A facile in situ sulfur deposition route to obtain carbon-wrapped sulfur composite cathodes for lithium-sulfur batteries

Article abstract of DOI:10.1016/j.electacta.2012.06.002

An in situ sulfur deposition route has been developed for synthesizing sulfur-carbon composites as cathode materials for lithium-sulfur batteries. This facile synthesis method involves the precipitation of elemental sulfur at the interspaces between carbon nanoparticles in aqueous solution at room temperature. The product has been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, charge-discharge measurements, and electrochemical impedance spectroscopy. The sulfur-carbon composite cathode with 75 wt.% active material thus obtained exhibits a remarkably high first discharge capacity of 1116 mAh g^-1 with good cycle performance, maintaining 777 mAh g^-1 after 50 cycles. The significantly improved electrochemical performance of the sulfur-carbon composite cathode is attributed to the carbon-wrapped sulfur network structure, which suppresses the loss of active material during charging/discharging and the migration of the polysulfide ions to the anode (i.e., shuttling effect). The integrity of the cathode structure during cycling is reflected in low impedance values observed after cycling. This facile in situ sulfur deposition route represents a low-cost approach to obtain high-performance sulfur-carbon composite cathodes for rechargeable Li-S batteries.

Full text of DOI:10.1016/j.electacta.2012.06.002

Electrochimica Acta 77 (2012) 272–278
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journal homepage: www.elsevier.com/locate/electacta
A facile in situ sulfur deposition route to obtain carbon-wrapped sulfur composite
cathodes for lithium–sulfur batteries
∗
Yu-Sheng Su, Arumugam Manthiram
Electrochemical Energy Laboratory & Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
An in situ sulfur deposition route has been developed for synthesizing sulfur–carbon composites as cath-
ode materials for lithium–sulfur batteries. This facile synthesis method involves the precipitation of
elemental sulfur at the interspaces between carbon nanoparticles in aqueous solution at room tempera-
ture. The product has been characterized by X-ray diffraction, scanning electron microscopy, transmission
electron microscopy, charge–discharge measurements, and electrochemical impedance spectroscopy.
The sulfur–carbon composite cathode with 75 wt.% active material thus obtained exhibits a remarkably
Received 22 March 2012
Received in revised form 16 April 2012
Accepted 2 June 2012
Available online 9 June 2012
Keywords:
Lithium–sulfur batteries
Lithium batteries
Cathode materials
Sulfur–carbon composites
In situ sulfur deposition
−1
−1
high first discharge capacity of 1116 mAh g with good cycle performance, maintaining 777 mAh g
after 50 cycles. The significantly improved electrochemical performance of the sulfur–carbon composite
cathode is attributed to the carbon-wrapped sulfur network structure, which suppresses the loss of active
material during charging/discharging and the migration of the polysulfide ions to the anode (i.e., shut-
tling effect). The integrity of the cathode structure during cycling is reflected in low impedance values
observed after cycling. This facile in situ sulfur deposition route represents a low-cost approach to obtain
high-performance sulfur–carbon composite cathodes for rechargeable Li–S batteries.
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
formation of intermediate polysulfide ions, which dissolve easily in
the electrolyte during the charge/discharge process and result in an
irreversible loss of active material during cycling [2–6]; the higher-
Lithium-ion batteries have revolutionized the portable elec-
2−
tronics industry over the past two decades, and there is immense
interest to adopt lithium-ion batteries for electric vehicle applica-
tions and stationary storage of renewable energies such as solar
and wind energy. For employing the lithium-ion battery technol-
ogy for these applications, several important criteria such as energy
density, power density, rate capability, cycle life, cost, and safety
need to be considered. The current lithium-ion battery technology
is based on insertion compound cathodes and anodes and is lim-
ited in energy density. They also suffer from safety concerns arising
from the chemical instability of the oxide cathodes under condi-
tions of overcharge and the high cost associated with expensive
transition metals like Co [1–3]. Accordingly, there is immense inter-
est to develop alternate cathode materials for lithium-ion batteries.
In this regard, sulfur is an attractive cathode candidate as it offers
order polysulfides (Li Sn , 4 ≤ n ≤ 8) produced during the initial
2
stage of the discharge process are soluble in the electrolyte and
move toward the lithium metal anode, where they are reduced to
lower-order polysulfides [7,8]. The formation of the final discharge
product Li S involves the platting of dense layers on the cathode
2
surface, which blocks the penetration of lithium ions and the trans-
fer of electrons or even breaks the integrity of the cathode structure,
leading to poor cyclability [9,10]. Moreover, sulfur is an insulator
−
30
−1 ◦
with a resistivity of 5 × 10
S cm at 25 C [11], resulting in a
poor electrochemical utilization of the active material. Although
the addition of conductive carbon could improve the overall elec-
trode conductivity, the core of sulfur particles which have little or
no contact with conductive carbon will still be highly resistive. In
this regard, improving the conductivity by increasing the fraction
of sulfur in contact with carbon is critical. Additionally, the loss
of active material from the dissolution of polysulfides should be
avoided or suppressed during the charge/discharge process.
Several approaches have been pursued to overcome these prob-
lems, such as forming sulfur–carbon composites with carbon black
or nanostructured carbon that have led to much improved perfor-
mances [12–21]. For example, a mesoporous carbon framework
filled with amorphous sulfur with the addition of polymer has
been found to exhibit a high reversible capacity of approximately
−
1
an order of magnitude higher theoretical capacity (1675 mAh g
)
−
1
than the currently employed cathodes (<200 mAh g ) and oper-
ates at a safer voltage range (1.5–2.5 V) [2,3]. In addition, sulfur is
abundant and inexpensive.
However, the major issue facing the sulfur cathode is its poor
cycle life. The discharge process of sulfur cathodes involves the
∗ Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681.
E-mail address: rmanth@mail.utexas.edu (A. Manthiram).
−
1
1000 mAh g
after 20 cycles [15]. However, most traditional
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.06.002

Y.-S. Su, A. Manthiram / Electrochimica Acta 77 (2012) 272–278
273
methods to synthesize sulfur–carbon composites include process-
ing by a sulfur melting route [12,13,15–17,19–21], resulting in
high manufacturing costs due to additional energy consumption.
Also, several reports have noted that the sulfur content in the
sulfur–carbon composites synthesized by the sulfur melting route
is limited to a relatively low value in order to obtain acceptable elec-
trochemical performance, leading to a lower overall capacity of the
cathodes [12,13,16,17,20]. Moreover, synthesizing homogeneous
sulfur–carbon composites through conventional heat treatment
is complicated. In the conventional synthesis of sulfur–carbon
composites, sulfur is first heated above its melting temperature,
and the liquid sulfur is then diffused to the surface or into the
pores of carbon substrates to form the sulfur–carbon composites
all the sulfur volatilizes and the sulfur content could be obtained
from the observed weight loss since carbon black does not exhibit
any weight loss in this temperature range. The sulfur–carbon com-
posites with 75 wt.% sulfur as confirmed by the TGA data were
employed for further experiments.
Wang et al. [22] used a similar solution-based reaction to obtain
sulfur, but the carbon was attached to the surface of the sulfur par-
ticles by an extra mixing process after the precipitation of sulfur.
In other words, only part of the sulfur was slightly connected with
the conductive carbon, resulting in a low utilization of the active
material. In contrast, the in situ sulfur deposition route used in our
study ensures that sulfur contacts with the carbon black closely
due to the heterogeneous nucleation of sulfur. In order to have a
comparison, pure sulfur was also synthesized by the same reaction
route, but carbon black was added after sulfur was precipitated.
[
12,13,15–17,19–21]. A subsequent high-temperature heating step
is then required to remove the superfluous sulfur on the surface of
the composites [12,13,16,19], leading to a waste of some sulfur.
Thus, the conventional synthesis by the sulfur melting route may
not be a practical scale-up approach to obtain a uniform industry-
level sulfur–carbon composite although good performances have
been realized with them.
A sulfur deposition method to synthesize a core–shell car-
bon/sulfur material for lithium–sulfur batteries has been reported
by Wang et al. [14]. Although it exhibited good cyclability and rate
capability, their sulfur deposition process is very sensitive and need
to be carefully controlled during synthesis; otherwise, a compos-
ite with poor electrochemical performance is produced. Therefore,
the objective of our study is to develop an easily scalable chemical
synthesis approach to synthesize sulfur–carbon composites with
low manufacturing cost. Accordingly, we present here a facile sul-
fur deposition route to synthesize sulfur–carbon composites, which
not only offers a low-cost approach for large-scale production but
also produces high-purity active material.
2.2. Structural and microstructure characterizations
The synthesized samples were characterized with a Philips X-
ray Diffractometer (PW 1830 + APD 3520) with Cu K␣ radiation
◦
◦
◦
between 10 and 70 at a scan rate of 0.04 /s. The microstructure
and morphology of the samples were examined with a JEOL JSM-
5610 and a FEI Quanta 650 scanning electron microscope (SEM)
and a JEOL JEM-2010F transmission electron microscope (TEM). The
composition of the sulfur–carbon composite was also determined
with an energy dispersive spectrometer (EDS) attached to the TEM
instrument.
2.3. Cell assembly
The sulfur–carbon composite thus obtained was individually
mixed with 10 wt.% of Super P carbon and 10 wt.% of polyvinyli-
dene fluoride (PVDF, Kureha) binder in an N-methylpyrrolidinone
(
NMP, Sigma–Aldrich) solution. The well-mixed slurry was tape-
2
. Experimental
casted onto a sheet of aluminum foil and the film was dried in
a convection oven at 50 C for 24 h, followed by pressing with a
roller and punching out circular electrodes of a 0.5 in. diameter.
The cathode electrode disks were dried in a vacuum oven at 50 C
◦
2.1. In situ sulfur deposition synthesis of sulfur–carbon
composites
◦
for an hour before assembling the cell. Similar electrodes with
the same overall amount of Super P carbon and binder were also
fabricated with the as-synthesized pure sulfur under the same con-
The synthesis process for the sulfur–carbon composite by the
in situ sulfur deposition route is illustrated in Scheme 1, and the
in situ sulfur deposition in aqueous solution involves the following
reaction:
ditions. Next, 1.0 M LiCF SO3 (Acros Organics) salt was added to a
3
mixture of 1,2-Dimethoxyethane (DME, Acros Organics) and 1,3-
Dioxolane (DOL, Acros Organics) (1:1, v/v) and stirred for 5 min to
prepare the electrolyte. The CR2032 coin cells were then assem-
bled with the prepared cathode disks, prepared electrolyte, Celgard
polypropylene separators, lithium foil anodes, and nickel foam cur-
rent collectors. The cell assembly was conducted in a glove box
filled with argon.
Na S O + 2HCl → 2NaCl + SO + H O + S ↓
2
2
3
2
2
First, 0.02 mole of sodium thiosulfate (Na S O , Fisher scien-
2
2
3
tific) was completely dissolved in 750 mL of deionized (DI) water
by stirring. Then, 0.1 g of commercial conductive carbon black with
a diameter of 30–60 nm (Super P) was suspended in the above solu-
tion by adding a small amount of isopropyl alcohol (C H O, Fisher
3
8
scientific) under ultrasonic vibrations. The reason for adding iso-
propyl alcohol is that it enhances the wetting of the hydrophobic
carbon nanoparticles in the aqueous solution. A 2 mL of hydrochlo-
ric acid (HCl, Fisher Scientific) was then slowly added to the solution
to precipitate the sulfur onto the surface and into the interspaces
of the nano-sized carbon black. During the in situ sulfur deposition
reaction, the sulfur particles grow to a thermodynamically favored
size and the carbon nanoparticles in the composite self-assemble,
become interconnected with each other, and finally wrap the sul-
fur. After allowing the reaction mixture to stir for 24 h, the product
was filtered and washed several times with DI water, ethanol, and
acetone. The sulfur–carbon composite thus collected was dried in
2.4. Electrochemical characterizations
The cyclic voltammetry (CV) data were collected with a Volta-
Lab PGZ 402 Potentiostat at a scan rate of 0.05 mV s
−
1
between
+
3.5 and 1.0 V with lithium metal as the reference electrode (Li/Li ,
−3.045 V vs. NHE). The charge–discharge profiles, cyclability, and
rate capability were assessed with an Arbin battery cycler. All cells
were rested for 30 min before electrochemical cycling. The cells
were then discharged to 1.5 V and charged to 2.8 V for one full
cycle. An additional charge condition of achieving a capacity of
−
1
1C (C = 1675 mAh g , the mass is based on the sulfur content in
cathodes) was set for the pure sulfur cathode to avoid the infinite
charging because the charge plateau would never reach 2.8 V
due to the severe shuttle effect [9,10]. After cycling, coin cells
were opened in the glove box to retrieve the cycled cathodes and
examined by SEM to investigate their morphological changes.
◦
an air-oven at 50 C for 24 h. The sulfur content in the compos-
ite was determined by thermogravimetric analysis (TGA) with a
Perkin-Elmer TGA 7 Thermogravimetric Analyzer at a heating rate
◦
◦
of 5 C/min from 30 to 300 C with flowing air. During this process,

2
74
Y.-S. Su, A. Manthiram / Electrochimica Acta 77 (2012) 272–278
Scheme 1. Scheme illustrating the in situ sulfur deposition route to obtain the sulfur–carbon composite.
Electrochemical impedance spectroscopy (EIS) measure-
ments were performed with Solartron Impedance
Analyzer (SI 1260 + SI 1287) in the frequency range of
in which sulfur particles are uniformly distributed throughout
the network structure formed by carbon black. Carbon black par-
tially embeds in the sulfur, and the remainder wraps around the
matrix sulfur as a protective layer. This network structure con-
firms close contact between the conductive carbon and sulfur,
providing not only excellent electron pathways for the insulat-
ing sulfur but also many adsorbent points to avoid the loss of
the soluble polysulfides into the electrolyte. The TEM images in
Fig. 3(a) and (b) show that the carbon black nanoparticles in the
sulfur–carbon composite are chain-like, which effectively enhances
the conductivity of the composite material [13]. The elemental
analysis of the sulfur–carbon composite carried out by EDS is
shown in Fig. 3(c), demonstrating the existence of both sulfur and
carbon.
a
1
5
MHz to 100 mHz with an AC voltage amplitude of
mV.
3
. Results and discussion
3.1. XRD, SEM, TEM and EDS studies
Fig. 1 compares the X-ray diffraction (XRD) patterns of the
pure sulfur, sulfur–carbon composite, and carbon black. The Super
P carbon black, showing no sharp crystalline peaks, has amor-
phous structure. The pure sulfur and sulfur–carbon composite
exhibit peaks perfectly matching with those of pure orthorhom-
bic sulfur (JCPDS 00-008-0247). The sulfur–carbon composite
shows much higher peak intensities than the pure sulfur as
the dispersed nanoparticles of carbon black act as numerous
deposition sites for elemental sulfur, leading to a favorable
precipitation environment. This in situ sulfur deposition route
thus provides an efficient means to produce high-purity sulfur
composites.
The microstructure of the carbon black, pure sulfur, and
sulfur–carbon composite is shown in Fig. 2. The particle size
of spherical carbon black is less than 100 nm. The pure sul-
fur contains glue-like particles with a diameter of few microns.
Fig. 2(c) presents the structure of the sulfur–carbon composite
Pure sulfur
Sulfur-carbon composite
Carbon black
10
20
30
40
50
60
70
2
(degree)
Fig. 2. SEM images of the (a) carbon black, (b) pure sulfur, and (c) sulfur–carbon
Fig. 1. XRD patterns of the pure sulfur, sulfur–carbon composite, and carbon black.
composite with carbon-wrapped sulfur network structure.

Y.-S. Su, A. Manthiram / Electrochimica Acta 77 (2012) 272–278
275
(a)
1.5
1.0
0.5
0.0
Pure sulfur
st
1
2
3
cycle
cycle
cycle
nd
rd
-
-
0.5
1.0
1
.0
1.5
2.0
2.5
3.0
IV
3.5
Potential (V)
(b)
1.5
1.0
0.5
0.0
Sulfur-carbon composite
st
1
2
3
cycle
cycle
cycle
III
nd
rd
Fig. 3. (a) Low and (b) high magnification TEM images of the sulfur–carbon com-
posite and (c) EDS analysis of the region in (b), showing the existence of both sulfur
and carbon.
-
0.5
I
II
3.2. Electrochemical properties
1.0
1.5
2.0
2.5
3.0
3.5
To understand the reduction/oxidation reactions of the sulfur-
containing cathodes in Li–S batteries, a CV test was conducted
with the CR2032 coin cells. For the pure sulfur cathode, two sharp
cathodic peaks located at 2.3 and 2.0 V are observed in the first
discharge process in Fig. 4(a), corresponding to the reduction of
elemental sulfur to soluble polysulfides and then to the insoluble
Potential (V)
Fig. 4. Cyclic voltammetry plots of the cathodes fabricated with the (a) pure sul-
fur and (b) sulfur–carbon composite at a scan rate of 0.05 mV s under a voltage
−
1
+
window of 1.0–3.5 V (vs. Li /Li).
Li S and Li S, respectively [18]. Several anodic peaks occur contin-
2
2
2
uously with similar current density from 2.3 to 3.0 V as the potential
scans to the charging voltage. These oxidation peaks occurring in a
broad voltage range show the consumption of much more energy
than that of regular cases, suggesting poor charging efficiency and
severe polarization due to the shuttle effect in Li–S batteries. In
the subsequent cycles, both the reduction peaks shift to lower
potential ranges compared to that in the first cycle, indicating a
discharge overpotential after recharging. The current densities of
both the reduction peaks also drop in the second and third cycles,
indicating the irreversible capacity fade of the as-synthesized pure
sulfur cathode. The CV profile of the sulfur–carbon composite cath-
ode synthesized by the in situ sulfur deposition route is shown in
Fig. 4(b). The CV patterns in the first three cycles almost overlap
each other in contrast to that found with the pure sulfur cathode
in Fig. 4(a), indicating the excellent cyclability of the sulfur–carbon
composite cathode. A small anodic shift of the first reduction peak
discharge profile, the two discharge plateaus (Plateau I and II) are
related to the two peaks (Peaks I and II) mentioned with the CV data.
In the charge profile, the two plateaus (Plateau III and IV) of the
sulfur–carbon composite cathode correspond to the two oxidation
reactions exhibited in the CV plots as well. The terminal states of
the charge process in the pure sulfur and sulfur–carbon composite
cathodes are quite distinct. The charge process in the sulfur–carbon
composite cathode ends with a sharp voltage raise when the cell
voltage reaches 2.8 V. In contrast, the charge process in the pure sul-
fur cathode shows a typical shuttle behavior even after the charge
capacityreachesover1C[7–9], leadingto poorchargeefficiencyand
loss of active material. The sulfur in the pure sulfur cathode resem-
bles glue-like particles, in which sulfur connects to one another
and forms many inactive regions without contact with conductive
carbon. These inactive portions dissolve easily in the electrolyte
without much confinement within the carbon black during cycling.
The adsorption of the polysulfides in the carbon-wrapped sulfur
network structure of the sulfur–carbon composite appears to pre-
vent the soluble polysulfides from migrating toward the anode
region, thereby efficiently suppressing the shuttle effect at a low
current density (C/20) during charging.
(
Peak I) is observed in the second and third cycles compared to that
in the first cycle. This may be due to the higher adsorbing energy
between carbon black and sulfur in the first cycle compared to that
in the subsequent cycles [12]. The oxidation reaction can be divided
into two overlapping peaks (Peak III and IV), representing the for-
mation of Li Sn (n > 2) and the final oxidized active sulfur (Li S or
Fig. 5(b) displays the discharge profiles at various cycle num-
bers of the pure sulfur and the sulfur–carbon composite cathodes.
The upper discharge plateau of the pure sulfur cathode continu-
ously shrinks as the cycle number increases, which is consistent
2
2 8
S), respectively [18].
The first charge/discharge profiles of the pure sulfur and
sulfur–carbon composite cathodes are shown in Fig. 5(a). In the

2
76
Y.-S. Su, A. Manthiram / Electrochimica Acta 77 (2012) 272–278
(
a)
1250
(a)
2
2
2
2
1
1
.75
.50
.25
.00
.75
.50
Pure sulfur
Pure sulfur
Sulfur-carbon composite
Sulfur-carbon composite
1000
IV
I
III
750
II
5
00
50
2
0
0
10
20
30
40
50
0
250
500
750
1000 1250 1500
Cycle Number
Specific Capacity (mAh/g - sulfur)
1250
(b)
C/20
C/10
C/5
C/4
(
b) 2.50
Pure sulfur
1000
2
2
1
1
.25
.00
.75
.50
7
50
00
5
3
0
3 2
1
2
2
2
1
1
.50
.25
.00
.75
.50
Sulfur-carbon composite
250
0
0
10
20
30
40
50
Cycle Number
3
0
32 1
1000
Specific Capacity(mAh/g - sulfur)
0
250
500
750
Fig. 6. Comparison of the cyclability of the (a) pure sulfur and sulfur–carbon com-
posite cathodes at a rate of C/20 and the (b) sulfur–carbon composite cathode at
various C rates.
Fig. 5. (a) First charge/discharge profile and (b) discharge curves at various cycles of
the pure sulfur and sulfur–carbon composite cathodes cycled at 1.5–2.8 V (vs. Li /Li)
at a rate of C/20.
capacity after 50 cycles still remains at 697 mAh g 1, representing a
82% capacity retention. This excellent cycle performance makes the
sulfur–carbon composite cathode a promising candidate for high
rate practical Li–S batteries.
−
+
with the diminished reduction peaks in Fig. 4(a). This indicates the
irreversible loss of active material in the pure sulfur cathode. After
the 30th cycle, the discharge capacity is less than half of the initial
capacity, showing poor cyclability. In contrast, the sulfur–carbon
composite cathode has overlapping upper plateaus in the first three
cycles, showing excellent electrochemical reversibility. The dis-
charge capacity of the sulfur–carbon composite cathode after the
3.3. SEM and EIS analysis of cycled electrodes
To investigate the morphological change of the cathode after the
charge/discharge process, the cycled cathodes were removed from
coin cells after 25 cycles. Fig. 7(a) and (b) illustrates the morphol-
ogy of the pure sulfur and sulfur–carbon composite cathodes before
cycling. The sulfur particles are fairly evenly distributed on the flat
cathode surface. Fig. 7(c) and (d) shows the surface microstructure
of the pure sulfur and sulfur–carbon composite cathodes after the
25th cycle. The sulfur–carbon composite cathode still maintains
a relatively flat surface, implying the electrochemical process has
limited impact on the cathode structure during cycling. This result
indicates that the reduction/oxidation process of the active sulfur is
effectively localized to the carbon-wrapped sulfur network struc-
ture. In contrast, a porous structure is formed in the case of the
pure sulfur cathode after 25 cycles. The pore size resembles the
particle size of the as-synthesized pure sulfur, indicating that the
active sulfur continuously leaches out during the charge/discharge
process and pores are gradually formed in the cathode structure.
These pores could develop into macroscopic cracks after many
3
0th cycle has a retention rate of 78%, which is much higher than
that found with the pure sulfur cathode.
The cyclabilities of the pure sulfur and sulfur–carbon composite
cathodes are compared in Fig. 6(a). The sulfur–carbon composite
−
1
cathode has a higher first discharge capacity of 1116 mAh g com-
−1
pared to 1006 mAh g for the pure sulfur cathode, implying that
improved active material utilization can be achieved when sul-
fur is well-distributed in the carbon network structure due to the
increased contact area between conductive carbon black and insu-
lating sulfur. The reversible discharge capacity of the sulfur–carbon
−
1
composite cathode after the 50th cycle is 777 mAh g . This
reversible capacity value largely exceeds that of the pure sulfur
cathode, indicating the superior cyclability of the sulfur–carbon
composite cathode. The cycle life plots of the sulfur–carbon com-
posite cathode at various rates are shown in Fig. 6(b). As previously
encountered, the first discharge capacity decreases with increasing
current density or C rate. At a rate of C/4, the reversible discharge
cycles due to the irreversible Li S plating on those areas, caus-
2
ing structural failure [10]. In other words, sulfur particles have
been distributed throughout the cathode by a conventional mixing

Y.-S. Su, A. Manthiram / Electrochimica Acta 77 (2012) 272–278
277
Fig. 7. SEM images of the (a) pure sulfur and (b) sulfur–carbon composite cathodes before cycling and the (c) pure sulfur and (d) sulfur–carbon composite cathodes after
cycling at C/5 rate for 25 cycles.
process with the carbon black in case of the pure sulfur cathode, and
Pure sulfur cathode
1
1
50
00
this structure cannot prevent the dissolution of polysulfides, result-
ing in poor electrochemical performance. In contrast, a conductive
carbon-wrapped sulfur network structure produced by the in situ
sulfur deposition route in the case of the sulfur–carbon composite
not only maintains the structural integrity but also suppresses the
migration of soluble polysulfides from the carbon matrix.
th
st
0th cycle
1th cycle
25
cycle
50 cycle
50
0
To understand the reason for the superior electrochemical per-
formance of the sulfur–carbon composite synthesized by the in
situ sulfur-deposition route, EIS measurements were carried out
with the coin cells. The Nyquist profiles of the pure sulfur and
sulfur–carbon composite cathodes and the equivalent circuits are
shown in Fig. 8. Re refers to the resistance of electrolyte, Rct refers
to the charge transfer resistance between the interfaces of car-
bon/sulfur/electrolyte, Wo refers to the Warburg impedance, and
CPE refers to the constant phase element [23]. The resistance of
electrolyte was estimated from the intersection of the front end of
Sulfur-carbon composite cathode
1
1
50
00
50
0
th
st
0th cycle
1th cycle
25
cycle
50 cycle
Re
Rct
Wo
CPE
0
100
200
300
ꢀ
semicircles with the Z axis, which is similar for both the cathodes.
Z' (ohm)
The diameter of the impedance semicircles is related to the charge
transfer resistance, which is a measure of the difficulty involved
for charges crossing the boundary between the electrode and elec-
trolyte. Before cycling, the sulfur–carbon composite cathode has
a slightly lower charge transfer resistance value than the pure sul-
fur cathode, which is expected considering its higher first discharge
capacity compared to that of the pure sulfur cathode. The close con-
tact between the conductive carbon black and the insulating sulfur
lowers the resistance for electrons transferring across the interface
between them. In the subsequent cycles (1st, 25th, and 50th), the
charge-transfer resistance of the pure sulfur cathode grows much
larger than that found with the sulfur–carbon composite cathode.
The main reason for this is the porous structure of the cycled pure
sulfur cathode. Electrons passing across the boundary between con-
ductive carbon and active material are impeded by the irreversible
Fig. 8. Electrochemical impedance spectra of the pure sulfur and sulfur–carbon
composite cathodes before and after cycling at C/5 rate.
thus reveal that the sulfur–carbon composite cathode exhibits bet-
ter electronic conductivity and lithium-ion transport than the pure
sulfur cathode due to the stable network structure of carbon black
wrapping around the sulfur. The impedance of the sulfur–carbon
composite after 50th cycles does not increase much, suggesting that
the network structure maintains its integrity during the cycling
process.
4. Conclusions
An in situ sulfur deposition route has been developed by the
heterogeneous nucleation of sulfur onto conductive carbon black
formation of the Li S layer in the pores [10]. The EIS measurements
2

2
78
Y.-S. Su, A. Manthiram / Electrochimica Acta 77 (2012) 272–278
(
Super P) from a solution-based reaction of sodium thiosulfate
[4] H. Yamin, E. Peled, Journal of Power Sources 9 (1983) 281.
[
5] R.D. Rauh, K.M. Abraham, G.F. Pearson, J.K. Surprenant, S.B. Brummer, Journal
of the Electrochemical Society 126 (1979) 523.
with hydrochloric acid. This facile synthesis method can produce
high-purity sulfur–carbon composites for large-scale manufactur-
ing. The sulfur–carbon composite thus produced exhibits discharge
capacities of, respectively, 1116 mAh g
7
[
6] K. Kumaresan, Y. Mikhaylik, R.E. White, Journal of the Electrochemical Society
155 (2008) A576.
−1
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in the first cycle and
−
1
77 mAh g after 50 cycles. Even after 50 cycles at a higher rate
[
8] Y.V. Mikhaylik, J.R. Akridge, Journal of the Electrochemical Society 151 (2004)
A1969.
−1
−1
of C/4 (419 mA g ), the reversible capacity is still 697 mAh g ,
which translates to a capacity retention of 82%. The excellent cycle
performance of this sulfur–carbon composite can be attributed to
the conductive carbon-wrapped sulfur network structure, which
not only decreases the charge transfer resistance but also helps
maintaining the integrity of the electrode structure during cycling.
The carbon black matrix also plays a protective role as an adsor-
bent agent to keep the soluble polysulfides within the electrode
structure, avoiding the unwanted shuttle effect during charging.
We believe this facile in situ sulfur deposition route to obtain
sulfur–carbon composites possessing good electrochemical perfor-
mance could enhance the feasibility of practical Li–S batteries.
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Acknowledgment
[18] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Angewandte
Chemie International Edition 50 (2011) 5904.
[
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19] W. Zheng, Y.W. Liu, X.G. Hu, C.F. Zhang, Electrochimica Acta 51 (2006) 1330.
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This work was supported by Seven One Limited.
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