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Materials Chemistry and Physics 140 (2013) 659e664

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Synthesis and characterization of olivine phosphate cathodematerial with different particle sizes for rechargeablelithium-ion batteries

Raza Shahid, Sevi Murugavel*

Department of Physics and Astrophysics, University of Delhi, Delhi 110007 India

h i g h l i g h t s

� A facile method for the synthesis of LiFePO4 with different particle sizes.� The consequences of phonon confinement are discernible in the vibrational spectra of LiFePO4.� The variation of the optical band gap and dc electrical conductivity with particle size is due to the size effect.

a r t i c l e i n f o

Article history:Received 19 December 2012Received in revised form19 March 2013Accepted 19 April 2013

Keywords:Inorganic compoundsPowder diffractionElectron microscopyElectrical conductivity

* Corresponding author.E-mail address: murug@physics.du.ac.in (S. Murug

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.04.020

a b s t r a c t

A different solid state preparation of undoped cathode material LiFePO4 (LFP) olivine compound has beensynthesized at low temperatures with an emphasis toward understanding the size dependent structuraland transport properties. The phase purity, size and structural properties of as prepared sample havebeen characterized by various analytical techniques. These studies confirm that the obtained LFP ofdifferent particle sizes are without any secondary phases. Additionally, the spectroscopic studies on thesesamples reveal that the observed vibrational modes are characteristic part of LFP samples and theconsequences of phonon confinement are discernible when the particle size becomes less than 200 nm.We substantiate that the measured optical band gap and electronic conductivity are the intrinsic part ofLFP samples. The room temperature electronic conductivity and optical band gap shows the systematicdependence on particle size. We ascribe the variation of the optical band gap and dc electronic con-ductivity with particle size is due to the quantum confinement effects.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The lithiated olivine phosphates are widely attracted new-generation of cathode material for Li-ion rechargeable batteriesowing to its numerous advantages such as high energy density, lowcost, and environmental friendliness [1]. Within the family ofolivine phosphate structures, the LiFePO4 (LFP) is the mostcommonly preferred cathode material than others because of itshigh gravimetric energy density (theoretical specific charge ca-pacity of 170 mAh g�1), which enables it to substitute the toxic andexpensive LiCoO2 material usually employed in commerciallithium-ion batteries. During lithium-ion extraction and insertion,the active material undergoes a two-phase transition betweenLiFePO4 and FePO4, with a flat voltage profile of about 3.4 V versusLi/Liþ. From the safety point of view, the LFP has more technologicaladvantages than LiCoO2 and other metal-oxide cathode materials.

avel).

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Additionally, the LFP cathode materials lie in its tolerance to over-charge/discharge and less prone to thermal runaway [2,3]. In spiteof its superior performances, still LFP material has to overcomesome of the barriers for the practical applications. Among thevarious issues, the most important concern is its low intrinsicelectronic conductivity as well as slow lithium-ion diffusion acrossthe LiFePO4/FePO4 phase boundary during charge/discharge pro-cess limits the rate performance of these materials. Both the deli-thiated and lithiated phases are wide-gap insulator that limits thehopping transport of electrons between the Fe2þ sites. The slowdiffusion rate of Li-ion has been attributed to the variety of mate-rials properties such as large miscibility gap, dimensionality, andnature of defect sites [4e8].

In order to improve the electronic conductivity, various ap-proaches like carbon coating around the particles, nanosizing, off-stoichiometric synthesis and, aliovalent doping into the olivinestructure has been carried out [9e13]. It is known that in LFP thepresence of trace amount of carbon/Fe2P/FeP can also lead toenhanced electronic conductivity [11,14]. However, in this context,

R. Shahid, S. Murugavel / Materials Chemistry and Physics 140 (2013) 659e664660

it is worth to mention that only very few reports have revealedabout the presence of impurity phases and others not [11e15].Additionally, various synthesis procedures are developed to mini-mize the particle size and to obtain a uniform size distributionwitha minimal impurities [16e23]. In spite of different synthesismethods developed recently, still the most commonly preferredroute is solid state synthesis due to its straightforward procedures.In fact, some of the reported methods are unrealistic to expand tolarge scale applications owing to complicated synthesis conditionsinvolved in it [16e23]. Therefore, it is critical to develop facile andefficient synthesis routes for the practical application of LFP ma-terials. Recently, it has been reported that the LFP prepared in asingle step at lower temperatures resulted with a significantamount of impurity phases of Fe3O4 [24].

In this contribution, we report the facile synthesis of LFP sam-ples in a single step by solid state reaction without any traceamount of secondary phases with different particle sizes. In orderto reach better performance characteristics of LFP at ambienttemperature, the work has been focused toward obtaining nano-sized particle as in this regime various new properties emerges.Nevertheless, the reliability of the electronic transport in LFP re-quires elimination of secondary phases completely through theappropriate synthesis procedures, as we do here. A simple corre-lation between optical band gap, electronic conductivity and theparticle size has been observed. The variation of the optical bandgap and dc electronic conductivity with particle size is attributed tothe quantum confinement effects.

2. Experimental

LiFePO4 was synthesized by a facile and modified solid statesynthesis procedure using lithium carbonate (Li2CO3) (SigmaAldrich, 99.997%), iron (II) oxalate dehydrate (FeC2O4$2H2O) (SigmaAldrich, >99.99%) and ammonium di-hydrogen phosphate(NH4H2PO4) (Sigma Aldrich, 99.999%) in stoichiometric ratio. Theprecursors were weighed, thoroughly mixed and grinded in anagate mortar and pestle without any additive for 4 h to ensure thehomogeneity of mixture. The fine powder thus obtained was keptin a furnace at 550 �C for 12 h in a reducing atmosphere (N2 with10% H2) and then cooled down to the room temperature and it gaveus average particle size 150 nm. Additionally, we have obtainedthree other particle sizes (450 nm, 750 nm and 1000 nm) bychanging synthesis parameter and are listed in Table 1. The powderthus obtained was of light brown in color showing the absence ofFe3þ impurities in the present investigation.

Structural analysis and phase formation of all the four sampleswere confirmed by a powder X-Ray Diffractometer (XRD) usingRigaku MINIFLEX II series equipped with Cu Ka radiation source(l ¼ 1.5406�A) in steps of 0.02� with a scan rate of 3� min�1. To gainfurther insight into site occupancies and to extract appropriatemicrostructural information from the diffraction pattern, carefulRietveld analysis of XRD data were carried out with the FullProfsuite software. Further phase purity of the samples was confirmed

Table 1The synthesis temperature, lattice parameters and R-factors as obtained fromrefinement for different particle sizes.

Particle size(nm)

Temperature(K)

Lattice parameters (�A) R-factors(%)

a b c Rwp RB

150 823 10.31196 5.99527 4.68829 19.8 1.19450 973 10.30415 5.99521 4.69336 17.8 1.48750 1023 10.30259 5.99432 4.69293 17.6 1.191000 1023 10.31090 5.99730 4.69299 18.3 1.20

by Raman and FTIR Spectroscopy. The room temperature Ramanspectra of LFP samples were recorded in the 180� back scatteringgeometry, using a 785 nm excitation of air-cooled Argon Ion laser(Renishaw InVia Reflex Micro Raman Spectrometer). The spec-trometer was equipped with a single monochromator with aPeltier-cooled CCD (Renishaw InVia Reflex). Spectral resolution ofthe instrument was better than 1 cm�1 and the incident power wasadjusted to 3 mW for all the samples. Fourier Transform Infra-RedSpectroscopy (FTIR) was recorded with Perkin Elmer FTIR systemspectrum GX in transmittance mode. The IR spectra’s were run onKBr Pellets with weight ratio of sample to KBr of 1:100 and thespectral resolution of 4 cm�1. The morphology and size of all thefour samples were characterized by the FEI Tecnai� TransmissionElectron Microscope (TEM) (TECNAI G2 T30 U-TWIN) and had beenattached with a double-tilt holder (�70�) along with an EDXanalyzer under an accelerating voltage of 300 kV (with the reso-lution of 0.19 nm). The images were recorded using a CCD camera(Gatan) and Fourier transform (FT) patterns have been conductedusing a digital micrograph (Gatan).

The electrical conductivity measurements were carried out onpellets of 13 mm diameter and thickness of 1 mm sputtered withsilver target onto each surfaces by using a Novocontrol a-S high-resolution dielectric analyzer in a frequency range from 10 mHzto 1 MHz. Diffuse reflectance measurement was done using aThermoscientific Absorption Spectrophotometer in the wavelengthrange of 200e1100 nm to calculate the optical band gap and tovalidate the obtained electrical conductivity data of different par-ticle sizes.

3. Results and discussion

3.1. XRD studies

Fig. 1 shows the typical XRD pattern of the LFP sample withvarying particle sizes in the present investigation. The preparedsamples are almost free from any impurity phases and the com-positions that are close to the stoichiometric, which show onlyreflections due to the single-phase LFP. Further information hasbeen obtained by carrying out the Rietveld-refinement analysis andextracted the unit cell parameters of all the particle sizes are listedin Table 1. In Fig. 2, we present the experimental powder XRDpattern (filled square) of 1000 nm and 750 nm sample comparedwith its Rietveld-refined profile (solid line) and the difference

Fig. 1. Room temperature XRD pattern of LFP samples with different particle sizes.Increase in the peak sharpness is observed with increase in particle size.

Fig. 2. The experimental powder XRD pattern (filled square) of LFP with particle size of(a) 1000 nm and (b) 750 nm sample compared with its Rietveld-refined profile (solidline) and the difference curve (bottom curve) taken at room temperature. The verticalmarkers below the diffraction pattern indicate positions of possible Bragg reflections.

Table 2Crystallite size calculated using Scherrer formula for various particle sizes along fourdifferent facets.

Particle size (nm) Crystallite size (nm)

(101) Facet (111) Facet (211) Facet (311) Facet

150 33 30 32 30450 40 41 41 42750 50 45 46 461000 58 56 56 59

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curve. All the XRD patterns are entirely indexed in the space groupof Pnma and we obtained lattice constants a ¼ 10.310(9) �A,b ¼ 5.997(3) �A, c ¼ 4.692(9) �A for 1000 nm particle size. The esti-mated lattice constants are in excellent agreement with the liter-ature and it corresponds to the orthorhombic structure [25]. Theparticle size dependence can also be seen on the obtained XRDpattern, where an increase in particle size leads to decrease in thepeak width suggesting that the growth of grains size [26,27]. Inaddition, the crystallite size have been calculated by using theScherrer relation along the four different planes (viz. 101, 111, 211,311) for each particle and are listed in Table 2.

The surface morphology and shape of all the particle sizes havebeen studied by TEM and we present representative of twodifferent particle sizes in Fig. 3. Based on the TEM analysis, theaverage particle sizes were identified as of 1000 nm, 750 nm,450 nm and 150 nm. Additionally, selective area electron diffraction(SAED) pattern has been carried out on the two representativesamples (150 and 450 nm) and are illustrated along with TEMimages. The bright spots represent the diffraction pattern andconfirm the crystallinity of our as prepared samples. In addition, it

is clear that particles are oriented along a particular zone axis. Theestimated crystallite size variations are consistent with the particlesize revealed by TEM measurements. Thus, we confirm that theobtained particle sizes are homogeneously distributed and esti-mated lattice parameters are similar to those reported in theliterature [25].

3.2. FTIR studies

In order to obtain the local molecular structure and to indentifythe presence of any impurity phases in LFP, we have characterizedall the particle sizes by FTIR in the spectral range of 150e1300 cm�1

shown in Fig. 4. It is interesting to note that the observed vibra-tional bands are analogous to the earlier work on LFP [28,29]and confirms to be intrinsic vibrational modes of LFP materialexpect minor changes in the intensity level. Additionally, we noticethat a systematic shift in band positions at low/high frequencieswhile varying particle size. It is known that the bands in therange 400e650 cm�1 are bending modes (n2 and n4) involve mainlyOePeO symmetric and anti-symmetric part as well as Li-ion vi-brations. In particular, the band at 396 and 547 cm�1 corresponds totranslations of vibration of lithium-ions within the cage of PO4

3�

ions, which shows the pronounced changes in the band widthwhile varying the particle size. On the other hand, the stretchingmodes (n1 and n3) involve the symmetric and anti-symmetric partof the PeO bonds and this accounts for the spectra observed in theregion 900e1200 cm�1. In this case, the vibrational modes areoriginated from the intra-molecular vibrations of the PO4

3� anioni.e. the internal vibrations of phosphate units and the displacementinvolve with the oxygen atoms. Furthermore, we do not observeany other vibrational bands that are associated with Li3PO4, P2O7 orP3O10, which reflect the absence of phosphate related impurityphases in the present investigation. It is interesting to note from theIR spectra that the vibrations associated with internal modesbecome sharper with decreasing the particle size. In contrast, weobserve opposite behavior in the external optic modes where theybecome broadened. Therefore, the effect of confinement on theoptical phonons clearly shows distinct behavior in the internal andexternal modes.

The effect of confinement on internal and external modes canalso be observed in FTIR spectra with different particle sizes. Thehigh frequency region corresponding to internal vibrations of PO4

3�

anion and do not show any change or shift in the band positionswith particle size. On the other hand, the bands in lower frequencyregime showa significant change in band position.With decrease inparticle size, the band at 547 cm�1 shows the pronounced broad-ening and suggest that the lesser interactions of Li-ions which canbe accounted only if Li-ions have increased number of defects orinterstitials which in turn will help in increasing ionic conductivityaspect as already suggested by Islam and Ceder [6,8]. Furthermore,the band at 396 cm�1 is involved with Li-ion motion and it showssignificant shift of about 10 cm�1 for 150 nm samplewith respect to1000 nm particle size. These results indicate that energy require-ment for Li-ionmotions become reduced and thereby enhancement

Fig. 3. TEM images and SAED pattern of representative two different particle sizes, (a) 450 nm and (b) 150 nm.

R. Shahid, S. Murugavel / Materials Chemistry and Physics 140 (2013) 659e664662

in the Li-ion conductivity, a positive aspect with respect to cathodematerial at reduced particle size. Similarly, other two low frequencybands viz. 348 and 193 cm�1 corresponds to Fe2þ motion can beseen in 1000 nm particle size but it diminishes with decreasing the

Fig. 4. FTIR transmittance spectra of LFP samples with different sizes recorded at roomtemperature.

particle sizes. Since, the Fe2þ related defects increaseswith decreasein the particle size, which could have negative impact on thelithium-ionmotion for particle size below200 nm. These results arein accord with the theoretical model predictions that iron relateddefect sites would block the Liþ conduction pathway [8]. The mostinteresting feature emerge from the FTIR data with lower particlesizes that the band at 175 cm�1 corresponding to phonon vibrationsof LFP and it disappears with increasing the particle size [29]. Withan enhanced thermal vibration for the lower particle size can lead tothe generation of more surface defects, which could be of formationof LieFe antisite defect pairs.

3.3. Raman spectroscopic studies

The Raman spectroscopic studies on the investigated samplesconfirm the absence of any impurity phases as well as any otherprecursors in the obtained samples. The room temperature Ramanspectra of LFP with different particle sizes are illustrated in Fig. 5a.The nature and behavior of the vibrational modes for the typicalolivine structure have been studied and described earlier [29e35].The olivine structure belongs to spectroscopic group of D2h

16, wherethe primitive cell is centrosymmetric with formula units of PO4

3�

and all the cations are distributed on octahedral 4a, 4c and tetra-hedral 4c sites (Wyckoff notation), respectively. In olivine phos-phates, the Raman active vibrational modes can be divided into twoclasses, i.e. internal and external modes with respect to the PO4

3�

unit. The internal modes are mainly due to the intra-molecularvibrations of PO4

3� units, which correlate between the motions ofthe phosphate anions within the unit cell. Thus, the resultingRaman spectra is highly dominated by the stretching and bendingmodes of PO4

3� units and are being located in the high frequencypart of the spectrum. Experimentally, we observe in the presentstudy the intense peak at 950 cm�1 is due to the symmetricstretching vibrations, while the two other bands at 998 and1075 cm�1, respectively, and are correspond to an anti-symmetric

Fig. 5. a. The Raman spectra of different sized LFP samples at room temperature. Thepeaks corresponding to FeeOeP and PO4

3� group are marked. b. Comparison of Ramanspectra of LFP samples in a narrow scale showing the change in peak width as afunction of particle sizes.

Fig. 6. The room temperature dc electronic conductivity and an optical band gap of LFPsample with different particle sizes.

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stretching vibration. It is known that the confinement results inasymmetric broadening and shift of the optical phonon Raman line,as we observe here.

At low frequencies, the vibrational modes are due to theexternal one, or lattice vibrations, occur below 700 cm�1. Thesemodes are the vibrations of the entire lattice as observable at lowfrequencies z400 cm�1 mainly due to the translational motions ofPO4

3� and Fe2þ ions [28,29]. The vibrational bands at 575 and630 cm�1correspond to FeeO stretching and 395 and 450 cm�1

represent the FeeOeP bending modes show the significant differ-ence in the intensity with particle size. It has been suggested thatthe motion of Li-ions gives very weak contributions at the lowfrequencies; however, we cannot simply ignore the strengtheningof band at 300 cm�1 with decreasing the particle size. The decreaseof particle size leads to increase of surface energy contributions,which leads to the formation of more number of defects and areavailable for the lithium-ions. Therefore, we suggest that the bandat 300 cm�1 is due to the lithium-ion motions coupled with PO4

3�

ions and it increases with decreasing the particle size. Thus, allthese features suggesting that the broadening of Raman bands(Fig. 5b) and growth of band at 300 cm�1 with decrease in particlesize provides an evidence of decrease in lifetime of phonons and an

existence of defects in the lattice sites within the LFP crystallites.Additionally, it is important to note that the presence of carbon inthe samples can be seen by the existence of small humpz1350 cm�1 and its intensity increases with decrease in the par-ticle size. However, the carbon content does not penetrate or makeany chemical bond with LFP, as we see from the dc electrical con-ductivity or optical band gap studies on the LFP sample because thetotal amount of carbon is itself less than 1 wt% of LiFePO4.

3.4. Particle size dependent electronic properties

In Fig. 6 we illustrate the measured dc electronic conductivity ofLFP samples at 313 K for four different particles and it varies be-tween 3 � 10�11 and 2 � 10�10 S cm�1. The measured dc conduc-tivity values are in agreement with some of the earlier works[10,11,36] and deviate with others [37]. The reason for the differ-ence in the dc electronic conductivity of LFP sample can be attrib-uted to various factors such as carbon content, metallic impurityphases etc. However, in the present work we do not observe anysuch contribution to the measured dc electronic conductivity of LFPsamples, which is then intrinsic property of LFP. In olivine phos-phates, the charge transport takes place by small polaron migrationgenerated by hopping of charge carrier results the Fe2þ to Fe3þ.

It is clear from the Fig. 6 that the dc conductivity decreases withdecreasing the particle size. Additionally, to support our electricalconductivity measurements on LFP samples, we have measuredoptical band gap by using diffuse reflectance spectroscopy. Themeasured optical band gap of LFP samples with different particlesize is depicted in Fig. 6. Remarkably, it is observed that the opticalband gap increases with decreasing the particle size and iscorroborate with dc electrical conductivity data. Furthermore, it isworth to mention that the measured optical band gap values aresmaller than those reported in the literature [38,39]. Thus, both theelectrical conductivity and optical band gap obtained from thepresent investigation are an intrinsic property of LFP sample andtheir variation with particle size is ascribed to quantum confine-ment effect.

Based on the above structural information, the present investi-gation allows us to provide more insights into the dependence ofthe electronic conductivity of the samples with their particle size.The correlations between the particle size and transport propertieswould shed more lights into the design of newer material/composition with improved performance characteristics. We sug-gest that the charge (electron/hole) transport in LFP is ascribed inthe form of localized small polarons rather delocalized characterdue to the hopping of charge carrier results the Fe2þ to Fe3þ.

R. Shahid, S. Murugavel / Materials Chemistry and Physics 140 (2013) 659e664664

It is likely that the electron transport involved in LFP is not delo-calized character rather localized small polarons. More traditionally,the polaron problem has been investigated in three-dimensionalsolid state system based on Holstein model [40]. However, in lowdimensional systems, due to the strong confinement effect on elec-trons and phonon vibrational modes, the polaron effects becomesmore significant and quite different from those of the bulkmaterials.In the present study, we find that effect on confinement on phononsismore pronounced (DEgopty 1.1 eV) thanpolarons effects (0.5 orderchange in the conductivity). Since, the transition of electron from thevalence band to conduction band is phonon-assisted Fe2þ / Fe3þ

intra-atomic transition at the origin of the optical gap, allowing forthe local distortion in the lattice uponchanging the valence state of Fesite. Recently, theoretical investigations have been carried on thepolarons inn-dimensional crystals and suggest the polaron effectwillbe enhanced as the dimensionality of the system reduces [41]. Theseresults thus suggest that going down in particle size for pure olivineLiFePO4 is not beneficial for the electronic conductivityaspect as it ledto the decrease in conductivity. To conclude more detail about thecharge transport mechanism in LFP sample with their particle sizedependence can only be cleared by studying defect related experi-mental study along with temperature dependent conductivitymeasurements.

4. Conclusions

A facile method has been developed for the synthesis of LFPwith different particle sizes. The prepared samples are found to bedevoid of any secondary phases, which is essential in under-standing the transport mechanism of olivine phase material. TheFTIR and Raman spectroscopy results reveal the observed vibra-tional modes are intrinsic part of LFP samples. Remarkably, theeffect of confinement on the optical phonons is intrinsic and itshows distinct behavior in the internal and external modes withrespect to (PO4)3� ions. The results obtained by impedance spec-troscopy and diffuse reflectance measurement reflect the effect ofparticle size on the measured dc conductivity and band gap and arecomplement with each other. The variation of dc conductivity andoptical band gap reveals the systematic trend and their dependenceon particle size are ascribed to the quantum confinement effect.

Acknowledgments

One of the authors, RS thanks CSIR for NETeJRF fellowship. Wealso thank the Department of Science and Technology (DST), Indiaand R&D Scheme of Delhi University for the financial support. Wethank Dr. R. Nagarajan, Department of Chemistry, University ofDelhi for valuable discussions at various stages and timely help inmaking diffuse reflectance measurements on LFP samples. Weacknowledge the M. Tech (Nanoscience) and USIC, University ofDelhi for providing the various characterizations facility during thecourse of work.

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