s12274-017-1870-2

Nano Research
https://doi.org/10.1007/s12274-017-1870-2
Polyaniline-coated selenium/carbon composites
encapsulated in graphene as efficient cathodes
for Li-Se batteries
Biwu Wang1, Jingjing Zhang1 (), Zhigang Xia2 (), Meiqiang Fan1, Chunju Lv1, Guanglei Tian1, and Xiaona Li3
1
Department of Materials Science and Engineering, China Jiliang University (CJLU), Hangzhou 310018, China
College of Metrology and Measurement Engineering, China Jiliang University (CJLU), Hangzhou 310018, China
3
Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, University of Science and Technology of
China, Hefei 230026, China
2
Received: 2 August 2017
ABSTRACT
Revised: 7 September 2017
In this work, we developed a polyaniline (PANI)-coated selenium/carbon
nanocomposite encapsulated in graphene sheets ([email protected]/C-G), with excellent
performance in Li-Se batteries. The [email protected]/C-G nanostructure presents
attractive properties as cathode of Li-Se batteries, with a high specific capacity
of 588.7 mAh·g–1 at a 0.2C (1C = 675 mA·g−1) rate after 200 cycles. Even at a high
rate of 2C, a high capacity of 528.6 mAh·g–1 is obtained after 500 cycles. The
excellent cycle stability and rate performance of the [email protected]/C-G composite
can be attributed to the synergistic combination of carbon black (as the conductive
matrix for Se) and the double conductive layer comprising the uniform PANI
shell and the graphene sheets, which effectively improves the utilization of
selenium and significantly enhances the electronic conductivity of the whole
electrode.
Accepted: 27 September 2017
© Tsinghua University Press
and Springer-Verlag GmbH
Germany 2017
KEYWORDS
lithium-selenium batteries,
selenium/carbon
composites,
polyaniline coating,
graphene,
efficient cathodes
1
Introduction
The rapidly growing demand for energy has generated
increasing interest in lithium-sulfur (Li-S) batteries,
which are considered promising candidates as nextgeneration power sources for electric and hybrid
electric vehicles, due to their high theoretical energy
density and capacity [1–4]. However, the commercialization of Li-S batteries is still hindered by the
highly insulating nature of S and the dissolution of
intermediate polysulfide species during cycling [5–9].
Selenium, as a homologue of sulfur with similar
redox chemistry, has been proposed as an alternative
potential cathode material for rechargeable lithium
batteries, due to its inherent advantages over the Li-S
system, including its higher electrical conductivity
(10−5 S·cm−1 vs. 5 × 10−30 S·cm−1), comparable volumetric
capacity (3,253 Ah·L–1 based on a density of 4.82 g·cm−3,
Address correspondence to Jingjing Zhang, [email protected]; Zhigang Xia, [email protected]
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vs. 3,467 Ah·L−1 based on 2.07 g·cm−3), and weaker
shuttle effect [10–12]. Nevertheless, similar to sulfur,
the selenium cathodes also suffer from low utilization
of selenium, due to a certain degree of polyselenide
dissolution, along with a drastic volume variation and
poor lithium ion and electron transport properties
during the repeated lithium insertion/deinsertion
processes [12, 13].
To overcome these issues and enhance the electrochemical performance of Li-Se batteries, promising
results have been achieved by confining Se in a
conductive network, e.g., conducting polymers, various
porous carbons, and graphene [13–25]. In this regard,
various porous carbon-based strategies have been
extensively utilized to improve the low utilization
of selenium. For example, upon dispersing Se into
metal-organic framework-derived porous carbon
microcubes, a Se/CMCs cathode retains a reversible
capacity of 425.2 mAh·g−1 after 100 cycles at 0.2C (1C =
675 mA·g–1) [18]. Guo et al. reported a Se micromesoporous carbon sphere composite with enhanced
cycling stability (540 mAh·g−1 at 0.1C after 100 cycles)
[19]. Moreover, interconnected porous hollow carbon
bubbles, previously prepared by our group, have also
proved to be a promising host for Se [16]. However,
the synthesis of these porous carbon systems is often
energy-intensive and time-consuming, which limits
their production on a commercial scale [26]. In addition,
the open channels of porous carbon materials fail
to completely trap the dissolved polyselenide intermediates, resulting in poor cycling stability, especially
at high rates [27]. Two strategies can be applied
to overcome the various drawbacks affecting the
Se-porous carbon cathode. The first involves using
commercial carbon black, which possesses excellent
conductivity and good adsorption properties, as an
effective host to confine Se. The second strategy involves
surface coatings with good compatibility and high
conductivity, such as polymers and graphene [23, 28,
29]. In particular, the combination of polyaniline (PANI)
and graphene oxide (GO) may provide unexpected
benefits in Li-Se batteries. In our previous work,
graphene-encapsulated selenium/PANI core-shell nanowires have been investigated as cathodes for Li-Se
batteries, and remarkable synergistic effects of graphene
and PANI were observed [23].
These promising results motivated us to adapt our
previous approach and design a novel PANI -coated
selenium/carbon nanocomposite encapsulated in
graphene sheets ([email protected]/C-G). In this study, commercial carbon black was used as a conductive matrix
to prepare Se/C composites by a simple melt-diffusion
method, followed by coating a conductive PANI layer
on the surface of the Se/C composites by an in situ
oxidative polymerization method. Finally, the positively
charged [email protected]/C composites were encapsulated
in graphene sheets by electrostatic interactions. Compared with our previous work, the [email protected]/C-G
composite showed higher specific capacity and better
stability, maintaining a large reversible capacity of
588.7 mAh·g−1 at a rate of 0.2C (1C = 675 mA·g–1) after
200 cycles. Moreover, the [email protected]/C-G composite
exhibited high electrochemical stability at high rate
(528.6 mAh·g−1 at 2C (1C = 675 mA·g–1) after 500 cycles).
The method developed here could be effectively
applied for the large-scale development of cathode
materials for Li-Se batteries.
2
Experimental
2.1 Material synthesis
2.1.1 Preparation of Se/C composites
All chemical reagents employed in this work were of
analytical grade and used without further purification.
The Se/C composites were fabricated via a meltdiffusion method, in which 0.6 g selenium and 0.4 g
commercial conductive carbon black (Ketjenblack
EC600JD) were thoroughly mixed in a ball mill
(300 rpm, 10 h) and then heated to 260 °C in a sealed
vessel under argon atmosphere, with a heating rate
of 1 °C·min−1,. After 12 h, the Se/C composites were
obtained.
2.1.2 Preparation of [email protected]/C-G composites
GO was synthesized by a modified Hummers method
[30]. The [email protected]/C composites were prepared by in
situ oxidative polymerization in an ice bath [28]. First,
the obtained Se/C composites (0.15 g) were dispersed
into a mixed solution (27 mL distilled water and 3 mL
acetone) in a round-bottomed flask (150 mL) under
sonication, to form a uniform black suspension. Then,
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aniline monomer (0.02 g) and a dilute hydrochloric
acid solution (10 mL, 2 M) were added into the
suspension and the mixture was stirred vigorously
for 2 h. Subsequently, an aqueous solution of ammonium
persulfate, ((NH4)2S2O8, 0.07 g dissolved in 10 mL
distilled water) as oxidant was added dropwise to
the above mixture under continuous stirring. After
10 h, 30 mL GO (0.1 mg·mL−1 in H2O) was added
into the above suspension. [email protected]/C-GO was then
obtained after centrifugation, and washed repeatedly
with distilled water and absolute ethanol. The obtained
[email protected]/C-GO was re-dispersed in distilled water
(100 mL) under slow stirring; then, hydrazine (1 mL,
85%) was added into the suspension to reduce GO.
One hour later, the [email protected]/C-G product was collected
by centrifugation, washed three times with water
and ethanol, and then dried under vacuum at 60 °C
overnight, for subsequent use. The Se contents of the
Se/C, [email protected]/C, and [email protected]/C-G composites were
determined to be 55.4 wt.%, 52.8 wt.%, and 51.9 wt.%,
respectively, by thermogravimetric analysis (TGA,
Fig. S1 in the Electronic Supplementary Material
(ESM)) [23, 28].
2.3
Electrochemical measurements
Electrochemical measurements were performed using
coin-type 2016 cells in an argon-filled glove box with
lithium foil as the anode, Celgard 2400 as the separator,
and a solution of 1.0 M LiPF6 in ethylene carbonate
(EC)/diethyl carbonate (DEC) (1:1 , v/v) as the electrolyte.
The samples were mixed with carbon black and
carboxymethyl cellulose binder in a weight ratio of
80:10:10, to form a slurry. The obtained slurry was
coated on an aluminum foil and dried at 80 °C for
12 h in vacuum. The loading of active material in the
electrode was approximately 2–3 mg·cm−2. Galvanostatic
charge/discharge tests were conducted using a
LANDCT2001A battery tester with a voltage window
of 1.0–3.0 V at different rates. Cyclic voltammetry
(CV) experiments were carried out with a potential
range of 1.0–3.0 V (vs. Li+/Li) at a scan rate of 0.1 mV·s−1
using a CHI 600A potentiostat at room temperature.
Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI 660D electrochemical
workstation in the 100 kHz–0.01 Hz frequency range.
The specific discharge/charge capacities were calculated
based on the mass of elemental selenium.
2.2 Characterization of the materials
The structure of the synthesized materials was
characterized by X-ray diffraction (XRD), using a
Philips X’pert diffractometer operated with Cu K
radiation (λ =1.541874 Å), and FTIR spectroscopy
(Bruker IFS-85). The morphologies of the samples
were investigated by field-emitting scanning electron
microscopy (SEM, JEOL JSM-6700F) and transmission
electron microscopy (TEM, H7650). High-resolution
transmission electron microscopy (HRTEM) images
were recorded on a JEOL 2010 transmission electron
microscope with an accelerating voltage of 200 kV.
An energy-dispersive X-ray (EDX) spectrometer was
attached to the JEOL 2010F microscope. X-ray photoelectron spectroscopy (XPS) measurements were carried
out with an ESCALAB 250 instrument. The selenium
content of the composites was determined by TGA
(Shimadzu TGA-50H) under Ar atmosphere at a
heating rate of 10 °C·min−1. Specific surface areas were
determined according to the Brunauer-Emmett-Teller
(BET) method, using a Tristar II 3020M apparatus.
3
Results and discussion
The full synthesis of [email protected]/C-G composites is
illustrated in Scheme 1. First, commercial carbon black,
with excellent conductivity and good adsorption properties, was used as a conductive matrix to prepare
selenium/carbon (Se/C) composites by ball-milling
Scheme 1 Schematic illustration of the formation of the [email protected]/
C-G composite.
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and subsequent heat treatment. The conductive PANI
was then closely covered onto the surface of the Se/C
composites via in situ oxidative polymerization,
resulting in the novel [email protected]/C core/shell structure.
After mixing the GO solution with the aqueous
dispersible [email protected]/C suspension, the GO sheets
could wrap the [email protected]/C composites to form the
hybrid structure, due to the electrostatic interactions
between the negatively charged hydrophilic GO nanosheets and positively charged PANI layers on the
Se/C surface. Finally, the [email protected]/C-G structure was
obtained after chemically reducing GO.
SEM and TEM images were recorded to inspect
changes in morphology upon formation of [email protected]/
C-G (Fig. 1). After the infiltration of Se, the characteristic
loosely packed nanostructure of conductive carbon
black (Fig. S2 in the ESM) became denser (Figs. 1(a)
and 1(d)) and no bulk selenium materials were observed
in the external region, indicating that all selenium
species were located in the pores of the carbon black
matrix, forming a homogenous Se/C nanocomposite.
This was also confirmed by the analysis of the specific
surface area, which showed that the BET surface
area and total pore volume drastically decreased to
23.5 and 0.043 cm3·g−1 from 843.7 and 0.86 cm3·g−1,
respectively (Fig. S3 in the ESM). Figures 1(b) and 1(e)
show that the Se/C composite is uniformly covered by
undulating PANI layers, forming a core/shell structure
with a thickness of ~6 nm. Furthermore, the SEM image
of [email protected]/C-G (Fig. 1(c)) shows that few layers
of wrinkled graphene sheets are tightly wrapped on
the surface of the [email protected]/C composite, as further
illustrated by the TEM analysis (Fig. 1(f)). As shown
in Fig. 1(f), the [email protected]/C composite was uniformly
encapsulated in the graphene sheets. In addition, the
elemental mapping images in Fig. 1(g) were used to
study the distribution of Se, N, and C elements in
[email protected]/C-G, clearly revealing that Se is uniformly
distributed in the matrix. These results confirm
that a core/shell structure consisting of [email protected]/C
encapsulated in graphene sheets was successfully
fabricated; this structural arrangement could enhance
Figure 1 SEM and TEM images of Se/C ((a) and (d)), [email protected]/C ((b) and (e)), and [email protected]/C-G ((c) and (f)). (g) Elemental mappings
of [email protected]/C-G (C, N, and Se signals are colored green, yellow, and red, respectively).
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the electrical conductivity of the composite and
effectively prevent lithium polyselenides from dissolving
into the electrolyte.
The XRD patterns and FTIR spectra shown in
Figs. 2(a) and 2(b), respectively, were used to analyze
the structural characteristics of the as-prepared Se/C,
[email protected]/C, and [email protected]/C-G samples. The sharp
diffraction peaks in the XRD spectrum of selenium
(Fig. 2(a)) illustrate its trigonal crystalline state [16].
Notably, the characteristic trigonal Se peaks in Se/C
disappear after heating, and only the broad peak of
carbon around 26.0° can be observed, which denotes
a good dispersion of amorphous Se in carbon black,
consistent with the morphology analysis. The XRD
patterns of the [email protected]/C and [email protected]/C-G composites are similar to that of Se/C, although the intensity
of the broad carbon peak was reduced, especially for
the [email protected]/C-G composite, due to the introduction
of the PANI and graphene layers [31].
Figure 2(b) displays the typical FTIR spectra of
the samples. Se/C does not exhibit obvious specific
absorption bands in the region examined, which
indicates the absence of chemical interactions between
carbon and selenium atoms [32]. Therefore, the bands
observed for [email protected]/C correspond to PANI absorptions only. The PANI spectra showed the characteristic
peaks at 1,577 cm−1 for the C=N stretching mode of
the quinonoid rings, 1,495 cm−1 for the C=C stretching
mode of benzenoid rings, 1,301 cm−1 for the C–N
stretching mode, and 1,247 cm−1 for the C=N stretching
vibrations. Moreover, the vibrational bands at 1,142
and 821 cm−1 are attributed to the in-plane and out-of
plane C–H bending modes of aromatic rings [33]. The
spectrum of [email protected]/C-G is also almost identical to
that of PANI. The analysis confirms that PANI could
be formed on the surface of the Se/C composites,
similar to what previously found for PANI in S/C
composites [28]. However, in the case of the [email protected]/
C-G hybrid, the intensity of the band around 1,142 cm−1
was much stronger, which could be indicative of
electron delocalization [34], suggesting the combination
of the PANI and graphene layers [23, 31]. The strong
interaction between these layers would facilitate a
charge-transfer process between the two components,
contributing to increasing the conductivity of the
nanocomposites. The presence of these interactions was
further confirmed by the shift of the characteristic
peaks to higher frequencies [31, 35].
XPS measurements were carried out to further
confirm the existence of graphene, PANI, and selenium
in the [email protected]/PANI composite, as shown in Fig. 3.
Figure 3(a) displays the C 1s, N 1s, and Se 3d signals.
In the Se 3d spectrum of the [email protected]/C-G hybrid
(Fig. 3(b)), the Se 3d3/2 (56.2 eV) and 3d5/2 (55.35 eV)
binding energies, with a spin-orbit splitting of 0.85 eV,
are attributed to metallic selenium [16]. The C 1s
spectrum was deconvoluted into four peaks at 284.6,
285.9, 287.0, and 288.4 eV (Fig. 3(c)), which can be
assigned to the C=C, C–C, and C–O/C–N bonds in
conductive carbon black and graphene, and HO–C=O
on the surface of graphene, respectively [27, 31, 36].
The N 1s core level spectrum can also be deconvoluted
into four peaks (Fig. 3(d)). The binding energies
centered at 399.1, 400.0, 400.3, and 401.8 eV can be
assigned to the quinoid imine [=N–], benzenoid amine
[–NH–], cationic nitrogen atoms [=NH+], and protonated
amine units [–NH+] in PANI, respectively [37].
Coin-type half cells were assembled to evaluate the
Figure 2 (a) XRD patterns and (b) FTIR spectra of the Se/C, [email protected]/C, and [email protected]/C-G composites.
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Figure 3 XPS analysis of the [email protected]/C-G composite, showing the overall spectrum (a), as well as the Se 3d (b), C 1s (c), and N 1s
(d) regions.
electrochemical performances of the products. The
CV curves of the Se/C, [email protected]/C, and [email protected]/
C-G composites in the first three cycles are shown
in Fig. 4. Only one pair of reversible redox peaks can
be observed for all samples, indicating the one-step
reversible reaction of [email protected], in agreement with
previous reports [13, 16, 17]. In comparison, Se/C
(Fig. 4(a)) and [email protected]/C (Fig. 4(b)) show a larger
shift of the cathodic peaks than [email protected]/C-G (Fig. 4(c)),
denoting a relatively weak polarization in the
[email protected]/C-G composite during the initial cycles [38].
Moreover, the [email protected]/C-G composite extends the
reduction and oxidation peak areas, which indicate
higher capacity. The oxidation and reduction peaks
of [email protected]/C-G overlap well from the second cycle
onward, demonstrating a relatively good capacity
retention.
Figure 5(a) shows the voltage profiles of Se/C,
[email protected]/C, and [email protected]/C-G at 0.2C (1C = 675 mA·g−1).
[email protected]/C-G exhibits a higher reversible capacity
(628.1 mAh·g−1) than Se/C and [email protected]/C (357.6
and 555 mAh·g−1, respectively) between 1.0 and 3.0 V,
further demonstrating the higher utilization of active
selenium by [email protected]/C-G. When the cells are cycled
at 0.2C, the discharge capacity decreases significantly
for Se/C and [email protected]/C, but remains high in the
case of [email protected]/C-G (Fig. 5(b)). After 200 cycles, the
[email protected]/C-G cathode retains a reversible capacity of
588.7 mAh·g−1 (Fig. 5(b)).
The corresponding Coulombic efficiency shows a
rapid increase after the second cycle, and is then
stabilized at ~100% (dark blue dots in Fig. 5(b)). However,
the reversible capacities of Se/C and [email protected]/C
drop to 227.4 and 458.5 mAh·g−1, respectively, under
the same conditions, and the corresponding Coulombic
efficiencies also tend to be stable after the second
cycle (dark blue dots in Fig. S6 in the ESM). This result
indicates that selenium is more chemically stable in
[email protected]/C-G than in Se/C and [email protected]/C.
The improved stability of selenium in [email protected]/
C-G should be ascribed to the synergistic effect of
coating Se/C by both PANI and graphene, which
suppresses the formation and dissolution of intermediates arising from the selenium reduction. This
was confirmed by inspecting the color changes of the
three electrodes immersed in carbonate-based electrolyte
after 100 and 200 cycles at 0.2C (Fig. S4 in the ESM).
After 100 cycles, no color change was observed for
the [email protected]/C and [email protected]/C-G electrodes, while
the Se/C sample changed from colorless to orange,
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Figure 4 CVs of the Se/C (a), [email protected]/C (b), and
[email protected]/C-G (c) samples at a scan rate of 0.1 mV·s−1 between
1.0 and 3.0 V.
Figure 5 (a) Initial charge/discharge potential profiles. (b) Cycle
performance at a current density of 0.2C between 1.0 and 3.0 V.
(c) Rate capability at various current densities between 1.0 and
3.0 V for Se/C, [email protected]/C, and [email protected]/C-G.
indicating the dissolution of intermediates. The
[email protected]/C-G electrode remained colorless for up to
200 cycles, whereas the [email protected]/C one changed to
orange.
Figure 5(c) shows the rate performances of the
three cathodes from 0.2C to 5C. It can be seen that
[email protected]/C-G exhibits much higher rate capability
than Se/C and [email protected]/C. For example, [email protected]/
C-G exhibited reasonable capacity retention with
increasing cycling rates: 529.2 (2C), 514.2 (3C), 499.8
(4C), and 475.6 mAh·g−1 (5C), whereas the measured
capacity values were only 184.7 (2C), 164.2 (3C), 139.4
(4C), and 125.4 mAh·g−1 (5C) for Se/C and 463.6 (2C),
438.2 (3C), 420.8 (4C), and 397.5 mAh·g−1 (5C) for
[email protected]/C. Most importantly, after the rate capability
test, [email protected]/C recovers its average discharge capacity
of 585.6 mAh·g−1 at 0.2C, whereas the discharge
capacities of Se/C and [email protected]/C dropped significantly
to 198.3 and 464.5 mAh·g−1, respectively. The improved
rate capability of [email protected]/C-G is most likely related
to its higher electrical conductivity, resulting from the
double conductive layers comprising the PANI coating
and graphene sheets. This was further confirmed
by EIS measurements (Fig. S5 in the ESM). The EIS
spectra show a depressed semicircle in the highfrequency region followed by an inclined line at low
frequencies. The EIS spectra were analyzed and fitted
with an equivalent circuit model, in which Re represents
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the impedance of the electrolyte, Rct is the charge
transfer resistance, and a constant phase element (CPE)
is used to represent the capacitance of the electrical
double layer. The analysis shows that [email protected]/C-G
has a much smaller Rct (56 Ω) than Se/C (111 Ω) and
[email protected]/C (69 Ω), which denotes its lowest charge
transfer resistance and fastest Li+ diffusion behavior
[10, 23].
Figure 6 shows the typical cycle performance
of [email protected]/C-G during high-rate discharge (2C and
5C). After a long cycling period (500 cycles), the
[email protected]/C-G composite retains a reversible capacity
of 528.6 mAh·g−1 at 2C, and its Coulombic efficiency
(represented as dark blue dots in Fig. 6) remains
consistently at ~99%. The [email protected]/C-G composite
maintains high cycling stability also at the highest
current density (5C), retaining a reversible capacity
of 403.2 mAh·g−1 after 500 cycles. A comparison of
the electrochemical performance of the present
[email protected]/C-G composite and previously reported Se/C
composites is shown in Table 1. The [email protected]/C-G
composite exhibits significantly improved performances
compared with most other composites, which could
be attributed to the synergistic effect of the good
conductivity of both the porous carbon black matrix
and the PANI/graphene coating layers. The porous
conductive carbon black matrix could effectively
disperse and sequester Se, to accommodate volume
variations and alleviate the shuttle effects of polyselenide species during repeated lithium insertion/
deinsertion processes. The PANI coating and graphene
layers not only provide an electronic conductive
network (which contributes to the improved rate
capability of the cathode), but also further suppress
the dissolution of intermediates, which plays an
Figure 6 Long-term cycling performance of the [email protected]/C-G
composite cathode at high current densities of 2C and 5C.
important role in the improved cycling stability of
the cathode.
4
Conclusions
In conclusion, we prepared a superior nanocomposite
consisting of a PANI-coated selenium/carbon structure
encapsulated in graphene sheets, with potential application as efficient cathode for Li-Se batteries. In the
synthesized composite, selenium is highly dispersed
in the conductive porous carbon black matrix, and
its dissolution is hindered by the uniform PANI
shell deposited on the surface of the Se/C composite.
Moreover, the combination of graphene and PANI
significantly facilitates electronic/ionic transport, leading
to improved cyclic stability and rate capability of the
selenium cathode.
Acknowledgements
The authors would like to appreciate the financial
support from the Natural Sciences Fund of Zhejiang
Province (No. LQ17B010003) and the National Natural
Science Foundation of China (NSFC) (No. 11604319).
Electronic Supplementary Material: Supplementary
material (further material characterization,such as TGA,
SEM, TEM, BET; detailed electrochemical analysis;
Table of comparison between [email protected]/C-G (this
work) and the reported selenium/carbon composites)
is available in the online version of this article at
https://doi.org/10.1007/s12274-017-1870-2.
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