Manipulation of adipic acid application on the electrochemical properties of LiFePO4 at high rate performance

Article abstract of DOI:10.1016/j.jallcom.2010.10.009

High performance nanocrystalline LiFePO₄ was successfully synthesized utilizing a solid-state method with adipic acid as the carbon source. Different molar ratios of adipic acid to total metal ions were analyzed, which included the mol ratios 0, 0.05, 0.1, and 0.2. Among the different concentrations tests, 0.1 mol adipic acid-treated LiFePO₄ exhibited superior performance in terms of capacity profile and a stable discharge behavior of ～150 mAh/g at room temperature. The thickness of the carbon coating layer was confirmed at approximately 5 nm by transmission electron microscopy. Rate capability studies were also performed from 1.0 to 37 C for 0.1 mol adipic acid-treated LiFePO₄. These studies clearly showed excellent capacity retention over 73 mAh/g at 37 C rate.

Full text of DOI:10.1016/j.jallcom.2010.10.009

Journal of Alloys and Compounds 509 (2011) 1279–1284
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Manipulation of adipic acid application on the electrochemical properties of
LiFePO4 at high rate performance
C.G. Sona, H.M. Yanga, G.W. Leea, A.R. Choa, V. Aravindana, H.S. Kimb, W.S. Kimc, Y.S. Leea,∗
a Faculty of Applied Chemical Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea
b Korea Electrotechnology Research Institute, Changwon 641-120, Republic of Korea
c Daejung EM Co. LTD., Incheon 405-820, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
High performance nanocrystalline LiFePO4 was successfully synthesized utilizing a solid-state method
with adipic acid as the carbon source. Different molar ratios of adipic acid to total metal ions were
analyzed, which included the mol ratios 0, 0.05, 0.1, and 0.2. Among the different concentrations tests,
0.1 mol adipic acid-treated LiFePO4 exhibited superior performance in terms of capacity profile and a
stable discharge behavior of ∼150 mAh/g at room temperature. The thickness of the carbon coating layer
was confirmed at approximately 5 nm by transmission electron microscopy. Rate capability studies were
also performed from 1.0 to 37 C for 0.1 mol adipic acid-treated LiFePO4. These studies clearly showed
excellent capacity retention over 73 mAh/g at 37 C rate.
Received 13 August 2010
Received in revised form 5 October 2010
Accepted 7 October 2010
Available online 14 October 2010
Keywords:
Adipic acid
LiFePO4
Solid-state synthesis
Carbon coating
Rate capability
TEM
Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction
LiFePO4 cathodes [11,12]. Numerous carbon source materials have
been explored for coating LiFePO4 and noteworthy among them are
The application of lithium iron phosphate (LiFePO4) as a poten-
tial cathode material for rechargeable lithium batteries were first
demonstrated by Padhi et al. [1] in 1997. Since then, LiFePO4 has
offered several unique benefits when compared to conventional
cathode materials such as LiCoO2, LiNiO2 and LiMn2O4.LiFePO4
exhibits a flat discharge profile, good thermal stability, high
theoretical capacity (170 mAh/g) and environmentally friendly
properties [2]. However, a significant problem with the use of this
material lies in its conductivity. This is due to the one-dimensional
diffusion of Li+ ions along the b axis formed by edge-shared LiO6
octahedra, compounded by poor electronic and ionic conductiv-
ity from the LiFePO4–FePO4 interface [3]. This leads to poor rate
performance of the cell. Two methods have been employed to cir-
cumvent the poor conductivity issue. The first is to reduce the grain
size of the cathode particles, which would shorten the diffusion
path length for both electrons and Li+ ions. The second is the use of
surface-modified LiFePO4 with a conductive matrix made of carbon
[4–6], copper [7,8] and silver [9], or doping with some guest species
[10]. Of these methods, surface-modified LiFePO4 with carbon has
proved to be an excellent alternative in the search for improved
the carboxylic acids reported by Fey et al. [13].
Several methods have been proposed for the synthesis of olivine
LiFePO4, including microwave processing, solid-state, sol–gel,
hydrothermal, spray pyrolysis, precipitation, mechano-chemical,
and emulsion drying [14–20]. Of the synthetic methods, solid-state
and solution-based routes are primarily appealing for large scale
production [20]. For the sol–gel method, synthesis is carried out
in two steps which are precursor preparation and powder crystal-
lization. In addition, a final heating step is often carried out at high
temperatures under an inert or reductive atmosphere to prevent
iron oxidization [18]. The precursor is prepared in the liquid phase,
by the subsequent evaporation of the solvent. During this pro-
cess, care must be taken to prevent Fe2+ ions from being oxidized
upon exposure to air. Therefore, the solution phase is less effective,
although it offers uniform sized nanoparticles with a homogeneous
coating. When compared with solution phase methods, the solid-
state method provides an easier and more effective approach for
mass production [14–20].
We recently reported on the synthesis of LiFePO4 using adipic
acid as a chelating agent by using the sol–gel method [21], and pro-
vided conclusive evidence of the enhanced and stable discharge
profile at a high rate capability. Previously, we also reported on a
new type of LiFePO4 material using adipic acid as a carbon source
that had been successfully synthesized by a solid-state method and
∗
Corresponding author. Tel.: +82 62 530 1904; fax: +82 62 530 1909.
E-mail address: leeys@chonnam.ac.kr (Y.S. Lee).
0925-8388/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.10.009

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C.G. Son et al. / Journal of Alloys and Compounds 509 (2011) 1279–1284
Fig. 1. Thermogravimetric and differential thermal analysis (TG-DTA) start-
ing materials used for the synthesis of LiFePO4 (a) Li2CO3, FeC2O4·2H2O, and
(NH4)2HPO4 and (b) Li2CO3, FeC2O4·2H2O, (NH4)2HPO4, and C6H10O4.
Fig. 2. X-ray diffraction patterns of LiFePO4 obtained by different molar ratios of
adipic acid to total metal ions: (a) without adipic acid; (b) 0.05 mol adipic acid; (c)
0.1 mol adipic acid and (d) 0.2 mol adipic acid.
presented superior cycle performance under harsh conditions in
communication [22]. In this study, we present the synthetic pro-
cess and electrochemical properties of the LiFePO4 material in more
detail. Our results show the excellent battery performance results
obtained to date of LiFePO4 using adipic acid as a carbon source
synthesized by a solid-state approach.
The first stage comprising multiple endothermic events starts at
∼100 ◦C and ends at ∼300 ◦C for both cases. The initial weight losses
observed are attributed to the dehydration and decomposition of
acetate groups from the starting material, as well as to the melting
of AA. The second melting event starts at 300–450 ◦C and can be
recognized by decomposition of the remaining reactants with the
carbonization of AA, which is the beginning of the crystallization of
LiFePO4. The final stage containing the slow and gradual weight loss
above 450–650 ◦C corresponds to decomposition of the remain-
ing reactants and oxidation of carbon to produce either carbon
monoxide or carbon dioxide [23–25]. The major difference between
the two materials is that the weight loss of the LiFePO4 precur-
sor with AA is larger than that of the precursor without AA, which
resulted from the acceleration of the chemical reaction owing to
the decomposition of adipic acid used as the carbon source. The
thermal analysis suggested that the precursor was heat-treated at
temperatures above 650 ◦C for the synthesis of a pure LiFePO4 phase
with high crystallinity.
The X-ray diffraction pattern of the LiFePO4 compound prepared
by the solid-state reaction method with different molar contents
of AA is presented in Fig. 2. The occurrence of broad and well-
defined Bragg peaks demonstrated the formation of size controlled
and highly crystallized LiFePO4 grains. The Miller indices (h k l)
of these peaks were indexed as per the JCPDS Card No. 40-1499,
corroborating the existence of an orthorhombic structure with a
Pnma space group. The lattice parameter values are presented in
Table 1. The lattice parameters of the parent LiFePO4 were consis-
tent with those reported in the literature [1,14]. The carbon-coated
LiFePO4 materials showed a slightly lower lattice parameter value
than the uncoated material due to the absence of impurity phases.
Furthermore, the absence of a peak at 2ꢀ = 27 and 41◦ indicated the
phase purity of the synthesized material without impurities such as
Li3Fe2(PO4)3, FeP, and Fe2S [8,13,25,26]. The XRD pattern revealed
that carbonization of AA not only provides the carbon coating, but
also provides a reducing atmosphere that favors the formation of
phase pure LiFePO4.
2. Experimental
The LiFePO4 material was synthesized from Li2CO3, FeC2O4·2H2O, (NH4)2HPO4,
and C6H10O4 (Sigma–Aldrich, USA) using a conventional solid-state method. Stoi-
chiometric ratios of the Li:Fe:P source materials at various molar ratios of adipic acid
to total metal ions (0, 0.05, 0.1, and 0.2) were used for the synthesis. The starting
materials were finely ground in a mortar and precalcined at 400 ◦C for 3 h to enable
carbonate and oxalate decomposition. The product was finely ground again in a mor-
tar and fired at 670 ◦C for 5 h under an Ar atmosphere (99.9%) to achieve the resultant
nanocrystalline LiFePO4. The same conditions and procedure were conducted for
different molar ratios of adipic acid.
Thermal studies were carried out by means of thermogravimetric-differential
thermal analysis (TG-DTA) using a thermal analyzer system (STA 1640, Stanton Red-
croft Inc., UK) with a thin Pt plate used as the sample holder. The powder was
heated at 5 ◦C/min and cooled at 10 ◦C/min. Structural analysis was performed by
powder X-ray diffraction (XRD, Rint 1000, Rigaku, Japan) using CuK␣ radiation.
The carbon content of the synthesized LiFePO4 particles was determined using
an elemental analyzer (CHN Flash EA series, CE Instruments, Italy). The surface
morphology of the resultant compound was observed using a field emission scan-
ning electron microscope (FE-SEM, S-4700, Hitachi, Japan). Particle size distribution
was carried out through a dynamic light scattering system (DLS-7000[AL], Otsuka
electronics, Japan). The internal structure of the samples was observed by trans-
mission electronic microscopy (TEM, TECNAI, Philips, Netherlands). Electrochemical
impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed using
a Bio-Logic electrochemical work station (SP-150, Biologic, France) with a three
electrode cell configuration. In the three electrode configuration, metallic lithium
served as the counter and reference electrodes. The composite cathode was fab-
ricated with exactly 20 mg of active material, 3 mg of Ketzen black, and 3 mg of
conductive binder (2 mg of Teflonized acetylene black (TAB) and 1 mg of graphite).
It was pressed on a 200 mm2 stainless steel mesh, serving as the current collector
under a pressure of 300 kg/cm2 and dried at 130 ◦C for 5 h. The cell was comprised
of a cathode and a metallic lithium anode separated by a porous polypropylene
film (Celgard 3401, USA). A 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbon-
ate (DMC) (1:1, v/v, Techno Semichem Co., Ltd., Korea) mixture was used as the
electrolyte. The charge/discharge current density was 0.1 C (1 C refers to a capacity
of 170 mA g−1 in one hour) with a cut-off voltage of 2.8–4.0 V and various current
densities (1–37 C) at room temperature.
Surface morphology features of the particles were analyzed by
scanning electron microscopy and each image is shown in Fig. 3.
Smooth surfaces were observed for uncoated and 0.1 mol AA-
coated LiFePO4, whereas flaky appearances were seen for 0.05 and
0.2 mol AA-coated LiFePO4 materials. Uniform and smooth surface
morphologies are a prerequisite for commercial cathode materi-
als [27]. All the carbon-treated materials showed reduced particle
sizes compared to uncoated LiFePO4, suggesting that formation of
3. Results and discussion
Thermogravimetric analysis (TGA) was used to ascertain the
temperature conditions for the synthesis of the nanocrystalline
LiFePO4 material. Fig. 1 shows the TG trace performed on the
LiFePO4 precursor both with and without adipic acid (referred to
as AA from here on), and is clearly separated by three main stages.

C.G. Son et al. / Journal of Alloys and Compounds 509 (2011) 1279–1284
1281
Fig. 3. SEM images of LiFePO4 obtained by different molar ratios to total metal ions: (a) without adipic acid; (b) 0.05 mol adipic acid; (c) 0.1 mol adipic acid and (d) 0.2 mol
adipic acid.
Fig. 4. Particle size distribution of LiFePO4 obtained for different molar ratios to total metal ions by solid-state method: (a) without adipic acid; (b) 0.05 mol adipic acid; (c)
0.1 mol adipic acid and (d) 0.2 mol adipic acid.

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C.G. Son et al. / Journal of Alloys and Compounds 509 (2011) 1279–1284
Fig. 5. TEM images of LiFePO4 for (a) without adipic acid and (b) 0.1 mol adipic acid.
a carbon layer hinders particle growth at high temperatures [28].
Dynamic light scattering experiments were performed to study the
distribution of the synthesized particles by the solid-state method
and this is graphically illustrated in Fig. 4. For the parent LiFePO4,
the majority of the particles lie between 200 and 400 nm. The
size of the particles was drastically reduced after the introduction
of the carbon coating, with the major proportion of the parti-
cles between 150–300 nm, 100–200 nm, and 150–400 nm for 0.05,
0.1, and 0.2 mol AA-treated LiFePO4, respectively. This might be
due to coated carbon preventing the growth of LiFePO4 particles
during synthesis. This result is in agreement with Konorova and
Taniguchi’s report [29] on the suppression of particle size during
the synthesis process. In the case of 0.2 mol adipic acid-treated
LiFePO4 comprises two separate range of particle distribution
∼150–250 nm and ∼400 nm region. Appearance of ∼400 nm region
is due to the presence of few larger size particles (based on the parti-
cles distribution) along with smaller ones that could be the reason
for the two separate regions in the particle size distribution. The
0.1 mol AA-treated LiFePO4 has a smaller particles size compared
to other AA coating concentrations and this may be reflected in the
electrochemical performance of the cell via the improved Li+ ion
diffusion kinetics and high surface area.
Fig. 6. The first charge/discharge curves of the Li/LiFePO4 cell obtained for different
molar ratios to total metal ions: (a) without adipic acid; (b) 0.05 mol adipic acid; (c)
0.1 mol adipic acid and (d) 0.2 mol adipic acid.
Furthermore, elemental analysis was also performed
to gauge carbon content in LiFePO4 by the solid-state
method. As shown in Table 1, carbon contents of 0.26, 1.02,
0.92, and 0.54 wt.% were observed for 0, 0.05, 0.1, and 0.2 AA
contents against the total metal ions present in LiFePO4, respec-
tively. Very small amounts of carbon present in the untreated
LiFePO4 may be due to decomposition of the starting materials,
such as Li2CO3 and FeC2O4·2H2O. At the higher concentration of
0.2 mol the carbon content was found to be lower, which may
be attributed to the fact that the carbon was highly oxidized and
could form CO and CO2 at high temperatures [23]. It was expected
that a carbon content of between 0.9 and 1.0% weight was good for
the electrochemical performance of the cell. If the carbon content
in the LiFePO4 was higher or lower than 1.0% weight, this lead to
poor cell performance at a calcination temperature of 670 ◦C. This
result is in agreement with Konorova and Taniguchi’s study on
carbon analysis [28].
Fig. 5 shows transmission electron microscopy (TEM) images of
pristine and 0.1 mol AA-treated LiFePO4 particles. A clear difference
was observed between the pristine and carbon-coated materials.
From this, it is apparent that the 0.1 mol AA is well formed as the
layer of carbon on the surface of the LiFePO4 particles. The carbon
is coated on the LiFePO4 particles at a thickness of approximately
5 nm, suggesting that the carbon layer also plays a crucial role in
particle growth of LiFePO4 during the synthetic process [29].
The first charge/discharge curves of the Li/LiFePO4 cells at 0.1 C
with different levels of carbon treatment are presented in Fig. 6.
Table 1
Powder properties of LiFePO4 materials obtained by different molar ratios of adipic acid to total metal ions.
3
Molar ratio of adipic acid
˛ (◦)
ˇ (◦)
ꢁ (◦)
Carbon content (wt.%)
˚
a (A)
˚
b (A)
˚
c (A)
˚
V (A)
0
10.347
10.332
10.339
10.329
6.019
6.010
6.011
6.009
4.699
4.663
4.663
4.638
90
90
90
90
90
90
90
90
90
90
90
90
2.923
2.916
2.919
2.913
0.26
1.02
0.92
0.54
0.05
0.1
0.2

C.G. Son et al. / Journal of Alloys and Compounds 509 (2011) 1279–1284
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Scheme 1. Dual phase nature of LiFePO4.
Fig. 8. Cyclic voltammograms of (a) without adipic acid and (b) 0.1 mol adipic acid.
The metallic lithium can act as reference and counter electrodes between 2.4 and
4.2 V with the scan rate of 0.02 mV/s.
The charge/discharge profiles of LiFePO4 show long and flat voltage
plateaus near the 3.4–3.5 V region, revealing the two-phase nature
of the lithium extraction and insertion reaction between LiFePO4
and FePO4 [1,30]. The dual phase nature is schematically repre-
sented in Scheme 1. It is evident that 0.1 mol AA-treated LiFePO4
exhibits the highest discharge capacity profile compared with the
additional concentrations tested. The initial discharge capacity val-
ues of carbon-coated LiFePO4 were 127, 142, 144, and 135 mAh/g
for the 0, 0.05, 0.1, and 0.2 mol concentrations of AA, respectively. As
shown in Fig. 7, the 0.1 mol AA-treated LiFePO4 showed an increase
in discharge capacity until the sixth cycle, a larger value than the
0.05 mol AA, even though it showed almost the same discharge
capacity in the first cycle. Furthermore, a small voltage difference
between charge and discharge plateaus of 0.1 and 0.05 mol AA-
treated LiFePO4 indicated improved kinetics compared to other
concentrations. This was relevant after considering the low elec-
trochemical diffusion rate of lithium ions in the solid-phase as well
as its poor electronic conductivity [13,16].
cycles are required for the complete participation of active mate-
rial used to deliver such stability, which was the reason to increase
in discharge capacity during first few cycles. In particular, sim-
ilar stability was also observed for 0.2 mol AA-treated, but with
marginally lower capacities than the 0.1 mol treated material. This
may be due to the residual carbon tending to form an inactive layer
on the surface of the active material, thus diluting the active par-
ticle distance and leading to the reduced discharge capacity of the
LiFePO4. Furthermore, the excess amount of carbon also acceler-
ates the formation of Fe2P from Fe and P and adversely affects
cell cycling performance [20,29]. The 0.05 mol AA-carbon-coated
LiFePO4 also exhibited stable discharge behavior with a capac-
ity significantly lower than the 0.1 mol AA-treated LiFePO4. This
decreased capacity may be attributed to the large particle size of
LiFePO4 compared to 0.1 mol AA-coated material, even though the
residual carbon content was higher than that of 0.1 mol AA-coated
LiFePO4. Thus, an optimization of carbon coating is necessary to
achieve higher and more stable discharge behaviors of LiFePO4. As
shown in these results, 0.1 mol AA concentrations are adequate to
attain the desired properties of LiFePO4 by solid-state synthesis.
Cyclic voltammograms of the pristine LiFePO4 and 0.1 mol AA-
treated LiFePO4 electrodes recorded at a scan rate of 0.02 mV/s are
presented in Fig. 8. Well-defined reduction and oxidation peaks of
LiFePO4 appear at 3.34 and 3.50 V, respectively, demonstrating that
the Fe2+/3+ redox pairs contribute to the gain and loss of electrons in
the LiFePO4 crystal structure during the lithiation and delithiation
process. In the 0.1 mol AA-treated LiFePO4 electrode, the reduction
and oxidation current is much higher than for uncoated LiFePO4
and this demonstrates improved conducting properties as well as
faster kinetics of the carbon-coated material. This also reveals that
the addition of carbon to LiFePO4 restrains its polarization. Further-
more, the same intensity values during extraction and insertion of
Li+ ions corresponded to the excellent phase purity and reversibility
of the material by adipic acid-intervened solid-state synthesis.
In order to study the effects of carbon coating on the electro-
chemical properties of LiFePO4, a.c. impedance measurements were
carried out for the pristine LiFePO4 and 0.1 mol AA-treated LiFePO4
cathodes. Fig. 9 shows the impedance spectra of the LiFePO4
and 0.1 mol AA-treated LiFePO4 electrodes. Both spectra showed
a semicircle in the middle frequency region and a 45◦ inclined
straight line in the low frequency region. The semicircle corre-
sponds to the charge-transfer (CT) process, and the straight line
is attributed to the diffusion of Li+ ions into the host material, also
referred to as the alleged Warburg tail [31]. The CT resistance of
Carbon-coated LiFePO4 cells showed stable discharge behavior
for observed cycles, whereas uncoated LiFePO4 showed a con-
tinuous capacity fading due to sluggish kinetics and inherent
conducting behavior. Of the carbon-coated materials, the 0.1 mol
AA-treated LiFePO4 delivered a superior performance with a dis-
charge capacity of 150 mAh/g for 50 cycles. However, initial few
Fig. 7. Cycle performance of the Li/LiFePO4 cells obtained by different molar ratios
to total metal ions: (a) without adipic acid; (b) 0.05 mol adipic acid; (c) 0.1 mol adipic
acid and (d) 0.2 mol adipic acid.

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C.G. Son et al. / Journal of Alloys and Compounds 509 (2011) 1279–1284
carbon-coated LiFePO4 by adipic acid using solid-state synthesis
is simple, convenient and very easily reproduced on an industrial
scale.
4. Conclusions
Nanocrystalline LiFePO4 was successfully synthesized by a
solid-state reaction method using various concentrations (0–0.2) of
adipic acid proportional to total metal ions. Powder X-ray diffrac-
tion measurements demonstrated a well-developed LiFePO4 phase
and crystallinity of the synthesized materials. A carbon layer of
approximately 5 nm was coated onto the synthesized particles from
the carbonization of adipic acid at high temperatures, which was
confirmed by TEM. The 0.1 mol adipic acid-treated LiFePO4 exhib-
ited superior performance over other concentrations of carbon
coating. The cell presented an excellent cycle performance until
37 C.
Fig. 9. Electrochemical impedance traces of (a) bare LiFePO4 and (b) 0.1 mol adipic
acid-treated LiFePO4 at ambient temperature.
Acknowledgement
This work was supported by a grant (Code#: 2010K000329)
from the Center for Nanostructured Materials Technology, under
the 21st Century Frontier R&D programs of the Ministry of Educa-
tion, Science and Technology, Korea.
References
[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144
(1997) 1188.
[2] M. Thackeray, Nat. Mater. 1 (2002) 81.
[3] A.S. Andersson, J.O. Thomas, J. Power Sources 97–98 (2001) 498.
[4] A.K. Shukla, T.P. Kumar, Curr. Sci. 94 (2008) 314.
[5] Z. Chen, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A1184.
[6] H. Huang, S.C. Yin, L.F. Nazar, Electrochem. Solid State Lett. 4 (2001) A170.
[7] F. Croce, A. D’Epifanio, J. Hassoun, A. Deptula, T. Olczac, B. Scrosati, Electrochem.
Solid State Lett. 5 (2002) A47.
[8] S.B. Lee, S.H. Cho, J.B. Heo, V. Aravindan, H.S. Kim, Y.S. Lee, J. Alloys Compd. 488
(2009) 380.
[9] K.S. Park, J.T. Son, H.T. Chung, S.J. Kim, C.H. Lee, K.T. Kang, H.G. Kim, Solid State
Commun. 129 (2004) 311.
[10] S.Y. Chung, J.T. Bloking, Y.M. Chiang, Nat. Mater. 1 (2002) 123.
[11] F.K. Hsu, S.Y. Tsay, B.J. Hwang, J. Mater. Chem. 14 (2004) 2690.
[12] M. Takahashi, H. Ohtsuka, K. Akuto, Y. Sakurai, J. Electrochem. Soc. 152 (2005)
A899.
[13] G.T.K. Fey, T.L. Lu, F.Y. Wu, W.H. Li, J. Solid State Electrochem. 12 (2008) 825.
[14] M.S. Song, H.S. Kwon, J. Power Sources 166 (2007) 260.
[15] A. Manthiram, A.V. Murugan, A. Sarkar, T. Muraliganth, Energy Environ. Sci. 1
(2008) 621.
Fig. 10. Rate capability performance of the Li/LiFePO4 cell at room temperature. The
molar ratio of adipic acid to total metal ions is 0.1 and the current densities are (a)
1 C, (b) 8 C, (c) 11 C, and (d) 37 C.
[16] Z. Li, D. Zhang, F. Yang, J. Mater. Sci. 44 (2009) 2435.
[17] K. Saravanan, M.V. Reddy, P. Balaya, H. Gong, B.V.R. Chowdari, J.J. Vittal, J. Mater.
Chem. 19 (2009) 605.
[18] D. Jugovic, D. Uskokovic, J. Power Sources 190 (2009) 538.
[19] C. Gouri, Lithium Iron Phosphate—A Promising Cathode-Active Material for
Lithium Secondary Batteries, Trans Tech Publishers, 2008.
[20] S.S. Zhang, A.L. Allen, K. Xu, T.R. Jow, J. Power Sources 147 (2005) 234.
[21] S.B. Lee, S.H. Cho, S.J. Cho, G.J. Park, S.H. Park, Y.S. Lee, Electrochem. Commun.
10 (2008) 1219.
[22] H.H. Lim, I.C. Jang, S.B. Lee, K. Karthikeyan, V. Aravindan, Y.S. Lee, J. Alloys
Compd. 495 (2010) 181.
[23] L.N. Wang, X.C. Zhan, Z.G. Zhang, K.L. Zhang, J. Alloys Compd. 456 (2008) 461.
[24] Y.L. Cao, L.H. Yu, T. Li, X.P. Ai, H.X. Yang, J. Power Sources 172 (2007) 913.
[25] G.X. Wang, S. Bewlay, J. Yao, J.H. Ahn, S.X. Dou, H.K. Liu, Electrochem. Solid-State
7 (2004) A503.
[26] K. Shiraishi, K. Dokko, K. Kanamura, J. Power Sources 146 (2005) 555.
[27] A.S. Aricò, P. Bruce, B. Scrosati, J.-M. Tarascon, W. van Schalkwijk, Nat. Mater. 4
(2005) 366.
[28] M. Konarova, I. Taniguchi, Mater. Res. Bull. 43 (2008) 3305.
[29] M. Konorova, I. Taniguchi, Powder Technol. 191 (2009) 111.
[30] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930.
[31] G.X. Wang, L. Yang, Y. Chen, J.Z. Wang, S. Bewlay, H.K. Liu, Electrochim. Acta 50
(2005) 4649.
the LiFePO4 electrode was much larger than those treated with AA.
Consequently, the improved battery performance of the LiFePO4
electrode may be due to the decrease in CT resistance of the elec-
trode.
Capacity retention is an important factor in determining the
capability and structural stability of the prepared material. In order
to study capacity retention, the 0.1 mol AA carbon-coated LiFePO4
cell was further investigated. The rate capability and capacity
retention of the Li/LiFePO4 cell was analyzed at different current
densities from 1 to 37 C at room temperature and this is illus-
trated in Fig. 10. The cell delivered a discharge capacity of 143 and
73 mAh/g for 1 C and the extremely high 37 C rates, respectively.
This excellent rate capability performance may be ascribed to the
superior phase purity and effective coating of carbon derived from
adipic acid by the solid-state method used. The discharge capacity
values were much higher than the reported values using solid-state
synthesis [20]. Therefore, it was concluded that the synthesis of