High sulfur loaded carbon aerogel cathode for lithium-sulfur batteries

Article abstract of DOI:10.1039/c5ra01436k

An attempt made to improve sulfur loading in a template-free porous carbon to the extent of 70 wt% has led to two interesting observations being inferred, viz., 60 wt% sulfur loading exhibits superior electrochemical properties in the S@C composite form and an enhanced 73 wt% sulfur loading leads to the formation-cum-stabilisation of sulfur in its monoclinic phase. Carbon aerogel with tunable properties, derived from the catalyst-aided cross linking of resorcinol and formaldehyde, possessing a specific surface area of 1395 m² g^-1 and pore volume of 1.423 cm³ g^-1 has been exploited for sulfur loading to form a series of S@C composites. Interestingly, carbon aerogel prepared in the present study offers multifarious advantages such as conducting additive, potential host to accommodate higher concentrations of sulfur and better polysulfide confining matrix. The 60 wt% sulfur loaded carbon aerogel composite exhibits an appreciable specific capacity of 600 mA h g^-1 at a C/10 rate up to 100 cycles, 480 mA h g^-1 at a 1C and 422 mA h g^-1 at a 2C rate. Hence, the capability of carbon aerogel, synthesized through the present study in accommodating higher concentrations of sulfur, effective management of polysulfide shuttle, and provision of favorable electrode-electrolyte interface to facilitate extended cycling possibilities at different current densities has been demonstrated.

Full text of DOI:10.1039/c5ra01436k

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High sulfur loaded carbon aerogel cathode for
lithium-sulfur batteries
Cite this: DOI: 10.1039/x0xx00000x
K. Balakumar and N. Kalaiselvi*
Abstract: An attempt made to improve sulfur loading in a template free porous carbon to the
extent of 70 wt. % has inferred two interesting observations, viz., 60 wt. % sulfur loading
exhibits superior electrochemical properties in the S@C composite form and an enhanced 73
wt. % sulfur loading leads to the formation-cum-stabilisation of sulfur in its monoclinic phase.
Carbon aerogel with tunable properties, derived from the catalyst aided cross linking of
2
resorcinol and formaldehyde, possessing a specific surface area of 1395 m /g and pore volume
3
of 1.423 cm /g has been exploited for sulfur loading to form series of S@C composites.
Interestingly, carbon aerogel prepared in the present study offers multifarious advantages such
as conducting additive, potential host to accommodate higher concentration of sulfur and better
polysulfide confining matrix. The 60 wt. % sulfur loaded carbon aerogel composite exhibits an
appreciable specific capacity of 600 mAh/g at C/10 rate up to 100 cycles, 480 mAh/g at 1C and
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
4
22 mAh/g at 2C rate. Hence, the capability of carbon aerogel, synthesized through the present
study in accommodating higher concentration of sulfur, effective management of polysulfide
shuttle and provision of favorable electrode-electrolyte interface to facilitate extended cycling
possibilities at different current densities has been demonstrated.
www.rsc.org/
-
30
-1
1
. Introduction
(5x10
S
cm
at 25°C), electrochemically generated
solid/soluble reaction products, shuttling reaction between long
and small chain polysulfides, irreversible deposition of Li2S
and/or Li S , unavoidable volume changes of the cell upon
The ever increasing energy demands, depleting fossil fuels and
the need for greener and cleaner environment force the
direction of research on renewable energy and energy storage
systems as an impeccable choice. In this regard, lithium–ion
battery could be considered as a superior storage system for
hybrid electric vehicles by its light weight, high operating
voltage and high theoretical capacity than other secondary
2
2
8-10
cycling and the storage performance of sulfur.
So, sulfur
should be embedded in an electrically conductive, polysulfide
confining matrix that may buffer the volume changes associated
with the Li-S system. Towards this direction, engineering of
electrodes with required pore size, pore volume and surface
area to ensure better utilization of sulfur in the electrochemical
cell assembly is the potential approach to qualify lithium-sulfur
batteries for practical applications.
1
-2
batteries. However, to meet with the required energy density
for hybrid electric vehicle applications, lithium-ion batteries
pose problems arising from the inferior theoretical capacity of
heavier transition metal (Fe, Co) based cathodes in stabilizing
the structure and the number of electrons transferred
corresponding to the intercalation and de-intercalation of
Solution mediated infiltration of sulfur into activated
10
mesoporous carbon , confinement of sulfur in ordered porous
11
carbon by melt diffusion strategy with polymer coating ,
3
-5
lithium ions. Interestingly, lithium-sulfur system circumvents
these issues with the general aspects of low cost,
environmentally benign and abundant nature of sulfur, which is
used as a cathode. Considering the transfer of two electrons per
sulfur, the theoretical capacity of 1675 mAh/g and the high
theoretical energy density of 2600 Wh/Kg of lithium-sulfur
battery could be understood based on the simplified over all
infusion of sulfur into hollow carbon to ensure untroubled
charge transport even at high rated current and effective
trapping of polysulfides , hollow carbon fiber host for sulfur
12
guest in a small wall thickness of carbon for beneficial electron
13
and lithium ion movement , superior performance of graphite
14
plane intercalated sulfur in disordered carbon are the few
examples of carbon based sulfur electrodes that demonstrate the
superiority and suitability of the same for application in Li-S
batteries.
6
-7
reaction
16Li + S8
8Li2S
However, the incredible performance of lithium-sulfur
system is hindered by factors such as insulating nature of sulfur
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1
5
In addition to carbon, graphene-sulfur and carbon black aerogel, consisting of mesoporous carbon in larger proportion
1
6
attached graphene wrapped PEG covered sulfur to increase along with the co-existence of micropores.
the utility of sulfur are reported in the literature. Graphene with
1
7
several unique properties , despite being considered as a 2. Experimental Section
beneficial additive for lithium-ion batteries at high rate
1
8
15
2.1. Materials
applications , could not solve the issues of sulfur cathode and
it requires specific structures to ensure high utilization of
1
6
19
All the chemicals were obtained from Sigma-Aldrich/ Alfa
Aesar. Deionized water with a conductivity of 0.055 μS/cm has
been used. All the chemicals were used without any further
purification.
sulfur. Further, conducting polymers such as polypyrrole ,
2
0
21
polythiophene ,
poly(3,4-ethylenedioxy-thiophene) ,
2
2
23
polyaniline and poly(acrylonitrile) have also been deployed
to chemically and/or physically confine sulfur and polysulfides
to alleviate the undesired dissolution, apart from enhancing the
conductivity to improve the overall performance of Li-S
batteries. Similarly, polysulfide adsorbing insulating materials
2.2. Synthesis of SCAC
Resorcinol-formaldehyde (RF) gel was obtained using R:F =1:2
and R:C=50:1. For R:F= 1:2, the typical mole ratio corresponds
to 0.29 M of R and 0.58 M of F. The precursor was dissolved in
deionized water with stirring and the pH of the solution was
brought below 6 by the careful addition of dilute nitric acid,
without changing the temperature of the solution. The solid
content of the solution was maintained as 50 wt. %. After
stirring for half an hour, the solution was transferred to a
hydrothermal setup and maintained at 85 °C for a week. The
obtained deep red gel was washed well with deionized water to
remove the residual catalyst and kept in acetone bath at 50 °C
for 3 days for water replacement. The water replaced gel was
dried at 50 °C for 1 day and carbonized at 800 °C over a period
of 3 h at a ramp rate of 1.5 °C/min. The product thus obtained
contains 40 % RF by weight and it is designated as CA. Sulfur-
carbon aerogel composites (designated as SCAC-X where X=
40, 50, 60, 66, 73 wt. % of sulfur) have been obtained by melt
diffusion strategy. Required amount of sulfur was mixed well
with CA and kept at 280 °C for 12 h to obtain SCAC-X
composites. Subsequently, the product was cooled down to
room temperature at a cooling rate of 2 °C/min.
such as SiO2 and Al O3 are also reported to improve the
2
electrochemical performance of sulfur cathode in Li-S
2
5-26
system.
Among the existing cathodes, carbon based sulfur
composites are preferred, due to the provision of facile electron
channel to sulfur and better polysulfide confinement. Mostly,
such porous carbon materials for sulfur are obtained by means
of template methods, which involve number of sequences and
hence tedious. On the other hand, the present study deals with
the development of carbon aerogel, wherein porous nature has
been achieved by cross-linking during sol-gel condensation,
thus excluding porogen elimination. Such an aerogel assumes
importance for its ability to tailor the pores and morphology, by
tuning the synthesis conditions and accordingly the
impregnation and utilization efficiency of loaded sulfur could
also be modified. Herein, acid catalysed cross linkage
2
7-28
formation and the deployment of optimized pH
are taken in
to special consideration to obtain the carbon aerogel with large
surface area, desired pore size, distribution of pores and pore
volume. Because, it is well known that while microporous
carbon facilitates higher rate performance of sulfur cathode, the
mesoporous carbon enhances lithium transport properties,
besides ensuring the confinement of polysulfides and sulfide
ions obtained from the electrochemical reaction. In other words,
microporous carbon suffers from lower sulfur loading and the
mesoporous nature enhances the rate capability behavior of
sulfur cathode. Further, it is well known that the amount of
loaded and utilized sulfur in the cathode determines the
2.3. Electrode Fabrication and Coin Cell
Assembly
To prepare the electrode, the sulfur-carbon aerogel composite
(
SCAC-X) was mixed with super-P carbon black and
polyvinylidene fluoride binder in the weight ratio of 80:10:10
and N-methyl-2- pyrrolidone was used to obtain a slurry. This
slurry was coated over aluminium foil current collector and
then dried at 60 °C for 12 h. The electrochemical performance
of SCAC-X has been studied using 2032 coin cells (Hohsen
Corporation) by deploying the electrode punched out from the
coated foil with a diameter of 15.9 mm as working electrode.
Lithium metal was used as counter and reference electrode and
polypropylene separator has been used. The electrolyte used
was 1M lithium bis(trifluoromethanesulfonyl)imide and 0.1M
lithium nitrate, dissolved in a co-solvent of 1,3-dioxolane and
2
8
volumetric energy density of the cell.
Based on these grounds, mesoporous carbon aerogel from
acid mediated resorcinol-formaldehyde (RF) sol-gel reaction,
obtained at a pH of below 6 has been prepared with a specific
2
surface of 1395 m /g, which is found to be bestowed with the
possibility of accommodating higher amount of sulfur. The
currently prepared carbon aerogel containing 60 wt. % sulfur
supersedes the previous work with limited volumetric energy
2
9
density. The appreciable performance of S@C composites
containing different concentrations of sulfur loading has been
demonstrated. Higher sulfur loading and the maintenance of
appreciable capacity of 600 mAh/g upon extended cycling (up
to 100 cycles) are the twin advantages achieved from the
currently developed sulfur cathode, impregnated in a carbon
1,2-dimethoxyethane (1:1v/v ratio).
2.4. Characterization
2
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The prepared CA and sulfur-carbon aerogel composites carbon aerogel, where it gets solidified and the structural
SCAC-X) were analysed using the following instruments. growth upon heat treatment and consequent cooling is limited
(
XRD was recorded with the Bruker D8 Advance X-ray to the pores size of the carbon aerogel. The decrease in intensity
diffractometer (XRD) using Ni-filtered Cu Kα radiation. TGA of broad carbon peak indicates the presence of sulfur in a well
and Raman spectral studies of SCAC-X were performed using dispersed state. Interestingly, the absence of crystalline peaks
TA Instruments SDT Q600 thermogravimetric analyzer and corresponding to sulfur in CA (from SCAC-40 to SCAC-66)
Renishaw Invia Raman Microscope using He-Ne LASER at an indicates that CA can accommodate sulfur upto 66 wt. % in the
exciting wavelength of 633 nm respectively. N2 available pores (more discussion is available under N isotherm
2
adsorption/desorption
isotherm
was
obtained
from section). On the other hand, the solidification of sulfur shows
Quantachrome Instruments version 11.02 and XPS results from distinct and peculiar behavior for higher amount (>66 wt. %) of
MULTILAB 2000 Base system. Morphology changes after loading. In other words, the appearance of less intense
sulfur loading and elemental mapping were studied using crystalline peaks of SCAC-73 matches with the monoclinic
Gemini Field Emission Scanning Electron Microscopy system (JCPDS No: 79-1517) rather than the orthorhombic
(
FESEM), Tecnai 20 G2 (FEI make) Transmission Electron structure, thus evidencing the fact the carbon aerogel stabilizes
Microscopy (TEM) and STEM JEOL, JEM 2100 respectively. sulfur in its monoclinic phase at room temperature, especially
Cyclic Voltammetry (CV) and impedance measurements were when the loading concentration exceeds 66 wt. %, which is in
3
0
carried out using
a
VMP3 multichannel potentiostat– line with a similar report on carbon nanotube . i.e., 73 wt. % of
galvanostat system (Biologic Science Instrument). Charge– sulfur content is slightly higher than the maximum possible
discharge studies were carried out using Arbin charge– level of loading with respect to the currently prepared carbon
discharge cycler.
aerogel. Hence, one can understand that the carbon aerogel of
the present study can accommodate a sulfur content as high as
~70 wt. %, which is an interesting observation.
3
. Results and Discussion
Fig. 1 XRD pattern of sulfur, carbon aerogel, 40 wt. %(SCAC-
0), 50 wt. %(SCAC-50), 60 wt. %(SCAC-60), 66 wt.
(SCAC-66) and 73 wt. %(SCAC-73) sulfur loaded carbon
Fig. 2 Thermogravimetry analysis of sulfur-carbon aerogel
composites.
4
%
aerogel.
Thermogravimetry analysis was performed under nitrogen
atmosphere and upto 500 °C with a heating rate of 10 °C/ min.
From this measurement, -dW/dT versus temperature has been
plotted and is displayed in Fig. 2 along with the variation in
weight loss as a function of temperature. The amount of sulfur
in CA has been calculated based on the weight loss of the
composite at 500° C and these values are used to formulate and
denote the samples with a general representation SCAC-X,
where X stands for the sulfur content (e.g. SCAC-40 contains
XRD pattern of sulfur, carbon aerogel and the series of SCAC
composites is shown in Fig. 1. Herein, CA exhibits two broad
peaks around 2θ=24 and 44°, corresponding to the (002) and
(
101) graphite planes, thus indicating the amorphous nature of
carbon. The d-spacing value calculated for (002) plane is 0.370
nm. Crystalline peaks of sulfur indicate the fddd orthorhombic
structure, which are matching with the JCPDS pattern No: 08-
0248. From the XRD pattern of composites, it is clear that the
40 wt. % of sulfur). In other words, the adsorption ability of
melt diffused crystalline sulfur has been converted to the
amorphous state. Further, it provides an indication that more
likely the molten sulfur diffuses into the porous structure of
carbon aerogel for various amounts of sulfur loading can be
evaluated using TGA analysis. Since it is based on the
evaporation of sulfur from the pores of carbon aerogel, the
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Fig. 3 Raman spectrum of (a) carbon aerogel, (b) sulfur-carbon aerogel composites and (c) 50 wt. % sulfur loaded and sulfur
removed carbon aerogel composites.
2
region occupied by –dW/dT versus temperature curve indicates of pairs of sp carbon corresponding to E symmetry (G band)
the adsorption ability of CA vs. amount of sulfur loading.
2
g
1
0
-1
that appears at 1597 cm , defect or graphite stimulated six fold
From Fig. 2, it is evident that major weight loss occurs at 240° aromatic ring breathing vibration corresponding to A (D
1
g
-
1
C for SCAC-60 that occupies the highest region than other band) at 1320 cm along with the overtone of in-plane phonon
SCAC samples, thus inferring the need for more thermal energy related 2D peak centered at ~ 2581 cm and D+G peak at ~
to remove sulfur from CA. Hence, the relatively superior 2896 cm are evident. The presence of such a combination of
-
1
-
1
thermal stability of SCAC-60 composite could be understood.
D, G, 2D and D+G peaks confirms the presence of nano and
polycrystalline graphitic carbon tied with amorphous carbon in
the currently prepared carbon aerogel. The crystallite size La
can be calculated from the intensity ratio of D and G bands,
which is about 39.45 nm.
Raman spectra of chosen combination of sulfur-carbon
aerogel composites are shown in Fig. 3(b). It is quite interesting
3
1
32
D-
G-
Raman Raman
I /
IG
La
(nm)
D
ID
IG
shift
cm )
shift
(cm )
-
1
-1
(
CA
1320.23 1597.91 510.96 526.40 0.97 39.45
to note that the I
), as evident from the increasing intensity of broader D band
that surpasses the narrow G band. However, the I /I value
increases from SCAC-40 to SCAC-60 and decreases thereafter
with the increasing concentration of sulfur (such as SCAC-66
and SCAC-73). Hence, it is understood that the amount of
sulfur dispersed into the carbon aerogel could be quantified
D G
/I ratio increases upon sulfur loading (Table
1
SCAC-
D
G
1
1
1
315.58 1597.03 333.07 302.00 1.10 34.72
319.30 1601.46 312.45 266.58 1.17 32.67
321.16 1597.91 464.12 386.24 1.20 31.86
4
0
SCAC-
5
0
SCAC-
using the calculated I /IG values i.e., with the increasing
6
0
D
loading of sulfur, disorder also increases in the carbon aerogel
with an exception of SCAC-66 and SCAC-73. However,
Raman spectrum recorded for SCAC-73 is entirely different
from the rest of the composites, as it contains the co-existing
Raman signatory peaks due to sulphur and CA. This in turn is
an indication that a small portion of sulfur remains on the
surface of the CA, besides filling the pores of carbon aerogel.
From this observation, it is understood that a concentration of
SCAC-
1
1
316.51 1597.03 453.23 388.39 1.17 32.81
325.85 1604.12 298.73 271.16 1.10 34.76
6
6
SCAC-
7
3
Table 1 Crystallite size and other parameters derived from
Raman spectral behavior of carbon aerogel and S@C
composite.
73 wt. % sulfur is slightly excess than the maximum possible
concentration that could be accommodated in the pores of the
CA, thus helping us to conclude that nearly 70 wt. % sulfur
uptake is possible with the currently prepared CA that blocks
the surface of CA partially. This is in agreement with the XRD
results, wherein peaks corresponding to CA are not visible in
SCAC-73. The effect of sulfur loading in changing the graphitic
nature of CA towards amorphous form is well understood from
Raman microscopy is a non-destructive technique, which is
a powerful tool to analyse the nature of carbon materials such
as graphitic or amorphous. It is quite reasonable to study the
change in the structure of carbon in the carbon aerogel,
especially upon sulfur loading and its interaction using this
technique. In Fig. 3(a), the intense in-plane stretching vibration
4
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the comparison of Raman spectra in Fig. 3(c) of SCAC-50 and the highest theoretical loading possibility of sulfur, which has
the corresponding sulfur removed CA (sulfur has been removed been calculated to be 74.66% in CA. As expected, the pore
by heating SCAC-50 in N atmosphere for 2h at 500 °C and the volume and surface area of CA decrease with the increasing
2
temperature has been fixed based on TGA study). Such a sulfur loading, thus substantiating the assumption that the
typical and random analysis of Raman spectral behavior probable location of sulfur is in the pores. For 50 wt. %
confirms the fact that the added sulfur plays a vital role in loading, the specific pore volume and specific surface area are
3
2
changing the slightly pronounced amorphous nature of CA reduced to 0.707 cm /g and 331.046 m /g and for 60 wt. %
3
2
gradually. However, the co-existence of graphitic and loading, it further reduces to 0.237 cm /g and 16.830 m /g
amorphous nature of CA is always beneficial for sulfur cathode respectively. These values are consistent with the previous
in terms of electrical network (otherwise known as carbon inference related to the location of sulfur in CA host at the
wiring) and buffers the volume changes. Hence, Raman study pores. As a result, enhanced electrical conductivity and better
offers an indication that SCAC-60 may give rise to better buffering to admit the volume expansion during cycling
electrochemical behavior compared with the rest of the performance are expected with the currently designed sulfur
composites and SCAC-73 composite may not be suitable for impregnated carbon aerogel.
application in Li-S batteries (as inferred from XRD also).
Sample
Pore
Radius
Specific
surface
Specific
Pore
(
nm)
Area (m²/g)
Volume
(
cm3/g )
1.42
CA
9.68
9.63
9.60
1395.62
331.05
16.83
SCAC-50
SCAC-60
0.71
0.24
Table 2 Textural parameter of carbon aerogel and select
concentration of sulfur loaded carbon aerogel.
FESEM images of carbon aerogel at low and high
magnification are shown in Fig. 5(a) and (b). It is obvious from
the figure that the particles are properly interconnected with
each other to enable three dimensional electron conduction and
the particle size is found to be less than 100 nm. The available
pores of the interconnected particles render transport path to
sulfur, especially during the melt diffusion and aided the
confinement of sulfur in the porous structure. TEM image in
Fig. 5(c) also shows the interconnected network of CA, wherein
region that appears as grey corresponds to the overlap at the
edges. However, three or more particles also overlap at the
corners and give rise to slightly dark regions, wherein the
increased thickness is responsible for the observed darkness.
Further, the amorphous nature of the carbon aerogel is
understood from the corresponding SAED pattern appended as
an inset of Fig. 5(c). In contrast to Fig. 5(c), HRTEM image of
SCAC-60 in Fig. 5(d) contains periodically varying grey and
black colour patterns. Not obvious any overlap of two or more
particles in the black area and hence the black colour region is
believed to be due to the presence of impregnated sulfur while
the grey region corresponds to that of carbon aerogel. The
appearance of periodic pattern of black and grey area is due to
the homogeneous distribution of embedded sulfur into the pores
of carbon aerogel, thus evidencing the effective and uniform
confinement of sulfur in CA. The corresponding SAED pattern
in the inset of Fig. 5(d) confirms the amorphous nature of sulfur
impregnated in the carbon aerogel and the same has been
substantiated by the XRD results of S@C composites. TEM
lattice fringes with a spacing of 0.365 nm (Fig. 5(e)) coincide
with the d-spacing in XRD pattern of CA(d(002)=0.370 nm), thus
Fig. 4 (a) Nitrogen adsorption-desorption isotherm of carbon
aerogel and select sulfur-carbon aerogel composites and (b)
corresponding pore volume distribution.
Nitrogen adsorption-desorption isotherms were recorded at
77 K for the CA, SCAC-50 and SCAC-60 in the range of
relative pressure P/P from 0.1 to 1.0. The isotherm curve in
0
Fig. 4(a) corresponding to carbon aerogel exhibits type-IV
3
3
isotherm with H1 hysteresis and the capillary condensation is
centered at a relative pressure of 0.82. Similar behavior has
been noticed for SCAC-50 and SCAC-60, except the decrease
in adsorbed volume observed as a function of increasing sulfur
content. The decrease in adsorbed volume is an evidence that
the elemental sulfur occupies the available pores of CA. BET
surface area and related parameters of CA are calculated and
the values of surface area, maximum pore width and pore
volume are tabulated (Table 2). Surprisingly, higher specific
2
3
surface area and pore volume of 1395 m /g and 1.423 cm /g
have been calculated for CA, corresponding to a maximum
width of 9.676 nm of carbon aerogel, deduced from the pore
distribution curve (Fig. 4(b)). It is quite reasonable and impetus
to use such a carbon aerogel with an appreciable high surface
area to impregnate sulfur and to exploit the same as a potential
cathode for Li-S system. Such an idea gets validated based on
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title cathode with an unavoidable association of trace amount of
oxygen (which is not unusual).
In order to confirm the uniform distribution of sulfur in the
carbon aerogel, elemental mapping has been done by scanning
transmission electron spectroscopy (STEM) for SCAC-60
sample. The area selected for mapping is shown in Fig. 6(a).
STEM mapping suggests that sulfur is uniformly distributed in
the carbon aerogel and the cumulative mapping results in Fig.
6(d) further ensures the intimate contact of sulfur with the
carbon aerogel.
Fig. 5 (a) Low and (b) high magnification FESEM images of
carbon aerogel (c) TEM image (inset-SEAD pattern) of carbon
aerogel (d) Low and (e) high magnification HRTEM images
(
inset-SEAD pattern) and (f) EDX spectrum of 60 wt. % of
sulfur loaded carbon aerogel.
Fig. 7 (a) Survey spectra, (b) 1s carbon spectra of CA and
validating the correlation of TEM based results with those of SCAC-60 and (c) and (d) correspond to 2s and 2p spectra
XRD. The discontinued lattice fringe patterns observed containing fitted photoelectron lines of sulfur in SCAC-60
throughout the TEM image confirms the co-existence of respectively.
disordered and graphitic nature of CA, which coincides with the
results of Raman studies. Energy dispersive X-ray spectrum of
The elements present and the corresponding oxidation state
SCAC-60 (Fig. 5(f)) shows pronounced peaks for carbon and of loaded sulfur in CA can be identified using XPS analysis,
sulfur and negligibly small signature for oxygen, thus which in turn could be used to substantiate the observations
confirming the elemental composition and stoichiometry of the made from Raman studies. Fig. 7(a) shows survey spectra of
CA and SCAC-60 and the individual scans of identified peaks
are shown in Fig. 7(b)-(d). Carbon and elemental oxygen are
identified in CA and carbon, sulfur and oxygen are present in
SCAC-60 as per Fig 7(a). The oxygen found in the XPS
analysis may be an adsorbed atmospheric oxygen in CA. The
peak appeared at 284.14 and 284.17 eV for CA and SCAC-60
in Fig 7(b) reveals that the binding energy of 1s electron of the
3
4
C-C bond in carbon aerogel, corresponding to the full width at
half maximum (FWHM) of CA is 1.43 eV, while it is 1.73 eV
for SCAC-60. Sulfur 2s peak appeared at 222.80 eV (Fig. 7(c))
and the 2p peaks of sulfur contain more information about the
bonding nature of sulfur in carbon aerogel. As presented in Fig.
7
(d), the 2p raw spectrum of sulfur can be deconvoluted into
four component peaks. The peak at 165.50 eV corresponds to
p1/2 spin-orbital coupling of 2p electrons and the other peak
2p -I, 2p -II and 2p -III) values at 165.0, 164.30 and
2
(
3/2
3/2
3/2
1
63.80 eV correspond to the presence of S
═
O, 2p3/2 spin-orbital
3
5-36
Fig. 6 (a) Selected area of SCAC-60 composite subjected to coupling and C─S bond respectively.
Hence, it is confirmed
elemental mapping (b) carbon (c) sulfur and (d) cumulative from XPS analysis that some of the sulfur atoms are chemically
carbon elemental mapping image. bonded to the carbon aerogel.
6
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different. Moving from SCAC-50 to 66, the anodic peaks shift
towards the cathodic direction and SCAC-50 shows exact
th
nd
confinement of 10 cycle with that of the 2 cycle, except the
reduced overall current that slightly shifts the peak current
position at 2.38 V. On the other hand, SCAC-60 cathode shows
a small shift in the peak position with almost negligible change
in peak current. The appearance of pronounced and well
distinguished peak at 2.38 V appears for SCAC-66 cathode
also, which is not significant as that of SCAC-60.The
appearance of an additional anodic peak at 2.38 V observed
during extended cycling is more pronounced in SCAC-60
cathode, compared with that of SCAC-66 cathode. Such an
appearance of additional peak arises, when the sulfur forms an
1
1
intimate contact with the carbon aerogel. So, an overall
improved performance has been observed for SCAC-60
cathode due to the availability of desired quantity of unfilled
pores to buffer the volume expansion. The thermal distribution
of sulfur in CA scaffold is found to get electrochemically
redistributed, which reduces the polarization in oxidation peaks.
Fig. 8 Second and tenth CV cycle behavior of (a) SCAC-50 (b) The background current of CV curve shows the absence of
SCAC-60 and (c) SCAC-66 composite cathode recorded at a unwanted red-ox reactions in the system, which is noteworthy.
scan rate of 0.1 mV/s
The overall cycling performance observed in CV could be
best understood and correlated using the following equations.
Cyclic voltammetry behavior of 50 (Fig. 8(a)), 60 (Fig 8(b))
and 66 (Fig. 8(c)) wt. % sulfur loaded CA cathodes shows
typical and characteristic peaks of sulfur with a slight
difference between initial and progressive CV cycles. Two
sharp and distinguishable cathodic peaks are observed, which
2Li + S ↔ Li S (overall reaction)
(1)
(2)
(3)
(4)
(5)
2
2Li + S ↔ Li S
2 8
8
2Li + Li S ↔ 2Li S
2 4
2
8
2Li + Li S ↔ 2Li S (insoluble)
2
4
2 2
2Li + Li S ↔ 2Li S (insoluble)
2
2
2
evidence the disproportional electrochemical reduction of In short, reactions (2)-(3) correspond to the first plateau and the
3
8
elemental sulfur into higher order polysulfides and their last two reactions to the second plateau.
subsequent cathode reduction to Li S/Li S upon extended Electrochemical performance of different concentration of
2
2 2
cathodic potential. The first reduction peak appears at a sulfur loaded CA cathodes has been evaluated using
potential of 2.35, 2.35 and 2.33 V corresponding to SCAC-50, galvanostatic charge/discharge studies performed at C/10 rate
60, 66 cathodes during the initial cycle and the same is shifted (C=1675 mA/gsulfur) (Fig. 9) and the current calculation has
respectively to 2.36, 2.35 and 2.34 V upon progressive cycling been made, based on the weight of sulfur in the electrode.
th
(
10 CV cycle). The shift in potential upon progressive cycling Typical two plateau electrochemical reduction behavior of
is found to be admissibly small for SCAC-50 and SCAC-60 voltage profile has been exhibited by all the SCAC-X
cathodes, while the difference in SCAC-66 cathode is little electrodes (Fig. 9(a)), thus evidencing the involvement of same
significant. The utilization of sulfur is found to be higher only reduction mechanism in all the chosen cathode formulations.
upon progressive cycling with a special reference to SCAC-66 Reduction begins from OCV, which ranges from 2.44 to 2.30 V
cathode, which is an indication that the initial volume and exhibits the presence of first and second reduction plateau,
expansion of sulfur in carbon aerogel hampers the complete found to present closer to each other. This is an indication that
2
4
utilization of active material. Second reduction peak potential more sulfur is physically adsorbed (free S8). The significant
of SCAC-(50, 60, and 66) cathodes appears at 2.06, 2.07 and reduction of lower order polysulphides in the potential range
nd
2
1
.07 V and 2.06, 2.06 and 2.06 V corresponding to the 2 and below 2.1 V could be observed for SCAC-40 and 50 cathodes,
th
0 cycle respectively. No obvious change in peak potential has due to the dissolution of higher amount of sulfur in the
been noticed for all the three chosen concentration of sulfur. electrolyte. Single plateau oxidation occurs during the initial
However, the peak current value is found to get reduced upon charging of all the cathodes. However, reasons such as higher
extended cycling with an exception of SCAC-60 cathode. In electrolyte contact area on lower loading and the non
other words, more surface area of SCAC-50 cathode active availability of sufficient pores to buffer volume changes on
material that gets exposed to the electrolyte available in unfilled more loading of sulfur are believed to be the reasons for the
3
7
pores enhances the dissolution of active material.
appearance of second oxidation plateau that has been observed
Subsequent anodic scan exhibits one broader peak for single for all the cathodes, with an exception of 60 wt. % loading (in
th
step conversion of Li S /Li S into Li S /S . But the CV peak 50 cycle). Further, the length of the second plateau
2
2
2
2
8
8
pattern pertinent to the initial and extended cycles is entirely corresponds to 32- 37 % of the total charging capacity of the
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Fig. 9 (a) Comparative galvanostatic charge/discharge (b) cycle life (c) selective cycle life behavior of all sulfur-loaded carbon
aerogel composites under a current rate of C/10 and (d) extended cycle life study of SCAC-60 cathode
individual cathodes, which is an indication that the resultant
capacity arises from the reduction of higher order polysulfide to
elemental sulfur. In other words, dissolved polysulfides (liquid
state) are converted into solid sulfur, which gets accumulated
on the surface of carbon rather than accommodating the pores
of CA. The lesser polarization behavior exhibited by SCAC-60
composite cathode substantiates the recommendation of the
same as the optimum composition of SCAC-X cathode, suitable
for Li-S system. Further, the absence of second plateau
observed with the SCAC-60 cathode is an evidence that the
loaded sulfur has occupied the available pores of CA and no
undesirable accumulation of sulfur could be seen on the surface
of CA to reduce the effective utilization of sulfur. As a result,
SCAC-60 cathode with negligible polarization change has been Fig. 10 Experimental and fitted Impedance spectra of the as
concluded as the better performing cathode through the present fabricated cell containing SCAC-60 cathode and the cell after
study.
completing 1 and 100 cycles (Inset- corresponding equivalent
Cycleability studies presented in Fig. 9(b) display initial circuit).
discharge capacity values of 879, 763, 918, 636 and 533 mAh/g
with respect to SCAC-X cathodes where X= 40, 50, 60, 66 and
State
R (Ω)
e
R (Ω)
ct
7
3 respectively. Among the chosen cathodes, the lowest
Before cycling
1.68
2.93
50.86
23.27
33.25
capacity has been observed for SCAC-66 and 73 cathodes. It is
found that the capacity gets stabilized invariably after 10 cycles
and the capacity of SCAC-X (X-40, 50, 60, 66, and 73)
cathodes after 50 cycles is found to be 450, 413, 649, 435 and
st
After 1 cycling
th
After 100 cycling 5.61
335 mAh/g, which is about 51, 54, 71, 68, 63 % of the respecti
ve initial capacity value. An appreciable and higher capacity
value of 649 mAh/g with a better retention behavior has been
shown by SCAC-60 cathode, as evident from Fig. 9(c). Based
on these observations, SCAC-60 cathode has been subjected
Table 3 EIS parameters of SCAC-60 cathode at different state
of cycling.
8
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Fig. 11 (a) High (1C) rate cycling behavior (inset- selective discharge/charge curves) (b) discharge/charge curves (c) retention of
discharge plateaus and (d) cycling and retention behavior of SCAC-60 cathode under the influence of different current rates.
further to extended cycleability, high rate performance and rate cell and it shows 50.86 Ω before cycling, which is found to
st
th
capability analyses.
decrease to the extent of 23.27 and 33.25 Ω after 1 and 100
Extended cycleability of SCAC-60 cathode (in Fig. 9(d)) at cycle respectively. As discussed in CV, the decrease in the
C/10 rate shows a nominal capacity of 600 mAh/g after 100 charge transfer resistance after cycling is most likely due to the
cycles with an acceptable degradation of 0.37 % per cycle. At fact that sulfur gets rearranged in the pores and facilitates easy
th
the 100 cycle, it exhibits a capacity of 600 mAh/g, which is 87 access to both electron and lithium ions. The slight increase in
th
th
%
of 30 cycle and 93 % of 50 cycle capacity. Similarly, the the charge transfer resistance indicates the superior conductive
coloumbic efficiency is found to be 90-95 % during the first 10 and buffering nature of CA to improve the electrochemical
cycles and increased subsequently to 97 % upon extended performance of sulfur. The observed inclined line in lower
cycling, due to improved stability.
Experimentally observed and fitted impedance spectra of electrode, which also follows the same trend for both 1 and
the as fabricated cell containing SCAC-60 cathode and the cell 100 cycle. Hence, the lowest charge transfer resistance of the
frequency region corresponds to the diffusion kinetics of
st
th
after completing 1 and 100 cycles are furnished in Fig. 10 with cell and the same trend observed in lower frequency region are
an inset containing the corresponding equivalent circuit model. due to the electrochemically favorable interface properties of
Closer resemblance of observed and calculated values is CA with the electrolyte.
evident from the figure (Fig. 10). The fitted equivalent circuit
The role of CA as a conductive matrix to improve the
contains various circuit elements such as R , Q //R , Q //R and performance of sulfur, formation of Li S and the related
e
s
s
ct
ct
2
Z . Here, R is the electrolyte resistance, R is the surface polysulfide retention in SCAC-60 cathode has been
W
e
s
resistance of lithium and SCAC-60 electrode, R is the charge investigated under a high rated current of 1C, which is shown in
ct
transfer resistance, Q and Q are the constant phase elements Fig. 11(a). It demonstrates an initial capacity of 632 mAh/g
s
ct
3
9
and ZW is the Warburg impedance . From Table 3, the smaller with a coloumbic efficiency of 96 % and retention of 480
th
R value of 1.68 Ω that has been observed before cycling mAh/g at the 50 cycle, which is about 76 % of initial capacity
e
th
exhibits small variation in electrolyte resistance even after and 86 % of 25 cycle capacity with a coloumbic efficiency of
completing 100 cycles. Such an observation indicates that 92 %. This reveals the advantage of CA skeleton in improving
cycling has not much altered the R of the cell and the small the electrochemical behavior of sulfur even at high current rates
e
changes observed with respect to the R of the cell before and and it emphasizes the polysulfide confinement at 1C rate.
e
after cycling is believed to be due to the small amount of Hence, CA prepared in the current study offers multifarious
dissolution of loosely bound sulfur at CA. Depressed semicircle benefits such as conducting additive, potential host for higher
at higher-middle frequency range relates to the R value of the
sulfur loading and better polysulfide confining matrix, which is
ct
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superior than the S@C composite, wherein carbon acts as a intermediately formed polysulfides along with those of
4
0
conducting additive only.
Li S/Li S and Li S respectively. Among the selected S@C
2 2 2 2 8
The voltage profile of SCAC-60 cathode under the composites, SCAC-60 cathode exhibits an appreciable and
influence of different current rates, viz., C/7, C/5, C/2, 1C and progressive capacity of 600 mAh/g upto 100 cycles, with a
2C has been displayed in Fig. 11(b). While the polarization capacity retention of 71 %. Further, an acceptable capacity of
calculated against 50 % of the individual capacity is found to be 480 mAh/g at 1C and 422 mAh/g at 2C rate has been exhibited
small (155 mV) and similar for C/7 and C/5 rates, it is found to by SCAC-60 cathode with an excellent coulombic efficiency of
increase slightly (320 mV) for 2C rate. Such smaller > 95 %. The carbon aerogel can house the reversible
polarization changes demonstrate the intimate contact of the disproportional reaction of sulfur and lithium and offers
sulfur with CA. However, part of the discharge capacity at 2C favourable electrode-electrolyte interface for cycling, as
is limited by lower cut-off voltage, which is unavoidable. Even understood from EIS measurements.
though SCAC-60 cathode provides a significant capacity of 422
mAh/g under the influence of 2C rate, the capacity limiting Acknowledgements
region in the discharge curve can be identified by evaluating the
One of the authors K. Balakumar gratefully acknowledges
contribution of each region to the total capacity and the
Department of Science and Technology, India for offering
corresponding plot is given in Fig. 11(c). Contribution of first
INSPIRE fellowship (Code. No: IF120555). We acknowledge
region on discharge curve (corresponds to conversion of S to
8
financial support of CSIR through MULTIFUN project (CSC-
Li S ) to the total capacity is 33, 36, 39, 44 and 52% for C/7,
2
6
0101), Dr. N Lakshminarasimhan for BET analysis and Mr.
C/5, C/2, 1C and 2C rates respectively and the capacity
retention of this region with respect to C/7 rate are 95, 92, 84,
and 78% for C/5, C/2, 1C and 2C rates respectively. Capacity is
mostly limited by the slow reduction kinetics of the second
Gopala Ram Bhadu and Dr. Divesh N. Srivastava for HRTEM
and STEM analyses.
4
1
Notes
region.
Rate capability of the SCAC-60 cathode was
evaluated under C/7 rate for first five cycles and subsequently
at C/5, C/2, 1C, 2C rates for every other six cycles. Finally, the
cell at 2C rate was brought back to C/7 discharge condition to
understand the capacity retention behavior. From the rate
capability study in Fig. 11(d), capacity values of 882, 782, 667,
Electrochemical Power System Division, CSIR-Central ElectroChemical
Research Institute, Karaikudi- 630 006, India. E-mail: kalaiselvicecri
@
gmail.com
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