Electrochemical and Solid-State Letters, 12 共6兲 A111-A114 共2009兲
A111
1099-0062/2009/12共6兲/A111/4/$25.00 © The Electrochemical Society
Effect of LiI Amount to Enhance the Electrochemical
Performance of Carbon-Coated LiFePO4
H. T. Kuo,a T. S. Chan,a N. C. Bagkar,a R. S. Liu,a,z C. H. Shen,b D. S. Shy,b
X. K. Xing,b,* and J.-F. Leec
a
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
SYNergy ScienTech Corporation, Science Tech-based Industrial Park, Hsinchu 300, Taiwan
National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
b
c
We used the wet chemistry method with different amounts of LiI to prepare LiFePO4 and carbon-coated LiFePO4 共LiFePO4 /C兲
samples. The lithiation process of LiI for preparing the LiFePO4 is proposed. We found that the amount of LiI greatly affected the
purities of products. The LiFePO4 /C sample showed constant values of current density during potential cycling up to 35 cycles as
compared to LiFePO4, suggesting that the LiFePO4 /C has better electrochemical performance. The obtained maximum capacity
for LiFePO4 /C can be approached to the theoretical capacity of 170 mAh/g. Moreover, Brunauer–Emmett–Teller measurements of
LiFePO4 and LiFePO4 /C showed a surface area of 6.7 and 50 m2 /g, respectively. It is reasonable to believe that the excellent
performance of the compound developed in our work can be attributed to the smaller particle size coated with conductive carbon
achieved by the controlling LiI in the sol-gel method.
© 2009 The Electrochemical Society. 关DOI: 10.1149/1.3095673兴 All rights reserved.
Manuscript submitted May 9, 2008; revised manuscript received February 10, 2009. Published March 30, 2009.
Recently, lithium iron phosphate 共LiFePO4兲 based on the olivine
structure has shown better stability and is considered to be a potential cathode material for rechargeable lithium-ion batteries.1,2
LiFePO4 offers an advantage as a cathode material for lithium-ion
batteries due to its high specific capacity 共⬃170 mAh/g兲, benign
environmental effect, and ease of fabrication.3,4 Unfortunately, the
poor conductivity affects the electrochemical performance and
makes it difficult to utilize it extensively in lithium-ion batteries.
Hence, some modifications are made for the material to improve the
low intrinsic electronic conductivity or minimize lithium-ion diffusion length across the LiFePO4 /FePO4 boundary. For example, the
work of Huang et al.5 showed that the reduction of the grain size can
enhance the performance of LiFePO4 and make it feasible as a cathode material in high-power-density lithium-ion batteries. It has been
proposed that conductivity can be improved by using carbon-coated
LiFePO4 materials.6 Moreover, 90% of the theoretical capacity at
the 0.5 C rate in carbon-coated LiFePO4 composites at room temperature has been achieved by using 15 wt % carbon gels as a carbon source.7 The same capacity value was obtained by reducing the
carbon content to nearly 3 wt %. Chen and Dahn8 also reported that
by reducing the carbon contents nearly 3 wt %, the same capacity
could be obtained as observed by Huang et al. via a new carbon
incorporation method. Recently, sol-gel synthesis of highly crystalline, microsized porous particles of LiFePO4 /C composites with different amounts of carbon has been reported.9,10 Porous particles
could be described as a channel for lithium supply, and the distance
between the pores determines the materials kinetics. These results
indicate that reduction of the grain size of carbon-coated LiFePO4
composites is an effective means to make LiFePO4 materials practically applicable. However, it is well known that LiFePO4 can be
prepared by many methods which may result in different materials
properties. The preparation of LiFePO4 by addition of LiI using the
sol-gel method was first demonstrated by Prosini et al.11 Kim et al.12
described solid-state synthesis of LiFePO4 by Li2CO3 using various
amounts of Li content. However, none of these researchers gave
many details about the lithiation effects and process.
The objective of this study is to describe the lithiation effects and
process of LiI addition using the sol-gel route and investigate suitable synthesis conditions to prepare the highly crystalline LiFePO4.
White sugar is added to the reactants before firing in a reductive
atmosphere, which results in the formation of carbon-coated
* Electrochemical Society Active Member.
z
E-mail: [email protected]
LiFePO4 particles with a cavity architecture exhibiting improved
physical, electrical, and electrochemical characteristics.
Experimental
The LiFePO4 sample was prepared via the sol-gel method. The
reactant materials of Fe共NH4兲2共SO4兲2 and NH4H2PO4 were 7.45
and 2.17 g, separately dissolved in 500 mL deionized water and
mixed together in a 1:1 共v/v兲 in stoichiometric amounts. The oxidizing reagent of H2O2 共35 wt %; 3 mL兲 was then added to produce
white precipitate of amorphous FePO4. The precipitate was filtered
and washed several times with deionized water. Finally, it was dried
in the oven at 400°C for 24 h. The amorphous phase of LiFePO4
was obtained by the reaction between the above 0.5 g FePO4 and
the reducing reagent of LiI dissolved in 50 mL CH3CN with different molar concentrations 共1.0, 2.5, 4.0, and 4.5 M兲. The reaction was
carried out at ambient temperature for 24 h followed by filtering and
washing several times with CH3CN and then drying under vacuum.
The crystalline LiFePO4 was obtained by sintering amorphous
LiFePO4 at 550°C for 1 h under 5% H2 /Ar atmosphere. The crystalline powder of carbon-coated LiFePO4 was prepared by sintering
with white sugar 共10 wt %兲 in the same experimental process.11
Preparation of cathode films and coin cells.— Cathode
films
were made by mixing 0.5000 g active material 共80 wt %兲, 0.0625 g
conductor 共10 wt % carbon, Super P兲, and 0.6250 g binder 共polyvinylidene fluoride兲 dissolved in 0.5625 g N-methyl-2-pyrrolidinone
共NMP兲 共10 wt %兲. The slurry mixture was stirred in ambient temperature for 2 h and then coated on aluminum foil as the electronic
collector. The prepared cathode film was heated for 4 h at 70°C in
an oven and then cold pressed by a roller with a 0.04 ␮m thickness.
Finally, the pressed cathode film was completely dried in a vacuum
at 110°C for at least 12 h to remove the trace NMP and absorbed
water in the electrode lamella. The cathode lamella was cut into a
circular disk with a diameter of 1 cm. The amount of active materials was ⬃10–20 mg/cm. The cell consisted of a cathode film and
a lithium metal anode separated by a porous polyethylene film. The
coin cell was assembled in a glove box filled with argon gas. The
electrolyte was a 1 M solution of LiPF6 in a 1:1 共v/v兲 mixture of
ethylene carbonate 共EC兲 and dimethyl carbonate 共DMC兲. The electrochemical testing of coin cells was performed using a Maccor
battery cycling system.
Characterization.— The sample purity was characterized by
X-ray diffraction 共XRD兲 using a PANalytical X’Pert PRO diffractometer with Cu K␣ radiation. The specific surface area was measured
by using a Brunauer–Emmett–Teller 共BET兲 apparatus 共TriStar 3000兲
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Electrochemical and Solid-State Letters, 12 共6兲 A111-A114 共2009兲
A112
(d) LiI = 4.5 M
* *
*
Intensity (a. u.)
(c) LiI = 4.0 M
(b) LiI = 2.5 M
(a) LiI = 1.0 M
Results and Discussion
Li3PO4 (ICSD #: 77095)
Fe2P2O7 (ICSD #: 036208)
LiFePO4 (ICSD #:072545)
20
tiostat PGSTAT20 with a frequency response analyzer. The frequency range is between 1 MHz and 0.1 Hz. To determine actual
carbon content in the LiFePO4 /C composite, elemental CHN analysis was performed on “Haraeus CHN-O Rapid” analyzer. The elementary CHN analysis result for carbon content 共%兲 was tested.
The exact value of carbon content was around 3.7% in the sintered
sample with white sugar.
30
40
50
2θ (degree)
60
70
Figure 1. XRD patterns of LiFePO4 with different amounts of LiI: 共a兲 LiI
= 1.0 M, 共b兲 LiI = 2.5 M, 共c兲 LiI = 4.0 M, and 共d兲 LiI = 4.5 M. The impurity phase of Fe2P2O7 is marked by arrows, and the impurity phase of Li3PO4
is marked by stars.
in N2 at −196°C 共liquid N2 surrounding兲. The X-ray absorption
experiments were carried out at the Wiggler beam line BL17C of the
National Synchrotron Radiation Research Center, Taiwan. The electron storage ring was operated at energy of 1.5 GeV with a beam
current of 300 mA. A Si共111兲 double-crystal monochromator was
employed for energy selection with a resolution of around 2
⫻ 10−4. The X-ray absorption near-edge spectrum 共XANES兲 at the
Fe K-edge was recorded at room temperature in transmission mode
using gas-filled ionization chambers to measure the intensities of the
incident and transmitted X-ray beams. After background subtraction,
the XANES spectra were normalized with respect to the edge jump.
The cyclic voltammetry 共CV兲 measurements were obtained by a
cyclic voltammeter 共digital potentiostat DP 8R and Autolab model
PGSTAT20兲 in the open circuit voltage range from 2.0 to 4.5 V vs
Li+ /Li at a 0.5 mV s−1 scan rate. The electrochemical impedance
spectrum 共EIS兲 measurement was obtained by an Autolab poten-
X-ray diffraction.— Figure 1 shows the XRD patterns of
LiFePO4 synthesized with different amounts of LiI. It can be seen
that a well crystalline, single phase of LiFePO4 can be obtained with
2.5 M LiI. The impurity phase of Fe2P2O7, as shown by arrows in
Fig. 1, was found in the products when the amount of LiI was less
than 2.5 M. The impurity phase might originate from the decomposition of some amounts of unreacted amorphous FePO4 phase when
the amount of LiI is not enough for the reaction.13,14 Moreover,
Li3PO4 impurity phase was found above 4.0 M, which may be due
to the phosphatic ions reacting with an excess amount of LiI, as
shown by the stars in Fig. 1.12 These results indicate that the sample
purity is affected by different amounts of LiI.
Lithiation mechanism studies.— In the following discussion, we
propose the lithiation process and discuss the suitable synthesis conditions to prepare highly crystalline LiFePO4. The lithiation process
and possible proposed chemical reaction equations are listed below
and are indicated by the rectangles with the dashed line in Fig. 2.
The different amounts of LiI dissolved in CH3CN turned into the
Li+ and I− ions as shown in Eq. 1. Afterward we added LiI solutions
drop by drop to the amorphous FePO4 to prepare LiFePO4. The
addition of LiI to FePO4 will change the color gradually to dark-red
because of the generation of I2 in CH3CN solution, as shown in Eq.
2. The grass-green color is the lithiation product of amorphous
FePO4 due to Li+ ions and e−, as shown in Eq. 3. When the amount
of LiI was less than 2.5 M, the unreacted amorphous phase of
FePO4 is reduced to Fe2P2O7 in the reduction conditions,13,14 as
shown in Eq. 4. When the amount of LiI was above 4.0 M, the
excess phosphatic ion will react with the LiI to form the Li3PO4
impurity phase.12 All the different amounts of LiI 共1.0, 2.5, 4.0, and
4.5 M兲 are according to the molar amount of FePO4
2LiI共CH3CN兲 → 2Li共+CH3CN兲 + 2I共−CH3CN兲
关1兴
Figure 2. 共Color online兲 The probable
lithiation processes with different amounts
of LiI used to prepare LiFePO4.
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Electrochemical and Solid-State Letters, 12 共6兲 A111-A114 共2009兲
A113
Figure 3. NyQuist plots of LiFePO4 and LiFePO4 /C.
2I共−CH3CN兲
→ I2共CH3CN兲 + 2e
−
关2兴
Figure 4. CV curve of Li/LiPF6 共EC/DMC兲/cathode over the 2.0–4.5 V
voltage range for LiFePO4 and LiFePO4 /C samples. The current density of
each sample measured for each oxidation peak as a function of cycle number
is also shown in the inset. Scan rate 0.5 mV s−1.
4FePO4共s兲 + 2Li共+CH3CN兲 + 2e− → 2LiFePO4共s兲 + 2FePO4共s兲 关3兴
2FePO4共s兲 → Fe2P2O7共s兲 + 1/2O2
关4兴
Charge–discharge studies.— The obtained discharge capacities
are 38.6, 141.6, 109.7, and 73.5 mAh/g at 0.1 C for LiFePO4 prepared with different amounts of LiI, 1, 2.5, 4.0, and 4.5 M, respectively. The results indicated that the lithiation effect of LiI and the
existence of impurities of Fe2P2O7 and Li3PO4 play a major role in
the electrochemical performance. As shown in Fig. 1, both LiFePO4
samples prepared using 2.5 and 4.0 M of LiI, respectively, appear to
be single phase, but the discharge capacities of both samples exhibit
values of 141.6 and 109.7 mAh/g, respectively. We have observed
more aggregation with increasing the amount of LiI. This may be
the reason for the decreasing capacity for samples prepared with 2.0
and 4.0 M LiI. Hence, we have selected the higher capacity sample
with 2.5 M LiI to prepare the carbon-coated LiFePO4 共LiFePO4 /C兲.
AC impedance analysis.— Figure 3 shows the NyQuist plots of
LiFePO4 and LiFePO4 /C samples. The high frequencies are shifted
in the negative direction on the real axis of the LiFePO4 /C sample.
All plots exhibit a depressed semicircle in the high-frequency region, which is attributed to the enhancement of the charge-transfer
process.
to determine the oxidation state of Fe;15 hence, the pre-edge and
near-edge features of LiFePO4 /C were compared at different voltages as shown in Fig. 5. The Fe pre-edge part 共labeled A in the inset兲
around 7112 eV corresponds to the 1s-3d transition, which is an
electric-dipole-forbidden process but quadrupole allowed.16 A weak
absorption at this energy is due to the mixing of the Fe 3d orbitals
with 4p orbitals 共3d-4p mixing兲 of the surrounded oxygen atoms,
which split the 3d orbital into t2g and eg states. The observed splitting of the 3d state can be assigned to the FeO6 octahedral crystal
field splitting of the t2g and eg energy level in the LiFePO4 /C structure. Comparison of the pre-edge feature of LiFePO4 /C during discharge with that of the standard LiFePO4 before discharge clearly
shows that the oxidation state of Fe in LiFePO4 has shifted from a
higher to a lower energy. It indicates that the reduction of Fe has
occurred in LiFePO4 /C during the lithium-ion insertion into the
crystal structure, which is in line with our assumption of intercalation of Li+ into the crystal structure. The near-edge part 共labeled B
in the inset兲 at 7114 eV as shown in Fig. 5 also shifted from higher
to lower values.
CV analysis.— Figure 4 shows the cyclic voltammograms of the
LiFePO4 and LiFePO4 /C samples at a scan rate of 0.5 mV s−1. The
current density is also shown in the inset of Fig. 4, which is measured for each oxidation peak as a function of cycle number. The
electrochemical properties of LiFePO4 and LiFePO4 /C were performed in the range of 2.0–4.5 V vs Li+ /Li at a 0.5 mV s−1 scan rate
for the 35 cycles. LiFePO4 without carbon showed much loss in
electrochemical activity during potential cycling up to 35 cycles.
However, LiFePO4 /C showed constant values of current density,
indicating the better and stable electrochemical performance of the
cathode film.
In situ XANES analysis.— Figure 5 shows room-temperature in
situ XANES results of LiFePO4 /C at different voltages used to
study the structural and electronic modification occurring around the
Fe center during the discharge process of LiFePO4 /C. Because
XANES can give valuable information about the local environment
around the Fe during potential cycling, we have recorded in situ
XANES at the Fe K-edge during the discharge process in order to
determine the electronic modifications occurring during the discharge of LiFePO4 /C. It is generally assumed that the pre-edge and
near-edge 共normalized absorption value 0.5兲 features are sufficient
Figure 5. In situ Fe K-edge XANES spectrum of LiFePO4 /C during the
discharge process.
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Electrochemical and Solid-State Letters, 12 共6兲 A111-A114 共2009兲
A114
4.5
4.0
3.5
200
3.0
Capacity (mAh/g)
Voltage (V)
0.1 C
LiFePO4
LiFePO4/C
2.5
2.0
LiFePO4
LiFePO4/C
0.2 C
150
100
50
processes are also investigated in detail. BET analysis showed that
the particle size of LiFePO4 /C was significantly reduced by addition
of carbon. The enhanced electrochemical activity can be attributed
to the decreased particle size and the formation of cavity architecture with carbon coating in the LiFePO4 /C. The improved charge
transfer due to carbon was also confirmed by CV, EIS, and a galvanic profile test. These results suggest that the carbon formed a
cavity architecture with carbon-coating in the LiFePO4 sample,
causing improvement in the electrochemical performance of cathode
materials. Such a modification route for reducing the particle size
and forming the cavity architecture of LiFePO4 could be useful for
its use as an alternative cathode material in reversible lithium batteries.
0
5
1.5
10
15
20
25
Cycle life (n)
0
20
40
60
Acknowledgments
80
100 120 140 160 180
Capacity (mAh/g)
Figure 6. Discharge profile of LiFePO4 and LiFePO4 /C at 0.1 C. The electrochemical cycling of LiFePO4 and LiFePO4 /C materials at 0.2 C is also
shown in the inset.
This work was supported by the National Science Council of
Taiwan under grant no. NSC 97-2113-M-002-012-MY3 and no. 962113-M-002-014-MY2.
National Taiwan University assisted in meeting the publication costs of
this article.
References
Galvanic profile analysis.— Figure 6 shows the discharge profiles from 2.0 to 4.2 V of LiFePO4 and LiFePO4 /C samples. The
electrochemical cycling at 0.2 C is also shown in the inset. A typical
flat plateau was observed at 3.45 V, characteristic of LiFePO4. The
discharge capacity for LiFePO4 and LiFePO4 /C samples at 0.1 C
was found to be 142 and 165 mAh/g, respectively. The LiFePO4 /C
sample showed capacity comparable to the maximum theoretical
capacity, indicating the enhancement of electrochemical properties
for the LiFePO4 sample by using the sol-gel route to get a smaller
particle size with cavity architecture. Electrochemical cycling of
these materials was performed during the discharge cycles at 0.2 C
rates. The stability of LiFePO4 /C was retained even after 25 cycles
but LiFePO4 decreased after 20 cycles, and the capacity of
LiFePO4 /C was higher than in the LiFePO4 sample. Thus, the
LiFePO4 /C sample was found to be highly stable at the same current
densities. It is reasonable to believe that the excellent performance
of the compound developed in our work can be attributed to the
unique microstructure, enhanced charge-transfer kinetics, and the
smaller particle size achieved by the sol-gel method.
Conclusion
A pure and highly crystalline form of LiFePO4 was prepared via
the sol-gel method by controlling the LiI amount. Probable lithiation
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