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Synthesis and characterization of in situ Fe2O3-coated FeF3 cathode materials
for rechargeable lithium batteries
Wei Zhang, Lin Ma, Hongjun Yue and Yong Yang*
Received 6th July 2012, Accepted 28th September 2012
DOI: 10.1039/c2jm34391f
Here, a novel architecture of a core–shell structured FeF3@Fe2O3 composite with particle size of
100–150 nm and tunable Fe2O3 content is synthesized by a simple heat treatment process utilizing FeF3
with fine network structure as precursor. The structure, morphology and electrochemical performance
of the pristine FeF3 and the FeF3@Fe2O3 composites are studied by XRD, SEM, TEM and discharge–
charge measurements. XRD results show that the Bragg peaks of the FeF3@Fe2O3 composites are well
indexed to FeF3 and Fe2O3. SEM and TEM images reveal the core–shell structure of the composites.
The comparison of the electrochemical performance between the pristine FeF3 and the FeF3@Fe2O3
composites reveals that the in situ Fe2O3 coating (even with small amount, 0.6–5.2 wt%) has great
influence on the improvement of electrochemical performance.
1. Introduction
Li-ion batteries have been widely used in portable electronic
devices and started many demonstration tests in electric vehicles,
smart-grid energy storage and load-leveling.1–7 Cathode materials
based on intercalation reactions (LiCoO2, LiFePO4 and
LiMn2O4, etc.) exhibit good electrochemical performance such as
cycling stability and rate capability; however, they provide limited
specific capacities which are determined by their crystal vacancies
for lithium intercalation without structural collapse. In contrast,
cathode materials based on conversion reactions show very large
specific capacities, because, unlike intercalation reactions, all of
the oxidation states of the active materials can be utilized.1–3,5
Theoretical specific energy density of the material is the
product of operating voltage and capacity. Of all the MxNy (M ¼
Co, Fe, Ni, Cu, etc.; N ¼ F, O, S, N, etc.) conversion materials,
only metal fluorides give high operating voltages and can be used
as cathode materials.6 The utilization of metal fluoride as
cathode materials can be traced back to the late 1960s.5 Up to
now, a great number of metal fluoride based cathode materials
have been studied;2,5,8–11 among which FeF3 is of particular
interest and is considered as one of the most promising class of
fluorides to maximize the energy density of rechargeable electrochemical cells. Because iron-based materials present distinct
advantages of low cost and low toxicity and the utilization
of three lithium ions resulting in high theoretical capacity of
712 mA h g1.5–7,12 The first application of FeF3 in rechargeable
lithium batteries was reported by Arai.8 The reported discharge
capacity of FeF3 compound was not high; about 140 mA h g1
State Key Laboratory of Physical Chemistry of Solid Surfaces and
Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen, 361005, China. E-mail:
[email protected]; Tel: +86-592-218-5753
This journal is ª The Royal Society of Chemistry 2012
for the first cycle and 80 mA h g1 for the second cycle between
2.0 and 4.5 V with a constant-current density of 0.2 mA cm2,
equivalent to 60% and 34% of the theoretical 237 mA h g1
respectively. The poor electrochemical performance is as a result
of the poor electronic conductivity of FeF3, associated with the
large band gap (5.96 eV (ref. 13)). Consequently, almost all
subsequent efforts on the improvement of the electrochemical
performance of FeF3-based materials either in electrochemical
reversibility or in rate capability are focused on increasing the
electronic conductivity and reducing the particle size.2–5,7,9,10,14–16
For example, Badway and co-workers3 reported a high-energy
mechanical ball-milling method by which metal fluorides were
mixed with conductive additives to form composites with smaller
crystallite size and an ex situ coating layer. Available conductive
additives include carbon materials (graphite, carbon black, activated carbon, etc.), metal oxides and metal sulfides (V2O5, MoO3,
MoS2, etc.). Their method has been applied successfully for the
synthesis of FeF3/C,3 BiF3/C,2 CuF2/C,10 FeF3/V2O5,9 CuF2/
MoO3,10 and BiF3/MoS2.10 The reversible capacity of the FeF3/C
composite obtained by Badway et al.3 was 367 mA h g1 at 22 C or
660 mA h g1 at 70 C in the 1.5–4.5 V region with a current rate of
7.58 mA g1. Besides this mechanical ball-milling method, several
methods based on wet chemistry have also been explored. Li15,16
reported a nanosized FeF3$0.33H2O material with a mesoporous
morphology prepared by an ionic-liquid-based synthesis. The
FeF3$0.33H2O material delivered about 140 mA h g1 at a current
rate of 35 mA g1 in the 4.5–1.6 V region. Kim4 reported a FeF3(H2O)x/CNTs composite by a HF-solution based synthesis utilizing
carbon nanotubes (CNTs) as conductive additive. Mechanical ballmilling with carbon black was carried out to further enhance the
electronic conductivity of the FeF3(H2O)x/CNTs composite.
However, the methods based on wet chemistry appear not appropriate for the synthesis of anhydrous FeF3-based materials.
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Previous work on FeF3/C (ref. 3) suggests that the ex situ
coating layer of conductive additives is essential for the synthesis
of FeF3/C composites with good electrochemical performance.
However, could we do an in situ coating layer of conductive
additives on FeF3? How about the effects of this coating method?
Apparently, such exploration is worthwhile in the development
of high-performance FeF3 cathode materials. In this paper, the
synthesis and characterization of an anhydrous core–shell
structured FeF3@Fe2O3 composite with in situ Fe2O3 coating are
reported with the results showing that the coating Fe2O3 layer is
effective for the improvement of electrochemical performance.
To the best of our knowledge, this is the first report on in situ
oxide coating on metal fluoride as cathode material. Due to the
fact that Fe2O3 is a semiconductor with lower band gap (2.0–2.1
eV (ref. 17)) and higher electronic conductivity than those of
FeF3, the coating is expected to improve the electronic conductivity of the FeF3@Fe2O3 composite materials.
2. Experimental
on Tecnai F30 (Philips-FEI, Netherlands), operated at an
accelerating voltage of 300 kV.
Electrochemical characterization
Electrochemical characterization of the cathode materials was
carried out with CR2025 coin-type electrochemical cells. The
cathodes were prepared by mixing FeF3@Fe2O3 composite,
acetylene black, and poly(vinylidene fluoride) in the weight ratio
of 62.5 : 25 : 12.5. 1-Methyl-2-pyrrolidinone was used as the
solvent. The slurry was pasted on aluminum current collector
and dried at 120 C for 1.5 h. The coin cells were assembled in an
argon-filled glove box with the prepared cathode, lithium anode,
Celgard 2300 separator, and 1 M LiPF6 in EC/DMC (1 : 1, v/v)
electrolyte. Cell testing was carried out at a current rate of 50 mA
g1 at 30 C using a Land CT2001A system. The charge–
discharge capacities are based on the mass of the total FeF3@Fe2O3 composites. The electrochemically induced structural
development was investigated by ex situ XRD.
Synthesis of FeF3 and FeF3/Fe2O3
A sol–gel process was adopted to synthesize the Fe2O3 precursor.
Briefly, 0.02 mol Fe(NO3)3$(H2O)9 and 0.04 mol citric acid were
dissolved in 25 ml de-ionized (DI) water and 7.2 ml NH4OH
solution (25%) was dropped slowly into the solution to adjust the
pH to ca. 7. The solution was then dried at 60 C for 2 days and
at 135 C for another 2 days to finally obtain a dark gel. The gel
was initially heated at 180 C for 2 h under air and then heated at
500 C for another 5 h to obtain the red Fe2O3. The Fe2O3
precursor was then fluorinated into FeF3 completely at 475 C
for 5 h under an inert gas containing fluorine. Finally, by heating
the FeF3 at 500 C under air for 15 to 480 s, the surface FeF3 was
transformed into Fe2O3 and FeF3@Fe2O3 core–shell composites
with in situ Fe2O3 coating were obtained. For convenience, the
FeF3@Fe2O3 composites obtained by heating for 15 s, 30 s, 60 s,
120 s, 240 s and 480 s are labeled as C15, C30, C60, C120, C240
and C480, respectively. All the reagents used in the experiment
were of analytical purity and purchased from Sinopharm
Chemical Reagent Co., Ltd. (SCRC) and used without further
purification.
3. Results and discussion
The XRD patterns and full pattern refinement of pure FeF3 are
shown in Fig. 1. All the Bragg peaks are well indexed to rhombohedral phase FeF3 (space group R
3c, PDF no. 00-033-0647).
No impurity phases were detected in our synthesized samples.
The lattice parameters refined with the GSAS program are a ¼ b
c ¼ 13.2794 A,
and V ¼ 313.105 A
3. The
¼ 5.2178 A,
morphology of the pure FeF3 is shown in Fig. 2; FeF3 particles
are connected with each other forming a fine network structure
with single particle size of about 100–150 nm. Such structure
favours gas diffusion and enables the FeF3 particles to full
contact with air, which is essential for the subsequent heat
treatment processes during the synthesis of the FeF3@Fe2O3
composites.
The light green color of pure FeF3 gradually turns to brown
when heated at 500 C under air. Fig. 3 shows the colors of the
FeF3@Fe2O3 composites at different heating time. Obviously,
Structural and morphological characterization
The XRD patterns for the FeF3@Fe2O3 composites with
different amounts of Fe2O3 were obtained by Panalytical X-pert
diffractometer (PANalytical, Netherlands) with Cu Ka radiation
operated at 40 kV and 30 mA using a step size of 0.00835 with a
counting time of 30 s per step. The ex situ XRD was accomplished by disassembling the electrochemical cell in an argonfilled glove box and rinsing the cathode with dimethyl carbonate
(DMC). The ex situ XRD patterns for the cathodes was obtained
by Ultima IV X-ray diffractometer (Rigaku, Japan) with Cu Ka
radiation operated at 35 kV and 15 mA with a scan speed of 10
min1. Rietveld refinement was performed using the General
Structure Analysis System (GSAS) program to obtain the crystal
structure parameters. Scanning electron microscopy (SEM)
studies were performed on S-4800 (HITACHI, Japan).
Transmission electron microscopy (TEM) and high resolution
transmission electron microscopy (HRTEM) were performed
24770 | J. Mater. Chem., 2012, 22, 24769–24775
Fig. 1 Rietveld refinement of pure FeF3. Experimental XRD pattern
(black cross) compared with the Rietveld-refined profile (red line) and
difference curve (blue line).
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Fig. 2 SEM images of the FeF3@Fe2O3 composites: (a and b) FeF3, (c)
C30, (d) C60, (e) C120 and (f) C240.
Fig. 4 XRD patterns of the FeF3@Fe2O3 composites. FeF3 and Fe2O3
phases are denoted by (hkl) and (hkl) respectively. ICDD standards for
FeOF (PDF no. 01-070-1522), Fe2O3 (PDF no. 01-086-0550) and FeF3
(PDF no. 00-033-0647) are shown for reference.
that the FeF3@Fe2O3 composites simply consist of several phases, i.e., FeF3, Fe2O3 and FeOF (the clue to the existence of
FeOF phase will be discussed in the following sections). The
reactions that happened are as follows:
Fig. 3 Coloration for the FeF3@Fe2O3 composites at different heating
time: (a) 0 s (FeF3), (b) 15 s, (c) 30 s, (d) 60 s, (e) 120 s, (f) 240 s, (g) 480 s
and (h) 5 h (Fe2O3).
the longer the heating time, the darker the color of the
composites. In fact, at 5 h heating time, the FeF3 is totally
transformed into red-brown Fe2O3, confirmed by XRD patterns
(see Fig. 4). The patterns of the FeF3@Fe2O3 composites change
systematically with the heating time. The pattern for C15 and
C30 are similar to that of pure FeF3; yet no Fe2O3 Bragg
reflections were observed. This is mainly due to the fact that the
amount of Fe2O3 is too small (see Table 1). For C60 or C120, the
Bragg peak (110) of Fe2O3 is clearly visible, while the others, such
as (012), (104), (116), are heavily overlapped with the peaks of
FeF3. For C240 and C480, all the Bragg peaks of Fe2O3 are
visible. The most important point to note is that irrespective of
the heating time, there is no change in the positions of all the
Bragg peaks but a significant decrease/increase in the relative
intensities, suggesting that there is no solid solution formed and
This journal is ª The Royal Society of Chemistry 2012
4FeF3 + 3O2 / 2Fe2O3 + 6F2
(1)
FeF3 + Fe2O3 / 3FeOF
(2)
The transformation of FeF3 to Fe2O3 has been demonstrated
by XRD (see Fig. 4). It has been reported that FeOF can be
synthesized with FeF3 and Fe2O3 by reaction (2) under high
temperature.18 The mass fractions of FeF3, Fe2O3 and FeOF of
as-prepared FeF3@Fe2O3 composites can be calculated by a
Table 1 The amount of each component of the FeF3@Fe2O3 composites
at different heating time
Sample
Heating time/s
FeF3 (wt%)
Fe2O3 (wt%)
FeOF (wt%)
FeF3
C15
C30
C60
C120
C240
C480
Fe2O3
0
15
30
60
120
240
480
5 ha
100
99.4
98.3
94.6
88.1
74.6
52.6
0
0
0.6
1.7
5.0
10.7
23.6
45.5
100
0
—
—
0.4
1.2
1.8
1.9
0
a
h denotes hour.
J. Mater. Chem., 2012, 22, 24769–24775 | 24771
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simple way. The data that we need for the calculation is the mass
of raw FeF3 (m) and the mass of as-prepared FeF3@Fe2O3
composite (n). Then the mass fractions of FeF3 and Fe2O3 can be
obtained by following the below formula based on reaction (1):
0
u1 ¼
2M1
M2
m
m
$ ¼ 3:420 2:420
2M1 M2 2M1 M2 n
n
0
0
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u2 ¼ 1 u1
0
(3)
(4)
0
where M1 and u1, M2, and u2 denote the molecular weights and
0
mass fractions of FeF3 and Fe2O3 respectively. Note that u1 and
0
u2 are obtained without considering the existence of FeOF. If the
mass fraction of FeOF (u3, see the following section) is taken
0
0
into account, u1 and u2 should be revised as follows:
0
u1 ¼ u1 0
u2 ¼ u2 M1
0
u3 ¼ u1 0:414u3
M1 þ M2
M2
0
u3 ¼ u2 0:586u3 ¼ 1 u1 u3
M1 þ M2
(5)
(6)
where u1 and u2 denote the revised mass fractions of FeF3 and
Fe2O3. The mass fractions of FeF3, Fe2O3 and FeOF of
FeF3@Fe2O3 composites are shown in Table 1.
The morphologies of the obtained FeF3@Fe2O3 composites are
examined by SEM and TEM. Fig. 2b–f show the SEM images of
the FeF3@Fe2O3 composites. It can be seen that there is no
significant change in the surface morphology of the materials until
the heating time is longer than 60 s when the surface is fully covered
with small knobs. The formation of these knobs may have their
roots in the lattice density difference between FeF3 (3.60 g cm3,
PDF no. 00-033-0647) and Fe2O3 (5.51 g cm3, PDF no. 01-0860550); the volume shrinks when the lattice changes from FeF3 to
Fe2O3 at 500 C in atmosphere. High-resolution TEM of C30 (see
Fig. 5) reveals that there is an outer layer with thickness of 5 nm on
the surface of the core FeF3; the outer layer is made of the knobs.
The two insets at top of Fig. 5 show the fast Fourier transform
(FFT) pattern of the HRTEM image and the selected area for
HRTEM. Fig. 6 shows the electrochemical performance of the
FeF3@Fe2O3 composites in the 4.5–2.0 V region at a current rate of
50 mA g1. The amount of the Fe2O3 coated layer has an impact on
the discharge capacities and cycling performance; the discharge
capacity of FeF3 without Fe2O3 coating is only 78 mA h g1 while
that of C15, C30 and C60 are 128 mA h g1, 174 mA h g1 and
201 mA hg1 respectively. However, for the FeF3@Fe2O3
composites with larger amount of Fe2O3, such as C120, C240 and
C480, the discharge capacity decreases with the heating time
because of the electrochemically inactive nature of Fe2O3 in the
4.5–2.0 V region, as shown in Fig. 6c. Fig. 6b and d show the cycling
performance of the FeF3@Fe2O3 composites. The discharge
capacity of FeF3 without Fe2O3 coating fades quickly, decreasing
to 26 mA h g1 after 10 cycles while those of the FeF3@Fe2O3
composites with Fe2O3 coatings are all higher than 70 mA h g1.
Moreover, the amount of the coated Fe2O3 also impacts on the
initial discharge potential plateaus of the FeF3@Fe2O3 composites;
the larger the amount, the higher the plateau. It is the improvement
of electronic conductivity that results in reduced electrochemical
polarization. Nevertheless the conductivity improvement of the
composite seems having some limitations; because for those
24772 | J. Mater. Chem., 2012, 22, 24769–24775
Fig. 5 HRTEM image of the FeF3@Fe2O3 composite (C30). The FFT
pattern of the HRTEM image (top left) and the selected area for
HRTEM (top right) are shown in the insets.
obtained by heating for more than 60 s, there was no difference in
the initial discharge plateaus. FeF3 and C15 exhibit poor capacity
and have plateaus around 2.35 V, which might be attributable to
the poor electronic conductivity of the materials. The reason for the
presence of the plateau around 2.35 V is still not clear, and a similar
observation has been reported in Badway’s work too.9 For the C60,
C120, C240 and C480, there was a ‘new’ slope at 2.2 V with corresponding capacities of 1.5 mA h g1, 4.1 mA h g1, 6.2 mA h g1
and 6.5 mA h g1, respectively. These capacities (denoted as O1,
O2, O3, O4) cannot be ascribed to the Fe2O3 layer because Fe2O3
is electrochemically inactive in the 4.5–2.0 V region (see Fig. 6c).
Fig. 6 (a and c) Discharge profiles of the FeF3@Fe2O3 composites in the
first cycle in the 4.5–2.0 V region at 50 mA g1 with (b and d) corresponding cycling performance.
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We therefore hypothesize that there is a transition layer of FeOF
existing between the FeF3 core and the Fe2O3 shell; the formation
of FeOF might take place as in reaction (2).18
FeOF is electrochemically active with a capacity of 350 mA h
g1 in the 2.2–2.0 V region,7 and its amount is calculated by using
following formula:
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u3 ¼
D
350 mA h$g1
(7)
where u3 denotes the mass fraction of FeOF in the FeF3@Fe2O3
composites. The values of u3 for C60, C120, C240 and C480 are
0.004, 0.012, 0.018 and 0.019, respectively (see Table 1); they
increase with the heating time and converge at 0.019. However, it
is difficult to determine the existence of FeOF by experiments
directly due to its small amount. In building a spherical model
and assuming that the size of the FeF3@Fe2O3 composites is
150 nm with equal densities of FeF3, Fe2O3, and FeOF, the
thickness of the FeOF layer can be calculated to be 1.4 nm. It has
been reported that nanosized FeOF decomposes into Fe2O3 and
FeF3 at much lower temperature (300 C (ref. 7)) than the
temperature reported by Brink (635 C (ref. 18)); in other words,
nanosized FeOF is more unstable. This is consistent with our
hypothesis that there might be FeOF but not in large quantity.
Fig. 7 shows the electrochemical performance of the FeF3@Fe2O3 composites discharged down to 1.2 V at a rate of 50 mA
g1. Obviously, the Fe2O3 coating layer improves the discharge
capacities and cycling performance. FeF3 has two plateaus: one
at 1.8 V and another at 1.6 V. C15 also has two plateaus at 1.9 V
and 1.7 V, about 0.1 V higher than those of FeF3. However, there
is only one plateau at 1.9 V for C60 and C480. The 1.9 V plateau
of C15 is not likely to be caused by the introduction of Fe2O3,
because such small amount of Fe2O3 (0.6 wt%) cannot deliver
such a large capacity. Badway9 and Yamakawa6 described the
discharge process as:
FeF3 + Li / LiFeF3
(8)
LiFeF3 + 2Li / 3LiF + Fe
(9)
Fig. 7 Discharge profiles of the FeF3@Fe2O3 composites in the first
cycle in the 4.5–1.2 V region at 50 mA g1. The inset shows the cycling
performance.
This journal is ª The Royal Society of Chemistry 2012
By reaction (8), the theoretically released capacity from the
complete reaction of FeF3 with Li in the 4.5–2.0 V region, is
237 mA h g1. In fact, the capacities of pure FeF3 and C15 are 78
mA h g1 and 128 mA h g1, equivalent to 33% and 54% of the
theoretical value respectively. This means that a large amount of
FeF3 does not participate in the chemical reaction (8) and that
only part of FeF3 has been transformed into LiFeF3; as a result,
the subsequent reaction (9) is constrained. Based on the above
analysis, the 1.8 V plateau of FeF3 and the 1.9 V plateau of C15
might have come from the reduction of LiFeF3 to Li and Fe. The
corresponding capacities for the initial 3.1–3.2 V and 1.8–1.9 V
plateaus are marked as ‘l’ and ‘l0 ’ respectively in Fig. 7. For FeF3
and C15, the ratios of l0 /l are both about 0.62, indicating that
only 62% of LiFeF3 can be easily reduced into LiF and Fe. The
rest 38% may be reduced under the lower 1.5–1.2 V region.
Reactions (8) and (9) in this region are expected to further deliver
some capacities. The discharge–charge profiles of C120 and C480
are more complicated because Fe2O3 exhibits electrochemical
activity when discharged down to 1.2 V. The detailed reaction
mechanism needs further investigations for these complex
systems.
In order to understand the structural development of the
cathode during discharge–charge, an ex situ XRD was performed. C480 contains 46.6 wt% Fe2O3 and was chosen as the
cathode material because its high content of Fe2O3 will be
sensitive to XRD detection. Fig. 8 shows the XRD patterns of
the C480 cathode at different states of discharge–charge in the
4.5–2.0 V region at a rate of 50 mA g1. The intensities of the
Bragg peaks (111) and (200) of aluminum vary unsystematically
due to the difference in the thickness of the cathode material on
aluminum current collector, implying that the intensities
obtained experimentally will not be appropriate to be used for
comparison directly. However, it is found that the Bragg peak
(110) of Fe2O3 seldom overlaps with other peaks and its intensity
is basically unchanged during discharge–charge in the 4.5–2.0 V
region (indicating that Fe2O3 is intact in the discharge–charge
process), as shown in Fig. 8c. Consequently, the intensities of the
various Bragg peaks can be compared with each other if the
intensity of Bragg peak (110) of Fe2O3 is used as a reference. In
addition, in Fig. 8, the peaks (104), (110) of FeF3 are hidden
because they are too close and tiny compared to those of Fe2O3.
The positions and the intensities of the peaks (113), (116), (024)
and especially (012) of the lithiated FeF3 change a lot during the
discharge–charge. This observation is consistent with Badway’s
work. The possible reason for such a big change in intensity of
the peak (012) (see Fig. 8c) is attributed to the possibility of
significant alteration of the Fe array in the lattice rather than the
intercalation of lithium because lithium is a light scatterer of
X-rays.3 In the 3.15–2.0 V region, there is less change in the
intensities of the peaks (012) and (113) but a significant shift in
their positions to lower 2q angles, suggesting an expansion of the
unit cell. Further studies on the structural evolution of FeF3
during cycling by using in situ X-ray absorption fine structure
(XAFS) technique are under way in our laboratory. When the
C480 cathode is re-charged to 4.5 V, all the Bragg peaks are
indexed to Fe2O3 with no apparent change in their positions and
intensities.
Fig. 9 shows the XRD patterns for the C480 cathode at
different states of discharge–charge in the 4.5–1.2 V region at a
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Fig. 8 (a–c) Ex situ XRD patterns for the C480 cathode at different
states as shown in (d) the corresponding discharge–charge curve in the
4.5–2.0 V region. Lithiated FeF3 and Fe2O3 phases are denoted by (hkl)
and (hkl) respectively. ICDD standards for Fe2O3 (PDF no. 01-0860550), FeF3 (PDF no. 00-033-0647) and Al (PDF no. 00-001-1179) are
shown for reference.
rate of 50 mA g1. During the discharge from 2.0 V to 1.2 V, the
decrease in most of the peak intensities is offset by an increase in
the intensity of the Bragg peaks at 37 , 43 and 45 , suggesting
the presence of new phases. Previous studies on the discharge–
charge of FeF3 have not reported the presence of peaks at 37
and 43 .3,6,9 Therefore, they must be derived from the lithiation
of Fe2O3. Thackeray19 reported that lithiation of Fe2O3 caused
the anion array to transform from hexagonal to cubic close
packing and that the structure of the lithiated Fe2O3 phase
(Li1.7Fe2O3) was the intermediate between spinel and rocksalt;
with characteristic peaks at 36.7 and 42.7 (calculated based on
the lattice parameters reported). The intensities of the peaks at
37 and 43 reach maximum when discharged to 1.2 V but
decrease when charged to 4.5 V, indicating that there is some
degree of reversibility when Fe2O3 is cycled. Close inspection of
the broadened peak at 45 reveals that it is decomposed to three
peaks: (110) of Fe, (200) of LiF and (200) of aluminum. Nanosized Fe and LiF are the products of conversion reaction (9).3,6,9
The peak at 45 does not appear until discharged to lower than
1.7 V with maximum intensity at 1.2 V, then decreases in intensity gradually during charge, in accordance with the mechanism
revealed by the reactions (8) and (9). During the charge from 1.2
to 4.5 V. the intensities of the peaks at 43 and 45 decrease
gradually but they do not disappear, while that of peak (012)
increases at first and then disappears when charged to 3.9 V. The
reason for the disappearance of peak (012) is uncertain, with the
possibility of being stemmed from the alteration of the Fe array
24774 | J. Mater. Chem., 2012, 22, 24769–24775
Fig. 9 (a–c) Ex situ XRD patterns for the C480 cathode at different
states as shown in (d) the corresponding discharge–charge curve in the
4.5–1.2 V region. Fe, LiF and lithiated Fe2O3 phases are denoted by
(hkl) , (hkl) and (hkl) respectively. ICDD standards for LiF (PDF no.
01-089-3610), Fe (PDF no. 00-006-0696), Fe2O3 (PDF no. 01-086-0550),
FeF3 (PDF no. 00-033-0647) and Al (PDF no. 00-001-1179) are shown
for reference.
in the lattice, just like that happening at the initial stage of the
discharge. In summary, it is found that the structural development of the FeF3@Fe2O3 composite electrode is the combination
of those of FeF3 and Fe2O3 when discharged–charged in the
4.5–1.2 V region, respectively. Herein, we should point out that
the improvements of electrochemical performance of FeF3
resulting from an in situ Fe2O3 coating is inferior to that by
mixing with carbon-based conductive additives. Nevertheless,
the in situ metal oxide coating, even in small amounts of Fe2O3
(0.6–5.2 wt%), has great influence on the improvement of electrochemical performance. Further studies are under way in our
laboratory to improve the performance with other coating layers
with higher electronic conductivity.
4. Conclusions
This paper reports an in situ oxide coating on metal fluoride as
cathode material for the first time. The synthesized FeF3 is of
100–150 nm size with fine network structure; allowing the FeF3
particles to fully react with hot air; essential for the synthesis of
the FeF3@Fe2O3 composites with uniform Fe2O3 coating. XRD
results show that the obtained FeF3 is rhombohedral in nature
with space group R
3c and that the Bragg peaks of the FeF3@Fe2O3 composites are well indexed to FeF3 and Fe2O3. SEM and
TEM images demonstrated the core–shell structure of the
composites. By comparing the electrochemical performance of
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the pristine FeF3 and the FeF3@Fe2O3 composites, it was found
that the in situ Fe2O3 coating, even with small amounts of Fe2O3
(0.6–5.2 wt%), had a beneficial impact on electrochemical
performance due to the improvement of electronic conductivity.
In addition, based on the analysis of the charge–discharge
profiles, there is a transition layer of FeOF between the FeF3 core
and the Fe2O3 shell. The results of ex situ XRD also show that
the electrochemically induced structural development of the
composites in the 4.5–1.2 region is quite different to that in the
4.5–2.0 V region because the Fe2O3 is electrochemically active
when discharged down to 1.2 V.
Acknowledgements
Financial support from the National Basic Research Program of
China (973 program, Grant no. 2011CB935903) and the
National Natural Science Foundation of China (Grant no.
21021002 and 20873115) is gratefully acknowledged. Special
thanks are due to Prof. Jinxiao Mi for helpful discussions.
Mr Ibrahim Abdul-Rauf is thanked for editorial assistance.
Notes and references
1 M. Armand and J. M. Tarascon, Nature, 2008, 451, 652–657.
2 M. Bervas, F. Badway, L. C. Klein and G. G. Amatucci, Electrochem.
Solid-State Lett., 2005, 8, A179–A183.
3 F. Badway, N. Pereira, F. Cosandey and G. G. Amatucci,
J. Electrochem. Soc., 2003, 150, A1209–A1218.
This journal is ª The Royal Society of Chemistry 2012
4 S.-W. Kim, D.-H. Seo, H. Gwon, J. Kim and K. Kang, Adv. Mater.,
2010, 22, 5260–5264.
5 G. G. Amatucci and N. Pereira, J. Fluorine Chem., 2007, 128, 243–
262.
6 N. Yamakawa, M. Jiang, B. Key and C. P. Grey, J. Am. Chem. Soc.,
2009, 131, 10525–10536.
7 N. Pereira, F. Badway, M. Wartelsky, S. Gunn and G. G. Amatucci,
J. Electrochem. Soc., 2009, 156, A407–A416.
8 H. Arai, S. Okada, Y. Sakurai and J.-i. Yamaki, J. Power Sources,
1997, 68, 716–719.
9 F. Badway, F. Cosandey, N. Pereira and G. G. Amatucci,
J. Electrochem. Soc., 2003, 150, A1318–A1327.
10 F. Badway, A. N. Mansour, N. Pereira, J. F. Al-Sharab, F. Cosandey,
I. Plitz and G. G. Amatucci, Chem. Mater., 2007, 19, 4129–
4141.
11 H. Li, G. Richter and J. Maier, Adv. Mater., 2003, 15, 736–
739.
12 R. Prakash, C. Wall, A. K. Mishra, C. K€
ubel, M. Ghafari,
H. Hahn and M. Fichtner, J. Power Sources, 2011, 196, 5936–
5944.
13 A. Lachter, J. Salardenne and A. S. Barriere, Phys. Status Solidi B,
1978, 90, 147–150.
14 L. Liu, M. Zhou, X. Wang, Z. Yang, F. Tian and X. Wang, J. Mater.
Sci., 2012, 47, 1819–1824.
15 C. Li, L. Gu, S. Tsukimoto, P. A. van Aken and J. Maier, Adv.
Mater., 2010, 22, 3650–3654.
16 C. Li, L. Gu, J. Tong, S. Tsukimoto and J. Maier, Adv. Funct. Mater.,
2011, 21, 1391–1397.
17 L. A. Marusak, R. Messier and W. B. White, J. Phys. Chem. Solids,
1980, 41, 981–984.
18 F. J. Brink, R. L. Withers and J. G. Thompson, J. Solid State Chem.,
2000, 155, 359–365.
19 M. M. Thackeray, W. I. F. David and J. B. Goodenough, Mater. Res.
Bull., 1982, 17, 785–793.
J. Mater. Chem., 2012, 22, 24769–24775 | 24775