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energies
Article
Electrochemical Performance of Graphene-Modulated Sulfur
Composite Cathodes Using LiBH4Electrolyte for
All-Solid-State Li-S Battery
Tarun Patodia 1,2,3, Mukesh Kumar Gupta 4, Rini Singh 5, Takayuki Ichikawa 5, Ankur Jain 1,2,6,*
and Balram Tripathi 7, *
Citation: Patodia, T.; Gupta, M.K.;
Singh, R.; Ichikawa, T.; Jain, A.;
Tripathi, B. Electrochemical
Performance of Graphene-Modulated
Sulfur Composite Cathodes Using
LiBH4Electrolyte for All-Solid-State
Li-S Battery. Energies 2021,14, 7362.
https://doi.org/10.3390/en14217362
Academic Editors: Carlos
Miguel Costa and Alvaro Caballero
Received: 16 August 2021
Accepted: 1 November 2021
Published: 5 November 2021
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
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Attribution (CC BY) license (https://
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4.0/).
1Natural Science Centre for Basic Research and Development, Hiroshima University,
Hiroshima 739-8530, Japan; 1987tkp@gmail.com
2School of Applied Science, Suresh Gyan Vihar University, Jaipur 302017, India
3Department of Physics, S.S. Jain Subodh College of Global Excellence, Jaipur 302022, India
4Department of Electrical Engineering, Suresh Gyan Vihar University, Jaipur 302017, India;
mukeshkr.gupta@mygyanvihar.com
5Graduate School of Engineering, Hiroshima University, Hiroshima 739-8527, Japan;
rinisingh.1910@gmail.com (R.S.); tichi@hiroshima-u.ac.jp (T.I.)
6Centre for Renewable Energy & Storage, Suresh Gyan Vihar University, Jaipur 302017, India
7Department of Physics, S.S. Jain Subodh P.G. (Autonomous) College, Jaipur 302004, India
*Correspondence: ankur.j.ankur@gmail.com (A.J.); balramtripathi1181@gmail.com (B.T.)
Abstract:
All-solid-state Li-S batteries (use of solid electrolyte LiBH
4
) were prepared using cathodes
of a homogeneous mixture of graphene oxide (GO) and reduced graphene oxide (rGO) with sulfur
(S) and solid electrolyte lithium borohydride (LiBH
4
), and their electrochemical performance was
reported. The use of LiBH
4
and its compatibility with Li metal permits the utilization of Li anode
that improves the vitality of composite electrodes. The GO-S and rGO-S nanocomposites with
different proportions have been synthesized. Their structural and morphological characterizations
were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the results
are presented. The electrochemical performance was tested by galvanostatic charge-discharge
measurements at a 0.1 C-rate. The results presented here demonstrate the successful implementation
of GO-S composites in an all-solid-state battery.
Keywords: Li-S battery; all-solid-state battery; solid electrolyte; LiBH4; reduced graphene oxide
1. Introduction
Lithium-sulfur (Li-S) batteries have attracted attention all over the world for the future
energy needs due to the material’s low cost, easy availability, non-hazardous nature and
high specific capacity of about 1675 mAh/g with a high theoretical specific energy density
of 2600 Wh/kg [
1
,
2
]. However, Li-S batteries are progressing slowly due to polysulfide
(PS) and dendrite formation, which significantly abbreviates the cycling life and raises
safety issues for Li-S batteries [
3
–
8
]. To overcome these issues, the trapping of sulfur inside
the porous carbon material matrix with different arrangements (for example, graphene,
graphene oxide, porous carbon, and carbon nanotubes) were employed [
9
–
12
]. Out of
these, GO and rGO have attracted extensive consideration [
13
–
17
] due to their stability,
lightweight, enormous surface territory, high electrical conductivity, incredible adaptability,
and mechanical properties compared to other carbon materials [
18
,
19
]. GO can easily be
deposited on the surface of the material and binds it due to its hydrophilic nature as it
contains more oxygen contents as compared to graphene. The rGO can be obtained from
GO by washing it with hydrogen hydrate, which reduces the oxygen content in rGO as com-
pared to GO, which makes it more conductive and is helpful to achieve better performance
in Li-ion battery. The unique construction of GO-S nanocomposite electrodes improves
Energies 2021,14, 7362. https://doi.org/10.3390/en14217362 https://www.mdpi.com/journal/energies
Energies 2021,14, 7362 2 of 12
the performance of Li-S batteries by adjusting volume expansion during electrochemical
testing. Moreover, rGO, with its vast surface region alongside pervasive cavities, can set
up better electronic contact with sulfur to prevent conglomeration for the smooth flow of
ions through the material. Peng Yu et al. [
20
], in their experimental study, prepared an
electrode material by reducing graphene through a solution-based technique to encapsulate
sulfur inside reduced graphene sheets, which resulted in better charging–discharging of a
cell with good cycle ability and capacity. Yu-Yun Hsieh et al. [
21
] supported that carbon
materials provide a major contribution to overcoming shuttle phenomena by trapping
polysulfides and also boost the cathode’s conductivity. This was accomplished by syn-
thesizing three-dimensional graphene with oxygen functionalization to form a composite
with sulfur, which demonstrates excellent electrochemical performance as compared to
previous works.
Different types of polymer electrolytes and composite electrolytes with perovskite and
oxides are drawing attention for their application in Li-S batteries to overcome the safety
issue of liquid electrolytes. In context to this, LiBH
4
—a notable hydrogen storage material—
has been studied as a promising electrolyte material for the fabrication of all-solid-state
batteries. The phase transition of LiBH
4
at ~115
◦
C provides high ionic conductivity
(~1 mS/cm
), which allows for its use as a solid electrolyte that supports the flow of lithium
ions through it [
22
–
26
]. LiBH4 is safer to be used in comparison to liquid flammable
electrolytes. It is quite stable up to 450
◦
C, after which it decomposes to LiH and B. In
this work, LiBH
4
is employed with the GO-S and rGO-S composite cathode material and
lithium foil as an anode. The use of GO and rGO as an additive with sulfur in this work is
expected to accommodate the volume expansion as a result of the expanded surface area
and cushioning effect due to the GO and rGO network.
2. Materials and Methods
2.1. Synthesis of GO-S and rGO-S Nanocomposites
Hummer’s method [
27
] was used to synthesize the powdered GO, whereas rGO was
obtained by chemical reduction of GO (washing of obtained GO with hydrogen hydrate to
reduce oxygen content). The GO-S and rGO-S composites were synthesized via ball-milling
for 1gm batch of composites that had different amounts of sulfur and GO/rGO, i.e., (i) S
99% and GO 1%, (ii) S 90% and GO 10%, (iii) S 99% and rGO 1%, and (iv) S 90% and rGO
10%. The milling was carried out for 24 h at 300 rpm using 20 stainless steel (SS) balls of
7 mm diameter.
2.2. Electrode Material Preparation
The electrode (Cathode) composite materials of GO-S and rGO-S were synthesized
via ball-milling with LiBH
4
and acetylene black (AB) in 40:30:30 weight proportion for 2 h,
resting for 30 min after 1 h of milling under an argon environment. For the synthesis of a
200 mg sample (80 mg active material, i.e.,S-GO/rGO composite, 60 mg solid electrolyte
LiBH
4
and 60 mg AB), 10 SS balls were utilized at 370 rpm in a Fritsch P7 processing
machine. LiBH
4
and AB were both dried using a dynamic vacuum at 200
◦
C for 24 h before
utilizing them to prepare the cathode.
2.3. Coin Cell Preparation
To test the electrochemical performance of the above-mentioned electrode materials,
coin cells were fabricated with Li-foil as the anode, LiBH
4
as the electrolyte, and the above-
mentioned composites (S 99% and GO 1%, S 90% and GO 10%, S 99% and rGO 1%, and
S 90% and rGO 10% with LiBH
4
and AB) as the cathode layer. To fabricate the cell, a
three-layer pellet was prepared by the following procedure. First, Li-foil (thickness of
0.14 mm) purchased from Honjo metal Co. Ltd., Osaka, Japan, in a circular shape was
spread onto an SS plate as the first layer (thickness approx. 0.63 mm), then dried LiBH
4
powder (~80 mg) was sprinkled and pressed under 10 MPa for 5 min using a hydraulic
press machine. It was followed by spreading composite powder (~10 mg) as the 3rd layer
Energies 2021,14, 7362 3 of 12
and was pressed under 40 MPa for 5 min. The prepared 3-layered pellet was put in a coin
cell case using a perfluoroalkoxy (PFA) gasket. The samples were kept completely isolated
from atmospheric conditions throughout the investigation. All the handling was conducted
inside a high-purity argon-filled glove box (oxygen and moisture content
<0.1 ppm
) to
prevent the samples from contamination of oxygen.
2.4. Material Characterization and Electrochemical Measurements
X-ray diffraction (XRD) using the Rigaku-RINT 2500 with CuK
α
(
λ
= 1.5406 Å) as
the radiation source was performed for all the samples within a 2
θ
range of 5
◦
–50
◦
at the
scan speed 5
◦
/min. The Debye Scherer formula was used to calculate the crystallite size
D = 0.9
λ
/
β
cos
θ
, where
λ
= X-ray wavelength,
θ
= Bragg diffraction angle,
β
= full width at
half maximum (FWHM).
Scanning electron microscopy (JEOL JSM-6380, JEOL Ltd., Tokyo, Japan) was used
for surface morphology and energy dispersive spectroscopy (EDS) for elemental analysis.
It is to note here that SEM/EDS observations were made on the top of the surface of the
3-layered pellet, which means that all the signals are from the cathode composite and
not from the electrolyte layer (LiBH
4
) or the anode layer (Li foil). A charge–discharge
analyzer (HJ1001SD8, Hokuto Denko Co.) was used for the measurement of electrochemical
performance of the graphene modulated GO-S and rGO-S composite cathode with LiBH
4
electrolyte at 0.1 C-rate and 120 ◦C temperatures.
3. Results and Discussion
3.1. X-ray Diffraction
Figure 1a,b shows X-ray diffraction spectra of GO-S and rGO-S composites with acety-
lene black (AB) and LiBH
4
. The individual curves are also shown in the Supplementary
Materials for better clarity (Figures S1–S5). The diffraction peaks at 2
θ
= 22.9
◦
, 25.9
◦
, and
28.0
◦
, which were indexed as (222), (026), and (040) planes, respectively. These represent the
orthorhombic structure (JCPDS card no. 001-0478) of sulfur with high crystallinity [
18
]. The
graphene oxide shows diffraction peaks at 2
θ
= 10.9
◦
(001), 25.6
◦
(002), and 43
◦
(100). The
GO-S and rGO-S composite with different wt% of GO and rGO showed all diffraction peaks
as that for pristine sulfur, which confirms no phase transformation occurred due to GO and
rGO in the composites. The detected characteristic peaks of S-GO and S-rGO composites
match very well, demonstrating the orthorhombic structure. The presence of LiBH
4
was
confirmed in the composite material by the presence of peaks at 2
θ
= 17.7
◦
,23.68
◦
, 24.48
◦
,
25.24
◦
, 26.78
◦
, 28.8
◦
, and 40.32
◦
. The average crystallite size of composites was 28 nm, as
calculated by the Debye Scherer formula [11].
Energies 2021, 14, x FOR PEER REVIEW 3 of 12
powder (~80 mg) was sprinkled and pressed under 10 MPa for 5 min using a hydraulic
press machine. It was followed by spreading composite powder (~10 mg) as the 3rd layer
and was pressed under 40 MPa for 5 min. The prepared 3-layered pellet was put in a coin
cell case using a perfluoroalkoxy (PFA) gasket. The samples were kept completely iso-
lated from atmospheric conditions throughout the investigation. All the handling was
conducted inside a high-purity argon-filled glove box (oxygen and moisture content <0.1
ppm) to prevent the samples from contamination of oxygen.
2.4. Material Characterization andElectrochemical Measurements
X-ray diffraction (XRD) using the Rigaku-RINT 2500 with CuKα (λ = 1.5406 Å ) as the
radiation source was performed for all the samples within a 2θ range of 5°–50° at the scan
speed 5°/min. The Debye Scherer formula was used to calculate the crystallite size D =
0.9λ/βcosθ, where λ = X-ray wavelength, θ = Bragg diffraction angle, β = full width at half
maximum (FWHM).
Scanning electron microscopy (JEOL JSM-6380, JEOL Ltd., Tokyo, Japan) was used
for surface morphology and energy dispersive spectroscopy (EDS) for elemental analysis.
It is to note here that SEM/EDS observations were made on the top of the surface of the
3-layered pellet, which means that all the signals are from the cathode composite and not
from the electrolyte layer (LiBH4) or the anode layer (Li foil). A charge–discharge ana-
lyzer (HJ1001SD8, Hokuto Denko Co.) was used for the measurement of electrochemical
performance of the graphene modulated GO-S and rGO-S composite cathode with LiBH4
electrolyte at 0.1C-rate and 120 °C temperatures.
3. Results and Discussion
3.1. X-ray Diffraction
Figure 1a,b shows X-ray diffraction spectra of GO-S and rGO-S composites with
acetylene black (AB) and LiBH4. The individual curves are also shown in the Supple-
mentary Materials for better clarity (Figures S1–S5). The diffraction peaks at 2θ = 22.9°,
25.9°, and 28.0°, which were indexed as (222), (026), and (040) planes, respectively. These
represent the orthorhombic structure (JCPDS card no. 001-0478) of sulfur with high
crystallinity [18]. The graphene oxide shows diffraction peaks at 2θ = 10.9° (001), 25.6°
(002), and 43° (100). The GO-S and rGO-S composite with different wt% of GO and rGO
showed all diffraction peaks as that for pristine sulfur, which confirms no phase trans-
formation occurred due to GO and rGO in the composites. The detected characteristic
peaks of S-GO and S-rGO composites match very well, demonstrating the orthorhombic
structure. The presence of LiBH4 was confirmed in the composite material by the pres-
ence of peaks at 2θ = 17.7°,23.68°, 24.48°, 25.24°, 26.78°, 28.8°, and 40.32°. The average
crystallite size of composites was 28 nm, as calculated by the Debye Scherer formula [11].
Figure 1. XRD Plots of (a) electrode materials with (i) S, (ii) GO, (iii) LiBH4, (iv) AB, (v)
(S100%)40%+(LiBH4)30%+(AB)30%, (vi) (S99%GO1%)40%+(LiBH4)30%+(AB)30%, and (vii)
(S90%GO10%)40%+(LiBH4)30%+(AB)30%. (b) Electrode materials that have (i) S, (ii) rGO, (iii) LiBH4, (iv)
Figure 1.
XRD Plots of (
a
) electrode materials with (i) S, (ii) GO, (iii) LiBH
4
, (iv)
AB, (v) (S
100%
)
40%
+(LiBH
4
)
30%
+(AB)
30%
, (vi) (S
99%
GO
1%
)
40%
+(LiBH
4
)
30%
+(AB)
30%,
and (vii)
(S
90%
GO
10%
)
40%
+(LiBH
4
)
30%
+(AB)
30%
. (
b
) Electrode materials that have (i) S, (ii) rGO, (iii) LiBH
4
,
(iv) AB, (v) (S
100%
)
40%
+(LiBH
4
)
30%
+(AB)
30%
, (vi) (S
99%
rGO
1%
)
40%
+(LiBH
4
)
30%
+(AB)
30%,
and (vii)
(S90%rGO10%)40%+(LiBH4)30%+(AB)30%.
Energies 2021,14, 7362 4 of 12
3.2. SEM Analysis
Figure 2a,b shows the SEM images of the top surface of the three-layer pellet, having
a cathode composite part on the top, followed by the LiBH
4
layer and the Li-foil layer of
pristine and cycled (30 cycles) Li/GO-S/LiBH
4
systems, respectively. Several major, as
well as minor, cracks have been observed in the cycled system, which might be attributed
due to the volume expansion and contraction during charging and discharging of the cell.
The dynamics of the system, which further affects the battery performance, can be verified
from Galvanostatic charge–discharge profiles. The system showed good capacity initially;
however, the performance was degraded with the beginning of crack formation.
Energies 2021, 14, x FOR PEER REVIEW 4 of 12
AB, (v) (S100%)40%+(LiBH4)30%+(AB)30%, (vi) (S99%rGO1%)40%+(LiBH4)30%+(AB)30%, and (vii)
(S90%rGO10%)40%+(LiBH4)30%+(AB)30%.
3.2. SEM Analysis
Figure 2a,b shows the SEM images of the top surface of the three-layer pellet, having
a cathode composite part on the top, followed by the LiBH4 layer and the Li-foil layer of
pristine and cycled (30 cycles) Li/GO-S/LiBH4 systems, respectively. Several major, as
well as minor, cracks have been observed in the cycled system, which might be attributed
due to the volume expansion and contraction during charging and discharging of the
cell. The dynamics of the system, which further affects the battery performance, can be
verified from Galvanostatic charge–discharge profiles. The system showed good capacity
initially; however, the performance was degraded with the beginning of crack formation.
Figure 2. SEM images of (a) Li/GO-S/LiBH4 system and (b) Cycled Li/GO-S/LiBH4 system (30 cy-
cles).
Figure 3a–d shows EDS images of the GO-S composite electrode system(SEM image
shown in Figure 2a), from which the uniform distribution of sulfur, carbon, and boron
was seen with mapping percentages of 72.9%, 16.1%, and 6.1%, respectively, as calcu-
lated from Figure 3e. However, EDS spectra reflect Li in very small quantities in com-
parison to the expected content from LiBH4, which is due to the low energy of character-
istic radiation of Li.
Figure 3. EDS images of the electrode with GO-S showing elements: (a) Sulfur, (b) Lithium, (c) Boron, and (d) Carbon. (e)
Mapping graph of the electrode with GO-S.
Figure 2.
SEM images of (
a
) Li/GO-S/LiBH4 system and (
b
) Cycled Li/GO-S/LiBH4 system
(30 cycles).
Figure 3a–d shows EDS images of the GO-S composite electrode system(SEM image
shown in Figure 2a), from which the uniform distribution of sulfur, carbon, and boron was
seen with mapping percentages of 72.9%, 16.1%, and 6.1%, respectively, as calculated from
Figure 3e. However, EDS spectra reflect Li in very small quantities in comparison to the
expected content from LiBH
4
, which is due to the low energy of characteristic radiation of
Li.
Energies 2021, 14, x FOR PEER REVIEW 4 of 12
AB, (v) (S100%)40%+(LiBH4)30%+(AB)30%, (vi) (S99%rGO1%)40%+(LiBH4)30%+(AB)30%, and (vii)
(S90%rGO10%)40%+(LiBH4)30%+(AB)30%.
3.2. SEM Analysis
Figure 2a,b shows the SEM images of the top surface of the three-layer pellet, having
a cathode composite part on the top, followed by the LiBH4 layer and the Li-foil layer of
pristine and cycled (30 cycles) Li/GO-S/LiBH4 systems, respectively. Several major, as
well as minor, cracks have been observed in the cycled system, which might be attributed
due to the volume expansion and contraction during charging and discharging of the
cell. The dynamics of the system, which further affects the battery performance, can be
verified from Galvanostatic charge–discharge profiles. The system showed good capacity
initially; however, the performance was degraded with the beginning of crack formation.
Figure 2. SEM images of (a) Li/GO-S/LiBH4 system and (b) Cycled Li/GO-S/LiBH4 system (30 cy-
cles).
Figure 3a–d shows EDS images of the GO-S composite electrode system(SEM image
shown in Figure 2a), from which the uniform distribution of sulfur, carbon, and boron
was seen with mapping percentages of 72.9%, 16.1%, and 6.1%, respectively, as calcu-
lated from Figure 3e. However, EDS spectra reflect Li in very small quantities in com-
parison to the expected content from LiBH4, which is due to the low energy of character-
istic radiation of Li.
Figure 3. EDS images of the electrode with GO-S showing elements: (a) Sulfur, (b) Lithium, (c) Boron, and (d) Carbon. (e)
Mapping graph of the electrode with GO-S.
Figure 3.
EDS images of the electrode with GO-S showing elements: (
a
) Sulfur, (
b
) Lithium, (
c
) Boron, and (
d
) Carbon.
(e) Mapping graph of the electrode with GO-S.
Energies 2021,14, 7362 5 of 12
Figure 4a,b shows SEM images of the top surface of the 3-layer pellet, having the
cathode composite part on the top, followed by the LiBH
4
layer and the Li-foil layer of
pristine and cycled (42 cycles) Li/rGO-S/LiBH
4
systems, respectively. Although there is no
direct evidence as SEM analysis was performed only after several cycles, by combining the
SEM analysis and the cyclic charge-discharge profiles, it can be speculated that the majority
of crack formation occurs during the initial cycling of the cell (the cell initially delivered
better performance, which started degrading immediately as the crack formation occurred).
On further cycling, these cracks started expanding more, thus resulting in fast capacity
decay, as seen from electrochemical testing. These cracks affect the flow of Li-ions through
the electrodes. As compared to the GO-S electrode, the rGO-S electrode exhibits less crack
formation and suggests that the electrode may benefit in terms of the capacity and cycling
performance of a battery. The addition of any carbon material is to accommodate the
volume expansion and act as a binder for sulfur. The smaller cracks in rGO are due to the
fact that it has a larger surface area than GO, so it can bind the electrode material (sulfur)
more efficiently compared to GO.
Energies 2021, 14, x FOR PEER REVIEW 5 of 12
Figure 4a,b shows SEM images of the top surface of the 3-layer pellet, having the
cathode composite part on the top, followed by the LiBH4 layer and the Li-foil layer of
pristine and cycled (42 cycles) Li/rGO-S/LiBH4 systems, respectively. Although there is
no direct evidence as SEM analysis was performed only after several cycles, by combin-
ing the SEM analysis and the cyclic charge-discharge profiles, it can be speculated that
the majority of crack formation occurs during the initial cycling of the cell (the cell ini-
tially delivered better performance, which started degrading immediately as the crack
formation occurred). On further cycling, these cracks started expanding more, thus re-
sulting in fast capacity decay, as seen from electrochemical testing. These cracks affect the
flow of Li-ions through the electrodes. As compared to the GO-S electrode, the rGO-S
electrode exhibits less crack formation and suggests that the electrode may benefit in
terms of the capacity and cycling performance of a battery. The addition of any carbon
material is to accommodate the volume expansion and act as a binder for sulfur. The
smaller cracks in rGO are due to the fact that it has a larger surface area than GO, so it can
bind the electrode material (sulfur) more efficiently compared to GO.
Figure 4. SEM images of (a) the Li/rGO-S/LiBH4 system and (b) the cycled Li/rGO-S/LiBH4 system
(42 cycles).
EDS images of the rGO-S composite system (Figure 5a–d) also show the uniform
distribution of S, C, and B, having mapping percentages of 70.8%, 17.4%, and 6.5%, re-
spectively (Figure 5e).The SEM image of the same composite electrode without cycling is
shown in Figure 4a.From the mapping percentage of the composites, it can be seen that
the percentage of carbon in the cathode of the rGO-S system is more when compared to
the GO-S cathode system, which may be due to more homogenous mixing of the carbon
content that might have played an important role in cycling, as it increased the storage
capacity of the electrode.
Figure 4.
SEM images of (
a
) the Li/rGO-S/LiBH
4
system and (
b
) the cycled Li/rGO-S/LiBH
4
system
(42 cycles).
EDS images of the rGO-S composite system (Figure 5a–d) also show the uniform
distribution of S, C, and B, having mapping percentages of 70.8%, 17.4%, and 6.5%, re-
spectively (Figure 5e).The SEM image of the same composite electrode without cycling is
shown in Figure 4a.From the mapping percentage of the composites, it can be seen that
the percentage of carbon in the cathode of the rGO-S system is more when compared to
the GO-S cathode system, which may be due to more homogenous mixing of the carbon
content that might have played an important role in cycling, as it increased the storage
capacity of the electrode.
Energies 2021,14, 7362 6 of 12
Energies 2021, 14, x FOR PEER REVIEW 6 of 12
Figure 5. EDS images of the electrode with rGO-S, showing elements: (a) Sulfur, (b) Carbon, (c) Boron, and (d) Lithium.
(e) Mapping graph of the electrode with rGO-S.
Figure 6a,b shows the Galvanostatic charge–discharge profiles of two GO-S compo-
site electrodes with 1%GO-99%S and 10%GO-90%S as active materials, which were per-
formed in the potential range 0.2 V to 4 V at a C-rate of 0.1 C-rate and temperature 120 °C.
The capacities shown here are calculated with respect to sulfur content. It is clear that the
first discharge cycle of 1%GO-99%S composite delivered a capacity of 1100 mAh/g, which
is lower than the theoretical capacity. Conversely, during the charging cycle, it showed a
capacity of 1700 mAh/g. This must be due to the simultaneous thermochemical reaction
between S and LiBH4 (present as a component in the electrode layer), as suggested in our
previous works on M2S3 (M = Bi and Sb) [28–31]. Actually, during the initial heating
process, sulfur partially reacted with LiBH4 (present as a component in the electrode
layer) and formed Li2S thermo chemically. Then, during the discharge cycle, the re-
maining sulfur further reacted with Li-ions (coming from Li-foil at the anode side) elec-
trochemically and converted into Li2S. This is the reason why the initial capacity was
observed lower than the theoretical capacity. When it comes to the charging cycle, Li2S
starts releasing Li-ions and forms S again. However, this freshly formed S reacts with
LiBH4 (present as a component in the electrode layer) thermo chemically again and is
converted into Li2S. At this time, the charging process is also going on, so the thermo
chemically formed Li2S keeps releasing Li-ions. These simultaneous reactions of S–LiBH4
(thermo chemical) and the Li2S to S conversion (electrochemical charging) continue until
the consumption of LiBH4. During the first charging cycle, the cell delivers a capacity
more than the theoretical (1675mAh/g) capacity during charging, which might be due to
the reaction between sulfur with solid electrolyte LiBH4 (present as a component in the
electrode layer) acting as a Li source. The capacity in the first charging cycle was ob-
served higher than the corresponding discharge cycle. A similar behavior was also ob-
served for the other composite (Figure 6b).The first cycle suggests a coulombic efficiency
(CE) ~65% for the 1%GO-99%S composite, whereas the cell that has GO10%S90%composite
material showed ~84% coulombic efficiency. It can be seen that CE is less, which resem-
bles the parasitic side reaction occurring between S and LiBH4 and irregular Li-ion
movements as a result of the shapes and values of the cell voltages’ plateau changing
dramatically in subsequent cycles. The discharge reaction of the cell was confirmed by
comparing the XRD profile before and after the discharge cycle (Figure S6), where the
peaks corresponding to sulfur are found in the as-prepared cell. In addition to sulfur
peaks, small peaks corresponding to the Li2S phase are also observed, which confirms our
Figure 5.
EDS images of the electrode with rGO-S, showing elements: (
a
) Sulfur, (
b
) Carbon, (
c
) Boron, and (
d
) Lithium. (
e
)
Mapping graph of the electrode with rGO-S.
Figure 6a,b shows the Galvanostatic charge–discharge profiles of two GO-S composite
electrodes with 1%GO-99%S and 10%GO-90%S as active materials, which were performed
in the potential range 0.2 V to 4 V at a C-rate of 0.1 C-rate and temperature 120
◦
C. The
capacities shown here are calculated with respect to sulfur content. It is clear that the first
discharge cycle of 1%GO-99%S composite delivered a capacity of 1100 mAh/g, which is
lower than the theoretical capacity. Conversely, during the charging cycle, it showed a
capacity of 1700 mAh/g. This must be due to the simultaneous thermochemical reaction
between S and LiBH
4
(present as a component in the electrode layer), as suggested in
our previous works on M
2
S
3
(M = Bi and Sb) [
28
–
31
]. Actually, during the initial heating
process, sulfur partially reacted with LiBH
4
(present as a component in the electrode layer)
and formed Li
2
S thermo chemically. Then, during the discharge cycle, the remaining sulfur
further reacted with Li-ions (coming from Li-foil at the anode side) electrochemically and
converted into Li
2
S. This is the reason why the initial capacity was observed lower than the
theoretical capacity. When it comes to the charging cycle, Li
2
S starts releasing Li-ions and
forms S again. However, this freshly formed S reacts with LiBH
4
(present as a component
in the electrode layer) thermo chemically again and is converted into Li
2
S. At this time, the
charging process is also going on, so the thermo chemically formed Li
2
S keeps releasing
Li-ions. These simultaneous reactions of S–LiBH
4
(thermo chemical) and the Li
2
S to S
conversion (electrochemical charging) continue until the consumption of LiBH
4
. During
the first charging cycle, the cell delivers a capacity more than the theoretical (1675mAh/g)
capacity during charging, which might be due to the reaction between sulfur with solid
electrolyte LiBH
4
(present as a component in the electrode layer) acting as a Li source. The
capacity in the first charging cycle was observed higher than the corresponding discharge
cycle. A similar behavior was also observed for the other composite (Figure 6b).The first
cycle suggests a coulombic efficiency (CE) ~65% for the 1%GO-99%S composite, whereas
the cell that has GO
10%
S
90%
composite material showed ~84% coulombic efficiency. It can
be seen that CE is less, which resembles the parasitic side reaction occurring between S
and LiBH
4
and irregular Li-ion movements as a result of the shapes and values of the
cell voltages’ plateau changing dramatically in subsequent cycles. The discharge reaction
of the cell was confirmed by comparing the XRD profile before and after the discharge
cycle (Figure S6), where the peaks corresponding to sulfur are found in the as-prepared
cell. In addition to sulfur peaks, small peaks corresponding to the Li
2
S phase are also
Energies 2021,14, 7362 7 of 12
observed, which confirms our hypothesis of a reaction between S and LiBH
4
during the
initial heating. These peaks corresponding to Li
2
S becoming stronger after discharging for
both the composites, and all the peaks corresponding to sulfur phase disappear.
Energies 2021, 14, x FOR PEER REVIEW 7 of 12
hypothesis of a reaction between S and LiBH4 during the initial heating. These peaks
corresponding to Li2S becoming stronger after discharging for both the composites, and
all the peaks corresponding to sulfur phase disappear.
Figure 6.Galvanostatic charge–discharge profiles of (a) the S99%GO1%electrode and (b) the S90%GO10%electrode.
Figure 7a shows the cycling performance of the Li-S cell investigated at a 0.1 C-rate
for 30 cycles with a cut-off voltage between 0.2 V and 4.0 V. The reduction in discharge
capacity was observed for both composites. The cell with the 1%GO-99%S composite
showed a lower initial capacity in comparison to the other composite (10%GO-90%S), but
it could be stable and could work up to 30 cycles. The cell with 10%GO-90%S composite
stopped working after 16 cycles but delivered a higher capacity of around 400 mAh/g.
Figure 7b shows the cyclic voltammetry of the full Li/S cell scanned at 0.1 mV/s operated
between 0.2 V to 4 V, which confirms the reversibility of the reaction and is used to de-
cide the potential window for charging-discharging cycles. The CV curve shows one
oxidation peak during discharge at 0.86 V, which corresponds to S to Li2S conversion. In
contrast, the curve during reduction shows two peaks, which is unusual. It can be ex-
plained on the basis of a thermochemical reaction that generated two different species of
sulfur/Li2S (structurally same, but kinetically different). Thus, the splitting in the CV
curve is due to the different kinetics associated with these.
Figure 6.
Galvanostatic charge–discharge profiles of (
a
) the S
99%
GO
1%
electrode and (
b
) the
S90%GO10%electrode.
Figure 7a shows the cycling performance of the Li-S cell investigated at a 0.1 C-rate
for 30 cycles with a cut-off voltage between 0.2 V and 4.0 V. The reduction in discharge
capacity was observed for both composites. The cell with the 1%GO-99%S composite
showed a lower initial capacity in comparison to the other composite (10%GO-90%S), but
it could be stable and could work up to 30 cycles. The cell with 10%GO-90%S composite
stopped working after 16 cycles but delivered a higher capacity of around 400 mAh/g.
Figure 7b shows the cyclic voltammetry of the full Li/S cell scanned at 0.1 mV/s operated
between 0.2 V to 4 V, which confirms the reversibility of the reaction and is used to decide
the potential window for charging-discharging cycles. The CV curve shows one oxidation
peak during discharge at 0.86 V, which corresponds to S to Li
2
S conversion. In contrast,
the curve during reduction shows two peaks, which is unusual. It can be explained on
the basis of a thermochemical reaction that generated two different species of sulfur/Li
2
S
(structurally same, but kinetically different). Thus, the splitting in the CV curve is due to
the different kinetics associated with these.
Figure 8a,b shows galvanostatic charge–discharge profiles of Li/S cells containing
1%rGO-99%S and 10%rGO-90%S composite electrodes in the potential range of 0.2 V to
4.0 V
at a 0.1 C-rate and 120
◦
C temperature. The cell with the1%rGO-99%S composite
material showed a discharge capacity of 1104 mAh/g (lower than the theoretical capacity)
and a charge capacity of 1698 mAh/g (higher than the discharge capacity) in the initial cycle
with ~65% coulombic efficiency, which reduced to a constant value around 150 mAh/g
after 42 cycles (Figure 8a). The cell with 10%rGO-90%S composite material showed an
initial charge capacity of 1309 mAh/g and a discharge capacity of 1165 mAh/g with an
~89% coulombic efficiency. The behavior is quite similar to that of the GO-added samples
but with a better capacity. Besides these findings, the second discharge cycle in all the cases
showed a higher discharge capacity, which is also associated with the thermochemical
reaction between S and LiBH
4
. In the first cycle, most of the LiBH
4
is already exhausted,
which leaves more sulfur unreacted and is available to participate in the electrochemical
reaction with Li-ions (see above—initially, sulfur reacted with LiBH
4
during heating, which
reduced the available sulfur content to participate in electrochemical cycling).
Energies 2021,14, 7362 8 of 12
Energies 2021, 14, x FOR PEER REVIEW 7 of 12
hypothesis of a reaction between S and LiBH4 during the initial heating. These peaks
corresponding to Li2S becoming stronger after discharging for both the composites, and
all the peaks corresponding to sulfur phase disappear.
Figure 6.Galvanostatic charge–discharge profiles of (a) the S99%GO1%electrode and (b) the S90%GO10%electrode.
Figure 7a shows the cycling performance of the Li-S cell investigated at a 0.1 C-rate
for 30 cycles with a cut-off voltage between 0.2 V and 4.0 V. The reduction in discharge
capacity was observed for both composites. The cell with the 1%GO-99%S composite
showed a lower initial capacity in comparison to the other composite (10%GO-90%S), but
it could be stable and could work up to 30 cycles. The cell with 10%GO-90%S composite
stopped working after 16 cycles but delivered a higher capacity of around 400 mAh/g.
Figure 7b shows the cyclic voltammetry of the full Li/S cell scanned at 0.1 mV/s operated
between 0.2 V to 4 V, which confirms the reversibility of the reaction and is used to de-
cide the potential window for charging-discharging cycles. The CV curve shows one
oxidation peak during discharge at 0.86 V, which corresponds to S to Li2S conversion. In
contrast, the curve during reduction shows two peaks, which is unusual. It can be ex-
plained on the basis of a thermochemical reaction that generated two different species of
sulfur/Li2S (structurally same, but kinetically different). Thus, the splitting in the CV
curve is due to the different kinetics associated with these.
Figure 7.
(
a
) The cycling performance of the GO-S composite electrode in a potential range2 V to 4 V. (
b
) Cyclic voltammetry
of the GO-S composite electrode scanned at 0.1 mV/s.
Energies 2021, 14, x FOR PEER REVIEW 8 of 12
Figure 7. (a) The cycling performance of the GO-S composite electrode in a potential range2 V to 4 V. (b) Cyclic voltam-
metry of the GO-S composite electrode scanned at 0.1 mV/s.
Figure 8a,b shows galvanostatic charge–discharge profiles of Li/S cells containing
1%rGO-99%S and 10%rGO-90%S composite electrodes in the potential range of 0.2 V to
4.0 V at a 0.1 C-rate and 120°C temperature. The cell with the1%rGO-99%S composite
material showed a discharge capacity of 1104 mAh/g (lower than the theoretical capacity)
and a charge capacity of 1698 mAh/g (higher than the discharge capacity) in the initial
cycle with ~65% coulombic efficiency, which reduced to a constant value around 150
mAh/g after 42 cycles (Figure 8a). The cell with 10%rGO-90%S composite material
showed an initial charge capacity of 1309 mAh/g and a discharge capacity of 1165 mAh/g
with an ~89% coulombic efficiency. The behavior is quite similar to that of the GO-added
samples but with a better capacity. Besides these findings, the second discharge cycle in
all the cases showed a higher discharge capacity, which is also associated with the ther-
mochemical reaction between S and LiBH4. In the first cycle, most of the LiBH4 is already
exhausted, which leaves more sulfur unreacted and is available to participate in the
electrochemical reaction with Li-ions (see above—initially, sulfur reacted with LiBH4
during heating, which reduced the available sulfur content to participate in electro-
chemical cycling).
Figure 8. Galvanostatic charge–discharge profiles of (a) the S99%rGO1%electrode and (b) the S90%rGO10%electrode.
The specific discharge capacity of cells prepared with GO-S and rGO-S composite
cathode material showed competitiveness with earlier reported work in which research-
ers utilized different carbon composite with sulfur by adopting different procedures for
composite making and electrolyte. Table 1 shows a list of a few composite materials and
their initial performance compared to the results of the present work. However, from
Table 1, it can be seen that innovative work of the Li-S battery is, as of yet, incipient and
shows that lots of exertion are needed for the commercialization of the battery.
Table 1. Comparison of the initial discharge capacity of various types of carbon–sulfur composites as cathode materials
reported earlier for the Li-S battery of the present study.
Composite
Sulfur Content
Electrolyte
Initial Discharge
Capacity (mAh/g)
C-Rate
Ref.
Graphene sheet-sulfur
67
Organic electrolyte
600
0.1 C
[32]
Graphene-sulfur
87
1M LiTFSI*/DOL*/TEGDME*
705
0.33 A/g
[33]
rGO-S
65
1M LiTFSI/DOL/DME*
724.5
1 C
[34]
Nafion-coated FGS
~72
1M LiTFSI/DOL/DME
839
0.2 C
[17]
Figure 8. Galvanostatic charge–discharge profiles of (a) the S99%rGO1%electrode and (b) the S90%rGO10%electrode.
The specific discharge capacity of cells prepared with GO-S and rGO-S composite
cathode material showed competitiveness with earlier reported work in which researchers
utilized different carbon composite with sulfur by adopting different procedures for com-
posite making and electrolyte. Table 1shows a list of a few composite materials and their
initial performance compared to the results of the present work. However, from Table 1, it
can be seen that innovative work of the Li-S battery is, as of yet, incipient and shows that
lots of exertion are needed for the commercialization of the battery.
Figure 9a shows the cyclic performance of the Li-S coin cell containing the rGO-S
composite electrodes within a potential range of 0.2 V to 4.0 V at a 0.1 C-rate and 120
◦
C. It
has been observed that the cycling and stability of composite having 1%rGO-99%S is better
than that of composite having 10%rGO-90%S. It is also noted here that the addition of both
GO and rGO is helpful to obtain better stability in comparison to pure sulfur (Figure S7),
where the battery works only for two cycles.
Energies 2021,14, 7362 9 of 12
Table 1.
Comparison of the initial discharge capacity of various types of carbon–sulfur composites as cathode materials
reported earlier for the Li-S battery of the present study.
Composite Sulfur Content Electrolyte Initial Discharge
Capacity (mAh/g) C-Rate Ref.
Graphene
sheet-sulfur 67 Organic electrolyte 600 0.1 C [32]
Graphene-sulfur 87 1M LiTFSI */DOL
*/TEGDME * 705 0.33 A/g [33]
rGO-S 65 1M
LiTFSI/DOL/DME
*724.5 1 C [34]
Nafion-coated FGS ~72 1M
LiTFSI/DOL/DME
839 0.2 C [17]
Expanded
graphite-
embedded
sulphur
80 1M
LiTFSI/DOL/DME
854 0.28 A/g [35]
3D S-CNT–rGO 60 1M
LiTFSI/DOL/DME
921 0.1 C [36]
Functionalized
graphene–sulfur
(FGS) ~72 1M
LiTFSI/DOL/DME
950 0.1 C [17]
CP rGO-S 60 1M
LiTFSI/DOL/DME
988.9 0.2 C [37]
Graphene-sulfur-
PEG 70 1M
LiTFSI/DOL/DME
1000 0.2 C [8]
3D
Sulfur-graphene
foam 63 1M
LiN(CF
3
SO
2
)
2
/DOL–
DME 1008 0.1 C [38]
Graphene-sulfur 73 1M
LiTFSI/DOL/DME
1053 0.1 C [39]
rGO-S 67 1M
LiTFSI/DOL/DME
1093 0.2 C [40]
S-CNT 65 1M
LiTFSI/DOL/DME
1100 0.1 C [41]
S-CNT 60 1M LiTFSI/DIOX
*/DME 1100 0.1 C [42]
Pristine sulphur 100 Li3PS41100 0.1 C [43]
S-Ketjein
Black(KB)-
Maxsorb 70 LiBH41140 0.05 C [44]
S-KB 70 LiBH4950 0.02 C [44]
S-KB (29:1) 29:1 LiBH4910 0.02 C [45]
S-GO 99wt%S
1% GO LiBH41100 0.1 C Present paper
S-rGO 99wt%S
1% rGO LiBH41104 0.1 C Present paper
* LiTFSI: Lithium bis(trifluoromethanesulfonyl)imide, DOL: 1,3-dioxolane, DME: 1,2-dimethoxyethane, TEGDME: tetra ethyleneglycol
dimethyl ether, and DIOX: Dioxolane.
Figure 9b shows consistent CV curves of the rGO-S cathode material used and ex-
amined at 0.1 mV/s starting from an open-circuit voltage of 4.0 V to 0.2 V against Li/Li
+
.
Sulfur-reducing CV peaks at ~3.33 V, ~2.81 V, and ~2.27 V can be seen in the reduction scan,
which is very similar to that of the GO-containing composites. In the anodic scan from
0.2 V
to 4.0 V, two oxidative peaks at ~0.88 V and a smaller peak at ~2.08 V can be seen and
connected to the oxidation of PS to S. The appearance of two peaks affirms the overlapping
of peaks.
Energies 2021,14, 7362 10 of 12
Energies 2021, 14, x FOR PEER REVIEW 9 of 12
Expanded graphite-embedded
sulphur
80
1M LiTFSI/DOL/DME
854
0.28 A/g
[35]
3D S-CNT–rGO
60
1M LiTFSI/DOL/DME
921
0.1 C
[36]
Functionalized graphene–sulfur
(FGS)
~72
1M LiTFSI/DOL/DME
950
0.1 C
[17]
CP rGO-S
60
1M LiTFSI/DOL/DME
988.9
0.2 C
[37]
Graphene-sulfur-PEG
70
1M LiTFSI/DOL/DME
1000
0.2 C
[8]
3D Sulfur-graphene foam
63
1M LiN(CF3SO2)2/DOL–DME
1008
0.1 C
[38]
Graphene-sulfur
73
1M LiTFSI/DOL/DME
1053
0.1 C
[39]
rGO-S
67
1M LiTFSI/DOL/DME
1093
0.2 C
[40]
S-CNT
65
1M LiTFSI/DOL/DME
1100
0.1 C
[41]
S-CNT
60
1M LiTFSI/DIOX*/DME
1100
0.1 C
[42]
Pristine sulphur
100
Li3PS4
1100
0.1 C
[43]
S-Ketjein Black(KB)-Maxsorb
70
LiBH4
1140
0.05 C
[44]
S-KB
70
LiBH4
950
0.02 C
[44]
S-KB (29:1)
29:1
LiBH4
910
0.02 C
[45]
S-GO
99wt%S
1% GO
LiBH4
1100
0.1 C
Present paper
S-rGO
99wt%S
1% rGO
LiBH4
1104
0.1 C
Present paper
* LiTFSI: Lithium bis(trifluoromethanesulfonyl)imide, DOL: 1,3-dioxolane, DME: 1,2-dimethoxyethane, TEGDME: tetra
ethyleneglycol dimethyl ether, and DIOX: Dioxolane.
Figure 9a shows the cyclic performance of the Li-S coin cell containing the rGO-S
composite electrodes within a potential range of 0.2 V to 4.0 V at a 0.1 C-rate and 120°C. It
has been observed that the cycling and stability of composite having 1%rGO-99%S is
better than that of composite having 10%rGO-90%S. It is also noted here that the addition
of both GO and rGO is helpful to obtain better stability in comparison to pure sulfur
(Figure S7), where the battery works only for two cycles.
Figure 9. (a) Cycling performance of the rGO-S composite electrode in a potential range 0.2 V to 4 V. (b) Cyclic voltam-
metry of the rGO-S composite electrode scanned at 0.1 mV/s.
Figure 9b shows consistent CV curves of the rGO-S cathode material used and ex-
amined at 0.1 mV/s starting from an open-circuit voltage of 4.0 V to 0.2 V against Li/Li+.
Sulfur-reducing CV peaks at ~3.33 V, ~2.81 V, and ~2.27 V can be seen in the reduction
scan, which is very similar to that of the GO-containing composites. In the anodic scan
from 0.2 V to 4.0 V, two oxidative peaks at ~0.88 V and a smaller peak at ~2.08 V can be
seen and connected to the oxidation of PS to S. The appearance of two peaks affirms the
overlapping of peaks.
Figure 9.
(
a
) Cycling performance of the rGO-S composite electrode in a potential range 0.2 V to 4 V. (
b
) Cyclic voltammetry
of the rGO-S composite electrode scanned at 0.1 mV/s.
4. Conclusions
All-solid-state Li-S batteries using two different composites of sulfur with GO and rGO
were prepared, and their electrochemical performance was investigated. The mechanism of
electrochemical charging/discharging is proposed herein by bringing several experiments
together. This work suggests a successful implementation of sulfur in all-solid-state batter-
ies using LiBH4 as a solid electrolyte. However, it is observed that sulfur also reacts with
LiBH
4
thermochemically, which negatively impacts the stability. It is important to examine
the performance at lower temperatures (<80
◦
C) to avoid the thermochemical reaction
between sulfur and LiBH
4
. This can be achieved by using a LiBH
4
composite with high
conductivity at low temperatures. Since the addition of graphene-modulated additives
provided a better path for electron movements, it significantly enhanced the performance
of the all-solid-state batteries studied in this work in comparison to pure S, as the sulfur cell
without additives did not work for more than two cycles. From the results, it is concluded
that the cycling ability and capacity are better with a cathode composite material with
rGO (42 cycles) compared to GO (30 cycles) and pristine sulfur (two cycles). The treated
composites have shown a great ion transport network with an expanded surface area
due to the addition of GO and rGO, which counter the volume expansion of the material
during charge and discharge. The results provide insight into a new composite material for
enhancing the performance of the Li-S battery.
Supplementary Materials:
The following are available online at https://www.mdpi.com/article/
10.3390/en14217362/s1, Figure S1: XRD plot of the as-prepared S-LiBH4-AB composite electrode,
Figure S2. XRD plot of the (S99%-GO1%)-LiBH4-AB composite electrode, Figure S3. XRD plot of the
(S90%-GO10%)-LiBH4-AB composite electrode, Figure S4. XRD plot of the (S99%-rGO1%)-LiBH4-AB
composite electrode, Figure S5. XRD plot of the (S90%-rGO10%)-LiBH4-AB composite electrode,
Figure S6. XRD plot of cells before and after discharge, Figure S7. Charge—discharge profile of the
S-LiBH4-AB composite electrode without any additive (GO or rGO).
Author Contributions:
Conceptualization, B.T. and A.J.; methodology, A.J. and M.K.G.; validation,
A.J.; formal analysis, T.P. and R.S.; investigation, T.P. and R.S.; resources, T.I.; data curation, T.P. and
B.T.; writing—T.P.; writing—review and editing, B.T. and A.J.; supervision, A.J. and B.T.; All authors
have read and agreed to the published version of the manuscript.
Funding:
This work is financially supported by the research grant from SERB-DST under ECR
scheme (Grant No. ECR/000655/2017) and DBT star scheme (BT/HRD/023/11/2019), Govt. of
India, New Delhi.
Data Availability Statement:
Data is available with authors and it may be provided on proper
request with valid reasons.
Energies 2021,14, 7362 11 of 12
Acknowledgments: Authors would like to thank Tomoyuki Ichikawa for his technical help.
Conflicts of Interest: The authors declare no conflict of interest.
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