In situ sodium chloride template synthesis of cobalt oxide hollow octahedra for lithium-ion batteries

Article abstract of DOI:10.1039/c4ra15551c

A novel template synthesis route is developed to synthesize cobalt oxide hollow octahedra. The sodium chloride templates are formed in situ in the reaction system by controlling the supersaturation of sodium chloride in the N,N-dimethylformamide solution. Transmission electron microscopy indicates that the porous shells of hollow octahedra are composed of ultra small particles. Electrochemical tests show that the discharge capacity of the cobalt oxide hollow octahedra is about 965 mA h g^-1 after 30 cycles at 0.2C.

Full text of DOI:10.1039/c4ra15551c

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In situ sodium chloride template synthesis of cobalt
oxide hollow octahedra for lithium-ion batteries†
Cite this: RSC Adv., 2015, 5, 23326
Fei Wang,* Danfei Cheng, Wengang Wang, Yuting Wang, Mingshu Zhao,
Shengchun Yang, Xuegang Lu and Xiaoping Song
A novel template synthesis route is developed to synthesize cobalt oxide hollow octahedra. The sodium
chloride templates are formed in situ in the reaction system by controlling the supersaturation of sodium
chloride in the N,N-dimethylformamide solution. Transmission electron microscopy indicates that the
porous shells of hollow octahedra are composed of ultra small particles. Electrochemical tests show that
Received 1st December 2014
Accepted 24th February 2015
DOI: 10.1039/c4ra15551c
ꢀ1
www.rsc.org/advances
the discharge capacity of the cobalt oxide hollow octahedra is about 965 mA h g after 30 cycles at 0.2C.
2
3,24
onto removable templates.
The templates, such as poly-
1
. Introduction
25
26
27
styrene, silica and cuprous oxide, are prepared in advance
Lithium-ion batteries (LIBs) are the predominant power source and removed via chemical etching or thermal decomposition
for portable electronics at present due to their high energy aer deposition. Therefore, these method are usually tedious,
1
density, long cycle life, etc. However, further applications of LIBs costly and instability because of the multistep procedures
in high energy areas, such as electric vehicles (EVs) and large- involving template preparation, shell deposition and template
scale energy storage systems, are still restricted. Therefore, with removal. In this paper, we present a facile template approach to
3 4
the growing demand for higher capacity and safety, synthesize uniform Co O hollow octahedrons. The used NaCl
numerous efforts have been made to develop alternative high- templates are formed in situ during reaction process and can be
2,3
performance electrode materials for next-generation LIBs.
3 4
easily removed by distilled water. The obtained Co O hollow
Recently, nano-sized transition metal oxides have been widely octahedrons exhibit high reversible capacity and superior
investigated as promising anodes for LIBs due to their higher cycling performance.
theoretical capacities than conventional graphite materials
ꢀ
1
3–14
(
3 4
<372 mA h g ). For example, the theoretical capacity of Co O
ꢀ1
reach to 892 mA h g . However, the severe volume variation that 2. Experimental
occurs during repeated lithiation/delithiation processes is a big
challenge to obtain excellent cycling performance.
effective strategies to alleviate these problems is to design hollow
micro-/nanostructured electrode materials instead of conven-
tional solid micro-/nanostructures.
area of the hollow structures can lead to increased electrode-
electrolyte contact area and more lithium storage sites. The
thin shells of them can signicantly reduce the diffusion paths
for both Li ions and electrons, leading to the faster reaction
kinetics at the electrode surface. Moreover, the hollow interior
can provide additional free space to alleviate the structural strain
associated with repeated lithium uptake/removal processes and
thus lead to improved cycling performance.
In a typical synthesis, 0.1 g poly-(vinyl pyrrolidone) (PVP), 1.0 g
15–18
One of the
poly(ethylene glycol) (PEG), 0.71 g CoCl $6H O and 0.3 g NaCl
2
2
were added to a three-neck round-bottom ask containing 45 ml
N, N-dimethylformamide (DMF). The mixture was maintained at
19–22
The larger specic surface
ꢁ
40
C, followed by the addition of 15 ml DMF solution con-
taining 0.16 g sodium borohydride (NaBH
4
) with stirring. The
ꢁ
nal solution was then kept at 40 C for 1 h. Aer the reaction,
the resulting suspension was washed and NaCl templates were
simultaneously removed by distilled water and ethanol. The
precipitate was separated from the mixture by centrifugation at
ꢁ
5
000 rpm for 2 min and dried at 60 C for 12 h. Finally, the dried
ꢁ
powders were heat-treated in air for 2 h at 500 C.
The structure and morphology of the products were charac-
terized by X-ray diffraction (XRD, Bruke D8-Advance, Cu-Ka,
l ¼ 0.15406 nm), eld emission scanning electron microscope
The general route for preparing hollow nanostructures is
based on the controlled deposition of the designed materials
(FESEM, JEOL JSM-7100F) and transmission electron micros-
School of Science, Key Laboratory of Shaanxi for Advanced Functional Materials and
Mesoscopic Physics, State Key Laboratory for Mechanical Behavior of Materials,
Xi'an Jiaotong University, Xi'an 710049, ShaanXi, People's Republic of China.
E-mail: feiwang@mail.xjtu.edu.cn; Fax: +86 29 82665995; Tel: +86 29 82663034
copy (TEM, JEOL JEM-2100). Energy dispersive X-ray (EDX)
analysis was obtained with an Oxford INCA detector installed on
the FESEM.
3 4
Electrochemical properties of Co O hollow octahedrons
were tested using two-electrode Swagelok cells with lithium
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra15551c
1
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metal as the counter and reference electrodes. The working
electrodes consisted of 80 wt% active materials (Co O ), 10 wt%
3
4
conductive materials (acetylene black), and 10 wt% binder
polyvinylidene uoride, PVDF). The electrolyte was 1 M LiPF in
(
6
a mixture of 50 vol% ethylene carbonate (EC) and 50 vol%
diethylene carbonate (DEC). Test cells were assembled in argon

lled glove box (Mikrouna Super 1220/750). The galvanostatical
charge–discharge measurement was carried out on Arbin
BT2000 battery testing system in the voltage range of 0.02 V-3.0
V (vs. Li/Li+). The cyclic voltammograms (CV) were tested on
Ametek VMC-4 electrochemical testing system at a scan rate of
3 4
Fig. 2 Schematic illustration of the formation of hollow Co O
octahedrons.
ꢀ1
+
0.5 mV s between 0 V and 3.0 V (vs. Li/Li ).
The formation of the solid octahedrons can be schematically
illustrated by Fig. 2. At the rst stage, the Co small crystallites
were deposited on the surface of the in situ formed NaCl crystals,
Fig. 1 shows the electron microscopic data of the reaction which act as the template for the formation of the octahedrons. At
products only briey washed by ethanol in order to maintain the the stage II, the deposited Co small crystallites would attach to
3 4
O
3. Results and discussion
3 4
O
NaCl templates. All particles in the SEM images show the typical each other because of the Oswald ripening occurring in organic
solid octahedron morphology with the size range of 2–4 mm. The solvent, which leads to the formation of shell layer on the surface
inset pattern of Fig. 1a shows the XRD analysis of the solid of NaCl crystals.
octahedrons. All diffraction peaks in it can be indexed to cubic
When the solid octahedron particles were washed with
NaCl structure (JCPDS no. 05-0628). The formation of NaCl distilled water, the NaCl templates were removed while the
crystals is ascribed to the limited solubility of NaCl in DMF cobalt oxide can be reserved due to the different water solubility
solution. Thus, the NaCl concentration plays an important role between ionic compounds and oxides (the stage III of Fig. 2).
to obtain the octahedral NaCl template. Only when the NaCl EDX pattern (the inset pattern of Fig. 3b) indicates that there are
concentration over a certain range of its saturated solubility, the only Co and O elements existing. SEM images (Fig. 3a and b)
octahedral NaCl templates can be well formed. Thus, the addi- show that the octahedron morphology has been well maintained
tional NaCl (0.3 g) is needed besides the NaCl resultant from the aer the removing of the NaCl templates. The hollow structure
3 4
chemical reaction (eqn (1)). Otherwise, the products are the can be clearly observed in the TEM image of an individual Co O
dispersive spherical nanoparticles (Fig. S1†). From the EDX octahedron (Fig. 3c). Comparing with the solid octahedrons with
spectrum (the inset pattern of Fig. 1b) we can see that Co and O NaCl templates, the hollow octahedrons have an obvious
elements also exist besides Na and Cl. The absence of the cobalt shrinkage and the size of the octahedrons is reduced to the
oxide diffraction peaks in the XRD pattern should be attributed range of 1–2 mm. The rough shells composed of the interlaced
to the poor crystallinity of the cobalt oxide. Thus, the diffraction nanosheets (Fig. 3c and d) should be ascribed to the shrink of
peaks of cobalt oxide are submerged in the sodium chloride the shell materials during the removing of the NaCl templates.
diffraction peaks due to the striking contrast of the diffraction
intensity. The formation of the cobalt oxide can be described as
two-step reaction processes, which are the reduction of Co(II)
4
ions to metallic Co (0) by NaBH and the further oxidation of
metallic Co to cobalt oxide, as shown in eqn (1) and (2).
CoCl
2
+ NaBH
4
/ Co + H
2
+ NaCl + BCl
3
(1)
(2)
Co + O
2
3 4
/ Co O
Fig. 3 The representations of the untreated hollow Co
octahedrons with templates. drons: (a, b) SEM images; (c) TEM image of an individual octahedron;
The inset of (a) is the XRD pattern and the inset of (b) is the EDX pattern (d) TEM image of the octahedron shell. The inset of (b) is the EDX
3 4
O octahe-
3 4
Fig. 1 (a, b) SEM images of the Co O
3
of the Co O
4
octahedrons with templates.
pattern of the hollow Co
3
O
4
octahedrons.
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of the small particles about 5 nm due to the atom diffusion in
high temperature. The corresponding SEAD characterization
shows the ring-like diffraction pattern (Fig. 4c), which suggests
that the hollow octahedrons are poor crystallinity even aer the
ꢁ
heat treatment at 500 C. All the diffraction rings could be
3 4
assigned to the spinel-type Co O (JCPDS No. 42-1267), which is
consistent with the XRD analysis (the inset pattern of Fig. 4a).
The size of the particles and holes can be observed more clearly
in high-resolution TEM image (Fig. 4d). The interplanar spacing
marked in it is about 0.24 and 0.47 nm, corresponding to the
(
311) and (111) plane of the cubic Co O , respectively.
3 4
The electrochemical reaction processes are investigated by
cyclic voltammograms. Fig. 5a shows the CV curves of the Co
3
O
4
ꢁ
hollow octahedrons heat treated at 500 C. During the rst
cathodic scan, only one broad peak near 0.5 V could be observed.
Compared to the initial cycle, a decrease of peak intensity and a
shi of the potential to the positive direction are revealed in the
subsequent cycles. This cathodic peak corresponds to a multistep
electrochemical reaction, which is generally attributed to the
Fig. 4 The representations of the hollow Co
treated at 500 C for 2 h: (a) SEM image; (b) TEM image of the octa-
hedron shell; (c) the corresponding SAED pattern of (b); (d) HRTEM
3
O
4
octahedrons heat-
ꢁ
28
reduction of Co(III) to Co(II) and metal Co. In the anodic scans,
image of the octahedron shell. The inset of (a) is the XRD pattern of the one peak at about 2.18 V is recorded, which are ascribed to the
ꢁ
3
hollow Co O
4
octahedrons heat-treated at 500 C for 2 h.
reversible oxidation of metal Co to cobalt oxide. Except for the rst
cycle, the following CV curves are similar, which demonstrate the
3 4
good cycle stability of Co O hollow octahedrons during the
To improve the structure stability and the lithium storage discharge–charge cycles. The electrochemical reaction of Li with
2
9–31
properties of cobalt oxide, the hollow octahedrons have been Co O can be expressed as eqn (3).
3
4
ꢁ
heat-treated at 500 C for 2 h before the testing as anode
+
ꢀ
Co
3
O
4
+ 8Li + 8e 4 4Li
2
O + 3Co
(3)
materials for LIBs. Comparing with the untreated hollow octa-
hedrons, it is similar in both octahedron morphology and size
aer the heat treatment (Fig. 4a). However, from the TEM image
Fig. 5b shows the cycling performance and coulomb effi-
ciency of the Co O hollow octahedrons heat treated at 500 C.
The initial discharge and charge capacities are 2058 and
(
Fig. 4b) we can see clearly that obvious changes have taken
ꢁ
3
4
place in the shell. Aer the heat treatment, the interlaced
nanosheets have transformed into porous structure composed
ꢁ
Fig. 5 Cyclic voltammogram curves (a), cycling performance (b) and rate capability (c) of the hollow Co
for 2 h; (d) cycling performance of the Co spherical nanoparticles.
O
3 4
octahedrons heat-treated at 500 C
3 4
O
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ꢀ
1
1
213 mA h g , respectively. The initial irreversible capacities
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28,32
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3 4
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5
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ꢀ1
9
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ꢀ1
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ꢁ
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ꢀ1
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. Conclusions
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3 4
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3
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