Microstructures and the Charge-Discharge Characteristics
of Advanced Al-Si Thin Film Materials
Chao-Han Wu
1, Fei-Yi Hung
2;*, Truan-Sheng Lui
1and Li-Hui Chen
1 1Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, R. O. China
2Institute 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 Al-Si film negative electrodes and the effect of pre-sputtered Al thin film on the charge-discharge capacity characteristics are discussed. The pre-sputtered 40 nm Al thin film not only reduced the resistivity of the composite negative electrode film, but also prevented peeling between the Al-Si films and Cu foils. In addition, annealing in the vacuum led to an improvement on the index of crystalline (IOC) of the negative electrode matrix and enhanced the diffusion of the pre-sputtered Al film. The annealed Al-Si film with diffused Al film saw an enhancement in the bonding characteristics at the interface stability and the charge-discharge cycling life at high temperature (55C). [doi:10.2320/matertrans.M2010070]
(Received February 24, 2010; Accepted August 3, 2010; Published September 25, 2010)
Keywords: aluminum-silicon, negative electrode material, charge-discharge
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-Si,7) Li-Si8)). Notably, Si-based intermetallic compounds (IMC) possess excellent capacity (Li4:4Si: 4200 mAh/g) and
con-tinue to be investigated. In addition to Si, Al has become attractive gradually owing to its excellent capacity (Li2:25Al:
2235 mAh/g). When the ratio of Li/Al is 2.25, the storage of Li ions for Al is at its maximum. This means that Al has less restraint for the insertion of Li ions, even less than that of Si. But the cycleability and irreversible capacity of Al still are lower without any doping. So, doping the second element in the Al negative electrode is able to improve the reliability. Some Al-based systems like Al-C,9) Al-Fe10) and Al-Sn11)
have been studied so far. However, the Al-Si system has still not been examined.
The present anode material was prepared using Al-based binary film on the Cu foil. Si was adopted as the second element because of its good capacity. According to the literature,8)the violent volume variation of Li22Si5compound
is greater than that of LiAl compounds during main lithiation/delothiation (LiSi is 4.4:1 and LiAl is 1:1). Although Si is not as good a choice for the negative electrode matrix material as Al, it is a suitable choice for the second element in the Al-Si active-active negative electrode system (low volume variation at a single voltage). In addition, one study of Al film12)revealed that heat treatment can improve
the interface bonding property between Al and Cu foil, and also increase the vertical conductivity. Thus, this paper not only investigates the charge-discharge characteristic of Al-Si negative electrodes, but also studies the effects of pre-sputtered Al film and the annealed behavior, so as to further understand the potential of the cycling life at high temperature (55C) for use as a thin film negative electrode
material.
2. Experimental Procedures
AlSi composite film of 400 nm was sputtered on 10mmCu foil without any treatment. The 360 nm AlSiAl film
(pre-sputtered 40 nm Al film between the AlSi composite film and Cu foil) was compared with the AlSi film. Some AlSiAlfilms
were annealed at 200C for 1 h in a vacuum and were
designated as AlSiAl-H. The cell configuration was showed
(Fig. 1). The structure was conformed to the standard and the interelectrode distance was at 25mm. Each film was cut for charge-discharge testing. The composition of the electrolyte was LiPF6+EC+DEC (EC:DEC¼1 : 1vol.).
The micro-morphology and interface characteristics of the AlSi films were investigated by SEM-EDX and FIB (focused ion beam). The phases and IOC of the un-annealed and annealed films were analyzed by thin-film XRD. The angle of incidence was 1. The velocity of scanning was 4/min and
the range was from 20to 100. ESCA was adopted to detect the variation of elements distributed over the interface after annealing. A constant current was used for electrochemical
Fig. 1 Cell configuration and fabrication.
*Corresponding author, E-mail: fyhung@mail.mse.ncku.edu.tw
[image:1.595.319.534.318.506.2]testing with 50 cycles. The voltage was limited to the range 0.01 V1.5 V with a constant current of 0.1C at 25C (room temperature) and 55C (high temperature). In addition, the resistivity of the thin film was measured using a four-point probe and each datum was average of at least35test results.
3. Results and Discussion
3.1 Structure characteristics
Figure 2 shows cross-section images of AlSi film and AlSiAl film. The 40mmpre-sputtered Al film in Fig. 2(b) is
not obvious because the thin film had a low IOC which resulted in low conductibility. EDS analysis of the surface of the AlSi film and AlSiAl film is shown in Table 1. The later
sample, AlSiAl film, with pre-sputtered Al film possessed a
greater fraction of Al than the former did. The measured resistivities of both samples are shown in Table 2. The resistivity of AlSiAl is lower than that of AlSi due to the
diffusion of some Al atoms from the pre-sputtered Al film. Another reason is that when the Si fraction of the AlSiAlfilm
is more than 30 at%, the resistivity increased substantially. After pre-sputtering Al thin film then performed the anneal-ing, this not only stabilize the IMCs, but also made the film matrix uniformly. Notably, the annealed treatment also improved the conductivity of AlSiAl film.
Thin-film XRD showed peaks for the Al layer and Cu foil but no peaks for Si appear (Fig. 3). After annealing, a Si peak for AlSiAl-H film was still not observed. In fact, the same
thing has been noted in other studies. We can say with a fair degree of certainty that Si within the film matrix not only was nanoscaled,13)but also had an amorphous structure.14,15)
After annealing, only the crystallization of Al was improved. This is why the Al peak of the AlSiAl-H film was higher than
that of the AlSiAlfilm.
Before and after test, the Al peaks didn’t shift revealing the value of2didn’t change during testing (the aluminum lattice had no obvious change). Based on the Al-Cu phase diagram, it was believed that the IMC layers appeared at the interface between the pre-sputtered Al layer and the Cu foil. Notably,
(a)
(b)
Pt
AlSi
Cu foil
Pt
Cu foil
AlSi
Al
[image:2.595.56.284.70.522.2]Fig. 2 SEM photographs of cross section: (a) AlSi film and (b) AlSiAlfilm.
Table 1 EDS analysis of thin films.
Al at% Si at% O at%
AlSi 45.39 36.76 17.86
AlSiAl 52.85 29.69 17.47
[image:2.595.303.550.84.141.2]AlSiAl-H 52.53 30.16 17.31
Table 2 Resistivity of thin films.
Thin film AlSi AlSiAl AlSiAl-H
(-cm) 4:1102 2:8104 2:5104
0 20 40 60 80 100
2θ
Intensity
AlSiAl
Al
Cu
AlSiAl-H
[image:2.595.316.539.152.501.2]the thickness of Cu/Al IMC layers was very thin (<3wt%) and there was no corresponding peak of IMCs in the XRD pattern.
3.2 Effect of pre-sputtered 40 nm Al film
Figure 4 shows the charge-discharge curves of the AlSl
film and AlSiAl film with the 1st, 5th and 30th cycles at
25C. Based on the Refs. 15), 16), the lithiation/delithiation voltage of Al is 0.2 V/0.48 V and that of Si is 0.05 V/0.3 V. It is clear that Si didn’t react with Li ions in the film and merely played the role of buffer. In Fig. 4(a), there are two voltage plateaus present at the 1st cycle, but none at others. We suggest that the reaction SEI film was performed during the initial cycles. After the charge-discharge reaction began to stabilize gradually (SEI reaction had finished), a voltage plateau appeared at 0.2 V in the lithiation curve. At a higher number of testing cycles (Fig. 4(b)(c)), the plateau width of AlSi and AlSiAl were different (from AlSiAlAlSi to
AlSiAl>AlSi).
According to reports,16,17)the voltage had rose when the reaction AlþLi!LiAl was performed. In other words, more LiAl phases formed in the AlSiAl film causing a wider
plateau at 0.2 V. Figure 5 shows the charge-discharge capacities as a function of cycles at 25C. During the 30 testing cycles, the capacity of AlSi reduced considerably (983!235mAh/g), but that of AlSiAl remained constant
(663!598mAh/g). Obviously, the pre-sputtered 40 nm Al film offered an enhancement to the charge-discharge cycling life.
3.3 Annealing mechanism and high temperature cycling life
Due to the AlSiAlfilm possessing a better charge-discharge
cycling life, annealing was performed to understand the effect of crystallization. After annealing, the cross-section characteristics of AlSiAl-H were observed using a FIB as
shown in Fig. 6. Comparing Fig. 2(b), the pre-sputtered Al film became thicker (diffused layer form) after annealing. This means that the IOC of the matrix had been raised and
(a)
0 1000 2000 3000 4000 5000 0
1 2 3 4
AlSiAl
AlSi
V
o
ltage,
V
/ V
Capacity, C / mAhg-1
(b)
0 400 800 1200 1600 0
0.4 0.8 1.2 1.6
AlSiAl
AlSi
V
oltage,
V
/ V
Capcity, C / mAhg-1
0 200 400 600 800 0
0.4 0.8 1.2 1.6
AlSiAl
AlSi
(c)
Capacity, C / mAhg-1
Volatge,
V
/ V
Fig. 4 Charge and discharge curves of AlSi film and AlSiAl film with various cycles: (a) 1st, (b) 5th and (c) 30th.
0 10 20 30
Cycles
0 200 400 600 800 1000
AlSiAl
AlSi
Discharge Capacity,
Dc
/ mAhg
-1
[image:3.595.64.272.67.761.2] [image:3.595.318.535.73.269.2]Al diffused toward both the Cu foil and Al-Si film, and the diffusion path was20nm. So, EDS analysis of the surface of the AlSiAl-H film and AlSiAl film yielded similar results
(Table 1). In addition, the diffraction peak of Al (AlSiAl-H
film) was higher and wider than prior to annealing (AlSiAl
film). We may say that the IOC was improved after annealing. This is why the resistivity of AlSiAl-H was lower
than the other films (Table 2).
Based on one Ref. 18), the IOC and the resistivity of a film are inversely proportional. Table 2 and Fig. 3 show that the AlSiAl-H film possessed a higher IOC and lower resistivity
than those of AlSiAl. In order to examine the crystallization
effect of annealing on the charge-discharge characteristics, the charge-discharge curves (Fig. 7) and the capacity as a function of cycles (Fig. 8) at 25C were compared under
different cycles. Notably, the charge-discharge performance of the AlSiAl-H film and the AlSiAlfilm were similar at room
temperature. To understand the high temperature mechanism and the contribution of the diffusion layer, a charge-discharge cycling test was carried out at 55C. Figures 9 and 10 show
the charge-discharge curves and the capacities as a function of cycles for the AlSiAl-H film and AlSiAl film at the 10th
cycle. Clearly, when the reaction started, the high temper-ature cycling life of the AlSiAl film decreased dramatically.
In the other hand, the voltage plateaus of both lithiation and delithiation of the AlSiAl-H film were much wider than those
of the AlSiAl film (Fig. 9). Compared with the AlSiAl film,
the AlSiAl-H film had better structure and had a more stable
charge-discharge cyclic performance (Fig. 10).
In fact, there was no evident showed the short circuit resulted from the dendrite in this study. When a full cell was constituted with the Al-Si alloy negative electrode and a positive electrode (ex: LiMn2O4), the effect of dendrite
would not happen. This is the main reason that the presented paper was able to prevent the problem of dendrite. In addition, annealing not only induced the Al diffused layer and raised the degree of crystallization, but also increased the high temperature cycling life. So, the pre-sputtered Al film given annealing treatment was able to improve the interface
Pt
Al-Si
Al
Cu foil
Fig. 6 Cross section image of AlSiAl-H film.
0 1000 2000 3000 4000 5000 0
1 2 3 4
AlSiAl
AlSiAl-H
(a)
Capacity, C / mAhg-1
Voltage,
V
/ V
0 400 800 1200 1600 0
0.4 0.8 1.2 1.6
AlSiAl
AlSiAl-H
(b)
Capcity, C / mAhg-1
Voltage,
V
/ V
0 200 400 600 800 0
0.4 0.8 1.2 1.6
AlSiAl AlSiAl-H
(c)
Capacity, C / mAhg-1
Voltage,
V
/ V
[image:4.595.321.528.71.760.2] [image:4.595.54.285.73.282.2]bonding characteristics. In addition, the pre-sputtered Al layer had formed stable IMCs to enhance the interface bonding after annealed. So, the diffused layer (interface zone) was able to resist the excessive stress between copper current collector and alloy film.
For applied science fields, ESCA of the AlSiAl-H film was
[image:5.595.64.280.70.274.2]performed to understand the variation of elements distributed over the interface after annealing (Fig. 11). It revealed that the Al diffused layer was at 80 nm (before annealed, the pre-sputtered Al film was at 40 nm). This high temperature stable diffused layer enhanced the Al/Cu and Al-Si/Al interface bonding and allowed the matrix to maintain performance in a difficult environment.
Figure 12 shows the discharge capacities of all films after cyclic testing (30 cycles at 25C; 10 cycles at 55C). At room
temperature, the pre-sputtered 40 nm Al film considerably improved the discharge capacities obviously. At high temperature, the capacity of the un-annealed AlSiAl film
0 10 20 30
Cycles
0 200 400 600 800 1000
AlSiAl
AlSiAl-H
Discharge Capacity,
Dc
/ mAhg
-1
Fig. 8 Discharge capacities as a function of cycle number for AlSiAland AlSiAl-H at 25C.
0 400 800 1200 0
0.4 0.8 1.2 1.6
AlSiAl
AlSiAl-H
Capacity, C / mAhg-1
Voltage,
V
/ V
Fig. 9 High temperature Charge and discharge curves of AlSiAlfilm and AlSiAl-H film at 10th cycles (55C).
0 2 4 6 8 10
Cycles
0 400 800 1200
AlSiAl
AlSiAl-H
Discharge Capacity,
Dc
/ mAhg
[image:5.595.324.528.73.266.2]-1
Fig. 10 Discharge capacities as a function of cycle number for AlSiAlfilm and AlSiAl-H film at 55C.
0 8 12
Sputter Time (min)
0 4000 8000 12000 16000
Intensity
Al Si O
Diffused Layer: 80 nm
[image:5.595.325.528.328.525.2]4
Fig. 11 ESCA analysis of AlSiAl-H film.
0 200 400 600 800 1000
AlSi AlSiAl AlSiAl-H AlSiAl AlSiAl-H
25°C 55°C
Discharge Capacity,
Dc
/ mAhg
-1
[image:5.595.65.273.328.525.2] [image:5.595.322.530.397.747.2] [image:5.595.324.529.573.757.2]did not improve the cycling life. After annealing, the high degree of crystallization and the Al diffused layer led to stable and excellent charge-discharge characteristics (high temperature life increase 280%).
4. Conclusion
Pre-sputtered 40 nm Al film was able to enhance the charge-discharge characteristics at room temperature. After annealing of 200C-1 h, the conductibility and crystallization
of the film had increased, which induced a high temperature stable Al layer. The AlSiAl-H negative electrode not only
had higher IOC but also possessed better interface bonding characteristics. At higher temperatures, this complex matrix conduced to a stable and excellent charge-discharge life.
Acknowledgements
The authors are grateful to National Cheng Kung Uni-versity, the Center for Micro/Nano Science and Technology (D98-2700) and NSC 99-2622-E-006-132 for the financial support.
REFERENCES
1) W. Pu, X. He, J. Ren, C. Wan and C. Jiang: Electrochim. Acta50 (2005) 4140–4145.
2) N. Tamura, R. Ohshita, M. Fujimoto, S. Fujitani, M. Kamino and I.
Yonezu: J. Power Sources107(2002) 48–55.
3) K. D. Kepler, J. T. Vaughey and M. M. Thackeray: J. Power Sources 81–82(1999) 383–387.
4) G. X. Wang, L. Sun, D. H. Bradhurst, S. X. Dou and H. K. Liu: J. Alloy. Compd.299(2000) L12–L15.
5) J. Chouvin, J. Olivier-Fourcade, J. C. Jumas, B. Simon and O. Godiveau: Chemical Physics Lett.308(1999) 413–420.
6) L. M. L. Fransson, J. T. Vaughey, R. Benedek, K. Edstrom, J. O. Thomas and M. M. Thackeray: Electrochem. Commun.3(2001) 317– 323.
7) H. Sakaguchi, H. Honda, Y. Akasaka and T. Esak: J. Power Sources 119–121(2003) 50–55.
8) M. Inaba, T. Uno and Akimasa: J. Power Sources146(2005) 473–477. 9) Z. Chen, J. Qian, X. Ai, Y. Cao and H. Yang: Electrochim. Acta54
(2009) 4118–4122.
10) M. J. Lindsay, G. X. Wang and H. K. Liu: J. Power Sources119–121 (2003) 84–87.
11) R. Z. Hu, L. Zhang, X. Liu, M. Q. Zeng and M. Zhu: Electrochem. Commun.10(2008) 1109–1112.
12) S. Chen, F. Ke, M. Zhou and Y. Bai: Acta Mater.55(2007) 3169– 3175.
13) H. Guo, H. Zhao, C. Yin and W. Qiu: Mater. Sci. Eng. B131(2006) 173–176.
14) H. J. Ahn, Y. S. Kim, W. B. Kim, Y. E. Sung and T. Y. Seong: J. Power Sourc.163(2006) 211–214.
15) L. B. Chen, J. Y. Xie, H. C. Yu and T. H. Wang: Electrochim. Acta53 (2008) 8149–8153.
16) Y. Hamon, Y. Brousse, F. Jousse, P. Topart, P. Buvar and D. M. Schleich: J. Power Sourc.97–98(2001) 185–187.
17) M. J. Lindsay, G. X. Wang and H. K. Liu: J. Power Sourc.119–121 (2003) 84–87.
