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Journal of the Electrochemical Society 152(4) A653-A657 (200


Journal of The Electrochemical Society, 152 4 A653-A657 2005
0013-4651/2005/152 4 /A653/5/$7.00 ? The Electrochemical Society, Inc.

A653

Preparation and Characteriz

ation of Thin Film Li4Ti5O12 Electrodes by Magnetron Sputtering
C.-L. Wang,a,b Y. C. Liao,a,c,z F. C. Hsu,a,b N. H. Tai,b and M. K. Wua,c,d
a

Materials Science Center, bDepartment of Materials Science and Engineering, and cDepartment of Physics, National Tsing Hua University, Hsinchu, Taiwan d Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan This paper reports that spinel-phase Li4Ti5O12 thin ?lms were successfully grown by radio frequency rf magnetron sputtering on an Au/Ti/SiO2 /Si substrate. In this process, the buffer layer of gold serves as a template for the texture growth of Li4Ti5O12 ?lm. The growth temperature affects the microstructure and electrochemical characteristics of the deposited ?lms. In our study, the spinel phase of Li4Ti5O12 appears at deposition temperatures above 500°C. The redox peaks in the cyclic voltammetry of the Li/Li4Ti5O12 cell approach the typical value of 1.55 V as raising the deposition temperature. Moreover, the in?uences of the surface morphology of the ?lm on the capacity were studied. They show that a columnar structure with high porosity was obtained in the ?lm deposited above 650°C. The columnar grains with good crystallinity of the deposited Li4Ti5O12 enhance the capacity of the electrode. In this work, the capacity of 53 Ah/cm2 m can be attained for the ?lm with a thickness of 230 nm deposited at 700°C. This study sheds light on the realization of a solid-state thin ?lm battery and provides a possible solution of electrical power for a mobile integrated circuit chip. ? 2005 The Electrochemical Society. DOI: 10.1149/1.1861193 All rights reserved. Manuscript submitted May 17, 2004; revised manuscript received September 25, 2004. Available electronically February 10, 2005.

Solid-state thin-?lm rechargeable batteries have great advantages over other types of batteries due to their ?exibility, safety, and miniaturization. There are many potential applications, such as smart cards, complementary metal oxide semiconductor CMOS -based integrated circuits, and microelectromechanical system MEMS devices. Lithium-transition-metal-oxide thin ?lms have long been recognized as good candidates for battery electrode materials. For example, layered-phase LiCoO2,1 LiNiO2,2 and spinel-phase LiMn2O43 with high voltage and stability were successfully used as the positive electrode in lithium ion batteries. Thackeray et al.4 proposed that the ionic conductor Li4Ti5O12 can also be a good electrode material for rechargeable lithium ion batteries. This material can be used as the negative electrode in the cell combined with other high voltage materials, such as LiCoO2 and LiMn2O4.5,6 The theoretical capacity of Li4Ti5O12 is 175 mAh/g 60 Ah/cm2 m according to the following reaction suggested by Ohzuku et al.7 Li
8a

Li1/3,Ti5/3

16dO4

+ e? + Li+ → Li2

16c

Li1/3,Ti5/3

16dO4

Based on this equation, during the insertion process, lithium ions are in the tetrahedral 8a sites and the guest lithium ions move to the octahedral 16c sites, thus the total insertion capacity is determined by the number of free octahedral sites. The merits of adopting spinel Li4Ti5O12 include its ?at electrical potential, nearly zero volume change, and excellent reversibility during the insertion/extraction process of Li ions. Li4Ti5O12 thin ?lm prepared by the sol-gel process for lithium battery electrodes has been reported in the past.8-10 The sol-gel growth method is known to be dif?cult to incorporate into the conventional semiconductor process. This article reports the successful growth of spinel Li4Ti5O12 thin ?lms using a radio frequency RF magnetron sputtering technique. Through a series of examinations of the crystallinity, surface morphology, and electrochemical properties of the high quality Li4Ti5O12 thin ?lm, this paper demonstrates the great potential of Li4Ti5O12 used as the material of the electrodes of solid-state thin-?lm batteries. Experimental Li4Ti5O12 thin ?lms were deposited by rf magnetron sputtering from a 2 in. diameter target onto Au 100 nm / Ti 10 nm /SiO2 /Si substrate maintained at various temperatures in the range of 500-700°C. The substrate was adhered to the surface of

the heater by a silver paste, and the temperature was determined by the thermocouple in the heater. All substrates were cleaned in an organic solvent acetone, methanol, isopropanol using a ultrasonic cleaner. The background pressure of the chamber before the heating of the substrate was less than 10?5 Torr. Au ?lm functions as a current collector, while the Ti layer is the buffer layer that improves the adhesion between Au and SiO2. These two metal layers were deposited by standard dc magnetron sputtering. The Li4Ti5O12 target was prepared by the solid-state reaction of TiO2 and Li2CO3 powders. The mixed powder was calcined at 800°C. Then it was reground, cold pelletized, and sintered at 950°C in the ambient air. The X-ray diffraction XRD patterns showed a pure spinel phase with the space group Fd ? m. Before the deposition, the target was 3 presputtered for about 20 min. The ?lms were deposited at the pressure of 30 mTorr with the mixed Ar/O2 3:2 gas, and the power density was estimated to be 4 W/cm2. All ?lms have a thickness around 230 nm. The crystal structure was examined by an X-ray diffractometer MAC Science employing Cu K line. The surface morphologies of Li4Ti5O12 ?lms deposited at various temperatures were observed with a JEOL 6500F scanning electron microscope SEM . The electrochemical properties of the oxide ?lms were measured in a two-electrode cell at room temperature. The cell uses an oxide ?lm as the working electrode combined with a lithium metal foil as the counter electrode. In the cell, the electrolyte was prepared by adopting 1 M LiPF6 dissolved in a solution of ethylene carbonate EC and ethylmethyl carbonate EMC with the volume ratio of 1:1. All cells were assembled inside the argon-?lled glove box. For galvanostatic cycling testing, cells were discharged and charged at the constant current density of 10 A/cm2 between 1.0 and 2.0 V. Cyclic voltammetry CV was performed at a sweep rate of 0.5 mV/s for the characterization of the ?lm electrode. Results Texture and crystallinity of the as-deposited thin ?lms.—Figure 1 shows the X-ray diffraction XRD patterns of the Li4Ti5O12 thin ?lms grown on the Au/Ti/SiO2 /Si substrate at different deposition temperatures. The well-crystallized thin ?lm can be obtained at the deposition temperatures above 500°C, and it exhibits a texture growth in the 111 plane. The texture growth along certain directions is bene?cial to the performance of the thin-?lm electrode.11 These thin ?lms are colorless insulators, as expected. As the substrate temperature is increased, the crystallinity of the ?lms is sub-

z

E-mail: ycliao@phys.nthu.edu.tw

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A654

Journal of The Electrochemical Society, 152 4 A653-A657 2005 of 0.006 ?. This value is only 0.81% less than the bulk value 8.358 ? of Li4Ti5O12. Perhaps it is the strain of gold 2a0 = 8.158 ?, a0 is the cubic lattice constant of gold that leads to this result. To conclude the discussion on the XRD data, the gold layer on the substrate can promote the texture growth of Li4Ti5O12 along 111 . The evolvement of surface morphology of Li4Ti5O12 ?lm with the deposition temperature is shown in Fig. 3. There exists a transition of surface morphology around the growth temperature of 650°C. At the deposition temperature of 600°C, the Li4Ti5O12 ?lm is smooth and shows densely packed grains Figs. 3a and 4a . Above 650°C, more dispersed island-like grains emerge in the ?lm, as shown in the cross-sectional SEM image of the sample deposited at 700°C Fig. 4b , and the ?lm exhibits a rougher surface. The length scale of this porous structure is around 0.1-0.2 m. This kind of structure does not exist in the Li4Ti5O12 ?lm grown on the SiO2 /Si substrate at the same growth temperature, which is demonstrated in Fig. 4c. The formation of these grain structures should be attributed to the presence of islands on the Au layer. These islands, which form preferentially along the 111 plane, serve as the nucleation sites for the depositing Li4Ti5O12 materials. Consequently, the preferred oriented Li4Ti5O12 grains with good crystallinity and the island-like structure appear only on the Au-buffered substrate. This is consistent with our XRD results. Electrochemical measurement of the as-deposited thin ?lms.—Figure 5 shows the CVs obtained from the Li4Ti5O12 ?lms grown at various substrate temperatures. All cyclic voltammogram measurements were operated in the potential range between 1.0 and 2.0 V at a scan rate of 0.5 mV/s. The measurement results indicate that the primary insertion and extraction potential of Li ion are in a range between 1.5 and 1.6 V, which have been suggested resulting from the coexistence of the spinel phase and the rock-salt phase during the extraction and insertion processes of Li+ ions.12 The CV diagrams clearly show that the shape and peak current density of redox peaks depend on the growth temperature. As increasing the deposition temperature, the difference in the peak potential and the width of the redox peak reduce gradually. This reveals the better crystallinity of the ?lm grown at a higher temperature. The value of the potential, which is 1.54 and 1.59 V in Li insertion and extraction of the ?lm deposited at 700°C respectively, agrees with the typical value of Li4Ti5O12.13 This result indicates that the insertion and extraction of lithium ions are easier to accomplish in the ?lm synthesized under higher deposition temperature. The observation is consistent with the previous data that the ?lms grown at higher temperature exhibits better crystallinity and preferred orientation. These ?lms provide more reversible channels for Li ions to diffuse in the three-dimensional framework of Li4Ti5O12.14 Figure 6 shows the discharge behaviors between 1.0 and 2.0 V of the ?lms deposited at various temperatures at the constant current density of 10 A/cm2. All these as-grown ?lms show the potential plateau around 1.55 V, which is the typical redox value of spinelphase Li4Ti5O12. The discharge capacity for the ?lms deposited at 700°C is about 53 Ah/cm2 m, and it is much greater than the ?lms deposited at 600°C. These observations are consistent with the results of cyclic voltammograms, where the peak current density relating to the capacity is enhanced signi?cantly from the deposition temperature of 600 to 650°C. The capacity of the deposited thin ?lm increases substantially as the deposition temperature over 650°C. However, the crystallinity does not change drastically above 650°C. This suggests that the crystallinity of the as-grown ?lm is not the sole reason responsible for the large energy capacity. The plot of both 2 of 111 diffraction peak and the discharge capacity con?rms this suggestion further. In the left axis of Fig. 7, it shows that the crystallinity of Li4Ti5O12 ?lm improves gradually with raising the growth temperature while there is a signi?cant enhancement of discharge capacity above 650°C, as shown in the right axis. The transitions of electrochemical properties coincide with the transition of surface morphology Fig. 3 and 4 . It is apparent that the surface

Figure 1. XRD patterns of the Li4Ti5O12 ?lms deposited on Au/Ti/SiO2 /Si at various deposition temperatures. It shows that both Li4Ti5O12 ?lm and the gold layer have the preferred orientation 111 .

stantially improved, and it shows a highly preferred orientation along the 111 , which is the major diffusion channel of Li ion. As mentioned earlier, the Au layer functions as the current collector for the electrode. Surprisingly, we ?nd that this Au layer enhances the crystallinity of the as-grown thin ?lms, thus gold acts as a buffer layer between the substrate and the Li4Ti5O12 ?lm as well. As shown in Fig. 1, the Au layer also exhibits the preferred 111 orientation at the deposition temperature. The preferred orientation of the Au buffer layer provides a better template to grow 111 oriented Li4Ti5O12 ?lms. This statement is proved by comparing the XRD patterns between the Li4Ti5O12 ?lms deposited on the substrates Au/Ti/SiO2 /Si and SiO2 /Si Fig. 2 . The 111 peak of Li4Ti5O12 ?lm grown at 700°C is enhanced by using the Au/Ti/SiO2 /Si substrate, while the amorphous SiO2 layer did not act as a good template to deposit Li4Ti5O12 ?lm. The preferred orientation along 111 in the Au layer also plays an important role here. Our experimental results indicate that there is no preferred orientation in the Li4Ti5O12 ?lm deposited on a gold foil without a speci?c texture. This gives further support to the above statement. The lattice constant of Li4Ti5O12 ?lm deposited on Au/Ti/SiO2 /Si, calculated from the XRD data, does not show any speci?c trend with the deposition temperature. The average of cubic lattice constants of the four samples is 8.291 ? with a standard deviation

Figure 2. XRD patterns of the Li4Ti5O12 ?lms on the two substrates Au/Ti/SiO2 /Si and SiO2 /Si grown at 700°C. The ?lm on Au/Ti/SiO2 /Si has a much better crystallinity than the ?lm on SiO2 /Si.

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Journal of The Electrochemical Society, 152 4 A653-A657 2005

A655

Figure 4. SEM image of the cross-sectional structure of Li4Ti5O12 thin ?lms deposited at a 600 and b 700 on Au/Ti/SiO2 /Si and c 700°C on SiO2 /Si. The ?lm on Au/Ti/SiO2 /Si grown at 700°C has a disperse-like grain structure while the same one deposited at 600°C shows the closepacked grains. This is attributed to the effect of the gold layer because the ?lm grown on SiO2 /Si at 700°C did not have a columnar structure.

Figure 3. SEM images of the surface morphology of Li4Ti5O12 thin ?lms deposited on Au/Ti/SiO2 /Si at a 600, b 650, and c 700°C. The surface morphology transits to a rougher one at the deposition temperature above 650°C.

morphology of the ?lm also plays an important role on the capacity. Owing to the ?nite diffusion length of Li ions in Li4Ti5O12, Li ions cannot fully penetrate into the grains. Those island-like grains that exist in the ?lms deposited at higher temperatures provide much more effective area for the insertion of Li ions. This statement is in

agreement with the previous study on the relationship between the charge capability and the particle size of Li4Ti5O12.15 In that paper, Kavan et al. showed that the charge capacity is proportional to the surface area of Li4Ti5O12 powder before the particle size reaches to few tens of nanometers. Therefore, the high capacity of Li4Ti5O12 ?lm deposited on Au/Ti/SiO2 /Si at 700°C results from both the rougher island-like grains and the good crystallinity, and consequently, it has sharp redox peaks and a large capacity. Conclusions Spinel-phase Li4Ti5O12 thin ?lms are successfully grown by rf magnetron sputtering on Au/Ti/SiO2 /Si substrate. The deposi-

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A656

Journal of The Electrochemical Society, 152 4 A653-A657 2005

Figure 6. Initial discharge curves of Li4Ti5O12 thin ?lms deposited on Au/Ti/SiO2 /Si at various temperatures. The substantial increase of charge capacity indicates that there should be some transition around the deposition temperature of 650°C. The capacity of the ?lm grown at 700°C reaches to nearly 90% of the theoretical value 60 Ah/cm2 m .

tion temperature in?uences the physical and electrochemical characteristics of the ?lms profoundly. Li4Ti5O12 ?lms grown on Au/Ti/SiO2 /Si can possess good crystallinity and proper surface morphology for the application in an electrode of a thin ?lm battery. Using the optimized Li4Ti5O12 thin ?lm, the test cell of Li/Li4Ti5O12 exhibits sharp redox peaks and a large capacity. The capacity of this ?lm estimated by the discharge curve is 53 Ah/cm2 m, and this value is comparable to those electrodes prepared by other methods. Both CV diagrams and discharge curves show that thin ?lm Li4Ti5O12 /Au/Ti/SiO2 /Si deposited by sputtering can be used as an excellent negative electrode in a lithium thin-?lm battery. The results of this study demonstrate the potential for the realization of lithium-based solid-state thin-?lm batteries. Acknowledgments The authors thank Chen-En Wu for help taking the SEM images. We also thank Phillip Wu for help editing the English writing. This work is supported by the Taiwan National Science Council grant no. NSC 91-2112-M-007-056.

Figure 5. Cyclic voltammograms of Li4Ti5O12 thin ?lms deposited on Au/Ti/SiO2 /Si at various deposition temperatures a 600, b 650, and c 700°C in 1 M in LiPF6 /EC + EMC at 0.5 mV/s. The peak current density is signi?cantly enhanced above 650°C. The potentials of reduction peak Li+ insertion and oxidization peak Li+ extraction agree with other studies.

Figure 7. The plot of both 2 of 111 diffraction peak left axis and the discharge capacity right axis . It indicates that the enhancement of capacity of Li4Ti5O12 thin ?lm does not totally result from the improvement of crystallinity. Because the discharge curve of the ?lm deposited at 500°C did not have any observable plateau, the discharge capacity of this ?lm is nominally zero in our measurement.

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Journal of The Electrochemical Society, 152 4 A653-A657 2005
National Tsing Hua University assisted in meeting the publication costs of this article.

A657

7. T. Ohzuku, A. Ueda, and N. Yamamoto, J. Electrochem. Soc., 142, 1431 1995 . 8. Y. H. Rho, K. Kanamura, M. Fujisaki, J. Hamagami, S. Suda, and T. Umegaki, Solid State Ionics, 151, 151 2002 . 9. L. Kavan and M. Gr?tzel, Electrochem. Solid-State Lett., 5, A39 2002 . 10. Y. H. Rho, K. Kanamura, and T. Umegaki, Chem. Lett., 2001, 1322. 11. K.-F. Chiu, F. C. Hsu, G. S. Chen, and M. K. Wu, J. Electrochem. Soc., 150, 503 2003 . 12. S. Scharner, W. Weppner, and P. Schmid-Beurmann, J. Electrochem. Soc., 146, 857 1999 . 13. D. Peramunage and K. M. Abraham, J. Electrochem. Soc., 145, 2609 1998 . 14. C.-M. Shen, X.-G. Zhang, Y.-K. Zhou, and H.-L. Li, Mater. Chem. Phys., 78, 437 2002 . 15. L. Kavan, G. Procházka, T. M. Spitler, M. Kalbá?, M. Zakalová, T. Drezen, and M. Gr?tzel, J. Electrochem. Soc., 150, 1000 2003 .

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