Controlled synthesis of uniform ultrafine CuO nanowires as anode material for lithium-ion batteries

Article abstract of DOI:10.1016/j.jallcom.2011.07.109

A simple solution route is used to synthesize ultrafine Cu(OH)₂ nanowires by restraining the morphology transformation of early formed 1D nanostructure. The obtained ultrafine nanowires can be well preserved at a low temperature structure transformation in solid state. As anode material for lithium-ion batteries, the ultrafine CuO nanowires exhibit high reversible capacity, superior cycling performance and improved rate capability. The improved electrochemical properties of CuO nanowires are ascribed to their ultrafine size which lead to the reduced over-potential, extra reversible reactions at low potentials and improved interface performance between the electrode and electrolyte.

Full text of DOI:10.1016/j.jallcom.2011.07.109

Journal of Alloys and Compounds 509 (2011) 9798–9803
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Controlled synthesis of uniform ultrafine CuO nanowires as anode material for
lithium-ion batteries
Fei Wang∗, Weizhe Tao, Mingshu Zhao, Minwei Xu, Shengchun Yang, Zhanbo Sun, Liqun Wang,
Xiaoping Song∗∗
MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong University, Xi’an 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2011
Received in revised form 18 July 2011
Accepted 30 July 2011
Available online 4 August 2011
A simple solution route is used to synthesize ultrafine Cu(OH)2 nanowires by restraining the morphology
transformation of early formed 1D nanostructure. The obtained ultrafine nanowires can be well preserved
at a low temperature structure transformation in solid state. As anode material for lithium-ion batteries,
the ultrafine CuO nanowires exhibit high reversible capacity, superior cycling performance and improved
rate capability. The improved electrochemical properties of CuO nanowires are ascribed to their ultrafine
size which lead to the reduced over-potential, extra reversible reactions at low potentials and improved
interface performance between the electrode and electrolyte.
Keywords:
Copper oxide
Ultrafine nanowires
Anode
© 2011 Elsevier B.V. All rights reserved.
Lithium-ion battery
1. Introduction
facile synthesis is still a challenge for well-designed CuO/C nano-
composites.
Lithium-ion batteries (LIBs) are widely used in portable elec-
tronic equipments. Now, they are becoming a good choice for
electric vehicles and hybrid electrical vehicles, but their perfor-
mance still lies behind the increasing demands of the consumer.
It’s necessary to explore new electrode materials or design novel
nanostructures of electrode materials to meet these demands
[1–6]. Recently, nano-sized transition metal oxides (MO, where
M is Fe, Co, Ni, and Cu) have been widely investigated as
promising anodes for LIBs since they were first reported by Taras-
con et al. [2]. Among them, copper oxide (CuO) has attracted
much attention due to its high theoretical capacity (670 mAh g−1),
non-toxic, low cost and facile synthesis. However, the severe vol-
ume expansion/contraction during lithiation/delithiation causes
the pulverization of electrodes and leads to rapid deterioration
in capacity. To overcome this intrinsical drawback, an effective
approach is adopted to fabricate CuO/C nano-composites [7–13].
The carbon materials used in composites can not only improve
the electronic conductivity but also prevent the breakdown of
electrode materials during Li+ insertion and extraction [8]. Nev-
ertheless, this strategy usually sacrifices the reversible capacity of
CuO due to the introduction of carbon. Moreover, the reliable and
Because the electrochemical properties of CuO can be greatly
affected by morphologies, intensive research has focused on the
controlled synthesis of various CuO nanostructures over the past
several years [14–23]. Improved cycling performance has been
obtained in most of them. However, it is still a great challenge to
achieve high rate capability in pure CuO nanostructures. In gen-
eral, it is well accepted that a smaller size of CuO can lead to
higher capacity and higher rate capability. This reduces the over-
potential and allows faster reaction kinetics at the electrode surface
[5]. Therefore, the synthesis of CuO nanostructures with ultrafine
size is a promising approach to obtain improved rate capability in
pure CuO nanostructures.
Polycrystalline CuO nanowires with tens of nanometers in diam-
eter have been investigated by others as anode materials for LIBs,
which exhibited superior cycling performance [15]. In this paper,
we focused on the controlled synthesis of uniform CuO nanowires
with ultrafine size and good dispersion by a simple solution route
[24–28], and investigated their lithium storage capacity, rate capa-
bility as well as cycling performance for LIB anode materials.
2. Experimental
In a typical experiment, 0.2 mol NaOH was dissolved in 100 ml distilled water
and 1 mmol CuCl2·2H2O was dissolved in 10 ml distilled water. Then, the CuCl2
solution was dropwise added into the NaOH solution under magnetic stirring at
20 ◦C. After reaction for 20 min, the mixed solution turned turbid. The blue products
were harvested by high-speed centrifugation and thoroughly washed with distilled
water and ethanol. The obtained blue precipitates were dried in oven for 2 h at 60 ◦C.
∗
Corresponding author. Tel.: +86 29 82663034; fax: +86 29 82667872.
Corresponding author. Tel.: +86 29 82665892; fax: +86 29 82667872.
E-mail addresses: feiwang@mail.xjtu.edu.cn (F. Wang), xpsong@mail.xjtu.edu.cn
∗∗
(X. Song).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.07.109

F. Wang et al. / Journal of Alloys and Compounds 509 (2011) 9798–9803
9799
for the sample. Fig. 2a and b presents the typical SEM and TEM
images of as-prepared Cu(OH)2 nanostructure, which show the
uniform ultrafine nanowires with about 5 nm (the inset HRTEM of
Fig. 2b) in diameter and several hundreds nanometers in length. The
nanowires exhibit good dispersion in spite of a slight aggregation
due to their high surface energy.
The reactions occurred in the solution of NaOH and CuCl2 can
be summarized as Eqs. (1) and (2). In highly basic solution, Cu2+
prefers square planar coordination by OH− to form [Cu(OH)4]2−
instead of Cu(OH)2 precipitates. The nucleation of Cu(OH)2 should
start from localized regions with relatively high concentrations of
[Cu(OH)4]2−. Once the nucleus are formed, 1D Cu(OH)2 nanostruc-
ture are easily formed because the growth along [1 0 0] direction is
much faster than other directions [24,29]. Whereas, the 1D Cu(OH)2
nanostructures are unstable in basic solution, which can transform
2
to 2D nanostructures through the coordination of OH− to dz of
Cu2+ and further transform to 3D nanostructures by weak hydro-
gen bond interactions [29]. In this research, to obtain ultrafine
Cu(OH)2 nanowires, the Cu2+ concentration and reaction time were
strictly controlled to restrain the transformation of 1D nanostruc-
tures formed in early stage. From Fig. 3 we can see clearly that both
prolonging the reaction time and increasing the Cu2+ concentration
can lead to two-dimensional aggregation growth.
Fig. 1. XRD pattern of ultrafine Cu(OH)2 nanowires.
Finally, the dried powders were heat-treated in a furnace for 2 h at 150 ◦C. In addi-
tion, the turbid solution mentioned above was heat-treated in water bath at 75 ◦
for 15 min. The black precipitates were harvested and dried.
C
Cu2+ + 4OH− → [Cu(OH)4]2−
(1)
(2)
The structure and morphology of the products were characterized by X-ray
diffraction (XRD, Bruke D8-Advance, Cu-K␣, ꢀ = 0.15406 nm), field emission scan-
ning electron microscope (FESEM, JEOL JSM-7100F) and transmission electron
microscopy (TEM, JEOL JEM-2100).
[Cu(OH)4]2− → Cu(OH)2 ↓ +2OH−
To investigate the electrochemical properties of ultrafine
nanowires as anode materials for LIBs, the as-prepared ultrafine
Cu(OH)2 nanowires should be transformed to CuO structure with-
out obvious morphological changes. Fig. 4a shows the SEM image of
the sample heat-treated at 150 ◦C for 2 h. The morphology of ultra-
fine nanowires is well preserved. XRD pattern (Fig. 4c) shows that
the crystal structure is transformed to monoclinic CuO (JCPDS No.
05-0661). The weak and broad diffraction peaks indicate the fine
grains of CuO. By using the Scherrer’s formula and the full width
Electrochemical properties of CuO electrodes were tested using two-electrode
Swagelok cells with lithium metal as the counter and reference electrodes. The
working electrodes consisted of 70 wt.% active materials (CuO), 20 wt.% conduc-
tive material (acetylene black), and 10 wt.% binder (polyvinylidene fluoride, PVDF).
The electrolyte was 1 M LiPF6 in a mixture of 50 vol.% ethylene carbonate (EC) and
50 vol.% diethylene carbonate (DEC). Test cells were assembled in argon filled glove
box. The galvanostatical charge–discharge measurement was carried out by using
an Arbin BT2000 battery testing system in the voltage range of 0.02–3.0 V (vs. Li/Li+).
The cyclic voltammograms (CV) were tested on Ametek VMC-4 electrochemical
testing system at a scan rate of 0.2 mV s−1 between 0 V and 3.0 V (vs. Li/Li+).
¯
at half maximum (FWHM) data of (1 1 1) peak, the mean crystallite
3. Results and discussion
size of CuO nanowires can be calculated to be 3.7 nm. For com-
parison, the ultrafine Cu(OH)2 nanowires were also heat-treated in
the solution at 75 ◦C for 15 min. Bowknot-like products (Fig. 4b) are
obtained instead of ultrafine nanowires. From XRD pattern (Fig. 4d)
we can see that the products are also monoclinic CuO. As is well
know, Cu(OH)2 is a metastable phase which easily transforms into
more stable CuO in both solid state and solution. Although the
The XRD pattern of the sample prepared by using 1 mmol
CuCl2·2H2O and the reaction time of 20 min (see Section 2) is
shown in Fig. 1. All the diffraction peaks are well consistent
with the orthorhombic Cu(OH)2 (JCPDS No. 13-0420). The rela-
tively broad diffraction peaks indicate the small crystallite size
Fig. 2. SEM image (a) and TEM image (b) of ultrafine Cu(OH)2 nanowires. The inset of (b) is the HRTEM image of ultrafine Cu(OH)2 nanowires.

9800
F. Wang et al. / Journal of Alloys and Compounds 509 (2011) 9798–9803
Fig. 3. (a) SEM image of Cu(OH)2 nanostructure prepared by using 1 mmol CuCl2·2H2O and the reaction time of 3 h; (b) SEM image of Cu(OH)2 nanostructure prepared by
using 4 mmol CuCl2·2H2O and the reaction time of 20 min.
main feature of two different routes is the formation of square
planar entities CuO4 giving rise to CuO [28,30], the morphology
changes are quite different. There is a little change in solid trans-
formation but a great change in solution which associated with
the formation of sheet-like structure and further self-assembling
into bowknot-like architectures through an oriented aggregate
mechanism [31].
Fig. 5a and b shows the first two CV curves of ultrafine CuO
nanowires and bowknot-like CuO at a scan rate of 0.2 mV s−1
.
Two kinds of CuO electrodes show similar cathodic and anodic
peaks in whole scan processes, but some differences can also be
found between them. Compared to the bowknot-like CuO, the
cathodic peaks of the ultrafine CuO nanowires locate at relatively
high potentials and anodic peaks locate at relatively low poten-
Fig. 4. SEM images of ultrafine CuO nanowires (a) and bowknot-like CuO (b); XRD patterns of ultrafine CuO nanowires (c) and bowknot-like CuO (d).

F. Wang et al. / Journal of Alloys and Compounds 509 (2011) 9798–9803
9801
Fig. 5. CV curves of ultrafine CuO nanowires (a) and bowknot-like CuO (b); voltage–capacity curves of ultrafine CuO nanowires (c) and bowknot-like CuO (d).
are 680 and 470 mAh g−1, respectively. Increasing the current den-
sity to 0.3 C, the discharge capacity of ultrafine CuO nanowires can
also be maintained at about 660 mAh g−1 after 70 cycles (Fig. 6c).
Improved cycling performance of CuO nanowires can be attributed
to the ultrafine size and good dispersion which is beneficial for
accommodating the strain associated with the severe volume vari-
ations during Li+ insertion and extraction. Fig. 6d shows the rate
capability of two kinds of CuO electrodes at different current
densities. Compared with bowknot-like CuO, the ultrafine CuO
nanowires exhibit higher rate performance. It can be observed
that a charge capacity of about 760 mAh g−1 is obtained at 0.1 C
tials. The potential difference between the cathodic and anodic
peaks can be considered as the polarization of the electrode. The
reduced over-potential in ultrafine CuO nanowires shows faster
reaction kinetics. The large surface areas of ultrafine CuO nanowires
make the Li+ insertion/extraction take place mostly at the sur-
face rather than in the bulk, which greatly reduce the Li+ diffusion
distance. Hence, the electrochemical reaction of lithium is easily
achieved in the ultrafine CuO nanowires. Furthermore, a relatively
strong cathodic peak at low potentials near 0 V can be found in
the ultrafine CuO nanowires electrode. The cathodic peak at low
potentials can lead to an extra capacity besides the reversible Li-
driven decomposition of the transition-metal oxide, which should
be associated with the formation of a gel-like film on the sur-
face of oxides particles. The formation and dissolution of gel-like
film was proposed as being the result of a catalytically enhanced
electrolyte reduction [32]. Therefore, the ultrafine CuO nanowires
should exhibit better catalytic effect and thus achieve a higher
reversible capacity at low potentials. Fig. 5c and d shows the
first two voltage–capacity curves of ultrafine CuO nanowires and
bowknot-like CuO at 0.1 C (1 C = 670 mA g−1). The discharge capac-
ity of ultrafine CuO nanowires in the second cycle can achieve
790 mAh g−1, while it is only 660 mAh g−1 for bowknot-like CuO.
From black straight line marked in Fig. 5c and d we can see
that the capacity difference between two kinds of CuO electrodes
mainly locates blow 0.7 V, which is corresponding well with the CV
results.
after 10 cycles, and this value is slowly reduced to 730 mAh g−1
,
680 mAh g−1, 590 mAh g−1, 420 mAh g−1 at 0.2 C, 0.4 C, 0.8 C and
1.6 C, respectively. Furthermore, the charge capacity can return
to the original value and gradually increase after 50 cycles when
the current density turns back to 0.1 C. The improved rate capa-
bility indicates a fast transferring of electrons and Li+ in ultrafine
CuO nanowires because of their large electrode-electrolyte contact
area and shortened Li+ diffusion distance. The gradual increase of
the excess capacity after 50 cycles should be attributed that not
all surface, such as the internal surface between the nanowires,
is covered with polymer layer after several cycles at relatively
rapid charge–discharge process. Shaju et al. [33] have suggested
that the growth of polymer layer on the internal surface is lim-
ited to some extent because of slower mass transport of the
constituents that make up the polymer layer. Therefore, the poly-
mer layer on internal surface builds-up slowly, over a number of
cycles.
The cycling performances of the two CuO electrodes at 0.1 C are
illustrated in Fig. 6a and b. The discharge capacities at 30th cycle

9802
F. Wang et al. / Journal of Alloys and Compounds 509 (2011) 9798–9803
Fig. 6. (a) Cycling performance of ultrafine CuO nanowires at 0.1 C; (b) cycling performance of bowknot-like CuO at 0.1 C; (c) cycling performance of ultrafine CuO nanowires
at 0.3 C; (d) cycling performances of ultrafine CuO nanowires and bowknot-like CuO at different current densities.
4. Conclusions
[5] M.G. Kim, J. Cho, Adv. Funct. Mater. 19 (2009) 1497–1514.
[6] R. Liu, J. Duay, S.B. Lee, Chem. Commun. 47 (2011) 1384–1404.
[7] S.F. Zheng, J.S. Hu, L.S. Zhong, W.G. Song, L.J. Wan, Y.G. Guo, Chem. Mater. 20
(2008) 3617–3622.
[8] B. Wang, X.L. Wu, C.Y. Shu, Y.G. Guo, C.R. Wang, J. Mater. Chem. 20 (2010)
10661–10664.
[9] S. Venkatachalam, H.W. Zhu, C. Masarapu, K.H. Hung, Z. Liu, K. Suenaga, B.Q.
Wei, ACS Nano 3 (2009) 2177–2184.
[10] J.Y. Xiang, J.P. Tu, Y.F. Yuan, X.L. Wang, X.H. Huang, Z.Y. Zeng, Electrochim. Acta
54 (2009) 1160–1165.
[11] J.Y. Xiang, J.P. Tu, J. Zhang, J. Zhong, D. Zhang, J.P. Cheng, Electrochem. Commun.
12 (2010) 1103–1107.
[12] C. Li, W. Wei, S.M. Fang, H.X. Wang, Y. Zhang, Y.H. Gui, R.F. Chen, J. Power Sources
195 (2010) 2939–2944.
[13] H.B. Wang, Q.M. Pan, J.W. Zhao, W.T. Chen, J. Alloys Compd. 476 (2009) 408–413.
[14] X.P. Gao, J.L. Bao, G.L. Pan, H.Y. Zhu, P.X. Huang, F. Wu, D.Y. Song, J. Phys. Chem.
B 108 (2004) 5547–5551.
The ultrafine CuO nanowires (∼5 nm) with good dispersion
are successfully synthesized by a simple solution route com-
bined with a heat treatment. Electrochemical studies indicate
that the obtained ultrafine CuO nanowires exhibit high capac-
ity, superior cycling performance and improved rate capability.
The discharge capacity after 70 cycles can be maintained at about
660 mAh g−1 at 0.3 C. Therefore, synthesizing nanostructures with
ultrafine size is an effective way to improve the electrochemi-
cal properties of CuO and other transition metal oxides anode
materials.
[15] L.B. Chen, N. Lu, C.M. Xu, H.C. Yu, T.H. Wang, Electrochim. Acta 54 (2009)
4198–4201.
Acknowledgements
[16] S.Q. Wang, J.Y. Zhang, C.H. Chen, Scripta Mater. 57 (2007) 337–340.
[17] S.Q. Wang, J.Y. Zhang, N. Ding, C.H. Chen, Scripta Mater. 60 (2009)
1117–1120.
[18] J.C. Park, J. Kim, H. Kwon, H. Song, Adv. Mater. 21 (2009) 803–807.
[19] Q. Pan, H. Jin, H. Wang, G. Yin, Electrochim. Acta 53 (2007) 951–956.
[20] H.B. Wang, Q.M. Pan, H.W. Zhao, G.P. Yin, P.J. Zuo, J. Power Sources 167 (2007)
206–211.
[21] J. Morales, L. Sanchez, F. Martin, J.R. Ramos-Barrado, M. Sanchez, Electrochim.
Acta 49 (2004) 4589–4597.
[22] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, J. Power Sources 195
(2010) 313–319.
This work was supported by National Natural Science Foun-
dation of China (51002117 and 50871081), Specialized Research
Fund for the Doctoral Program of Higher Education of China
(20100201120049) and Fundamental Research Funds for the Cen-
tral Universities (0109-08141012, 08143017).
References
[1] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997)
[23] Q.T. Pan, K. Huang, S.B. Ni, F. Yang, S.M. Lin, D.Y. He, J. Alloys Compd. 484 (2009)
322–326.
1395–1397.
[2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000)
[24] C.H. Lu, L.M. Qi, J.H. Yang, D.Y. Zhang, N.Z. Wu, J.M. Ma, J. Phys. Chem. B 108
(2004) 17825–17831.
496–499.
[3] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. Van Schalkwijk, Nat. Mater. 4
[25] X.J. Zhang, G.F. Wang, X.W. Liu, J.J. Wu, M. Li, J. Gu, H. Liu, B. Fang, J. Phys. Chem.
C 112 (2008) 16845–16849.
(2005) 366–377.
[4] P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Ed. 47 (2008)
2930–2946.
[26] Z.H. Yang, J. Xu, W.X. Zhang, A.P. Liu, S.P. Tang, J. Solid State Chem. 180 (2007)
1390–1396.

F. Wang et al. / Journal of Alloys and Compounds 509 (2011) 9798–9803
9803
[27] H.L. Xu, W.Z. Wang, W. Zhu, L. Zhou, M.L. Ruan, Cryst. Growth Des. 7 (2007)
2720–2724.
[31] Y.J. Zhang, S.W. Or, X.L. Wang, T.Y. Cui, W.B. Cui, Y. Zhang, Z.D. Zhang, Eur. J.
Inorg. Chem. 16 (2009) 168–173.
[28] D.P. Singh, A.K. Ojha, O.N. Srivastava, J. Phys. Chem.
3409–3418.
C
113 (2009)
[32] S. Laruelle, S. Grugeon, P. Poizot, M. Dolle, L. Dupont, J.M. Tarascon, J. Elec-
trochem. Soc. 149 (2002) A627–A634.
[29] X.G. Wen, W.X. Zhang, S.H. Yang, Nano Lett. 2 (2002) 1397–1401.
[30] Y. Cudennec, A. Lecerf, Solid State Sci. 5 (2003) 1471–1474.
[33] K.M. Shaju, F. Jiao, A. Débart, P.G. Bruce, Phys. Chem. Chem. Phys. 9 (2007)
1837–1842.