JournalofPowerSources240(2013)328e337
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JournalofPowerSources
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Articlehistory:
Received23January2013Receivedinrevisedform13March2013
Accepted7April2013
Availableonline18April2013Keywords:
PolyanilinechlorideLiFePO4
Polymerpyrolysis
CarbonenitrogencoatingCrosslinking
Solidstatesynthesis
abstract
Afacileandef?cientsolidstatesynthesisofcarbonandnitrogencoatedlithiumironphosphate(LiFePO4/CN)cathodematerialisachievedviapolymer-pyrolysismethodusingpolyaniline-chloride(PANI-Cl).ThecurrentinvestigationiscomparativelyanalyzedwiththeresultsofthecompositeofLiFePO4/C(LFP/C)synthesizedusingsucroseascarbonprecursor.TheoptimizedLiFePO4/CN(LFP/CN)compositeissyn-thesizedat700??Cusing10wt.%PANI-Cl.Thecompositeexhibitsremarkableimprovementincapacity,cyclabilityandratecapabilitycomparedtothoseofLFP/C.Thespeci?cdischargecapacitiesashighas164mAhgà1(theoreticalcapacity:170mAhgà1)at0.1Cand100mAhgà1at10CrateswereachievedwithLFP/CN.Inaddition,thecompositeexhibitsalong-termcyclingstabilitywiththecapacitylossofonly10%after1000cycles.PANI-Clshiftsthesizedistributionofthecompositetonanometerscale(approximately150nm),howevertheadditionofsucrosedoesnothavesuchaneffect.LFP/CNcontains1.6wt.%nitrogenand15.8wt.%carbon.LFPparticlesaremostlycoatedwithafewnanometersthickCeNlayerformingacoreeshellstructure.ThepossiblecrosslinkingmechanismofPANI-Cluponpyrolysisonsizereductionandformationofuniformcarbon/nitrogencoatingonLFParealsodiscussed.
ó2013ElsevierB.V.Allrightsreserved.
1.Introduction
LiFePO4(LFP)isconsideredtobeoneofthemostpromisingcathodematerialforthenextgenerationhigh-powerlithiumionbatteriesduetoitslowcost,highsafety,non-toxicityandacompetitivetheoreticalcapacity[1,2].Theyareenvisionedtobeusedinelectricvehiclesandstationaryenergystoragesystemsforstoringsolarandwindpower.Unfortunately,lowelectroniccon-ductivityandslowdiffusionoflithiumionsacrossthetwo-phaseboundaryofLFPseriouslylimititsratecapability[3e6].Toover-cometheseintrinsiclimitations,manyeffortshavebeencarriedoutincludingparticlesizereduction[7],dopingwithsupervalentcat-ions[8,9],coatingwithconductivepolymers[6,10e12]andforming
*Correspondingauthor.Tel.:t902626772772;fax:t902626772309.E-mailaddresses:ercan.avci@tubitak.gov.tr,eavci07@yahoo.com(E.
Avci).0378-7753/$eseefrontmatteró2013ElsevierB.V.Allrightsreserved.http://dx.doi.org/10.1016/j.jpowsour.2013.04.030
acarbonlayer(LFP/C)[5,6,13e18].Uniformcarboncoatinggreatlyenhancesspeci?ccapacity,ratecapacityandcyclingperformanceofLFP[5,6].Thisisduetothepyrolyticcarbonintheproduct,whichsuppressestheparticlegrowthandthusleadingbothparticlesizeminimizationandintimatecarboncontact[19].However,itisquitedif?culttoobtainahomogeneouslycoatedcarbonshellonLFPparticlesduringtheheattreatment.Incaseofpartialformationofcarbonshell,aninsuf?cientelectronicallyconductingnetworkwouldleadtodecreaseintheratecapabilityofthematerial.
PreviousstudiesmostlyfocusedonthesynthesisofLFP/Ccom-positematerialsbyusinglowweightorganicprecursors[20,21],suchassucrose,glucoseorcitricacid,asthecarbonsource.Althoughthepolymerpyrolysismethodisarelativelysimpleandeffectivewaytoformcarbon-coatedmaterialssuchasceramic/carboncom-posites[22,23],ithasbeenrarelyappliedforcoatingtheLFPparti-cles[12,15,19].Insuchaprocess,pyrolizedpolymerscanleadtotheformationofreducedsizeLFP/Cparticlescoateduniformlywitha
E.Avcietal./JournalofPowerSources240(2013)328e337
329
Table1
ExperimentalconditionsforthesynthesisofLFP/CandLFP/CNcompositesandcarbonandnitrogencontentsdeterminedbyelementalanalysis.
NotationMaterial/carbonSinteringCarbonContents(wt.%)
sourcetemperature(??C)source(wt.%)
CNN/CLFP/C
LFP/Csucrose700108.4eeLFP/CN1LFP/CPANI-Cl6501016.12.10.13LFP/CN2LFP/CPANI-Cl7001015.81.60.10LFP/CN3LFP/CPANI-Cl7501015.51.60.10LFP/CN4LFP/CPANI-Cl700612.91.20.09LFP/CN5LFP/CPANI-Cl7001421.32.50.12e
PANI-Cl
e
e
59.812.3
0.21
fewnanometerthickcarbonlayers[15,19].Itis,however,importanttochoosetheproperpolymerprecursorfortailoringthepropertiesofLFP/Ccomposite.Recently,Nienetal.[15]preparedLFP/CsamplesformedbycalcinatingamorphousLFPwithvariouspolymerssuchaspolystyrene,polyethyleneoxide,polybutadieneat600??C.Theyre-portedthatpolystyrene(5wt.%)derivativewithfunctionalizedar-omaticgroupsexhibitedanimprovedperformance.Theyachievedacapacityof147mAhgà1and90mAhgà1at0.1Cand3Crates,respectively.Additionally,YuandFang[18]announcedthatLFP/Ccompositepreparedusingpolystyrenenano-spheres(7wt.%)sin-teredat800??Cdisplayedadischargecapacityof167mAhgà1and150mAhgà1at0.1Cand1C,respectively.
Polyaniline(PANI)exhibitsaninterestingbehavioruponpyrol-ysiscomparedtomanyotherpolymersbyforminganetworkstructureasaresultofcrosslinkingofpolymerchains[24e29].Mentusetal.reportedthatadisorderednitrogen-containinggraphitestructureisformeduponcarbonizationofsulfuricaciddopedPANIwithconductivityhigherthanthatofpristinePANI[26].Inthisarticle,weintroducethechloridedopedPANI(PANI-Cl)asacarbonenitrogensourceforthesolidstatesynthesisofLFP/CNcompositewhichhasverygoodpropertiesascathodematerialinhighpowerlithiumionbatteries.Thiscompositeseemstobea
Fig.1.XRDpatternsofLFP/CandLFP/CNcompositessynthesizedatdifferentcondi-tions(seeTable1
).
promisingalternativerouteforanindustrial-scaleproductionduetoitsfacilefabricationandlowcost.AmixtureofnanometerandmicrometersizeLFP/CNparticlesareformedasaresultofthepy-rolysisofPANI-Cl.Itisbelievedthatthecrosslinkedpolymerchainsinhibitthegrowthofcrystallinegrains.LFP/CNcompositesynthe-sizedat700??CusingPANI-Cl(10wt.%)exhibitssuperiorelectro-chemicalperformancesinwideoperationdischargeraterangecomparedtoLFP/Csynthesizedusingsucroseasacarbonprecursor.2.Experimental
2.1.SynthesisofLiFePO4/CandLiFePO4/CNcomposites
Asthestartingmaterialsforthesolid-statesynthesis,stoichio-metricamountsofanalyticalgrade2.24gofLi2CO3(Alfa-Aesar),10.99gofFeC2O4.2H20(Aldrich),6.98gof(NH4)2HPO4(Carlo-Erba)aswellas2.25gofPANI-Clor2.25gofsucrose(Carlo-Erba)ascarbonsourceprecursorswerethoroughlyplanetarymilledinmortarfor2h(300rpm)inthemixturesolventsof30mlofethanoland30mlofN-methylpyrrolidine(NMP,Merck).Theresultinggelwasdriedat100??Cinafurnaceandthenheatedto350??Cfor6hin
Fig.2.Thermogravimetriccurvesof(a)sucroseandPANI-ClunderN2?ow;(b)LFP/CNsinteredat650,700,750??Cunderair,(c)LFP/CNsynthesizedusing6,10,14wt.%PANI-ClandLFP/Csinteredat700??Cunderair.
330E.Avcietal./JournalofPowerSources240(2013)328e337
Aratmosphere.ThedecomposedmixturewaspressedintopelletsandsinteredunderAr?owat650,700,or750??Cfor12hinordertoobtainthecrystallinephasecomposites.LFP/Cprecursorhaving10wt.%sucrosecontentwassinteredat700??C(donatedasLFP/C).ThreeLFP/CNsamplesweresinteredat650??C,700??C,and750??Cbyusing?xed10wt.%PANI-CldonatedasLFP/CN1,LFP/CN2,andLFP/CN3,respectively.OthertwoLFP/CNsamplesweresinteredat700??Cbyusing6wt.%and14wt.%weightcontentsofPANI-Cl(namedasLFP/CN4,andLFP/CN5,respectively)asshowninTable1.
PANI-ClusedinLFP/CNmaterialswassynthesizedbyoxidativepolymerizationofaniline-hydrochloride(Sigma-Aldrich)withammoniumperoxydisulfate(APS,Sigma)at4:5molmolà1ratioandexcessamountofhydrochloricacid(HCl).Atypicalprocessisdescribedasfollows.First,aniline-hydrochloride(12.9g)wasdis-solvedin400mlofdistilledwaterandstirredwhilekeepingthemixtureat0e5??C.AprecooledAPS(28.4g)aqueoussolution(400ml)wasaddedintotheabovesolutiontogetherwith0.1MHCl(200ml)solution.Themixturewasreactedfor2hat0e5??C.Theresultingdarkgreenprecipitatewaswashedseveraltimeswithdeionizedwater,ethanolandacetone,consecutivelyandthendriedovernightat75??Cinavacuumoven.Underthepresentcondition,thesynthesisofhighdegreecrystallineofPANI-Clhavingmolecularweightaround85000isexpected[30].2.2.Characterizationofcathodematerials
ThecrystallographicstructuralcharacterizationwasperformedbyX-raypowderdiffraction.XRDoftheLFP/CandLFP/CNcom-positeswerecarriedoutonaRigakuMini?ex600diffractometerequippedwithCu-Karadiationofl?0.15405nmintherangeof0??<2q<80??.ThemicromorphologyoftheLFP/CpowderswasobservedusingaJEOLJSM6510-LVscanningelectronmicroscope(SEM).Transmissionelectronmicroscopy(TEM)measurementswereperformedusingJEOLJEM2100HRTEMoperatingat200kV
(LaB6?lament)withOxfordInstrumentsX-Sight6498energydispersivespectroscopy(EDS)andselected-areaelectrondiffrac-tion(SAED)systems.ThecarboncontentwasestimatedfromaLeco(TruSpec)elementalanalyzerandthermogravimetricanalysis(Mettler-ToledoTGA-851).Particlesizeanalysis(PSA)ofthecom-positeswasinvestigatedbydispersingthematerialsinwaterusingMalvernMastersizer-2000.
2.3.Cellfabricationandelectrochemicalmeasurements
ThecrystallinecathodeactivematerialsofLFP/CNorLFP/CweremixedandgroundedwithSuperP(12wt.%,TimcalCo.)andpoly-vinylidene?uoride(PVdF,8wt.%)asabinderdissolvinginNMPfor30min.Theresultingmixturewascasteduniformlyontoanaluminumfoilanddriedat100??C.Electrodeswerepunchedintheformofadischaving3e4mgcmà2ofactivematerial,pressedanddriedat100??Cfor4h.Thecoincells(CR2032)wereassembledinanargon-?lledglove-box(Vigor)withaLimetaldiscasanodeandWhatmanGF/Dglass-?berasseparatorand1MLiPF6inamixtureofethylenecarbonate(EC)anddiethylcarbonate(DEC)(1:1invol.ratio)astheelectrolyte.Cyclicvoltammograms(CV)(between2.0and4.3V)andgalvanostaticcharge/dischargemeasurementswerecarriedout(between2.2and4.2V)withPARVersaSTATMultichannelpotentiostat/galvanostat.Aftereachchargingstep,thecellswerefurtherrechargedholdingat4.0Vfor10mintorecovertheirfullcapacitybeforeapplyingasubsequentdischargestep.3.Resultsanddiscussion3.1.Materialcharacterization
XRDpatternsoftheLFP/CpowderspreparedusingdifferentconcentrationsofsucroseandPANI-Clatvaryingsinteringtem-peraturesareshowninFig.1.Allpeaksoneachcurvecan
be
Fig.3.(a)DehydrochlorinationofPANI-Cl,(b)crosslinkingchemicalreactionofdehydrochlorinatedPANI-Clchainsduringheattreatment[24,26,27].
E.Avcietal./JournalofPowerSources240(2013)328e337331
indexedasasinglephasewithanorderedorthorhombicolivinestructurewithaspacegroupPmnb(ICDD(PDF-2/Release2011RDB)DBcardnumber:01-074-9597)indicatingthesuccessfulsynthesisofphase-pureLiFePO4(latticeparameters:a?10.321??b?6.008??A,
A,c?4.692??A).Crystallitesizescalculatedaccordingto
Williamson-Hallmethodvarybetween25and28nm.
SucroseandPANI-ClwerethermallyanalyzedinordertoexaminetheeffectoftheirpyrolysisontheformationofLFP/CandLFP/CNcompositesasshowninFig.2(a).Thesampleswereheatedfrom25??Cto900??Catarateof10??Cminà1underN2?ow(40mlminà1).Thepyrolysisofsucrosetakesplaceatsinglestepstartingat200??Candsucroselosses75%ofitsweighttill700??C.Ontheotherhand,thepyrolysisofPANI-Clproceedsatfoursteps.The?rststepattemperaturesupto150??Cispossiblyduetothelossofunboundwater(w10wt.%).Thesecondstepinthetemperaturerangeof150e350??CisattributedtothelossofHCl(dehydro-chlorination,w20wt.%),ammoniaandboundwater[26,28,29].ThereactionschemeofdehydrochlorinationofPANI-CltakingplaceatthisheatingstepisshowninFig.3(a).Around200??C,dehydrochlorinatedPANI-Clchainsundergobothinter-macromolecularandintramacromolecularcrosslinkingprocesses[24e29].Asaresultofcrosslinking,nearlycompleteconversionofquinoidringstobenzenoidringstakesplaceasshowninFig.3(b)[24,27]Theweightlossatthethirdstep(w21wt.%)between350and600??Cmightbetheresultofthedegradationofthecross-linkedstructures,polymerchainbreakintoextendedaromaticfragments(benzene,aniline,diphenylamine,othergreateroligo-mers[31])aswellasammoniaremoval.Theforthstepatthetemperaturesover600??C(w5wt.%)maybeduetothedegra-dationofthecrosslinkedstructuresandpolymerbackbone[26,32].
Todeterminetheamountofcarboninthecomposites,TGAwascarriedoutinair(40mlminà1)ataheatingrateof10??Cminà1(Fig.2(b,c))inadditiontotheelementalanalysis.Thepercentcarbonamountsdeterminedbyelementalanalysis(Table1)areclosetothoseofobtainedfromTGAexperiments.Atthispoint,itisworthnotingthattheoxidationofFe(II)toFe(III)(formationofironoxides)whichisobservedattemperaturerangesbetween350and500??Cleadstoanincreaseof4.8wt.%fortheretainedmassofcomposites[33,34].ConsideringbothelementalanalysisandTGA,carboncontentsofLFP/CandLFP/CN2weremeasuredas8.4and15.8wt.%,respectively.ItisalsoimportanttonotethattheweightlossofsucroseandPANI-Clduringtheactivematerialpreparation(w62wt.%forLFP/C,w25wt.%forLFP/CN2)islowerthanthatduringthepyrolysisofthesecarbonsourcesduringTGAexperiment(w75wt.%forsu-crose,w55wt.%forPANI-Cl).ThisindicatesthatsomeofvolatilepyrolysisproductsofsucroseandPANI-Clsticktothecompositewallwithoutleavingthematerialandthismighthelpthefor-mationofmoreuniformcarbonorcarbonenitrogenshellaroundparticles.Anotherimportantpointtonoticeisthelowratioofresidualweightofnitrogenoverresidualweightofcarbon(N/C)inLFP/CNcompositescomparedtothatofpristinePANI-Cl(Table1).Thisshowsthattheremovalofammoniaandothernitrogencontainingpyrolysisproductstakesplacemorereadilycomparedtothecarboncontainingproducts.
Fig.4showstheparticlesizedistributionsofpyrolizedformsofcarbonsources,LFP/CandLFP/CNsamples.ThedistributionofpyrolizedPANI-Clisnarrowandcenteredonw10mm,whereasthedistributionforthepyrolizedsucroseisbroadandcenteredonw100mm(Fig.4(a)).Thedistributionshiftstosmallersizes(centeredonw3mm)forLFP/Ccompositecomparedtothatofpyrolizedsucrose.SizedistributionsofLFP/CNcompositespre-paredatdifferentconditionsexhibitaninterestingbehaviorlocalizingattworegions(Fig.4(b,c)).Onedistributionregionis
Fig.4.Particlesizedistributionsof(a)sucroseandPANI-Clpyrolizedat350??C(6h),700??C(12h)underAr?owandLFP/C;(b)LFP/CNsinteredat650,700,750??C,(c)LFP/CNsynthesizedusing6,10,14wt.%PANI-Cl.
centeredonw150nm.Ontheotherhand,anotherregioniscenteredonw2mm.Thebordersofthoseregionsdonotchangewiththesynthesisconditions,suchassinteringtemperatureortheweightcontentofPANI-Clintheprecursor.However,thenumberofparticlesintheseregionsstrictlydependsonthesynthesisconditions.ThenumberratioofsubmicrometerLFP/CNparticlesincreaseswithsinteringtemperature(Fig.4(b))anddecreasessurprisinglywiththecontentofPANI-Clintheprecursor(Fig.4(c)).ThedistributionsclearlyrevealthatPANI-Clrestrictsthegrowthofthecompositeparticles,butthisresultdoesnotmeanthathighercontentofPANI-Clwouldleadtotheformationofsmallerparticles.Approximately,40vol.%ofLFP/CN2particlesarebelow500nmindiameter,whereasmostofLFP/Cparticlesarelargerthan1mmindiameter.
ThemorphologyofthecompositeswascharacterizedbySEM.Fig.5showstheSEMimagesofLFP/CNandLFP/C
composites
332E.Avcietal./JournalofPowerSources240(2013)328e337
Fig.5.SEMimagesof(aec)LFP/CNcompositessinteredat650,700,750??C(10wt.%PANI-Cl);(d,e)LFP/CNsynthesizedusing6,14wt.%PANI-Clsinteredat700??C,respectively;and(f)LFP/Csinteredat700??C.
synthesizedatdifferentconditions.Themicrographsexhibitmostlyasimilarcharacteristicasthoseoftheparticlesizeanalysis(PSA,Fig.4).LFP/CNcompositesappeartohavebothnanometerandmicrometergrainsizewithirregularmorphology(Fig.5(aee)).However,LFP/Cparticlesaremostlyinmicrometersize(Fig.5(f)).AsthesinteringtemperatureofLFP/CNincreasesfrom650to750??C,formationofnanometersizeparticlesbecomesdominantasshownFig.5(aec).Thisindicatesthattheparticlesbecomemorepulverizedwithtemperature.SimilartoPSAresults,highercontentofPANI-Clresultsinlargercompositeparticleasobservedinthemicrographs(Fig.5(b,d,e)).BothPSAandSEMresultsrevealthatcarbonenitrogencoatingviapyrolysisofPANIClplaysanimportantroleincontrollingthegrowthofparticles.ThereducedparticlesizeresultsinshorteningofthediffusionpathofLitandhencebetterelectrochemicalperformanceofLFP/CNcomparedtothatofLFP/C.Thisisfurthersupportedbytheelectrochemicalmeasurementsdescribedbelow.
Tocon?rmhowthecarbonisdispersedwithinLFPpowder,thenanoscalemicrostructuresofLFP/CandLFP/CN2compositesarefurtherstudiedusingTEM,SAEDandEDSmethodsasshowninFig.6andFig.7,respectively.TherearediscretecarbonlayerscoatingonbothLFPparticlesasshowninFig.6(aed)andFig.7(aed).OneofthecommonfeaturesoftheTEMimagesofbothsamplesisthatlargecarbonsegmentsarelocatedbetweenlargeLFPparticles(Fig.6(a)andFig.7(a)).TheEDS/SAEDspectrainFig.6(eeh)andFig.7(eeh)indicatethepresenceofcrystallineLiFePO4(darkerregions)andabundantamorphouscarbonphases.However,afewweakspotsinadditiontohollowringobservedintheSAEDpatternofLFP/CN(Fig.7(f))indicatethepresenceofsomecrystallinecarbonphase.Thiscrystallinecarbonphaseispossiblyduetothearomaticpyrolysisproducts.Theimagesathighermagni?cation(Fig.6(d)andFig.7(d))demonstratethatthecoatingisextendedoverthesurfacesofLFPparticlesformingacoreeshellstructure.Thicknessesoftheshellsaremeasuredas3.1nmand1.6nmforthesamplesofLFP/CandLFP/CN2,respec-tively.Thecompletecarboncoatingaroundorbetweenthepar-ticlesisexpectedtocontributetotheimprovedelectrochemicalpropertiesofthecomposites.
ConsideringtheresultsderivedfromPSA,SEMandTEMmethods,thepossibleexplanationforthesizereductioneffectofPANI-ClcanbegiveninFig.8.Inthisscheme,PANI-Clwas?rstmixedwithLFPprecursors,followedbyheatingat350??Cfor6h.Duringthisheatingperiod,calcinationoftheprecursorsandthecrosslinkingofPANIchainsoccur.Uponpyrolysis,formationoflowdegreeandhighdegreecrosslinkedofPANInetworkstructureisexpected.Theproductwasfurthersubjectedtoheattreatmentsintherange650??Ce750??CunderN2atmospheretosintertheLFPandcarbonizePANI.Atthisheatingstage,PANIchainsbreak
into
E.Avcietal./JournalofPowerSources240(2013)328e337333
Fig.6.(aed)TEMandHRTEMimagesofLFP/Ccompositeparticles;(e,f)SAEDpatternsfortheparticles;(g,h)EDSanalysesforthe
particles.Fig.7.(aed)TEMandHRTEMimagesofLFP/CN2compositeparticles;(e,f)SAEDpatternsfortheparticles;(g,h)EDSanalysesfortheparticles.
334E.Avcietal./JournalofPowerSources240(2013)328e337
Fig.8.SchematicdiagramofformationofnanometerandmicrometersizeLFP/CN
composite.
aromaticfragments,whilelowerweightvolatilefragmentsleave
thecomposite,heaviervolatilefragmentsarenotabletoleavethe
materialandpossiblysticktothewallofthecomposite.Addi-
tionally,asthehighdegreecrosslinkedPANIproductsformrela-
tivelylargersizeCeNsegmentsaroundmicrometersizeLFP
particles,lowdegreecrosslinkedproductsmostlysurroundrela-
tivelysmallersizeLFPparticlesformingnanometersizeLFP/CN
particles.
3.2.Electrochemicalmeasurements
Fig.9showsthecyclicvoltammogram(CV)curvesofLFP/CN
sinteredatdifferenttemperaturesanddifferentconcentrationsof
PANI-Clprecursorsscannedatarateof0.1mVsà1.Allsamples,
exceptthesampleofLFP/CN5synthesizedusingthehighestPANI-Cl
weightcontent,revealthesingle-electronreactionmechanism
indicatingthecharacteristicredoxbehaviorofLiFePO4cathode
material.However,incomparisonwiththesampleshavingless
PANI-Clcontent(LFP/CN2andLFP/CN4),LFP/CN5composite
(21.8wt.%carbon)possesstwobroadoxidationpeakslocatedat3.6
and4.0Vandasinglebroadreductionpeakat3.2V.
Fig.10showstheresultsofcyclabilitytestsofLFP/CandLFP/CN
electrodesatthedischargingraterangingfrom0.2Cto10C
(1C?170mAhgà1).InFig.10(a),theeffectofvarioussintering
temperatures(650,700and750??C)oncyclabilityofLFP/CNelec-
trodesisdemonstrated(using10wt.%PANI-Cl).Clearly,thehighest
dischargecapacitiesatallC-ratesareobtainedfromtheelectrodeof
LFP/CN2sinteredat700??C(0.2C:163mAhgà1,1C:148mAhgà1,
10C:100mAhgà1),whereasthelowestcapacitiesareobservedat
theelectrodematerialsinteredat750??C(LFP/CN3).AlthoughLFP/
CN3containsalmostthesameamountofcarbonandnitrogen(see
Table1)aswellasitsparticlesaresmaller(Fig.4(b),Fig.5(b,c))
comparedtoLFP/CN2material,itexhibitsalowerelectrochemical
performance.Thereasonbehindthiscontradictioncanbeattrib-
utedtotheblockingin1DchannelswheretheLitionsmovein
LiFePO4crystalasaresultoftheformationofundesiredimpurities
suchasFe2P,Fe2O3,Li3Fe2(PO4)3orantisitedefects[35].Whenthe
XRDpatterns(Fig.1)areexaminedclosely,impurityphasesarenot
encountered.Therefore,theformationofantisitedefectsasaresult
ofthehigherreactiontemperatureseemstobethestrongerreason
behindtheformationofblockedchannelsaswellaslowerelec-
trochemicalperformance.To?ndouttheeffectofconcentrationof
PANI-ClonthecyclabilityofLFP/CNelectrodes,thesinteringtem-
peratureis?xedat700??Candthreedifferentprecursorshaving6,10and14wt.%PANI-Cl(LFP/CN4,LFP/CN2andLFP/CN5,respec-tively)wereinvestigated(Fig.10(b)).Substantially,theperfor-mancewasnotnecessarilybetterwithhighercarboncontents.Itisclearthat10wt.%PANI-Clcontentintheprecursoristheoptimalvalueamongthepreparedsamplesandresultsinthehighestdischargecapacitiesattheraterangingfrom0.2Cto10C.Thesamplehavingthehighestcarboncontent(LFP/CN5)hasthelowestcapacityatlowrates,butsurprisinglyhasrelativelyhighercapacityathigh10Crate.Therapiddecreaseofperformanceatlow
ratesFig.9.CVcurvesofLFP/CNcomposites(a)sinteredat650,700,and750??C,(b)using6,
10,and14wt.%PANI-Clsinteredat700??Catscanrateof0.1mVsà1.
E.Avcietal./JournalofPowerSources240(2013)328e337335
Fig.10.Ratecapabilityof(a)LFP/CNcomposites(10wt.%PANI-Cl)sinteredat650,700,and750??C,(b)LFP/CNcompositessynthesizedusing4,10,and14wt.%PANI-Clsin-teredat700??C,and(c)LFP/CandLFP/CN2(10wt.%)sinteredat700??C.
canbeattributedtotheformationofexcessiveamorphouscarbonphasedilutingthedensityofthecrystallineLFPphaseandthesuppressionoftheformationofcrystallineLFPphasebyexcesscarbon[36].
InFig.10(c),thecyclabilityperformancesofLFP/CandLFP/CN2electrodesarecomparedbasedonvaryingdischargingrates.IncomparisontoLFP/Ccomposite,LFP/CN2compositedemonstratesapromisingcapacity,cyclability,andrateperformance.Thespeci?cdischargecapacitiesashighas163mAhgà1at0.2C,100mAhgà1at10Crateswitha2%capacityfadingafter100cycleswereobtainedforLFP/CN2.Ontheotherhand,LFP/Cexhibitsthespeci?cdischargecapacitiesof149mAhgà1at0.2C,60mAhgà1at10Crateswiththecapacityfadingof5%after100cycles.Thisexcellentelectro-chemicalperformanceofLFP/CN2impliesthatthecoatingofLFPsurfacewithpyrolyzedPANI-Clfacilitatestheelectrochemicalinsertion/extractionprocessofLition,especiallyathighrate.
Cyclicvoltammetrywasusedtoexaminethekineticsoflithiumintercalationandde-intercalation.Fig.11(a)and(b)showstheCVcurvesofLFP/CandLFP/CN2atanincreasingscanratefrom0.1to2mVsà1,respectively.Itisclearthattheintensityandtheareaundertheredoxpeaksforbothcompositesincreaseswiththescanrate.UsingRandlesSevcikequation,ip?2.69?105n3/2AD1/2Cn1/2,thelineardependenceofpeakcurrent(ip)ofCVofsamplesonthesquarerootofscanrate(n1/2)canbeobservedinFig.11(c).ConsideringtheparametersofA(electrodearea),C(concentrationofLit)andn(numberofelec-tronsinvolvedintheredoxprocess)are?xedvaluesforbothelectrodes,thediffusioncoef?cientoflithium(DLi)ofLFP/CN2isfoundtobe1.35timeshigherthanthatofLFP/C.ThisindicatesthatthecoatingofLFPwithcarbonandnitrogenoriginatingfromPANI-ClismoreuniformanditimprovestransportationofLiionsduringcell
operation.
Fig.11.CVcurvesof(a)LFP/Cand(b)LFP/CNsinteredat700??C,atvariousscanrates.(c)Therelationshipofpeakcurrent(ip)andthesquarerootofscanrate(n1/2)forboth
samples.
336E.Avcietal./JournalofPowerSources240(2013)328e337
Fig.12(a,b)presentsthegalvanostaticchargeedischargevoltagepro?lesofcellscontainingLFP/CandLFP/CN2attheincreasingratefrom0.2Cto10Cbetween2.2and4.2Vvs.Lit/Li.Bothsamplesexhibittypical?atcharge-dischargeplateausatapproximately3.4VimplyingtheFe2t/Fe3tredoxreaction.AsshowninFig.12(a),theinitialdischargecapacityofLFP/Cisaround150mAhgà1at0.2Crate,whereasLFP/CN2exhibitsahigherdischargecapacityof163mAhgà1atthesamecondition.AthigherC-rates,againLFP/CN2compositedemonstratesasuperiorperformancecomparedtotheLFP/Csample.AsshowninFig.12(c),itisclearthatLFP/CN2demonstratesaslowerenlargementinvoltagepolarizationcomparedtotheLFP/CsampleastheC-rateincreases.
LongtermcyclingcapabilityofLFP/CN2sampleistestedatvaryingC-ratesasshowninFig.13.Theinitialdischargecapacityofthecompositeis164mAhgà1at0.1Candfoundtodecaygraduallywithcontinuouscyclingatincreasingrates,retaining148mAhgà1at0.1Cafter1000cyclessufferingonly10%capacityloss.Suchasuperiorelectrochemicalperformancesurelycansatisfythepowerrequirementsofelectricvehicles[37].
Fig.12.Chargeedischargepro?lesof(a)LFP/Cand(b)LFP/CN2(10wt.%PANI-Cl)sinteredat700??C.(c)ThedependenceofvoltagedifferencebetweencharginganddischargingathalfcapacityonC-rateforboth
samples.
Fig.13.Cyclabilityperformance(1000cycle)ofLFP/CN2synthesizedusing10wt.%PANI-Clsinteredat700??C.
4.Conclusions
Withasimpleandef?cientway,themixtureofnanometerandmicrometersizeLFP/CNcompositewassuccessfullypreparedusingPANI-Clascarbonnitrogenprecursor.CrosslinkingofPANI-Cluponpyrolysispossiblyleadstosizereductionandformationofcom-pletecarbonenitrogencoatingonLFP.TheamountofPANI-Clintheprecursorandthesinteringtemperaturehassigni?canteffectsonthephysicalandelectrochemicalpropertiesofthecomposites.TheoptimizedLFP/CNispreparedusing10wt.%PANI-Clandsinteredat700??C,whichcontains1.6wt.%nitrogeninadditionto15.8wt.%carbon.TheLFP/CNcompositeexhibitsahigherelectrochemicalperformancecomparedwithLFP/Ccompositesynthesizedusingsucrose.LFP/CNparticlesaremostlycoveredwithafewnanometerthickcarbonenitrogenlayer,formingacoreeshellstructure.Thespeci?cdischargecapacitiesof164mAhgà1at0.1C,100mAhgà1at10Crateswitha2%capacityfadingafter100cycleswereachieved.Besides,thecompositeexhibitsalong-termcyclingstabilitywiththecapacitylossofonly10%beyond1000cycles.Acknowledgments
TheauthorsthanktoTheScienti?candTechnologicalResearchCouncilofTurkey(TUBITAK)forthe?nancialsupportundertheCOSTprogramme(contractno.111T567).ThanksarealsotoM.Sc.SerkanGürbüzforXRDmeasurementsandM.Sc.BerrinEnginforTGAexperiments.References
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