preparation of fe2o3 submicro - flowers by a hydrothermal approach

A vailable online at www.sciencedirect.com Electrochimica Acta 53 (2008) 4213–4218 Preparation of ␣-Fe 2 O 3 submicro-flowers by a hydrothermal approach and their electrochemical performance in lithium-ion batteries Yanna NuLi a,b , Peng Zhang a , Zaiping Guo a,∗ , P. Munroe c , Huakun Liu a a Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia b Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China c Electron Microscopy Unit, University of New South Wales, Kensington, NSW 2052, Australia Received 21 September 2007; received in revised form 3 December 2007; accepted 24 December 2007 Available online 6 January 2008 Abstract Uniform ␣-Fe 2 O 3 submicron-sized flowers have been synthesized by a simple hydrothermal process conducted at 160 ◦ C for 24 h. The crystalline structure and morphology of the as-synthesized powder have been characterized by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and field emission scanning electron microscopy (FE-SEM). The results revealed that the highly crystalline ␣-Fe 2 O 3 submicro- flowers were composed of nanospheres with an average size of 20–30 nm. The electrochemical performance as anode material for lithium-ion batteries was further evaluated by cyclic voltammetry (CV) and by electrochemical impedance and charge–discharge measurements. It was demonstrated that the material could provide an initial reversible capacity of 959.6 mAh/g at a current density of 20 mA/g over the voltage range from 0.01 to 3.0 V. The capacity retention upon the 50th cycle was 44.4 and 35.9% at 60 and 100 mA/g, respectively. The superior electrochemical performance may be resulted from the high surface area and the small and uniform grain size. © 2008 Elsevier Ltd. All rights reserved. Keywords: Iron oxide; Hydrothermal method; Submicro-flowers; Anode materials; Lithium-ion batteries 1. Introduction Among the various methods for the preparation of oxide pow- ders, such as the sol–gel process [1], solid-state transformation [2], chemical vapour deposition [3], physical vapour deposition [4], spray pyrolysis [5], microwave-induced hydrolysis [6], and emulsion [7] and hydrothermal processes [8], the hydrothermal technique occupies a unique place, owing to its advantages over conventional technologies. Various crystalline powders with high purity, phase homogeneity, controlled morphology, narrow particle size distribution, little or no macroscopic agglomeration, and excellent reproducibility can be produced by this technique, due to a combination of simple equipment with easy sintering and less time consumption than in other methods [9,10]. As the most stable iron oxide, hematite (␣-Fe 2 O 3 ) has been extensively used in the production of pigments, catalysts, gas ∗ Corresponding author. Tel.: +61 2 4221 5225; fax: +61 2 4221 5731. E-mail address: zguo@uow.edu.au (Z. Guo). sensors, and raw materials for hard and soft magnets, due to its low cost, environmental friendliness, and high resistance to corrosion [11]. Furthermore, it has also been shown to act as a rechargeable electrode material that reacts with six Li per formula unit, exhibiting higher capacity than the carbonaceous substances (e.g. maximum of 372 mAh/g for graphite) that are used currently in commercial lithium-ion batteries [12].Ithas been considered that most of these functions depend strongly on the structure and particle size of the materials, and nano-␣-Fe 2 O 3 exhibits better electrochemical performance than micro-sized samples [13]. It is evident that the purpose of preparing ␣-Fe 2 O 3 in various sizes and shapes has been driven by the strong inter- est in their novel properties and potential applications [14]. Several methods have been successfully developed for fabri- cation of hematite nanoparticles, such as the sol–gel process, template methods, chemical precipitation, the microemulsion technique, the gas–solid reaction technique, the forced hydroly- sis method, and the hydrothermal approach [15–24].Inviewof the homogeneous nucleation and grain growth, the hydrother- 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.12.067 4214 Y. NuLi et al. / Electrochimica Acta 53 (2008) 4213–4218 mal method shows advantages over conventional methods. It has been reported that hydrothermal reactions involving an aqueous iron nitrate solution over a wide concentration range produce porous hematite nanocrystals (40–80 nm) containing non-intersecting 5–20 nm pores [25]. Single-phase hematite can be formed in ferric chloride hydrothermal systems when the fer- ric concentration is kept as low as 0.02–0.04 M [26]. It is clear that nanocrystals can be obtained by adjusting the hydrother- mal conditions, but nanocrystals formed in this manner tend to agglomerate. Zhao et al. have successfully synthesized monodis- perse ␣-Fe 2 O 3 nanoparticles with different particle sizes by a hydrothermal method [27]. ␣-Fe 2 O 3 nanotubes and nanorods have also been selectively synthesized through a hydrothermal method using Span80 or L113B as a soft template [17]. Wan et al. have proposed a soft-template-assisted hydrothermal route to prepare single crystal nanorods with an average diameter of 25 nm and length of 200 nm at 120 ◦ Cin20h[24].Sev- eral workers have prepared nanoparticles using both aqueous and non-aqueous solvents, with or without surfactants, under hydrothermal conditions. The experimental temperature ranges from 180 to 250 ◦ C in most of the cases, and the typical size of the products varies from 20 to 200 nm [28–30]. It has been shown that the preparation conditions, such as concentration, reaction temperature, and time, are the main factors in determining the morphologies and structures of the ␣-Fe 2 O 3 nanocrystals pre- pared under hydrothermal conditions. Nevertheless, developing the hydrothermal method for the preparation of ␣-Fe 2 O 3 nano- materials, as well as the modification of their size, morphology, and porosity, has been intensively pursued, not only for their fundamental scientific interest, but also for many technological applications. Herein, we describe an easy route to synthesize ␣-Fe 2 O 3 submicro-flowers via a low-temperature hydrothermal method and their study as attractive material for lithium-ion batteries. Poly(ethylene glycol) with an average molecular weight of 600 was employed as a soft template. PEG is a typical non-toxic, non-immunogenic, non-antigenic, and protein-resistant poly- mer reagent with long polymer chains [31], which was used here as a coordination and linking reactant, a stabilizer, and a structure-directing agent. The electrochemical properties of the ␣-Fe 2 O 3 submicro-flowers as anode material for lithium-ion batteries were investigated using cyclic voltammetry, electro- chemical impedance, and galvanostatic methods. It was found that as-prepared submicro-flowers, characterized by uniform size and shape with a high specific surface area, exhibited supe- rior electrochemical activity, with an initial discharge capacity of 905.7 mAh/g and reversible capacity retention of 35.9% after 50 cycles at 100 mA/g at ambient temperature. 2. Experimental 2.1. Preparation of α-Fe 2 O 3 submicro-flowers All the chemical reagents were analytically pure and used without further purification. The ␣-Fe 2 O 3 submicro-flowers were prepared by a PEG-precursor route. In a typical experimen- tal procedure, PEG-600 (Aldrich) was dissolved in methanol to form a 1 M solution. A 1 M aqueous solution of FeCl 3 (BDH Laboratory Supplies, England) with equivalent molar number to the PEG-600 was added dropwise under continuous stirring at room temperature to obtain a homogeneous solution. The solu- tion was kept at 50 ◦ C for 12 h to form crystals, which were collected as the precursor. A stoichiometric proportion of the precursor and NaOH (4 M) were added under stirring to a 15 mL Teflon-lined autoclave, which was filled to one-third by volume. The autoclave was sealed and heated to 160 ◦ C, then held at that temperature for 24 h. After the reaction, the autoclave was cooled down naturally. The resulting product was separated and collected by centrifugation, washed with ethanol and distilled water to ensure total removal of the inorganic ions, and then dried under vacuum at 80 ◦ C for 4 h. 2.2. Sample characterizations X-ray powder diffraction analysis was conducted on a Philips 1730 X-ray diffractometer (XRD) using Cu K␣ radi- ation (λ = 1.54056 ˚ A), with 2θ ranging from 20 ◦ to 80 ◦ ,to analyse the structure of the expected product. Both a JEOL JSM 6460A scanning electron microscope (SEM) and a Hitachi S4500 field emission scanning electron microscope (FE-SEM) were employed to examine the morphology of the sample. Electrodes were prepared by drying a slurry (composed of 70 wt.% active material, 15 wt.% carbon black, and 15 wt.% polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidinone) on a copper foil (1 cm 2 )at100 ◦ C for 4 h under vacuum. It was compressed at a rate of about 150 kg/cm 2 , and then weighed. The electrochemical behaviour of the test material was examined via CR2025 coin cells with a lithium metal counter electrode, Cel- gard 2700 membrane separator, and an electrolyte consisting of 1 M LiPF 6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight ratio). The cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany). The charge (delithiation)–discharge (delithiation) mea- surements were carried out at ambient temperature on a multi-channel battery cycler at different current densities in the range of 20–100 mA/g, with voltage cut-offs of 0.01 and 3.0 V. Cyclic voltammetry (CV) and electrochemical impedance mea- surements were obtained using a CHI instrument. The scanning rate for CV was 0.1 mV/s, and the amplitude of the alternating voltage signal in impedance measurements was 5 mV over the frequency range between 10 5 and 0.1 Hz. The electrochemical impedance measurements were performed at the open-circuit voltage (OCV) before and after the CV experiments. 3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of as-synthesized powder prepared using the hydrothermal method. As observed, the positions of the characteristic peaks of the product are consis- tent with the standard values for the hexagonal ␣-Fe 2 O 3 phase (JCPDS number 33-0664), suggesting that there are no other compounds present in the final product. The sharp peaks confirm the high crystallinity of the product. Y. NuLi et al. / Electrochimica Acta 53 (2008) 4213–4218 4215 Fig. 1. XRD pattern of the as-prepared ␣-Fe 2 O 3 sample. By using Scherrer’s equation [32], D = kλ/β cos θ, where λ is the X-ray wavelength, β the half-peak width, θ the Bragg angle in degrees, and k is the shape factor (often assigned a value of 0.89), the primary crystal shape of the hematite can be estimated from different crystallographic directions [33].It has been reported that a large intensity difference for diffraction peaks (1 0 4) and (1 1 0) could indicate that the particles have an ellipsoidal shape, while comparable intensities for both peaks are related to a spherical shape. No large intensity differences were observed for these peaks in this work. The crystal sizes of the sample as evaluated by the Scherrer’s equation from several typical peaks, (1 0 4), (1 1 0), (0 2 4), and (1 1 6), are shown in Table 1. It can be seen that the crystal sizes along these four crys- tallographic directions were similar, indicating that the crystals in the sample are close to spherical in shape. Fig. 2(a) and (b) shows SEM images of the as-prepared sam- ple at different magnifications. Fig. 2(a) is a low-magnification view of the product, showing that the sample is composed of uni- form spheres with an average diameter of about 200 nm.Fig. 2(b) contains a SEM image at a higher magnification, from which it can be seen that there are some features on the surface of every sphere. The micro-structure of the sample was further investi- gated by FE-SEM, and typical images are presented in Fig. 2(c) and (d). It can be seen that the particles are actually flower-like structures composed of nanospheres 20–30 nm in size, which is consistent with the results from XRD, indicating that the product could possess a high surface area. Fig. 3 shows cyclic voltammograms of electrode made from the as-prepared sample. It can be seen that there is a substantial difference between the first and the subsequent cycles. In the first cycle, a spiky peak appears at about 0.4 V in the cathodic process, which can be associated with the irreversible reduction reaction of electrolyte and the reversible conversion reaction of Table 1 The crystal sizes along four crystallographic directions for the ␣-Fe 2 O 3 sample D (104) (nm) 24.25 D (110) (nm) 29.90 D (024) (nm) 19.83 D (116) (nm) 22.50 Fig. 2. SEM images (a) and (b) and FE-SEM images (c) and (d) of the as-prepared ␣-Fe 2 O 3 sample. 4216 Y. NuLi et al. / Electrochimica Acta 53 (2008) 4213–4218 Fig. 3. Cyclic voltammograms of the ␣-Fe 2 O 3 electrode at a sweep rate of 0.1 mV/s. Fe 2 O 3 with metallic lithium into amorphous Li 2 O and metal- lic iron [12]. Meanwhile, an anodic peak is recorded at about 1.8 V, corresponding to the reversible oxidation of Fe 0 to Fe 3+ . In the subsequent cycles, the peak potentials shift to 0.7 and 1.85 V, respectively. The peak intensity gradually decreases dur- ing the first four cycles, and then remains almost steady, showing the reversible reduction and oxidation of the material, which suffered some irreversible capacity loss in the first few cycles. Fig. 4 presents typical Nyquist plots obtained before and after the CV experiments on the same electrode. The plots are similar to each other in shape, with a semicircle appearing in the high frequency domain and a straight line in the low fre- quency region. Before the CV experiments, the semicircle is about 220  cm 2 in terms of resistance. The semicircle become smaller, and the resistance value decreases to about 180  cm 2 after the CV experiments, suggesting an easier reaction pro- cess after several CV cycles. The electrode could then possess higher reactivity and lower polarization. On the other hand, the changes in impedance are also related to the component modi- fications of the electrode. After CV cycling, the electrolyte can soak into the particles of the electrode, and the pristine Fe 2 O 3 changes into lower oxidation state iron oxides and Li 2 O, so some irreversibility should also be taken into account. Fig. 4. Electrochemical impedance results on the ␣-Fe 2 O 3 electrode obtained before and after CV measurements. Fig. 5. The first and secondcharge–discharge curves of the ␣-Fe 2 O 3 electrode at different current densities: (a) 20 mA/g, (b) 40 mA/g, (c) 60 mA/g, (d) 80 mA/g, and (e) 100 mA/g. Fig. 5 shows the charge–discharge curves of the electrode in the first and second cycles at different current densities. Com- pared with the charge process, the differences in the voltage trends for the discharge are more obvious at every current den- sity. In the discharge curves of the first cycle, there is an obvious potential plateau at 0.9–0.8 V, which decreases to 0.4–0.3 V with increasing current density, followed by a gradual decrease in the potential down to 0.01 V. For the second cycle, the voltage increases, and the amplitude of the plateau is markedly reduced, so that only a discharge slope is observed, with a decrease in the discharge capacity. The initial discharge capacities of the material are 1248.1, 1225.7, 1171.1, 1020.7 and 905.8 mAh/g, and the initial coulombic efficiencies are 75.9%, 75.3%, 74.7%, 73.8% and 72.7% at current densities of 20, 40, 60, 80 and 100 mA/g, respectively. The corresponding reversible discharge capacities are 959.6, 939.7, 883.2, 751.3 and 669.8 mAh/g. It can be seen that not only the discharge capacity, but also the coulombic efficiency, decreases with increasing current density, which apparently results from the higher polarization at a larger current density. Fig. 6 presents the cycling behaviour of the ␣-Fe 2 O 3 electrode measured at different current densities. It can be seen that 46.9% of the reversible capacity can be maintained over 50 cycles at 40 mA/g, and the electrode exhibits 35.9% capacity retention Y. NuLi et al. / Electrochimica Acta 53 (2008) 4213–4218 4217 Table 2 Comparison of the electrochemical properties of ␣-Fe 2 O 3 submicro-flower electrodes fabricated in this work with those of ␣-Fe 2 O 3 with different particle sizes reported in literatures Samples Current density Potential range (V vs. Li/Li + ) Initial capacity (mAh/g) Initial coulombic efficiency (%) Capacity retention after 50 cycles References ␣-Fe 2 O 3 submicro-flowers 20 mA/g 3.0–0.01 1248.1 75.9 52.1% This work 40 mA/g 1225.7 75.3 46.9% 60 mA/g 1171.1 74.7 44.4% 80 mA/g 1020.7 73.8 41.2% 100 mA/g 905.8 72.7 35.9% Nanometric (20 nm) ␣-Fe 2 O 3 C/5 Discharge to 0 V 1400 – – [12,13] Micrometric (0.5 ␮m) ␣-Fe 2 O 3 C/5 Discharge to 0 V 1200 – – [12,13] Micrometric ␣-Fe 2 O 3 0.5 mA/cm 2 3.0–0.5 1000 70 Very poor [34] Fig. 6. The discharge capacity vs. cycle number curves of the ␣-Fe 2 O 3 electrode at different current densities. up to 50 cycles at a current density as high as 100 mA/g. For comparison purpose, the electrochemical properties of the ␣- Fe 2 O 3 submicro-flowers fabricated in this work and those of ␣-Fe 2 O 3 with different particle sizes reported in literatures are summarized in Table 2. It can be found that the effect of particle size on the electrochemical performance of ␣-Fe 2 O 3 . Although the ␣-Fe 2 O 3 submicro-flower electrode has slightly lower ini- tial capacity compared with the nano-sized sample, the capacity retention of the ␣-Fe 2 O 3 submicro-flowers prepared in this work are better than that of micrometric sample. It is well known that a large surface area is important for the improvement of reac- tion performance, considering the introduction of lithium ions into the holes of the hematite surface. The capacity and affinity will be greatly enhanced when the surface area is high, since the diffusion lengths of the lithium ions are greatly shortened [35]. Moreover, the volume variation of nanometer particles is much smaller than large size particles during charge–discharge cycles, and nanometer materials often have higher plasticity and deformability. Those factors may take effect to avoid the crack and pulverization of the electrode [36]. It is reasonable to believe that the excellent performance of the ␣-Fe 2 O 3 submicro-flowers, which are apparently composed of primary nanoparticles, are resulted from the small particle size and high surface area, and the high uniformity and good dispersion also share some credit. 4. Conclusions Herein, we reported the feasible synthesis of hematite submicro-flowers by a convenient hydrothermal method. SEM and FE-SEM measurements showed that the flower-like parti- cles were composed of nanospheres 20–30 nm in size. Probably due to the fine particle size and large surface area, the material shows superior electrochemical performance, with a high initial capacity of 905.8 mAh/g and good capacity retention of 35.9% up to the 50th cycle at 100 mA/g. Acknowledgement This work is financially supported by the Australian Research Council through a Linkage project (LP0775456). References [1] X.W. Wang, Zh.Y. Zhang, Sh.X. Zhou, Mater. Sci. Eng. B 86 (2001) 29. [2] S. Dash, M. Kamruddin, P.K. Ajikumar, A.K. Tyagi, B. Raj, Thermochim. Acta 363 (2000) 129. [3] D.A. Everest, I.G. Sayce, B. Selton, J. Mater. Sci. 6 (1971) 218. [4] R.W. Siegel, S. Ramasamy, H. Hahn, L. Zongquan, L. Ting, R. Gronsky, J. Mater. Res. 3 (1988) 1367. [5] C.R. Bickmore, K.F. Waldner, R. Baranwal, T. Hinklin, D.R. Treadwell, R.M. Laine, J. Eur. Ceram. Soc. 18 (1998) 287. [6] D. Dong, P. Hong, S. Dai, Mater. Res. Bull. 30 (1995) 531. [7] R.J. Davey, T. Hirai, J. Cryst. Growth 17 (1997) 1318. [8] Ch.H. Lu, W.Ch. Lee, Sh.J. Liou, G.T.K. Fey, J. Power Sources 81–82 (1999) 696. [9] K. Byrappa, T. Adschiri, Prog. Cryst. Growth Charact. Mater. 53 (2007) 117. [10] W.J. Li, E.W. Shi, T. Fukuda, Cryst. Res. Technol. 38 (2003) 847. [11] C. Gong, D. Chen, X. Jiao, Q. Wang, J. Mater. Chem. 12 (2002) 1844. [12] D. Larcher, D. Bonnin, R. Cortes, I. Rivals, L. Personnaz, J M. Tarascon, J. Electrochem. Soc. 150 (2003) A1643. [13] D. Larcher, C. Masquelier, D. Bonnin, Y. Chabre, V. Masson, J.B. Leriche, J M. Tarascon, J. Electrochem. Soc. 150 (2003) A133. [14] T. Sugimoto, K. Sakata, J. Colloid Interf. Sci. 152 (1992) 587. [15] K. Woo, H.J. Lee, J.P. Ahn, Y.S. Park, Adv. Mater. 15 (2003) 1761. [16] J. Chen, L.N. Xu, W.Y. Li, X.L. Gou, Adv. Mater. 17 (2005) 582. [17] L. Liu, H.Z. Kou, W.L. Mo, H.J. Liu, Y.Q. Wang, J. Phys. Chem. B 110 (2006) 15218. [18] L. Suber, D. Fiorani, P. Imperatory, S. Foglia, A. Montone, R. Zysler, Nanostruct. Mater. 11 (1999) 797. [19] X.Q. Liu, S.W. Tao, Y.S. Shen, Sens. Actuators B: Chem. 40 (1997) 161. 4218 Y. NuLi et al. / Electrochimica Acta 53 (2008) 4213–4218 [20] W.T. Dong, C.Sh. Zhu, J. Mater. Chem. 12 (2002) 1676. [21] W.T. Dong, S.X. Wu, D.P. Chen, Chem. Lett. 5 (2000) 496. [22] M. Vasquez-Mansilla, R.D. Zysler, C. Arciprete, J. Magn. Magn. Mater. 226–230 (2001) 1907. [23] C.J. Jia, L.D. Sun, Z.G. Yan, L.P. You, F. Luo, X.D. Han, Y.C. Pang, Z. Zhang, C.H. Yan, Angew. Chem. Int. Ed. 44 (2005) 4328. [24] J. Wan, X. Chen, Z. Wang, X. Yang, Y.T. Qian, J. Cryst. Growth 276 (2005) 571. [25] A.A. Burukhin, B.R. Churagulov, N.N. Oleynikov, A.V. Knotko, in: Y. Kazumichi, F. Qi (Eds.), Proceedings of the Joint 6th International Sym- posium on Hydrothermal Reactions and 4th International Conference on Solvo-Thermal Reaction, Nishimura Tosha-Do Ltd., Kochi, Japan, 2P-54, 2001. [26] M.P. Morales, T.G. Carreno, C.J. Serna, J. Mater. Res. 7 (1992) 2538. [27] T. Zhao, D.H. Chen, D.R. Chen, Chin. Chem. Lett. 14 (2003) 323. [28] Zh.H. Jing, Sh.H. Wu, Mater. Lett. 58 (2004) 3637. [29] Y. Zheng, Y. Cheng, Y. Wang, F. Bao, J. Cryst. Growth 284 (2005) 221. [30] G.S. Li, R.L. Smith Jr., H. Inomata, K. Arai, Mater. Res. Bull. 37 (2002) 949. [31] J.M. Harris, in: J.M. Harris (Ed.), Poly(ethylene glycol) Chemistry, Biotechnical and Biomedical Application, Plenum Press, New York, 1992, Ch. 1. [32] H.P. Klug,L.E. Alexander, X-rayDiffraction Proceduresfor Polycrystalline and Amorphous Materials, Wiley, New York, 1974, p. 618. [33] M. Ocana, M.P. Morales, C.J. Serna, J. Colloid Interf. Sci. 212 (1999) 317. [34] H. Morimoto, S I. Tobishima, Y. Iizuka, J. Power Sources 146 (2005) 315. [35] S.Y. Zeng, K.B. Tang, T.W. Li, J. Colloid Interf. Sci. 312 (2007) 513. [36] H. Li, X.J. Huang, L.Q. Chen, Zh.G. Wu, Y. Liang, Electrochem. Solid State Lett. 2 (1999) 547. . A vailable online at www.sciencedirect.com Electrochimica Acta 53 (2008) 4213–4218 Preparation of ␣-Fe 2 O 3 submicro-flowers by a hydrothermal approach and. through a hydrothermal method using Span80 or L113B as a soft template [17]. Wan et al. have proposed a soft-template-assisted hydrothermal route to prepare