Carbon-coated copper-tin alloy anode material for lithium ion batteries

Article abstract of DOI:10.1016/j.jallcom.2008.11.159

Carbon-coated copper-tin alloy powders were prepared by heating of mixtures of thermoplastic poly(vinyl alcohol) and nano-sized copper-tin alloy particles in argon atmosphere. The products were characterized by X-ray diffraction (XRD), thermogravimetric a

Full text of DOI:10.1016/j.jallcom.2008.11.159

Journal of Alloys and Compounds 478 (2009) 694–698
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Carbon-coated copper–tin alloy anode material for lithium
ion batteries
Sheng Liua, Qi Lia,∗, Yuxi Chenb, Fengju Zhangb
a Institute of Batteries and Polymer Materials, Hunan University,
Changsha, 410082, PR China
b College of Materials Science and Engineering, Hunan University,
Changsha, 410082, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon-coated copper–tin alloy powders were prepared by heating of mixtures of thermoplastic
poly(vinyl alcohol) and nano-sized copper–tin alloy particles in argon atmosphere. The products
were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission elec-
tron microscopy (TEM), and scanning electron microscopy (SEM). The electrochemical properties of the
carbon-coated copper–tin alloy powders as anode materials for lithium batteries were studied by cyclic
voltammetry (CV) and galvanostatic method. The anode showed high charge capacity up to 460 mAh g−1
and stable cyclic performance even after 40 cycles. This could be ascribed to the existence of inactive
matrix Cu which buffered the large volume change in the course of Li–Sn alloying–dealloying process,
and the presence of carbon layer of alloy particles which enhanced dimensional stability during Li–Sn
alloying–dealloying electrochemical process.
Received 28 October 2008
Received in revised form
22 November 2008
Accepted 25 November 2008
Available online 13 December 2008
Keywords:
Copper–tin alloy
Carbon coating
Anode material
Lithium ion battery
© 2009 Published by Elsevier B.V.
1. Introduction
change which accompany the electrochemical process [11]. It has
been demonstrated that active/inactive nano-composite materials
Due to great demand on high specific energy density and safety
of the lithium ion battery for diverse application, considerable
efforts have been made to explore different types of materials to be
employed as anode materials [1-4]. Among them, tin-based mate-
rials have been widely studied as alternative anode materials to
carbon for lithium ion batteries, because 4.4 Li atoms can react
with Sn to deliver a specific capacity of 993 mAh g−1. However, a
large specific volume change usually occurs during the charging
and discharging processes [5], which leads to rapid pulverization of
the electrode [6,7] and decrease of electrical conductivity between
active material particles. As a result, these alloy materials exhibit
poor cycle performance.
exhibit longer cycleability than that of pure tin electrode. However,
capacity fading of the active/inactive nano-composite materials is
inevitable [12]. Therefore, keeping or enhancing their dimensional
stability during cycling is the most critical factor in solving the
problem.
In this study, nanometer copper–tin alloy particles were encap-
sulated by amorphous carbon layer through carbonization of
poly(vinyl alcohol) (PVA) at high temperature. The amorphous car-
bon layer can enhance dimensional stability of copper–tin alloy
particles during Li–Sn alloying–dealloying electrochemical process.
Hence, the anode exhibit good cycling performance.
It is believed that pulverization effects may be reduced by
using smaller active particles and by using multi-phase systems or
alloys [8,9]. The active/inactive nano-composite concept has been
particularly investigated [10]. The concept involves the intimate
mixture of two or more materials, one (e.g., Sn) electrochemically
reacting with lithium, whereas the others act as electrochemi-
cally inactive confining buffers to accommodate the large volume
2. Experimental
2.1. Preparation of nano-sized Cu–Sn alloy powders and carbon-coated Cu–Sn
alloy powders
Nano-sized Cu–Sn alloy powders were synthesized by reductive precipitation
of metal chlorides from aqueous solution with NaBH4. To produce the Cu6Sn5 alloy
powders, SnCl2·2H2O, CuCl2·2H2O and C6H5Na3O7·2H2O were mixed together with
molar ratios of 1:1.2:3.2, and solved in distilled water to form a 0.2 M solution
of Sn2+. The solution was added drop-wise to 0.4 M alkalineNaBH4 aqueous solu-
tion 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 copper–tin alloy powders in the aqueous solution were aged
6 h in water bath with a constant temperature of 80 ◦C [13]. Then, the solu-
∗
Corresponding author. Tel.: +86 731 8821040; fax: +86 731 8821040.
E-mail addresses: liusheng1209@163.com (S. Liu), liqi@hnu.cn (Q. Li).
0925-8388/$ – see front matter © 2009 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2008.11.159

S. Liu et al. / Journal of Alloys and Compounds 478 (2009) 694–698
695
tion was filtered and the product was washed thoroughly using distilled water
3. Results and discussion
and acetone three times, respectively. The black product was dried at 105 ◦C for
24 h under vacuum. To encapsulate the copper–tin alloy nano-particles with the
amorphous carbon, 2 g poly(vinyl alcohol) was dissolved in distilled and deoxy-
3.1. Morphology and structure of samples
genated water (40 ml) under magnetic stirring at 60 ◦C until
a clear solution
Fig. 1 presents SEM and TEM pictures of nano-sized Cu–Sn
alloy powders and their carbon-coated particles. In Fig. 1(a),
irregular particles (about 30 nm) and their agglomerates can be
observed. Fig. 1(b) shows Cu–Sn alloy particles with about 40 nm
in size are dispersed homogeneous in amorphous carbon. From
Fig. 1(c), it can be seen there is a distinct contrast difference
in the image, which confirms existence of two phases, amor-
phous carbon and copper–tin alloy particles which indicates the
copper–tin alloy particles are coated by amorphous carbon layer.
The content of carbon in the carbon-coated sample can be deter-
mined by TGA results. TGA results of the sample are displayed
in Fig. 2, which shows the weight loss of the powder termi-
nated at about 600 ◦C. The weight loss below 600 ◦C is mainly
due to the carbon in the powder. It can be assessed that carbon
content in the powder is about 30% in mass. With temperature
increasing up to 900 ◦C, TGA shows the weight of the powder
increased slightly, which may correspond to the oxidation of Cu–Sn
3 alloy.
was observed. The nano-sized copper–tin alloy powders (0.4 g) were dispersed
in the solution. Then the viscous fluid was sent into electric tube furnace under
argon atmosphere and heated up to 900 ◦C. It was then held for 6 h (heating rate
to the programmed temperature was 5 ◦C min−1 and a flow rate of Ar gas was
60 ml min−1). Thus the carbon-coated copper–tin alloy powders were obtained. The
crystal structure of obtained materials was detected by X-ray diffraction (D/adv-
8) with Cu-K radiation. The morphology and microstructure of the powders were
observed by field-emission scanning electron microscopy (FE-SEM: JSM-6700F)
and transmission electron microscopy (H-800). The content of carbon was deter-
mined from the ignition loss of the sample heated at 900 ◦C for 1 h in air in a TGA
apparatus.
2.2. Cells assembling and electrochemical test
Composite electrodes were prepared by pasting a slurry, which consists of
80 wt.% Cu–Sn alloy powder, 10 wt.% acetylene black and 10 wt.% LA132 binder (aque-
ous emulsion containing a copolymer of acrylamide, AM; lithium methacrylate,
LiMAA; acrylonitrile, AN) dispersed in de-ionized water, onto a copper foil sub-
strate. The electrodes were dried in a vacuum oven at 105 ◦C for 24 h prior to use.
Test cells were assembled in an argon-filled glove box using Celgard2400 as the sep-
arator, 1 mol l−1 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
BT-2000 battery tester at a constant current density of 50 mA g−1 between 0.0 V and
2.0 V. Cyclic voltammetry measurements were initially conducted at a scanning rate
of 0.5 mV s−1 over the potential range from 2.00 V to 0.00 V on the Arbin BT-2000
battery tester.
Fig. 3 shows XRD patterns of the as-prepared nanometer Cu–Sn
alloy and carbon-coated nanometer Cu–Sn alloy particles. The
diffraction pattern of nano-sized Cu–Sn alloy powders synthe-
sized by reductive precipitation method consists of two phases:
Fig. 1. SEM images of nano-sized Cu–Sn alloy (a), carbon-coated nano-sized Cu–Sn alloy (b) and TEM images of carbon-coated nano-sized Cu–Sn alloy (c).

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S. Liu et al. / Journal of Alloys and Compounds 478 (2009) 694–698
Fig. 2. TGA curve of carbon-coated Cu–Sn alloy power.
the major phase of Cu6Sn5 and the impure phase of Cu3Sn. But
another impure phase of SnO2 appears in carbon-coated pow-
ders, indicating that there was some oxidization in carbon coating
process.
3.2. Electrochemical performance of carbon-coated nanometer
Cu–Sn alloy particles
To investigate the electrochemical properties, cyclic voltam-
metry measurements were first conducted at a scanning rate of
0.5 mV s−1 over the potential range from 2.00 V to 0.00 V. The initial
five scanning cycles are shown in Fig. 4. From Fig. 4(a), it can be seen
that, during the first scanning cycle, there are two reduction peaks
and two oxidation peaks. The first reduction peak in the potential
range 0.5–1.0 V is ascribed to the formation of solid electrolyte inter-
face (SEI) film on the surface of electrode [14], which disappears in
subsequent cycles; the second reduction peak close to 0.0 V versus
Li/Li+ is attributed to the formation of Li4.4Sn. From Fig. 4(b) it can
be seen that the shape of curves is almost similar to that in Fig. 4(a)
except that the fourth scanning cycle and the fifth are nearly over-
lapping, indicating that the carbon-coated nano-sized Cu–Sn alloy
electrode shows better cycling performance. From the second scan-
ning cycle, the reduction peak close to 0.0 V versus Li/Li+ divides into
two peaks, corresponding to two processes: lithium ions insert-
ing Cu6Sn5 to form Li2CuSn (0.4–0.2 V) and further formation of
Fig. 4. Cyclic voltammetry profiles of nano-sized Cu–Sn alloy (a) and carbon-coated
copper–tin alloy (b).
Li4.4Sn (0.2–0.0 V). This is in accordance with the result of Guo
et al. [15].
Fig. 5 presents the discharge and charge profiles of nano-sized
Cu–Sn alloy and carbon-coated nano-sized Cu–Sn alloy composite
electrodes for the 1st, 20th, and 40th cycles.
In the discharge curve, it is generally accepted that discharge
voltage above 0.7 V is attributed to irreversible reaction of oxide
impurity reduction, electrolyte decomposition and solid elec-
trolyte interphase film formation, discharge voltage above 0.2 V
is attributed to lithium ions insert in Cu6Sn5 to form Li2CuSn,
while discharge voltage between 0.0 V 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 V and 0.8 V correspond to
the reversibly formation of the Li2CuSn and Cu6Sn5, respectively
[16]. A plateau at 0.1 V which does not appear in Cu–Sn alloy elec-
trode, appears in the charge curve of the carbon-coated Cu–Sn alloy
electrode. It is possible that the amorphous carbon coating layer
affects the overall change in phase-change rate. Significant differ-
ence among voltage curves can be seen in Fig. 5(a), indicating that
capacity fading happens in cyclic process. From Fig. 5(b), though the
initial charge capacity is far less than the initial discharge capac-
ity, the 20th and 40th charge and discharge curves almost overlap,
showing that there is no obvious capacity fading in the following
cycles. There is high irreversible capacity at the first recharge. It can
be attributed to two main reasons. The one is the formation of SEI
film on the surface of electrode which is inevitable. The second is
the initiation of SnO2 in carbon coating process and previous stud-
Fig. 3. XRD patterns of nano-sized Cu–Sn alloy and carbon-coated nanometer Cu–Sn
alloy particles.

S. Liu et al. / Journal of Alloys and Compounds 478 (2009) 694–698
697
Fig. 6. Cycle performance and columbic efficiency curves of nano-sized copper–tin
alloy (a) and its carbon-coated powders (b).
the cycling columbic efficiency of the electrode, is under additional
investigation.
Fig. 5. Voltage profiles of copper–tin nano-particles (a) and carbon-coated
copper–tin alloy particles (b).
4. Conclusion
ies have shown that SnO2 led to high irreversible capacity at the
first recharge [17]. In this study, the initial high irreversible capac-
ity can be minimized by modifying the carbon coating process, such
as increasing vacuum degree of oven, using more pure protecting
gas.
Carbon-coated copper–tin alloy powders were prepared by
heating of mixtures of thermoplastic poly(vinyl alcohol) and nano-
sized copper–tin alloy particles in argon atmosphere. Despite the
inactive matrix Cu which buffers the larger volume change in the
course of Li–Sn alloying–dealloying process, the external carbon
layer of coated particles can enhance dimensional stability during
Li–Sn alloying–dealloying electrochemical process. So, this material
shows excellent capacity retention, 460 mAh g−1 retention after 40
cycles. The carbon-coated Cu–Sn alloy powders have great poten-
tial as anode materials for improving the energy density of lithium
secondary batteries.
Fig. 6 shows cycling performance and coulomb efficiency of the
two samples. In Fig. 6(a), the first discharge and charge capac-
ity of the nano-sized Cu–Sn alloy electrode is 620 mAh g−1 and
450 mAh g−1, respectively. Further cycling leads to a rapid capacity
decay to 56 mAh g−1after 40 cycles. It can be seen that the coulomb
efficiency of the nano-sized Cu–Sn alloy powder in initial cycles is
low, only 70% for the first cycle and reaching 97% till to the 20th
cycle. Although the existing of inactive matrix Cu can buffer the
large volume change in the course of alloying process, quick fad-
ing still happens. In contrast, in Fig. 6(b), the first discharge and
charge capacities of the carbon-coated Cu–Sn alloy particle elec-
trode are 1015 mAh g−1 and 600 mAh g−1, respectively. And after 40
cycles, there is 460 mAh retention. Columbic efficiency for carbon-
coated Cu–Sn alloy particles increases rapidly at the second cycle
and then gradually with increasing cycle number, tends to saturate
to about 98%. This high performance of carbon-coated Cu–Sn alloy
powders is supposed to attribute to the presence of carbon layer of
alloy particles, which enhances dimensional stability during Li–Sn
alloying–dealloying electrochemical process. So, the cyclic perfor-
mance of the carbon-coated Cu–Sn alloy powders is superior to the
Cu–Sn alloy systems. This aspect, which may rise some concern on
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