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

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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