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Influence of sintering atmosphere on the physical and electrochemical of
LiFePO4/C
Yan Lin1, a, Jianbo Wu2, b, Aijiao Xu3, c and Jinyong Xu4, d
1 College of Physics and Electronic Engineering, Taizhou University, Taizhou, Zhejiang, China
2 College of Physics and Electronic Engineering, Taizhou University, Taizhou, Zhejiang, China
3 College of Physics and Electronic Engineering, Taizhou University, Taizhou, Zhejiang, China
4 College of Physics and Electronic Engineering, Taizhou University, Taizhou, Zhejiang, China
aemail: linyan@tzc.edu.cn,
bemail: wujb@tzc.edu.cn,
cemail: xaj3270@126.com,
demail:
xjy@tzc.edu.cn
Keywords: LiFePO4/C; Fe3+
; reductive atmosphere; Fe2P
Abstract. LiFePO4/C was synthesized from a gel precursor with ferric iron and an organic chelating
agent as carbon source. Reductive atmosphere of N2 + H2 with H2 content of 0-20 vol % was used in
the sintering process of LiFePO4/C composites. The microstructures of the obtained LiFePO4/C
particles were characterized by X-ray diffraction, field emission scanning electron microscopy,
element analysis and particle size analysis. The results showed that suitable reductive sintering
atmosphere was needed to get pure LiFePO4/C phase, but too strong reducibility led to the formation
of iron phosphides, most of which was Fe2P. The amount of Fe2P increased with the increase of H2
content in the sintering atmosphere. The rate capability of LiFePO4/C was improved when the
sintering atmosphere became more reductive, while the discharge capacity of 0.1C decreased, which
was probably due to the appearance of Fe2P phase.
Introduction
Lithium iron phosphate LiFePO4 is a promising alternative cathode material for lithium ion battery
due to its environmental benignity, low price, high cycling stability and high temperature capability
[1]. The theoretical capacity of LiFePO4 is 170 mAh/g and its charge/discharge voltage is about 3.4 V
versus Li/Li+. However, the low ionic and electronic conductivity (~ 10
-9 S·cm
-1) of LiFePO4 has
greatly limited its practical application. Improvements have been achieved on the rate performance by
means of reducing the particle size [2, 3], coating an electronic conductive layer on the particle [4, 5]
or doping metals [6, 7]. Especially, the important finding by Nazar and his co-workers [8] that the
metal-rich phosphides such as Fe2P could improve the electronic conductivity of LiFePO4 in a factor
of about 108, which made a great step forward in the application of LiFePO4 as cathode material.
Reductive conditions are always needed to get pure LiFePO4 in the synthesis process, especially
when ferric iron source is used [9,10]. Barker et al. [9] used Fe2O3 as iron source and pointed out that
the addition of carbon which was a high surface area carbon black to the precursor could reduce Fe3+
to Fe2+
, and this method for the synthesis of LiFePO4 is known as CTR (carbothermal reduction).
When the ferrous iron is used as iron source, reductive condition such as carbon addition before
sintering or H2 addition in the sintering atmosphere is still needed, for the Fe2+
is easily oxidized
during the synthesis process [11,12]. Arnold et al. [13] pointed out that different reductive sintering
atmospheres (containing 1% H2 and 7% H2) had an effect on the color of LiFePO4 powder and they
thought it might be ascribed to the generation of Fe2P. However, the influence of sintering
atmospheres on the microstructure and electrochemical properties of LiFePO4 had not yet been
systematically studied.
Advanced Materials Research Vols. 287-290 (2011) pp 1308-1313Online available since 2011/Jul/04 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.287-290.1308
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 132.177.228.65, University of New Hampshire, Durham, United States of America-12/03/13,10:24:24)
In this work, ferric nitrate (Fe3+
) which was cheaper than ferrous oxalate is chosen as iron source.
We systematically study the effect of sintering atmospheres of N2 containing 0 to 20 vol % H2 on the
physical and electrochemical performance of LiFePO4/C synthesized by sol-gel method. And the
generation conditions of Fe2P were investigated.
Experimental
A sol-gel method was used to prepare LiFePO4/C sample. The starting materials were citric acid,
which was used as carbon source and chelating agent, Fe(NO3)3·9H2O (ferric nitrate), LiNO3·2H2O
(lithium nitrate) and NH4H2PO4 (dihydrogen ammonium phosphate). Mixture of equal mole of iron
nitrate, dihydrogen ammonium phosphate and lithium nitrate was dissolved in an aqueous solution of
citric acid. The resulting solution was heated at 80 ºC to obtain a gel, subsequently dried at 120ºC for
48 h, then ball-milled for 30 min, to form black powder. The powder was further sintered at 700 ºC for
10 h in a flow of the mixing gas of hydrogen and nitrogen containing 0 vol %, 5 vol %, 10 vol %, 15
vol % and 20 vol % of H2, respectively. The gas flow rate was set at 60 ml·min-1
.
The phase identification of the synthesized LiFePO4/C products was carried out by X-ray
diffraction (XRD, Thermo X’ TRA). XRD data collected by step-scanning method over an angular 2θ
range from 10º to 80º with a step interval of 0.04º and a count time of 3s per step were analyzed by the
Rietveld method using a RIETAN97 software to obtain the phase abundance. A field emission
scanning electron microscope (SEM, Sirion-100, Philips-FEI) was used to observe the particle
morphologies. The carbon content in the LiFePO4/C composites was detected by an element analysis
apparatus of Flash EA1112 (ThermoFinnigan, Italy). The particle size distribution was determined by
a Zetasizer3000HSA LS Particle Size Analyzer (U. K.).
The electrochemical performance of the cathodes was evaluated using a coin-type cell (size 2025)
with a lithium metal anode. The cathode consisted of the active material, synthetic graphite (Super P,
Timcal Ltd.), acetylene black (Timcal Ltd.) and polyvinylidene fluoride (PVDF) with weight ratio of
80:6:6:8. The electrolyte was a mixed solvent of ethylene carbonate and dimethyl carbonate (1:1)
containing 1 M LiPF6. The typical loading of active material on the cathode was between 3 and 5 mg
in each cell. Cells were cycled galvanostatically at current values of 0.1 C to 15 C (1C = 170 mA/g),
with cutoff voltages set to be 2.5 and 4.2 V vs. Li/Li+ at room temperature. The charge/discharge
capacities of the LiFePO4/C samples in this study were calculated on the basis of materials including
residual carbon from the organic compound and impurities in-situ formed in the samples.
Results and discussions
Figure 1 shows the XRD patterns of the samples sintered in the N2 + H2 mixing gases containing
different H2 content. And Table 1 lists the results of Rietveld analysis and carbon content. It can be
seen from Fig. 1 that LiFePO4 was the main phase of the products and the crystallization of LiFePO4
particles was well except that of the sample sintered in the atmosphere of pure N2. It had 35 wt % of
Fe2P and only 50 wt % of the electrochemical active material LiFePO4 (Table 1). Therefore, it can be
deduced that the reductive atmosphere caused by the decomposition of organic compound during the
sintering process reduced Fe3+
to Fe2+
, which is coincided with the findings of Barker [9]. The sample
with high purity of LiFePO4 was obtained when the sintering atmosphere contained 5% and 10% H2.
The diffraction peaks of Fe2P (2θ = 40.28º; 44.20º; 47.29º) appeared when the content of H2 of the
sintering atmosphere was increased to 15%.The amount of Fe2P increased (14 wt %) and Li3PO4
formed when the content of H2 further increased to 20%, as can be seen from Fig. 1 and Table 1.
As to the formation of Fe2P, there should be some differences between the samples sintered in the
atmospheres of pure N2 and N2 + 15-20 % H2. When the dried gel was sintered in pure N2 and the
temperature reached to the decomposition of the organic carbon net, the sudden strong in-situ
reductive gas may likely result in the reduction of Fe3+
and P5+
neighboring, and thus favored the
formation of Fe2P. For the samples sintered in the atmosphere containing 5-10% H2, hydrogen
reduced Fe3+
to Fe2+
prior to the gas resulted from the decomposition of the organic carbon net. And at
Advanced Materials Research Vols. 287-290 1309
the temperature of the decomposition of the organic carbon net amorphous LiFePO4 may already form
and it was not reductive enough to reduce LiFePO4 to Fe2P. Only when the sintering atmosphere was
more reductive, such as 15% and 20% H2, could the powder be further reduced to form Fe2P.
20 30 40 50 60
♦ ♦ • •
♦ ♦ ♦
♦ ♦ ♦
Inte
nsity /
a.u
.
♦
+ Li3PO
4
+ +
♦ Fe2P
♦
♦
2θ / (°)
0%
20%
15%
10%
5%
+
• FeP
Fig. 1 XRD patterns of the LiFePO4/C sintered in various gases.
Table 1 Results of Rietveld analysis and carbon content of samples sintered in different atmospheres
From the elemental analysis of the samples listed in Table 1, we can see that the carbon content
increased with increasing the hydrogen content when the samples sintered in reductive atmosphere
containing H2. However, the sample sintered in pure N2 had almost the same carbon content as the
sample sintered in atmosphere containing 15% H2. Excluding the impurities in the LiFePO4/C
products, net amount of LiFePO4 was roughly calculated and shown in Table. 1. It can be seen that
sample sintered in pure N2 contained the lowest amount of electrochemically active LiFePO4 material
and the amount of LiFePO4 decreased with the increase H2 content in the sintering atmosphere.
Samples
Phase abundance [wt.%]
Fitting parameters
C
content
[wt. %]
Net
LiFePO4
[wt. %]
Theoretical
capacity
[mAh/g] LiFePO4 Fe2P FeP Li3PO4
N2 50.4 34.6 / 15.0 Rp=6.8 Rwp=8.5
χ2=2.8
18.0 41.3 70
5% H2 100 / / / Rp=6.4 Rwp=8.2
χ2=2.6
10.1 89.9 152
10% H2 100 / / / Rp=7.7 Rwp=9.9
χ2=3.6
15.7 84.3 143
15% H2 96.6 3.4 / / Rp=9.8 Rwp=12.7
χ2=7.0
17.7 79.5 135
20% H2 81.5 14.0 2.4 2.1 Rp=11.3 Rwp=14.3
χ2=8.5
21.2 64.2 110
1310 Applications of Engineering Materials
Fig. 2 SEM images of the samples sintered in different atmospheres: (a) N2; (b) 20 vol % H2
Morphologies of the samples sintered in atmosphere containing H2 were similar. Figure 2 shows
SEM micrographs of the samples sintered in pure N2 and 20% H2, respectively. We can see obvious
LiFePO4 particles of irregular shape in Fig.2 (b) and there are no such particles in Fig.2(a), indicating
that the samples sintered in pure N2 had severe agglomoration.
The corresponding particle size distribution of the samples sintered in pure N2 and 20% H2 is
displayed in Fig. 3. It demonstrates that the distributions exhibited large variations of the samples.
Plateaus in the plots indicate the absence of corresponding particle size. The relative span of the
particle size distribution defined as mean/D(1,0) decreased from 19.4 to 10.7 and the median size
decreased from 1.758 to 0.446 µm when the sample was sintered in 20% H2. This shows that LiFePO4
sintered in 20% H2 had narrower size distribution and smaller particle size than the sample sintered in
pure N2.
0 1 2 3 4 5 60
20
40
60
80
100
Particle size / µm
Cum
ula
tive s
ize d
istr
ibu
tio
n /
%
20% H2
N2
Fig. 3 Particle size distribution of samples sintered from the dried gel precursor in N2 and mixed gas
with 20% H2, respectively.
Figure 4 shows discharge capacity of the LiFePO4/C products at different discharge rates.
Apparently, the sample sintered in pure N2 demonstrates the lowest discharge capacity at various
rates. When the discharge rate is 0.1C - 5C, the sample sintered in 5% H2 displays the highest
capacity. However, with increasing the discharge current density to 10C, samples sintered in 15% and
20% H2 show higher discharge capacity and the rest have little discharge capacity. Furthermore, the
sample sintered in 20% H2 still has a capacity of 33 mAh/g at 15C, exhibiting the best rate
performance.
LiFePO4
particles
Advanced Materials Research Vols. 287-290 1311
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Dis
charg
e c
apa
city (
mA
h/g
)
Discharge rate (C)
N2
5%H2
10%H2
15%H2
20%H2
Fig. 4 Discharge capacity of Li/1M LiPF6, EC-DMC/LiFePO4 cells at 0.1C, 1C, 5C, 10C and 15C.
At low discharge rates, 0.1C and 1C, the discharge capacity is proportional to the amount of active
material, for carbon, Fe2P and Li3PO4 are electrochemically inert and do not have lithium ions for
intercalation/deintercalation. However, when the discharge rate is 10C, the discharge capacity is
proportional to the content of H2 (Fig. 4) and the sample sintered in atmosphere containing 20%H2
displays the highest capacity, though it contains the least amount of LiFePO4 among the samples
sintered in atmosphere containing H2.
From the fact that carbon contents of samples sintered in gases with 5% and 10% H2 are about 10
wt % and 15 wt %, respectively, and the increase of carbon content makes little improvement in the
rate performance between the samples, it may be deduced that the improvement of the rate
performance of samples sintered in 15% and 20% H2 is probably due to the increase content of Fe2P.
Nevertheless, the sample sintered in pure N2 which also contained Fe2P showed poor electrochemical
performance. Nazar et al. [8] has reported that a nano-network of metal-rich phosphides, which was
mainly the Fe2P, was responsible for the enhanced conductivity. And the high rate discharge
capability of the sample sintered in 20% H2 was well improved due to the existence of Fe2P. One
reason why the sample sintered in pure N2 with large amount of Fe2P (34.6 wt %) showed poor rate
performance may be that it contained small amount of active LiFePO4 (41.3 wt %) and poor
crystallized LiFePO4 particles. The other may be the uneven particle size distribution of the sample,
which can be seen from the results of particle size analysis.
Consequently, it can be presumed that besides uniform particle size distribution and well-grown
LiFePO4 crystals, the formation of Fe2P in the mixed gas also favors the improvement of rate
capability of LiFePO4, though it will cause certain loss of discharge capacity at 0.1C. Further study
should be done on the synthesis of LiFePO4/C with suitable amount of Fe2P and the composite should
possess good rate performance without the loss of capacity.
Conclusions
Phase pure LiFePO4/C synthesized from a gel precursor with Fe(NO3)3·9H2O as iron source was
obtained under reductive sintering atmosphere, 5 vol % H2 or 10 vol % H2 in N2, though the carbon
derived from citric acid can reduce Fe3+
to Fe2+
when sintered in pure N2. Reductive gas containing
more hydrogen (15-20%) lead to the further reduction of Fe2+
and P5+
to generate Fe2P, and the
sample synthesized in 20 vol % H2 + N2 showed the best rate performance.
Fe2P phase could apparently improve the rate capability of LiFePO4/C, while LiFePO4/C has
well-grown LiFePO4 crystals and uniform particle size distribution. However, Fe2P phase is a
two-edged sword, for it is electrochemically inert and too much of it will decrease the capacity of
LiFePO4/C. Therefore, the amount of Fe2P in LiFePO4 material should be seriously controlled, and
our work showed some new insights into the formation condition of Fe2P.
1312 Applications of Engineering Materials
Acknowledgements
This work was financially supported by the Taizhou Science and Technology Plan Project
(102XCP07), Scientific Research Project of Zhejiang Provincial Department of Education
(Y201016923).
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Advanced Materials Research Vols. 287-290 1313
Applications of Engineering Materials 10.4028/www.scientific.net/AMR.287-290 Influence of Sintering Atmosphere on the Physical and Electrochemical of LiFePO4/C 10.4028/www.scientific.net/AMR.287-290.1308
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