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J. Cent. South Univ. Technol. (2011) 18: 978984DOI: 10.1007/s1177101107907

Electrochemical performance of LiFePO4/(C+Fe2P) composite

cathode material synthesized by sol-gel method

CHEN Quan-qi()1, 2, LI Xiao-shuan()1, WANG Jian-ming()2

1. Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,School of Chemistry, Xiangtan University, Xiangtan 411105, China;

2. Department of Chemistry, Zhejiang University, Hangzhou 310027, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: A LiFePO4/(C+Fe2P) composite cathode material was prepared by a sol-gel method using Fe(NO 3)39H2O, LiAcH2O,NH4H2PO4and citric acid as raw materials, and the physical properties and electrochemical performance of the composite cathode

material were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy(TEM) and electrochemical tests. The Fe2P content, morphology and electrochemical performance of LiFePO4/(C+Fe2P) compositedepend on the calcination temperature. The optimized LiFePO4/(C+Fe2P) composite is prepared at 650 C and the optimizedcomposite exhibits sphere-like morphology with porous structure and Fe2P content of about 3.2% (mass fraction). The dischargecapacity of the optimized LiFePO4/(C+Fe2P) at 0.1C is 156 and 161 mAh/g at 25 and 55 C, respectively, and the correspondingcapacity retentions are 96% after 30 cycles; while the capacity at 1Cis 142 and 149 mAh/g at 25 and 55 C, respectively, and thecapacity still remains 135 and 142 mAh/g after 30 cycles at 25 and 55 C, respectively.

Key words:LiFePO4/(C+Fe2P) composite; sol-gel; sphere-like morphology; electrochemical performance

1 Introduction

Lithiated transition metal polyanion cathode

materials based on 34PO have been attractive for their

stable frameworks, relative high operating voltage, good

lithium ion transport and large theoretical capacity.

Presently, the studied polyanion cathode materials

mainly involve LiFePO4 [17], Li3V2(PO4)3 [810] and

LiVPO4F [1113], while LiFePO4 has been considered

as the most promising cathode material because of its

relatively low cost and environmental friendliness.

However, the intrinsically low electronic conductivity, on

the order of 109 S/cm, of pristine LiFePO4degrades its

electrochemical performance and impedes its

applications. Previous reports have demonstrated that it

is an effective method to improve the electrochemical

performance of LiFePO4 by enhancing its electronic

conductivity. The solutions to improve the electronic

conductivity of cathode materials frequently include

adding conductive materials [5, 1415], coating particles

with a thin carbon layer [16] and doping heteroatoms [3,

1719]. Fe2P, a metal phosphide with high electron

conductivity of about 101 S/cm usually generated by

carbothermal reduction at high temperatures, has been

considered as an effective conductive additive to

significantly improve the electrochemical performance of

LiFePO4. More recently, LiFePO4/(C+Fe2P) composite

with improved electrochemical performance was

prepared by solid-state method using Fe(NO3)39H2O

[14] and Fe2O3[15] as iron sources at high temperature.

Whereas, the solid-state reaction needs high calcination

temperature and results in larger particles of LiFePO4,

unfavourable for improving the electrochemical

performance of LiFePO4.

In this work, sphere-like LiFePO4/(C+Fe2P) was

prepared at relatively lower calcination temperature by a

sol-gel method using cheap Fe(NO3)39H2O, LiAcH2O,

NH4H2PO4 and citric acid as raw materials, and the

effects of calcination temperatures on the content of Fe2P

and the physical and electrochemical performance of

LiFePO4/(C+Fe2P) composite cathode material were

investigated.

2 Experimental

Stoichiometric amount of cheap Fe(NO3)39H2O,

LiAcH2O, NH4H2PO4and certain amount of citric acid

Foundation item:Project(50571091) supported by the National Natural Science Foundation of China; Project(09C947) supported by the Scientific ResearchFund of Hunan Provincial Education Department, China

Received date:20100612; Accepted date: 20101028Corresponding author: CHEN Quan-qi, Associate Professor, PhD; Tel: +8673158292206; E-mail: [email protected], [email protected]


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J. Cent. South Univ. Technol. (2011) 18: 978984 979

were dissolved in de-ionized water solution, and then the

mixed solution was stirred vigorously at 80 C in

thermostatic bath for 10 h to get the resulted gel. The

obtained gel was dried in a vacuum oven at 80C for

12 h, and then the precursor was pelletized and heated at

300C in a tubular furnace with flowing argon gas for

8 h to allow NH3and H2O to evolve. After slow cooling

to the room temperature, the product was ground and

pelletized again, and heated to 600850C at a ramp rate

of 2 C/min under a stream of argon gas for 8 h, then

slowly cooled to room temperature to obtain the

LiFePO4/(C+Fe2P) composite product.

During the synthesis of LiFePO4/(C+Fe2P)

composite material, the citric acid as a chelating agent

for iron facilitates the formation of the homogenous

precursor gel; moreover, its decomposition at higher

temperatures in an inert atmosphere may provide welldispersed carbon which was mainly used as the selective

reduction agent for Fe(III) and conductive additive.

Thermogravimetric (TG) analysis and differential

scanning calorimetry (DSC) were carried out using a

NETZSCHSTA 409 PG/PC thermal analyzer for heating

from room temperature to 950 C at a rate of 10 C/min

in N2 atmosphere. The crystallinity and structure of the

samples were determined using a D/Max III X-ray

diffractometer (XRD) with Cu K radiation (=1.541 8

). The carbon content of samples was analyzed by a

carbon-sulfur analyzer (Mlti EA2000). The surfacemorphology of the samples was observed using a

SIRION-100 (FEI) scanning electron microscope (SEM).

The microstructure was examined using a JEM 1010

transmission electron microscope (TEM).

The electrochemical tests of the samples were

carried out using two-electrode or three-electrode cells.

In all cells, lithium metal served as the counter and

reference electrodes, Celguard2300 was used as the

separator, and the electrolyte was 1 mol/L LiPF6solution

in a mixture of ethylene carbonate/dimethyl carbonate

(1:1 in volume). The cathode was prepared by casting aslurry of the prepared sample, acetylene black and

polyvinylidene fluoride (PVDF) in a mass ratio of

80:10:10 on an aluminium foil current collector. After

dried at 120 C in a vacuum oven for 24 h, the resulting

electrodes with an active material loading of about

6 mg/cm2were transferred to an Ar-filled glove box to

assemble testing cells. Galvanostatic chargedischarge

measurements were carried out in two-electrode cells

using a PCBT-143-320 battery programme-control test

system (Lixing, Wuhan, China) in the voltage range of

2.5

4.5 V vs Li

+

/Li at room temperature. Cyclicvoltammetry (CV) was conducted in three-electrode cells

at a scanning rate of 0.05 mV/s on a PARSTAT 2273

electrochemistry work station.

3 Result and discussion

Figure 1 shows the TG-DSC curves of the driedprecursor gel. On the DSC curve near 109.3 C, there isan obvious endothermic peak, associated with the massloss on the TG curve, which is related to the fastdehydration of precursor gel. There are three exothermic

peaks near 222.9, 489.3, and 556.7 C, possiblyassociated with the evolution of NH3 and H2O,decomposition of organic compound and formation ofLiFePO4, respectively. When the calcination temperatureis higher than 600 C, TG curve shows that the massalmost remains constant, implying that the formation ofLiFePO4phase begins at this temperature. Based on theabove analysis, the calcination temperature for the dried

precursor gel is adjusted in the range of 600850 C.

Fig.1TG-DSC curves of dried gel in nitrogen atmosphere

The XRD patterns of samples synthesized atdifferent temperatures are presented in Fig.2. It can beseen that the XRD patterns of all samples except thesample calcined at 600 C comprise the phases ofLiFePO4 and Fe2P, in agreement with the olivine-typeLiFePO4 (PDF 83-2092) and Fe2P (PDF 51-0943),

respectively. There exists only olivine-type LiFePO4(PDF 83-2092) in the XRD pattern of sample calcined at600 C. The existence of Fe2P with higher electronicconductivity can enhance the electrochemical

performance of LiFePO4 [15]. A quantitative analysis ofFe2P is performed by the direct comparison method forthe integrated intensity of reflection of LiFePO4and Fe2P.Quantitative analysis for a particular substance was

possible because the intensities of the diffraction linesascribed to one phase of the mixture depend on the

proportion of that phase in the sample [15]. The mass

ratio of Fe2P to LiFePO4 is approximately calculated byJade 5.0, an X-ray diffraction data analysis softwaresystem. The carbon content of samples determined usinga carbon-sulfur analyser (Mlti EA2000) and the Fe2P


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J. Cent. South Univ. Technol. (2011) 18: 978984980

contents are listed in Table 1. It is clear that the carboncontent decreases with increasing the calcinationtemperature and the Fe2P content increases withincreasing the calcination temperature, suggesting thathigher calcination temperature favors the formation ofconductive Fe2P. It is found that adequate amount of Fe2Pcan significantly improve the conductivity of LiFePO4

cathode material, resulting in better electrochemicalperformance of LiFePO4[1415].

Typical SEM images for the samples prepared at650, 750 and 850 C are shown in Fig.3. It can be seenthat the sample prepared at 650C has smaller particlesize than the samples prepared at 750 and 850 C and thesurface of the sample calcined at 650C is rougher and

Fig.2 XRD patterns of samples synthesized at different temperatures (a) and magnified XRD patterns of samples synthesized at

different temperatures in 2range of 3941 (b)

Table1Lattice parameters of LiFePO4, carbon content and Fe2P content of composites prepared at different temperatures

Calcination temperature/C a/nm b/nm c/nm V/nm3 w(C)/% w(Fe2P)/%

600 1.031 3 0.600 5 0.469 4 0.290 7 9.5 0

650 1.032 9 0.600 6 0.469 5 0.291 3 7.0 3.2700 1.033 1 0.600 5 0.469 6 0.291 3 6.5 8.8

750 1.033 4 0.600 9 0.469 5 0.291 5 6.0 13.2

850 1.033 5 0.601 2 0.469 4 0.291 7 3.9 17.4

Fig.3 SEM images of samples synthesized

at 650

C (a), 750

C (b) and 850 C (c)


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porous, while the surfaces of samples calcined at 750 and850C are smooth. The porous structure is in favor of

penetration of electrolyte and the smaller particle size ishelpful for the reduction of lithium ion diffusion pathwayin LiFePO4, suggesting that the composite sample

prepared at 650 C may have better electrochemicalperformance. In order to further understand themicrostructure of composite samples prepared at 650,750 and 850 C, the microstructures were observed withTEM and the images are presented in Fig.4. Figure 4(c)demonstrates that LiFePO4 and Fe2P phases can bediscerned by SAD (selected area diffraction), and Fe2Poccurs as solid sphere with a diameter of about 100 nm,similar to the previous report [15]. Figure 4(b) revealsthat a few smaller Fe2P particles locate on the surface ofthe composite synthesized at 750 C. However, it is

difficult to discriminate Fe2P from LiFePO4 in Fig.4(a),and the composite prepared at 650 C exhibits sphere-like morphology with porous structure, implying that thesphere-like Fe2P particles may be dispersively embeddedinto the composite and result in better electric contact of

particles. The contact of larger highly conductive Fe2Pwith LiFePO4 nanocrystal just like Fig.4(c) is noteffective to improve the electric conductivity ofcomposite. Compared with Figs.4(b) and 4(c), porousstructure and dispersive Fe2P could significantly improvethe conductivity of the LiFePO4/(C+Fe2P) compositesynthesized at 650 C, resulting in better electrochemical

performance.Figure 5 illustrates the initial chargedischarge

profiles of LiFePO4/(C+Fe2P) composite cathodematerials prepared at different calcination temperaturesat 0.1C(17 mA/g) in the voltage range of 2.54.5 V vsLi+/Li. The capacity of the cathode is calculated based onthe mass of LiFePO4 active material. It can be seen thatthe discharge capacity of the LiFePO4/(C+Fe2P)composites increases with the calcination temperature

increasing from 600 to 650 C, then decreases with thecalcination temperature increasing from 700 to 850 C.The possible reason for this change tendency ofdischarge capacity is that lower temperature isunfavourable for the growth of LiFePO4crystal and the

Fig.4TEM images of samples synthesized

at 650 C (a), 750 C (b) and 850C (c)

and SAD patterns of Fe2P (d) and LiFePO4

(e)


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Fig.5 Initial chargedischarge profiles of samples synthesized

at different temperatures at 0.1C

formation of highly conductive Fe2P and carbon, whilehigher sintering temperature facilitates the formation oflarger LiFePO4 particles and the larger amount ofnon-electroactive Fe2P, resulting in the poorer

electrochemical performance. The LiFePO4/(C+Fe2P)composite synthesized at 650 C exhibits the highestdischarge capacity of 156 mAh/g, close to the theoretical

capacity of LiFePO4 (170 mAh/g), higher than that ofthe LiFePO4/(C+Fe2P) reported by XU et al [14] andKIM et al [15].

The cyclic voltammograms of the LiFePO4/(C+Fe2P)composites prepared at 650, 750 and 850 C aresummarized in Fig.6. All composites exhibit a couple ofredox peaks, corresponding to the characteristics ofelectrochemical lithium insertion/extraction reactions ofLiFePO4[1]. The voltage differences of redox peaks forLiFePO4/(C+Fe2P) composite prepared at 650, 750 and850 C are 0.14, 0.22 and 0.3 V, respectively, suggestingthat the composite synthesized at 650 C has better

reversibility, resulting in better electrochemicalperformance. The cycle performances of theLiFePO4/(C+Fe2P) composites synthesized at different

temperatures shown in Fig.7 reveal that the compositeprepared at 650 C exhibits the highest dischargecapacity of 156 mAh/g at a rate of 0.1Cand a capacityretention rate of 96% after 30 cycles.

In order to investigate the effects of charge/

discharge current rates and environmental temperature onthe electrochemical performance of LiFePO4/(C+Fe2P)composite, the composite synthesized at 650 C wasgalvanostatically charged/discharged at different rates at

25 and 55 C, respectively, and the initial chargedischarge profiles and cycle performances of composite

cathode are presented in Fig.8 and Fig.9, respectively.Figure 8 shows that the discharge capacity at 25 Cdecreases from 156 mAh/g (0.1C) to 142 mAh/g (1C)and the capacity at 55 C declines from 161 mAh/g

Fig.6Cyclic voltammograms of LiFePO4/(C+Fe2P) composites

synthesized at different temperatures at scanning rate of0.05 mV/s

Fig.7 Cycle performances of LiFePO4/(C+Fe2P) composites

synthesized at different temperatures at rate of 0.1C

Fig.8Initial chargedischarge profiles of LiFePO4/(C+ Fe2P) at

different rates at 25 and 55 C

(0.1C) to 149 mAh/g (1C). The reduction of capacity

with increasing current rates is resulted from the larger

polarization at higher current rate, and the increase of


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Fig.9 Cycle performances of LiFePO4/(C+ Fe2P) at different

rates at 25 and 55C

capacity with increasing environmental temperature may

be attributed to the fact that elevated temperature can

effectively improve the diffusion of lithium-ion in

LiFePO4/(C+Fe2P) composite and results in better

electrochemical performance.

The cycle performances presented in Fig.9 reveal

that the LiFePO4/(C+Fe2P) composite has better cycle

performance at different environmental temperatures.

The capacity retention rates of cathode composite are

96% (0.1C, 25 C), 96% (0.1C, 55 C), 95% (1C, 25 C)

and 95% (1C, 55 C) after 30 cycles, respectively. It isnoted that the LiFePO4/(C+Fe2P) composite still remains

the capacity at 1Cof 135 and 142 mAh/g after 30 cycles

at 25 and 55 C, respectively, higher than that of the

previously reported LiFePO4/(C+Fe2P) [1415]. From

the above results, it can be concluded that the sol-gel

method is more effective than the solid-state method to

prepare LiFePO4/(C+Fe2P) with better electrochemical

performance.

4 Conclusions

1) LiFePO4/(C+Fe2P) composite cathode material

can be prepared by a sol-gel method using cheap

Fe(NO3)39H2O as the iron source and citric acid as the

chelating agent and carbon source.

2) The Fe2P content of the LiFePO4/(C+Fe2P)

composite increases with the calcination temperature

increasing from 600 to 850C, and the morphology of the

LiFePO4/(C+Fe2P) composite changes with the increase

of calcination temperature. The porous sphere-like

morphology and about 3.2% of non-active Fe2P with

high conductivity can significantly improve theelectrochemical performance of the composite, which

leads to the LiFePO4/(C+Fe2P) composite synthesized at

650 C to exhibit better electrochemical performance

than that of other composites prepared at the other

temperatures.

3) Higher environmental temperature favors the

improvement of the electrochemical performance of the

LiFePO4/(C+Fe

2P) composite. At 55 C, the LiFePO

4/

(C+Fe2P) composite delivers capacities of 161 and

148 mAh/g at 0.1Cand 1C, respectively; while at 25 C,

it exhibits capacities of 156 and 142 mAh/g at 0.1Cand

1C, respectively. The LiFePO4/(C+Fe2P) composite

shows excellent cycle performances at 25 and 55 C, and

its capacity retention rate is not less than 95% after 30

cycles at the current rates of 0.1Cand 1C.

References

[1] PADHI A K, NANJUNDASWAMY K S, GOODENOUGH J B.Phospho-olivines as positive-electrode materials for rechargeable

lithium batteries [J]. Journal of the Electrochemical Society, 1997,

144(4): 11881194.

[2] BELHAROUAK I, JOHNSON C, AMINE K. Synthesis andelectrochemical analysis of vapor-deposited carbon-coated LiFePO4

[J]. Electrochemistry Communications, 2005, 7(10): 983988.

[3] ZHANG Bao, LI Xin-hai, LUO Wen-bin, WANG Zhi-xing.Electrochemical properties of LiFe1xMgxPO4 for cathode materials

of lithium ion batteries [J]. Journal of Central South University:

Science and Technology, 2006, 37(6): 10941097. (in Chinese)

[4] GABERSCEK M, DOMINKO R, JAMNIK J. Is small particle sizemore important than carbon coating? An example study on LiFePO 4

cathodes [J]. Electrochemistry Communications, 2007, 9(12):22782283.

[5] MI C H, CAO Y X, ZHANG X G, ZHAO X B, LI H L. Synthesis andcharacterization of LiFePO4/(Ag+C) composite cathodes with

nano-carbon webs [J]. Powder Technology, 2008, 181(3): 301306.

[6] CHANG Zhao-Rong, L Hao-Jie, TANG Hong-Wei, LI Hua-Ji,YUAN Xiao-Zi, WANG Hai-jiang. Synthesis and characterization of

high-density LiFePO4/C composites as cathode materials for

lithium-ion batteries [J]. Electrochimica Acta, 2009, 54(20):

45954599.

[7] DING Y, JIANG Y, XU F, YIN J, REN H, ZHUO Q, LONG Z,ZHANG P. Preparation of nano-structured LiFePO4/graphene

composites by co-precipitation method [J]. ElectrochemistryCommunications, 2010, 12(1): 1013.

[8] HUANG H, YIN S C, KERR T, TAYLOR N, NAZAR L F.Nanostructured composites: A high capacity, fast rate

Li3V2(PO4)3/carbon cathode for rechargeable lithium batteries [J].

Advanced Materials, 2002, 14(21): 15251528.

[9] YIN S C, GRONDEY H, STROBEL P, ANNE M, NAZAR L F.Electrochemical property: Structure relationships in monoclinic

Li3yV2(PO4)3[J]. Journal of the American Chemical Society, 2003,

125(34): 1040210411.

[10] CHEN Quan-qi, WANG Jian-ming, TANG Zhen, HE Wei-chun,SHAO Hai-bo, ZHANG Jian-qing. Electrochemical performance of

the carbon coated Li3V2(PO4)3 cathode material synthesized by a

sol-gel method [J]. Electrochimica Acta, 2007, 52(16): 52515257.

[11] ZHONG Shen-kui, YIN Zhou-lan, WANG Zhi-xing, CHEN Qi-yuan.Synthesis and characterization of triclinic structural LiVPO4F as

possible 4.2 V cathode materials for lithium ion batteries [J]. Journal


7/7


of Central South University of Technology, 2007, 14(3): 340343.

[12] BARKER J, SAIDI M Y, SWOYER J L. Electrochemical insertionproperties of the novel lithium vanadium fluorophosphate, LiVPO4F

[J]. Journal of the Electrochemical Society, 2003, 150(10):

A1394A1398.

[13] BARKER J, GOVER R K B, BURNS P, BRYAN A, SAIDI M Y,SWOYERB J L. Performance evaluation of lithium vanadium

fluorophosphate in lithium metal and lithium-ion cells [J]. Journal of

the Electrochemical Society, 2005, 152 (9): A1776A1779.

[14] XU Yan-bin, LU Ying-jun, YAN Lan, YANG Zheng-yin, YANGRu-dong. Synthesis and effect of forming Fe2P phase on the physics

and electrochemical properties of LiFePO4/C materials [J]. Journal of

Power Sources, 2006, 160(1): 570576.

[15] KIM C W, PARK J S, LEE K S. Effect of Fe2P on the electronconductivity and electrochemical performance of LiFePO4

synthesized by mechanical alloying using Fe3+ raw material [J].

Journal of Power Sources, 2006, 163(1): 144150.

[16] WANG L N, ZHAN X C, ZHANG Z G, ZHANG K L. A softchemistry synthesis routine for LiFePO4-C using a novel carbon

source [J]. Journal of Alloys and Compounds, 2008, 456(1/2):

461465.

[17] MICHAEL T. An expected conductor [J]. Nature Materials, 2008,1(2): 8182.

[18] SURENDRA K M, JUDITH G, ORTAL H, ELLA Z, THIERRY D,JAMES H M, IVAN E, ANDREAS K, BORIS M, AURBACH D.

LiMn0.8Fe0.2PO4: An advanced cathode material for rechargeable

lithium batteries [J]. Angewandte Chemie International Edition, 2009,

48: 85598563.

[19] YIN Xiong-ge, HUANG Ke-long, LIU Su-qin, WANG Hai-yan,WANG Hao. Preparation and characterization of Na-doped

LiFePO4/C composites as cathode materials for lithium-ion batteries

[J]. Journal of Power Sources, 2010, 195(13): 43084312.

(Edited by PENG Chao-qun)

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