Nickel-cobalt oxides/carbon nanoflakes as anode materials for lithium-ion batteries

Article abstract of DOI:10.1016/j.materresbull.2008.03.018

Novel nickel-cobalt oxides/carbon nanoflakes with Ni/Co molar ratio = 1:1 and 1:2 have been synthesized by a convenient hydrothermal method followed by a simple calcination process. X-ray diffraction results showed that the composites were composed of NiO

Full text of DOI:10.1016/j.materresbull.2008.03.018

Materials Research Bulletin 44 (2009) 140–145
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Nickel–cobalt oxides/carbon nanoflakes as anode materials for lithium-ion
batteries
b
b
a
a
Yanna NuLi a,b, , Peng Zhang , Zaiping Guo
, Huakun Liu , Jun Yang , Jiulin Wang
*
b,**
a Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
b Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
Novel nickel–cobalt oxides/carbon nanoflakes with Ni/Co molar ratio = 1:1 and 1:2 have been
synthesized by a convenient hydrothermal method followed by a simple calcination process. X-ray
diffraction results showed that the composites were composed of NiO, Co3O4, and carbon. Scanning
electron microscope measurements demonstrated that the composites were flakes less than 100 nm in
thickness, and the corresponding energy dispersive spectroscopy mapping showed that the carbon was
distributed homogeneously in the composites. The electrochemical results showed that the composite
electrodes exhibited low initial coulombic efficiency and excellent charge–discharge cycling stability.
Additionally, the effect of different Ni/Co molar ratios on the electrochemical properties of the
composites was investigated, and better performance was obtained for the sample with a Ni/Co molar
ratio of 1:2.
Received 21 November 2007
Received in revised form 3 March 2008
Accepted 13 March 2008
Available online 21 March 2008
Keywords:
A. Oxides
A. Nanostructure
C. X-ray diffraction
C. Electrochemical measurement
D. Electrochemical properties
ß 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, binary Ni–Co oxides have been developed as
alternative electrode materials for supercapacitors, and their
A worldwide effort has been made to search for materials with
larger capacities and better cycling performances in order to find a
better replacement for conventional graphite anode for lithium-
ion batteries [1,2]. It was found that transition-metal oxides (MO,
where M is Co, Ni, Cu, or Fe) can provide a large reversible capacity
(700 mAh/g), good cycling performance and high recharging rates
[3]. The reaction mechanism of these metal oxides with lithium
involves the reversible reduction and oxidation of metal (M)
nanosized particles dispersed into a lithia matrix (Li2O), followed
by the formation/decomposition of a solid electrolyte interface
(SEI). Although the average lithium removal voltage for transition-
metal oxides was above 1.0 V vs. Li, the Li-ion cells using them as
anode still can deliver 100% of their capacity at voltages >1.2 V, a
voltage anticipated to be the cut-off discharge voltage of
tomorrow’s electronics [4]. However, it has been found that the
electrochemical properties of the electrodes are restricted by the
large volume change, the serious aggregation or pulverization of
the active particles during charge–discharge, and the poor
conductivity of the transition metal oxides [5].
electrochemical properties in terms of oxygen evolution reaction
(OER) have also been studied widely in an alkaline medium [6].
Synergistic electrocatalytic behaviour has been found and depends
on the composition of the oxides [7]. Moreover, it has also been
found that the composition of binary Ni–Co oxides has effects on
* Corresponding author at: Department of Chemical Engineering, Shanghai Jiao
Tong University, Shanghai 200240, PR China. Tel.: +86 21 5474 5887;
fax: +86 21 5474 1297.
** Corresponding author. Tel.: +61 2 4221 5225; fax: +61 2 4221 5731.
E-mail addresses: nlyn@sjtu.edu.cn (Y. NuLi), zguo@uow.edu.au (Z. Guo).
Fig. 1. XRD patterns of as-prepared nickel–cobalt oxide/C powders for Ni/Co molar
ratio = (a) 1:1 and (b) 1:2.
0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2008.03.018

Y. NuLi et al. / Materials Research Bulletin 44 (2009) 140–145
141
their capacitive properties [8]. However, few scientists have inves-
tigated the electrochemical properties of binary Ni–Co oxides as
anode materials for lithium-ion batteries and the effects of the
composition of the oxides on the properties.
methodfollowedbycalcination. Poly(ethyleneglycol), atypicalnon-
toxic, non-immunogenic, non-antigenic, and protein-resistant
polymer reagent with long polymer chains, was employed as the
carbon source. Carbon in the composite provides a good conductive
matrix, which not only maintains the integrity of the electrode, but
also decreases the polarization, thus enhancing capacity retention. It
was found that the composition of binary oxides in the composites
was controlled easily and accurately, and the effect of different Ni/Co
molar ratios on their electrochemical properties as anode materials
for lithium-ion batteries was further investigated.
It has been reported that the preparation methods for binary Ni–
Co oxides include thermaldecomposition [9,10], solution deposition
[11], combinatorial sputtering deposition [12], single-target sputter
deposition [12], hydroxide co-precipitation [13], and so on. Binary
Ni–Co oxides/carbon nanotube (CNT) composite electrodes were
also prepared by the electrodeposition method [14]. However, it was
difficult to accurately control the composition of the binary oxides.
It is well known that the hydrothermal method is an idea
technique for processing very fine powders having high purity,
controlled stoichiometry, narrow particle size distribution, high
crystallinity, controlled morphology, etc. [15]. In the work reported
here, we describe an easy route to isochronously synthesize nickel–
cobalt oxides/C nanoflakes via a low-temperature hydrothermal
2. Experimental
2.1. Preparation of nickel–cobalt oxide/carbon nanoflakes
All the chemical reagents were analytically pure and used
without further purification. In a typical experimental procedure,
Fig. 2. SEM images of as-prepared nickel–cobalt oxide/C nanoflakes for Ni/Co molar ratio = (a), (b) 1:1 and (c), (d) 1:2 at different magnifications. TEM images of as-prepared
nickel–cobalt oxide/C nanoflakes for Ni/Co molar ratio = (e) 1:1 and (f) 1:2.

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Y. NuLi et al. / Materials Research Bulletin 44 (2009) 140–145
1 M Ni(NO3)2 (Aldrich) and 1 M Co(NO3)2 (Aldrich) (with the Ni/Co
molar ratio 1:1 and 1:2, respectively) were added dropwise
to equivalent molar number 1 M PEG-600 (Aldrich) methanol
solution under continuous stirring at room temperature to obtain a
homogeneous precursor solution. A stoichiometric proportion of
the precursor and NaOH (4 M) were added under stirring to a 125-
mL Teflon-lined autoclave, which was filled to one-fourth by
volume. Then, the autoclave was sealed and heated to 160 8C for
24 h. After the reaction, the autoclave was cooled down naturally.
The resulting products were washed with ethanol and distilled
water to ensure total removal of the inorganic ions, and then dried
under vacuum at 80 8C for 4 h as the intermediate product. After
that, the intermediate product was calcined at 300 8C for 2 h in
argon.
2.2. Sample characterizations
X-ray powder diffraction (XRD) analysis was conducted using
a Philips 1730 X-ray diffractometer with Cu K
˚
(l = 1.54056 A). Scanning electron microscopy (SEM) was per-
a radiation
formed using a JEOL JSM 6460A instrument with JEOL energy
dispersive spectroscopy (EDS) and EDS-X-ray mapping systems. A
JEOL 2011 200 keV transmission electron microscope (TEM) was
further employed to examine the morphology of the products.
Fig. 3. A SEM image of as-prepared nickel–cobalt oxide/C nanoflake for Ni/Co molar ratio = 1:1 (a) and the corresponding EDS mapping for the elements Ni (b), Co (c), O (d) C
(e) and the EDS spectrum (f).

Y. NuLi et al. / Materials Research Bulletin 44 (2009) 140–145
143
A typical slurry was obtained by grinding a mixture of active
material, carbon black, and poly(vinylidene fluoride) (PVDF)
dissolved in N-methyl-2-pyrrolidinone (NMP) with a weight ratio
of 70:15:15. It was then pasted onto a copper foil (1 cm2) to form
the electrode. After the electrode was dried at 100 8C for 4 h under
vacuum, it was compressed and then weighed. The electrochemi-
cal behaviour of the materials was examined via CR2025 coin-type
cells with lithium metal as the counter electrode, Celgard 2400
membrane as the separator, and 1 M LiPF6 in a mixture of ethylene
carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) as
the electrolyte. The cells were assembled in an argon-filled glove
box (Mbraun, Unilab, Germany). The charge–discharge measure-
ments were conducted at ambient temperature on a multi-channel
battery cycler in the voltage range between 0.01 V and 3.0 V at a
current density of 40 mA/g. Cyclic voltammetry (CV) measure-
ments were performed using a CHI instrument at a scanning rate of
1 mV/s.
Fig. 4. Cyclic voltammograms of the electrode made from nickel–cobalt oxide/C
nanoflake for Ni/Co molar ratio = 1:1.
3. Results and discussion
about 1.6 V and 2.4 V can be attributed to the reverse process
where metals are reoxidized to oxides and Li2O decomposes. In the
subsequent cycle, the reduction peak becomes broad and shifts up
to around 0.8 V, while the oxidation peaks remain at the same
potential with a slow decrease in peak intensity.
Fig. 5(a) and (b) shows the charge/discharge curves of nickel–
cobalt oxide/C nanoflake electrodes for Ni/Co molar ratios of 1:1
and 1:2, respectively. For both samples, the potential rapidly falls
and reaches a long voltage plateau at around 0.85 V in the first
discharge process, followed by a gradual decrease to 0.01 V,
798.5 mAh/g and 1058.2 mAh/g initial discharge capacity was
obtained, respectively. The first charge process exhibits a higher
and more sloping voltage profile, with two inconspicuous plateaus
Fig. 1(a) and (b) shows the X-ray diffraction patterns of the as-
synthesized powders for Ni/Co molar ratio = 1:1 and 1:2, respec-
tively. The diffraction peaks at 31.278, 36.268, 44.198 and 65.238 can
be indexed as the (2 2 0), (3 1 1), (4 0 0), and (4 4 0) crystal planes
of Co3O4 (JCPDS No. 09-0418), respectively. Peaks at 37.408, 43.508,
63.008 and 75.408 corresponds to the (1 1 1), (2 0 0), (2 2 0), and
(3 1 1) crystal planes of NiO (JCPDS No. 04-0835). The broad
diffraction peak at about 248 is related to the carbon. The results
indicate the products are composites composed of carbon and
nickel–cobalt oxides, with more NiO present for a Ni/Co molar ratio
of 1:1 and more Co3O4 for a Ni/Co molar ratio of 1:2.
Fig. 2(a)–(d) show SEM images of the as-prepared samples at
different magnifications for Ni/Co molar ratio = 1:1 and 1:2,
respectively. It can be seen that there are no obvious differences
between the two samples. The low-magnification views demon-
strate that the samples are composed of flakes embedded in a thin
silk-like material, and the flakes shown in the higher magnification
images are uniform, with an average diameter of about 0.5
mm and
a thickness of less than 100 nm. Fig. 2(e) and (f) further shows TEM
images of as-prepared samples for Ni/Co molar ratio = 1:1 and 1:2,
respectively. It can be seen that fine particles, which appear dark,
are distributed in a brighter matrix. Based on EDS analysis, it was
found that the fine particles are the oxides, while the brighter
matrix is carbon. Further high-resolution imaging is required to
fully characterize the distribution of carbon within the oxide
particles.
However, SEM-EDS analysis indicated
a uniform carbon
concentration, as shown in Fig. 3. Fig. 3(a) is a SEM image of the
sample with a Ni/Co molar ratio of 1:1, and Fig. 3(b)–(e) shows the
corresponding EDS mappings for the elements of Ni, Co, O, and C,
respectively. The bright spots correspond to the presence of each
element. It is obvious that the distribution of carbon in the
composite is homogeneous. It is reasonable to believe that the
oxides and carbon particles were thoroughly amalgamated. The
representative EDS analysis (Fig. 3(f)) indicates that the Ni/Co ratio
in the composite is 0.97, near the ratio of the reaction reagents,
while the carbon content is 11.56 mol%. The same measurement
was also conducted for the sample with the Ni/Co molar ratio of
1:2, and the results show that the Ni/Co ratio in the composite is
0.51 and the carbon content is as high as 20.10 mol%.
Fig. 4 shows the cyclic voltammetric curves of the nickel–cobalt
oxide/C nanoflake electrodes for a Ni/Co molar ratio of 1:1. A
reduction peak at about 0.2 V in the first cycle corresponds to the
decomposition of the oxides into Ni and Co, and the formation of
amorphous Li2O and the SEI. The two oxidation peaks located at
Fig. 5. Charge–discharge curves for the first three cycles of the electrodes made
from nickel–cobalt oxide/C nanoflakes for Ni/Co molar ratio = (a) 1:1 and (b) 1:2.

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Y. NuLi et al. / Materials Research Bulletin 44 (2009) 140–145
Table 1
Comparison of the electrochemical properties of nickel–cobalt oxide/carbon nanoflakes with Ni/Co molar ratio = 1:1 and 1:2 that were fabricated in this work with those of
NiO and Co3O4 with different shapes reported in the literatures
Samples
Current
density
Potential range
(V vs. Li/Li+)
Initial capacity
(mAh/g)
Initial coulombic
efficiency (%)
Capacity retention
Reference
Nickel–cobalt oxides/carbon
nanoflakes with Ni/Co molar ratio = 1:1
Nickel–cobalt oxides/carbon
nanoflakes with Ni/Co molar ratio = 1:2
Nanosized NiO
40 mA/g
3.0–0.01
798.5
82.1
75.0
555.8 mAh/g after 30 cycles
699.4 mAh/g after 30 cycles
This work
1058.2
14.36 mA/g
100 mA/g
25 mA/g
50 mA/g
C/5
3.0–0.01
3.0–0.02
3.0–0.01
3.0–0.01
3.0–0.01
3.0–0.01
3.0–0.01
3.0–0.01
About 1000
997.4
About 610
1300
About 65
About 400 mAh/g after 30 cycles
About 500 mAh/g after 30 cycles
200 mAh/g after 20 cycles
410 mAh/g after 30 cycles
913 mAh/g after 20 cycles
About 460 mAh/g after 30 cycles
550 mAh/g after 10 cycles
550 mAh/g after 25 cycles
[3]
NiO nanopowder
64.9
Low
–
[16]
[17]
[18]
[19]
[20]
[21]
[22]
NiO nanotube
Spherical NiO nanoshaft
Nanosized Co3O4
1411
34
–
Nanosize Co3O4 powders
Nanosized Co3O4
20 mA/g
0.1C
780
1380
67
–
Co3O4 microspheres
50 mA/g
–
and lower capacities of 655.2 mAh/g for a Ni/Co molar ratio of 1:1
and 793.5 mAh/g for a Ni/Co molar ratio of 1:2. It can be calculated
that the initial coulombic efficiency is 82.1% and 75.0%, respec-
tively. In the second discharge process, the voltage plateau appears
at a higher voltage of about 1.3 V, while the amplitude of the
plateau is reduced. A reversible capacity of 648.8 mAh/g and
792.5 mAh/g can be achieved, respectively. There are no obvious
differences in subsequent cycles.
charge–discharge. It has been proposed that mixed transition
metal oxide can react reversibly with a larger amount of lithium
[23] and exhibits improved electrode performance compared to
pure oxides, as a consequence of the synergistic effects of both
transition metal elements [24]. In this work, carbon further
provides a good conductive matrix, which not only maintains the
integrity of the electrodes, but also decreases the polarization, thus
enhancing the capacity retention. Moreover, the good interface
affinity between the oxides and the carbon particles ensures
structural stability during cycling and results in the excellent
electrochemical performance of the composites. The higher
amounts of Co3O4 and carbon lie behind the better electrochemical
performance for the composite with a Ni/Co molar ratio of 1:2.
Fig. 6 presents the cycling behaviour of the nickel–cobalt oxide/
C nanoflake electrodes for Ni/Co molar ratio = 1:1 and 1:2,
respectively. It can be seen that higher capacity and better cyclic
retention are obtained for the higher amount of Co3O4. The
capacity after 30 cycles is maintained at 555.8 mAh/g and
699.4 mAh/g, which is about 85.7% and 88.3% of the reversible
capacity, respectively. There is no serious capacity fading,
especially for the sample with the higher amount of Co3O4,
suggesting that no observable structural degradation of the
nanoflakes takes place during repeated cycling. For comparison
purposes, the electrochemical properties of nickel–cobalt oxide/
carbon nanoflakes with Ni/Co molar ratio = 1:1 and 1:2 that were
fabricated in this work and those of NiO and Co3O4 materials
reported in the literatures are summarized in Table 1. Although the
composite electrodes prepared in this work have slightly lower
initial capacity compared with some pure NiO or Co3O4 samples,
the initial coulombic efficiencies are higher than those of the
individual oxides, and the capacity retention is better than that of
NiO. As mentioned above, transition-metal oxides show poor
conductivity, and the electrodes suffer large volume change, with
serious aggregation or pulverization of active particles during
4. Conclusions
Nickel–cobalt oxide/C nanoflakes for Ni/Co molar ratio = 1:1
and 1:2 were successfully prepared by the hydrothermal method,
followed by calcination in argon at 300 8C for 2 h. At a current
density of 40 mA/g, 648.8 mAh/g and 792.5 mAh/g reversible
capacity can be obtained, and the capacity retention was 85.7% and
88.3% after 30 cycles, respectively. It is notable that the initial
coulombic efficiency reached as high as 82.1% and 75.0%,
respectively. The excellent electrochemical performance of the
nanoflakes could be mainly attributed to the high distribution of
oxide particles within the carbon matrix and the good interface
affinity between oxide and carbon particles, which resulted from
the in situ preparation of the oxides and carbon. The higher
amounts of Co3O4 and carbon lie behind the better electrochemical
performance for the composite with a Ni/Co molar ratio of 1:2.
Acknowledgement
This work was financially supported by the Australian Research
Council through a Linkage Project (LP0775456).
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