Decoration of sulfur with porous metal nanostructures: An alternative strategy for improving the cyclability of sulfur cathode materials for advanced lithium-sulfur batteries

Article abstract of DOI:10.1039/c3cc41875h

Porous Pt nanostructure decorated sulfur microparticles (Pt@S) are fabricated using sulfur as the template. The Pt@S electrode shows a higher volumetric specific capacity of 520 mA h cm^-3 and improved cyclability with only 15% capacity fading a

Full text of DOI:10.1039/c3cc41875h

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Decoration of sulfur with porous metal nanostructures:
an alternative strategy for improving the cyclability of
sulfur cathode materials for advanced lithium–sulfur
batteries†
Cite this: DOI: 10.1039/c3cc41875h
Received 13th March 2013,
Accepted 28th March 2013
DOI: 10.1039/c3cc41875h
Xinyong Tao, Feng Chen, Yang Xia, Hui Huang, Yongping Gan, Xiaorong Chen
and Wenkui Zhang*
www.rsc.org/chemcomm
Porous Pt nanostructure decorated sulfur microparticles (Pt@S) are fabri-
cated using sulfur as the template. The Pt@S electrode shows a higher
volumetric specific capacity of 520 mA h cmÀ3 and improved cyclability
with only 15% capacity fading after 80 cycles at 0.1 C (167.5 mA gÀ1).
Lithium–sulfur (Li–S) batteries have attracted intense scrutiny for over
two decades due to their high theoretical capacity (1675 mA h gÀ1
)
and energy density (2600 W h kgÀ1).1,2 Compared with conventional
lithium ion batteries, Li–S batteries exhibit 5 times greater energy
densities at a significantly lower cost. Furthermore, Li–S batteries
are competitive in terms of their safety, environmental friendliness
and wide temperature range of operation.3,4 Nevertheless, wide-
scale commercial use is so far limited because of some crucial
technical problems that must be tackled: poor electrical conductivity
of sulfur (5 Â 10À30 S cmÀ1 at 25 1C)5 and shuttling of higher-order
polysulfides during charging. The low electrical conductivity of
sulfur limits its activity and utilization. The shuttling process
decreases utilization of the overall active material, leading to poor
cyclability, and reduces the columbic efficiency of the battery.6
In order to solve these problems, besides new electrolytes7 and
binders,8 researchers modified the sulfur cathode materials to
improve the cyclability of Li–S batteries. Nearly all the strategies
can be divided into two categories: (i) to fabricate carbon–sulfur
composites using mesoporous carbon,2,9 carbon spheres,10
carbon fibers,11 graphene,12 carbon nanotubes13,14 and so on;
(ii) to manufacture polymer–sulfur composites by adding poly-
aniline,15 polypyrrole,16 polyvinyl pyrrolidone17 etc. Besides these,
we wonder whether there are other efficient strategies for improv-
ing the electrochemical performance of sulfur electrodes.
Scheme 1 The reactions and schematic illustration of preparation of Pt@S.
improved energy storage performance in comparison with their
pristine S counterpart. We believe that this effective decoration
method can be generalized to other metallic porous nanostructures
to improve the electrochemical performance of batteries.
Scanning electron microscope (SEM) images (Fig. 1a and b) show
that the fresh sample consists of abundant Pt@S with diameters
ranging from 2 to 4 mm. In order to demonstrate that Pt@S is more
stable than pristine S at room temperature, we performed an aging
experiment for 50 days. Pt@S after long term aging (inset in Fig. 1b)
In this work, we report an alternative strategy for improving the
cyclability of lithium–sulfur batteries. Pt@S is fabricated by dec-
orating pristine S with porous Pt nanostructures (Scheme 1).
Benefiting from the unique structure, the Pt@S electrodes exhibit
College of Chemical Engineering and Materials Science, Zhejiang University of
Technology, Hangzhou 310014, China. E-mail: msechem@zjut.edu.cn;
Fax: +86 571 88320394; Tel: +86 571 88320394
Fig. 1 (a) and (b) SEM images of the fresh Pt@S sample. The inset in (a) shows
the corresponding TGA curves. The inset in (b) shows the SEM image of Pt@S
after aging. (c) and (d) SEM images of pristine S sample after aging.
† Electronic supplementary information (ESI) available: Experimental details,
XRD, TEM, EDX, EIS. See DOI: 10.1039/c3cc41875h
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is similar to the freshly prepared sample. However, there is obvious
agglomeration and a significant particle size increase in the pristine
S sample after aging (Fig. 1c and d). The Ostwald ripening mecha-
nism can be used to explain the particle size increase in pristine S
after aging. The decrease in surface energy is usually assumed as the
driving force for the Ostwald ripening. There are abundant small
sulfur microparticles with higher surface energy in the pristine
sample, which are less stable. As the system tries to lower its overall
energy, small sulfur particles will attach to the surface of larger
particles. Therefore, the number of the small sulfur microparticles
will be shrinking, while larger particles continue to grow. However,
due to the fact that Pt has a high melting point and remarkable
chemical stability, the Pt nanostructures prevent the S micro-
particles from agglomeration and the grain growth during long
term aging. The inset in Fig. 1a shows the thermogravimetric
analysis (TGA) curve of Pt@S. As is well known, sulfur is evaporated
below 350 1C,18 while the reaction temperature of S and Pt to form
PtS2 is higher than 650 1C.19 Therefore, the mass of sulfur can be
derived from the TGA curve, which shows that the composite
contains approximately 76 wt% of sulfur. The X-ray diffraction
(XRD) pattern (Fig. S1a, ESI†) reveals that the as-prepared sulfur
microparticles can be indexed to the orthorhombic crystal type
(JCPDS No. 78–1889). Compared with pristine S, Pt@S has a broad
peak at 401 (Fig. S1b, ESI†), corresponding to the (111) plane of Pt.
To provide a further demonstration of the stabilization effect of
the Pt nanostructure, pristine S and Pt@S are examined using high-
energy (300 keV) electron beam irradiation (EBI). We found that it was
very difficult to obtain a high quality bright field transmission electron
microscope (TEM) image of the pristine S microparticle because of its
rapid sublimation within 0.5–2 s under EBI. In contrast, as shown in
Fig. S2a (ESI†), we can obtain a distinct bright field TEM image of the
Pt@S under EBI. Only a small fraction of sulfur microparticles shown
in the blue frame was sublimed after 20 s of EBI (Fig. S2b, ESI†). With
further reduction of the S microparticles, the contour of the Pt
nanostructures can be clearly identified (Fig. S2c and d, ESI†). After
5 min of EBI, the high-energy electron beam not only sublimes S but
also destroys the Pt structure (Fig. S2d, ESI†). Besides the heat effect
resulting from inelastic scattering during EBI, many other factors also
contribute to the sublimation of sulfur. One is the displacement of
atoms caused by the incident electrons.20 The other is the charging of
the specimen system.21 Based on these, the pristine S can be
sublimed rapidly due to its poor electrical conductivity and thermal
conductivity. However, due to the high electrical conductivity, thermal
conductivity and atomic mass of Pt, the Pt nanostructure could
prevent the S particle from rapid sublimation. But the electron beam
with high kinetic energy will destroy the Pt nanostructure and sublime
more S under long time EBI over 5 min (Fig. S2d, ESI†). Compared
with the pristine S counterpart, the unique Pt nanostructure could
stabilize the S microparticles vividly, as proved by the EBI tests.
Fig. 2a shows a bright field TEM image of Pt@S. Fig. 2b–d show
the corresponding high resolution TEM (HRTEM) images of the Pt
particles at different areas. The measured fringe spacing values of Pt
particles are 0.227 and 0.202 nm, corresponding to (111) planes and
(200) planes of Pt, respectively. The corresponding Fast-Fourier-
Transform (FFT) patterns of the HRTEM images with sharp diffrac-
tion spots imply that the Pt is crystallized (inset in Fig. 2b and c).
Apparently, the Pt nanostructure is constructed by Pt nanograins
Fig. 2 (a) TEM image of the Pt@S particle. (b)–(d) HRTEM image of Pt particles.
with diameter ranging from 3 to 8 nm (Fig. 2b–d). In addition, some
obvious mesopores between the nanograins can be observed
(Fig. 2b–d). As shown in Fig. 2, the Pt nanograins grow in different
crystallographic orientations with various grain boundaries such as
coherent twin boundary (CTGB), coherent grain boundary (CGB)
and incoherent grain boundary (ICGB). The EDX spectrum (Fig. S3,
ESI†) collected from the interface of the S particle and Pt nano-
structure shows that the Pt@S material contains S and Pt. The Cu
signal comes from the TEM grid.
Fig. 3a and b show the initial cyclic voltammogram (CV) curves of
pristine S microparticles and Pt@S. The two reduction peaks at about
2.3 and 2.0 V correspond to the conversion of, respectively, high-order
lithium polysulfides (e.g., Li2S8) to low-order lithium polysulfides
Fig. 3 CV curves: (a) pristine S electrode and (b) Pt@S electrode. Discharge–
charge voltage profiles of (c) pristine S and (d) Pt@S electrodes at a current
density of 0.02 C. (e) Cycling performance of S and Pt@S at a current density of
0.1 C. (f) Rate capabilities of the S and Pt@S.
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(Li2Sx, 8 > x Z 4) and lithium polysulfides to solid-state Li2S2/Li2S.22 attributed to the Pt nanostructures, which decorate the S particle and
When the voltage sweep was reversed, the CV plot exhibited a broad provide facile electronic transport and avoid the shuttle effect. In
peak at 2.45 V with a shoulder at 2.55 V. This indicates that two addition, electrochemical impedance spectra (Fig. S4, ESI†) of pristine
oxidative peaks exist and overlap with each other, which corresponds S electrodes and Pt@S electrodes indicate that the resistant of pristine
to the reverse reactions of Li2S2/Li2S to low-order lithium polysulfides S electrodes is almost 2 times that of the Pt@S electrodes, implying
and low-order lithium polysulfides to high-order lithium poly- that the Pt nanostructure can promote the electronic transport. Based
sulfides.4 As shown in Fig. 3b, all the reduction–oxidation peaks on the long term aging, EBI, and electrochemical results, it is clear
remain rather constant upon cycling, implying the excellent electro- that the unique porous conductive Pt nanostructures could stabilize
chemical stability of the Pt@S composite. Nevertheless, the CV of the the S microparticles and improve the energy storage performance.
pristine sulfur electrode shows shifting peaks and presents poor Due to the high cost of Pt, we also fabricated Ni nanostructure
electrochemical reversibility. In this work, the Pt@S sample has a decorated sulfur microparticles, which also show enhanced energy
porous conductive Pt nanostructure, which has two advantages: on storage performance (Fig. S6, ESI†).
the one hand, it could reduce the shuttling phenomenon during the
In this work, we demonstrate an alternative strategy for improv-
charge–discharge cycles and suppress the loss of active materials, as a ing the cyclability of lithium–sulfur batteries. Porous conductive
result, the structural integrity of the sulfur cathode can be maintained metal nanostructure decorated S particle have been successfully
well. On the other hand, owing to this nanostructure constructed by fabricated by a facile wet chemical method using pristine S micro-
Pt nanoparticles, they have good electrical conductivity, which could particles as the template. The metal nanostructure with unique
enhance the electrochemical reaction kinetics and improve the porous structure stabilizes the S effectively, provides good electronic
electrochemical reversibility.
conductivity and suppresses the shuttle effect of sulfur in the
The charge–discharge profiles of pristine S and Pt@S at the same charge–discharge cycles. As a result, the metal nanostructure deco-
current density of 0.02 C (33.5 mA gÀ1 during activation step) are rated S particles show remarkable electrochemical performance with
shown in Fig. 3c and d. Two typical plateaus for both S and Pt@S improved cyclability. We believe that this facile and effective decora-
electrodes in the discharge process can be assigned to the two-step tion method can be extended to other metallic porous nanostruc-
reaction of sulfur with lithium, as shown in Fig. 3a and b. The shuttle tures to improve the electrochemical performance of Li–S batteries.
phenomenon in the pristine sulfur sample caused a serious over-
This work was supported by the National Natural Science
charge,6 leading to a low initial charge and discharge efficiency of Foundation of China (No. 51172205 and 51002138) and New
only 39% (Fig. 3c). However, the initial coulombic efficiency of Pt@S Century Excellent Talents in University (NCET 111079).
reached as high as 98% (Fig. 3d), suggesting that the Pt nanostructure
Notes and references
can inhibit overcharging effectively. The reason why both the capa-
cities of pristine S and Pt@S increased after the initial discharge
could be attributed to two aspects. On the one hand, the sulfur
changes from a solid state to the dissolved poly-sulfide state. On the
other hand, it takes some time for the electrolyte to infiltrate the
sulfur deeply due to the scale of the sulfur particle. Similar pheno-
mena have been reported by He et al.8 The discharge cyclic perfor-
mance of pristine S and Pt@S cathodes at a current rate of 0.1 C
(167.5 mA gÀ1) is displayed in Fig. 3e. The reversible capacity of
pristine S drastically faded to only about 100 mA h gÀ1 after 80 cycles.
In contrast, benefiting from the unique Pt nanostructure, the Pt@S
electrode showed better cyclic durability than the pristine sulfur elec-
trode. The specific capacity of the Pt@S composite (the mass refers to
the active sulfur, similarly hereafter) was about 680 mA h gÀ1, which
was 85% of the initial capacity. It is noticeable that the Pt@S sample
has a higher tap density (0.72 g cmÀ3) than conventional mesoporous
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This journal is The Royal Society of Chemistry 2013
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