Electrochemical properties of NiO/Co-P nanocomposite as anode materials for lithium ion batteries

Article abstract of DOI:10.1016/j.jallcom.2010.12.117

NiO/Co-P nanocomposite is prepared by an electroless cobalt plating technique. The as-prepared composite is characterized by means of X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. SEM and TEM images reveal that the NiO particles are about 200 nm in size, which are modified by Co-P nanoparticles of about 30 nm. The electrochemical properties as anode materials for lithium ion batteries are examined by cyclic voltammetry (CV) and discharge-charge tests. The results show that, compared with the bare NiO without electroless cobalt plating, NiO/Co-P nanocomposite exhibits a smaller polarization and a better rate capability, which is attributed to the Co-P nanoparticles.

Full text of DOI:10.1016/j.jallcom.2010.12.117

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Journal of Alloys and Compounds 509 (2011) 3425–3429
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Electrochemical properties of NiO/Co–P nanocomposite as anode materials for
lithium ion batteries
X.H. Huanga,∗, Y.F. Yuanb, Z. Wanga, S.Y. Zhanga, F. Zhoua,∗
a Academy of Frontier Science, Nanjing University of Aeronautics and Astronautics, 29#, Yudao Street, Nanjing 210016, China
b College of Machinery and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China
a r t i c l e i n f o
a b s t r a c t
Article history:
NiO/Co–P nanocomposite is prepared by an electroless cobalt plating technique. The as-prepared com-
posite is characterized by means of X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS),
scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. SEM and
TEM images reveal that the NiO particles are about 200 nm in size, which are modified by Co–P nano-
particles of about 30 nm. The electrochemical properties as anode materials for lithium ion batteries are
examined by cyclic voltammetry (CV) and discharge–charge tests. The results show that, compared with
the bare NiO without electroless cobalt plating, NiO/Co–P nanocomposite exhibits a smaller polarization
and a better rate capability, which is attributed to the Co–P nanoparticles.
Received 25 September 2010
Received in revised form
11 December 2010
Accepted 15 December 2010
Available online 23 December 2010
Keywords:
NiO
Electroless cobalt plating
Composite
© 2010 Elsevier B.V. All rights reserved.
Anode
Lithium ion battery
1. Introduction
Ni on the surface of the host particles is a dense film. Although this
dense Ni film can improve the electronic conductivity, it may be
3d transition-metal oxides (e.g. FeO, CoO, NiO, Cu2O, and their
high-valence oxides) as anode materials of lithium ion batteries
have received much attention since they were first proposed by
Tarascon and co-workers [1–5]. These oxides can deliver reversi-
ble capacities twice higher than carbon-based materials even at
high discharge–charge current densities [6–18], and among these
oxides, NiO has some advantages, e.g. stable, easy to prepare, and
inexpensive. Recently, more research work on NiO anode materials
focuses on how to improve their cycling stability and many met-
hods have been developed. For example, forming composites with
the conductive materials such as metals and carbon is an effec-
tive approach. These metals and carbon can remarkably improve
the conductivity of active materials, facilitate the lithiation and
delithiation process, and meanwhile buffer the volume changes to
alleviate the pulverization of active materials [19–25].
unfavorable to the transport of lithium ions. It is believed that the
granular plating materials should be more favorable for the contact
between the electrolyte and active materials as well as the pas-
sing of lithium ions. Therefore, in the present work, granular cobalt
plating is introduced to the surface of the NiO anode materials by
electroless plating technique, and the effects of the cobalt plating
on the electrochemical properties are investigated in detail.
2. Experimental
2.1. Electroless cobalt plating
Table 1 lists the compositions of the electroless cobalt plating bath. Cobalt sulfate
was used as the source of cobalt, and sodium hypophosphite as the reducing agent.
NiO powders, prepared by calcining the precipitation from Ni(NO3)2 and NH4HCO3
solutions, were used as the starting material. NiO powders were first pretreated
by a sensitization–activation process in a mixed solution containing SnCl2·2H2O
(30 g L−1), PdCl2 (0.5 g L−1), NaCl (160 g L−1), and concentrated HCl (60 mL L−1), and
subsequently transferred into the electroless cobalt plating bath with a powder
load of 10 g L−1. The pH value was adjusted to 10 by the addition of NaOH solution.
The bath was stirred vigorously at 55 ◦C for 1 h. Finally, the powders were washed
repeatedly with distilled water and dried at 90 ◦C in vacuum for 12 h.
The electroless plating is an effective method to improve the
electrochemical properties of the electrode materials, because this
technique readily forms a uniform metallic deposition on the
surface of the materials, which is favorable to enhance their con-
ductivity. According to the previous research [26,27], the plating
2.2. Characterization of materials
The structure of the composite was characterized by X-ray diffraction (XRD,
Rigaku D/max-rA with Cu K␣ radiation). The component analysis of the composite
was conducted using energy dispersive X-ray spectroscopy (EDS, Thermo Noran)
equipped on a scanning electron microscope (SEM, FEI Sirion-100). The morpho-
∗
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).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.12.117

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Table 1
Ni
Compositions of the electroless plating bath.
element
O K
P K
Co K
Ni K
wt %
18.1
1.07
10.51
70.32
at %
44.5
1.36
7.01
47.12
Component
Concentration (g L−1
)
CoSO4·7H2O
NaH2PO2·2H2O
Na3C6H5O7·2H2O
H3BO3
25
25
52.3
25
O
logy was observed by scanning electron microscopy (SEM, Hitachi S-4800) and
transmission electron microscopy (TEM, JEOL JEM200CX).
Ni
Co
2.3. Electrochemical tests
Ni
P
NiO or NiO/Co–P nanocomposite (80 wt.%) used as the anode active mate-
rials was mixed with acetylene black (12 wt.%), and polyvinylidene fluoride (PVDF,
8 wt.%). N-methyl pyrrolidinone (NMP) was added to the mixed powders, conti-
nuously stirred until the slurry became homogeneous. The slurry was uniformly
incorporated to copper foam current collector to fabricate working electrodes. The
electrodes were dried at 95 ◦C for 12 h in vacuum and pressed under 15 MPa. Coin
cells of 2025 type were assembled in an argon-filled glove box, using lithium metal
foil as the counter electrode, polypropylene film as the separator, and a solution
of 1 mol L−1 LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by
weight) as the electrolyte.
0
500
1000
Energy (eV)
1500
2000
Fig. 1. EDS pattern of the NiO powders after electroless cobalt plating.
The cells were cyclically tested on a LAND CT2001A battery test instrument using
different current densities over a voltage range of 0.02–3 V. Cyclic voltammetry (CV)
tests of the electrodes were carried out on a CHI660D electrochemical workstation
at a scan rate of 0.1 mV s−1 between 0 and 3 V.
3. Results and discussion
3.1. Preparation and characterization of NiO/Co–P
nanocomposite
NiO does not have self-catalysis activity for electroless cobalt
plating, and needs some noble metal nanoparticles to stimu-
late the deposition of cobalt. Therefore, the pretreatment of
sensitization–activation is necessary. If the NiO particles are not
pretreated, the electroless cobalt plating does not occur at all.
During the sensitization–activation process, metallic Pd nanopar-
ticles are anchored on the surface of NiO particles and act as the
nucleation centers of cobalt at the subsequent electroless plating
process. This activation reaction is Eq. (1). Sodium hypophosphite
is used as the reducing agent for the electroless plating because its
reaction condition is milder than those of other reducing agents,
e.g. hydrazine hydrate and sodium borohydride. The electroless
cobalt plating reaction mainly proceeds according to Eq. (2). In addi-
tion, there are also some side reactions, including the evolution of
hydrogen (Eq. (3)) and phosphorus (Eq. (4)).
Fig. 2. XRD patterns of (a) bare NiO and (b) electroless cobalt-plated NiO powders.
Co–P depositions [28,29], and a weak peak at 51.5◦ corresponds to
the (2 0 0) reflection of Co (JCPDS No. 15-0806).
Fig. 3 is the SEM image of NiO/Co–P composite. Lots of nano-
particles with the sizes of about 30 nm are uniformly deposited
on the surface of the larger host particles. TEM images further
show the morphology changes of NiO before and after the elec-
troless cobalt plating (Fig. 4). The well-crystallized NiO particles
are about 200 nm, and the surface is smooth (Fig. 4(a)). After the
Pd2+ + Sn2+ → Pd + Sn4+
(1)
(2)
(3)
(4)
Co2+ + H2PO2− + 3OH− → Co + HPO32− + 2H2O
H2PO2− + OH− → HPO32− + H2
3H2PO2− → HPO32− + OH− + 2H2O + 2P
Therefore, the use of sodium hypophosphite will introduce a
small quantity of P to the deposition, leading to the formation of
Co–P alloy.
EDS pattern of the NiO powders after electroless cobalt plating
is shown in Fig. 1. Besides the main elements of Ni and O coming
from NiO, the composite also contains 7.01 at% of Co and 1.36 at%
of P. This confirms that the deposition is Co–P alloy.
Fig. 2 compares the XRD patterns of NiO before and after electro-
less cobalt plating. The bare NiO (pattern a) exhibits five diffraction
peaks at 37.2◦, 43.3◦, 62.9◦, 75.4◦, and 79.4◦, which 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 (JCPDS No. 47-1049). After electroless cobalt plating, besides
the strong NiO reflection peaks, a broad band of weak diffraction
appears at 40–50◦ (pattern b), which is related to the amorphous
Fig. 3. SEM image of electroless cobalt-plated NiO.

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X.H. Huang et al. / Journal of Alloys and Compounds 509 (2011) 3425–3429
3427
Fig. 4. TEM images of (a) the fresh prepared bare NiO, (b) after the sensitization and activation treatment, (c) after electroless cobalt plating, and (d) SAED pattern of NiO/Co–P
nanocomposite.
sensitization–activation treatment (Fig. 4(b)), NiO are uniformly
a
2.25 V
1
0
modified by numerous nanoparticles less than 10 nm. The elec-
troless cobalt plating process makes most NiO particle entirely
modified by many Co–P nanoparticles with the sizes of about 30 nm
(Fig. 4(c)). Selected area electron diffraction (SAED) pattern of the
electroless cobalt-plated NiO exhibits {1 1 1}, {2 0 0}, {2 2 0} reflec-
tion rings of NiO, and a {2 0 0} reflection ring of Co (Fig. 4(d)),
coinciding with XRD results.
1st
2nd
1.55 V
3rd
3rd
-1
-2
-3
2nd
1st
1.1 V
3.2. Electrochemical properties of NiO/Co–P nanocomposite
Electrochemical properties of the NiO/Co–P nanocomposite as
anode materials for lithium ion batteries are investigated. We mea-
sure the cyclic voltammograms (CV) of NiO and NiO/Co–P between
0 and 3 V at a scan rate of 0.1 mV s−1 (Fig. 5). The theoretical poten-
tial of the reaction (NiO + Li → Ni + 2Li2O) is 1.794 V [4], but the
real reduction and oxidation peaks deviate from this value, due
to polarization. For bare NiO (Fig. 5(a)), there is a strong reduction
peak around 0.35 V during the first reduction scan, which is related
to the first electrochemical process of the electrode. This process
involves one major reaction (NiO + 2Li → Ni + Li2O), and some side
reactions. The main side reaction is the formation of the gel-like
solid electrolyte interphase (SEI) layer, which consists of ethylene-
oxide-based oligomers, LiF, Li2CO3, and lithium alkyl carbonate
(ROCO2Li) [30]. Two oxidation peaks appear in the first oxidation
scan. The weak one locates at about 1.55 V, which is due to the dis-
solution of the gel-like SEI layer during the charge process [3,5]. The
main oxidation peak at about 2.25 V is stronger, and corresponds to
the charge reaction (Ni + Li2O → NiO + 2Li). After the 1st cycle, the
CV curves almost coincide with each other in shape, and the reduc-
tion peaks locate at 1.1 V, while the oxidation peaks locate at 1.55
and 2.25 V. This indicates that the electrode reactions become more
reversible after the 1st cycle. For NiO/Co–P composite (Fig. 5(b)),
the first reduction peak shifts to 0.6 V, the reversible reduction
peaks shift to 1.25 V, and the main oxidation peaks shift to 2.2 V.
0.35 V
0
1
2
3
Potential (V vs. Li/Li+)
b
1
0
2.2 V
3rd
2nd
1st
1.55 V
3rd
2nd
-1
-2
1.25 V
1st
0.6 V
0
1
2
3
Potential (V vs. Li/Li+)
Fig. 5. CV curves of (a) bare NiO and (b) NiO/Co–P nanocomposite at a scan rate of
0.1 mV s−1. The cycle numbers are marked in the graph.

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3.0
2.5
2.0
1.5
1.0
0.5
0.0
1200
1000
800
600
400
200
0
1
discharge
charge
3
50
2
1
2
50
3
0
200
400
600
800
1000
0
10
20
30
40
50
Specific capacity (mAh g–1)
Cycle number
Fig. 7. Cycling performance of the NiO/Co–P nanocomposite at a current density of
100 mA g−1
Fig. 6. The discharge and charge curves of the NiO/Co–P nanocomposite at a current
density of 100 mA g−1. The cycle numbers are indicated in the graph.
.
Obviously, the polarization of NiO/Co–P composite is smaller than
that of bare NiO, meaning that the electrode conductivity has been
enhanced by electroless cobalt plating.
capacity is 680 mAh g−1, and the second discharge capacity is
690 mAh g−1. After 50 cycles, the discharge and charge capacities
remain 560 mAh g−1 and 540 mAh g−1, respectively. The discharge
and charge capacities of the 1–50th cycles are plotted in Fig. 7. It can
be seen that about 80% of the reversible capacity (charge capacity)
is maintained after 50 cycles.
Co–P alloy may also react with lithium, according to Xiang et al.’s
report [31], forming Co and Li3P. Nevertheless, the capacity contri-
buted by Co–P alloy nanoparticles should be very low in this work,
because the content of P in the composite is only about 1 wt.%, and
thus the capacity provided by Li3P is negligible.
The rate capability of NiO/Co–P nanocomposite was further
evaluated as shown in Fig. 8. The bare NiO without electroless
cobalt plating is also put together for a comparison. Both of the NiO
and NiO/Co–P nanocomposite were tested between 0.02 and 3 V
at various current densities from 100 to 1000 mA g−1. It is obvious
that NiO/Co–P nanocomposite exhibits better rate capability than
bare NiO. At 100 mA g−1, the NiO and NiO/Co–P nanocomposite
exhibit similar final charge capacities, which are 635 mAh g−1
for NiO, and 625 mAh g−1 for NiO/Co–P nanocomposite,
Fig. 6 shows the galvanostatic discharge–charge curves for the
first three cycles and the 50th cycle of the NiO/Co–P composite
at a current density of 100 mA g−1. The discharge–charge pla-
teaus all coincide well with the cathodic and anodic peaks in CV
curves. An obvious long plateau located at about 0.6 V appears
in the first discharge curve, but the first charge plateau is not
so obvious, and the main slope is around 2.2 V. After the 1st
cycle, the curves of each cycle are similar in shape, indicating
that the electrode reactions become more reversible. The slope
in each discharge curve is around 1.25 V, and in each charge
curve is around 2.2 V, respectively. NiO/Co–P nanocomposite deli-
vers a first discharge capacity of 1050 mAh g−1 (determined on
basis of the mass of NiO and Co–P), much higher than the theo-
retical value (718 mAh g−1) of pure NiO that is calculated from
the electrode reaction (NiO + 2Li ꢀ Ni + 2Li2O). The extra capacity
results from the irreversible side reactions including the forma-
tion of the SEI layer during the first discharge. The first charge
Fig. 8. Comparison of rate capabilities of NiO and NiO/Co–P nanocomposite.