J. Cent. South Univ. Technol. (2008) 15: 531-534

DOI: 10.1007/s11771-008-0100-1

Preparation of LiFePO4 for lithium ion battery using Fe2P2O7 as precursor

HU Guo-rong(胡国荣), XIAO Zheng-wei(肖政伟), PENG Zhong-dong(彭忠东),

DU Ke(杜 柯), DENG Xin-rong(邓新荣)

(School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China)

Abstract：In order to obtain a new precursor for LiFePO₄, Fe₂P₂O₇ with high purity was prepared through solid phase reaction at 650 ℃ using starting materials of FeC₂O₄ and NH₄H₂PO₄ in an argon atmosphere. Using the as-prepared Fe₂P₂O₇, Li₂CO₃ and glucose as raw materials, pure LiFePO₄ and LiFePO₄/C composite materials were respectively synthesized by solid state reaction at 700 ℃ in an argon atmosphere. X-ray diffractometry and scanning electron microscopy(SEM) were employed to characterize the as-prepared Fe₂P₂O₇, LiFePO₄ and LiFePO₄/C. The as-prepared Fe₂P₂O₇ crystallizes in the  space group and belongs to β-Fe₂P₂O₇ for crystal phase. The particle size distribution of Fe₂P₂O₇ observed by SEM is 0.4-3.0 μm. During the Li⁺ ion chemical intercalation, radical  is disrupted into two  ions in the presence of O^2-, thus providing a feasible technique to dispose this poor dissolvable pyrophosphate. LiFePO₄/C composite exhibits initial charge and discharge capacities of 154 and 132 mA·h/g, respectively.

Key words: lithium ion battery; cathode material; preparation; precursor; LiFePO₄; Fe₂P₂O₇

1 Introduction

The original work of PADHI et al^[1] for LiFePO₄ has aroused researchers’ interest in this new cathode material for lithium ion batteries. With the advantages of cheap raw materials, environmental friendliness, high safety, good cycle stability and appreciable specific capacity of 170 mA·h/g, LiFePO₄ has established itself as a potent competitor in cathode material field and challenged the widely used LiCoO₂ and other cathode material candidates such as LiMn₂O₄, LiMnO₂ and LiNiO₂^[2-4]. In particular based on the listed characteristics, LiFePO₄ has highly potential application in hybrid electric vehicles(HEVs)^[5].

Presently, the work of the preparation of LiFePO₄ has centered on different synthetic methods: solid state reactions at high temperature^[6], hydrothermal synthesis^[7] and sol-gel methods^[8] etc. In most cases, lithium salts or lithium hydroxide, ferrous or ferric compounds and phosphates are used as raw materials to obtain LiFePO₄ either in solid or liquid state^[9-11]. Here, enlightened by the concept of precursor Ni_1/3Co_1/3Mn_1/3CO₃ with molar ratio n(Ni)?n(Mn)?n(Co)= 1?1?1 for LiNi_1/3Co_1/3Mn_1/3O₂, a new precursor Fe₂P₂O₇ for LiFePO₄ was presented. The compound has the following characteristics.

1) Besides O^2-, the component includes only Fe and P with molar ratio of 1?1.

2) The valences of elements Fe and P in Fe₂P₂O₇ are the same as those in LiFePO₄, namely +2 and +5 respectively, and during the synthesis of LiFePO₄ they stay unchanged.

In this work, the precursor Fe₂P₂O₇ with high purity was prepared and then chemically intercalated with Li⁺ ion to successfully synthesize LiFePO₄ through solid state reaction.

2 Experimental

Fe₂P₂O₇ was prepared by a solid state reaction using FeC₂O₄ and NH₄H₂PO₄ as raw materials. The detailed procedure was as follows: the stoichiometric amount of raw materials FeC₂O₄ and NH₄H₂PO₄ were mixed and milled in a planetary mill at a speed of 300 r/min for 4 h in acetone to get them thoroughly mixed. The slurry was then dried at 80 ℃ to remove acetone. The dried mixture was heated to 650 ℃ in an argon atmosphere to start the reaction of FeC₂O₄ and NH₄H₂PO₄, and finally Fe₂P₂O₇ was prepared.

Fe₂P₂O₇, stoichiometric Li₂CO₃ and some glucose were ball-milled at a speed of 300 r/min for 4 h in acetone to ensure a homogenous mixing. After being dried at 80 ℃ overnight, the mixture was ground and then sintered at 700 ℃ under a flowing argon gas. Composite cathode material LiFePO₄/C(simply denoted as LiFePO₄/C) was synthesized in the sintering procedure. Carbon content of LiFePO₄/C was determined as follows: dissolve LiFePO₄/C in hydrochloric acid, filtrate the solution, wash the indissoluble carbon thoroughly, finally calculate mass fraction of the dried carbon.

X-ray powder diffraction of Fe₂P₂O₇ and LiFePO₄ was performed on Philips X’pert powder diffractometer with Cu K_α radiation. The morphology of Fe₂P₂O₇ was characterized using scanning electron microscope(SEM) (JSM-5600LV, JEOL).

The electrochemical performance test of LiFePO₄/C was carried out with coin cell 2025 which was assembled in an argon atmosphere glove-box. The electrolyte was  1 mol/L LiPF₆ in a 1?1 (volume ratio) mixture of ethylene carbonate(EC) and dimethyl carbonate(DEC). Membrane used is Celgard2300 and the metal lithium was used as the anode electrode. N-methyl pyrrolidinone was used as organic solvent, and the cathode electrode was made, which consisted of 80% LiFePO₄, 10% PVDF (polyvinylidence fluoride) binder and 10% carbon black (in mass fraction) coated onto an aluminum foil current collector. The cathode was dried under vacuum at 120 ℃ for 24 h before cell assembly procedure. The coin cells were galvanostatically charged and discharged at 0.1C rate and cut-off of 2.5-4.1 V.

3 Results and discussion

3.1 Preparation and characterization of Fe₂P₂O₇

The preparation of Fe₂P₂O₇ using FeC₂O₄ and NH₄H₂PO₄ as raw materials in an argon atmosphere can be described as follows:

                                                                                                        2NH₄H₂PO₄+2FeC₂O₄→

Fe₂P₂O₇+2NH₃↑+3H₂O↑+2CO₂↑+ 2CO↑  (1)

Actually, the divalent metal pyrophosphates M₂P₂O₇ (M=Fe, Mn, Co, Ni, Cu, Zn) are polymorphic, including α, β and γ phases. Radical  is the common ion in pyrophosphates, among which the P—O—P bond angles are different from each other in the range of 120?-180?, and the bond length of P—O in P—O—P is longer than that in tip P—O (Fig.1). HOGGINS^[12] reported that P—O—P bond angle of Fe₂P₂O₇ is linear as a result of extensive p bonding with the p orbital on the bridge oxygen atom.

Fig.1 Plane configuration of structure of radical

The X-ray diffraction pattern of Fe₂P₂O₇ prepared is shown in Fig.2. According to Ref.[13], the as-prepared Fe₂P₂O₇ crystallizes in the  space group and belongs to β-Fe₂P₂O₇ for crystal phase, which is isostructural with high temperature form of β-Mg₂P₂O₇.

Fig.2 X-ray diffraction pattern of Fe₂P₂O₇ prepared at 650 ℃

In this experiment, fully crystallized Fe₂P₂O₇ was made at 700-900 ℃ using FeC₂O₄ and NH₄H₂PO₄ as raw materials in an argon atmosphere. Here, it must be mentioned that according to the experiment results, there is, to a certain extent, no difference in electrochemical performance between LiFePO₄/C samples with same carbon content using either Fe₂P₂O₇ prepared at 650 ℃ as precursor or fully crystallized ones obtained at higher temperature.

The SEM image of the as-prepared Fe₂P₂O₇ is shown in Fig.3. The powders have a narrow particle size distribution ranging from 0.4 to 3.0 μm. An average primary crystal size can be roughly estimated using Schererr equation^[14]:

                                 (2)

where  d is the average crystal size, 0.89 is Schererr constant, λ=0.154 nm is the wavelength of X-ray, B is the  width (in radian) of the X-ray diffraction peak at half maximum intensity and θ is Bragg diffraction angle. With the reference of the X-ray diffraction data, the calculated crystal size is 91 nm. The small primary crystallinity and fine particles of the as-prepared Fe₂P₂O₇ allow its easily homogenous mixing and reaction with Li₂CO₃, ensuring a thorough Li⁺ chemical intercalation in the synthesis procedure of LiFePO₄.

Fig.3 SEM image of Fe₂P₂O₇ powder prepared at 650 ℃ using FeC₂O₄ and NH₄H₂PO₄ as raw materials

3.2 Synthesis of LiFePO₄ and mutual transformation between PO₄^3- and P₂O₇^4-

The synthesis of LiFePO₄ using Fe₂P₂O₇ and Li₂CO₃ as raw materials in an inert atmosphere can be expressed by the following reaction:

Fe₂P₂O₇+Li₂CO₃→2LiFePO₄+CO₂↑             (3)

It should be noticed that in the procedure of synthesis of Fe₂P₂O₇ and thereafter LiFePO₄ an exciting mutual conversion between  and  is realized: →→In this work,  is converted into  in the reaction of NH₄H₂PO₄ and FeC₂O₄. There are some other cases for this conversion. Generally, Fe₂P₂O₇ is prepared by the reduction of FePO₄ in a reductive atmosphere containing H₂ via a long and strict process^[15]. Some divalent metal pyrophosphates can be obtained by heating their corresponding ammoniated or acidic phosphates. For example:

2NH₄NiPO₄?6H₂O→Ni₂P₂O₇+2NH₃↑+13H₂O     (4)

In aqueous solution, dilute phosphoric acid H₃PO₄ becomes viscid when heated to 422 K and then dehydrates into H₄P₂O₇ when heated to 485-486 K, and finally pure H₄P₂O₇ is obtained at 528-533 K.

For the transformation from  to  however, there are few cases reported ever. In this work, pure LiFePO₄ was prepared by disrupting the  radical into two  ions in the presence of O^2- at elevated temperature in an inert atmosphere. Though Fe₂P₂O₇ can be dissolved in acidic solution, it is sluggish in kinetics. Due to the high dissolubility of LiFePO₄ in acidic solution, the structural conversion from  to  provides a feasible technique to dispose those pyrophosphates of low dissolubility.

3.3 XRD characterization of pure LiFePO₄ and LiFePO₄/C

The X-ray powder diffraction patterns of pure LiFePO₄ prepared using the as-prepared Fe₂P₂O₇ and Li₂CO₃ and LiFePO₄/C obtained using Fe₂P₂O₇, Li₂CO₃ and glucose are shown in Fig.4. The two diffraction lines are indexed to an orthorhombic crystal structure (space group Pnma). The peaks in both curves are very sharp and perfect, suggesting a high degree of crystallinity. The patterns agree well with that of phospho-olivine LiFePO₄, and no impurity phase is detected. For the diffraction of LiFePO₄/C, no peaks of carbon are revealed, indicating the amorphous property of the electron conductor.

Fig.4 X-ray diffraction patterns of pure LiFePO₄ (a) and LiFePO₄/C (b)

3.4 Electrochemical test of LiFePO₄/C

The charge/discharge curves of LiFePO₄/C (Fig.5) reveal a flat potential plateau of 3.45 V (vs Li⁺/Li). At this voltage, the lithium extraction/insertion proceeds at the room-temperature in a two-phase reaction between LiFePO₄ and FePO₄ that belongs to the same space group. A 0.08 V voltage difference between charge and discharge plateaus is shown in Fig.5, which represents very small polarization in electrodes and electrolyte of coin cell during charge and discharge process.

The first charge and discharge capacities of LiFePO₄/C are 154 and 132 mA·h/g, accounting for 90.6% and 77.6% of the theoretical value (170 mA·h/g), respectively. Here, it can be concluded the ecumenic electrochemical performance and low efficiency of the initial cycle (85.7%) of the LiFePO₄/C are ascribed to the non-optimal coating of carbon and calcining temperature and time. Furthermore, the appearance of LiFePO₄/C is a medium to dark grey, and according to Ref.[16], the conductivity of the as-prepared LiFePO₄/C in this color is not high enough to ensure an appreciable electrochemical performance. More evidence about the poor conductivity of LiFePO₄/C is the low efficiency of the initial charge/discharge. Further work will be carried out to study charge/discharge capacity and improve initial charge/discharge efficiency of LiFePO₄/C.

Fig.5 Initial charge/discharge curves of LiFePO₄/C (mass fraction of carbon is 9%)

4 Conclusions

1) β-Fe₂P₂O₇ with high purity is prepared through solid phase reaction at 650 ℃ using starting materials of FeC₂O₄ and NH₄H₂PO₄ in an argon atmosphere. Pure LiFePO₄ and LiFePO₄/C can be synthesized by solid state reaction at 700 ℃ in an argon atmosphere using the as-prepared Fe₂P₂O₇ and Li₂CO₃.

2) Radical  is disrupted into two  ions in the presence of O^2- during the synthesis procedure of LiFePO₄, thus an important mutual transformation is realized: →→ which offers a feasible technique to deal with this hardly dissolvable pyrophosphate.

3) Composite material LiFePO₄/C shows appreci- able initial charge and discharge capacities of 154 and 132 mA·h/g, respectively.

References

[1] PADHI A K, NANJUNDASWAMY K S, MASQUELIER C, OKADA S, GOODENOUGH J B. Effect of structure on the Fe³⁺/Fe²⁺ redox couple in iron phosphates [J]. Journal of Electrochemical Society, 1997, 144(5): 1609-1613.

[2] ZHANG Bao, LI Xin-hai, ZHU Bing-quan. Low temperature synthesis and electrochemical properties of LiFePO₄/C cathode [J]. Journal of Central South University: Science and Technology, 2006, 37(3): 505-508. (in Chinese)

[3] JOONGPYO S, KATHYN A S. Cycling performance of low-cost lithium ion batteries with natural graphite and LiFePO₄ [J]. Journal of Power Source, 2003, 119/121: 955-958.

[4] BELHAROUAK I, JOHRSON C, AMINE K. Synthesis and electrochemical analysis of vapor-deposited carbon-coated LiFePO₄ [J]. Electrochemistry Communications, 2005, 7(10): 983-988.

[5] STRIEBEL K, SHIM J, SIERRA A. The development of low cost LiFePO₄-based high power lithium-ion batteries [J]. Journal of Power Source, 2005, 146(1/2): 33-38.

[6] SCACCIA S, CAREWSKA M, WISNIEWSKI P, PROSINI P P. Morphological investigation of sub-micron FePO₄ and LiFePO₄ particles for rechargeable lithium batteries [J]. Materials Research Bulletin, 2003, 38(7): 1155-1163.

[7] YANG S F, ZAVALIJ P Y, STANLEY W M. Hydrothermal synthesis of lithium iron phosphate cathodes [J]. Electrochemistry Communication, 2001, 3(9): 505-508.

[8] HU Y Q, DOEFF M M, KOSTECKI R, FINONES R. Electro- chemical performance of sol-gel synthesized LiFePO₄ in lithium batteries [J]. Journal of Electrochemical Society, 2004, 151(8): A1279-A1285.

[9] AMINE K, LIU J, BELHAROUAK I. High-temperature storage and cycling of C-LiFePO₄/graphite Li-ion cells [J]. Electrochemistry Communication, 2005, 7(7): 669-673.

[10] SINGHAL A, SKAMDAN G, AMATUCCI G, BADWAY F, YE N, MANTHIRAM A, YE H, XU J J. Nanostructured electrodes for next generation rechargeable electrochemical devices [J]. Journal of Power Source, 2004, 129(1): 38-44.

[11] ALAN D S, GIRTS V, JOHN R O. A solution-precursor synthesis of carbon-coated LiFePO₄ for Li-ion cells [J]. Journal of Electrochemical Society, 2005, 152(12): A2376-A2382.

[12] HOGGINS J T, SWINNEA J S, STEINFINK H. Crystal structure of Fe₂P₂O₇ [J]. Journal of Solid State Chemistry, 1983, 47(3): 278-283.

[13] PARADA C, PERLES J, SAEZ-PUCHE R, RUIZ-VALERO C, SNEJKO N. Crystal growth, structure, and magnetic properties of a new polymorph of Fe₂P₂O₇ [J]. Chemistry Material, 2003, 15(17): 3347-3351.

[14] WU Da-xiong, WU Xi-jun, LU Yan-fei, WANG Hui. Synthesis and room temperature conductivity of nano-LaF₃ bulk material [J]. Trans Nonferrous Met Soc China, 2006,16(4): 828-832.

[15] MAMORU A, KYOJI O. Effects of the method of preparing iron orthophosphate catalyst on the structure and the catalytic activity [J]. Applied Catalysis A: General, 1999, 180(1/2): 47-52.

[16] CHUNG S Y, BLOCKING J T, CHIANG Y M. Electronically conductive phospho-olivines as lithium storage electrodes [J]. Nature Material, 2002, 1(2): 123-128.

Foundation item: Project(50604018) supported by the National Natural Science Foundation of China

Received date: 2007-12-18; Accepted date: 2008-03-09

Corresponding author: HU Guo-rong, Professor, PhD; Tel: +86-731-8830474; E-mail: hgrhsj@263.net

(Edited by CHEN Wei-ping)