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 C 1 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 P2O74 − is disrupted into two PO34− ions in the presence of O2−, 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

the same as those in LiFePO4, namely +2 and +5

1 Introduction respectively, and during the synthesis of LiFePO4 they
stay unchanged.
The original work of PADHI et al[1] for LiFePO4 has In this work, the precursor Fe2P2O7 with high purity
aroused researchers’ interest in this new cathode material was prepared and then chemically intercalated with Li+
for lithium ion batteries. With the advantages of cheap ion to successfully synthesize LiFePO4 through solid
raw materials, environmental friendliness, high safety, state reaction.
good cycle stability and appreciable specific capacity of
170 mA·h/g, LiFePO4 has established itself as a potent 2 Experimental
competitor in cathode material field and challenged the
Fe2P2O7 was prepared by a solid state reaction using
widely used LiCoO2 and other cathode material
FeC2O4 and NH4H2PO4 as raw materials. The detailed
candidates such as LiMn2O4, LiMnO2 and LiNiO2[2−4]. In
procedure was as follows: the stoichiometric amount of
particular based on the listed characteristics, LiFePO4
raw materials FeC2O4 and NH4H2PO4 were mixed and
has highly potential application in hybrid electric
milled in a planetary mill at a speed of 300 r/min for 4 h
vehicles(HEVs)[5].
in acetone to get them thoroughly mixed. The slurry was
Presently, the work of the preparation of LiFePO4
then dried at 80 ℃ to remove acetone. The dried mixture
has centered on different synthetic methods: solid state was heated to 650 ℃ in an argon atmosphere to start the
reactions at high temperature[6], hydrothermal synthesis[7] reaction of FeC2O4 and NH4H2PO4, and finally Fe2P2O7
and sol-gel methods[8] etc. In most cases, lithium salts or was prepared.
lithium hydroxide, ferrous or ferric compounds and Fe2P2O7, stoichiometric Li2CO3 and some glucose
phosphates are used as raw materials to obtain LiFePO4 were ball-milled at a speed of 300 r/min for 4 h in
either in solid or liquid state[9−11]. Here, enlightened by acetone to ensure a homogenous mixing. After being
the concept of precursor Ni1/3Co1/3Mn1/3CO3 with molar dried at 80 ℃ overnight, the mixture was ground and
ratio n(Ni)‫׃‬n(Mn)‫׃‬n(Co)= 1‫׃‬1‫׃‬1 for LiNi1/3Co1/3Mn1/3O2, then sintered at 700 ℃ under a flowing argon gas.
a new precursor Fe2P2O7 for LiFePO4 was presented. The Composite cathode material LiFePO4/C(simply denoted
compound has the following characteristics. as LiFePO4/C) was synthesized in the sintering
1) Besides O2−, the component includes only Fe and procedure. Carbon content of LiFePO4/C was determined
P with molar ratio of 1‫׃‬1. as follows: dissolve LiFePO4/C in hydrochloric acid,
2) The valences of elements Fe and P in Fe2P2O7 are filtrate the solution, wash the indissoluble carbon

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
532 J. Cent. South Univ. Technol. (2008) 15: 531−534
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 Kα 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 Fig.2 X-ray diffraction pattern of Fe2P2O7 prepared at 650 ℃
was made, which consisted of 80% LiFePO4, 10% PVDF
(polyvinylidence fluoride) binder and 10% carbon black to β-Fe2P2O7 for crystal phase, which is isostructural
(in mass fraction) coated onto an aluminum foil current with high temperature form of β-Mg2P2O7.
collector. The cathode was dried under vacuum at 120 ℃
In this experiment, fully crystallized Fe2P2O7 was
for 24 h before cell assembly procedure. The coin cells
made at 700−900 ℃ using FeC2O4 and NH4H2PO4 as
were galvanostatically charged and discharged at 0.1C
raw materials in an argon atmosphere. Here, it must be
rate and cut-off of 2.5−4.1 V.
mentioned that according to the experiment results, there
is, to a certain extent, no difference in electrochemical
3 Results and discussion
performance between LiFePO4/C samples with same
3.1 Preparation and characterization of Fe2P2O7 carbon content using either Fe2P2O7 prepared at 650 ℃
The preparation of Fe2P2O7 using FeC2O4 and as precursor or fully crystallized ones obtained at higher
NH4H2PO4 as raw materials in an argon atmosphere can temperature.
be described as follows: The SEM image of the as-prepared Fe2P2O7 is
shown in Fig.3. The powders have a narrow particle size
2NH4H2PO4+2FeC2O4→ distribution ranging from 0.4 to 3.0 µm. An average
Fe2P2O7+2NH3↑+3H2O↑+2CO2↑+ 2CO↑ (1) primary crystal size can be roughly estimated using
Actually, the divalent metal pyrophosphates M2P2O7 Schererr equation[14]:
(M=Fe, Mn, Co, Ni, Cu, Zn) are polymorphic, including 0.89λ
α, β and γ phases. Radical P2 O 74− is the common ion in d= (2)
B cos θ
pyrophosphates, among which the P—O—P bond angles
are different from each other in the range of 120˚−180˚, where d is the average crystal size, 0.89 is Schererr
and the bond length of P—O in P—O—P is longer than constant, λ=0.154 nm is the wavelength of X-ray, B is the
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 π bonding with the p orbital on the bridge
oxygen atom.

Fig.1 Plane configuration of structure of radical P2 O 74−

The X-ray diffraction pattern of Fe2P2O7 prepared is

shown in Fig.2. According to Ref.[13], the as-prepared Fig.3 SEM image of Fe2P2O7 powder prepared at 650 ℃ using
Fe2P2O7 crystallizes in the C 1 space group and belongs FeC2O4 and NH4H2PO4 as raw materials
J. Cent. South Univ. Technol. (2008) 15: 531−534 533
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.

3.2 Synthesis of LiFePO4 and mutual transformation

between PO43− and P2O74−
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 PO 34− and P2 O 74− is
realized: PO 34− → P2 O 74− → PO 34− . In this work, PO 34−
is converted into P2 O 74− 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:
Fig.4 X-ray diffraction patterns of pure LiFePO4 (a) and
2NH4NiPO4·6H2O→Ni2P2O7+2NH3↑+13H2O (4) LiFePO4/C (b)
In aqueous solution, dilute phosphoric acid H3PO4
becomes viscid when heated to 422 K and then and perfect, suggesting a high degree of crystallinity. The
dehydrates into H4P2O7 when heated to 485−486 K, and patterns agree well with that of phospho-olivine LiFePO4,
finally pure H4P2O7 is obtained at 528−533 K. and no impurity phase is detected. For the diffraction of
For the transformation from P2 O 74− to PO 34− , LiFePO4/C, no peaks of carbon are revealed, indicating
however, there are few cases reported ever. In this work, the amorphous property of the electron conductor.
pure LiFePO4 was prepared by disrupting the P2 O 74−
radical into two PO 34− ions in the presence of O2− at 3.4 Electrochemical test of LiFePO4/C
elevated temperature in an inert atmosphere. Though The charge/discharge curves of LiFePO4/C (Fig.5)
Fe2P2O7 can be dissolved in acidic solution, it is sluggish reveal a flat potential plateau of 3.45 V (vs Li+/Li). At
in kinetics. Due to the high dissolubility of LiFePO4 in this voltage, the lithium extraction/insertion proceeds at
acidic solution, the structural conversion from P2 O 74− the room-temperature in a two-phase reaction between
to PO 34− provides a feasible technique to dispose those LiFePO4 and FePO4 that belongs to the same space group.
pyrophosphates of low dissolubility. A 0.08 V voltage difference between charge and
discharge plateaus is shown in Fig.5, which represents
3.3 XRD characterization of pure LiFePO4 and very small polarization in electrodes and electrolyte of
LiFePO4/C coin cell during charge and discharge process.
The X-ray powder diffraction patterns of pure The first charge and discharge capacities of
LiFePO4 prepared using the as-prepared Fe2P2O7 and LiFePO4/C are 154 and 132 mA·h/g, accounting for
Li2CO3 and LiFePO4/C obtained using Fe2P2O7, Li2CO3 90.6% and 77.6% of the theoretical value (170 mA·h/g),
and glucose are shown in Fig.4. The two diffraction lines respectively. Here, it can be concluded the ecumenic
are indexed to an orthorhombic crystal structure (space electrochemical performance and low efficiency of the
group Pnma). The peaks in both curves are very sharp initial cycle (85.7%) of the LiFePO4/C are ascribed to the
534 J. Cent. South Univ. Technol. (2008) 15: 531−534
non-optimal coating of carbon and calcining temperature
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