Journal of New Materials for Electrochemical Systems 6, 259-262 (2003)
© J. New. Mat. Electrochem. Systems
A New Synthetic Method for Preparing LiFePO
with Enhanced Electrochemical Performance 4
Z.P. Guoa, H. Liub, S. Bewlaya, H.K. Liua, and S.X. Doua
aInstitute
for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia
of Materials Science and Engineering, Sichuan University, Chengdu 610065, China
bCollege
(Received June 2, 2003; received revised form November 6, 2003)
Abstract: The paper describes the synthesis and the properties of LiFePO4 cathode material prepared by a novel modified solid-state reaction. The
novel aspect of the synthesis is based on the addition of a growth inhibitor (citric acid) to the precursor. The citric acid addition does not affect the
structure of the cathode but considerably improves its electrochemical performances because small particle size powders are obtained. A cell
discharged at 17 mA/g was able to deliver a specific capacity of 167 mAh/g with good capacity retention. EIS experiments verified that the charge
transfer has been improved compared to the sample prepared by the conventional solid-state reaction.
Key words: LiFePO4, Citric acid, growth inhibitor
In this study, we describe a novel modified solid-state reaction
to prepare nanosized LiFePO4 with enhanced electrochemical
performance in terms of the specific capacity and charge/
discharge rate. The basic idea is that citric acid added to the
precursors could limit the growth of the LiFePO4 particles and
thus help in obtaining samples with low and uniform particle
sizes. The modified solid-state reaction has been described
previously by this group as a method for preparing other
electroactive materials such as LiCoO2, LiMnO2, etc. [5, 6].
We believe the modified solid-state method provides an
extremely promising general method for the mass production
of lithiated transition metal compounds.
1. INTRODUCTION
Since the pioneering study of Padhi et al. [1], lithium
transition metal phosphates have become of great interest as
storage cathodes for rechargeable lithium batteries. Among
these materials, LiFePO4 is one of the most promising
compounds due to its high density, the low cost of the starting
materials, the relative absence of toxicity, and the fact that the
intermediate voltage value (3.45V vs Li) at which it operates
makes it stable in most of the organic electrolytes usually
employed. However, this cathode was strictly related to the
current density used, associated with diffusion -controlled
kinetics of the electrochemical process. Based on previous
studies, a reduction in the grain size could be one possible
route to improve the performance of LiFePO4, as the slow
lithium ion diffusion in LiFePO4 is the main cause of the poor
electrochemical performance [2-4].
2. EXPERIMENTAL
2.1. Powder preparation
The LiFePO4 compounds were prepared using a modified
solid-state
reaction.
Stoichiometric
Fe(COO)2.2H2O,
(NH4)2HPO4, Li2CO3, and citric acid with Li:citric acid =1:1
were ball milled for 12 h in a planetary mill with agate
vessels. The resulting product was first decomposed at 350 °C
*To whom correspondence should be addressed: E-mail:
[email protected]; Fax: 61 2 4221 5731
259
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Z.P. Guo et al. / J. New. Mat. Electrochem. Systems 6, 259-262 (2003)
for 10h to expel the gases, then the mixture was ground and
pressed into pellets. The pellets were then fired in a purified
Ar flow at various temperatures from 650°C to 800°C for 24 h.
011
020
Phase analysis was carried out by powder X-ray diffraction
(XRD) with Cu Kα radiation in a Philips 1730 X-ray
diffractometer. Scanning electron micrographs (SEM) were
obtained to examine the morphology of the powder. The
Brunauer, Emmett, and Teller (BET) method was used to
measure the surface area of powders.
120
111
121
021
200
041
211
031
101
800oC
131
221
140
012
112
022
700oC
600oC
15
20
25
30
35
40
45
2 - T het a / d eg r ee
2.2. Electrochemical measurements
3. RESULTS AND DISCUSSION
3.1. Crystal structure
The main limitation with respect to this compound is its very
poor electronic conductivity which limits the cycling
capability. Several methods have been proposed to solve this
problem, such as carbon coating or thin metal particle
dispersion. But the amount of carbon recover the theoretical
capacity (~20 %) and the metal used (copper or silver) are
much more expensive than the cathode material itself [7,8].
LiFePO4 compounds synthesized by the modified solid-state
reaction could overcome the disadvantages above, as well as
provide impressive electrochemical performance.
Figure 1a. X-ray diffraction patterns of LiFePO4 prepared by modified solid state
reaction and sintered at 600°C, 700°C and 800°C.
Intensity / Arb. unit
A cathode was prepared by mixing LiFePO4 powder with
20 wt% carbon black and 5 wt% PVDF (polyvinylidene
fluoride) solution. The LiFePO4 and carbon black powders
were first added to a solution of PVDF in N-methyl-2pyrrolidinone (NMP) to make a slurry with appropriate
viscosity. Al foil was then used to coat the mixture. After the
electrode was dried at 140 °C for 2 h in vacuum, it was
compressed at a rate of about 150 kg/cm2. Teflon test cells
were assembled in an argon filled glove box, where the
counter electrode was Li metal and the electrolyte was 1M
LiPF6 dissolved in a 50/50 vol% mixture of ethylene
carbonate (EC) and dimethyl carbonate (DMC). These cells
were cycled between 2.0 and 4.4 V at room temperature to
measure the electrochemical response. In order to measure the
electrochemical impedance response, an EG&G Model 6310
electrochemical impedance analyzer was used, and
electrochemical impedance software (Model 398) was used to
control a computer for conductivity and stability
measurements. After the electrode attained an steady-state
potential, electrochemical impedance measurements were
carried out by applying an ac voltage of 5 mV over a
frequency range from 1 mHz to 100 kHz.
350oC
10
20
30
40
50
60
70
2-Theta / degree
Figure 1b. X-ray diffraction pattern of LiFePO4 prepared by modified solid state
reaction and sintered at 350°C.
Although a single phase of olivine was obtained (from the xray patterns) at various temperatures, the cathode performance
depended strongly on the sintering temperatures. Samples
synthesized above 800 °C show good performance as shown
in Figure 2. The reversible capacity of 164 mAh/g (167 mAh/
g for initial discharge) for compounds sintered at 800 °C is
almost equal to the theoretical capacity of LiFePO4 (170 mAh/
g), while samples sintered at 600 °C and 700 °C
had
capacities of 97 and 149 mAh/g, respectively. To explain this
phenomenon from the structural aspect, the diffraction
patterns were refined based on an ordered olivine structure
200
18 0
16 0
14 0
12 0
Figure 1 shows the x-ray diffraction data collected for
LiFePO4 materials. X-ray analysis shows that the sample heat
treated at 350 °C is completely amorphous. Further heated the
samples from 600 °C to 800 °C, the x-ray diffraction patterns
show a series of diffraction peaks, which indicate that a
transformation from an amorphous to a crystalline phase
occurred. Single phase LiFePO4 compounds were obtained at
temperatures from 600 to 800 °C.
10 0
80
60
40
20
0
500
550
600
650
700
750
Sint er ing T emp er at ur e/
800
o
850
900
950
C
Figure 2. Change in capacity (1st discharge) as a function of sintering temperature.
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4
comprising PO4 tetrahedra and distorted FeO6 octahedra,
which produce a two-dimensional pathway for lithium-ion
diffusion. The refined relative cell parameters for the LiFePO4
samples are plotted versus the synthesis temperature (Fig. 3).
The relative lattice parameters are normalized to the
respective values of the sample synthesized at 600 °C, in
order to illustrate the temperature dependency more clearly.
The absolute lattice parameters for this reference material are
a = 10.298 Å, b = 5.993 Å, c = 4.685 Å. The lattice parameters
a and c decrease slightly with increased synthesis
temperature, whereas the lattice parameter b is unaffected.
This result may indicate the existence of an unidentified
stoichiometric or structural deviation from the ideal olivine
type LiFePO4, which depends on the synthesis temperature.
On the other hand, based on Yamada’s theory [9], samples
sintered below 800 °C could contain Fe3+ impurity phase,
which appeared amorphous or nano-particulate because it was
not detectable by x-ray diffraction. The existence of trivalent
Fe, which might be formed by a small amount of oxygen
included in the argon flow and/or residual air trapped in the
small pores of the particles, decreases the charge-discharge
capacity, as shown in Figure 2.
1.0001
1.0000
Relative lattice dimension
0.9999
0.9998
0.9997
0.9996
0.9995
0.9994
o
a/a(600 C)
o
b/b(600 C)
o
c/c(600 C)
0.9993
0.9992
0.9991
0.9990
600
650
700
750
800
Synthesis temperature
Figure 3. Influence of the synthesis temperature on relative lattice parameters of
different samples, sintered at different temperatures. For clarity, the lattice constants
are normalized to the respective values for the material synthesized at 600°C. The
absolute values for this reference material are a=10.298Å, b=5.993Å, c=4.685Å.
It was also reported that LiFePO4 particles would have an
abrupt increase in size when the sintering temperature is
above 600 °C, which could result in a sharp decrease in
material capacity [9]. However, we did not observe this
phenomenon because the citric acid added to the precursors
works as a growth inhibitor, which significantly reduced the
particle size. There is no obvious increase in particle growth
even at 900 °C.
Figure 4. SEM micrograph of LiFePO4 obtained by modified solid-state reaction.
3.2. Electrochemical characteristics
Figure 5 shows the voltage profiles of a LiFePO4 sample as a
function of the specific capacity for several discharge rates.
The cell was discharged galvanostatically under different
specific currents, ranging from C/10 to 3C. The discharge and
charge cut-off voltages are 2.0 and 4.5 V, respectively. The cell
was always recharged at the same specific current of 17 mA/g
to assure identical initial conditions for any discharge. At the
lowest specific discharge current used (17 mA/g), the cell was
able to deliver a specific capacity of 167 mAh/g based on the
active material weight only. By increasing the current density
the utilization of the active material decreased, and about 145
mAh/g was delivered at a specific current of 3C. This result is
very good when compared with iron phosphate synthesized
by traditional solid state chemistry, in which an increase in the
discharge current results in severe capacity fading. This effect
was claimed to have a kinetic origin because the full capacity
was recovered on returning to a lower current density. The
excellent performance of the samples is due to the small
particle sizes achieved by the modified solid-state synthesis.
Small particles lead to a large surface area of LiFePO4 (29 m2/
g), which makes better contact between the particles and
electrolyte, and it helps to accelerate the lithium diffusion in
5
4.5
4
3.5
3
2.5
2
3C
C/ 10
1
0.5
0
0
Figure 4 is a typical SEM micrographs for samples sintered at
800 °C. The materials are characterized by a globular
structure with a grain size of about 100 nm. BET analysis
gives a specific surface area of 29 m2/g.
1C
1.5
20
40
60
80
100
120
140
160
180
Sp ecif ic cap acit y / A h/ Kg
Figure 5. Rate capability test of LiFePO4 compounds, synthesized by modified solid
state reaction.
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Z.P. Guo et al. / J. New. Mat. Electrochem. Systems 6, 259-262 (2003)
the LiFePO4 structure [10]. Long–term cycling of the LiFePO4
compounds confirmed the excellent reversibility of the
insertion reaction. As shown in Figure 6, the cycle retention
rate of LiFePO4 is 92 % after 50 charge-discharge cycles.
180
160
140
120
100
80
60
40
20
0
1
3
5
7
9
11
13
15
17
19
21 23
25
27
29
31 33
35
37
39
41 43
45
47
49
4. CONCLUSION
The results reported demonstrate that the electrode response
of the phosphor-olivine LiFePO4 cathode may be enhanced by
a novel modified solid-state reaction synthesis procedure.
Citric acid addition does not affect the structure of the
cathode but it appears to favor the growth of small particles.
This in turn improves the electrochemical performance of the
LiFePO4 electrode. A cell discharged at 17 mA/g was able to
deliver a specific capacity of 167 mAh/g with good capacity
retention. EIS experiments verified that the charge transfer has
been improved compared to the sample prepared by
conventional solid-state reaction. Work is in progress in our
laboratory to understand the mechanism of lithium insertion
into this very promising cathodic material for lithium ion
batteries.
C ycle numb er
Figure 6. Cycling test of LiFePO4 synthesized by the modified solid state reaction
(sintered at 800 °C, charged-discharged at a C/10 rate).
To investigate the kinetics of the electrode process, AC
impedance measurements were conducted. EIS experiments
were performed on working electrodes in the OCV state.
Compared to the LiFePO4 sample prepared by conventional
solid state reaction, the EIS experiments performed on
LiFePO4 samples prepared by modified solid state reaction
(Fig. 7) show clearly that the charge transfer has been
improved. The charge transfer resistance varies from 500 Ω
(conventional sample) to 200 Ω (sample prepared by modified
solid state reaction).
More measurements are needed to understand the mechanism
of lithium insertion into this very promising cathodic material
for lithium ion batteries. Recent work performed in our
laboratory involving synthesis of substituted-LiFeO4 (such as
Mg) using the modified solid-state reaction is very promising
in terms of enhancement of the conductivity of the material.
This should allow very good electrochemical performance.
The details will be published elsewhere.
-1000
-800
Zim / ohm
-600
-400
-200
0
0
200
400
600
800
1000
ZR / ohm
Figure 7. EIS spectra of LiFePO4 cathodes, o: prepared by conventional solid state
reaction; ∆: prepared by modified solid state reaction.
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