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J. Phys. Chem. C 2008, 112, 4550-4556
Novel Microwave Synthesis of Nanocrystalline SnO2 and Its Electrochemical Properties V. Subramanian,† William W. Burke,‡ Hongwei Zhu,† and Bingqing Wei*,† Department of Mechanical Engineering, UniVersity of Delaware, Newark, Delaware 19716, and Department of Electrical and Computer Engineering, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 ReceiVed: December 7, 2007; In Final Form: January 4, 2008
We report a sol-gel-assisted microwave synthesis of nanocrystalline tin oxide with a particle size of 15-18 nm. The microwave synthesis reaction can be completed in only a few minutes. In situ formation of amorphous carbon from the decomposition of citric acid used as a chelating agent provided both high microwave susceptibility for the faster formation reaction of SnO2 and also prevented agglomeration of nanoparticles maintaining the size as small as 15 nm. Microwave reaction time can be optimized for the presence of a small amount of carbon, which helped in improving the electrochemical performance of the synthesized SnO2 when used as an electroactive material in lithium ion battery with a very high reversible capacity ∼ 900 mA h/g. The synthesis method could be extended for producing other metal oxide nanoparticles with controlled sizes.
Introduction Lithium ion batteries are considered as one of the most promising and efficient power sources for a variety of applications ranging from consumer electronics to electric vehicles.1 The versatility of lithium ion batteries greatly depends on the performance of the electrode materials used. Most of the present day commercial cells use carbonaceous materials as anode materials, while lithium cobalt oxide is being strongly considered as cathode material.2,3 However, using carbonaceous materials limits the possibility of extracting higher energy density from the battery, preventing its use for larger applications such as in electric vehicles. Hence, the search for alternative anode materials and an effective and efficient way of synthesizing them in nanoscale architecture is an ongoing pursuit in the lithium battery research. Using a nanocrystalline metal oxide as an anode material has many advantages because of the larger surface area and higher reactivity.4,5 Various nanocrystalline oxides, such as TiO2, SnO2, Co3O4, NiO, have been closely envisaged as anode materials in lithium ion batteries.6-9 Of the various anode materials strongly considered for the replacement of carbonaceous anode materials in lithium ion batteries, tin oxide enjoys a place of pride because of its very high specific capacity (>600 mA h/g) compared to that of graphite (372 mA h/g). Tin-based amorphous oxides were first reported as a battery-active material by scientists of Fuji Photo Film Co. in 1997.10 Nanostructured tin oxides have been synthesized by a variety of techniques, such as sol-gel, coprecipitation, hydrothermal/solvothermal, and surfactantassisted methods.11-14 Dahn and his group studied extensively tin oxide and its related materials using in situ and ex situ techniques and established a mechanism for the lithium storage in this material.15,16 Li et al. reported template-synthesized nanocrystalline SnO2 showing high capacity and high rate capability.7 * Corresponding author. E-mail:
[email protected]. † University of Delaware. ‡ Louisiana State University.
The synthesis route employed to prepare a material has a greater impact on the electrochemical performance when used as an electrode material in a lithium ion battery. The main advantage of the solution-assisted synthesis is the molecular level mixing of the starting materials, which leads to a very high degree of homogeneity in the synthesized material. Usually, the solution-assisted methods utilize a carboxylic acid based complexing agent to prevent any segregation of intermediate phase, and a high-temperature treatment is then required to remove the carboxylic acid used. The temperature and time of sintering are very critical for the particle size of the synthesized material and also for the complete removal of any residual carbon. Microwave synthesis is an interesting technique for the synthesis of oxide materials.17 Battery-active materials have been synthesized in remarkably short time under microwave irradiation.18,19 The choice of starting materials which have good microwave susceptibility is very important for the shorter synthesis time of the respective oxide materials. Utilizing microwave energy for the thermal treatment generally leads to a very fine particle in the nanocrystalline regime because of the shorter synthesis time and a highly focused local heating. However, the idea of utilizing the complexing or chelating agent as an in situ microwave susceptor has not been exploited so far. In this paper, we report, as a typical case, the sol-gelassisted microwave synthesis of nanoscrystalline SnO2 and its electrochemical performance as a lithium ion battery anode. This method can be extended to many oxide materials in many applications, as the procedure is very simple and has no limitation with respect to the starting materials. The structural and electrochemical performance of the synthesized materials were correlated and are discussed. Experimental Methods Synthesis. Nanocrystalline SnO2 was synthesized using a sol-gel-assisted microwave technique. The starting materials, SnCl2‚H2O and citric acid, were separately dissolved in water and then mixed slowly without any precipitation. The SnCl2‚
10.1021/jp711551p CCC: $40.75 © 2008 American Chemical Society Published on Web 03/06/2008
Microwave Synthesis of Nanocrystalline SnO2
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Figure 1. Schematics of the formation of SnO2 nanoparticles in microwaves.
2H2O:citric acid was taken in a mole ratio of 3:5. The mixture was loaded into an oven preheated at 80 °C. The water slowly evaporated, leading to the formation of a sol that subsequently became a gel. The gel was introduced into a microwave reactor (Thermwave; operating frequency, 2.4 GHz; output power, 1.25 kW) and was treated for different times with close observation to record the reaction progress. The microwave reaction was done for 6, 12, 18, and 25 min, respectively. During the microwave irradiation, the gel became more viscous and resembled a fibrous resin. On continuous microwave irradiation, there has been an observed burning, leading to the formation of a very high surface area and fluffy powder. In order to compare the structural aspects of the formed tin oxide, the fluffy mass was further heated at 500 °C for 1 h in a conventional furnace. Characterizations. The synthesized powders both in microwave and conventional furnace were studied using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) analysis. The phase purity of the synthesized material was characterized with XRD using a Siemens D5000 diffractometer in the 2θ range between 10° and 70°. HRTEM measurements were made to investigate the crystalline structures and the pore morphology of the materials. The powder samples were dispersed in methanol and transferred to a holey carbon grid, and the HRTEM and selected area electron diffraction measurements were obtained on a JEOL JEM2010 (200 kV) TEM. EDX measurements to estimate the carbon content in the samples were carried out on a holey SiO2/molybdenum grid. The particle size distribution profiles for each sample were obtained using ImageJ software to analyze three TEM images with 256 pixels. The amount of amorphous carbon residue in each sample was estimated from the EDX measurements on the samples prepared on the holey SiO2/molybdenum grid. For the TEM studies of the cycled electrode material, a similar procedure was adopted. XPS was employed to identify the valence state of tin in the synthesized materials. XPS studies were performed using an AXIS 165 high-performance multitechnique surface analyzer (XPS/Auger). The electrochemical performance of the nanocrystalline tin oxide synthesized was studied by assembling two-electrode HS-test cells (Hohsen
Corp.) with lithium metal as an anode in a 1 M LiPF6 in EC: DEC (1:1 in volume) electrolyte (Ferro Corp.). The electrode materials were prepared by mixing the active material with 10% PVdF binder and 10% conducting carbon black additive in NMP. The well-mixed slurry was coated onto a copper foil using doctor blade method. The coated foil was allowed to dry in an oven at 110 °C for 1 h and 30 min and then uniaxially pressed for better contact of the coated material and the copper current collector. Circular disc electrodes were punched from the foil and used as cathode for assembling the test cells. All manipulations were performed in a glovebox filled (Unilab, MBraun) with purified argon. The moisture content and oxygen level were less than 5 ppm inside the glovebox. The assembled cells were discharged and charged galvanostatically at 0.1 °C rate between 2.0 and 0.005 V in an Arbin BT4 and/or Autolab potentiostat/ galvanostat (PGSTAT30). Results and Discussions As mentioned, one of the advantages of the solution-based synthesis is the molecular level mixing of the starting materials. In the present case, the starting tin forms a chelating complex with citric acid, which when heated at 80 °C subsequently led to the formation of a sol and then a gel. Introduction of the gel into the microwave reactor first led to the removal of water, leading to the formation of a resin-type thickened gel, which subsequently started burning. The burning was observed when the microwave irradiation time was just about 1.5 min. The main reason for this behavior is the decomposition of citric acid to form a high surface area amorphous carbon, which is very good microwave susceptor and eventually provides the local heating for the formation of the tin oxide. Similar results have been observed for the formation of nanocrystalline molybdenum oxide from molybdenum chloride:sucrose:triethanolamine (TEA) mixture when heated in a conventional furnace.20 The initial heating under the influence of microwave was provided by water present in the gel, a good microwave susceptor, which leads to decomposition of the citric acid, resulting in the formation of amorphous carbon. Once the system is dried up completely, the presence of the amorphous carbon not only supports the faster formation reaction of SnO2 but also prevents the ag-
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Figure 2. TEM images of tin oxide nanoparticles: (a) sol-gel microwave tin oxide (6 min), (b) sol-gel microwave tin oxide (6 min) + 500 °C /1 h heating in furnace, (c) sol-gel microwave tin oxide (10 min), (d) sol-gel microwave tin oxide (15 min), (e) sol-gel microwave tin oxide (20 min), (f) sol-gel microwave tin oxide (25 min), (g) highresolution TEM image taken from figure (a), and (h) electron diffraction from figure (a).
glomeration of the individual SnO2 nanoparticles, resulting in very fine nanocrystalline SnO2 particles. The schematic representation of the formation of the nanocrystalline SnO2 particles is shown in Figure 1. It is obvious that without the presence of amorphous carbon there would not be such a fast formation reaction of SnO2. In the case of conventional furnace heating, the formation of SnO2 and the decomposition reaction happen with a relatively large time lag, leading to the formation of SnO2 particles with larger particle size because of the agglomeration. The combined effect of faster heating in the microwave field along with the amorphous carbon preventing the agglomeration of SnO2 nanoparticles led to the formation of very fine SnO2 nanoparticles with a particle size between 15 and 18 nm (Figure 2). As the heating reaction in the microwave was continued for different intervals of time such as 10, 12, 15, 18, 20, and 25 min, there was an increase in the particle size of the formed SnO2 up to ∼50 nm. The fundamentals behind the microwave susceptibility of different materials are dealt with extensively
Subramania et al. in the open literature17 and it is a little beyond the scope of the present paper. The TEM images of SnO2 prepared under different conditions are shown in Figure 2. It can be seen that the particle size of the as prepared SnO2 in microwaves for 6 min is between 15 and 18 nm. Increasing further the microwave heating time to 10, 12, and 25 min led to an increase in the particle size to ∼50 nm (Figure 3). This is mainly due to the fact that, as the microwave heating is continued in air for a longer time, the amorphous carbon would be removed, inducing SnO2 particleparticle contact and leading to the agglomeration. Similar particle sizes have been observed for the postcalcined SnO2 in a conventional furnace at 500 °C for 1 h (Figure 2b). Moreover, the HRTEM image in Figure 2g clearly shows lattice fringes, indicating that the materials are crystalline. SAED studies (Figure 2h) also corroborated the results from the XRD and HRTEM studies on the crystallinity of the synthesized materials. The effect of microwave heating time on the SnO2 particles and effective carbon content is shown in Figure 3. The carbon content is inversely proportional to the microwave irradiation time. There has been one previous report for the rapid synthesis of SnO2 using SnCl4 and urea in microwave field.21 However, there was a requirement for postformation heating at 800 °C to get a pure phase nanocrystalline material.21 In the present method, the in situ formation of an amorphous carbon provides the local heating, leading to the formation of nanocrystalline SnO2 without any further heat treatment. This is a major advantage, as the total reaction time is just 6 min, which very much improves the cost effectiveness of the synthesis technique. Typical X-ray diffraction patterns of as prepared SnO2 with microwave heating for 6 min and that followed by a heating in a conventional furnace at 500 °C for 1 h are shown in Figure 4. For SnO2 prepared under different synthesis conditions, all the peaks can be indexed to the pure SnO2 with a cessiterite structure (JCPDS 41-1445); there is no major difference observed among the two diffractograms shown. The as prepared SnO2 show broad peaks, indicating the nanocrystalline nature of the synthesized particles. The diffractograms for the further heated SnO2 show comparatively sharper peaks than the 6 min microwave (MW) SnO2 due to the relatively larger grain size. The possible reason for the absence of any peak corresponding to the coexisting carbon in the microwave only sample may be due to the amorphous nature of the formed carbon and a relative small amount of carbon content. The crystallinity of SnO2 is very high, and a minor contribution from the carbon would have been completely masked. This indicates that further heating increases the particle size and removes further the carbon content of the formed tin oxide, as evidenced by TEM/EDX studies (Figure 3b). The main objective to heat the 6 min MW SnO2 sample in the conventional furnace is to compare the effect of heat treatment on the particle size as well as on the exact Sn valence state. Both the tin oxides prepared at 6 min microwave time and that followed by the conventional heating at 500 °C for 1 h showed almost identical profiles. Similar XRD patterns have been observed for the SnO2 prepared at different microwave times (not shown). Although X-ray and SAED studies confirmed that the formed compounds under all conditions are SnO2, further exploration with XPS will give a more accurate understanding of the exact valence state of Sn. This is very critical, as the electrochemical performance of any material depends on the oxidation state of the metal ion in the oxide. In other words, it is critical to see whether the as prepared material exhibits the same valence state as that after calcination. The Sn 3d spectra recorded for the
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Figure 3. Variation of the (a) particle size and (b) carbon content of the synthesized SnO2 particles at different microwave irradiation time. The average particle sizes of SnO2 prepared under different conditions are given along with the figure labels.
as-prepared SnO2 and the SnO2 calcined in the furnace are shown in Figure 4c. It is clear from the figure that both exhibit identical binding energy, 487.15 eV, which agrees well with that reported for SnO2.22 Reduction in oxygen stoichiometry leads to the shifting of the O 1s spectra of SnO2 to lower binding energy.23 Hence, it is clear that both the tin oxide prepared with 6 min microwave irradiation and that after calcination treatment have Sn in +4 state. Surface area measurements were conducted using N2-adsorption-desorption studies for the 6 min SnO2 sample and for the 500 °C heated SnO2 sample. The BET surface area of the former was 285 m2/g, while that of the latter showed a surface area of 123 m2/g. This clearly agrees with the TEM results on the increase in the particle size when conventional heating was employed. Apart from the size effect, the main reason behind the difference in the surface area of the two different SnO2 is the amount of amorphous carbon present in the respective samples. In other words, in the case of heat-treated sample, the amorphous carbon that formed because of the decomposition of the citric acid is almost completely removed, as evidenced by the EDX studies (Figure 3b). However, in the case of the SnO2 prepared by microwave irradiation for 6 min, there is ∼11 wt % (Figure 3b) of amorphous carbon residue. This amorphous carbon has a high surface area and also contributes to the overall specific surface area of the 6 min synthesized SnO2. The surface area difference of the samples has a direct impact on the reversible and irreversible reactions of lithium, as evidenced in Figure 6.
The electrochemical charge-discharge curves at 0.1 C rate for the first cycle is shown in Figure 5. During the first cycle, there is a very high irreversible capacity that has been reported well in the literature for SnO2. The fundamentals behind the electrochemical performance of SnO2 have been extensively studied by Dahn and his co-workers.15,16 The reaction mechanism for lithium insertion and deinsertion during the first cycle can be expressed as follows.15,16
Insertion (Discharge): Irreversible reduction of SnO2 to metallic Sn 4Li+ + 4e- + SnO2 f 2Li2O + Sn
(1)
Deinsertion (Charge): Reversible alloying/dealloying of Li with metallic Sn xLi+ + xe- + Sn f LixSn (0 e x e 4.4)
(2)
During the first cycle, lithium insertion, the main reaction leading to a very high specific capacity is the irreversible reduction of SnO2 to metallic Sn and Li2O, as shown in eq 1. In subsequent lithium deinsertion and in further cycles, the main reaction is the alloying and dealloying between metallic Sn and Li, which is responsible for the reversible capacity. Theoretically, 4.4 Li atoms can be stored per atom of Sn, which would amount to a very high theoretical charge-storage capacity of 781 mA h/g. This value is much higher when compared to the
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Figure 4. X-ray diffractograms of nanocrystalline SnO2 prepared under different conditions (a) under microwave (MW) for 6 min and (b) 6 min MW followed by calcination at 500 °C/1 h. (c) XPS spectra for as prepared nanocrystalline SnO2 (MW 6 min) and SnO2 MW 6 min + calcined at 500 °C for 1 h.
Figure 5. Typical first cycle charge-discharge curves for nanocrystalline SnO2 prepared in microwave irradiation for 6 min. Inset: TEM picture of cycled SnO2 electrode; the circle shows the formed Sn nanograin as per eqs 1 and 2, and the inset shows the electron diffraction pattern recorded for the Sn nanograin.
traditional graphitic anode, which exhibits a theoretical capacity of 372 mA h/g. However, large volume changes in bulk tin oxide samples during the alloying/dealloying process are a major
disadvantage that resulted in a faster fade in capacity.24,25 In order to check the formation of Sn nanograins, the cycled electrode material was subjected to TEM studies and is shown
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Figure 6. Cycling performance of the SnO2 synthesized under different conditions. The cycling was recorded galvanostatically between 2.0 and 0.005 V in a 1 M LiPF6-based nonaqueous electrolyte.
in the inset of Figure 5. The formation of nanograins of Sn is confirmed from the SAED pattern, supporting the lithium insertion-deinsertion mechanism shown in eqs 1 and 2. In the present study, the first cycle discharge capacity (DC) was 2156 mA h/g and the corresponding charge capacity (CC) was 898 mA h/g. In the second cycle, the DC and CC were 957 and 821 mA h/g, respectively. There was a considerable decrease in the irreversible capacity as mentioned in eqs 1 and 2. In general, the specific capacity values of SnO2 synthesized via other methods are lower than that observed in the present study. For instance, the specific capacity for mesoporous SnO2 prepared by an anionic surfactant showed a specific capacity of ∼450 mA h/g.11 Even for an improved SnO2 with an amorphous AlPO4 coating, the reversible capacity values were in the range of
