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Nano Res

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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Nano Research

DOI 10.1007/s12274-014-0646-1

TABLE OF CONTENTS (TOC)



in Li-Ion Batteries

Seung Ho Choi1,2, Jung-Kul Lee2,*, Yun Chan Kang1,*

[1] Department of Materials Science and Engineering,

Korea University, Republic of Korea.

[2] Department of Chemical Engineering, Konkuk

University, Republic of Korea.

The new three-dimensional (3D) porous graphene-metal oxide

composite microspheres are prepared by a one-pot spray pyrolysis

process. 3D porous SnO2-graphene microspheres selected as the first

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.



in Li-Ion Batteries

Seung Ho Choi1,2, Jung-Kul Lee2(), and Yun Chan Kang1()

Received: day month year

Revised: day month year

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

ABSTRACT

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.

1. Introduction

Two-dimensional (2D) graphene-metal oxide

composites with unique combinations of electronic,

chemical, and mechanical properties are considered

promising materials for various applications such

as batteries, supercapacitors, sensors, and catalysts

[1-9]. In continuous and large-scale production of

2D material by liquid solution processes,

graphene-based composites are subject to serious

aggregation between the graphene sheets owing to

van der Waals forces [10-14]. A change to

three-dimensional (3D) graphene from 2D graphene

sheets is not only a good solution for aggregation

phenomenon, but also introduces desirable

characteristics of fast ionic and electronic transport,

high specific area and strong mechanical properties

[10-20].

In energy storage fields, various 3D

graphene-metal oxide structures, such as porous

graphene film, graphene foam, and graphene ball

hybrids, are more significant than the 2D forms. 3D

graphene structures maintain the superior intrinsic

properties of graphene sheets, such as large surface

areas, novel physical properties and excellent

electrochemical properties, and can further exhibit

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Nano Res.

improved functions through addition of metal

oxides [21-34]. For example, a 3D graphene

structure with porous morphology has many

advantages such as easy electrolyte penetration and

fast Li+ diffusion for lithium ion batteries [26-34].

Also, 3D graphene sheets with their outstanding

electrical conductivity and flexibility act as an

excellent support, and as a buffer layer that

mitigates volume changes during Li-ion

insertion/extraction by absorption of stress, thus

improving the structural stability and cyclability of

electrode materials [26-34]. In particular, 3D

graphene-metal oxide composites have been

reported to possess excellent electrochemical

properties through a range of energy storage

applications [21-34].

Choice of a synthesis method of 3D

graphene-metal oxide architectures for facile,

continuous, and large-scale production is extremely

important. In previous reports, 3D graphene-metal

oxide composite structures were mainly prepared

using a chemical vapor deposition (CVD) process or

a multistep solution process [15-18,35-39]. CVD

synthesized graphene-metal oxide is commonly

grown on a flat metal foil or thin film greatly

limiting its application in energy storage due to low

production yield [16-18]. Compared with the CVD

method, solution processes, including

hydrothermal, self-assembly, and freeze-drying

techniques, are large scale and less costly, and the

resulting 3D graphene-metal oxide architectures

have desirable features of high porosity and pore

structure with meso and macropores [35-39].

However, obstacles to use of the solution process

are the multiple steps, long reaction times, and use

of a toxic reducing agent for graphene oxide

reduction [35-39]. As potential commercial products

for industrial applications, 3D graphene-metal

oxide spherical powders with regular morphology

and non-aggregating properties are attractive.

Therefore, development of a new continuous and

one-step process for 3D graphene-metal oxide

spherical hybrids remains a large and essential

challenge. In this study, we first synthesized the

new structured 3D graphene-metal oxide spherical

powders with macroporous structure for anode

applications of LIBs. 3D porous graphene

microspheres are aggregates of individual hollow

graphene nanospheres formed by graphene sheets.

Metal oxide nanocrystals were uniformly

distributed over the graphene sheets forming the

microspheres. The hollow graphene nanosphere

structure accommodates the volume change of

metal oxides during repeated lithium insertion and

extraction and prevents growth of active material

crystals during cycling.

In this study, 3D graphene-SnO2 microspheres

with porous structure were selected as the first

target material because SnO2 nanostructured

materials with various morphologies have been

widely studied as anode materials for LIBs.

Polystyrene (PS) nanobeads were used as sacrificial

templates in creating a porous structure [14,29]. 3D

porous graphene-SnO2 composite microspheres

showed high reversible capacity and excellent

cycling and fast rate performance. Our approach to

fabricating a 3D porous graphene structure

powders may be valuable in developing this

application for energy storage devices.

2. Experimental

2.1 Synthesis of 3D porous graphene-SnO2

composite microspheres.

Graphene oxide (GO) was synthesized using a

modified Hummer’s method [40]. 3D porous

graphene-SnO2 microspheres were directly

prepared by ultrasonic spray pyrolysis at 800oC; a

schematic of the apparatus is shown in Figure S10.

A quartz reactor of length 1200 mm and diameter 50

mm was used, with a nitrogen flow rate (carrier

gas) of 10 L min-1. The as-obtained GO was

redispersed in distilled water and exfoliated by

ultrasonication to generate GO sheets. 500 mL of the

exfoliated GO solution (1 mg ml-1) was added to 1.4

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

Nano Res.

g of Sn oxalate (13 mM) in H2O2 solution. Then, 4.2

g of 100-nm PS nanobeads was added into the

solution of GO sheets and Sn. Subsequently, the

prepared solution with Sn oxalate salt, PS

nanobeads, and GO sheets was dispersed in

distilled water by ultrasonication.

2.2 Characterizations

The crystal structures of the powders were

investigated by X-ray diffractometry (XRD), (X’pert

PRO MPD), using Cu Kradiation (= 1.5418 Å ).

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 areas of the 3D porous

graphene-SnO2 composite microspheres were

calculated from a Brunauer–Emmett–Teller (BET)

analysis of nitrogen adsorption measurements

(TriStar 3000). The powders were also investigated

using X-ray photoelectron spectroscopy (XPS),

(ESCALAB-210) with Al K radiation (1486.6 eV).

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.

2.3 Electrochemical Measurements

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

(EC/DMC) with 5% fluoroethylene carbonate.

Charge-discharge characteristics of the samples

were determined through cycling in the potential

range 0.001–3.0 V at various fixed current densities.

Scheme 1. Schematic diagram for the formation process of 3D porous graphene-SnO2 composite microsphere.


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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,

as shown in Figure S1.

Morphologies of the 3D porous SnO2-graphene

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

and graphene sheets, respectively, uniformly


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Figure 2. TEM images and elemental mapping images of the

3D porous graphene-SnO2 composite microspheres prepared

from spray solution with (a-d) low and (e-h) high concentration

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.

distributed over the 3D porous SnO2-graphene

composite microspheres. Figure 2 show the

morphologies and elemental mapping images of the

graphene-SnO2 composite microspheres prepared

from the spray solutions with low (8 mM) and high

(18 mM) concentrations of tin oxalate. The

graphene-SnO2 composite microspheres had 3D

porous structure and uniform distribution of

ultrafine SnO2 nanocrystals over the microspheres

regardless of SnO2 contents in the composite

powders. Morphologies of the 3D porous graphene

microspheres, not containing SnO2, prepared by the

one-pot spray pyrolysis method from a spray

solution of PS nanobeads and graphene oxide sheets,

are shown in Figure 3. The graphene microspheres

had similar structure to the

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

3e shows sets of bright spots and faint spots,

indicating that the sample consists of randomly

oriented graphene layers [44,45].

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

graphene microspheres shows a broadened

diffraction peak around 23o similar to reduced

graphene oxide, while the SnO2-graphene

microspheres exhibits pure crystal structure of

rutile SnO2 (JCPDS card no. 41-1445) [46]. The mean

crystallite size of SnO2 nanocrystals dispersed on

graphene microspheres calculated from the (110)

peak using Scherer’s formula was an ultrafine 4 nm.

The pore-size distributions of the 3D porous

SnO2-graphene and graphene microspheres were

investigated by nitrogen isothermal adsorption, and

the results are shown in Figure 4b. Both samples

had similar pore size distributions, including both

mesopores (2–50 nm) and macropores. BET surface

areas of the SnO2-graphene and graphene

microspheres were 120 and 200 m2 g-1, respectively.


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The graphene content in the 3D porous

SnO2-graphene microspheres was evaluated to be

19.5 wt% by thermogravimetric analysis (Figure S3).

Figures 4c and 4d show the XPS profiles of C1s

acquired from the 3D porous graphene

microspheres (Figure 4c) and 3D porous

graphene-SnO2 microspheres (Figure 4d). The C1s

peak in the XPS profile could be attributed to sp2

bonded carbon (C–C), epoxy and alkoxy groups

(C–O) and carbonyl and carboxylic (C=O) groups,

which corresponded to peaks at 284.6, 286.6, and

288.1 eV, respectively [47]. The XPS profiles showed

sharp peaks at around 284.6 eV, which could be

assigned to graphitic carbon [47]. The relative

carbon content of the sp2 bonded carbon at 286.4 eV

was 83% in the 3D porous SnO2-graphene

microspheres. The carbonyl and carboxylic (C=O)

groups were not observed in the XPS C1s profile of

the graphene-SnO2 composite microspheres due to

the complete reduction of GO sheets into reduced

GO sheets. Analysis by XPS indicated that the

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

porous graphene microspheres

without SnO2 nanocrystals, and

bare SnO2 microspheres, both

prepared by spray pyrolysis under

the same conditions; the bare SnO2

microspheres had a spherical shape and dense

structure as shown in Figure S5. Figure 5a shows

the cyclic voltammogram (CV) curves for the first 5

cycles of the 3D porous SnO2-graphene

microspheres at a scan rate of 0.1 mV s-1. In the first

cathodic step, the apparent reduction peak at 0.9 V

is attributed to form Li2O and Sn metal and solid

electrolyte interphase (SEI) layers when SnO2

nanocrystals react with Li+ [48-54]. Low-potential

peaks (at < 0.6 V) corresponding to the LixSn alloy

formation are also observed [43,48-54]. The

oxidation peaks at 0.25 V and 0.6 V during the

anodic scan are related to dealloying of LixSn

[43,48-54]. In the second cycle, the decomposition

peak of SnO2 at 0.9 V disappeared and the related

peaks of Li-Sn alloy and dealloying reactions were

seen repeatedly. Reduction peaks at 1.5 V in the first

cathodic step were observed in the CV curves for

3D porous SnO2-graphene microspheres and 3D

porous graphene microspheres and the peaks

Figure 4. Properties of the 3D porous graphene and graphene-SnO2 composite

microspheres: a) XRD patterns, b) pore size distributions, c) XPS spectrum of C1s of 3D

porous graphene microspheres, and d) XPS spectrum of C1s of 3D porous graphene-SnO2

composite microspheres.


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disappeared from the second cycles as shown in

Figures 5a and S6. The reduction peak at 1.5 V in

the first cathodic step is attributable to irreversible

lithium insertion into graphene layers of the 3D

porous structure. Figure 5b shows the initial charge

and discharge curves of the 3D porous

SnO2-graphene and graphene microspheres and

bare SnO2 powders. The operating cut-off voltages

were 0.001 and 3 V at a current density of 2 A g-1.

The initial discharging and charging specific

capacities of the 3D porous graphene microspheres

were 1244 and 389 mA h g-1, respectively.

Coulombic efficiency (CE) of the graphene

microspheres observed in the first cycle was low at

32%, which is an unavoidable phenomenon in

reduced graphene oxide materials [55,56].

Electrochemical properties of reduced graphene

oxide are affected by synthesis methods, and

various lithium storage properties have been

previously reported [6-8]. However, most reduced

graphene oxide has low initial CE due to the high

production of SEI layers resulting from the high

specific surface area and to irreversible lithium ion

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, with a

corresponding initial CE

of 67%. The irreversible

capacity loss of the bare

SnO2 powders is

attributed to the

formation of Li2O from SnO2 and SEI layers at the

electrode–electrolyte interface [43,48-54].

Cycling performances of the 3D porous

SnO2-graphene and graphene microspheres and

bare SnO2 powders at a constant current density of

2 A g-1 are shown in Figure 5c. In comparison, after

500 cycles the 3D porous SnO2-graphene and

graphene microspheres and bare SnO2 powders

delivered discharge capacities of 1009, 196, and 52

mA h g-1, respectively, and the corresponding

capacity retentions measured after the first cycles

were 96, 47, and 5%. Coulombic efficiency of 3D

porous SnO2-graphene microspheres reached

approximately 99% after the tenth cycle and

remained at this value in subsequent cycles. The

average Coulombic efficiency from 100th to 500th

cycles of these SnO2-graphene microspheres was

99.8%. The increase in the discharge capacities of

the 3D porous SnO2-graphene microspheres after

100 cycles had been attributed to the formation of a

gel-like reversible polymer film on the microsphere

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.


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surfaces [57-59]. The 3D porous SnO2-graphene

composite microspheres have excellent

electrochemical properties regardless of SnO2

contents (see Figure S7a). The long-term cycling

performance of 3D porous SnO2-graphene

microspheres at a high value of the current density,

5 A g-1, is shown in Figure S7b. The 2nd and 1000th

discharge capacities of the 3D porous

graphene-SnO2 microspheres were 905 and 730 mA

h g-1, and the Coulombic efficiency was reached at

99.7% after 50 cycles. The discharging and charging

rate performance of the 3D porous graphene-SnO2

microspheres are shown in Figure 5d. As can be

seen, the average reversible discharge capacities

were 1020, 875, 785, 716, and 660 mA h g-1 at current

densities of 1, 3, 5, 7, and 9 A g-1, respectively. After

high-rate charge-discharge cycling, discharge

capacity recovered to a value as high as 941 mA h

g-1 for a lower current density of 1 A g-1.

TEM images as shown in Figure S8 showed that

3D porous graphene-SnO2 microspheres maintained

their overall morphology after cycling, but bare

SnO2 powders lost their original morphologies after

the 200th cycle. The 3D graphene backbone of the

graphene-SnO2 microspheres was maintained, as

shown in the TEM images. The Sn component

remained uniformly dispersed over the

microspheres without aggregation even after

cycling, as shown in the elemental mapping images

in Figure S8b. The 3D porous graphene structure

suppressed the aggregation of SnO2 nanocrystals

during repeated cycling. The graphene increases the

electrical conductivity of the electrode and is able to

accommodate the strain induced by volume change

of SnO2. It also facilitated fast transportation of

electrons and lithium ions, which is responsible for

the good cycling stability and rate capability [59-61].

On the other hand, changes in the bare SnO2

powders caused disconnection between active

material, conducting carbon, and copper foil,

resulting in rapid capacity fading upon cycling. The

good conductivity and morphological uniqueness

of 3D porous graphene-SnO2 microspheres

contribute to their excellent electrochemical

performance, and were confirmed by

electrochemical impedance spectroscopy (EIS)

measurements. Nyquist plots of the electrodes

consisted of a semicircle in the medium frequency

region and an inclined line at low frequencies. The

medium-frequency semicircle was attributed to

charge-transfer resistance between active material

and electrolyte, and the low frequency portion of

the trace corresponded to the lithium diffusion

process within the electrodes [62-64]. The charge

transfer resistance of 3D porous graphene-SnO2

microspheres was much smaller than that of bare

SnO2 powders before cycling, as shown in Figure

S9a. Figure S9b shows the relationship between the

real part of the impedance spectrum Zre and ω-1/2

(where ω = 2πf is angular frequency) in the

low-frequency region, before cycling. The gradual

low-frequency slope (the Warburg impedance

coefficient) of Zre versus ω-1/2 indicates high lithium

ion diffusivity for 3D porous graphene-SnO2

microspheres [63,64]. The unique structure of 3D

porous graphene-SnO2 microspheres resulted in

low charge transfer resistance and high lithium ion

diffusivity. Figures S9c and S9d show the Nyquist

plot and the Zre - ω-1/2 relationship after 200 cycles.

The microspheres continued to have low charge

transfer resistance and high lithium ion diffusivity

even after 200 cycles. On the other hand, the charge

transfer resistance of bare SnO2 powders increased

after cycling. The excellent electrochemical

performance of 3D porous graphene-SnO2

microspheres is due to their structural stability and

graphene’s synergy effects.

4. Concluisons

In this study, electrochemical properties of 3D porous

graphene-SnO2 microspheres were compared with


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those of 3D porous graphene microspheres (without

SnO2) and bare SnO2 powders, prepared by the same

process. 3D porous graphene-SnO2 microspheres

were formed from single droplets using the one-pot

spray pyrolysis method. Ultrafine SnO2 nanocrystals

were found to be uniformly distributed within the

individual graphene nanospheres, which consisted of

graphene layers. The size of individual graphene

nanospheres forming the 3D porous microspheres,

and the content of SnO2 active material, were easily

controlled through the size of polystyrene (PS)

nanobeads used as sacrificial templates, and the

concentration of tin salt dissolved in the spray

solution, respectively. 3D porous graphene-SnO2

composite microspheres showed high reversible

capacity and good cycling performance at high

current density. The good conductivity and

morphological uniqueness of 3D 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.

Acknowledgements

This work was supported by the National Research

Foundation of Korea (NRF) grant funded by the

Korea government (MEST) (No.

2012R1A2A2A02046367). This work was supported

by the Energy Efficiency & Resources Core

Technology Program of the Korea Institute of

Energy Technology Evaluation and Planning

(KETEP), granted financial resource from the

Ministry of Trade, Industry & Energy, Republic of

Korea (201320200000420).

Electronic Supplementary Material: Supplementary

material (Schematic diagrams of 3D graphene,

HR-TEM, TG, and XPS data of the 3D porous

graphene-SnO2 composite microsphere, SEM image

of the bare SnO2 powders, CV curves of the 3D

porous graphene, Cycling performances of the 3D

porous graphene-SnO2 composite, TEM images of the

porous graphene-SnO2 composite after cycling, EIS

data of the 3D porous graphene-SnO2 composite and

the bare SnO2, Schematic diagram of spray pyrolysis

process, TEM image of the 3D porous graphene-SnO2

composite microspheres prepared using 40-nm PS

nanobeads, SEM image of the crushed 3D porous

graphene-SnO2 composite microspheres) is available

in the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher).

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