Simple synthesis of surface-modified hierarchical copper oxide spheres with needle-like morphology as anode for lithium ion batteries

Article abstract of DOI:10.1016/j.electacta.2009.10.073

Hierarchical, nanostructured copper oxide spheres were synthesized in a stirred solution of cupric acetate and ammonium hydroxide. Cetyltrimethylammonium bromide (CTAB) was used as a surfactant to modify the surface morphology of CuO spheres. Ordered nano-needle arrays can be formed on the surface of the CuO spheres (instead of disordered nano-leaves) in the presence of CTAB. Each CuO sphere is about 2 μm in diameter and possesses a large number of nano-needles that are about 20-40 nm in width and more than 300 nm in length. The needle-like hierarchical structure can greatly increase the contact area between CuO and electrolyte, which provides more sites for Li⁺ accommodation, shortens the diffusion length of Li⁺ and enhances the reactivity of electrode reaction, especially at high rates. After 50 cycles, the reversible capacity of the prepared needle-like CuO can sustain 62.4% and 56.4% of the 2nd cycle at a rate of 0.1C and 1C, respectively.

Full text of DOI:10.1016/j.electacta.2009.10.073

Electrochimica Acta 55 (2010) 1820–1824
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Simple synthesis of surface-modified hierarchical copper oxide spheres with
needle-like morphology as anode for lithium ion batteries
J.Y. Xiang, J.P. Tu∗, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi
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:
Received 29 August 2009
Received in revised form 28 October 2009
Accepted 28 October 2009
Available online 3 November 2009
Hierarchical, nanostructured copper oxide spheres were synthesized in a stirred solution of cupric acetate
and ammonium hydroxide. Cetyltrimethylammonium bromide (CTAB) was used as a surfactant to modify
the surface morphology of CuO spheres. Ordered nano-needle arrays can be formed on the surface of the
CuO spheres (instead of disordered nano-leaves) in the presence of CTAB. Each CuO sphere is about
2 ␮m in diameter and possesses a large number of nano-needles that are about 20–40 nm in width and
more than 300 nm in length. The needle-like hierarchical structure can greatly increase the contact area
between CuO and electrolyte, which provides more sites for Li+ accommodation, shortens the diffusion
length of Li+ and enhances the reactivity of electrode reaction, especially at high rates. After 50 cycles,
the reversible capacity of the prepared needle-like CuO can sustain 62.4% and 56.4% of the 2nd cycle at a
rate of 0.1C and 1C, respectively.
Keywords:
Copper oxide
Hierarchical nanostructure
Anode
Lithium ion battery
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
shortens the diffusion length of Li+ [15]. However, most nanos-
tructured CuO is prepared within a relatively long time or under
Since Tarascon and co-workers [1] first reported the high capac-
ity and excellent cycling capacity retention of CoO, many oxides of
3d transition metals (Fe, Co, Ni, and Cu) have been widely investi-
gated as promising anodes for lithium ion batteries [2–5]. Among
these transition metal oxides, CuO is proposed as an attractive
anode because it is inexpensive, non-toxic, easily produced, readily
stored and has a high theoretic capacity (670 mAh g−1). The elec-
trochemical reaction mechanism of CuO with Li+ is as follows:
a complex preparation condition such as hydrothermal synthesis
[12,14,16–18].
In the present work, we develop a very simple method to syn-
thesize hierarchical, nanostructured CuO in a stirred solution of
cupric acetate and ammonium hydroxide. Cetyltrimethylammo-
nium bromide (CTAB) was used as surfactant to modify the surface
morphology of the CuO spheres. The morphology modification
mechanism under the effect of CTAB is discussed, and the improved
electrochemical performance of CuO spheres obtained in the pres-
ence of CTAB is investigated.
CuO + 2Li+ + 2e ⇔ Cu + Li2O
(1)
Different morphologies and sizes of CuO can greatly influence
its electrochemical properties [6]. Hence, in the past few years,
many efforts have been devoted to synthesize CuO with vari-
ous morphologies including hollow spheres, nanotubes, nanorods,
nanoflowers and the like [7–13]. For instance, Gao et al. fabri-
cated fine and polycrystalline CuO nanorods, which can deliver a
reversible capacity of 550 mAh g−1 for up to 100 cycles [8]. Chen
et al. reported CuO nanowires can exhibit a reversible capac-
ity of more than 600 mAh g−1 even after 100 cycles [12]. Pan
et al. demonstrated that sheaf-like CuO can sustain a reversible
capacity of 580 mAh g−1 after 40 cycles [14]. The improved elec-
trochemical performances of CuO are due to the nanostructure,
which increases the contact area between CuO and electrolyte and
2. Experimental
Hierarchical structured CuO particles were obtained from a
stirred solution of Cu(Ac)2·H2O and NH3·H2O. In a typical proce-
dure, 0.4 g Cu(Ac)2·H2O (0.01 M) was first dissolved in aqueous
solution. Ammonia was used to adjust the pH of the solution to
11. Several minutes later, flocky precipitates appeared in the blue
solution. Next, the solution was stirred at a temperature of 90 ◦C for
1 h. Subsequently, 0.1 g CTAB was added to the mixed solution as a
surfactant. After stirring at 90 ◦C for 12 h, the blue solution became
black and turbid. The precipitates were finally separated by cen-
trifugation, washed with deionized water and ethanol for several
times, and then dried in vacuum at 60 ◦C for 12 h.
The structure and morphology of the prepared precursor and
CuO particles were characterized by X-ray diffraction (XRD, Philips
PC-APD with Cu K␣ radiation), field emission scanning electron
∗
Corresponding author. Tel.: +86 571 8795 2573; fax: +86 571 8795 2856.
E-mail address: tujp@zju.edu.cn (J.P. Tu).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.10.073

J.Y. Xiang et al. / Electrochimica Acta 55 (2010) 1820–1824
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microscopy (FESEM, FEI SIRION) and high-resolution transmission
electron microscopy (HRTEM, JEOL JEM-2010F), respectively.
Electrochemical performance of the CuO particles was investi-
gated with two-electrode coin-type cells (CR 2025). The working
electrodes were prepared using a slurry coating procedure. The
slurry consisted of 80 wt.% CuO particles, 10 wt.% acetylene black
and 10 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methyl
pyrrolidinone (NMP). The slurry was incorporated on a piece of
nickel foam with diameter of 12 mm. After drying at 90 ◦C for 24 h
in vacuum, the foam was pressed under a pressure of 20 MPa. Test
cells were assembled in an argon-filled glove box with the metallic
lithium foil as both the reference and counter electrodes, 1 M LiPF6
in ethylene carbonate (EC)–dimethyl carbonate (DME) (1:1 in vol-
ume) as the electrolyte and a polypropylene (PP) micro-porous film
(Cellgard 2300) as the separator.
The galvanostatic charge–discharge tests were conducted on a
LAND battery program-control test system at rate of 0.1C and 1C
(1C = 670 mA g−1) in the voltage range of 0.02–3.0 V (versus Li/Li+)
at room temperature (25 1 ◦C). Cyclic voltammetry (CV) was per-
formed on a CHI660C electrochemical workstation at a scan rate of
0.1 mV s−1 from 0 to 3.0 V.
3. Results and discussion
Fig. 1a shows the XRD pattern of the precursor precipitated
in the blue solution before heat treatment. All the diffraction
Fig. 2. TEM images of Cu(OH)2 precursor precipitated in the solution before heat
treatment.
peaks correspond well with orthorhombic Cu(OH)2 (Space group
Cmcm(63), JCPDS 35-0505). Broadening in the peaks of Cu(OH)2
indicates the formation of a Cu(OH)2 nanostructure. The XRD pat-
terns of the powders obtained from the solutions after stirring at
90 ◦C for 12 h are shown in Fig. 1b. In samples prepared with and
without CTAB, the samples exhibit similar diffraction peaks that can
be perfectly indexed to monoclinic CuO (Space group C2/c, JCPDS
48-1548) with lattice constants a = 4.68 Å, b = 3.42 Å, and c = 5.13 Å.
The reactions for CuO precipitation can be summarized as follows:
+
Cu2+ + 2NH3·H2O → Cu(OH)2↓ + 2NH4
(2)
(3)
◦
90
C
Cu(OH)2 −→ CuO + H2O
Fig. 2 shows the TEM images of Cu(OH)2 precursor appearing in
the mixed solution before heat treatment. It is noticed that Cu(OH)2
precursor exhibits a highly dispersed nanowire morphology. The
nanowires are 10 nm in diameter and 300–400 nm in length. The
SEM images of CuO particles prepared without CTAB in the solution
are shown in Fig. 3a and b. The CuO spheres has a “bird nest” appear-
ance and exhibits a hierarchical system, in which the skeleton is
made up of spheres with diameters of 2–4 ␮m and possessing thou-
sands of 2D leaves. The nano-leaves are about 200 nm in width and
20 nm in thickness. For the CuO particles obtained in the existence
of CTAB, the diameter of each sphere is also about 2–4 ␮m, and
when viewed under low magnification, the morphology is similar
to that synthesized without CTAB, as shown in Fig. 3c. However, the
nanostructure on the surface of the hierarchical CuO sphere is quite
different. As shown in Fig. 3d, ordered nano-needle arrays appear
on the surface of CuO spheres instead of disordered nano-leaves.
Each needle is about 20–40 nm in width and more than 300 nm in
length.
Fig. 1. XRD patterns of (a) Cu(OH)2 precursor precipitated in the solution before
heat treatment and (b) CuO particles obtained after heat treatment at 90 ◦C for 12 h.

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J.Y. Xiang et al. / Electrochimica Acta 55 (2010) 1820–1824
Fig. 3. SEM images of the prepared CuO particles: (a, b) in the absence of CTAB and (c, d) in the presence of CTAB.
Obviously, the varying morphology of CuO sphere should be
significantly upon addition of CTAB into the solution. CTAB is a typi-
cal ionic compound that can ionize completely in aqueous solution.
The resulting cation is a positively charged tetrahedron with a long
hydrophobic tail, which can be absorbed onto the surface of some
nanocrystals [19]. In this work, as CTAB is added into the stirring
solution, the cations are absorbed to the Cu(OH)2 nanowires at the
outer surface of the assembled spheres. Under the electrostatic and
stereochemical effects, the Cu(II) ions are dispersed in an orderly
way at the outward end of the sphere (Step D). After dehydration,
CuO grains grow perpendicularly to the surface of the sphere rather
than growing out of order. In addition, the existence of CTAB can
attributed to the surfactant effect of CTAB. Fig. 4 presents a
schematic of the formation of CuO without and with CTAB. Sev-
eral minutes after the Cu(Ac)2·H2O solution is mixed with a proper
amount of NH3·H2O, Cu(OH)2 nanowires are first formed (Step A)
and then assemble closely parallel to their length and become leaf-
shaped sheets (Step B). Subsequently, these nano-leaves are prone
to building up a hierarchical structure (Step C). In the absence of
CTAB, the CuO grains then grow disorderly on the surface, and a
bird nest-like sphere is finally formed after dehydration (Step E1).
However, the growth of CuO on the surface of the sphere changes
Fig. 4. Schematic illustration of the formation of CuO with and without CTAB.

J.Y. Xiang et al. / Electrochimica Acta 55 (2010) 1820–1824
1823
Fig. 5. Cycling performance of the CuO electrodes at different rates (from the 2nd
cycle to the 50th cycle).
also reduce the Gibbs free energy of the surface of the hierarchical
CuO sphere [20]. Thus, the hierarchical sphere with nano-needle
arrays on the surface is formed by using CTAB as surfactant (Step
E2).
The cycling performance of the obtained CuO particles at dif-
ferent rates is shown in Fig. 5. After 50 cycles, the bird nest-like
CuO and needle-like CuO spheres can sustain 59.6% (392 mAh g−1
)
and 62.4% (441 mAh g−1) capacity of the 2nd cycle at a rate of 0.1C,
respectively. The high reversible capacity of both the CuO elec-
trodes at 0.1C can be attributed to their hierarchical nanostructure.
A large number of nano-leaves and nano-needles, which make up
the CuO spheres, can provide sufficient contact between CuO and
the electrolyte, and offer a great deal of sites to accommodate Li+,
leading to high-discharge capacity. As the discharge–charge rate
increases to 1C, the discharge capacity of the CuO electrode synthe-
sized without CTAB maintains only 39.5% (183 mAh g−1) capacity
of the 2nd cycle, while the CuO obtained by using CTAB maintains
56.4% (303 mAh g−1) capacity of the 2nd cycle after 50 cycles. The
capacity fade of both electrodes is a little faster than cycling at 0.1C.
There are two causes for the relatively fast capacity fade. One is
that CuO is a typical semiconductor with poor conductivity, and the
charge transfer between each CuO sphere is not as sufficient when
discharged at a lower rate. The other is that the indirect contact
of the CuO/current collector and supplementary inactive interfaces
via the traditional slurry coating procedure will also limit the elec-
tron transfer between the CuO and the current collector, especially
at high rates. Even though, the capacity fade of the pure needle-like
CuO spheres is much less influenced by the discharging rate when
compared with the bird nest-like CuO sphere. The good capacity
retention can be attributed to the needle-like surface morphology
of CuO formed under the effect of CTAB, which provides more con-
tact area between the CuO and the electrolyte and greatly enhances
the reactivity of electrode reaction, especially at higher rate.
The SEM images of the CuO electrodes after cycling are shown
in Fig. 6. The CuO spheres prepared without CTAB adhere to each
other, and the bird nest morphology can hardly be identified
(Fig. 6a). However, for the CuO spheres obtained by using CTAB,
the needle-like morphology is still sustained, though the needles
on the surface become a little shorter after cycling (Fig. 6b). The
relatively stable structure and morphology also indicates better
capacity retention of the needle-like CuO.
Fig. 6. SEM images of the CuO electrodes prepared (a) in the absence of CTAB and
(b) in the presence of CTAB after 20 cycles at 0.1C.
0.7–0.9 V appear in the first discharge curves, which corresponds
to the reductive reaction from CuO to the intermediate compos-
ite copper oxide phase, to Cu2O and further decomposition into
Cu and Li2O, respectively [21]. In the first charge curves, only two
inclined plateaus near 2.5 V and 1.5 V can be observed, correspond-
ing to the process 2Cu + Li2O → Cu2O + 2Li and the partial oxidation
of Cu2O to CuO, respectively. However, there are still some differ-
ences in the discharge–charge curves. For instance, the needle-like
CuO exhibits higher first discharge capacity (1047 mAh g−1) than
the bird nest-like CuO (958 mAh g−1). The extra capacity compared
with the theoretic capacity (670 mAh g−1) results from the forma-
tion of SEI during the first discharging process [21]. Hence, it is also
indicated that a larger area of SEI is formed in the needle-like CuO
during the initial cycle due to larger specific surface of the sphere.
The SEI film is known as a gel-like layer, which contains ethy-
lene oxide based oligomers, LiF, Li2CO3, and lithium alkyl carbonate
(ROCO2Li) [22]. The incomplete decomposition of SEI is always the
main reason for the low coulombic efficiency and large capacity loss
during the initial cycle, which happens for all 3d transition metal
oxides, including CuO, NiO, Co3O4 [23–26]. However, as seen from
Fig. 5, the needle-like CuO exhibits a higher initial coulombic effi-
ciency (65%) than the bird nest-like CuO (60%) though a larger area
of SEI is formed on the surface of the needle-like CuO. This may also
be due to the better contact between the CuO nano-needles and the
electrolyte, making the Li ions transfer quickly and efficiently at the
CuO/electrolyte interface, which greatly enhances the reactivity of
electrode reaction. Therefore, the decomposition of SEI is relatively
efficient of needle-like CuO during the initial cycle.
Fig. 7 displays the first three discharge–charge curves for bird
nest-like and needle-like CuO electrodes between 0.02 V and 3.0 V
at rate of 0.1C. Both electrodes exhibit similar discharge behav-
iors. Three plateaus at the potential of 1.6–1.8 V, 1.0–1.2 V and
The first cyclic voltammogram curves of CuO electrodes at a
scan rate of 0.1 mV s−1 are shown in Fig. 8. The electrochemical

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val between the cathodic and anodic peaks can be suggested as the
polarization of the electrode. Therefore, it is considered that the
needle-like CuO electrode exhibits a smaller polarization than the
bird nest-like CuO. As is well known, the transferring delay of elec-
trons and lithium ions on the active material/electrolyte interface is
one of the main causes for the polarization in lithium ion batteries
[27]. Therefore, the results of the CV curves show that the electrons
and lithium ions can transfer more actively in needle-like CuO due
to the better contact between the CuO needles and electrolyte.
4. Conclusions
Hierarchical, nanostructured CuO spheres with needle-like
morphology were successfully prepared in a stirred solution of
Cu(Ac)2·H2O and NH3·H2O by using CTAB as a surfactant. The
hierarchical nanostructure of CuO has some influence on the elec-
trochemical performance. The needle-like CuO obtained in the
presence of CTAB shows better high rate properties than that of
bird nest-like CuO prepared without CTAB. It is attributed to the
larger specific surface area of needle-like CuO, leading to sufficient
contact area for CuO/electrolyte, shortened the diffusion length of
Li+, and enhanced reactivity of electrode reaction during cycling.
In addition, the needle-like morphology can be sustained well
after cycling. Therefore, the hierarchical structured CuO sphere
with needle-like morphology synthesized through this simple and
low-cost method is an extremely promising anode for lithium ion
batteries.
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peaks correspond well with the discharge–charge plateaus in Fig. 7.
Compared to the bird nest-like CuO, the cathodic peaks of the
needle-like CuO are located at relatively high potentials. The anodic
peaks of both electrodes are almost at the same locations. The inter-