Multilayered cobalt oxide platelets for negative electrode material of a lithium-ion battery

Article abstract of DOI:10.1149/1.2987945

Layer-controllable CoO and Co₃O₄ platelets were prepared by calcination of hexagonal Β-Co (OH)₂, which was synthesized via a surfactant-free hydrothermal method. As negative electrode material for lithium-ion batteries, CoO and Co₃O₄ platelets demonstrated high reversible capacity (more than 800 mAhg for CoO and 600 mAhg for Co₃O₄) and excellent electrochemical cycling stability. The multilayered CoO platelets showed larger capacity and much better cycling performance than the monolayer CoO platelets and CoO nanoparticles. The effect of dimension and morphology of CoO particles on the electrode behavior was discussed.

Full text of DOI:10.1149/1.2987945

Journal of The Electrochemical Society, 155 ͑12͒ A903-A908 ͑2008͒
A903
0
013-4651/2008/155͑12͒/A903/6/$23.00 © The Electrochemical Society
Multilayered Cobalt Oxide Platelets for Negative Electrode
Material of a Lithium-Ion Battery
,
z
Wenli Yao, Jun Yang,* Jiulin Wang, and Yanna Nuli
Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240 China
Layer-controllable CoO and Co O platelets were prepared by calcination of hexagonal ␤-Co͑OH͒ , which was synthesized via a
3
4
2
surfactant-free hydrothermal method. As negative electrode material for lithium-ion batteries, CoO and Co O platelets demon-
3
4
strated high reversible capacity ͑more than 800 mAh/g for CoO and 600 mAh/g for Co O ͒ and excellent electrochemical cycling
3
4
stability. The multilayered CoO platelets showed larger capacity and much better cycling performance than the monolayer CoO
platelets and CoO nanoparticles. The effect of dimension and morphology of CoO particles on the electrode behavior was
discussed.
©
2008 The Electrochemical Society. ͓DOI: 10.1149/1.2987945͔ All rights reserved.
Manuscript submitted May 15, 2008; revised manuscript received August 20, 2008. Published October 7, 2008.
High lithium storage capacity, coulombic efficiency, and long
cycling life are still the major challenges for designing electrode
room temperature. The Co͑OH͒ gel precursor was then transferred
2
into a Teflon-lined autoclave with 75–80% filling ratio in argon and
hydrothermally treated at 120°C for 4 h without agitation. After the
autoclave was cooled down to room temperature, the ␤-Co͑OH͒2
product can be obtained by filtering and drying under vacuum at
1,2
materials for rechargeable lithium batteries. Although graphite-
based anode materials are widely used in commercial lithium-ion
batteries due to the excellent charge and discharge cycling behavior,
the theoretical Li-storage capacity of graphite is limited to
7
0°C. The corresponding CoO and Co O platelets were finally ob-
3 4
3
,4
3
72 mAh/g. Various materials have been intensively investigated
tained by calcining the ␤-Co͑OH͒ precursor respectively at 550°C
2
5-9
for the anode application. Transition metal oxides, such as cobalt
in Ar flow for 2 h and at 500°C in air for 2 h. The precursor for
CoO or Co O nanoparticles was prepared as follows: 4.47 g
oxides ͑CoO, Co O ͒, are attractive due to the high Li-storage ca-
3
4
5
,10-13
3
4
pacities about three times larger than those of graphite.
How-
Co͑NO ͒ ·6H O were dissolved into 100 mL of isopropyl alcohol-
3
2
2
ever, the large irreversible capacity in the first cycle and poor capac-
ity retention during charge and discharge cycling restrict their
water ͑1:1, v/v͒ solution in a three-necked round bottom. Then
20 mL of aqueous solution containing 2.4 g of NH4HCO3 was
added into the above solution and aged for 2 h to form the precursor
at room temperature under Ar. Finally, the precursor was obtained
by filtering and drying under vacuum at 70°C. The CoO and Co3O4
nanoparticles were obtained by calcining the dried precursor, respec-
tively, at 550°C in Ar flow and at 500°C in air for 2 h.
12-15
practical applications.
As reported, the electrochemical properties of as-synthesized
CoO and Co O strongly depended on their structures and
3
4
5
,8,10
morphologies.
For example, Co O4 nanotube synthesized via
3
the anodic aluminum oxide template showed the capacity about
8
5
00 mAh/g after 100 cycles at the current density of 50 mA/g. Lou
The resulting samples were analyzed by X-ray diffraction ͑XRD͒
on a Rigaku diffractometer D/MAX-2200/PC at a scanning rate of
et al. reported a one-step self-supported topotactic transformation
approach for synthesis of needlelike Co O nanotubes, which deliv-
3
4
5
°/min with a 2␪ ranging from 20–80°, using Cu K␣ radiation
ered high capacities ϳ1000 mAh/g for 30 cycles and 400 mAh/g
͑
1.5406 Å͒. Morphology of the powders was observed by scanning
14
after 80 cycles. Although these synthesized Co O nanotubes
3
4
electron microscopy ͑SEM͒. The prepared sample was mixed with
ethanol under sonication to form dispersion, and a small amount of
the dispersion was dropped on an aluminum sheet. After the solvent
evaporation, SEM micrographs were recorded with a JEOL field-
emission microscope ͑JSM-7401F͒. Specific surface area was deter-
mined by gas adsorption on ASAP 2010 M + C ͑Micromeritics Inc.
USA͒.
showed higher capacities and better capacity retention than those of
corresponding nanoparticles, their cycling performance should be
still further improved for the next-generation of lithium-ion batter-
ies. Thus far, there has been no report on the synthesis and applica-
tion of microcobalt oxides platelets for lithium-ion batteries.
In this study, we report the preparation of layer-controllable CoO
and Co O platelets through thermal decomposition of as-
3
4
Electrodes were fabricated by coating a slurry containing
synthesized ␤-Co͑OH͒ precursors. The as-synthesized ␤-Co͑OH͒
2
2
8
0 wt % active material, 10 wt % acetylene black, and 10 wt %
samples with different dimensions were synthesized by a “green”
hydrothermal route without organic solvents, toxic reagents, and
surfactants. As negative electrode material for lithium-ion batteries,
polyvinylidene fluoride binder dissolved in N-methyl-2-
pyrrolidinone, on a copper foil and drying under vacuum at 120°C
over 3 h. A typical electrode disk contained active material of
the resulting mesoporous CoO and Co O4 samples exhibit larger
−2
3
1
.0–1.5 mg cm . Electrochemical performance of the composite
capacity and much better cycle performance than CoO and Co O
3
4
materials was examined via CR2016 coin cells with lithium metal
counter electrode, Celgard 2700 membrane separator, and electro-
nanoparticles. The possible reason for the remarkable improvement
has been discussed.
lyte with 1 M LiPF dissolved in the mixture of ethylene carbonate
6
and dimethyl carbonate ͑DMC͒ ͑1:1, v/v͒. The test cells were as-
sembled in an argon-filled glove box containing Ͻ1 ppm each of
oxygen and moisture. Cyclic voltammogram measurements were
performed using a CHI660A electrochemical workstation at a scan-
ning rate of 0.2 mV/s. Galvanostatic charge ͑delithiation͒ and dis-
Experimental
All reaction reagents, purchased from Sinopharm Chemical Re-
agent Co. ͑China͒, are analytic grade reagents and directly used
without further treatment. In a typical reaction, 0.56–4.47 g of
Co͑NO ͒ ·6H O were dissolved into 100 mL of aqueous solution in
+
3
2
2
charge ͑lithiation͒ was cycled between 3 and 0.01 V vs Li /Li on
a three-necked round bottom. This fresh solution was stirred for
.5 h under an argon flow. Then 20 mL of aqueous solution contain-
ing 0.5–2.0 g of ammonia ͑25 wt % NH ·H O͒ was added into the
LAND CT2001A cycler at a current density of 100 mA/g at 25°C.
0
Results and Discussion
3
2
suspending solution and aged for 10 min to form the Co͑OH͒ gel at
The phase purity and crystallinity of CoO and Co O platelets
2
3
4
were characterized using XRD. Figure 1a shows the XRD pattern of
CoO samples. All the diffraction peaks at ͑111͒, ͑200͒, and ͑220͒ can
be indexed as the cubic symmetry of CoO phase ͑space group
Fm3hm, JCPDS card no. 48-1719͒. The clear diffraction peaks in
*
Electrochemical Society Active Member.
E-mail: yangj723@sjtu.edu.cn
z
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A904
Journal of The Electrochemical Society, 155 ͑12͒ A903-A908 ͑2008͒
Figure 1. XRD pattern of as-synthesized cobalt oxide platelets: ͑a͒ CoO and
b͒ Co O .
͑
3
4
Fig. 1b represent a typical character of the crystalline face-centered-
cubic Co O phase ͑space group Fm3hm, JCPDS card no. 78-1970͒.
3
4
No peaks related to ␤-Co͑OH͒ phase are observed from CoO or
2
Co O products, indicating the complete decomposition of hydrox-
3
4
ides under the corresponding experimental conditions.
The ␤-Co͑OH͒ precursor, synthesized from 0.128 M Co͑NO ͒
2
3 2
solution, can be converted into CoO or Co O under different cal-
3
4
cination conditions. Figure 2 exhibits SEM images of ␤-Co͑OH͒2,
Figure 2. SEM images of different samples: ͑a͒ ␤-Co͑OH͒2 precursor, ͑b͒
CoO, and ͑c͒ Co O .
CoO, and Co O4 platelets. A typical SEM image of the obtained
3
4
3
␤
-Co͑OH͒ sample shown in Fig. 2a indicates a multilayered hex-
2
Figure 3. N2 adsorption/desorption iso-
therm and BJH pore size distributions of
cobalt oxides: ͑a͒ CoO and ͑b͒ Co O .
3
4
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Journal of The Electrochemical Society, 155 ͑12͒ A903-A908 ͑2008͒
A905
with transition-metal oxides, such as cobalt oxides, mostly involves
a displacement redox reaction ͑widely called “conversion reaction”
in the lithium battery community͒ as follows
lithiation
CoO + 2Li
ꢀ
Co + Li O
͓1͔
2
delithiation
Figure 4. Cyclic voltammograms of cobalt oxide platelets: ͑a͒ CoO and ͑b͒
Co O , scan rate: 0.2 mV/s.
3
4
agonal platelet structure with average discal size of 15 ␮m diam and
␮m thick. The corresponding SEM images of the obtained CoO
and Co O samples ͑Fig. 2b and c͒ indicate that the original features
6
3
4
are fully preserved. The representative N absorption/desorption iso-
2
therms shown in Fig. 3 and the corresponding Barret–Joyner–
Halenda ͑BJH͒ pore size distribution curves ͑inset͒ reveal the poros-
ity of CoO and Co O4 platelets after thermal treatments for 2 h.
3
Both samples exhibit bimodal pore distribution with pore sizes of
about 2–4 nm range and average mesopore sizes of ϳ30 nm. In
contrast, the porous character of the ␤-Co͑OH͒2 precursor is not
obvious. Brunauer–Emmett–Teller ͑BET͒ surface areas of the corre-
sponding CoO and Co O platelets after calcinations process are
3
4
2
2
9
.88 and 9.32 m /g, respectively, much larger than 3.16 m /g for
the initial ␤-Co͑OH͒ samples.
2
Figure 4 shows cyclic voltammograms of electrodes made from
CoO and Co O platelets at a scan rate of 0.2 mV/s. In the first
3
4
cycle, a large cathodic current peak appears at 0.53 V for CoO and
at 0.82 V for Co O4 respectively, representing electrochemical
3
lithiation of these oxides. The cathodic peak at 0.5–0.80 V is also
partly related to the formation of a polymer/gel-like film, which
Figure 5. The electrochemical behaviors of the cobalt oxide electrodes at a
current rate of 100 mA/g: charge and discharge profiles of CoO platelets ͑a͒
and Co O platelets ͑b͒, the capacity-cycle number curves of platelike CoO
16,17
occurs below 0.8 V.
Anodic peaks at 2.08 and 2.06 V corre-
3
4
spond to the oxidation ͑delithiation͒ of CoO and Co O . As Poizot et
and Co O ͑c͒. Solid and hollow symbols in ͑c͒, respectively, represent
3
4
3
4
5,6
al. have proposed, the electrochemical reaction mechanism of Li
lithium insertion and extraction.
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A906
Journal of The Electrochemical Society, 155 ͑12͒ A903-A908 ͑2008͒
Figure 6. SEM images of ͑a͒ CoO-1
platelets, ͑b͒ CoO-2 platelets ͑c͒ CoO-3
platelets, and ͑d͒ CoO nanoparticles.
2
3-26
lithiation
terms of a heterogeneous interfacial storage mechanism,
where
Co O + 8Li
ꢀ
3Co + 4Li O
͓2͔
3
4
2
excess lithium is accommodated at the Co/Li O interface which is
formed in the course of the heterogeneous solid-state reaction. The
coulombic efficiencies for CoO and Co O platelets rapidly rise
2
delithiation
Compared to the first cycle, the main cathodic peak is shifted to
.26 V for CoO and 1.04 V for Co O4 in the second cycle. The
significant difference in the lithiation voltage may be related to the
3
4
1
3
from 74 and 75.4% in the first cycle to 99 and 90% in the second
one, respectively, and then an efficiency of mostly Ͼ98% is main-
tained in the following cycles for both the samples. Two possibilities
can explain why the charge capacity is lower than the discharge
capacity. First, side reactions such as electrolyte decomposition
could consume a small amount of lithium for the formation of the
interfacial film. Second, the formation nanodispersed metallic Co
during the discharge process is not completely transformed back into
CoO or Co O upon the following charge sweep. Even after
00 cycles, the reversible capacity of the CoO sample is still kept at
00.8 mAh/g ͑Fig. 5c͒. The capacity fade is rapid during initial
cycling for Co O , but its following cycles remain stable and the
capacity keeps at ca. 600 mAh/g for the 100th cycle. The multilay-
ered CoO platelets show larger capacity and better charge and dis-
charge performance than the multilayer Co O platelets.
changes in the CoO and Co O phase structure and the interfacial
3
4
property. The decrease in the following peak intensity and integral
areas results from the incomplete conversion reaction and partly
irreversible lithium loss for the formation of the polymer/gel-like
The peak feature of the third cycle is closer to that of the
second one, indicating that the electrochemical reversibility is
gradually set up after the initial cycle.
The electrochemical performance of as-synthesized CoO and
Co O platelets was investigated and compared. Figures 5a and b
show the discharge ͑lithiation͒ and charge ͑delithiation͒ profiles for
CoO and Co O platelets. The voltage trends are well indicative of
1
8,19
film.
3
4
1
8
3
4
3
4
3
4
5
,12
typical characteristics of CoO and Co O electrodes,
long voltage plateau at about 0.8 V for CoO ͑Fig. 5a͒ and 1.07 V for
Co O ͑Fig. 5b͒, followed by a sloping curve down to the cutoff
namely, a
3
4
3
4
3
4
The CoO platelets with different dimension in Fig. 6 are used as
electrode material for electrochemical lithium insertion and extrac-
tion. To study the dimension effect of reaction phase, CoO nanopar-
ticles in average size of 50 nm are compared ͑see Fig. 6d͒. In Fig. 6,
platelike CoO-1, CoO-2, and CoO-3 respectively, represent the CoO
samples synthesized from starting solutions containing 0.032, 0.064,
voltage of 0.01 V during the first discharge step. A small plateau at
.7 V for the first Li insertion into Co O in Fig. 5b is probably
1
3
4
2
0,21
related to the formation of the intermediate Li Co O phase.
The
x
3
4
first specific lithiation capacity is 1107 mAh/g for CoO and
1
134.4 mAh/g for Co O platelets, respectively, higher than their
3 4
theoretical values ͑715 and 890 mAh/g͒. The extra capacity may
result from the formation/dissolution of the polymer/gel-like
film.
and 0.128 M of Co͑NO ͒ . The average discal size and thickness of
3 2
1
0,18,19
Indeed, such polymers that have been intensively studied
CoO-1, CoO-2, and CoO-3 platelets are summarized in Table I. In
spite of the same calcinating temperature and time for the material
preparation, the cycling performance is considerably different for
the three samples. As shown in Fig. 7, the capacity turns to be stable
turn out to form from a multistep degradation mechanism including
22
electrochemical and chemical processes. Moreover, the excess re-
versible capacity in the sloped regime has also been explained in
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Journal of The Electrochemical Society, 155 ͑12͒ A903-A908 ͑2008͒
A907
2
8
5
0 cycles ͑not shown here͒. A similar result related to nanosized
Table I. Average discal size and BET surface area of different
cobalt oxide materials.
1
2
CoO and Co O electrodes was also observed by Wang et al.
3
4
It has been revealed that the electrochemical lithiation of CoO
Average
diameter
͑␮m͒
Average
thickness
͑␮m͒
BET
leads to the formation of Li O and nanodispersed metallic Co, along
2
1
8,19
surface area
with the formation of the interfacial polymer/gel-like film.
More
2
Sample
͑m /g͒
finely divided active phase will result in more formation of polymer/
gel-like film. The formation of this special film eventually consumes
some charges and electrolyte solvent for the polymerization, and
simultaneously the outer layer of the active phase may take part in
side reactions due to highly reactive nanodispersed Co, leading to
the irreversibility. It has been reported that a fast capacity fading
would be favored by small particles or agglomerates due to poor
electronic conductivity of the metal-electrolyte interphase and the
CoO nanoparticles
CoO-1
CoO-2
0.035
2
8
15
15
—
0.1
2
6
6
20.13
15.09
11.36
9.88
CoO-3
Co O
9.32
3
4
2
7,29
lost contact between nanograins.
Another disadvantage of nano-
after the first cycle for the platelike CoO-3 electrode but declines
noticeably within 10 cycles for platelike CoO-2. Particularly, rapid
capacity drop occurs within 20 cycles for CoO-1 platelets and CoO
nanoparticles. With the increase of the dimension for CoO platelets,
the electrode behavior becomes better. The multilayered CoO plate-
lets with average discal size of 15 ␮m diam and 6 ␮m thick pre-
sents larger capacity and much better charge and discharge perfor-
mance than the monolayer CoO platelets and CoO nanoparticles.
Similarly, the great influence of the particle size of 3d-metal oxides
on the electrochemical performances was also proved by the experi-
sized active particles for electrode application is the particle aggre-
gation driven by electrochemical insertion/extraction reactions.
5
,30
For these reasons, the nanodivided CoO granularity is unfavorable
for the electrochemical stability. However, in view of the volume
effect of CoO during lithiation/delithiation and the limited reaction
kinetics, a simple enhancement of the reactive phase dimension can-
not improve the electrode performance. The platelike CoO in a suit-
+
able size not only ensures a short Li diffusion depth, but also sup-
plies a balance for the incompatible two sides mentioned above. In
addition, the mesoporous CoO platelets ͑Fig. 8a͒ can accommodate
part of the volume expansion on lithium insertion by means of the
free space inside and thus ensure the structural integrity of CoO
electrode over many discharge–recharge cycles. These may explain
the effect of the dimension and morphology of CoO particles on the
electrode behavior. As shown in Fig. 8b, the original dimension and
morphology of the CoO platelet can be maintained even after
27
ments carried out on copper oxides. As for the multilayered Co O
3
4
platelets, the electrode can remain stable with the capacity of ca.
6
00 mAh/g for 100 cycles ͑see Fig. 5c͒. By contrast, nano-Co O
3 4
electrode shows very high capacity of ϳ1000 mAh/g for initial
cycles, but the capacity decreases rapidly to 300 mAh/g after
5
0 cycles. Note that, for these SEM experiments, the delithiated
CoO platelet electrodes were taken out from cells, washed with
DMC, and then dried in an argon dry box. A similar effect of the
dimension and morphology on the electrochemical performance has
also been observed for Co O samples.
3
4
Conclusions
Layer-controllable CoO and Co O platelets can be prepared by
3
4
thermal treatment of highly crystalline ␤-Co͑OH͒ templates, which
2
was synthesized by a surfactant-free hydrothermal method. The
electrochemical reversibility of the CoO and Co O reactants toward
3
4
lithiation and delithiation is strongly dependent on the dimension
and morphology. The multilayered CoO sample presents larger ca-
pacity and much better charge and discharge performance than the
monolayer CoO platelets and CoO nanoparticles. In addition, the
CoO platelet is superior to the corresponding Co O in the electro-
3
4
chemical reversibility. The mesoporous and multilayered CoO
sample with average discal size of 15 ␮m diam and 6 ␮m thick
delivers a stable specific capacity of 800 mAh/g for a life extending
Figure 7. Capacity-cycle number curves of platelike CoO-1, CoO-2, CoO-3,
and CoO nanoparticles. Solid and hollow symbols, respectively, represent
lithium insertion and extraction.
Figure 8. SEM images of ͑a͒ a single
CoO platelet and its enlarged one ͑inset͒,
and ͑b͒ CoO platelets after 50 cycles.
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A908
Journal of The Electrochemical Society, 155 ͑12͒ A903-A908 ͑2008͒
1
1
3. Y. Liu, C. Mi, L. Su, and X. Zhang, Electrochim. Acta, 53, 2507 ͑2008͒.
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1
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This work was supported by National 973 Program ͑grant no.
1
1
2
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͑
Shanghai Jiao Tong University assisted in meeting the publication costs
5
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