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COMMUNICATION
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Carbon nanotube-amorphous FePO4 core–shell nanowires as cathode
material for Li ion batteriesw
Sung-Wook Kim,z Jungki Ryu,z Chan Beum Park* and Kisuk Kang*
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Published on 08 September 2010 on http://pubs.rsc.org | doi:10.1039/C0CC02524K
Received 13th July 2010, Accepted 23rd August 2010
DOI: 10.1039/c0cc02524k
Carbon nanotube (CNT)-amorphous FePO4 core–shell nanowires are synthesized by aqueous solution-based mineralization
through sequential adsorption of Fe3+ and PO43 ions onto the
CNT surface. The hierarchical nanostructure with FePO4 shell
directly grown on the CNT core exhibits excellent electrochemical properties and performance as a cathode material for
Li ion batteries.
Rechargeable Li ion batteries have attracted great amount of
interest with the modern appeal to renewable green energy
sources. Moreover, the Li ion battery is perceived as an
efficient energy storage device as it has the ability to store and
release electric energy with high density and reversibility.1–4
Although the current status of Li ion battery technology seems
sufficient to power small portable devices such as mobile
phones and lap-top computers, improvements in safety as well
as specific capacity and power are still required for further
applications such as in electric and hybrid electric vehicles.3
Conventional cathode materials such as lithium transition
metal oxides (LiMOx, where M is a transition metal) possess
intrinsic chemical instability at overcharged state. They release
oxygen from the crystal structure or experience irreversible
phase transformation at elevated temperature, which consequently
raises safety concerns during operation.5,6 In this respect,
numerous studies have been carried out in order to find a safe
and stable cathode material. Especially, transition metal compounds containing polyanions such as PO43 are intensively
investigated as the strong P–O covalent bond is believed to
stabilize lattice oxygen even at highly charged state.7–15
Since the pioneering work done by Padhi et al.,7 many
researchers have examined olivine structured LiFePO4 as the
next generation cathode material due to its atop safety, environmental friendliness, affordability, as well as its comparatively
reasonable electrochemical performance.10–13 Li ions diffuse
out of olivine LiFePO4 through one-dimensional channel.10
This one-directional Li ion diffusion can be greatly restricted
by the defects located on the channel. For example, antisite defect (i.e. site interchange between Li+ and Fe2+ ions)
could greatly affect olivine LiFePO4 performance.11 Hence,
controlling the defect formation is one of the most crucial and
sensitive issues of its synthesis. Furthermore, it has been
Department of Materials Science and Engineering, KAIST,
335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea.
E-mail: [email protected], [email protected];
Fax: (+82)42-350-3310; Tel: (+82)42-350-3340, 3341
w Electronic supplementary information (ESI) available: Experimental
details, Fourier Transform Infrared spectroscopy, Raman spectroscopy, and X-ray diffraction of CNTs before and after mineralization.
See DOI: 10.1039/c0cc02524k
z These authors contributed equally to this work.
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The Royal Society of Chemistry 2010
repeatedly reported that a slight change in the synthesis
conditions can significantly alter the electrochemical performance of LiFePO4.12
On the other hand, in the amorphous phase, which is
considered conceptually defect-free phase, Li ion diffusion
cannot be restricted by any defects. Therefore, it is not defectsensitive. Hong et al. previously reported on an amorphous
FePO4 cathode, synthesized by heat treatment of hydrated
FePO42H2O.13 They observed that the reversible Li ion
insertion and extraction could occur without the formation
of any crystalline phases; however, the insertion and extraction
were kinetically limited due to low electronic conductivity.
Okada et al. reported that carbon-coated amorphous FePO4
via mechanical ball milling improved the specific capacity by
about 35% with high rate capability and cyclability.14 In our
recent work,15 we have fabricated carbon-coated amorphous
FePO4 nanotubes by mineralization of a self-assembled
peptide template. The template left an amorphous carbon
layer inside the nanotube after the heat treatment. Moreover,
the unique morphology of the nanotube improved Li ion and
electron transport resulting in a high specific capacity.
In this communication, we first report on the synthesis of
carbon nanotube (CNT)-amorphous FePO4 core–shell nanowires and their application as a cathode material for Li ion
batteries. CNT is an attractive material for hybridization with
electrochemically active materials because it ensures a fast
electron conduction path and a nanosize framework for
coating materials resulting in improved electrochemical
performance.16–18 CNT-amorphous FePO4 core–shell nanowires were fabricated by functionalizing multi-walled CNTs
with carboxylic groups (–COOH) via acid treatment and also
by mineralizing them with FePO4 in aqueous phase under
ambient conditions. We found that CNTs coated with an
ultrathin amorphous FePO4 shell (only a few nm thick)
exhibited remarkable battery performance even at high current
rates. For example, based on the weight of active cathode
material (or total electrode), they showed a specific capacity of
125 (100) and 100 (80) mAh g1 at a current rate of 500 and
1000 mA g1, respectively. We attribute the good performance
of CNT-amorphous FePO4 hybrid nanowires to the CNTbased interconnected core–shell nanostructure of the cathode.
The CNT core provides facile electron transport path and
structural template for active material (i.e., FePO4) while the
ultrathin amorphous FePO4 shell rapidly stores and releases
Li ions.
Fig. 1a schematically illustrates the core–shell structure of
CNT-amorphous FePO4 hybrid nanowires. Li ions could
readily diffuse into and out of the amorphous FePO4 shell
which is an active material, due to its nanometre scale dimension and large surface area. Electrons could also be effectively
Chem. Commun., 2010, 46, 7409–7411
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Fig. 1 (a) Schematic illustration of the CNT-amorphous FePO4
core–shell nanowire. (b) HRTEM image of the fabricated core–shell
nanowire. (c) Lower magnitude image and (d) corresponding EDS
mapping images of Fe (green) and C (orange). (e) EDS spectra of the
core–shell nanowire.
supplied into the insulating amorphous FePO4 shell through
the CNT core during the operation. It is expected that the
improved Li ion and electron transport can enhance battery
performance especially at high rates. We fabricated the
core–shell nanowires by mineralization of amorphous FePO4
along CNTs. Prior to mineralization, the sidewalls of CNTs
were functionalized with carboxyl groups to be used as
adsorption sites. The functionalized CNTs were then mineralized
by sequential adsorption and vacuum filtration of aqueous
solutions containing (i) Fe3+ and (ii) PO43 ions followed by
heat treatment. The detailed description of the mineralization
process is included in the Electronic Supplementary Information, ESI.w After mineralization, the CNT core was fully
covered with a thin amorphous shell (only a few nm thick)
(Fig. 1b–d). The high resolution transmission electron microscopy (HRTEM) image in Fig. 1b clearly identifies the
amorphous nature of the outer shell. Moreover, the energy
dispersive X-ray spectroscopy (EDS) mapping analysis shows
that the outer shell is composed of Fe (green color in Fig. 1d)
while C is mostly populated in the inner part of the wire
(orange color in Fig. 1d). The Fe/P ratio was found to be
1.12 0.26 on average, indicating that the outer shell is
amorphous FePO4 (Fig. 1e). The formation of the amorphous
FePO4 shell was further investigated by various spectroscopic
analyses such as Fourier Transform Infrared spectroscopy,
Raman spectroscopy, and X-ray diffraction. The detailed
description is included in the ESI.w These results clearly
proved the formation of amorphous FePO4 after mineralization. The weight fraction of CNT in the core–shell nanowire
was about 20 wt% measured by carbon/sulfur determinator.
Accordingly, we electrochemically characterized the
CNT-amorphous FePO4 core–shell nanowires to test their
feasibility as a cathode for Li ion batteries. We expected high
specific capacity as well as high rate capability due to the facile
Li ion and electron transport in the hybrid nanowires. In
addition, we anticipated that the complex CNT network and
close adhesion between CNT and amorphous FePO4 would
prevent the inter-wire disconnection and electrical isolation of
electrochemically-active amorphous FePO4, which is induced
by the volume change during operation. The volume change
will strongly affect the capacity fading upon cycling, especially
on nanostructured materials.2 Test cells were assembled into
CR2016-type coin cells composed of the core–shell nanowire
working electrode, a Li metal counter electrode, a polymer
membrane separator, and an organic electrolyte containing a
Li salt. These test cells were then galvanostatically swept at a
voltage in the range of 2.0–4.5 V.
Fig. 2a shows charge–discharge profiles of the initial 5 cycles
of the core–shell electrode at a current rate of 20 mA g1.
While the crystalline olivine LiFePO4 cathode exhibits a clear
potential plateau near 3.4 V due to the well-known two-phase
behavior between Li-rich Li1xFePO4 and Li-poor LixFePO4
(xB0) phases,10 the potential in amorphous FePO4 increases
(or decreases) smoothly as a function of state of charge
(or discharge). This behavior has an advantage in monitoring
the state of charge during battery operation. The discharge
capacity based on the total weight of the electrode material
(CNT and amorphous FePO4 core–shell nanowires) was
about 149 mAh g1 at the first cycle and then it saturated to
140 mAh g1 after several cycles with high reversibility
(indicated by the lower x-axis in Fig. 2a). The negligible
irreversible capacity at the first cycle indicates that the
core–shell nanowires have only few surface defects, where Li
Fig. 2 Electrochemical performance of CNT-amorphous FePO4 core–shell nanowires. (a) Charge–discharge profiles of the initial 5 cycles at a
current rate of 20 mA g1. (b) Specific capacity depending on the number of cycles at current rates of 20, 100, 200, 500, and 1000 mA g1.
(c) Comparison of the specific capacity of the core–shell nanowires and the simple mixture at a current rate of 100 mA g1.
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ions could be trapped. In contrast, many nanostructured
electrodes often suffer from the highly irreversible reaction at
the first cycle mainly due to the increased number of defects on
the enlarged surface area.2 More interestingly, the specific
capacity based only on the weight of the amorphous FePO4
shell was about 175 mAh g1 (indicated by the upper x-axis in
Fig. 2a), which is almost comparable to the maximum theoretical capacity of FePO4 cathode (178 mAh g1). In general, it
is hard to obtain the theoretical capacity in a bulk or submicrometre sized particle due to its low electronic conductivity
and slow Li ion diffusion rate. Facile Li ion diffusion through
the outer shell and fast electron transport through the core are
to be responsible for the observed high Li-storage capacity.
We further investigated high-rate performance of the
core–shell nanowires to examine the feasibility of our strategy.
The high-rate capability is essential for future battery applications such as in electric vehicles that require high power
density during operation. Fig. 2b shows the specific capacity
of core–shell nanowires at increased current rates. The left
x-axis represents the specific capacity based on the total weight
of the electrode material (CNT and amorphous FePO4
core–shell nanowires), whereas the right x-axis represents the
specific capacity based on the weight of the active component
only (i.e., amorphous FePO4 shell). High specific capacity of
amorphous FePO4 shell was still sustained with good capacity
retention upon cycling even at high current rates up to
1000 mA g1. The reversible capacity was about 160 (or 128),
133 (or 107), 125 (or 100) and 100 (or 80) mAh g1 based on
the weight of the amorphous FePO4 shell (or based on the
total weight of the core–shell nanowires) at a current rate of
100, 200, 500, and 1000 mA g1 respectively. The reversible
capacity was well retained upon cycling at all rates tested, due
to the high structural stability of the core–shell nanowire
network. For comparison, we also examined the electrochemical performance of a simple mixture of CNTs and
amorphous FePO4 at 100 mA g1 in Fig. 2c. The simple
mixture was prepared by mechanical mixing CNTs and
FePO42H2O followed by heat treatment. According to the
results, the specific capacity of the core–shell nanowires is
much greater than the specific capacity of the simple mixture.
Again, this indicates the facile transport capability of Li ions
and electrons into amorphous FePO4 in the core–shell structure, which is essential for the improvement of the electrochemical performance.
To summarize, we have successfully fabricated hybrid nanowires composed of a highly conductive CNT core and highly
active nanosized amorphous FePO4 shell via mineralization
onto the CNT surface. The CNT core with complex network
does not only act as a template for the synthesis of organic/
inorganic hybrid nanowires, but also plays the role of electron
transport path and structural support during operation. The
amorphous FePO4 shell coated onto the surface of CNT
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ensures facile transport of Li ions as a consequence of its
small dimension and high surface area. The core–shell nanowires exhibited excellent electrochemical performance, i.e.,
high specific capacity comparable to the theoretical value
and high rate capability with good capacity retention upon
cycling. Our results demonstrate that CNT-amorphous
core–shell nanowires are a promising cathode for Li ion
batteries. The environmentally-benign and simple mineralization route of their fabrication is also beneficial to the industrialscale synthesis of various CNT-based hybrid nanomaterials.
This research was supported by Energy Resource Technology Development program funded (2008-E-EL11-P-08-3-010)
and Energy Resources Technology R&D program
(20092020100040) under the Ministry of Knowledge Economy,
Republic of Korea. This research was also partially supported
General Research program (2009-0094219), Converging
Research Center program (2009-0082069 and 2009-0082276),
National Research Laboratory program (R0A-2008-00020041-0), and Engineering Research Center program
(2008-0062205) through the National Research Foundation
(NRF) funded by the Ministry of Education, Science, and
Technology.
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