Pillow-shaped porous CuO as anode material for lithium-ion batteries

Article abstract of DOI:10.1016/j.inoche.2010.09.025

Pillow-shaped porous cupric oxide (CuO) was prepared by a template-free method. Highly crystallized CuC₂O₄ precursor with micron-size in high dispersivity was prepared through a hydrothermal method first. Thermal treatment was carried out with the obtained precursor to produce porous cupric oxide. Thermogravimetry and differential thermal analysis (TG-DTA), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and galvanostatic cell cycling were employed to characterize the structure and electrochemical performance of the porous cupric oxide. The porous CuO powder exhibited high average coulombic efficiency (98.3%) and capacity retention (83.3% of the discharge capacity of the second cycle after 50 cycles) at a current rate of 0.1 C.

Full text of DOI:10.1016/j.inoche.2010.09.025

Inorganic Chemistry Communications 14 (2011) 38–41
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Pillow-shaped porous CuO as anode material for lithium-ion batteries
Mei Wan a, Dalai Jin b, Ran Feng c, Limin Si a, Mingxia Gao c, Linhai Yue a,
⁎
a
Chemistry Department, Zhejiang University, Hangzhou 310027, People's Republic of China
Center of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, People's Republic of China
b
c
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Pillow-shaped porous cupric oxide (CuO) was prepared by a template-free method. Highly crystallized
CuC2O4 precursor with micron-size in high dispersivity was prepared through a hydrothermal method first.
Thermal treatment was carried out with the obtained precursor to produce porous cupric oxide.
Thermogravimetry and differential thermal analysis (TG-DTA), X-ray powder diffraction (XRD), scanning
electron microscopy (SEM) and galvanostatic cell cycling were employed to characterize the structure and
electrochemical performance of the porous cupric oxide. The porous CuO powder exhibited high average
coulombic efficiency (98.3%) and capacity retention (83.3% of the discharge capacity of the second cycle after
50 cycles) at a current rate of 0.1 C.
Received 26 August 2010
Accepted 20 September 2010
Available online 29 September 2010
Keywords:
Porous cupric oxide
Thermal decomposition
CuC2O4 precursor
© 2010 Elsevier B.V. All rights reserved.
Electrochemical performance
Lithium-ion batteries are currently the dominant power sources
for portable appliances such as mobile phones and notebook
computers. And they also have potential appliances in electric
vehicles. Graphite is used as the main anode material of lithium-ion
batteries currently in commercial. Thus, the graphite electrode has its
theoretical capacity limit of 372 mAh g−1. In order to meet the
increasing demands of lithium-ion batteries, the exploration of new
electrode materials with superior capacity is an important issue in
lithium-ion batteries development.
reversible capacity as high as 625 mAh g−1. Yang and his co-workers
[14] have fabricated CuO nanotube film electrode which demonstrated a
reversible capacity over 417 mAh g−1 after 30 cycles. Owning to the
high surface area, porous materials have promising application in high-
efficiency battery materials [15,16].
Herein, novel pillow-shaped porous CuO has been prepared through
a template-free approach from copper oxalate precursor. Electrochem-
ical reaction of lithium was under research taking this porous active
material as the anode material. It is confirmed to be a promising method
to prepare excellent porous electrode materials for Li-ion batteries.
All the reagents used were of analytical grade, and were used
without further purification. In a typical synthesis process, 0.0125 M
K2C2O4 solution and 0.0125 M CuSO4 solution were prepared first.
Then 40 mL K2C2O4 solution was slowly added into CuSO4 solution
with the equal volume under constant stirring at room temperature.
The obtained blue solution was transferred into a 100 mL Teflon-lined
autoclave and sealed. The autoclave was heated at 100 °C for 4 h and
cooled down naturally to room temperature.
In recent years, transition-metal oxides have become promising
candidates for anode materials of lithium-ion batteries due to their huge
specific capacity [1–7]. Among these transition-metal oxides, copper
oxide is of particular interest for its inexpensiveness and nontoxicity.
The electrochemical process of the CuO electrode is expected that is
evolved in the following reaction: CuO+2Li++2e_ Cu+Li2O. For the
conversion reaction of CuO with two-electron transfer, the theoretical
capacity can be calculated as (96,500 C mol−1×2)/(80 g mol−1)=
2412.5 C g−1=670 mAh g−1
.
Various researches prove that the morphology and the size of CuO
have significant influences on its electrochemical performances [8–10].
Many efforts have been demonstrated to enable CuO and CuO-based
composites with diverse morphologies, in order to improve the
electrochemical capability of Li-ion batteries. For example, dendrite-
shaped CuO [11] and nanoflower-like CuO/Ni film [12] have been
fabricated via Kirkendall-effect-based approach and direct oxidation
method, respectively. Morales et al. [13] have prepared nanostructured
CuO thin films and found that the CuO electrodes could deliver a
The blue precipitates were collected by centrifugation, washed with
distilled water for several times and dried at 80 °C for 12 h. The obtained
precursor was calcined at 350 °C for 4 h to obtain a final cupric oxide
powder sample.
Morphology of the fabricated sample was characterized by field-
emission scanning electron microscopy (SEM, Hitachi S-4700)
operated at an acceleration voltage of 5 kV. Powder X-ray diffraction
(XRD, Philips X'Pert MPD diffractometer, Cu Kα radiation) was used
for phase identification. Scan rate of 4°min−1 and step size of 0.04°
were applied to record the pattern in the 2θ range 10–80°. The
operation voltage and current were 40 kV and 45 mA, respectively.
TG-DTA curves were determined by a WCT-1 apparatus at a heating
rate of 10 °C/min in air atmosphere.
⁎
Corresponding author. Tel./fax: +86 571 87952403.
E-mail address: zjchem_yue@126.com (L. Yue).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.025

M. Wan et al. / Inorganic Chemistry Communications 14 (2011) 38–41
39
To evaluate the electrochemical discharge/charge properties of the
porous pillow-shaped CuO as anode material for lithium ion batteries, a
2025-type half-cell was assembled. The CuO electrode was prepared by
spreading the 1-methyl-2-pyrrolidone slurry onto a nickel foil, which
containing 80 wt.% active material (the synthesized porous CuO), 10 wt.
% polyvinylidene fluoride (PVDF) binder and 10 wt.% acetylene black. It
was then dried in vacuum at 120 °C for 14 h. The active material loaded
on the nickel foil with a surface area of 1.54 cm2 was around 5 mg. The
coin cell of CR2025 type containing electrode, separator, electrolyte and
lithium foil as anode was assembled in an argon-filled glove box in
which the moisture and the oxygen levels were less than 1 ppm. The
electrolytewasa solution of 1 M LiPF6 in a mixture ofethylene carbonate
(EC) and dimethyl carbonate (DMC) (1:1, v/v).
The electrochemical test was carried out using Land battery test
system (CT-2001A). The cell was aged for 12 h before measurement,
and was cycled galvanostatically at a current rate of 0.1 C. All the
electrochemical tests were performed at room temperature.
Fig. 1 shows the XRD patterns of the precursor and the as-prepared
sample. The sharp diffraction peaks of the precursor (Fig. 1a) can be
indexed as a pure monoclinic phase of copper oxalate and match well
with the reported data (JCPDS No.21-0297). No characteristic peaks of
other impurities were detected, indicating the high purity of the
precursor. The crystalline structure of the copper oxalate is the Pnnm
space group with the lattice parameters a=5.403 Å, b=5.571 Å and
c=2.546 Å (orthorhombic).
Fig. 1b is the XRD pattern of the as-prepared sample. The
diffraction peaks can be indexed to the (110), (111), (020), (202),
(220), (311) and (004) reflections of a monoclinic CuO phase
correspondingly according to the standard card JCPDS No.72-0629.
The thermal behavior of the precursor CuC2O4 crystal was
illustrated in Fig. 2. The small amount of weight loss before 230 °C
Fig. 2. TG-DTA curves of copper oxalate.
can be attributed to the emission of H2O in the existence of physical
and chemical absorption. There is a large weight loss at 230 °C–300 °C
accompanied with a sharp exothermic DTA peak at 300 °C. It could be
attributed to the decomposition of the oxalate. The weight loss
(47.02%) agrees approximately with the expected value (47.52%) for
the following reaction, 2CuC2O4 +O2 →2CuO+4CO2.
The morphology of the precursor as well as the sample was
detected by SEM measurement. SEM images are shown in Fig. 3. It can
be seen that the precursor (Fig. 3a and b) exhibits square pillow-
shaped morphology and imperfect round shape (similar to pillow
shape), and the diameter is about 3–4 μm. The obtained cupric oxide
particles almost maintain the appearance morphology of the
precursor after annealing treatment. Pillow-shaped architecture can
be observed, as shown in Fig. 3c and d. It means that the framework of
the precursor keeps well during calcinations process. Further
observation shows that these CuO particles were porous and
constructed by nanocrystals of about 30 nm.
Apparently, copper oxalate can self-aggregate into pillow-shaped
complex morphology in the absence of any specific additives or
templates. It is known that crystal growth mainly occurs by two different
mechanisms: classical model of crystallization and aggregation-mediated
crystallization. Aggregation-mediated crystallization appears to be
prominent for solids, such as copper oxalates [17] and copper oxides
[18], which contains metal ions that readily undergo hydrolytic
polymerization and cluster formation in aqueous solution. In our case,
growth of copper oxalate may occur by aggregation mechanism.
Aggregation-mediated crystallization produces particle assemblies,
which can be ordered or disordered. Crystal growth by aggregation is
explained by complex formation mechanisms [19–23]. According to the
micro-mechanical mode developed by Hounslow et al. [21,23], the
aggregation process is governed by the collision efficiency and rate, and
low ionic strength in solution may facilitate the ordered assembly of
building block. The ionic concentration in our reaction system is
0.0125 M, which may be low enough to result in regular assembly of
the building blocks and cause the formation of the pillow shape.
Fig. 4a demonstrates the charge–discharge curves of the pillow-
shaped CuO electrode at a current rate of 0.1 C (C is defined as 2 Li+
per hour; 670 mAh g−1). These voltage-capacity profiles of the porous
CuO electrode are similar to those of CuO materials reported in
literatures [13,24,25]. The first discharge and charge capacities were
measured to be 770.3 and 374.2 mAh g−1, respectively. Unprospec-
tively, the initial capacity of the porous CuO is higher than the
theoretical specific capacity, which could be attributed to the
formation of a solid electrolyte interphase (SEI) film during the
discharge process [26]. However, the discharge capacity decreases
greatly from 770 mAh g−1 to around 400 mAh g−1 during the second
Fig. 1. XRD patterns of the precursor and as-prepared sample: (a) precursor; and (b) as-
prepared sample.

40
M. Wan et al. / Inorganic Chemistry Communications 14 (2011) 38–41
Fig. 3. SEM images of the copper oxalate precursor and as-prepared CuO sample: (a–b) copper oxalate; and (c–d) cupric oxide.
charge/discharge cycle, which can be due to the irreversible insertion/
deinsertion of lithium into the pore structure during the cycle.
Fig. 4b shows the cycling behavior of the CuO/Li Teflon cell. The
discharge capacity of the obtained porous CuO electrode maintains
350 mAh g−1 after 30 cycles and 320 mAh g−1 after 50 cycles, which
is better than that of spherical CuO nanoparticles reported by Zhang,
300 mAh g−1 after 30 charge/discharge cycles [27]. The average
coulombic efficiency is 98.3%. The coulombic efficiency is about 49% in
the first cycle, while it increases to about 99.2% in the second cycle and
maintains at this value for the rest cycles. After 50 cycles, the porous
CuO can sustain 83.3% capacity of the second cycle at a current of 0.1 C.
The reversible capacity of the CuO can be attributed to the porous
pillow-shaped morphology, which provides large specific surface area
for the intercalation of lithium ions and contains many structural
defects to accommodate lithium ions [28].
In summary, pillow-shaped porous cupric oxide has been prepared
by a template-free method. Micron-size CuC2O4 crystal was first
prepared by a simple hydrothermal chemical route. Porous cupric
oxide was then obtained by the controlled thermal decomposition of
the precursor CuC2O4. The CuO particles are porous and pillow-shaped
(about 3.4 μm), which are constructed by nanocrystals with diameter
about 30 nm. The fine and polycrystallized pillow-shaped CuO with
porous structure was detected its electrochemical performance as
anode material for lithium-ion batteries. It exhibits an initial discharge
capacity of 770.3 mAh g−1 with the average coulombic efficiency of
98.3% and 83.3% retention of the discharge capacity of the second
cycle after 50 cycles.
Acknowledgements
This work was supported by Science and Technology Program of
Zhejiang Province, P.R. China (No. 2009C34012), and Natural Science
Foundation of Zhejiang Province, P.R. China (No. Y4080190).
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