Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 2665–2673
Synthesis, characterization and electrochemical performance of
mesoporous FePO4 as cathode material for
rechargeable lithium batteries
Z.C. Shi 1 , A. Attia 2 , W.L. Ye, Q. Wang, Y.X. Li, Y. Yang ∗
Department of Chemistry and the State Key Lab for Physical Chemistry of Solid Surfaces,
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China
Received 1 April 2007; received in revised form 8 June 2007; accepted 25 June 2007
Available online 10 July 2007
Abstract
Mesoporous FePO4 could deliver enhanced specific capacity of 160 mAh g−1 at first discharge process, 90% of theoretical capacity of pure
FePO4 , and 135 mAh g−1 in the following cycles at 0.1 C rate. At 1 and 3 C rates, the capacities are 110 and 85 mAh g−1 , respectively, which
is much higher than that of previously reported for modified FePO4 materials. Electrochemical impedance spectroscopy (EIS) tests proved that
mesoporous structure in FePO4 materials enhanced the lithium ion intercalation/deintercalation kinetics as indicated by smaller charge transfer
resistance (Rct ) of these materials. These results revealed that this mesoporous electrode material can be a potential candidate for high-power energy
conversion devices.
© 2007 Published by Elsevier Ltd.
Keywords: Iron phosphate; Mesoporous materials; Cathode materials; Electrochemical performance; Energy conversion devices
1. Introduction
In recent years, nanostructured electrode materials, such as
mesoporous materials, have shown promising applications in
energy conversion devices [1–5]. Fe-based cathode materials,
such as Fe(III)PO4 and LiFe(II)PO4 , are attractive lithium intercalation electrode materials for their low cost, environmentally
friendly and high theoretical specific capacity [6–12]. FePO4
shows a discharge process from 3.5 V down to 2.5 V and a theoretical specific capacity of 178 mAh g−1 upon 1 mol of lithium
intercalation [6]. However, the practical specific capacity of
FePO4 is quite low due to the poor kinetics of lithium intercalation/deintercalation process. To improve the electrochemical
performance of FePO4 , Croce et al. added RuO2 to quartz FePO4
to enhance its specific capacity with a higher material interparticle electronic conductivity [8]. Both amorphous hydrated and
∗
Corresponding author. Fax: +86 592 218 5753.
E-mail address: [email protected] (Y. Yang).
1 Current address: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada.
2 On leave from Department of Physical Chemistry, National Research Centre,
El-Tahrir St., Dokki 12622, Cairo, Egypt.
0013-4686/$ – see front matter © 2007 Published by Elsevier Ltd.
doi:10.1016/j.electacta.2007.06.079
anhydrous FePO4 were also found to have better electrochemical performance than that of pure crystalline FePO4 [9–11]. The
specific capacity of FePO4 is still incomparable to its reported
theoretical one.
We first reported that importing mesoporous structure into
FePO4 , by a surfactant (EO20 -PO70 -EO20 , Pluronic P123)
self-assembly method resulted in enhancement of the electrochemical performance of FePO4 [2]. The preparation conditions
and electrochemical performance of the mesoporous FePO4
were also primarily optimized. Afterwards, mesoporous FePO4
with smaller pore diameter acted as a good storage material
especially as lithium ion intercalation cathode material [13]. By
using cetyltrimethyl ammonium bromide (CTAB) as template,
this mesoporous FePO4 had a mesostructure with a pore diameter of 3–4 nm as confirmed by small angle X-ray diffraction
compared with the results of Guo et al. [14], where the pore diameters were not large enough for the fast transport of electrolyte
and failed to improve the kinetics of lithium ion intercalation
with a result of relative low capacity.
In this work, systematic studies on synthesis, characterization and electrochemical performance of mesoporous FePO4
were conducted, aimed at improving FePO4 electrochemical
performance.
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the samples were suspended in ethanol solution by the help of
ultrasonic bath, then mounted on a carbon holder, and left to dry
before examination.
2. Experimental
2.1. Samples preparation
FePO4 samples were synthesized by a surfactant (EO20 PO70 -EO20 , Pluronic P123) self-assembly method similar to that
reported in our previous work [2]. After solvent evaporation,
precursor gels were firstly dried at 80 ◦ C for at least 10 h, and
then calcinated in a muffle furnace in air for 10 h. We synthesized FePO4 samples with different P123/(Fe + P) molar ratio
of 0.013, 0.026, 0.039, and 0.052 (named as 1P, 2P, 3P, and 4P)
at different calcination temperatures of 400, 450, 500, 600, and
700 ◦ C. Therefore, we got three series of FePO4 samples named
as follows—(1) 1P-FPO samples: 1P-400, 1P-450, 1P-500, 1P600, and 1P-700; (2) 3P-FPO samples: 3P-400, 3P-450, 3P-500,
3P-600, and 3P-700; (3) 450 ◦ C-FPO samples: 1P-450, 2P-450,
3P-450, and 4P-450.
The residual carbon in FePO4 samples was analyzed
by elemental analysis using an EA1110 (ThermQuest Italia
S.P.A., Italy) instrument, the precision of the measurements
was ±0.3%. X-ray diffraction (XRD) experiments were carried out by a Panalytical X-pert diffractometer (PANalytical,
The Netherlands), using Cu K␣ radiation (λ = 1.54059 Å).
Fourier transform infrared (FTIR) spectra were recorded on
an Avatar 360 spectrophotometer (Nicolet, USA) and the
resolution of spectra collected at 2 cm−1 interval over the
range measured of wavenumber. The mesoporous structure
information (specific surface area and pore diameter distributions) of FePO4 samples were obtained from N2 sorption
isotherm plot on TriStar3000 (Micromeritics, USA) based on
Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda
(BJH) equations, the error in determination of BET surface area
and porosity was in the range of ±10%. The structure and morphology of FePO4 samples were further characterized by high
resolution transmission electron microscopy (HRTEM) technique by using Tecnai F30 (Philips-FEI, Netherlands) where
2.2. Cell fabrication and testing
The electrochemical performance of FePO4 samples were
assessed using CR2025 coin cells. The preparation of the electrodes and the assembly of coin cells are the same as that reported
in Ref. [2], in which the electrolyte was 1 M LiPF6 dissolved
in EC + DMC (1:1, v/v) and all potentials mentioned in this
work were recorded versus Li/Li+ electrode. Charge–discharge
experiments were performed between 1.5 and 4.0 V at various
current densities (0.1, 1, 3 and 0.1 C) consecutively using a
LAND CT2001A Battery Cycler (Wuhan, China). Electrochemical impedance spectroscopy (EIS) experiments of these cycled
coin cells were done using an impedance/gain-phase analyzer
(Solartron SI 1260) combined with an electrochemical interface
(Solartron SI 1287) at an equilibrium charge/discharge state. The
impedance spectra were obtained by applying a 10 mV potential amplitude excitation over a frequency range from 106 to
10−3 Hz. Impedance data acquisition and analysis were performed by using the electrochemical impedance software ZPlot
and Zview (version 2.90, Scribner Associates Inc., USA).
3. Results and discussion
3.1. Samples synthesis and phase analysis
Table 1 shows the residual carbon in all synthesized samples
and surface properties of the samples. Table 1 shows that the
residual carbon is inversely proportional to the temperature used
for calcination of samples, i.e., by increasing the temperature
from 400 to 500 ◦ C, the amount of residual carbon decreased
from 1.48 to 0.94 wt.% in 1P samples, and similiarly for 3P
samples, the residual carbon decreased upon increasing the tem-
Table 1
Residual carbon and surface properties of FePO4 samples
Properties
P123/(Fe + P) molar ratio
400 ◦ C
Residual carbon content (wt.%) in FePO4 samples
1P
2P
3P
4P
1.48
N/A
1.79
N/A
Specific surfaces area (m2 g−1 ) of FePO4 samples
1P
2P
3P
4P
101
N/A
108
N/A
Pore volume (cm3 g−1 ) of FePO4 samples
1P
2P
3P
4P
0.28
N/A
0.38
N/A
Pore diameter (nm) of FePO4 samples
1P
2P
3P
4P
4.8
N/A
7.5
N/A
N/A signifies that these samples were not tested.
450 ◦ C
1.30
1.67
1.74
1.72
87
108
113
127
0.26
0.27
0.40
0.50
5.0
6.2
9.0
10.0
500 ◦ C
600 ◦ C
700 ◦ C
0.94
N/A
1.38
N/A
0.09
N/A
0.09
N/A
0.06
N/A
0.06
N/A
61
N/A
99
N/A
2.7
N/A
1.9
N/A
1.4
N/A
1.7
N/A
0.20
N/A
0.37
N/A
0.02
N/A
0.01
N/A
0.01
N/A
0.01
N/A
7.0
N/A
11.0
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
Z.C. Shi et al. / Electrochimica Acta 53 (2008) 2665–2673
perature from 400 to 500 ◦ C from 1.79 to 1.38 wt.%. Low carbon
content can be found in the samples synthesized at higher temperature of 600 and 700 ◦ C in our case. Accordingly, the specific
capacities of FePO4 samples have been corrected considering the
carbon content.
Analogously, as mentioned in our previous work [2], strong
peaks of trigonal FePO4 (ICDD 29-0715) can only be indexed
in XRD pattern of the FePO4 samples synthesized at 600 and
700 ◦ C (shown in supporting information files, Figs. 1–3), which
indicates that the crystallization of trigonal FePO4 occurred
at the temperature between 500 and 600 ◦ C, however, below
600 ◦ C, no crystalline peaks detected and amorphous phases
obtained instead [15]. This crystallization process proved to be
an endothermal peak on the DTA curve (shown in supporting
information files, Fig. 4). Some of phosphorous resources might
be lost during the synthesis process because of the possibility of
presence of some impurities such as Fe2 O3 and Fe3 PO7 in the
crystalline sample, where Fe2 O3 is apt to form easily in most
of the conditions including ours and Fe3 PO7 is detected in the
XRD (shown in supporting information files, Figs. 1 and 2) at
600 and 700 ◦ C.
It is well known that the vibrational motions of ionic crystal can be classified into internal and external optic modes.
In FePO4 , the internal modes come from the intramolecular
vibrations of the PO4 3− polyanion [16], and the external ones
originate in the lattice vibrations. Fig. 1 shows the FTIR spectra of 1P-FPO samples in the 1500–400 cm−1 range. Here, we
mainly discuss the internal modes of 1P-FPO samples. Similar to that of olivine FePO4 and other iron phosphate materials
[15,17,18], FTIR spectrum (Fig. 1, curve e) of trigonal FePO4
(1P-FPO-700) exhibited a broad maximum between 900 and
1200 cm−1 which can be assigned to the P–O vibrations of
PO4 3− polyanion. Band of 932 cm−1 came from the symmetric
vibration of P–O bond and the other bands from the asymmetric vibrations of P–O bond. In the range of 500–700 cm−1 ,
Fig. 1. FTIR spectra of the 1P-FePO4 samples calcinated at
(b), 500 ◦ C (c), 600 ◦ C (d), and 700 ◦ C (e) for 10 h.
400 ◦ C
(a),
450 ◦ C
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two medium bands at 595 and 633 cm−1 , and several weak
bands were all attributed to the asymmetric vibrations of O–P–O
bond. Band at 432 cm−1 and its shoulder at the right side were
attributed to the symmetric vibrations of O–P–O bond.
In the case of amorphous FePO4 , the bonding between all
kinds of atoms was only partially reserved, which resulted in
poor symmetric structure. Therefore, IR inactive vibrations of
trigonal FePO4 became active in amorphous FePO4 . Consequently, the two absorbed bands at about 1000 and 600 cm−1 in
amorphous mesoporous FePO4 prepared at 400, 450 and 500 ◦ C
(Fig. 1, curves a–c) were broadened. Moreover, the maximum
frequency of the broad band shifted from 1027 to 1060 cm−1 ,
which indicates a new dominating vibration of P–O bond at
1060 cm−1 in the amorphous mesoporous FePO4 . The FTIR
spectra of 3P-FPO samples showed the same result as that of
1P-FPO samples (shown in supporting information files, Fig.
5).
3.2. Mesoporous structure of FePO4 samples
The nano-structured characteristics of mesoporous FePO4 are
largely affected by preparation conditions. The FePO4 samples
prepared at or below 500 ◦ C show typical type IV isotherm plots
in the N2 sorption experiments (shown in supporting information
files, Figs. 6–8), indicating the existence of mesoporous structure
of the FePO4 samples. Figs. 2 and 3 show the pore diameter
distribution plot of 3P-FPO samples and all FPO samples at
450 ◦ C, respectively. Table 1 shows also the specific surface
area, pore volume and pore diameter of all FePO4 samples under
different conditions.
When the calcination temperature of 1P-FPO samples
increased from 400 to 500 ◦ C, the specific surface area decreased
from 101 to 61 m2 g−1 , and pore volume decreased from 0.28 to
0.20 cm3 g−1 . On the contrary, the pore diameter increased from
4.8 to 7.0 nm. Mesoporous information could not be obtained for
1P-FPO-600 and 1P-FPO-700 samples, and the specific surface
area decreased significantly to 2.7 and 1.4 m2 g−1 , respectively.
This indicated that the mesoporous structure collapsed dur-
Fig. 2. Pore diameter distribution plots of FePO4 samples calcinated at 400 ◦ C
(), 450 ◦ C (), 500 ◦ C (), 600 ◦ C (), and 700 ◦ C (♦) for 10 h with
P123/(Fe + P) = 0.039(3P).
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Fig. 3. Pore diameter distribution plots of FePO4 samples calcinated at 450 ◦ C
for 10 h with P123/(Fe + P) = 0.013 (, 1P-450), 0.026 (, 2P-450), 0.039 (,
3P-450) and 0.052 (, 4P-450). The average pore diameters are 5.0, 6.2, 9.0,
and 10.0 nm, respectively.
ing the calcination process in the temperature between 500
and 600 ◦ C, which accompanied with the formation of trigonal
FePO4 .
For 3P-FPO samples calcinated at temperature from 400
to 500 ◦ C, the specific surface areas decreased from 108 to
99 m2 g−1 , respectively; which is expected due to the anticipated
crystal growth of the FePO4 particles upon temperature increase.
The pore volumes were found to be 0.38 and 0.37 cm3 g−1 ,
while the pore diameters were 7.5 and 11.0 nm, respectively.
Mesoporous information could not be obtained for samples 3PFPO-600 and 3P-FPO-700 (Fig. 2). It is obvious from the above
data and data in Table 1 that, at the same P123/(Fe + P) molar
ratio, the specific surface area and the pore volume decreased
with calcination temperature, and the pore diameter distribution
broadened at the same time. Moreover, samples prepared at 600
and 700 ◦ C did not show any mesoporous information because
the mesoporous structure collapsed upon the crystallization of
FePO4 and growth of the particles between 500 and 600 ◦ C.
As for 450-FPO samples, with the P123/(Fe + P) molar ratio
increased from 1P to 2P, 3P and 4P, the specific surface area
increased from 87 to 108, 113 and 127 m2 g−1 , respectively,
Fig. 4. TEM images of mesoporous FePO4 samples calcinated at 450 ◦ C for 10 h with P123/(Fe + P) = 0.013 (a, 1P-450), 0.026 (b, 2P-450), 0.039 (c, 3P-450), and
0.052 (d, 4P-450).
Z.C. Shi et al. / Electrochimica Acta 53 (2008) 2665–2673
while the pore volume from 0.26 to 0.27, 0.40 and 0.50 cm3 g−1 ,
respectively and the pore diameter from 5.0 to 6.2, 9.0 and
10.0 nm, respectively as shown in Table 1. Therefore, under the
same calcination condition of temperature and time, the specific
surface area, pore volume and pore diameter were enlarged with
the increase of the surfactant ratio, although the pore diameter
distribution got broadened and the uniformity of pore structure
decreased (Fig. 3).
Fig. 4 shows the HRTEM images of mesoporous FePO4 samples prepared at 450 ◦ C with different four P123/(Fe + P) ratios.
The particle sizes were found to be in the range of 100–200 nm
and meso-structure can be observed in all of the four samples,
however, the mesoporosity was not of organized structure as
indicated by HRTEM (Fig. 4). 1P-450 and 2P-450 samples
synthesized under smaller P123/(Fe + P) molar ratio, showed
smaller pore diameters than the other two samples of 3P-450
and 4P-450 of higher P123/(Fe + P) molar ratio and they showed
more homogeneous pore distribution. However, HRTEM image
of sample 4P-450 revealed destructed pore structure. For the
highest P123/(Fe + P) molar ratio, it seemed difficult for inorganic compounds in the pore wall to support the mesostructure
with large pore diameter.
3.3. Electrochemical performance of mesoporous FePO4
cathodes
In our electrochemical tests, the typical voltage profile of
all FePO4 samples, with an average discharge voltage at 3.0 V
(shown in supporting information files, Figs. 9–11), is similar
to that results of reported FePO4 materials [8–11]. Figs. 5 and 6
show the specific discharge capacity upon cycling of 1P and
3P FePO4 samples calcinated at different temperatures and at
different current density. All FePO4 samples prepared at 600
and 700 ◦ C delivered low capacity of 40–50 mAh g−1 (shown
in supporting information files, Fig. 12). This inactive behavior might be attributed to the presence of a glassy surface
phase [9,19] or due to presence of some impurities such as
Fe2 O3 and Fe3 PO7 , where Fe2 O3 is possibly formed easily
Fig. 5. Specific discharge capacity upon cycling of 1P-FePO4 samples (1P-400
(), 1P-450 (), 1P-500 () and 1P-600 ()) at 0.1, 1, 3 and 0.1 C.
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Fig. 6. Specific discharge capacity upon cycling of different FePO4 samples
(3P-400 (), 3P-450 (), 3P-500 () and 3P-600 ()) at 0.1, 1, 3 and 0.1 C.
in our conditions in small concentration below the detection
limit of XRD technique, and Fe3 PO7 was detected by XRD
(Figs. 1 and 2 in supporting information files) and might be
responsible for the poor lithium ion intercalation/deintercalation
kinetics in the crystalline trigonal FePO4 .
In agreement with our recent results [2], the specific capacities of 1P-FPO samples were not highly improved (Fig. 5). 1P450 and 1P-500 samples showed initial capacity of 140 mAh g−1
at 0.1 C at the first discharge progress, which suffered an
irreversible capacity of about 40 mAh g−1 and decreased to
approximately 100 mAh g−1 at the subsequent cycles.
In comparison with 1P-FPO samples, 3P-FPO samples
showed better initial capacity and less irreversible capacity upon
subsequent cycling. This behavior can be associated to the higher
specific surface area, larger pore volume and larger pore diameter, which obtained upon increasing the P123/(Fe + P) molar
ratio (Fig. 6). Among 3P samples, mesoporous 3P-450 samples delivered the highest discharge capacity of 159 mAh g−1 at
0.1 C rate, which reached 93.5% of the theoretical capacity of
FePO4 . Even though, 3P-450 sample showed initial discharge
capacity less than that of 3P-400. In the subsequent cycles, the
average discharge capacity of 3P-450 was 135 at 0.1 C rate, 110
at 1 C rate, and 85 mAh g−1 at 3 C rate which is higher than
that of RuO2 -added quartz FePO4 (110 mAh g−1 at C/3 rate) [8]
and of amorphous FePO4 (70 mAh g−1 at 1 C rate) [9,10,15,20].
3P-400 samples showed a much lower capacity, because the calcination temperature was not high enough for completing the
reaction. Although 3P-500 samples had a mesoporous structure,
they delivered a lower capacity which might be a result for its
smaller specific surface area and pore volume.
From the above results, we can conclude that the best electrochemical properties obtained for the P123/(Fe + P) ratio was at
temperature of 450 ◦ C among 3P materials and it was of 500 ◦ C
for the 1P materials. This implied that the ideal calcination temperature for synthesizing mesoporous FePO4 with the highest
specific capacity was around 450–500 ◦ C and is affected by the
molar ratio of P123/(Fe + Fe).
In order to distinguish the impact of surfactant molar ratio on
the mesostructure of FePO4 samples, we synthesized a series of
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Fig. 7. Specific discharge capacity upon cycling of different FePO4 samples
(1P-450 (), 2P-450 (), 3P-450 () and 4P-450 ()) at 0.1, 1, 3 and 0.1 C.
mesoporous FePO4 at 450 ◦ C with different P123/(Fe + P) molar
ratios. Then the cathode performance of these samples with different specific surface area and pore diameter were investigated
by charge/discharge measurements at various rates as shown in
Fig. 7. Cell tests proved that 3P-450 sample delivered the highest capacity of 135 mAh g−1 at 0.1 C rate, while 2P-450 and
4P-450 samples only delivered a capacity of 115 mAh g−1 , and
the capacity of 1P-450 decreased to 90 mAh g−1 . On further
cycling at high rates of 1 C and 3 C, 3P-450 sample delivered
the highest capacity of 110 and 85 mAh g−1 , respectively, while
when cycled at 0.1 C rate again, all samples could be recovered the original high capacity, which confirmed the stability of
mesoporous structure during cycling. 3P-450 sample exhibited
the best cathode performance which might be explained due to
its larger specific surface area and pore diameter which might
facilitate the fast transport of electrolyte and lithium ion intercalation/deintercalation, than that of 1P-450 and 2P-450 samples.
Even sample 4P-450 possessed the highest specific BET surface
area, pore volume and pore diameter, it is not understandable
why it has lower capacity than that of 3P-450 sample. However, 3P-450 showed better electrochemical performance when
cycled compared to the rest of the other materials. At this point,
even it is speculative; one can think about that there is ideal pore
diameter which allows the fastest kinetics of lithium intercalation/deintercalation, after it, the lithium intercalation kinetics
decreases again.
3.4. EIS characterization of lithium batteries based on
mesoporous FePO4 cathodes
One of the advantages of EIS is that it can study the
surface of the electrodes in question compared to cyclic voltammetry which can only study the bulk of the electrode. EIS
as non-transient technique not alter the electrode-interface
which make this technique more precise one. In this work,
to study the effect of mesoporous structure on the lithium
intercalation/deintercalation process of FePO4 cathodes, EIS
Fig. 8. Typical EIS of (a) Li/meso-FePO4 cell from 106 to 10−3 Hz at 25 ◦ C,
insets show its high-medium frequency segments and (b) equivalent circuit of
impedance for this cell ( experimental data; — fit result of experimental data).
experiments carried out on the above coin cells at equilibrium
state after 20 charge/discharge cycles. Fig. 8(a) presents a typical
EIS of Li/mesoporous FePO4 coin cells at 2.87 ± 0.01 V after
20 charge/discharge cycles. The impedance behavior included
three semicircles followed by linear part. The first semicircles
were at high frequency range of 106 to 5000 Hz and the second
semicircle at 5000–30 Hz (Fig. 8(a), lower inset) while the third
semicircle at 30–0.2 Hz (Fig. 8(a) upper inset) followed by a
linear part at very low frequency of 0.2–0.01 Hz.
The fitting circuit used in this work is shown in Fig. 8(b),
where Re represented the electrolyte resistance; CPE1 and R1
represented the elements of the interfacial layer between the
FePO4 -carbon composite and the electrolyte and they signify
the capacitive and resistive contribution from the surface film;
while CPE2 and R2 represented the bulk capacitive and resistive
contribution; and the CPE3 and R3 represented the double layer
capacitance and charge transfer resistance, respectively. Since
the diffusion coefficient was out of the interest in this discussion,
we did not fit the linear part of the impedance. Fig. 8(a), showed
that the non-linear curve fitting were in high accordance with
the experimental data and that the χ2 and weighted Σ 2 were of
0.000419 and 0.0528, respectively (Table 1 in supporting information files).
True capacitance and constant phase element (CPE) are
related to each other according to the following equation:
C = Q0 (max )n−1
(1)
The true capacitance is equal to CPE (its magnitude represented by Q0 if n = 1), and CPE capacitance properties decreases
when n approaches zero, and ωmax is the angular frequency at
which maximum imaginary impedance attains.
Z.C. Shi et al. / Electrochimica Acta 53 (2008) 2665–2673
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Fig. 9. EIS of fresh Li/Li coin cells after two cycles () and seven cycles ()
of cyclic voltammetry between 0.5 and −0.5 V at frequency range of 1–5 mHz.
The reasons of having depressed semicircles may be
attributed to non-flat electrode surface as early discussed by the
pioneer work of de Levie [21], fractal geometry of the electrode
was proposed recently as another reason for this depressed semicircle [22] which in consent of the early work of de Levie. The
existence of depressed semicircle in this work is highly probably due to the mesoscopic nature of FePO4 material. Besides,
since the electrolyte decomposition cannot be excluded, it may
contribute to this behavior as well. Other contribution can originate from the formation of solid electrolyte interface (SEI) which
can be formed, as reported previously, very fast on carbonaceous
materials [23], and on oxide materials [24–26] especially at the
early stages of the lithium intercalation or deintercalation [25].
SEI formation can be a direct interpretation of why Rct increase.
The formation of SEI is likely to be multilayer of compact and
porous in nature and can be formed during cycling [27,28]. To
ascertain that the main contribution of the passivity and charge
transfer resistance is mainly originated from the LiFePO4 and
not from the lithium anode, a symmetrical coin cell containing
Li as working and reference electrodes where used and studied
by the EIS (Fig. 9) after different cycles of cyclic voltammetry.
Fig. 9 shows that the contribution of the impedance from Li/Li
coin cell is negligible compared to that one of the total impedance
of Li/FePO4 coin cells (Fig. 8), which support that the main
impedance is originated from the mesoporous Li/FePO4 but not
from Li anode.
Figs. 10 and 11 evaluate the EIS of Li/mesoporous FePO4
(450-FPO) cell to that of Li/crystalline FePO4 (3P-700) cell at
2.86 ± 0.01 V after 20 cycles in the frequency range from 106
to 10−3 Hz at 25 ◦ C. Fitting results of all the EIS data using
the appropriate equivalent circuit mentioned (Table 1 in supporting information files). The χ2 and Σ 2 of fitting were low
enough for us to confirm our appropriate choice of the equivalent
circuit. Moreover, all Re (electrolyte resistance) were smaller
than 10 with small difference, which could be due to the
tiny difference of cell assembly and the interaction between the
electrolyte and the dendritic formation of SEI after 20 cycles
of charging–discharging and all CPE1 and CPE2 were on the
same magnitude of 10−5 F because all of them originated from
the same source, i.e., surface and bulk of electrode materials,
respectively, while all CPE3 reached the magnitude of 10−3 F
because it represented the charge transfer process ensued inside
active cathode materials.
Similarly to that of Li/mesoporous FePO4 (3P-450 sample)
cell, Li/non-mesoporous crystalline FePO4 (3P-700 sample) cell
Fig. 10. (a) EIS of Li/FePO4 cell based on 3P-450 () and 3P-700 () samples
at 2.86 V (open-circuit voltage) from 105 to 5 × 10−3 Hz at 25 ◦ C and (b) shows
their high-medium frequency segments.
also showed an EIS which had the characteristic of two depressed
semicircles and straight lines (Fig. 10(a)). Due to the large specific surface area (113 m2 g−1 ) and residual carbon (1.72 wt.%),
many surface reactions with electrolyte arose on the particle surface of 3P-450 mesoporous FePO4 sample, which hence resulted
Fig. 11. EIS of Li/FePO4 cell based on 1P-450 (), 2P-450 (), 3P-450 ()
and 4P-450 (䊉) samples at 2.86 ± 0.01 V (open-circuit voltage) at 25 ◦ C: (a)
shows the full spectra from 106 to 10−3 Hz and (b) shows their high-medium
frequency segments.
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Z.C. Shi et al. / Electrochimica Acta 53 (2008) 2665–2673
Table 2
Fitting results of EIS in Figs. 10 and 11
Sample
Chi-square
Sum-square
Re ()
R1 ()
CPEl-T
CPE1-P
R2 ()
CPE2-T
CPE2-P
R3 ()
CPE3-T
CPE3-P
1P-450
2P-450
3P-450
4P-450
3P-700
2.0E−3
1.5E−3
0.5E−3
0.4E−3
0.6E−3
0.31
0.23
0.07
0.08
0.08
3.2
3.6
9.4
5.6
8.9
6.1
5.2
11.0
8.4
14.8
1.2E−5
0.3E−5
1.6E−5
5.4E−5
5.5E−5
0.93
0.91
0.78
0.72
0.69
271
228
73
152
22
1.3E−5
1.2E−5
2.5E−5
1.8E−5
1.8E−5
0.75
0.82
0.86
0.85
0.87
1015
426
210
338
625
1.7E−3
1.7E−3
1.5E−3
1.9E−3
2.0E−3
0.70
0.79
0.80
0.75
0.72
in a larger surface impedance on the surface layer (Fig. 10(b)).
Simultaneously, the mesoporous structure and carbon residual in
those mesoporous FePO4 samples enhanced lithium ion intercalation kinetics, which resulted in a smaller Rct than that of
3P-700 sample. At the same time, the increase of the temperature
resulted in growth of the particles which can enhance the resistivity according to lose of the interconnectivities to the carbon
material.
The mesoporous FePO4 samples enhanced electrochemical
performance because of a small Rct during lithium intercalation/deintercalation process which probably due to the addition
of carbon which can enhance the FePO4 electronic conductivity.
To ascertain that the mesoporous is the key factor of enhancement of the electronic conductivity and not the residual carbon
in the samples, we recall that P-450 samples, the carbon content
were similarly with minor differences (Table 1), especially the
last three samples (2P-450, 3P-450 and 4P-450) where carbon
content were 1.67, 1.74 and 1.72 wt.%, respectively. Moreover,
additional carbon of 20 wt.% in the form of carbon black was
mixed with 75 wt.% FePO4 and 5 wt.% PVDF during the preparation of FePO4 composite electrode. Therefore, the difference
of carbon content in these samples gets more negligible. The
composite carbon is therefore, took a part in improving the conductivity between the particles of the FePO4 while the added
carbon during the electrode fabrication was to enhance the
conductivity between the current collector and the composite
material. On the other hand, the mesoporosity was the main feature responsible for enhancement of conductivity of the FePO4
material. Fig. 11(a) shows the EIS of Li/mesoporous FePO4
cells based on 1P-450, 2P-450, 3P-450 and 4P-450 samples at
2.86 ± 0.01 V in the frequency range from 106 to 10−3 Hz at
25 ◦ C. From the high-medium frequency segments (Fig. 11(b)),
we can notice that 3P-450 sample has the smallest impedance,
as a result of its improved electrochemical performance. Fitting
results of these EIS data indicated that FePO4 cathodes with different mesoporous structure had different impedance response
(Table 2), with the surfactant ratio increased, specific surface
area, pore volume and pore diameter of mesoporous FePO4
samples were increased (Table 1), which initiated the Rct to
lessen because large specific surface area could provide large
active sites for lithium ion intercalation/deintercalation, large
pore channels could facilitate fast transport of electrolyte containing lithium ion, and large pore volume could accommodate
more electrolyte and can lead to thin pore wall which reduce the
resistance of lithium ion intercalation/deintercalation into/from
inner active FePO4 materials. For 4P-450 sample, though possessed the largest specific surface area, pore volume and pore
diameter, it exhibited larger Rct than that of 3P-450 sample
because of its less homogeneous pore distribution. Therefore, in
general, large specific surface area, pore sizes, pore volume, and
homogeneous pore distribution in the FePO4 cathode material
will improve aptly its electrochemical performance (Table 1).
Our results of impedance, even it is for two-electrode system, resemble that of the three-electrode system especially after
charging–discharging. This observation was in agreement with
the work of Zane et al. [29] where they showed that the only
difference between three-electrode system and two-electrode
system is only a small shift in the Rct values, being lower for the
three-electrode system.
4. Conclusions
In summary, poor lithium intercalation/deintercalation kinetics of FePO4 could be improved by importing a mesoporous
structure, which could be tuned by changing template ratio and
calcination temperatures. EIS tests proved that this mesoporous
structure could enhance their electrochemical performance,
especially at high current density, as indicated by low values
of Rct of FePO4 cathodes. The results demonstrated the role of
mesoporous structure in improving the fast transport and intercalation kinetics of lithium ions in the FePO4 materials. Moreover,
this work presented a new method for preparing new materials
with good rate of performance for batteries and other energy
storage devices such as electrochemical supercapacitors.
Acknowledgments
The financial supports from the National Natural Science
Foundation of China (Nos. 29925310 and 20021002) and the
Ministry of Science and Technology of China (2001CB610506)
are acknowledged.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.electacta.2007.06.079.
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