preparation of fe2o3 submicro - flowers by a hydrothermal approach

The electrochemical performance as anode material for lithium-ion batteries was further evaluated by cyclic voltammetry CV and by electrochemical impedance and charge–discharge measureme

Electrochimica Acta 53 (2008) 4213–4218

and their electrochemical performance in lithium-ion batteries

Yanna NuLia,b, Peng Zhanga, Zaiping Guoa,∗, P Munroec, Huakun Liua

aInstitute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia

bDepartment of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

cElectron 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␣-Fe2O3submicron-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␣-Fe2O3 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 (␣-Fe2O3) 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] It has been considered that most of these functions depend strongly on the structure and particle size of the materials, and nano-␣-Fe2O3 exhibits better electrochemical performance than micro-sized samples[13]

It is evident that the purpose of preparing ␣-Fe2O3 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] In view of 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

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␣-Fe2O3nanoparticles with different particle sizes by a

hydrothermal method [27] ␣-Fe2O3 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◦C in 20 h [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␣-Fe2O3nanocrystals

pre-pared under hydrothermal conditions Nevertheless, developing

the hydrothermal method for the preparation of␣-Fe2O3

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 ␣-Fe2O3

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 polypoly-mer chains [31], which was used

here as a coordination and linking reactant, a stabilizer, and a

structure-directing agent The electrochemical properties of the

␣-Fe2O3 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 ␣-Fe2O3 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 FeCl3 (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 cm2) at 100◦C for 4 h under vacuum It was compressed at a rate of about 150 kg/cm2, 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 LiPF6dissolved 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 105and 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 1shows 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␣-Fe2O3phase (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

Fig 1 XRD pattern of the as-prepared ␣-Fe 2 O 3 sample.

By using Scherrer’s equation[32], D = k λ/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 inFig 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 3shows 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

Fig 2 SEM images (a) and (b) and FE-SEM images (c) and (d) of the as-prepared ␣-Fe O sample.

Fig 3 Cyclic voltammograms of the ␣-Fe 2 O 3 electrode at a sweep rate of

0.1 mV/s.

Fe2O3 with metallic lithium into amorphous Li2O and

metal-lic iron[12] Meanwhile, an anodic peak is recorded at about

1.8 V, corresponding to the reversible oxidation of Fe0to Fe3+

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, showdur-ing

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 cm2in terms of resistance The semicircle become

smaller, and the resistance value decreases to about 180 cm2

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 Fe2O3

changes into lower oxidation state iron oxides and Li2O, 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 second charge–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 5shows 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 6presents the cycling behaviour of the␣-Fe2O3electrode 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

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

(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

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

␣-Fe2O3 submicro-flowers fabricated in this work and those of

␣-Fe2O3with different particle sizes reported in literatures are

summarized inTable 2 It can be found that the effect of particle

size on the electrochemical performance of␣-Fe2O3 Although

the␣-Fe2O3submicro-flower electrode has slightly lower

ini-tial capacity compared with the nano-sized sample, the capacity

retention of the␣-Fe2O3submicro-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 introducreac-tion 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␣-Fe2O3submicro-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.

[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-ray Diffraction Procedures for 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.