methode

Electrochimica Acta52(2007)

4922–4926

A promising sol–gel route based on citric acid to synthesize

Li3V2(PO4)3/carbon composite material for lithium ion batteries

Yuzhan Li,Zhen Zhou,Xueping Gao,Jie Yan?

Institute of New Energy Material Chemistry,Nankai University,Tianjin300071,China

Received13September2006;received in revised form6January2007;accepted16January2007

Available online27January2007

Abstract

Li3V2(PO4)3/carbon composite material was synthesized by a promising sol–gel route based on citric acid using V2O5powder as a vanadium source.Citric acid acts not only as a chelating reagent but also as a carbon source,which enhance the conductivity of the composite material and hinder the growth of Li3V2(PO4)3particles.The structure and morphology of the sample were characterized by TG,XRD and TEM measurements. XRD results reveal that Li3V2(PO4)3/carbon was successfully synthesized and has a monoclinic structure with space group P21/n.TEM images show Li3V2(PO4)3particles are about45nm in diameter embeded in carbon networks.Galvanostatic charge/discharge and cyclic voltammetry measurements were used to study its electrochemical behaviors which indicate the reversibility of the lithium extraction/insertion processes. Li3V2(PO4)3/carbon performed in a voltage window(3.0–4.8V)exhibits higher discharge capacity,better cycling stability and its discharge capacity maintains about167.6mAh/g at a current density of28mA/g after50cycles.

?2007Elsevier Ltd.All rights reserved.

Keywords:Li3V2(PO4)3/carbon;Sol–gel route;Cathode material;Lithium ion batteries

1.Introduction

There are great interests in lithium insertion materials as cathode for rechargeable lithium ion batteries.Generally,cath-ode materials are selected from layer LiCoO2,LiNiO2,spinel LiMn2O4and their substituting derivative compounds,but alter-native cathode materials have been pursued to replace the oxidative unstable lithium transition metal oxide.Recently,con-siderable investigations have been performed to the lithium conducting phosphates such as LiFePO4[1–7],LiMnPO4 [8–10],LiCoPO4[11–14],Li3V2(PO4)3[15–25],etc.The sub-stitution of the larger polyanion phosphate,instead of the smaller O2?ions,in an open3D framework helps to stabilize the struc-ture and allow a faster ion migration.LiFePO4was?rst proposed in1997which was a more stable compound.But in LiFePO4, a separation of the FeO6octahedra by polyanions reduces the electronic conductivity,which is polaronic in the mixed-valent state[1].So,many researchers tried to improve its conductivity through conductive carbon coating.LiFePO4had been prepared ?Corresponding author.Tel.:+862223503623;fax:+862223502604.

E-mail address:yanjie@https://www.doczj.com/doc/a92158363.html,(J.Yan).by solid-state reaction[1–5]and sol–gel route[6,7]which indi-

cated carbon existence played an important role in improving the

charge/discharge performance and cyclic stability of LiFePO4.

Among the above mentioned phosphates,monoclinic lithium

vanadium phosphate Li3V2(PO4)3,is a highly promising cath-

ode material for Lithium ion batteries.Li3V2(PO4)3has

monoclinic structure(space group P21/n)and theoretical

discharge capacity of197mAh/g.Li3V2(PO4)3with three-

dimensional(3D)framework contains three independent lithium

sites.During charge/discharge process,it can extract/insert2.0

Li ion reversibly between3.0and4.3V based on the V3+/V4+

redox couple.When charged to4.8V,3.0Li ion may be com-

pletely extracted.Li3V2(PO4)3has low conductivity due to the

polarization of V–O bond as same as LiFePO4.So Li3V2(PO4)3

has been synthesized by solid-state method[15]and sol–gel

method based on V2O5·H2O hydro-gel[17]by carbon dop-ing.Carbon doping may help to improve the conductivity and

electrochemical properties of Li3V2(PO4)3.

But to our knowledge,the report about the sol–gel route

based on citric acid to synthesize of Li3V2(PO4)3/carbon com-

posite material has been limited until now.In this work,

Li3V2(PO4)3/carbon was synthesized by a sol–gel process,in

which citric acid acted as not only a chelating reagent but also

0013-4686/$–see front matter?2007Elsevier Ltd.All rights reserved. doi:10.1016/j.electacta.2007.01.019

Y.Li et al./Electrochimica Acta52(2007)4922–49264923

a carbon source.Structure and electrochemical performance of Li3V2(PO4)3/carbon were investigated in detail.

2.Experimental

Li3V2(PO4)3/carbon composite material was synthesized by a sol–gel route based on citric acid using V2O5powder as a vanadium source in this study.Citric acid was employed as both a chelating reagent and a carbon source in the sol–gel process. Traditionally,the chelating reagent only provided the mixing of anion at the molecular level in a sol–gel process.Here cit-ric acid was also used as a carbon source,which can prevent the oxidation of vanadium ions and afford the network struc-ture of carbon for electronic conduction.Starting materials were the following analytical pure reagents.Stoichiometric ratio of V2O5,Li2CO3and NH4H2PO4were?rst dissolved in the deion-ized water with magnetic stirring.To this solution,a saturated aqueous solution of citric acid was added slowly with mag-netic stirring.In all experiments,the molar ratio of chelating reagent to the total metal ions was unity.The mixtures were heated gently with continuous stirring to remove the excess water at80?C.The color of the sol became blue from brown at the beginning and kept blue invariably during heating pro-cess.The resulting gel precursor was dried in an air oven at 80?C.After drying,the precursors were decomposed at300?C for4h in?owing argon.Obtained powder was slightly ground, pressed into pellets,and then sintered at750?C for4h in?owing argon.

TG measurement was performed on the gel precursors using a Perkin-Elmer analyzer in the?owing nitrogen atmosphere. The TG spectra were acquired in the temperature range from 25to850?C at a heating rate of10?C min?1.X-ray diffraction pattern of the sample was characterized and carried out with2θbetween3?and50?by using a D/Max III diffractometer with CuK?radiation,at the scan rate of8?min?1,and voltage of the40kV.The nanoscale microstructure was examined using a transmission electron microscope(TEM,JEOL,JEM1010). The sample was dispersed into the anhydrous ethanol and the suspension solution was dropped onto a standard copper TEM grid.

Te?on-type testing cells were assembled for electrochemical characterization of Li3V2(PO4)3/carbon electrodes.The test-ing electrodes were made by mixing85wt%active materials, 10wt%carbon black and5wt%colloidal PTFE binder.This mixture was pressed resulting in a circular pellet electrode whose diameter was8mm.The pellets were then dried at100?C for 24h.The cells were assembled in an argon?lled glove-box by keeping both the oxygen and moisture levels at less than1ppm (Nanjing University,China)with the positive electrode and Li metal as negative electrode.The electrolyte was1mol L?1LiPF6 in a mixture of ethylene carbonate(EC)and dimethyl carbon-ate(DMC)(1:1,v/v).The charge–discharge measurements and cyclic voltammetry(CV)tests were done using Land battery tester(CT2001A,Wuhan university)and CHI600A electro-chemical analyzer(Chenhua,Shanghai,China).The cells were galvanostatically charged and discharged in the voltage range of 3.0–4.8V versus Li metal counter electrode.Cyclic voltamme-try(CV)were measured between3.0and4.8V at a scan rate of 0.05mV s?1to characterize the redox reactions of the electrode.

3.Results and discussion

Thermogravimetric(TG)spectra for the gel precursor obtained from V2O5based on citric acid process operated in the temperature range from25to850?C at a heating rate of10?C min?1is shown in Fig.1.About55%weight loss is observed during the temperature sweep to600?C.Above 600?C,the change of weight loss becomes insigni?cant when the temperature is further increased to850?C.The weight loss process can be divided into three regions100–200,200–400and 400–600?C.It can be seen that in the?rst weight loss range of 100–200?C,the weight loss is about26wt%resulted from the evaporation of NH3and water.It can be observed that the sec-ond weight loss region of200–400?C,the weight loss is around 22wt%which is arisen from the pyrolysis of citric acid.Sub-sequently,about7wt%weight loss is shown due to pyrolysis of the remaining organic compounds occurs in the temperature range from400to600?C The?nal product remains at about 45wt%Li3V2(PO4)3/carbon composite cathode material,which was consistent with the theoretical yield of Li3V2(PO4)3(Li: 1.6wt%,V:8.1wt%,PO43?:22.7wt%)and the amount of the residual carbon(12.5wt%)in the composite material measured by elementary analysis.

Fig.2shows the X-ray diffraction pattern of the Li3V2(PO4)3/carbon sample sintered for4h at750?C.The phase of the synthesized material is similar to the previous reports[15,16].The diffraction peaks of the sample correspond to a single-phase and indexed with monoclinic structure with space group P21/n and no other secondary phases are detected by XRD analysis.Carbon left in Li3V2(PO4)3/carbon compos-ite material is not detected which is undoubtedly because the residual carbon is amorphous or the thickness of the residual carbon on the Li3V2(PO4)3/carbon powders is too thin[5].The grain size of Li3V2(PO4)3/carbon power calculated from the powder X-ray pattern using the Debye–Scherrer equation is in the range of45nm which is much smaller than those

prepared Fig.1.TG spectra for the gel precursor in N2at the heating rate10?C min?1.

4924Y.Li et al./Electrochimica Acta 52(2007)

4922–4926

Fig.2.X-ray diffraction pattern of Li 3V 2(PO 4)3/carbon composite material.

through conventional solid-state reaction.The grain size of the Li 3V 2(PO 4)3is smaller which indicate that the formed carbon can provide a network structure to impede the grain growth of the Li 3V 2(PO 4)3.

TEM investigations were conducted to examine the parti-cle size and the morphology of Li 3V 2(PO 4)3/carbon powder which is shown in Fig.3.From the TEM image,it can be seen that Li 3V 2(PO 4)3/carbon particles consist of many nano-sized Li 3V 2(PO 4)3grains coated with the formed carbon.Fig.3shows the TEM light region images of the formed carbon network.In this image,the dark grey 45nm spots represent the Li 3V 2(PO 4)3grains.The size of the Li 3V 2(PO 4)3grain is in good agreement with the crystalline size obtained from the X-ray diffraction pat-tern.Nano-sized material appears because of the existence of the formed carbon which can provide good electronic contact between the synthesized Li 3V 2(PO 4)3particles.

Electrochemical performance of the compound in the cell con?guration Li/Li 3V 2(PO 4)3has been evaluated in the

voltage

Fig.3.TEM images of Li 3V 2(PO 4)3/carbon composite

material.

Fig.4.First charge/discharge pro?les of Li 3V 2(PO 4)3/carbon composite mate-rial.

range 3.0–4.8V at room temperature function of the cycle num-ber for LAND CT 2001A cell is shown in Fig.4.Fig.4shows the ?rst charge/discharge pro?les of Li 3V 2(PO 4)3/carbon compos-ite material sintered at 750?C for 4h.The sample exhibits four charge ?at plateaus around 3.61,3.69,4.10and 4.56V and three discharge ?at plateaus around 3.57,3.65and 4.01V ,which has been identi?ed as the two-phase transition process during charge and discharge process [16].From Fig.4,a signi?cant over-voltage can be seen at 4.56V ,thinking the extracting the third lithium needs higher energetics and exists irreversibility dur-ing charge.However,the Li 3V 2(PO 4)3sample exhibits a higher charge capacity about 194.4mAh/g and discharge capacity about 189.1mAh/g at the current density of 28mA/g.Furthermore,the sample also shows higher coulombic ef?ciency 97.3%.

The results of charge/discharge cycles at current density of 28mA/g in the voltage range 3.0–4.8V at room temperature is shown in Fig.5for the Li 3V 2(PO 4)3/carbon composite material.From Fig.5,it can be seen clearly that the sample exhibits higher discharge capacity after about 50cycles,meaning that the sam-ple have better cyclic capacity and electrochemical stability.The initial discharge capacity of the Li 3V 2(PO 4)3/carbon is rather high,194.4mAh/g.But the discharge capacity of the ?rst

10

Fig.5.Discharge capacity vs.cycles of Li 3V 2(PO 4)3/carbon composite material at the current density of 28mA/g.

Y.Li et al./Electrochimica Acta52(2007)4922–4926

4925

Fig.6.Discharge capacity vs.cycles of Li3V2(PO4)3/carbon composite material at different current density.

cycles decreases more sharply about15mAh/https://www.doczj.com/doc/a92158363.html,ly,in the Li3V2(PO4)3system,the?rst charge/discharge process forms thick surface?lm which makes impendence larger and irre-versibility greater.Subsequently insigni?cant discharge capacity fading is observed after10cycles and remains the discharge capacity of167.6mAh/g which is higher than those of the solid-state reaction after50cycles[15].In the paper reported by Saidi,Li3V2(PO4)3showed the discharge capacity of about 140mAh/g.

Li3V2(PO4)3/carbon had good rate capability which can be seen from Fig.6.At a current density of70mA/g,the discharge capacity reaches to152.8mAh/g after50cycles.The capac-ity loss observed on Li3V2(PO4)3/carbon is about17%after 50cycles but capacity fading becomes slow along with cycles. When the current density is increased to140mA/g,the dis-charge capacity still attains135.3mAh/g after50cycles.From above data,it can be seen that the sol–gel route greatly increases the discharge capacity because of carbon presence.A higher carbon content would provide a better carbon structure for elec-tron conductivity and closer contact between the Li3V2(PO4)3 particles.

As for cyclic voltammetry,the potential interval between anodic peak and cathodic peak is an important parameter to value the electrochemical reaction.Cyclic voltammetry has been recorded for the sample,Li3V2(PO4)3/carbon using Li metal as counter and reference electrode in stimulant cell con?guration is shown in Fig.7.The curves indicate the potential range in which the lithium extraction/insertion occurs during this process.For the?rst cycle,the extraction processes occurs at3.62,3.68, 4.11and4.59V,and insertion occurs at3.55,3.63and3.93V, respectively.The second cycle reduces the extraction potential to 3.61,3.67,4.10and4.58V and increases the insertion potential to3.58,3.66and3.97V,which indicate that the overpotential for the extraction/insertion process was reduced.The second corresponding potential interval is less than the?rst potential interval which is because the solid electrolyte interface(SEI) formed on the active particles to block the insertion of solvent molecules,decrease the polarization to increase reversibility. The well-de?ned peaks and smaller value of potential

interval Fig.7.CV curves recorded at scan rate of0.05mV s?1between3.0and4.8V potential range for Li3V2(PO4)3/carbon composite material.

showed the enhancement of electrode reaction reversibility.The reduced difference between the potentials of the two processes further shows a greater reversibility for the electrode processes. The larger currents seen in the cyclic voltammetry agreed with charge/discharge capacities for the?rst cycle.For example,at a current density of28mA/g,the charge/discharge capacities were194.4and189.1mAh/g,respectively for the?rst cycle. For the second cycle,they are183and180.9mAh/g,respec-tively.The larger irreversibility of the materials in the?rst cycle was re?ected in the voltammograms.

Li3V2(PO4)3/carbon has been successfully synthesized by solid-state method and a sol–gel route based on V2O5·n H2O hydro-gel,and offered some encouraging performances because of carbon existence.In this investigation,Li3V2(PO4)3/carbon prepared by sol–gel method based on citrid acid shows a dis-charge capacity about167.6mAh/g after50cycles;moreover, which is required lower calcining heat and shorter calcining time;especially nano-sized composite material obtained through a sol–gel method has a carbon network which is bene?cial to the electrochemical performances.

4.Conclusions

Li3V2(PO4)3/carbon composite material was successfully synthesized by a sol–gel route based on citric acid using V2O5 powder as a vanadium source.The subsequent sintering tem-perature and time are750?C and4h,respectively which are lower than those of solid-state reaction(temperature was850?C and time was24h).XRD results show that a pure single phase was obtained and Li3V2(PO4)3material is monoclinic structure.In XRD pattern,the peaks of carbon do not appear because carbon on Li3V2(PO4)3particle is amorphous and its sizes is smaller.The Li3V2(PO4)3/carbon particle is nano-sized particle in diameter coated by a thin layer carbon.Excess carbon is remained in the structure which can suppress the growth of the Li3V2(PO4)3particle during the sintering pro-cess.The electrochemical properties of the Li3V2(PO4)3/carbon composite material are in?uenced by the current density.At the current density of28,70and140mA/g,the discharge

4926Y.Li et al./Electrochimica Acta52(2007)4922–4926

capacities after50cycles are167.6,152.8and135.3mAh/g, respectively.

Acknowledgement

This work was supported by the973Program (2002CB211800)in China.

References

[1]A.K.Padhi,K.S.Nanjundaswamy,J.B.Goodenough,J.Electrochem.Soc.

144(1997)1188.

[2]A.Yamada,S.C.Chung,J.Electrochem.Soc.148(2001)A960.

[3]A.Yamada,M.Hosoya,S.C.Chung,Y.Kudo,K.Hinokuma,K.-Y.Liu,Y.

Nishi,J.Power Sources119–121(2003)232.

[4]A.Yamada,S.C.Chung,K.Hinokuma,J.Electrochem.Soc.148(2001)

A224.

[5]H.C.Shin,W.I.Chob,H.Jang,J.Power Sources159(2006)1383–1388.

[6]M.M.Doeff,Y.Q.Hu,F.McLarnon,R.Kosteckib,Electrochem.Solid State

Lett.6(2003)A207.

[7]Y.Q.Hu,M.M.Doeff,R.Kostecki,R.Finonesa,J.Electrochem.Soc.151

(2005)A1279.

[8]G.Li,H.Azuma,M.Tohda,Electrochem.Solid State Lett.5(2002)A135.

[9]C.Delacourt,P.Poizot,M.Morcrette,J.M.Tarascon,C.Masquelier,Chem.

Mater.16(2004)93.

[10]S.-Y.Chung,J.T.Bloking,Y.M.Chiang,Nat.Mater.1(2002)123.[11]O.Le Bacq,A.Pasturel,Phys.Rev.B69(2004)245107.

[12]K.Amine,K.Yasuda,M.Yamachi,Electrochem.Solid State Lett.3(2000)

178.

[13]J.M.Lloris,C.P.Vicente,J.L.Tirado,Electrochem.Solid State Lett.5

(2002)A234.

[14]W.C.West,J.F.Whitacre,B.V.Ratnakumar,J.Electrochem.Soc.150

(2003)A1660.

[15]M.Y.Saidi,J.Barker,H.Huang,J.L.Swoyer,G.Adamson,J.Power

Sources119–121(2003)266–272.

[16]H.Huang,S.-C.Yin,T.Kerr,N.Taylor,L.F.Nazar,Adv.Mater.14(2002)

1525.

[17]Y.Z.Li,Z.Zhou,M.M.Ren,X.P.Gao,J.Yan,Electrochim.Acta51(2006)

6498–6502.

[18]D.Morgan,G.Ceder,M.Y.Saidi,J.Barker,J.Swoyer,H.Huang,G.

Adamson,Chem.Mater.14(2002)4684.

[19]S.Patoux,C.Wurm,M.Morcrette,G.Rousse,C.Masquelier,J.Power

Sources119–121(2003)278.

[20]S.-C.Yin,P.S.Strobel,H.Grondey,L.F.Nazar,Chem.Mater.16(2004)

1456.

[21]S.-C.Yin,H.Grondey,P.Strobel,M.Anne,L.F.Nazar,J.Am.Chem.Soc.

125(2003)10402.

[22]M.Sato,S.Tajimi,H.Okawa,K.Uematsu,K.Toda,Solid State Ionics

152/153(2002)247.

[23]J.Barker,M.Y.Saidi,J.L.Swoyer,J.Electrochem.Soc.150(2003)A684.

[24]M.Higuchi,H.Katayama,Y.Azuma,M.Yukawa,M.Suhara,J.Power

Sources119–121(2003)258.

[25]M.Dutreilh,C.Chevalier,M.El-Ghozzi,D.Avignant,J.Solid State Chem.

142(1999)1.