Phase transition of hollow-porous α-Fe2O3 microsphere based anodes for lithium ion batteries during high rate cycling

Article abstract of DOI:10.1039/c6ta07131g

In the present paper, hollow-porous α-Fe₂O₃ microspheres are prepared via cation etching of zinc citrate microspheres and subsequent thermal treatment. The superior performance of the as-obtained α-Fe₂O₃ microspheres as an anode material for lithium ion batteries is evaluated. After 1000 cycles, the capacity still remains more than 1100 mA h g^-1 at a current rate of 1 A g^-1. Meanwhile, the crystal size induced phase transition of Fe₂O₃ microspheres (α → γ → β) is observed during cycling by the measurements of ex situ XRD and TEM, which is responsible for their abnormal performance fluctuation.

Full text of DOI:10.1039/c6ta07131g

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Phase transition of hollow-porous α-Fe2O3 microspheres based
anodes for lithium ion batteries during high rate cycling
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
Shiji Hao , Bowei Zhang , Sarah Ball c *, Junsheng Wu d, *, Madhavi Srinivasan a, b and Yizhong
Huang a, b, *
DOI: 10.1039/x0xx00000x
In the present paper, the hollow-porous α-Fe2O3 microspheres are prepared via cation etching of zinc citrate
microspheres and subsequent thermal treatment. The superior performance of the as-obtained α-Fe2O3 microspheres is
evaluated as anode materials for lithium ion batteries. After 1000 cycles, the capacity still remains more than 1100 mAhg-1
www.rsc.org/
at current rate of 1 Ag-1. Meanwhile, the crystal size induced phase transition of Fe2O3 microspheres (α
γ
β) is
observed during cycling by the measurements of ex-situ XRD and TEM, which is responsible for their abnormal
performance fluctuation.
Moreover, the channels built by the unique porous and hollow
structures could offer a high rate of Li+ ions diffusion
process.10, 11 Based on these considerations above, recently, a
large quantity of porous and hollow Fe2O3 nanomaterials with
1 Introduction
Currently, numerous efforts have been devoted to the
investigation of iron oxide (Fe2O3) based nanomaterials due to
their promising application as anode materials for lithium ion
batteries (LIBs).1-3 Compared with other anode materials,
Fe2O3 exhibits much better compatibility and applicability, such
as low cost, high theoretical capacity (1005 mAhg-1) and
environmental friendliness.4 However, The Fe2O3 electrodes in
LIBs generally are subject to the electrochemical process of
conversion reactions (i.e. the reduction and oxidation between
lithium and iron oxide (with 6 Li+ per Fe2O3 molar)), which
causes extremely large volume expansion and electrode
destruction during cycling.5, 6 In order to address a series of
triggered problems such as the loss of electrical contacts and
active materials pulverization, the best strategy is to design
and synthesize Fe2O3 based anode materials with porous and
hollow nanostructures. In the light of some related studies, the
perfection of the porous and hollow nanostructures could
bring some obvious merits to the performance improvement
of the Fe2O3 electrode. First, the interval of Fe2O3 primary
particles acts as buffer layer for the volume change during
cycling.7 Second, the intrinsic porosity increases the specific
superior performance have been successfully prepared.
In spite of the high performance of Fe2O3 anode materials
achieved by various techniques, the detailed electrochemical
reaction process during cycling still needs to be investigated.
The electrochemical reaction process has been widely
accepted following the equation of Fe2O3 + 6 Li+
2 Fe
+ 6 Li2O.12-14 However, it is well known that the anode
materials usually undergo abnormal performance fluctuation,
in which the initial capacities progressively decrease and then
ramp up during cycling. The phenomenon is generally
attributed to the formation of solid electrolyte interface (SEI)
film and the reversible formation of the polymer-like film on
the surface of active materials.15-17 In fact, the detailed
mechanism on the performance fluctuation still remains
unclear. The investigation on the phenomenon will bring a
further perspective on their performance fading and
nanostructure designation.
In the present work, we synthesized the hollow-porous α-
Fe2O3 microspheres by a facile strategy. The method involves
cation etching of zinc citrate microspheres template and
annealing treatment. The as-obtained Fe2O3 electrode provides
the capacity of more than 1100 mAhg-1 at current rate of 1 Ag-1
after 1000 cycles in the electrochemical test. The phase
transition process of Fe2O3 nanoparticles initiated by the
crystal size reduction, during high rate and long term cycling,
has been demonstrated to contribute this excellent
performance revealed by using ex-situ high-resolution TEM
(transmission electron microscope) technique along with XRD
(X-ray diffraction).
active area for the lithiation/delithiation reaction.8,
9
a
Energy Research Institute @ NTU, Nanyang Technological University, Singapore
639798, Singapore
b
School of Materials Science & Engineering, Nanyang Technological University,
Singapore 639798, Singapore
c Johnson Matthey Technology Centre, Reading, RG4 9NH, UK
d
Institute of Advanced Materials and Technology, University of Science and
Technology Beijing, Beijing, 100083 (China)
* Corresponding Author. Email: yzhuang@ntu.edu.sg; wujs76@163.com
Electronic Supplementary Information (ESI) available: template SEM and precursor
XRD pattern. See DOI: 10.1039/x0xx00000x
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suspension changes from green to red, inVdieicwaAtritnicgle Ontlhinee
2 Experimental
DOI: 10.1039/C6TA07131G
2.1 Materials synthesis
All chemicals were purchased from Sigma Aldrich. Zinc citrates
were prepared through
previously4. 0.392 g of zinc nitrate hexahydrate, 0.110 g of
sodium citrate tribasic dihydrate and 0.105 of
hexamethylenetetramine (HMT) were dispersed into 50 ml DI
water to form uniform suspension. Thereafter, the
suspension was kept reacting under 90 with water bath for
a modified method reported
g
a
℃
20 min. The obtained zinc citrate templates were subsequently
separated by centrifugation, washed with DI water and
ethanol and dried at 80
℃ for 12 h. To synthesize how and
porous α-Fe2O3 microspheres, 0.02g of zinc citrate templates
was dispersed into 20 ml of 0.05 M ferrous sulfate solution and
then the reaction was maintained for 24hrs under soft stirring.
After washed with DI water and ethanol, the product was
Fig. 1 SEM image of the precursor (a) and SEM images (b, c) and XRD
pattern (d) of the as-obtained α-Fe2O3 microspheres.
transferred to the box furnace and heated to 600
℃ for 5hrs (2
oC min-1) in the air.
formation of ferrous hydroxide and the subsequent conversion
to ferric hydroxide. As shown in Fig. 1a, the precursor appears
the hollow microsphere structure with an average diameter of
1 µm. In addition, its amorphous phase structure is revealed
from the XRD pattern (as shown in Fig. S2). The formation
mechanism of the hollow structure could be described as
following: when zinc citrate templates are immersed in an
aqueous solution of ferrous sulfate, the hydrolysis reaction of
Fe2+ is spontaneously carried on around the surface of the zinc
citrate with generation of ferrous hydroxide. Simultaneously,
the hollow structures are built while zinc citrate templates are
being etched by H+. As the reaction goes on, ferrous hydroxide
progressively turns to be ferric hydroxide due to the oxidation
reaction of air. The detailed reaction process is summarized as:
2.2 Characterization
The phase structure of the as-obtained product was
characterized by powder X-ray diffraction (XRD, Bruker D8 with
Cu Kα radiation). The morphology and microstructure were
investigated by scanning electron microscopy (SEM, JEOL
7600) and Transmission electron microscopy (TEM, JEOL 2100).
The Brunauer-Emmett-Teller (BET) specific surface areas of the
α-Fe2O3 particles were measured by using the
Micromeritics ASAP Tristar II 3020.
2.3 Electrochemical Measurement
Fe2+
(1)
+
2
H2O
Fe(OH)2
+
2
H+
CR-2016 coin cells were assembled in glove box under N2
atmosphere, with lithium metal foils and Celgard 2400
membranes as the counter electrodes and separators,
respectively. The electrolyte employed was 1M LiPF6 in 1:1
mixture of EC and DEC. The working electrodes were made of
slurry consisting of the active materials (70 wt%), acetylene
black (20 wt%) and polyvinylidene fluoride (PVDF) binder (10
wt%) casted on the copper foil. The galvanostatic charge-
and
4Fe(OH)2
(2)
+
O2
+2H2O
4Fe(OH)3
After annealing in the air, the porous and hollow α-Fe2O3
microspheres (as shown in Fig. 1 b and c) were obtained. Such
hollow microsphere structure of the as-obtained α-Fe2O3
nanoparticles is apparently derived from the precursor. In
addition, the porous surface of the obtained nanoparticles is
clearly observed from their high-resolution SEM (scanning
electron microscope) image (Fig. 1c). Fig. 1d shows the X-ray
diffraction (XRD) pattern collected from the α-Fe2O3
microspheres. All diffraction peaks are indexed (ICSD 40142)
and generated from the α-Fe2O3 particles demonstrating their
high purity of α-Fe2O3 phase as attributed to the complete
chemical etching reactions.
discharge cycling test was conducted at 25
℃ between 0.01 V
and 3.0 V under different current densities in a Neware battery
test system (Neware, BTS-5V10mA, China). Cyclic voltammetry
(CV) test was performed by using an electrochemical
workstation (autolab, PGSTAT302N) with a scan rate of 0.5
mVs-1 over 0.1 V to 3.0 V (vs. Li/Li+).
To reveal the interior crystalline structure, the α-Fe2O3
microspheres, TEM was employed. Fig. 2a is a bright field TEM
image, which clearly approves the hollow and porous
3 Results and discussion
The chemical reaction of Fe2+ ions with zinc citrate templates nanostructure of the α-Fe2O3 microspheres in good agreement
(Fig. S1) was employed to prepare the precursor of the hollow- with the SEM images (Fig. 1b, c). The nanocrystalline structure
porous α-Fe2O3 microspheres. In this process, the color of the of microspheres is identified from the selected area electron
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diffraction (SAED) pattern (inset of Fig. 2a), which consists of demonstrated in the profile of the corresponding CV test
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as indicated from multiple discrete rings. In Fig. 2b, it can be conducted at a rate of 0.5 mV s-1.18-21 InDtOhIe: 1c0a.1t0h3o9/dCic6TpAr0o7c13e1sGs
observed that the porous structure is formed by the randomly of the first cycle, a strong and three weak peaks are viewed
Fig. 3 BET isotherm plots and corresponding BJH pore size distributions
(insets) of the α-Fe2O3 microspheres
and they are located at 0.62 V, 1.60 V, 1.33V and 0.95 V,
respectively. The strong peak is attributed by the lithiation
reaction of Fe2O3 and the irreversible decomposition of
electrolyte. The other three small peaks are sequentially
Fig. 2 (a, b) TEM and (c, d) HRTEM images of the as-synthesized α-Fe2O3
microspheres.
associated with the further lithium intercalation, the reduction
process of Fe3+ to Fe2+ and the complete reduction reaction of
Fe2+ to Fe0. However, in the subsequent cycle, the cathodic
peak appears much broader and is shifted to 0.75 V, indicating
the occurrence of the irreversible phase transformation and
electrolyte decomposition. In the anodic part, a broad anodic
peak is observed and located at 1.7 V, corresponding to the
delithiation and the oxidation reaction of Fe0 to Fe3+. With
more cycles, the oxidation peak position and intensity almost
remain unchangeable, demonstrating the high reversibility of
the oxidation reaction. According to previous studies, the
detailed electrochemical reaction is processed through the
piling up of nanocrystals with average size of 30 nm.
Furthermore, the high-resolution TEM (HRTEM) images, Fig. 2c
and d (magnified from the square part in Fig. 2c), show a single
nanocrystal of α-Fe2O3 which can be confirmed by the fast
fourier transform (FFT) pattern (inset of Fig. 2d), in accordance
with the XRD result (Fig. 1d). As shown in Fig. 2d, the fringes
are shown to be separated by 0.29 nm, corresponding to the
spacing between (210) planes of α-Fe2O3 nanocrystals.
Brunauer-Emmett-Teller gas-sorption measurement was
performed to investigate the porous nature and specific areas
of the as-synthesized α-Fe2O3 microspheres. The typical type
Ⅳ
nitrogen adsorption-desorption isotherms and the inset of
following equation12-14
:
+
corresponding pore size distribution profile are shown in Fig. 3.
The observed sharp cappilary condensation step from high
Fe2O3
(1)
+
6Li+
6e-
2Fe0
+
3Li2O
pressure (p/p0
＞0.8) and the H1 type hysteresis loop indating
The profile of galvanostatic discharge-charge was performed
to evaluate the electrochemical performance of the hollow-
porous α-Fe2O3 microspheres. The Fig. 4b shows the first 3
discharge-charge profiles of the as-obtained α-Fe2O3 anode
electrode under the current density of 0.2 C (200 mA g-1). At
the first discharge, there exists plateaus at potentials of 1.47 V,
1.08 V and 0.88 V, respectively, followed by a gradual dropping
of voltage, corresponding to the reduction of metal oxides and
electrolyte decomposition. In the subsequent cycles, the
discharge curves only show a wide slope between 0.8 - 1.5 V.
The drastic change demonstrates the occurrence of the
irreversible reduction reaction and electrolyte decomposition
in the first cycle.5, 22 In the charge process, a steady wide slope
is located between 1.2 V and 2.5 V which is attributed by the
reversible oxidation reaction of the active material. The test
results above are in well agreement with Fig. 4a.23, 24 The
the both existence of the macro- and mesopores. The BET
surface area of the α-Fe2O3 microspheres is calculated to be
around 22.2 m2 g-1. According to the analysis of Barrett-Joyner-
Halenda (BJH), the pore size distribution shows the obvious
property of the multimodal porosity. The relatively narrow
distribution located centured at 10 nm is believed to be
contributed by the presence of the mesoporous shell.
Whereas, the wide distribution of large pores (45 nm-80 nm) is
assumed to be associated with the interspace among the
integrated α-Fe2O3 particles. The result above can also be
confirmed by the image of SEM and TEM.
The electrochemical property of the as-obtained α-Fe2O3
microsphere was evaluated by the measurements of cyclic
voltammetry (CV) and galvanostatic discharge-charge using the
assembled CR2016 coin cells. As shown in Fig. 4a, the detailed
electrochemical reaction process can be thoroughly
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repective capacities of the initial discharge and charge are The excellent rate capability of the α-Fe2O3 electrode was
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measured to be 1477.22 mAh g-1 and 1125.33 mAh g-1. The further assessed by the multiple-step galvanostatic charge-
DOI: 10.1039/C6TA07131G
loss of the irreversible capacity (351.89 mAh g-1) in the first discharge test. In Fig. 4d, the reversible capacities are
cycle is likely attributed to the formation of SEI film and the measured to be around 1200, 1150, 1000, 820, 650 and 450
mAh g-1 at six rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C,
respectively. The reversible capacity is still above 372 mAh g-1,
even when the rate is high up 5 C. However, the reversible
capacity immidiately increases to 897 mAh g-1 and then
reaches more than 1100 mAh g-1 after the rate is downs back
to 0.1 C. Furthermore, the high rate capability under the
current density of 1 C is demonstrated in Fig. 4e. After 1000
cycles, the retention of the reversible capacity is as high as
1150 mAh g-1. It is worth to note that the capacity rapidly goes
down at the beginning to around 550 mAh g-1 within 110
cycles and and then slowly rises up to more than 1000 mAh g-1
subject to more cycles This unususal phenomenon has been
widely observed in different kinds of transition metal oxide
based anode materials for LIBs. According to previous studies,
this abnormal performance fluctuation is generally due to the
reversible formation of the polymeric/gel-like films, which
gradually proceeds in the active materials’ porous structure
with the increase of cycle times.26, 27 However, the detailed
explaination for the electrochemical reaction process is
needed to be further provided.
In order to reveal the structural change of the Fe2O3
microspheres under high rate current density, ex-situ XRD
measurement was performed to the electrodes obtained after
110 (labeled as sample 110) and 500 (labeled as sample 500)
Fig. 4 (a) First five cyclic voltammogram (CV) curves (at a scan rate of 0.5
mV·s-1), (b) discharge and charge curves, (c) low r cycling performance (0.1C),
(d) rate capability and (e) high rate cycling performance (1C) of the as-
prepared the as-obtained α-Fe2O3 microspheres.
cycles at 1 C. As illustrated in Fig. 5, both samples display weak
peaks from 10° to 45°, reflected from the nanosized
constituents of the active materials but strong peaks at
43.0°,50.0° and 73.9°, generated from the metalic copper
substrate indicating the occurrence of the strcuture
destruction during cycling. By zooming in the patterns from
10° to 45°, the weak peaks of sample 110 at 23.9°, 26.3° and
30.4°, diffracted from (106), (116), (206) lattice planes, are
clearly visible which is indexed to be γ-Fe2O3 (ICSD 172906)
with lattice parameters of a = 8.33 Å and c = 25.11 Å.
Likewise, β- Fe2O3 (ICSD 36281) with lattice parameters of a=
5.56 Å and c=22.55 Å is identified from the three peaks at
20.0°, 32.3° and 35.5° diffracted from the (0 1 2), (1 1 -1), (1 1 -
4) lattice planes. Based on the analysis above, it is believed
that the crytal structure of the Fe2O3 microspheres undergoes
reduction of oxide to metallic Fe with Li2O matrix.25 The
perfection of the as-synthesized α-Fe2O3 electrode’s cyclability
at a low rate (0.2 C) is shown in Fig. 4c. In the range of the
whole 50 cycles, the coloumbic efficiency is kept stable above
98% except the initial one (76.18%). Meanwhile, the capacities
remain a steady value of around 1150 mAh g-1, which is much
higher than the theoretical capacity of the α-Fe2O3 (1005 mAh
g-1)4. In the light of previous studies26-28, the extra capacity is
usually contributed to two parts. Firstly, the polymeric/ gel-like
films could be reversibly formed/dissolved over the surface of
active materials by the decomposition of electrolyte. Secondly,
the electrocatalysing redcution reacion of C4+ from carbonate
salt to other C sepcies with low valences could be triggered by
the fresh metal nanocrystals surface formed during the
lithiation reaction. The great cycling perfomance is mainly
contributed by the merits of the unique structure. On the one
hand, the invervals of the numerous nanocrystals consisting of
the porous shell and the interior cavity of the microsphere
offer the short diffusion length of the Li+ ions and provide the
reserved space for the volume change during lithiation-
delithiation reaction. On the other hand, the porous and
hollow structure significantly increases the specific surface
area of the active materials and consequently improves the
reversible reaction.
the transformation in the order of α-Fe2O3
γ-Fe2O3
β-
Fe2O3 with the increase of the cycling times.
The morphological and crystalline structural changes of
microspheres subject to charge-discharge cycles were revealed
using ex-situ TEM. One of microsphere from sample 110
shown in Fig. 6a is seen to be covered by a thick SEI layer. The
microspherical particle appears amorphous-like structure but
is embedded with some nanoparicles (Fig. 6b). These
nanoparticles are tiny with the size of ~ 7-8 nm but crystalline
(Figs. 6c and d). With the increase of cycling up to 500 times,
the SEI film that covers the surface of the integrated
microsphere becomes much thicker as seen from Fig. 6e.
Meanwhile, the average size of the nanocrystals embedded in
the microsphere (Fig. 6f) is reduced to 3 - 4 nm. It is worth to
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note that the respective HRTEM images of Fig. 6c and Fig. 6g of the electrode increase after 110 cycles, which VisiewveArrtiicflieeOdnlbinye
are taken from Fig 6b and Fig. 6f in the squared areas as the diameter changes of the secondDOseI:m10i.c1i0r3c9le/C6aTnAd071t3h1Ge
marked. Fig. 6d and Fig. 6h are their corresponding atomic decreasing line slope. The increasing resistance is likely
images after performing inverse FFT process. As indicated from attributed to the thickening SEI film during cycling which
Fig. 5 XRD patterns of the electrodes after (a) 110 cycles (sample 110) and (b)
500 cycles (sample 500).
Fig. 6c, the spacing of the lattice fringes is measured to be
0.32 nm, which can be indexed as planes 1 3 1 of γ-Fe2O3
along the zone axis of <1 1 4>. Meanwhile, the HRTEM image
of sample 500 (Fig. 6g) shows a clear crystal lattice of β-Fe2O3
imaged along the <1 0 1> zone axis, based on the indexing of
the inset FFT pattern.
Using ex-situ TEM, the phase transition among the three Fe2O3
phases (α
γ
β) during high rate cycling as anode for
lithium ion batteries was observed in the present study, which
has been hardly reported elsewhere. This abnormal phase
transition phenomenon is believed to be ascribed to the
structural disorder with the decrease of the particle size29.
Considering the effect of surface (or interface) energy
contribution to the free energy for nanoparticles, smaller
particle size causes the lattice modification30, which is in
agreement with an earlier interpretation that describes the
size-induced lattice deformations using the protohematite-
hydrohematite-Stoichiometric Hematite (SH) two-dimensional
map31. Therefore, the phase transition (α
γ
β) occurs
at the structural disorder site under the driving force
(electrochemical cycling).
In the present paper, the effect of Fe2O3 microspheres phase
transition on their electrochemical performance as anode
materials in lithium ion batteries was also revealed using
Fig. 6 TEM, HRTEM and the FFT inverse images of the 110 (a-d) and 500 (e-h)
electrochemical impedance spectroscopy (EIS). Fig.
7
sample.
represents the Nyquist plot of the as-obtained Fe2O3
microspheres electrode along with different cycles. The EIS
figures of all the electrodes are composed of one depressed
semicircle in the high and intermediate frequency, comprising
of two semicircles, and a straight line, which is attributed to
the diffusion resistance. The semicircles in the high and
intermediate frequency can be assigned to the migration of
the lithium ion through the interfacial film and the charge
transfer reaction on the electrode/electrolyte interface. It is
evident that the charge transfers and ion diffusion resistance
increases the lithium ions diffusion and electron exchange
speed. However, when the 500th cycle is achieved on the
electrode, it is interestingly seen that the charge transfer
resistance drops to an even smaller number than the one of
fresh electrodes and the diffusion resistance gets slightly
recovered from the value at 110th cycle, even though, the SEI
film of electrode after 500 cycles turns to be thicker, compared
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fluctuation during high rate cycling. The hole, interlayer and
with the one after 110 cycles (Fig. 6e).
It has been well known that the intrinsic crystal structure tunnel’s variation of the crystal structure allows
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DOI: 10.1039/C6TA071t3h1Ge
shows the great effect on their lithium intercalation penetration of more lithium ions for intercalation process in
performance, where the lithium ions can intercalate into the the electrochemical reaction.
Fig. 8 Crystal structures of (a) α-Fe2O3, (b) γ-Fe2O3, (c) β-Fe2O3 and their
corresponding observation of the first crystal layer.
Fig. 7 Nyquist plots of the as-obtained α-Fe2O3 microspheres before cycling
and after 110 and 500 cycles.
4 Conclusions
interlayer, the tunnels, and the holes in the crystals32.
In summary, we have rationally designed and fabricated
According to Stokes–Einstein principle, the stokes radius of
hollow-porous α-Fe2O3 microspheres by a facile cation etching
approach. Owing to merits from their intrinsic nanostructure,
lithium ions (rLi, +) in organic electrolyte (EC, DEC or DMC) is
eff
calculated to be around 3.44 ~ 3.63 Å which is required to
the α-Fe2O3 microspheres exhibit excellent electrochemical
stablize the crystal tunnels33-35. The crystal structures of α, γ
performance (1100 mAhg-1 at 1 C) as anode materials for
and β phases Fe2O3 are shown in Fig. 8. For the hematite
structure (Fig. 8a), each α-Fe2O3 crystal consists of basic FeO6
lithium ion batteries. Importantly, the occurrence of Fe2O3
phase transition process (α
γ
β) along with the
octahedral units, which seems to be a rhombohedrally
centered hexagonal structure of the corundum type with a
closed-packed lattice. Although there are no obvious interlayer
spacings and tunnels through the whole structure, there exist
hexagonal holes (4.01 Å in diameter) on the cystal surface,
which are suitable for cation accommodation. Fig. 8b
represents the crystal structure of γ-Fe2O3, which indicates the
typical structure of spinel type. In the unit cell, the eight
abnormal performance fluctuation during high rate cycling is
observed using ex-situ TEM and XRD. The transformation
process is believed to be activated by crystal size reduction
followed by the generation of β phase, which not only
increases the electrode’s ion conductivity but also decreases
the lithium ions diffusion path. This finding provides an
effective approach to explain the activation process of
transition metal oxides based anode materials. It may also be
extended to open up new opportunities to construct novel
anode materials with high performance.
tetrahedral sites are occupied by Fe3+ cations. Meanwhile,
13
⁄
Fe3+ cations occupy the 16 octahedral sites. Similar to α-
Fe2O3, the crystal holes of γ-Fe2O3 only exist in the surface
lattice layer where edge-sharing tetrahedral units (2.98 Å) and
octahedral units (3.17 Å) are linked to form trapezoid and
triangle holes. Due to their narrow hole size, the progress of
cation intercalation is conducted in γ-Fe2O3 worse than in α-
Fe2O3. The present of steric hindrance will increase the
resistance of the electrochemical reaction as confirmed by EIS
tests (Fig. 7). With regard to β-Fe2O3 (Fig. 8c), the hexagonal
crystal structure has pace group P3 (143 lattice structure with
the parameters of a=5.56 Å and c=22.55 Å, which is comprised
of distorted octahedral and tetrahedral units. In this structure,
one dimensional tunnel structures (3.2 × 7.5 Å) is viewed along
the [1 0 0] direction which allows the rapid migration of
lithium ions and consequently increases the specific contact
area between eletrolyte and active materials, giving rise to the
higher performance after 500 cycles. As a result, we can
emphasize that the crystal size induced Fe2O3 electrode phase
Acknowledgements
This research was support by SUG (Start-up funding in NTU),
Tier 1 (AcRF grant MOE Singapore M401992), Tier 2 (AcRF
grant MOE Singapore M4020159) and the Chinese Natural
Science Foundation (Grant 60906053, 62174118 and
51308050309).
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