Antimony-Doped SnO2 Nanopowders with High Crystallinity for

Jun 10, 2009 - For the desired purpose of a battery electrode material, such a micrometer ... of the film to approximately 80 °C to spur hydrolysis/c...
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3202 Chem. Mater. 2009, 21, 3202–3209 DOI:10.1021/cm9007014

Antimony-Doped SnO2 Nanopowders with High Crystallinity for Lithium-Ion Battery Electrode Yude Wang,*,†,‡ Igor Djerdj,§ Bernd Smarsly,‡ and Markus Antonietti*,‡ †

Department of Materials Science and Engineering, Yunnan University, 650091 Kunming, China, :: ‡ Max Planck Institute of Colloids and Interfaces, Research Campus Golm, Am Muhlenberg 1, 14476 Potsdam-Golm, Germany, and §Laboratory for Multifunctional Materials, Department of Materials, :: :: ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland Received March 12, 2009. Revised Manuscript Received May 14, 2009

Antimony-doped SnO2 (ATO) nanopowders with high crystallinity were obtained by a polymer-assisted sol-gel process based on a novel amphiphilic block-copolymer (“KLE” type, poly(ethylene-co-butylene)block-poly(ethylene oxide) and simple tin reagents (SnCl4 and Sb(OC2H5)3). As-synthesized samples were analyzed by Thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron micrographs (TEM), N2 adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS). The results showed that the particles were the high crystalline ATO nanopowders of 5-8 nm primary particle size and the Sb was indeed incorporated into the SnO2 crystal structure (cassiterite SnO2). The as-prepared samples were used as negative electrode materials for lithium-ion batteries, whose charge-discharge properties, cyclic voltammetry, and cycle performance were examined. A high initial discharge capacity about 2400 mA h g-1 was observed at a constant discharge current density of approximately C/5 in a potential range of 0.005-3.0 V. A highly stable capacity of 637 mA h g-1 after 100 cycles is substantially higher than that of most previously reported SnO2 nanostructures. The high reversible capacity for ATO nanopowders may be due to the presence of Sb for Sn, leading to an improved formation of metals with respect to structure and formation dynamics from ATO. Introduction Tin oxide is an attractive material as a potential substitute for the conventional graphite anode in lithiumion batteries, because the theoretical capacity of SnO2 (783 mA h g-1) has been estimated to be superior to that of graphite (372 mA h g-1) 1 but also owing to the reasonably low potentials and high volumetric and gravimetric capacities.2 However, the practical use of SnO2 powders as a negative electrode for lithium-ion batteries is still hindered by several major problems: poor cycleability, resulting from the severe volume expansion and contraction during the alloying-dealloying cycles with Li+ ions and the associated charge transfer process,3 and the pulverization and the agglomeration of primitive particles, which drastically reduces the total entrance/exit sites available for Li+ ions.4 On the other hand, the poor electronic conductivity hinders the reaction with lithium during the first discharge.5 It is believed that both the particle size and its size distribution play important roles in the electrochemical performance. Generally, smaller particles and a *To whom correspondence should be addressed. E-mail: [email protected] (Y.D.W.), [email protected] (M.A.).

(1) Uchiyama, H.; Hosono, E.; Honma, I.; Zhou, H. S.; Imai, H. Electrochem. Commun. 2008, 10, 52. (2) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395. (3) (a) Winter, M.; Besenhard, J. O. Electrochim. Acta 1999, 45, 31. (b) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (4) Wang, Y.; Lee, J. Y.; Zeng, H. C. Chem. Mater. 2005, 17, 3899. (5) Santos-Pe na, J.; Brousse, T.; Sanchez, L.; Morales, J.; Schleich, D. M. J. Power Source 2001, 97-98, 232.

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more uniform distribution of the particles tend to experience more moderate volume changes and a stable microstructure of the textured powder, thereby giving improved performance.6 SnO2 is an n-type semiconductor oxide with a wide energy gap (Eg = 3.62 eV at 300 K), and the charge carrier concentration of SnO2 and thus the conductivity is further increased by extrinsic dopants. Antimony is a common n-type dopant in SnO2. The free electron concentration in SnO2 strongly increases when it is doped with Sb.7 As an important member of the transparent conductive oxides (TCOs), antimony-doped tin oxide (ATO) exhibits high optical transmission, electrical conductivity, good stability, and low cost. The synthesis and preparation of ATO are of great technological and scientific interest owing to the use of these materials as transparent electrodes, heat mirrors, displays, electrochromic windows, gas sensors, catalysts, rechargeable Li batteries, and energy storage devices. ATO therefore has potential uses in photovoltaic and optoelectronic devices.8 (6) Fan, J.; Wang, T.; Yu, C.; Tu, B.; Jiang, Z.; Zhao, D. Adv. Mater. 2004, 16, 1432. (7) Stjerna, B.; Olsson, E.; Granqvist, C. G. J. Appl. Phys. 1994, 76, 3797. (8) (a) Rockenberger, J.; zum Felde, U.; Tisher, M.; Troger, L.; Hasse, M.; Weller, H. J. Chem. Phys. 2000, 112, 4296. (b) Zhang, J. R.; Gao, L. Inorg. Chem. Commun. 2004, 7, 91. (c) Shukla, S.; Ludwig, L.; Parrish, C.; Seal, S. Sens. Actuators, B 2005, 104, 223. (d) Sun, K.; Liu, J.; Browning, N. D. J. Catal. 2002, 205, 266. (e) Santos-Pena, J.; Brousse, T.; Sanchez, L.; Morales, J.; Schleich, D. M. J. Powder Sources 2001, 97-98, 232. (f ) Emons, T. T.; Li, J. Q.; Nazar, L. F. J. Am. Chem. Soc. 2002, 124, 8516. (g) Gamard, A.; Babot, O.; Jousseaume, B.; Rascle, M. C.; Toupance, T.; Campet, G. Chem. Mater. 2000, 12, 3419. (h) Orel, Z. C.; Orel, B.; Hodoscek, M.; Kaucic, V. J. Mater. Sci. 1992, 27, 313.

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Indeed it is expected that particle sizes in the nanoregime and specific crystal morphologies enhance the performance and allow a fine-tuning of the properties.9 Decreasing the primary particle size to a few nanometers while retaining a homogeneous size distribution however still remains a crucial issue for most of the applied synthesis techniques.10 A variety of techniques has been usually used to synthesize oxides nanoparticles including hydrothermal, coprecipitation, combustion, sol-gel, solvothermal, and microemulsions routes.11 In those techniques, it is however difficult to obtain a small polydispersity in particle size or low degree of agglomeration for several reasons: these methods require a very stringent control of various processing parameters, together with a low production yield,12 or they do not produce materials with a high surface area. On the other hand, it is also necessary that the morphology of the materials withstands postcalcination at higher temperatures, which is needed to incorporate Sb atoms into the SnO2 lattice.12b Usually, such treatment inevitably results in growth and further agglomeration of nanoparticles. ATO therefore has been synthesized as thin films for some of the aforementioned applications, while the synthesis of powder of ATO nanoparticles has been relatively unexplored. This paper describes a method to prepare ATO nanocrystalline powder with high surface area and high conductivity. On the basis of polymer-assisted sol-gel templating and simple chemical reagents, metal and metal oxide nanoparticles have been previously prepared.13 The polymer not only provides favorable site for the growth of the particulate assemblies, but it also influences the formation process, including nucleation, growth, coagulation, and flocculation.14 Our method is based on a novel amphiphilic block-copolymer (“KLE” type, poly(ethylene-co-butylene)-block-poly(ethylene oxide)15 (9) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2004, 43, 4345. (10) (a) Feldmann, C.; Jungk, H. O. Angew. Chem., Int. Ed. 2001, 40, 359. (b) Trifonova, A.; Wachtler, M.; Wagner, M. R.; Schriettner, :: H.; Mitterbauer, C.; Hofer, F.; Moller, K. C.; Winter, M.; Besenhard, J. O. Solid State Ionics 2004, 168, 51. (11) (a) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893. (b) Dusastre, V.; Williams, D. E. J. Phys. Chem. B. 1998, 102, 6732. (c) Koivula, R.; Harjula, R.; Lehto, J. Microporous Mesoporous Mater. 2002, 55, 231. (d) Li, L. L.; Mao, L. M.; Duan, X. C. Mater. Res. Bull. 2006, 41, 541. (e) Posthumus, W.; Laven, J.; de With, G.; van der Linde, R. J. Colloid Interface Sci. 2006, 304, 394. (f) Zhang, J. R.; Gao, L. Mater. Res. Bull. 2004, 39, 2249. (g) Zhang, J. R.; Gao, L. Inorg. Chem. Commun. 2004, 7, 91. (12) (a) Zhang, J. R.; Gao, L. Mater. Res. Bull. 2004, 39, 2249. (b) Pereira, A. G.; Batalha, L. A. R.; Porto, A. O.; de Lima, G. M.; Silva, G. G.; Ardisson, J. D.; Siebald, H. G. L. Mater. Res. Bull. 2003, 38, 1805. (c) Wang, Y.; Lee, J. Y.; Deivaraj, T. C. J. Mater. Chem. 2004, 14, 362. (d) Jeon, Y. A.; No, K. S.; Choi, S. H.; Ahn, J. P.; Yoon, Y. S. Electrochim. Acta 2004, 50, 907. (e) Zhang, J.; Gao, L. Mater. Chem. Phys. 2004, 87, 10. (13) (a) Wang, Y. D.; Ma, C. L.; Sun, X. D.; Li, H. D. Inorg. Chem. Coumm. 2002, 5, 751. (b) Kluson, P.; Kacer, P.; Cajthanml, T.; Kalaji, M. J. Mater. Chem. 2001, 11, 644. (c) Jing, Z. H.; Wu, S. H.; Zhang, S. M.; Huang, W. P. Mater. Res. Bull. 2004, 39, 2057. (d) Gui, Z.; Fan, R.; Mo, W. Q.; Chen, X. H.; Yang, L.; Hu, Y. Mater. Res. Bull. 2003, 38, 169. (e) Chen, D. H.; Hsieh, C. H. J. Mater. Chem. 2002, 12, 2412. (14) Dixit, S. G.; Mahadeshwar, A. R.; Haram, S. K. Colloids Surf., A 1998, 133, 69. (15) Thomas, A.; Schlaad, H.; Smarsly, B. M.; Antonietti, M. Langmuir 2003, 19, 4455.

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and simple tin reagents (SnCl4 and Sb(OC2H5)3). ATO was obtained with small particle size and good crystallinity. The as-prepared samples were used as negative electrode materials for lithium-ion battery, whose charge-discharge properties, cyclic voltammetry, and cycle performance were examined. Experimental Section Preparation of ATO Nanoparticles. All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification. ATO nanopowders with a 7.5% molar ratio of Sb were prepared because the ratio was found to be the most promising for the electrochemical performance and the conductivity in our incipient experiments. The initial solutions were prepared by dissolving 600 mg of SnCl4 and 7.5% molar ratio Sb(OC2H5)3 in 2 mL of EtOH. The isotropic solutions containing 36 wt % of the amphiphilic block-copolymer (“KLE” type, poly(ethyleneco-butylene)-block-poly(ethylene oxide)), with respect to the amount of tin oxide formed, were dissolved in the proper amount of EtOH-THF. The resulting sol of mixed inorganic precursors and polymer template was stirred for 24 h before it was used to prepare the nanoparticles. The sol was poured into a Petri dish with a diameter of 9 cm and dried in air at room temperature for 24 h so that most of the solvent was gone via evaporation. The resulting material was heated at 150 °C for 24 h. Afterward, the remaining substrates were scratched from the Petri dish. The product was heated at 300 °C with a ramp of 1 °C/min for 2 h. Finally, the dried piece of the mixed material was calcined at 500 °C for 2 h with a ramp of 2 °C/min to remove all the organics. The oven was cooled down to room temperature automatically without any additional cooling system. Characterization of ATO Nanoparticles. Powder X-ray diffraction (XRD) data were carried out with a D8 diffractometer from Bruker instruments (Cu KR radiation) in reflection mode. The sample was scanned from 20° to 60° (2θ) in steps of 0.05°. The crystallite domain sizes (D) have been examined from XRD peaks based on the Scherrer’s equation: D = 0.9λ/(ΔW cos θ), where λ is the wavelength of X-ray (λ = 0.15418 nm), θ is the Bragg’s diffraction angle, and ΔW is the true half-peak width of the X-ray diffraction lines. Thermogravimetric analysis (TGA) curves were obtained in flowing air on NETZSCH STA 449 C with a temperature increasing rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) was measured at room temperature on an ESCALAB 250. During XPS analysis, the Al KR X-ray beam was adopted as the excitation source, and the power was set to 250 W. Vacuum pressure of the instrument chamber was 1  10-7 Pa as read on the panel. Measured spectra were decomposed into Gaussian components by a least-squares fitting method. Bonding energy was calibrated with reference to the C 1s peak (285.0 eV). The transmission electron micrographs (TEM) were made with on a Zeiss EM 912Ω instrument at an acceleration voltage of 120 kV, while high-resolution transmission electron microscopy (HRTEM) characterization was done using a Philips CM200-FEG microscope (200 kV, Cs = 1.35 mm). The samples for TEM were prepared by dispersing the final powders in ethanol; this dispersing was then dropped on carbon-copper grids. Scanning electron microscopy (SEM) images of the morphology of particles were obtained from a LEO440 instrument equipped with an InLens detector (acceleration voltage: 2.0 kV). The samples for SEM were prepared by dispersing the final powders in the conductive glue; this dispersing was then sprayed with gold. N2 adsorption-desorption

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isotherms at 77 K were recorded on a Micromeritics ASAP 2010 automated sorption analyzer. The samples were outgassed 20 h at 150 °C before the analysis. Conductivity Measurement. Conductivity measurement was performed on ATO pellets (13 mm in diameter) prepared from about 160 mg of powder in an evacuated press under a pressure of 15 tons. The cold-pressed pellet was directly used for the measurements without any additional thermal treatment. The pellet was contacted between the gold-plated cylindrical electrodes acting both as voltage and current leads. Resistance across the pellet was measured by a digital multimeter (Keithley Instruments, Model 2000) in a four-probe mode for eliminating the undesired resistance of the measuring circuit. The specific conductivity S was calculated from the measured resistance R as S = H/RA, where A is the electrode area and H is the pellet thickness. Electrochemical Characterization. Electrochemical experiments were performed using Swagelok-type cells. The working electrodes were prepared by mixing the TNHCs, carbon black, and poly(vinyl difluoride) (PVDF) at a weight ratio of 70:15:15 and were pasted on pure Cu foil (99.6%, Goodfellow). Glass fiber (GF/D) from Whatman was used as a separator. The electrolyte consists of a solution of 1 M/L LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, in volume) obtained from UbeIndustries Ltd. Pure lithium foil (Aldrich) was used as the counter electrode. The discharge and charge measurements were carried on an Arbin MSTAT system. The cells were assembled in an argon-filled glovebox. The cyclic voltamogram experiment was performed on a Voltalab 80 electrochemical workstation at a scan rate of 0.2 mV s-1.

Results and Discussion Synthesis is performed in two steps: first an appropriately structured intermediate is synthesized by nonaqueous sol-gel reaction, and then this intermediate is calcined toward the final ATO particles. Thermogravimetric analysis (TGA) of the as-synthesized intermediate (after drying in air at room temperature for 24 h) under air shows an approximately 66 wt % total weight loss in two apparent steps on heating to 500 °C (Figure 1). The first occurs over the temperature range from room temperature to approximately 380 °C, the second over the temperature range from 380 °C to approximately 480 °C. Presumably, the first step is attributable to the release of chemically bound residual solvent, adsorbed water, and surface hydroxylated species and the second to desorption and decomposition of the polymer KLE.2,6 As the weighed-in polymer content was only 36 wt %, approximately 30 wt % mass loss has to be attributed to the removal of organic solvents, mainly EtOH and THF, bound within the amorphous ATO intermediate. Little further weight loss in the TG curve was observed at a temperature above 500 °C, indicating the completion of any reaction involving a weight change. As a result of the much lower heating rate in the synthesis (TGA measurements were performed with 10 °C /min, in contrast to only 1 °C /min and 2 °C /min heating in the calcination program which in addition contained 2 h holding steps at 300 and 500 °C), it is therefore safe to say that the organics have been removed in the calcination step.

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Figure 1. TGA diagram of sample heated at 150 °C for 24 h.

Figure 2. X-ray diffraction analysis of ATO nanoparticles. The experimental data is shown in red, the calculated patterns in black, and the difference curves in blue. The short vertical bars in green represent the positions of the Bragg reflections.

X-ray diffraction analysis of the sample treated at 150 °C in air supported the formation of an amorphous phase (Figure S1 in Supporting Information), while complete crystallization of tin oxide occurred after the final heat treatment at 500 °C for 2 h (Figure 2). The XRD pattern of the final blue powder revealed well-developed reflections of cassiterite (ICDD PDF No. 88-0287), without any indication of crystalline byproducts such as Sb2O3 or Sb2O5. This finding implies that the antimony doping most probably occurs by substituting tin atoms in the crystal structure. The assignment is further confirmed by the refinement of the diffraction pattern with the Rietveld method16 using the program FULLPROF.17 The structural parameters calculated from the Rietveld profile refinement are presented in Table 1 of the Supporting Information. The difference curve shows that the calculated and experimental XRD patterns are in satisfactory agreement, although the significant line broadening due to the small crystallite sizes and microstrain brought in by the heterosubstitution influenced the final agreement between experimental and calculated patterns. The mean grain size (D) of the as-prepared ATO nanoparticles was calculated by using the Scherrer equation to (16) McCusker, L. B.; Von Dreele, R. B.; Cox, D. E.; Louer, D.; Scardi, P. J. Appl. Crystallogr. 1999, 32, 36. (17) Rodriguez-Carvajal, J. FullProf, version 1.9c; LLB: CEA/Saclay, France, 2001.

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Figure 3. SEM images of the calcined sample show the surface structure and morphology (a and b), a typical energy dispersive X-ray spectrum (EDX) of ATO nanopowders (c), and nitrogen adsorption/desorption isotherms of ATO nanopowders (d).

the (110) plane diffraction peak (2θ =26.6°). The mean Scherrer grain size is about 2.9 nm. The scanning electron microscopy (SEM) image (Figure 3) of the sample revealed that the macroscopic texture of the product consists of interconnected spherical hollow particles forming a spongy structure with typical macropores and structural units in the range of 1.502.0 μm. In some parts, broken pieces of the inorganic network were observed. For the desired purpose of a battery electrode material, such a micrometer scale transport system turned out to be extremely favorable as it improves the dynamic rate behavior.18 The elemental composition of the as-prepared ATO nanoparticles was determined by using energy dispersive X-ray (EDX) spectroscopy (Figure 3c). The EDX spectrum performed on the nanocrystals confirmed the simultaneous presence of Sn, O, and Sb. EDX measurements on different spots of the samples always gave the same patterns, which proved the good homogeneity of the samples. Nitrogen sorption measurements were applied to determine the surface area of the ATO nanoparticles (Figure 3d). Gas sorption meaurements indicate at high (18) (a) Hu, Y. S.; Adelhelm, P.; Smarsly, B. M.; Hore, S.; Antonietti, M.; Maier, J. Adv. Funct. Mater. 2007, 17, 1873. (b) Adehelm, P.; Hu, Y. S.; Chuenchom, L.; Antonietti, M.; Smarsly, B. M.; Maier, J. Adv. Mater. 2007, 19, 4012.

relative pressures the presence of the macropore network of the powder assembly (which was already observed in SEM) but in addition a pronounced interparticle mesoporosity as reflected by the hysteresis loop in the intermediary pressure range. The Brunauer-Emmett-Teller (BET) specific surface area of the as-prepared sample was approximately 120 m2 g-1. This means that the single nanoparticles within the spongy microstructure are not really glued together but are accessible from the outside via a secondary mesopore system. The electrical conductivity measured on pressed and pelletized ATO nanopowders was approximately 0.08 S 3 cm-1. In comparison to the reported literature results (7.5 wt % Sb, 18 nm mean particle size, conductivity 0.0058 S 3 cm-1 19), the conductivity of our ATO samples is almost one order of magnitude higher. The reason is that single ATO nanoparticles are not really glued together but are accessible from the outside via mesopore wall. The amount of the grain boundary was significantly reduced and led to the enhancement of the electrical conductivity. Transmission electron microscopy (TEM) images (Figure 4a) clearly show the presence of slightly aggregated (19) Bai, F. F.; He, Y.; He, P.; Tang, Y. W.; Jia, Z. J. Mater. Lett. 2006, 60, 3126.

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Figure 4. (a) TEM image of ATO nanoparticles, (b) selected area electron diffraction, (c) HRTEM image of ATO nanoparticles, and (d) the power spectrum (Fourier transform) of HRTEM image (region A) with its IFT (inverse Fourier transform) image as the inset.

nanoparticles of 5-8 nm primary particle size. In contrast to the previously reported oxide template with the KLE polymer, especially thin films (EISA process), evidently no long-range nanostructural order is present in these ATO powders. The precursors of antimony oxide and tin oxide possess a different hydrolysis and condensation behavior, and thus the sol-gel process is strongly sensitive toward the external humidity and the evaporation rate of the solvent. Note that tin compounds usually show a slow rate of hydrolysis, so that so far mesoporous SnO2 was mainly synthesized as thin films, as described recently:20 It was recently confirmed that the generation of well-ordered mesoporous SnO2 requires a very distinct post-treatment procedure.21 The EISA process for mesoporous SnO2 is successful by using a diluted precursor solution and rapid evaporation during and after the dip-coating step, requiring immediate exposure of the film to approximately 80 °C to spur hydrolysis/condensation. Consequently, block (20) Brezesinski, T.; Fischer, A.; Iimura, K.; Sanchez, C.; Grosso, D.; Antonietti, M.; Smarsly, B. Adv. Funct. Mater. 2006, 16, 1433. (21) Urade, V. N.; Hillhouse, H. W. J. Phys. Chem. B 2005, 109, 10538.

copolymer templating of tin oxide and ATO in the form of a powder is aggravated by insufficient hydrolysis/ condensation and the removal of solvent from the inside of grains. Thus, heat treatment of the hybrid results in disruption of the still fragile, weakly condensed network. The addition of Sb further increases the complexity of the sol-gel process and thus finally leads to massive deformation of the mesostructure, creating a disordered nanostructure, which nevertheless has a high surface area. Figure 4b displays the corresponding selected-area electron diffraction (SAED) pattern of an ensemble of particles. The spotted diffraction rings from inside to outside can be indexed to the (110), (101), (200), (211), and (301) planes of rutile SnO2, respectively. These indexed patterns are in good accordance with the XRD reflections described above. To get further insight into the atomic order of the ATO nanoparticles, high-resolution images were recorded. As shown in Figure 4c, each ATO nanoparticle was well crystallized, with an average particle size of 6 nm. The particle size, as determined from the extension of lattice planes in the TEM image, is obviously by a factor 2-3 bigger than the apparent crystallite size

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To evaluate the potential applicability in high-power Li batteries, we investigated fundamental electrochemical properties of the as-synthesized ATO microspheres with respect to Li insertion/extraction. The cyclovoltammogram (CV) in the potential range of 5 mV-3.0 V vs Li/Li+ is usually used to analyze the active voltage range for the ATO electrodes. The CV curve (Figure 6) for the ATO electrodes clearly indicated a reaction during the first discharge with a reduction peak at 0.85 V. The profiles were similar to those observed for SnO2 22 and antimony doped SnO2.5 The strong reduction peak at 0.85 V corresponded to the formation of both the Sn and Sb metals and the Sn-Sb alloy in the Li2O matrix (eqs 1 and 2), which only happens in the first discharge cycle:5 Figure 5. High-resolution XPS spectra of superposed Sb 3d5/2 and O 1s for ATO nanopowders. Inset is the analysis for Sb valency at the Sb 3d3/2 peak. The experimental data is shown in red, the fitted patterns in black, the Sb 3d5/2 curves in blue, the O 1s curves in green, and the fitting curve of Sb 3d3/2 in magenta.

determined from XRD experiments. We attribute this to the heterosubstitution effects and strain effects being not correctly considered by the X-ray analysis; that is, heavy antimony doping results in locally strained crystal lattices, which are however long-range periodic within every nanoparticle. Also, the TEM images should be analyzed with care, because the particles seen in TEM could be composed of smaller primary particles of approximately 2-3 nm in size. For further elucidation of the crystallinity, filtering of HRTEM images was performed. Figure 4d shows the power spectrum (Fourier transform) of the part of the image denoted by the square revealing spots which are assigned to (110) lattice planes, and no peculiarities are found. The composition of the ATO nanocrystals was further analyzed by XPS. Apart from the C 1s peak positioned at 284.0 eV, which originated from spurious amounts of surface carbon of the decompsed KLE polymer template adsorbed onto the inorganic framework of ATO nanoparticles, XPS spectra (Figure S2, Supporting Information) confirmed the high chemical purity of the ATO nanoparticles, consisting solely of Sn, Sb, and O. Figure 5 shows the high-resolution XPS spectra of the ATO nanoparticles, revealing two peaks of Sn 3d5/2 and Sn 3d3/2 at 486.8 and 495.2 eV with a better symmetry, and they were assigned to the lattice tin in SnO2. The distance between these two peaks was 8.4 eV, being in good agreement with the energy splitting reported for SnO2. The values correspond to the 3d binding energy of Sn(IV) ions (indexed Standard ESCA Spectra of the Elements and Line Energy Information, Φ Co., U.S.A.). The corresponding Sb 3d5/2 and Sb 3d3/2 spinorbital spectra were assigned to binding energies 530.8 and 540.1 eV. The evaluation of the areas of the Sn 3d5/2 and Sb 3d5/2 emission lines using suitable sensitivity factors (4.89 for Sn 3d5/2 and 4.80 for Sb 3d5/2) resulted in an atomic Sb-to-Sn ratio of 6.43, which is only slightly smaller as compared to the starting recipe.

Sbx Sn1 -x O2 þ Li w ð1 -xÞSn þ xSb þ Li2 O Sbx Sn1 -x O2 þ Li w Sbx Sn1 -x þ Li2 O

ð1Þ ð2Þ

The oxidation peaks at 0.62 and 1.27 V during charging can be attributed to the formation of LixSn and LiySb alloys (0 e x e 4.4, 0 e y e 3). Theoretically, a complete discharge process correlates to the reduction reactions: Sn þ xLi w Lix Sn

ð3Þ

Sb þ yLi w Liy Sb

ð4Þ

Snx Sby þ xyLi w Lix Sn þ Liy Sb

ð5Þ

The first cycle charge-discharge curve at a constant current of approximately C/5 with a cutoff voltage window of 5 mV to 3 V is shown in Figure 7a. A discharge capacity of 2406 mA h g-1 and a charge capacity of 1286 mA h g-1 in the first cycle were achieved, which indicates only the principal potential of a nanostructured ATO as a high capacity anode for lithium-ion rechargeable batteries. These initial discharge and charge capacities are in fact much larger than those of tin oxide anodes reported previously.23 However, the material also suffers from a large amount of irreversible capacity in the first discharge-charge cycle. The initial capacity decreased after 100 cycles to a still high value of 637 mA h g-1, corresponding to 49.5% of the initial specific capacity. Also this value is substantially higher than that of most previously reported SnO2 nanostructures (e.g., SnO2 nanoparticles 67 mA h g-1, SnO2 nanotubes 525 mA h g-1,4 SnO2 hollow nanospheres about 490 mA h g-1,24 SnO2 nanoparticles about (22) Ng, S. H.; dos Santos, D. I.; Chew, S. Y.; Wexler, D.; Wang, J.; Dou, S. X.; Liu, H. K. Electrochem. Commun. 2007, 9, 915. (23) (a) Subramanian, V.; Gnanasekar, K. I.; Rambabu, B. Solid State Ionics 2004, 175, 181. (b) Nam, S. C.; Yoon, Y. S.; Cho, W. I.; Cho, B. W.; Chun, H. S.; Yun, K. S. Electrochem. Commun. 2001, 3, 6. (c) Yuan, L.; Guo, Z. P.; Konstantinov, K.; Liu, H. K.; Dou, S. X. J. Power Sources 2006, 159, 345. (24) Lou, X. W.; Wang, Y.; Yuan, C. L.; Lee, J. Y.; Archer, L. A. Adv. Mater. 2006, 18, 2325.

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Figure 6. CV of the ATO nanoparticles with the scan speed of 0.2 mV s-1.

100 mA h g-1, and nanotubes about 300 mA h g-1, nanowires about 310 mA h g-1),25 Sb doped SnO2 thin film (250 mA h g-1),5 and mesoporous SnO (350 mA h g-1).26 Very recently, Wang and Lee27 reported 700 mA h g-1 for SnO2 nanostructures with a high aspect ratio. Also in this case very high initial capacities of up to 1100 mA h g-1 were observed, which was partially attributed to morphological changes upon cycling. It should be noted that in these studies different scan rates were used for charging/decharging, thus impeding a direct quantitative comparison. Moreover, it was found that the electrochemical performance of SnO2 greatly depends on its electronic conductivity,19 while of course the resistance of ATO nanoparticles is much lower than those of the corresponding pure SnO2 structures.25 The mode of formation of metallic tin and antimony from ATO is an important factor for the reversible capacity. The high reversible capacity for nanostructured ATO may be due to the presence of about 7 mol % Sb for Sn, leading to an improved formation of metals with respect to structure and formation dynamics from ATO. The very high reversible capacity mainly depends on the alloying of Li with metals, and hence, the improved availabilibilty of Sn and Sb via structure and surface area will obviously increase the capacity. Different scan rates were applied to study the influence of the high surface area and the hierachical pore transport system on the electrochemical properties of SnO2 here. As it might have been expected from the structural characterization, our nanocrystalline ATO showed good rate capabilities. The corresponding results are shown in Figure 8. The cell was first cycled at 0.2C and, after 10 cycles, the rate was increased in stages to 3.0C. A specific charge capacity of around 791.0 mA h g-1 was obtained at a rate of 0.2C after 10 cycles; this value decreased to 700.6 mA h g-1 at 0.5C, 603.9 mA h g-1 at 0.75C, 558.3 mA h g-1 at 1.0C, 460.8 mA h g-1 at 1.5C, 385.3 mA h g-1 at 2.0C, 324.8.3 mA h g-1 at 2.5C, and (25) Park, M. S.; Kang, Y. M.; Wang, G. X.; Dou, S. X.; Liu, H. K. Adv. Funct. Mater. 2008, 18, 455. (26) Yu, A. S.; Frech, R. J. Power Sources 2002, 104, 97. (27) Wang, Y.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 17832.

Figure 7. Galvanostatic voltage profiles between 5 mV and 3 V for the first cycle (a) and the discharge/charge capacity profiles between 5 mV and 3 V voltage window and C/5 up to the hundredth cycle (b).

finally, 254.1 mA h g-1 at 3.0C. Returning after this complete dynamic loading cycle to a rate of 0.2C for a second time, a specific charge capacity of 794.1 mA h g-1 was obtained, indicating the excellent cycling stability of our ATO nanostructure even after rather harsh loading/ deloading rates. Such good rate capability and reversibility conceptually results from the optimization of the nanostructure with its macroporous transport and mesoporous exchange pores, thus enabling an improved dynamics for both electronic and Li+ transport as well as a high electrode-electrolyte contact area. In summary, Sb doped SnO2 nanoparticles were synthesized by a polymer-assisted sol-gel process and were analyzed by XRD, SEM, TEM, XPS, and electrochemical measurements. The TEM, XRD, and XPS results showed that the Sb was indeed incorporated into the SnO2 crystal structure (cassiterite SnO2). The electrochemical characterization revealed surprisingly good properties. In particular, a high initial discharge capacity of about 2400 mA h g-1 was observed at a constant discharge current density of approximately C/5 in a potential range of 0.005-3.0 V. We believe that the nanostructure reduces the possibility of aggregation of

Article

Chem. Mater., Vol. 21, No. 14, 2009

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but also leads to improved mechanical reversibility of charging/decharging changes. Significant morphological changes are avoided, because mechanical stress is balanced by the large free volume, enabling a rearrangement without structural changes or breakdown. A highly stable capacity of 637 mA h g-1 was found after 100 cycles. On the basis of this promising capacity, further studies will address improving the performance of nanostructured ATO at higher rates by further structural and compositional variations.

Figure 8. Cycling and rate performance of ATO nanopowders electrode cycled at C/5, C/2, 3C/4, 1C, 1.5C, 2.0C, 2.5C, and 3.0C.

metallic tin toward larger entities and accommodates the volume change of the tin phase during Li alloying/ dealloying. As shown recently, SnO2 can undergo drastic changes in the crystalline morphology upon (de)charging.28 Thus, a nanostructured ATO with interstitial porosity not only features a higher surface area and thereby facilitated diffusion of electrolyte and charges (28) Seong, H. K.; Kim, M. H.; Choi, H. J.; Choi, Y. J.; Park, J. G. Metal. Mater. Int. 2008, 14, 477.

Acknowledgment. The authors thank Xinglong Wu and Professor Yuguo Guo (Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing China) for the Electrochemical Characterization. One of the authors (Y.D. Wang) acknowledges the financial support from the Alexander Humboldt Society in the form of fellowship. Supporting Information is available online from Wiley Inter Science or from the authors. Supporting Information Available: Structural data and refinement parameters for ATO nanoparticles calculated by Rietveld refinement of the experimental XRD powder pattern; XRD pattern of as-prepared material heated at 150 °C for 24 h; XPS survey spectrum of ATO nanopowders; and SEM image of the ATO nanopowders after 100 cycles (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.