Electrosprayed porous Fe O /carbon microspheres as anode ... · Nano Res. 2018, 11(2): 892–904 893 1 Introduction In order to address large-scale environmental deterioration, lithium-ion
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Electrosprayed porous Fe3O4/carbon microspheres as anode materials for high-performance lithium-ion batteries
1 Engineering Laboratory for Next Generation Power and Energy Storage Batteries, and Engineering Laboratory for Functionalized
Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China 2 School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 3 Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay,
bright spots from iron are homogeneously dispersed
in the carbon background, therefore indicating a
uniform distribution of Fe3O4 nanoparticles in PFCMs.
We can also distinctly observe that numerous voids
are distributed in the matrix uniformly, which are
formed by decomposition of the polymers and
accumulation of the CNTs and KB particles.
Figure 2 (a) and (b) SEM, (c) TEM and (d) HRTEM images of PFCMs; (e) STEM image of PFCMs and (f)–(h) EDX elemental mappings of (f) iron, (g) oxygen and (h) carbon.
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897 Nano Res. 2018, 11(2): 892–904
The crystal structure of as-prepared PFCMs was
investigated by XRD. As shown in Fig. 3(a), all the
diffraction peaks of PFCMs agree well with the standard
XRD data of cubic-phase Fe3O4 (JCPDS card, file no.
89-4319) with the exception of a broad peak at around
26°, which corresponds to the (002) reflection of the
carbon matrix. The XRD results indicate that a pure
Fe3O4 component is formed in the carbon matrix by
decomposing Fe(acac)2 under the selective annealing
conditions.
Nitrogen adsorption/desorption isotherm measure-
ments were carried out to characterize the porous
structure of PFCMs (Fig. 3(b)). The BET specific surface
area and the pore volume of PFCMs are calculated
to be 190 m2·g–1 and 0.61 cm3·g–1, respectively. The
relative pore size distribution based on Barrett–Joyner–
Halenda (BJH) desorption is also shown in the inset
of Fig. 3(b). We note that the nanopores deliver a
hierarchical size distribution, mainly ranging from 2
to 10 nm and centering around 3.5 nm. The hierarchical
pore structure can not only shorten the lithium ion
pathway by facilitating electrolyte infiltration, but also
can accommodate the volume changes Fe3O4 undergoes
during cycling.
TGA was performed from 30 to 1,000 °C with a
heating rate of 10 °C·min–1 in an air atmosphere to
verify the actual Fe3O4 content of PFCMs. The weight
loss curve of PFCMs is shown in Fig. 3. The initial
weight-loss of PFCMs below 200 °C is small and
attributed to moisture volatilization. For temperature
increases from 200 to 800 °C, the Fe3O4 component was
entirely oxidized to Fe2O3, while the carbon matrix
was completely pyrolyzed to CO2/CO gases. This
results in a drastic weight loss for the PFCMs. According
to the TGA curve, the contents of Fe3O4 and carbon
can be calculated to be 36.7 wt.% and 63.3 wt.%, res-
pectively (further details are provided in the ESM).
3.3 Electrochemical properties of PFCMs
The electrochemical characteristics of the discharge–
charge, CV, and EIS curves for PFCM anodes are
shown in Fig. 4. The discharge–charge profiles of
PCFMs are shown in Fig. 4(a) for the voltage range of
0.01–3.0 V at different current densities (from 0.1 to
Figure 3 (a) XRD pattern, (b) nitrogen adsorption/desorption isotherm and pore size distribution, and (c) TGA curve of PFCMs.
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898 Nano Res. 2018, 11(2): 892–904
5 A·g–1). The various cycles at 0.1 A·g–1 are shown
in Fig. 4(b). All the discharge–charge profiles present
obvious voltage plateaus, which is essential for the
industrial application of electrode materials. This
therefore indicates that Fe3O4 active materials are well
supported in the PFCM hybrids. As shown in Fig. 4(b),
the PFCM anode exhibits an initial charge capacity
of 999 mAh·g–1 and an initial discharge capacity of
1,795 mAh·g–1, the latter of which is considerably higher
than the theoretical specific capacity of Fe3O4. The
ultrahigh initial discharge capacity may mainly result
from irreversible processes such as the reaction of
lithium with oxygen-containing functional groups, the
inevitable decomposition of the electrolyte to form an
SEI layer, and irreversible Li+ trapping in the matrix
[24, 39]. Thus, the irreversible capacity loss of the first
discharge–charge process could be attributed to the
formation of a SEI layer via electrolyte decomposition
and trapped Li+ inserted in the abundant inner holes
of PFCMs [40, 41]. These processes are ubiquitous for
the redox reactions of most anode materials in LIBs.
Figure 4(c) shows the CV profiles of the PFCM
electrode for the initial 10 cycles. These were recorded
in the potential window of 0.01–3.0 V at a scan rate of
0.1 mV·s–1. There are two obvious peaks in the initial
discharge process. The strong peak at 0.76 V reveals
the reduction reaction Fe3O4 + Li+ → Fe0 + Li2O, and the
small peak at 0.93 V may be ascribed to the irreversible
reactions of the electrolyte to form the SEI layer [42,
43]. For the charge process, two oxidation peaks occur
at around 1.58 and 1.85 V and can be attributed to the
oxidation of Fe0 to Fe2+ and Fe3+ during the delithiation
reactions [44–46]. During the subsequent cycles, the
reduction and oxidation peaks are relatively unaltered,
therefore indicating the presence of a stable SEI film
on the PFCM surface and an excellent reversibility
for the lithiation/delithiation process.
EIS measurements were conducted in the frequency
range of 0.1 Hz to 100 kHz to further reveal the
electrochemical characteristics of the PFCM electrode.
The impedance spectra of PFCMs in the initial state,
as well as the fully charged states (after the 3rd and
100th cycles at 1 A·g–1) are both shown in Fig. 4(d).
The depressed semicircle in the high-medium frequency
region indicates a charge transfer process, while the
inclined line in the low frequency region suggests the
diffusion of lithium ions into the electrode materials.
It is noted that the semicircle radius after three cycles
Figure 4 Discharge–charge profiles of PFCM anode at (a) different current densities and (b) a current density of 0.1 A·g–1 for different cycles; (c) CV curves of PFCM electrode at a scan rate of 0.1 mV·s–1; (d) impedance spectra of PFCM electrode before and after cycling.
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899 Nano Res. 2018, 11(2): 892–904
is slightly smaller than in the initial state due to the
activation reaction. The slight increase of resistance
after the 100th cycle may be related to the stable SEI
film formed on the outside surface of PFCMs during
cycling [26, 47].
To better understand the aforementioned phenomena,
we further investigate the electrochemical capacities
and rate performances of PFCM anodes in detail. The
cycle performance of PFCMs in a voltage range of
0.01–3.0 V vs. Li/Li+ at 0.1 A·g–1 is shown in Fig. 5(a).
After activating for a few cycles, the PFCMs have a
stable Coulombic efficiency (above 97%) and the
reversible capacity is maintained at ~ 1,000 mAh·g–1.
Notably, the specific capacity of PFCMs keeps increasing
through almost the entire discharge/charge processes,
rising to 1,317 mAh·g–1 at the 130th cycle. Such a high
and stable capacity at a low current density (0.1 A·g–1)
resulted from the optimized and stabilized micros-
tructures of PFCMs. CNTs, KB, and amorphous carbon
are intertwined together to construct an electrically
conductive skeleton, which greatly promotes the
electron/ion transportation. Additionally, coating the
confined Fe3O4 nanoparticles with carbon layers can
efficiently avoid particle agglomeration and achieve
fast reaction kinetics. Furthermore, abundant mesopores
can buffer the volume changes of Fe3O4 during
lithiation/delithiation processes. As a result, a stable
and thin SEI layer can be generated on the surface of
Fe3O4/carbon composite. Meanwhile, consumption of
electrolyte is effectively suppressed [44, 48–50].
It is worth noting that the capacity of the PFCM
electrode clearly increased under the current density
of 0.1 A·g–1. The gradual increase of capacity might
be attributed to the formation of stable organic
polymeric/gel-like layers resulting from kinetically
activated electrolyte degradation [35]. These organic
polymeric/gel-like layers can repeatly absorb and
release lithium ions, resulting in a slight increase
of capacity during cycling. This effect is especially
pronounced under low current densities [24, 26, 33,
38, 45, 51–53]. For instance, Lou and co-authors
fabricated carbon coated α-Fe2O3 hollow nanohorns
on CNT backbones. As an anode in LIBs, the capacity
of the α-Fe2O3/carbon composite shows a gradual
increase from 660 to 820 mAh·g–1 after 100 cycles at a
current density of 500 mA·g–1 [24]. Lu et al. prepared
carbon-coated Fe3O4 nanotubes, which delivered a
capacity increase up to 1,155 mAh·g–1 during the initial
90 cycles under 0.2 A·g–1 [34]. Furthermore, while using
in-situ TEM to study the structural evolution of lithium
insertion/extraction in electrochemically prelithiated
Fe2O3 nanoparticles confined in CNTs, Cheng’s group
also observed this phenomenon [26]. Most of these
researchers ascribed this phenomenon to the reversible
growth of a polymeric gel-like film resulting from
in CNTs during lithium insertion/extraction by in situ
TEM. They observed a high reversible capacity of
2,071 mAh·g−1 for the encapsulated Fe2O3 nanoparticles
in CNTs [9]. It was also demonstrated that the capacity
increase might relate to the following factors: extra
lithium storage from the two-phase capacitive behavior
of the Li2O/Fe interface confined in the pores of the
carbon matrix, which allows for the storage of Li+ on
the Li2O compound side and electrons on the Fe side
[62, 63]; and a reversible conversion reaction of LiOH to
form LiH and Li2O on the organic polymeric/gel-like
layers [2, 26, 64].
It is well-known that, compared to gravimetric
capacity, the volumetric capacity for porous materials
is a more appropriate index for determining the
practicality of using these materials for industrial
applications. Based on the BET and TGA results, as
well as the specific capacities shown in Figs. 5(a) and
5(b), the reversible volumetric capacities of PFCM
anodes are more than 1,000 mAh·cm–1 at 0.1 A·g–1,
which is higher than that of graphite anodes (about
760 mAh·cm–3). The detailed calculation process is
provided in the ESM.
Remarkably, the PFCM electrode exhibits long
lifespans of over 300 cycles at the high current densities
of 1 and 5 A·g–1 (Fig. 5(c)). Its initial charge capacities
are 766 and 616 mAh·g–1 at 1 and 5 A·g–1, respectively.
After 300 cycles, the discharge capacities still remained
at 746 and 525 mAh·g–1 under the current densities of
Figure 5 (a) Cycling performance and Coulombic efficiency of PFCMs anode at a current density of 0.1 A·g–1; (b) rate capability and Coulombic efficiency of PFCMs anode under different current densities; (c) cycling performances and Coulombic efficiency at high current densities of 1 and 5 A·g–1.
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901 Nano Res. 2018, 11(2): 892–904
1 and 5 A·g–1, respectively. These display the corres-
ponding capacity retentions of 97% and 85%. The
superior rate performance and cycling stability of
PFCMs are fairly competitive compared to other
Fe3O4-based anodes reported in recent literatures
(Table S1 in the ESM). The excellent electrochemical
performances of PFCMs should therefore be attributed
to: (1) the well-dispersed and encapsulated Fe3O4
nanoparticles; (2) the support, protection, and high
electronic conductivity provided by the porous carbon
matrix; and (3) the high mechanical strength, abundant
pore volume, and hierarchical pore distribution.
Cycling performances of the carbon matrix anode
without Fe3O4 and the PFCM anode with a lower
concentration of CNTs (adding 0.05 g CNTs into
precursor solution, while the masses of the other
components are unchanged) is shown in Fig. S3 in
the ESM. These studies were conducted in order to
investigate the roles of both the entire carbon matrix
and CNTs in the electrochemical performances of
PFCMs. Figure S3(a) in the ESM shows the cycle
performance of the porous carbon matrix anode obtained
by etching Fe3O4 in PFCMs with hydrochloric acid. It
can be seen that the carbon matrix exhibits capacities
of 419 and 370 mAh·g–1 at 0.1 and 1 A·g–1 after 200 cycles
without significant capacity decay from the 50th to the
200th cycle. This therefore indicates that the stable
carbon matrix plays a crucial role in the electrochemical
performance of our PFCM composite anode. As shown
in Fig. S3(b) in the ESM, the reversible specific capacities
of PFCMs with lower CNT content start to clearly
decrease after 70 cycles at current densities of both
0.1 and 1 A·g–1, which further confirms the skeleton
support and conductive effects of CNTs on the whole
composite material.
The morphology and inner microstructure of the
PFCM electrode after 100 cycles at 1 A·g–1 are shown
in Fig. S5 in the ESM. The spherical structure of the
cycled PFCMs (Figs. S5(a) and S5(c) in the ESM) is
maintained and indicates a good structural stability
of the PFCM anode materials during electrochemical
cycling. Furthermore, the STEM image (Fig. S5(d)
in the ESM) and EDX elemental mapping images
(Figs. S5(e)–S5(g) in the ESM) show that the Fe3O4
nanoparticles are still homogeneously dispersed in
the carbon matrix without obvious agglomeration. The
superior structural stability of the PFCM electrode
can be ascribed to the robust support and protection
of the porous carbon matrix composed of CNTs, KB,
and amorphous carbon, which can effectively prevent
the aggregation of Fe3O4 nanoparticles, accommodate
their volumetric change, and maintain their structural
integrity during cycling, thus, leading to the excellent
electrochemical performances of the PFCM anode of
LIBs.
4 Conclusion
PFCMs with superior electrical conductivity and
mechanical strength were successfully fabricated based
on a facile electrospray synthesis. The PFCMs show
excellent electrochemical performances as the anode
of LIBs. The composite features with homogeneously
dispersed Fe3O4 nanoparticles were confined in the
highly conductive carbon matrix composed of CNTs,
KB, and amorphous carbon. The porous structure
provides enough space to accommodate the volume
change of Fe3O4 nanoparticles. Also, the interlaced
networks of interconnecting pores and carbon
framework provide perfect channels and pathways
for electrolyte penetration and electron conduction.
As a result, the PFCM electrodes exhibit high specific
capacities of 1,317 mAh·g–1 at 0.1 A·g–1 after 130 cycles,
and 746 mAh·g–1 at 1 A·g–1 and 525 mAh·g–1 at 5 A·g–1
after 300 cycles. This work has provided a facile
method to fabricate metal oxide electrodes with a
well-designed structure for high-performance energy
storage.
Acknowledgements
This work was supported by the National Basic
Research Program of China (No. 2014CB932400), Joint
Fund of the National Natural Science Foundation
of China and the China Academy of Engineering
Physics (Nos. U1330123 and U1401243), the National
Natural Science Foundation of China (No. 51232005),
and Shenzhen Technical Plan Project (No. JCYJ
20150529164918735).
Electronic Supplementary Material: Supplementary
material (calculation methods of Fe3O4 content and
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902 Nano Res. 2018, 11(2): 892–904
volumetric capacity; SEM images of various as-sprayed
precursor microspheres before annealing; electro-
chemical performances of carbon matrix and PFCM
with less CNTs; SEM, TEM and EDX elemental
mapping images of the PFCM anode after cycling;
and a comparison of electrochemical performances of
Fe3O4-based anodes for LIBs) is available in the online
version of this article at https://doi.org/10.1007/
s12274-017-1700-6.
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