Influence of the preparation conditions on the morphology and electrochemical performance of nano-sized Cu-Sn alloy anodes

Article abstract of DOI:10.1016/j.jallcom.2006.08.074

Nano-sized Cu-Sn alloy powders were prepared by reductive precipitation method combining with the aging treatment in constant temperature water bath at 80 °C. The microstructure, morphology and electrochemical property of synthesized Cu-Sn alloy powders w

Full text of DOI:10.1016/j.jallcom.2006.08.074

Journal of Alloys and Compounds 439 (2007) 249–253
Influence of the preparation conditions on the morphology and
electrochemical performance of nano-sized Cu–Sn alloy anodes
∗
Fei Wang , Mingshu Zhao, Xiaoping Song
Department of Materials Physics, School of Science, Xi’an Jiaotong University, Xi’an 710049, PR China
Received 13 May 2006; received in revised form 15 August 2006; accepted 20 August 2006
Available online 6 October 2006
Abstract
Nano-sized Cu–Sn alloy powders were prepared by reductive precipitation method combining with the aging treatment in constant temperature
◦
water bath at 80 C. The microstructure, morphology and electrochemical property of synthesized Cu–Sn alloy powders were evaluated by X-ray
diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and galvanostatical cycling tests. The results indicated that the aged
sample had uniform phase components, integrated particles, loose network-like structures and excellent electrochemical performance. When used
−
6 4 6 5
Sn and Cu Sn samples delivered reversible capacities of about 340 and 370 mAh g
1
as negative electrodes for lithium-ion batteries, the aged Cu
up to 20 cycles, respectively.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Nano-sized; Cu–Sn alloy; Aging treatment; Anode materials; Lithium-ion batteries
1
. Introduction
the electrochemical agglomeration, serious irreversible trap-
ping of insert ions and unstable solid electrolyte interphase
(SEI) film [1,2]. To overcome the above-mentioned disadvan-
tages, several methods, such as high-energy ball-milling [5–8],
vapor deposition [9], electrodeposition [10–12] and reductive
precipitation [13–18], have been tried to prepare nano-sized
alloy anodes with different morphologies, because the elec-
trochemeical performance of nano-sized alloy anode materi-
als could be altered greatly by changing their morphology
[3,4].
Among the above-mentioned methods for preparing nano-
sized alloy anode, the reductive precipitation is shown as a
simple and easy way to obtain good morphology and excellent
electrochemical performance. In this paper, we investigated the
influence of constant temperature water bath treatment (or aging
treatment) on the morphology and electrochemical performance
of alloy anode materials prepared by reductive precipitation.
The Cu–Sn alloy was chosen as the investigated anode mate-
rials for its several advantages including the strong structural
relationship existing between the parent binary intermetallic
electrode and its lithiated product [19–21], relatively inexpen-
sive elements and environmental friendliness. We found that the
optimized morphology and homogeneous composition of the
Cu–Sn alloy powders were achieved by proper aging treatment,
which could be adjusted simply by changing the aging time.
New anode materials with high energy density and specific
capacity are constantly high in demand compared with the con-
ventional anode materials of graphite for lithium-ion batteries.
In recent years, Li/metal alloys, such as Al, Sn, Sb and Si, have
aroused much attention as negative electrodes for lithium-ion
batteries, because they can offer an extremely large specific
and volumetric capacity. However, they undergo severe phase
changes during the lithiation and delithiation processes with
severe volume expansion and contraction. This greatly limits the
mechanical stability and cycle life of the electrode. A number
of approaches have been attempted to overcome this problem,
of which the most promising one is the preparation of superfine
or nano-sized alloy anode materials [1–4].
Making the particle size of the electrode materials into
nanometer scale can reduce the absolute volume variation and
ion diffusion length during the lithiation and delithiation pro-
cesses and thus, improve the cycle life and enhance the kinet-
ics of the electrochemical process. However, nano-sized alloy
anode materials also have some disadvantages, for instance,
∗
Corresponding author. Tel.: +86 29 82675055; fax: +86 29 82667872.
E-mail address: feiwang@mail.xjtu.edu.cn (F. Wang).
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.074

2
50
F. Wang et al. / Journal of Alloys and Compounds 439 (2007) 249–253
More importantly, the electrochemical performance of the sam-
ple was improved largely by proper aging treatment. Our work
shows that the aging treatment is a simple and efficient method
to improve the morphology and electrochemical property of the
alloy anode materials prepared by reductive precipitation.
2
. Experimental
Nano-sized Cu–Sn alloy powders were synthesized by reductive pre-
cipitation of metal chlorides from aqueous solution with NaBH4. To pro-
duce the Cu6Sn5 and Cu6Sn4 alloy powders, SnCl2·2H2O, CuCl2·2H2O and
C6H5Na3O7·2H2O were mixed together with molar ratios of 1:1.2:3.2 and
1
:1.5:3.5, respectively, and solved in distilled water to form a 0.1 M solution of
2
+
Sn . Themixedaqueoussolutionwasaddeddrop-wiseto0.2 MalkalineNaBH4
aqueous solution (pH > 12) under strong magnetic stirring at room temperature.
The superfluous NaBH4 solution was used to ensure a complete reduction of
the metal ions. After co-precipitation, the Cu–Sn alloy powders in the aqueous
solution were aged for 20 and 300 min in water bath with a constant temperature
◦
of 80 C. Then, the solution was filtered and the product was washed thoroughly
using distilled water and acetone. The black product was dried at 105 C for 10 h
6 4
Fig. 2. XRD patterns of Cu Sn alloy powder prepared by reductive precipita-
◦
◦
tion method: (a) aging at 80 C for 300 min and (b) non-aging.
under vacuum.
The crystal structure of obtained materials was detected by X-ray diffraction
D/Max-3A) with Co-K radiation. The morphology and particle size of the pow-
ders were observed by means of field-emission scanning electron microscopy
FE-SEM: JSM-6700).
Composite electrodes were prepared by pasting a slurry, which consists of
0 wt.% Cu–Sn alloy powder, 10 wt.% acetylene black and 10 wt.% polyvinyli-
a little Cu2O, as shown in Fig. 1(c). The presence of impure
phases of Cu3Sn and Cu2O in the non-aged sample indicates
that it also contains a small amount of stannous impure phase
simultaneously, which is not detected by XRD here. So, the
distributing of Sn in non-aging sample is asymmetric. When
(
(
8
dene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP), onto a copper
◦
◦
the sample was aged at 80 C, phase transformation happens
foil substrate. The electrodes were dried in a vacuum oven at 110 C for 12 h
gradually, which will decrease the production of impure phase.
Fig. 1(b) shows that Cu O phase disappears and Cu Sn phase
prior to use. Test cells were assembled in an argon-filled glove box using Cel-
−1
gard 2400 as the separator, 1 mol l LiPF6/ethylene carbonate (EC)–diethyl
carbonate (DEC) (1:1) as the electrolyte, and Li foil as the counter electrode.
The cells were cycled on the arbin BT2000 battery tester at a constant current
2
3
decreases greatly after 20 min aging. When aging time reaches
◦
3
00 min at 80 C, the impure phase of the sample is eliminated.
−2
density of 0.2 mA cm between 0.02 and 1.30 V.
As depicted in Fig. 1(a), pure Cu Sn phase has been achieved.
6
5
TheX-raydiffractionpatternsofCu Sn4 alloypowderareshown
6
3
. Results and discussion
Fig. 1 shows the X-ray diffraction patterns of Cu Sn alloy
in Fig. 2, which is similar to Fig. 1. The non-aged Cu Sn4 sam-
6
◦
ple has several impure phases, but the sample aged at 80 C
6
5
for 300 min only has two phases of Cu Sn and Cu3Sn. The
6
5
powder synthesized by reductive precipitation method. The
diffraction pattern of non-aged sample consists of three phases:
the major phase of Cu Sn and the impure phases of Cu3Sn and
XRD measurement shows that the aging treatment improves the
homogeneity of phase composition, which is due to the complete
atom dispersion in aging treatment process. The homogeniza-
tion of the component must be propitious to Li-ion transferring
and structural maintenance during the lithiation and delithiation
processes.
6
5
From the peak position and the half-height width of the X-
ray diffraction peak, the mean crystallite size can be calculated
using the Scherrer equation. The results are shown in Table 1.
It is observed that the mean crystallite size of Cu Sn alloy
6
5
◦
powder grow up markedly after aging at 80 C. The particle size
observed from FE-SEM images has the similar tendency which
is illustrated as follows.
The FE-SEM images of nano-sized Cu Sn samples with
6
5
different aging time are shown in Fig. 3(a–c). It can be seen
Table 1
The mean crystallite size of Cu6Sn5 alloy powder
Sample
Mean crystallite size (nm)
Cu6Sn5 (non-aging)
Cu6Sn5 (aging at 80 C for 20 min)
Cu6Sn5 (aging at 80 C for 300 min)
11.0
14.5
22.3
Fig. 1. XRD patterns of Cu6Sn5 alloy powder prepared by reductive precipita-
◦
◦
◦
◦
tion method: (a) aging at 80 C for 300 min, (b) aging at 80 C for 20 min and
c) non-aging.
(

F. Wang et al. / Journal of Alloys and Compounds 439 (2007) 249–253
251
that the morphology of three samples have dramatic differ-
ences. As shown in Fig. 3(a), the primary particles of non-aged
Cu Sn sample are too small to be identified and aggregate to
6
5
large aggregations randomly. It can be contributed to that the
strong reductant NaBH4 in combination with an intense stirring
during the reductive process creates a large number of nuclei,
which limited the growth of grains. The small particles with
high surface energy tend to aggregate spontaneously to decreas-
ing their system energy. When the non-aged sample is aged at
◦
8
0 C for 20 min, some smaller primary particles dissolve and
bigger ones grow gradually. But the smaller primary particles
cannot dissolve completely in this period. The residual smaller
particles aggregate on the surface of bigger ones to form large
secondary particles which also have slight aggregation, as shown
◦
in Fig. 3(b). When aging at 80 C for 300 min, the primary par-
ticles and the aggregation of particles disappear completely. A
loose network-like structure appears instead, which can be seen
clearly in Fig. 3(c). The network formed between the particles is
similar to the sinter-neck formed in the powder metallurgy (PM)
incompact sintering process. The loose density of the sample
◦
aged at 80 C for 300 min is far less than that of the non-aged
one because of the loose network-like structure. The size of the
particleswithnetwork-likestructureisabout50 nmandhasgood
uniformity.
Fig. 4 shows that the discharge capacities of the aged sam-
ple are markedly higher than that of the non-aged one except
the first cycle. The aged Cu Sn4 and Cu Sn samples delivered
6
6
5
−1
reversible capacities of about 340 and 370 mAh g up to 20
cycles, respectively. The high reversible capacities of the aged
sample are contributed to the low first cycle irreversible capaci-
ties and good cycling performance, as shown in Figs. 5 and 6.
Fig. 5 shows the typical voltage versus specific capacity
curves of Cu Sn samples for the first cycle. Although the exact
6
5
reaction mechanism of Cu Sn with Li is still not clearly under-
6
5
stand, it is generally accepted that discharge voltage above 0.7 V
is attributed to irreversible reaction of oxide impurity reduction,
electrolyte decomposition and solid electrolyte interphase (SEI)
film formation, discharge voltage above 0.2 V is attributed to
lithium ions insert in Cu Sn to form Li2CuSn, while discharge
6
5
Fig. 3. FE-SEM images of Cu6Sn5 alloy powder prepared by reductive precip-
◦
itation method: (a) non-aging, (b) aging at 80 C for 20 min and (c) aging at
◦
8
0 C for 300 min.
Fig. 4. Cycling performance of Cu6Sn4 and Cu6Sn5 electrodes.

2
52
F. Wang et al. / Journal of Alloys and Compounds 439 (2007) 249–253
energy of nano-sized particle is very high and the system tends
to reduce its free energy by decreasing the surface energy as
much as possible. Therefore, almost all primary particles in the
non-aged sample aggregate spontaneously to decreasing the sur-
face energy. So the contact between the primary particles is very
close but instability. In lithiation process, the aggregated primary
particles can be bonded directly to form larger electrochemical
agglomerations when Li-ions are extracted electrochemically.
When larger agglomerated particles formed, it cannot be sepa-
rated in subsequent lithiation process. The agglomeration of the
particles will destroy the stability of SEI film. Some SEI films
are embedded within the agglomerations and thus deteriorate
the electrical conductivity of the alloy particle. Furthermore,
the intrinsic and extrinsic defects with high density and high
energy exist in the non-aged sample, which also results in low
Coulomb efficiency at the initial several cycles by inducing more
irreversible trappings of insert Li-ions [1,2].
Fig. 5. Initial voltage profiles of Cu6Sn5 electrodes.
On the contrary, the cycling performance is remarkably
◦
improved at initial several cycles by aging at 80 C for 300 min.
voltage between 0.0 and 0.2 V corresponds to the formation
of the fully lithiated Sn phase (i.e., Li4.4Sn). In charge curve,
the charge plateaus at about 0.5 and 0.8 V correspond to the
The following three aspects can explain this phenomenon.
Firstly, the aging treatment promotes the growth of the particles
and reduces the surface area, which results in the decreas-
ing of the first cycle irreversible reaction of oxide impurity
reduction, electrolyte decomposition and SEI film formation.
Secondly, the high integrity and uniformity of the particles of
the aged sample can largely reduce the intrinsic and extrinsic
defects. Thus, the irreversible trappings of insert Li-ions become
weaken. Finally, particle aggregation disappears completely and
a loose network-like structure forms after aging, which greatly
reduces the possibility of electrochemical agglomeration when
Li-ions are electrochemically extracted from the host lattice and
improves the stability of SEI film.
Fig. 6 also shows that the aged and the non-aged samples
exhibit good cycling performance at the subsequent cycles fol-
lowing the initial several ones. The existing of inactive matrix
Cu buffers the larger volume change in the course of alloying
process. Accordingly, the Cu–Sn alloy anodes provide a much
more stable cycling performance than pure tin metal. Nano-
scale of the samples induces small absolute volume variation and
short Li-ion diffusion length during the lithiation and delithia-
tion processes, which must be propitious to improve the cycling
performance. Moreover, more Li2O in non-aged sample and
uniform phase components in aged sample can, respectively,
improve their cycling performance, too.
reversibly formation of the Li2CuSn and Cu Sn , respectively.
6
5
From Fig. 5 we can see that the voltage trends of two sam-
ples are similar. However, the non-aged sample shows more
discharge capacities and less charge capacities than that of the
aged one, which indicates that the non-aged sample has more
first cycle irreversible capacities. It should be noted that the
more discharge capacities of the non-aged sample mainly appear
above 0.7 V in the discharge curve, which is attributed to the
more irreversible reaction because smaller particle and larger
surface area exists in the non-aged sample. In charge curve of
the non-aged sample, the absence of the obvious charge plateau
around 0.8 V, which caused by more irreversible trapping of Li-
ions, results in less charge capacities.
Fig. 6 shows that the non-aged samples of Cu Sn4 and
6
Cu Sn have relatively low Coulomb efficiency at the initial sev-
6
5
eral cycles. There are following possible factors resulting in this
phenomenon. In the perspective of thermodynamics, the surface
4. Conclusions
In this work we have extended the investigation of Cu–Sn
alloy anode by reductive precipitation method. It is proved
that the aging treatment is a simple and efficient method to
improve the morphology and electrochemical property of Cu–Sn
alloy anode materials prepared by reductive precipitation. XRD
patterns and FE-SEM images show that the samples aging at
◦
8
0 C for 300 min has uniform phase components, integrated
particles and loose network-like structures. The aged sample
exhibits excellent electrochemical performance, such as low first
cycle irreversible capacities, high reversible capacities and good
Fig. 6. Coulomb efficiency of Cu6Sn4 and Cu6Sn5 electrodes.

F. Wang et al. / Journal of Alloys and Compounds 439 (2007) 249–253
253
cycling performance. When cycled at a constant current density
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−
2
2
of 0.2 mA cm between 0.02 and 1.30 V, the aged Cu Sn4 and
6
[
[
Cu Sn samples deliver reversible capacities of about 340 and
6
5
1
−1
3
70 mAh g up to 20 cycles, respectively.
Acknowledgement
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The authors would like to acknowledge Introduction of Tal-
ents Fund of Xi’an Jiaotong University (Grant No: 090071181)
for providing financial support to this work.
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