Template-directed preparation of two-layer porous NiO film via hydrothermal synthesis for lithium ion batteries

Article abstract of DOI:10.1016/j.materresbull.2012.04.018

A two-layer porous NiO film is prepared by hydrothermal synthesis method through self-assembled monolayer polystyrene spheres template. The substructure of the NiO film is composed of ordered close-packed hollow-sphere array and the superstructure is made up of randomly NiO nanoflakes. The electrochemical properties are measured by galvanostatic charge/discharge tests and cyclic voltammetric analysis (CV). As anode material for lithium ion batteries, the two-layer porous NiO film exhibits high initial coulombic efficiency of 75%, high reversible capacity and rather good cycling performance. The discharge capacity of the two-layer porous NiO film is 501 mAh g^-1 at 0.5 C after 50 cycles. The two-layer porous architecture is responsible for the enhancement of electrochemical properties.

Full text of DOI:10.1016/j.materresbull.2012.04.018

Materials Research Bulletin 47 (2012) 1987–1990
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Template-directed preparation of two-layer porous NiO film via hydrothermal
synthesis for lithium ion batteries
*
Z. Chen , A. Xiao, Y. Chen, C. Zuo, S. Zhou, L. Li
College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde, Hunan 415000, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A two-layer porous NiO film is prepared by hydrothermal synthesis method through self-assembled
monolayer polystyrene spheres template. The substructure of the NiO film is composed of ordered close-
packed hollow-sphere array and the superstructure is made up of randomly NiO nanoflakes. The
electrochemical properties are measured by galvanostatic charge/discharge tests and cyclic voltam-
metric analysis (CV). As anode material for lithium ion batteries, the two-layer porous NiO film exhibits
high initial coulombic efficiency of 75%, high reversible capacity and rather good cycling performance.
The discharge capacity of the two-layer porous NiO film is 501 mAh gꢀ1 at 0.5 C after 50 cycles. The two-
layer porous architecture is responsible for the enhancement of electrochemical properties.
ß 2012 Elsevier Ltd. All rights reserved.
Received 1 February 2012
Received in revised form 27 March 2012
Accepted 11 April 2012
Available online 17 April 2012
Keywords:
A. Oxides
A. Thin films
B. Energy storage
B. Electrochemical properties
1. Introduction
surface area, large surface-to-volume ratio, and favorable struc-
tural stability. To date, lots of nanostructured NiO including
Lithium ion batteries have attracted considerable attention due
to increasing demands for portable electronic devices [1]. High
lithium storage capacity, high coulombic efficiency, and long
cycling life are still the major challenges for high-performance
lithium ion batteries [2]. Over the past decades, great efforts have
been dedicated to searching for alternative anode materials of
lithium ion batteries for improving their energy density and safety.
Among the numerous explored systems, 3d transition metal oxides
such as nickel oxide (NiO), cobalt oxide (Co3O4), and copper oxide
(CuO) have been demonstrated with high reversible capacities
(about three times larger than those of graphite) at a relative low
potential, which greatly spurs the rapid development in this field
[3–6].
Since the pioneering work of Tarascon’s group [7], NiO is
considered as one of the most promising anode candidates with a
theoretic capacity of 718 mAh gꢀ1. However, large irreversible
capacity in the first cycle and poor capacity retention during
charge/discharge cycling restrict its practical applications. The
undesirable performance is ascribed to the volume change and
subsequently particle pulverization during the charge/discharge
process. Extensive efforts have been dedicated to overcoming
these problems by either designing new types of active materials
or modifying traditional electrodes. An effective strategy is to
construct NiO with high porous nanostructures possessing high
nanotube [8], nanowall [9,10], nanosheet [11], nanobowel [12],
nanocone[13], and ordered mesoporous structure [14], have been
prepared by various methods and enhanced performances are
obtained in these systems compared to the bulk counterparts due
to their fast electron/ion transfer and morphological stability.
In recent years, controlled synthesis of active material for
lithium ion batteries by template technique has attracted
tremendous interest [15]. Nano/microscaled materials with
different morphologies such as Fe3O4 nanorod [16], V2O5 nanotube
[17], three-dimensional (3D) Co3O4 [4], and 2D CuO [18]
nanostructures can be obtained through the template synthesis
method [15]. These template-synthesized materials normally
exhibit small crystalline size, high surface area, large surface-to-
volume ratio, and favorable structural stability, leading to
enhanced performance. Previously, Yuan et al. [12] reported a
2D ordered NiO macrobowl array prepared by an electrodeposition
method through monolayer polystyrene (PS) spheres template and
its improved properties for lithium ion batteries. Besides, our
group reported a porous-structured NiO film prepared by a
chemical bath deposition through polystyrene spheres template
[19]. In the present work, we report a two-layer 3D porous NiO film
via hydrothermal synthesis method based on monolayer PS
spheres template and apply it as anode material for lithium ion
batteries. The two-layer porous NiO film exhibits high initial
coulombic efficiency of 75%, high reversible capacity and rather
good cycling performance. The discharge capacity of the two-layer
porous NiO film is 501 mAh gꢀ1 at 0.5 C after 50 cycles. The NiO
film electrodes exhibit noticeable electrochemical performance
* Corresponding author. Tel.: +86 736 78186715; fax: +86 736 78186716.
E-mail address: xiaoanguo123@126.com (Z. Chen).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.04.018

1988
Z. Chen et al. / Materials Research Bulletin 47 (2012) 1987–1990
template has been already described in previous works
[10,12,20]. In a typical synthesis of precursor film, 5 mmol of
nickel nitrate (Ni(NO3)2) and 25 mmol urea (CO(NH2)2) were
dissolved in 50 ml distilled water to form homogeneous solution.
Then the homogeneous solution was transferred into Teflon-lined
stainless steel autoclave liners. After that, the template electrode
above was immersed into the reaction solution. The liner was
sealed and maintained at 120 8C for 1 h in an electric oven. The
reactions for precursor film involved in the hydrothermal synthesis
could be illustrated as follows:
∀
NiO
Ni (111)
Ni (200)
Ni (220)
∀
∀
∀
H2NCONH2 þ H2O ! 2NH3 þ CO2
NH3ꢂ3H2O ! NH4þ þ OHꢀ
Ni2þ þ 2OHꢀ ! NiðOHÞ2
(1)
(2)
(3)
20
30
40
50
60
70
80
2θ/ degree
Afterwards, the sample was immersed in toluene for 24 h to
remove the PS spheres template. Finally, the as-prepared sample
was annealed at 400 8C for 1.5 h. The loading weight of the NiO is
Fig. 1. XRD pattern of two-layer porous NiO film.
approximately 0.9 mg cmꢀ2
.
with good cycle life and high capacity due to the highly porous
structures.
The morphology and microstructure of the sample were
characterized by a field emission scanning electron microscopy
(FESEM, Hitachi S-4700), transmission electron microscopy (TEM,
JEM 200 CX 200 kV), and X-ray diffraction (XRD, Philips PC-APD
2. Experimental
with Cu Ka radiation).
All solvents and chemicals were of reagent quality and were
used without further purification. The monodispersed PS spheres
with particle sizes of 500 nm in diameter were purchased from Alfa
Aesar Corporation. They were well dispersed in deionized water
and prepared as a suspension with concentration of 2.5 wt% before
fabricating colloidal monolayers.
Test coin cells were assembled in an argon-filled glove box
using the nickel-supported NiO films as working electrode, Li foil
as counter-electrode, polypropylene film as separator, and an
electrolyte of 1 M LiPF6 in a 50:50 (w/w) mixture of ethylene
carbonate and diethyl carbonate. The galvanostatic charge–
discharge tests were conducted on LAND battery program-control
test system from 0.02 to 3.0 V (versus Li/Li+) at room temperature
(25 ꢃ 1 8C). Cyclic voltammetry (CV) tests were carried out using the
Clean nickel foil with a size of 2.5 ꢁ 2.5 cm2 was used as the
substrate. The fabrication process of monolayer PS spheres
Fig. 2. SEM images of (a) top and side views of the monolayer PS spheres template (side view presented in inset); (b) and (c) top and side views of two-layer porous NiO film
(side view presented in inset); (d) TEM image of individual NiO nanoflake (SAED pattern presented in inset).

Z. Chen et al. / Materials Research Bulletin 47 (2012) 1987–1990
1989
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
CHI660C electrochemical workstation at
0.1 mV sꢀ1
a scanning rate of
.
3. Results and discussion
1.56 V
2.34 V
Anodic process
Fig. 1 shows the XRD pattern of the as-annealed film. Except for
three representative peaks from Ni foil, all the other diffraction
peaks can be indexed to cubic NiO, which is consistent with the
value in the standard card (JCPDS 47-1049). SEM image of the as-
fabricated monolayer PS spheres template is presented in Fig. 2a.
PS spheres are well-organized into an ordered monolayer array
with a hexagonal close-packed alignment on the substrate.
Interestingly, the as-synthesized NiO film exhibits a two-layer
porous structure. The upper part is randomly porous net-like NiO
nanoflakes with a thickness of 25 nm and the lower part is close-
packed monolayer NiO hollow-sphere with a diameter of about
500 nm (Fig. 2b and c). Combining these results, it is reasonable
that the NiO film with a two-layer porous structure could be
successfully prepared via hydrothermal synthesis method based
on monolayer PS spheres template. The individual flake form the
film presents a smooth appearance (Fig. 2d). In addition, all
diffraction rings in the selected area electronic diffraction (SAED)
pattern of the nanoflake can be indexed to the cubic NiO phase
(JCPDS 47-1049), indicating that the NiO film is polycrystalline in
nature.
0.2
Cathodic process
0.4
1.32 V
0.6
0.85 V
0.8
1.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
+
Potential (vs. Li/Li) / V
Fig. 4. CV curve of the two-layer porous NiO film at a scanning rate of 0.1 mV sꢀ1 at
the second cycle.
Fig. 4. In the cathodic process, the main reduction peak at 0.85 V
and weak reduction peak at 1.32 V correspond to the reduction of
NiO to metallic Ni nanoparticles and the formation of a partially
reversible SEI layer [3,12,24], respectively. Wang and coworkers
reported that SEI layer had already started above 2.0 V [25], and
Tarascon and co-workers also observed the formation of SEI layer
by ex situ TEM at the sloping potential range of 1–0.02 V [26].
During the following anodic process, the oxidation peak at 1.56 V is
associated with the partial decomposition of the SEI layer, and
another oxidation peak at 2.34 V corresponds to the decomposition
of Li2O leading to the formation of NiO [3,12,24]. The peaks in the
CV curve are consistent with the plateaus or sloping potential
ranges in the voltage-capacity profiles.
Fig. 5 shows the capacity retention properties of the two-layer
porous NiO film electrode. The two-layer porous NiO film is found
to sustain and deliver 570 mAh gꢀ1 at 0.1 C, 501 mAh gꢀ1 at 0.5 C,
and 409 mAh gꢀ1 at 2 C after 50 cycles, corresponding to 79%, 70%,
57% of the theoretical value, respectively, demonstrating the
capability for a high cycling rate. These values are comparable to
those obtained from other hierarchically porous NiO films
[3,12,14], and higher than the single-layer porous NiO film with
a capacity of about 300 mAh gꢀ1 at 2 C after 50 cycles [27]. The
enhanced performance is ascribed to the following morphological
benefits. First, the free-standing NiO nanoflakes with open spaces
allow easier electrolyte penetration to every part of the particle
and provide shorten diffusion paths for both electrons and lithium
ions within the oxides. Second, the macropores in the monolayer
The electrochemical performances of the as-prepared two-layer
porous NiO film are evaluated by standard method based on NiO/Li
cells. Fig. 3 shows the discharge/charge curves of the two-layer
porous NiO film electrode measured between 0.02 and 3.0 V versus
Li/Li+ at 0.1 C rate (1C = 718 mA gꢀ1). The long plateau at 0.63 V in
the first discharge process is due to the conversion from NiO to Ni.
The discharge plateau becomes higher in the second cycle. The
two-layer porous NiO film shows a first discharge capacity of
932 mAh gꢀ1
, which is higher than the theoretical value
(718 mAh gꢀ1). The extra capacity could be attributed to the
formation of solid electrolyte interphase (SEI) in the first discharge
process. This phenomenon happens in other 3d transition metal
oxides including CuO, Co3O4 and FeO [4,5,21,22]. The first charge
capacity of the two-layer porous NiO film is approximately
703 mAh gꢀ1, much less than that of the first discharge. The
irreversible capacity loss between the first discharge and charge is
attributed to the incomplete decomposition of both of the SEI and
Li2O [1]. The corresponding initial coulombic efficiency is about
75%, higher than that of the power electrode (65%) [23], and
comparable to other highly porous NiO films [3,12].
To understand the electrochemical process in detail, the CV
profile of the two-layer porous NiO film electrode is recorded in
1000
2nd
1st
3.0
0.1 C
0.5 C
2 C
800
2.5
2.0
1.5
1.0
0.5
600
400
200
0
2nd
1st
1000
0.0
0
200
400
600
800
-1
0
10
20
30
40
50
Capacity / mAh g
Cycle number
Fig. 3. The discharge/charge curves for the two-layer porous NiO film.
Fig. 5. Cycling performances of the two-layer porous NiO film at different rates.

1990
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In summary, a two-layer porous NiO film has been successfully
prepared via hydrothermal synthesis method based on monolayer
PS spheres template. As anode of lithium ion batteries, the two-
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good cycling performance. The enhanced electrochemical perfor-
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shorter diffusion length for lithium ion and electrons, leading to
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Acknowledgement
This work was supported by the construct program of the key
discipline in Hunan province (Applied Chemistry).
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