Carbon nanofiber-sulfur composite cathode materials with different binders for secondary Li/S cells

Article abstract of DOI:10.1016/j.electacta.2012.01.051

A sulfur-coated carbon nanofiber (CNF-S) composite cathode material was prepared by a chemical deposition method in an aqueous solution. This CNF-S material was evaluated as the cathode material in lithium/sulfur cells with three different binders. The results of the SEM and TGA measurements reveal that CNF-S has a typical core-shell structure, containing 75.7 w/o sulfur coated uniformly on the surface of the CNFs. The effects of different binders on the potential profiles, electrode capacity and capacity retention with cycling were investigated. The electrode prepared with CMC + SBR binder has the best performance compared with PVdF and PEO binders, exhibiting a specific capacity of up to 1313 mAh g^-1 S at the initial discharge and a specific capacity of 586 mAh g^-1 S after 60 cycles.

Full text of DOI:10.1016/j.electacta.2012.01.051

Electrochimica Acta 65 (2012) 228–233
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Carbon nanofiber–sulfur composite cathode materials with different binders for
secondary Li/S cells
Mumin Raoa,b,1, Xiangyun Songb, Honggang Liaoc, Elton J. Cairnsa,b,∗
a Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720, USA
b Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
c Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 August 2011
Received in revised form 11 January 2012
Accepted 13 January 2012
Available online 23 January 2012
A sulfur-coated carbon nanofiber (CNF–S) composite cathode material was prepared by a chemical depo-
sition method in an aqueous solution. This CNF–S material was evaluated as the cathode material in
lithium/sulfur cells with three different binders. The results of the SEM and TGA measurements reveal
that CNF–S has a typical core–shell structure, containing 75.7 w/o sulfur coated uniformly on the sur-
face of the CNFs. The effects of different binders on the potential profiles, electrode capacity and capacity
retention with cycling were investigated. The electrode prepared with CMC + SBR binder has the best per-
formance compared with PVdF and PEO binders, exhibiting a specific capacity of up to 1313 mAh g−1 S at
the initial discharge and a specific capacity of 586 mAh g−1 S after 60 cycles.
Keywords:
Sulfur
Carbon nanofiber
Binder
© 2012 Elsevier Ltd. All rights reserved.
Cathode material
Lithium cell
1. Introduction
to increase the electronic conductivity in the sulfur cathode and
supply electrochemical reaction sites for the sulfur [11–14].
High specific energy lithium ion rechargeable batteries are being
developed as important power sources for clean and efficient
energy storage and conversion technologies, especially for electric
vehicles (EVs) and hybrid electric vehicles (HEVs) [1–3]. Existing
Li-ion cells have a specific energy of up to 200 Wh kg−1, which
is not high enough for a 200+ mile range EV. The Li/S cell offers
the opportunity for a much higher specific energy. Elemental sul-
fur possesses a high theoretical specific capacity of 1675 mAh g−1
In this work, we report a carbon nanofiber–sulfur (CNF–S) com-
posite material prepared by a chemical deposition method in an
aqueous solution. The carbon nanofibers (CNFs) are an attrac-
tive choice as the electronic conductor for the sulfur electrode
because they can provide a more effective electronically con-
ductive network than other conductive additives, such as carbon
black (CB), acetylene black (AB) and graphite. The linear carbon
materials provide an effective conduction path and a network-
like structure that forms a stable structure for the sulfur electrode
during the charge–discharge process [15–17]. The electrochemi-
cal performance of the CNF–S composites as cathode material for
rechargeable lithium batteries is investigated. The binder material
has been found to play a very import role in stabilizing the elec-
trode structure [18–20]. Because the sulfur expands by 76% as it is
converted to Li2S, it is important that the binder has elastomeric
properties in order to accommodate the significant volume change
that accompanies charge/discharge. As reported here, the effects of
three different binders for the carbon nanofiber–sulfur material on
both the potential profiles and capacity were investigated.
and the Li/S cell has a theoretical specific energy of 2600 Wh kg−1
.
Moreover, sulfur is inexpensive, abundant and nontoxic. There-
fore, sulfur is an attractive positive active material for high-specific
energy Li/S cells [4–6]. However, the electrical insulating nature of
sulfur and the high solubility of lithium polysulfides as intermedi-
ate products generated during the discharge process in traditional
organic electrolyte have resulted in very poor cycle life [7–10]. Sev-
eral approaches have so far been used to tackle this problem. One
approach is to reduce the sulfur particle size to improve the utiliza-
tion of sulfur. Another approach is using different kinds of carbon
∗
Corresponding author at: Department of Chemical and Biomolecular Engineer-
2. Experimental
ing, University of California, Berkeley, CA 94720, USA. Tel.: +1 510 486 5028;
fax: +1 510 486 7303.
2.1. Preparation of CNF–sulfur composites
E-mail address: ejcairns@lbl.gov (E.J. Cairns).
Visiting researcher from College of Materials Science and Engineering, South
1
CNF–sulfur composites were prepared by a chemical deposi-
tion method as discussed in our previous report [21,22]. Carbon
China University of Technology, Guangzhou 510641, PR China (Supervisor: Prof.
Weishan Li).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.01.051

M. Rao et al. / Electrochimica Acta 65 (2012) 228–233
229
Fig. 1. SEM images of CNFs (a) and CNF–S composite (b–d); EDX spectra of CNF–S composite (e).
nanofibers (CNFs) were purchased from Sigma–Aldrich. 0.58 g of
sodium sulfide (Na2S, anhydrous, Aldrich) was added into a flask
that has been filled with 20 ml distilled water to form a Na2S
solution, then 0.72 g elemental sulfur was suspended in the Na2S
solution and stirred with a magnetic stirrer for about 2 h at room
temperature to obtain a sodium polysulfide (Na2Sx) solution. The
color of the solution changed slowly to orange-yellow as the sul-
fur dissolved. After dissolution of the sulfur, a sodium polysulfide
(Na2Sx) solution was obtained:
2 h. After reaction, the resulting suspension was filtered, and the
precipitate was repeatedly rinsed with acetone and distilled water
to remove surfactant. Finally, the precipitate was dried in vacuum
at 50 ◦C for 24 h:
Sx2− + 2H+ → (x − 1)S + H2S
2.2. Preparation of the CNF–sulfur cathode
The CNF–S electrode was prepared from a mixture of 70 wt.%
of CNF–S, 20 wt.% of carbon black, 10 wt.% of binder. The resulting
slurry was coated onto a carbon-coated Al foil substrate using a
doctor blade. The solvent was allowed to evaporate overnight at
ambient temperature. The resulting cathode film (ca. 50 ␮m thick)
was used to prepare the cathodes by punching circular discs having
an area of 0.9 cm2. These discs were dried at 50 ◦C under vacuum
for 48 h before use.
Na2S + (x − 1)S → Na2Sx
0.2 g CNFs were dispersed in 100 ml distilled water and sonicated
for 10 h to achieve a homogenous suspension. An appropriate
volume of 0.1 mol L−1 NaOH solution was added to the above solu-
tion to adjust its pH > 8, and then the sodium polysulfide (Na2Sx)
solution and surfactant hexadecyl trimethyl ammonium bromide
(CTAB) were added and the mixture was sonicated for 1 h to prepare
Solution (I).
Cathode with PVDF binder: 70 wt.% of CNF–S, 20 wt.% of carbon
black, and 10 wt.% of polyvinylidenefluoride (PVDF). N-methyl-2-
pyrrolidone (NMP) served as dispersant.
Solution (I) was titrated into 100 ml of 2 mol L−1 formic acid
solution (HCOOH) at a rate of 30–40 drops/min with stirring for

230
M. Rao et al. / Electrochimica Acta 65 (2012) 228–233
Fig. 2. TEM images of CNFs (a and b) and CNF–S composite (c and d).
Cathode with PEO binder: 70 wt.% of CNF–S, 20 wt.% of carbon
black, and 10 wt.% of polyethylene oxide (PEO). Acetonitrile served
as dispersant.
Cathode with CMC + SBR binder: 70 wt.% of CNF–S, 20 wt.%
of carbon black, 10 wt.% of carboxy methyl cellulose (CMC) and
styrene butadiene rubber (SBR) (CMC:SBR = 2:3, by weight). Water
served as dispersant.
the electrodes were implemented with the use of a Solartron 1480.
The impedance spectroscopy sweeps were conducted from 10 kHz
to 100 mHz at an amplitude of 5 mV. The discharge–charge cycling
was carried out with a Maccor cycle life tester using the voltage
range 1.5–2.8 V. The specific capacity was calculated on the basis
of the weight of sulfur in the positive electrode.
3. Results and discussion
2.3. Preparation of electrolyte
3.1. SEM and TEM
N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)
imide (PYR14TFSI), poly(ethylene glycol) dimethyl ether (PEGDME,
Fig. 1 shows the SEM morphological characterization of the CNFs
and the CNF–S composite. Fig. 2 shows the TEM images of CNFs and
CNF–S composite. It can be seen from Figs. 1(a) and 2(a) and (b) that
the diameter of the CNFs is about 100 nm with a hollow structure.
Elemental sulfur has a low surface tension on the carbon surface.
While the sulfur is precipitating from the aqueous solution the CNFs
serve as the host surface, nucleating the deposit. Sulfur deposits
on the CNF inside and outside surfaces, then continues depositing
to wrap the whole CNF, this also reported in our previous porous
CNFs–S study [22]. It can be seen in Figs. 1(b)–(d) and 2(c) and (d)
that the sulfur coating on the surface of the CNFs is very uniform.
The diameter of CNF–S nanofibers is about 130 nm, which means
the sulfur layer is about 15 nm thick (the uncoated CNF has a diame-
ter of 100 nm). The energy-dispersive X-ray (EDX) microanalysis in
Fig. 1(e) confirms the existence of S in the CNFs–S composite. This
core–shell structure of CNF–S is beneficial for constructing a sturdy
three-dimensional carbon network to accommodate the mechani-
cal stresses induced by volume changes caused by the reaction of
sulfur during the discharge–charge cycles. In addition, the use of
Mw = 250)
and
lithium
bis(trifluoromethylsulfonyl)imide
(LiTFSI) were purchased from Sigma–Aldrich and used with-
out further treatment. 1 M kg−1 LiTFSI in PYR14TFSI + PEGDME
(PYR14TFSI:PEGDME = 1:1, by weight) was used [23,24].
2.4. Characterization and measurements
The morphology of the samples was investigated by scanning
electron microscopy (SEM-JEOL JSM 7500F) coupled with an energy
dispersive X-ray spectrometer (EDX) and transition electron micro-
scope (TEM: 200 kV FEI monochromated F20 UT Tecnai). The sulfur
content in the CNF–S composite was ascertained by thermogravi-
metric analysis (NETZSCH STA 409 PC/PG).
The test cells, CR2032-type coin cells, used lithium foils
as the negative electrodes, Cellgard 2400 microporous mem-
brane as separator and 1 M kg−1 LiTFSI in PYR14TFSI + PEGDME
(PYR14TFSI:PEGDME = 1:1, by weight) as the electrolyte. The cells
were assembled in an Argon-filled glove box. EIS measurements of

M. Rao et al. / Electrochimica Acta 65 (2012) 228–233
231
Fig. 5. Charge and discharge curves of a lithium/sulfur cell with PEO binder: 1.0
1 M kg−1 LiTFSI in PY14TFSI + PEGDME (1:1, by weight) as the electrolyte, charge and
discharge rate: 0.05 C (C = 1645 mA g−1), capacity normalized to sulfur mass.
Fig. 3. TGA curves for CNF–S composite (a), pure sulfur (b) and CNFs (c) from room
temperature to 400 ◦C at a heating rate of 5 ◦C min−1 under N2 atmosphere.
an elastomeric binder improves the ability of the electrode struc-
ture to expand and contract in order to accommodate the volume
changes associated with the conversion of sulfur to Li2S.
The discharge curves with different binders all show two dis-
tinct plateaus at about 2.4 V (short upper plateau), which is due
to the reduction of elemental sulfur (S8) to lithium polysulfides
(Li2Sn, 2 < n < 8), and about 2.0 V (long lower plateau), which is
due to the further reduction polysulfides to lithium sulfide, as
reported previously [25–28]. The detailed mechanisms for oxida-
tion and reduction of sulfur, polysulfides and lithium sulfide during
discharge–charge were already reported [6,8,29].
From Figs. 4–6 it can be seen that the initial discharge capac-
ity of the CNF–S electrodes is 1000 mAh g−1, 1060 mAh g−1 and
1313 mAh g−1, for electrodes with PVDF, PEO and CMC + SBR binder,
respectively. The CNF–S nanocomposite we report here increases
the capacity retention of the sulfur electrode as compared to sulfur
electrodes of more conventional construction [8,9]. The reason for
the improved capacity retention may be the very fine dispersion of
sulfur on the CNFs and the adsorption (intimate contact) of sulfur
species on the extended surface of the CNFs.
It is shown in Fig. 4 that the initial discharge capacity of the
CNF–S cathode with PVDF binder is 1000 mAh g−1 sulfur, and the
lower plateau voltage is about 1.95 V. There is decrease in the lower
discharge plateau voltage and an increase of the charge voltage of
the CNF–S electrode with cycling. The discharge plateau voltage
is 1.9, 1.8, 1.8 and 1.75 V for the 2nd, 10th, 20th and 30th cycles,
respectively. Interestingly, from Figs. 5 and 6 it can be seen that the
discharge plateau voltage of the electrodes with PEO and CMC + SBR
3.2. TGA analysis
The sulfur content of the CNF–S composite was ascertained by
thermogravimetric analysis (TGA), under a N2 atmosphere, from
room temperature to 400 ◦C at a heating rate of 5 ◦C min−1. Fig. 3
shows the results obtained from TGA on the as prepared CNF–S
composite, pure sulfur and CNFs. It can be seen from Fig. 3, curve (a)
that the weight of the CNF–S composite decreases with increasing
temperature from 150 ◦C and the weight loss is continuous until the
CNF–S is heated to over 280 ◦C, which indicates that the reduction
in weight of the CNF–S composite is due to evaporation of sulfur.
The sulfur content was 75.7% in the CNF–S composite. The inter-
pretation of the weight loss for the CNF–S composite is confirmed
by the TGA curves of pure sulfur and CNFs, shown in Fig. 3 curve
(b) and (c), and as discussed in another report [12].
3.3. Cell performance
In order to investigate the performance of CNF–S electrodes for
lithium/sulfur cells, three kinds of binders were used to prepare
the electrodes. The room-temperature discharge–charge curves of
CNF–S electrodes with different binders are shown in Figs. 4–6.
Fig. 4. Charge and discharge curves of a lithium/sulfur cell with PVDF binder: 1.0
1 M kg−1 LiTFSI in PY14TFSI + PEGDME (1:1, by weight) as the electrolyte, charge and
discharge rate: 0.05 C (C = 1645 mA g−1), capacity normalized to sulfur mass.
Fig. 6. Charge and discharge curves of a lithium/sulfur cell with CMC + SBR binder:
1.0 1 M kg−1 LiTFSI in PY14TFSI + PEGDME (1:1, by weight) as the electrolyte, charge
and discharge rate: 0.05 C (C = 1645 mA g−1), capacity normalized to sulfur mass.

232
M. Rao et al. / Electrochimica Acta 65 (2012) 228–233
Fig. 7. The specific capacity of CNF–S electrodes with different binders for 60 cycles:
1.0 1 M kg−1 LiTFSI in PY14TFSI + PEGDME (1:1, by weight) as the electrolyte, charge
and discharge rate: 0.05 C (C = 1645 mA g−1), capacity normalized to sulfur mass.
was higher than that of the PVDF binder. The long discharge plateau
for the electrodes with PEO and CMC + SBR binder was close to 2 V,
and remained almost unchanged with cycling. This indicates that
the rate of electrode polarization increase with cycling is much
reduced with the PEO and CMC + SBR binder.
Fig. 7 shows the specific capacity as a function of cycle num-
ber for CNF–S electrodes prepared with the various binders. It can
be seen from Fig. 7 that all the CNF–S electrodes with different
binders have a capacity of more than 500 mAh g−1 after 30 cycles.
This indicates that good capacity retention for the lithium/sulfur
cell can be achieved by ameliorating the polysulfide dissolution
problem (through the use of the ionic-liquid + PEGDME electrolyte)
and providing good electronic contact to the sulfur. These results
may also imply that the CNF–S retards the movement of the
polysulfides away from the electrode by adsorbing polysulfides
physically within the porous structure of the sulfur electrode.
However, the CNF–S electrode with CMC + SBR binder shows a
better capacity retention than that of the electrodes with PEO or
PVDF binder. In the case of CNF–S electrode with PVDF, it is clear
that the discharge capacity decreased with increasing cycle num-
ber. After 60 cycles, the discharge capacity of CNF–S electrodes
Fig. 8. Impedance plots of lithium/sulfur cells with CNF–S cathode with dif-
ferent binders (a) initial and (b) after the 30th discharge, frequency range:
100 kHz–10 mHz.
with CMC + SBR binder shows lower interfacial impedance than the
CNF–S electrodes with PEO and PVDF binder. This is further evi-
dence indicating a lower impedance of the CNF–S electrode with
CMC + SBR binder, which results in better cell performance.
After 30 cycles, the impedance responses of CNF–S cathodes
with different binders had changed. Two obvious semicircles are
shown in Fig. 8(b). The semicircle at high frequencies is related
to the resistance and capacitance of solid-state interface layers on
the surfaces of the electrodes; and the semicircle at medium fre-
quencies is related to the charge-transfer resistance and its related
double-layer capacitance; similar results were also reported in
[32–34]. From Fig. 8(b) we can see that the (intermediate fre-
quency) charge-transfer resistances now are prominent in the
impedance spectra. As expected, the charge transfer impedance is
largest for the electrode with PVdF binder, and smallest for the
electrode with CMC + SBR binder, since the CMC + SBR binder is an
elastomer that provides a binding force that accommodates the
volume change of the sulfur as it is converted to Li2S, and back
to S with cycling, thus maintaining a maximum interfacial area for
charge transfer. Note also that the high-frequency semicircle for
the CMC + SBR electrode has become smaller, consistent with the
idea of maintaining a large interfacial area between the active sul-
fur and the electrolyte. This active area was probably increased by
cycling of the cell. The electrode with PEO binder exhibits interme-
diate behavior, and the PVdF electrode shows the highest interfacial
and charge-transfer impedances, presumably due to its inability to
accommodate the sulfur volume changes while maintaining good
bonding properties.
with PVDF and PEO decreased to 350 mAh g−1 and 420 mAh g−1
,
respectively. The cycling performance of the CNF–S electrode with
CMC + SBR was better and gave a reversible capacity of 586 mAh g−1
after 60 cycles. The structure of CNF–S electrode with CMC + SBR
appears to be sturdy enough to retain the matrix structure sub-
stantially unchanged. Capacity fading from the 4th cycle to the
60th cycle was gradual without any abrupt change, remaining at
about 586 mAh g−1. Due to the use of CNFs the sulfur exists in the
form of nano-sulfur in intimate contact with the conductive car-
bon nanofibers. The overall matrix structure of CNF–S material was
maintained without gross damage even after 60 cycles.
3.4. Electrochemical impedance spectroscopy
Fig. 8(a) shows Nyquist plots of uncycled CNF–S electrodes with
different binders. It can be seen from Fig. 8(a) that the Nyquist
plots of the CNF–S electrodes with different binders before dis-
charge are composed of a semicircle at high frequencies and
medium frequencies and an inclined line in the low frequency
region. The high-frequency intercept on the real axis represents
the ohmic resistance of the cell, including the electrolyte and elec-
trode resistances. The semicircle at high to medium frequencies
is attributable to the surface layers and interfacial impedance of
the electrodes, and the inclined low-frequency region is indicative
of mass transport [30,31]. Before discharge, the CNF–S electrode
These results point out the importance of intimate contact
between the sulfur and the conductive carbon, and the use of an

M. Rao et al. / Electrochimica Acta 65 (2012) 228–233
233
elastomeric binder to accommodate the volume changes during
cycling. CNF–S cathodes with CMC + SBR binder are suitable for use
in lithium/sulfur rechargeable cells.
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4. Conclusions
A CNF–S composite material with a network structure was
prepared by a chemical deposition method in aqueous solution.
This material was used to prepare electrodes using three different
binders. These electrodes were evaluated in lithium/sulfur cells.
SEM examination of the CNF–S material shows that the sulfur
is dispersed uniformly on the surface of the conductive CNF’s.
The CNFs with good conductivity provide a chain-like electron
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CNF–S composite material exhibits good electrochemical prop-
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Acknowledgement
M.R. was partially supported by the China Scholarship Council
while visiting the Department of Chemical & Biomolecular Engi-
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