J.

of Supercritical Fluids 55 (2011) 1027–1037

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

journal homepage:www.elsevier.com/locate/supflu

Small capacity decay of lithium iron phosphate (LiFePO 4) synthesized continuously in

supercritical water: Comparison with solid-state method
a,c b,c a,∗ b
Seung-Ah Hong , Su Jin Kim , Jaehoon Kim , Kyung Yoon Chung ,
b c
Byung-Won Cho , Jeong Won Kang
a Supercritical Fluid Research Laboratory, Clean Energy Center, Energy Division, Korea Institute of Science and Technology (KIST), 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of
Korea
b c
Advanced Battery Center, Korea Institute of Science and Technology (KIST), 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
Department of Chemical and Biological Engineering, Korea University, 5-1 Anam Dong, Seongbuk-gu, Seoul 136-701, Republic of Korea

article info abstract

Article history: Nanosize lithium iron phosphate (LiFePO4 ) particles are synthesized using a continuous supercriti-cal hydrothermal synthesis
Received 10 July 2010 ◦
method at 25 MPa and 400 C under various flow rates. The properties of LiFePO 4 synthesized in supercritical water
Received in revised form 8 September 2010 including purity, crystallinity, atomic composition, particle size, surface area and thermal stability are compared with those of
Accepted 17 September 2010 particles synthesized using a conven-tional solid-state method. Smaller size particles ranging 200–800 nm, higher BET
2 −1
surface area ranging 6.3–15.9 m g and higher crystallinity are produced in supercritical water compared to those of the
Keywords: 2 −1
solid-state synthesized particles (3–15 m; 2.4 m g ). LiFePO4 synthesized in supercritical water exhibit higher discharge
Lithium iron phosphate −1 −1
Cathode active material capacity of 70–80 mAh g at 0.1 C after 30 cycles than that of the solid-state synthesized LiFePO 4 (60 mAh g ), which is
−1
Supercritical hydrothermal synthesis attributed to the smaller size particles and the higher crystallinity. Smaller capacity decay at from 135 to 125 mAh g is
Solid-state method observed during the 30 cycles in carbon-coated LiFePO 4 synthesized using supercritical water while rapid capacity decay
−1
from 158 to 140 mAh g is observed in the carbon-coated LiFePO4 synthesized using the solid-state method.

Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction layer coating on the LiFePO 4 particles by either adding conduc-tive carbon to
presynthesized LiFePO4 particles [13,14] or adding carbon precursors to
An olivine-structured phosphate LiFePO4 has received much attention
LiFePO4 ingredients followed by cosynthe-sizing carbon-coated LiFePO 4 (C-
since the last decade as a promising alternative to cobalt-oxide based
materials in large-scale battery applications such as plug-in hybrid electric LiFePO4 ) [11,15]. The enhanced electronic conductivity of carbon-coated
vehicles (PHEV) and backup power systems [1,2]. This is due to the LiFePO4 results in the improved electrochemical performance. By optimising
beneficial properties of LiFePO 4 over LiCoO4 including low starting material the amount and the structure of the carbon in the C-LiFePO 4 composite, up to
cost, nontoxicity, and better tol-erance at abusive conditions [3]. However, 95% theoretical capacity of LiFePO 4 can be achieved at room tem-perature
−10
inherent drawbacks of LiFePO4 such as low electronic conductivity (∼10 [5].
−1
S cm ) Particle size, morphology, crystallinity, and electrochemical properties of
+
[3] and sluggish Li ion diffusivity through the olivine structure LiFePO4 are strongly dependent on preparation meth-ods and conditions
[4] make it difficult to achieve the full theoretical capacity of
−1 [16,17]. Current LiFePO4 synthesis method is either solid-state or solution-
170 mAh g . Numerous attempts have been made to enhance based method. Although the solid-state method can produce highly crystalline
electrochemical properties of LiFePO 4 . This includes control of syn-thetic and hydroxyl-free LiFePO4 , large particle size, high energy consumption,
parameters to adjust particle size and morphology to reduce the transport path several time-consuming batch-wise synthesis steps (10–24 h), require-ment of
+
length of Li [5–8], doping with supervalent cations in the lithium or iron high-temperature sintering (500–800
◦
C), and impurity incorporation
sites to enhance electronic conductiv-ity [3,9], and surface modification with
associated with ball milling are major concerns [5]. The solution-based
metal nanoparticle coatings such as silver and copper [10–12]. One of the
techniques include spray pyrolysis [18,19], hydrothermal [20,21], co-
most promising way to enhance electrochemical properties of LiFePO 4 is thin precipitation [22], sol–gel [23,24], and emulsion drying [25]. These methods
carbon often suffer from long synthe-sis time (6–24 h), low production rate associated
with batch-wide running mode, generation of large chemical and aqueous
wastes, and requirement of an additional high-temperature sintering step
∗
Corresponding author. Tel.: +82 2 958 5874; fax: +82 2 958 5205. E-mail
address: jaehoonkim@kist.re.kr (J. Kim).

0896-8446/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.supflu.2010.09.026
1028 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

Fig. 1. A schematic diagram of the supercritical hydrothermal synthesis system. R, reactor; PH, water pre-heater coil; HF, heating furnace; M, mixing tee; F, filter; C, cooler; BPR, back pressure
regulator; C, collection vessel; L1, LiOH solution container; L2, FeSO4 ·H3 PO4 solution container; L3, distilled and deionized water container; P, high pressure pump; N, nitrogen cylinder.

to enhance crystallinity. Therefore, there are still considerable efforts materials result in high quality, ultrafine nanoparticles in a high rate (∼10 s).
underway to develop more reliable, simpler, less chemical-intensive and less The advantages of SHS have also been utilized to syn-thesize cathode active
expensive techniques that can produce high quality LiFePO 4 for ultimate materials such as LiCoO2 [29,30], LiMn2 O4
large-scale battery applications. [31] and LiNi1/3 Co11/3 Mn1/3 O2 [32] using either batch or continu-ous
Supercritical hydrothermal synthesis (SHS) is a very promising method to reactor system. More recently, Lee et al. showed that LiFePO 4 with particle
produce high quality, highly crystalline and nanosize metal oxide particles size in the range of 1–2 m can be synthesized using the batch SHS method
[26–28]. In addition, continuous SHS is an environmental-friendly, fast and
[33,34] and Xu et al. showed that LiFePO 4 particles with 45–120 nm in
continuous method that are read-ily scalable. Nanosize metal oxide diameter can be obtained using the continuous SHS method [35]. However,
nanoparticles including CeO2 , AlOOH, Fe2 O3 , TiO2 , CuO, ZnO, and electrochemical properties such as charge–discharge capacity of the
complex metal oxides have been widely synthesized continuously in continuously synthesized LiFePO4 were not studied. Later, Aimable et al.
supercritical water [26]. High diffusivity of reactants in supercritical water, studied continuous synthesis of LiFePO 4 particles and their electrochemical
fast reaction rate, and high degree of supersaturation due to low solubility of properties
starting [36]. Primary particles with 100–200 nm in diameter aggregated

Table 1
Synthesis conditions of LiFePO4 using the continuous SHS method and the BET surface areas.

Sample No. Pre-heater coil Mixing tee Reactor furnace Reactor Feed Flow rates Residence BET surface Crystallite
−1 2 −1 a
temperature temperature temperature temperature concentration (g min ) time (s) area (m g ) size (nm)
◦ ◦ ◦ ◦
( C) ( C) ( C) ( C) (M)
LiOH FeSO4 H3 PO4 LiOH FeSO4 /H3 PO4 H2 O
E1 485 400 405 400 0.09 0.03 0.03 1.7 1.7 9 38 6.3 25.3
E2 533 400 435 400 0.09 0.03 0.03 3.0 3.0 9 30 12.1 22.4
E3 595 400 450 400 0.09 0.03 0.03 3.0 3.0 18 18 15.9 18.4
a Estimated using the XRD patterns and the Sherrer’s equation.
 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

Fig. 2. XRD patterns of the LiFePO4 particles and the carbon-coated LiFePO 4 particles (L:F/P:H; the ratio of flow rate). (a) Sample E1, (b) sample E2, (c) sample E3, (d) sample E1C, (e) sample S,
and (f) sample SC ( , Fe3 O4 ; , Fe2 O3 ).

and formed secondary particles with 4–40 m in diameter. The ini-tial 2.2. Continuous SHS apparatus
−1
discharge capacity at 0.1 C rate was in the range 70–75 mAh g and that at
−1 Continuous SHS of nanosize LiFePO4 was conducted in a custom-built,
0.01 C rate was 90 mAh g . The low discharge capacity compared to the
continuous flow reactor apparatus. A schematic of the apparatus was shown in
−1
theoretical value (170 mAh g ) was attributed to fast agglomeration of the Fig. 1. It consists of a tubular reactor (R), a water pre-heater coil (PH), a
particles, thus not all of the active material was electronically connected. mixing tee (M), two heat furnaces (HF), two metal filters (F), a cooler (C), a
In this work, physicochemical and electrochemical properties of nanosize back pressure regulator (BPR), a LiOH solution container (L1), FeSO 4 /H3
LiFePO4 particles synthesized using the continuous SHS method were studied PO4 solution con-tainer (L2), a DDI water container (L3), three high-pressure
in detail and the properties were compared with those of LiFePO 4 synthesized pumps (P1, P2, P3) and a nitrogen cylinder (N). The reactor (R) is cylindri-cal
in shape with an inside diameter (I.D.) of 14 mm and an inside height of 300
using the conventional solid-state method. Herein, we show that the 3
electrochemical properties of the continuously synthesized particles can be mm, giving a volume of 47 cm . The water preheater coil (PH) is a 7-m
−1
enhanced up to 135 mAh g at 0.1 C rate using a simple carbon coating length coiled tubing with I.D. of 1.75 mm. The reactor and the preheater coil
were made from stainless steel (SS 316). A modified mixing tee (M) from
method. The sections that follow describe the continuous SHS apparatus and ◦
process, the effect of precursor solution and water flow rate on the particle commercially available 3/8 inch Swagelok-type 90 tee (T-union) was used, as
properties including particle size, morphology, crys-tallinity, surface area and shown in Fig. 1b. The size of connecting part between the reactor and the
atomic composition. The property of carbon coated on LiFePO 4 was also mixing tee remained 3/8 inch while the size of connecting part between pre-
heater coil and the mixing tee was modified to 1/8 inch. During the
characterized.
modification the central axis of the scH 2 O pre-heater tube was shifted from
that of the exit tube. It is note that mixing tee design in the continuous
2. Experimental supercritical hydrothermal synthesis plays a signif-icant role in determining
particle properties (size, size distribution, crystallinity, etc.) and in controlling
2.1. Materials plugging problem [37–39]. In general, mass flow rate of scH2 O is 3–6 times
higher than that of precursor solution to increase the precursor solution
Lithium hydroxide monohydrate (LiOH·H2 O, purity of ≥98 wt.%), iron temper-ature rapidly to a supercritical water temperature. In the current
experimental condition, the Reynolds number (Re) of the precur-sor solution
sulfate heptahydrate (FeSO4 ·7H2 O, purity of ≥99 wt.%), phosphoric acid
was 291 (laminar) and Re of supercritical water was 15,677 (turbulent). When
(H3 PO4 , purity of ≥98 wt.%), lithium carbonate (Li2 CO3 , purity of ≥99 wt. the conventional T-union is used as the mixing tee, the convergence of these
%), iron oxalate dehydrate (FeC2 O4 ·2H2 O, purity of ≥99 wt.%), ammonium two flows with significantly different momentums and temperatures causes
hydrogen phos- complicated circu-lation zone at the mixing point (see Computational Fluid
phate ((NH4 )2 H·PO4 , purity of ≥98 wt.%) and sucrose (C12 H22 O11 , Dynamics (CFD) simulation result in Fig. 1c. Simulation condition; a
◦ −1
purity of ≥99 wt.%) were purchased from Sigma–Aldrich (St. Louis, precursor solutions temperature of 30 C and a flow rate of 3.5 g min were
MO, USA) and used as received. Nitrogen (purity of ≥99.9%) and argon with ◦
mixed with scH2 O at a temperature of 490 C and flow rate of 9 g min
−1
.
5% hydrogen (purity of ≥99.999%) were obtained from Shinyang Sanso Co. ◦
(Seoul, Korea). Distilled and deionized (DDI) water was prepared using a The average fluid temperature was 400 C and the pressure was 25 MPa. A
number of computational cell was 357,137. Other simulation conditions
Milli-Q Ultrapure water purification system with a 0.22 m filter (Billerica,
including mass, momentum, energy equa-
MA, USA). Poly(vinylidene difluoride) (PVDF, Kureha Chem. Co., Tokyo,
Japan), acetylene back (DENKA Co. Ltd., Tokyo, Japan) and 1-methyl-2-
pyrrolidinone (NMP, purity of ≥98 wt.%, Alfa-Aesar, MA, USA) were used
as received.
1030 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

Table 2
Elemental compositions, the BET surface areas, the conductivity, and the carbon content of LiFePO 4 and the carbon-coated LiFePO4.
a b
Sample No. BET surface Crystallite size Mole ratio Sucrose:LiFePO4 Actual carbon Conductivity
2 −1 −1
area (m g ) (nm) weight ratio contents in (S cm )
c
C-LiFePO4 (wt.%)
Li Fe P
d −9
S 2.4 37.6 0.49 0.51 0.52 – – 2.2 × 10
e −4
SC 10.6 33.6 0.51 0.52 0.49 4:1 2.08 5.5 × 10
f −9
E1 6.3 25.3 0.50 0.59 0.46 – – 1.1 × 10
g −5
E1C 13.2 20.2 0.49 0.61 0.51 4:1 1.97 1.3 × 10
g −4
E1C2 38.5 19.8 0.52 0.60 0.50 12:1 5.91 7.1 × 10
a b
Estimated using the XRD patterns and the Sherrer’s equation. Analyzed by ICP-MS.
c
Analyzed by EA.
d
LiFePO4 synthesized using the solid-state method.
e
Carbon-coated LiFePO4 synthesized using the solid-state method.
f
LiFePO4 synthesized using the supercritical hydrothermal method.
g
Carbon-coated LiFePO4 synthesized using the supercritical hydrothermal method.

−1
tion model (steady RANS) and turbulence model (RNG –ε) are the same with ±0.2 g min . At the mixing tee and the reactor, LiFePO 4 formed by the
the previous work [39]). This inhomogeneous flow mix-ing often leads to reaction:
particles with broad size distribution [39]. Eddy flow inside the mixing tee
can be also generated by the inhomo-geneous flow mixing, as shown in Fig. 3LiOH + H3 PO4 + FeSO4 → LiFePO4 + Li2 SO4 + 3H2 O
1c. The mixed flow at the downstream of the mixing tee was turbulent (Re = The molar concentration ratio of LiOH H 2 O:H3 PO4 :FeSO4 7H2 O was
139,960). In the continuous supercritical hydrothermal synthesis of LiFePO 4 maintained to 3:1:1 to keep solution pH to ∼8 because the precip-itation of
using the conventional T-union tee geometry, frequent plugging in the LiFePO4 resulted at neutral or slightly basic conditions [40]. After a desired
precursor solution inlet part was observed. period of synthesis, the temperature of the reactor was then decreased to room
temperature and the synthe-sized particles in the metal filters were collected.
The temperatures of the reactor, the water pre-heater coil, and the mixing The obtained particles were purified by dispersing in DDI water, sonicating,
tee were controlled using the heat furnaces (HF), manufactured by Daepoong and separating using centrifugation at 3000 rpm for 30 min. The proce-dure of
Industries (Seoul, Korea) and a DX-7 temperature controller, manufactured the purification was carried out in triplicate and the purified particles were
by Hanyoung Precision Co. (Seoul, Korea). The temperatures of the reactor ◦
dried at 60 C in a vacuum oven for 24 h. The synthe-sis conditions are listed
and the mix-ing tee were monitored by inserting type-K thermocouples inside
in Table 1.
of the units, manufactured from Omega Engineering, Inc. (Stam-ford, CT).
The thermocouples were connected to a multichannel recorder (model DR
240, Yokogawa, Japan). In order to maintain experimentally desired
◦
2.4. Solid-state synthesis of LiFePO4
temperature of 400 C at the mixing tee and the reactor, the heating energy of
the water pre-heater and the reactor furnaces increased at the higher flow rate A mixture containing precursors of 0.5 mol Li 2 CO3 , 1 mol (NH4 )2
conditions (see Table 1).
H·PO4 and 1 mol FeC2 O4 ·2H2 O were placed in a zirconia bowl and the
mechanochemical activation was carried out with zirconia balls for 3 h using a
planetary mill (FRITSCH Pulverisette 5, Pauley Equipment Solutions,
The high-pressure pumps for the water and the precursor solu-tion
Antrim, England) in air. The rotat-ing speed was set to 250 rpm and the ball-
transport into the reactor were model HKS-1000 digital pumps, manufactured to-powder weight ratio was set to 20:1. The activated powders were then heat-
by Hanyoung Precision Co. (Seoul, Korea). The pres-sure of the reactor was ◦
treated at 600 C using a heat furnace with a flow of 5% hydrogen in argon at
controlled using a 26-1700 Series model back pressure regulator,
−1
manufactured by Tescom, Co. (MN, USA). The particles formed in the 100 ml min for 10 h.
reactor were separated from the solu-tion using two of 0.5 m metal filters (F),
manufactured by Mott, Co. (Farmington, USA). 2.5. Carbon coating

A carbon source, sucrose, was dissolved in 2 ml DDI water to pre-pare 4

2.3. SHS procedure wt.% or 12 wt.% solution. Four grams of LiFePO 4 particles and the sucrose
solution was then well-mixed. The LiFePO4 –sucrose mixture was then dried
Prior to each experiment, the precursor solutions and the DDI water were ◦
purged with nitrogen at least 1 h to remove dis-solved oxygen and to prevent at 80 C in a vacuum oven for 3 h to evaporate water. The sucrose-coated
◦
2+ 3+ LiFePO4 particles were then calcinated at 600 C with a flow of 5% hydrogen
oxidation of Fe to Fe . The DDI water was then introduced into the
−1
reactor system using the high-pressure pump at an experimentally desired in argon at a flow rate of 100 ml min for 3 h. During the calcinations,
pressure of 250 bar. The pressure of the reactor was controlled using the sucrose was carbonized and the produced carbon was coated on surface of the
back-pressure regulator. The temperatures of the reactor and the pre-heater LiFePO4 particles.
were then increased to experimentally desired values using the heat furnaces.
After experimentally desired temperature and pressure reached, the precursor 2.6. Characterization
solutions were introduced into the reactor system. Typically, the temperature
◦
of the mixing tee and the reac-tor can be maintained at 400 ± 5 C over the The structure of the particles was characterized by X-ray diffrac-tion
length of the reactor and the pressure can be maintained at 250 ± 1 bar over (XRD) using a D/Max-2500 V/PC Rigaku X-ray diffractometer (Tokyo,
the entire reaction time. The flow fluctuation during the synthesis was within Japan). The morphology of the particles was observed using a Hitachi S-4100
field emission scanning electron microscope (FE-SEM, Tokyo, Japan). The
Brunauer–Emmett–Teller (BET) surface
 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

Fig. 3. SEM images of (a) sample E1, (b) sample E2, (c) sample E3, (d) sample S, (e) sample E1C, and (f) sample SC (L:F/P:H; the ratio of flow rate).

area of the particles was measured using a Belsorp-max mini II apparatus −2

5–7 mg cm . The cathodes were incorporated into cell with a lithium foil
(BEL Inc., Osaka, Japan). The thermal properties of the samples were anode and a Celgard 2500 microporous membrane sep-arator (Celgard LLC.,
examined using thermal gravimetric analysis (TGA, TA instruments TGA
Charlotte, NC). The electrolyte was 1 M LiPF 6 in ethylene carbonate/dimethyl
2050, DE, USA). The carbon content of the carbon-coated sample was
measured by elemental analysis (EA, model TC-136, Leco Corporation, MI, carbonate/ethylmethyl carbonate (EC:DMC:EMC = 1:1:1) solvent. The cells
were assembled in a dry room. The electrochemical characterization was
USA). The structure and the composition of individual particle were
performed in stan-dard 2032 coin cell configuration using a commercial
characterized using Nicolet almega XR dispersive Raman spectrometer
multichannel galvanostatic charge–discharge cycler (Maccor Series 4000
(Thermo Fisher Scien-tific Inc., MA, USA). Elemental analyses of Li, Fe, and Battery Test System, Oklahoma, USA) at room temperature. The cells were
P contents in the synthesized particles were carried out using inductively + −1
coupled plasma-mass spectroscopy (ICP-MS) (ELAN 6100 series, Perkin- cycled between 2.5 and 4.3 V versus Li /Li at 0.1 C rate (17 mAh g ).
Elmer, NY, USA). The morphology of the carbon-coated samples was
3. Results and discussion
2
observed with a Tecnai-G high-resolution transmission electron microscopy
(HR-TEM) (FEI Co. Ltd., OR, USA). Fig. 2 shows XRD patterns of the LiFePO4 particles produced using the
SHS method at various flow rates, of the LiFePO 4 particle synthesized using
the solid-state method, and of the carbon-coated LiFePO4 (C-LiFePO4 ) using
2.7. Electrochemical measurements the carbon coating technique described in Section 2. The main diffraction
patterns of the SHS’d and the solid-state synthesized particles can be indexed
For the electrochemical test of the LiFePO4 particles, the active material to orthorhombic LiFePO4 olivine-type phase (JCPDS PDF number 40-1499).
(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 The sharp peaks in the patterns indicate that the LiFePO4 particles prepared
◦
Kaisha LTD. Tokyo, Japan). The slurry was then coated onto Al foil. After using SHS retain highly crystalline structure. In fact, the peak near 62 , which
◦ are (0 4 0)(5 1 2), (1 1 3)(2 0 3) and (6 2 0), are better split for the SHS’d
vacuum drying at 80 C for 24 h, the electrode discs with diameter of 1.77
2 samples than those of the solid-state synthesized
cm were punched and weighed. Typical cathode loadings were in the range
1032 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

Fig. 4. The discharge curves of (a) sample E1, (b) sample E2, (c) sample E3, (d) sample S, and (e) the cycling performance of the LiFePO 4 particles prepared using SHS method at various flow rates
and solid-state method (closed symbol; charge, opened symbol; discharge).

samples, indicating that the structure of the former samples is bet-ter ordered 2S), the color of LiFePO4 synthesized at the lower flow rate is light grey,
than the latter one. In other words, the SHS’d samples have higher which is very similar to the color of the solid-state synthe-sized LiFePO 4 . In
crystallinity than solid-state synthesized samples. As shown in Fig. 2a–c, the
−1 −1 contrast, LiFePO4 synthesized at higher flow rates was light brown, indicating
increase in the precursor solution flow rate from 1.7 g min to 3 g min
−1 larger amount of red-colored Fe2 O3 was co-produced. LiFePO4 synthesized
(sample E2) or the increase in the water flow rate from 9 g min to 18 g
−1 ◦
using the solid-state method did not show the impurity peaks associated with
min (sample E3) resulted in slightly less split peaks near 62 indicating a
the iron oxides, indi-cating highly phase-pure LiFePO 4 particles were
slight decrease in the LiFePO4 crystallinity. The average crystallite size of produced (sample S, see Fig. 2e). The iron oxide formation may result from
2+ 3+
LiFePO4 , esti-mated form the XRD data and Sherrer’s equation, decreased oxida-tion of the Fe (FeSO4 ) to Fe species in the reagent container and/or
from 25.3 to 18.4 nm as the flow rate incrased (see Table 1). These crys-tallite during the synthesis. Oxygen concentration of water can be reduced from 4.1
size are smaller compared to that of the particle produced by the solid-state ppm to 0.13 ppm in during the 1 h nitrogen purg-ing. Further nitrogen purging
method (37.6 nm). up to 8 h did not decrease the oxygen content below 0.1 ppm. Thus the
residual oxygen in water and in the reagent aqueous solution may be
Fig. 2a–c shows that LiFePO4 particles synthesized in supercrit-ical water 3+
contained some impurities. By comparing of the XRD patterns with iron responsible for the formation of the Fe species. The impurities are known to
◦ ◦ ◦
oxides, the impurity peaks at 2 of 33 , 41 and 54 were identified to Fe2 O3 be detrimental to the electrochemical properties of LiFePO 4 [41]. The amount
3+ ◦ ◦ ◦ 2+
(Fe ), and the peaks at 2 of 30 , 43 and 57 were identified to Fe3 O4 (Fe 3+
3+ of the Fe species decreased after carbon coating (E1C, see Fig. 2d),
/Fe ) (see Fig. 1S in the Supplementary data). At higher flow rate, the indicating that hydrogen and carbon generated from the pyrolysis of sucrose
intensity of the impu-rity peaks increased, indicating larger amount of Fe 2 O3 3+ 2+
play a role in the reduction of Fe to Fe [42,43]. The XRD pattern of
and Fe3 O4 were produced. In fact, as shown in the Supplementary data (Fig. SHS’d and carbon-coated sample with higher car-
 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

Fig. 5. TGA of (a) sample E1, (b) sample E1C, (c) sample E1C2, (d) sample S, and (e)
sample SC.
Fig. 6. Raman spectra of (a) sample E1, (b) sample E1C, (c) sample E1C2, (d) sample S, and (e)
sample SC.

bon content (E1C2, see Table 2, carbon content will be discussed later) is
very simlar to that of E1C (see Fig. S3). −1 −1
as the water flow rate increased from 9 g min to 18 g min . The
Table 2 lists the elemental analysis and the BET surface area results of the production of smaller size particles at higher flow rate in super-critical
LiFePO4 particles synthesized using the SHS method and the solid-state hydrothermal synthesis is due to rapid heat transfer and improved mixing
between the precursor solution and supercrit-ical water at the nucleation stage
method. It can be seen that ∼10% excess amount of Fe was present in the
[27]. The decrease in particle size with an increase in flow rate was also
particles synthesized using the SHS method. The excess Fe may contribute to
observed in the syn-thesis of LiFePO 4 [35,36], TiO2 [27], and ZnO [49]. Fig.
the iron oxide impurities that were observed in the XRD pattern. As discussed
3+ 3d shows the morphology of the LiFePO4 particles synthesized using the
previously, the peaks associated with the Fe species completely disappeared
solid-state method. Primary particles in the range of 200–300 nm highly
in the XRD spectrum of the C-LiFePO4 samples. However, 10% excess aggregated and formed larger size secondary particles in the range of 3–15 m.
amount of Fe still persisted in the C-LiFePO 4 samples (see Table 2). Reac-tor The particle aggregation may result from the sinter-ing at a high temperature
◦
corrosion and iron from the reactor material (SS316) did not contribute the (600 C) for an extended period (10 h). As a result, the BET surface area of
iron excess in the SHS’d LiFePO 4 (see Table S1 in the Supplemetary data). the particles synthesized using the solid-state method is 3–7 times lower (2.4
3+ 2 −1
Previous reports showed that LiFePO4 can be synthesized from Fe m g ) compared to those prepared using the SHS method. As shown in Fig.
precursors (such as Fe2 O3 ) with an addi-tion of carbon during carbothermal 3e–f, the morphology of the particles did not change much after the car-bon
◦
reduction [44,45]. At higher temperatures, the carbothermal reduction resulted coating, indicating the mild carbonization condition (600 C for 3 h) did not
in the forma-tion of iron phosphides (Fe 2 P, Fe3 P), iron carbides (Fe3 C, Fe2 cause severe interparticle aggregation. As shown in Fig. S4 in the
C) and/or iron phosphocarbides (Fe xPyCz) [46]. It was also found that Supplementary data, morphology of C-LiFePO4 with higher carbon content
3+ 2+ (E1C2) is very similar to C-LiFePO 4 with lower carbon content (E1C). Table
complete reduction of Fe impurities remained in LiFePO4 into Fe during
the carbonization reaction always generated the iron phosphides species 2 lists the BET surface area of the C-LiFePO 4 samples. Higher BET surface
[22,46]. The carbon oxidizes to form CO or CO 2 during the reaction. These areas of C-LiFePO4 than those of the bare LiFePO4 is because the carboneous
studies may imply that the forma-tion of iron phosphides/iron carbides/iron material coated on the surface of the particles contribute to the increase in the
phosphocarbides during the carbonization might be responsible for the excess surface area.
Fe species in the carbon-coated LiFePO 4 . In contrast to the SHS’d samples,
the expected LiFePO4 stoichiometry was obtained in the parti-cles prepared
Fig. 4 shows the electrochemical properties of the LiFePO 4 par-ticles
using the solid-state method and C-LiFePO4 (see Table 2).
prepared using the SHS method and the solid-state method. The bare LiFePO 4
Fig. 3 shows SEM images of the LiFePO4 particles prepared using the samples showed sloping plateaus, indicating that the active materials are not
SHS method and the solid-state method. The size of particles synthesized at
−1 properly utilized. The SHS’d LiFePO4 (samples E1–E3) exhibited smaller
the lower precursor solution flow rate (1.7 g min ) is in the range of 200–
800 nm (Fig. 3a). More uniform and smaller size particles with less initial capacity compared to the solid-state synthesized LiFePO 4 (sample S),
aggregation were produced at the higher precursor solution flow rate (3 g 3+
which may be because the presence of the Fe impurities in the SHS’d
−1 −1
min ) (Fig. 3b). When the water flow rate increased from 9 g min to 18 g LiFePO4 (see Table 2). However, the E1 sample showed larger capacities
−1
min , the particles are smaller and rod-shape particles seem to increase (Fig. after 6 cycles and the E2 and E3 samples showed larger capaci-ties after 16–
3+ 20 cycles over the S sample. The charge–discharge capacity of the E1–E3
3c). Larger amount of the Fe species was produced at higher flow rate, as −1
samples is in the rage of 70–80 mAh g after 30 cycles at 0.1 C rate. This
dis-cussed in the previous section. Note that the morphology of SHS’d Fe 2 O3 −1
is spherical in shape [47,48]. Thus, the rod-like morphology resulted at higher value is higher compared to that of the S sample (60 mAh g ). The better
3+ cycle performance of the SHS’d LiFePO 4 particles can be due to more
flow rate may not be related to the Fe species. The effect of flow rate on the
particles size was characterized quan-titatively by measuring BET surface homogeneous utiliza-tion of the active materials and shorter diffusion length
area. As listed in Table 1, the BET surface area increased two times from 6.3 +
of Li ions associated with the smaller size particles [5–8]. In addition, higher
2 −1 2 −1
m g to 12.1 m g as the precursor solution flow rate increased from 1.7 g crystallinity of the SHS’d LiFePO4 particles may be beneficial to the better
−1 −1 2 −1 2 −1
min to 3 g min , and further increased from 12.1 m g to 15.9 m g cycling performance by retarding dissolution of iron from the cathode material
in the electrolyte [50]. Therefore, even though some degree of impurities was
present, smaller particle size and
1034 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

Fig. 7. HR-TEM images of (a) sample E1, (b) sample S, (c) sample E1C2, (d) sample SC.

higher crystallinity of the SHS’d LiFePO 4 showed better cycling performance which is because carbon coated on the particles lost during the heating. A
compared to the solid-state synthesized LiFePO4 . significant weight change of the E1C2 sample (prepared with sucrose to
As shown in Fig. 4e, the charge–discharge capacity decreased as the LiFePO4 weight ratio of 12:1) compared to the E1C and S samples (prepared
−1 −1
solution flow rate increased 1.7 g min to 3 g min (samples E2). As with sucrose to LiFePO4 weight ratio of 4:1) indicate that larger amount of
discussed in the previous section, the amount of impurity increased when the carbon was present in the E1C2 sample. The carbon content in the C-LiFePO 4
solution flow rate increased. Thus, it is very likely that the larger amount of
impurities formed at higher flow rate resulted in lower discharge capacities. samples, measured using the elemental analysis, is listed in Table 2.
−1 Approximately 2 wt.% carbon was present in the SC and the E1C samples,
The particles synthe-sized at the higher water flow rate of 18 g min (sample
E3) exhibit better cycling performance over sample E2, even though larger and 6 wt.% carbon was present in the E1C2 sample. The carbon coating can
amount of impurities were present in sample E3. The smaller par-ticle size of enhance electronic conductivity of LiFePO4 . As shown in Table 2, the C-
sample E3 may be responsible for the better discharge capacity. LiFePO4 samples have 4–5 orders of magnitude higher elec-tronic
−4 −5 −1 −9
conductivity (∼10 to 10 S cm ) compared to the bare LiFePO4 (10 S
−1
cm ).
To improve the electrochemical performance of LiFePO 4 , the particles The structure of carbon in the C-LiFePO 4 samples was further
were coated with a carbon layer using sucrose as a cheap and environmentally investigated using Raman spectroscopy, as shown in Fig. 6. The sharp and
benign carbon source. The carbon coated on the particles was characterized −1 +
strong peaks at 220 and 292 cm correspond to the Li motion of LiFePO4
◦ −1
by heating C-LiFePO4 up to 800 C in oxygen atmosphere with temperature [52]. The peaks between 450 and 800 cm are assigned to mostly 4 vibration
◦ −1
ramping of 5 C min , as shown in Fig. 5. It is well-known that oxidation with some contribution from 2 vibration of LiFePO4 [52,53]. The peaks at
state purity of LiFePO 4 can be determined by heating the material in oxygen −1
630, 985 and 1040 cm are assigned to 4 , 1 , and 3 intramolecular stretching
◦
atmosphere up to 600 C. A complete oxidation of all the iron in LiFePO4 modes of symmetric PO4 in LiFePO4 [52,53]. The peak at 1350 cm is
−1
2+ 3+
from ferrous state (Fe in LiFe(II)PO4 ) to ferric state (Fe in LiFe(III)PO4 3 −1
assigned to D band of the sp coordinated carbon, and the peak at 1590 cm
(OH)) causes a weight gain. If all the iron in the LiFePO 4 sample is the 2
is assigned to G band of the sp coordinated carbon. The Raman spectra of the
ferrous state, then the theoretical weight gain is known to be ∼5.1% [51]. The
bare LiFePO4 particles did not show the D and G bands, indicating that
weight gain of the LiFePO 4 prepared using the solid-state method is 5.1% that carbon materials were not present (Fig. 6a and d). After the carbon coating,
is identical to the theo-retical value (Fig. 5d). In contrast, the weight gain of broad and intense D peak and rela-tively weak G peak were observed (Fig. 6b,
the particle prepared using the SHS method is 3.95%, a value smaller than c and e). Note that lower Raman D/G band ratio, thus higher amount of
graphene carbons, is beneficial to the electronic conductivity and
that of the solid-state synthesized particle (Fig. 5a). As shown in Fig. 5b, c
electrochemical per-formance of C-LiFePO 4 [42]. Factors to increase G band
and e, lower weight gain was observed in the C-LiFePO 4 samples, of coated
 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

carbon such as carbon sources, coating method, and sintering tem-peratures

are being studied.
Fig. 7 shows the TEM images of bare LiFePO4 and C-LiFePO4 par-ticles.
The dark region is the LiFePO4 particle and the light grey region is the
carbon. Factors determining electrochemical proper-ties of carbon-coated
LiFePO4 can include carbon content, property of carbon (graphite or
amorphous carbon), and coating morphol-ogy, surface coverage and
uniformity. Larger amount of graphite carbon, uniform surface coverage with
an island structure can result in good electrochemical performance of C-
LiFePO4 [54]. It can be seen in Fig. 7c that the carbon was loosely connected
to the surface of the SHS’d LiFePO 4 particles. In addition, some por-tions of
the particles were not covered by the carbon. In contrast, thin carbon layer
with thickness in the range of 1–2 nm and with better surface coverage was
coated on the solid-state synthesized LiFePO 4 (Fig. 7d). This surface
coverage and morphology difference may result in an order of lower
electronic conductivity of the SHS’d C-LiFePO 4 than that of the solid-state
synthesized C-LiFePO4 at the same carbon coating condition (SC and E1C
samples, see Table 2).
Fig. 8 shows the discharge curves and the cycle performances of the
LiFePO4 and the C-LiFePO4 samples prepared by the SHS and the solid-state
methods. The carbon coated LiFePO 4 showed larger capacities with the
clearer flatness of the plateau at 3.4–3.5 V region. The E1C, E1C2, and SC
−1
samples delivered discharge capac-ities of 126, 135, and 158 mAh g ,
respectively (Fig. 8a and b). This indicates that the carbon-coated LiFePO 4
was utilized more effectively than the bare LiFePO4 due to the enhanced
electronic conductivity (see Table 2). Similar enhancement of the
charge/discharge capacity after the carbon coating has been observed in
previous works [11,13]. The C-LiFePO4 prepared by the solid-state method
(sample SC) showed larger capacities than those of C-LiFePO4 synthesized in
supercritical water (samples E1C and E1C2) during the 30th cycle. As
discussed in the previous sec-tion, the SC sample did not contain impurities
while ∼10% excess Fe impurities were present in the E1C and E1C2 samples.
In addi-tion, the SC sample retained better surface coverage with carbon. The
absence of the impurities and the better carbon coating may be responsible for
the higher discharge capacity of the solid-state synthesized C-LiFePO 4
sample.

Capacity decay can be observed in the C-LiFePO 4 samples pre-pared by

the SHS and the solid-state methods. The capacities of the SC sample
−1
increased from 143 to 158 mAh g for the first 9th cycles, and rapidly
−1
decayed to 140 mAh g for the last 30th cycles (11% decay of the initial
capacity). The capacity increases up to 9th cycle can be due to incomplete
−1
activation of the cell. Capac-ity decay from 126 to 112 mAh g was Fig. 8. The discharge curves of (a) samples El, ElC, and E1C2 and (b) samples S and SC, and
observed in the E1C sample (11% decay of the initial capacity), and capacity (c) the cycling performance of samples E1, ElC, E1C2, S, and SC (closed symbol; charge,
−1 opened symbol; discharge).
decay from 135 to 125 mAh g (7% decay of the initial capacity) was
observed in the E1C2 sample. An increase in polarization of electrode by
dissolu-tion of active material may be responsible for the observed capacity lization of the active materials. This observation clearly suggests that the SHS
decay [50]. method is a very promising alternative to the con-ventional solid-state
Lastly, when the carbon content increased by using higher sucrose method. In addition, rapid production rate (within 1 min) and continuous
concentration, the discharge capacity at 0.1 C rate after 30th cycles increased running mode of the SHS method is amenable to commercial scale production
−1 of the active material.
from 126 to 135 mAh g . Higher amount of carbon coating layer increased
the electronic conductivity by an order of magnitude (see Table 2) and
induced better utilization of the active materials, resulting in higher discharge 4. Conclusions
capacity and better capac-ity retention. It is noted that factors determining
discharge capacity were not optimized in this study. The factors including the Nanosize LiFePO4 particles with size in the range of 200–800 nm were
purity of the particles and the optimization of the carbon coating are being synthesized using the fast and continuous SHS method at the reaction time of
studied to improve discharge capacity. 40 s. The SHS’d particle size was much smaller than that of the particle
produced by the conventional solid-state method (3–15 m). However, ∼10%
Even though the initial capacities of LiFePO 4 and C-LiFePO4 syn- 3+
Fe impurities were present in the SHS’d LiFePO4 . The discharge capacity
thesized by the SHS method were smaller than those synthesized by the solid- −1
of the SHS’d LiFePO4 particles were in the range of 70–80 mAh g at 0.1 C
state method, the discharge curve profile and better capacity retention suggest
after 30 cycles depending on the synthesis conditions. These values were
that the nano-sized LiFePO4 cathode material synthesized by the SHS method −1
induces a better uti- higher compared to that of solid state synthesized LiFePO 4 (60 mAh g
1036 S.-A. Hong et al. / J. of Supercritical Fluids 55 (2011) 1027–1037

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