Large-scale fabrication of porous carbon-decorated iron oxide microcuboids from Fe-MOF as high-performance anode materials for lithium-ion batteries

Article abstract of DOI:10.1039/c4ra11900b

A facile, cost-effective and environmentally friendly route has been developed to synthesise porous carbon-decorated iron oxides on a large scale via annealing iron metal-organic framework (MOF) precursors. The as-prepared C-Fe₃O₄ particles exhibit microcuboid-like morphologies that are actually composed of ultrafine nanoparticles and show a greatly enhanced lithium storage performance with high specific capacity, excellent cycling stability and good rate capability. The C-Fe₃O₄ electrodes demonstrate a high reversible capacity of 975 mA h g^-1 after 50 cycles at a current density of 100 mA g^-1 and a remarkable rate performance, with capacities of 1124, 1042, 886 and 695 mA h g^-1 at current densities of 100, 200, 500 and 1000 mA g^-1, respectively. The satisfactory electrochemical performance was attributed to the hierarchical architecture, which benefitted from the synergistic effects of the high conductivity of the carbon matrix, the cuboid-like secondary particles on the microscale, and the ultrafine primary nanoparticles on the nanoscale. This low-cost and simple method provides the possibility to prepare anode materials on a large scale and hence may have great potential applications in energy storage and conversion. This journal is

Full text of DOI:10.1039/c4ra11900b

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Large-scale fabrication of porous carbon-
decorated iron oxide microcuboids from Fe–MOF
as high-performance anode materials for lithium-
ion batteries
Cite this: RSC Adv., 2015, 5, 7356
Minchan Li,ab Wenxi Wang,b Mingyang Yang,ab Fucong Lv,b Lujie Cao,b Yougen Tang,a
c
b
*
Rong Sun and Zhouguang Lu
A facile, cost-effective and environmentally friendly route has been developed to synthesise porous
carbon-decorated iron oxides on a large scale via annealing iron metal–organic framework (MOF)
precursors. The as-prepared C–Fe3O4 particles exhibit microcuboid-like morphologies that are actually
composed of ultrafine nanoparticles and show a greatly enhanced lithium storage performance with
high specific capacity, excellent cycling stability and good rate capability. The C–Fe3O4 electrodes
demonstrate a high reversible capacity of 975 mA h gꢀ1 after 50 cycles at a current density of
100 mA gꢀ1 and a remarkable rate performance, with capacities of 1124, 1042, 886 and 695 mA h gꢀ1 at
current densities of 100, 200, 500 and 1000 mA gꢀ1, respectively. The satisfactory electrochemical
performance was attributed to the hierarchical architecture, which benefitted from the synergistic effects
of the high conductivity of the carbon matrix, the cuboid-like secondary particles on the microscale, and
the ultrafine primary nanoparticles on the nanoscale. This low-cost and simple method provides the
possibility to prepare anode materials on a large scale and hence may have great potential applications
in energy storage and conversion.
Received 7th October 2014
Accepted 9th December 2014
DOI: 10.1039/c4ra11900b
www.rsc.org/advances
urgent task in building next-generation LIBs in order to meet
the ever-growing performance demands.4 Among the various
1. Introduction
anode materials for LIBs, iron oxides have been proven to be
appealing candidates because of their high theoretical capac-
ities (ꢁ1000 mA h gꢀ1), widespread availability, and environ-
mental benignity.5–7 Nonetheless, the rapid fading of capacity,
which is mainly caused by the structural reorganisation and
volumetric variation upon charge/discharge, has impeded their
practical applications.8,9 Many efforts have been devoted to
synthesising nanostructured porous materials. In fact, it is
expected that nanomaterials accommodate the mechanical
strain of Li+ insertion/extraction much better than bulk coun-
terparts and provide shorter path lengths for the transport of
electrons and lithium ions.7,10–14 Meanwhile, sufficient internal
porosity ensures maximum electrode stability by the large
volume variation of the electrode upon cycling.15–18 However, the
potential disadvantages associated with the development of
nanostructured anodes include the possibility of signicant
side-reactions with the electrolyte due to the high external
surface-to-volume ratio and large surface free energy. These
undesirable side reactions could cause the formation of thick
solid electrolyte interphase (SEI) lms, leading to safety
concerns and poor calendar life.19–21 These problems may be
solved with secondary nanomaterials, which typically contain
particles with micrometre dimensions but have internal
The continuously rising crude-oil prices and the aggravation of
global warming has spurred a great effort to develop clean and
efficient energy storage technologies. Owing to their high
energy density, long cycle lifespan, exible and lightweight
design, and environmental benignancy, lithium-ion batteries
(LIBs) have become promising candidates among the existing
rechargeable battery technologies.1,2 To satisfy the ever-
increasing demands for high-performance LIBs for large-scale
energy applications including electric vehicles (EV), hybrid
electric vehicles (HEV) and stationary electrical energy storage,
the development of advanced electrode materials with low cost,
improved safety, high energy density, and easy synthetic and
processing approaches has attracted considerable attention.3
Graphite, the anode material currently used in commercial
LIBs, has a relatively low Li+ storage capacity of 370 mA h gꢀ1
.
Thus, the search for alternative anode materials has become an
aSchool of Chemistry & Chemical Engineering, Central South University, Changsha,
Hunan 410083, China
bDepartment of Materials Science and Engineering, South University of Science and
Technology of China, Shenzhen, Guangdong 518055, China. E-mail: luzg@sustc.
edu.cn
cShenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen,
P.R. China
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nanometre-sized regions; these materials might reduce side excellent electrochemical performance including
reactions with the electrolyte, leading to considerably enhanced reversible capacity and excellent rate capabilities.
electrochemical properties. Consequently, proper structures
a high
(hollow/porous structure, suitable size, surface feature, and so
on) could buffer the volume variation to sustain structural
integrity and promote lithium storage reactions, resulting in
2. Experimental section
2.1. Synthesis of FeC2O4$2H2O
fast ion/electron transfer and sufficient contact between the
active materials and electrolyte. Thus, improved electrode
performance such as higher overall capacity, better high-rate
capability, and longer cycling life can be expected.
Ferrous oxalate dehydrate was synthesised according to the
literature.35 First, oxalic acid dihydrate (2521 mg, 20 mmol) and
potassium oxalate dihydrate (3684 mg, 20 mmol) were dissolved
in 180 ml deionised water with magnetic stirring. Then, ferrous
oxalate heptahydrate (2780 mg, 10 mmol) was added to the
Metal organic frameworks (MOFs), rst dened by Yaghi and
co-workers in 1995 (ref. 22 and 23) following the pioneering
works of Hoskins and Robson,24–26 are crystalline porous hybrid
materials comprising coupling units (metal ions or metal-oxo
units) coordinated by electro-donating organic ligands. MOFs
demonstrate the highest specic surface areas reported to date
for porous materials and large internal pore volumes with well
organized pore sizes.27,28 They have emerged as a promising
class of porous materials in a wide range of extended applica-
tions including gas storage and separation,29 catalysis,30
imaging and sensing.31 Recently, a new burgeoning direction in
the eld of MOFs is their utilisation as templates to synthesise
hollow/porous-structured inorganic nanomaterials via ther-
molysis. For example, by using MIL-88-Fe MOFs as templates,
spindle-like mesoporous a-Fe2O3 has been synthesised for high-
rate LIBs.32 Hollow octahedral CuO has also been obtained by
the thermal treatment of Cu-based MOFs.33 Nevertheless, to the
best of our knowledge, there has been no report about the
design and preparation of carbon-decorated iron oxide micro-
cuboids from Fe-MOFs. Meanwhile, ferrous oxalate dehydrate,
so-called Humboldtine in the eld of mineralogy, is one of the
simplest MOFs; it was found in brown coal about a century ago
and is also available in soil. It has a one-dimensional (1D) chain
composed of ferrous ions and oxalate, as shown in Fig. 1a.34,35
In this work, we present a simple method for the large-scale
preparation of iron oxides using FeC2O4$2H2O as a template to
fabricate porous a-Fe2O3 and carbon-decorated Fe3O4 (C–Fe3O4)
microcuboids. In recent years, carbon coating has oen been
shown to improve the electrochemical performances of anode
and cathode materials due to its high electronic conductivity,
good lithium permeability, and electrochemical stability.36
When evaluated as anode materials for LIBs, the as-synthesised
porous carbon-decorated material iron oxides exhibited
ꢂ
stirred solution. The mixture was kept at 90 C in an oil bath
pan for one hour. A yellow precipitate was collected by ltration
and washed three times with water. The obtained yellow powder
ꢂ
was dried at 50 C overnight.
2.2 Synthesis of mesoporous carbon-decorated Fe3O4
microcuboids
In order to obtain mesoporous Fe3O4 carbon-decorated micro-
cuboids, the as-prepared FeC2O4$2H2O microcuboids precursor
ꢂ
was transferred into a tube furnace and calcined at 400 C for
2 hours under owing N2. Aer cooling, the FeC2O4$2H2O
microcuboids precursor changed from yellow to black, indi-
cating that the FeC2O4$2H2O microcuboids transformed into
Fe3O4 microcuboids.
2.3 Synthesis of mesoporous a-Fe2O3 microcuboids
The single-crystalline mesoporous a-Fe2O3 microcuboids were
prepared by the following procedure. The atmosphere was
changed, and the microcuboids were heated at 400 ꢂC for
2 hours under air in muffle furnace. As a result, we obtained
a-Fe2O3 microcuboids as a red powder.
2.4 Characterisations
The phases of samples were identied with powder X-ray
diffraction (XRD) using a Rigaku D/Max-2400 X-ray diffractom-
˚
eter with Cu-Ka radiation (l ¼ 1.54056 A). The Raman spectra of
samples were obtained on a Lab RAM HR 800 Raman micro-
scope with an excitation laser beam wavelength of 633 nm. The
morphologies and microstructures of samples were observed by
scanning electron microscopy (SEM; FEI, Nova Nano SEM 450).
Transmission electron microscopy (TEM) and high resolution
TEM were conducted on a Tecnai G2 F20 TEM with an accel-
erating voltage of 200 kV.
2.5 Electrochemical measurements
All electrochemical measurements were carried out using CR
2016-type coin cells assembled in an argon-lled glovebox
(MBRAUN). The working electrode was fabricated by mixing the
electroactive materials (Fe3O4, a-Fe2O3), acetylene black and
polyvinylideneuoride (PVDF) with a mass ratio of 70 : 20 : 10
in N-methyl-2-pyrrolidone (NMP) to form a slurry, which was
subsequently pasted on a pure Cu foil and dried overnight at
120 ꢂC in a vacuum oven. A pure lithium foil was used as both
the counter and reference electrodes. LiPF6 (1 M) in ethylene
Fig. 1 (a) Crystal structure of ferrous oxalate dehydrate and (b) XRD
patterns of (1) FeC2O4$2H2O, (2) a-Fe2O3, and (3) Fe3O4.
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carbonate (EC)–dimethyl carbonate (DMC) (1 : 1 in volume) was
the electrolyte. The galvanostatic charge/discharge tests of the
assembled cells were carried out using an Arbin-BT2000 battery
in the voltage range of 0.05–3 V at room temperature. Cyclic
voltammetry (CV) measurements were performed at the same
voltage window at a scan rate of 0.1 mV sꢀ1 on a BioLogic-VMP3
electrochemical workstation. Electrochemical impedance
spectra (EIS) were also recorded on the electrochemical work-
station in the frequency range of 100 mHz to 100 kHz at room
temperature.
3. Results and discussion
3.1 Structure and morphology
Fig. 3 SEM images of the as-prepared samples: (a, d) FeC2O4$2H2O;
(b, e) a-Fe2O3; and (c, f) Fe3O4.
Fig. 1 shows the XRD patterns of the precursor and the products
aer calcination. In Fig. 1(b-1), all the diffraction peaks can be
indexed to FeC2O4$2H2O (JCPDS no. 72-1305). No other impu-
rity peaks are observed, and the strong and sharp diffraction
peaks indicate the highly crystalline nature. Aer calcining the
iron oxalate precursor in air and inert atmospheres, phase-pure
a-Fe2O3 (JCPDS no. 33-0664) and Fe3O4 (JCPDS no. 19-0629) are
obtained, as shown in Fig. 1(b-2) and (b-3), respectively.
benecial for achieving better electronic conductivity. Ther-
mogravimetric analysis (TGA) was carried out in oxygen at a
heating rate of 10 ꢂC minꢀ1 to determine the content of carbon
(Fig. 2b). The sample was heated to 800 ꢂC to oxidize the Fe3O4
to Fe2O3 and the carbon to CO2. According to the remaining
weight, the content of carbon was around 1.6 wt%.
The morphologies and microstructures of the FeC2O4$2H2O
precursor and the a-Fe2O3 and C–Fe3O4 products derived from
the pyrolysis of the Fe-MOFs were investigated by SEM and TEM
(Fig. 3 and 4). The representative SEM images of the products
shown in Fig. 3 reveal uniform microcuboid-like morphologies
with an average diameter of about 5 mm and a length of about
30 mm. Moreover, the surfaces of the precursor FeC2O4$2H2O
microcuboids appear smooth (Fig. 3d). Aer annealing in air
and N2 at 400 ꢂC for 2 h, the microcuboid morphology was
maintained, while the surfaces became much rougher (Fig. 3e
and f). Meanwhile, supercial cracks developed in the restricted
direction, and porous structures were generated from the
release of gas molecules. The only difference is that the a-Fe2O3
microcuboids remained aggregated together, whereas the
C–Fe3O4 microcuboids separated from each other. From the
corresponding TEM image shown in Fig. 4a, it can be clearly
seen that the precursor FeC2O4$2H2O microcubids are actually
assembled from nanoplates with sizes of about 50–100 nm. The
TEM image in Fig. 4b indicates that the a-Fe2O3 microcuboids
are composed of porous nanoplates, which in turn consist of
The obtained Fe3O4 product was further analysed by Raman
spectroscopy, as presented in Fig. 2a. The spectrum shows the
characteristic peaks of magnetite at 331, 532 and 661 cmꢀ1
,
which can be assigned to the Eg, Tg and A1g vibration modes of
Fe–O,37 respectively, conrming that the as-synthesized product
is composed of a single phase (magnetite Fe3O4). Meanwhile,
two distinguishable peaks can be observed at about 1348 cmꢀ1
and 1590 cmꢀ1, which can be ascribed to the D-band and
G-band of carbon, respectively. The D-band is associated with
the disorder coordination of carbon, allowing zone edge modes
of the graphite structure to become active due to the lack of
long-range order in amorphous and quasi-crystalline forms of
residual carbon in the C–Fe3O4 composites. The G-band corre-
sponds to the E2g stretching vibrations in the basal plane of the
ordered crystalline graphite. The peak intensity ratio between
the D- and G-bands (ID/IG) provides a useful index for comparing
the degree of crystallinity of various carbon materials; the
smaller the ratio of ID/IG, the higher the degree of ordering in
the carbon composition.20,38 The ID/IG ratio for the C–Fe3O4
microcuboids was calculated to be 1.03, demonstrating that the
carbon formed is not very well ordered compared to reports in
the literature.37 Meanwhile, the existence of the carbon matrix is
Fig. 4 TEM and HRTEM images of the as-prepared samples: (a, d)
Fig. 2 (a) Raman spectrum and (b) TGA profile of the as-prepared FeC2O4$2H2O; (b, e) a-Fe2O3; and (c, f) Fe3O4. The insets of (d–f) show
C–Fe3O4.
the corresponding SAED patterns.
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ultrane nanoparticles with sizes of less than 10 nm. The TEM especially for the discharge branch. There are two well-dened
image in Fig. 4c reveals that the C–Fe3O4 microcuboids peaks observed at 0.86 and 0.67 V (vs. Li+/Li) in the rst cathodic
obtained in insert atmosphere are actually assembles consisting sweep, which indicates the two steps of the lithiation reaction of
of ultrane nanoparticles with sizes of about 20 nm linked C–Fe3O4 (step 1, Fe3O4 + 2Li+ + 2eꢀ / Li2(Fe3O4); and step 2,
together by the carbon matrix. Fig. 4d–f show the high resolu- Li2(Fe3O4) + 6Li+ + 6eꢀ / 3Fe + 4Li2O)39–41 as well as the
tion TEM images and corresponding SAED patterns of the occurrence of side reactions with electrolyte on the electrode
FeC2O4$2H2O and iron oxides. Lattice fringes are clearly visible surface. Meanwhile, two peaks are observed at approximately
with a spacing of 0.30 nm in Fig. 4d, which is in accordance with 1.66 and 1.85 V in the anodic process, which are attributed to
the (ꢀ402) planes of FeC2O4$2H2O. The crystallinity of a-Fe2O3 the oxidation of Fe to Fe2+ and Fe3+, respectively. During the
is poor, while the SAED pattern shown in the inset of Fig. 4e following two cycles, the peak intensity drops signicantly,
displays distinct diffraction rings that can be ascribed to the indicating the occurrence of some irreversible reactions via the
(110) and (220) planes of a-Fe2O3. The clear lattice fringes with formation of a SEI lm. It is noteworthy that aer the rst cycle,
spacings of 0.29 nm and 0.25 nm as well as the corresponding the CV curves almost overlapped, which indicates the formation
SAED images in Fig. 4f indicate that the obtained C–Fe3O4 of a stable SEI lm and a good electrochemical reversibility of
particles are well crystallised. Furthermore, the results clearly the mesoporous C–Fe3O4.
show the amorphous carbon coating on the surface of the
The detailed electrochemical behaviour of the a-Fe2O3
highly crystalline Fe3O4, which is quite consistent with the microcuboids was evaluated by CV tests (Fig. 5b). A spiky peak is
aforementioned Raman results.
observed at 0.71 V with two small peaks appearing at 1.08 V and
1.65 V, indicating the following three lithiation steps: a-Fe2O3 +
xLi+ + xeꢀ / a-LixFe2O3; a-LixFe2O3 + (2 ꢀ x)Li+ / Li2Fe2O3
(cubic); and Li2Fe2O3 (cubic) + 4Li+ + 4eꢀ / 2Fe + 3Li2O. First,
at the initial stage of lithium intercalation, a small amount of
lithium can be inserted into the crystal structure of a-Fe2O3 to
form hexagonal a-LixFe2O3 followed by a transformation to
cubic Li2Fe2O3.42–45 The sharp peak corresponds to the complete
reduction of iron from Fe2+ to Fe0 and the decomposition of
electrolyte. In the anodic process, on the other hand, two broad
peaks are recorded at about 1.66 V and 1.91 V, corresponding to
the oxidation of Fe to Fe2+ and further oxidization to Fe3+. The
subsequent cycles are signicantly different from the rst cycle
as only the cathodic peak appears with decreased peak intensity
owning to an irreversible phase transformation from hexagonal
a-LixFe2O3 to cubic Li2Fe2O3 and SEI lm formation.
3.2 Electrochemical performance
Fig. 5a shows the rst three CV curves of the C–Fe3O4 micro-
cuboids at room temperature between 0.05 and 3.0 V at a scan
rate of 0.1 mV sꢀ1. It is apparent that the CV curve of the rst
cycle is quite different from those of the subsequent cycles,
Fig. 5c shows representative discharge/charge voltage proles
of C–Fe3O4 microcuboids at a current density of 100 mA gꢀ1
between 0.05 and 3.0 V. It can be seen that the sample delivers a
very high lithium storage capacity of 1719 mA h gꢀ1 during the
initial discharge process, but a relative low reversible capacity of
1212 mA h gꢀ1 is achieved, leading to an initial Coulombic effi-
ciency of around 70%. The relatively low initial Coulombic effi-
ciency may be caused by the irreversible capacity loss including
the inevitable formation of SEI lms and the decomposition of
the electrolyte, which are common in most anode materials.39–44
In the second cycle, however, the Coulombic efficiency increases
rapidly to 94%. The discharge and charge capacities of the 2nd
cycle are 1212 and 1286 mA h gꢀ1, respectively. Even aer the
50th cycle, the discharge capacity of the C–Fe3O4 microcuboids is
still maintained at 975 mA h gꢀ1, and the Coulombic efficiency
steadily reaches around 95%, which further indicates that the
C–Fe3O4 microcuboids are extraordinarily stable during cycling.
To explore the merits of the as-synthesised iron oxide
Fig.
5 Electrochemical performance of C–Fe3O4, a-Fe2O3,
commercial Fe3O4 nanoparticles and Fe3O4 derived from commercial
FeC2O4 : CV curves of (a) C–Fe3O4 and (b) a-Fe2O3 in the voltage microcuboids as anode materials for LIBs, we tested the cycling
range of 0.05 to 3.0 V (vs. Li+/Li) with a scan rate of 0.1 mV sꢀ1; (c)
performance of the products at 100 mA gꢀ1. For comparison,
commercial Fe3O4 nanoparticles together with the Fe3O4 prod-
ucts derived from commercial FeC2O4 were also investigated
under the same conditions. All results are compiled in Fig. 5d.
Apparently, the as-prepared C–Fe3O4 electrode demonstrates a
voltage profiles plotted for the 1st, 2nd, 25th, and 50th cycles of the
C–Fe3O4 electrode at a current density of 100 mA h gꢀ1; (d) cycle
performance of all samples at a current density of 100 mA h gꢀ1; and
(e) comparison of the rate capabilities of all samples at various current
densities between 100 mA h gꢀ1 and 1000 mA h gꢀ1
.
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much better cycle retention and a much higher reversible the Nyquist plots for C–Fe3O4, a-Fe2O3, commercial Fe3O4
capacity of 975 mA h gꢀ1 than other samples aer 50 cycles. nanoparticles and Fe3O4 products derived from commercial
Compared to the C–Fe3O4 microcuboids, the commercial Fe3O4 FeC2O4 using fresh coin cells; the plots share the common
particles and Fe3O4 derived from commercial FeC2O4 quickly feature of
a high-frequency depressed semicircle and a
lose their activity in the rst few cycles and deliver signicantly medium-frequency depressed semicircle followed by a linear
lower capacities at the end of 50 cycles. For the bare a-Fe2O3, tail in the low frequency region. The common equivalent circuit
though a capacity of 608 mA h gꢀ1 is observed aer 50 cycles, it descriptions of these features are as follows: the intercept on the
still shows poor cycling performance. This demonstrates the Z0 axis at the high-frequency end is the electrolyte resistance
remarkable positive effect of the unique structure of C–Fe3O4 (Rs); Rf and CPE1 are the resistance and capacitance of the solid-
from Fe-MOFs. For the C–Fe3O4 microcuboids, the carbon state interface layer formed on the surface of the electrodes,
coating has been shown to considerably improve the electro- respectively, which correspond to the semicircle at high
chemical performance of anode materials due to its high frequencies; Rct and CPE2 are faradic charge-transfer resistance
conductivity, good lithium permeability, and capability to and its relative double-layer capacitance, respectively, which
withstand the volume change to hold the structure integrity. It correspond to the semicircle at medium frequencies; and the
is well known that the lithium storage capability of Fe3O4 is inclined line at the low-frequency region represents the War-
mainly achieved through the reversible conversion reaction burg impedance (Zw) related to a combination of the diffusional
between the lithium ion and Fe3O4, which forms well dispersed effects of lithium ion on the interface between the active
Fe nanocrystals in the Li2O matrix. In addition, the carbon shell material particles and the electrolyte. The low Rct is benecial to
can prevent the formed Fe nanocrystals from catalysing the enhancing the electron kinetics in electrode materials.48,49
decomposition of the outer SEI. In turn, this allows for the Apparently, the diameter of the semicircle for the C–Fe3O4
growth of a stable SEI on the surface of the carbon shell and electrode is signicantly smaller than other samples, indicating
prevents the continuous rupturing and re-forming of the SEI. that the C–Fe3O4 composite electrodes possess the lowest
Aer the formation of a stable SEI, the high capacity of iron contact and charge-transfer impedances, which can lead to
oxide is very well maintained, and the C–Fe3O4 microcuboid rapid electron transport during the electrochemical lithium
anode thus exhibits an exceedingly excellent cycling perfor- insertion/extraction reaction and thus signicantly improve the
mance.20,46 However, for the bare a-Fe2O3 and commercial Fe3O4 rate performance (Fig. 6).
nanoparticles, the SEI will rupture due to the mechanical strain
Based on the above characterisation results and electro-
generated by the volume expansion/contraction during the chemical investigations, the C–Fe3O4 composite electrode
lithium extraction process, and the electrode surface will thus displays extremely satisfactory electrochemical performance
be cyclically exposed to the electrolyte. The cyclical rupture and and structural stability. These outstanding properties can be
growth of the SEI can cause low Coulombic efficiency, a higher attributed to the distinct structure and morphology of C–Fe3O4,
resistance to ionic transport, and low electronic conductivity of which offer the following benets: (1) the C–Fe3O4 micro-
the entire electrode. The Fe3O4 products derived from cuboids with mesoporous structures enhance the contact area
commercial FeC2O4 lack the unique structure derived from the between the electrode and electrolyte, allowing better electrolyte
synthesis of Fe–MOFs; therefore, their capacity deceased penetration, and the porous structure may shorten the trans-
rapidly, and their cycle performance is much worse than that of port path for both electrons and lithium ions, leading to good
the C–Fe3O4 composites.47
conductivity and fast charge/discharge rates; and (2) the carbon
As expected, the C–Fe3O4 composites exhibit excellent rate framework in intimate contact with the Fe3O4 particles not only
capability (Fig. 5e). When the current density is increased from
100 mA gꢀ1 to 1000 mA gꢀ1, the C–Fe3O4 simultaneously
displays promising specic capacities and rate capabilities. The
discharge and charge capacities remain stable and decrease
regularly with an increased rate. The reversible capacities at
100, 200, 500 and 1000 mA gꢀ1 are about 1124, 1042, 886 and
695 mA h gꢀ1, respectively. Moreover, when the current rate is
nally returned to its initial value of 100 mA gꢀ1, a capacity of
976 mA h gꢀ1 is still retained. This clearly demonstrates that the
unique mesoporous structure along with the carbon framework
of the C–Fe3O4 microcuboids play important roles in relieving
the impact of volume changes during the repeated charge–
discharge processes, which is in good agreement with previous
reports.20,36,46 Thus, these C–Fe3O4 microcuboids present
remarkable cyclic stability and rate performance.
EIS, which are promising tools for investigating electron and
ion diffusion, were collected at frequencies ranging from
Fig. 6 EIS spectra of fresh cells of all the samples in the frequency
range from 100 mHz to 100 kHz. The inset is an equivalent circuit
100 kHz to 100 mHz to investigate the charge transfer and Li+
diffusion kinetics of various electrode materials. Fig. 5d shows model.
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provides more reactive sites on the surface, but also acts as a
very efficient conducting network to facilitate the fast charge
transfer and shorten the diffusion paths for rapid Li+ insertion/
extraction. It is worth pointing out that the as-prepared C–Fe3O4
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