Journal of Power Sources 192(2009)660–667
浦东公租房Contents lists available at ScienceDirect
Journal of Power
Sources
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j p o w s o u
r
Formation mechanism of LiFePO 4/C composite powders investigated by X-ray absorption spectroscopy
Kuei-Feng Hsu a ,b ,Shao-Kang Hu a ,b ,Chinh-Hsiang Chen a ,Ming-Yao Cheng a ,Sun-Yuan Tsay b ,T-Chuan Chou b ,Hwo-Shuenn Sheu c ,Jyh-Fu Lee c ,Bing-Joe Hwang a ,c ,∗
a
不相上下的意思Department of Chemical Engineering,National Taiwan University of Science &Technology,43Keelung Road,Sec.4,Taipei 106,Taiwan b
Department of Chemical Engineering,National Cheng Kung University,No.1,Ta-Hsueh Road,Tainan 701,Taiwan c
National Synchrotron Radiation Rearch Center,Hsinchu 300,Taiwan
a r t i c l e i n f o Article history:
Received 30December 2008
Received in revid form 19February 2009Accepted 24February 2009Available online 9March 2009Keywords:Li ion battery LiFePO 4
Mechanism Cathode XAS
a b s t r a c t
The local structure and oxidation states for both the precursors and the LiFePO 4/C composite powders were investigated by X-ray absorption spectroscopy (XAS)to provide a deep insight into their formation mechanism.It was found that the local structure and oxidation states of the precursors and the syn-thesized LiFePO 4/C powders as well as the electrochemical properties of the synthesized powders were strongly influenced by the R ratio (R :molar ratio of citric acid to total metal ions).The oxidation states of iron ions of the precursors for R =1and 0.75consist mainly of Fe(II)and traces of Fe(III).However,the oxi-dation state of iron ions of the precursor for R =0.5compris mainly of Fe(III).The oxidation state of iron ions of all the synthesized powders is Fe(I
I).The structure of the precursors and the synthesized powders for R =1and 0.75is more ordering than that for R =0.5.It is in good agreement with the obrvation of the cation mixing obtained from the Riteveld analysis of the XRD data.The better the electrochemical perfor-mance is,the more ordering the structure or the less the cation mixing.However,the effect of the R values on the carbon content is also esntial for the electrochemical properties of the synthesized LiFePO 4/C composite powders.Increasing the carbon content leads to the increa in the electronic conductivity but impedes the Li +ion diffusion of the composite materials.Conquently,the powders synthesized at the optimal R ratio of 0.75exhibited the highest initial capacity,about 150mAh g −1when cycled at 1/40C rate at room temperature.The structural scheme of the precursors and the synthesized powders and the formation mechanism of the LiFePO 4/C composite powders are also addresd in this work.
©2009Elvier B.V.All rights rerved.
1.Introduction
Since the demonstration of electrochemically reversible lithium inrtion–extraction for LiFePO 4in 1997[1],lithium transition metal phosphate with an ordered olivine structure,LiMPO 4(M =Co,Ni,Mn,F
e,Cu)has attracted much attention as promising new cathode materials for rechargeable lithium batteries [2–7].One of the most promising candidates for rechargeable lithium batteries is LiFePO 4.Moreover,lithium can be extracted from LiFePO 4and inrted into FePO 4along with a flat potential plateau at 3.5V vs.Li/Li +together with a theoretical specific capacity of 170mAh g −1[1].LiFePO 4is inexpensive,nontoxic,nonhygroscopic,and environ-mentally friendly.It occurs in nature as the mineral triphylite,has an orthorhombic unit cell (space group Pnma).Both Li and Fe atoms are
∗Corresponding author at:Department of Chemical Engineering,National Taiwan University of Science &Technology,43Keelung Road,Sec.4,Taipei 106,Taiwan.Tel.:+886227276624.
E-mail address:ust.edu.tw (B.-J.Hwang).in octahedral sites with Li located in the 4a and Fe in the 4c positions.The O sites form a nearly tetrahedral arrangement about P sites and also form an approximately octahedral arrangement about each Fe site.It is apparent that there are channels along the b -axis which accommodate mobile Li +ions [8,9].When Li +ions and electrons are removed from LiFePO 4,the remaining FePO 4framework has same structure,with a small (7%)reduction in volume [10,11].The volumetric expansion and the structural charge during Li +interca-lation are low compared to other intercalation compounds,which is advantageous for the ac
hievement of a high cycle life.
XAS is a powerful technique to probe both electronic struc-ture and local structure of new materials.Information about the valence state of the investigated element and its electron configura-tion can be obtained from the X-ray absorption near-edge structure (XANES)region,whereas the extended X-ray absorption fine struc-ture (EXAFS)region can provide the local structure of absorbing atom.Recently,some groups have investigated the oxidation state and local structure of the LiFePO 4powders by means of X-ray absorption spectroscopy (XAS)[12–15].The features in the region of the 1s →3d transition have been shown to be very nsitive to the
0378-7753/$–e front matter ©2009Elvier B.V.All rights rerved.doi:10.1016/j.jpowsour.2009.02.076
K.-F.Hsu et al./Journal of Power Sources192(2009)660–667661
oxidation state and geometry of the iron atom in the XANES region [16].X-ray absorption spectroscopy has been ud to determine the structural variation of Li x FePO4electrode material during cycling. They have reported that Fe ions are octahedrally coordinated and in the Fe2+state.Even after repeated charging and discharging, the structure of the LiFePO4cycled electro
des did not change.The preparation conditions may change the local structure of precur-sors and LiFePO4powders which will affect their electrochemical properties significantly.Unfortunately,no report has studied the oxidation state and local structure of precursors and LiFePO4pow-ders prepared at various conditions to our knowledge.It is also of great interest to understand the relationship between the structure of synthesized powders and their corresponding precursors.
The LiFePO4/carbon composite cathode materials were success-fully synthesized by our developed sol–gel process[17,18],which is not only easy but also low cost.The much cheaper precursor,iron powder instead of iron salt was employed in our developed process. The citric acid in the developed process plays the role not only as a complexing agent but also as a carbon source,which improves the conductivity of the composites and hinders the growth of LiFePO4 particles.The nano-sized LiFePO4particles without the impurity pha have been successfully synthesized.
It is of great importance for the further improvement of the performance of LiFePO4powders to investigate its formation mech-anism in our developed process.Meanwhile,it is also interesting to study the role of the molar ratio of citric acid to total metal ions(R) in the process.This work provides a deep insight into the forma-tion mechanism bad on the local structure and oxidation states of not only the precursors but also the LiFePO4/C composite powders which are prepared at various R val
ues are investigated by employ-ing X-ray absorption spectroscopy(XAS).The effects of the R values on the local structure and electrochemical properties of LiFePO4/C are also discusd.
2.Experimental
2.1.Synthesis
A sol–gel route using iron powders as starting materials was employed to obtain the LiFePO4cathode materials.The detailed procedures ud in this work for the sol–gel preparation of the materials have been described in our previous papers[17–19]. LiFePO4samples were synthesized by a sol–gel method using cit-ric acid as a chelating agent[17,18].Stoichiometric amounts of iron powder and lithium nitrate were dissolved into an aqueous solu-tion of saturated citric acid with continuous stirring.Then10ml of a saturated aqueous solution of ammonium dihydrogen phosphate was added.The mixtures were heated gently with continuous stir-ring for4h to remove excess water.The resulting gel precursor was dried in a circulation oven for a week at60◦C.The precursors were further calcined at850◦C in99.999%nitrogen atmosphere for2h. The heating rate of the furnace was10◦C min−1.The precursors and the calcined powders were prepared at various molar ratios of cit-ric acid to total metal ions(R)of1,0.75,and0.5for the further XAS investigation.
2.2.Characterization
2.2.1.Carbon content
To determine the carbon content,elemental analysis was per-formed(EA,Heraeus CHN-O Rapid Analyzer)for the powders prepared at various R values.
2.2.2.XRD
X-ray diffraction patterns of the synthesized powders were col-lected for structural analysis.The XRD data were obtained over an angular2Ârange from10◦to60◦with a step size of0.01◦and a constant counting time of5s per step using a powder X-ray diffractometer(Cu K␣radiation).X-ray Rietveld refinements were performed using the GSAS(General Structure Analysis System)pro-gram to obtain the crystal structure parameters[20].
2.2.
3.X-ray absorption spectroscopy measurement
X-ray absorption spectra(XAS)have been recorded at the beam line BL17C of National Synchrotron
Radiation Rearch Center (NSRRC)at Hsinchu,Taiwan.The storage ring was operated with electron energy of1.5GeV and a current between100and200mA. Data collection was carried out in the transmission mode with a Si(111)double crystal monochromator.High order harmonics was eliminated by adjusting the parallelism of the monochroma-tor crystals.The intensities of incident and transmitted beams were monitored using ionization chambers as detectors.The edge jump of the samples was properly adjusted to improve the S/N ratio of spectra.The measurement for energy calibration was performed in each scan using the Fe foil as a reference which was positioned in front of the window of the third ionization chamber.During XAS measurements,the beam size was limited by the horizontal and vertical slits with the area of2mm×2mm.
2.2.4.EXAFS data analysis
Standard procedures were followed to analyze the EXAFS data. First,the raw data undergo background subtraction and normal-ized.The normalized EXAFS data( (E))was Fourier transformed from energy space to k-space,where k is the photoelectron wave vector.The (k)data described the oscillation of the backscatter-ing wave through the local environment of about∼10Å,which give us the surrounding information for the central atom.For the LiFePO4material,the k3-weighted EXAFS spectra,k3 (k),for the lected absorber Fe,were calculated to compensate the damping of the EXAF
S oscillations in high-k region.Subquently,k3-weighted (k)data in the k-space ranging from3.8to10.6Å−1for Fe K-edge, respectively,was Fourier transformed(FT)to r-space in order to p-arate the EXAFS contributions from different coordination shells.A nonlinear least-squares algorithm was applied for curvefitting of EXAFS in r-space between0.8and4.2Åfor Fe K-edge,respectively.
The structure parameters such as coordination numbers(N), bond distances(R)and Debye–Waller factor were extracted by curvefitting analysis bad on Winxas2.3.The theoretical EXAFS parameters,the backscattering amplitude and the pha shift,were calculated by FEFF7code[21]for all possible scattering paths which were generated from the crystallographic model of the known structures.The amplitude reduction factor S2
was scaled to afixed value of0.72after preliminary refinements.The structure parame-ters describing the local environment for Li1−x FePO4were obtained from the EXAFS data with Winxas2.3and the FEFF7code.In all cas,the values of Residual factor defined by the following equa-tion are less than10,indicating that thefitting error is quite small.
Residual factor(%)=
N
i=1
|y exp(i)−y theo(i)|
N
i=1
|y exp(i)|
×100
where y exp and y theo are experimental and theoretical data points, respectively.
2.2.5.Electrochemical properties
Electrochemical characterization was carried out with coin-type cells.The electrode was prepared by using83%of LiFePO4/carbon active material,11%Super P carbon black,and6%polyvinylidene difluoride(PVdF),as binder,dissolved in N-methyl-2-pyrrolidinone (NMP)solvent.The obtained slurry was then cast on an Al current collector and dried for2h in an oven at100◦C.The resulting elec-
662K.-F.Hsu et al./Journal of Power Sources192(2009)
世界上最诡异的地方660–667
Fig.1.Normalized absorption of Fe K-edge for LiFePO4precursors at various R values, along with the references of FePO4and FeC2O4.
trodefilm was subquently presd and punched into a circular disc.The electrodefilms are prerved in an argon-filled glove box (Unilab,Mbruan).The coin cell was fabricated using lithium metal as a counter electrode.The electrolyte ud consisted of a1M solu-tion of LiPF6in a mixture1:1by volume of ethylene carbonate(EC) and diethyl carbonate(DEC).The parator(Celgard2400,Hoechst Celene Corp.)was soaked in an electrolyte for24h prior to u. All the weighing procedure and coin cell asmbly were performed in an argon-filled glove box by keeping both oxygen and mois-ture level less than1ppm.The charge–discharge measurements were performed on the coin cells using a programmable battery tester(Maccor2300)at different C-rates over a potential range of 3.0–4.0V.
3.Results and discussion
3.1.XANES of the precursors
Here the pre-edge peak of X-ray absorption spectroscopy is the most uful feature to determine the oxidation state and symmetry of Fe sites.The Fe K-edge XANES spectra of the LiFePO4precursors
prepared at various R values and the references of FePO4as well as FeC2O4are shown in Fig.1.According to the literature[16,22],the Fe pre-edge peak around7112eV corresponds to the1s→3d tran-sition,which is a dipole forbidden process for the Fe site of LiFePO4.
A weak absorption peak at this energy is due to the hybridization of Fe3d orbital with O4p orbital(3d–4p mixing).It is sufficient to determine the oxidation state of Fe from its pre-edge feature[22]. Fig.1shows the peak energy for the precursors of R=1and0.75is similar and clo to that of FeC2O4,indicating that their iron oxida-tion state consist mainly of Fe(II)and traces of Fe(III).However,
the Fig.2.k3-Weighted FT spectra of Fe K-edge for LiFePO4precursors at various R values.
higher peak energy(∼7114eV)of the precursor of R=0.5,indicating its iron oxidation state compris mainly of Fe(III).
3.2.EXAFS of the precursors
Fig.2shows the radial structure functions obtained from Fourier transformation(FT)of the k3 (k)functions in the k-space range between3.8and10.6Å−1for the precursors at various R values.The radial structure function indicates the scattering contribution of different atomic shells around the X-ray absorbing iron atoms.The first strong peak corresponds to the scattering process of the ejected electron at the oxygen-containing coordination shell,and the c-ond and the third peaks are contributed from the scattering process of the ejected electron at the phosphorous-and iron-containing coordination shells,respectively.The features of the peaks con-tributed from the Fe–O,Fe–P,and Fe–Fe shells are almost identical for the precursors of R=1and0.75,implying that the local environ-ment of Fe in the both samples are nearly the same.It is suggested that the formation reactions of the precursors are alike during the sol–gel process for R=1and0.75.For R=0.5,the features of the first three peaks change considerably.The FT corresponding to Fe–O shell for R=0.5i
s much higher than that of R=1and0.75,indicating that the local environment around the iron ion is changed dramati-cally for the precursor of R=0.5.It implies that the iron environment for the precursor of R=0.5might be completely different from that for the precursor of R=1and0.75.
The coordination atom of thefirst and the cond shells are con-sidered to be oxygen,and that of the third and the fourth shells are considered to be phosphorous and iron,respectively in thefitting model.The structural parameters of the LiFePO4precursors shown in Table1are obtained from the FEFF-fit analysis of their corre-sponding k3 (k)functions using possible scattering paths in the
Table1
Structural parameters of Fe K-edge for LiFePO4precursors at various R values simulated by four-shell model.
R value Sphere Shell Coordination number(N)Radius R j(Å)Debye–Waller factor 2
种菠萝j
(×10−3Å2)Residual% 11st Fe–O13 1.9110.7 3.53 2nd Fe–O23 2.09 6.6
3rd Fe–P1 3.15 2.9
4th Fe–Fe1 3.758.5
0.751st Fe–O13 1.9211.3 2.35
2nd Fe–O23 2.08 6.2
3rd Fe–P1 3.16 5.0
4th Fe–Fe1 3.738.8
0.51st Fe–O14 1.928.0 3.69
2nd Fe–O24 2.03 5.6
3rd Fe–P1 3.18 4.4
4th Fe–Fe00.000.0
K.-F.Hsu et al./Journal of Power Sources192(2009)660–667
663
Fig.3.FEFF7fit the radial structure function for LiFePO4precursors. model.The experimental Fourier-filtered curve matches very well with the simulated one for the LiFePO4precursor at various R val-ues,as shown in Fig.3.It implies that the propod model is suitable to describe the structure of the precursor.
According to the Pourbaix diagram for iron[23],iron powder would be oxidized to Fe2+while hydrogen ions are being reduced to hydrogen in the citric acid solution in which pH is about2.The generated Fe2+ions will be chelated by citric acid immediately and form a stable complex(Cit3−–Fe2+)if the R value is high enough. However,if the R value is too low,the generated Fe2+ions will be further oxidized to the ferric ions associated with the reduction reaction of the dissolved oxygen and then a Cit3−–Fe3+complex formed instead of a Cit3−–Fe2+one.This formation process of the precursor is depicted in Fig.4.
According to Table1,the Fe–O1,F–O2,Fe–P and Fe–Fe distances are1.91,2.09,3.15and3.75Å,respectively and their corresponding coordination numbers are3,3,1and1,respectively,for the precur-sor of R=1.It indicates that the iron site is octahedrally coord
inated by6oxygen ions.However,the octahedral coordination is distorted with3short Fe–O1and3long Fe–O2bonds.There is one P and one Fe in the third and fourth shells,respectively.The structural param-eters and the coordination number(N)for the precursor of R=1and 0.75are almost identical,as shown in Table1,indicating their local structure is similar.However,the magnitude of the FT peak cor-responding to the Fe–O shell becomes larger for the precursor of R=0.5compared to R=1.0and0.75.The coordination number of the Fe–O1,Fe–O2,Fe–P and Fe–Fe for the precursor of R=0.5are
4,4,1and0,respectively,indicating the coordination number
of
Fig.4.Schematic reprentation of the formation
process.
Fig.5.Normalized absorption of Fe K-edge for LiFePO4/C composite powders at兵马俑简介
various R values,along with the references of FePO4and FeC2O4.
the Fe–O increas but the coordination of the Fe–Fe disappears.
The Fe atoms for the precursor of R=1and0.75are surrounded
by six oxygen ions but by eight oxygen ions for the precursor of
R=0.5.Although a iron complex with8-coordinate geometry is rare,
the square antiprism and dodecahedron are common in transition
metal complexes,and there are some8-coordinate iron complexes
such as almandine-rich garnet[Fe(EDTA)(H2O)2]+etc.[24,25].It is
suggested that the iron ion in the precursor for R=0.5must have
more space to accommodate eight ligands than in the other sam-
ples.Meanwhile,the distortion in the precursor of R=0.5is more
rious than that in the precursor of R=1.0and0.75.
3.3.XANES of the synthesized powders
As shown in Fig.5,the peak in the pre-edge region reprents an s–d like transition which is originally dipole forbidden,but it
becomes partially allowed by mixing of the d-states of the transi-
tion metal with the p-states of the surrounding oxygen atoms.Its
energy position depends mainly on the Fe oxidation state,whereas
its intensity depends on the geometry around[22,25–27],so that
it will be virtually zero in ca of regular octahedral symmetry
(O h)around the absorber,and it will reach its maximum in ca of
tetrahedral coordination(T d).The XANES of the LiFePO4powders
重庆火锅介绍
prepared at various R values is shown in Fig.5.To facilitate compari-
son,the pre-edge region of the corresponding spectra here has been
magnified in Fig.5.The pre-edge bands of iron(II)and iron(III)com-
plexes have been extensively investigated in the literature[22,25].
Meanwhile,the pre-edge bands of LiFePO4and FePO4were also
well-discusd[14].A full explanation of the pre-edge features is
beyond the aim of the prent work.Briefly,the occurrence of this
peak is related to the nature of1s→3d transition,which is electric
dipole forbidden but quadrupole allowed.The pre-edge intensity
is the most nsitive to site centrosymmetry with the most cen-
trosymmetric Fe coordination having the lowest intensity.From the
obrvation of Fig.5it reveals that Fe is octahedrally coordinated
by oxygen in the LiFePO4powders.The reference FePO4(purchad
from Merck)has a trigonal structure with a space group of P13121.
The symmetry of the iron site in the reference FePO4is tetrahedral,
which is different from an octahedral site in the fully charged sam-
ple reported in the literature[14].The pre-edge region is assigned
to be1s→3d transition,and weak intensity in this region indicates
an octahedral coordination as oppod to the tetrahedral coordi-
nation from which a strong pre-edge intensity(FePO4)is found.
Another interesting feature of the XANES spectra shown in Fig.5is
the1s–3d pre-edge peak obrved for all three LiFePO4-bad sam-
ples for Fe(II)is at7112.4eV which is at lower energies than that for
664K.-F.Hsu et al./Journal of Power Sources 192(2009)
660–667
Fig.6.k 3-Weighted FT spectra of Fe K-edge for LiFePO 4/C composite powders at various R values.
山西历史名人Fe(III)in FePO 4where the pre-edge peak is at 7113.8eV,indicating
the oxidation state of Fe in the synthesized LiFePO 4powders is +2.The local structure and oxidation state of the precursors and the synthesized LiFePO 4/C composite powders were investigated by X-ray absorption spectroscopy (XAS)to provide deep insights into their formation mechanism.Although Fe 3+ions exist for the precursor of R =0.5,the oxidation state of iron in all the synthesized powders is +2.It implies that the iron ions would be completely reduced to ferrous ions even the precursor consisting of Fe 3+in the developed process.
3.4.EXAFS of the synthesized powders
The Fourier transform (FT)of the Fe K-edge EXAFS spectra for the LiFePO 4powders is shown in Fig.6.The first strong peak is ascribed to the scattering process of the ejected electron at the oxygen-containing coordination shell,and the cond and the third peaks reprent the scattering process of the ejected electron at the phosphorous-and iron-containing coordination shells,respec-tively.For the synthesized powders with R =1and 0.75,the shape and intensity of the Fe–O,Fe–P,and Fe–Fe peaks are almost same,implying that the local environment of the iron in the two sam-ples is identical.For R =0.5,the feature of the oxygen coordination shell is unlike with that of the other two
samples.The splitting of the first peak was obrved,indicating that the Fe environment for the powder of R =0.5is different from that of the other two samples.The shape of peak strongly depends on each sample due to either the crystallographic symmetry or the disordering environment of iron environments.
In the four-shell model,the coordination atom of the first and the cond shells are oxygen,and that of the third and the fourth shells
Fig.7.FEFF7fit the radial structure function for LiFePO 4/C composite powders.
are phosphorous and iron,respectively.The experimental Fourier-filtered and simulated data are fitted very well for the synthesized LiFePO 4powders with various R values,as shown in Fig.7.The structural parameters were obtained by the fitting the EXAFS data with the four-shell model considering possible scattering paths,as shown in Table 2.
It was found that the structural parameters for the LiFePO 4pow-ders of R =1and 0.75are similar,as shown in Table 2.However,the coordination number of Fe–Fe and Fe–P shells for the powder of R =0.5are decread to be 3and 4,respectively.It indicates that the local environment of Fe sites in the powders of R =1.0and 0.75is different from that in the powders of R =0.5.Considering first the evoluti
on of the Fe–O,Fe–P,and Fe–Fe coordination number,in a perfect LiFePO 4crystal,the Fe atoms are surrounded by six O,five P,and four Fe atoms [28].It implies that the structure of the LiFePO 4powders of R =1and 0.75is more ordering than that of the powder of R =0.5.Bad on our experimental obrvations it is suggested that the LiFePO 4powder with better structural ordering can be achieved from the precursor with better structural ordering.On the contrary,the disordered powders would be obtained if the structure of their corresponding precursor is distorted.3.5.XRD of the synthesized powders
The X-ray diffraction patterns recorded for the synthesized powders with various R values are shown in Fig.8.Single-pha structure of LiFePO 4was obrved for all the samples.No impu-rity of Fe 2O 3or Fe 3O 4can be identified in Fig.8,implying that Fe(III)does not exist in the synthesized powders.It is consistent
Table 2
Structural parameters of Fe K-edge for LiFePO 4/C composite powders at various R values simulated by four-shell model.R value Sphere Shell Coordination number (N )Radius R j (Å)Debye–Waller factor 2j (×10−3Å2)Residual %1
1st Fe–O14 2.067.8 6.52
2nd Fe–O22 2.339.03rd Fe–P 5 3.25 4.94th Fe–Fe 4 3.949.60.751st Fe–O14 2.038.2 4.85
2nd Fe–O22 2.2810.33rd Fe–P 5 3.23 5.14th Fe–Fe 4 3.929.40.51st Fe–O13 1.98 1.69.88
2nd Fe–O23 2.160.03rd Fe–P 4 3.21 5.54th
Fe–Fe
3
3.94
8.2威风的近义词
K.-F.Hsu et al./Journal of Power Sources 192(2009)660–667
665
Fig.8.XRD pattern for LiFePO 4/C composite powders prepared at different R ratios:(a)R =1,(b)R =0.75,(c)R =0.5.
with the obrvation from the XANES feature of the synthesized powders.Since the carbon is formed during the pyrolysis of citric acid,the Fe(III)in the precursor can be completely reduced to Fe(II)by the oxidation of the formed carbon and the formed Fe(II)ions further react with lithium and phosphate ions to form LiFePO 4pow-der [17,18].The grain size (t )is calculated from XRD patterns using the Scherrer formula t =0.9 /(B cos Â),where is the wavelength
of the X-ray ud,B the width at an intensity equal to half of I max ,and Âthe Bragg angle of the diffraction peak considered.The grain size is found to be 42,44,and 47nm for R =1,0.75,and 0.5,respec-tively.The grain size of the LiFePO 4is incread insignificantly with R value in the developed process.It is obviously obrved that the growth of the LiFePO 4grains is inhibited by the formed carbons.This phenomenon is in good agreement with our previous report [17,18],indicating that the nano-sized LiFePO 4and its derivatives can be obtained by the developed sol–gel process.
The XRD data have been normalized with respect to the inten-sity of (311)peak,as shown in Fig.8.The normalized intensity of (121)is incread with a decrea in the R values.Since the struc-ture factor of (121)plane is only contributed from lithium and oxygen ions,partial interchange of occu
pancy of the Li +ions and Fe 2+ions among the sites will lead to increa in the I (121)/I (311)peak ratio.Therefore,the normalized intensity will increa if the cation mixing between Li +and Fe 2+increas.It indicates that the cation mixing of the obtained powders decreas with an increa in the R values.Further study on the structural characterization of the olivine was undertaken by Rietveld refinement of the XRD data,using the GSAS software suite.The structural model and parameters for olivine LiFePO 4propod by Streltsov [28]were utilized as the starting attempt.Fig.9shows the result of fitting and the difference between the obrved and calculated data.The obrved and cal-culated patterns match very well.The final structural parameters for all the samples are collected in Table 3.The reliability factors (R wp )for all the samples were quite ,around 6%.It was found that the lattice parameters of the synthesized LiFePO 4are influenced by the R values.Decreasing the R values leads to expand the a -,b -and c -axes,as well as increa the orthorhombic unit cell volume from 290.03to 291.92Å3.Cho et al.[29]have suggested that the lattice parameters increa if the iron ion is reduced from Fe 3+(0.64Å)to Fe 2+(0.77Å)in their synthesized powders.How-ever,there is no change in the valence state of the iron ions in their
samples from their Fe K-edge XANES.The variation of the lattice parameters is probably from the other reason.We suggest that the unit cell volume will increa if partial cation mixing occurs even th
ere is no change in the valence state of iron ions.It is consistent with the data reported by Yang et al.[30–32].The reason is the ionic radius of divalent iron (Fe 2+,0.77Å)is somewhat larger than that of lithium ion (Li +,0.68Å).Therefore,the lattice parameters will increa if the Li +and Fe 2+ions occupy 4c and 4a sites,respectively.The simulation model propod by Isiam et al.[33]shows that the most favorable intrinsic defect is the Li–Fe “anti-site”pair in which a Li +(on the M1site)and an Fe 2+(on the M2site)are interchanged.As the R value is incread from 0.5to 1,the cation mixing ratio decrea from 6.4to 2.8%.It demonstrates the molar ratio of citric acid to metal ions plays an important role in the synthesis process.The R value is therefore a nsitive indicator of a change in local environment or a local distortion around the central atom.3.6.Electrochemical properties
The cycling behaviors of the LiFePO 4/carbon cells were tested at a C-rate of 1/40between 3.0and 4.0V.The cycle-life plots of R =1,0.75and 0.5are shown in Fig.10.The first discharge spe-
Fig.9.Results of fitting the X-ray powder diffraction data corresponding to olivine LiFePO 4for LiFePO 4/C composite powders (R =1).
