J. of Supercritical Fluids 73 (2013) 70–79
Contents lists available at SciVerse ScienceDirect
The Journal of Supercritical Fluids
journal homepage: www.elsevier.com/locate/supflu
Continuous synthesis of lithium iron phosphate (LiFePO4 ) nanoparticles in
supercritical water: Effect of mixing tee
Seung-Ah Hong a , Su Jin Kim b , Kyung Yoon Chung b , Myung-Suk Chun c , Byung Gwon Lee a ,
Jaehoon Kim a,d,∗
a
Supercritical Fluid Research Laboratory, Clean Energy Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic
of Korea
b
Center for Energy Convergence, KIST, Republic of Korea
c
Sensor System Research Center, KIST, Republic of Korea
d
Green School, Korea University, 5-1 Anam Dong, Seongbuk-gu, Seoul 136-701, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 27 September 2012
Received in revised form
13 November 2012
Accepted 13 November 2012
Keywords:
Hydrothermal synthesis
Lithium iron phosphate
Mixing tee geometry
Nanoparticles
Supercritical water
a b s t r a c t
Continuous supercritical hydrothermal synthesis of olivine (LiFePO4 ) nanoparticles was carried out using
mixing tees of three different geometries; a 90◦ tee (a conventional Swagelok® T-union), a 50◦ tee, and
a swirling tee. The effects of mixing tee geometry and flow rates on the properties of the synthesized
LiFePO4 , including particle size, surface area, crystalline structure, morphology, and electrochemical performance, were examined. It was found that, when the flow rate increased, the particle size decreased;
however, the discharge capacity of the particles synthesized at the high flow rate was lower due to the
enhanced formation of Fe3+ impurities. The use of a swirling tee led to smaller-sized LiFePO4 particles
with fewer impurities. As a result, a higher discharge capacity was observed with particles synthesized
with a swirling tee when compared with discharge capacities of those synthesized using the 90◦ and
50◦ tees. After carbon coating, the order of initial discharge capacity of LiFePO4 at a current density of
17 mA/g (0.1C) and at 25 ◦ C was swirling tee (149 mAh/g) > 50◦ tee (141 mAh/g) > 90◦ tee (135 mAh/g).
The carbon-coated LiFePO4 synthesized using the swirling tee delivered 85 mAh/g at 20C-rate and
at 55 ◦ C.
Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction
In response to decreasing petroleum reserves and growing
concern about global climate change, there is great enthusiasm
regarding the development of rechargeable lithium secondary
batteries (LIBs) for powering more sustainable methods of transportation and renewable energy storage devices, for example,
plug-in hybrid electric vehicles, electric vehicles, and electric
energy storage systems. For such applications, it is essential for
LIBs to offer an excellent safety profile, high energy/power densities, excellent cyclability, and low cost [1,2]. The ordered olivine
lithium iron phosphate (LiFePO4 ) has been considered as one of
most promising cathode materials for large-scale application in
LIBs [3–5]. Its advantageous features include a high theoretical
capacity of 170 mAh/g, a flat voltage profile at ∼3.4 V versus Li+ /Li,
∗ Corresponding author at: Supercritical Fluid Research Laboratory, Clean Energy
Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea. Tel.: +82 2 958 5874;
fax: +82 2 958 5205.
E-mail address: jaehoonkim@kist.re.kr (J. Kim).
the low cost of the starting materials, environmental benignity,
high tolerance to overcharge, and high thermal stability [6–9].
However, there are some drawbacks of LiFePO4 that need to be
addressed in order to improve its potential for use in practical applications. These include low electronic conductivity (∼10−10 S/cm)
and sluggish Li+ ion diffusivity (10−14 to 10−17 cm2 /S) through the
olivine structure [10,11]; as a result, a significant decrease in capacity at higher discharge rates is often observed, making LiFePO4
unsuitable for use in high-power battery applications. Recently,
considerable efforts have been directed toward improving the rate
performance of LiFePO4 : reduction of particle size or control of
particle porosity to shorten the transport path length of Li+ [12,13],
a conductive layer coating (e.g., carbon, conducting polymer,
metal oxides) [14–17], or doping with cations/anions [9,18] to
enhance the intrinsic electron and/or Li+ conductivity. Another
obstacle to the commercial use of LiFePO4 is the need to develop
reliable and economically viable large-scale production methods.
Although various methods have been proposed to synthesis
LiFePO4 that includes solid-state, sol–gel, co-precipitation, hydrothermal/solvothermal, molten state, spray solution, microwave,
emulsion drying and so forth, only a few of them are currently
applied in the commercial production of LiFePO4 [5].
0896-8446/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.supflu.2012.11.008
S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
Supercritical hydrothermal synthesis (SHS) is a very promising
alternative to the conventional techniques for producing cathode
and anode active materials [19]. The unique physical properties of supercritical water, including extremely low viscosity,
high reactant diffusivity, zero surface tension, high reactivity, and
high supersaturation ratio of reaction intermediates, make it a
promising medium for the production of highly crystalline and
nanosized particles [20–22]. In addition, the supercritical hydrothermal method is environmentally friendly, fast, simple, and
readily scalable by the employment of continuous operation. Various types of metal oxide nanoparticles have been produced by
SHS, including CeO2 , CuO, TiO2 , Fe2 O3 , NiO, ZrO2 , and ZnO; the
morphology and size distribution of fine particles can be controlled by adjusting pH, metal salt concentration, temperature,
and pressure [22,23]. In recent years, the advantages of SHS
have led to it being widely utilized to synthesize active materials for use in LIBs. This includes LiCoO2 [24,25], LiMn2 O4 [26],
LiNi1/3 Co1/3 Mn1/3 O2 [27], LiFePO4 [28–33], and Li4 Ti5 O12 [34–36].
In 2010, the first commercial plant for the production of LiFePO4
using continuous SHS was constructed in Korea [17]. This plant can
continuously produce LiFePO4 and has a capacity of 1000 tons per
year.
In a typical continuous SHS, the design of the mixing tee plays
a critical role in determining the properties of the synthesized
nanoparticles (e.g., particle size, particle size distribution, particle
agglomeration) and in allowing continuous operation without particle deposition on the mixing zone [37–42]. A stream of aqueous
metal salt solution and a stream of supercritical water, which have
significantly different fluid and flow properties (e.g., temperature,
viscosity, density, flow velocity, flow Reynolds number) meet at the
mixing tee. As the temperature of the aqueous solution approaches
that of near-critical water, the OH− concentration increases; this
in turn accelerates the formation of nanoparticle by hydrolysis,
followed by dehydration steps. When a conventional Swagelok®
T-union is used as the fluid mixer, asymmetric flow convergence
and frequent particle agglomeration are often observed due to an
insufficient mixing rate and heterogeneous nucleation followed by
growth at the wall of the mixer [37,42]. Therefore, considerable
efforts are still being made to develop a mixing tee with the capacity
for more reliable, more uniform mixing and less particle agglomeration, which will result in the production of high-quality metal
oxide nanoparticles various nanoparticle applications. To date, various types of mixing tees have been developed, including the central
collision-type micromixer [37], the T-type micromixer [38], the
cross-type mixer [39], the swirling micromixer [40], and the nozzletype mixer [42]. The use of these mixing tees has resulted in the
production of smaller-sized particles with more uniform particle
distribution, when compared with those produced by the conventional T-union mixer, owing to the improved mixing and more rapid
heat transfer.
In our previous studies, in which we utilized a Swagelok®
T-union (90◦ tee) and a home-made 50◦ tee, we analyzed the
effects of various process parameters (temperature, flow rates,
concentration) on the properties of LiFePO4 and the effects of
carbon coating on the electrochemical performance of LiFePO4
[32,33]. The carbon-coated LiFePO4 (C-LiFePO4 ) synthesized using
the aforementioned tees exhibited a rather low discharge capacity
of ∼135 mAh/g. In the present study, a new mixing tee geometry, a swirling-type mixing tee, was evaluated for the production
of LiFePO4 nanoparticles with enhanced electrochemical performance. The following sections describe the effects of flow rate
and mixing tee geometry on particle properties including particle
size, surface area, morphology, crystallinity, and electrochemical performance. The explanation for the improved discharge
capacity of the C-LiFePO4 synthesized using the swirling mixer is
discussed.
71
2. Experimental
2.1. Materials
Lithium hydroxide monohydrate (LiOH·H2 O, purity of >98 wt%),
iron sulfate heptahydrate (FeSO4 ·7H2 O, purity of > 99 wt%), phosphoric acid (H3 PO4 , purity of >98 wt%), and sucrose (C12 H22 O11 ,
purity of ≥99 wt%) were purchased from Sigma–Aldrich (St. Louis,
MO, USA) and used as received. Nitrogen (purity of >99.9%) and
argon with 5% hydrogen (purity of >99.999%) were obtained from
Shinyang Sanso Co. (Seoul, Korea). Distilled and deionized (DDI)
water was prepared using a Milli-Q® Ultrapure water-purification
system with a 0.22 ␮m filter (Billerica, MA, USA). The cellulose ester
membrane filter with a pore size of 0.45 ␮m was purchased from
Toyo Roshi Kaisha Ltd. (Tokyo, Japan). Polyvinylidene difluoride
(PVDF; Kureha Chem. Co., Tokyo, Japan), acetylene black (DENKA
Co. Ltd., Tokyo, Japan) and 1-methyl-2-pyrrolidinone (NMP; purity
of ≥98 wt%, Alfa-Aesar, MA, USA) were used as received.
2.2. Continuous SHS apparatus and process
The continuous tubular high-pressure and high-temperature
apparatus was built in the Supercritical Fluid Research Laboratory of the Korea Institute of Science and Technology for carrying
out research into metal and metal oxide nanoparticle synthesis in
supercritical water or in supercritical alcohols [43–46]. The geometries of the mixing tees tested in this work are presented in Fig. 1.
The 90◦ tee is the commercially available Swagelok® T-union. In the
50◦ tee, the flow of precursor solutions at room temperature meets
the flow of supercritical water at a cross angle of 50◦ . In the swirlingtype mixing tee, the supercritical water flows are introduced from
two different directions, and each supercritical water flow mixes
with the precursor solution flow at a cross angle of 60◦ . Shifting
the supercritical water flow lines to a distance of 1.8 cm from the
precursor-solution flow line generates a swirling flow in the mixing
tee.
The connector between the mixing tee and the reactor has the
same inner diameter of 6.5 mm and the whole volume of mixing fluids was introduced into the reactor. The inner volumes of
the mixing tees were 1.5 cm3 (90◦ tee), 2.25 cm3 (50◦ tee), and
4.5 cm3 (swirling tee). The flow pattern at the mixing tee was
calculated by computational fluid dynamics modeling using the
FLUENT program (version 6.2). Details of the simulation conditions are given in Table S1 and the temperature distribution in
the mixing tees are given in Fig. S1 (supplementary data). In the
synthesis under investigation, the mass flow rate of supercritical
water is 3–6 times higher than that of the precursor solution. The
Re of the precursor solution flows was in the range 229–952 (laminar), and that of the supercritical water flows was in the range
15,100–16,400 (turbulent), as listed in Table 1. The mixed flow of
the combined precursor solution and supercritical water downstream of the mixing tee was turbulent with a Re in the range
140,000–269,000. Typically, the temperature of the reactor and
mixing tee was maintained at 400 ± 5 ◦ C, the pressure of the whole
system was kept at 25 ± 0.1 MPa, and the flow fluctuation was
within ± 0.2 g/min during the synthesis. The mole concentration
ratio of LiOH/H3 PO4 /FeSO4 was maintained at 0.09:0.03:0.03 in
order to keep the pH of the solution at ∼8 and maintain neutral
or slightly basic conditions [28]. The obtained particles were purified by being dispersed in DDI water, sonicated, and decanted using
centrifugation at 3000 rpm for 30 min. The purification procedure
was carried out in triplicate and the purified particles were dried
at 60 ◦ C in a vacuum oven for 24 h to remove the moisture in the
LiFePO4 particles. The synthesis conditions are listed in Table 1.
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S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
Fig. 1. Computational fluid dynamics simulation results of the three mixing tees. (a) 90◦ tee, (b) 50◦ tee and (c) swirling tee.
Table 1
Synthesis conditions for LiFePO4 .
Sample code
E90-H
E90-M
E90-L
E50-H
E50-M
E50-L
ES-H
ES-M
ES-L
Tee design
90◦ tee
90◦ tee
90◦ tee
50◦ tee
50◦ tee
50◦ tee
Swirling tee
Swirling tee
Swirling tee
Flow rates (g/min)
Residence Time (s)
LiOH
FeSO4 ·H3 PO4
H2 O
3
3
1.7
3
3
1.7
3
3
1.7
3
3
1.7
3
3
1.7
3
3
1.7
18
9
9
18
9
9
18
9
9
Reynolds number (Re)
Precursor solution
18
31
37
18
32
38
18
32
38
560
590
291
952
937
345
352
384
229
Supercritical water
16,400
15,100
15,700
16,400
15,100
15,600
16,300
15,100
15,400
After the mixing
tee (total fluids)
269,000
167,000
140,000
269,000
168,000
140,000
269,000
168,000
140,000
S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
2.3. Carbon coating
In order to layer a carbon coating on the LiFePO4 particles,
sucrose (as a carbon source) was dissolved in 1.3 ml DDI water
to prepare a 12 wt% solution. Four grams of LiFePO4 particles was
mixed into the sucrose solution and the mixture was dried at 80 ◦ C
in a vacuum oven for 24 h to evaporate the water. After the dried
powders had been ground and strained using a 5-␮m size sieve,
the powder was sintered at 600 ◦ C with a flow of 5% hydrogen in
argon at 100 ml/min for 3 h (heating rate of 5 ◦ C/min). During the
heat treatment under the reducing condition, a carbon layer formed
from the precursor that was coated on the surface of the LiFePO4
particles.
2.4. Characterization
The structure of the particles was characterized by X-ray diffraction (XRD) using a D/Max-2500 V/PC X-ray diffractometer (Rigaku,
Tokyo, Japan). The morphology of the particles was observed using a
Hitachi S-4100 field emission scanning electron microscope (SEM;
Tokyo, Japan) and a Tecnai-F20 G2 high-resolution transmission
electron microscope (HR-TEM; FEI Co. Ltd., OR, USA). The carbon
distribution of the C-LiFePO4 samples was observed using energydispersive X-ray spectroscopy (EDX; model FP6595/05, FEI Co. Ltd.,
OR, USA). A copper grid coated with a silicon monoxide film was
used to ensure that any carbon detected originated from the samples. The carbon contents of the C-LiFePO4 samples were measured
by elemental analysis (model TC-136, LECO Corporation, MI, USA).
The Brunauer–Emmett–Teller (BET) surface area of the particles
was measured using a BELSORP mini II apparatus (BEL Inc., Osaka,
Japan). Elemental analyses of Li, Fe, and P in the samples were
carried out using inductively coupled plasma mass spectroscopy
(ICP-MS; ELAN 6100 series, Perkin-Elmer, NY, USA).
2.5. Electrochemical measurements
For the electrochemical test of the bare LiFePO4 and C-LiFePO4
samples, the active material (85 wt%), acetylene black as a conducting material (10 wt%), and PVDF as a binder (5 wt%) in NMP were
well mixed using a homogenizer (Nihonseiki Kaisha Ltd., Tokyo,
Japan). The cathodes were incorporated into cells with a lithium foil
anode and a Celgard 2500 microporous membrane separator (Celgard LLC, Charlotte, NC, USA). The electrolyte was 1 M LiPF6 in an
ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl
carbonate (EMC) solvent with an EC/DMC/EMC volume ratio of
1:1:1. The cells were assembled in a dry room. Electrochemical characterization was performed in a standard 2032 coin
cell configuration using a commercial multichannel galvanostatic
charge–discharge cycler (model WBCS 3000, WonATech Corp.,
Korea) at temperatures of −20, 25, and 55 ◦ C. The cells were cycled
between 2.5 V and 4.3 V versus Li+ /Li at a 0.1–30C-rate (which corresponds to current densities of 17–5100 mAh/g).
3. Results and discussion
Fig. 2 shows the XRD patterns of the LiFePO4 particles synthesized using the three different mixing tees at the various flow rates.
The main diffraction patterns of the LiFePO4 can be indexed to
orthorhombic LiFePO4 olivine-type phase (JCPDS PDF number 401499). The profiles of the peaks in the patterns are quite sharp and
narrow, indicating that the LiFePO4 particles prepared using SHS
retained a highly crystalline structure without additional calcination. When the continuous hydrothermal synthesis of LiFePO4 was
carried out in the subcritical water condition (300 ◦ C and 25 MPa),
the particles were highly agglomolated and the LiFePO4 crystalline
phase did not form. Some impurity peaks of relatively low intensity
73
can be observed in each sample; the peaks at 2 values of 33◦ , 41◦ ,
and 54◦ can be assigned to the (1 0 4), (1 1 3), and (1 1 6) diffraction
planes of Fe2 O3 (Fe3+ ), and the peaks at 2 values of 30◦ and 43◦
can be assigned to the (2 2 0) and (4 0 0) diffraction planes of Fe3 O4
(Fe2+ /Fe3+ ) phase. This indicates that some portion of the Fe2+ precursor (FeSO4 ) is oxidized to the Fe3+ species during the SHS. It is
worth noting that the whole reactor, the DDI water, and the aqueous precursor solution were purged with high-purity nitrogen for
1 h prior to the synthesis, and the precursor solution and water
reservoir were continuously purged during the synthesis to minimize oxygen content. During the synthesis, the oxygen content in
the precursor solution and water reservoir was measured to be very
small (∼0.13 ppm). Thus, the formation of the Fe3+ impurities may
be due to an inherent preference of the Fe2+ precursor for oxidizing
in the supercritical water condition. In fact, the particles obtained
from the SHS using the iron(II) precursor (FeSO4 ·7H2 O) were found
to be mixed phase of magnetite (Fe3 O4 ) and hematite (Fe2 O3 ), as
shown in Fig. S2. The formation rate of iron oxide species in supercritical water is known to be very fast owing to the low intermediate
solubility and high supersaturation ratio [38,47]. This may cause the
formation of the Fe3+ impurity phase during SHS of LiFePO4 .
The peak intensity ratio of the (1 0 4) Fe2 O3 phase or the (4 0 0)
Fe3 O4 phase to the (1 1 1) LiFePO4 phase can serve as an indicator of the amount of Fe3+ impurities in the sample. As listed in
Table 2, a relatively lower amount of impurities was present in the
samples synthesized using the swirling tee when compared with
those synthesized using the 50◦ and 90◦ tees at the condition of
medium-to-low flow rate (M and L samples). The better mixing
between the precursor solution flow and supercritical water flow
in the swirling tee can induce more rapid nucleation of the particles due to the lower intermediate solubility. Thus, the precursors
experienced similar stage of nucleation, which may lead to better
chance to form LiFePO4 with lower amount of impurities. In addition, the use of the swirling tee resulted in particles with higher
crystallinity; the (0 4 0)(5 1 2), (1 1 3)(2 0 3), and (6 2 0) peaks of the
LiFePO4 phase are clearly better split for the samples prepared using
the swirling tee when compared with those for the samples prepared using the 50◦ or 90◦ tees; this difference is more profound
at the high-flow-rate condition. When the flow rate increased, the
peak intensity ratios of (1 0 4) Fe2 O3 phase to (1 1 1) LiFePO4 phase
and/or of (4 0 0) Fe3 O4 phase to (1 1 1) LiFePO4 phase increased,
except the 50◦ case. The Fe content in each sample, measured by
ICP-MS, also increased with an increment in the flow rate. This
suggests that larger amounts of the Fe3+ impurities formed at the
high-flow-rate condition. Similar trend was observed in the continuous synthesis of CoFe2 O4 in hot-temperature water. Amount
of the impurity (Fe2 O3 ) was increased from 3% to 6% when the residence time decreased from 19 to 11 s at 295 ◦ C and 24 MPa [48].
The increase in iron oxide phase at high flow rate condition may
be due to much lower solubility and much higher supersaturation
ratio of iron intermediates when compared to other species (e.g.,
Co intermediates, PO4 intermediates). For example, the formation
rate of iron oxide nanoparticles in supercritical water is extremely
fast (0.002 s) and the conversion was very high (97.9%) due to the
low intermediate solubility in scH2 O [38]. Thus at the high flow
rate condition, iron precursor may precipitate in forms of Fe2 O3
and Fe3 O4 when the other intermediate species still remain in the
fluid phase.
Fig. 3 shows SEM images of the LiFePO4 particles synthesized
using the three different mixing tees at different flow rates. At
the high flow rate, the use of the swirling tee resulted in much
smaller sized particles (ES-H) when compared with the particles
synthesized using the 90◦ tee (E90-H) and the 50◦ tee (E50-H).
This leads to a larger BET surface area for ES-H than for E50-H and
E90-H, as shown in Table 2. The size distribution of the samples
was rather broad (E90-H: 200–600 nm; E50-H: 300–700 nm; ES-H:
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S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
Fig. 2. X-ray diffraction patterns of the LiFePO4 particles synthesized at LiOH/(FeSO4 /H3 PO4 )/H2 O flow rates of: (a) 3:3:18 g/min, (b) 3:3:9 g/min and (c) 1.7:1.7:9 g/min (•:
Fe3 O4 ; : Fe2 O3 ).
100–400 nm), suggesting that each particle experienced a different
nucleation and growth stage during its formation. At lower flow
rates, the samples synthesized using the three different mixing tees
retained very similar particle size. Indeed, the BET surface areas of
the samples synthesized at the low flow rate (E90-L, E50-L, ES-L)
were very similar, in the range 6.2–7.3 m2 /g. Regardless of mixing
tee geometry, the size of the particles synthesized at the higher flow
rate is much smaller than that of those synthesized at the lower flow
S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
75
Table 2
Elemental composition, BET surface area, conductivity, carbon content, and initial/30th discharge capacity of LiFePO4 and carbon-coated LiFePO4 .
Sample code
E90-H
E90-M
E90-L
CE90-L
E50-H
E50-M
E50-L
CE50-L
ES-H
ES-M
ES-L
CES-L
a
b
c
Mole ratioa
BET surface
area (m2 /g)
Li
Fe
P
0.49
0.55
0.50
0.52
0.31
0.53
0.49
0.52
0.39
0.33
0.49
0.47
0.72
0.69
0.59
0.60
0.65
0.66
0.61
0.60
0.63
0.62
0.59
0.59
0.50
0.52
0.46
0.50
0.45
0.51
0.51
0.50
0.45
0.46
0.49
0.48
15.9
7.5
6.2
38.5
12.2
6.3
7.0
38.5
17.4
10.4
7.3
41.2
Peak intensity ratio
(1 0 4 )
Fe2 O3 /(1 1 1)
LiFePO4
(4 0 0)
Fe3 O4 /(1 1 1)
LiFePO4
0.1698
0.1379
0.1247
–
0.1159
0.1253
0.1204
–
0.1530
0.1029
0.0689
–
0.0984
0.0425
0.0435
–
0.0917
0.0363
0.0441
–
0.0873
0.0407
0.0458
–
Carbon content in
C-LiFePO4 b (wt%)
Conductivity
(S/cm)
Initial/30th
discharge capacityc
(mAh/g)
0
0
0
5.91
0
0
0
5.90
0
0
0
6.26
–
–
1.1 × 10−9
7.1 × 10−5
–
–
1.1 × 10−9
7.1 × 10−5
–
–
1.2 × 10−9
6.5 × 10−5
52/43
65/55
85/73
135/125
66/57
76/67
100/85
141/135
78/67
88/72
100/87
149/143
Analyzed by ICP-MS.
Analyzed by EA.
Initial and 30th discharge capacities at 0.1C.
rate. Several research groups have observed a reduction in particle
size at higher flow rates with continuous SHS, for example CeO2
[23], ␥-AlO(OH) [49], NiO [41], and ZnO [50]. As listed in Table 1,
Re at the high-flow-rate condition (E90-H, E50-H, ES-H) is approximately two times higher than Re at the low-flow-rate condition
(E90-L, E50-L, ES-L). The reduction in particle size at the higher flow
rate can be attributed to improved mixing of fluids and rapid heat
transfer in the mixing region at the nucleation stage [42]. Thus,
reaction intermediates can be consumed at the nucleation stage,
resulting in smaller size particles. At the low flow rate condition
Fig. 3. Scanning electron microscope images of the LiFePO4 particles synthesized using the three mixing tees at various flow rates.
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S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
Fig. 4. The cycling performance and the charge–discharge curves of the LiFePO4 samples synthesized at the LiOH/(FeSO4 /H3 PO4 )/H2 O flow rates of (a), (d) 3:3:18 g/min; (b),
(e) 3:3:9 g/min; and (c), (f) 1.7:1.7:9 g/min (closed symbol: charge; open symbol: discharge).
(long reaction time), particle size is not dependent on the types
of mixing tee, suggesting dissolution and recrystallization may be
dominant particle formation mechanism.
Fig. 4 shows the electrochemical properties of the bare LiFePO4
samples measured at 25 ◦ C. Table 2 lists the initial and 30th discharge capacities of each sample. All of the bare LiFePO4 samples
synthesized using the three different mixing tees show sloping
voltage plateaus and low discharge capacities of equal to or less
than 100 mAh/g at 0.1C-rate, indicating that the active materials are not properly utilized without carbon coating. In general,
higher discharge capacities are observed when the LiFePO4 particles have higher crystallinity, smaller size, and a lower amount
of impurities [51]. Under the higher-flow-rate condition (Fig. 4a
and b), the discharge capacities were in the order swirling tee > 50◦
S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
77
Fig. 5. High-resolution transmission electron microscope images and energy-dispersive X-ray spectroscopy analysis results of the LiFePO4 particles synthesized using (a),
(b), (e) the 90◦ tee; and (c), (d), (e) the swirling tee at the LiOH/(FeSO4 /H3 PO4 )/H2 O flow rate of 1.7:1.7:9 g/min.
tee > 90◦ tee. Smaller-sized particles and higher crystallinity in the
samples synthesized using the swirling tee may result in higher discharge capacities because each sample retained a similar amount
of impurities (Table 2). On the other hand, under the low-flow-rate
condition (Fig. 4c), the order of discharge capacities was swirling
tee ≥ 50◦ tee > 90◦ tee. ES-L and E50-L exhibited similar discharge
capacity patterns until the 15th cycle while ES-L showed slightly
larger capacities than E50-L after the 16th cycle. Since the particle sizes and crystallinity of each sample synthesized at the low
flow rate were very similar, the impurities may play a role in the
difference in discharge capacity. In fact, as shown in Table 2, the
peak intensity ratio of (1 0 4) Fe2 O3 phase to (1 1 1) LiFePO4 phase
was relatively lower for ES-L when compared with those of E90L and E50-L. The lower amount of impurities in ES-L may also be
responsible for its smaller polarization; at a capacity of 40 mAh/g,
ES-L showed a much smaller voltage difference between charge
and discharge curves (0.09 V) when compared with that of E50-L
(0.2 V), even though the initial capacity of E50-L was similar to that
of ES-L.
To improve the electrochemical performance of the bare
LiFePO4 samples, the particles were coated with a carbon layer
using sucrose as the carbon source. As discussed in our previous work, carbon content, carbon thickness, and carbon structure
play an important role in determining the electrochemical performance of C-LiFePO4 synthesized by the supercritical hydrothermal
method as well as the solid-state method [33]. The optimum carbon
content for enhanced discharge capacity was found to be ∼6 wt%.
Therefore, in this work, the carbon content of the C-LiFePO4 was
fixed at ∼6 wt% by adjusting the sucrose concentration. As shown in
Table 2, the 6 wt% carbon coating on the LiFePO4 led to a four orders
of magnitude increase in conductivity from ∼10−9 to ∼10−5 S/cm.
Fig. 5 shows HR-TEM images, EDX results, and selected area
electron diffraction patterns of the C-LiFePO4 samples synthesized
using the 90◦ tee (CE90-L) and the swirling tee (CES-L). In the
HR-TEM images, the LiFePO4 particles appear as dark regions and
carbon as light gray regions, as confirmed by the EDX analysis.
Both of the C-LiFePO4 samples revealed individual LiFePO4 particles
embedded into the carbon network. Observation of the interface
between the LiFePO4 particle and the carbon layer showed that the
carbon distribution around the particle is not very uniform [33].
The XRD patterns of the C-LiFePO4 samples shown in Fig. S4 reveal
that the Fe3+ impurities in each sample disappeared after the carbon coating. This may be because the Fe3+ impurities changed to
iron phosphides (FeP, Fe2 P, Fe3 P) and/or iron carbide species (Fe3 C,
Fe2 C) during the carbothermal reduction [33,52,53].
Fig. 6 shows the charge–discharge curves and cycling performances of the C-LiFePO4 samples at 0.1 C-rate in the potential range
2.5–4.3 V at 25 ◦ C. The discharge capacities of the bare LiFePO4 samples are shown in the figure for comparison purposes; the discharge
capacities of the C-LiFePO4 samples are significantly improved
when compared with those of the bare LiFePO4 particles, indicating that the carbon-coated active materials were utilized more
effectively than the uncoated samples owing to their enhanced
electronic conductivity. After carbon coating, CES-L exhibited an
extremely flat voltage curve shown at ∼3.4 and 3.5 V during discharge and charge while CE50-L and CE90-L exhibited sloping
voltage profiles. In addition, the discharge capacity of the C-LiFePO4
prepared using the swirling tee was higher than those of the samples prepared using the 50◦ and the 90◦ tees; the initial discharge
capacities of the CES-L, CE50-L, and CE90-L samples were 135, 141,
and 149 mAh/g, respectively, and their discharge capacities after
the 30th cycle were 117, 134, and 143 mAh/g, respectively. Again,
78
S.-A. Hong et al. / J. of Supercritical Fluids 73 (2013) 70–79
C-rates from 0.1 to 30C-rate for 55 cycles and then again at 0.1C-rate
for 10 cycles. The discharge capacity of CES-L was very stable at each
current rate and at each temperature. At the high 20C-rate (which
means that it takes 3 min to charge and discharge), CES-L demonstrated a relatively high discharge capacity of 85 mAh/g at 55 ◦ C
and 73 mAh/g at 25 ◦ C, and a low discharge capacity of 20 mAh/g at
−20 ◦ C. The higher discharge capacity of LiFePO4 observed at high
temperature is due to the faster lithium diffusion rate in LiFePO4
[54]. When returning to 0.1C-rate after 55 charge–discharge cycles,
the discharge capacities were 150, 143, and 115 mAh/g at 55, 25,
and −20 ◦ C, respectively, indicating only ∼3% capacity loss from
the initial discharge capacities. This indicates that the high crystalline olivine phase of CES-L can retain its structural integrity even
during the high Li+ ion intercalation and de-intercalation process.
The results observed in this study may indicate that the use of the
swirling tee in SHS could be a promising method for the production
of LiFePO4 as a cathode material for lithium ion batteries.
4. Conclusion
Fig. 6. (a) Charge–discharge voltage profiles and (b) cycling performance of the
bare LiFePO4 and C-LiFePO4 synthesized at the LiOH/(FeSO4 /H3 PO4 )/H2 O flow rate
of 1.7:1.7:9 g/min (closed symbol: charge; open symbol: discharge).
the lower amount of impurities in the samples synthesized using
the swirling tee under the low-flow-rate condition may be responsible for the higher discharge capacity.
To investigate potential high-power, outdoor applications, the
rate capabilities of CES-L were measured at various current densities and different temperatures, and the results are shown in Fig. 7.
The sample was progressively charged and discharged at various
Mixing tees of three different geometries, 90◦ , 50◦ , and swirling
tees, were evaluated in continuous SHS for the production of
nanosized LiFePO4 particles with improved electrochemical properties. Use of the swirling tee resulted in smaller-sized particles
with higher discharge capacities and lower polarization under the
high-flow-rate condition. When the flow rate decreased, particle
size increased. Even though similarly sized particles in the range
400–900 nm were produced using the three mixing tees at the low
flow rate, a smaller amount of impurities was present in the particles produced by the swirling tee when compared with those from
the 90◦ and 50◦ tees. As a result, a higher discharge capacity was
observed with samples produced with the swirling tee. After carbon coating, the discharge capacities of C-LiFePO4 at 0.1C-rate and
after 30 cycles, measured at 25 ◦ C, were 143 mAh/g (swirling tee),
135 mAh/g (50◦ tee), and 125 mAh/g (90◦ tee). When the discharge
capacity of the sample produced with the swirling tee was measured at the higher temperature of 55 ◦ C, the value was 155 mAh/g
(which corresponds to 92% of theoretical value) at 0.1C-rate and
103 mAh/g at 10C-rate. These results, showing highly crystalline
LiFePO4 particles with smaller amounts of impurities and better
discharge capacities, suggest that the swirling tee may be a very
promising alternative to conventional tees in the SHS of LiFePO4 .
Acknowledgments
This research was supported by the KIST Young Fellow Program
of the Korea Institute of Science and Technology. The authors also
acknowledge support from the Global Research Lab (GRL) Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (MEST) (grant
number: 2011-00115) and the NRF of Korea Grant funded by the
Korean Government (MEST) (2012, University-Institute cooperation program).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.supflu.2012.11.008.
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Fig. 7. Rate performance of CES-L at different C-rates and at different temperatures
(closed symbol: charge; open symbol: discharge).
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