Journal of Power Sources158(2006)
543–549
Preparation and characterization of high-density spherical
Li0.97Cr0.01FePO4/C cathode material for lithium ion batteries Jierong Ying∗,Min Lei,Changyin Jiang,Chunrong Wan,Xiangming He,
Jianjun Li,Li Wang,Jianguo Ren
Institute of Nuclear and New Energy Technology,Tsinghua University,P.O.Box1021,Beijing102201,PR China
Received15August2005;received in revid form30August2005;accepted30August2005
Available online25October2005
Abstract
LiFePO4is the new generation cathode material for lithium ion batteries.To improve the powders’pile density is considered as the important rearching direction.One effective way is to prepare powders compod of spherical particles.Spherical amorphous FePO4·x H2O powders were synthesized by controlled crystallization method,using Fe(NO3)3,H3PO4and NH3as the raw materials.The FePO4·x
H2O powders were pre-heat treated at520◦C for20h in air to obtain spherical hexagonal FePO4powders.The FePO4powders were homogeneously mixed with Li2CO3, Cr(NO3)3and sucro with certain molar ratios,and then sintered at800◦C for16h in N2.The spherical olivine Li0.97Cr0.01FePO4/C powders were finally obtained through carbothermal reduction process.The composition,structure,morphology,and physicochemical properties of FePO4·x H2O, FePO4and Li0.97Cr0.01FePO4/C powders were characterized in detail by DTA/TGA,ICP,XRD,SEM,XPS,lar particle size analysis,and tap-density testing.It is obrved the tap-density of the spherical Li0.97Cr0.01FePO4/C powders is as high as1.8g cm−3,which is remarkably higher than the non-spherical LiFePO4powders reported.At current of0.005,0.05,0.1,0.25and1.0C,the composite cathode materials have initial discharge specific capacity of163,151,142,131and110mAh g−1,respectively.The material also shows excellent cycling performance.The high-density spherical Li0.97Cr0.01FePO4/C cathode material can be ud in the lithium ion batteries to greatly increa the batteries’energy density.To further improve the material’s pile density and rate capability is considered as the rearching direction.
©2005Elvier B.V.All rights rerved.
Keywords:Lithium ion batteries;Controlled crystallization method;Li0.97Cr0.01FePO4/C;High-density;
Spherical
1.Introduction
Recently,olivine-structured LiFePO4propod by Padhi et al.[1]is gaining particular interest as a candidate cathode mate-rial for lithium ion batteries.Comparing to the commercially ud LiCoO2,LiNiO2,LiMn2O4and their derivatives,LiFePO4 cathode materials have the outstanding advantages of low cost, excellent heat stability,satisfactory safety and low toxicity,etc.
However,there are two main obstacles preventing LiFePO4 to be put into commercially ud.One is the poor electronic conductivity,which leads to initial capacity loss and poor rate capability.The other is the low pile density,which leads to low volumetric specific capacity.
∗Corresponding author.Tel.:+861082780860/89796085;
fax:+861069771464/89796031.
E-mail address:yingjr@mail.(J.Ying).
To improve the electronic conductivity,veral effective ways
have been propod,including synthesis of LiFePO4/electronic
抢课conductor composites[2–5](carbon or metal nanoparticles),
substitution of a small quantity of Li+by supervalent metal ions
[6,7](Mg2+,Al3+,Cr3+,Zr4+,Ti4+,Nb5+,W6+),preparation of
礼记
powders withfine particles[8],etc.
Unfortunately,little attention has beenfixed on improving the
pile density of LiFePO4so far.The LiFePO4powders are usually
prepared via conventional solid state reaction of mechanically
mixed lithium compounds(typically Li2CO3or LiOH·H2O),
iron compounds(typically FeC2O4·2H2O or Fe(OOCCH3)2), and phosphates(typically NH4H2PO4or(NH4)2HPO4).The
obtained LiFePO4powders always show irregular particle mor-
phology with broad particle size distribution.According to our
绣球花的花语test,the tap-density of the powders is usually1.0–1.4g cm−3,
which is much lower than the tap-density of commercially ud
LiCoO2(typically2.4–2.6g cm−3).The low density of LiFePO4
cathode materials leads to the low volumetric specific capacity,
0378-7753/$–e front matter©2005Elvier B.V.All rights rerved. doi:10.1016/j.jpowsour.2005.08.045
544J.Ying et al./Journal of Power Sources158(2006)543–549
thus riously limiting the energy density of lithium ion batter-ies.Since great progress has been made to improve the electronic conductivity of LiFePO4,how to improve the powders’pile density,usually the tap-density,becomes more important and urgent.
As has been reported in our previous publications[9,10],the tap-density of the powders is almost deci
ded by the powders’particle morphology,besides the materials’theoretical density. The powders compod of spherical particles have higher den-sity than the powders compod of irregular particles.Thus,to obtain high density LiFePO4cathode material,preparing spher-ical powders is expected as an effective way.
In our laboratory,the high-density spherical LiCoO2 and LiNi0.8Co0.2O2cathode materials have been pre-pared via a controlled crystallization—solid-state reaction method[9,10].We have also reported a novel controlled crystallization—carbothermal reduction method to synthesize spherical carbon-coated LiFePO4cathode material in which the carbon content is about6wt.%[11].The spherical carbon-coated LiFePO4has the high tap-density of1.6g cm−3.How-ever,the material has the initial discharge capacity of only 129mAh g−3at0.1C and unsatisfactory rate capability,result-ing from the low electronic conductivity[11].Shi et al.
[7]reported that substitution of a small quantity of Li+in LiFePO4by Cr3+could greatly enhance the material’s elec-tronic conductivity,thus the material’s reversible capacity and rate capability could be obviously improved.In this work, we synthesized spherical Li0.97Cr0.01FePO4/C composite cath-ode material with the inexpensive Fe(NO3)3as iron source and sucro as reductive agent and carbon source via con-trolled crystallization—carbothermal reduction method.Com-pared with the pr
evious spherical carbon-coated LiFePO4[11], the tap-density,reversible capacity and rate capability of the spherical Li0.97Cr0.01FePO4/C were obviously improved.
2.Experimental
First,spherical amorphous FePO4·x H2O powders were syn-thesized by controlled crystallization method,using Fe(NO3)3, H3PO4and NH3as the raw materials,according to the reaction: Fe(NO3)3+H3PO4+3NH3+x H2O
=FePO4·x H2O+3NH4NO3(1) The reactor is illustrated in Fig.1.The spherical FePO4·x H2O was synthesized as the follows.The mixed solution of Fe(NO3)3 and H3PO4was pumped continuously into the reactor.At the same time,the solution of NH3was also pumped into the reac-tor to control the pH of the mixture.The concentration of the two solutions,average rest time(or the feed-in velocity),agitat-ing intensity,temperature,and pH of the mixture being agitating vigorously in the reactor should be controlled carefully.Thus, the growth of FePO4·x H2O particles in the reactor could be controlled effectively.The irregular particles changed gradu-ally into spherical particles after enough time of reaction and agitation.The mixture in the reactor wasfiltered,washed and dried.Thus,the spherical FePO4·x H2O powders were
obtained.Fig.1.Schematic diagram of the reactor for controlled crystallization process. In this work,the controlled crystallization parameters were as follows.The concentration of the Fe(NO3)3and H3PO4solution were both1.0mol L−1.The concentration of the NH3solution was3.0mol L−1.The agitating intensity was50–60W L−1.The average rest time was8–12h.The temperature was45◦C.The pH was2.1.
大学城美食
The spherical amorphous FePO4·x H2O powders were pre-heat treated at520◦C for20h in air to obtain spherical anhydrous FePO4powders.
To synthesize spherical carbon-coated Li0.97Cr0.01FePO4 powders,we will mix spherical FePO4precursors,Li2CO3, Cr(NO3)3and sucro(C12H22O11)uniformly.However,if we u the traditional mechanically mixing methods,such as ball milling,the uniform spherical FePO4particles will be broken to pieces.Thus,to keep FePO4precursor and Li0.97Cr0.01FePO4/C product particles as ideal spheres,we have to u special mixing methods.In this work,Li2CO3,Cr(NO3)3,sucro(C12H22O11) and deionized water(H2O)werefirstly mixed in a molar ratio of Li2CO3:Cr(NO3)3:C12H22O11:H2O=0.485:0.01:0.1:2and ball milled for4h in a planetary miller.A kind of uniform slurry was obtained.Then,we added spherical FePO4powders in a molar ratio of FePO4:Li2CO3=1:0.485into the slurry and agitated the mixture vigorously.In the eventually obtain
ed homoge-neous slurry,the FePO4particles kept the original spheres,while thefine particles of Li2CO3,Cr(NO3)3,and C12H22O11were uniformly coated on the surface of spherical FePO4particles orfilled up the vacancies among the spherical FePO4parti-cles.The mixed slurry was dried and then sintered at800◦C for16h in N2.The spherical carbon-coated Li0.97Cr0.01FePO4 powders werefinally obtained through carbothermal reduction process.During the sintering process,the reactions may be very complex.We assume the carbothermal reaction can be approxi-mately written as the formula below(omit the small quantity of Cr(NO3)3).According to the formula,the obtained composite will be Li0.97Cr0.01FePO4/0.7C.The calculated residual carbon
J.Ying et al./Journal of Power Sources158(2006)543–549545
content is about5wt.%.
0.5Li2CO3+FePO4+0.1C12H22O11
总是拉肚子是什么原因=LiFePO4+0.5CO2+0.5CO+0.7C+1.1H2O(2) The Fe/P molar ratio of the precursors synthesized by con-trolled crystallization method was analyzed by ICP.DTA/TGA of FePO4·x H2O was ud to direct the pre-heat treatment.
Powder X-ray diffraction(XRD,D/max-rB)using Cu K␣radiation was ud to identify the crystalline pha and crystal lattice parameters of the FePO4·x H2O,FePO4and Li0.97Cr0.01FePO4/C powders.The sample morphology was obrved byfield emission scanning electron microscopy (SEM,JSM6301F).The surface elements’content of spherical Li0.97Cr0.01FePO4/C powders were determined by X-ray pho-toelectron spectroscopy(XPS,PHI-5300ESCA).The powders’particle size distribution was identified by lar particle size ana-lyzer(OMEC LS-POP(III)).The tap-density and carbon content of the powders were tested using the method described in Ref.
[11].
Experimental test cells for measurements ud the cathode with the composition of80wt.%Li0.97Cr0.01FePO4/C,10wt.% carbon black,and10wt.%PTFE.The parator was a Celguard 2400microporous polypropylene membrane.The electrolyte was1M LiPF6EC+DEC(1:1by volume).A lithium metal anode was ud in this study.The cells were asmbled in a glove boxfilled with argon gas.The charge–discharge cycling was galvanostatically performed at a current of0.005,0.05,0.1, 0.25and1.0C with cut-off voltages of2.5–4.2V(versus Li/Li+) at20◦C.
3.Results and discussion
3.1.The Fe/P molar ratio of the precursors in relation to
pH during controlled crystallization process
湛蓝的海
In order to obtain spherical Li0.97Cr0.01FePO4/C powders, the stoichiometric FePO4powders are considered as the nec-essary precursor according to our previous experiments.It is very important to insure the Fe/P molar ratio of the precursors synthesized by controlled crystallization method is1.
It is well known that Fe3+is easy to hydrolyze to Fe(OH)2+, Fe(OH)2+and Fe(OH)3when the solution’s pH increas,while PO43−is easy to hydrolyze to HPO42−,H2PO4−when the solu-tion’s pH decreas.Thus,different pH of the mixture in the reactor will lead to different products during controlled crys-tallization process,using Fe(NO3)3,H3PO4and NH3as the raw materials.Fig.2shows the Fe/P molar ratio of the pre-cursors in relation to pH.When pH is less than1.95,the Fe/P molar ratio is less than1.When pH is larger than2.25,the Fe/P molar ratio is larger than1.When pH is between1.95 and2.25,the Fe/P molar ratio is very clo to1.Bad on the results,in this work,wefixed the pH on2.1.The formula of the precursor obtained at pH2.1can be written as FePO4·x H2
O.
Fig.2.The Fe/P molar ratio of the precursors prepared at different pH. 3.2.Pre-heat treatment of the FePO4·xH2O precursor
The spherical FePO4·x H2O precursor obtained at pH2.1is amorphous.The water content of FePO4·x H2O is notfixed. According to our analysis,the value of x is about2.5,but varies slightly with drying condition,particle size distribution,etc.To insure the accuracy of the proportion of raw materials batch to batch,the anhydrous stoichiometric FePO4is preferred to.The anhydrous stoichiometric FePO4can be obtained from pre-heat treatment of FePO4·x H2O.
Fig.3shows the TGA–DTA curves of the FePO4·x H2O pow-ders with a heating rate of10◦C min−1from room temperature to580◦C in air.On the DTA curve near178◦C,there is a very strong endothermic peak,associating with the sharply weight loss on the TGA curve,which is related to the quickly dehydra-tion of FePO4·x H2O.During178–500◦C,the TGA curve indi-cates the slowly elimination of residual H2O in FePO4·x H2O. When the temperature is high than500◦C the TGA curve indi-cates the weight remains constant.We can conclude that the dehydration of FePO4·x H2O willfinish and the powders’com-position will be confirmed when the pre-heat treating tempera-ture is
high than500◦C.On the DTA curve near475◦C,there is a strong exothermic peak,which is related to the transformation of the amorphous FePO4to hexagonal FePO4crystal.
梦到自己哭了Bad on the above analysis,we pre-heat treated the FePO4·x H2O powders at520◦C for20h in air
to
Fig.3.DTA/TGA curves of the FePO4·x H2O.
546J.Ying et al./Journal of Power Sources 158(2006)543–549
obtain anhydrous hexagonal FePO 4powders,which were ud as the precursors to synthesize Li 0.97Cr 0.01FePO 4/C powders.
3.3.XRD analysis of FePO 4·xH 2O,FePO 4and Li 0.97Cr 0.01FePO 4/C powders
Fig.4shows the XRD patterns of the FePO 4·x H 2O,FePO 4and Li 0.97Cr 0.01FePO 4/C powders.There are no identifiable peaks on the XRD spectra of FePO 4·x H 2O powders,indicat-ing FePO 4·x H 2O synthesized by controlled crystallization pro-cess is amorphous.There are strong and sharp peaks on the FePO 4powders’XRD spectra,which is almost the same as the XRD spectra of anhydrous hexagonal structured FePO 4(JCPDS card no.29-0715).The crystal lattice parameters calculated by
the XRD data are a =5.034˚A,
b =5.034˚A,
c =11.246˚A.The spectra proves the amorphous FePO 4·x H 2O powders pre-heat treate
d at 520◦C for 20h wer
象棋桥e well crystallized into pha-pure anhydrous hexagonal FePO 4powders.The spectra o
f Li 0.97Cr 0.01FePO 4/C is almost the same as the spectra of pure ordered orthorhombic olivine structured LiFePO 4(JCPDS card no.40-1499).The abnce of any other signals indicates there are no unwanted impurity phas,such as Li 3PO 4and Fe 3+related compounds.There is no evidence of diffraction peaks for carbon,indicatin
g the residual pyrolytic carbon in prod-uct is amorphous.The crystal lattice parameters calculated by
the XRD data of the material are a =6.008˚A,
b =10.328˚A and
c =4.693˚A.
According to X-ray diffraction analysis,FePO 4and Li 0.97Cr 0.01FePO 4/C have similar structure.Du
ring sinter-ing the mixture of FePO 4,Li 2CO 3,Cr(NO 3)3and sucro in N 2,the FePO 4framework approximately holds the line,while the Li +and Cr 3+diffu into FePO 4spheres and inrt into the FePO 4framework.At the same time,the sucro will pyroly.The hydrogen and carbon generated from sucro can produce a strong reductive atmosphere for the reduction of Fe 3+to Fe 2+,resulting the synthesis of Li 0.97Cr 0.01FePO 4.The residual pyrolytic carbon will coat on the spherical Li 0.97Cr 0.01FePO 4particles to form the composite Li 0.97Cr 0.01FePO 4
/C.
Fig.4.XRD patterns of the FePO 4·x H 2O,FePO 4,Li 0.97Cr 0.01FePO 4/C pow-ders.
3.4.Morphology of FePO 4·xH 2O,FePO 4and Li 0.97Cr 0.01FePO 4/C powders
The morphology of FePO 4·x H 2O and FePO 4powders is very similar.The powders are both compod of well-disperd spherical particles,as shown in Fig.5(a)and (c).Each of the spherical particles is made up of a large number of small grains,as shown in Fig.5(b)and (d).Fig.5(e)shows the Li 0.97Cr 0.01FePO 4/C powders are mainly compod of spher-ical particles similar to the FePO 4precursors,although there are slight agglomeration and a small quantity of fragments.Fig.5(f)shows the spherical Li 0.97Cr 0.01FePO 4/C particle is wholly coated by some substance who composition is mainly carbon,proved by EDS and XPS analysis.The crystalline grains cannot be obrved becau of the coated carbon layer.There are also some carbon fragments adhering to the spherical particle.3.5.XPS analysis of the Li 0.97Cr 0.01FePO 4/C powders The carbon content of the Li 0.97Cr 0.01FePO 4/C powders is about 6wt.%,determined by the method described in Ref.[11].In other words,the molar ratio of C:Li 0.97Cr 0.01FePO 4is about 0.8.The tested carbon content (6wt.%)is clo to the calculated data (5wt.%),indicating the assumed carbothermal reduction formula mentioned in the experimental part is correct in the rough.However,according to XPS analysis,on the surface of Li 0.97Cr 0.01FePO 4/C powders,the molar ratio of C:Li:Fe:P is about 40:
1:1:1.The result indicates the surface composition is mainly the carbon.The pyrolytic carbon is coated the surface of spherical Li 0.97Cr 0.01FePO 4particles rather perfectly,which accords with the SEM analysis.
3.6.Particle size distribution and tap-density of the
spherical FePO 4·xH 2O,FePO 4and Li 0.97Cr 0.01FePO 4/C powders
As shown in Table 1,the tap-density of the spherical Li 0.97Cr 0.01FePO 4/C powders prepared in this work is as high as 1.8g cm −3,which is remarkably higher than the non-spherical LiFePO 4powders reported,who tap-density is usual 1.0–1.4g cm −3.Compared with the previous spherical carbon-coated LiFePO 4powders (the tap-density is 1.6g cm −3)[11],the spherical Li 0.97Cr 0.01FePO 4/C powders prepared in this work have higher tap-density,mainly becau of the better spherical quality.The high-density spherical Li 0.97Cr 0.01FePO 4/C cath-ode material can be ud in the lithium ion batteries to greatly increa the batteries’energy density.According to our expe-Table 1
The particle size distribution and tap-density of the spherical FePO 4·x H 2O,FePO 4and Li 0.97Cr 0.01FePO 4/C Spherical powders D 10(m)D 50(m)D 90(m)Tap density (g cm −3)FePO 4·x H 2
O 4.410.819.1 1.1FePO 4
4.29.91
5.8 1.5Li 0.97Cr 0.01FePO 4/C
3.2
8.0
14.4
1.8
J.Ying et al./Journal of Power Sources158(2006)543–549
547
Fig.5.SEM images of the spherical FePO4·x H2O,FePO4and Li0.97Cr0.01FePO4/C:(a)FePO4·x H2O powders,(b)a spherical FePO4·x H2O particle,(c)FePO4 powders,(d)a spherical FePO4particle,(e)Li0.97Cr0.01FePO4/C powders and(f)a spherical Li0.97Cr0.01FePO4/C particle.
rience,the powders’tap-density can be further improved by two possible ways.One is to optimize the precursors’spheri-cal quality and particle size distribution through adjusting the controlled crystallization parameters.The other is to improve the pyrolytic carbon’s coating quality,especially to eliminate the carbon fragments adhering to the spherical particles.The carbon sources,mixing procedure,and the carbothermal reduction pro-cess should be optimized.We will report the results elwhere.
3.7.Electrochemical performance of the spherical
Li0.97Cr0.01FePO4/C cathode material
Fig.6prents the initial and100th charge discharge curves of the Li0.97Cr0.01FePO4/C composite at current of0.1C.Aflat charge discharge curve around3.45V over a large compositional range implie
s that the two-pha redox reaction proceeds via a first-order transition between LiFePO4and FePO4[1].The small voltage difference between the charge and discharge plateaus is reprentative of its good kinetics.The composite cathode mate-rial has afirst cycle charge capacity of152mAh g−1followed by a discharge capacity of142mAh g−1,and the rather high initial charge discharge efficiency of93.4%.After100cycles, the reversible discharge capacity is138mAh g−1,showing
the Fig. 6.Charge–discharge curves of the initial and100th cycle of Li0.97Cr0.01FePO4/C at0.1C.
