Synthesis of two-dimensional transition-metal phosphates with highly ordered mesoporous structures for lithium-ion battery applications

Article abstract of DOI:10.1002/anie.201404615

Materials with ordered mesoporous structures have shown great potential in a wide range of applications. In particular, the combination of mesoporosity, low dimensionality, and well-defined morphology in nanostructures may exhibit even more attractive features. However, the synthesis of such structures is still challenging in polar solvents. Herein, we report the preparation of ultrathin two-dimensional (2D) nanoflakes of transition-metal phosphates, including FePO₄, Mn₃(PO₄)₂, and Co₃(PO₄)₂, with highly ordered mesoporous structures in a nonpolar solvent. The as-obtained nanoflakes with thicknesses of about 3.7nm are constructed from a single layer of parallel-packed pore channels. These uniquely ordered mesoporous 2D nanostructures may originate from the 2D assembly of cylindrical micelles formed by the amphiphilic precursors in the nonpolar solvent. The 2D mesoporous FePO₄ nanoflakes were used as the cathode for a lithium-ion battery, which exhibits excellent stability and high rate capabilities. Ultrathin two-dimensional (2D) nanoflakes of transition-metal phosphates, including FePO₄ (see TEM image), Mn ₃(PO₄)₂, and Co₃(PO ₄)₂, with highly ordered mesoporous structures have been successfully synthesized in a nonpolar solvent. The use of the 2D mesoporous FePO₄ nanoflake as a cathode in a lithium-ion battery resulted in excellent stability and high rate capabilities.

Full text of DOI:10.1002/anie.201404615

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Angewandte
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DOI: 10.1002/anie.201404615
Lithium-Ion Batteries Very Important Paper
Synthesis of Two-Dimensional Transition-Metal Phosphates with
Highly Ordered Mesoporous Structures for Lithium-Ion Battery
Applications**
Dan Yang, Ziyang Lu, Xianhong Rui, Xiao Huang, Hai Li, Jixin Zhu, Wenyu Zhang,
Yeng Ming Lam, Huey Hoon Hng, Hua Zhang,* and Qingyu Yan*
Abstract: Materials with ordered mesoporous structures have
shown great potential in a wide range of applications. In
particular, the combination of mesoporosity, low dimension-
ality, and well-defined morphology in nanostructures may
exhibit even more attractive features. However, the synthesis of
such structures is still challenging in polar solvents. Herein, we
report the preparation of ultrathin two-dimensional (2D)
nanoflakes of transition-metal phosphates, including FePO4,
Mn3(PO4)2, and Co3(PO4)2, with highly ordered mesoporous
structures in a nonpolar solvent. The as-obtained nanoflakes
with thicknesses of about 3.7 nm are constructed from a single
layer of parallel-packed pore channels. These uniquely ordered
mesoporous 2D nanostructures may originate from the 2D
assembly of cylindrical micelles formed by the amphiphilic
precursors in the nonpolar solvent. The 2D mesoporous FePO4
nanoflakes were used as the cathode for a lithium-ion battery,
which exhibits excellent stability and high rate capabilities.
sion, and storage devices, etc.[1] The synthetic methods for
preparing the ordered mesoporous materials can be catego-
rized into two general types, known as the hard-template and
soft-template methods. The former, which commonly uses
siliceous materials as the template, involves the tedious
removal of the template with HF or NaOH solution, and thus
is not applicable to materials that are sensitive to acid or
alkali.[2] The use of different structure-directing agents in
latter method allows the generation of ordered mesoporous
structures through charge density mapping, hydrogen bond-
ing, or dative coordinate bonding interactions.[3] The polar
solvent, such as water, ethanol, formamide, or methanol,
normally used in the soft-template methods plays an impor-
tant role in the formation of ordered mesoporous structure, as
it can provide sufficient electrostatic shielding and effective
hydrogen bonding, and the solvent evaporation can some-
times assist the self-assembly of molecules. Unfortunately,
nonpolar solvents, especially those with high boiling points,
have rarely been considered in the synthesis of ordered
mesoporous structures because of their negligible interaction
with the molecules in the solution and the high temperature
necessary to remove them.[4] However, many important
functional nanomaterials, for example, PbSe and CdSe, are
formed in the oil phase, that is, a nonpolar solvent.[5]
Therefore, the extension of the synthesis of mesoporous
materials to nonpolar solvents would broaden the scope of
materials with ordered mesoporous structures and allow more
interesting applications to be explored.
C
ontrol over the morphology, composition, and pore
structures of mesoporous materials has proved to be crucial
for their applications in catalysis, biosensors, energy conver-
[*] D. Yang, Dr. Z. Y. Lu, Dr. X. H. Rui, X. Huang, Dr. H. Li, W. Y. Zhang,
Prof. Y. M. Lam, Prof. H. H. Hng, Prof. H. Zhang, Prof. Q. Y. Yan
School of Materials Science and Engineering
Nanyang Technological University
50 Nanyang Aveue, Singapore 637819 (Singapore)
E-mail: hzhang@ntu.edu.sg
alexyan@ntu.edu.sg
In particular, the combination of porous channels with
defined structure and morphology, for example, one-dimen-
sional (1D) nanowires/nanotubes, two-dimensional (2D)
nanosheets etc., may induce attractive features for various
applications,[6] which, however, have mainly been demon-
strated for non-ordered porous structures. Herein, we report
a general approach for the synthesis of ultrathin 2D nano-
flakes of amorphous transition-metal phosphates, including
FePO4, Mn3(PO4)2, and Co3(PO4)2, with ordered pore chan-
nels. The ordered pore channels are aligned with their long
axis parallel to the primary surfaces of the nanoflakes.
Tetradecylphosphonic acid (TDPA) served as both the
structure-directing agent and reactant to generate phosphates
of the corresponding organic metal complex (i.e. metal
oleate). A vacuum annealing process allows ordered pore
channels with an average inner diameter of about 2.5 nm to be
prepared in the ultrathin flakes. As a proof-of-concept
application of the amorphous metal phosphates, the electro-
chemical properties of mesoporous FePO4 as a cathode for
D. Yang, Dr. J. X. Zhu, Prof. Q. Y. Yan
TUM CREATE, 1 CREATE Way, no. 10-02 CREATE Tower
Singapore 138602 (Singapore)
Prof. Q. Y. Yan
Energy Research Institute@NTU, Nanyang Technological University
Singapore 637459 (Singapore)
[**] This work was supported by MOE under AcRF Tier 2 (ARC 26/13,
no. MOE2013-T2-1-034), AcRF Tier 1 (RG2/13, RG 61/12, RGT18/
13, RG5/13), A*STAR SERC grant 1021700144, MPA 23/04.15.03
grant, a Start-Up Grant (M4080865.070.706022) in Singapore, and
Singapore National Research Foundation under the CREATE
program: EMobility in Megacities. This research is also conducted
by NTU-HUJ-BGU Nanomaterials for Energy and Water Manage-
ment Programme under the Campus for Research Excellence and
Technological Enterprise (CREATE) that is supported by the
National Research Foundation, Prime Minister’s Office, Singapore.
Supporting information for this article (including complete details
of the synthesis of metal phosphates with ordered mesoporous
structures and characterization methods) is available on the WWW
under http://dx.doi.org/10.1002/anie.201404615.
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lithium-ion batteries (LIBs) was studied, which delivered
a large discharge capacity of 160 mAhgÀ1 for the first cycle
and good charge/discharge cycling stability.
strates the size of the flakes to be in the range of several tens
to hundreds of nanometers, and the thickness of a single flake
to be about 3.7 nm (Figure 1c,d). The low-angle X-ray
diffraction (XRD) pattern for the sample shows four resolved
diffraction peaks (Figure 1e). The d values for the four peaks
were calculated from Braggꢀs law (2dsinq = nl) to be
approximately 3.77, 1.85, 1.28, and 0.96 nm. The ratio of
these d values is close to 1:1/2:1/3:1/4. Therefore, the four peaks
can be accordingly indexed as (100), (200), (300), and (400)
reflections, thus indicating the highly ordered structure along
the [100] direction. As shown in the inset of Figure 1e, the
magnified TEM image reveals that the distance between the
centers of two neighboring channels (defined as Dcenter) is
3.72 nm, which is highly consistent with the d value (3.77 nm)
calculated from the low-angle XRD pattern. Importantly,
both the d and Dcenter values are close to the thickness of the
flake, as determined by AFM (3.7 nm, Figure 1d), which
indicates the FePO4 flakes may consist of only one single
layer of pore channels. The wide-angle XRD pattern shows no
apparent diffraction peaks (see Figure S4 in the Supporting
Information), thus indicating the amorphous nature of the
products.
The FePO4 flakes yield type-IV nitrogen adsorption and
desorption isotherms with a hysteresis in the middle relative
pressures (see Figure S5 in the Supporting Information),
which is characteristic of uniform mesopores.[9] The BET
surface area is 108 cm2 gÀ1 and the pore size distribution was
calculated by the Barrett–Joyner–Halenda (BJH) model,
which showed a narrow peak with a mean value of 2.5 nm (see
inset in Figure S5 in the Supporting Information). This value
corresponds to the diameter of the channel-shaped pores
(defined as dpore). From the values of dpore and Dcenter, the wall
thickness of the mesopores, that is, (DcenterÀdpore)/2, was about
0.6 nm.
As a typical experiment, iron oleate (Fe(OA)3) was
prepared by reaction of FeCl3 with sodium oleate (NaOA).
Then, 1 mmol Fe(OA)3 and 1 mmol TDPA powder were
added to 5 mL 1-octadecene (ODE) solution and the reaction
was kept at 3008C for 1 h. The precipitate was washed several
times with hexane, and dried at room temperature. The
resulting product was then further annealed in a gas mixture
of H2/Ar at 4008C for 5 h under vacuum (ca. 10À2 torr) to
remove excessive surfactant.
To investigate the composition of the obtained product,
X-ray photoelectron spectroscopy (XPS) was carried out (see
Figure S1 in the Supporting Information). The peaks corre-
sponding to the binding energy of Fe2p appear at 712.6 and
725.7 eV (see Figure S1a in the Supporting Information),
which indicates an FeIII state.[7] The P2p (see Figure S1b in the
Supporting Information) and O1s spectra (see Figure S2 in the
Supporting Information) show the feature peaks at 531.5 and
133.3 eV, respectively, which match the characteristic peaks of
(PO4)3À very well.[7] The Fe/P atomic ratio was 1.05:1 as
measured by inductively coupled plasma (ICP) analysis.
These results indicate that the as-obtained sample is FePO4.
Furthermore, the XPS spectrum for C1s (see Figure S2b in the
Supporting Information) and Raman spectra (see Figure S3 in
the Supporting Information) confirmed the presence of
carbon in the composites. The carbon content was determined
to be approximately 30% on the basis of a previously
reported method that involved dissolving the sample in
a hydrochloric acid solution and weighing the remains.[8]
As shown in the transmission electron microscope (TEM)
image (Figure 1a), the obtained FePO4 was composed of
interconnected flakes. The ordered continuous channels with
uniform diameters in each flake are clearly observed (Fig-
ure 1a,b). Atomic force microscopy (AFM) analysis demon-
Our method is very general. Besides FePO4, this synthetic
procedure can be extended to the preparation of manganese
and cobalt phosphates, by using Mn(OA)2 and Co(OA)2 as
precursors, respectively. XPS results showed the metals are in
the MnII and CoII states, with the representative peak located
at 641.2 and 780.2 eV, respectively (see Figures S6 and S7 in
3À
the Supporting Information). The presence of PO4 was
confirmed by the binding energy of P2p being found at around
133 eV (see Figures S6b and S7b in the Supporting Informa-
tion). ICP measurements showed the Mn/P and Co/P ratio to
be 1.47:1 and 1.52:1, which corresponds to the stoichiometric
ratios of Mn3(PO4)2 and Co3(PO4)2, respectively. The low-
magnification TEM images (see Figures S8 and S9 in the
Supporting Information) showed both samples to have an
ultrathin-flake morphology. The shape of the flakes is
irregular and their sizes are also not very uniform, ranging
from tens to hundreds of nanometers. AFM images (see
Figures S10 and S11 in the Supporting Information) showed
that the thickness of the Mn3(PO4)2 and Co3(PO4)2 nanoflakes
is 4.0 and 5.2 nm, respectively. The high-magnification TEM
images (Figure 2a,b) revealed pore channels in both samples,
with their long axes aligned parallel to the primary surfaces of
the thin flakes. The Dcenter values were 3.53 and 3.44 nm for
Mn3(PO4)2 and Co3(PO4)2, respectively. The low-angle XRD
patterns (Figure 2c,d) showed a series of peaks at 2.46, 4.83,
Figure 1. a) Low-magnification TEM image showing the 2D morphol-
ogy of FePO4 flakes (scale bar: 50 nm). b) Magnified TEM image of
a typical FePO4 nanoflake (scale bar: 20 nm). c) Low-magnification
AFM image of FePO4 flakes (scale bar: 1 mm). d) AFM image and
corresponding height profile of a typical FePO4 nanoflake (scale bar:
20 nm). e) Low-angle XRD pattern of FePO4 flakes (inset: magnified
TEM image of FePO4 nanoflake, showing the distance between the
neighboring channels is 3.72 nm).
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Figure 3. a) Cycling performance of mesoporous FePO4 nanoflakes at
0.1 C. b) Cycling performance of mesoporous FePO4 nanoflakes at
different C rates.
value.[10c] The rate capability of the ordered porous thin flakes
of FePO4 was also tested (Figure 3b). The discharge capacity
was 149.1, 140.7, 130.6, 120.0, 116.5, 104.9, and 89.4 mAhgÀ1
when the battery was cycled at 0.1, 0.2, 0.5, 1, 2, 5, and 10 C,
respectively. Such a good retention of capacity even at a high
C rate indicates the excellent rate capabilities of the amor-
phous FePO4. This excellent cyclic performance may be
related to structural features including the ultrathin thickness
of the amorphous FePO4 and the ordered mesoporous
structure with long channels along its primary surface.
Importantly, the TEM image (see Figure S17 in the Support-
ing Information) showed that the well-defined mesoporous
structures of the electrode material were preserved after the
cycling test, thus indicating the excellent structure stability of
the as-prepared FePO4 flakes. The formation of the ordered
mesoporous structure is proposed to originate from the
cooperative assembly of composite micelles formed by TDPA
and Fe(OA)3, as a result of their amphiphilic molecular
structure.[11] When the temperature is high enough to break
Figure 2. a,b) TEM images (scale bar: 25 nm for (a) and 20 nm for
(b)) and c,d) low-angle XRD patterns for the Mn3(PO4)2 (a,c) and
Co3(PO4)2 (b,d) mesoporous structures. Insets in (c) and (d): magni-
fied images of Mn3(PO4)2 and Co3(PO4)2 samples.
7.13, and 9.488 for Mn3(PO4)2, and peaks at 2.72, 5.32, and
7.918 for Co3(PO4)2, which correspond to the d spacings of
3.58, 1.82, 1.24, and 0.93 nm for Mn3(PO4)2 and 3.24, 1.66, and
1.12 nm for Co3(PO4)2. The peaks are indexed as (100), (200),
(300) ,and (400) for Mn3(PO4)2, and (100), (200), and (300) for
Co3(PO4)2. Low-angle XRD only showed a (100) reflection
and its multiples, which indicates that these products contain
highly oriented 2D mesostructures, and are in good agree-
ment with the TEM (Figure 2a,b) and AFM observations (see
Figures S10 and S11 in the Supporting Information). Wide-
angle XRD showed that Mn3(PO4)2 and Co3(PO4)2 samples
are also amorphous (see Figures S12 and S13 in the Support-
ing Information). The BET surface areas were determined on
the basis of the nitrogen adsorption and desorption isotherms
(see Figures S14 and S15 in the Supporting Information) to be
112.8 m2 gÀ1 for Mn3(PO4)2 with a channel diameter of
2.43 nm, and 104.8 m2 gÀ1 for Co3(PO4)2 with a channel
diameter of 2.23 nm.
Amorphous FePO4 with a controllable shape and size has
attracted great interest as an alternative cathode material for
lithium-ion batteries (LIBs), and such a lithium-free cathode
is expected to spur the development of LIBs with Li-
containing anodes.[10] As a proof of concept, we investigated
the cathode performance of amorphous FePO4 with an
ordered mesoporous structure for LIBs. Galvanostatic dis-
charge/charge profiles (see Figure S16 in the Supporting
Information) revealed a flat plateau around 3.2 V, in accord-
ance with the previous reports.[10a] The first cycle/discharge
capacity is 160 mAhgÀ1 and the corresponding charge
capacity is 153 mAhgÀ1, which corresponds to a coulombic
efficiency of 95.6% (1 C is equal to 170 mAgÀ1). The cycling
stability of the sample was studied by charging/discharging
the FePO4 electrode at 0.1 C over 100 cycles (Figure 3a). The
discharge capacities stabilized at 140 mAhgÀ1 during the
100th cycle, which corresponds to 82.3% of the theoretical
À
the Fe O bond, Fe(OA)3 can react with TDPA to generate
FePO4 (see Scheme S1 in the Supporting Information).
To study the formation of the mesoporous structure, the
solution was heated to 2008C, which is lower than the reaction
temperature but high enough to ensure the precursors are
completely dissolved (the DSC results are shown in Fig-
ure S18 in the Supporting Information). The as-heated
solution was then cooled to room temperature, but no
precipitate could be observed. The structure of the micelles
in the solution was studied by small-angle X-ray scattering
(SAXS; Figure 4a), with four resolved peaks being detected.
The q values were located at 1.68, 3.47, 5.14, and 6.43 nmÀ1.
The d values are calculated from the equation q = 2p/d to be
3.74, 1.81, 1.22, and 0.98 nm, respectively, which are very close
to the d values of the amorphous FePO4 flakes measured by
low-angle XRD (Figure 1d). There are two possible struc-
tures to explain the SAXS results: 1) the micelles in the
solution form lamellar structures with regular periodical
spacing,[12] and 2) there is a 2D assembly of cylinder-shaped
micelles with mainly the (100) facets exposed. On the basis of
the TEM observations discussed above, the second structure
is more convincing. The mesoporous FePO4 flakes are likely
evolved from the 2D assembly of the cylinderical micelles
composed of Fe(OA)3 and TDPA in ODE. Within each
cylindrical micelle, the electric dipole could be formed due to
the presence of the positively charged Fe3+ ion and the
deviation of the pore shape from that of a perfect cylinder
(Figure 4c–e).[13] As ODE is a nonpolar solvent, there should
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be no ions such as H+ or OHÀ (as in the polar solvent) to
interact with the charged groups in the micelles and thus
affect the micelle–micelle interaction. Hence, the interaction
between each micelle would be more likely related to dipole
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their ordered mesoporous structure (Figure 4b), the metal
phosphate nanoflakes are amorphous in nature, thus their 2D
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graphene sheets.[15] With the aforementioned demonstration
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process in a nonpolar organic solution, it is expected that the
scope of mesoporous materials could be further expanded by
similar systems, which may bring about more surprising
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Received: April 23, 2014
Published online: July 2, 2014
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Keywords: lithium-ion batteries · mesoporous materials ·
nonpolar solvents · transition-metal phosphates · ultrathin flakes
.
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