Self-assembled sandwich-like NiO film and its application for Li-ion batteries

Article abstract of DOI:10.1016/j.jallcom.2010.12.151

NiO film with sandwich-like morphology is fabricated on nickel foam by a simple ammonia-evaporation process. Ammonia plays a major role in controlling the final geometry during this template- and surfactant-free synthesis. The obtained NiO film is constructed by regular triangular prisms with side length of 500 nm. Each triangular prism is self-assembled by single crystalline platelets. As an anode for lithium ion battery, this NiO film electrode exhibits high discharge capacity and excellent cycling performance. The reversible capacity of the sandwich-like NiO film sustains 400 mAh g^-1 even after 50 cycles at 2 C, much higher than that of the dense NiO film prepared by eletrodeposition (198 mAh g^-1). The high rate capability and reversibility of this NiO film can be attributed to its unique sandwich-like architecture.

Full text of DOI:10.1016/j.jallcom.2010.12.151

Journal of Alloys and Compounds 509 (2011) 3889–3893
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Self-assembled sandwich-like NiO film and its application for Li-ion batteries
J. Zhong, X.L. Wang∗, X.H. Xia, C.D. Gu, J.Y. Xiang, J. Zhang, J.P. Tu∗
State Key Laboratory of Silicon Materials and 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
Article history:
NiO film with sandwich-like morphology is fabricated on nickel foam by a simple ammonia-evaporation
process. Ammonia plays a major role in controlling the final geometry during this template- and
surfactant-free synthesis. The obtained NiO film is constructed by regular triangular prisms with side
length of 500 nm. Each triangular prism is self-assembled by single crystalline platelets. As an anode
for lithium ion battery, this NiO film electrode exhibits high discharge capacity and excellent cycling
performance. The reversible capacity of the sandwich-like NiO film sustains 400 mAh g−1 even after 50
cycles at 2 C, much higher than that of the dense NiO film prepared by eletrodeposition (198 mAh g−1).
The high rate capability and reversibility of this NiO film can be attributed to its unique sandwich-like
architecture.
Received 26 September 2010
Received in revised form
17 December 2010
Accepted 17 December 2010
Available online 28 December 2010
Keywords:
Nickel oxide
Self-assembled film
Anode
© 2010 Elsevier B.V. All rights reserved.
Li-ion battery
1. Introduction
promising for their applications in Li-ion batteries. To the best of
our knowledge, there is little literature about NiO films with such
3d transition metal oxides (M-O, M is Fe, Co, Ni, and Cu) have
been widely studied as anodes for lithium ion batteries due to their
high theoretic capacity, excellent reversibility of electrode reac-
tion and low cost [1–10]. Among these transition metal oxides,
NiO has been paid much attention due to its attractive advantages
such as high theoretic capacity (718 mAh g−1), nontoxicity and low
material cost.
In the past decades, many methods have been reported for the
synthesis of NiO particles and films with various morphologies
[11–37], which included nanoplates, nanorods, nanowires, nan-
otubes, 3D-rose like and hollow microsphere particles, and dense,
porous, network structure films. Recently, self-assembled synthe-
sis of hierarchical structured NiO particles have been developed
[38–42]. For instance, concave polyhedron shaped NiO particles
obtained by Zhou’s group show potential application for batteries
[43]. These particles look like regularly twisted triangular prisms
and exhibit better reversibility than the nanoplates as electrode
materials. However, in the case of powder materials for Li-ion bat-
teries, the active materials should be mixed with ancillary materials
such as carbon black and polymer binder. This process is tedious
and may negate the benefits associated with the reduced particle
size and introduce undesirable interfaces [44]. Therefore, the direct
growth of hierarchical NiO films on current collector would be more
structure.
In the present work, nickel foam-supported NiO film composed
of sandwich-like regular triangular prisms was firstly synthesized
by a facile ammonia-evaporation-induced method. The potential
application of the as-prepared NiO film in Li-ion batteries was
investigated and the effects of the sandwich-like regular triangu-
lar prism structure on the reversibility and rate capability of the
electrode were also discussed.
2. Experimental
40 ml aqueous ammonia (25–28%) was added to 20 ml aqueous solution of
Ni(NO3)2·H2O (0.5 M) with vigorous stirring. The resulting homogeneous solution
was transferred into a Petri dish. Nickel foam substrate pressed to thin plate was
immersed in the above reaction solution. After heated at 90 ◦C for 3 h, the nickel
foam substrate was covered with a green film. This green precursor film was
washed with deionized water and ethanol for several times, and dried. Then the
film was heated in a tube furnace at 350 ◦C for 2 h in flowing argon. The loading
density of the film after heat treatment is 0.92 mg cm−2 calculated from the weight
increment. A blank test was performed in a solution without nickel salt. After the
reaction, the weight of the nickel foam did not change, indicating the nickel foam
do not react with the solution. For comparison, a dense NiO film was also prepared
on nickel foam substrate by an electrodeposition method reported by our group
before [12].
The structure and morphology of the film were characterized by X-ray diffrac-
tion (XRD, RigakuD/max-3B), scanning electron microscopy (SEM, FEI Sirion-100)
and transmission electron microscopy (TEM, JEOL JEM200CX). Electrochemical
performances of the film were investigated with two-electrode coin-type cells
(CR2025). The cells were assembled in an argon-filled glove box using the nickel
foam-supported NiO films as working electrodes, Li foils as counter-electrodes,
polypropylene films as separators, and an electrolyte of 1 M LiPF6 in a 1:1 (w/w)
mixture of ethylene carbonate and diethyl carbonate. The cells were galvanostati-
cally discharged and charged at a rate of 2 C (1 C = 718 mA g−1) in the voltage range
∗
Corresponding authors. Tel.: +86 571 87952856; fax: +86 571 87952573.
E-mail addresses: wangxl@zju.edu.cn (X.L. Wang), tujp@zjuem.zju.edu.cn,
tujp@zju.edu.cn (J.P. Tu).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.12.151

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J. Zhong et al. / Journal of Alloys and Compounds 509 (2011) 3889–3893
of 0.01–3.0 V (vs. Li+/Li) at 25 1 ◦C. Cyclic voltammetry (CV) test was carried out
using the CHI660C at a scanning rate of 0.5 mV s−1 between 0 and 3 V.
*
Ni(OH)
NiO
*
2
3. Results and discussion
Fig. 1 shows the XRD patterns of the as-prepared nickel foam-
supporting films before and after heat treatment. For the sample
before heat treatment, besides the peaks of the Ni substrate, all
the diffraction peaks in the pattern (Fig. 1a) correspond to well-
crystallized ␤-phase hexagonal nickel hydroxide (JCPDS Card No.
14-0117). The diffraction peak at 19.3◦, which is indexed to (0 0 1)
plane of ␤-Ni(OH)2, is much stronger than the others. It implies the
preferable growth of ␤-Ni(OH)2 along ꢀ0 0 1ꢁ direction. In Fig. 1b,
the diffraction peaks of the film after heat treatment can be well
assigned to cubic NiO (JCPDS Card No. 47-1049). The reaction pro-
cesses can be expressed below [40]. As is seen, the formation of NiO
is resulted from the competition balance between reactions (1) and
(2) [46]. The high concentration of dissolved ammonia in the ini-
Ni
Ni
Ni
a
*
Ni
Ni
*
* *
*
*
Ni
b
10
20
30
40
50
60
70
80
2 theta
Fig. 1. XRD patterns of nickel foam-supported films (a) before and (b) after heat
tial solution brings excess NH4 and NH3, which react with Ni2+ to
+
treatment.
form Ni(NH3)62+. When heated at 90 ◦C, ammonia begins to evap-
Fig. 2. SEM images of nickel foam-supported Ni(OH)2 and NiO films: (a) bare nickel foam substrate, (b) overview of Ni(OH)2 film, (c) top view of Ni(OH)2 film, (d) a magnified
image of a Ni(OH)2 particle, (e) a magnified image of a NiO particle, and (f) the dense NiO film prepared by eletrodeposition.

J. Zhong et al. / Journal of Alloys and Compounds 509 (2011) 3889–3893
3891
Fig. 3. TEM images and SAED patterns of (a) and (b) front view of the flakes of Ni(OH) 2, (c) and (d) NiO, and (e) side view of the flakes of NiO film.
orate and the hydrolysis of Ni(NH3)62+ occurs, forming Ni(OH)2. At
last, NiO is obtained by the thermal decomposition of Ni(OH)2.
image in Fig. 2d shows that each prism is composed from regular
stacking of triangular platelets like a sandwich. The average side
length of the triangular prism is about 500 nm. Fig. 2e shows the
SEM image of NiO film. It can be seen that the morphology of NiO
film is quite similar to that of the Ni(OH)2 film, except for the rough
surfaces and edges of the prisms.
TEM images of the flakes of the as-prepared films in Fig. 3a and
b further confirm the triangular shape of Ni(OH)2 and NiO. The
corresponding SAED pattern of the Ni(OH)2 displays a hexagonal
symmetry, revealing that every Ni(OH)2 triangular prism is a single
crystal and the surface of the stacks is (0 0 1) plane of the hexago-
nal ␤-phase (Fig. 3b). It indicates that the Ni(OH)2 nanoplates pile
up together and grow along [0 0 1] direction, namely the c-axis
of ␤-Ni(OH)2. It is consistent with the (0 0 1) preferred orienta-
Ni2+ + 6NH3 → Ni(NH3)6
(1)
(2)
(3)
2+
◦
Ni(NH3)62+ + 2OH−9−0→ Ni(OH)2 ↓ +6NH3
C
◦
350
C
Ni(OH)2 −→ NiO + H2O
Typical SEM images of the Ni(OH)2 and NiO films are shown
in Fig. 2. The nickel foam substrates are fully covered by plenty of
homogeneous small particles (Fig. 2a and b). Fig. 2c is a top-view of
the Ni(OH)2 precursor film, revealing that this film is constructed by
uniform and well-organized regular triangular prisms. The prisms
are perpendicular to the surface of the substrate. The magnified

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J. Zhong et al. / Journal of Alloys and Compounds 509 (2011) 3889–3893
3.0
1000
800
600
400
200
0
100
50th
2.5
2nd
Coulombic Efficiency
1st
80
60
40
20
0
Sandwich-like NiO film
Dense NiO film
2.0
1.5
1.0
0.5
0.0
2nd
50th
1st
0
10
20
30
40
50
Cycle Number
0
200
400
Capacity (mAh g-1)
600
800
Fig. 4. Cycling performances of (a) the sandwich-like NiO film and (b) the dense NiO
film; (c) coulombic efficiency of the sandwich-like NiO film during cycling.
Fig. 5. Discharge/charge curves of the NiO film electrode at 1436 mA g−1 (2 C rate).
tion revealed by XRD. The SAED pattern of the NiO shows that
each prism is constructed by single-crystalline NiO nanoplates with
[1 1 1] axis perpendicular to the triangle face (Fig. 3d). The single-
crystalline nature of the NiO nanoplates can be understood as a
result of the conversion from Ni(OH)2 to NiO with a relationship of
Ni(OH)2 [0 0 1] || NiO [1 1 1], since the symmetry of the lattice array
in (1 1 1) plane of the fcc NiO is the same as that in (0 0 1) plane of
the hexagonal ␤-Ni(OH)2, and d-spacing of (2 0 0) plane (0.136 nm)
in Ni(OH)2 is close to that of (2 2 0) plane (0.147 nm) in NiO [47].
TEM image of the side view of the NiO flakes further verifies the
lamellar feature (Fig. 3e). It is evident that each prism is resulted
from the piling up of hundreds of thin plates. The thickness of a
layer is less than 3 nm. These layers become not so uniform after
the dehydration, explaining the rough and blurry edges of NiO.
It is considered that the formation of triangular prism is resulted
from the self-assembled growth of Ni(OH)2. In a typical Ni(OH)2
molecule, the metal cations Ni2+ are located in the central spaces
of oxygen octahedral from six hydroxyl groups. These octahedral
then share their edges to form two dimensional (2D) sheets simi-
lar to the well-know mineral compound brucite Mg(OH)2 [48]. In
this experiment, ammonia controls the precipitation and the pas-
sivation of crystal surfaces. The nanostructures grow in a dynamic
process with a gradual decreasing of pH value and ammonia con-
centration in the solution. And the final hierarchical nanostructure
forms because of the different affinity that ammonia with different
crystal plane [45].
appear at about 1.2 V and 2.2 V, respectively and the polarization
occurs. Polarization also exists at rates of 0.1 C and 1 C. It can be
seen that the difference of the polarization between 1 C and 2 C are
not obvious. And the polarization in 0.1 C is smaller.
Cyclic voltammogram (CV) curves give complementary
data to the galvanostatic cycling (Fig. 7). During the second
cathodic/anodic scanning, the reduction peaks around 1.34 V
and 1.0 V correspond to the formation of the SEI and the initial
reduction of NiO to metallic Ni, respectively. The oxidation peak
around 1.47 V and 2.28 V correspond to the partial decomposition
of SEI and the decomposition of Li2O, respectively. These voltage
values are in agreement with NiO nanoparticle reported in pre-
cious literature [16]. Compared with the dense NiO film reported
before [48], the separation between cathodic and anodic peaks of
the sandwich-like NiO film in this work is smaller, indicating the
weaker polarization and better reversibility.
The good electrochemical performances of sandwich-like NiO
film can be attributed to the following reasons: first, the open space
between the neighboring prisms and the layered structure can
offer larger electrode/electrolyte contact area and shorter diffusion
length of lithium ions. It can also accommodate the strain induced
by the volume change during the charge and discharge process. Sec-
ond, every prism contacts directly with the foam nickel substrate,
which can enhance the conductivity of the electrode. Third, the
stability of the large single-crystalline domains may enhance the
As anode for lithium ion batteries, the electrochemical per-
formances of the NiO film are evaluated by galvanostatically
discharged–charged cycling and CV tests. Fig. 4 shows the capac-
ity retention properties of the NiO film electrodes. The specific
capacity for the sandwich-like NiO film electrode after 50 cycles
is 400 mAh g−1 at a discharge current density of 1436 mA g−1 (2 C
rate), which retains 78% of that in the 2nd cycle. Moreover, the
coulombic efficiency of the electrode keeps over 98% after the 2nd
cycle. But for the dense NiO film, the specific capacity after 50 cycles
is only 198 mAh g−1. By comparison, the sandwich-like NiO film
exhibits much better capacity retention than the dense one.
Fig. 5 shows the discharge/charge curves of the NiO film elec-
trode at 1436 mA g−1 (2 C rate). During the first discharge process,
the voltage decreases steeply to 0.5 V where a plateau region sets in
and continues until a discharge capacity of 630 mAh g−1 is reached.
Another slope is observed at 0.4 V, with a total first discharge capac-
ity of 793 mAh g−1. The first charge capacity is 511 mAh g−1, leading
to the initial columbic efficiency of about 64.4%. Fig. 6 shows the
second discharge/charge curves of the NiO film electrode at rates
of 0.1 C, 1 C and 2 C. During this cycling at 2 C, the voltage plateaus
3.0
2.5
2 C
2.0
1 C
0.1 C
1.5
1.0
0.5
0.0
0
200
400
600
800
-1
Capacity (mAh g )
Fig. 6. The second discharge/charge curves of the NiO film electrode at rates of 0.1 C,
1 C and 2 C.

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3893
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NiO film with regular triangular prisms was grown on the
nickel foam substrate by a facile ammonia-evaporation method.
The single-crystalline NiO nanoplates pile up together and form
sandwich-like structures with the bottom sticking to the substrate.
The ammonia plays the major role in controlling the sandwich-like
structure during the self-assembly growth process. The resultant
NiO film exhibits high rate capability and coulombic efficiency, as
well as good cycling performance, indicating that this sandwich-
like film would be a potential electrode for the next-generation
lithium ion batteries. Moreover, the ammonia-evaporation fabri-
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to preparing 3d transition metal hydroxide/oxide thin-film elec-
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