High reversible capacity of SnO2/graphene nanocomposite as an anode material for lithium-ion batteries

Article abstract of DOI:10.1016/j.electacta.2011.01.126

A gas-liquid interfacial synthesis approach has been developed to prepare SnO₂/graphene nanocomposite. The as-prepared nanocomposite was characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller measurements. Field emission scanning electron microscopy and transmission electron microscopy observation revealed the homogeneous distribution of SnO₂ nanoparticles (2-6 nm in size) on graphene matrix. The electrochemical performances were evaluated by using coin-type cells versus metallic lithium. The SnO₂/graphene nanocomposite prepared by the gas-liquid interface reaction exhibits a high reversible specific capacity of 1304 mAh g^-1 at a current density of 100 mA g^-1 and excellent rate capability, even at a high current density of 1000 mA g^-1, the reversible capacity was still as high as 748 mAh g^-1. The electrochemical test results show that the SnO₂/graphene nanocomposite prepared by the gas-liquid interfacial synthesis approach is a promising anode material for lithium-ion batteries.

Full text of DOI:10.1016/j.electacta.2011.01.126

Electrochimica Acta 56 (2011) 4532–4539
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
High reversible capacity of SnO /graphene nanocomposite as an anode material
2
for lithium-ion batteries
a
b
a
a
b
a,∗
Peichao Lian , Xuefeng Zhu , Shuzhao Liang , Zhong Li , Weishen Yang , Haihui Wang
a
School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou 510640, China
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A gas–liquid interfacial synthesis approach has been developed to prepare SnO /graphene nanocom-
posite. The as-prepared nanocomposite was characterized by X-ray diffraction, field emission scanning
2
Received 11 December 2010
Received in revised form 19 January 2011
Accepted 31 January 2011
electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller measurements.
Field emission scanning electron microscopy and transmission electron microscopy observation revealed
the homogeneous distribution of SnO2 nanoparticles (2–6 nm in size) on graphene matrix. The
electrochemical performances were evaluated by using coin-type cells versus metallic lithium. The
SnO2/graphene nanocomposite prepared by the gas–liquid interface reaction exhibits a high reversible
Available online 24 February 2011
Keywords:
Graphene sheets
SnO2 nanoparticles
Nanocomposite
Anode material
Lithium-ion batteries
−
1
−1
specific capacity of 1304 mAh g at a current density of 100 mA g and excellent rate capability, even
at a high current density of 1000 mA g 1, the reversible capacity was still as high as 748 mAh g−1. The
electrochemical test results show that the SnO2/graphene nanocomposite prepared by the gas–liquid
interfacial synthesis approach is a promising anode material for lithium-ion batteries.
−
©
2011 Elsevier Ltd. All rights reserved.
1
. Introduction
causes cracking and crumbling, which will result in electrical
disconnection of active material with current collector and conse-
quently limit the cycling capability of electrodes [6–8,17–20]. The
agglomeration of primitive particles drastically reduces the total
entrance/exit sites available for Li+ ions and creates even more
severe mechanical stresses in the surface region of a particulate
aggregate, leading to poor electrochemical performance [13,23,25].
In order to alleviate the volume changes and agglomeration, two
common strategies have been pursued. One strategy is to design
Lithium-ion batteries are currently the dominant power
source for portable electronic devices and viewed as the
promising power source of electrical/hybrid vehicles. Graphite
is the widely commercial anode material, but its theoretical
specific capacity is only 372 mAh g
tion compounds (LiC )), which cannot fulfill the increasing
demand for high performance lithium-ion batteries [1]. SnO2 is
concerned as a promising anode material for lithium-ion bat-
teries due to the following two reasons [2,3]. Firstly, based
−
1
(by forming intercala-
6
hollow or porous structure SnO anode materials [6–8,13,15,20,27].
2
Another one is to prepare SnO /C composites by loading SnO2
2
+
−
on an alloying mechanism, SnO + 4Li + 4e → 2Li O + Sn and
nanoparticles into a ductile carbon matrix [9–12,17,18,21,25,26].
The second strategy is very effective to improve the electrochemi-
2
2
+
Sn + xLi + xe ꢀ LixSn (0 ≤ x ≤ 4.4), its maximal theoretical capac-
−1
ity is 782 mAh g
[4,5], which is more than twice that of the
cal performance of SnO material. The carbon in SnO /C composites
2
2
−1
already-commercialized graphite (372 mAh g ) [6–8]. Secondly,
the potential of the SnO2 anode is higher than that of graphite,
which reduces the potential safety problems with metallic lithium
deposition on the host anode during rapid charge [1,3]. However,
its application in practical lithium-ion batteries is still hampered
by the poor cycling performance arising from the severe aggre-
gation and huge volume changes of SnO2 particles during Li
insertion/extraction process [6–27]. The large volume changes of
SnO2 particles during lithium ions insertion/extraction process
can not only buffer the volume changes of SnO2 nanoparticles
and prevent them from aggregating to large particles but also
increase electronic conductivity of composites due to its good
electronic conductivity [5,9,10,12,16,23,25], which favors high rate
performance.
Graphene, a monolayer of graphite, exhibits a number of
intriguing unique properties [28,29], such as superior electronic
2
−1
conductivity, high surface area (theoretical value 2620 m g ),
large surface-to-volume ratio, good mechanical properties. There-
fore, graphene is an ideal carbon nanostructure which can be
used to design high performance SnO /carbon nanocomposite
2
∗ Corresponding author. Tel.: +86 20 87110131; fax: +86 20 87110131.
E-mail address: hhwang@scut.edu.cn (H. Wang).
electrodes. So far, there have been a few reports about the prepara-
tion of SnO2/graphene nanocomposite [30–36]. For example, Paek
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.01.126

P. Lian et al. / Electrochimica Acta 56 (2011) 4532–4539
4533
a
Interface
NH
3
nanoparticle
graphene sheets
Sn4+
EG
3 2
NH ·H O
b
SnO
2
nanoparticle
depositing
graphene sheets
2
SnO /graphene nanocomposite
Fig. 1. (a) Schematic illustration for the reaction system and (b) synthesis process of SnO2/graphene nanocomposite.
et al. [30] synthesized SnO /graphene nanocomposite with three-
2. Experimental
2
dimensionally delaminated flexible structure by mechanically
mixing SnO2 nanoparticles with graphene dispersions. The as-
2.1. Materials preparation
prepared SnO /graphene electrode material exhibited an enhanced
2
cycle performance and lithium storage capacity. As claimed, the
All chemicals were of analytical grade and used as received,
without further purification. Graphene sheets were prepared via
a thermal exfoliation route involving graphite oxidation, following
by rapid thermal expansion under nitrogen atmosphere. Detailed
preparation procedure can be found in our previous paper [39].
−
1
reversible capacity could remain 570 mAh g
after 30 cycles
that is about 70% retention of the original reversible capacity
−1
at a current density of 50 mA g . Yao et al. [31] reported the
in situ chemical synthesis of the SnO /graphene nanocomposite.
2
The obtained SnO /graphene nanocomposite exhibits a reversible
capacity of 765 mAh g in the first cycle and remains 520 mAh g
SnO /graphene nanocomposite and SnO2 nanoparticles were pre-
pared by a gas–liquid interfacial reaction, as shown in Fig. 1. Briefly,
2
2
−1
−1
after 100 cycles. Besides, some groups [32–35] prepared the
in a 20-mL beaker, 0.3647 g of SnCl ·5H O (Tianjin Kermel Chemical
4
2
SnO /graphene nanocomposite via in situ chemical synthesis
Reagent Co., Ltd., China) was dissolved in 10 mL of ethylene glycol
(EG) (Beijing Chemical Reagent Co., Ltd., China), and then 0.0476 g of
graphene sheets were added and sonicated for 6 h to yield a homo-
geneous suspension. Then the beaker was placed into a 100-mL
Teflon-lined autoclave that contained 14 mL of ammonia solution
(Guangzhou Chemical Reagent Factory, China). Then the autoclave
2
route based on an oxidation–reduction reaction. The prepared
SnO /graphene nanocomposite also shows enhanced electrochem-
2
ical performance. However, the mechanically mixing method limits
the homogeneous dispersion of SnO2 nanoparticles and the sep-
aration of graphene sheets [31], and all those in situ chemical
synthesis methods need extra thermal treatment, which makes
the synthesis process complicated and time-consuming. Further-
◦
was sealed and placed in a drying oven preheated to 180 C and
held at this temperature for 12 h. After cooling and centrifugation,
washing with ethanol (Beijing Chemical Reagent Co., Ltd., China)
more, these SnO /graphene nanocomposites only show relatively
2
low reversible capacity. Therefore, it is still necessary to explore
for several times, the collected black solid product was dried at
◦
facile synthesis techniques for the preparation of SnO /graphene
100 C in vacuum to obtain the desired SnO /graphene nanocom-
2
2
nanocomposite.
posite. The bare SnO2 nanoparticles were synthesized under the
same condition without the addition of graphene sheets for com-
parison.
Recently, Cui et al. [37] prepared Fe O4 nanoparticles by
a facile gas–liquid interfacial reaction. Lian et al. [38] devel-
oped the gas–liquid interfacial synthesis approach and prepared
3
Fe O /graphene nanocomposite. The Fe O /graphene nanocom-
2.2. Materials charaterization
3
4
3
4
posite prepared by the gas–liquid interfacial synthesis approach
exhibited a very high reversible capacity. In this study, the
facile gas–liquid interfacial synthesis approach was firstly used
The structure and morphology of the as-prepared
SnO /graphene nanocomposite and SnO2 nanoparticles were
2
to prepare SnO /graphene nanocomposite. The as-prepared
characterized by X-ray diffraction (XRD, Bruker D8 Advance), scan-
ning electron microscopy (SEM, Quanta 200F) and high-resolution
transmission electron microscopy (HRTEM, FEI, Tecnai G2 F30
S-Twin). N2 adsorption/desorption isotherms were measured
using a Micromeritics ASAP 2010 (USA) analyzer at liquid nitrogen
temperature. Elemental analysis was carried out on vario EL III
2
nanocomposite as an anode material for lithium-ion batteries
exhibits an unprecedented high reversible specific capacity of
−1
1
304 mAh g after 150 cycles. Such excellent performance could
be attributed to its porous structure and the enhanced surface elec-
trochemical reactivity due to the large surface-to-volume ratio of
SnO2 nanoparticles [30].
elementar (Germany) by burning the SnO /graphene nanocom-
2

4
534
P. Lian et al. / Electrochimica Acta 56 (2011) 4532–4539
posite to form carbon dioxide. The content of carbon (i.e., graphene
sheets) in SnO /graphene nanocomposite was calculated accord-
ing to the mass of carbon dioxide. The content of graphene in
2
110
101
2
11
SnO /graphene nanocomposite was determined to be 22.76 wt.%.
2
To reveal the reaction mechanism of SnO /graphene nanocom-
2
200
a
b
posite with Li, both of the fresh SnO /graphene electrode sheet
2
and fully charged (3.0 V) one after 150 cycles were characterized
by X-ray diffraction. The bare SnO2 nanoparticles electrode sheets
were also characterized for comparison.
2.3. Electrochemical measurement
002
The electrochemical measurements were carried out using the
coin-type cells. The working electrode was prepared by mixing
c
active material (SnO2 nanoparticles or SnO /graphene nanocom-
2
posite) with poly(vinylidene fluoride) (PVdF, Kureha, Japan) and
Super P carbon at a weight ratio of 75:10:15 in N-methyl-2-
pyrrolidone (NMP, Tianjin Kermel Chemical Reagent Co., Ltd.,
China) to form a slurry. Then, the resultant slurry was uniformly
1
0
20
30
40
50
60
2
<theta> / degree
◦
pasted on Cu foil with a blade, dried at 120 C in a vacuum oven
Fig. 2. XRD patterns of the as-synthesized (a) SnO2/graphene nanocomposite and
b) SnO2 nanoparticles, they are identified as tetragonal SnO2 (JCPDS file card no.
(
DZF-6020, Shanghai Qi Xin Scientific Instrument Co., Ltd., China)
(
and pressed under a pressure of 20 MPa. The active material load-
72-1147). (c) XRD pattern of graphene sheets.
−2
ing density of the electrode is ca. 1.0 mg cm . The Celgard 2325
microporous membrane was used as separator. The electrolyte
is necessary for forming a homogeneous suspension. Secondly, EG
−
1
was 1 mol L LiPF6 dissolved in a mixture of ethylene carbonate
EC) and dimethyl carbonate (DMC) (1:1 by volume) (Beijing Insti-
4+
is not a good solvent for NH , the reaction between NH3 and Sn
3
(
was limited at the gas/liquid interface [37].
tute of Chemical Reagents, China). The lithium sheet was used as
both counter and reference electrodes. CR2025-type coin cells were
assembled in an argon-filled glove box and galvanostatically dis-
charged and charged in a voltage range from 3.0 to 0.01 V using
a Battery Testing System (Neware Electronic Co., China). Cyclic
voltammetry (CV) measurements were carried out on an elec-
trochemical workstation (Zahner IM6ex) over the potential range
Fig. 2a and b shows XRD patterns of as-prepared SnO /graphene
2
nanocomposite, SnO2 nanoparticles. The diffraction peaks of crys-
talline SnO2 nanoparticles are clearly distinguishable. All strong
diffraction peaks can be indexed to the standard tetragonal SnO2
phase (JCPDS file card no. 72-1147), indicating crystalline SnO2
nanoparticles could be formed by the gas/liquid interfacial reaction.
All peaks in the XRD pattern of SnO2 nanoparticles are also in good
agreement with those reported in the literature [30,31]. The broad
+
−1
.
0
.01–3.0 V vs. Li/Li at a scanning rate of 0.2 mV s
diffraction peaks of the bare SnO2 and SnO /graphene nanocom-
2
3
. Results and discussion
posite suggest that the nanoparticles are very small in size. There
is a weak (0 0 2) diffraction peak in the XRD pattern of graphene
sheets as shown in Fig. 2c. This result indicates that graphene
sheets stacked into multilayers, leading to loss of their high sur-
face area and intrinsic chemical and physical properties [31,33,34].
However, no obvious diffraction peak attributed to graphite in the
3.1. Microstructural characterization
As illustrated in Fig. 1, the beaker-in-autoclave setup is
designed to confine the synthesis at the gas/liquid interface
4+
inside a sealed space [37,38]. SnCl4 (Sn species) and graphene
sheets in EG solution were stored in the beaker, while aque-
ous ammonia solution (NH ·H O) was placed in the autoclave
XRD pattern of SnO /graphene nanocomposite was observed. This
2
result indicates that the SnO nanoparticles deposited on graphene
2
sheets can prevent them from stacking into multilayers, favoring
the maintenance of high surface area and intrinsic chemical and
physical properties of graphene sheets [31,33,34]. The average crys-
tal size of SnO nanoparticles in the SnO /graphene nanocomposite
3
2
liner outside the beaker. At room temperature, the beaker sep-
arates the two solutions. During the reaction, the system was
◦
heated to 180 C quickly and then kept at that temperature for
2
2
1
2 h. Such a temperature increase results in the evaporation
is around 3.7 nm calculated by Scherrer’s formula based on the
(1 1 0) peak.
4+
of ammonia, which reacts with Sn
at the gas/liquid inter-
face to produce Sn(OH)4 in situ deposited onto the graphene
The SEM and TEM micrographs of SnO nanoparticles, graphene
2
sheets [8,30]. The produced Sn(OH)4 could be quickly decomposed
sheets and SnO /graphene nanocomposite are shown in Fig. 3.
2
◦
to SnO2 at 180 C, and the SnO /graphene nanocomposite was
As shown in Fig. 3a and e, the SnO2 nanoparticles prepared by
the gas–liquid interfacial reaction aggregated into large particles.
The resulting large particles could be easily pulverized owing to
an asymmetrical volume change during Li+ insertion/extraction
process, resulting in capacity decay as an anode material for
lithium-ion batteries [13,23,25]. Fig. 3b presents the representa-
tive SEM image of graphene sheets from the top view, showing
the layered platelets composed of curled nanosheets, which is
accordant to the weak (0 0 2) diffraction peak in the XRD pat-
tern of the bare graphene sheets. Fig. 3c shows the SEM image of
2
4+
formed in the beaker. Because all of the NH , Sn and graphene
3
sheets are homogeneously distributed at the gas/liquid interface,
the formation of SnO /graphene nanocomposite is uniform and
2
simultaneous at the interface. Furthermore, the reaction is lim-
ited at the interface, so that the particle growth is localized, which
is important for synthesizing monodispersed nanoparticles with
a narrow size distribution [37]. During the reaction, the convec-
tion inside the beaker will cause the graphene sheets to become
immersed in the liquid and return back to the interface along the
convection flow [37], which helps to achieve a homogeneous load-
ing of SnO2 nanoparticles on all the graphene sheets. It is very
important to choose EG as the solvent in the beaker-in-autoclave
setup. Firstly, EG is a good dispersant for graphene sheets, which
SnO /graphene nanocomposite prepared by the facile gas–liquid
2
interfacial synthesis approach. Obviously, the deposition of SnO2
nanoparticles onto the surfaces of graphene sheets is homoge-
neous. The SnO2 nanoparticles are uniformly distributed on 2D

P. Lian et al. / Electrochimica Acta 56 (2011) 4532–4539
4535
Fig. 3. SEM and TEM observation of SnO2 nanoparticles, graphene sheets and SnO2/graphene nanocomposite: SEM micrographs of (a) SnO2 nanoparticles, (b) graphene
sheets (inset shows the edge-side of graphene sheets) and (c) SnO2/graphene nanocomposite. Low-magnification TEM images of (d) SnO2/graphene nanocomposite. High-
magnification TEM images of (e) SnO2 nanoparticles, (f) SnO2/graphene nanocomposite.
graphene sheets in as-prepared SnO /graphene nanocomposite,
tion between SnO2 nanoparticles and ultrathin graphene sheets
can form highly conducting 3D graphene electronic conductive
2
as shown in Fig. 3d. From Fig. 3c and d, it can be observed that
the graphene sheets are distributed between the SnO2 nanopar-
ticles and the nanoporous composite with large amount of void
spaces was formed [30]. Such porous nanocomposite can possess
excellent cycle performance as an anode material for lithium-ion
batteries due to the large amount of void spaces which could
buffer large volume changes of SnO2 nanoparticles during lithium
ions insertion/extraction process [7,8,12,16,18,30,38]. Moreover,
the graphene sheets distributed between the SnO2 nanoparticles
can prevent the aggregation of these nanoparticles to a certain
extent [9,14,25,32,38], which can be of great benefit to cycle life.
The nanoparticles deposited on graphene sheets can also pre-
vent them from stacking into multilayers [31,33,34], matching
well with the result that no obvious diffraction peak attributed
network and porous structure of the SnO /graphene nanocom-
2
posite [38]. The formed highly conducting 3D graphene electronic
conductive network could increase electronic conductivity of the
nanocomposite and the porous structure can facilitate liquid elec-
trolyte diffusion into the bulk materials, which is beneficial for
achieving high rate performance [7,8,30,31,34]. The HRTEM images
(Fig. 3e and f) revealed that the average particle size of SnO2
nanoparticles in the SnO /graphene nanocomposite is compara-
2
ble to that of bare SnO2 nanoparticles. Both of them are less than
6 nm. The average particle size of SnO2 nanoparticles (ca. 4 nm)
in the SnO /graphene nanocomposite observed from the HRTEM
2
image is also well consistent with that estimated by the Scherrer
equation. The small SnO nanoparticles can provide short diffusion
2
to graphite in the XRD pattern of SnO /graphene nanocomposite
was observed. It should be pointed out that the random hybridiza-
length for lithium ions insertion, resulting in good rate capabil-
ity [7,38]. More importantly, even after a long time of sonication
2

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536
P. Lian et al. / Electrochimica Acta 56 (2011) 4532–4539
3
.2. Electrochemical properties of SnO /graphene nanocomposite
2
a
0.012
0.010
0.008
0.006
0.004
0.002
0.000
and SnO nanoparticles
2
4
3
2
1
00
00
00
00
0
The electrochemical properties of the SnO /graphene nanocom-
2
posite and the bare SnO2 nanoparticles were investigated using
coin-type cells. The first 100 discharge/charge cycles were tested
at a current density of 100 mA g , and the corresponding dis-
charge/charge curves are shown in Fig. 5a. The initial small plateaus
and long slope profiles of SnO /graphene nanocomposite is similar
to that of bare SnO nanoparticles. Their first discharge voltage pro-
−
1
0
50
100
150
200
2
Pore diameter / nm
2
files both show classic plateaus in the potential ranging from 0.8 to
1.5 V, which was mainly due to the conversion reaction between
2
-1
SBET= 211.4 m g
+
−
SnO2 and Li, SnO + 4Li + 4e → 2Li O + Sn, leading to in situ for-
2
2
3
-1
Vtotal= 0.56 cm g
mation of Sn–Li O nanocomposites [13,33,41,42].
2
As shown in Fig. 5b, the initial reversible specific capacity of
0.0
0.2
0.4
0.6
0.8
1.0
−1
the SnO2 nanoparticles is 603.4 mAh g . After the first cycle, the
Relative pressure (P/Po)
−1
charge specific capacity increases to 639.9 mAh g in the 3rd cycle,
indicating an activation process in the material [9]. The activation
process is due to the aggregation of SnO2 nanoparticles, which
result in that only part of active material is involved in the ini-
tial reaction. During the first Li insertion/extraction process, the
aggregated SnO2 particles were pulverized into small particles
due to their large volume changes [13,30,31], resulting in more
active materials are involved in the 2nd and 3rd cycles. However,
the pulverization of in situ formed Sn particles during the first
three cycles destroyed the electronic conducting network between
the active material and current collector [5,7,33], resulting in the
reversible capacity decay from the 4th cycle. The reversible capac-
1
1
20
00
0.005
b
0.004
0.003
0.002
0.001
8
6
4
2
0
0
0
0
0
0.000
0
50
100
150
Pore diameter / nm
−
1
2
-1
ity of bare SnO2 nanoparticles remains only 198.2 mAh g after
00 cycles. As shown in Fig. 3c and d, SnO2 nanoparticles are
tightly attached to the graphene sheets and spatially separated by
SBET= 122.2 m g
1
3
-1
Vtotal= 0.13 cm g
the graphene sheets in the SnO /graphene nanocomposite, which
2
are favorable for improving the cycling performance [9,31,33]. In
addition, the nanosized SnO2 particles and the elastic graphene
0
.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
sheets in the porous SnO /graphene nanocomposite also help to
2
accommodate the volume changes and prevent the pulveriza-
tion of SnO2 to a certain extent during discharge/charge cycles
[13,30–34], which would lead to excellent cycle capability. As
Fig. 4. Nitrogen adsorption/desorption isotherms of (a) SnO2/graphene nanocom-
posite and (b) SnO2 nanoparticles, insets show the pore-size distribution by original
density functional theory model.
shown in Fig. 5b, the SnO /graphene nanocomposite indeed shows
2
enhanced electrochemical performances as expected. The charge
−
1
capacity of the nanocomposite is as high as 936.4 mAh g
in
−
1
during the preparation of the TEM specimen, the SnO2 nanoparti-
cles were still strongly anchored on the surface of graphene sheets
with a high density (shown in Fig. 3d and f), implying the strong
interaction between SnO2 nanoparticles and graphene sheets [38].
N2 adsorption/desorption isotherms were employed to inves-
the first cycle and decreases to 732.2 mAh g in the 20th cycle.
Nevertheless, the reversible capacity begins increasing from 21st
−
1
cycle and increases to 1156.4 mAh g
in the 100th cycle. The
phenomenon, the reversible specific capacity decreases to a low
value then gradually increases, has never been reported for SnO /C
2
tigate the pore structures of SnO /graphene nanocompos-
composites. But the similar phenomenon has been reported for
Fe O /C composites [43]. The decay of reversible capacity of the
2
ite and the bare SnO2 nanoparticles, as shown in Fig. 4.
The Brunauer–Emmett–Teller (BET) specific surface area of
SnO /graphene and SnO nanoparticles are about 211.4 and
3
4
SnO /graphene during the first 20 cycles could be due to the
2
2
2
pulverization of original SnO2 and in situ formed Sn nanoparti-
cles during Li insertion/extraction process, which lead to loss of
electrical connectivity between neighboring particles [17]. Dur-
ing discharge/charge cycles, the in situ formed Sn nanoparticles
became smaller and smaller due to electrochemical milling effect
[44] and attached on the graphene sheets tightly. It is reported
that activation energies for solid-state double decomposition reac-
tions decreased with decreasing reagent particle size [45]. Thus, the
2
−1
1
22.2 m g , respectively. Obviously, the BET specific surface area
of SnO /graphene nanocomposite is higher than that of SnO2
2
nanoparticles, which could lead to its increased electrochemical
reactive activity [30,40,41]. The SnO /graphene nanocomposite
2
possesses a much better porosity than bare SnO nanoparticles. The
2
3
−1
)
total pore volume of SnO /graphene nanocomposite (0.56 cm g
2
3
−1
is much larger than that of the SnO2 nanoparticles (0.13 cm g ).
The increased pore volume could arises primarily from the for-
mation of secondary pores between the SnO2 nanoparticles and
graphene sheets as well as the close stacking of graphene sheets
distributed between the SnO2 nanoparticles [38]. The nanopores
+
−
conversion reaction, SnO + 4Li + 4e → 2Li O + Sn, which is usu-
2
2
ally reported to be irreversible [9,41], can become reversible in the
SnO /graphene nanocomposite due to the very small Sn nanopar-
2
ticles [5,27,46–49]. It has been reported that the Sn–Li alloying and
+
in SnO /graphene nanocomposite could act as buffering spaces
dealloying reactions only occur below 1.0 V (vs. (Li/Li )) [4,10,50]
2
against the volume changes of SnO2 nanoparticles during lithium
ion insertion/extraction process [7,8,13,14,18,38], which would
lead to enhanced cycling stability as an anode material for lithium-
ion batteries.
and the Li–O bonds are not stable when the charge potential
is above 1.3 V [50]. As shown in Fig. 5a, there is a plateau at
around 1.3 V at the charge curves of SnO /graphene nanocompos-
2
+
−
ite, indicating the reaction SnO + 4Li + 4e → 2Li O + Sn indeed
2
2

P. Lian et al. / Electrochimica Acta 56 (2011) 4532–4539
4537
4
3
2
1
0
be pointed out that although a plateau above 1.0 V was observed
in both the first and third charge curves of bare SnO2 nanopar-
ticles, it completely disappeared in the 20th cycle. This result
indicates that the reaction between bare SnO2 nanoparticles and
SnO /graphene nanocomposite
2
a
SnO nanoparticles
2
1
00 20
1 3 20
3 1
100
+
−
Li, SnO + 4Li + 4e → 2Li O + Sn, is partially reversible during the
2
2
Charge
first several cycles, but the reversibility decrease with increas-
ing cycle number due to the decreasing electrochemical reactivity
because of the aggregation of in situ formed Sn nanoparticles
[
5,13,24,25]. In contrast, a charge plateau above 1.0 V constantly
existed during all the charge/discharge cycles of the SnO /graphene
2
nanocomposite, indicating that the reaction between SnO and
2
Discharge
+
−
Li, SnO + 4Li + 4e → 2Li O + Sn, is constantly reversible. The pos-
2
2
sible reason is that the graphene sheets in the SnO /graphene
2
1
00
20
3
20
800
3
100
1
1
nanocomposite can prevent the aggregation of in situ formed
Sn nanoparticles [10,22]. The initial coulombic efficiency of the
0
400
1200
1600
SnO /graphene is 59%, but it is above 94% after 5 cycles. The large
2
Specific capacity / mAh g-1
irreversible capacity during the first cycle can be attributed to irre-
versible lithium loss due to the formation of thick solid-electrolyte
interface (SEI) layer on the electrode surface [5,17,35], and the
fact that some Sn(0) cannot be re-oxidized back to Sn(IV) [9,19].
Table 1 shows a comparison of the reversible specific capacity at a
1
1
600
200
b
Discharge
Charge
low current density between the SnO /graphene nanocomposites
2
reported before and in this work. Obviously, the SnO /graphene
2
SnO /graphene nanocomposite
2
nanocomposite prepared by the facile gas–liquid interfacial syn-
thesis approach possesses higher reversible specific capacity and
better cycle performance than those reported previously.
8
4
00
00
0
After 100 cycles, the same cell was further evaluated for
rate capability as shown in Fig. 5c. When the same cell was
SnO nanoparticles
2
−
1
tested at various current densities from 100 to 1000 mA g , the
SnO /graphene nanocomposite exhibits an excellent rate capabil-
2
−
1
ity. At the high current densities of 300, 500 and 1000 mA g
,
the SnO /graphene nanocomposite can still exhibit high reversible
2
−
1
0
20
40
60
80
100
capacities of 1072, 976 and 748 mAh g , respectively. All of these
values are more than twice the theoretical specific capacity of
the commonly used graphite anode material (372 mAh g ). Such
excellent rate capability should be attributed to the highly con-
ducting 3D graphene electronic conductive network and porous
Cycle number
−
1
c
1
1
1
600
200
Discharge
Charge
-
-1
100 mA g
structure of the SnO /graphene nanocomposite. The highly con-
2
1
00 mA g
ducting 3D graphene electronic conductive network could increase
electronic conductivity of the nanocomposite and the porous
structure can facilitate liquid electrolyte diffusion into the bulk
-1
300 mA g
-1
5
00 mA g
-
1
materials [7,30,34,35,41]. In addition, the shorten path length
3
2
1
0
1000 mA g
8
4
00
00
0
for Li+ transport due to nanosized SnO2 particles in as-prepared
SnO /graphene nanocomposite can also favor the high rate capa-
2
bility [7,41]. Remarkably, when the current density returns to the
−
1
initial 100 mA g
after more than 140 cycles, a high reversible
−
1
specific capacity of 1304 mAh g
cycle. The reversible capacity of 1304 mAh g is much higher than
can be obtained in the 150th
0
400
800
1200
−1
-
1
Specific capacity / mAh g
the theoretical specific capacity of the SnO /graphene nanocom-
2
−
1
100
110
120
130
140
150
posite (858 mAh g
)
calculated according to the theoretical
specific capacity of SnO2 (782 mAh g 1 based on the conven-
−
Cycle number
−
1
tional alloying mechanism) and graphene sheets (1116 mAh g
[38,39]. This result gives assistant information that the con-
version reaction of SnO in the SnO /graphene nanocomposite,
)
Fig. 5. (a) Discharge/charge profiles of SnO2/graphene nanocomposite and SnO2
nanoparticles at a current density of 100 mA g . (b) Capacities versus cycle number
between 3 V and 0.01 V at the current density of 100 mA g . (c) Cycling performance
of SnO2/graphene nanocomposite at various current densities (the same cell after
cycled for 100 cycles shown in b), inset shows the 150th discharge/charge voltage
profiles.
−1
2
2
−
1
+
−
SnO + 4Li + 4e → 2Li O + Sn, should be reversible to a certain
2
2
extent. Such a result can be related to the enhanced surface elec-
trochemical reactivity due to its large BET specific surface area and
nanocrystalline nature [30,40,41,48]. Based on the novel conver-
+
−
sion and alloying mechanism, SnO + 4Li + 4e ꢀ 2Li O + Sn and
2
2
+
−
became partially reversible in the SnO /graphene nanocomposite.
The plateau above 1.0 V in the 100th charge curve is longer than
that at the 20th charge curve, suggesting that the electrochemical
reactivity increased due to the reduced Sn and Li O nanoparticles
with increasing cycle number. It is very possible that the increas-
ing electrochemical reactivity results in the increase of reversible
Sn + xLi + xe ꢀ LixSn (0 ≤ x ≤ 4.4) [25], the maximum theoretical
2
−
1
specific capacity of SnO2 is 1493 mAh g , and the theoretical spe-
cific capacity of SnO /graphene nanocomposite should be as high
2
−
1
as 1407 mAh g . This theoretical value is quite close to the exper-
imental one of 1304 mAh g
2
−
1
.
The cyclic voltammetry of the as-prepared SnO /graphene
2
capacity of SnO /graphene nanocomposite after 20 cycles. It should
nanocomposite is shown in Fig. 6a. In the cathodic polarization pro-
2

4
538
P. Lian et al. / Electrochimica Acta 56 (2011) 4532–4539
Table 1
Reversible specific capacities of SnO2/graphene nanocomposites reported before and in this work.
Current density/mA g−1
Initial capacity/mAh g
−1
Cycle number
Remaining capacity/mAh g
−1
Ref.
5
5
6
64
5
0
5
7
810
765
978
786
862
541
936
30
100
30
50
50
570
520
840
558
665
377
1156
[30]
[31]
[32]
[33]
[34]
[35]
This work
2
0
2
1
00
00
35
100
cess of the first cycle, two obvious peaks were observed at 0.90 and
.03 V. The peak at 0.90 V could be ascribed to the formation of solid
electrolyte interface (SEI) layer on the surface of active material
9,31,40], reduction of SnO2 to Sn and the synchronous formation
Meanwhile, in the anodic polarization process, two peaks were
recorded at about 0.60 and 1.26 V. The peak at 0.60 V represents the
de-alloying process of Li+ ions [48] as described in Eq. (4):
0
[
LixSn ꢀ Sn + xLi+ + xe− (0 ≤ x ≤ 4.4)
(4)
of Li O [32,48] as described in Eq. (1):
2
The following peak at 1.26 V could be ascribed to in part
reversible reaction of the Eq. (1) [31]. After the first cycle, the
peak intensity of CV curve in the 4th cycle becomes lower than
that in the 2nd cycle. However, the peak intensity of the CV
curve in the 150th is still very high, and the integral area is also
very big, revealing the good cycling stability. Note that a peak at
SnO + 4Li + 4e− → 2Li O + Sn
+
(1)
2
2
The peak at 0.90 V is in good agreement with the first dis-
charge voltage plateau during the discharge/charge tests. The peak
at 0.03 V corresponds to alloying of Sn with Li and the reversible
reaction between lithium and graphene sheets [17,31,32,38,39], as
described in Eqs. (2) and (3) respectively:
1
.32 V at the 150th CV curve of SnO /graphene nanocomposite
2
is observed, which indicates that the Eq. (1) is at least partially
reversible in the SnO2/graphene nanocomposite [5,17,27]. This
result is in good agreement with that of galvanostatically dis-
charged and charged tests. For the bare SnO2 nanoparticles in
Fig. 6b, there are still characteristic peaks at 0.91 and 0.10 V,
corresponding to SEI layer formation and the alloying process,
which also correspond to the reaction of lithium with SnO2 dur-
ing the first discharge process [5,9]. Although the CV curves of the
bare SnO2 nanoparticles in the 4th and 2nd cycles almost over-
lap, the peak intensity and integral area of the CV curve in the
100th cycle are obviously decreased, implying poor capacity reten-
tion. This result matches well with that of the discharge/charge
tests.
Sn + xLi+ + xe− ꢀ LixSn + (0 ≤ x ≤ 4.4)
(2)
(3)
2
C + Li+ + e− ꢀ LiC2
a
0
0
.4
.0
-
-
-
0.4
0.8
1.2
To further confirm that the conversion reaction of SnO2 in
1
2
st
the SnO /graphene nanocomposite is partially reversible, the
2
nd
XRD patterns of electrode sheets before and after being cycled
are shown in Fig. 7. As shown in Fig. 7c, the diffraction peaks of
fully charged bare SnO2 nanoparticles electrode sheets after 100
4th
50th
1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Δ
+
Potential vs. (Li/Li ) / V
Δ: Cu Substrate
1
0
0
.0
.5
.0
b
#
: SnO2
: Sn
∗
Δ
Δ
a
#
#
#
#
b
c
-
-
-
0.5
1.0
1.5
1
2
4
1
st
nd
th
∗
∗
∗
d
00th
10
20
30
40
50
60
70
80
0.0
0.5
1.0
1.5
2.0
2.5
3.0
2
<theta> / degree
+
Potential vs. (Li/Li ) / V
Fig. 7. XRD patterns of electrode sheets for (a) SnO2 nanoparticles before being
cycled, (b) SnO2/graphene nanocomposite before being cycled, (c) SnO2 nanoparti-
cles after 100 cycles, and (d) SnO2/graphene nanocomposite after 150 cycles.
Fig. 6. Cyclic voltammograms (CV) of (a) SnO2/graphene nanocomposite and (b)
−1
SnO2 nanoparticles at a scanning rate of 0.2 mV s
.

P. Lian et al. / Electrochimica Acta 56 (2011) 4532–4539
4539
cycles match well with characteristic peaks of Sn (JCPDS file card
Acknowledgements
no. 04-0673), indicating that reaction mechanism of bare SnO2
nanoparticles with Li is the usually reported alloying mechanism.
In comparison, a diffraction peak at around 30.05 ascribed to
This work was financially supported by the National Natural
Science Foundation of China (no. 20936001) and the Fundamental
Research Funds for the Central Universities, SCUT (2009220038).
◦
SnO2 (JCPDS file card no. 29-1484) is observed in the XRD pattern
of fully charged SnO /graphene nanocomposite electrode sheet
2
after 150 cycles, suggesting that the reaction mechanism of SnO2
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