by huge volume expansion during
lithiation process, which is also found for other
alloy-type anode materials. This leads to the
pulverization phenomenon of the active material,
electrical disconnection between active material
particles and a current collector, deterioration of the
solid electrolyte interphase (SEI) layer on the
electrode, and, eventually, significant degradation of
the electrochemical performance.
So far, some strategies have been supposed to
solve the drawbacks of SnS2. They include
fabrication of nanostructure,
[4]
fabrication of
composites of SnS2 with other active/inactive
materials,
[5]
fabrication of doped SnS2
[6]
and
structure design for SnS2-based materials.
[7]
In this work, we report a facile routine to
synthesize the composite of SnS2 and CMK-3, a
kind of ordered mesoporous carbon, with the aim of
application as high performance-anode material for
LIBs. For this designed composite, SnS2 nanosheets
are filled inside and deposited partially outside
mesopores of CMK-3 framework, which works as
nanocage to confine the SnS2 nanosheets. Owing to
high porosity and high stable structure of CMK-3,
the volume expansion of SnS2 nanosheets during
Vietnam Journal of Chemistry Le Thi Thu Hang et al.
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 623
lithiation process can be alleviated. Accordingly, the
composite exhibits remarkable lithium storage
behaviors in term of cyclability, rate capability, and
capacity.
2. MATERIALS AND METHODS
2.1. Synthesis of CMK-3/SnS2 composite
CMK-3/SnS2 composite was fabricated via incipient
wetness impregnation technique, following by heat
treatment at high temperature. Where, CMK-3 as
supporting material was synthesized by hard
template method, which was described in the
literature.
[8]
In detail, CMK-3 powder (0.5 g) was
added in 5 mL of a 50v/50v ethanol/water mixture
containing SnCl4 and ultrasonicated for 30 mins.
After drying at 80 C for 5 h, the resultant powder
continued to be mixed well with 2 g of NaBH4, and
then storage in the humid air for 48 h. Because
NaBH4 is a highly hygroscopic substance in
combination with the presence of oxygen in the air,
the CMK-3/SnO2 composite could be easily formed
according to chemical reactions as follows:
[9]
SnCl4 + 4NaBH4 → Sn +4NaCl +2B2H6 +2H2 (1)
Sn + O2 → SnO2 (2)
After that, the CMK-3/SnO2 sample was rinsed,
and dried at 100
o
C, followed by a heat treatment
process in nitrogen atmosphere at 350
o
C for 3 h.
Prior to the heat treatment, CMK-3/SnO2 powder
was put into a small alumina crucible, which was
surrounded by a certain amount of thiourea in a large
covered alumina crucible. At such a high
temperature, thiourea was completely decomposed
to convert SnO2 to SnS2 via the following
reactions:
[10,11]
(NH2)2CS → NH3 + HNCS (3)
(NH2)2CS → H2S + C(NH)2 (4)
SnO2 + 2H2S → SnS2 + 2H2O (5)
Next, the sample was washed thoroughly by
mixture of ethanol and deionized water to remove
the undesired byproducts. Finally, the CMK-3/SnS2
composite was obtained by drying at 120
o
C for 24 h
under vacuum.
2.2. Microstructure characterizations
Morphological, chemical and structural
characterizations were performed on a scanning
electron microscope (SEM, Hitachi S-4700/EX-200,
Japan), a X-ray photoelectron spectrometer (XPS,
Multilab 2000, VG, UK) and a high-resolution X-ray
diffractometer (D/MAX Ultima III, Japan),
respectively. The thermal stability of the sample was
measured in a temperature range, up to 900
o
C in the
air atmosphere. This thermogravimetric analysis
(TGA) was conducted on a thermogravimetric
analyzer (TGA-50, Shimadzu, Japan).
2.3. Electrochemical characterization
Lithium storage behaviors of the CMK-3/SnS2
composite were measured using CR2032-coin cells.
The working electrodes were fabricated via casting
method. A mixture of CMK-3/SnS2 (electrode active
material), carbon super P (conducting agent) and
lithium polyacrylate (binder) with a mass ratio of
8:1:1 was mixed well in deionized water to form a
black slurry, which was accompanied by a casting
process using a doctor blade on a copper foil. After
being dried at 100
o
C at a vacuum oven, the foil was
cut into dishes with a 14-mm diameter. Each coin
cell was comprised of a working electrode, a Li
metal foil as counter/reference electrode and a
separator. The electrolyte used in the cell was an
organic 50v/50v mixture solution of dimethyl
carbonate (DMC) and ethylene carbonate (EC)
containing 1 M LiPF6 and 5 wt% fluoride ethylene
carbonate (FEC) additive. The cell assembly process
was implemented in an argon gas-tight glove box.
The loading mass of the working electrodes was
fixed around 1.0 mg cm
-2
. The specific capacities of
the electrodes were calculated, based on the mass of
the active material.
Two electrochemical measurement techniques
including cyclic voltammetry (CV) and
galvanostatic charging-discharging (GCD) were
employed to investigate the lithiation/delithiation
mechanism as well as discharge-charge response of
the synthesized materials. The CV measurements
were preceded on a Gamry Potentiostat instrument
while GCD measurements were conducted on a
battery cycler (WonATech, WBCS 3000).
3. RESULTS AND DISCUSSION
To verify the crystalline structure as well as phase
purity of the synthesized CMK-3/SnS2 composite,
XRD analysis was used. Figure 1 showed XRD
pattern of CMK-3/SnS2. For comparison, the XRD
pattern of CMK-3 supporting material was included.
Obviously, the reflection peaks of CMK-3/SnS2 all
matched well to the standard lines of SnS2 phase
with the hexagonal structure (JCPDS card No. 01-
089-2358, space group: P-3m1). It is noted that both
two typical diffraction peaks located at 24
o
and 44
o
of CMK-3
[12]
were not recognized in the XRD
pattern of CMK-3/SnS2. This probably stems from
Vietnam Journal of Chemistry Fabrication and lithium storage performances
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 624
the covering of SnS2 nanosheets on the surficial
CMK-3, as well as the low content of CMK-3
present in the composite, all of which were further
confirmed by SEM and TGA results as below.
To investigate the morphology of CMK/3-SnS2,
the samples were examined by SEM. Figure 2
presents SEM images of the resultant CMK-3/SnS2
composite. As seen in figures 2a,b, CMK-3 was
composed of uniformly short nanorods. The length
of the nanorods was around 1 µm. Each nanorod
was supposed to be constructed by primary carbon
particles with sizes of ~10 nm. These particles were
arranged in a well-ordered structure (inset in figure
1b) to form a typical hexagonal shape, which is
regarded as the reversible replication of SBA-15
hard template.
[13]
Figure 1: XRD patterns of CMK-3 and
CMK-3/SnS2 composite
Figure 2: Low- and high-resolution SEM images of
CMK-3 and CMK-3/SnS2 composite
After loading SnS2, the surface of nanorods seem
to be covered by SnS2 nanosheets, which possessed
the thickness of about 30 nm (figures 2c,d). Due to
the presence of SnS2 nanosheets, the voids between
the adjacent nanorods were formed to be relatively
large. This suggests that such a structure of the
CMK-3/SnS2 induce the contact of electrolyte with
the active substance to become easier.
[14]
In order to determine the thermal stability as
well as the practical composition of CMK-3/SnS2,
TGA was implemented in the air atmosphere at a
heating rate of 10
o
C min
-1
. The obtained TGA result
is displayed in figure 3. It is clear to observe that
there were four main weight losses. The first weight
loss of 3.2 wt.%, occurred at around the temperature
of 95
o
C. This process is associated to the
vaporization of water molecules absorbing
physically at the surface of the sample. The second
weight loss started at 415
o
C, corresponding to the
oxidation process of SnS2 in the CMK-3/SnS2
composite.
[3,15]
The further weight losses recorded at
an onset temperature range of 527
o
C was assigned
to the combustion of CMK-3 present in the CMK-
3/SnS2 composite.
[16]
At a temperature range of 527-
660
o
C, in addition to the combustion reaction of
CMK-3, the oxidation process of SnS2 still occurred
to form SnO2.
[3]
At temperatures above 660
o
C, the
combustion and oxidation processes completed, and
the weight of the sample almost unchanged.
Figure 3: TGA plot of CMK-3/SnS2 composite in
the air atmosphere
C + O2 → CO2 (6)
SnS2 +3O2→ SnO2 + 2SO2↑ (7)
Based on the TGA result, the content of SnS2
present in the composite was calculated to be 73.41
wt.%.
XPS is a powerful method widely used for the
surface analysis of materials. In the present work,
XPS enabled verifying the chemical state of the
synthesized CMK-3/SnS2 composite. As shown in
figure 4a, wide survey XPS spectrum, the presence
of the constituent elements of CMK-3/SnS2 such as
C, Sn, and S was detected. Remarkably, the
appearance of O and N elements was recorded as
well. The existence of O element can be explained
by the surface functionalization of CMK-3 during
Vietnam Journal of Chemistry Le Thi Thu Hang et al.
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 625
the synthesis process. Meanwhile, the presence of N
element was caused by the doping N process
occurring when thiourea was decomposed at the
high temperature in the synthesis process of the
CMK-3/SnS2 composite. The result obtained in this
study is totally consistent with those reported
previously.
[17,18]
In figure 4b, the Sn 3d-core-level
XPS spectrum was deconvoluted into two peaks of
Sn 3d5/2 at 487.0 eV and Sn 3d3/2 at 495.5 eV, which
are characteristic of Sn
4+
. Furthermore, a satellite
peak was observed at 497.5 eV. On the other hand,
the S 2p spectrum in figure 4c illustrates the
appearance of S 2p3/2 at a binding energy of 162.1
eV and that of S 2p1/2 at a binding energy of 163.3
eV. These results proved the presence of SnS2
component in the synthesized composite.
[14]
In figure
4d, the C 1s-core-level XPS spectrum was separated
into three components at binding energies of 284.6,
286.4 and 288.7 eV. These peaks represent C=C,
C–N/C–O, and C=N/C=O functional groups.[12,19]
This is evidence of the doping N atoms into C
networks of CMK-3 frame work. According to the
deconvoluted N 1s XPS in figure 4e, there were
three bonding types between N and C. In particular,
the peaks at 399.1 eV and 400.6 eV are associated to
pyridinic N and pyrrolic/pyridonic N
Figure 4: XPS spectra of CMK-3/SnS2 composite: (a) Wide survey, (b) Sn3d, (c) S 2p, (d) C 1s, (e) N 1s,
(f) Illustration of N bond types in the C network of CMK-3
bonds, respectively (figure 4f).
[20]
According to the
previous reports,
[21-22]
doping N heteroatoms into C
networks would increase the number of active sites
for lithium storage as well as the electrical
conductivity for the carbonaceous materials. Hence,
the doping N was expected to benefit for the
electrochemical performances of the synthesized
CMK-3/SnS2 composite.
For investigation of Li insertion/extraction
behavior, the CMK-3 and CMK-3/SnS2 composite
electrodes were tested by CV method at the potential
between 0.01 and 3 V vs. Li
+
/Li at a scan rate of 0.1
mV s
-1
for five cycles, with open circuit potential
(OCP) as a starting point. The measured results are
presented in Figure 5. As seen in Figure 5a, in the
first cycle, the CMK-3 electrode has two cathodic
peaks at 1.65 V and 0.7 V vs. Li
+
/Li. In the
following cycles, these peaks totally disappeared,
suggesting the irreversible formation of a SEI layer
on the surface of the electrode as well as the
presence of other irreversible parasitic reactions.
[12]
From the second cycle onwards, the shapes of the
CV plots appeared similar. In addition, these CV
plots in this work are the same to those of the
previous report.
[19]
In particular, the discharge
capacity of CMK-3 attained came from two parts: (i)
The discharge capacity is related to lithium storage
in cavities formed naturally, boundaries between
graphite crystallite domains in turbostratic structure
as well as in the diversity defects of pores, nanorods
of CMK-3 carbon.
[23]
This part was attained at the
potential above 0.45 V vs. Li
+
/Li; (ii) The another
came from the underpotential deposition
process of Li metal.
[12]
This contribution was gained
Vietnam Journal of Chemistry Fabrication and lithium storage performances
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 626
at the potential below 0.45 V vs. Li
+
/Li. As for the
anodic scan direction, delithiation process occurred
within a wide potential range from 0.01 to 3.0 V vs.
Li
+
/Li with the appearance of two broadening anodic
peaks located at 0.14 and 1.16 V vs. Li
+
/Li. The
former was characteristic of delithiation from
nanosized graphite-like domains. The latter
represented delithiation from pore structure and/or
defect sites within the carbon wall.
Figure 5: CV plots of CMK-3 and CMK-3/SnS2 composite
Figure 6: Discharge-charge voltage profiles of (a) CMK-3 and (b) CMK-3/SnS2 at a current density of 100
mA g
-1
. (c) Cyclability and (d) rate response of CMK-3 and CMK-3/SnS2 composite. (e) Long-term
cyclability of CMK-3/SnS2 at different rates of discharge-charge for 500 cycles
As for the CMK-3/SnS2 composite electrode, the
Li insertion/extraction was complex. At the first
cycle, five cathodic peaks appeared (figure 5b). The
first peak at 2.53 V vs. Li
+
/Li is assigned to the
formation of the SEI layer on the electrode surface.
Four next peaks at 1.85, 1.29, 0.32 and 0.13 V vs.
Li
+
/Li correspond to insert Li into SnS2 of the
composite as described below:
[24]
Li + electrolyte + xe
-
→ SEI layer (8)
SnS2 + xLi
+
+ xe
-
→ LixSnS2 (9)
LixSnS2 + 4xLi
+
+ (4-x)e
- ⇋ Sn + 2Li2S) (10)
Sn + 4.4Li
+
+ 4.4e
-
⇋ Li4.4Sn (11)
In the following cycles, the peaks corresponding
to the formation of the SEI layer and LixSnS2
compound disappeared. This indicates that these
processes were irreversible and only occurred at the
first discharge. Meanwhile, the rest cathodic peaks
exhibited a slight shift because of the structural
change of the active material after the first cycle.
For the anodic scan direction, two distinct anodic
peaks (0.52 V, and 1.89 V vs. Li
+
/Li) were recorded
for the five cycles of scanning. These peaks imply
Vietnam Journal of Chemistry Le Thi Thu Hang et al.
© 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 627
the dealloying of Li4.4Sn to form Sn, and the
oxidation of Sn to SnS2.
[25]
In addition, in
comparison with figure 5a, two characteristic anodic
peaks of CMK-3 were found in the CV plot of the
CMK-3/SnS2 composite.
Figures 6a,b illustrate the characteristic
galvanostatic discharge-charge curves of the CMK-3
and CMK-3/SnS2 composite electrodes at a current
density of 100 mA g
-1
for 100 cycles. As seen in
Figure 6a, as for the first cycle, the CMK-3 electrode
delivered specific discharge/charge capacities of
1276.8/628.8 mA h g
-1
. Accordingly, its initial
coulombic efficiency (CE) was calculated to be 49.2
%. This obtained low CE resulted from the
irreversible formation of the SEI layer and parasitic
reactions. Within first ten cycles, the specific
capacity had tendency to decrease rapidly for both
discharge/charge processes due to the marginal
structure deterioration of porous CMK-3.
[12]
In the
subsequent cycles, the microstructure stabilized
gradually. Accordingly, the discharge-charge
voltage curves almost overlapped, demonstrating
highly stable cyclability of the CMK-3. Noticeably,
no discharge/charge plateaus were observed for the
CMK-3 electrode.
In contrast, the CMK-3/SnS2 composite electrode
exhibited two discharge plateaus (~1.4 V and ~0.4
V) and two charge plateaus (~0.5 V and ~1.8 V).
The obtained results are accordance with the CV
analysis results above. In the first cycle, the CMK-
3/SnS2 could offered discharge/charge capaciti