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Synthesis of LiFePO4/C cathode materials with both high-rate capability andhigh tap density for lithium-ion batteries

Xiaoming Lou and Youxiang Zhang*

Received 4th October 2010, Accepted 4th January 2011

DOI: 10.1039/c0jm03331f

LiFePO4/C microspheres composed of many densely compact nanoplates were synthesized by a simple

rheological phase method using nanoplate assembled quasi-microspheres of FePO4$2H2O as raw

materials. The quasi-sphere FePO4$2H2O precursors were synthesized via a sodium dodecylsulfate

assisted hydrothermal process. Both the LiFePO4/C composite and the FePO4$2H2O precursors were

characterized by XRD, TG, SEM, TEM, and Raman spectroscopy. The FePO4$2H2O quasi-spheres

had a size distribution of about 1 mm and were composed of nanoplates with a 30 nm thickness. The

LiFePO4/C microspheres were also composed of the same sized nanoplates with an �2 nm thick

amorphous carbon layer coating at the surface. The as-synthesized LiFePO4/C composite showed

excellent high-rate capability, with discharge capacities reaching 116, 96 and 75 mAh g�1 at 10 C, 20 C

and 30 C current rates, respectively. Furthermore, the LiFePO4/C material composed of microspheres

had a high tap density (1.4 g cm�3). Therefore, this LiFePO4/C material can be the cathode material for

large-scale applications such as electric vehicles and plug-in hybrid electric vehicles.

1. Introduction

Energy is the lifeblood of modern society. Global warming, finite

fossil-fuel supplies and city pollution conspire to make the use of

renewable energy, together with electric transportation,

a worldwide imperative. There is a pressing need to design elec-

trical-energy-storage systems to satisfy the electric markets

demands and the upcoming plug-in hybrid electric vehicles

(PHEVs) or electric vehicles (EVs). Secondary lithium batteries

have fast become the attractive power source for portable elec-

tronic devices, as well as electrical vehicles since their commercial

introduction in the early 1990s. Moreover, owing to its particular

advantages with regard to low cost, nontoxicity, environmental

friendliness, and high safety, intensive studies have focused on

the LiFePO4 material with an olivine structure, which was first

reported as a positive electrode for rechargeable Li-ion batteries

in 1997 by J. B. Goodenough and co-workers.1 This material has

a theoretical capacity of 170 mAh g�1 with a flat voltage profile at

3.4 V versus Li+/Li.

Although LiFePO4 possesses many advantages, the inherent

very poor electronic conductivity (�10�9 S cm�1) and Li-ion

diffusion coefficient (�1.8 � 10�14 cm2 s�1) at room temperature2

brings difficulties for high-rate battery applications. These

problems may be surmounted through two ways. One is coating

the LiFePO4 particles with an electron-conducting layer or

doping with supervalent ions.3–8 H. M. Xie and co-workers

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan,430072, China. E-mail: yxzhang04@whu.edu.cn; Fax: +86-27-6875-4067;Tel: +86-27-6578-3395

4156 | J. Mater. Chem., 2011, 21, 4156–4160

designed a core/shell LiFePO4 which had a high volume energy

density and displayed excellent discharge capacity.3 S. Y. Chung

et al. increased the electronic conductivity of LiFePO4 by a factor

of �108 by doping with aliovalent ions into the Li 4a site,4

although there has been some debate about the true reason for

the increase in electronic conductivity.5 X. L. Wu et al.6 and L. Q.

Sun et al.8 coated the LiFePO4 with a nano-structure network to

improve the electronic conductivity. Another way is to shorten

the Li+ diffusion distance by minimizing the particle size9–11 or

making it porous12–16 through controlling the synthesis condi-

tions. Most nano-LiFePO4 materials currently reported have

a tap density about 1.0 g cm�3.16 Dominko et al. reported

a porous LiFePO4 with a much higher tap density (ca. 1.9 g

cm�3), but unfortunately this material did not show any excellent

high-rate performance.13

Another interesting and furious research realm of the LiFePO4

material is the synthesis method. During the past decades,

tremendous efforts have been made to develop a favorable way

to obtain nanostructured LiFePO4 particles, such as hard-

template (anodized alumina oxide, AAO) synthesis17 and a sol–

gel approach using citric acid.18 The hydrothermal method is one

of the primary synthesis methods in research labs. M. S. Whit-

tingham’s group synthesized LiFePO4 by the hydrothermal

method first;19–21 S. Franger and co-workers also attempted the

hydrothermal and mechanochemical activation synthesis routes

for enhanced electrochemical performance of LiFePO4;22 X. Qin

et al. reported the formation mechanism of LiFePO4 platelets

under hydrothermal conditions.23 K. Kanamura et al. discussed

the relationship between LiFePO4 particle morphology and the

concentration of the Li source and the pH of the precursor.24

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 XRD patterns of FePO4$2H2O precursors (a) and as-synthesized

LiFePO4/C (b).

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Z. Liu et al.25 and K. Saravanan et al.26 studied the relationship

between the morphology and size of LiFePO4 and the sol-

vothermal parameters. Y. Guo’s group27 and I. Honma’s group28

made LiFePO4 nanoarchitectures using the solvothermal

method. A. V. Murugan et al. compared solvothermal and

hydrothermal methods to synthesis LiFePO4.29 Ionothermal

synthesis of tailor-made LiFePO4 was innovative work by J. M.

Tarascon’s group.30 These groups successfully synthesized

LiFePO4 with various wonderful shapes or excellent electro-

chemical performance, but could not have both excellent elec-

trochemical performance and high tap density (volumetric

energy density) which is also very important for lithium battery

applications.

Herein, based on the preceding outstanding work, by using

inexpensive and stable Fe3+ as the iron source, we have developed

a method to synthesize LiFePO4 microspheres which are

composed of many densely compact nanoplates with an �2 nm

thick amorphous carbon layer evenly coated at the surface. This

LiFePO4/C composite not only showed excellent high-rate

performance, but also had a high tap density.

2. Experimental

2.1 Synthesis of nano-sized FePO4$2H2O

In a typical procedure, sodium dodecylsulfate (SDS, 0.5 g) was

dissolved in deionized water (100 ml), and stirred about 30 min.

4 mmol of H3PO4 was added to the solution and stirred until

a homogeneous solution was formed. Then Fe(NO3)3$9H2O

(0.202 g) was added into the solution. The mixture was sealed in

a teflon-lined stainless steel autoclave, and heated at 170 �C for

4 h. After cooling to room temperature, the products were

separated by centrifugation for 5 min and washed with ethanol

and water several times. After heating the product at 100 �C for

4 h in air, nano-sized FePO4 was obtained.

2.2 Synthesis of LiFePO4/C composite

A rheological phase method was employed to synthesize the

LiFePO4/C composite. First, a stoichiometric amount of nano-

sized FePO4$2H2O and LiOH were mixed in a mortar to get

a homogeneous mixture which was mixed with polyethylene

glycol (PEG-10000), following the proportion of 50 g PEG per 1

mol FePO4 until a solid-liquid mushy rheological body was

formed. Finally, the slurry-like mixture was calcined at 650 �Cfor 10 h in an Ar flow. After cooling to room temperature, the

LiFePO4/C composite was obtained.

All the regents used in the experiment were analytical grade.

2.3 Characterization

X-Ray Diffraction (XRD) was performed on a Bruker

D8 Advance X-ray diffractometer with Cu-Ka radiation

(l ¼ 1.5406 �A). The powders were observed using a JSM-

5610LV scanning electron microscope (SEM, JEOL Ltd. Japan)

and a JEM 2010-FEF transmission electron microscope

(HRTEM, JEOL Ltd. Japan). The microstructure and particle

size of the produced powders could be obtained based on SEM

images. The thermal analysis was determined by a Netzsch STA

449C (Germany) in oxygen at a heating rate of 10 �C min�1

This journal is ª The Royal Society of Chemistry 2011

from room temperature to 700 �C. Raman spectra were

obtained from a RM-1000 Renishaw confocal Raman micro-

spectroscope with 514.5 nm laser radiation at a laser power of

0.48 mW in the range of 100–2000 cm�1. The electrochemical

measurements were carried out using CR2016 coin cells with

lithium metal as the counter electrode. The working electrode

was fabricated by compressing a mixture of the active material

(LiFePO4/C)/acetylene black/polyvinylidene fluoride (PTFE)

with a weight ratio 75 : 20 : 5. The active materials of a working

electrode were about 5.0 mg cm�2. The electrolyte was 1 M

LiPF6 in a 1 : 1 mixture of ethylene carbonate (EC)/diethyl

carbonate (DEC) and the separator was Celgard 2300 micro-

porous film. The cell was assembled in a glove box filled with

high purity argon gas. The cells were galvanostatically charged

and discharged between 2.0 and 4.4 V versus Li+/Li on a battery

cycler (Neware TC481 China).

3. Results and discussion

Fig. 1 shows the XRD patterns of the as-synthesized

FePO4$2H2O precursor (a) and LiFePO4/C (b). As shown in

Fig. 1a, the XRD pattern matches with standard data JCPDS

card No.33-666 (FePO4$2H2O, phosphosiderite, monoclinic,

space group: P21/n, a ¼ 0.53290 nm, b ¼ 0.9798 nm, c ¼ 0.8710

nm). Two peaks (2q ¼ 35.44, 41.30) can be found corresponding

to the other phase of FePO4$2H2O (strengite, No. 33-667,

orthorhombic, space group: Pcab, a¼ 1.0122 nm, b¼ 0.9886 nm,

c ¼ 0.8723 nm). The diffraction peaks of the as-synthesized

LiFePO4 (Fig. 1b) can be well indexed to pure LiFePO4 with an

orthorhombic structure (JCPDS card No. 83-2092). No impu-

rities can be found and the peaks are strong and narrow, indi-

cating the high crystallinity of the LiFePO4 sample. The

crystallite sizes calculated using Jade 5.0 software from the most

intense peaks in Fig. 1a and Fig. 1b are 38.6 and 36.2 nm,

respectively.

Fig. 2 shows the SEM and TEM images of the as-synthesized

FePO4$2H2O (a–c) and LiFePO4/C (d–f). Fig. 2a presents the

panorama image of the FePO4$2H2O synthesized by the SDS-

assisted hydrothermal process at 170 �C for 4 h. The particles are

quasi-spheres, with the sizes between 0.5 to 2 mm. Fig. 2b and 2c

show very clearly that the FePO4$2H2O microspheres are

composed of many densely compact nanoplates which have

J. Mater. Chem., 2011, 21, 4156–4160 | 4157

Fig. 2 SEM (a, b, c, and d) and TEM (e and f) images for synthesized FePO4$2H2O precursors and the as-synthesized LiFePO4/C. The SEM images a, b

and c are for panorama, individual and partial FePO4$2H2O quasi-spheres, respectively. SEM image d shows a microsphere of LiFePO4/C. High

resolution TEM images e and f show the whole (e) and a part (f) of a LiFePO4/C nanoplate; image f is the area circled with an ellipse in image e.

Fig. 3 TG and DTA curves of the LiFePO4/C sample.

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about a 100 nm size and 30 nm thickness. These results are

consistent with the values calculated from the XRD pattern.

Fig. 2d shows the SEM image of a typical LiFePO4 particle

synthesized by the simple rheological phase method using the

FePO4$2H2O microspheres as the raw material. Comparison

between the FePO4$2H2O microsphere (Fig. 2b) and the

LiFePO4 particle (Fig. 2d) shows that there are thin carbon

layers on the surface of the LiFePO4 microspheres. The high-

resolution TEM image of the LiFePO4 microspheres (Fig. 2e)

shows that the microspheres are composed of nanoplates with

the same sizes as the nanoplates in the FePO4$2H2O micro-

spheres. The interplane distances of 0.3763 and 0.3446 nm can be

found from the nanoplates (Fig. 2f), which are very consistent

with the interplane distance values of (011) and (201) in ortho-

rhombic LiFePO4. In addition, it can be clearly seen from the

high resolution TEM image of the nanoplates (Fig. 2f) that there

is an �2 nm thick amorphous carbon layer coating the LiFePO4

nanoplates evenly.

The carbon content of the LiFePO4/C microspheres was

measured by the thermogravimetric (TG) method. In a typical

TG process in air for LiFePO4, the olivine LiFePO4 can be

oxidized to Li3Fe2(PO4)3 and Fe2O3 in the temperature range of

200 to 500 �C, with a weight gain of 5.07 wt%, based on eqn (1):31

LiFePO4 + 1/4O2 ¼ 1/3Li3Fe2(PO4)3 + 1/6Fe2O3 (1)

While for the LiFePO4/C composite, the equation will be

LiFePO4 + xC + 1/4O2 + xO2 ¼ 1/3Li3Fe2(PO4)3 + 1/6Fe2O3

+ xCO2 (2)

where x denotes the carbon content in the composite. According

to the TG and DTA curves of our as-synthesized LiFePO4/C

sample in oxygen (Fig. 3), the LiFePO4/C sample starts to be

oxidized to Li3Fe2(PO4)3 and Fe2O3 at about 300 �C with the

mass of the sample starting to grow. At about 420 �C, the carboncontent in the sample starts to be oxidized to CO2 gas making the

mass of the sample grow much slower. At about 550 �C, the

4158 | J. Mater. Chem., 2011, 21, 4156–4160

LiFePO4/C sample finishes both the oxidizing reactions in air

and the mass of the sample remains constant. The total weight

gain in the TG curve is 2.86 wt% and thus the carbon content is

calculated to be 2.21 wt% (5.07 wt% � 2.86 wt%).31

Raman spectra can detect the surface components due to the

limited penetration depth of the laser into the sample, and is

often used to examine the carbon component of LiFePO4/C

composites by calculating the intensity ratio (ID/IG) in Raman

shift spectra.17 In the Raman spectra of our LiFePO4/C micro-

spheres sample (Fig. 4), there are two peaks presented in the

spectra, one at 1351 cm�1 which is attributed to the D band

(disordered carbon, sp3) and the other at 1605 cm�1 which is due

to the G band (graphite, sp2). The calculated peak intensity ratio

of ID/IG is 1.25, indicating that there is less graphitized carbon

than disordered carbon. There are also two small peaks at 950

cm�1 and 620 cm�1, which are attributed to the intramolecular

stretching modes of PO43�, indicating that the carbon coating

layer is thin enough to allow the detecting laser beam to pene-

trate.

Fig. 5 exhibits the typical electrochemical performance curves

of the synthesized LiFePO4/C microspheres composite at

current rates from 0.1 C to 30 C. Fig. 5a shows the typical

This journal is ª The Royal Society of Chemistry 2011

Fig. 4 Raman spectra of the LiFePO4/C sample.

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voltage–capacity curves and Fig. 5b shows the specific capacities

at different current rates. At a low rate of 0.1 C (17 mA g�1), the

LiFePO4/C delivers a discharge capacity of 158 mAh g�1, cor-

responding to 93% of the theoretical capacity of LiFePO4.

Considering that there is 2.21 wt% carbon in the composite

material, the specific capacity calculated from the pure LiFePO4

weight is as high as 162 mAh g�1. The remarkable advantage of

this material is its high-rate capability. The discharge capacities

of the LiFePO4/C sample can reach 116 and 96 mAh g�1 at high

rates of 10 C (1700 mA g�1) and 20 C (3400 mA g�1). Even when

the current is increased to 30 C (5100 mA g�1) the discharge

capacity can still reach 75 mAh g�1. Considering the carbon

content mass in the composite, the discharge capacities reach

119, 98, and 77 mAh g�1 for 10 C, 20 C and 30 C, respectively.

The cycling capability of the LiFePO4/C sample is shown in

Fig. 5c. In the 100th cycle the capacity losses are only 2.31%,

Fig. 5 (a) Typical charge-discharge curves at various current rates in the pot

rates; (c) the high rates capacity retention for the LiFePO4/C sample.

This journal is ª The Royal Society of Chemistry 2011

2.57% and 6.36% from their first cycle for 10 C, 20 C and 30 C

rates, respectively.

These nanoplate (primary particles) packed LiFePO4/C

microspheres (secondary particles), with the sizes between 0.5 to

2 mm, can form compact powders and have a tap density as high

as 1.4 g cm�3. This tap density is much higher than most

LiFePO4/C nanoparticles that have superior high-rate electro-

chemical performance. Generally the tap density of LiFePO4

with nanoparticles and an irregular carbon coating is less than

1.0 g cm�3. So our synthesis strategy can produce LiFePO4/C

cathode electrode material that can offer excellent high-rate

electrochemical performance without sacrificing the volume

energy density.

4. Conclusion

LiFePO4/C microspheres composed of many densely compact

nanoplates are synthesized by a simple rheological phase method

using FePO4$2H2O quasi-microspheres as the raw materials. The

nanoplate assembled quasi-microspheres of the FePO4$2H2O

precursors are fabricated via a SDS-assisted hydrothermal

process. The LiFePO4/C microsphere composites are composed

of nanoplate primary particles, which have about a 100 nm size

and about a 30 nm thickness, and show excellent high-rate

capability as the cathode electrode materials for lithium-ion

batteries. The discharge capacities can reach 116, 96 and 75 mAh

g�1 at 10 C, 20 C and 30 C current rates, respectively. The

materials also have a high tap density (1.4 g cm�3), and thus can

offer high rate performance without sacrificing the volume

energy density as the cathode material for large-scale applica-

tions such as electric vehicles and plug-in hybrid electric vehicles.

ential window 2.0-4.4 V (vs. Li+/Li); (b) the specific capacities at different

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Acknowledgements

The authors thank the Center for Electron Microscopy at

Wuhan University for help in taking the TEM and high-resolu-

tion TEM images for the materials. This study was supported by

the National Science Foundation of China (grant No. 20901062)

and the Fundamental Research Funds for the Central Univer-

sities (grant No. 2082002).

References

1 K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough,J. Electrochem. Soc., 1997, 144, 1188.

2 L. Wang, G. C. Liang, X. Q. Qu, X. K. Zhi, J. P. Zhang and J. Y. Cui,J. Power Sources, 2009, 189, 423.

3 H. M. Xie, R. S. Wang, J. R. Ying, L. Y. Zhang, A. F. Jalbout,H. Y. Yu, G. L. Yang, X. M. Pan and Z. M. Su, Adv. Mater., 2006,18, 2609.

4 S. Y. Chung, J. T. Bloking and Y. M. Chiang, Nat. Mater., 2002, 1,123.

5 A. Yamada, H. Koizumi, S. I. Nishimura, N. Sonoyama, R. Kanno,M. Yonemura, T. Nakamura and Y. Kobayashi,Nat.Mater., 2006, 5,357.

6 X. L. Wu, L. Y. Jiang, F. F. Cao, Y. G. Guo and L. J. Wan, Adv.Mater., 2009, 21, 2710.

7 Y. H. Huang and J. B. Goodenough, Chem. Mater., 2008, 20, 7237.8 L. Q. Sun, M. J. Li, R. H. Cui, H. M. Xie and R. S. Wang, J. Phys.Chem. C, 2010, 114, 3297.

9 P. P. Prosini, M. Carewska, S. Scaccia, P. Wisniewski andM. Pasquali, Electrochim. Acta, 2003, 48, 4205.

10 M. Konarova and I. Taniguchi, J. Power Sources, 2010, 195, 3661.11 Y.Wang, J. Wang, J. Yang and Y. Nuli,Adv. Funct. Mater., 2006, 16,

2135.12 R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel,

J. M. Goupil, S. Pejovnik and J. Jaminik, J. Power Sources, 2006,153, 274.

4160 | J. Mater. Chem., 2011, 21, 4156–4160

13 R. Dominko, M. Bele, J. Goupil, M. Gaberscek, D. Hanzel, I. Arconand J. Jamink, Chem. Mater., 2007, 19, 2960.

14 S. Lim, C. S. Yoon and J. Cho, Chem. Mater., 2008, 20,4560.

15 C. M. Doherty, R. A. Caruso, B. M. Smarsly and C. J. Drummond,Chem. Mater., 2009, 21, 2895.

16 J. Qian, M. Zhou, Y. Cao, X. Ai and H. Yang, J. Phys. Chem. C,2010, 114, 3477.

17 F. Teng, S. Santhanagopalan, R. Lemmens, X. Geng, P. Patel andD. D. Meng, Solid State Sci., 2010, 12, 952.

18 K. F. Hsu, S. Y. Tsay and B. J. Hwang, J. Mater. Chem., 2004, 14,2690.

19 S. Yang, Y. Song, P. Y. Zavalij and M. S. Whittingham, Electrochem.Commun., 2002, 4, 239.

20 J. Chen and M. S. Whittingham, Electrochem. Commun., 2006, 8,855.

21 J. Chen, S. Wang and M. S. Whittingham, J. Power Sources, 2007,174, 442.

22 S. Franger, F. L. Cras, C. Bourbon and H. Rouault, Electrochem.Solid-State Lett., 2002, 5, A231.

23 X. Qin, X. Wang, H. Xiang, J. Xie, J. Li and Y. Zhou, J. Phys. Chem.C, 2010, 114, 16806.

24 K. Dokko, S. Koizumi, H. Nakano and K. Kanamura, J. Mater.Chem., 2007, 17, 4803.

25 S. Yang, X. Zhou, J. Zhang and Z. Liu, J. Mater. Chem., 2010, 20,8086.

26 K. Saravanan, P. Balaya, M. V. Reddy, B. V. R. Chowdari andJ. J. Vital, Energy Environ. Sci., 2010, 3, 457.

27 H. Yang, X. L. Wu, M. H. Cao and Y. G. Guo, J. Phys. Chem. C,2009, 113, 3345.

28 D. Rangappa, K. Sone, T. Kudo and I. Honma, J. Power Sources,2010, 195, 6167.

29 A. V. Murugan, T. Muraliganth and A. Manthiram, J. Phys. Chem.C, 2008, 112, 14665.

30 N. Recham, L. Dupon, M. Courty, K. Djellab, D. Larcher,M. Armand and J. M. Tarascon, Chem. Mater., 2009, 21, 1096.

31 I. Belharouak, C. Johnson and K. Amine, Electrochem. Commun.,2005, 7, 983.

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