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J. Phys. Chem. C 2009, 113, 21258–21263
Mesoporous Anatase TiO2 with High Surface Area and Controllable Pore Size by F--Ion Doping: Applications for High-Power Li-Ion Battery Anode Hun-Gi Jung,† Chong Seung Yoon,‡ Jai Prakash,§ and Yang-Kook Sun*,† Department of Energy Engineering and DiVision of Materials Science and Engineering, Hanyang UniVersity, Seoul 133-791, South Korea, and Department of Chemical and EnVironmental Engineering, Center for Electrochemical Science and Engineering, Illinois Institute of Technology, 10 West 33rd Street, Chicago, Illinois 60616 ReceiVed: September 9, 2009
A pioneering effort has been made to develop F--ion-doped anatase TiO2 with a spherical shape and mesoporous structure synthesized by a unique, low-temperature urea-assisted hydrothermal synthesis. The electrochemical anode performance of pristine TiO2 for lithium-ion batteries was improved significantly by utilizing F--ion doping. TiO2 doped with 10 mol % fluorine ions achieved a reversible capacity of 157 mA h/g after 100 cycles. In addition, the rate capability of the F--ion-doped TiO2 was encouraging, with a stabilized capacity of 144 mA h/g at 30C (C rating) of charge (Li extraction). Introduction Considerable attention has been paid to electrochemical energy storage devices, especially rechargeable lithium-ion batteries, with both high energy and high power densities because of their potential applications in electronic vehicles and portable electronic devices. Despite remarkable growth and widespread applications for these types of batteries, the use of Li-ion batteries for high-power applications is often criticized because of kinetic problems associated with the electrode materials. As such, the demand for novel electrode materials with superior electrochemical performance is inevitable. Recently, metal oxides have gained popularity as anodes for Liion battery applications because of their structural stability and diversity.1-3 However, complexities related to volume change and irreversible capacity loss have hampered the utilization of most metal oxides in lithium battery applications.3,4 Because anode materials should be able to endure repeated lithium insertion and extraction processes without structural damage for a long rechargeable lifetime, many transition-metal oxides have been subjected to doping or surface modification to avoid such structural deterioration.3,5,6 Construction of size-reduced or nanometric grains has also been shown to enhance the electrode performance of metal oxides.1-3 Among the transition-metal oxides, TiO2 has considerable potential as an electro-active material because of its high activity, strong oxidation capability, and chemical stability. The electrochemical performance of TiO2 relies strongly on its crystallinity, crystallite size, crystal structure, specific surface area, and thermal stability.7-9 Furthermore, among the polymorphs of TiO2, anatase is generally considered to be the most electroactive Li-insertion host.10 Because a TiO2 electrode material with a large active surface area would allow for the use of high electrochemical reaction rates per unit volume, the preparation of TiO2 electrode materials with highly porous structures is becoming of paramount importance in battery applications.11,12 Also, because a large internal surface area * To whom correspondence should be addressed.
[email protected]. † Department of Energy Engineering, Hanyang University. ‡ Division of Materials Science and Engineering, Hanyang University. § Illinois Institute of Technology.
enhances diffusion kinetics by reducing the diffusion pathway for electronic and ionic transport, which favors an enhanced power density, mesoporous TiO2 with pore sizes ranging from 2 to 50 nm have become increasingly important. Therefore, the electrochemical properties of TiO2, triggered by the presence of mesoporosity and the magnitude of the surface area, greatly depend on the synthesis methodology employed.13 In addition to high porosity and specific surface area, cationic or anionic doping with an alternate element has also been found to be very effective in enhancing the properties of TiO2.14,15 In this article, we report on the electrochemical performance of anatase-phase F--ion-doped mesoporous TiO2 that exhibits superior electrochemical properties compared to pristine TiO2. Two different ratios of F to Ti (hereafter referred to as FD1 and FD2 for 10 and 20 nominal atomic %, respectively) were selected, and the physical and electrochemical behaviors are discussed in detail. Experimental Section Pristine and F--ion-doped mesoporous anatase-phase TiO2 submicrospheres were synthesized using a hydrothermal method. Details of the synthetic process for pristine spherical mesoporous TiO2 were previously reported by our group.16 For F--ion doping, high-purity TiCl4 was added dropwise to a distilled water/ethanol mixture submerged in an ice water bath. To this homogeneous solution were added a calculated quantity of TiF4 serving as the source of F- ions and urea with continuous stirring until the solution became clear. When this happened, a calculated quantity of ammonium sulfate was added, and the mixture was allowed to stir for 2-4 h. The resulting transparent solution was transferred to a Teflon-lined autoclave and heated to a temperature of 393 K for 24 h. After this incubation, the reaction mixture was allowed to cool to room temperature, and the resulting slurry was filtered and washed in ethanol. The powder obtained after the filtering and ethanol wash was vacuum-dried and sintered at 400 °C for 5 h in air. The morphology and crystalline structure of the synthesized material were analyzed by field-emission scanning electron microscopy (FE-SEM; JSM-6400, JEOL) and high-resolution transmission electron microscopy (HR-TEM; JEM-2010, JEOL).
10.1021/jp908719k 2009 American Chemical Society Published on Web 11/12/2009
F--Ion-Doped Anatase TiO2 for Lithium-Ion Batteries
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Figure 1. FE-SEM images of pristine TiO2 with different concentrations of urea and ammonium sulfate: (a) 1 mol/L urea and 0.05 mol/L ammonium sulfate, (b) 2.5 mol/L urea and 0.05 mol/L ammonium sulfate, (c) 5 mol/L urea and 0.05 mol/L ammonium sulfate (inset shows the surface of the synthesized TiO2), and (d) 5 mol/L urea and no ammonium sulfate.
Powder X-ray diffraction (XRD) measurements were carried out using a Rikaku Rint-2000 instrument with Cu KR radiation. Nitrogen sorption measurements were performed using a Quantachrom Autosorb-1 apparatus after the sample had been degassed at 200 °C for 4 h. The fluorine doping level was determined by X-ray photoelectron spectroscopy (XPS; ESCALAB 220-I, VG). To evaluate the electrochemical anode ability for use in lithium-ion batteries, the electrodes were made of either the pristine TiO2, FD1, or FD2 active materials with carbon black conducting agent (super P and KS6) and a poly(vinylidene fluoride) (PVDF) binder in a weight ratio of 80:10:10. After these materials had been thoroughly mixed in an N-methyl-2pyrrolidone (NMP) solution, the prepared slurry was coated on Cu foil to a thickness of approximately 70 µm. The coated electrode was dried at 110 °C for 1 h and was subsequently roll-pressed. Preliminary cell tests were completed with a 2032 coin-type cell, prepared in an argon-filled glovebox, using Li metal as the counter electrode and a microporous polyethylene separator. The electrolyte solution was 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 vol %, Ukseung Chemical Co., Ltd.). Results and Discussion Figure 1 shows FE-SEM images of the TiO2 materials synthesized with different concentrations of urea and ammonium sulfate. The TiO2 particles synthesized with 1 mol/L urea had very smooth surfaces without obvious granular features (Figure 1a). As the concentration of urea was increased to 5 mol/L, it was observed that the primary crystal size increased, and the surface of the TiO2 became rough (Figure 1b,c) without a secondary particle size change. In this reaction system, urea was added to control the rate of hydrolysis, where the ammonia produced by the hydrolysis reaction (eq 1) of urea controlled the pH of the reaction at temperatures higher than 80 °C17
(NH2)2CO + 3H2O f 2NH4+ + 2OH- + CO2
(1)
Apart from controlling the rate of hydrolysis, the ammonia evolved from the urea also played a vital role in imparting porosity to the particles.18 As shown in Figure 1c, the synthesized TiO2 materials contained nanocrystals (about 7-15 nm) and pores (about 4-7 nm) that were observed over the surface of the particle. As compared in parts c and d of Figure 1, it was found that the spherical shape of the synthesized TiO2 particle was stabilized by even small concentrations (0.05 mol/L) of ammonium sulfate. The role of ammonium sulfate in stabilization is known because of its zeta potential modification of the spherical polycondensed TiO2 particles generated at the initial stage of the hydrolysis reaction.19,20 Therefore, the ammonium sulfate-dissolved EtOH/H2O solution allowed for the formation of mesoporous TiO2 with exactly spherical shapes. On the basis of this experimental evidence, it was concluded that the mesoporosity and spherical shape of the TiO2 are brought about by the combined mechanism of ammonium sulfate and urea. The hydrolysis of TiCl4 assisted by urea in the presence of ammonium sulfate resulted in the formation of mesoporous pristine and F--ion-doped TiO2 with exactly spherical shapes. The particle morphology and internal crystalline structure of the pristine TiO2 and FD1 and FD2 materials were observed by FE-SEM and HR-TEM analysis, as shown in Figure 2a-f. The presence of exactly spherical secondary particles constructed by networks of large numbers of nanometric primary spheres, which provided the porous nature of the material, is clearly visible in the micrographs. A primary particle size of about 10-20 nm and a secondary particle size of about 0.5-1.0 µm were identified from the FE-SEM and HR-TEM images. The presence of microscopic voids in the HR-TEM images authenticated the highly mesoporous nature of the synthesized materials. Also, a notable increase in primary particle size and pore size of the pristine TiO2, upon doping with F- ions, was apparent in the TEM images. The XRD patterns of the pristine, FD1, and FD2 materials (Figure 3) substantiated the presence of a pure, anatase-phase TiO2 with an I 41/amd (141) space group and a tetragonal crystal lattice arrangement.21 The slight augmentation in the calculated unit cell values and the cell volume (Table 1) of the FD1 and FD2 materials were attributed to the effective incorporation of the dopant F- ion into the octahedral site of the pristine TiO2 matrix. The mean crystallite size, D101, calculated along the (101) plane using Scherrer’s formula (Table 1) indicates that the crystallite size and relative crystallinity (peak intensity) increased as a function of dopant F--ion concentration, which is in accordance with the HR-TEM analysis (Figure 2) and the results reported in the literature.15 The selected-area electron diffraction (SAED) patterns (insets in Figure 2d-f) showed an increase in crystallinity upon doping of the pristine TiO2 with F- ions, which is consistent with the results derived from XRD analysis.15 The specific surface area and pore size distributions of the pristine and F--ion-doped TiO2 materials were measured by nitrogen gas adsorption and desorption isotherms (Figure 4). The standard multipoint Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. Pore size distributions were obtained from the isotherm adsorption branches based on the Barrett-Joyner-Halenda (BJH) model. Nitrogen adsorption/desorption isotherms (Figure 4a) of the pristine, FD1, and FD2 materials exhibited a type-IV isotherm (hysteresis loop between H1 and H2 types), indicating the presence of highly mesoporous particles.22 The large hysteresis loops shown by the adsorption/desorption isotherms, with shapes that were intermediate between representative H1- and H2-type isotherms, were observed for the mesoporous pristine, FD1, and
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Figure 2. (a-c) FE-SEM and (d-f) HR-TEM images and corresponding SAED patterns (insets) of the (a,d) mesoporous pristine TiO2, (b,e) FD1, and (c,f) FD2 materials.
Figure 3. XRD patterns of calcined pristine TiO2 and F--ion-doped TiO2 (FD1 and FD2) at 400 °C.
TABLE 1: Crystalline Information on Pristine TiO2 and FD1 and FD2 Synthesized by the Urea-Assisted Hydrothermal Method sample
a (Å)
c (Å)
pristine 3.7820 9.5020 FD1 3.7853 9.5044 FD2 3.7868 9.5062
cell volume (Å3) dXRDa (nm) dTEMb (nm) 135.912 136.187 136.318
14 16 19
10 12 18
a Crystal size calculated by applying the Scherrer equation to the (101) anatase peak. b Crystal size measured from the corresponding HR-TEM images (Figure 2d-f).
FD2 materials.23 The strong hysteresis is believed to be related to the capillary condensation associated with the large pore channels, which can also be attributed to the modulation of the channel structure.23 The BJH pore size distributions measured for the pristine, FD1, and FD2 materials also demonstrate the presence of well-developed mesoporosity with very narrow poresize distributions, in addition to an average pore diameter ranging from 6 to 9 nm. It is worth mentioning that only these mesopores allow rapid filling of electrolyte within the TiO2 particle during the electrochemical charge/discharge reaction and enhance the rate capability of the material. The BET surface areas calculated for the synthesized samples are summarized in Table 2. FD2 demonstrated a higher surface area (161.4 m2/g) than did the pristine (116.5 m2/g) or FD1 (139.9 m2/g) samples, irrespective of the potential enhancement in average crystallite
Figure 4. (a) Nitrogen sorption isotherms and (b) corresponding pore diameter distributions obtained for the adsorption branch of calcined pristine and F--ion-doped mesoporous TiO2 at 400 °C. The isotherms of 10 and 20 mol % F--ion-doped TiO2 were shifted by 100 and 200 cm3/g, respectively.
size upon F--ion doping. This can be explained by the fact that doping of F- ions into the TiO2 lattice structure not only increases the primary particle size but also increases the pore volume (Table 2), which contributes to the favorable increase in surface area. The surface composition and the corresponding binding energy values of the respective elements (Ti, O, and F) present in the pristine and F--ion-doped TiO2 materials were analyzed by X-ray photoelectron spectroscopy (XPS). Figure 5 shows high-resolution XPS plots of the anatase-phase mesoporous pristine, FD1, and FD2 powders calcined at 400 °C. The XPS pattern shows that the synthesized pristine and F--ion-doped
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TABLE 2: Physical Properties from N2 Sorption Studies of Pristine TiO2 and FD1 and FD2 Synthesized by the Urea-Assisted Hydrothermal Method sample pore diameter (nm) pore volume (cm3/g) pristine FD1 FD2
4.30 6.56 7.78
0.1842 0.2453 0.2615
specific surface area (m2/g) 116.5 139.9 161.4
TiO2 materials contained only Ti, O, and F elements without any trace impurities. The peaks observed at 459 and 465 eV (Figure 5a) for Ti 2p3/2 and Ti 2p1/2, 531 eV for O 1s (Figure 5b), and 684.1 and 688.8 eV for F 1s (Figure 5c) and their corresponding binding energy values were used for quantitative measurements of the elements present in the material. The main contribution from 684.1 eV, analogous to the F 1s region, was attributed to the F- ions physically adsorbed on the surface of TiO2. An additional and subsidiary peak at 688.2 eV, exhibited by FD2 (Figure 5c), was assigned to the octahedral arrangement or bulk substitution of F- ion in the crystal lattice of the TiO2 matrix, which is in agreement with previously reported literature.15 It is interesting to note that the small peak at 688.2 eV was seen only when the concentration of F- ions exceeded a specific value. When the total doping ratio of F to Ti reached 20 atom %, this peak started to emerge, which can be correlated with the saturation of the surface F--ion concentration leading to bulk substitution. The approximate composition of the surface was determined by dividing the individual peak area, after appropriate background subtraction, by the respective atomic sensitivity factors (ASFs). Accordingly, the calculated Ti/O/F atomic ratios of the pristine, FD1, and FD2 materials were 1:2.1:0, 1:2.0:0.07, and 0.9:2.0:0.21, respectively, which corroborates with the nominal atomic composition. The calculated F--ion concentration in FD1 was 7%, whereas FD2 displayed a F--ion concentration of 21%, where 17% was adsorbed on the surface and 4% was substituted in the bulk. In general, nanostructured TiO2 is well-known for its use as an anode material because of its rapid and low potential lithium insertion/extraction ability, without any volume change and capacity loss.6,14 Figure 6a shows the first-cycle charge/discharge profiles of the pristine and F--ion-doped TiO2 materials in the 1.5-3.0 V (Li/Li+) voltage range with a current density of 17 mA/g [0.1C (C rating)]. All three compounds exhibited a plateau at ∼1.8 V, and there was little difference in the capacities of the pristine and F--ion-doped TiO2 anodes. The charge capacities (lithium extraction) of the samples were about 195 mA h/g (pristine), 207 mA h/g (FD1), and 200 mA h/g (FD2), corresponding to x ) 0.58, 0.62, and 0.60, respectively, for
Figure 6. Electrochemical charge discharge studies of pristine TiO2, FD1, and FD2 anodes, including (a) voltage vs capacity plot and (b) Nyquist plot of the anodes measured at the highly charged (0.1C) state.
LixTiO2. The amount of inserted Li for LixTiO2 was larger than x ) 0.5 reported for the ordinary anatase phase.24 The larger charge capacity resulted from the high surface area of the nanostructured materials and has already been investigated in mesoporous TiO2 and nanosized TiO2.11,25 The charged-state Nyquist plots of the pristine and F--iondoped TiO2 materials are shown in Figure 6b. For all samples, a well-defined semicircle at high frequencies (>1.81 Hz) and a near-vertical straight region at low frequencies (
