Journal of Crystal Growth ] (]]]]) ]]]–]]]
Contents lists available at ScienceDirect
Journal of Crystal Growth
journal homepage: www.elsevier.com/locate/jcrysgro
Self-assembly of LiFePO4 nanodendrites in a novel system
of ethylene glycol–water
Fei Teng a,c, Sunand Santhanagopalan a, Anjana Asthana a, Xiaobao Geng a, Sun-il Mho b,
Reza Shahbazian-Yassar a, Dennis Desheng Meng a,n
a
Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, MI 49931, USA
Division of Energy System research, Ajou University, Suwon 443-749, Korea
c
School of Environmental Science and Engineering, Nanjing University of Information Science and Technology Nanjing 210044, P.R. China
b
a r t i c l e in fo
abstract
Article history:
Received 24 March 2010
Received in revised form
9 July 2010
Accepted 2 September 2010
Communicated by M. Schieber
In this work, a novel system of ethylene glycol/water (EG/W) was employed to synthesize LiFePO4, in
which dodecyl benzene sulphonic acid sodium (SDBS) was used as soft template to control particle
morphology. The samples were characterized by X-ray diffractometer (XRD), field emission scanning
electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), selected
area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX). The LiFePO4 sample
obtained by the reported method displays interesting hierarchical nanostructure (i.e. nanodendrites),
which was constructed by nanorods of 3–5 mm in length and 50 nm in diameter. The EG/W ratio,
amount of SDBS added, hydrothermal temperature and duration played important roles in the assembly
of the hierarchical nanostructures. A formation mechanism was proposed and experimentally verified.
It is concluded that the nanodendrites were formed due to the end-to-end self-assembly of nanorods.
Compared to previously reported methods, the reported approach shows obvious advantages of onestep synthesis, environmental friendliness and low cost, to name a few. The nanodendrites as a cathode
material have a higher capacity, compared with the other samples.
& 2010 Published by Elsevier B.V.
Keywords:
A1. Crystal morphology
A1. Crystal structure
A1. Nanostructures
A2. Self-assembly
B1. LiFePO4
1. Introduction
Since the pioneering work of Padhi et al. [1], LiFePO4 has been
intensively investigated as one of the most promising cathode
materials in rechargeable lithium batteries [2,3]. The main advantages of LiFePO4 include flat voltage plateau (3.4 V versus Li + /Li),
moderate theoretical specific capacity (170 mAh g 1), high thermal
stability, long cycling life, environmental friendliness and abundance
of iron (Fe) resources in nature. Moreover, LiFePO4 is considered to
be much safer than other cathode materials (e.g., LiCoO2 and
LiMn2O4) due to its outstanding stability upon overcharging and
overdischarging [1–3]. For example, metastable LiCoO2 can easily
lose oxygen while overcharging, which increases the probability of
overheating and electrolyte decomposition. In contrast, due to the
strong P–O covalent bonds in PO3
polyanion, phospho-olivine
4
LiFePO4 has a very stable three-dimensional framework, which
greatly reduces the risk of oxygen liberation. Hence, LiFePO4 is
expected to be the most promising cathode material for large-size
lithium-ion batteries in electrical vehicles (EVs) and hybrid electric
vehicles (HEVs) that demand both fast charging and strict safety
regulation [1,4]. However, bulk LiFePO4 electrodes are limited on its
n
Corresponding author. Tel./fax: + 1 906 4873551.
E-mail address: [email protected] (D.D. Meng).
rate capability, due to its intrinsic low electrical conductivity and
limited Li ion diffusion rate [2,3,5–7]. Its electrical conductivity
(10 9–10 10 S cm 1) is by several orders of magnitude lower than
that (e.g., ca. 10 3 S cm 1) of LiCoO2 cathode materials, which
prevents achievement of full theoretical capacity (170 mAh g 1) at
very high rates [3,7]. To date, most attempts have been carried out to
overcome the electronic and lithium-diffusion limitations, such as
coating of carbon [3,7,8], cationic supervalent substitution [3,7,9,10]
and addition of conductive materials (Cu, Ag, carbon black) [11–13].
During the last decade, it has also been demonstrated that size
reduction of LiFePO4 micro- or nanoparticles can effectively enhance
Li diffusion rate and significantly improve performances [3,14].
Another significant challenge is the large-scale synthesis of highpurity LiFePO4 nanoparticles with uniform morphology, which is
essential for their commercial applications. To date, various methods
have been proposed to synthesize LiFePO4, such as solid-state
reaction [15], sol–gel [16], hydrothermal [17,18], co-precipitation
[7], vapor deposition [8], microwave [19], spraying technology [20],
to name a few. Of these, hydrothermal synthesis is an effective
method to obtain well-crystallized materials with well-defined
morphologies, where no additional high-temperature annealing is
needed [17,21]. To date, synthesis of di-element materials with
various morphologies by a hydrothermal route has been a fairly
common practice. Nevertheless, morphosynthesis of multi-element
LiFePO4 represents significant challenges and thus has been reported
0022-0248/$ - see front matter & 2010 Published by Elsevier B.V.
doi:10.1016/j.jcrysgro.2010.09.005
Please cite this article as: F. Teng, et al., J. Crystal Growth (2010), doi:10.1016/j.jcrysgro.2010.09.005
(311)
(113)
(412)
(331)
(430)
(022)
(421)
(222)
#: Fe4(PO4)3(OH)3
(121)
(410)
(221)
(112)
(020) (211)
(301)
(101)
(210)
(011)
(200)
*: Li3PO4
(d)
Intensity/a.u.
(c)
(b)
* *#
(a)
##
20
40
30
50
60
70
2 Theta/degree
(020)
only scarcely. This situation has been attributed to the fact that the
different precursor chemicals generally have different reactivities,
which is not favorable for crystal growth or formation of pure
products. In the past years, Nazar et al. investigated the influence of
hydrothermal conditions and additives on the morphology of
LiFePO4 crystals [18]. Chen et al. [22] prepared the large diamondshaped LiFePO4 sample by the hydrothermal method. Sides et al.
[23] synthesized LiFePO4 nanofibers with polycarbonate membranes
template. They reported that the nanofibers showed an excellent
rate capacity because the nanofiber morphology mitigates the
problem of slow Li ion-transport in solid state. Vittal et al. also
synthesized LiFePO4 nanoplates with high rate capability [24]. Their
results showed that particle morphology or architecture has a great
influence on the performances of materials [25]. Although significant efforts have to be made to synthesize LiFePO4 nanostructures,
designing or controling new and advanced nanostructures with
enhanced performances is always of most interest to materialists.
Recently, Rangappa et al. [26] synthesized LiFePO4 flowers using the
solvothermal method. However, the preparation was performed at a
very high temperature (400 1C) and pressure (40 MPa), which limits
its practical production. Yang et al. [27] synthesized LiFePO4
dumbbell-like microstructures composed of nanoplates via a
solvothermal route, which showed an excellent cycling stability. In
their work, nevertheless, expensive benzyl alcohol and LiI were used,
and a long reaction time (48 h) was required. Hence, the preparation
is costly and time-consuming. Much effort is still needed to develop
a more economically efficient route to synthesize LiFePO4 with welldefined morphology. Although the hydrothermal method has shown
its potential to provide such a route, it has not been reported as a
feasible approach to synthesize LiFePO4 nanodendrites.
In this work, a novel system of ethylene glycol/water (EG/W)
was developed to synthesize LiFePO4 nanodendrites under hydrothermal conditions. Lithium hydroxide was used as a precursor and
dodecyl benzene sulphonic acid sodium (SDBS) was used as a soft
template to control crystal growth. LiFePO4 samples have been
prepared under various synthesis conditions to reveal the influences of EG/W volumetric ratio, amount of SDBS added, hydrothermal temperature and time. The samples were characterized by
XRD, FE-SEM, HRTEM, ED and EDX. A formation mechanism of
LiFePO4 nanodendrites was proposed based on the time-dependent
results. The reported approach shows significant advantages over
the state of the art, such as one-pot synthesis, environmental
friendliness and low cost, which are mainly attributed to the
employed inexpensive precursors and solvent, as well as the short
(111) (201)
F. Teng et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]
(d)
Intensity/a.u.
2
(c)
29.67
(b)
26
27
28
29
30
31
32
2 Theta/degree
Fig. 1. XRD patterns of the as-prepared samples at different hydrothermal
temperatures corresponding to those in Table 1. (A) scanning in the range of
15–701; (B) slow scanning in the range of 26–321: (a) S1-1; (b) S1-2; (c) S1-3;
(d) S1-4.
Table 1
Effects of hydrothermal temperature and duration on particle shape, crystal structure and sizea of the sample.
Sample
EG/Wb
SDBS/Fec
S1-1
1/1
1/1
140/6
S1-2
S1-3
S1-4
S1-5
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
160/6
180/6
200/6
160/2
S1-6
S1-7
S1-8
1/1
1/1
1/1
1/1
1/1
1/1
160/4
160/12
160/24
T/t (1C/h)
Crystal phases
Particle shaped
LiFePO4
+ Li3PO4
+ Fe4(PO4) 3(OH)3
LiFePO4
LiFePO4
LiFePO4
LiFePO4 (Little)
+ Fe4(PO4) 3(OH)3
+ Unknown phases
LiFePO4
LiFePO4
LiFePO4
–
a
Crystal size calculated by the Sherrer equation basing on [0 2 0] plane.
Ethylene glycol/deionized water volumetric ratio.
c
Molar ratio; SDBS stands for dodecyl benzene sulphonic acid sodium.
d
Observed by FE-SEM or HRTEM.
b
Please cite this article as: F. Teng, et al., J. Crystal Growth (2010), doi:10.1016/j.jcrysgro.2010.09.005
Nanodendrites
Irregular
Irregular
Nanoparticles
Nanorods
Nanodendrites
Nanodendrites
F. Teng et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]
3
(113)
(412)
(331)
(430)
(022)
(421)
(222)
(112)
(121)
(410)
(221)
(311)
(211)(020)
(301)
2.2. Preparation of samples
*: Unknown
#: Fe4(PO4)3(OH)3
Intensity/a.u.
(210)
(011)
(200)
(101)
(111)(201)
Fig. 2. FE-SEM micrographs of the as-prepared samples corresponding to those in Table 1: (a) and (b) S1-2 at different magnifications, (c) S1-3 and (d) S1-4.
(e)
(d)
(c)
(b)
*
20
*
#
#
30
#
40
50
2 Theta/degree
*
60
(a)
70
Fig. 3. XRD patterns of the as-prepared samples at different hydrothermal
durations correspond to those in Table 1: (a) S1-5, (b) S1-6, (c) S1-2, (d) S1-7
and (e) S-8.
process time ( 6 h). The novel nanodendrites are expected to
present excellent electrochemical properties compared to other
nanostructures, e.g., near-spherical nanoparticles.
2. Experimental section
2.1. Chemicals
In this work, all chemicals were used as purchased without
further purification. Deionized (DI) water treated in-house by an
ion-exchange system was used in the experiment. FeCl2 7H2O,
LiOH, H3PO4 (85 wt% aqueous solution), L( + )-ascorbic acid,
ethylene glycol (EG) and dodecyl benzene sulphonic acid sodium
(SDBS, C18H29SO3Na) were purchased from Sigma Aldrich.
The LiFePO4 nanodendrites were prepared by a simple hydrothermal process, during which L-ascorbic acid was added as a mild
reducing agent to prevent the oxidation of Fe(II). Typically, the
appropriate quantities of LiOH, FeCl2 7H2O, H3PO4, L-ascorbic acid
and SDBS with the molar ratios of 3:1:1:3:1 were added into 35 mL
of EG/W mixture (1/1 volumetric ratio). After intensive magnetic
stirring for 1 h at room temperature, a homogeneous solution was
formed. The solution was then transferred into a 50 mL Teflons
-lined stainless steel autoclave, which was heated to 160 1C and
kept at this temperature for 6 h. After being cooled naturally to
room temperature, the product was centrifuged, washed at least
six times with DI water and ethanol, and finally dried at 80 1C in a
vacuum for 24 h. In order to investigate the effects of preparation
conditions on the samples, several experimental parameters, such
as the volumetric ratio of EG to W, the amount of SDBS added,
hydrothermal time and temperature, were varied.
2.3. Characterization
Scanning electron microscopy (SEM) images were taken with a
Hitachi S-4700 field emission scanning electron microscope
(FE-SEM). Before FE-SEM inspection, the samples were coated
with 5-nm-thick platinum/palladium layer by direct current
(DC) sputtering. The acceleration voltage was 15 keV, and the
acceleration current was 1.2 nA. The morphology, crystalline
properties, surface structure and element composition of the
samples were determined by using high-resolution transmission
electron microscopy (HRTEM). We employed a JEOL JEM-4000FX
system equipped with electron diffraction and energy dispersive
X-ray spectroscopy (EDX) attachments with an acceleration
voltage of 200 kV. The powders were first ultrasonically dispersed
in ethanol, and then deposited on a thin amorphous carbon film
supported by a copper grid. The crystal structures of the samples
were characterized by an X-ray powder diffractometer (XRD,
Rigaku D/MAX-RB), using graphite monochromatized Cu Ka
Please cite this article as: F. Teng, et al., J. Crystal Growth (2010), doi:10.1016/j.jcrysgro.2010.09.005
4
F. Teng et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]
electrode slurry was composed of 75 wt% LiFePO4 powder, 15 wt%
acetylene black, 10 wt% polyvinylidene fluoride (PVDF) binder.
N-methyl-2-pyrrolidinon (NMP) was used as solvent. The slurry
was cast onto the aluminum current collector and achieved a
loading of 1 mg cm 2. The electrode was then dried overnight
under vacuum at 120 1C. A Teflon Celgard separator (#2400) was
used to separate the working electrode and a lithium foil counterelectrode. The electrolyte consisted of a solution of 1 M LiPF6 in
ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, in wt%) obtained from Ferro Corporation.
Charge/discharge test of the assembled coin cell was performed
on an Arbin BT2000 system in the voltage range of 2.0–4.2 V at
different C rates of 0.1C–5C (C¼170 mA g 1) at 25 1C.
3. Results and discuss
3.1. Influences of hydrothermal temperature and duration
Fig. 4. HRTEM micrographs of the as-prepared samples at different reaction times
corresponding to those in Table 1: (a) t ¼2 h (S1-5), (b) t¼ 4 h (S1-6), (c) t ¼6 h
(S1-2), (d) t ¼12 h (S1-7), (e) the tip of an individual particle for sample S1-2,
(f) the inset of SAED image, (g) lattice fringe for sample S1-2 and (h) energy
dispersive X-ray spectra (EDX) for sample S1-2.
radiation (l ¼0.154 nm), operating at 40 kV and 50 mA. The XRD
patterns were obtained in the range of 15–701 (2y) at a scanning
rate of 51 min 1. A nitrogen adsorption isotherm was performed
at 77 K and o10 4 bar on a Micromeritics ASAP2010 gas
adsorption analyzer. Each sample was degassed at 200 1C for 5 h
before the measurement. Surface area and pore size distribution
were calculated by the BET (Brunauer–Emmett–Teller) method.
2.4. Electrochemical measurements
Electrochemical measurements were performed using a CR2032
coin cell assembled in an argon-filled glove box. The working
First, we investigated the influence of hydrothermal temperature on the sample (Table 1 S1-1 to S1-4). The hydrothermal
temperature was varied from 140 to 160, 180 and 200 1C, while
the other processing parameters were maintained the same. Fig. 1
gives the typical XRD patterns of the as-synthesized samples.
After being processed at 140 1C for 6 h, no single-phase LiFePO4
sample was obtained (S1-1). Instead, the sample contained
significant amounts of impurity crystals (e.g., Li3PO4 and
Fe4(PO4)3(OH)3). It could be assumed that the higher energy is
necessary for the formation of LiFePO4 crystals. Above 160 1C, the
diffraction peaks of all the samples (S1-2, S1-3 and S1-4 in
Fig. 1(A)) can be well indexed to single-phase LiFePO4 with an
orthorhombic olivine structure (JCPDS card no. 81-1173). Moreover, the diffraction peaks become stronger and narrower with
the increase of temperature. The mean crystallite size (D020) was
further calculated using Scherrer’s equation from the [0 2 0]
diffraction peak (Fig. 1(B)). The mean D020 values for S1-2, S1-3
and S1-4 are 25.5, 37, and 42 nm, respectively. It is obvious that
the crystals have grown larger with the increase in hydrothermal
temperature. These results show that the reaction temperature
has a significant influence on crystal growth. Further, the
morphologies of the as-prepared samples (S1-2, S1-3, and S1-4)
were characterized by FE-SEM. Interestingly, the as-prepared
sample (S1-2) at 160 1C displays a hierarchical structure
(i.e. nanodendrites), as illustrated in Fig. 2(a) and (b). When
observed at a low magnification (Fig. 2(a)), the particles displayed
fairly uniform morphology, and each particle features a nanodendrite of 4–5 mm in length. As revealed in Fig. 2(b), the individual
nanodendrite consists of nanorods of 100 nm in diameter and
2–5 mm in length. These nanorods seem to be attached end-to-end
to form an ordered architecture. This novel hierarchical architecture of LiFePO4 nanorods has not been reported previously. At
higher temperatures (180 and 200 1C), however, the as-prepared
samples display irregular morphologies (Fig. 2(c) and (d)). It is
obvious that the hydrothermal temperature has a significant
influence on the morphology and structure of the products. It
could be concluded that there exists a critical temperature at which
the nucleation and growth of crystals are too fast to form crystals
with well-defined morphology. It is clear that the hydrothermal
temperature played a key role in the formation of high crystallinity
LiFePO4 crystals. Our results have shown that an appropriate
hydrothermal temperature (160 1C) can be chosen to synthesize
LiFePO4 nanodendrites. This mild temperature of the reported
process is considered to be a major advantage over existing
methods employing higher processing temperatures.
Hydrothermal time was then varied from 2, 6, 12 and 24 h to
investigate the effect of reaction time while the hydrothermal
Please cite this article as: F. Teng, et al., J. Crystal Growth (2010), doi:10.1016/j.jcrysgro.2010.09.005
F. Teng et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]
5
Table 2
Effect of reaction medium composition (EG/W) on particle shape and crystal structure of the sample.
Sample
EG/Wa
SDBS/Feb
T/t (1C/h)
Crystalline phases
Particle shapec
S2-1
S1-2
S2-3
S2-4
S2-5
0/1
1/1
2/1
4/1
1/0
1/1
1/1
1/1
1/1
1/1
160/6
160/6
160/6
160/6
160/6
LiFePO4 + Fe4(PO4) 3(OH)3 (Little)
LiFePO4
LiFePO4
LiFePO4 + Li3PO4 Fe4(PO4) 3(OH)3
LiFePO4 + Li3PO4 Fe4(PO4) 3(OH)3
Plates
Nanodendrites
Large shuttles
Large rods
Irregular particles
a
b
c
Volumetric ratio.
Molar ratio.
Observed by FE-SEM.
*: Li3PO4
#: Fe4(PO4)3(OH)3
Intensity/a.u.
#
#
*#
* *#
#
(d)
(c)
#
(b)
#
20
(a)
#
30
40
50
60
70
2 Theta/degree
Fig. 5. XRD patterns of the as-prepared samples at different volumetric ratios of
EG/W corresponding to those in Table 2: (a) S2-1, (b) S2-3, (c) S2-4 and (d) S2-5.
temperature was kept at 160 1C (Table 1, S1-2, and S1-5 to S1-8).
Fig. 3 shows XRD patterns of the as-prepared samples. After being
processed for 2 h, no LiFePO4 was evidenced, but the weak peaks of
Fe4(PO4)3(OH)3 and unknown phases can be observed (Fig. 3(a)).
When the reaction time was extended to 4 h, the peaks of both
Li3Fe2(PO4)3 and Li3PO4 disappeared (Fig. 3(b)). At the same time,
all the diffraction peaks, although relatively weak, can be well
indexed to single-phase LiFePO4 with an orthorhombic olivine
structure. When the reaction time was extended to 6 h, all the
diffraction peaks of the sample turned out to be strong, and can be
well indexed to high-purity LiFePO4 structure (Sample S1-2).
Further increasing reaction time to 12 and 24 h results in stronger
all diffraction peaks for samples S1-7 and S1-8. The crystallite size
(D020) was calculated using Scherrer’s equation (D¼0.9l/b cos y)
from the full-width-at-half-maximum (b) of the strong and wellresolved diffraction peak [0 2 0]. The calculated mean D020 values
for S1-6, S1-2, S1-7 and S1-8 are 18.8, 25.5, 27.1 and 27.5 nm,
respectively. The results showed that after the processing time was
longer than 6 h, the crystals grew at a fairly smaller extent. It is
therefore concluded that the crystal growth slowed down after 6 h.
An even longer processing time will have minor influence on the
crystal size and morphology of the reported LiFePO4 nanodendrites.
Further, HRTEM was performed to investigate the variation of
LiFePO4 particle morphology with processing time, as shown in
Fig. 4. It is revealed that after being processed for 2 h at 160 1C, the
sample (S1-5) was composed of 10-nm-large nanoparticles
(Fig. 4(a)). On the other hand, when the processing time is
extended to 4 h, the sample obtained (S1-6) turned out to be
LiFePO4 nanorods that are 100 nm in length and 10–20 nm in
diameter (Fig. 4(b)). After being processed for 6 h, the HRTEM
images further confirm that the as-obtained sample consists of
many well-dispersed nanodendrites with uniform morphology.
The length of those structures is again confirmed as of 4–5 mm
(Fig. 4(c)). The results consistently agree with the FE-SEM images
presented in Fig. 2(a) and (b). It is worth mentioning that these
nanodendrites are sufficiently stable to withstand ultrasonic
treatment for 30 min (or even longer) without being broken into
fragments. We also observed that after being processed for 12 or
24 h, the nanodendrite architectures maintained their morphologies (Fig. 4(d)). The TEM image for sample S1-8 (24 h) is not shown
herein. To exemplify the morphology of the sample obtained, the
high-magnification image of an individual particle is shown in
Fig. 4(d). The spiky structure on the edge of the particle indicates
that the particles are composed of nanorods with a diameter of
about 100 nm (Fig. 4(e)). The inset of ED patterns (Fig. 4(f))
indicates the single-crystalline nature of the nanorods. A HRTEM
image (Fig. 4(g)) taken on the tip of an individual nanorod displays
clear crystal lattices with d-spacing of 0.29 nm, corresponding to
the (0 2 0) plane of orthorhombic LiFePO4 crystals. The result
further confirmed the single-crystalline nature of the nanorods,
and suggested that the crystal preferentially grow along the {0 2 0}
direction. Fig. 4(h) provides the EDX spectra of the sample (S1-2).
The peaks of copper are obviously caused by the copper grid, which
is the carrier of the HRTEM samples. The peaks of Fe and P can be
observed clearly, but the peaks of lithium did not appear in EDX
spectra due to the light mass of lithium. The above time-dependent
experiments suggest that at the beginning stage, the nucleation
process takes place under hydrothermal conditions, leading to the
formation of nanoparticles; then the nanoparticles grow into
nanorods through a crystallization–dissolvation––recrystallization
process. Eventually, the nanorods organize into the hierarchical
structures through an end-to-end self-assembly process. Extensive
future research is still needed to clarify the details of the selfassembly process.
3.2. Influence of reaction medium composition
We have also investigated the effect of reaction medium on the
samples. EG/W volumetric ratio was varied at 0/1, 1/1, 2/1, 4/1 and
1/0 while other preparation parameters were maintained the same
(Table 2). The typical XRD patterns and FE-SEM images of the asprepared samples are shown in Figs. 5 and 6, respectively. At the
EG/W ratios of 0/1, the sample obtained is mainly composed of
LiFePO4 crystals accompanied by a very small amount of Fe4(PO4)
3(OH)3 impurity crystals (Fig. 5(a)). At the EG/W ratios of 1/1 and
2/1, single-phase LiFePO4 crystals were obtained, while no other
impurity crystals were observed (Fig. 5(b), S1-2 in Fig. 2). At the
EG/W volumetric ratio of 0/1, the obtained sample is composed of
plates with a thickness of 100–150 nm and a width of 1 mm (S2-1
in Fig. 6). The nanodendrite structure was obtained at an EG/W
volumetric ratio of 1/1, as shown in Fig. 2. When the volumetric
ratio of EG/W was changed to 2/1, the sample obtained consists of
Please cite this article as: F. Teng, et al., J. Crystal Growth (2010), doi:10.1016/j.jcrysgro.2010.09.005
6
F. Teng et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]
Fig. 6. FE-SEM images of the as-prepared samples at different volumetric ratios of EG/W corresponding to those in Table 2: (a) S2-1, (b) S2-3, (c) S2-4 and (d) S2-5.
Table 3
Effect of the SDBS addition amount on particle shape and crystal structure of the sample.
Sample
EG/Wa
SDBS/Feb
T/t (1C/h)
Crystalline phases
Particle shapec
S3-1
S1-2
S3-3
S3-4
1/1
1/1
1/1
1/1
0/1
1/1
2/1
4/1
160/6
160/6
160/6
160/6
LiFePO4
LiFePO4
LiFePO4
LiFePO4
Large spheres + irregular particles
Nanodendrites
Rods +dumbbells
Large bones
a
b
c
Volumetric ratio.
Molar ratio.
Observed by FE-SEM.
large spindle-shaped particles with a length of 2 mm and a center
diameter of 1 mm (S2-3 in Fig. 6). When the volumetric ratio of EG
to W was further increased to 4/1 and 1/0, high-purity LiFePO4
crystals were not obtained. Instead, the impurity crystals (Li3PO4,
FeFe3(PO4)3(OH)3 and unknown phases) started to appear in these
samples. FE-SEM images show that both samples consist of large
rods and irregular particles (S2-4 and S2-5 in Fig. 6). It is obvious
that reacting medium composition has a significant influence on
phase composition and morphology of the samples. Two aspects of
the new mixture medium are assumed to be very important in this
process. First of all, a high enough solubility of the precursor
chemicals in the EG/W medium needs to be guaranteed to ensure
the formation of proper LiFePO4 crystal structure. As the EG content
in the mixture increases, the solubility of the precursor chemicals
decreases. As a result, the precursor chemicals cannot be mixed
homogeneously at higher EG content, which does not favor the
formation of pure LiFePO4 crystals. On the other hand, it is well
known that the viscosity of EG (Z ¼ 21 mPa s, 20 1C) is much higher
than that of water (Z ¼1.0087 10 3 mPa s, 20 1C). The mobility or
reactivity of ions in solvent has an important influence on the
crystal growth. Too higher EG content may lead to very slow
mobility or reactivity of ions in the medium, which does not favor
to form the final product. At an appropriate content of EG (i.e. 1/1
volumetric ratio of EG to W), the reactivity of the precursors may
match one another, which does favor to form the final product.
Although those two factors can explain our experimental results
well, further investigations are still needed to confirm our
hypothesis and reveal the fundamental physical and chemical
interactions in our process.
3.3. Influence of the amount of surfactant added
To investigate the influence of the added amount of SDBS on
the self-assembly behavior of LiFePO4 nanostructures, this
particular processing parameter (SDBS amount) was changed
while others were maintained the same, as shown in Table 3.
Fig. 7 indicates high-purity LiFePO4 structures of the as-prepared
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(113)
(412)
(331)
(430)
(022)
(421)
(222)
(112)
(311)
(121)
(410)
(221)
(211) (020)
(301)
Intensity/a.u.
(210)
(011)
(200)
(101)
(111) (201)
samples with different amounts of SDBS. Fig. 8 shows the FE-SEM
images of these samples. When no SDBS was added to the system,
large nearly spherical particles were obtained (Fig. 8(a)). In this
case, neither the hierarchically assembled structure (i.e. nanodendrites) nor the nanorod building blocks were observed,
although the final samples seem to be composed of high-purity
LiFePO4 crystals. Under an SDBS/Fe molar ratio of 1/1, the optimal
synthesis conditions in our research, a number of well-defined
hierarchical structures of nanorods are observed, as exemplified
by sample S1-2 in Fig. 2. When the molar ratio of SDBS/Fe was
(c)
7
further increased to 2/1, relatively larger dumbbells of 10 mm in
length are observed, consisting of 3–4 mm long microrods (S3-3 in
Fig. 8(b)). When the volumetric ratio of SDBS/Fe was further
increased to 4/1, coarse bone-shaped microparticles of 10 mm in
length were obtained (S3-4 in Fig. 8(c) and (d)). The blow-up view
of those nanorods in S3-4 is shown in Fig. 8(d). It appears that
they are assembled by nanorods, similar to the building blocks of
nanodendrites in S1-2. These observations clearly establish that
the presence of SDBS plays a key role in the formation of nanorod
building blocks, as well as the self-assembly process. Additionally,
the amount of SDBS is a crucial factor for the control of the
hierarchical structures. We further conjecture that SDBS may act
as a soft template to direct the growth of nanoparticles into
nanorods at the early stage of the process through a preferential
bonding to certain crystal planes. Later on, these nanorods are
further aligned tightly and assembled end-to-end under the
guidance of the soft template to form various forms of hierarchical
structures (e.g., nanodendrites or bones).
3.4. Formation mechanism of nanodendrites
(b)
(a)
20
30
40
50
60
70
2 Theta/degree
Fig. 7. XRD patterns of the as-prepared samples corresponding to those in Table 3:
(a) S3-1, (b) S3-3 and (c) S3-4.
From the results obtained in this research, it can be concluded
that the EG/W mixture system and SDBS are essential for the
formation of novel hierarchical LiFePO4 structures. First of all, the
reactivity of all the precursor chemicals can be reduced in this
mixture solvent medium. At an optimal EG content (EG/W¼1/1 in
our experiment), the differences among the reactivity of the
precursors would be reduced significantly so that their reactivity
can match each other, which will facilitate the formation of the
desired olive-structure crystal instead of the impurity crystals,
such as Li3PO4 and Fe4(PO4)3(OH)3. Secondly, the solubilities of
the precursors in EG/W would be smaller than those in deionized
Fig. 8. FE-SEM images of the as-prepared samples corresponding to those in Table 3: (a) S3-1, (b) S3-3, (c) S3-4 and (d) the tip of the particles (indicated with arrow in (c)).
Please cite this article as: F. Teng, et al., J. Crystal Growth (2010), doi:10.1016/j.jcrysgro.2010.09.005
8
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4.5
4.5
4.0
4.0
3.5
Voltage (V)
Voltage (V)
Fig. 9. The formation mechanism of the nanodendrites: (a) particle growth and (b) self-assembly by an end-to-end mode.
3.0
S1-2
S3-1
S3-3
S3-4
2.5
2.0
0
20
40
3.5
0.1C
0.5C
1C
5C
3.0
2.5
2.0
60
80
100
120
Capacity (mAh
g-1
140
160
180
0
200
20
40
80
100 120 140 160 180 200
Capacity (mAh g-1)
)
Fig. 10. The initial charge/discharge curves of LiFePO4 samples (labeled in Table 3)
at 0.1C rate between 2.0 and 4.2 V.
180
150
Capacity (mAh g-1)
water, i.e. the precursor chemicals in EG/W could have a higher
degree of supersaturation than they do in deionized water if the
same amounts of precursors are used. Consequently, the EG/W
system would favor the nucleation and growth processes of
crystals. At too high an EG content, the precursors cannot be
mixed homogeneously due to the lower solubility of the
precursors. For the process reported herein, the mixture system
turns out to be a promising reaction medium to provide wellcontrolled crystallization. Moreover, a proper surfactant present
in the system can be used to adjust the size and morphology of
the particle being produced by binding SDBS molecules onto the
newly formed surfaces during crystal growth [18,28–31]. The
formation mechanism of the hierarchical structures was proposed
and described in Fig. 9. The time-dependent experiment results
presented in Table 1 and Fig. 4 reveal the evolution of the
hierarchical nanostructures, while the results at various SDBS
content (Table 3 and Fig. 8) have revealed the critical role played
by this surfactant in the formation of the hierarchical structure. As
we concluded in the previous section, SDBS, an anionic surfactant,
was used to both guide the growth of the crystalline LiFePO4
nanorods and template their self-assembly. During crystal
growth, the surfactant acted as strong coordinating agents by
binding to some crystal faces, and accordingly inhibited the
crystal growth along the other defined crystal plane. As a result, at
the early stages of the process, SDBS directs crystals to grow along
the crystal direction whose crystal plane weakly bonds with the
surfactant molecules. As a result, nanorods are formed, as was
60
120
90
60
30
0
0
10
20
30
Cycling number
40
50
Fig. 11. (a) Charge/discharge curves at different discharge rates from 0.1C to 5C
and (b) discharge capacity vs. cycling number at a capacity rate of 0.1C of LiFePO4
nanodendrites between 2.0 and 4.2 V.
observed and described in the former parts. In comparison, in the
absence of SDBS, no nanorods building blocks were found in the
final product (Fig. 8(a)). In addition, it is conjectured that the SDBS
molecules also act as a soft template to assemble the nanorod
building blocks into the final hierarchical structures, in which the
nanorods are tightly attached together by their ends. We assume
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F. Teng et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]]
that the end-to-end self-assembly results from the Van der Waals
attraction of hydrophobic interaction of the surfactant molecules
bonded to the end of the nanorods. Since surfactant molecules
preferentially and strongly adsorbed on the nanorod side, the
stronger electrostatic force exists on the nanorod side than that
on the end. It seems that there is a balance between electrostatic
repulsion interaction and hydrophobic attraction interaction.
Hence the electrostatic repulsion interaction on the side is
stronger than that on the end between nanorods, which refrains
the side-by-side attachment. The long hydrophobic chains of the
SDBS molecules boned on the nanorods will be attracted to one
another through hydrophobic interaction. As a result, the
nanorods are attached with each other by their ends to form
hierarchical structure. Note that this hypothetic formation
mechanism still needs to be confirmed by direct proof. Summarily, the reported approach demonstrates obvious advantages,
such as one-step synthesis, environmental friendliness, and low
cost. The methodology will also inspire a similar route to prepare
other phospholivines, e.g., LiMnPO4. Most importantly, nanodendrites have been recently identified as one of the most promising
morphologies for ultrahigh electrochemical/catalytic activity [32].
The synthesized LiFePO4 nanodendrites could be a promising
electrode material for lithium-ion batteries.
9
thus be confirmed that the nanodendrites show good rate
capacity. Fig. 11(b) gives the cycling stability of the nanodendrites
at a 0.1C rate. There was almost no capacity fading after 50 cycles,
indicating its good cycling stability. The good capacitive behavior
of the hierarchical LiFePO4 nanodendrites demonstrates its
potential application as an excellent cathode material for
lithium-ion batteries.
4. Conclusions
LiFePO4 nanodendrites composed of nanorod building blocks
have been successfully synthesized in the EG/W system using
SDBS as the surfactant. The results indicate that both reaction
medium (EG/W) and SDBS played an important role in the selfassembly of hierarchical nanostructures. Evidences are shown
that the nanodendrite hierarchical structures are formed through
the self-assembly of nanorods by their end-to-end attachment.
The reported approach is simple and economical, which has
simplified the traditional multi-step methods and will encourage
the future applications of LiFePO4 crystals, especially in lithiumion batteries.
Acknowledgments
3.5. Electrochemical properties
The electrochemical properties of LiFePO4 nanodendrites were
investigated and compared with other three typical samples.
Fig. 10 shows the charge/discharge profiles of the samples in the
cutoff voltage range of 2.0–4.2 at 0.1C (17 mA g 1). A flat plateau
at 3.4 3.5 V can be observed obviously during the charge/
discharge process, representing the typical electrochemical Li +
insertion/extraction behavior of LiFePO4. This flat plateau is
attributed to the two-phase reaction as follows (1) [33]:
LiFePO42(1 x) LiFePO4 +xFePO4 + xLi + + xe (1)
At 0.1C, the discharge capacity of LiFePO4 nanodendrites (S1-2
sample) is determined to be 154 mA h g 1 for the first cycle,
which is much higher than those (103, 107, 110 mA h g 1) of the
S3-1, S3-3 and S3-4 samples. This difference may be explained by
their different microstructures or textural properties. To further
confirm it, the BET areas of the S1-2 and S3-4 samples have been
tested with the isothermal sorption of nitrogen. The BET area of
LiFePO4 nanodendrites is determined as 32.6 m2 g 1, much larger
than that (3.4 m2 g 1) of the S3-4 sample. The large surface area
of the active material facilitates its sufficient contact with the
electrolyte, resulting in high Li ion mobility and low chargetransfer resistance. Moreover, the higher BET area of the
nanodendrites may also confirm the hierarchical microstructures
constructed by the nanorods. These small nanorods may facilitate
the intercalation processes of lithium ions, which may have
improved the charge and discharge kinetics. Martin et al. have
reported that the nanofiber morphology mitigates the slow
transport problem of Li ions, because the diffusion distance of Li
ions within the electrode material is minimized [34]. Therefore,
the higher discharge capability of LiFePO4 nanodendrites can be
mainly ascribed to the shorter and easier diffusion path of Li ions
in the material with small grain size [33,35]. Fig. 11a gives the
charge/discharge curves of LiFePO4 nanodendrites at different
discharge rates from 0.1C to 5C. Its initial discharge capacity
almost maintained 154 mA h g 1 at 0.5C. Furthermore, the
discharge capacities at the rates of 1C and 5C decreased to 145
and 141 mA h g 1, respectively, corresponding to about 93% and
90% capacity retention, compared with that at a rate of 0.1C. It can
We would like to thank Mr. Nate Kroodsma for English
improvement.This work was financially supported by the Michigan Tech Faculty Startup Fund, Michigan Tech Research Excellent
Fund, and the Korea Research Foundation grant (KRF-2007-412J04003).
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