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Journal of Crystal Growth 274 (2005) 214–217

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Synthesis and electrochemical performance of amorphoushydrated iron phosphate nanoparticles

Xiong Wanga, Xiaohong Yangb, Huagui Zhenga, Jiayi Jina, Zude Zhanga,�

aDepartment of Chemistry, University of Science and Technology of China, Hefei 230026, ChinabDepartment of Chemistry, Chizhou Normal School, Chizhou 246700, China

Received 29 July 2004; accepted 21 September 2004

Communicated by M. Schieber

Available online 28 October 2004

Abstract

Amorphous hydrated iron (III) phosphate nanoparticles with average particle size of 80 nm were synthesized by a

facile coprecipitation method at ambient temperature. The obtained sample was characterized by X-ray powder

diffraction (XRD), thermogravimetrical analysis (TGA), and scanning electron microscopy (SEM). The electro-

chemical performances of the samples as cathode active materials were investigated in the 3.8–2.0V range at a current

density of 0.4mA cm�2. The results show that the obtained discharge capacity was improved for amorphous

FePO4 � 2H2O.

r 2004 Elsevier B.V. All rights reserved.

PACS: 61.66.Fn; 81.07.Bc; 82.45.Yz

Keywords: A1. Nanostructures; B1. Inorganic compounds; B1. Phosphates

1. Introduction

With the advancement of nanotechnology, thereis an interest in the replacement of conventionalelectrode materials by nanomaterials. Reducingthe particle size of electrode materials can enhancethe specific capacity of a lithium ion battery [1–3].

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.09.076

ng author. Tel: +86 551 3607752; fax:

2.

ss: libchem@mail.ustc.edu.cn (Z. Zhang).

Since the commercialization of lithium cobaltoxide by SONY in the early nineties, alternativecathode materials have been pursued to improvethe electrochemical performances [4–7]. Amongthe several cathode materials, Fe-based materialshave attracted extensive interest due to the lowcost and environmental benignity [8–15].Iron phosphate, FePO4 has been used in the

steel and glass industries [16,17]. At normalpressure, FePO4 adopts a berlinite (AlPO4) struc-ture related to a-quartz with each iron and

d.

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phosphorus atom tetrahedrally bonded to fouroxygen atoms [18].In this paper, we reported the synthesis of

amorphous iron phosphate (FePO4 � 2H2O) nano-particles through a facile coprecipitation route.And the electrochemical performances of FePO4nanoparticles as the cathode material for lithiumion battery were investigated in the 3.8–2.0 V rangeat a current density of 0.4mA cm�2.

Fig. 1. TGA curve of the obtained amorphous hydrated iron

phosphate.

2. Experimental procedure

All the reagents are of analytical purity grade,and have been received from commercial sources.Appropriate amount of Fe(NH4)2(SO4)2 � 6H2Osolution was added into a stoichiometric solutionof NH4H2PO4 in a 1:1 volume ratio. After atransparent solution was obtained, a concentratedhydrogen peroxide solution (30%) was addeddropwise under vigorous stirring at room tem-perature. Immediately, a white precipitate wasformed and aged for several hours. Then thesample was collected by filtration, washed severaltimes with distilled water, and dried undervacuum. For comparison, part of the productwas heated to 600 1C with a ramping rate of 5 1Cmin�1 and then was calcined at 600 1C in air for2 h.The crystalline phase was identified by powder

X-ray diffraction (XRD) using a Philips X’PertPro-Super diffractometer with graphite monochro-matized Cu Ka radiation (l=1.54178 A). SEMimages were taken on a Hitach (X-650) scanningelectron microscope. Thermal gravimetric analysis(TGA) of the as-synthesized sample was carriedout on a Shimadzu TA-50 thermal analyzer at aheating rate of 10Kmin�1 from room temperatureto 700 1C in air.Electrochemical tests were conducted with

metallic lithium as anode. The positive electrodeswere fabricated by pasting slurries of iron phos-phate (75wt%), carbon black (Super P, 20wt%)and poly(vinylidenefluoride) (PVDF, 5wt%) dis-solved in N-methyl-pyrrolidinone (NMP) on Alfoil strips by doctor blade technique. The stripswere then dried at 140 1C for 24 h in an air oven,pressed under 20MPa pressure and kept at 100 1C

for 12 h in a vacuum. The electrolyte was 1MLiPF6 in a 1:1 mixture of ethylene carbonate (EC)/diethyl carbonate (DEC); the separator wasCelgard 2500. The cells were assembled in theglove box filled with highly pure argon gas. Thecells were galvanostatically cycled in the 3.8–2.0 Vrange at a current density of 0.4mA cm�2.

3. Results and discussion

The TGA curve of the obtained sample is shownin Fig. 1. There is a main weight loss from roomtemperature to 700 1C, which corresponds to theemission of crystalline water. The weight loss of19.34% corresponds to 2 molecules water performula unit, suggesting the general formulaFePO4 � 2H2O.Fig. 2 displays the XRD patterns of the as-

prepared FePO4 � 2H2O and the sample afterheating at 600 1C. With increasing the reactiontemperature, amorphous FePO4 � 2H2O was trans-formed into a crystalline phase. All the reflectionpeaks (Fig. 2b) can be indexed as the hexagonalFePO4 with lattice parameters a=5.035,c=11.32 A, which are in agreement with thereported values (JCPDS 29-0715). No otherimpurities were detected.The morphologies of the samples were examined

with scanning electron microscopy (SEM). Fig. 3a

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shows the SEM image of amorphous hydratediron phosphate, indicating a sponge-like structurewith average particle size of 80 nm, while anaverage diameter of 150 nm for the crystalline ironphosphate nanoparticles (Fig. 3b).Electrochemical tests were conducted to inves-

tigate the electrochemical performance of the as-prepared amorphous FePO4 � 2H2O, as well as forthe crystalline FePO4 for comparison. Fig. 4exhibits the discharge curves for the two samplesat a voltage window of 3.8–2.0 V. At the firstdischarge, about 0.87 Li+/Fe was intercalated intothe active material at the current density. During

Fig. 2. XRD patterns of the as-prepared (a) amorphous

FePO4 � 2H2O and (b) crystalline FePO4.

Fig. 3. SEM images of the resulting samples. (a) Am

the subsequent cycles, ca. 0.67 Li+/Fe was able tointercalate and extract reversibly. It can beattributed that crystalline water remaining in theactive material host reacts with the lithiuminserted, which may cause a higher first dischargecapacity and a large degradation in the specificcapacity in the subsequent cycles. The wholeprocesses can be described as follows:

Liþ þ FePO4 þ e� Ð LiFePO4; (1)

2Liþ þ 2H2Oþ 2e� ! 2LiOHþH2: (2)

orphous FePO4 � 2H2O; (b) crystalline FePO4.

Fig. 4. Discharge curves for amorphous FePO4 � 2H2O (J) and

crystalline FePO4 (n) nanoparticles at a current density of

0.2mAcm�2 at a voltage window of 3.8–2.0V.

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The irreversible capacity of about 0.2 Li+/Fecan be associated with the reaction between theremaining water with lithium. From the TGanalysis (Fig. 1), about 0.2mol crystalline waterper FePO4 � 2H2O remained in the host after heat-treating at 140 1C, which was consistent with theresults of the electrochemical test. The reversibledischarge capacity of amorphous FePO4 � 2H2Onanoparticles is about twice as high as that ofcrystalline FePO4 (0.31 Li+/Fe) due to theamorphous nature and the reduced particle sizeof the as-prepared FePO4 � 2H2O nanoparticles.Such a situation is in agreement with otheramorphous materials with better electrochemicalperformance than their crystalline homologs[19,20].

4. Conclusions

Amorphous hydrated iron phosphate nanopar-ticles were fabricated through a facile coprecipita-tion approach at ambient temperature. Thesamples were characterized by a variety oftechniques. The electrochemical performances ofiron phosphate nanoparticles were investigated inthe voltage range of 3.8–2.0V at a current densityof 0.4mA/cm�2.

Acknowledgements

Financial support from the Ministry of Scienceand Technology of China is greatly appreciated.

References

[1] S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P.

Poizot, J.-M. Tarascon, J. Electrochem. Soc. 148 (2001)

A285.

[2] A. Singhal, G. Skandan, G. Amatucci, F. Badway, N. Ye,

A. Manthiram, H. Ye, J.J. Xu, J. Power Sources 129 (2004)

38.

[3] S. Panero, B. Scrosati, M. Wachtler, F. Croce, J. Power

Sources 129 (2004) 90.

[4] R. Koksbang, J. Barker, H. Shi, M.Y. Saidi, Solid State

Ionics 84 (1996) 1.

[5] X. Wang, X. Chen, L. Gao, H. Zheng, M. Ji, T. Shen, Z.

Zhang, J. Crystal Growth 256 (2003) 123.

[6] P. Arora, B.N. Popov, R.E. White, J. Electrochem. Soc.

145 (1998) 807.

[7] Y. Gao, J.R. Dahn, J. Electrochem. Soc. 143 (1996) 1783.

[8] X. Wang, X. Chen, L. Gao, H. Zheng, M. Ji, C. Tang, T.

Shen, Z. Zhang, J. Mater. Chem. 14 (2004) 905.

[9] K.S. Park, J.T. Son, H.T. Chung, S.J. Kim, C.H. Lee,

H.G. Kim, Electrochem. Commun. 5 (2003) 839.

[10] Y. Song, S. Yang, P.Y. Zavalij, M.S. Whittingham, Mater.

Res. Bull. 37 (2002) 1249.

[11] X. Wang, L. Gao, F. Zhou, Z. Zhang, M. Ji, C. Tang, T.

Shen, H. Zheng, J. Crystal Growth 265 (2004) 220.

[12] M. Takahashii, S. Tobishima, K. Takei, Y. Sakurai, J.

Power Sources 97–98 (2001) 508.

[13] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J.

Electrochem. Soc. 114 (1997) 1188.

[14] P. Prosini, M. Carewska, S. Scaccia, P. Wisniewski, M.

Pasquali, Electrochim. Acta 48 (2003) 4205.

[15] A.S. Andersson, B. Kalska, P. Eyob, D. Aernout, L.

Haggstrom, J.O. Thomas, J. Power Sources 140 (2001) 63.

[16] C.A. Boras, R. Romagnoli, R.O. Lezna, Electrochim. Acta

45 (2000) 1717.

[17] A. MogusMilankovic, D.E. Day, G.J. Long, G.K.

Marasinghe, Phys. Chem. Glasses 37 (1996) 57.

[18] N. Rajic, R. Gabrovsek, V. Kaucic, Thermochim. Acta

359 (2000) 119.

[19] J.J. Xu, A.J. Kinser, B.B. Owens, W.H. Smyrl, Electro-

chem. Solid State Lett. 1 (1998) 1.

[20] B. Yebka, C.G.A. Nazri, Mater. Res. Soc. Symp. Proc. 548

(1999) 229.