Hydrothermal synthesis of lithium iron phosphate cathodes

Electrochemistry Communications 3 2001) 505±508
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Hydrothermal synthesis of lithium iron phosphate cathodes
Shoufeng Yang, Peter Y. Zavalij, M. Stanley Whittingham *
Department of Chemistry, Institute for Materials Research, Binghamton University, Binghamton, NY 13902-1600, USA
Received 8May 2001; received in revised form 24 May 2001
Abstract
Hydrothermal methods have been successfully applied to the synthesis of lithium iron phosphates. Li 3 Fe 2 …PO 4 † 3
was synthesized
by heating at 700°C LiFePO 4 …OH†, formed hydrothermally in an oxidizing environment. Crystalline LiFePO 4 was formed in a
direct hydrothermal reaction in just a few hours, and no impurities were detected. This result leads to the possibility of an easy scaleup
to a commercial process. The samples were characterized by X-ray di€raction, thermogravimetric analysis and scanning electron
microscopy. Both phosphates were tested as the cathode in lithium batteries and showed results comparable to those formed by
conventional high-temperature synthesis. Ó 2001 Elsevier Science B.V. All rights reserved.
Keywords: Hydrothermal synthesis; Lithium iron phosphate; Redox couple; Cathode; Olivine
1. Introduction
Since the chalcogenide material TiS 2 was discovered
[1] in the early 1970s to be able to intercalate lithium
ions into its structure, other materials such as MoS 2 [2],
LiCoO 2 [3], and LiMn 2 O 4 [4] have been investigated and
commercialized. But these materials either provide rather
low discharge capacity, high cost or poor cyclability,
so other compounds have been investigated
extensively, especially Fe 2‡ =Fe 3‡ redox couple-based
materials such as NASICON sodium super ionic conductor)
and olivine compounds [5±8].
For the NASICON compounds A 3 Fe 2 …XO 4 † 3
A ˆ Li, Na; X ˆ P, As, S), its 3D framework is made up
of the XO 4 tetrahedra and the FeO 6 octahedra. Each
FeO 6 octahedron shares its corners with six tetrahedra
and each XO 4 tetrahedron shares its corners with four
octahedra. This structure allows lithium intercalation
and extraction, making it a promising cathode material
for lithium batteries [5]. Since the FeO 6 octahedron is
separated from other octahedra by XO 4 tetrahedra, it
reduces the conductivity for this material, so the compound
has a large polarization e€ect during the cycling,
as shown later during the discussion. Of these NAS-
ICON compounds, Li 3 Fe 2 …PO 4 † 3
was reported by
* Corresponding author.
E-mail address: stanwhit@binghamton.edu M. Stanley Whittingham).
Goodenough's group [6] to generate about 2.8V vs
lithium while maintaining excellent capacity retention.
Due to the Fe 3‡ =Fe 2‡ redox couple, 2 mol of lithium can
be removed and reversibly intercalated into
Li 3 Fe 2 …PO 4 † 3
, delivering a capacity of 128Ah/kg, which
is comparable to that of the LiCoO 2 cathode, where
only half the lithium can be cycled in practical cells, and
it is much lower in cost.
LiFePO 4 is of the olivine formula MNXO 4 , where M
and N are cations with di€erent sizes. Its structure is
composed of PO 4 tetrahedra and FeO 6 octahedra. Each
FeO 6 octahedron shares edge with one tetrahedron
along c-axis and two corners in the ab-plane. This
compound is environmentally benign and cheap, making
it ideal for battery applications. Padhi et al. [7] reported
that 0.8mol of lithium can be reversibly
extracted at 0:05 mA=cm 2 . The polyanion group stabilizes
the structure and lowers the Fermi level of this
redox couple through the Fe±O±X inductive e€ect, thus
providing a higher voltage. Its discharge voltage is 3.5 V,
almost 0.7 V higher than Li 3 Fe 2 …PO 4 † 3
. Since its formula
has 1 mol of lithium, it should be charged ®rst and
its theoretical capacity is 170 Ah/kg. During the recharge,
its volume shrinks only about 6.8% [8], comparable
to that of LiTiS 2 , thus minimizing mechanical
decrepitation and the resulting loss of electrical contact
during cycling.
In this paper, we exploited the advantages of hydrothermal
synthesis, quick easy synthesis at low energy
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 1 3 88- 2 4 81 0 1 ) 00200-4

506 S. Yang et al. / Electrochemistry Communications 3 2001) 505±508
cost and readily scalable. We report the rapid formation
of several lithium iron phosphates including crystalline
LiFePO 4 , where no impurities were observed, making it
an excellent alternative to high-temperature synthesis.
Previously, our group synthesized a series of manganese
and vanadium compounds such as K x MnO 2 yH 2 O [9],
Li 0:6 V 2 d O 4 d H 2 O [10], MnV 2 O 5 [11], etc. This method
involves reaction at low temperature 100±200°C) for a
short period 1 h to 3 days), which is very good for
commercialization of these compounds.
2. Experiment
2.1. Synthesis of LiFePO 4 …OH† and Li 3 Fe 2 …PO 4 † 3
FeCl 2 4H 2 O Aldrich) and LiOH Aldrich) with
molar ratio 1:2 were dissolved in deionized water separately
and mixed to get an Fe…OH† 2
precipitate. The
slurry was ®ltered and washed, then transferred to a 100
ml beaker; 10 ml 15% H 2 O 2 J.T. Baker) was added
slowly to the slurry while stirring for 3 min. P 2 O 5
Fisher) and LiOH molar ratio 1:3) were dissolved in
another beaker with warm water 50°C) to get a lithium
phosphate solution. Then the two samples were mixed
together and stirred for 5 min. The solution was transferred
to the Te¯on-lined Parr reactor and reacted at
170°C for 3 days. The ®nal solution was cooled and
®ltered, the bright yellow precipitate was air-dried at
40°C overnight. Then the sample was transferred into a
crucible and heated to 700°C in a furnace in air for 12 h.
2.2. Synthesis of LiFePO 4
LiFePO 4 was prepared by direct hydrothermal synthesis
of FeSO 4 98%, Fisher), H 3 PO 4 Fisher) and
LiOH Aldrich) in the stoichiometric ratio 1.0:1.0:3.0.
FeSO 4 and H 3 PO 4 solution were mixed ®rst to avoid
Fe…OH† 2
because it is easily oxidized to FeIII), then
LiOH solution was added to the mixture with stirring
for 1 min. pH was 7.56 and the solution was quickly
transferred to Parr reactor for up to 5 h at 120°C. After
the sample was cooled, the pH was 6.91. The light green
precipitate was ®ltered and air-dried at 40°C for 2 h. For
several similar syntheses, pH was slightly di€erent and it
had no e€ect on the ®nal product. Several attempts had
been tried with FeII) salt and Li 3 PO 4 , all of them had
some impurities. When FeCl 2 Aldrich) and
…NH 4 † 2
Fe…SO 4 † 2
Fisher) were used instead of FeSO 4 ,
the same compound was synthesized. Due to the easy
oxidation of FeII) salt, sometimes reddish impurities
will be produced, especially when the iron salt is not
new, but they can be easily separated from LiFePO 4 in a
centrifuge.
2.3. Analysis of LiFePO 4 and LiFePO 4 …OH†
X-ray di€raction patterns were collected for
LiFePO 4 …OH†, LiFePO 4 and Li 3 Fe 2 …PO 4 † 3
on a Scintag
XDS 2000 di€ractometer with CuKa radiation and
GeLi) solid-state detector. Thermogravimetric analysis
was performed on a Perkin Elmer model TGA7 to determine
the nature of the products formed. Fig. 1 shows
the TGA for LiFePO 4 …OH† in oxygen, indicating a
weight decrease of 4.4% consistent with the expected
weight loss of 5.1% for the reaction
6LiFePO 4 …OH† !2Li 3 Fe 2 …PO 4 † 3
‡ Fe 2 O 3 ‡ 3H 2 O
XRD analysis con®rmed the formation of Li 3 Fe 2 …PO 4 † 3
and Fe 2 O 3 .
In order to determine the purity of LiFePO 4 , standard
K 2 Cr 2 O 7 solution was used to titrate the amount of
Fe 2‡ in the product. Each time, about 100 mg LiFePO 4
was used and dissolved in the mixture of 40 ml 1 M
H 2 SO 4 and 20 ml 5 M H 3 PO 4 , and phenylamine sulfonic
acid was used as the indicator. For each titration, we
calculated the amount of Fe 2‡ based on the titration and
on the assumption of 100% LiFePO 4 , respectively. The
ratio of these two will be denoted as g. If no impurities
exist in the sample, g should be 1.000.
2.4. Electrochemical testing for LiFePO 4 and Li 3 Fe 2 …PO 4 † 3
To test these materials as cathodes in lithium cells,
the lithium iron phosphate, carbon black, Te¯on were
mixed in a mortar for 20 min. For Li 3 Fe 2 …PO 4 † 3
, the
weight ratio was 75:15:10, and for LiFePO 4 , the material
was coated with carbon by dissolving it in sucrose solution,
weight ratio for LiFePO 4 and sucrose was 9:1. It
was ®red at 700°C for 3 h [12]. The ®nal brown material
was used as the cathode. The weight ratio was 70:20:10.
The sample was ground and crushed into a thin sheet
and put on an Exmet stainless steel grid and hot pressed
at 100°C for 30 min. A lithium sheet was used as the
anode and the electrolyte was 1 M LiPF 6 dissolved in
ethylene carbonate/dimethyl carbonate EC:
Weight, %
101
100
99
98
97
96
95
100 200 300 400 500 600 700
Temperature,˚C
Fig. 1. TGA of LiFePO 4 …OH† in oxygen.

DMC ˆ 1:1, EM Industries) and the separator was
Celgard 2400 Hoechst Celenese Corp). The electrodes
were submerged in the electrolyte and put in a plastic
bag. The assembling of this battery was done in a glove
box ®lled with pure helium. For the Li 3 Fe 2 …PO 4 † 3
, the
current density was about 0:5 mA=cm 2 and the cut-o€
voltage was 1.9 and 4.1 V. For the LiFePO 4 , the current
density was about 0:14 mA=cm 2 and cut-o€ voltage was
2.0 and 4.5 V. The battery performance was collected on
a MacPile system galvanostatically at 25°C MacPile A-
3.10, Biologic, Claix, France).
S. Yang et al. / Electrochemistry Communications 3 2001) 505±508 507
Fig. 4. Electrochemical performance of Li 3 Fe 2 …PO 4 † 3
.
3. Results and discussion
LiFePO 4 …OH† was synthesized as the precursor for
Li 3 Fe 2 …PO 4 † 3
, it was indexed in the triclinic system with
space group P 1 and the cell parameters: a ˆ 5:367…1† A,
b ˆ 7:305…1† A, c ˆ 5:131…1† A. a ˆ 109:31…1†°, b ˆ
97:84…1†°, c ˆ 106:37…5†°. Li 3 Fe 2 …PO 4 † 3
was indexed as
follows: space group is P2 l =n with a ˆ 8:579…2† A,
b ˆ 8:622…2† A, c ˆ 12:041…3† A, b ˆ 90:46…2†°.
Fig. 2 shows the X-ray powder di€raction patterns
for LiFePO 4 …OH† a) and Li 3 Fe 2 …PO 4 † 3
b). For
LiFePO 4 …OH†, the arrows point to the peaks of an unknown
impurity.
In the case of LiFePO 4 , we performed a Rietveld
re®nement using the WinCSD crystallographic software.
It was indexed in the orthorhombic system with space
group Pnma and cell parameters: a ˆ 10:381…7† A,
b ˆ 6:013…5† A, c ˆ 4:716…3† A. Fig. 3 shows the high
crystallinity and purity of the material formed.
Several attempts were made to test the cathodic behavior
of LiFePO 4 …OH†, but no capacity was observed
between 1.9 and 4.1 V. However, its thermal product
Li 3 Fe 2 …PO 4 † 3
showed highly reversible capacity as indicated
in Fig. 4. This capacity at 0:5 mA=cm 2 is shown
per formula weight of Li 3 Fe 2 …PO 4 † 3
after backing out
the weight of the ferric oxide. The capacity is about 91
Ah/kg, consistent with that reported by Masquelier [6].
The morphology for LiFePO 4 was determined on a
JEOL 8900 SEM see Fig. 5). The average particle size is
about 3 lm, and this is smaller than the 20 lm average
Fig. 2. XRD pattern for LiFePO 4 …OH† a), and Li 3 Fe 2 …PO 4 † 3
b).
Fig. 3. Experimental dotted), calculated line) and the di€erence
bottom) for LiFePO 4 , the vertical lines show the re¯ections.
Fig. 5. Morphology of LiFePO 4 .

508 S. Yang et al. / Electrochemistry Communications 3 2001) 505±508
hydrothermal reaction. The crystal size was smaller and
the Fe 2‡ purity was higher compared to high-temperature
synthesis. After a rough carbon coating of this
material, a capacity of 100 A h/kg was obtained at
0:14 mA=cm 2 . We will optimize the carbon coating
process for these hydrothermally synthesized materials,
and are presently exploring microwave assisted hydrothermal
as an even more rapid synthetic approach.
Acknowledgements
size of LiFePO 4 reported by Yamada et al. [8]. The
FeII) purity was determined by titration, which indicated
that the g value, was always between 0.99 and
1.00, demonstrating that the purity for this material was
almost 100%.
Fig. 6 shows the cycling pro®le for LiFePO 4 with a
small amount of unoptimized carbon coating. About 0.6
mol lithium can be inserted reversibly at 0:14 mA=cm 2 .
Raising the temperature to 60°C allowed the rate to be
increased to over 0:5 mA=cm 2 whilst maintaining a
similar capacity. Our earlier results without carbon
coating showed only 0.4 mol of lithium intercalation/
deintercalation at 0:05 mA=cm 2 with the same cell
con®guration. Optimized carbon coatings on high-temperature
LiFePO 4 permit capacities of 0:8Li=LiFePO 4
to be attained [13].
4. Conclusions
Fig. 6. Electrochemical performance of LiFePO 4 .
We have synthesized Li 3 Fe 2 …PO 4 † 3
by a combination
of the hydrothermal method and high-temperature
process. The initial discharge capacity was about 91 Ah/
kg. This method has to be improved due to the impurities.
LiFePO 4 was very readily synthesized by a direct
We thank the US Department of Energy, O ce of
Transportation Technologies, for support of this work
through the BATT program at Lawrence Berkley National
Laboratory of this work. Initial results of this
work have been reported in the cognizant DOE reports.
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