Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
Hydrothermal Processing and Characterization of
LiFePO4 Crystalline Particles
Ryota Isomura, Makoto Moriya, Wataru Sakamoto and Toshinobu Yogo
EcoTopia Science Institute, Nagoya University, Nagoya, Japan
Abstract: LiFePO4 particles have been synthesized via the hydrothermal process from LiOH･H2O,
FeSO4･7H2O and H3PO4 as starting materials and characterized as cathode materials for lithium ion secondary batteries. LiFePO4 particles were successfully prepared by controlling several processing conditions. Particle size of the hydrothermally synthesized LiFePO4 can be controlled by using the planetary
ball milling. Furthermore, the charge-discharge capacity was improved by the planetary ball milling followed by the annealing at 400℃ in N2.
Keywords: Hydrothermal process, Lithium iron phosphate, Charge-discharge performance, Particle size
2.2. Electrochemical characterization
The charge-discharge capacities were characterized by
fabricating a coin-type cell. LiFePO4 powder was mixed
with carbon (LiFePO4:carbon = 2:1.3) and binder solution
(polyvinylidene fluoride (PVDF):N-methylpyrrolidone =
2:23). The resulting paste was cast on an Al foil to prepare a cathode. Li-metal was used as an anode material.
The liquid electrolyte used was 1M LiPF6 dissolved in a
3:7 mixture of ethylene carbonate (EC) and diethylene
carbonate (DEC). The coin-type cell was assembled using
these electrodes and electrolyte. The charge-discharge
measurements were carried out in the 3.0-4.3V ranges at
a current density of 5 mA/g.
3. RESULTS AND DISCUSSION
Fig. 1(a) shows the XRD patterns of products synthesized for a constant time of 24 h. LiFePO4 particles were
synthesized at 150℃. When the treatment temperature
was higher than 170℃, the diffraction lines of unknown
phases began to emerge clearly with LiFePO4 phase. On
the other hand, Fig. 1(b) shows the XRD patterns of
products synthesized at a constant temperature of 150℃.
Although the diffractions of unknown phases were
● LiFePO4 ○ Li3PO4 ▼ unknown
(b)
● LiFePO4 ▼ unknown
(a)
48h
Intensity (arb. units)
Intensity (arb. units)
190ºC
170ºC
▼▼
▼▼
24h
15h
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30
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5h
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40
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●●
50
●
2θ (deg.)
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●
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● ●●
60
70
CuKα
▼
●
80
10
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● ● ●○ ▼
▼▼
○
20
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8h
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20
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150ºC
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10
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2. EXPERIMENTAL
2.1. Synthesis and material characterization
LiOH･H2O, FeSO4･7H2O and H3PO4 were selected as
starting materials. The mixing ratio (Li:Fe:P) before
hydrothermal treatment was set at 3:1:1. First of all,
FeSO4 aqueous solution and H3PO4 were mixed together.
LiOH aqueous solution was then added to the mixed solution, yielding a slate-blue suspension. The resulting
suspension was transferred in a Teflon-lined stainless
steel autoclave and hydrothermally treated under various
conditions. After the hydrothermal treatment, the precipitate was filtered and the resulting precipitate powder was
rinsed with acetone. Then, the obtained powder was dried
at 80℃ for 15 h. The annealing treatment was carried out
at 400℃ for 2 h under N2 atmosphere in order to improve
the crystallinity. In addition, hydrothermally prepared
LiFePO4 powder was ball-milled from 30 min to 1 h in
ethanol to reduce its particle size.
X-ray diffraction (XRD) measurement using CuKα radiation with a monochromator was performed to identify
crystallographic phases of resultant products. The mor-
phology of the samples was observed by scanning electron microscopy (SEM). The specific surface areas of
LiFePO4 powders were measured by Brunauer-Emmett-Teller (BET) technique.
●
1. INTRODUCTION
LiCoO2 has well been utilized as a cathode material for
lithium-ion secondary batteries. However, LiCoO2 has
many problems such as high cost and amount of global
resources. Recently, lithium iron phosphate (LiFePO4)
has been receiving great attention under the consideration
of cost and resource issues [1]. Moreover, LiFePO4 has a
three dimensional framework due to strong P-O covalent
bonds in (PO4)3- polyanion, which prohibit the dissociation of oxygen in its structure. This provides an excellent
safety and stable operation of battery even under unusual
conditions [2].
In this study, hydrothermal process was employed for
the preparation of LiFePO4, because this method has advantages such as simple aqueous system and low energy
consumption [3]. The additional heat treatment in inert
gas atmosphere was examined to improve the
charge-discharge performance of LiFePO4. Moreover,
size-controlled LiFePO4 particles were prepared and the
effect of the particle size on several properties was also
investigated.
30
○ ●● ●
40
●
50
2θ (deg.)
● ●
●
60
70
80
CuKα
Fig. 1. X-ray diffraction patterns of products after hydrothermal treatment,
(a) for 24 h at 150ºC, 170ºC and 190ºC and (b) for 5 h, 8 h, 15 h, 24 h and 48 h
at 150ºC.
Corresponding author: W. Sakamoto, [email protected]
123
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
(A)
effect of reduction of particle size by the planetary ball
milling. As the particle size becomes smaller, the reactive
area for diffusion of lithium ions increases effectively
improving charge-discharge properties. However, the
charge-discharge capacity of the LiFePO4 powder synthesized in this study was not sufficiently high compared
with the theoretical value (170 mAh/g) [1] and those reported by other researchers [5,6]. The formation of the
unknown secondary phase and the low capacities may be
due to the fluctuation of lithium content in synthesized
LiFePO4 particles. Therefore, further optimization of the
synthesis conditions, particularly precise control of the
chemical composition (Li:Fe:P in LiFePO4), is required to
improve the electrochemical properties and it is currently
under investigation.
4.3
4.1
V oltage (V )
slightly observed, LiFePO4 particles were obtained when
the duration time of treatment was longer than 8 h.
Fig. 2 shows SEM images of LiFePO4 powders synthesized under various hydrothermal conditions. When
the treatment temperature was raised from 150℃ to
190 ℃ , no significant change in the morphology of
LiFePO4 particles was observed. On the other hand, increase of the treatment time leads to grain growth. However, when the treatment time was longer than 24 h,
LiFePO4 particles having similar morphology were reproducibly observed. LiFePO4 particles synthesized at
150℃ for 24 h had the particle size of about 5 μm and
the specific surface area of around 3 m2/g. The
charge-discharge performance was examined after annealing at 400℃ for 2 h in a N2 flow. However, this sample showed a poor charge-discharge capacity (10-12
mAh/g). It may be attributable to the difficulty of the
lithium ion diffusion through the LiFePO4/FePO4 interfaces in large particles [4].
(C)
(B)
3.9
3.7
3.5
3.3
1μm
1st
2nd
3.1
2.9
(c)
(b)
(a)
1μm
1μm
0
10
20
30
40
50
60
70
Capacity (mAh/g)
5μm
5μm
Fig. 4. Charge-discharge curves of LiFePO4 powder
after planetary milling and annealing at 400ºC in N2.
5μm
Fig. 2. SEM images of LiFePO4 powders after hydrothermal treatment,
for 24 h at (A) 150ºC, (B) 170ºC and (C) 190ºC , for (a) 15 h, (b) 24 h
and (c) 48 h at 150ºC.
Next step, the planetary ball milling was carried out in
order to reduce particle size of LiFePO4 powders. Fig. 3
shows SEM images of LiFePO4 powers before and after
the planetary ball milling for 1 h. LiFePO4 particles
well-ground by the planetary ball milling were observed.
The average particle size was found to be below 1 μm.
The milling time of 1 h was sufficient to decrease the
particle size around 1 μm maintaining the LiFePO4 phase.
The specific surface area of about 13 m2/g was obtained
and increased by more than four times. The
charge-discharge property was examined after annealing
at 400℃ for 2 h in N2.
(a)
(b)
5μm
5μm
Fig. 3. SEM images of LiFePO4 powders,
(a) before and (b) after planetary milling for 1 h in ethanol.
Fig. 4 shows the charge-discharge curves of LiFePO4
powder from 3.0 to 4.3V. The charge-discharge capacity
of LiFePO4 particles after milling was improved compared to that before milling treatment. This is due to the
4. CONCLUSIONS
LiFePO4 particles were successfully synthesized via
the hydrothermal treatment using LiOH･H2O, FeSO4･
7H2O and H3PO4 as starting materials. LiFePO4 particles
were synthesized at 150℃ for 24 h having the particle
size of around 5 μm and the specific surface area of about
3 m2/g. The specific surface area of LiFePO4 powders increased by more than four times through the planetary
ball milling for 1 h. The charge-discharge capacity of the
LiFePO4 was found to be improved by this milling treatment followed by the annealing at 400℃ in N2.
ACKNOWLEDGEMENT
The authors would like to express their grateful thanks
to the material development group members of NEC
Lamilion Energy, Ltd. for their support in the preparation
of coin-type cell and the charge-discharge cell capacity
measurement.
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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
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