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Electrochimica Acta 53 (2007) 1939–1944

Preparation and characterization of nano-particle LiFePO4 andLiFePO4/C by spray-drying and post-annealing method

Fei Gao ∗, Zhiyuan Tang, Jianjun XueSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

Received 24 May 2007; received in revised form 22 August 2007; accepted 23 August 2007Available online 30 August 2007

bstract

Pure, nano-sized LiFePO4 and LiFePO4/C cathode materials are synthesized by spray-drying and post-annealing method. The influence of theintering temperature and carbon coating on the structure, particle size, morphology and electrochemical performance of LiFePO4 cathode materials investigated. The optimum processing conditions are found to be thermal treatment for 10 h at 600 ◦C. Compared with LiFePO4, LiFePO4/Carticles are smaller in size due to the inhibition of crystal growth to a great extent by the presence of carbon in the reaction mixture. And thathe LiFePO /C composite coated with 3.81 wt.% carbon exhibits the best electrode properties with discharge capacities of 139.4, 137.2, 133.5 and

4

27.3 mAh g−1 at C/5, 1C, 5C and 10C rates, respectively. In addition, it shows excellent cycle stability at different current densities. Even after 50ycles at the high current density of 10C, a discharge capacity of 117.7 mAh g−1 is obtained (92.4% of its initial value) with only a low capacityading of 0.15% per cycle.

2007 Published by Elsevier Ltd.

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eywords: LiFePO4; Nano-particle; Discharge capacity; Carbon coating; Sinte

. Introduction

Since the first report on the electrochemical properties ofiFePO4 by Padhi et al. [1], LiFePO4 is emerging as a promis-

ng cathode material for lithium-ion batteries because of lowost and environmental compatibility. In addition, LiFePO4 haslarge theoretical capacity of 170 mAh g−1, a flat discharge

otential of 3.4 V versus Li/Li+, the good cycle stability, and thexcellent thermal stability. In spite of these attractive features,iFePO4 requires further modifications to overcome limita-

ions such as poor electronic conductivity and slow lithium-ioniffusion. Conductivity is enhanced appreciably by coatingiFePO4 particles with electrically conductive materials likearbon [2–4], metal and metal oxides [5–8]. Minimizing thearticle size of LiFePO4 material to provide more surface areaas also been investigated as a means to enhance ion diffusion

9–11].

To ensure highly homogeneous sub-micro-sized particlesnd pure crystallization, several techniques have been applied

∗ Corresponding author. Tel.: +86 20 61920385; fax: +86 20 61981111.E-mail address: gaofei220@hotmail.com (F. Gao).

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013-4686/$ – see front matter © 2007 Published by Elsevier Ltd.oi:10.1016/j.electacta.2007.08.048

emperature

o prepare LiFePO4, such as the sol–gel, co-precipitation andydrothermal methods [12–15]. The prepared LiFePO4 powdersy the above approaches have achieved the good electrochemicalroperties to some extent. In this work, nano-particle LiFePO4nd LiFePO4/C cathode materials were synthesized by thepray-drying and post-annealing method, which has been widelysed to synthesize ultra-fine LiCoO2 and sub-sized LiMn2O416,17]. The effects of carbon coating and thermal processingonditions on the phase-purity, particle size, morphology andlectrochemical performance of the materials are investigated.he cycle performance of lithium-ion batteries with these activeaterials as cathodes is also evaluated.

. Experimental

The precursors of CH3COOLi·2H2O (99.9% Aldrich),eC2O4·2H2O (98% Aldrich), (NH4)2HPO4 (99.5% Aldrich)ere mixed in stoichiometric amounts. First, the precursors wereround for 8 h by wet ball-milling in the aqueous solution mixed

ith polyvinyl alcohol (PVA) (mass ratio of PVA and watereing 3:100) to decrease the particle size of the reactants andnsure intimate and homogeneous mixing. Then the resultingtable suspension was dried to form a mixed dry precursor via

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spray-dryer. The suspension was atomized via a sprinkler atn air pressure of 0.2 MPa, and was dried in the spray-dryery hot air. The inlet air temperature was 220 ◦C, and the exitir temperature was 110 ◦C. The as-prepared precursor powdersith pellets ranging from 50 to 100 �m, were sintered at 300 ◦C

or 5 h, and then to the elevated temperature of 500, 600 and00 ◦C for 10 h in nitrogen atmosphere. In the end, the samplesere reground for 8 h by the ball-milling after they were cooled

o room temperature and the final products of LiFePO4 werebtained. For preparing LiFePO4/C, a mixture of the above rawaterials and 5.1 wt.% of glucose (99.9% Aldrich) was used;

he other processing steps were the same as that for LiFePO4.The structure and phases of the LiFePO4 and LiFePO4/C

amples were identified by X-ray diffraction (XRD, RU-00B/RINT, Rigaku, Rotaflex using monochromatic Cu K�adiation). The morphology of the samples was recorded bysing a scanning electron microscopy (SEM, Hitachi S800). Thectual carbon content in the LFePO4/C materials was measuredy the high frequency IR carbon–sulfur analyzer (Eltra CS800).he specific surface area of the powder particles was mea-ured by the Brunauer–Emmett–Teller (BET) method (Gemini-2380).

The electrodes were manufactured for electrochemical test-ng of the samples by casting on an aluminum current collector

N-methylpyrrolidone (NMP) slurry of the LiFePO4 activeaterial (82 wt.%) mixed with a carbon black (Super-P, Timcalraphite Corp.) conductive additive (10 wt.%) and a polyvinyli-ene fluoride (Kynar2801, Eif Atochem) binder (8 wt.%). TheseiFePO4 electrodes with the thickness of 120 �m were assem-led in a nitrogen filled glove box using superfluous mesocarbonicrobeads (MCMB, Osaka-gas Chemical) anode as counter

lectrode. The MCMB anode electrode was prepared from slurryf MCMB with 3 wt.% of synthetic flake graphites (SFG15, Tim-al Graphite Corp.) and 3 wt.% poly(vinylfluoride) dissolved inMP. The slurry was spread on the copper foil and dried under

he vacuum circumstance. The thickness of the anode electrodeas about 90 �m. The electrolyte was 1 mol L−1 LiPF6 in a mix-

ure of ethylene carbonate (EC) and dimethyl carbonate (DMC)1:1, v/v). The charge and discharge characteristics of the cath-des were examined between 2.0 and 4.2 V using a cylindrical

attery (18650 type).

Cyclic voltammetry (CV) measurements were performedsing a voltalab system with VM4 (Radiometer Analytical SAS.GZ301) between 2.0 and 4.2 V at a scan rate of 0.05 mV s−1.

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able 1he lattice parameters, calculated crystallite size (D) of the (1 3 1) diffraction peaks an

emperatures of 500, 600 and 700 ◦C

amples Sintering temperature (◦C) The lattice parameters

a (nm) b (nm)

iFePO4

500 1.032 0.5996600 1.031 0.5998700 1.031 0.6001

iFePO4/C500 1.031 0.6001600 1.032 0.6002700 1.032 0.6002

ig. 1. XRD spectra of (a) LiFePO4, (b) LiFePO4/C samples prepared at differ-nt temperatures of 500, 600 and 700 ◦C.

hese tests were performed using a three-electrode cell. Thelectrodes of LiFePO4 were used as working electrodes. Theounter and reference electrodes were lithium foil.

. Results and discussion

Fig. 1 shows the effect of sintering temperature on the XRDpectra of LiFePO4 and LiFePO4/C. The samples were pre-ared by heating for 10 h at different temperatures of 500, 600nd 700 ◦C. By increasing post-calcination temperature up to00 ◦C, it is appeared an impurity phase (marked by arrow inig. 1) which is considered to be iron(II, III) pyrophosphatesr phosphates (perhaps Li3Fe2(PO4)3 or Li3PO4), and when theintering temperature increases to 600 or 700 ◦C, all the diffrac-ion peaks can be indexed on the orthorhombic structure with thepace group Pnmb and there are no impurity phase peaks. Theharacteristic peaks of LiFePO4 are formed at the temperaturef 600 ◦C, which is close to the crystallization temperature ofiFePO4 that has been reported to be ∼567 ◦C [18].

The lattice parameters and the calculated crystallite sizeD1 3 1) based on the (1 3 1) diffraction peaks for six samplesre summarized in Table 1. The crystallite size was calculated

sing the Scherer formula: D = kλ/β cos θ, where k is 0.9, λ is.15406 nm and β is the full width at the half maximum length ofhe diffraction peak on a 2θ scale, respectively. Though the lat-ice parameters of all the samples have almost the same values,

d particle size range for LiFePO4 and LiFePO4/C samples prepared at different

Crystallite size D1 3 1 (nm) Particle size (nm)

c (nm)

0.4689 38 90–2300.4687 49 120–3500.4686 75 150–800

0.4691 32 80–1700.4688 41 80–3000.4687 56 100–500

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he D1 3 1 values of LiFePO4 prepared at 500, 600 and 700 ◦C

re 38, 49 and 75 nm, respectively, and 32, 41 and 56 nm forhe corresponding LiFePO4/C samples. It is found that the crys-allite size increases as sintering temperature and the crystalliteize of the LiFePO4/C sample is smaller than that of the corre-

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ig. 2. SEM of samples prepared at different temperatures (a)–(c) LiFePO4 at 500espectively.

cta 53 (2007) 1939–1944 1941

ponding LiFePO4 sample, which conforms that carbon inhibits

he particle growth to a considerable extent.

Fig. 2 shows SEM images of the powders. Near-spherical,ano-meter sized particles with minimal agglomeration arebtained for both LiFePO4 and LiFePO4/C. The particle sizes

, 600 and 700 ◦C, respectively; (d)–(f) LiFePO4/C at 500, 600 and 700 ◦C,

1942 F. Gao et al. / Electrochimica Acta 53 (2007) 1939–1944

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Table 2Carbon content in various LiFePO4/C composite powders prepared at 600 ◦C

Added glucose content (wt.%) Residual carbon amount (wt.%)

0 01.7 1.153.4 2.625.1 3.816.8 5.508.5 7.18

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crlbon deliver high initial discharge capacities of similar value.The LiFePO4/C material coated with 3.81 wt.% carbon givesthe highest capacity, viz., 137.2 mAh g−1.

ig. 3. Initial charge and discharge capacities of LiFePO4 and LiFePO4/C sam-les prepared at different temperatures (a) 500 ◦C, (b) 600 ◦C and (c) 700 ◦C at/5 rate.

estimated from SEM analysis, given in Table 1) of the LiFePO4amples obviously increase as sintering temperature from 90o 230 nm at 500 ◦C, 120–350 nm at 600 ◦C to 150–800 nmt 700 ◦C. Compared with LiFePO4, LiFePO4/C particles aremaller in size due to the inhibition of crystal growth to a greatxtent by the presence of carbon in the reaction mixture, and thearbon coating in LiFePO4/C provides a comparatively rougherurface texture for the particles. The measured residual carbonontent is 3.81 wt.%, which originates from pyrogenation oflucose and PVA. The specific surface area of the LiFePO4/Camples prepared at 500, 600 and 700 ◦C is 22.5, 19.6 and3.7 m2 g−1, respectively. The decrease of specific surface areas the increasing temperature results from the greater particleizes.

The electrochemical performance in terms of the initialharge–discharge capacities of LiFePO4 and LiFePO4/C pre-ared at different sintering temperatures are compared in Fig. 3.he LiFePO4/MCMB batteries were measured at room tem-erature and at C/5 rate. All the samples exhibit a flat voltagelateau at ∼3.3 V, which is the main characteristic of thewo-phase reaction in the electrode. The discharge capaci-ies for LiFePO4 prepared at 500, 600 and 700 ◦C are 86.2,24.5 and 110.6 mAh g−1, respectively, and 113.1, 139.4 and32.5 mAh g−1 for the corresponding LiFePO4/C samples.iFePO4/C with conductive carbon coating exhibits higher per-

ormance compared with uncoated counterparts. The samplerepared at 600 ◦C, which has small particles with uniformorphology, is found to be the optimum. The 500 ◦C sample

erforms least, although it has a particle size comparable withhat of the 600 ◦C sample. The lower utilization of the 500 ◦Cample could be due to its lower level of crystallinity. The largerarticles of the 700 ◦C sample result in a lower performance thanhe 600 ◦C sample with the optimum particle size and purityevel.

By altering the heat treatment, it has been found that the resid-

al carbon from the precursor hinders the growth of particles.o extend the investigation of the effect of carbon content on theroperties of materials, the heat treatment at 600 ◦C has beenmployed with control of the glucose content in the precursor.

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ig. 4. Discharge capacities at 1C rate and tap densities of LiFePO4/C coatedith different carbon content.

ix samples were prepared with 0, 1.7, 3.4, 5.1, 6.8 and 8.5 wt.%lucose added to the raw materials. The amount of remainingarbon in the products is listed in Table 2.

Fig. 4 shows the effect of carbon content on the dischargeapacities at 1C rate and tap densities of LiFePO4/C. As theesidual carbon content increases, the tap densities becomeess and samples coated with 2.62, 3.81, and 5.50 wt.% car-

ig. 5. Cycling performance at 1C rate for LiFePO4/MCMB batteries.iFePO4/C composites as cathode materials are synthesized with: (a) 0 wt.%;

b) 1.7 wt.%; (c) 3.4 wt.%; (d) 5.1 wt.%; (e) 6.8 wt.%; (f) 8.5 wt.% glucose,espectively.

F. Gao et al. / Electrochimica Acta 53 (2007) 1939–1944 1943

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Fig. 7. Discharge curves of lithium-ion batteries with the 3.81 wt.% carbon-coating LiFePO4/C cathodes at different current densities (C/5, 1C, 5C and10C).

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dccaaiIin Table 3, the nano-sized LiFePO4/C composite synthesizedvia the spray-drying method exhibited comparable dischargecapacities even at higher rates in our experiments. It is con-sidered that the smaller particle size and intimate contact of

Table 3The discharge capacities at the high current densities from different sources ofLiFePO4/C

LiFePO4/C Rate Capacity (mAh g−1)

Franger et al. [3] 5C 125Meligrana et al. [13] 10C 115

ig. 6. The CV curves of the 3.81 wt.% carbon-coating LiFePO4/C electrodes.

Fig. 5 presents the cycle performance of LiFePO4/C com-osites prepared with 0, 1.7, 3.4, 5.1, 6.8 and 8.5 wt.% glucose.iFePO4/C materials prepared with 5.1 and 6.8 wt.% glucosexhibit the excellent reversibility during cycling duo to the bet-er electronic conductivity and lithium-ion diffusion. Samplesrepared from a precursor with a low glucose amount haveiminished capacity retention, especially when using a pre-ursor without glucose. In the absence of glucose, an initialischarge capacity of 95.2 mAh g−1 decreases 65.7 mAh g−1

fter 50 cycles with severe capacity fading. When glucose con-ent increases to 8.5 wt.%, a small discharge capacity fadingf LiFePO4/C is observed. A slightly reduced discharge capac-ty originates with the thicker carbon coating, which enhanceshe difficulty of lithium-ion diffusion through the thick carbon-oating layer and hinders electrolyte from penetrating intolectrode.

Further studies of LiFePO4/C with 3.81 wt.% carbon pre-ared at 600 ◦C have been undertaken. Fig. 6 shows CV curvef the LiFePO4/C electrode cycled between 2.0 and 4.2 V versusi+/Li at a scan rate of 0.05 mV s−1. During the anodic sweep,

ithium ions are extracted from the LiFePO4 structure. An oxida-ion peak is located at 3.52 V versus Li/Li+. When the potentials scanned from 4.2 to 2.0 V, a reduction peak occurs at 3.26 V,orresponding to lithium insertion into the LiFePO4 structure.he peak profiles are symmetric, and after 10 scanning cycles,

he redox peaks remain in similar magnitude, indicating goodtability of the electrode. However, it should be noticed that thehape of the peaks is less sharp at its summit and more rounded,ndicating the kinetic restriction of the lithium diffusion in thehick electrode.

The electrochemical rate performance and cycleability ofiFePO4/C are shown in Figs. 7 and 8. At C/5 rate, we canbtained the initial discharge capacity of 139.4 mAh g−1, whilet 1C rate, the capacity is still 137.2 mAh g−1 and for more severeonditions, we have, respectively, 133.5 and 127.3 mAh g−1

t 5C and 10C. With increasing current density, the dischargeapacity and the plateau voltage are rapidly reduced due to the

ntrinsic lithium-ion diffusion limitations of the material. At theeginning of discharging at 10C rate, the plateau voltage is dis-inctly reduced. As the discharge proceeds, the temperature ofhe batteries quickly increases, which enhances the lithium-ion

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ig. 8. Cycle performance of lithium-ion batteries with the 3.81 wt.% carbon-oating LiFePO4/C cathode at different current densities (C/5, 1C, 5C and 10C).

iffusion and raises the plateau voltage. Nevertheless, a goodycling property is shown at the different current densities. Theapacity retention of LiFePO4/C is almost 100% after 50 cyclest C/5 rate, even after 50 cycles at the high current density of 10C,discharge capacity of 117.7 mAh g−1 is obtained (92.4% of its

nitial value) with only a low capacity fading of 0.15% per cycle.n comparison to the results reported by other research groups

ominko et al. [15] 5C 120

ur samples 5C 13210C 127

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he LiFePO4 particles with carbon would result in a relativelyuperior cycleability and rate capability.

. Conclusions

Spray-drying and post-annealing method is used to synthe-ize nano-sized LiFePO4 and LiFePO4/C cathode materials.he influence of the sintering temperature on the structure,article size, morphology and electrochemical performance ofiFePO4 and LiFePO4/C cathode material is investigated. Theptimum processing conditions are found to be thermal treat-ent for 10 h at 600 ◦C. The effect of the amount of carbon

n the electrochemical properties of LiFePO4/C powders haslso been evaluated. Compared with LiFePO4, the 3.81 wt.%arbon-coating LiFePO4/C prepared at 600 ◦C displays highererformance and exhibits initial discharge capacities of 139.4,37.2, 133.5 and 127.3 mAh g−1 at C/5, 1C, 5C and 10C rates,espectively. In addition, it shows excellent cycle stability atifferent current densities. Even after 50 cycles at the high cur-ent density of 10C, a discharge capacity of 117.7 mAh g−1 isbtained (92.4% of its initial value) with only a low capacityading of 0.15% per cycle.

cknowledgements

We acknowledge Great Power Battery Co. Ltd., for supplyings experimental equipments. This work was supported by thecience and technology research item of Guangzhou, P.R. ChinaNo. 2007Z3-D0021).

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cta 53 (2007) 1939–1944

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