17832
J. Phys. Chem. B 2004, 108, 17832-17837
Molten Salt Synthesis of Tin Oxide Nanorods: Morphological and Electrochemical Features Yong Wang†,‡ and Jim Yang Lee*,†,‡ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, 119260, Singapore, and Singapore-MIT Alliance, National UniVersity of Singapore, 4 Engineering DriVe 3, 117576, Singapore ReceiVed: July 22, 2004; In Final Form: September 2, 2004
This article reports the nonhydrothermal synthesis of SnO2 nanorods at moderate temperatures (320 or 700 °C), and the application of these nanorods as a Li storage compound for Li-ion batteries. The 15 nm diameter high aspect ratio (∼50-100) SnO2 nanorods prepared at 700 °C were fabricated as Li-ion battery anodes and tested. They exhibited good capacity (∼700 mAh/g nominal in the 5 mV to 1 V range) and extended cyclability for a tin-based anode. More interestingly was the observation of a capacity of ∼1100 mAh/g in the 5 mV to 2 V window that exceeds the theoretical capacity of SnO2. SEM and TEM characterizations revealed substantial morphological changes in the nanorods during cycling. A few possible reasons for the high capacity and the morphological changes are provided.
1. Introduction
2. Experimental Methods
SnO2 has been identified in many reports as a possible substitute for graphite anodes in Li-ion batteries.1-7 SnO2 can be reduced to Sn by Li-ions in the first cycle and forms various alloys (LixSn) with Li subsequently.2 Based on this mechanism, SnO2 anodes can deliver a very high theoretical capacity of ∼790 mAh/g at the maximum x value of 4.4. This capacity is a significant improvement over the theoretical value of 372 mAh/g for graphite. Most of the published work on tin-related anodes is based on SnO2 thin films,3 SnO2 nanoparticles,6,7 Snbased alloys,4 and composites.2-5 There has not been any report on the use of SnO2 nanorods as a reversible lithium storage compound. In view of the success in using carbon nanotubes8 and more recently CuO nanorods9 to enhance the Li storage capacities of carbon and CuO, respectively, it is of interest to explore whether the nonspherical form of SnO2 nanoparticles can likewise deliver an improved application performance. 1D SnO2 nanomaterials have been prepared by a number of techniques10 and have been used in high-performance gas sensors.11,12 Most of the methods reviewed in ref 10 are either high-temperature syntheses (>750 °C) or solvothermal syntheses at reduced temperatures and high pressures (200 °C and about ∼8-10 MPa).13 This paper describes an alternative means of preparing 1D SnO2 nanomaterials using only moderate temperatures (320 or 700 °C) and atmospheric pressure. The obtained SnO2 nanorods were fabricated as Li-ion battery anodes, and their lithium storage capacity was experimentally determined. An extremely high specific capacity of ∼1100 mAh/g could be extracted from the SnO2 nanorod anodes in the 5 mV to 2 V voltage window. The capacity is higher than the theoretical capacity of SnO2 based on the maximum alloy constitution of Li4.4Sn as determined from the binary phase diagram of bulk Li-Sn alloys. Changes in the morphology of the SnO2 nanorods due to Li-ion insertion and extraction reactions in repeated charging and discharging operations were also followed.
Materials. Commercially available SnCl4 (Riedel-de Haen, 99%), NaBH4 (Fluka), 1,10-phenanthroline (phen, Nacalai Tesque), CH3COOK (KOAc, Merck), NaCl (Merck), KCl (Merck), and methanol (Merck) were used as received without further purification. Deionized water was purified by double distillation. Synthesis of SnO2 Nanorods. First, 10 mL of 0.05 M SnCl4 was added to a suspension of 0.1 g of phen in 10 mL of water (phen:SnCl4 ) 1:1). A clear solution was formed after stirring for a few minutes, indicating the formation of the coordination compound (phen)SnCl4. Next, 20 mL of 0.1 M NaBH4 aqueous solution was introduced dropwise (NaBH4:(phen)SnCl4 ) 4:1). The mixture turned to pale yellow, indicating the formation of a sol of 2-5 nm phen-capped Sn nanoparticles.14 The hydrosol was further stirred for 2 h before the Sn nanoparticles were spun down in an ultracentrifuge (15 000 rpm for 1 h). 0.1 g of phen-capped Sn nanoparticles was mixed with 0.7 g of KOAc or a mixture of 0.2 g of NaCl and 0.3 g of KCl, ground into a fine powder, and heated at 320 °C for 3 h or at 700 °C for 1 h in a furnace. The melt was then naturally cooled to room temperature. The solidified mass was washed several times with water and methanol and was dried at 130 °C for 3 h. Electrochemical Measurements. A N-methyl pyrrolidinone (NMP) slurry consisting of 70 wt % of the SnO2 nanorods, 20 wt % of conducting carbon black (Super-P), and 10 wt % of poly(vinylidene difluoride) (PVDF, Aldrich) was uniformly applied to a copper foil (0.7 mm). The coated foil was cut into disk electrodes 11 mm in diameter. The electrodes were vacuumdried overnight at 130 °C followed by compression at 2.0 × 106 Pa. They were then assembled into Li test cells using 0.75 mm lithium foil negative electrodes, microporous polypropylene separators, and an electrolyte of 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Cell assembly was carried out in a recirculating argon glovebox where both the moisture and the oxygen contents were below 1 ppm each. All cells were tested at the constant current density of 0.4 mA/cm2 and were charged (Li+ insertion) and discharged (Li+ extraction) between fixed voltage limits (1 V
* Corresponding author. Phone: (65) 6874-2899. Fax: (65) 6779-1936. E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering, NUS. ‡ Singapore-MIT Alliance, NUS.
10.1021/jp0467447 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/20/2004
Molten Salt Synthesis of Tin Oxide Nanorods
J. Phys. Chem. B, Vol. 108, No. 46, 2004 17833
to 5 mV or 2 V to 5 mV) on a Maccor Series 2000 battery tester. Cyclic voltammetry was performed on an EG&G model 273 potentiostat/galvanostat, using the SnO2 nanorods-coated disk as the working electrode and lithium as both the counter and the reference electrodes. A scan range of 0-1.5 V (vs Li) was used which was swept at the scan rate of 0.1 mV/s. Materials Characterizations. The materials were characterized by scanning electron microscopy (SEM, JEOL JSM5600LV), field emission scanning electron microscopy, energydispersive X-ray spectroscopy (FE-SEM/EDX, JEOL JSM6700F), transmission electron microscopy, selected area electron diffraction (TEM/SAED, JEOL JEM-2010F), high-resolution transmission electron microscopy, selected area electron diffraction (HRTEM/SAED, Philips FEG-CM300), and powder X-ray diffraction (XRD, Siemens D6000). 3. Results and Discussion The TEM images in Figure 1a-c show the whiskerlike SnO2 nanorods which were formed in abundance by heating the phencapped SnO2 nanoparticles in KOAc. The surface of the nanowhiskers was quite rough, and some remnants of nanoparticles could still be found. The diameters of the nanowhiskers were about 5 nm, the same as the diameter of the starting material, phen-capped Sn nanoparticles.14 KOAc was chosen as the flux material in this molten salt synthesis because of its low melting point (∼292 °C). Its functionality in the synthesis should be similar to that of NaCl used in other studies.14,15 The thermal decomposition of pristine KOAc occurred between 360 and 400 °C under the experimental conditions, and at the temperature of 320 °C used for nanorod synthesis, KOAc only underwent solid-liquid phase transformation without noticeable thermal decomposition (this was experimentally verified). In the TEM image of Figure 1b, a few nanoparticles could be seen embedded in different parts of the nanowhiskers. They were all of the same size as the phen-capped Sn nanoparticles, suggesting that the starting Sn particles were the building blocks of the nanowhiskers. It is likely that the nanowhiskers were formed by assembling the nanoparticles one at a time and were aligned by the combined action of the flux and the capping agent (phen). A few isolated Sn nanoparticles perhaps served as the initial growth sites. Directed growth ensued after other Sn nanoparticles were added to the growth sites in a stepwise manner. Because the temperature involved was rather moderate, the self-assembly impetus was not particularly strong and some SnO2 nanowhiskers still exhibited remnants of nanoparticles added during the various stages of growth. The HRTEM image in Figure 1c also shows some premature crystal structures of the product at this low assembly temperature. More regularly shaped SnO2 nanorods were obtained by molten salt synthesis using a mixture of NaCl and KCl (melting point: ∼650 °C) as the flux. Figure 2a is a typical SEM image of the product showing a large number of rodlike structures nanometers in diameters and several hundred nanometers to several micrometers in lengths. TEM examination under high magnifications (Figure 2b) revealed a very smooth external surface and diameters of about ∼15 nm for the nanorods. The selected area electron diffraction (SAED, Figure 2c) pattern taken from the tip of a nanorod showed the characteristics of a perfect single crystal where all of the diffraction spots could be indexed to the [020] zone axis of SnO2 with the rutile structure. Two neighboring tin oxide nanorods were also examined by HRTEM in Figure 2d (where the lighter areas were the boundaries between the rods). The (101) and (200) planes could
Figure 1. TEM images of whiskerlike tin oxide nanorods synthesized at 320 °C: (a) scale bar, 100 nm; (b) scale bar, 50 nm; and (c) HRTEM image of a free-standing tin oxide nanowhisker, scale bar, 2 nm.
be identified in each of the nanorods which grew along the [001] direction (c-axis). From the X-ray diffraction (XRD) pattern of the SnO2 nanorods in Figure 3, the nanorods were highly crystalline, and all of the diffraction peaks could be indexed to the tetragonal rutilestructured SnO2 according to JCPDS card 21-1250. Figure 4 shows the FTIR spectrum of the SnO2 nanorods. The two characteristic absorption peaks at 683.1 and 478.6 cm-1 could be assigned to the lattice vibrations of the Sn-O-Sn framework.
17834 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Wang and Lee
Figure 2. (a) SEM image, (b) TEM image, (c) SAED pattern, and (d) HRTEM image of the SnO2 nanorods prepared at 700 °C.
Figure 3. X-ray diffraction (XRD) patterns of SnO2 nanorods.
Figure 4. FTIR spectra of SnO2 nanorods.
The more orderly and nanoparticle-free SnO2 nanorods obtained by molten salt synthesis at 700 °C were tested as an anode material for Li-ion batteries. Figure 5a shows the first cycle voltammogram of the SnO2 nanorod electrode. The large cathodic peak around 0.8 V in the cathodic scan could be easily identified as the reduction of SnO2 (eq 1) and the formation of
Figure 5. (a) First cycle cyclic voltammogram of SnO2 nanorods at 0.1 mV/s and (b) the first charging (lithiation) and discharging (delithiation) curves of SnO2 nanorods.
a SEI (solid-electrolyte interface) layer on the nascent Sn surface.2,6 The same process was shown as a voltage plateau at about the same voltage in the charge (Li+ insertion) and discharge (Li+ extraction) curves of Figure 5b. After the metallic Sn formation reaction, Li+ insertion and extraction reactions
Molten Salt Synthesis of Tin Oxide Nanorods
J. Phys. Chem. B, Vol. 108, No. 46, 2004 17835
SCHEME 1: Illustrations Showing (a) the Formation of Shortened Nanorods, Isolated Nanoparticles, and Cavities on the Nanorod Surface and (b) the Expansion of Diameters Near the Ends of the Nanorods Relative to the Midsections
would then proceed reversibly according to eq 2 (alloy formation and de-alloying):
SnO2 + 4Li + 4e- f 2Li2O + Sn
(1)
Sn + xLi + xe- a LixSn (xmax ) 4.4)
(2)
The cycling performance of SnO2 nanorods shown in Figure 6 covered two different voltage windows: 2 V to 5 mV and 1 V to 5 mV at a constant current density of 0.1 mA/mg. Using the narrower voltage range of 5 mV to 1 V, a high specific capacity of ∼700 mAh/g and a very stable cycling performance could be achieved simultaneously. Good cyclability in a more restricted voltage window is a known phenomenon.3-5 It is evident from the same figure that the specific capacity of ∼1100 mAh/g could be obtained in the voltage range of 2 V to 5 mV, at least for the first five cycles. The capacity of 13 nm SnO2 nanoparticles (∼650 mAh/g) was also included in Figure 6 as a reference.7 Most notably, the specific capacity of 1100 mAh/g is higher than the theoretical capacity of SnO2 calculated by assuming the formation of a LixSn alloy with an xmax of 4.4. Such high capacity did decline quickly after the first few cycles, although the capacity fading was more moderate in subsequent cycles. The unusually high specific capacity of the nanorods is unique and has not been observed previously. It is tempting to
Figure 6. Cyclabilities of SnO2 nanorod anodes and a reference nanoparticle anode (0.4 mA/cm2, 5 mV to 1 or 2 V).
attribute the high capacity to Li-ion entrapment in the electrode in the first lithiation process, and the slow release of the Liions in the subsequent cycles. However, this alone cannot explain the persistence of the higher than theoretical capacity for up to the first 15 cycles (Figure 6). The morphology of the SnO2 nanorods cycled between 5 mV and 1 V after the first and the 60th cycle was examined via SEM and TEM. The TEM image of the nanorod electrode after one complete cycle of charging and discharging is shown in Figure 7. Amidst carbon black and PVDF, which were used as the electrode additives, were a few nanorods ∼300 nm in lengths and 15 nm in diameters. As compared to the starting pristine SnO2 nanorods in Figure 2b, the nanorods after the first cycle showed a substantially shortened length, while maintaining almost the same diameter. This indicates that the SnO2 nanorods were fractured by Li-Sn reactions occurring mostly on the walls of the rods than on the circular end faces. (It should be
Figure 7. TEM images of the nanorod anode after one complete cycle.
17836 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Figure 8. FE-SEM images of the SnO2 nanorod anode after 60 cycles showing discrete particles: (a) scale bar, 100 nm; (b) scale bar, 10 nm; and (c) the TEM image of a few isolated nanorods.
emphasized that ultrasonication was used in dispensing samples onto the TEM copper grids. The TEM image in Figure 2b shows that fracturing due to ultrasonication was minimal, if any.) It is assumed that there is no preference for the Li-ions to insert in the walls or in the end faces of the nanorods. The extent of insertion, however, is expected to be higher at the walls because the high aspect ratio (∼50-1000) rods offered more wall surface areas than end surface areas (shown in Scheme 1a). In the inset TEM image of Figure 7, slightly enlarged diameters were observed near the ends as compared to the midsections of the rods. The larger volume expansion occurring near the ends of the rods was due to two possible ways of inserting Li-ions
Wang and Lee there: through the walls of the rods and also through the end faces of the rods (Scheme 1b). A large number of nanoparticles 1-3 nm in diameters could also be found among the shortened nanorods. They were most likely the product of uncontrolled exfoliation from the nanorod surface induced by the Li-Sn reactions. When the mechanical stress caused by the volume change in the Li-Sn reactions could not be fully relieved by rupturing the rods to create a few new cross sections, cratering on the nanorod surface would likely occur. As the size of the detached particles was substantially smaller than the diameter of the SnO2 nanorods, it is surmised that the surface layer of the nanorod first expanded after Li-ion insertion while the core remained largely intact. If the rate of expansion were much faster than the relaxation time for the lattice points to adapt to their new positions, tin would begin to detach from the external surface in the form of isolated particles of very small size. Such a process is likely to be anisotropic, stochastic, and uncontrollable, resulting in the formation of very small craters on the nanorod surface and some detached nanoparticles (Scheme 1a). These roughened surfaces could store more Li-ions than a glazed surface similar to the ink pot effect known for the disordered carbon anodes.16 The other reason for the high capacity of SnO2 nanorods could be their excellent crystallinity combined with a very low dimensionality. Although the detailed mechanism has not been fully deliberated, several groups have reported that a more crystalline structure could lead to a higher storage capacity.6 It has also been suggested that energy density of 1D nanomaterials could be improved by facilitated Li+ diffusion because of the onedimensional (1D) topology. The heterogeneous kinetics of Li+ diffusion is faster with a higher surface-to-volume ratio.17-19 Li-ions could also be stored in the space within a stack of collapsed nanorods,17,20 again based on the abovementioned ink pot effect. Figure 8a,b shows the FE-SEM images of a SnO2 nanorod anode after 60 cycles in the 5 mV to 1 V voltage window. A large number of 8-10 nm particles could be found which were almost 5 times as big as the nanoparticles after the first cycle. Besides the nanoparticles, some nanorods could still be detected by TEM (Figure 8c). The cycling data in Figure 6 show a capacity fading of ∼75 mAh/g after 60 cycles. However, as the Sn nanoparticles were still quite small (8-10 nm) then, the extent of electrode pulverization was not excessive enough to lead to a catastrophic failure. Therefore, capacity fading was primarily due to factors such as gradual loss of electrical contact and/or mechanical integrity. Energy-dispersive X-ray spectroscopy (EDX) measurements showed that there was some trace oxygen in the FE-SEM samples of Figure 8a,b, possibly due to the presence of blanketing Li2O or inevitable surface oxidation of Sn during the sample preparation for FE-SEM examination. Most of the particles shown in Figure 8a,b were Sn nanoparticles, confirming that SnO2 had been successfully converted to Sn according to eq 1 and the resulting Sn was the active Li storage compound.2 4. Conclusions SnO2 nanorods could be prepared by the molten salt synthesis of phenathroline-capped Sn nanoparticles at moderate temperatures (320 or 700 °C) and atmospheric pressure. The 15 nm tin oxide nanorods synthesized at 700 °C were evaluated as an anode material for Li-ion batteries. The SnO2 nanorods showed a very high specific capacity of ∼1100 mAh/g in the 5 mV to 2 V window, which exceeds the theoretical capacity of SnO2 (∼790 mAh/g) based on the maximum stoichiometry of
Molten Salt Synthesis of Tin Oxide Nanorods Li4.4Sn. A few possible reasons for the high capacity were suggested on the basis of the observation of morphology changes in the nanorods during cycling. References and Notes (1) Bueno, P. R.; Leite, E. R.; Giraldi, T. R.; Bulhoes, L. O. S.; Longo, E. J. Phys. Chem. B 2003, 107, 8873. (2) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2943. (3) Li, N.; Martin, C. R.; Scrosati, B. Electrochem. Solid-State Lett. 2000, 3, 316. (4) Yang, J.; Wachtler, M.; Winter, M.; Besenhard, J. O. Electrochem. Solid-State Lett. 1999, 2, 161. (5) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045. (6) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (7) Wang, Y.; Lee, J. Y.; Chen, B. H. Electrochem. Solid-State Lett. 2003, 6, A19. (8) Shimoda, H.; Gao, B.; Tang, X. P.; Kleinhammes, A.; Fleming, L.; Wu, Y.; Zhou, O. Phys. ReV. Lett. 2002, 88, 015502. (9) Gao, X. P.; Bao, J. L.; Pan, G. L.; Zhu, H. Y.; Huang, P. X.; Wu, F.; Song, D. Y. J. Phys. Chem. B 2004, 108, 5547.
J. Phys. Chem. B, Vol. 108, No. 46, 2004 17837 (10) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5. (11) Kolmakov, A.; Zhang, Y.; Cheng, G.; Moskovits, M. AdV. Mater. 2003, 15, 997. (12) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. D. Angew. Chem., Int. Ed. 2002, 41, 2405. (13) Zhang, D. F.; Sun, L. D.; Yin, J. L.; Yan, C. H. AdV. Mater. 2003, 15, 1022. (14) Wang, Y.; Lee, J. Y.; Deivaraj, T. C. J. Phys. Chem. B 2004, 108, 13589. (15) Liu, Y. K.; Zheng, C. L.; Wang, W. Z.; Yin, C. R.; Wang, G. H. AdV. Mater. 2001, 13, 1883. (16) Tokumitsu, K.; Fujimoto, H.; Mabuchi, A.; Kasuh, T. Carbon 1999, 37, 1599. (17) Dominko, R.; Arcon, D.; Mrzel, A.; Zorko, A.; Cevc, P.; Venturini, P.; Gaberscek, M.; Remskar, M.; Mihailovic, D. AdV. Mater. 2002, 14, 1531. (18) Zhou, Y.; Cao, L.; Zhang, F.; He, B.; Li, H. J. Electrochem. Soc. 2003, 150, A1246. (19) Chen, J.; Tao, Z. L.; Li, S. L. Angew. Chem., Int. Ed. 2003, 42, 2147. (20) Arcon, D.; Zorko, A.; Cevc, P.; Mrzel, A.; Remskar, M.; Dominko, R.; Gaberscek, M.; Mihailovic, D. Phys. ReV. B 2003, 67, 125423.
