Microstructures and the Charge-Discharge Characteristics

of Advanced Al-Si Thin Film Materials

Chao-Han Wu1, Fei-Yi Hung2;*, Truan-Sheng Lui1 and Li-Hui Chen1

1Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, R. O. China2Institute 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 Althin film on the charge-discharge capacity characteristics are discussed. The pre-sputtered 40 nm Al thin film not only reduced the resistivity ofthe composite negative electrode film, but also prevented peeling between the Al-Si films and Cu foils. In addition, annealing in the vacuum ledto an improvement on the index of crystalline (IOC) of the negative electrode matrix and enhanced the diffusion of the pre-sputtered Al film. Theannealed Al-Si film with diffused Al film saw an enhancement in the bonding characteristics at the interface stability and the charge-dischargecycling life at high temperature (55�C). [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 replacegraphite as the anode in lithium rechargeable batteries dueto 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: 4200mAh/g) and con-tinue to be investigated. In addition to Si, Al has becomeattractive gradually owing to its excellent capacity (Li2:25Al:2235mAh/g). When the ratio of Li/Al is 2.25, the storage ofLi ions for Al is at its maximum. This means that Al has lessrestraint for the insertion of Li ions, even less than that of Si.But the cycleability and irreversible capacity of Al still arelower without any doping. So, doping the second element inthe 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 stillnot been examined.

The present anode material was prepared using Al-basedbinary film on the Cu foil. Si was adopted as the secondelement because of its good capacity. According to theliterature,8) the violent volume variation of Li22Si5 compoundis greater than that of LiAl compounds during mainlithiation/delothiation (LiSi is 4.4:1 and LiAl is 1:1).Although Si is not as good a choice for the negative electrodematrix material as Al, it is a suitable choice for the secondelement in the Al-Si active-active negative electrode system(low volume variation at a single voltage). In addition, onestudy of Al film12) revealed that heat treatment can improvethe interface bonding property between Al and Cu foil,and also increase the vertical conductivity. Thus, this papernot only investigates the charge-discharge characteristicof Al-Si negative electrodes, but also studies the effects ofpre-sputtered Al film and the annealed behavior, so as tofurther understand the potential of the cycling life at hightemperature (55�C) for use as a thin film negative electrodematerial.

2. Experimental Procedures

AlSi composite film of 400 nm was sputtered on 10 mm Cufoil without any treatment. The 360 nm AlSiAl film (pre-sputtered 40 nm Al film between the AlSi composite film andCu foil) was compared with the AlSi film. Some AlSiAl filmswere annealed at 200�C for 1 h in a vacuum and weredesignated as AlSiAl-H. The cell configuration was showed(Fig. 1). The structure was conformed to the standard and theinterelectrode distance was at 25 mm. Each film was cut forcharge-discharge testing. The composition of the electrolytewas LiPF6+EC+DEC (EC : DEC ¼ 1 : 1 vol.).

The micro-morphology and interface characteristics of theAlSi films were investigated by SEM-EDX and FIB (focusedion beam). The phases and IOC of the un-annealed andannealed films were analyzed by thin-film XRD. The angle ofincidence was 1�. The velocity of scanning was 4�/min andthe range was from 20� to 100�. ESCA was adopted to detectthe variation of elements distributed over the interface afterannealing. A constant current was used for electrochemical

Fig. 1 Cell configuration and fabrication.

*Corresponding author, E-mail: fyhung@mail.mse.ncku.edu.tw

Materials Transactions, Vol. 51, No. 10 (2010) pp. 1958 to 1963#2010 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

testing with 50 cycles. The voltage was limited to the range0.01V�1.5V with a constant current of 0.1C at 25�C (roomtemperature) and 55�C (high temperature). In addition, theresistivity of the thin film was measured using a four-pointprobe and each datum was average of at least 3�5 test results.

3. Results and Discussion

3.1 Structure characteristicsFigure 2 shows cross-section images of AlSi film and

AlSiAl film. The 40 mm pre-sputtered Al film in Fig. 2(b) isnot obvious because the thin film had a low IOC whichresulted in low conductibility. EDS analysis of the surface ofthe AlSi film and AlSiAl film is shown in Table 1. The latersample, AlSiAl film, with pre-sputtered Al film possessed agreater fraction of Al than the former did. The measuredresistivities of both samples are shown in Table 2. Theresistivity 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 AlSiAl filmis 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 thefilm matrix uniformly. Notably, the annealed treatment alsoimproved the conductivity of AlSiAl film.

Thin-film XRD showed peaks for the Al layer and Cu foilbut no peaks for Si appear (Fig. 3). After annealing, a Si peakfor AlSiAl-H film was still not observed. In fact, the samething has been noted in other studies. We can say with a fairdegree of certainty that Si within the film matrix not onlywas 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 thanthat of the AlSiAl film.

Before and after test, the Al peaks didn’t shift revealing thevalue of 2� didn’t change during testing (the aluminum latticehad no obvious change). Based on the Al-Cu phase diagram,it was believed that the IMC layers appeared at the interfacebetween the pre-sputtered Al layer and the Cu foil. Notably,

(a)

(b)

Pt

AlSi

Cu foil

Pt

Cu foil

AlSi

Al

Fig. 2 SEM photographs of cross section: (a) AlSi film and (b) AlSiAl film.

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

AlSiAl-H 52.53 30.16 17.31

Table 2 Resistivity of thin films.

Thin film AlSi AlSiAl AlSiAl-H

� (�-cm) 4:1� 10�2 2:8� 10�4 2:5� 10�4

0 20 40 60 80 100

2θ

Inte

nsi

ty

AlSiAl

Al

Cu

AlSiAl-H

Fig. 3 Thin-film XRD of specimens.

Microstructures and the Charge-Discharge Characteristics of Advanced Al-Si Thin Film Materials 1959

the thickness of Cu/Al IMC layers was very thin (<3wt%)and there was no corresponding peak of IMCs in the XRDpattern.

3.2 Effect of pre-sputtered 40 nm Al filmFigure 4 shows the charge-discharge curves of the AlSl

film and AlSiAl film with the 1st, 5th and 30th cycles at25�C. Based on the Refs. 15), 16), the lithiation/delithiationvoltage of Al is 0.2 V/0.48V and that of Si is 0.05V/0.3V.It is clear that Si didn’t react with Li ions in the film andmerely played the role of buffer. In Fig. 4(a), there are twovoltage plateaus present at the 1st cycle, but none at others.We suggest that the reaction SEI film was performed duringthe initial cycles. After the charge-discharge reaction beganto stabilize gradually (SEI reaction had finished), a voltageplateau appeared at 0.2V in the lithiation curve. At a highernumber of testing cycles (Fig. 4(b)(c)), the plateau widthof AlSi and AlSiAl were different (from AlSiAl � AlSi toAlSiAl > AlSi).

According to reports,16,17) the voltage had rose when thereaction Alþ Li ! LiAl was performed. In other words,more LiAl phases formed in the AlSiAl film causing a widerplateau at 0.2 V. Figure 5 shows the charge-dischargecapacities as a function of cycles at 25�C. During the 30testing 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 Alfilm offered an enhancement to the charge-discharge cyclinglife.

3.3 Annealing mechanism and high temperature cyclinglife

Due to the AlSiAl film possessing a better charge-dischargecycling life, annealing was performed to understand theeffect of crystallization. After annealing, the cross-sectioncharacteristics of AlSiAl-H were observed using a FIB asshown in Fig. 6. Comparing Fig. 2(b), the pre-sputtered Alfilm 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 50000

1

2

3

4

AlSiAl

AlSi

Volt

age,

V /

V

Capacity, C / mAhg -1

(b)

0 400 800 1200 16000

0.4

0.8

1.2

1.6

AlSiAl

AlSi

Volt

age,

V /

V

Capcity, C / mAhg -1

0 200 400 600 8000

0.4

0.8

1.2

1.6

AlSiAl

AlSi

(c)

Capacity, C / mAhg-1

Vo

latg

e, 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

Dis

char

ge

Cap

acit

y, D

c / m

Ah

g-1

Fig. 5 Discharge capacities as a function of cycle number for AlSi film and

AlSiAl film.

1960 C.-H. Wu, F.-Y. Hung, T.-S. Lui and L.-H. Chen

Al diffused toward both the Cu foil and Al-Si film, and thediffusion path was �20 nm. So, EDS analysis of the surfaceof the AlSiAl-H film and AlSiAl film yielded similar results(Table 1). In addition, the diffraction peak of Al (AlSiAl-Hfilm) was higher and wider than prior to annealing (AlSiAlfilm). We may say that the IOC was improved afterannealing. This is why the resistivity of AlSiAl-H was lowerthan the other films (Table 2).

Based on one Ref. 18), the IOC and the resistivity of a filmare inversely proportional. Table 2 and Fig. 3 show that theAlSiAl-H film possessed a higher IOC and lower resistivitythan those of AlSiAl. In order to examine the crystallizationeffect of annealing on the charge-discharge characteristics,the charge-discharge curves (Fig. 7) and the capacity as afunction of cycles (Fig. 8) at 25�C were compared underdifferent cycles. Notably, the charge-discharge performanceof the AlSiAl-H film and the AlSiAl film were similar at roomtemperature. To understand the high temperature mechanismand the contribution of the diffusion layer, a charge-dischargecycling test was carried out at 55�C. Figures 9 and 10 showthe charge-discharge curves and the capacities as a functionof cycles for the AlSiAl-H film and AlSiAl film at the 10thcycle. 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 anddelithiation of the AlSiAl-H film were much wider than thoseof the AlSiAl film (Fig. 9). Compared with the AlSiAl film,the AlSiAl-H film had better structure and had a more stablecharge-discharge cyclic performance (Fig. 10).

In fact, there was no evident showed the short circuitresulted from the dendrite in this study. When a full cell wasconstituted with the Al-Si alloy negative electrode and apositive electrode (ex: LiMn2O4), the effect of dendritewould not happen. This is the main reason that the presentedpaper was able to prevent the problem of dendrite. Inaddition, annealing not only induced the Al diffused layerand raised the degree of crystallization, but also increased thehigh temperature cycling life. So, the pre-sputtered Al filmgiven 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 50000

1

2

3

4

AlSiAl

AlSiAl-H

(a)

Capacity, C / mAhg-1

Vo

ltag

e, V

/ V

0 400 800 1200 16000

0.4

0.8

1.2

1.6

AlSiAl

AlSiAl-H

(b)

Capcity, C / mAhg -1

Vo

ltag

e, V

/ V

0 200 400 600 8000

0.4

0.8

1.2

1.6

AlSiAl

AlSiAl-H

(c)

Capacity, C / mAhg-1

Vo

ltag

e, V

/ V

Fig. 7 Charge and discharge curves of AlSiAl film and AlSiAl-H film

(annealed film) with various cycles: (a) 1st, (b) 5th and (c) 30th.

Microstructures and the Charge-Discharge Characteristics of Advanced Al-Si Thin Film Materials 1961

bonding characteristics. In addition, the pre-sputtered Allayer had formed stable IMCs to enhance the interfacebonding after annealed. So, the diffused layer (interfacezone) was able to resist the excessive stress between coppercurrent collector and alloy film.

For applied science fields, ESCA of the AlSiAl-H film wasperformed to understand the variation of elements distributedover the interface after annealing (Fig. 11). It revealed thatthe Al diffused layer was at 80 nm (before annealed, the pre-sputtered Al film was at 40 nm). This high temperature stablediffused layer enhanced the Al/Cu and Al-Si/Al interfacebonding and allowed the matrix to maintain performance ina difficult environment.

Figure 12 shows the discharge capacities of all films aftercyclic testing (30 cycles at 25�C; 10 cycles at 55�C). At roomtemperature, the pre-sputtered 40 nm Al film considerablyimproved the discharge capacities obviously. At hightemperature, the capacity of the un-annealed AlSiAl film

0 10 20 30

Cycles

0

200

400

600

800

1000

AlSiAl

AlSiAl-H

Dis

char

ge

Cap

acit

y, D

c / m

Ah

g-1

Fig. 8 Discharge capacities as a function of cycle number for AlSiAl and

AlSiAl-H at 25�C.

0 400 800 12000

0.4

0.8

1.2

1.6

AlSiAl

AlSiAl-H

Capacity, C / mAhg-1

Vo

ltag

e, V

/ V

Fig. 9 High temperature Charge and discharge curves of AlSiAl film and

AlSiAl-H film at 10th cycles (55�C).

0 2 4 6 8 10

Cycles

0

400

800

1200

AlSiAl

AlSiAl-HDis

char

ge

Cap

acit

y, D

c / m

Ah

g-1

Fig. 10 Discharge capacities as a function of cycle number for AlSiAl film

and AlSiAl-H film at 55�C.

0 8 12

Sputter Time (min)

0

4000

8000

12000

16000In

ten

sity

AlSiO

Diffused Layer: 80 nm

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

Dis

char

ge

Cap

acit

y, D

c / m

Ah

g-1

Fig. 12 Discharge capacities of specimens at final cycle as a function of

temperature.

1962 C.-H. Wu, F.-Y. Hung, T.-S. Lui and L.-H. Chen

did not improve the cycling life. After annealing, the highdegree of crystallization and the Al diffused layer led tostable and excellent charge-discharge characteristics (hightemperature life increase 280%).

4. Conclusion

Pre-sputtered 40 nm Al film was able to enhance thecharge-discharge characteristics at room temperature. Afterannealing of 200�C-1 h, the conductibility and crystallizationof the film had increased, which induced a high temperaturestable Al layer. The AlSiAl-H negative electrode not onlyhad higher IOC but also possessed better interface bondingcharacteristics. At higher temperatures, this complex matrixconduced 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 financialsupport.

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Microstructures and the Charge-Discharge Characteristics of Advanced Al-Si Thin Film Materials 1963