Download Preparation of Carbon Nano-Onions and Their

ARTICLE
pubs.acs.org/JPCC
Preparation of Carbon Nano-Onions and Their Application as Anode
Materials for Rechargeable Lithium-Ion Batteries
Fu-Dong Han, Bin Yao, and Yu-Jun Bai*
Key Laboratory for LiquidSolid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University,
Jinan 250061, P. R. China
bS Supporting Information
ABSTRACT: Carbon nano-onions (CNOs) were prepared at
600 °C by a simple reaction between copper dichloride hydrate
(CuCl2 3 2H2O) and calcium carbide (CaC2). The morphology
and structure of the obtained products were investigated by
field-emission scanning electron microscope, high-resolution
transmission electron microscopy, X-ray diffraction, Raman
spectrum, and nitrogen adsorption. Large quantities of CNOs
consisting of quasi-spherically concentric graphitic shells with
high purity and uniform size distribution (about 30 nm) were
obtained. The crystal water in CuCl2 3 2H2O plays an important
role in the formation of CNOs. The CNOs as-obtained exhibit
high capacity and excellent cycling performance as anode materials for lithium-ion batteries, which can deliver a reversible capacity of
391 mAh g1 up to 60 cycles.
’ INTRODUCTION
Carbon nano-onions (CNOs), which consist of concentric
graphitic shells, represent another new allotropic nanophase of
carbon materials. CNOs have already been shown to offer a
variety of potential applications such as solid lubrication,1
electromagnetic shielding,2 fuel cells,3 heterogeneous catalysis,4
gas and energy storage,5 and electro-optical devices6 owing to
their outstanding chemical and physical properties. According to
a recent study, CNOs can also be used to produce ultrahighpower micrometer-sized supercapacitors due to their accessible
external surface area for ion adsorption.7
Since the first observation of the onion-like structure by Iijima
in vacuum deposited amorphous carbon films in 1980,8 several
methods have been reported to prepare CNOs. For example,
Ugarte examined the formation of CNOs in a transmission
electron microscope (TEM) by irradiating carbon material
consisting of different carbon nanostructures (fullerenes, nanotubes, amorphous carbon).9 Kuznetsov et al. obtained CNOs by
high-temperature annealing of diamond nanoparticles under
vacuum.10 Cabioc’h et al. prepared CNOs by high-dose carbon
ion implantation into copper and silver.11 Sano et al. fabricated
CNOs by arc discharge between two graphite rods immersed in
water.12 Recently, Hou et al. reported the high-yield synthesis of
CNOs in counterflow diffusion flames.13,14 In addition, chemical
vapor deposition was also considered as a viable method to
synthesize CNOs by lots of scientists.1518 Other ways have also
been proposed to synthesize CNOs, such as laser irradiation,19
shock compression,20 and high-energy ball-milling.21 However,
most of the methods above-mentioned require high energy
input, and CNOs are sometimes of low yield or just as a
r 2011 American Chemical Society
byproduct.16,22 For applications such as fuel-cell electrodes, large
quantities (kilograms) of the material are desired. Nevertheless,
CNOs can only be produced in minute quantities by lots of
methods such as electron beam irradiation of carbon materials.
Moreover, it is not practical to use the CNOs obtained by some
methods requiring high investment and running costs for
ordinary applications such as solid lubrication. Therefore, how
to economically synthesize CNOs in large scale is of great
importance for their wide applications.
In this article, we reported a simple, efficient, and economical
route for large-scale preparation of CNOs with uniform morphology and size. The CNOs were obtained via the reaction
between CaC2 and CuCl2 3 2H2O at 600 °C. In addition, the
electrochemical properties of the as-obtained CNOs as anode
materials for lithium-ion battery have also been evaluated.
’ EXPERIMENTAL SECTION
Materials and Synthesis. The raw materials used are chemically pure copper dichloride hydrate (CuCl2 3 2H2O) and calcium carbide (CaC2). In a typical procedure, 10 g (0.059 mol)
CuCl2 3 2H2O and 3.2 g (0.05 mol) CaC2 were put into a stainless
steel autoclave of 30 mL capacity. Because the removal of
CuCl2 3 2H2O is easier than that of CaC2, the amount of CuCl2 3
2H2O was slightly in excess to ensure the complete reaction of
CaC2. The autoclave containing the raw materials was sealed
Received: January 24, 2011
Revised:
March 29, 2011
Published: April 14, 2011
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tightly and heated in an electric oven to 600 °C and maintained
for 10 h at this temperature. When the autoclave was cooled
naturally to ambient temperature, the products in the autoclave
were collected and primarily washed several times with a mixed
solution of ammonia (NH3 3 H2O) and carbon tetrachloride
(CCl4) to remove the byproduct copper23 and then rinsed
successively with dilute hydrochloric acid and deionized water
until all soluble materials were removed. After drying at 60 °C for
10 h, black powders were ultimately obtained.
Characterization. The morphology of the product was examined using a Hitachi SU-70 field-emission scanning electron
microscope (FESEM) with an energy-dispersive X-ray spectrometer (EDX) and a JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM). X-ray powder diffraction
(XRD) patterns were obtained on a Rigaku Dmax-rc diffractometer with Ni-filtered Cu KR radiation (V = 40 kV, I = 50 mA)
at a scanning rate of 4°/min. Raman spectra were collected on a
Renishaw confocal Raman microspectroscopy (Renishaw Co.
Ltd., Gloucestershire, U.K.) with a laser excitation wavelength of
780 nm. Nitrogen adsorption and desorption isotherms were
carried out at 77 K on a Quadrasorb (Supporting Information)
sorption analyzer. The samples were outgassed at 300 °C for 3 h
under a vacuum in the degas port of the analyzer. The specific
surface area was calculated with the BrunauerEmmettTeller
(BET) model, and the pore-size distribution was calculated from
the adsorption/desorption data by using the DFT method.
Electrochemical Measurement. Electrochemical experiments were carried out in 2025 coin-type cells. The working electrodes were prepared by coating the slurry of CNOs (85 wt %),
carbon black (5 wt %), and polyvinylidene fluoride (PVDF)
(10 wt %) dissolved in n-methyl pyrrolidinone (NMP) onto a Cu
foil substrate and dried in a vacuum oven at 120 °C for 12 h.
Lithium metal foil was utilized as the counter electrode, and
Celgard 2300 was used as the separator. The electrolyte was a
mixture of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl
carbonate (DMC) (1:1 by volume). Half-cells were assembled in
an argon-filled glovebox. The cell performance was estimated
galvanostatically at a current density of C/10 (one Li per six
formula units (LiC6) in 10 h)) for both charge (Li extraction)
and discharge (Li insertion) at room temperature. The cells were
cycled in the voltage range of 0.023 V (versus Li/Liþ).
’ RESULTS AND DISCUSSION
The morphology of the asobtained product was observed by
FESEM, as shown in part a of Figure 1. The product is comprised
of numerous quasi-spheres about 30 nm in diameter, and no
other carbon nanostructures such as carbon nanotubes can be
detected. Part b of Figure 1 is the EDX spectrum taken from the
spheres, from which there is no detectable metal catalyst (Cu) or
other impurities except for C and a little O resulting from
absorption, indicating the high purity of the spheres. The
examination by HRTEM (parts ce of Figure 1) demonstrates
that the spheres have an onion-like structure. The interplanar
spacing of 0.34 nm corresponds to that of the (002) plane of
graphite. The short-range order or turbostratic graphite structure
gives rise to concentric graphite layers around a hollow and
irregular core about 5 nm in diameter. The corresponding
selected area electron diffraction (SAED) pattern (insert in part
d of Figure 1) reveals the poor graphitization of the CNOs. It is
worthwhile to note that Cu-encapsulating CNOs can hardly be
observed in the product even by careful TEM examination,
ARTICLE
Figure 1. FESEM image (a), EDX spectrum (b), and HRTEM images
(ce) of the as-obtained CNOs. The insert in (d) is the corresponding
SAED pattern.
Figure 2. XRD pattern (a) and Raman spectrum (b) of the as-obtained
CNOs.
further confirming the high purity of CNOs, which may imply
that the formation of CNOs was not by virtue of encapsulating
Cu. In addition, a little amorphous carbon can be found on the
surface of CNOs, which may result from the deposition of vaporous carbon atoms around the CNOs, and this will be discussed
in detail.
To determine the structure of the as-obtained product, XRD
measurements were conducted, as shown in part a of Figure 2.
The main peak at 2θ = 25.8° can be attributed to the (002)
diffraction of hexagonal graphite. The broadening nature of the
peak is indicative of the long-range disorder structure of the
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the reaction between CaC2 and CuCl2. According to the molar
ratio of approximately 5.9:5 for CuCl2 3 2H2O to CaC2, the H2O
generated from the CuCl2 3 2H2O is excess enough to consume
the CaC2 thoroughly, so no CaC2 could be left for the further
reaction between CaC2 and CuCl2 under the experimental
conditions. With the increase of temperature, a redox reaction
between C2H2 and CuCl2 will happen, resulting in the formation
of carbon and Cu. The above-mentioned process can be expressed by the following reactions:
Figure 3. Nitrogen adsorption/desorption isotherms (a) and the DFT
pore-size distributions (b) of the as-obtained CNOs.
as-obtained CNOs due to their small sizes. Moreover, the
disorder structure can be also reflected by the indistinguishability
of the (100) and (101) peaks centered at 2θ = 43.7°.24 Raman
spectra were measured to identify the bonding and structure of
the CNOs as-obtained, as shown in part b of Figure 2. The two
broad peaks centered at about 1303 and 1571 cm1 are ascribed
to the D peak for disordered carbon and the G peak for graphite
carbon, respectively.25 The intensity ratio of ID to IG in Raman
spectra is a typical parameter to quantify the disorder degree of
carbon materials, that is a greater value of ID/IG ratio means a
higher disorder degree for graphite. As to the as-obtained CNOs,
the calculated value of 1.10 for ID/IG implies a low graphitization
degree resulted from the long-range disorder structure of the
CNOs, in good agreement with the HRTEM and XRD results.
Nitrogen adsorption/desorption isotherms and DFT pore
size distribution of the as-obtained CNOs are displayed in
Figure 3. From part a of Figure 3, the isotherms exhibit a typical
IV-type curve with a small hysteresis loop at higher relative
pressure, suggesting the diversity of pores and the presence of
micropores and mesopores in the product. The specific surface
area and pore volume are 101 m2 g1 and 0.16 cm3 g1,
respectively. The pore-size distribution shown in part b of Figure 3
indicates that the size of pores is primarily between 1 and 5 nm.
According to previous reports, CaC2 can be used as carbon
source to synthesize carbon spheres under appropriate
conditions.2629 CuCl2 is sometimes used as a promoter in the
formation of carbon nanostructures.30 In our experiment, however, CuCl2 3 2H2O acts as a reactant to form CNOs because Cu
can be detected in the products washed just by deionized water,
as confirmed by XRD analysis (Figure S1 of the Supporting
Information). For insight into the formation mechanism of the
CNOs, a comparative experiment was performed using CuCl2
without crystal water as the reactant. In contrast, few CNOs were
obtained; instead, some hollow structures with various sizes and a
few nanoparticles could be observed (Figure S2 of the Supporting Information). Although the reaction between CuCl2 and
1
CaC2 is both thermodynamically (ΔrGΘ
m = 508.2 kJ mol ,
Θ
1
ΔrHm = 515.5 kJ mol ) and experimentally feasible, as
expressed by eq 1, the formation of the CNOs cannot be just
ascribed to the simple redox reaction between CaC2 and CuCl2.
CuCl2 þ CaC2 ¼ Cu þ 2C þ CaCl2
ð1Þ
Thus, the crystal water in CuCl2 3 2H2O plays an important role
in the formation of the CNOs. On the basis of the information
acquired, a possible chemical reaction mechanism is proposed as
follows. The crystal water in CuCl2 3 2H2O will be lost at 110 °C
during heating and will immediately react with CaC2 to produce
acetylene (C2H2). The reaction between water and CaC2 occurs
quite readily even under ambient atmosphere and thus is prior to
CuCl2 3 2H2 O ¼ CuCl2 þ 2H2 O
ð2Þ
CaC2 þ 2H2 O ¼ CaðOHÞ2 þ C2 H2
ð3Þ
CuCl2 þ C2 H2 ¼ Cu þ 2C þ 2HCl
ð4Þ
CaðOHÞ2 þ 2HCl ¼ CaCl2 þ 2H2 O
ð5Þ
The overall reaction can be concluded according to the above
reactions:
CuCl2 3 2H2 O þ CaC2 ¼ CaCl2 þ Cu þ 2C þ 2H2 O
ð6Þ
As has been known, the chemical reactions described by eqs 2, 3,
and 5 can occur readily. According to free energy calculations,
eq 4 is also thermodynamically spontaneous (ΔrGΘ
m = 223.9
1
kJ mol1) and highly exothermic (ΔrHΘ
m = 191.92 kJ mol ),
indicating the feasibility of the above-mentioned chemical processes. Moreover, the thermodynamic calculation for the overall
1
Θ
reaction of eq 6 (ΔrGΘ
m = 631.393 kJ mol , ΔrHm = 353.539
1
kJ mol ) also demonstrates that the reaction between CuCl2 3
2H2O and CaC2 could happen under the reaction conditions.
The heat generated further prompts the progress of the reactions,
during which large quantities of carbon and Cu atoms are
simultaneously produced. It is well-known that Cu has a very
strong effect on graphitization of carbon.31,32 Accordingly, the
carbon atoms generated will grow into graphitic flakes under the
catalysis of Cu. These graphitic flakes obtained are the combination of pentagonal and heptagonal structures together with the
planar hexagonal structure of graphene so that they can accommodate the curvature of the CNOs.33 With continuous heating
treatment, the graphitic flakes aggregate into quasi-spherical
graphitic shells, giving rise to the formation of CNOs. The
driving force for the graphitic flake aggregation has been discussed in terms of a free energy minimization theory,34,35 that is
the highly stable spherical shape of carbon species and the
multiple shell structure are more energetically favorable than a
single or tubular shell. Meanwhile, this process can also result in
the elimination of dangling bonds at the edge of the flakes, which
can undoubtedly lower the energy of the system.9,36 However,
some defects such as dangling bonds also exist on the surface of
the CNOs. At the end of the growth of CNOs, these defects
would possibly act as nucleation sites for further deposition of
vaporous carbon atoms on the surface of CNOs, which could
account for the amorphous carbon observed on the surface of
CNOs under HRTEM.
Galvanostatic discharge/charge experiments were carried out
to evaluate the electrochemical performance of the as-obtained
CNOs. Part a of Figure 4 shows the discharge/charge curves of
the as-obtained CNOs for the first four cycles at a rate of C/10.
Surprisingly, at this current density, a first reversible capacity as
high as 501 mAh g1 can be observed, which is significantly
higher than the theoretical capacity of graphite (372 mAh g1).
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected], tel: þ8653188392677, fax:
þ8653188392315.
Figure 4. Galvanostatic discharge/charge curves (a) and cycling performance (b) of the as-obtained CNOs cycled at a rate of C/10.
Meanwhile, a large irreversible capacity of about 381 mAh g1
was also observed for the CNOs electrode during the first
discharge and charge process; however, after the fifth cycle, the
Coulombic efficiency is above 95%. In fact, the initial irreversible
capacity is an expected phenomenon in carbonaceous electrodes
in lithium-ion batteries.37 The irreversible capacity loss could
originate from the decomposition of the electrolyte, which leads
to the formation of solid electrolyte interphase films at the
electrode/electrolyte interface and/or from the irreversible
lithium insertion into special positions such as in the vicinity of
residual H atoms in the carbonaceous material.3840 The cycling
performance of the CNO electrode was examined under longterm cycling over 60 cycles, which demonstrated a good cyclic
performance and reversibility (part b of Figure 4). After 60 cycles,
the CNO anode still maintained a specific reversible capacity of
391 mAh g1, which also represents a more enhanced performance than that of graphite anodes. From these results, it can be
concluded that CNOs are very promising for use as anode
electrodes in lithium-ion batteries.
In the case of the as-obtained CNOs, the enhanced specific
capacity may be originated from the following reasons. (1) The
nanoscale carbon onions can provide a high electrode
electrolyte contact area (101 m2 g1) and a short pathway both
for electron and Liþ transport. (2) The unique graphitic multilayer structure of CNOs can afford more lithium storage sites
such as cavities and nanopores,37 leading to a higher capacity. (3)
The amorphous carbon around the CNOs may also contribute to
the enhanced specific capacity due to the fact that the disordered
carbon exhibits a higher capacity than that of graphite.41
’ CONCLUSIONS
In conclusion, large quantities of carbon nano-onions have
been successfully prepared by a simple and economical method
via the reaction between CuCl2 3 2H2O and CaC2. The formation
of the CNOs is greatly associated with the crystal water in
CuCl2 3 2H2O. The electrochemical performance testing gives
some evidence that the prepared CNOs with a unique graphitic
multilayer structure have intensive potential as a candidate of
anode materials with high reversible capacity and good cycling
performance for rechargeable lithium-ion batteries.
’ ASSOCIATED CONTENT
bS
Supporting Information. XRD pattern of the product
rinsed just by deionized water and without further purification by
the mixed solution of NH3 3 H2O and CCl4, TEM image of the
product obtained by the reaction of CaC2 and CuCl2 without
crystal water at 600 °C. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (No. 50972076, 50872072, and 50772061),
Shandong Provincial Natural Science Foundation, China
(Y2008F26 and Y2008F40), Science and Technology Development Project of Shandong Province (2009GG10003001,
2009GG10003003), and Special Fund for Postdoctoral Innovative
Project of Shandong Province (200702024).
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