Highly ordered LiFePO4 cathode material for Li-ion batteries templated by surfactant-modified polystyrene colloidal crystals

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Journal of Power Sources, 205, pp. 414-419, 2012-01-10

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Highly ordered LiFePO4 cathode material for Li-ion batteries templated

by surfactant-modified polystyrene colloidal crystals

Yim, Chae-Ho; Baranova, Elena A.; Abu-Lebdeh, Yaser; Davidson, Isobel

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JournalofPowerSources205 (2012) 414–419

ContentslistsavailableatSciVerseScienceDirect

Journal

of

Power

Sources

j o ur n a l ho m e p age :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

Highly

ordered

LiFePO

4 cathode

material

for

Li-ion

batteries

templated

by

surfactant-modified

polystyrene

colloidal

crystals

Chae-Ho

Yim

a

_,

_Elena

_A.

_Baranova

b

_,

_Yaser

_Abu-Lebdeh

a,∗

_,

_Isobel

_Davidson

a

a_National_Research_Council_of_Canada,₁₂₀₀_Montreal_Road,_Ottawa,_ON,_Canada_K1A_0R6

b_Department_of_Chemical_and_Biological_Engineering,_University_of_Ottawa,₁₆₁_Louis_Pasteur,_Ottawa,_ON,_Canada_K1N_6N5

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received8December2011 Accepted2January2012 Available online 10 January 2012 Keywords:

LiFePO4

Li-ionbattery Cathodematerial

Three-dimensionallyorderedmacroporous structure

Organictemplateassistedsynthesis

a

b

s

t

r

a

c

t

AnorganictemplateassistedsynthesiswasdevelopedtoobtainhighlyorderedporousLiFePO4cathode

material.Thedevelopedsynthesisenabledtheuseofpolystyrene(PS)astemplatebymodifyingitssurface

withasurfactant(Brij78)torenderithydrophilic.Thematerialwassynthesizedinhighyieldandpurity

asconfirmedbyX-raypowderdiffraction.TheTEMandSEMimagesclearlyconfirmedthepresenceofa

highlyorderedporousstructureandthelattershowedthatthestructurehasanaverageporediameter

of400nmandawallthicknessanddepthof∼100nm.Thermalscansandelementalanalysisshowed

thatthematerialcontainsahighamountofcarbonreaching23–28%byweight.Thesurfaceareawas

calculatedusingtheBETmethodandfoundtobe7.71m2_g−1_._Li/LiFePO

4halfcellsweretestedandgave

satisfactorydischargecapacities;aninitialcapacity(158mAhg−1₎_close_to_the_theoretical_value_and

recoverablecapacitiesathighC-rates(2–5C).

1. Introduction

Li-ionbatteriesarecurrentlyusedinmanyconsumerelectronic productsduetotheirsuperiorperformance(e.g.higherenergy den-sityandlongercyclelife)overotherbatterytechnologies[1].They arethereforeunderconsiderationforawidevarietyofapplications whereanenergystoragesystemisneededwithmostofthe atten-tionisfocusedontransportation[2]andtheelectricalgrid[3].This requiresafurtherimprovementintheperformanceofthebatteries, lowertheircostandenhancetheirsafety,whichcanbeachievedby enhancingthefunctionalityofcurrentlyusedmaterialsor introduc-ingnewones.CathodematerialsbasedonolivinestructureLiMPO4

(M:Mn,Fe,CoNi)arecurrentlyconsideredasaviablealternative tothosebasedontheLiMO2layeredoxidestructure,withmostof

thedevelopmentisfocusedonLiFePO4toreplaceLiCoO2.LiFePO4

showsahightheoreticalcapacity∼170mAhg−1_at_3.4_V._It_is_less

toxic,lesscostlyandmoreenvironmentalfriendlythanLiCoO2.It

isalsointrinsicallysaferduetothestrongP Ocovalent charac-terofthepolyanion(PO4−3)groupthatrendersthematerialless

pronetooxygenreleasethanlayeredoxides.Themajordrawback ofLiFePO4and olivinematerialsingeneralistheirlow intrinsic

electronic(∼10−9_S_cm−1₎_and_ionic_conductivity_{(uni-dimensional}

and/ornotnormaltothecrystallineplanes).Severalapproaches

∗ Correspondingauthor.Tel.:+16139494184;fax:+16139912384. E-mailaddresses:Yaser.Abu-Lebdeh@nrc-cnrc.gc.ca,yagal73@yahoo.ca

(Y.Abu-Lebdeh).

havebeensuggestedtoovercometheproblemsuchas introduc-ingcarbonasacoating[4]onthesurfaceoftheparticlesorasa composite[5],or“doping”withmetalpowders[6]orsupervalent cations[7]ortheformationofironphosphidenetworks[8].

Threedimensionallyorderedmacroporous(3DOM)materialsor inverseopalsisonetypeofhighlyporousmaterialsthatattracteda lotofattentioninrecentyearsandhavebeenusedinawiderange ofapplicationssuchasinbiosensors[9–11],photonics [12–14], andnanofabrications[15].Inbatteries,betterratecapabilitywas demonstratedwithawidevarietyofelectrodematerialsthathave beensynthesizedwitha3DOMstructureduetofasterionic mobil-itybyprovidingcontinuousandveryshortionicpathsandhigh surfaceareaforgreatercontactwiththeelectrolyte.Thisismostly doneby a soft chemical methodthat involves impregnationof theinorganicprecursorsolutionintotheorderedtemplatethatis usuallyorganicinnaturelikepolymers,synthesizedbyradical poly-merization,suchaspoly(styrene)(PS)orpoly(methylmethacrylate) (PMMA)[16],orinorganicsuchassilica[17].Also,whentheordered template is removeda structure withcloselypackeduniformly sized spherical pores (>10nm) and thick walls (10–100nm) is formed.Inthecasewhenanorganictemplateisuseditservestwo purposes:besidesactingasatemplatedirectingthestructureinto threedimensionalorder,itsthermaldecompositionduring syn-thesisleadstotheformationofaconductivecarbon coatingon thesurfaceofthesynthesizedelectrodematerial,which isvery advantageousinthecaseofolivines.

Recently,Dohertyetal.[18]synthesized3DOMLiFePO4using

a polymethyl methacrylate (PMMA) colloidal template and Lu

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Fig.1. SchematicoftheexperimentalprocedurewithandwithoutBrij78.

etal.[19]usingacopolymerofpoly(styrene-methyl methacrylate-acrylicacid). The use of PMMA as a colloidal templates was a naturalchoiceduetoitshydrophilicitythatallowsforthe infil-trationofLiFePO4aqueousprecursor(CH3COOLi,Fe(NO3)3,H3PO4)

solutionintoitsinterstitialvoidsbyinteractingwiththeionic acry-lategroupsofthepolymer.However,ithasbeendemonstratedthat theperformanceofLiFePO4asacathodematerialinLi-ion

batter-iesdependsontheamount,typeandmorphologyofthecarbon coatingthatinturniscontrolledbythechemistryofthecarbon source[20–22].Awidevarietyofcarbonsourceswerestudiedby otherstoimprovetheelectronicconductivityofolivinematerials [21,22].Nienetal.[22]studiedtheeffectofdifferenttypesof poly-mersandfoundthatPSasacarbonsourceresultedinthehighest capacityforLiFePO4.However,PSunlikePMMAistoo

hydropho-bictoallowfortheimpregnationoftheLiFePO4aqueousprecursor

solutionintoitsinterstitialvoids.Althoughionicresiduesfromthe initiatorafterpolymerizationcouldmakeitslightlyhydrophilicand allowforthesynthesis,hereinweproposeamoreeffectivewayto overcometheproblembasedontheuseofasurfactant.Brij78is anon-ionicsurfactantthatcontainsalonghydrophobic(aliphatic) andhydrophilic(etheric)chain.InfiltrationofBrij78intothePS templateallowsforthehydrophobicchaintointeractwiththe sur-faceofthePSandanchoringthesurfactantmoleculesleavingthe hydrophilicsidefreetointeract(complex)withtheionsinthe pre-cursorsolutionpossiblythroughtheionpairsofethericoxygens. Thiswillallowforafacileimpregnationoftheaqueousprecursor solutionintotheinterstitialvoidsofthetemplate.

Inthiswork,thesynthesisof3DOMLiFePO4assistedbyaPS

templatewassuccessfullypossiblewithveryhighyieldby modify-ingthesurfaceofPSwithanon-ionicsurfactant(Brij78)toenhance itsaffinity(hydrophilicity)totheaqueousprecursorsolution.The obtainedhighlyorderedporousLiFePO4materialwascharacterized

byelementalanalysis(EDXandXRF),SEM,TEM,TGA,andXRD.Its performancewasfullyinvestigatedinLi/LiFePO4halfcellbatteries

(Fig.1).

2. Experimental

2.1. Synthesisofpolystyrene(PS)spherescolloidaltemplate

PScolloids weresynthesized byemulsionpolymerization,as describedbyHollandetal.[23].Thestyrenemonomerwasobtained from Sigma–Aldrich (≥99% Reagentplus) and was purged with nitrogenpriortouse.Then,50mLofstyrenewasaddedto425mL ofde-ionized water,which wasalsopurgedwithnitrogen.The mixture was heated to 70◦_C. _To _initialize _the _{polymerization,}

nitrogen-purgedaqueoussolutionof0.1MK2S2O8(Sigma–Aldrich,

99%)wasaddedtothemixture.Thesolutionwaskeptat70◦_C_for

24hundernitrogen.ThePScolloidswerewashedwithethanol (CommercialAlcoholsInc.,95%)andcentrifugedat10000rpmfor 1htoobtaintheorderedorganictemplate,asshowninFig.2,Step1. Thisstepwasrepeatedthreetimestoremovetheinorganic impu-rities.Theresultingtemplatewasdriedinopenatmosphereprior totheinfiltrationofLiFePO4.

Fig.2.Generalsynthesisof3DOMmaterial.Step1:orderingorganiccolloids;Step2:renderinghydrophobicitywithBrij78;Step3:immersionwithprecursorsolution;and Step4:calcinationtoproduce3DOMmaterial.

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416 C.-H.Yimetal./JournalofPowerSources205 (2012) 414–419

2.2. SynthesisofLiFePO4 2.2.1. 3DOMLiFePO4

To infiltrate the hydrophilic LiFePO4 precursor solution,the

hydrophobicityofthePStemplate waschangedwithBrij78,as showninFig.2,Step2.First,0.01gmL−1_of_Brij₇₈_{(Sigma–Aldrich)}

solutionwaspreparedbyadding0.3gofBrij78to30mLofethanol. ThePStemplatewasimmersedintheBrij78solutionfor5minto makesuretheBrijisinfiltratedintothetemplate.Theexcess solu-tionwasremovedusingvacuumfiltration,andthetemplatewas transferredtoanaluminaboatforfurtherinfiltrationofthe olivine-basedprecursor solution.Thesolutionwasprepared bymixing LiNO3,H3PO4(BDHInc.,85%),andmetalchloride,FeCl2·4H2O(Alfa

Aesar,98%)indistilledwater.Theprecursorsolutionwasprepared toyield1goftheolivine-basedmaterial.Themetalchloridewas fullydissolvedin1gofwater,andH3PO4andLiNO3wereadded

andmixeduntilthesaltswerefullydissolved.Theprecursor solu-tionoftheolivine-basedmaterialwasinfiltratedintothetemplate. Apipettewasusedtoslowlycoatthetemplatewiththesolution, asshowninFig.2,Step3.Theinfiltratedtemplatewasallowedto dryovernight.Then,thedryorganictemplatewassinteredunder argon(Linde Canada,99.999%). Calcinationswere performedin twostagesunderargon:onceat260◦_C_for₃_h_to_remove_the_PS

templatefromtheolivine-basedmaterialsandagainat700◦_C_for

3htoobtaintheolivine-basedmaterialswitha carboncoating. TopreventthevolatiledecompositionofthePStemplate,aslow temperaturerateof2◦_C_min−1_was_used.

2.2.2. Standardnon-templatedLiFePO4/C

A solid-state reactionwas usedto synthesizeLiFePO4 (LFP).

Li2CO3 (Sigma–Aldrich,99%),Fe(C2H3O2)2·2H2O(RiedeldeHaen,

99%),andNH4H2PO4(Sigma–Aldrich,99%)weremixedinamolar

ratioof0.5:1:1.Citricacidwasaddedtothemixturetocarboncoat theLFP.Theamountofcitricacidwas5wt%offinalsynthesizedLFP. Themixturewassoftballmilledusing60mLofNALGENEjar(VWR) withzirconiabeads(TosohCorp.,5mmindiameter)for homoge-nousmixing.EnoughacetonetofillhalfoftheNALGENEjarwas addedtothemixturewith20zirconiabeads.Themixturewasball milledforthreedaysandthenacetonewasevaporatedunderthe fumehood.Theremainingmixturewassinteredunderargonat 700◦_C.

2.3. CharacterizationofPSand3DOMLiFePO4

PowderX-raydiffractionwascarriedoutbetween15◦_and₈₅◦

(2angles) usingaBrukerAXSD8diffractometerwitha CoK␣ sourcewith35kVand45mAofpower,adivergenceangleof0.3◦_,_a

stepsizeof0.027◦_,_and_an_acquisition_time_of₁_s_per_step._The

pat-ternswereanalyzedbytheRietveldrefinementmethod[24]using thesoftwareTOPASv4.2fromBrukerAXS[25].Themorphology ofthematerialwascharacterizedbyscanningelectronmicroscopy (SEM)usingaJEOL840A.Thermalgravimetricanalysis(TGA)was performedtoquantifytheamountofcarbonpresentinthe3DOM LiFePO4.ATGA2950TAinstrumentwasused.Thecompositewas

heatedat10◦_C_min−1_up_to₁₁₀◦_C_for₃₀_min_to_remove_moisture

containedinthematerialandthenheatedcontinuouslyupto800◦_C

inairinordertooxidizethecarbonfromthecomposite.

The elemental analysis technique, X-ray fluorescence (XRF) analysis,wasperformedusinganautomatedBrukerAXSS4 Pio-neer.TheS4pioneerisa4kWsequentialwavelength-dispersive XRFspectrometerusinganRh-anodeX-raytubewith75␮mthin Beendwindow.Allthemeasurementsweredoneundervacuum modeusinga28mmmaskanda0.23◦_collimator._The_sample_was

preparedbythelithiumboratefusionmethod.100mgofsample wasmixedwith7gofflux.ThetypeoffluxusedwasClaisse,which ismadeof66.67%ofLi2B4O7,32.83%ofLiBO2,and0.5%ofLiBr.The

mixtureswerefusedwithaClaisseM4three-positiongasfluxer producing32-mmglassbeadsat1000◦_C.

Transmission electron microscopy (TEM) analysis of 3DOM LiFePO4wascarriedoutusingaJEOLJEM2100FFETEMmicroscope

operatingat120kV.ThesamplewaspreparedforTEManalysisby sonicatingandsuspendingthe3DOMLiFePO4 inethanol.Copper

gridwasdippedtothesamplesolutionseveraltimesanddried onair.Bulkelementalcompositionoftheresulting3DOMLiFePO4

depositedoncarbonsupportwasstudiedusingenergydispersive X-rayanalysis(EDX).TheEDXanalysiswasperformedusingaJEOL JSM-7500Ffieldemissionscanningelectronmicroscopeequipped withanenergydispersiveX-rayspectrometer(OxfordInstrument). Thespectrawereacquiredatanaccelerationvoltageof20kV.

The specific surface area was estimated by nitro-gen adsorption–desorption porosimetry at 77K via the Brunauer–Emmet–Teller(BET)method.Theinstrumentemployed wasaMicromeriticsASAP2000system.Priortoeachmeasurement, thesamplewasevacuatedovernightat150◦_C_under_a_pressure,

10−6_Torr.

2.4. Lithiumbatteryperformanceof3DOMLiFePO4inLi-ioncell

3DOMLiFePO4 wastestedasa Li-ionhalf-cellusinga

2325-typecoincell.Theelectrodewaspreparedfromaslurrymadeof 75wt%activematerial,5wt%carbonSuperS(Timcalgraphiteand Carbon,Switzerland),5wt%carbongraphiteE-KS4(KG,LonzaG+T, Switzerland)and15wt%ofpolyvinylidenefluoride(PVDF,Kynar Flex 2800dissolvedinN-methyl-pyrrolidonefromAldrich)asa binder.Thecastwaspreparedusinganautomateddoctorblade spreaderanddriedovernightinaconvectionovenat85◦_C,_and

thenina vacuumovenovernightat80◦_C._Individual_electrodes

(Ø=12.5mm) werepunched out, dried at 80◦_C _under _vacuum

overnight,pressedunderapressureof0.5metricton,andwere usedaspositiveelectrodes.Alithiumdisk(Ø=16.5mm)wasused as a negative electrode (counter electrode and reference elec-trode).70␮Lof1MLiPF6 inEC:DMC(1:1byvolume)wasused

aselectrolyteandspreadoveradoublelayerofthepolypropylene separators(Celgard30).Thebatterieswereassembledinan argon-filleddrybox.ThecellswerecycledgalvanostaticallyusinganArbin batterycyclerandtestedusingcyclicvoltammetrytechniques.The cellsweretestedatdifferentC-ratesofcharge/discharge:C/12,C/6, C,2Cand5C.Fivefullcyclesofcharge/dischargeweremeasured foreachC-rate.

3. Resultsanddiscussion

AwhitepowderofPSsphereswasobtainedbycentrifugationof themilkysolutionattheendofpolymerizationreactionofstyrene, separationoforderedPStemplatefromthesolutionanddrying the PStemplate. Coating thepowder withBrij 78 followed by impregnationwithprecursorsolutionandcalcinationsgaveablack LiFePO4powder.Fig.3showstheRietveldrefinementofthe

synthe-sizedLiFePO4powderwiththereferencepeaksforLiFePO4(ISCD

161479)shownasblacklinesrightabovethex-axis.TheXRD pat-ternsshowa pureLiFePO4 phasewithnoevidenceofanyother

phasesorimpurities.ThespacegroupofLiFePO4wasidentifiedas

Pmnawithacrystalsizeof78.1nmandthelatticeparametersare determinedandlistedinTable1.Theparametervaluesandcrystal structureagreeverywellwiththeresultsdescribedinthe litera-ture[18,26–28].Thereferencepeaksareusedtosimulatethefitin ordertoevaluatethelatticeparameterandcrystalsizeusingthe softwareTOPASv4.2.

ThesurfaceareaofthecompositewasmeasuredusingtheBET method.Itwasfoundtobe7.71m2_g−1 _which_was_much_lower

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Fig.3.RietveldrefinementoftheX-raydiffractionpatternofthe3DOMLiFePO4.

Table1

LatticeparametersofsynthesizedandreferenceLiFePO4.

Latticeparameters Synthesized Ref.[32]

a( ˚A) 10.3331(5) 10.33

b( ˚A) 6.0106(3) 6.01

c( ˚A) 4.6953(3) 4.70

Crystalsize(nm) 78.1(2) N/A

[18],andalittlesmallerthantheLiFePO4synthesizedbyPhostech

LithiumInc.(12m2_g−1₎_[29]_._It_is_known_that_the_amount_of_carbon

inLiFePO4/CcompositeaffectsgreatlythesurfaceareaofLiFePO4;

thehighertheamountofcarbonthelowerthesurfacearea[18]. Thiscouldbeattributed,amongotherthings,tothefactthatthe carboncontentinthecompositeinterferesinthe3Dporous net-workandblockssomeofthepathshenceisolatinglargesurfaces fromtheaccessibletotalsurfaceareaofLiFePO4.

Fig.4showstheSEMmicrographsofthesynthesizedLiFePO4

withaBrij78-coatedPStemplateandtheunmodifiedPStemplate. Fromtheinsetimage,itcanbeseenthatPStemplatehas aver-agespherediametersintherangebetween300and500nmclosely packedinanorderlyfashion.Themainimageshowsa macrop-orousstructureofLiFePO4withoverlaidlayersandorderedpores

inasimplecubicstructurehavingadiameterthatmatchesthose ofthespheresofthePStemplate.Thethicknessandthedepthof

Fig.4.SEMmicrographofthesynthesized3DOMLiFePO4andPScolloids(inset).

Fig.5. TEMmicrographofthesynthesized3DOMLiFePO4.

thesynthesized3DOMLiFePO4aresimilarasitisthecaseforcubic

structuresandcanbeapproximatelyestimatedtobeintherange of100nm.Insomepartsoftheimage,rupturesinthestructure canbeeasilyseenthatcouldbeattributedtothevolatile decompo-sitionproductsofsomechainsofthePStemplate.Theprocedure wasrepeatedtoensurethatthesynthesisof3DOMLiFePO4could

bereproducedandthesamemorphology(SEM)andXRD diffrac-tionpatternwereobserved.Fig.5showsTEMmicrographsofthe synthesizedLiFePO4.Theimagesshowclearlyanopenstructure

witharegularvoidpatternthatresemblestheoneobservedinthe SEMimages.Theenlargedimageshowspartofthe3DOM struc-turewithLiFePO4 core(darkregion)surroundedbytheedgesof

thesphericalvoidscoveredbyacarboncoating(brightregion)that is50nmorlessthick.Thewhitedashedlinesshownarethetrace ofthesphericalvoids.ThesecondinsetisanEDXscanofthesame 3DOMLiFePO4sample.Thescanshowsalltheexpectedelementsin

thecomposites:Fe,P,O,C,exceptforLithatcouldnotbeobserved duetothelimitoftheEDX.Cufromthesampleholdercanalsobe seen.

Brij78 wasusedtorenderPStemplate hydrophilictoallow for theinfiltrationof theconcentratedaqueous precursor solu-tionthatisnecessarytoobtainhighyieldofthe3DOMmaterial. Thehighlyconcentratedprecursorsolution(1gofwater)has dif-ficulty infiltratingintothe interstitialvoids of thehydrophobic PStemplatewhennomodificationofsurfacewasmade.Infact, theprecursor solution formeda large convexmeniscus ontop of thePS template and when leftto dry turned intolumps of yellowspotsdistributedunevenlyatthesurface.TheBrij78 surfac-tantconsistsoflongetherichydrophilicandaliphatichydrophobic blocks.ThehydrophobicblocksboundwiththePScolloidswhile thehydrophilicblocksstandingoutofthecolloidstoallowfor infil-trationofthewater-basedprecursorsolution.Thiswasachievedby immersingthedryPStemplateinasolutionofBrij78inethanol. Thepowderbrokeintomillimeter-sizedparticlesassociatedwith smallbubblesoriginatingfromairtrappedamongtheinterstitial voidsofthetemplates.After2min,thefinebubblescouldnolonger beobserved,indicatingthecompletionoftheinfiltration.To com-pletetheinfiltrationoftheBrijsolution,itwasleftstillforfewmore minuteswithtotalinfiltrationtimeofapproximately5min.The excessBrijsolutionwasremovedusingvacuumfiltration.When

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418 C.-H.Yimetal./JournalofPowerSources205 (2012) 414–419

Fig.6.TGAanalysisofLiFePO4underoxygen.

Brij78wascoatedontothetemplate,theinfiltrationperiodforthe aqueousprecursorsolutionwasshortanditwashomogenously infiltratedthroughthePStemplate.

Two-stepcalcinationatveryslowheatingratewasperformed inordertoremovethePScolloidswhilemaintainingthe3DOM structurebypreventingthevolatiledecompositionofPScolloids andtogiveenoughtimefortheinorganicmaterialstocondense andreact.Inthefirststep,calcinationoccurredat260◦_C_for₃_h_in

ordertoremovethePStemplateandcondensethematerial,while inthesecondstepcalcinationoccurredat700◦_C_for_another₃_h_in

ordertosynthesizeandcrystallizetheLiFePO4.Attheendofthe

cal-cinationsthesynthesizedmaterialswerelefttocooldownslowly toroomtemperature.Thecalcinationswereperformedunderinert atmosphere(argon)topreventtheoxidizationofFe2+_to_Fe3+_,_and

alsotoallowfortheformationofcarboncoatingonthematerials insteadofforminggasses.

Fig.6shows thethermal gravimetricanalysis(TGA)curveof LiFePO4 heated inair. TGAwasperformedunder air tooxidize

anddecomposetheentirecarbonresidueinthematerial.TGAis commonlyusedforevaluatingthecarboncontent[27].Basedon theresults,thecommercialLiFePO4had3%weightgain,

suggest-ing1wt%ofcarbonintheactivematerial,when5%weightgain isassumedtobetheresultofLiFePO4oxidation.Thesynthesized

LiFePO4 inthisworkshowed19%weightloss,thatcorresponds

to23wt%ofcarboncontentintheLiFePO4.Athightemperatures

andpressure,Fe(II)inLiFePO4getsoxidizedtoFe(III)(Eq.(1)),as

suggestedbyBelharouaketal.[27]:

LiFePO4+(1/4)O2→ (1/3)Li3Fe2(PO4)3+(1/6)Fe2O3 (1)

Asaresultofthisoxidation,weightgainof5%wasobserveddue tothereactionofoxygenwithironcationinLiFePO4.Thesame

cal-culationwasmadeusingtheTGAresults.Astheoxidationoccurs athightemperaturethat followsthesamereactionpath asEq. (1),usingtheinitialandfinalweights,thecarboncontentsinthe materialswereevaluatedandfoundtobe23wt%.XRFwasusedto verifytheamountofcarboncontent.Duringthesample prepara-tion28wt%losswasobserved,duetothelossofcarbonresidues inthecompositeandthereforethetotalamountofcarboninthe sample.Thevalueisslightlyhighercomparedtotheoneobtained byTGA(23wt%).Also,theatomicratioofthecompositewas con-firmedtobe1:1:4forFe:P:O,whichistheexactcompositionratio ofLiFePO4.

Theeffectoftheamountandtypeofcarboncoatingonthe per-formanceofLiFePO4 hasnotyetbeenfullystudied.However,a

thin layerisonly necessaryin ordertoallowfor thediffusion/

Fig.7.Dischargecurvesof3DOMLiFePO4atdifferentC-rates.

migration of Liions in and outof LiFePO4 and lessthan 5wt%

seemssufficientevenathighcharge/dischargerates.The calcu-latedamountofcarbon(23wt%)inourmaterialisveryhighdue tothenatureofPSdecompositionwhere lessdepolymerization versusside chaindecomposition takesplaceat theinitialstage of calcination.This allows for theformation of higher molecu-lar weight carbon precursor instead of the loss of material by theformation of volatile low molecularweight,depolymerized monomer-likespecies.Also,inoursyntheticmethodthe decompo-sitionproductsoftheBrij78surfactantcontributetotheamount of carbon coating. Moreover,thetype of the carbon formedin ourcase is believed to behighly conductiveas pointed outby Nien et al. [22] who showed that PSmakes highly conductive carboncoatingsonLiFePO4duetothefactthatitcontainsan

aro-maticphenylgroup.Organic(molecularorpolymeric)compounds witharomaticmoietieshavebeeninvestigatedasacarbon precur-sorduringthesynthesisofLiFePO4duetotheirabilitytoforma

highlygraphitictypeofcarbonthatismoreelectronically conduc-tive[20,27].

Fig.7showsthedischargecurveofthesynthesizedLiFePO4that

wastestedasacathodeinahalf-cellwhilelithiummetalwasused asananodeelectrode.Itshowsthetypicalflatdischargecurveof LiFePO4indicativeoftwo-phasereactionofLiFePO4/FePO4

accord-ingto:LiFePO4→FePO4+Li++e−1.Theinitialdischargecapacityat

C/12wasfoundtobe158mAhg−1_that_is_close_to_the_theoretical

capacityof170mAhg−1_[26]_._The_first_discharge_curves_at

differ-entC-ratesarealsoshown.Itcanbeseenthatathighdischarge C-ratetheflatplateauisshortenedandtheinclineddropin poten-tialdominatesduetodiffusionpolarization(masstransportstep slowerthantheotherstepsintheelectrochemicalreaction).The presenceofahighlyporousstructureintheLiFePO4cathodeseems

tohavelittleeffectinmitigatingtheeffectofdiffusion polariza-tionontheelectrochemicalperformanceofLiFePO4whichisdue

tothelargecarboncontentinthematerial.Similarbehaviorwas observedbyDohertyetal.[18]whoreportedslightlylower capac-ities(140mAhg−1_at_C/10_and₇₀_mAh_g−1_at₅_C)_for_the_LiFePO

4

templatedbyPMMA(sphereswithdiameter270nm)andhas car-boncontentof7wt%synthesizedat700◦_C._They_suggested_that

excesscarbonseemstoblockLiionmovementthroughthe inter-face and theelectrolyte moleculesand ions through thepores. Similarbehaviorwasalsoreportedbymanyothersinnano-sized LiFePO4 particles (∼100nm)and it is widelyacceptedthat low

carboncontent(3.2wt%)withporousstructureandveryhigh con-ductivity(sp2_character)_is_sufficient_to_allow_for_electrons_to_reach

(7)

Fig.8. Specificcapacityof3DOMLiFePO4atdifferentC-rates.

highpowerapplications.Itisalsoworthmentioningthatthehigh amountofcarbonhasalsoalargenegativeeffectonthevolumetric energydensityofthebattery.Usingadensityvalueof2.2gcm−3

forcarbonand 3.6gcm−3_,_we_can_calculate_a _0.7_Wh_cm−3 _drop

ofenergydensity(from2.1to1.4Whcm−3₎_at_a_carbon_content

of23wt%.In general,wecan seethat thehighamountof car-bon,althoughofthemoreconductivegraphitictype,hasanegative effectontheelectrochemicalandbatteryperformanceofLiFePO4

but different approaches are under consideration to lower the amountofcarbonsuchas:sinteringthematerialindifferentgas compositioninhydrogenandsteamthatarecommonlyusedto purifycarbonnanotubes[30];orusingdifferentsizesofPSspheres [18].

Fig.8showsthespecificdischargecapacityofthesynthesized LiFePO4 tested atdifferentC-rates.Fivecycleswererepeatedat

eachC-rate:C/12,C/6,C,2C,5C,andbacktoC/12againtocheckfor anycapacityfadeafterhighC-ratecycling.Duringthefirst10cycles (atC/12andC/6),capacityfadeof6%wasobservedthatis com-monlyassociatedwithincreasedresistancesattheinterfaceand thebulkelectrode,withtheformerbeingmoresignificantinthis caseduetothehighsurfaceareaofthematerial[31].Afterthetenth cycle,thecapacitywasstabilizedandalsorecoveredatlowC-rate (C/12)witha12%lossfromtheinitialcapacity(158mAhg−1_)._For

comparison,standardnon-templatedcarbon-coatedLiFePO4was

synthesizedandtestedinahalfcellbatteryandtheresultsarealso showninFig.8.ThedischargecapacityatC/12for3DOMLiFePO4

hasbeenfittedusingthefirstandlastfivecyclesandwere calcu-latedbasedonweightofLiFePO4inthecomposite.Usingthefitted

linethecapacitycouldbecomparedtothestandardLiFePO4.As

showninthefigure,thecapacityis∼20mAhg−1_higher_for_3DOM

LiFePO4sampleduetothegreateraccessoflithiumintheporous

structure.

4. Conclusions

Threedimensionally orderedmacroporous LiFePO4 was

suc-cessfullysynthesizedbymeansofaneworganictemplate-assisted synthesis. The polystyrene polymer synthesized as spheres in 300–500nmdiameterswasorderedsuccessfullytomakethe tem-plate.ThesurfactantBrij78wasusedtorenderthesurfaceofthe sphereshydrophilictoallowfortheinfiltrationofprecursor solu-tion.Uponcalcinations,purephaseinhighyieldoftheLiFePO4with

acrystalsizeof78.1nmwassuccessfullyachievedasevidencedby XRDpatternsandtheorderedmacrostructurewasclearlyseenby SEMandTEMimages.Carboncontentwasestimatedat23–28wt% ofthematerial.Theresulting3DOMLiFePO4materialshoweda

sat-isfactorybatteryperformancethatwasclearlyaffectedbythehigh amountofcarbonyetstillhigherthanthenon-templatedsample;it showedadischargecapacityof158mAhg−1_recoverable_after_long

andhighC-ratecycling.

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