Chem. Mater. 2004, 16, 1991-1995
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Synthesis and Characterization of Sn Nanophases in a Ta2O5 Matrix Hyo-Jin Ahn, Kyung-Won Park, and Yung-Eun Sung* Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712, Republic of Korea Received November 18, 2003. Revised Manuscript Received March 9, 2004
To investigate the effect of Sn-Ta2O5 nanostructures on lithium insertion and extraction, size-controlled Sn-Ta2O5 nanostructured electrodes, consisting of Sn nanoparticles in an amorphous, porous Ta2O5 matrix, were designed and fabricated using a cosputtering system with Sn metal and Ta2O5 targets. Transmission electron microscopy revealed well-dispersed Sn particles with average particle sizes of ∼19 nm (sample A), ∼30 nm (sample B), and ∼50 nm (sample C). Both X-ray photoelectron spectroscopy and X-ray diffraction analysis of SnTa2O5 confirmed that the Sn particles exist as a metallic crystalline structure, whereas the Ta2O5 matrix was present in an amorphous state. The performance of the Sn-Ta2O5 nanostructured electrode was superior to that of a Sn thin-film electrode during lithium insertion and extraction. The high-performance in the Sn-Ta2O5 nanostructured electrode is due to the larger number of reactive sites of Sn nanoparticles and the facilitation of the movement of lithium ions through the porous Ta2O5 matrix.
Introduction Nanoscale materials are of considerable interest due to their unique physical, chemical, electronic, and optical properties, as compared with the same materials in the bulk.1-5 Although the effects of nanoparticles have been reported in several systems, questions concerning their properties and uses have still yet to be addressed. Nanostructured materials have previously found applications as additives in electrode materials used in energy-storage devices such as fuel cells, batteries, and solar cells.6-10 The key to the successful use of nanostructured electrodes in batteries is thought to be an ideal combination of a high number of reaction sites, a fast transport pathway for electrons and ions, and the cycle performance of the battery.8,9 For example, Martin et al. demonstrated the advantage of nanosized SnO2 particles within the anode of secondary lithium batteries during Li insertion.11 They proposed that the improved rate and cycling performance are related to the small * Corresponding author. Current address: School of Chemical Engineering and Research Center for Energy Conversion and Storage, Seoul National University, Seoul 151-744, Republic of Korea. E-mail:
[email protected]. (1) Timp, G. Nanotechnology; Springer-Verlag: New York, 1999. (2) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (3) Buffat, P.; Borel, J. P. Phys. Rev. A 1976, 13, 2287. (4) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (5) Goldstein, A. N. Handbook of Nanophase Materials; Marcel Dekker: New York, 1997. (6) Park, K.-W.; Ahn, K.-S.; Choi, J.-H.; Nah, Y.-C.; Sung, Y.-E. Appl. Phys. Lett. 2003, 82, 1090. (7) Park, K.-W.; Choi, J.-H.; Kwon, B.-K.; Lee, S.-A.; Sung, Y.-E.; Ha, H.-Y.; Hong, S.-A.; Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 1869. (8) Attand, G. S.; Elliott, J. M.; Bartlett, P. N.; Whitehead, A.; Owen, J. R. Macromol. Symp. 2000, 156, 179. (9) Li, N.; Martin, C. R.; Scrosati, B. J. Power Sources 2001, 97, 240. (10) Gra¨tzel, M. Nature 2001, 414, 338. (11) Li, N.; Martin, C. R. J. Electrochem. Soc. 2001, 148, 164.
domain size of the Sn grains within the SnO2 nanofibers. However, the exact relationship between the grain size and the electronic/electrochemical properties of the metal nanoparticles in battery systems is not yet fully understood. The demand for lithium rechargeable batteries has shown steady growth in recent times, leading to an increasing need for greater battery capacity. To enhance the energy density of lithium batteries, it is necessary to develop new anodic materials which have capacities greater than that of carbon (372 mAh/g).12 A metal capable of forming an alloy with lithium represents a more promising anode material than the current industry standards, carbon and graphite.13-17 Metallic Sn is known to have one of the highest theoretical capacities (990 Ah/kg) when used as a lithium storage electrode, corresponding to a limiting composition of Li4.4Sn. However, the use of metallic Sn for lithium insertion and extraction requires further study in order to address the enhanced initial performance, the observed mechanical instability due to the large volume expansion, and improved cycling performance. Thin-film electrodes are ideal models for understanding the relationship between electrode properties and electrochemical behavior because they do not usually contain a binder. Further, thin-film electrodes can be applied directly into thin-film batteries.18 Because the technology associated with thin-film electrodes in lithium (12) Tarascon J.-M.; Armand, M. Nature 2001, 414, 359. (13) Tamura, N.; Ohsita, R.; Fusimoto, M.; Fujitani, S.; Kamino, M.; Yonezu, I. J. Power Sources 2002, 107, 48. (14) Winter, M.; Basenhard, J. O. Electrochim. Acta 1999, 45, 31. (15) Kepler, K. D.; Vaughey, J. T.; Thackeray, M. M. Electrochem. Solid-State Lett. 1999, 2, 307. (16) Egashira, M.; Takatsuji, H.; Okada, S.; Yamaki, J. J. Power Sources 2002, 107, 56. (17) Anani, A.; Crouch-Baker, S.; Huggins, R. A. J. Electrochem. Soc. 1987, 134, 3098.
10.1021/cm030665a CCC: $27.50 © 2004 American Chemical Society Published on Web 04/23/2004
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rechargeable batteries is still in its infancy, the development of a suitable fabrication method is of great interest. The main points of this study concern the development of a nanostructured thin-film electrode utilizing the advantages of nanomaterials, and an investigation into the effects of nanoparticle size control with a view to improving the electrode performance during lithium insertion and extraction. Currently, thin-film electrodes are typically prepared by conventional sputtering methods using a single metal target. However, the corresponding electrode material often has a fixed composition with no nanostructure, making this fabrication method unsuitable for the formation of nanoparticle electrodes with controlled composition. On the other hand, a co-sputtering system, comprising sputter guns for both the metal and a porous oxide matrix, is a promising technique from the standpoint that it can produce a two-phase electrode consisting of a metal nanostructured electrode embedded within a matrix. Indepth information regarding the effect of the nanostructure on the electrode, however, has yet to be ascertained. Ta2O5 represents an interesting matrix material as it is known to be a solid electrolyte that facilitates the movement of Li ions during Li insertion and extraction. Moreover, it is also used as a buffer matrix to compensate for the expansion and aggregation of metallic Sn, thus preserving the electrical pathway.19,20 In this study, size-controlled Sn-Ta2O5 nanostructured electrodes have been designed and fabricated using a co-sputtering system comprising Sn and Ta2O5 targets. The performance of the Sn-Ta2O5 electrode has been compared with that of a Sn thin-film electrode, prepared without a Ta2O5 matrix, in terms of lithium insertion and extraction efficiency. Experimental Section Sn-Ta2O5 nanostructured electrodes were deposited on platinum-coated Si substrates using an RF magnetron sputtering system, at a base pressure of less than 5 × 10-6 Torr and a working pressure of 5 × 10-3 Torr, for all films prepared. Sputtering was performed under an atmosphere of inert Ar gas at 40 standard cubic cm per min (SCCM) at room temperature. The Sn-Ta2O5 nanostructured electrodes were subsequently deposited for 10 min at RF powers of 20 W (Sn target) and 20 W (Ta2O5 target) in the case of sample A, 50 W (Sn target) and 20 W (Ta2O5 target) in the case of sample B, and 70 W (Sn target) and 20 W (Ta2O5 target) in the case of sample C. The Sn single-phase electrode was also deposited on a Pt-coated substrate, at an RF power of 50 W for 10 min. All samples were fabricated with electrode areas of 1 × 1 cm and thicknesses of ca. 300 nm. Accordingly, size-controlled SnTa2O5 nanostructured electrodes were obtained by controlling the RF power of each target. Structural analyses of prepared samples were carried out by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). TEM bright field images and selected-area electron diffraction (SAED) patterns (1.1 × 106 magnification) were obtained using a Phillips CM20T/STEM electron microscope at an accelerating voltage of 200 kV (Cu grids were used as the substrate for the TEM analysis). XRD (Rigaku X-ray diffractometer equipped (18) Bates, J. B.; Dudney, N. J.; Neudecker, B.; Ueda, A.; Evans, C. D. Solid State Ionics 2000, 135, 33. (19) Frenning, G.; Nilsson, M.; Westlinder, J.; Niklasson, G. A.; Mattsson, M. S. Electrochim. Acta 2001, 46, 2041. (20) Mattsson, M. S.; Niklasson, G. A. J. Appl. Phys. 1999, 85, 8199.
Ahn et al. with a Cu KR source) analyses of the as-prepared electrodes were used to examine the structure and degree of crystallinity. Data were collected over the range of 20-60° in increments of 0.05° at room temperature. XPS analyses were performed using a VG Scientific (ESCALAB 250) X-ray photoelectron spectrometer, with an Al KR source (1486.6 eV) operated at 15 kV and 150 W, at a base pressure of 2 × 10-9 Torr. To understand the nature of the nanostructured electrodes prepared using the co-sputtering method, and to evaluate their performance, the electrochemical behavior for lithium insertion and extraction was investigated using a conventional twoelectrochemical system. Test cells fabricated with an asdeposited thin-film working electrode and a metallic Li anode were prepared in a dry room under the following conditions: dew point -71 °C and temperature 20 °C. A 0.75 M solution of LiCF3SO3 in PC/DME (1:2 by volume) was used as the electrolyte. The cell performance was evaluated by galvanostatically discharging and charging the cell at a constant current density of 20 µA/cm2 at room temperature using a WBCS 3000 battery tester system (Won-A Tech Corp., Korea).
Results and Discussion Figure 1 shows TEM images and SAED patterns for three size-controlled Sn-Ta2O5 nanostructured electrodes fabricated using the co-sputtering system, in which the Sn nanoparticles (dark region) are embedded within an amorphous, porous Ta2O5 matrix (bright region). As shown in Figure 1, the three samples (AC, images a-c, respectively) exhibit well-dispersed Sn particles with an average particle size of ∼19 nm (sample A), ∼30 nm (sample B), and ∼50 nm (Sample C). The spot image insets in Figure 1 correspond to fast Fourier transformations (FFTs) of the atomic arrangement of the corresponding ordered crystalline lattice. As the Sn particle size decreases, the SAED patterns reveal a gradual transformation from spot (bulk-scale) to ring-like (nanoscale) patterns, consistent with the formation of size-controlled Sn-Ta2O5 nanostructures with crystalline Sn in the range 19-50 nm. Figure 1d shows TEM images and SAED patterns for the microstructure of a pure Sn thin-film electrode fabricated using the conventional sputtering system. The pure Sn thin-film electrode fabricated without the oxide matrix shows agglomerated Sn rather than metallic Sn nanosized grains. In addition, SAED also reveals spot (bulkscale) patterns rather than the ring-like patterns associated with nanoscale formation. Observation of crystalline planes in the enlarged high-resolution TEM (HRTEM) image of sample A (Figure 1e), suggests that the Sn nanoparticles in the Ta2O5 matrix exhibit excellent crystallinity. In the (200) plane of the Sn-Ta2O5 nanostructured electrode, the distance between the Sn nanoparticles corresponds to 0.298 ( 0.010 nm, which is comparable with that (0.295 nm) observed in the (200) plane of the pure Sn thin-film electrode, indicating that the Sn nanoparticles are embedded in the amorphous Ta2O5 matrix following cosputtering. The FFT atomic arrangement of the embedded Sn nanoparticles, pictured in Figure 1d (inset), reveals an ordered crystalline lattice. This diffraction pattern corresponds to a similar pattern generated by theoretical simulation based on dynamic diffraction theory, confirming that the Sn nanoparticles in the co-sputtered Sn-Ta2O5 nanostructured electrode are present in a tetragonal Sn arrangement (space group I41/amd). XRD data were collected to further elucidate the structural properties of the Sn-Ta2O5 nanostructured
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Figure 1. TEM images of (a) sample A, (b) sample B, and (c) sample C of a Sn-Ta2O5 nanostructured electrode deposited using a co-sputtering system. The inset at the top right represents the selected-area electron diffraction (SAED) patterns. (d) Sn grains in a pure Sn thin-film electrode without oxide matrix. (e) Enlarged HRTEM images consisting of Sn nanoparticles and an amorphous Ta2O5 matrix for sample A (inset: fast Fourier transformation (FFT) of Sn crystalline nanoparticles in Sn-Ta2O5).
Figure 2. X-ray diffraction patterns of Sn, Ta2O5, and SnTa2O5 (sample B) thin-films deposited using sputtering or cosputtering methods.
electrodes. The presence of polycrystalline Sn nanoparticles and amorphous Ta2O5 was confirmed by XRD, as shown in Figure 2 (sample B). The XRD patterns for the Sn thin-films are in good agreement with those for Sn metal (2θ ) 30.6, 32.01, and 44.09) [JCPDS 04-0673]. Moreover, no features were observed in the XRD pattern of crystalline Ta2O5, confirming the formation of an amorphous phase, a result which was further corroborated by the lack of evidence for crystalline tanta-
lum oxide in the HRTEM analysis. Therefore, the SnTa2O5 two-phase electrode of sample B has structural properties corresponding to both polycrystalline Sn metal and amorphous Ta2O5, confirming the existence of two phases within the electrode layer. Adjusting experimental parameters such as the RF power level is believed to control the size of the Sn nanoparticles in the amorphous Ta2O5 matrix. The incorporation of nanosized electrodes into a battery system is reported to enhance the performance of lithium ion batteries.8,9,21,22 Accordingly, the cell performance is mainly dependent on the size of the nanoparticles, where controlled grain size is closely related to the cycle performance during lithium insertion and extraction. Therefore, an improved performance can be achieved via the use of Sn nanoparticles in a porous oxide matrix, thus facilitating the role of lithium ions in the lithium insertion and extraction processes. Both XRD and TEM analyses confirm that the Sn-Ta2O5 electrode fabricated by the cosputtering system comprises Sn nanoparticles embedded within a Ta2O5 matrix. The Sn 3d and Ta 4f XPS spectra for the co-sputtered Sn-Ta2O5 electrode are shown in Figure 3. A charge correction was applied to the C 1s (284.5 eV) signal, and (21) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.M. Nature 2000, 407, 495. (22) Aurbach, D.; Nimberger, A.; Markovsky, B.; Levi, E.; Sominski, E.; Gedanken, A. Chem. Mater. 2002, 14, 4155.
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Figure 3. XPS spectra of (a) Sn 3d and (b) Ta 4f in the SnTa2O5 nanostructured electrode (sample B).
all other XPS peaks in the C 1s, Sn 3d, and Ta 4f regions. The Sn 3d5/2 XPS spectrum exhibits signals consistent with the presence of metallic (485.0 eV) and oxidized (486.5 eV) Sn states. The oxidized state may have originated from the formation of a nanocomposite with the oxidized states of tantalum. The Sn metallic state was further confirmed by XRD analysis; however, no peaks relating to the formation of crystalline tin oxides were apparent (Figure 2). In the case of tin oxide, the XRD peaks indicated that 2θ ) 26.6, 33.9, and 51.7° [JCPDS 41-1445] (not shown here). The XRD patterns for the pure Sn thin-films and the Sn-Ta2O5 nanostructure are in good agreement with those for Sn metal. In addition, there is no evidence of crystalline Sn oxide in the HRTEM analysis. Considering the Sn nanoparticles in the Sn-Ta2O5 nanostructure are subject to oxidation, the oxidized Sn is likely to be present on the sample surface and therefore detectable by XPS. Accordingly, the oxidized Sn species in the Sn-Ta2O5 nanostructure are not present as crystalline, but as surface and subsurface oxidized states. Therefore, from XRD, HRTEM, and XPS analyses, we can infer that there is a short-range order of oxidized Sn within the Sn-Ta2O5 nanostructure. If we assume that the photoelectrons in the XPS study have an escape depth of about 2-3 nm, then the Sn in Sn-Ta2O5 is expected to have a predominantly metallic crystalline structure. The binding
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energy separation and area ratio between Ta 4f7/2 (26.4 eV) and Ta 4f5/2 (28.3 eV) were determined to be 1.9 eV and 1.37, respectively,23,24 confirming that tantalum oxide in Sn-Ta2O5 is fully oxidized. Figure 4a shows the voltage vs lithium content (x) curves for the Li/Sn cell containing size-controlled SnTa2O5 electrodes at a constant current density of 20 µA/ cm2. During the first lithium insertion, the potential falls rapidly, and then continues to decrease to 0.2 V in the voltage range 0.2-1.8 V. Lithium insertion reactions for samples A-D result in the subsequent formation of a number of intermetallic LixSn phases at room temperature. The lithium content curves reveal that the nanostructured electrode can be clearly distinguished from the Sn thin-film electrode during the first cycle. For sample A, Sn reacts with 4.2 lithium atoms per formula unit in the first insertion process. In addition, samples B and C show an increased performance with 4.04 and 3.2 lithium atoms per formula unit, respectively, compared to the pure Sn thin-film electrode (Li2.96Sn), which after subsequent charging results in removal of half of the lithium atoms per formula unit. Thus, the Sn-Ta2O5 nanostructured electrodes show Li insertion properties superior to that of the pure Sn thinfilms. This result is likely due to the higher number of reactive sites in the nanosized Sn, coupled with the readily accessible lithium ions that penetrate through the porous Ta2O5 matrix.20,21 Another advantage of SnTa2O5 nanostructured electrodes is the decrease in large absolute volume changes when smaller metallic Sn particles are used. An investigation into this phenomenon is currently in progress.14 To further investigate the optimum conditions for the Sn nanoparticles in the porous Ta2O5 matrix for varying average particle size, capacity retention data were collected for samples A-C. For the preliminary capacity retentions, the three Sn-Ta2O5 nanostructured electrodes showed higher initial performance and good stability compared to that of the pure Sn thin-film electrode. This is due to the higher number of reactive sites and also the porous Ta2O5 matrix, which is known to facilitate the movement of lithium ions in the insertion and extraction process, as shown in Figure 4b. Sample A shows higher performance and better stability compared to the two other samples. However, for the case of the pure Sn thin-film electrode, the reversible retention was determined to be unstable and observed to decay. Therefore, the Sn nanostructured electrodes exhibited greater stability than the pure Sn thin-film electrode, and demonstrated a decrease in stability with increasing particle size: sample A > B > C > pure Sn thin-film. Accordingly, a highly efficient thin-film electrode performance can be realized through the use of size-controlled Sn nanoparticles, thus permitting the preparation of better electrode materials. Conclusions Three Sn-Ta2O5 nanostructured electrodes, comprising size-controlled Sn nanoparticles embedded within a porous Ta2O5 matrix, were fabricated using a cosput(23) Ahn, K.-S.; Sung, Y.-E. J. Vac. Sci. Technol. A 2001, 6, 2840. (24) Mao, A. Y.; Son, K. A.; White, J. M.; Kwong, D. L.; Roberts, D. A.; Vrtis, R. N. J. Vac. Sci. Technol. A 1999, 17, 954.
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Figure 4. (a) Voltage vs. lithium content (x) curves for the Sn-Ta2O5 nanostructured electrode (samples A, B, and C), compared to the Sn thin-film electrode and (b) LixSn vs cycle number for the same cells under similar conditions for three size-controlled Sn-Ta2O5 nanostructured electrodes in which the cutoff voltages were 0.2 and 1.8 V.
tering system. The Sn nanoparticles and the amorphous Ta2O5 matrix were analyzed using TEM, XRD, and XPS. The thin-film nanostructured electrodes showed excellent performance during lithium insertion and extraction compared to that of the pure Sn thin-film electrode. This enhanced performance is attributed to the higher number of reactive sites, and the presence of the porous matrix which facilitates the lithium insertion and extraction process.
Acknowledgment. This work was supported by a grant (code # 04K1501-02121) from the Center for Nanostructured Materials Technology, under the 21st Century Frontier R&D Program of the Ministry of Science and Technology, KOSEF, through the Research Center for Energy Conversion and Storage, and the Brain Korea 21 project of the Ministry of Education. CM030665A
