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Preparation and characterization of electrospunLiFePO4/carbon complex improving rate performanceat high C-rate
Sei-Hyun Lee • Min-Jung Jung • Ji Sun Im •
Kyou-Yoon Sheem • Young-Seak Lee
Received: 15 December 2009 / Accepted: 6 February 2010 / Published online: 18 September 2010
� Springer Science+Business Media B.V. 2010
Abstract LiFePO4/carbon complexes were prepared by electrospinning to
improve rate performance at high C-rate and their electrochemical properties were
investigated to be used as a cathode active material for lithium ion battery. The
LiFePO4/carbon complexes were prepared by the electrospinning method. The
prepared samples were characterized by SEM, EDS, XRD, TGA, electrometer, and
electrochemical analysis. The LiFePO4/carbon complexes prepared have a contin-
uous structure with carbon-coated LiFePO4 and the LiFePO4 in LiFePO4/carbon
complex has improved thermal stability from carbon coating. The conductivity of
LiFePO4/carbon complex heat-treated at 800 �C is measured as 2.23 9 10-2 S
cm-1, which is about 106–107 times more than that of raw LiFePO4. The capacity
ratio of coin cell manufactured from raw LiFePO4 is 40%, whereas the capacity
ratio of coin cell manufactured from LiFePO4/carbon complex heat-treated at
800 �C is 61% (10 C/0.1 C). The improved rate performance of LiFePO4/carbon
complex heat-treated at 800 �C is due to the carbon coating and good electrical
connection.
Keywords High C-rate � Rate performance � LiFePO4/carbon complex �Electrospinning
S.-H. Lee
Department of Electrical and Electronic Engineering, Korea Polytechnic College,
Daejeon 300-702, Republic of Korea
M.-J. Jung � J. S. Im � Y.-S. Lee (&)
Department of Applied Chemistry and Biological Engineering, Chungnam National University,
Daejeon 305-764, Republic of Korea
e-mail: youngslee@cnu.ac.kr
K.-Y. Sheem
Energy Lab., CRD Center, Samsung SDI, Suwon, Gyeonggi-do 443-390, Republic of Korea
123
Res Chem Intermed (2010) 36:591–602
DOI 10.1007/s11164-010-0167-9
Introduction
Recently, lithium-ion batteries have drawn an interest to be used for electric vehicles
(EVs), hybrid electric vehicles (HEVs), dispersed energy storage systems, etc. [1].
Such applications require safety, high-rate performance, and energy density [2].
Especially good rate performance at high C-rate is essential for batteries of electric
vehicles [3]. Lithium-ion batteries consist of a cathode, an anode, and an electrolyte.
Among several components of the lithium-ion batteries, cathode materials are a
major concern for the improvement of rate performance at high C-rate. Cobalt or
manganese-based oxides, such as LiCoO2 and Li2MnO4, are generally used as
cathode materials of commercial lithium-ion batteries. The cobalt-based oxides have
disadvantages of high cost and toxicity [4], whereas manganese-based oxides have
advantages of low cost and low toxicity. However, they suffer from important
capacity fading during cycling, especially at high temperatures [5]. Since the study of
Padhi et al. [6], the olivine structure of lithium iron phosphate (LFP, LiFePO4) has
attracted great interest as a substitute for conventional cathode materials.
It has many advantages of high energy density, high theoretic capacity, low cost,
safety, and environmental friendliness [7]. However, LiFePO4 has poor conductiv-
ity. This disadvantage results from the low lithium-ion diffusion rate and low
electronic conductivity in the LiFePO4 phase. In order to improve the electronic
conductivity of LiFePO4, some methods have been studied, including coating
conductive carbon [8–10] or metals [11–13] on LiFePO4 surface or doping ions in
the crystalline lattice of LiFePO4 [14–16]. Above all, the carbon-coating method is
very efficient in increasing conductivity of LiFePO4 due to high electronic
conductivity of carbon, and the increase of the electronic conductivity of LiFePO4
surface is expected to provide a good rate performance at high C-rate [17–19].
The LiFePO4/carbon complex has been prepared by the sol–gel method,
hydrothermal method, freeze–drying method, spray-pyrolysis method, etc. [20–24].
For improvement of the rate performance at high C-rate, LiFePO4 and carbon must
be connected effectively. Therefore, the ameliorated method is necessary to
manufacture a well-connected LiFePO4/carbon complex. Electrospinning is well
known as a simple and effective method for fabrication of complex [25, 26]. It is
expected that LiFePO4/carbon complex is manufactured well-connected form of
carbon and LiFePO4.
In this study, LiFePO4/carbon complexes for a cathode active material of a lithium-ion
battery were prepared by the electrospinning method in order to improve rate perfor-
mance at high C-rate, and their electrochemical characteristics were also evaluated.
Materials and methods
Preparation of LiFePO4/carbon complexes by electrospinning method
Carbon precursor, PAN (polyacrylonitrile, mW 80,000, Aldrich, USA) was
dissolved in DMF (N,N-dimethyl formamide, Acros, USA) at 80 �C, and its
concentration was 10 wt%.
592 S.-H. Lee et al.
123
An active material of cathode, LiFePO4 (lithium iron phosphate (LFP), Postech,
Korea) was added to PAN solution to produce a mixture solution with a
PAN:LiFePO4 ratio of 1:4, and then the solution was spun out of fibers with the
electrospinning experiment. The electrospinning conditions were as followings;
3 ml/h of solution feeding rate and 15 kV of applied electric power. The details of
the electrospinning method have been presented elsewhere [27]. After electrospin-
ning, as-spun fibers were stabilized at a temperature of 250 �C for 5 h in air with a
heating rate of 1 �C/min. The stabilized fibers were then heated at 600 and 800 �C
for 1 h in nitrogen atmosphere (heating rate: 10 �C/min) to investigate electro-
chemical property changes with different heat-treatment temperatures. The prepared
samples were named as follows; r-LFP (raw LiFePO4), LFP/C600 (LiFePO4/carbon
complex heat-treated at 600 �C), and LFP/C800 (LiFePO4/carbon complex heat-
treated at 800 �C).
Characterizations of LiFePO4/carbon complexes
The morphologies of the samples were observed with field emission scanning
electron microscope (SEM, S-5500, Hitachi, Japan). Elemental mapping was carried
out with energy dispersive spectroscopy (EDS) images. Crystal structures of
samples were investigated by X-ray diffraction (XRD, D/MAX-2200 Ultima/PC,
Rigaku International Corporation, Japan) with a Cu cathode (CuKa radiation
k = 1.54056 A) at room temperature. Carbon content was evaluated by the
thermogravimetric analysis (TGA, 2960SDT, TA Instruments, USA) in air and
nitrogen from room temperature to 600 �C with a heating rate of 5 �C min-1. The
conductivity of LiFePO4/carbon complexes was measured in air at ambient
temperature with an electrometer (6514 system electrometer, Keithley, USA).
Electrochemical analysis of LiFePO4/carbon complex
LiFePO4/carbon complex electrodes were fabricated by mixing the prepared
electrospun LiFePO4/carbon complex (80 wt%) and polyvinylidene fluoride (PVDF,
Aldrich) binder (20 wt%). LiFePO4 electrode was fabricated by mixing r-LFP (70
wt%), PVDF (20 wt%), and carbon black (10 wt%, Super-P, MMM Carbon) as a
conductive material. The disc electrodes, having a diameter of 16 mm, were cut out
of the coating film and cast onto metal foil current collector using a punch. The coin
half cells (CR2016 standard size) were assembled in an argon-filled glove box at
room temperature. The electrolyte was prepared by mixing ethylene carbonate (EC,
Aldrich), ethyl methyl carbonate (EMC, Aldrich), and dimethyl carbonate (DMC,
Aldrich) with a ratio of EC:EMC:DMC = 3:3:4 to assemble the coin cells.
Electrochemical investigations were performed by using a CC/CV mode. The coin
cells were named as follows; C-rLFP (coin cell manufactured from r-LFP), C-
LC600 (coin cell manufactured from LFP/C600), and C-LC800 (coin cell
manufactured from LFP/C800). The ratio of coin cells with each sample are
presented in Table 1.
Preparation and characterization of electrospun LiFePO4/carbon complex 593
123
Results and discussion
Morphology of electrospun LiFePO4/carbon complexes
Figure 1 shows SEM images of r-LFP and LiFePO4/carbon complex samples
prepared with different heat-treatment temperatures. As shown in Fig. 1a, r-LFP is
observed with uniform oval-shaped particles. It confirms many clusters of small
particles that have an average particle size with a major axis of about 200 nm.
Manufactured LFP/C600 and LFP/C800 by electrospinning (Fig. 1b, c) are fibrous
with a rough surface. The average diameter of LiFePO4/carbon complexes is about
5 ± 0.8 lm. The surfaces of the LiFePO4/carbon complexes become rougher with
increasing heat-treatment temperature. The surface of LFP/C800 is more porous
than that of LFP/C600 because the PAN shrank during heat-treatment [28]. Since
PAN shrinks more at higher heat-treatment temperature [29], LFP/C800 is more
Table 1 The ratio of coin cells with each sample
The ratio of coin cell (wt%) Real weight of
LiFePO4 in cell (g)LiFePO4 Conducting material Binder
CB PBCa
C-rLFP 70.0 10.0 – 20.0 0.0060
C-LC600 71.1 – 8.9 20.0 0.0037
C-LC800 72.2 – 7.8 20.0 0.0047
a PAN-based carbon
Fig. 1 SEM images of a r-LFP, b LFP/C600, c LFP/C800, d a cross-section of LFP/C600 and e a cross-section of LFP/C800
594 S.-H. Lee et al.
123
porous than LFP/C600. It is expected to have more chances for electrolyte to be
absorbed into an internal electrode, improving rate performance.
The schematic diagrams of r-LFP, prepared LiFePO4/carbon complex, and
electron transfer pathway of the prepared complex in this study is suggested in
Fig. 2. r-LFP is observed as a form of clusters, without any connection between
each LiFePO4 particle. However, LiFePO4/carbon complexes are observed as a
continuous structure with LiFePO4 particles coated and linked with PAN-based
carbon. As shown in the cross section of LiFePO4/carbon complexes (Fig. 1d, e), it
is also observed that LiFePO4 particles are dispersed uniformly in LiFePO4/carbon
complex and coated and connected with carbon. In addition, LiFePO4/carbon
complexes have a high-density fiber structure due to the effect of the electrospinning
method. As shown in Fig. 2b, these carbon coatings and connections ensure that
LiFePO4 particles get electrons effectively, and could further alleviate this
polarization phenomenon [30]. In the EDS images (Fig. 3), carbon is observed as
white spots on the surface of LiFePO4/carbon complexes. This can be another
evidence for the existence of carbon coating on the LiFePO4/carbon complex
surface.
XRD analysis
Figure 4 shows the XRD patterns of r-LFP and LiFePO4/carbon complexes. The
XRD pattern of r-LFP (Fig. 4a) is identified as an orthorhombic system in Pnmaspace group (JCPDS card No. 81-1137) and it is almost similar to those of LiFePO4/
carbon complexes (Fig. 4b,c), not showing the presence of crystalline carbon. It is
hard to observe the crystalline carbon peak (002) in the XRD patterns of the
complexes due to the overlapping of the crystalline carbon peak with LiFePO4 peak
(111).
According to previous studies [31], XRD pattern of LiFePO4/carbon complex
treated at 800 �C exhibited the impurity peaks caused by reduction of LiFePO4,
such as Fe2P (22, 23�) or Li3PO4 (41, 44�). The presence of impurities causes the
decrease of ionic conductivity, capacity, and cycling rates [32–34]. In this study,
structural changes of LiFePO4 are not observed in the XRD patterns of the LiFePO4/
carbon complexes (Fig. 1b, c). This result suggests that carbon coating contributes
to increasing thermal stability of LiFePO4. It is expected that electrochemical
properties of LiFePO4 in LiFePO4/carbon complexes are not affected by impurities.
Fig. 2 The schematic diagrams of a r-LFP, b prepared LiFePO4/carbon complex, and c electron transferpathway of the LiFePO4/carbon complex
Preparation and characterization of electrospun LiFePO4/carbon complex 595
123
Fig. 3 EDS images of a LFP/C600, b LFP/C800, c C mapping of LFP/C600, and d C mapping of LFP/C800
20 30 40 50 60 70
313
213
701
040
430
331
412,
610
231
131
022
102,
221
321,
212
112
41012
131
1
301
020
111
01121
010
1
LiFePO4(c)
(b)
Inte
nsity
(a.
u.)
2θ (Degree)
(a)
200
Fig. 4 X-ray diffraction patterns of (a) r-LFP, (b) LFP/C600, and (c) LFP/C800
596 S.-H. Lee et al.
123
TGA and electrical conductivity analysis
Figure 5 shows TGA curves of r-LFP, LiFePO4/carbon complexes, and PAN-based
electrospun carbon fiber under air flow, and stabilized electrospun PAN fiber under
N2 flow. As shown in the TGA curve of r-LFP (Fig. 5a), it is observed that oxidation
of r-LFP started at around 250 �C. As it has been previously reported by Belharouak
[35], r-LFP is oxidized with two steps of weight increment and formatted two
phases, Li3Fe2(PO4)3 and Fe2O3. In addition, the TGA curve shows a weight gain of
2.2% for the r-LFP after oxidation. As shown in Fig. 4b and c, LiFePO4 in LiFePO4/
carbon complexes is also oxidized at or above 400 �C. The LiFePO4 oxidation peak
of LFP/C600 and LFP/C800 is shifted to a higher temperature than that of r-LFP,
which means that the carbon coating will be able to delay oxidation of LiFePO4 and
0
20
40
60
80
100(a)
(1)
(2)
(3)
(4)
(5)
Res
idua
l mas
s (%
)
Temperature (Co)
(b)
100 200 300 400 500 600 700 800
100 200 300 400 500 600
85
90
95
100
105(b)
(1)
(2)
(3)
Temperature (Co)
Res
idua
l mas
s (%
)
Fig. 5 TGA results of prepared samples (a) and expansion of selected area (b); (1) r-LFP observed in air,(2) LFP/C600 observed in air, (3) LFP/C800 observed in air, (4) PAN-based electrospun carbon fiberobserved in air, and (5) stabilized electrospun PAN fiber in observed nitrogen
Preparation and characterization of electrospun LiFePO4/carbon complex 597
123
improve thermal stability. The improved thermal stability of active material is a
very important factor to determine the safety of Li-ion batteries. This result is
similar to XRD patterns of the LiFePO4/carbon complexes, where no structural
changes of LiFePO4 were observed.
Below 400 �C, the weight loss of LFP/C800 is faster than that of LFP/C600
because LFP/C800 is more porous than LFP/C600, as explained above. Rapid
weight loss of LFP/C600 and LFP/C800 is observed at around 400 and 450 �C,
respectively. The starting temperature of rapid weight loss of LiFePO4/carbon
complexes increases with increasing heat-treatment temperature. Weight loss of
LiFePO4/carbon complexes occurs because of decomposition of existing carbon in
LiFePO4/carbon complexes. It is also observed in TGA curve of PAN-based
electrospun carbon fiber (Fig. 5d). A weight loss of LiFePO4/carbon complexes is
associated with oxidation of carbon and some organic compounds, which means that
carbon contents of the LiFePO4/C complex can be calculated from weight loss of
the complex and the amount of carbon coated on the LFP/C600 and LFP/C800 is
calculated to be around 11.1 and 9.8 wt%, respectively. The carbon amount of
LiFePO4/carbon complexes is calculated theoretically using TGA curve of
stabilized electrospun PAN fiber. According to the TGA curve of stabilized
electrospun PAN fiber (Fig 5e), the carbon yield of PAN in LFP/C600 and LFP/
C800 is estimated to be around 43.5 and 34.7 wt%, respectively. The theoretical
amount of carbon coated on the LFP/C600 and LFP/C800 is calculated to be around
9.8 and 7.9 wt%, respectively.
Even though LFP/C600 contains a larger percentage of carbon than LFP/C800,
crystallinity of carbon in LFP/C800 would be higher than that of carbon in LFP/
C600 because crystallinity and conductivity of carbon increases with increasing
heat-treatment temperature [36]. Therefore, it is expected that conductivity of LFP/
C800 has higher than that of LFP/C600. Actually, the conductivity of LFP/C600 and
LFP/C800 is measured at 5.62 9 10-4 and 2.23 9 10-2 S cm-1, respectively. LFP/
C800 has improved conductivity about 40 times more than LFP/C600. Therefore, it
suggests that increasing the heat-treatment temperature in this experiment led to an
increase in the conductivity. Compared to low electrical conductivity of LiFePO4
about 10-9–10-10 S cm-1 [37], conductivity of LFP/C800 is considerably increased
about 107–108 times. As each LiFePO4 particle is coated with carbon, a conductive
network is formed between the LiFePO4 particles. Therefore, conductivity of the
material can dramatically increase. It is expected to improve rate performance at a
high C-rate.
Electrochemical performance
Figures 6, 7, 8, and 9 and Table 2 show the discharge profiles of the coin cells
measured from 4.5 and 2.0 V vs. Li at different discharge rates of 0.1, 1, 5, and
10 C. The cells are cycled between 2.4 and 4.1 V. In case of C-LC600, it shows a
poor electrochemical property with the discharge capacities of 0.08 mAh g-1 at 0.1
C, which is much lower than the discharge capacities of C-rLFP, 153.18 mAh g-1 at
0.1 C. This is the conductivity difference between conductive materials used for cell
manufacturing. C-rLFP was manufactured with carbon black as a conductive
598 S.-H. Lee et al.
123
material, which usually has a conductivity of 10-1–102 S cm-1 [38], whereas
C-LC600 was manufactured with heat-treated PAN fibers at 600 �C, which has a
measured conductivity of 3.34 9 10-3 S cm-1. Therefore, due to the low
conductivity of heat-treated PAN fiber at 600 �C, the conductivity of C-LC600 is
2.4
2.8
3.2
3.6
4.0
Vol
tage
(V
vs.
Li/L
i+)
Capacity (mAh g-1)
C-rLFP C-LC600 C-LC800
0 20 40 60 80 100 120 140 160
Fig. 6 The discharge curvesat 0.1 C
0 20 40 60 80 100 120 140 160
2.4
2.8
3.2
3.6
4.0
Vol
tage
(V
vs.
Li/L
i+)
Capacity (mAh g-1)
C-rLFP C-LC600 C-LC800
Fig. 7 The discharge curvesat 1 C
2.4
2.8
3.2
3.6
4.0
Vol
tage
(V
vs.
Li/L
i+)
Capacity (mAh g-1)
C-rLFP C-LC600 C-LC800
0 20 40 60 80 100 120
Fig. 8 The discharge curvesat 5 C
Preparation and characterization of electrospun LiFePO4/carbon complex 599
123
expected to be significantly lower than that of C-rLFP, causing low discharge
capacity of C-LC600. The low conductivity of heat-treated PAN fiber at 600 �C is
due to the presence of a large amount of amorphous carbon in the fiber heat-treated
at relatively low temperature and the amorphous carbon obstructs movement of
lithium ion inside of the active materials.
The discharge capacities of C-rLFP are 153.18 mAh g-1 (0.1 C) and 61.06 mAh
g-1 (10 C), and the capacity ratio of 10 C/0.1 C is 40%. In case of C-LC800,
discharge capacities are 139.95 mAh g-1 (0.1 C) and 85.60 mAh g-1 (10 C), and
the capacity ratio of 10 C/0.1 C is 61%. The high rate performance reveals that the
LiFePO4/carbon complex would be suited for cathode material of a high-power
lithium-ion battery. The high rate performance in the high discharge rate condition
is attributed to dramatic conductivity improvement through carbon coating and
connecting.
At a high discharge rate, the lithium ion diffusion is restricted inside of active
materials [5]. For this reason, discharge capacity and rate performance of cell is
greatly influenced by surface conductivity of active materials and connection of
active materials at high discharge rate. There are continuities between restricting of
lithium-ion diffusion and change of the voltage plateau region [5]. As Figs. 6, 7, 8,
and 9 show, the voltage plateau of C-rLFP is changed dramatically with increasing
discharge rate. However, the C-LC800 has an excellent flat voltage plateau around
3.2–3.4 V in most discharge rates. This effect can be attributed to the improved
lithium-ion diffusion at active material surface, due to the carbon coating and
connecting.
0 20 40 60 80 100
2.4
2.8
3.2
3.6
4.0 C-rLFP C-LC600 C-LC800
Vol
tage
(V
vs.
Li/L
i+ )Capacity (mAh g-1)
Fig. 9 The discharge curvesat 10 C
Table 2 Discharge capacity
with different discharge rateDischarge capacity (mAh g-1)
0.1 C 1 C 5 C 10 C
C-rLFP 153.18 146.23 105.12 61.06
C-LC600 0.08 0.04 0.03 0.03
C-LC800 139.95 112.23 93.52 85.60
600 S.-H. Lee et al.
123
Conclusions
Electrospun LiFePO4/carbon complexes are manufactured as high dense fibers with
continuous structure of LiFePO4 particles which are linked with PAN-based carbon
coating. In this study, structural changes of LiFePO4 are not observed in the XRD
patterns of LiFePO4/carbon complexes. As shown in the TGA curves, the LiFePO4
oxidation peak of LiFePO4/carbon complexes are shifted to higher temperatures
than that of r-LFP. These results suggest that carbon coating contributes to
increasing thermal stability of LiFePO4. The amount of carbon coated on the LFP/
C600 and LFP/C800 is calculated to be around 11.1–9.8 wt%, respectively, from
TGA curves. The conductivity of LFP/C600 and LFP/C800 is measured to be
5.62 9 10-4 and 2.23 9 10-2 S cm-1, respectively. Even though LFP/C600 con-
tains a larger percentage of carbon than LFP/C800, crystallinity of carbon in LFP/
C800 would be higher than that of carbon in LFP/C600 because of crystallinity and
conductivity of carbon increase with increasing heat-treatment temperature.
Therefore, conductivity of LFP/C800 is higher than that of LFP/C600. The discharge
capacities of C-rLFP are 153.18 mAh g-1 (0.1 C) and 61.06 mAh g-1 (10 C), and
the capacity ratio of 10 C/0.1 C is 40%. In case of C-LC800, discharge capacities
are 139.95 mAh g-1 (0.1 C) and 85.60 mAh g-1 (10 C), and the capacity ratio of 10
C/0.1 C is 61%. The improved rate performance of C-LC800 can be explained with
increased conductivity of active materials and network between active materials
caused by carbon coating and connecting.
Acknowledgments This work was partially supported by Samsung SDI, Korea.
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