J. Phys. Chem. C 2008, 112, 13043–13046
13043
LiVOPO4 Hollow Microspheres: One-Pot Hydrothermal Synthesis with Reactants as Self-Sacrifice Templates and Lithium Intercalation Performances M. M. Ren, Z. Zhou,* X. P. Gao, L. Liu, and W. X. Peng Institute of New Energy Material Chemistry, Tianjin Key Laboratory of EnVironmental Remediation and Pollution Control, Nankai UniVersity, Tianjin 300071, China ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: June 12, 2008
Self-produced template (usually bubbles and carbon particles) methods have been adopted to obtain micrometerand nanometer-scaled hollow spheres. In this paper, we report the preparation of R-LiVOPO4 hollow spheres through a hydrothermal reaction of LiOH · H2O, V2O5, H3PO4, and N2H4 · H2O at 250 °C for 48 h. During this procedure the reactant, V2O5 particles, served as the template and was consumed without any residues when the hydrothermal reaction was completed. Therefore, such a self-sacrifice template method makes large-scale production of R-LiVOPO4 hollow spheres feasible. R-LiVOPO4 hollow spheres were exploited as both cathode and anode materials for lithium ion batteries. 1. Introduction Micrometer- and nanometer-scaled hollow spheres have been attracting much research interest because of their wide applications to catalysis, drug delivery and controllable release, optical materials, microcapsule reactors, chemical sensors, Li ion battery materials, etc. The common synthesis methods for hollow microor nanospheres are mostly template ones, which can be further divided into two categories: hard templates such as silica,1 polymer,2,3 or carbon particles,4 and soft templates such as vesicles,5,6 emulsions,7,8 micelles,9,10 and even gas bubbles.11 Through template routes, recently a lot of oxides, sulfides, nitrides, and metal/alloy hollow spheres have been synthesized and offer unique performances and potential applications in many fields.12–16 Some of them were exploited as anode materials for lithium ion batteries.15,16 Template routes exhibit complicated procedures and high cost, and they are not practical for Li ion batteries with mass production. Template-free methods are more prospective for such purpose. Until now, the reported template-free strategies, such as the self-produced template routes, have mainly focused on anode materials for lithium ion batteries, while cathode materials, both lithium transition metal oxides and phosphates, are significantly lacking.17 Lithium transition metal phosphates, such as LiMPO4,18–21 (M ) Fe, Mn, or Co), Li3M2(PO4)3 (M ) V, Fe, or Ti),22–27 and LiVPO4F,28,29 have been attracting a great deal of attention. These materials include both mobile Li ions and redox-active transition metals within a rigid phosphate network, displaying high electrochemical and thermal stability as well as comparable energy density. Another transition metal phosphate, vanadyl phosphate (VOPO4), exists in several crystal phases;30–36 the β- and ε-phases have a three-dimensional framework, and the RI-, RII-, δ-, γ-, ω-phases have a layered structure.32 Vanadyl phosphate can be used as cathode material for lithium ion batteries, with the theoretical Li ion intercalation capacity of 166 mA h g-1.30,32,35 Usually lithium vanadyl phosphate was synthesized by wet chemistry methods 30–36 or the carbothermal reaction route.31 In this work R-LiVOPO4 hollow microspheres were synthesized via a one-pot hydrothermal process with the * To whom correspondence should be addressed. E-mail: zhouzhen@ nankai.edu.cn.
reactant, V2O5 particles, as self-sacrifice templates, and electrochemical Li ion intercalation performances were also evaluated both as anodes and cathodes. 2. Experimental Section 2.1. Preparation of r-LiVOPO4. The starting materials were V2O5, LiOH · H2O, H3PO4, and N2H4 · 2H2O (Alfa-Aesar). First, V2O5 was dissolved in H3PO4 solution, and then N2H4 · 2H2O was added to the solution drop by drop under vigorous stirring at room temperature. Then LiOH · H2O was added to the mixed solution with continuous stirring for 10 min. Finally 20 mL of the precursor solution was transferred to a 30 mL Teflon vessel sealed in an autoclave and hydrothermally treated at 250 °C for 48 h. The molar ratio of Li:P:V in the mixed solution was 9:3:1, and the concentration of Li+ in the solution was 0.7 mol/ L. A green precipitate was obtained after the hydrothermal reaction, then washed with deionized water for at least 5 times, and finally dried in an oven at 100 °C for 8 h. During the synthesis of LiVOPO4, N2H4 · 2H2O is used to reduce V from V(V) to V(IV). 2.2. Sample Characterization. Structural and crystallographic analyses of the products were taken from the X-ray diffraction (XRD) using the D/MAXIII diffractometer with Cu KR radiation (λ ) 1.5418 Å). The samples were also observed using the Hitachi S-3500N scanning electron microscopy (SEM). 2.3. Galvanostatic Charge/Discharge Tests. Electrochemical performances of the samples were evaluated in Li test cells. The electrodes were prepared by mixing the products with acetylene black and polytetrafluoroethylene (PTFE) with a weight ratio of 85:10:5 in ethanol to ensure homogeneity. After the ethanol was evaporated, the mixture was rolled into a sheet and cut into circular strips of 8 mm in diameter. The strips were then dried at 100 °C for 10 h. Lithium metal was used as counter electrode. The electrolyte was composed of a 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ ethylene methyl carbonate (EMC) with the volume ratio of 2:1: 1. The three parts were assembled into test cells in an argonfilled dry glovebox, and then the cells were measured using a Land CT2001 battery tester. The electrochemical measurements were performed in a voltage range of 3.0-4.5 V or 0.01-3.5 V at 25 °C.
10.1021/jp804335b CCC: $40.75 2008 American Chemical Society Published on Web 07/25/2008
13044 J. Phys. Chem. C, Vol. 112, No. 33, 2008
Ren et al.
Figure 1. XRD pattern of the R-LiVOPO4 material synthesized by hydrothermal reaction at 250 °C for 48 h.
TABLE 1: Cell Parameters of r-LiVOPO4 Synthesized in This Work a (Å)
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
5.3376
7.2728
5.1208
109.114
97.404
106.654
174.42
Figure 3. XRD profiles for the R-LiVOPO4 materials synthesized via the hydrothermal route at 250 °C with different reaction times.
3. Results and Discussion 3.1. Sample Characterization. The XRD pattern of R-LiVOPO4 samples is shown in Figure 1. The diffraction patterns were refined based on a triclinic structure using space group P1j. It can be seen that no impurity phase was detected from our XRD measurement. Table 1 shows the cell crystallographic parameters of R-LiVOPO4. Figure 2 shows the SEM images of the R-LiVOPO4 sample synthesized at 250 °C for 48 h. From the low-magnification SEM images, the samples present typical spherical morphologies with diameters about 12 µm. The hollow structure of the spheres is clearly revealed in Figure 2c. The hollow spheres are built from 2D sheets with the wall thickness of 3-4 µm. 3.2. Formation Mechanism of r-LiVOPO4 Hollow Microspheres. In order to explore the growth mechanism of hollow spheres, the precursors were hydrothermally treated for different amounts of time. The products were examined by SEM and XRD. Parts a and b of Figure 4 show the images of the product with a 2 h reaction time. The observed particles are spherical but noncompact like cotton, with a particle size of about 2 µm. Figure 4. SEM images of R-LiVOPO4 synthesized at 250 °C for a hydrothermal reaction time of 2 h (a and b), 6 h (c and d), or 12 h (e and f).
Figure 2. SEM images of R-LiVOPO4 material synthesized at 250 °C for 48 h.
The XRD pattern in Figure 3 shows that the peaks of V2O5 are very clear at the reaction time of 2 h, and the product is a mixture of R-LiVOPO4 and V2O5. When the reaction time increased to 6 h, the morphology of the product did not change, but the particle size increased to around 6 µm (Figure 4c,d). At this status, R-LiVOPO4 still coexists with V2O5, seen from the XRD results (Figure 3); however, the peak intensity from V2O5 decreased apparently, indicating that V2O5 reacted with the solution and was consumed gradually. The particle diameters increased to 9 µm when the reaction time was further elongated to 12 h (Figure 4e,f), and all the peaks of the product can be indexed to R-LiVOPO4. According to the SEM images, when the reaction times were 2 and 6 h, the particles are solid spheres. However, when the reaction time was 12 h, the SEM images (Figure 4e,f) revealed the existence of both broken and intact hollow spheres. A longer reaction time brought an obvious
LiVOPO4 Hollow Microspheres
Figure 5. First charge/discharge profiles of R-LiVOPO4 at 10 mA g-1 in the potential range of 3.0-4.5 V at 25 °C.
J. Phys. Chem. C, Vol. 112, No. 33, 2008 13045
Figure 6. Cyclic performance of the R-LiVOPO4 in the potential range of 3.0-4.5 V at a current density of 10 mA g-1 at 25 °C.
SCHEME 1: Schematic Illustration of the Formation of r-LiVOPO4 Hollow Spheres
increase in particle size. When the reaction time was 48 h, the samples retained hollow spherical morphologies with the diameter of around 12 µm. Based on the above experimental results, we can deduce the formation procedure of R-LiVOPO4 hollow spheres. As illustrated in Scheme 1, first, V2O5 small particles were surrounded by H3PO4, N2H4 · 2H2O, and LiOH solution. At high temperature and pressure, V2O5 particles gradually reacted with the outer solution to form R-LiVOPO4, and the newly formed R-LiVOPO4 aggregated in the interface of V2O5 particles and solution. In accordance with the inner V2O5 consumption, the outer R-LiVOPO4 particles grew large during the subsequent hydrothermal process. There are pores among outer R-LiVOPO4 particles, and the solution can diffuse through the pores; V2O5 particles are surrounded by the reaction solution, and they react with the solution gradually. Finally, V2O5 particles were consumed completely after more than 12 h of hydrothermal reaction, and R-LiVOPO4 particles exhibited hollow interiors. Actually, this procedure is also a self-produced template route.17 Usually self-produced templates are bubbles and monodisperse carbon particles. In this work, the self-produced template is unique; the reactant, V2O5 particle, serves as the template. Moreover, the reactant templates are all consumed up without any residues when the hydrothermal reaction is completed. Therefore, such reactant templates can be called self-sacrifice ones. In a former report on the preparation of MoS2 hollow spheres, the intermediate K2NaMoO3F3 crystals served as a selfsacrifice template.37 3.3. Electrochemical Li Intercalation Performances of r-LiVOPO4 Hollow Microspheres. Figure 5 shows the initial charge/discharge curves for the Li/R-LiVOPO4 test cells at a current density of 10 mA g-1 in the voltage range of 3.0-4.5 V at 25 °C. The electrode shows two charge plateaus around 4.0 and 4.1 V, respectively, and a discharge plateau around 3.9 V. Two plateaus in the first charge may be due to the delithium process subdivided into two compositional regions, like the LiVPO4F cathode material reported by Barker and co-workers.28 The initial charge capacity of R-LiVOPO4 is 197.7 mA h g-1, exceeding its theoretical value (165 mA h g-1) because of the
Figure 7. First charge/discharge profiles of R-LiVOPO4 at 30 mA g-1 in the potential range of 0.01-3.0 V at 25 °C.
excessive lithium ions in the sample; the first discharge capacity is 93.8 mA h g-1. The cyclic performance of R-LiVOPO4 is shown in Figure 6. The initial discharge capacity of R-LiVOPO4 is 93.3 mA h g-1, and the discharge capacity is 85.6 mA h g-1 at the 30th cycle. The discharge capacity retention is 91.7%. The hollow-sphere morphology can increase the contact between the material and electrolyte and shorten the Li+ diffusion route. However, R-LiVOPO4 has lower electrical conductivity than other transition metal phosphates; therefore, its practical discharge capacity is lower than the theoretical value, and further efforts are needed to improve its electrochemical performances. Since the material has a higher discharge plateau around 3.9 V (vs Li/Li+), it can be used in high-voltage lithium ion batteries. Also, R-LiVOPO4 hollow spheres can be used as anode materials for lithium ion batteries. Similar to the report that LiFePO4 as an anode material exhibited a capacity 4-5 times higher than that as a cathode material,38 R-LiVOPO4 hollow spheres show similar results. Figure 7 presents the initial charge/ discharge curves for the Li/R-LiVOPO4 test cells at a current density of 30 mA g-1 in the potential range of 0.01-3.5 V at 25 °C. There are three obvious sloping potential ranges (2.5-2.0, 1.0-0.6, and 0.5-0.01 V vs Li/Li+) during the first discharge, and the initial discharge capacity reached 656.4 mA h g-1, which is approximately 6 times higher than the corresponding discharge capacity of R-LiVOPO4 used as cathode materials. However, after 20 charge/discharge cycles, the discharge capacity retained only 206.2 mA h g-1. Such cyclic performance was rather poor, compared with the samples used as cathode materials.
13046 J. Phys. Chem. C, Vol. 112, No. 33, 2008 4. Conclusion In summary, R-LiVOPO4 hollow spheres were synthesized through a one-step hydrothermal route at 250 °C for 48 h. In this reaction, V2O5 particles, the reactant, served as self-sacrifice templates, making large-scale production of R-LiVOPO4 hollow spheres feasible. The samples were exploited as both cathode and anode materials for lithium ion batteries. The discharge capacity of the R-LiVOPO4 cathode material was 93.8 mA h g-1 in the first cycle and 85.6 mA h g-1 in the 30th cycles at the current density of 10 mA g-1, with the 91.7% retention rate of discharge capacity. As anode material, R-LiVOPO4 showed an initial discharge capacity as high as 656.4 mA h g-1. Acknowledgment. This work was supported by the 973 National Basic Research Program (2002CB211800), the 863 National High Technology Research and Development Program (2007AA03Z225), and Tianjin Natural Science Foundation (06YFJMJC13300) in China. References and Notes (1) Salgueirino-Maceira, V.; Spasova, M.; Farle, M. AdV. Funct. Mater. 2005, 15, 1036. (2) Caruso, F.; Spasova, M.; Saigueirino-Maceira, V.; Liz-Marzan, L. M. AdV. Mater. 2001, 13, 1090. (3) Kawahashi, N.; Persson, C.; Matijevic, E. J. Mater. Chem. 1991, 1, 577. (4) Xia, Y. D.; Mokaya, R. J. Mater. Chem. 2005, 15, 3126. (5) Schmidt, H. T.; Ostafin, A. E. AdV. Mater. 2002, 14, 532. (6) Hubert, D. H. W.; Jung, M.; German, A. L. AdV. Mater. 2000, 12, 1291. (7) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001, 2028. (8) Hirai, T. S.; Hariguchi, I.; Komasawa, R.; Davey, J. Langmuir 1997, 13, 6650. (9) Fowler, C. E.; Khushalani, D.; Mann, S. J. Mater. Chem. 2001, 11, 1968. (10) Huang, J. X.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y. T.; Zhang, S. Y. AdV. Mater. 2000, 12, 808. (11) Du, F. L.; Guo, Z. Y.; Li, G. C. Mater. Lett. 2005, 59, 2563. (12) Sun, X. M.; Liu, J. F.; Li, Y. D. Chem.-Eur. J. 2006, 12, 2039. (13) Qian, H. S.; Lin, G. F.; Zhang, Y. X.; Gunawan, P.; Xu, R. Nanotechnology 2007, 18, 355602.
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