Synthesis of mesoporous NiO nanospheres as anode materials for lithium ion batteries

Article abstract of DOI:10.1016/j.electacta.2012.06.107

In this work, three-dimensional mesoporous NiO nanostructures have been synthesized by a simple ethylene glycol (EG)-mediated self-assembly route and subsequent calcination process. The synthesized nickel alkoxide precursors annealed at 300 and 500 °C exh

Full text of DOI:10.1016/j.electacta.2012.06.107

Electrochimica Acta 80 (2012) 140–147
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis of mesoporous NiO nanospheres as anode materials for lithium ion
batteries
Guanhua Zhang, Yuejiao Chen, Baihua Qu, Lingling Hu, Lin Mei, Danni Lei, Qing Li, Libao Chen,
Qiuhong Li∗, Taihong Wang
Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, and State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha
410082, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, three-dimensional mesoporous NiO nanostructures have been synthesized by a simple
ethylene glycol (EG)-mediated self-assembly route and subsequent calcination process. The synthe-
sized nickel alkoxide precursors annealed at 300 and 500 ◦C exhibit different surface area, crystallinity
and pore distribution, which have been characterized by scanning electron microscopy, transmission
electron microscopy, selected area electron diffraction, X-ray diffraction, Fourier transform infrared spec-
troscopy and Nitrogen adsorption/desorption isotherms. The electrochemical properties of these NiO
mesoporous nanostructures are investigated including the cycling and rate performance as anode mate-
rials for lithium-ion batteries. It is indicated that mesoporous NiO nanospheres synthesized at 500 ◦C
exhibit better electrochemical performance than that obtained at 300 ◦C. The NiO nanospheres annealed
at 500 ◦C present a reversible specific capacity of 518 mAh g−1 at a current density of 0.1 A g−1 after 60
cycles. With varying the rate from 0.1 to 8.0 A g−1, the capacity remains at 535 mAh g−1 at 2 A g−1 after 30
cycles and resumes to 582 mAh g−1 at 0.1 A g−1 after 60 cycles. The results indicate that our mesoporous
NiO nanospheres are promising anode materials for lithium ion batteries.
Received 16 April 2012
Received in revised form 28 June 2012
Accepted 29 June 2012
Available online 6 July 2012
Keywords:
Nickel oxide
Nanospheres
Lithium-ion batteries
Anode material
© 2012 Elsevier Ltd. All rights reserved.
1. Introduction
properties [21–23]. The improvement profits from their easy acces-
sibility for the electrolyte (facilitating Li+ ions transportation),
Lithium ion batteries (LIBs), a fast-developing technology in
electric energy storage, are the dominant power sources for a
wide range of portable electronic devices [1–3]. Great efforts are
underway to develop new electrode materials and new technolo-
gies for next generation LIBs. Recently, transition metal oxides
(such as Co3O4, Fe3O4 and NiO) have been extensively investigated
as alternative anode materials for LIBs owing to high theoret-
ical capacity (700–1000 mAh g−1) based on a novel conversion
mechanism [4–8]. Among them, NiO has attracted considerable
attention due to its high theoretical capacity (∼718 mAh g−1), low
cost, natural abundance and environmental friendliness. It has been
shown that the electrochemical performance of NiO nanostruc-
tures largely depends on their morphology and surface properties
[9–13]. Up to date, various NiO nanostructures have been used
as anode materials including nanoparticles [14], nanotubes [15],
hollow nanostructures [16,17], nanosheet assembled microspheres
[18,19], and nanoflakes [20].
which can accelerate phase transition and restrain the crumbling
and cracking of the electrode, offering more connecting opportu-
nities and positions with the electrode. In addition, the 3D porous
structures with large surface-to-volume ratio and short diffusion
length for lithium intercalation can lead to superior cycling and
rate performance as electrode materials [23,24]. Thus the design
and preparation of new electrode materials with 3D porous nanos-
tructures will provide a tremendous opportunity to improve their
electrochemical properties.
In this work, we report a simple and efficient approach to
prepare nickel oxide precursors through an EG-mediated self-
assembly process. Such a method has been adopted previously
for the preparation of hollow NiO spheres [16], flowerlike CeO2
[25], and SnO2 nanowires [26]. However, in these previous studies,
surfactants (such as polyvinylpyrrolidone) or other additives (for
instance tetrabutylammonium bromide and urea) are necessary in
the synthetic procedure; in this study we successfully synthesize
nickel alkoxide without any use of surfactants, or additives. Fur-
thermore, by calcinating at different temperatures, the as-obtained
nickel alkoxide is transformed to 3D mesoporous structured NiO.
The electrochemical properties of the NiO nanospheres are also
investigated as anode materials for lithium ion battery. The ini-
tial discharge capacities of NiO annealed at 300 ◦C and 500 ◦C are
Three-dimensional (3D) porous nanostructures have been
proved to be effective in improving their electrochemical
∗
Corresponding author. Tel.: +86 731 88822137; fax: +86 731 88822137.
E-mail address: liqiuhong2004@hotmail.com (Q. Li).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.06.107

G. Zhang et al. / Electrochimica Acta 80 (2012) 140–147
141
1026 and 1220 mAh g−1 at a current density of 100 mA g−1, respec-
tively. However, the NiO annealed at 300 ◦C exhibits a value of
222 mAh g−1 after 60 cycles, while the NiO annealed at 500 ◦C
shows better electrochemical performance of 518 mAh g−1. It indi-
cates that the porous NiO nanospheres annealed at 500 ◦C have
potential applications in lithium ion batteries.
2.2. Characterization
The as-prepared products were characterized by a powder X-ray
diffraction (XRD, Siemens D-5000) with Cu K␣ (ꢀ = 0.15418 nm).
The morphology of the synthesized samples was examined using
field-emission scanning electron microscopy (SEM, Hitachi S-4800)
and transmission electron microscopy (TEM, JEOL-2010) operated
at an accelerating voltage of 200 kV. Specific surface area and poros-
ity analysis for the samples were performed through measuring
Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption
isotherms with a Micromeritics ASAP2020 apparatus. Fourier trans-
form infrared (FT-IR) spectra were recorded for a KBr diluted
sample using a bruker vertex 70 spectrometer with a resolution
2. Experimental
2.1. Synthesis
All chemical reagents utilized in the study were of analytical
grade and used without further purification. In a typical prepara-
tion process, 5 mmol Ni(CH3COO)2·4H2O was dissolved in 100 mL
ethylene glycol under stirring. After 30 min, the obtained green
transparent solution was transferred into a 250-mL round flask
and then refluxed at 170 ◦C for 10 h with constant magnetic stir-
ring. After cooling to room temperature, the green precipitate was
collected by centrifugation, washed with deionized water and abso-
lute ethanol several times, and dried in an oven at 60 ◦C overnight.
Finally, the products were annealed at 300 and 500 ◦C for 4 h, here-
inafter designated as S-300 and S-500, respectively.
of 4 cm−1
.
2.3. Electrochemical measurements
A CR2025-type coin cell was assembled to investigate the elec-
trochemical properties of the products. The working electrodes
were prepared by mixing 80 wt% active material, 10 wt% acetylene
black (Super-P), and 10 wt% binder (LA133 and CMC) in distilled
water and absolute alcohol mixture. After coating the above slurries
Fig. 1. (a) XRD patterns of (I) as-synthesized NiO precursor, (II) S-300, (III) S-500; (b) SEM image of as-synthesized NiO precursors; (c, d) SEM image and magnified SEM
image of S-300, respectively; (e, f) SEM image and magnified SEM image of S-500, respectively.

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G. Zhang et al. / Electrochimica Acta 80 (2012) 140–147
Fig. 2. (a, b) TEM images of S-300 and S-500, respectively; (c, d) HR-TEM images of S-300 and S-500, with corresponding SAED patterns (inset), respectively.
on a copper foil, the electrodes were dried at 80 ◦C under vacuum
to remove the solvent. The cells were assembled in an argon-filled
glovebox with water and oxygen contents less than 1 ppm. A Cel-
gard 2400 microporous polypropylene membrane was used as a
separator. The electrolyte consisted of a solution of 1 M LiPF6 in
ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1,
in weight percent). Pure lithium foil was used as a counter elec-
trode. The discharge and charge measurements were carried out
on an Arbin BT2000 system with the cut off potentials being 0 and
3 V at different current densities from 0.1 to 8.0 A g−1. The CV exper-
iment was also tested on the Arbin battery test system at a scan rate
a basic shape of the precursor but the surface of the nanospheres
became rough, as shown in Fig. 1c. The high magnification SEM
image (Fig. 1d) reveals large quantities of grains which are agglom-
erated loosely on the surface of nanospheres. The morphology of
S-500 is displayed in Fig. 1e, which also exhibits basic shape of the
precursor but the size of spheres decreases. The nanospheres are
porous and constructed by aggregation of a lot of nanoparticles with
an average size about 20 nm as shown by the high magnification
SEM image (Fig. 1f).
The detailed morphological and structural features of the pow-
ders are also examined by TEM. Fig. 2a shows the TEM image of
S-300. It can be seen that the nanosphere consists of nanocrystals
with diameters of 5–10 nm, which arrange randomly and connect
with each other. Many pores can be detected at the edge of the
nanosphere. Fig. 2b shows the TEM image of S-500, the diameters of
the nanocrystals along with the nanopores grow larger, because of
the fusion between the nanocrystals at a higher annealing temper-
ature. In addition, the high resolution TEM (HR-TEM) image (Fig. 2c
and d) of an individual NiO nanosphere indicates that there are two
types of lattice fringes with lattice of 0.24 and 0.21 nm, correspond-
ing to the (1 1 1) and (2 0 0) crystalline plane of face-centered cubic
NiO. The selected area electron diffraction (SAED) pattern (the inset
of Fig. 2c and d) shows several Debye–Scherrer rings, corresponding
to the reflections of the NiO polycrystals.
For further investigation of the chemical composition of the as-
prepared NiO precursor, the FT-IR measurement was carried out.
Fig. 3 shows the IR spectroscopic spectrum of the precursor. The
vibrational bands of CH2 in the range of 2850–2950 cm−1 as well as
the stretching band of C O at 1061 cm−1 derived from the ethylene
glycol unit are observed. Simultaneously, the strong absorption
bands of C O in acetate ions at 1590 cm−1 are also detected. Thus,
it can be concluded that the as-synthesized nickel oxide precursor
is probably a kind of nickel alkoxide and that the structure might
be similar to those of nickel alkoxide and cerium alkoxide [16,25].
of 0.5 mV s−1
.
3. Results and discussion
Fig. 1a shows the X-ray diffraction (XRD) patterns of the as-
obtained nickel oxide precursor and the thermally decomposed
products at 300 and 500 ◦C. The XRD pattern of the nickel alkoxide
shows a strong low-angle reflection with characteristically strong
peaks at around 10◦. Such XRD features are also found on several
other EG-mediated metal glycolates. It was considered to be a typ-
ical feature from the coordination and alcoholysis of EG with the
center metal ions [25–28]. The XRD pattern of the calcined prod-
uct matches well with the single phase of face-centered cubic NiO
(JCPDS No. 47-1049). Furthermore, with increasing the calcination
temperature, the intensities of the diffraction peaks become intense
and sharp, which indicates that NiO nanoparticles obtained at a
higher calcination temperature are better crystallized with larger
crystallite sizes. Fig. 1b shows the typical SEM image of the nickel
oxide precursor. It is composed of many irregular spheres with a
diameter of 200–500 nm. The surface of the nanospheres is smooth
and no isolated nanoparticles can be detected. When the precur-
sor was calcined at 300 ◦C, the morphology of S-300 can maintain

G. Zhang et al. / Electrochimica Acta 80 (2012) 140–147
143
The textural properties of the NiO samples were further
investigated by nitrogen sorption and desorption isotherms and
corresponding pore size distribution (inset) are shown in Fig. 5. It
can be seen that the specific surface areas are calculated using the
BET method and are determined to be 80.5 and 15.8 m2 g−1 for S-
300 and S-500, respectively. When it comes to the temperature of
300 ◦C, the pore size distribution reveals with mean diameter of
26 nm (Fig. 5a, inset). With the annealing temperature increasing
to 500 ◦C, crystallites grow larger via thermal re-crystallinzation.
Meanwhile, some small mesopores gradually heal and diminish,
but some other mesopores may gradually merge and become big-
ger. As a result, the inter-crystallite pores become larger, the pore
size distribution reveals a bimodal nature with mean diameters
of 32 and 48 nm (Fig. 5b, inset). The porous structure cannot only
benefit the diffusion of solid-state Li-ion [34–38], but also accom-
modate volume changes of charge/discharge process to maintain
the structure integrity [39,40].
In order to investigate the lithium storage properties, the NiO
was used as positive electrode with a standard NiO/Li half-cell
configuration. Fig. 6 shows the charge–discharge curves for the
1st, 2nd, 10th, 30th and 60th cycle. The corresponding discharge
capacities of S-300 are 1026, 848, 611, 346, 222 mAh g−1, respec-
tively. Those discharge capacities of S-500 are up to 1220, 866, 772,
586, 518 mAh g−1, respectively. In general, these charge–discharge
curves present similar profiles. The initial discharge shows a clear
potential plateau at ∼0.6 vs. (Li/Li+)/V followed by a sloping curve,
which corresponds to the following convention reaction:
Fig. 3. FT-IR spectra of as-synthesized NiO precursor.
Based on the above results, we elucidate a plausible mechanism
responsible for the formation of NiO nanospheres (Fig. 4). Dur-
ing the synthesis process, EG first coordinated with Ni(CH3COO)2
to produce nickel alkoxide, which precipitated to become the
nuclei [16,29]. In the following secondary growth stage, the nickel
alkoxide underwent several steps of intermediate reactions and
eventually led to the formation of longer chains, which could
further self-assemble into nanospheres through van der Waals
interactions and hydrogen bonds [25,29]. Ultimately, the obtained
NiO precursor was thermally decomposed to NiO through anneal-
ing in air. Such a process is consistent with previous reports
of a so-called two-stage growth followed by a slow aggrega-
tion and crystallization of primary particles [16,25,29–33]. Further
work is underway to investigate the detail of the self-assembly
growth mechanism. According to the previous papers [16,25,33],
the related reaction equations can be described as follows:
NiO + 2Li → Ni + Li2O
(3)
The sloping part in the end of the discharge curve between
0.6 and 0 V corresponds to the formation of the solid electrolyte
interface (SEI) layer [41,42]. The charge and discharge plateaus are
not obvious since the second cycle. There are two slops around
1.6 and 2.3 V in each charge curve, and the slop in each discharge
curve is around 1.3 V. According to Fig. 6, it is apparent that the
charge/discharge performance and cycleability of S-500 are bet-
ter than S-300. In addition, the discharge capacity of S-500 after
60 cycles is 518 mAh g−1, which is about 1.5 times that of current
carbon-based electrode materials. This improved cycling perfor-
mance might be attributed to the robust mesoporous structure and
improved crystallinity [43–45]. The initial discharge capacities of
S-500 and S-300 are 1220 and 1026 mAh g−1. Both of them are sig-
nificantly higher than the theoretical capacity. The extra capacities
are mainly due to the decomposition of non-aqueous electrolyte
during the discharge process [46].
Ni(CH3COO)2 + HO(CH2)2OH → (CH3COO)Ni(O(CH2)2OH)
+ CH3COOH
(1)
(2)
2(CH3COO)Ni(O(CH2)2OH) + 9O2 → 2NiO + 8CO2 + 8H2O
The role of acetate was found to be very critical in this synthesis
method. In a control experiment, when nickel acetate was replaced
by nickel sulfate under the same reaction conditions, no apparent
precipitate was obtained. In our experiment, when EG served as a
ligand and coordinated with Ni2+ to form nickel alkoxide, it would
be associated with producing acid (H2SO4 or CH3COOH). Sulfuric
acid is a vigorous proton acid, the accumulation of H+ would inhibit
further nickel alkoxide formation with the elongated time. Nev-
ertheless, acetic acid is more difficult to ionize, thus allowing the
coordination reaction to be completed.
The lithium storage capacities of porous NiO nanospheres were
determined by galvanostatic charge/discharge cycling at the cur-
rent density of 100 mA g−1. As shown in Fig. 7, the capacity of S-300
decreases sharply as the cycle number increases. It can be seen that
the discharge capacity of S-500 is maintained at ∼447 mAh g−1 after
80 cycles, while S-300 exhibits a value of ∼148 mAh g−1. Poizot
Fig. 4. Schematic illustration of the preparation of the NiO nanospheres.

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G. Zhang et al. / Electrochimica Acta 80 (2012) 140–147
Fig. 5. N2 adsorption/desorption isotherm curve of NiO nanospheres and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset): (a) S-300 and (b) S-500.
et al. suggest that for each metal oxide system there is an opti-
mum particle size and hence the best electrochemical performance
[8,9]. It can be inferred that several factors (surface area, grain size,
crystallinity and structure stability) are competing with each other,
which may affect the Li-ion storage performance [43,47,48]. In the
previous reports, Zhu et al. summarized that the optimal grain size,
crystallinity and morphology codetermined the anode material’s
property [47]. Yan et al. also reached a similar conclusion and fur-
ther pointed out that well-crystalline material could maintain the
nanosize crystallite and activate the decomposition of Li2O, thus
increasing the discharge capacity [43]. For samples annealed at a
low temperature, their specific surface area is high but their crys-
talinity is poor. In this case, the diffusion of the Li in the grain is
not efficient. Therefore, the electrochemical enhancement of S-500
might be attributed to its appropriate grain size and robust porous
structure with improved crystallinity. In order to further explain
the electrochemical processes, the materials after 20 cycles have
been characterized by SEM. Fig. 8a and b shows the SEM images
of the resultant materials after cycling, respectively. By comparing
SEM images before and after cycling of the electrode, the spher-
ical structure maintains basically after cycling. Nevertheless, the
agglomeration and volume expansion of S-300 are more obvious,
which might be attributed to the reason that the grain size and pore
structure changed significantly during repeated charge/discharge
processes. To further understand the detailed influences of these
factors on the electrochemical performance, more in-depth inves-
tigations are still underway.
Compared with the reported performances of nanotube [49],
net structure [50], flower-like microspheres [18], nanosheet-based
microspheres [51], and spheres [52], the cyclic stability of our sam-
ple synthesized at 500 ◦C is much enhanced, as shown in Table 1.
As we all know, the diffusion length for lithium ion and electron
is especially important for cycling. What is more, the 3D porous
structures with large surface-to-volume ratio and short diffusion
length for lithium intercalation can lead to superior cycling and
rate performance of electrode materials [23,24]. For our structure,
the porous structure provides space for volume expansion during
Fig. 6. Galvanostatic lithium-insertion/extraction curves of NiO electrodes between
Fig. 7. Charge/discharge capacities vs. cycle number of NiO nanospheres at the same
0 and 3.0 V at a current density of 100 mA g−1: (a) S-300 and (b) S-500.
current density of 100 mA g−1 between 0 and 3.0 V.

G. Zhang et al. / Electrochimica Acta 80 (2012) 140–147
145
Fig. 8. SEM images of the electrodes after 20 cycles between 0 and 3.0 V at a current density of 100 mA g−1: (a) S-300 and (b) S-500.
lithiation. Moreover, these grains enlarge and adhere together to
form a compact layer, which have high stability and preserve the
integrity of the structure. Therefore, we propose that high stabil-
ity of the mesoporous structure is responsible for the improved
performances.
Fig. 9 shows the representative discharge/charge voltage pro-
files of the electrodes with the current densities ranging from
100 mA g−1 (0.14 C, 1 C = 718 mA g−1) to 8000 mA g−1 (11.2 C). With
increasing the discharge/charge rate, the discharge potential
decreases and the charge potential increases due to kinetic effects
of the material, rendering higher overpotential [53]. The rate capa-
bility of the two samples was demonstrated in Fig. 10. They have the
similar decreasing trends for the capacity along with the increas-
ing current rates. However, the capacity of cells with S-300 NiO
anode dropped more rapidly than that of S-500 NiO anode. The
capacity difference between these two samples is reduced at the
high current density (8000 mA g−1), which can be ascribed to the
structure collapse at higher current rates, making the gap between
them minish. When the current rate resumes to 0.14 C, it is impres-
sive to observe a specific storage capacity of 582 mAh g−1 for S-500
after 60-cycle discharge, while only 310 mAh g−1 is obtained for
S-300. The stark contrast between the two samples demonstrates
that mesoporous NiO spheres assembled by nanoparticles with bet-
ter crystallinity are suitable for electrodes of LIBs. As compared to
NiO net-structure and NiO hollow microspheres [15,50], the porous
NiO nanospheres exhibit better rate capability. Such considerable
improvement is believed to be caused by the high stability of the
unique 3D porous structure and shorter Li+ diffusion length.
The cyclic voltammetric (CV) curves of S-500 were tested over a
potential range from 0 to 3.0 V at a scan rate of 0.5 mV s−1. As shown
in Fig. 11, in the first cathodic scan, a large cathodic peak starts at
0.22 V corresponding to the initial reduction of Ni2+O to metallic
Ni, accompanying the formation of Li2O. During the first anodic
scan, two oxidation peaks are found at 1.63 and 2.26 V. The weak
Fig. 9. Representative discharge/charge voltage profiles of (a) S-300 and (b) S-500
peak at 1.63 V should correspond to the dissolution of the organic
SEI layer [53,54], and the strong peak at 2.26 V corresponds to the
at various current densities.
Table 1
Comparison of the electrochemical properties of NiO synthesized at 500 ◦C in this work with those of NiO reported in the literature.
Sample
Current density
Potential range
(V vs. Li/Li+)
Initial capacity
Capacity retention
(mAh g−1
Ref.
(mA g−1
)
(mAh g−1
)
)
NiO nanosphere
NiO nanotube
Net-structured NiO
Flower-like NiO
Nanosheet-based NiO
Spherical NiO
100
25
71.8
50
50
100
0.0–3.0
0.01–3.0
0.02–3.0
0.01–3.0
0.01–3.0
0.01–3.0
1220
∼610
1190
1276
1570
1147
518 after 60 cycles
200 after 20 cycles
178 after 40 cycles
219 after 40 cycles
100 after 30 cycles
367 after 50 cycles
This work
[49]
[50]
[18]
[51]
[52]

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G. Zhang et al. / Electrochimica Acta 80 (2012) 140–147
Furthermore, it is also noteworthy that this work may provide a
simple and efficient approach for the large-scale synthesis of other
3D complex metal oxide nanomaterials.
Acknowledgements
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Grant Nos. 21003041,
21103046), and Hunan Provincial Natural Science Foundation of
China (Grant Nos. 11FH003, 10JJ1011).
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NiO formation, accompanying the decomposition of Li2O [53,55].
During the anodic process, both the peak current and the inte-
grated area of the anodic peak are decreased, indicating capacity
loss during the charging process. In the subsequent 2nd and 3rd
cycles, the cathodic peak potentials shift to 1.02 and 1.03 V, respec-
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,
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