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Nano Research
Nano Res
DOI
10.1007/s12274-014-0646-1
Three-Dimensional Porous Graphene-Metal Oxide
Composite Microspheres: Preparation and Application
in Li-Ion Batteries
Seung Ho Choi1,2, Jung-Kul Lee2(), and Yun Chan Kang1()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0646-1
http://www.thenanoresearch.com on November 24 2014
© Tsinghua University Press 2014
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1
TABLE OF CONTENTS (TOC)
Three-Dimensional Porous Graphene-Metal Oxide
Composite Microspheres: Preparation and Application
in Li-Ion Batteries
Seung Ho Choi1,2, Jung-Kul Lee2,*, Yun Chan Kang1,*
[1] Department of Materials Science and Engineering,
The new three-dimensional (3D) porous graphene-metal oxide
Korea University, Republic of Korea.
composite microspheres are prepared by a one-pot spray pyrolysis
[2] Department of Chemical Engineering, Konkuk
process. 3D porous SnO2-graphene microspheres selected as the first
University, Republic of Korea.
target material showed superior electrochemical properties as anode
materials for lithium ion batteries. Discharge capacity of the 3D porous
SnO2-graphene microspheres after 500 cycles at a high current density
of 2 A g-1 is 1009 mA h g-1.
Nano Research
DOI (automatically inserted by the publisher)
Review Article/Research Article
Please choose one
Three-Dimensional Porous Graphene-Metal Oxide
Composite Microspheres: Preparation and Application
in Li-Ion Batteries
Seung Ho Choi1,2, Jung-Kul Lee2(), and Yun Chan Kang1()
Received: day month year
ABSTRACT
Revised: day month year
The use of new three-dimensional (3D) porous graphene-metal oxide
composite microspheres that has been studied as anode material for lithium
ion batteries (LIBs) is firstly introduced here. 3D graphene microspheres are
aggregates of individual hollow graphene nanospheres composed of sheet
graphene. Metal oxide nanocrystals are uniformly distributed over the
graphene surface of the microspheres. The 3D porous graphene-SnO2
microspheres were selected as the first target material for investigation
because of their superior electrochemical properties. The 3D porous
graphene-SnO2 and graphene microspheres, and bare SnO2 powders,
deliver discharge capacities of 1009, 196, and 52 mA h g-1, respectively, after
500 cycles at a current density of 2 A g-1. The 3D porous graphene-SnO2
microspheres have uniquely low charge transfer resistances and high
lithium ion diffusivities before and after cycling.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Graphene • Metal oxide •
Nanostructures
•
Electrode
material•
Batteries
1. Introduction
Two-dimensional
(2D)
graphene-metal
oxide
phenomenon,
but
also
introduces
desirable
composites with unique combinations of electronic,
characteristics of fast ionic and electronic transport,
chemical, and mechanical properties are considered
high specific area and strong mechanical properties
promising materials for various applications such
[10-20].
as batteries, supercapacitors, sensors, and catalysts
In
energy
storage
fields,
various
3D
[1-9]. In continuous and large-scale production of
graphene-metal oxide structures, such as porous
2D
processes,
graphene film, graphene foam, and graphene ball
graphene-based composites are subject to serious
hybrids, are more significant than the 2D forms. 3D
aggregation between the graphene sheets owing to
graphene structures maintain the superior intrinsic
van der Waals forces [10-14]. A change to
properties of graphene sheets, such as large surface
material
by
liquid
solution
three-dimensional
(3D)tographene
from [email protected];
2D graphene
areas, Lee,
novel
physical properties and excellent
Address correspondence
Yun Chan Kang,
Jung-Kul
[email protected].
sheets is not only a good solution for aggregation
electrochemical properties, and can further exhibit
Nano Res.
improved functions through addition of metal
spherical hybrids remains a large and essential
oxides [21-34]. For example, a 3D graphene
challenge. In this study, we first synthesized the
structure with porous morphology has many
new structured 3D graphene-metal oxide spherical
advantages such as easy electrolyte penetration and
powders with macroporous structure for anode
fast Li diffusion for lithium ion batteries [26-34].
applications
Also, 3D graphene sheets with their outstanding
microspheres are aggregates of individual hollow
electrical conductivity and flexibility act as an
graphene nanospheres formed by graphene sheets.
excellent support, and as a buffer layer that
Metal
mitigates
Li-ion
distributed over the graphene sheets forming the
insertion/extraction by absorption of stress, thus
microspheres. The hollow graphene nanosphere
improving the structural stability and cyclability of
structure accommodates the volume change of
electrode materials [26-34]. In particular, 3D
metal oxides during repeated lithium insertion and
graphene-metal
extraction and prevents growth of active material
+
reported
volume
to
changes
oxide
possess
during
composites
excellent
have
been
electrochemical
properties through a range of energy storage
applications [21-34].
Choice
of
graphene-metal
a
of
oxide
LIBs.
3D
porous
nanocrystals
were
graphene
uniformly
crystals during cycling.
In this study, 3D graphene-SnO2 microspheres
with porous structure were selected as the first
synthesis
target
material
because
SnO2
nanostructured
continuous, and large-scale production is extremely
widely studied as anode materials for LIBs.
important. In previous reports, 3D graphene-metal
Polystyrene (PS) nanobeads were used as sacrificial
oxide composite structures were mainly prepared
templates in creating a porous structure [14,29]. 3D
using a chemical vapor deposition (CVD) process or
porous graphene-SnO2 composite microspheres
a multistep solution process [15-18,35-39]. CVD
showed high reversible capacity and excellent
synthesized graphene-metal oxide is commonly
cycling and fast rate performance. Our approach to
grown on a flat metal foil or thin film greatly
fabricating
limiting its application in energy storage due to low
powders may be valuable in developing this
production yield [16-18]. Compared with the CVD
application for energy storage devices.
processes,
for
3D
materials with various morphologies have been
solution
architectures
of
facile,
method,
oxide
method
a
3D
porous
graphene
structure
including
hydrothermal, self-assembly, and freeze-drying
2. Experimental
techniques, are large scale and less costly, and the
2.1
resulting 3D graphene-metal oxide architectures
composite microspheres.
Synthesis
of
3D
porous
graphene-SnO2
have desirable features of high porosity and pore
Graphene oxide (GO) was synthesized using a
structure with meso and macropores [35-39].
modified Hummer’s method [40]. 3D porous
However, obstacles to use of the solution process
graphene-SnO2
are the multiple steps, long reaction times, and use
prepared by ultrasonic spray pyrolysis at 800 oC; a
of a toxic reducing agent for graphene oxide
schematic of the apparatus is shown in Figure S10.
reduction [35-39]. As potential commercial products
A quartz reactor of length 1200 mm and diameter 50
for industrial applications, 3D graphene-metal
mm was used, with a nitrogen flow rate (carrier
oxide spherical powders with regular morphology
gas) of 10 L min-1. The as-obtained GO was
and non-aggregating properties are attractive.
redispersed in distilled water and exfoliated by
Therefore, development of a new continuous and
ultrasonication to generate GO sheets. 500 mL of the
one-step process for 3D graphene-metal oxide
exfoliated GO solution (1 mg ml-1) was added to 1.4
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microspheres
were
directly
Nano Res.
g of Sn oxalate (13 mM) in H2O2 solution. Then, 4.2
analysis of nitrogen adsorption measurements
g of 100-nm PS nanobeads was added into the
(TriStar 3000). The powders were also investigated
solution of GO sheets and Sn. Subsequently, the
using X-ray photoelectron spectroscopy (XPS),
prepared
PS
(ESCALAB-210) with Al K radiation (1486.6 eV).
nanobeads, and GO sheets was dispersed in
Thermal gravimetric analysis (TGA, SDT Q600) was
performed in air at a heating rate of 10°C min-1 to
determine the amount of graphene in the composite
microspheres.
solution
with
Sn
oxalate
salt,
distilled water by ultrasonication.
2.2 Characterizations
The crystal structures of the powders were
investigated by X-ray diffractometry (XRD), (X’pert
2.3 Electrochemical Measurements
PRO MPD), using Cu Kradiation (= 1.5418 Å ).
Capacities and cycling properties of the powders
were determined using a 2032-type coin cell format.
The electrode was prepared from a mixture
containing 70 wt% of the active material, 15 wt% of
Super P, and 15 wt% of sodium carboxymethyl
cellulose (CMC) binder. Lithium metal and
microporous polypropylene film were used as
counter electrode and separator, respectively. The
electrolyte was 1 M LiPF6 in a 1:1 mixture by
volume of ethylene carbonate/dimethyl carbonate
The morphological features were investigated using
field-emission
scanning
electron
microscopy
(FE-SEM, Hitachi S-4800), and high-resolution
transmission
electron
microscopy
(HR-TEM,
JEM-2100F), at a working voltage of 200 kV. The
specific
surface
graphene-SnO2
areas
composite
of
the
3D
porous
microspheres
were
calculated from a Brunauer–Emmett–Teller (BET)
Scheme 1. Schematic diagram for the formation process of 3D porous graphene-SnO2 composite microsphere.
(EC/DMC) with
Charge-discharge
were determined
range 0.001–3.0 V
5% fluoroethylene carbonate.
characteristics of the samples
through cycling in the potential
at various fixed current densities.
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Cyclic voltammetry (CV) measurements were
carried out at a scan rate of 0.1 mV s-1. Dimensions
of the negative electrode were 1 cm × 1 cm and mass
loading was approximately 1.2 mg cm-2.
3. Results and discussion
3D
porous
graphene-metal
oxide
composite
microspheres were prepared by a one-pot spray
pyrolysis process. The SnO2 nanocrystals of active
material for lithium storage were distributed inside
individual hollow graphene nanosphere consisting of
graphene sheets. A schematic diagram for the
formation of 3D porous SnO2-graphene microspheres
by one-pot spray pyrolysis is shown in Scheme 1.
Droplets containing PS nanobeads, fragments of
graphene oxide sheets, and tin oxalate, were formed
by an ultrasonic nebulizer. Drying these droplets
produced
the
PS-tin
oxalate-graphene
oxide
composite powder, in which PS nanobeads were
uniformly distributed among the graphene oxide
sheets and tin oxalate. Graphene oxide binds with PS
nanobeads
in
water
through
a
hydrophobic
interaction [41,42]. In consequence, PS nanobeads
improve the stability of a colloidal solution of
graphene oxide sheets. Elimination of the PS
nanobeads by thermal decomposition into CO2 and
H2O resulted in 3D porous graphene microspheres
with numerous tiny hollow individual nanospheres.
Decomposition of tin oxalate and thermal reduction
of graphene oxide sheets resulted in SnO2-graphene
composite microsphere [40]. Overall, 3D porous
SnO2-graphene microspheres were formed from
single droplets by one-pot spray pyrolysis, where
process conditions included a short residence time of
3 s in a hot wall reactor maintained at 800°C under a
nitrogen atmosphere. Ultrafine SnO2 nanocrystals
were uniformly distributed over the 3D porous
graphene microspheres. The size of nanospheres
making up a 3D porous microsphere could be easily
manipulated by controlling the PS nanobeads’ size,
Figure 1. Morphologies of the 3D porous graphene-SnO2
composite microspheres: a,b) SEM images, c-e) TEM images, f)
SAED pattern, and g) elemental mapping images of Sn and C
components.
composite microspheres prepared by the one-pot
spray pyrolysis method described above are shown
in Figure 1. The concentration of tin oxalate was 13
mM. SEM images of the composite microspheres
show an embossed structure, and TEM images show
numerous tiny hollow graphene nanospheres. The
high resolution TEM image shown in Figure 1e
reveals graphene sheets with multiple layers forming
the
skin of individual
graphene
nanospheres.
Ultrafine SnO2 nanocrystals of several nanometers in
size were uniformly distributed over the graphene
sheets, as shown in Figure 1e. The Figure S2 exhibits
clear lattice fringes with d spacings of 0.34 nm and
0.26 nm, which can be attributed to the (110) and (101)
planes of rutile SnO2, respectively [29]. The selected
area electron diffraction (SAED) pattern of the
composite powders as shown in Figure 1f shows the
ring-like mode characteristic of polycrystalline SnO2
[43]. The elemental mapping images in Figure 1g
show that the uniform distributions of the Sn and C
components originated from the SnO2 nanocrystals
as shown in Figure S1.
Morphologies of the 3D porous SnO2-graphene
and graphene sheets, respectively, uniformly
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Figure 3. Morphologies and SAED pattern of the 3D porous
graphene microspheres: a,b) SEM images, c,d) TEM images,
and e) SAED pattern.
SnO2-graphene composite microspheres. The high
resolution TEM image in Figure 3d shows graphene
sheets with multiple layers forming the skin of
individual drupelets. The SAED pattern in Figure
Figure 2. TEM images and elemental mapping images of the
3e shows sets of bright spots and faint spots,
3D porous graphene-SnO2 composite microspheres prepared
indicating that the sample consists of randomly
from spray solution with (a-d) low and (e-h) high concentration
oriented graphene layers [44,45].
of Sn oxalate (8 mM and 18 mM): a-c) TEM images of low
SnO2, d) elemental mapping images of low SnO2, e-g) TEM
images of high SnO2, and h) elemental mapping images of high
SnO2.
Figure 4a shows the XRD patterns of both the 3D
porous graphene and graphene-SnO2 microspheres
prepared
directly
by
spray
pyrolysis
at
temperatures of 800 °C. The XRD pattern of the
distributed over the 3D porous SnO2-graphene
graphene
composite
diffraction peak around 23
microspheres.
Figure
2
show
the
microspheres
shows
o
a
broadened
similar to reduced
morphologies and elemental mapping images of the
graphene
graphene-SnO2 composite microspheres prepared
microspheres exhibits pure crystal structure of
from the spray solutions with low (8 mM) and high
rutile SnO2 (JCPDS card no. 41-1445) [46]. The mean
(18
The
crystallite size of SnO2 nanocrystals dispersed on
graphene-SnO2 composite microspheres had 3D
graphene microspheres calculated from the (110)
porous
of
peak using Scherer’s formula was an ultrafine 4 nm.
ultrafine SnO2 nanocrystals over the microspheres
The pore-size distributions of the 3D porous
regardless of SnO2 contents in the composite
SnO2-graphene and graphene microspheres were
powders. Morphologies of the 3D porous graphene
investigated by nitrogen isothermal adsorption, and
microspheres, not containing SnO2, prepared by the
the results are shown in Figure 4b. Both samples
one-pot spray pyrolysis method from a spray
had similar pore size distributions, including both
solution of PS nanobeads and graphene oxide sheets,
mesopores (2–50 nm) and macropores. BET surface
are shown in Figure 3. The graphene microspheres
areas
had
microspheres were 120 and 200 m2 g-1, respectively.
mM)
concentrations
structure
similar
and
of
tin
uniform
structure
oxalate.
distribution
to
the
of
oxide,
the
while
the
SnO2-graphene
SnO2-graphene
and
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thermal reduction of GO sheets
containing
oxygen
functional
groups into graphene occurred
when
the
preparation
temperature was set to 800 °C
[47]. The overall XPS spectrum
of the 3D porous SnO2-graphene
microspheres
(Figure
S4)
showed clear Sn 3d5/2 (487.3eV)
and Sn 3d3/2 (495.8eV) peaks [48].
The electrochemical properties
of the 3D porous SnO2-graphene
microspheres were evaluated by
assembling a coin-type half-cell.
In
addition,
we
tested
electrochemical properties of 3D
Figure 4. Properties of the 3D porous graphene and graphene-SnO2 composite porous
microspheres: a) XRD patterns, b) pore size distributions, c) XPS spectrum of C1s of 3D
SnO2
microspheres,
both
prepared by spray pyrolysis under
composite microspheres.
graphene
microspheres
without SnO2 nanocrystals, and
porous graphene microspheres, and d) XPS spectrum of C1s of 3D porous graphene-SnO2 bare
The
graphene
content
in
the
3D
the same conditions; the bare SnO2
porous
SnO2-graphene microspheres was evaluated to be
microspheres had a spherical shape and dense
19.5 wt% by thermogravimetric analysis (Figure S3).
structure as shown in Figure S5. Figure 5a shows
Figures 4c and 4d show the XPS profiles of C1s
the cyclic voltammogram (CV) curves for the first 5
acquired
cycles
from
the
3D
porous
(Figure
4c)
and
graphene
of
the
3D
porous
SnO2-graphene
porous
microspheres at a scan rate of 0.1 mV s-1. In the first
graphene-SnO2 microspheres (Figure 4d). The C1s
cathodic step, the apparent reduction peak at 0.9 V
peak in the XPS profile could be attributed to sp2
is attributed to form Li2O and Sn metal and solid
bonded carbon (C–C), epoxy and alkoxy groups
electrolyte interphase (SEI) layers when SnO2
(C–O) and carbonyl and carboxylic (C=O) groups,
nanocrystals react with Li+ [48-54]. Low-potential
which corresponded to peaks at 284.6, 286.6, and
peaks (at < 0.6 V) corresponding to the Li xSn alloy
288.1 eV, respectively [47]. The XPS profiles showed
formation
sharp peaks at around 284.6 eV, which could be
oxidation peaks at 0.25 V and 0.6 V during the
assigned to graphitic carbon [47]. The relative
anodic scan are related to dealloying of Li xSn
carbon content of the sp2 bonded carbon at 286.4 eV
[43,48-54]. In the second cycle, the decomposition
was
SnO2-graphene
peak of SnO2 at 0.9 V disappeared and the related
microspheres. The carbonyl and carboxylic (C=O)
peaks of Li-Sn alloy and dealloying reactions were
groups were not observed in the XPS C1s profile of
seen repeatedly. Reduction peaks at 1.5 V in the first
the graphene-SnO2 composite microspheres due to
cathodic step were observed in the CV curves for
the complete reduction of GO sheets into reduced
3D porous SnO2-graphene microspheres and 3D
GO sheets. Analysis by XPS indicated that the
porous graphene microspheres and the peaks
microspheres
83%
in
the
3D
porous
3D
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are
also
observed
[43,48-54].
The
Nano Res.
storage at highly active
defect sites [49-52]. 3D
porous SnO2-graphene
microspheres delivered
discharge and charge
capacities of 1586 mA h
g-1 and 1010 mA h g-1 in
the first cycle, and the
corresponding
initial
CE was 64%. Bare SnO2
delivered
discharge
and charge capacities
of 1397 mA h g-1 and
935 mA h g-1 in the first
cycle,
Figure 5. Electrochemical properties of the 3D porous graphene and graphene-SnO2 composite
microspheres and bare SnO2 powders: a) CV curves, b) initial charge/discharge curves at a current
density of 2 A g-1, c) cycling performances at a current density of 2 A g-1, and d) rate performance
and Coulombic efficiencies of 3D porous graphene-SnO2 composite microspheres.
with
of 67%. The irreversible
capacity loss of the bare
SnO2
powders
attributed
disappeared from the second cycles as shown in
a
corresponding initial CE
to
is
the
Figures 5a and S6. The reduction peak at 1.5 V in
formation of Li2O from SnO2 and SEI layers at the
the first cathodic step is attributable to irreversible
electrode–electrolyte interface [43,48-54].
lithium insertion into graphene layers of the 3D
Cycling
performances
of
the
3D
porous
porous structure. Figure 5b shows the initial charge
SnO2-graphene and graphene microspheres and
and
porous
bare SnO2 powders at a constant current density of
SnO2-graphene and graphene microspheres and
discharge
curves
of
the
3D
2 A g-1 are shown in Figure 5c. In comparison, after
bare SnO2 powders. The operating cut-off voltages
500 cycles the 3D porous SnO2-graphene and
were 0.001 and 3 V at a current density of 2 A g-1.
graphene microspheres and bare SnO2 powders
The initial discharging and charging specific
delivered discharge capacities of 1009, 196, and 52
capacities of the 3D porous graphene microspheres
mA h g-1, respectively, and the corresponding
were 1244 and 389 mA h g , respectively.
capacity retentions measured after the first cycles
Coulombic
were 96, 47, and 5%. Coulombic efficiency of 3D
-1
efficiency
(CE)
of
the
graphene
microspheres observed in the first cycle was low at
porous
32%, which is an unavoidable phenomenon in
approximately 99% after the tenth cycle and
reduced
graphene
oxide
materials
SnO2-graphene
microspheres
reached
[55,56].
remained at this value in subsequent cycles. The
Electrochemical properties of reduced graphene
average Coulombic efficiency from 100th to 500th
oxide are affected by synthesis methods, and
cycles of these SnO2-graphene microspheres was
various lithium storage properties have been
99.8%. The increase in the discharge capacities of
previously reported [6-8]. However, most reduced
the 3D porous SnO2-graphene microspheres after
graphene oxide has low initial CE due to the high
100 cycles had been attributed to the formation of a
production of SEI layers resulting from the high
gel-like reversible polymer film on the microsphere
specific surface area and to irreversible lithium ion
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surfaces [57-59]. The 3D porous SnO2-graphene
good conductivity and morphological uniqueness
composite
excellent
of
of
contribute
microspheres
electrochemical
properties
have
regardless
SnO2
3D
porous
to
graphene-SnO2
their
excellent
were
contents (see Figure S7a). The long-term cycling
performance,
and
performance
electrochemical
impedance
of
3D
porous
SnO2-graphene
microspheres
electrochemical
confirmed
by
spectroscopy
(EIS)
microspheres at a high value of the current density,
measurements. Nyquist plots of the electrodes
5 A g-1, is shown in Figure S7b. The 2nd and 1000th
consisted of a semicircle in the medium frequency
discharge
porous
region and an inclined line at low frequencies. The
graphene-SnO2 microspheres were 905 and 730 mA
medium-frequency semicircle was attributed to
h g , and the Coulombic efficiency was reached at
charge-transfer resistance between active material
99.7% after 50 cycles. The discharging and charging
and electrolyte, and the low frequency portion of
rate performance of the 3D porous graphene-SnO2
the trace corresponded to the lithium diffusion
microspheres are shown in Figure 5d. As can be
process within the electrodes [62-64]. The charge
seen, the average reversible discharge capacities
transfer resistance of 3D porous graphene-SnO2
were 1020, 875, 785, 716, and 660 mA h g at current
microspheres was much smaller than that of bare
densities of 1, 3, 5, 7, and 9 A g -1, respectively. After
SnO2 powders before cycling, as shown in Figure
high-rate
discharge
S9a. Figure S9b shows the relationship between the
capacity recovered to a value as high as 941 mA h
real part of the impedance spectrum Zre and ω-1/2
g-1 for a lower current density of 1 A g-1.
(where ω = 2πf is angular frequency) in the
capacities
of
the
3D
-1
-1
charge-discharge
cycling,
TEM images as shown in Figure S8 showed that
low-frequency region, before cycling. The gradual
3D porous graphene-SnO2 microspheres maintained
low-frequency slope (the Warburg impedance
their overall morphology after cycling, but bare
coefficient) of Zre versus ω-1/2 indicates high lithium
SnO2 powders lost their original morphologies after
ion diffusivity for 3D porous graphene-SnO2
the 200th cycle. The 3D graphene backbone of the
microspheres [63,64]. The unique structure of 3D
graphene-SnO2 microspheres was maintained, as
porous graphene-SnO2 microspheres resulted in
shown in the TEM images. The Sn component
low charge transfer resistance and high lithium ion
remained
dispersed
over
the
diffusivity. Figures S9c and S9d show the Nyquist
aggregation
even
after
plot and the Zre - ω-1/2 relationship after 200 cycles.
cycling, as shown in the elemental mapping images
The microspheres continued to have low charge
in Figure S8b. The 3D porous graphene structure
transfer resistance and high lithium ion diffusivity
suppressed the aggregation of SnO2 nanocrystals
even after 200 cycles. On the other hand, the charge
during repeated cycling. The graphene increases the
transfer resistance of bare SnO2 powders increased
electrical conductivity of the electrode and is able to
after
accommodate the strain induced by volume change
performance
of SnO2. It also facilitated fast transportation of
microspheres is due to their structural stability and
electrons and lithium ions, which is responsible for
graphene’s synergy effects.
uniformly
microspheres
without
cycling.
The
of
excellent
electrochemical
porous
graphene-SnO2
3D
the good cycling stability and rate capability [59-61].
On the other hand, changes in the bare SnO2
4. Concluisons
powders caused disconnection between active
material, conducting carbon, and copper foil,
In this study, electrochemical properties of 3D porous
resulting in rapid capacity fading upon cycling. The
graphene-SnO2 microspheres were compared with
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those of 3D porous graphene microspheres (without
of the bare SnO2 powders, CV curves of the 3D
SnO2) and bare SnO2 powders, prepared by the same
porous graphene, Cycling performances of the 3D
process. 3D porous graphene-SnO2 microspheres
porous graphene-SnO2 composite, TEM images of the
were formed from single droplets using the one-pot
porous graphene-SnO2 composite after cycling, EIS
spray pyrolysis method. Ultrafine SnO2 nanocrystals
data of the 3D porous graphene-SnO2 composite and
were found to be uniformly distributed within the
the bare SnO2, Schematic diagram of spray pyrolysis
individual graphene nanospheres, which consisted of
process, TEM image of the 3D porous graphene-SnO2
graphene layers. The size of individual graphene
composite microspheres prepared using 40-nm PS
nanospheres forming the 3D porous microspheres,
nanobeads, SEM image of the crushed 3D porous
and the content of SnO2 active material, were easily
graphene-SnO2 composite microspheres) is available
controlled through the size of polystyrene (PS)
in
nanobeads used as sacrificial templates, and the
http://dx.doi.org/10.1007/s12274-***-****-*
concentration of tin salt dissolved in the spray
(automatically inserted by the publisher).
solution, respectively. 3D porous graphene-SnO2
composite microspheres showed high reversible
capacity and good cycling performance at high
current
density.
morphological
The
good
conductivity
uniqueness
of
3D
and
porous
graphene-SnO2 microspheres contribute to excellent
electrochemical performance. The process developed
here could be applied in the preparation of various
types of 3D porous graphene-metal oxide composite
microspheres for a wide range of applications,
including energy storage devices.
the
online
version
of
this
article
at
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Effect
of
Morphological
Modification
on
the