Hollow microspheres of NiO as anode materials for lithium-ion batteries

Article abstract of DOI:10.1016/j.electacta.2010.08.039

NiO hollow spheres are prepared by heating the NiCl₂/resorcinol- formaldehyde (RF) gel in argon at 700 °C for 2 h, and subsequently in oxygen at 700 °C for 2 h. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are employed to characterize the structure and morphology of the as-prepared NiO hollow spheres. These hollow spheres have a diameter of about 2 μm, which are composed of NiO particles of about 200 nm. The electrochemical properties of these NiO hollow spheres are investigated to determine the reversible capacity and cycling performance as anode materials for lithium-ion batteries, and the advantages of their hollow spherical morphology to the electrochemical performance are discussed.

Full text of DOI:10.1016/j.electacta.2010.08.039

Electrochimica Acta 55 (2010) 8981–8985
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Hollow microspheres of NiO as anode materials for lithium-ion batteries
X.H. Huanga,∗, J.P. Tub, C.Q. Zhangb, F. Zhoua,∗
a Academy of Frontier Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
b Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
NiO hollow spheres are prepared by heating the NiCl2/resorcinol-formaldehyde (RF) gel in argon at 700 ◦C
for 2 h, and subsequently in oxygen at 700 ◦C for 2 h. X-ray diffraction (XRD), scanning electron microscopy
(SEM), and transmission electron microscopy (TEM) are employed to characterize the structure and mor-
phology of the as-prepared NiO hollow spheres. These hollow spheres have a diameter of about 2 ␮m,
which are composed of NiO particles of about 200 nm. The electrochemical properties of these NiO hollow
spheres are investigated to determine the reversible capacity and cycling performance as anode materials
for lithium-ion batteries, and the advantages of their hollow spherical morphology to the electrochemical
performance are discussed.
Article history:
Received 9 June 2010
Received in revised form 9 August 2010
Accepted 10 August 2010
Available online 17 August 2010
Keywords:
NiO
Hollow sphere
Anode
© 2010 Elsevier Ltd. All rights reserved.
Lithium-ion battery
1. Introduction
spheres. These NiO hollow microspheres are assembled with many
NiO particles, and their electrochemical properties are investigated
3d transition-metal oxides, including FeO, CoO, NiO, Cu2O, and
their high valence oxides or mixed oxides, exhibit capacities over
700 mAh g−1, which are twice higher than the traditional carbon
materials, are promising alternatives to carbon as anode materials
for lithium-ion batteries [1–7]. However, for many unmodified 3d
transition-metal oxides, their capacities often reduce very quickly
during the repeated charge–discharge cycling. So in recent years,
much research work is focused on improving their electrochemical
performance, and many methods are invented. For example, form-
ing composites with conductive materials such as carbon [8,9] is a
very common and useful method, but this method often reduces
the mass specific capacity. So preparing materials with special
morphology such as porous structure [10–13], nanoarrays [14,15],
microspheres [16–20] and some others [21] to enhance the electro-
chemical performance has attracted much attraction. For example,
hollow spherical materials often exhibit good electrochemical per-
formance, mainly due to their large specific surface area, short
diffusion length of Li+, good electrical contact and conductivity,
or good ability of buffering the volume change during the electro-
chemical reactions.
to determine the capacity and cycling performance as anode mate-
rials for lithium-ion batteries.
2. Experimental
The synthetic procedure is described in Scheme 1. In a typical
synthesis, 10 g NiCl2·6H2O, 6.5 g resorcinol, and 9 mL 37% formalde-
hyde was dissolved in 100 mL deionized water. Adjust pH with
concentrated hydrochloric acid until it reached 1.0. The mixture
was sealed with a layer of PE film and placed in a water bath at
85 ◦C for 3 h to produce a NiCl2/resorcinol-formaldehyde (RF) gel,
and then the gel was dried in an oven at 85 ◦C for 24 h. Subsequently,
the dried gel was transferred to a quartz tube furnace and calcined
at 700 ◦C for 2 h under flowing argon, and this carbonization process
produced Ni/carbon composite microspheres. When the Ni/carbon
composite was finally heated to 700 ◦C and maintained for 2 h under
oxygen atmosphere, the carbon spheres template was burnt out
completely and NiO hollow microspheres were obtained.
The structure and morphology of the materials were charac-
terized by X-ray diffraction (XRD, Rigaku D/max-rA), scanning
electron microscopy (SEM, FEI Sirion-100), and transmission elec-
tron microscopy (TEM, JEOL, JEM200CX).
The working electrodes were prepared by coating the slurry
consisted of 80 wt% active materials, 12 wt% acetylene black, and
8 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrro-
lidinone (NMP) on copper foil. The electrodes were dried at 95 ◦C
for 12 h in vacuum and then pressed under 20 MPa. Half cells were
assembled in an argon-filled glove box using Li foil as counter
electrode, and polypropylene film as separator. The electrolyte
Hollow spherical metal oxides can be prepared using carbon
spheres as the template [22,23]. In the present work, Ni/carbon
composite microspheres are prepared by calcining NiCl2/RF gel in
argon, and then used as the template to prepare NiO hollow micro-
∗
Corresponding authors. Tel.: +86 25 84893083; fax: +86 25 84893083.
E-mail addresses: huangxh@nuaa.edu.cn (X.H. Huang), fzhou@nuaa.edu.cn
(F. Zhou).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.08.039

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X.H. Huang et al. / Electrochimica Acta 55 (2010) 8981–8985
Scheme 1. Schematic representation of synthetic procedure.
was a mixed solution of ethylene carbonate (EC) and diethyl
carbonate (DEC) containing 1 mol L−1 LiPF6. The cells were galvano-
statically discharged and charged using different current densities
over a potential range of 0.02–3.0 V. Cyclic voltammetry (CV)
measurements of the electrodes were carried out on a CHI660C
electrochemical workstation at a scan rate of 0.1 mV s−1 between 0
and 3 V.
3. Results and discussion
3.1. Characterization of NiO hollow spheres
When NiCl2/RF gel was heated at 700 ◦C in argon, the gel was
undergoing a carbonization process and NiCl2 was reduced to Ni
metal. Fig. 1(a) shows the XRD pattern of the product after this
carbonization process. The peaks at 44.4◦, 51.7◦, and 76.3◦ can be
assigned to the (1 1 1), (2 0 0), and (2 2 0) reflections of cubic Ni,
respectively. After subsequently heated at 700 ◦C in oxygen, the
carbon was burnt out completely and Ni was oxidized to NiO. This is
confirmed in the XRD pattern (Fig. 1(b)), in which the peaks at 37.2◦,
43.3◦, 62.8◦, 75.5◦, and 79.4◦ can be assigned to the (1 1 1), (2 0 0),
(2 2 0), (3 1 1), and (2 2 2) reflections of cubic NiO, respectively.
Microspheres were obtained after the carbonization of NiCl2/RF
gel in argon, as shown in Fig. 2(a). These spheres should be Ni/C
composite spheres, and their diameter is about 2 ␮m. After cal-
cined in oxygen, the product remains the spherical morphology, as
shown in Fig. 2(b), and the diameter of these NiO spheres is still
about 2 ␮m, but their surface becomes much rougher. According to
the high magnified image of a sphere inserted in the lower right cor-
ner, it can be seen that the NiO sphere is constructed of many NiO
particles. From the TEM image of a NiO sphere, shown in Fig. 3(a), it
is obviously that the sphere is hollow, with the shell of 300–500 nm
in thickness. Fig. 3(b) shows the TEM image of some particles peeled
off from the spheres by an ultrasonic process. It can be seen that
the sizes of these NiO particles are about 200 nm.
3.2. Electrochemical performance of NiO hollow spheres
The as-prepared NiO hollow microspheres were used as anode
materials for lithium-ion batteries and their electrochemical per-
Fig. 1. XRD pattern of (a) Ni/C composite prepared by calcining NiCl2/RF gel in argon,
and (b) NiO prepared by calcining Ni/C composite in oxygen.

X.H. Huang et al. / Electrochimica Acta 55 (2010) 8981–8985
8983
Fig. 2. SEM images of (a) Ni/C composite spheres and (b) NiO spheres.
Fig. 3. TEM images of (a) a NiO hollow sphere and (b) NiO particles peeled off from
the spheres.
formance were tested. Fig. 4 shows the cyclic voltammograms of
a NiO/Li cell at a scan rate of 0.1 mV s−1 in the potential range of
0–3 V. In the first reduction process, a strong reduction peak at
0.6 V is observed, corresponding to the first electrochemical reac-
tion NiO + 2Li → Ni + Li2O, and the formation of the SEI layer, which
is a polymeric gel-like layer which contains ethylene-oxide-based
oligomers, LiF, Li2CO3, and lithium alkyl carbonate (ROCO2Li) [7].
In the first oxidation process, two oxidation peaks are found at 1.6
and 2.3 V. The weak peak at 1.6 V should correspond to the disso-
lution of the organic SEI layer [3,5], and the strong peak at 2.3 V
corresponds to the charge reaction Ni + Li2O → NiO + 2Li. However,
the second curve is different from the first one, in which the reduc-
tion peak becomes weaker, and shifts to 1.2 V, exhibiting a smaller
polarization. The curve of the third cycle is very similar to the sec-
ond one, indicating the reversibility of the cell gets better since the
second cycle.
NiO hollow spheres is 1100 mAh g−1, much higher than the theo-
retical value. The extra capacity should come from the growth of
the gel-like SEI layer on the surface of the particles during the first
discharge [6]. The first charge capacity is 620 mAh g−1, and the sec-
ond discharge capacity is 700 mAh g−1. Since the second cycle, the
curves are similar, but the capacity is gradually decreases. Until
the 45th cycle, the discharge and charge capacities remain 560 and
490 mAh g−1, respectively. The discharge and charge capacities for
each cycle are plotted in Fig. 6. It can be calculated that 80% of
Fig. 5 shows the discharge and charge curves for the first three
cycles and the 45th cycle of the NiO/Li cell measured between 0.02
and 3.0 V at a current density of 100 mA g−1. In the first discharge,
there is a quick potential drop to 0.6 V at first, followed by a long
flat plateau at 0.6 V, which corresponds to the convention reaction
NiO + 2Li → Ni + Li2O. The sloping part in the end of the discharge
curve between 0.6 and 0 V corresponds to the formation of the SEI
layer [6]. However, starting from the first charge, the charge and
discharge plateaus are not obvious. There are two slops around 1.6
and 2.3 V in each charge curve, and the slop in each discharge curve
is around 1.2 V. Smaller potential hysteresis indicates that the elec-
trode reactions become more reversible since the second cycle. It is
known that the theoretical capacity of NiO is 718 mAh g−1, which is
calculated from the electrode reaction NiO + 2Li ꢀ Ni + 2Li2O. How-
ever, it can be seen in the curve that the first discharge capacity of
Fig. 4. Cyclic voltammograms for NiO/Li cell at a scan rate of 0.1 mV s−1. The cycle
numbers are marked in the graph.

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X.H. Huang et al. / Electrochimica Acta 55 (2010) 8981–8985
Fig. 7. The specific capacities of NiO/Li cell cycled at different current densities.
Fig. 5. The discharge and charge curves of the NiO/Li cell at a current density of
100 mA g−1. The cycle numbers are indicated in the graph.
the reversible capacity remains after 45 cycles. The cycling perfor-
mance of these NiO hollow microspheres is much better than NiO
irregular particles and NiO solid spheres prepared in our previous
work [24,25].
well with each other, leading to good electric contact during the
electrochemical reaction. Furthermore, the hollow spheres have
good ability of accommodating the large volume change during
the discharge and charge reaction and thus alleviate the pulveriza-
tion process. The good conductivity and the alleviated pulverization
lead to the good cycling performance of NiO hollow spheres.
The rate capability of the NiO/Li cell was also investigated and
the results are shown in Fig. 7. When cycled at the current den-
sity of 200 mA g−1 for the first 10 cycles, the final discharge and
charge capacities are 635 and 535 mAh g−1, respectively. Then the
current density increases to 500 mA g−1, and the end capacities are
520 and 450 mAh g−1. After that, the current density reaches to
1000 mA g−1, and the capacities are 275 and 240 mAh g−1. Subse-
quently, the current reduces to 100 mA g−1, and the capacities are
up to 580 and 510 mAh g−1. When the current density returns back
4. Conclusions
NiO hollow spheres are prepared by calcining the NiCl2/RF gel
in argon and subsequently in oxygen. SEM and TEM images show
that these hollow spheres are assembled by NiO particles with sizes
of about 200 nm. Electrochemical test shows that these spheres
deliver discharge and charge capacities of 560 and 490 mAh g−1
after 45 cycles at the current density of 100 mA g−1, and also exhibit
good rate capability. The NiO hollow microspheres can offer large
specific surface area, good electric contact among the particles,
and good tolerance of volume change, which are benefit for their
electrochemical performance.
to 200 mA g−1 at last, the final capacities are 520 and 470 mAh g−1
.
As compared to many NiO irregular particles and NiO dense solid
spheres [18,24,25], the NiO hollow microspheres exhibit better
electrochemical performance. The hollow spherical morphology of
NiO has many advantages to its electrochemical performance. In the
hollow spheres, both the two sides of the shell can be immersed in
the electrolyte, leading to a larger specific surface area and a shorter
diffusion length of Li+. So the electrochemical reactions of the NiO
hollow spheres can proceed much more quickly and completely,
and thus they exhibit higher capacity and better rate capability.
According to the SEM and TEM images, the NiO hollow spheres
are assembled by many NiO particles, and these particles contact
Acknowledgements
This work is supported by China Postdoctoral Science Foun-
dation (no. 20090461109), Jiangsu Postdoctoral Science Research
Foundation (no. 0902015C), and Science Foundation of Nanjing
University of Aeronautics and Astronautics (no. 909388). We would
like to acknowledge them for financial support.
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