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Electrochemical Properties

Preparation and electrochemical properties of LiFePO4/Ccomposite with
network structure for lithium ion batteries


CHEN Han(陈 晗), YU Wen-zhi(于文志), HAN Shao-chang(韩绍昌), XUZhong-yu(徐仲榆)


College of Materials Science and Engineering, Hunan University,Changsha 410082, China


Received 14 March 2007; accepted 22 June 2007



Abstract: The bare LiFePO4 and LiFePO4/C composites with networkstructure were prepared by solid-state reaction. Thecrystalline
structures, morphologies and specific surface areas of thematerials were investigated by X-ray diffractometry(XRD),scanning
electron microscopy(SEM) and multi-point brunauer emmett andteller(BET) method. The results show that the LiFePO4/C
composite with the best network structure is obtained by adding 10%phenolic resin carbon. Its electronic conductivity increases to
2.86×10.2 S/cm. It possesses the highest specific surface area of115.65 m2/g, which exhibits the highest discharge specificcapacity
of 164.33 mA·h/g at C/10 rate and 149.12 mA·h/g at 1 C rate. Thedischarge capacity is completely recovered when C/10 rate is
applied again.


Key words: LiFePO4/C composite; lithium ion batteries;electrochemical properties; network structure




1 Introduction


LiFePO4 is an interesting alternative cathode
material for lithium ion batteries, due to its cheap starting
materials, good environmental compatibility, excellent
cycling stability and high temperature performance[1.2].
The main disadvantages are very low electronic
conductivity and diffusion coefficient of lithium ion,
which lead to its poor rate capability and hinder its
commercialization as cathode material for lithium ion
batteries[3.4]. Some efforts to increase its conductivity
have focused on reducing particle size[5.6], coating
carbon[7.8] and doping metal cation[9.10].

Carbon coating can not change the intrinsical
conductivity of LiFePO4, but it is very effective to
enhance the capacity and rate capability[11.12]. Carbon
black and sugar are used as main carbon resources. The
electronic conductivity of LiFePO4/C composite
prepared by BEWLAY et al[13] added with sucrose
reached 0.1 S/cm. PROSINI’s research[14] showed that
the practical capacity and rate capability of LiFePO4 can
be improved by adding 10% carbon black to the starting
materials. Carbon can control particles growth and
enhance its conductivity. Therefore higher capacity and
better rate capability can be obtained. At present, it is
necessary to optimize carbon source and prepare
LiFePO4/C composite with excellent electrochemical
properties, especially rate capability.

Phenolic resin was used as carbon resource by
YANG et al[15], the discharge capacity of LiFePO4/C
composite is only 65 mA·h/g at C/3 rate and its structure
has not been reported in their research. In this study,
LiFePO4/C composite with porous network structure was
prepared using phenolic resin as carbon resource.


2 Experimental


2.1 Materials preparation

LiFePO4/C composites were prepared by solid-state
reaction. Li2CO3 (AR, Taishan Chemical Plant),
FeC2O4·2H2O (AR, Fluka), and NH4H2PO4 (AR,
Mingfeng Reagent Corporation) were used as the starting
materials. Phenolic resin was added with different mass
fractions. The starting materials were weighed in stoichio-
metric ratio and homogenously mixed by ball grinding in


Foundation item: Project(50672024) supported by the NationalNatural Science Foundation of China; Project(06FJ2006) supported bythe Applied Basic
Research of Hunan Province, China

Corresponding author: HAN Shao-chang; Tel: +86-731-8822967;E-mail:


anhydrous ethanol. The mixture was solidified at 150 ℃
for 5 h. To decompose the oxalate and the phosphate, the
mixture was placed in a tubular furnace and treated at
350 for 5 h in argon flow. The resultant powders were ℃
cooled down to room temperature and reground, then
returned to the tubular furnace and calcined at 650 for ℃
18 h in argon flow. The last powders were cooled down
to room temperature. Thus the LiFePO4/C composites
containing various carbon amount were obtained.


2.2 Materials characterization

The crystalline structure was confirmed by a
powder X-ray diffractometer (D5000) with Cu Kα radia-
tion. The XRD data were obtained over an angular 2θ
range from 15° to 45° with the step size of 0.02° and
constant counting time of 0.2 s per step. The crystallite
size of samples was calculated by the Scherrer equation
D=Kλ/(β×cosθ) from the full width at half maximum (β)
of the (131) diffraction peaks, in which K value is 0.89.
The morphology of the composites was observed by
scanning electron microscopy (JSM.6700F). The
specific surface area of the powders was measured by the
multi-point Brunauer Emmett and Teller (NOVA.1000)
method. Conductivity measurement using four-probe
testing instrument (SX1934) was made on disc-shaped
pellet by four-point direct current method at room
temperature, the measured value was revised for the
thickness of pellet.


2.3 Electrochemical tests

Active material, conductive additive and
polytetrafluorethylene(PTFE) were mixed homo-
geneously in mass ratio of 75.20.5. The mixture was
rolled into a 0.1 mm thin sheet with uniform thickness,
from which pellets with 12 mm in diameter were cut.
The pellet was used as the cathode and the electrolyte
was 1 mol/L LiClO4 in ethylene carbonate and dimethyl
carbonate (1.1 in volume). Lithium foil was used for the
counter and reference electrodes. The separator was
Celgard 2400 microporous membrane. The cell was
assembled in an argon glove box. The galvanostalical
tests were run by Arbin instrument (BT-2000) between
2.5 V and 4.1 V versus Li/Li+ at various rates.


3 Results and discussions


3.1 XRD analysis

The XRD patterns of LiFePO4/C composites
containing various carbon amount and bare LiFePO4 are
shown in Fig.1. All peaks of the samples can be indexed
as olivine LiFePO4 phase without any observable
secondary phase, but the XRD patterns have somewhat
difference with the carbon amount increasing. The
position of diffraction peaks deviates to lower diffraction
angle, which shows that the distance between
neighboring crystal planes becomes larger according to
the Bragg equation 2dsinθ=λ. The intensity of diffraction
peaks turns weaker gradually. The practical temperatures
of composites are slightly lower than that of bare
LiFePO4, because of adsorption of heat during
decomposition of phenolic resin[16]. The lattice
parameters of the samples are calculated from the
Rietveld refinement on the basis of the Pmnb space
group and shown in Table 1. The crystallite size of
samples is calculated based on the strongest practical
peak (131). With the carbon amount increasing, the
lattice parameters of the composites become slightly
larger. When cell volume increases, the size (D131)
decreases gradually. When cell volume is too small, the



atoms around lithium ions will hider their diffusion. On
the contrary, when the cell volume is too large, the cell
will be destroyed due to extraction and insertion of
lithium ions. The lithium ions have to diffuse over larger
distances between the boundary and center of crystallite
grains due to larger crystallite size[17]. Therefore sample
c has a good compromise between cell volume and
crystallite size.


3.2 SEM micrographs analysis

The SEM micrographs of the composites are shown
in Fig.2. The porous network structure can be clearly
observed in Figs.2(b) and (d). When phenolic resin is
solidified, lots of polymeric webs form. Since the
starting materials with phenolic resin are uniformly
dispersed in anhydrous ethanol during solidification, the
starting materials are trapped into the polymeric network.
During the heat treatment, phenolic resin carbonizes, the
porous network structure forms, and LiFePO4 covers on
the carbon framework. This kind of structure
characterized in detail in other paper is entirely different
from carbon coating. The coexisted network structural
composite and LiFePO4 grains can be clearly observed in
Figs.2(a) and (b), which is due to inadequate phenolic
resin portion to the starting materials of LiFePO4. As
shown in Fig.2(c), the perfect network structure without
any LiFePO4 grains because of the enough carbon
framework adhering LiFePO4 grains is observed. As
shown in Figs.2(e) and (f), it is obvious that the porous
network structure disappears and the smooth grains pile
together because the nei***oring network incorporates
one another during the teat treatment, when the 20%
carbon is applied.


3.3 BET tests

Fig.3 shows the specific surface areas of the
samples with various carbon amount. As shown in Fig.3,
the specific areas of materials increase rapidly with
increasing the carbon amount. When 10% carbon is
applied, the sample exhibits the highest specific surface
area of 115.65 m2/g, nearly twenty times greater than that
of bare LiFePO4. The high specific surface area is mainly
attributed to porous network structure. When 20%
carbon is applied, the specific surface area decreases
abruptly because of the disappearance of porous network
structure and the formation of smooth grains.



Fig.3 Specific surface areas of samples with various carbon


3.4 Conductivity measurement

The electronic conductivity of LiFePO4/C
composites compared with bare LiFePO4 is shown in
Table 2. Phenolic resin carbon plays a significantly
important role to enhance electronic conductivity. The
electronic conductivities of LiFePO4/C composites are
much higher than that of bare LiFePO4. They increase
with increasing carbon amount. The lowest limit of
conductivity testing instrument (SX1934) is 10.6 S/cm,


and the electronic conductivity of bare LiFePO4 is cited
to compare with that of LiFePO4/C composite.


3.5 Electrochemical properties analysis

Fig.4 shows the charge and discharge curves of
LiFePO4/C composites and bare LiFePO4 at C/10 rate.
The calculation of specific capacity of LiFePO4/C
composites is based on LiFePO4. The potential difference
between the charge and discharge plateau of LiFePO4/C
composites is only 0.05 V, which shows the weaker
potential hystereresis compared with that of bare
LiFePO4 because of the formation of porous network
structure. The initial discharge specific capacity of
LiFePO4/C composites is much larger than that of the
bare LiFePO4. Especially, the LiFePO4/C composite
containing 10% carbon has the largest initial discharge
specific capacity (164.33 mA·h/g). The perfect porous
network structure increases the contact area between
carbon and LiFePO4, providing multi-dimension
channels for diffusion of lithium ions.



Fig.4 Charge and discharge curves of samples with various
carbon amount: (a) 0%; (b) 5%; (c) 10%; (d) 20%


The extended line of the potential plateau intersects
the tangents of discharge curves at points A and B, shown
in Fig.5. The capacity between points A and B is defined
as plateau capacity. The ratio of the plateau capacity to
the full discharge capacity is defined as plateau ratio. The
materials with higher plateau capacity and plateau ratio
can work longer. The comparison on the plateau
performance of bare LiFePO4 and LiFePO4/C composites
is listed in Table 3. When 10% carbon is applied, sample
c possesses the highest plateau capacity and considerably
high plateau ratio. But the plateau capacity of sample d
decreases due to the hindrance of excessive carbon to the
diffusion of lithium ions and the disappearance of porous
network structure.




Fig.6 shows the curves of rate and cycling
capability of all samples. To investigate the rate
capability, various current densities corresponding to
C/10, C/2 and 1 C are applied. All of them show
excellent cycling stability. The discharge specific
capacity of bare LiFePO4 at 1 C rate is only half as much
as that at C/10 rate. When the rate increases, the capacity
loss of LiFePO4/C composites containing 5% and 20%
carbon is much larger than that of the composite
containing 10% carbon. The composite containing 5%
carbon has fewer network structure and more bare
LiFePO4 grains. Likewise, when 20% carbon is applied,
the network structure disappears, the smooth grains
forms because of incorporation of neighboring network,
and the excessive carbon hiders the diffusion of lithium
ions, so the discharge specific capacity of LiFePO4/C
composite containing 5% and 20% carbon is 100 mA·h/g
at 1 C rate. The discharge specific capacity of LiFePO4/C
composite containing 10% carbon is 149.12 mA·h/g at
1 C rate, and its capacity loss is only 9.5%. The good rate
performance is attributed to perfect porous network
structure that provides multi-dimension channels for
diffusion of lithium ion and reduces the resistance for
diffusion of lithium ion. Moreover, the composite with
porous network structure can suck up electrolyte to
shorten enormously the diffusive distance of lithium ion.




Fig.6 Curves of rate and cyclic capability of samples with
various carbon amount: (a): 0%; (b) 5%; (c) 10%; (d) 20%


When C/10 is applied again, the discharge specific
capacity of all samples recovers completely, which
shows that their structure has not been broken, and the
capacity loss is due to stronger polarization at high rate.
Therefore the porous network structure plays a
significantly important role in improving the rate
capability of LiFePO4.


4 Conclusions


1) LiFePO4/C composites containing various
amount of carbon have well-crystallized olivine

2) The LiFePO4/C composite with excellent porous
network structure and the highest specific surface area is
obtained by adding 10% phenolic resin carbon, which
exhibits excellent cycling and rate capability.

3) The LiFePO4/C composite with porous network
structure is a promising cathode material for lithium ion




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(Edited by HE Xue-feng)

Preparation and Electrochemical Properties