Materials Transactions, Vol. 50, No. 2 (2009) pp. 381 to 387
#2009 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Microstructural Characteristics and the Charge-Discharge Characteristics
of Sn-Cu Thin Film Materials
Chao-Han Wu1 , Fei-Yi Hung2; * , Truan-Sheng Lui1; * and Li-Hui Chen1
1
Department of Materials Science and Engineering, Center for Micro/Nano Science and Technology,
National Cheng Kung University, Tainan, Taiwan 701, R.O. China
2
Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Technology,
National Cheng Kung University, Tainan, Taiwan 701, R.O. China
In this study, radio frequency magnetron sputtering was used to prepare Cu6 Sn5 film anodes. The effects of the thickness of the film and its
index of crystallinity (IOC) on the charge-discharge capacity characteristics are discussed. Increasing the thickness of the film anode from 500 to
1500 nm, not only raised the IOC, but also improved the migration of lithium ions and electrons because of the lower resistivity. So, the
cyclability of the as-adopted film was enhanced with increasing the film thickness. After recrystallization, the IOC rose and the resistivity fell.
However, cracks on the film induced by thermal strain increased the area of the passive film, resulting in reduced cyclability. Also, prolonging
the duration of sputtering (5000 nm) led to a deterioration in the charge-discharge capacity. [doi:10.2320/matertrans.MER2008291]
(Received August 25, 2008; Accepted November 14, 2008; Published January 25, 2009)
Keywords: tin-copper, negative electrode material, crystallization
1.
Introduction
Many alloy systems are being developed to replace
graphite as the anode in lithium rechargeable batteries due
to their better capacity (Sn-Cu,1–4) Li-Sn,5) Cu-Sb,6) Mg-Si7)
and Li-Si8)). Sn-based intermetallic compounds and their
oxides9,10) possess higher capacity, and numerous relevant
reports have investigated Sn-Cu,1–4) Sn-Mo,11) Sn-P,12)
Sn-Ni,13–15) Sn-Ca,16) Sn-Sb,17) Sn-S,18) Li-Sn,5) Ce-Sn,7)
and Sn-O.9,10) Notably, most of the above Sn-based anode
materials were prepared by mechanical alloying (ball milling),4,13,19,20) sintering,4,14) and chemical reduction8,11,17,21,22)
which tend to cause inhomogeneity and microsegregation.
In this study, the Sn-Cu alloy was prepared by refining in a
vacuum at a higher cooling rate. So the target for sputtering
was a homo-Sn-Cu intermetallic compound. Avoiding the
above mentioned problems, the Sn-Cu system has acceptable
cost, simple process conditions (low m.p., larger solid
solution limit, lower activity of elements and higher safety,
the annealing temperature of Sn-Cu is about 250 C, while
that of Sn-Ni is from 350 C to 600 C14,23)), environmental
friendliness (does not contain P, S, Co) and excellent
capacity, making it one of the best choices for an anode
material. So, the present work not only investigates the
microstrucural characteristics of Sn-Cu film anodes, but also
discusses feasible applications.
H. Each sputtering film was cut, after which charge-discharge
testing was performed. The composition of the electrolyte
was LiPF6 þ EC þ DEC (EC : DEC ¼ 1 : 1 vol).
The micro-morphology and interface characteristics of the
Sn-Cu film were investigated using a SEM and a Focused
Ion Beam (FIB). The phases and IOC of the unheated films
and heated films were analyzed by thin-film XRD. The two
main diffraction peaks (30.35 and 43.45 ) were selected to
analyze the index of crystalline (IOC). The IOC is defined
as the function, X ¼ ðIC =ISTD Þ 100%, the IC is the sum of
integrated values of the two peaks for measured samples, the
ISTD is that of the standard sample and X is the IOC. The
angle of incidence was 1 . The velocity of scanning was
4 /min and the range was from 20 to 100 . A constant
current was used for electrochemical testing with 50 cycles
of charge-discharge. The voltage was limited to the range
0:01 V1:5 V with a constant current of 0.1C at room
temperature. In addition, the resistivity of the thin film was
measured using a four-point probe and each datum was the
average of at least 35 test results. For comparison test, a
thicker film with a thickness of 5000 nm was prepared and
defined as SC5000. Cyclic voltammetry (CV) was used to
investigate the cycling efficacy. The electric potential was
limited to the range 0:01 V1:5 V and the velocity of
scanning was 0.5 mV s1 .
3.
2.
Results and Discussion
Experimental Procedures
A Sn-Cu target was prepared in a vacuum melting furnace.
A film of Sn-Cu was sputtered on 10 mm Cu foil without any
treatment. The film with a thickness of 500 nm is defined as
SC500; the 1500 nm is defined as SC1500. Some SC1500
films were annealed at 200 C for 1 h in a vacuum (the cooling
rate was approximately 0.1 C/s) and are designated SC1500*Corresponding
author, E-mail: [email protected], fyhung@
mail.mse.ncku.edu.tw
3.1 Thickness effect
Figure 1 shows the surface characteristics of SC500 and
SC1500. Finer grains are observed on the SC500 film.
However, the size of the grains on the SC1500 film had
grown because of the longer sputtering duration. EDS
analysis on the surface of the SC500 film and SC1500 film
are shown in Table 1. The compositions (Sn/Cu) of the two
films were close to that of Cu6 Sn5 . Thin-film XRD confirmed
that the microstructures of the SC500 film and SC1500 film
were both Cu6 Sn5 (Fig. 2). In other words, both the IOC and
382
C.-H. Wu, F.-Y. Hung, T.-S. Lui and L.-H. Chen
Intensity
Cu6Sn5
SC1500
(a)
SC500
20
30
50
40
60
70
90
80
100
2θ
Fig. 2
Table 2
Thin-film XRD analysis of specimens.
Index of crtstallization (IOC) and sheet resistivity (SR).
Thin film
SC500
SC1500
IOC (%)
27.5
84.0
SC1500-H
100
SR (-cm)
1:87 104
6:66 105
2:92 106
(b)
Fig. 1
SEM photographs of surfaces: (a) SC500 and (b) SC1500.
Table 1 EDS analysis of thin film (at%).
Element
Sn
Cu
SC500
45.37
54.63
SC1500
44.96
55.04
SC1500-H
45.04
54.96
grain size of the Cu6 Sn5 film had increased with increasing
the thickness of the film (Table 2).
The cross-section characteristics were observed using an
FIB (Fig. 3). The results show that the two films had
laminated constructions and the number of deposited cracks
increased as the thickness was raised. Notably, no cracks
were observed using a general polishing process. Furthermore, the surface of the SC500 film was less rough than that
of the SC1500 film (Fig. 3). This means that the SC500 film
not only possessed finer grains (Fig. 1), but also had a larger
surface area. Based on the above findings, charge-discharge
testing was performed on the SC500 and SC1500 films to
investigate the effects of film thickness, grain size, IOC and
deposited cracks.
The curves of the initial discharge capacities at 25 C are
shown in Fig. 4. Based on a reference,24,25) there are two level
zones at 0.4 V (equation: Cu6 Sn5 þ 10Li ! 5Li2 CuSn þ
Cu) and 0.2 V (equation: Li2 CuSn þ 2.4Li ! Li4:4 Sn þ Cu).
In Fig. 4, the curves also possess two level zones. For the
0.4 V zone, the longer SC1500 curve suggests that more
Li2 CuSn was produced during the discharge reaction. Due to
the effect of film thickness, the initial electric potential of the
SC1500 film was higher than that of the SC500 film. Figure 5
shows the charge and discharge capacities as a function
of cycle numbers for the SC500 film and SC1500 film at
room temperature. The first charge capacity of the SC1500
film (560 mAh/g) was higher than that of the SC500 film
(351 mAh/g). After 50 cycles, the SC1500 film still possessed a better charge-discharge cycling capacity.
According to previous research and relevant references,26,27) a sputtering film with higher IOC not only causes
resistivity to decrease, but also inhibits lattice deformation
during cycling. In addition, the lithium ions encounter fewer
obstacles during migration which improves the cycleability
of the charge-discharge. In order to avoid the effects of film
thickness, grain size and deposited cracks, the SC1500
film was subjected to recrystallization at 200 C for 1 h in a
vacuum, then cooled to room temperature with a cooling rate
of 0.5 C/min. In this way, a recrystallization film (named
SC1500-H) with a higher IOC was obtained.
3.2 Recrystallized behavior and compound phases
Figure 6 is the surface image of the SC1500-H film and
there is no obvious difference compared with the SC1500
Microstructural Characteristics and the Charge-Discharge Characteristics of Sn-Cu Thin Film Materials
383
2
SC500
Carbon
Cu6 Sn5
Voltage, V / V
1.6
SC1500
1.2
0.8
SC1500
0.4
Cu-foil
SC500
0
0
200
400
600
Capacity, C / mAhg-1
(a)
Fig. 4
Initial discharge curves of specimens with various thickness.
800
Discharge Capacity, Dc / mAhg-1
Carbon
Cu6Sn5
Cu-foil
SC500
SC1500
600
400
200
0
(b)
Fig. 3 Cross-section characteristics: (a) SC500 and (b) SC1500.
film (Fig. 1(b)). Comparing the XRD patterns of the
SC1500-H film and SC1500 film (Fig. 7), the two peaks
(32 and 45 ) disappeared after the heat treatment. Based on a
reference,24,25) the main reason why peaks disappear is that
some low-temperature Cu6 Sn5 phases (0 -Cu6 Sn5 ) transform
into high-temperature Cu6 Sn5 phases (-Cu6 Sn5 ). -Cu6 Sn5
can enhance the cycleability of charge-discharge. The IOC of
each film was calculated from the thin-film XRD pattern. The
SC1500-H film was defined to be a standard (IOC: 100%)
because it had the highest IOC of all the films (Fig. 8, shows
the SC500 film). The results confirm that IOCs were raised by
a recrystallization treatment, even though the film thickness
increased.
According to a reference,28,29) the IOC and resistivity have
an inverse tendency. The IOC and resistivity of each film are
shown in Table 2, which reveals that the SC1500-H film had
a higher IOC and lower resistivity. In order to understand the
0
5
10
15
20
25
30
35
40
45
50
Cycles
Fig. 5 Charge and discharge capacities as a function of cycle number for
SC500 and SC1500.
Fig. 6 SEM photographs of SC1500-H specimen.
384
C.-H. Wu, F.-Y. Hung, T.-S. Lui and L.-H. Chen
2
1.6
SC1500
Intensity
Voltage, V / V
Cu6Sn5
SC1500-H
1.2
0.8
0.4
SC1500-H
0
0
200
400
600
Capacity, C / mAhg -1
Fig. 9
Initial discharge curves of specimens.
SC1500
20
40
60
80
100
2θ
Fig. 7
GI-XRD diffraction patterns of SC1500-H and SC1500.
Index of crystallization, IOC / %
100
STD
80
84.0
60
40
Discharge Capacity, Dc / mAhg-1
800
SC1500
SC1500-H
600
400
200
0
0
5
10
15
20
25
30
35
40
45
50
Cycles
20
27.5
Fig. 10 Charge and discharge capacities as a function of cycle number for
SC1500 and SC1500-H.
0
SC500
SC1500
SC1500-H
Fig. 8 Comparison in index of crystallization (IOC).
relationship between IOC and the form of high-temperature
Cu6 Sn5 phase (-Cu6 Sn5 ), the first charge capacity (Fig. 9)
and the cycleability of charge-discharge (Fig. 10) of the
SC1500-H film and the SC1500 film were compared.
Notably, the first charge capacity and the cycleability of
the SC1500-H film were lower than those of the SC1500 film.
In fact, many cracks could be observed in the cross-section of
the SC1500-H film using an FIB (Fig. 11). It is clear that the
heat treatment (200 C-1 h) encouraged cracks to grow on the
film (thermal expansion coefficient of Cu6 Sn5 film and Cu
foil were different), resulting in a deterioration of the chargedischarge characteristics, even though this thermal effect
raised the crystallization.
Due to the thickness of the SC1500-H film being the same
as that of the SC1500 film, there was no apparent difference
in the first charge capacity (Fig. 9). The main reason is that
the number of lithium ions inserted was similar. However, the
area of passive film for SC1500-H increased due to the
growth of thermal cracks,28) which caused the insertion of
lithium ions to be inhibited and the cycleability to deteriorate
(Fig. 10). Previous studies have found that neither a decrease
in the heat treatment temperature nor an increase in the heat
treatment duration lead to an improvement in the IOC or an
increase in the amount of high-temperature CuSn compounds. Thus, the charge-discharge characteristics are not
enhanced, and this is why the aging mechanism of the coating
film needs further research.21,22) It should be concluded from
what has been said above that reducing the crystallizing
strain in the thin film is a topic which deserves further
discussion with regards to lithium anodes.
Microstructural Characteristics and the Charge-Discharge Characteristics of Sn-Cu Thin Film Materials
385
Fig. 12 The surface image of SC5000 film.
(a)
Carbon
Cu 6 Sn 5
Cu-foil
(a)
(b)
Fig. 11 Cross-section observation of SC1500-H film using FIB.
To increase the cycleability of the thin film, a thicker film
(SC5000 nm) was prepared to investigate the film structure
and capacity. Figure 12 shows the surface characteristics of
SC5000, revealing dendritic crystals. The cross section
observation is shown in Fig. 13(a). Notably, many deposited
defects were found on the film and a few Cu3 Sn phases
(Fig. 13(b), dark zones) formed in the Cu6 Sn5 film matrix
(bright zones). In other words, the microstructure of the
SC5000 thin film was composed of Cu6 Sn5 phases, Cu3 Sn
phases and solidification defects distributed in the film
matrix. To compare the capacities, the charge and discharge
capacities as a function of cycle numbers for the SC500 film,
SC1500 film and SC5000 film at 25 C are shown in Fig. 14,
and we see that the SC1500 film had a better cycleability than
the other films. In addition, cyclic voltammetry (CV) of the
(b)
Fig. 13 Cross-section observation of SC5000 film using FIB: (a) structural
image (b) magnified image.
386
C.-H. Wu, F.-Y. Hung, T.-S. Lui and L.-H. Chen
discharge capacities value
Discharge Capacity, Dc / mAhg-1
800
SC500
SC1500
SC5000
600
400
annealed
//
500 nm
200
5000nm
1500 nm
Sn-Cu: thickness increase
thin
thick
(a)
0
5
0
10
15
20
25
30
35
40
45
50
Cycles
defects
Cu6Sn5
Fig. 14 Charge and discharge capacities profiles.
Cu3Sn
0.004
SC5000
Cu foil
Current, I / Acm-2
0
(b)
-0.004
1st
2nd
3rd
4th
5th
Fig. 16 Charge and discharge capacities: (a) thick vs. annealed effects and
(b) compound phases effect.
-0.008
-0.012
0
0.4
0.8
1.2
1.6
Electric potential, E / V
Fig. 15 CV curve of SC5000 film from 1st to 5th.
200 C-1 h, the Sn-Cu film had a higher IOC and lower
resistivity. Due to the growth of deposited cracks and the area
of passive film increasing, the -Cu6 Sn5 (high temperature
phase) was not able to enhance the charge-discharge
cycleability. A thicker Sn-Cu film possessed Cu6 Sn5 phases,
Cu3 Sn phases and deposited defects that affected the
oxidation reduction.
Acknowledgements
SC5000 film was performed to plot the migration of lithium
ions (Fig. 15). Two peaks were found at 0.65 V (LiSn phases)
and 0.2 V (Li4:4 Sn phases). Notably, the 0:4 V position had
no peak: this is different from the results in Fig. 4 and Fig. 9.
It should be concluded, from what has been said above, that
the dendritic structure (microsegregation and defects) of the
SC5000 film restrained the oxidation reduction. A schematic
illustration of the charge and discharge capacity is shown in
Fig. 16. It is noteworthy that increasing the thickness of the
film or lengthening the duration of the annealing treatment
(200 C-1 h) did not enhance the charge-discharge characteristics of the Sn-Cu thin film materials.
4.
Conclusion
The IOC and grain size of Cu6 Sn5 film increased with
increasing the thickness of the film. After recrystallization at
This work has been granted by the Center for Frontier
Materials and Micro/Nano Science and Technology, National Cheng Kung Universuty, Taiwan (D97-2700) and NSC
97-2221-E-006-103-MY2/NSC 97-2221-E-006-018 for the
financial support.
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