Rapid and large-scale synthesis of bare Co3O4 porous nanostructures from an oleate precursor as superior Li-ion anodes with long-cycle lives

Article abstract of DOI:10.1039/c6dt02136k

In this study, we describe a rapid and environmentally friendly synthesis of bare Co₃O₄ nanocrystals derived from Co(ii) oleate complexes by calcination treatment. When directly used as anode materials for lithium-ion batteries (LIBs), the as-prepared nanocrystals could deliver a high reversible capacity of 980 mA h g^-1 after 250 cycles at a current density of 100 mA g^-1 and excellent cycling performance, which may be beneficial to promote the further development of the next generation of lithium ion batteries. The synthetic route can offer great advantages for the flash preparation of other metal oxide nanocrystals for energy storage application.

Full text of DOI:10.1039/c6dt02136k

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DOI: 10.1039/C6DT02136K
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ARTICLE
Rapid and large-scale synthesis of bare Co3O4 porous
nanostructures from oleate precursor as superior Li-ion anodes
with long-cycle life
Received 00th January 20xx,
Accepted 00th January 20xx
Danhua Ge,a Junjie Wu,a Genlong Qu,a Yaoyao Deng,a Hongbo Geng,a Junwei Zheng,b Yue Pan*a and
DOI: 10.1039/x0xx00000x
Hongwei Gu*a
www.rsc.org/
In this study, we described a rapid and environmentally friendly increasing the electrochemical performance via the high
synthesis of bare Co3O4 nanocrystals derived from Co(II) oleate physical stability and electron conductivity, the excessive
complexes by the calcination treatment. When directly used as addition could exercise a great influence on the specific
anode materials for lithium-ion batteries (LIBs), the as-prepared capacity of Co3O4.17 As we all know, nanomaterial-based
nanocrystals could deliver high reversible capacity of 980 mA h g-1 electrodes are used to availably facilitate the power densities
after 250 cycles at a current density of 100 mA g-1 and excellent and
charge/discharge
rates,
because their
large
cycling performance, which may be beneficial to promote the electrode/electrolyte contact areas and solid electrolyte
further development of the next generation of lithium ion interface (SEI) films can provide valid transfer pathways for Li
batteries. The synthenic route can offer great advantages for flash ions and electrons.18 Moreover, Co3O4 nanostructures
preparation of other metal oxides nanocrystals for energy storage provided with active sites and large specific surface areas have
application.
been developed to improve the anode capacities for Li
intercalation/deintercalation reactions.12,18 In the previous
reports, Xu et al. summarized that agglomerated Co3O4
nanoparticles derived from metal-organic frameworks (MOFs)
favored the enhanced capacity performance.19 More recently,
Wang et al. reported that Co3O4 nanoparticles could give rise
to high specific discharge capacity and excellent cycling
stability, when metal organic complexes were used as
precursors by the solvothermal method and calcination
treatment.20 However, the experiment processes are usually
very cumbersome. Therefore, a green method is still needed to
expeditious synthesize Co3O4 electrode with advanced
electrochemical performance.
Here, we describe a cheap and large-scale (gram-scale)
pathway to produce bare Co3O4 nanocrystals (NCs) from cobalt
oleate complexes. The electrochemical performance of the
prepared Co3O4 NCs is evaluated as anode materials in LIBs. To
the best of our knowledge, little report has been conducted to
design the nano-sized transition metal oxides electrodes via
the direct calcination treatment of metal oleate precursors.
Interestingly, it is found that the as-prepared Co3O4 NCs deliver
high specific capacity of 980 mA h g-1 after 250 cycles at a
current density of 100 mA g-1 with long cycle life. The result
suggests the potential application in negative anode materials
for LIBs.
Introduction
With the depletion of the earth's fossil-fuel resources and
increasingly serious environmental problems, rechargeable
lithium ion batteries (LIBs) have been regarded as
a
predominant candidate for energy storage devices in recent
years.1-8 The graphene is the conventional anode material in
lithium storage, which is limited by its low theory capacity (372
mA h g-1).2 In contrast, considerable attentions have been
attracted to transition metal oxides (TMOs) due to the high
specific capacity (500~1000 mA h g-1) and operating potential,
especially for Co3O4.7-10 However, poor electronic conductivity
and serious capacity decay derived from aggregation and
volume changes in the charge/discharge process hinder the
practical application of Co3O4 as anode materials.11
Some effective methods have been introduced to tackle this
problem, such as combining with conductive composites and
preparing various Co3O4 nanostructures.10,12-16 Although the
combination of conductive materials could be aiming at
Experimental
Materials
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All chemicals were used as received without further
purification. Cobalt (II) chloride hexahydrate (CoCl2·6H2O, 99%)
and sodium oleate (98%) were used as source materials.
Results and discussion
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As illustrated in Scheme 1, the obtainedDCOoI:3O104.1N03C9s/Cw6eDrTe02f1r3o6mK
a simple synthesis of cobalt oleate as precursor, followed by
the calcination treatment. Briefly, cobalt oleate precursors
were prepared by reacting CoCl2·6H2O and sodium oleate in
the mixed solvents of water, ethanol and hexane at 70 °C for 4
h, as with a typical method of preparing ferric oleate.21 Then,
the product was annealed at 350 °C with a high heating rate of
10 °C min-1 in N2 for 1 h and continuously at 600 °C in air for 3
h with a low heating rate of 1 °C min-1 to form Co3O4 NCs (see
the Supporting Information (SI) for details).
Synthesis of the Co3O4 NCs
The nanocomposites were simply prepared in two procedures:
the synthesis of cobalt oleate precursor was followed by
thermal decomposition. According to
a typical method,
CoCl2·6H2O (5 mM, 1.18 g) and sodium oleate (10 mM, 3.04 g)
were added in a mixture solution containing 6 mL of water, 8
mL of ethanol and 14 mL of hexane under vigorous stirring at
o
70 C for 4 h, respectively. The obtained layered solution was
separated in a separatory funnel and the upper organic layer
was washed three times with water. Then, the wine precursor
was dried at 80 oC under vacuum condition overnight. To
synthesize Co3O4 NCs, the cobalt oleate precursor was first
annealed at 350 oC with a high heating rate of 10 oC min-1 in N2
o
for 1 h, and continuously at 600 C in air for 3 h with a low
heating rate of 1 oC min-1.
Materials characterization
The morphologies and sizes were examined by scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM). SEM was performed on a Hitachi S-4700
cold field-emission scanning electron microscope operated at
30 KV, and TEM was performed on a TecnaiG220 microscope
(FEI, USA) fitted with a Gatan CCD794 camera operated at 200
KV. The as-prepared Co3O4 NCs were characterized by X-ray
diffraction (XRD) using an X'Pert-Pro MPD diffractometer
Scheme 1. The illustration for the synthesis route of Co3O4 NCs
derived from Co(II) oleate complexes.
(Netherlands PAN-alytical) with
a Cu KX-ray source
(1.540598Å). Thermogravimetric analysis (TGA) was carried
out on a Perkin Elmer TGA 4000 thermogravimetric analyser.
XPS data were obtained using a KRATOS Axis ultra-DLD X-ray
90 photoelectron spectrometer with monochromatic Mg KX-
ray (1283.3 eV). The specific surface area was calculated by the
Brunauer-Emmett-Teller (BET) method. The pore size
distribution was calculated by the Barret-Joyner-Halenda (BJH)
method.
Electrochemical measurement
The electrochemical measurements of the hollow and porous
products were carried out using a coin-type half cell (CR 2016).
The working electrode was prepared by mixing 80 wt% active
material (Co3O4 NCs), 10 wt% conductive material (acetylene
black, AB) and 10 wt% polymer binder (polyvinylidene
difluoride, PVDF) with the N-methyl-2-pyrrolidone (NMP)
solvent to form a homogeneous slurry, followed by spreading
onto copper foil. Finally, the copper foil was dried in a vacuum
Figure 1. a) SEM image; b) TEM image; c) HR-TEM image (insert:
TEM image) and d) XRD pattern of Co3O4 NCs.
The morphology and structure of Co3O4 NCs were observed
by means of scanning electron microscopy (SEM) as illustrated
in Figure 1. According to SEM images, it can be seen that the
sample is the NCs of uneven size in shape with diameters
ranging from 50 to 100 nm (Figure 1a). The morphologies were
further demonstrated by transmission electron microscopy
(TEM) in Figure 1b. It is declared that the obtained product can
be ascribed to the pure phase of Co3O4. Moreover, the high-
resolution TEM (HR-TEM) image further demonstrated the
highly crystalline nature of porous Co3O4 NCs, and the lattice
fringe spacing of 0.29 nm corresponds to the (220) planes of
o
at 120 C overnight. The electrolyte was 1M LiPF6 in a mixed
solution of ethylene carbonate (EC) and diethyl carbonate (DEC)
in the ratio of EC-DEC=1:1 (w/w, provided by Guotaihuarong,
Zhangjiagang, China). Cell assembly was performed in an
argon-filled glovebox in the presence of moisture and oxygen
below 0.1 ppm. The cells were charged and discharged
between 3.0 V and 0.01 V at room temperature.
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Co3O4 (Figure 1c).The crystallographic structure and phase eV with a spin-orbit splitting of 15.3 eV corresponding to Co
View Article Online
25
DOI: 10.1039/C6DT02136K
purity of Co3O4 NCs were examined by powder X-ray 2p1/2 and Co 2p3/2 spectra, respectively.
And The
diffraction (XRD) as shown in Figure 1d. All the dominant decomposed Co 2p spectra involves the contribution of Co3+
diffraction peaks are well matched with (220), (311), (511), and Co2+ ions. Figure 2d shows the O 1s XPS spectra with the
(440) planes of Co3O4 (JCPDS No. 43-1003).22 The porous low BE peak at 530 eV attributed to the oxygen species in
structure of the NCs was checked as a typical type-IV isotherm Co3O4, and the high BE peak at 533 eV indicates the presence
with H3 hysteresis loop by the N2 adsorption-desorption of surface adsorbed H2O, O2 and CO2.25 Such a unique Co3O4
isotherm (Figure S1a).12,23 And it is confirmed to be the with (220) orientated nanocrystal and many oxygen vacancies
mesoporous structure. The Brunauer–Emmett–Teller (BET) may be beneficial to deliver the higher specific capacity and
surface area and the pore volume are calculated to be 5.1 m2 rate performance of LIBs.
g-1 and 0.045 cm3 g-1, respectively. Importantly, the BJH pore
size distribution reveals that the well-defined pores are with
diameters in about 19 nm, which presents the porous Co3O4
NCs (Figure S1b). The porosity not only provides extra active
sites for Li+ storage but also effectively promote mass diffusion
and ion transport in insertion/extraction process.24
Figure 2. a) EDS of Co3O4 NCs; b) TGA curves under N2 and air
of cobalt oleate precursor; c) Co 2p and d) O 1s XPS spectra of
Co3O4 NCs.
Next, the chemical composition of the Co3O4 NCs was
characterized by energy dispersive X-ray spectroscopy (EDS).
The main peaks of O, Co are determined in EDS file (other
peaks originated from the ITO substrate), indicating a pure
Co3O4 material (Figure 2a). In addition, Figure 2b shows the
Figure 3. Electrochemical performances of Co3O4 NCs electrode:
a) Cyclic voltammograms (CV) of Co3O4 NCs for five sequential
cycles at a scanning rate of 0.1 mV s-1; b) voltage profiles of
Co3O4 NCs electrode at a current density of 100 mA g-1; c)
cycling performance at a current density of 100 mA g-1 of
Co3O4 NCs.
thermal stability of precursor cobalt oleate investigated by
thermogravimetric analysis (TGA) curves in the temperature
range 25-800 °C with a heating rate of 10 °C under N2 and air
atmosphere, respectively. It can be clearly seen from the TGA
curve under N2 atmosphere (red) that the first weight loss
between 10-350 °C may correspond well to the removal of
adsorbed water. The complete decomposition temperature of
cobalt oleate is about 500 °C under an air atmosphere (black).
On the basis of the TGA data, the as-synthesized precursor was
first annealed at 350 °C with a high heating rate of 10 °C min-1
in N2 for 1 h, then at 600 °C in air for 3 h with a low heating
rate of 1 °C min-1 to synthesize the as-prepared Co3O4 NCs. To
further illustrate the chemical composition, the X-ray
photoelectron spectrum (XPS) was carried out. Figure 2c
displays two peaks at binding energy (BE) of 795.4 and 780.1
Subsequently, we investigated the electrochemical
properties of as-prepared Co3O4 NCs as anode materials for
lithium-ion batteries by a half-cell test. Figure 3a presents the
cyclic voltammograms (CV) curves of the first five cycles of NCs
electrode at a scanning rate of 0.1 mV s-1 over the potential
range 0.01-3.0V at room temperature. In the first scan, the
sharp cathodic peak at 0.75 V can be assigned to the reduction
of Co3+/Co2+ to Co0 and formation of a SEI layer and
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amorphous Li2O, while there is a broad anodic peak around good but slightly poor at high discharge rate (1000 mA g-1).
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2.05 V corresponding to the reversible oxidation of Co0 to With the purpose of further insightDOinI:t1o0.10t3h9e/C6eDxTc0e2l1le36nKt
Co3+/Co2+.24,26 In the second scan, the cathodic and anodic electrochemical performance of the Co3O4 NCs from the Co-
peaks have been shifted to 1.01 V and 2.04 V, respectively. We oleate complexes, the SEM and TEM images after 250 cycles at
should note that the decrease in the intensity of reduction a current density of 100 mA g-1 have been provided (Figure S3).
peaks may be ascribed to the formation of the irreversible SEI It is found that most of the nanocrystals have been well
film and reduction reaction.24 These sequential cycles present preserved, which could prove the stability of the nanocrystals
the similar integral areas, which demonstrate high reversibility structures. In this study, the synthetic procedure of Co3O4
of lithium storage during the cycling process. The electrode directly from cobalt oleate complex exhibits some
electrochemical reactions as follows:27
Co3O4+ 8Li+ + 8e- ↔ 3Co + 4Li2O
CoO + 2Li+ + 2e- ↔ Co + Li2O
important advantageous features over that of the previous
reports. First, the simple synthetic process is economical and
environmentally friendly; second, the Co3O4 NCs could be
readily achieved by multi-grams; third, the porous Co3O4
electrode could effectively promote the transport of lithium
ions and electrons to improve reversible capacity.4 The
(1)
(2)
The Galvanostatic discharge and charge curves of Co3O4 NCs reversible capacity (980 mA h g-1), the cycling performance and
for different cycles are shown at a current density of 0.1 A g-1 cycle life (250 cycles) are much better than those previously
in a voltage between 0.01 and 3.0 V (versus Li/Li+) in Figure 3b. reported, such as conductive materials/Co3O4 and some
For the first discharge cycle, we observe a long characteristic unique Co3O4 structures.13,39-44
plateau about 1.0 V corresponding to the conversion reaction
(1), which are in accordance with previous reports.28-29 Then
Conclusions
the following sloping region to the cutoff voltage (0.01 V)
should be ascribed to producing surface polymeric layer.28,30
In summary, we report a green and rapid synthesis of the
The NCs electrode could deliver the higher initial discharge and
charge capacities of 1674 mA h g-1 and 1021 mA h g-1,
respectively, than that of the previously reported Co3O4@TiO2
electrode (1279 mA h g-1).31 We infer that the irreversible
capacity loss should be resulted from the consumption of Li
ions to form SEI film.28,32
bare porous Co3O4 nanocrystals derived from cobalt oleate
precursor via calcination treatment in large scale. Then the
electrochemical performance of the nanocrystals could be
tested as anode materials for lithium ion batteries (LIBs),
which exhibit high reversible capacity of 980 mA h g-1 after 250
cycles at a current density of 100 mA g-1, and excellent cycling
performance. Little research has been reported about the
calcination of cobalt oleate complexes to form nano-sized Co(II)
oxides. More importantly, this facile synthetic route provides
an alternative opportunity for developing metal oleate
complexes as a promising crucial precursor for other metal
oxides or bimetal oxides with excellent electrochemical
performance.
In order to further test the cycling performance of Co3O4
NCs electrode as anode materials in LIBs, the subsequent
cycles were carried out at a current density of 100 mA g-1
between 0.01-3V. As shown in Figure 3c, it is presented that a
gradual decrease of the reversible capacity in the first 20
cycles is followed by a slow increase to 1007 mA h g-1 at the
140th cycle. And more importantly, the high reversible
capacity of 980 mA h g-1 is maintained to the 250th cycle with
99% Coulombic efficiency from the 25th to the 250th cycle
attributed to the stable architecture. The result discovers
excellent cyclability of the Co3O4 NCs compared with those
previous reports.2,31,33 The decreasing capacity in the first
20cycles may be resulted from the just started activation step
and the formation of some compounds (such as LixCo3O4)
derived from Li intercalation into Co3O4.34 The increasing
specific capacity presented is a common phenomenon for
Co3O4 electrode and also other nanostructured metal oxide
anodes such as Fe3O4, NiO, CoO, which may be resulted from
the reversible growth of a polymeric gel-like film ascribed to
kinetically activated electrolyte degradation.34-38 In order to
further comprehend the electrochemical performance, rate
performance of the Co3O4 electrode was carried out. And the
nanocrystals could deliver the average capacities of 534, 444,
314 and 203 mA h g-1 when cycled at current densities of 100,
200, 500 and 1000 mA g-1, respectively (Figure S2). Importantly,
the specific capacity is able to be returned to 671 mA h g-1 and
gradual increasing when the current density is back to 100 mA
g-1. The cycle performance at a current density of 100 mA g-1 is
Acknowledgements
H.W.G. acknowledges financial support from National Natural
Science Foundation of China (No. 21373006). Y.P.
acknowledges financial support from National Natural Science
Foundation of China (No. 51402203), the Natural Science
Foundation of Jiangsu Province for Young Scholars
(BK20140326), the Natural Science Foundation of Jiangsu
Higher Education Institutions (14KJB430021). The project was
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD).
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DOI: 10.1039/C6DT02136K
The bare porous Co3O4 nanocrystals derived from Co(II) oleate complexes under the calcination
treatment by a facile and green method, exhibit excellent electrochemical performance including
high reversible capacity of 980 mA h g-1 after 250 cycles at a current density of 100 mA g-1 as
anode materials for lithium-ion batteries.