Nano SiO2 particle formation and deposition on polypropylene separators for lithium-ion batteries

(1)

Publisher’s version / Version de l'éditeur:

Journal of Power Sources, 206, 15 May 2012, pp. 325-333, 2011-11-06

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la

première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.1016/j.jpowsour.2011.10.130

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Nano SiO2 particle formation and deposition on polypropylene

separators for lithium-ion batteries

Fu, Dong; Luan, Ben; Argue, Steve; Bureau, Martin N.; Davidson, Isobel J.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=a8737a5c-3f65-4daf-afb1-805b0db5fd2b

https://publications-cnrc.canada.ca/fra/voir/objet/?id=a8737a5c-3f65-4daf-afb1-805b0db5fd2b

(2)

This article appeared in a journal published by Elsevier. The attached

copy is furnished to the author for internal non-commercial research

and education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling or

licensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of the

article (e.g. in Word or Tex form) to their personal website or

institutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies are

encouraged to visit:

(3)

JournalofPowerSources206 (2012) 325–333

ContentslistsavailableatSciVerseScienceDirect

Journal

of

Power

Sources

j ou rn a l h o m e pa 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

Nano

SiO

2 particle

formation

and

deposition

on

polypropylene

separators

for

lithium-ion

batteries

Dong

Fu

a

_,

_Ben

_Luan

a,∗

_,

_Steve

_Argue

b

_,

_Martin

_N.

_Bureau

c

_,

_Isobel

_J.

_Davidson

b

a_Center_for_Automotive_Materials_{Manufacturing,}_Industrial_Materials_Institute,_National_Research_Council_Canada,₈₀₀_Collip_Circle,_London,_Ontario_N6G_4X8,_Canada b_Institute_for_Chemical_Process_and_{Environmental}_Technology,_National_Research_Council_Canada,₁₂₀₀_Montreal_Road_Building_M-12,_Ottawa,_Ontario_K1A_0R6,_Canada c_Industrial_Materials_Institute,_National_Research_Council_Canada,₇₅_de_Mortagne_Boulevard,_{Boucherville,}_Quebec_J4B_6Y4,_Canada

a rt i cle i n fo

Articlehistory:

Received7October2011

Receivedinrevisedform27October2011 Accepted31October2011

Available online 6 November 2011

Keywords: Lithium-ionbatteries Separators Silicacoating Ratecapability Thermalshrinkage a b st ra ct

ThenoveltyofthisworkistheformationanddepositionofSiO2,asopposedtodepositionusing

com-merciallyavailableSiO2powdersuspensioninthesolution,toformceramiccoatingonpolypropylene

(PP)separatorsforlithium-ionbattery.TheformationofSiO2nanoparticleswithuniformparticlesize

isaccomplishedthroughdirecthydrolysisoftetraethylorthosilicate(TEOS),whilethedepositionofthe formedSiO2onPPseparatorswasconductedinthesamesolutioncontainingpolyvinylidene

fluoride-hexafluoropropylene(PVDF-HFP)asbindersandacetoneasthesolvent.Theeffectsoftheceramiccoating onthesurfacemorphology,tensilestrength,contactangles,electrolyteuptake,thermalshrinkageofthe PPseparatorsandthecellperformancessuchasbatteryratecapabilityandCoulombicefficiencywere investigated.Thecoatedseparatorsshowsignificantreductioninthermalshrinkageandimprovement intensilestrength,contactangles,electrolyteuptakeandbatteryperformanceascomparedtotheplain PPseparator.

1. Introduction

Lithium-ionbatteriesarewidelyusedforelectronicdevicessuch asmobilephones,laptopcomputers,anddigitalcamerasdueto theirhighenergydensityandlonglifecycle.Withthesearchfora solutionasalternativepropulsionsystem,lithium-ionbatteriesare alsoexpectedtobeanalternativepowersourceforPlug-inHybrid ElectricVehicle(PHEV),particularlywiththeheightenedneedsfor alternativeenergyandenvironmentprotection,duetothe advan-tagesinlongercyclelife,highervoltagesandenergydensitywhen comparedwithotherrechargeablebatteries[1–6].

Theseparatorinabatteryplays animportantroletoretain electrolyte,preventshortagebetweenthetwoelectrodeswhile maintaininghighionpermeation,andtoperformsafe deactiva-tionofthecellunderovercharge,abnormalheatingormechanical rupture conditions [7,8]. In general, separators in lithium-ion batteries are made of polyolefins, predominantly polyethylene (PE)orpolypropylene(PP).However,eventhoughthese microp-orouspolyolefinbasedseparatorshavemanyadvantagesinterms ofadded mechanicalintegrityand theadditional advantage of exhibitingthermalshut-downundersevereabuseconditions[7], thethermalshrinkageandmechanicalstrength arestillserious concerns overtheirability to maintainthenecessaryelectrical

∗ Correspondingauthor.Tel.:+15194307043;fax:+15194307032.

E-mailaddress:ben.luan@nrc-cnrc.gc.ca(B.Luan).

isolationbetweenelectrodeswhenconsideredforhigherpower outputforonboardelectricvehicleapplications[9].Another con-cernisrelatedtotheformationofdendritesformedduringthe charge/dischargecyclingofcellswhichcouldprotrudethroughthe separatorsandcreateshortcircuitingoftheelectrodeswhichposes aserioussafetyconcern[10–12].

Recently,variousapproachestoovercometheseshortcomings ofpolyolefinbasedseparatorshavebeenreported.Mostreports focusedonmanyalternativesofpolyolefinbasedseparatorssuchas non-wovenseparatorsandinorganiccomposite[13–18].Another recentapproachwasthecoatingofpolyolefinbased separators usinginorganicparticlesduetotheexcellentthermalstabilityand wettabilityofinorganicparticleswith organicelectrolytes.This approachsignificantlyimprovesthethermalshrinkageand elec-trochemicalperformanceoftheseparators[19–26].However,in thesestudies,commerciallyavailableinorganicpowders(suchas clay,SiO2,Al2O3,etc.)weresuspendedinacetonebasedsolutions

withpolyvinylidenefluoride-hexafluoropropylene(PVdF-HFP)as binderstoattachtheinorganicparticlesontheseparatorsurface toformnanocompositeseparators.Examplesofsuch nanocompos-itesarecharacterizedofSiO2nanoparticlesembeddedinapolymer

matrixhencetheestablishmentofahighlyorderednanoporous structure,reportedbyParkandJeongandLee[19,26].TheSiO2

nanoparticles are close-packed and interconnected by organic binders(PVdF-HFP) onboth sides ofa PE separator.However, theinorganicparticlesareeasytoformaggregatesor agglomer-ates.Consequently,sonicationandballmillinghadtobeusedto

(4)

Author's personal copy

326 D.Fuetal./JournalofPowerSources206 (2012) 325–333

distributetheinorganicparticlesevenlyinthesolventsto possi-blyformauniformsolution.However,thistreatmentrepresents anaddedinvestmentandprocessing.Also,wetballmillingleadsto unavoidableusageofhigherviscosesolutionsforhigherballmilling efficiencybecauseviscosityappearstohavethemostsignificant effectontherateofparticlesizereduction[27].Highersolution vis-cosityresultsinthickerthandesiredcoatingandconsequentlythe reductionofavailablespaceforactivematerialsinthecellandthe increaseofcellweight,botharedetrimentaltothespecificenergy densityofbatteries[26].Inaddition,excessivebuildupofcoating couldalsocloguptheporesintheseparatorwhichcompromises theionicconductivityandtheelectrochemicalkinetics.

Inthispaper,anewprocesswasdevelopedforthecoatingof separators,featuringsynthesisofSiO2nanoparticlesandits

depo-sitiononseparators.ItoughttobenotedthatthesynthesisofSiO2

wasbasedontheStöbermethod[28,29].Thedifferencesofour workincludes:(1)formationofSiO2inadifferentsolution.Stöber

method used methanol, ethanol,propanol and butanol as sol-vents,whileacetonewasusedinourwork;and(2)Stöbermethod focusedsolelyontheformationofparticlesbutdidnotaddress thedepositionoftheformedparticles.Inourwork,PVdF-HFPwas usedasabindertofacilitatethedepositionoftheformedSiO2

particles.Thisnewapproachofhydrolysisoftetraethyl orthosil-icate(TEOS)toform SiO2 andthedepositiononPPmembrane

separatorinPVDF-HFPacetonesolutionresultsinthecoatingof evenlydistributedparticlesofuniformsizedistribution.Allthis wasaccomplishedthroughasimpleyeteffectiveimmersionofthe separatormembraneinthesolutiondescribedabove.Thisprocess avoidedsonicationandballmillingforsuspensionand distribu-tionoftheceramicnanoparticlesinthesolution.Theeffectsof theceramiccoatingontheseparatorsurface morphology, ten-silestrength,contactangels,electrolyteuptake,thermalshrinkage, thecellcharge/discharge Coulombicefficiency, and the signifi-cantimprovementofbatteryratecapabilitywereinvestigated,and comparedwiththoseobtainedwithaplainPPseparatorwithout coating.

2. Experimental

2.1. Chemicalsandmaterials

Allchemicalswerepurchasedandusedasreceivedwithout fur-therpurificationormodification.Celgard®₂₅₀₀_separators_were

purchasedfromCelgardCompany(25␮mmicroporousmonolayer Polypropylenemembranewithaporosityof55%).

2.2. Coatingofseparators

Dissolving1gofPVdF-HFP(Aldrich)in160mlacetone,followed by theaddition of6ml de-ionized water and3ml ammonium hydroxide(30%Fisherscientific)intothesolutionwithvigorous stirringfor20min.Subsequently,6mlofTEOS(Aldrich)wasadded intothesolutionfollowedbyacontinuousagitationfor5huntilthe solutioncolorchangedfromcleartouniformlycloudy.Bothsides ofthePPseparatorscanthenbecoatedwithasimpleimmersion oftheseparatorintothepreparedsolution.Thecoatedseparators werethendriedatroomtemperaturetoevaporateacetone.

2.3. Characterizationsofseparators

Themorphologyofthemembranewas investigatedusing a

field emission scanning electron microscope (FE-SEM, S-4800, Hitachi).ThesampleswerealsoanalyzedwithX-rayphotoelectron spectroscopy(XPS) using a KratosAxis Ultra X-ray photoelec-tron spectrometer which probes thesurface of the sample to adepthof7–10nm,andhas detectionlimits rangingfrom 0.1

to 0.5 atomic percent depending on the elements. The

spe-cificsurfaceareas,poresizesandporevolumeweredetermined using a Micromeritics ASAP 2010 nitrogen adsorption appara-tus(Micromeritics,USA).Allthesamplesweredegassedat80◦_C

priortomeasurements.Thespecificsurfaceareawascalculated bytheBrunauer–Emmett–Teller(BET)method.Theporesizewas obtainedfromthemaximaoftheporesizedistributioncurve cal-culatedby theBarrett–Joyner–Halenda(BJH)methodusing the desorptionbranchoftheisotherm,andthetotalporevolumewas evaluatedbythesinglepointmethod.Thethermalshrinkageofthe SiO2coatedseparatorswasdeterminedbymeasuringthe

dimen-sionalchangeafterbeingsubjectedtoheattreatmentatvarious temperaturesfor30mininatemperaturechamber(TESTEQUITY 1000series).Theelectrolyteuptake(T)ofthemembranewas deter-minedbyimmersingthemembranein1MLiPF6(DMC/EC;1:1in

volume)for30minandobtainedby: T(%)=W2

−W₁ W1

×100 (1)

whereW1andW2arethemassofthedryandwetmembranes,

respectively.

Thesurfaceadvancingcontactanglesweremeasuredusinga contactanglemeter(Kernco Instruments,USA).Thesame elec-trolyteusedforbatteryassembling,1MLiPF6inDMC/EC(1:1in

volume),wasdroppedontothesampleusingamicrosyringeduring thetest.Apictureofthedropwascapturedafterthedropsetonto thesample.Thecontactangleswerecalculatedthroughanalyzing theshapeofthedrop.Thetensilestrengthofthefilmswasobtained fromtherecordedload–displacementcurveofspecimenswitha crosssectionwidthof10mmpulledataspeedof5mmmin−1_mm

usingatensionmachine(INSTRONmicrotensile,UK)witha2kN loadcell.Theelectrochemicalpropertiesoftheassembled2325 buttonstylelithiumtestcellswereinvestigatedusinga16Channel ArbinBT2043batterytestsystem.Thecathodesofthetestcells werepreparedbyslurrycoatingamixtureofLiCoO2,SuperS

car-bon,LonzaEKS-4graphitewithaPVDFbinderontohighpurity aluminumfoilwhichwasthendriedandpressed.Theanodeused washighpuritylithiummetal.Thetestcellswereassembledina dry,oxygenfreegloveboxusingasinglelayerofthetest separa-torsthathadbeenpreviouslydriedinavacuumovenat40◦_C_for

12h.A35␮laliquotof1MLiPF6inDMC/EC(1:1)wasplacedonto

thecathode,followedbythetestseparatorthathadbeenwetted withanother35␮laliquotoftheelectrolyteinaglassdishbefore placing.Afinal35␮laliquotoftheelectrolytewasthenaddedon topoftheseparatorpriortoplacingtheanodeandcompletingthe cellbuild.

3. Resultsanddiscussion

TEOScanbeeasilyhydrolyzedtoformSiO2particlesunder

catal-ysisofammoniumhydroxide,i.e.Stöbermethod,asshowninEq.

(2).

Si(OC2H5)4+H₂O−→NH3SiO₂+4C₂H₅OH (2)

ThemechanismofStöbermethodwithrespectstosilica par-ticleformation,growth,andparticlesize,distribution,andshape havebeeninvestigatedinthepastdecades[30–42].However,the mechanismsreportedbydifferentresearchersarecontradictory. Wangetal.summarizedthemechanismsfromliteratureswhich maybesortedintotwodifferentmodels:themonomeraddition modelandthecontrolledaggregationmodel[42].Themonomer additionmodelarguesthatafteralimitedperiodoftimefor nucle-ation,growthoccursthroughtheadditionofhydrolyzedmonomers totheoligomerssurfaceandgraduallybecomesthedominant

(5)

D.Fuetal./JournalofPowerSources206 (2012) 325–333 327

claimsthattheaggregationofsub-particlesleadstoparticle

forma-tion[31,32,37,40,41].Recentresultsreportedseemstosupportthe

monomeradditionmodel[42].

WhiletheabovereactionformsSiO2particles,depositionofthe

formedparticlesontotheseparatorsurfacetakesplaceinthesame solutionbecauseoftheco-existenceofPVdF-HFPinthesolution, accordingtoournovelelectrolyteformulation.Unlikereportedby others[19,26],ourmethodprovidesasimplifiedprocessofcoating, withouthavingtousecostlyandtime-consumingsonicationand ballmillingtosuspendanddistributethenanoparticlesproduced bythesuppliers.Toourknowledge,thissynthesisanddepositionof SiO2nanoparticlesinthePVDF-HFPacetonesolutionforseparator

coatingisreportedforthefirsttime.Inaddition,boththeparticle sizeandtheuniformityofparticlesizedistributioncanbeeasily controlledbyadjustingthesynthesisconditions.

XPSsurveyscananalyseswere carriedoutwith ananalysis areaof300␮m×₇₀₀_␮m_and_a_pass_energy_of₁₆₀_eV₍_Fig.₁_(a)). Highresolutionanalyseswerecarriedoutwithananalysisarea of300␮m×700_␮mandapassenergyof20eV.Fig.1(b)presents thehighresolutionSi2pspectrashowingasinglepeakatbinding energiesof103.6–103.7eVwhiletheO1sspectra(Fig.1(c)) show-ingasinglepeakat∼_533.0_eV._This_is_consistent_with_the_reported bindingenergiesofSi2pandO1sforSiO2are103.5and533.2eV,

respectively[43],thereforeconfirmstheformationofSiO2from

ourprocess.Inaddition,theoxygentosiliconratioof2.3isalso indicativeoftheformationofSiO2.Afewfactorscouldcontribute

totheexcessiveamountofoxygen:(1)accordingtotheCasaXPS manual2.3.15Rev1.2[44]andotherreportedwork[45,46],XPS atomiccompositionmeasurementhasascatter/errorofabout10%; (2)theexcessiveamountofoxygenwaslikelytoincludeasmall amountofoxygenboundtocarbon,possiblypickedupduringthe sampletransferinair,and(3)thecoatingmayalsocontainOH−

groupsandotherlowmolecularweightalkylsilicateoligomers.It oughttobenotedthattheseOH−_groups,_once_formed,_are

diffi-culttoremove[47]andmayadverselyaffectbatteryperformance becauseofitsreactivitytowardslithiummetal[48].Systematicand in-depthinvestigationneedtobeconductedinfurtherresearch.

Fig.2showscoatinggainsvsnumberofcoating/dippingtimes. Thecoatingweightpercentagegaindemonstratesalinear relation-shipwiththenumberofdipsinthesolution.Inotherwords,every singledipprovidesaconsistentamountofcoating.Thisisafluid dynamicsphenomenawhichcanbeunderstoodbyintroducingthe dipcoatingthicknessequation[49]:

h=0.94(U0)

2/3

1/6_(g)1/2 (3)

wherehiscoatingfilmthickness,issolutionviscosity,U0is

sub-stratewithdrawspeed, isliquid–vapourinterfacetension,is solutiondensity,gisaccelerationofgravity.Foragivensolution,, ,andgareconstants.Thecoatingfilmthicknessisthensolely determinedbythesubstratewithdrawspeed. Ifthespeedwas keptaconstantduringdipping,aconsistentgainofcoatingcan beobtained.Inourexperiments,thewithdrawspeedwaskeptat about4cms−1_._In_order_to_provide_a_visual_{understanding}_of_the

coatingthickness,Fig.3presentstwoexamplesofSEMcross sec-tionanalysisofthecoatedsamplescorrespondingtotheminimum (onedip, Fig.3(a))andthemaximum(sixdips, Fig.3(b)) coat-ingthickness,showingacoatingthicknessofabout1and6␮m, respectively.

ThetopsurfaceSEMimagesoftheuncoatedPPseparatorandthe coatedseparatorsareshowninFig.4.Incontrasttotheuncoated PPseparator(Fig.4(a)),close-packedSiO2nanoparticles

intercon-nectedbyPVdF-HFPbinderarepresentontheseparatorsurface (Fig.4(b–g))comprisingceramiccoatinglayerswithvarious coat-ingweightgains.TheoverallmorphologyoftheSiO2coatinglayers

1000 800 600 400 200 0 0 20000 40000 60000 80000 100000 120000 140000 CPS

Binding Energy (eV)

F 1s O 1s C1s Si 2s Si 2p O KLL F KLL

a

110 108 106 104 102 100 0 100 200 300 400 500 600 CPS

Binding Energy (eV)

Experimental Silica Background Fitting Si-2p

b

542 540 538 536 534 532 530 528 0 500 1000 1500 2000 2500 3000 CPS

Binding Energy (eV)

Experimental SiOx and Organic O Background Fitting

O-1s

c

Fig.1.XPSspectraofSiO2coatedseparator.(a)XPSsurveyspectrum;(b)the

high-resolutionSi-2pspectrum;(c)thehigh-resolutionO-1sspectrum.

issimilartothenanoparticlearrangementdrivenbyself-assembly (thespontaneousorganizationofmaterialsthroughnoncovalent interactionswithnoexternalintervention[50]),aswasreported byMasudaetal.[51,52].Themorphologyoftheseparatorwiththe highestamountofcoating(Fig.4(g))isdifferentwhencompared withtheothersamplesinthatthesurfaceismostlycoveredwith PVdF-HFPwhichformsanetworkofbinder.Nano-sizedparticles existwithinthebindernetwork,asisclearlyshowninthetwo

(6)

Author's personal copy

6 5 4 3 2 1 0 10 20 30 40 50 60 70 Coating gain (%) Dipping times R2 = 0.9856

Fig.2.Coatinggainvsdippingtimes.

bird-nest-likeinsetsinFig.4(g).Theexcessivebinderbuildupis probablycausedbytheaccumulationofthePVdF-HFPbinderwith repeateddipping,whichresultsintheformationofanobvious net-workduringdryingprocessduetotheviscosenatureofthebinder andthesurfacetension.

Understandingthecoatingcompositionisimportant,though areliabledirectmeasurementisdifficult.However,acetoneand ammoniumwaterarehighlyvolatilewithaboilingtemperatureof 56.5and37.7◦_C,_{respectively.}_TEOS_is_also_categorized_as_a_volatile

organiccompound(VOC)withaboilingpointof166–169◦_C,_much

belowtheVOChighlimittemperatureof250◦_C_under_an

atmo-sphericpressure of101.3kPa.Withthat,anyoftheseresiduals wouldevaporateduringdryingofthecoating,particularlywiththe largesurfaceareaduetotheporousnatureofthecoating.With that,thecoatingconsistsmainlyofSiO2 particleandPVdF-HFP

binder.Giventhatadirectandquantitativemeasurementofthe SiO2/PVdF-HFPratiointhecoatingisdifficult,anindirect

measure-mentwascarriedoutbycentrifugingtheoriginaldepositionbathat 3500rpmforonehour.BecausethecoatingbathformsSiO2

homo-geneouslyandthattheparticlesformedwereobservedtosuspend uniformlyforanextendedperiodoftimeofseveralhours,itis rea-sonabletobelievetheratiooftwoconstituentsinthecoatingand inthebathareconsistent.Thisisparticularlythecasegiventhatthe bathwasthoroughlyagitatedeverytimepriortothedepositionof coating.Theparticlescollectedwerethenwashedandcentrifuged usingfreshacetoneforthreetimestocleantheparticlesbefore drying.Thedriedparticleswerethenweighedandcomparedwith

Table1

Specificsurfacearea,porediameter,porevolumewithvariouscoatinggain. Coatinggain% Specificsurface

area(m2_g−1₎

Poresize(nm) Porevolume (cm3_g−1₎ 0 54.0 27.5 0.47 8 53.7 25.5 0.35 20 50.1 19.1 0.29 27 50.0 22.3 0.28 45 52.2 22.0 0.31 55 58.5 22.0 0.34 63 49.8 24.9 0.33

theinitialweightofPVdF-HFPbinderaddedintothebath.From ourmeasurement,thecoatingcompositionintermsoftheweight ratioofSiO2toPVdF-HFPis1.6–1.

Regardingporosityretentionaftercoating,thespecificsurface areas,poresizesandporevolumeswereexaminedandlistedin

Table1.AscanbeseenfromTable1,thespecificsurfaceareasand

poresizesarenotchangedsignificantlyaftercoatingascompared withplainseparator.However,fortheporevolumes,althoughitis notsignificantlydifferentamongthecoatedseparatorswith var-iouscoatinggains,theporevolumesdeclineapproximately30% forthecoatedseparatorsfrom0.47cm3_g−1_to_an_average_of_about

0.32cm3_g−1_.

Thewettabilityoftheseparatorusedinthelithiumbatteries playsacriticalroleinthebatteryperformancebecausethe separa-torwithgoodwettabilitycaneffectivelyretaintheelectrolyteand facilitateitsdiffusionwellintothecellassembly[7,8].Contactangle ()measurementisoneoftheimportantindicatorsforwettability: thelowerthecontactanglethemorewettable(hydrophilic)the surfaceis.However,itoughttobeclarifiedthatonaroughsurface, thecontactanglemeasurementisaffectedbythesurface rough-nessevenwhenthesurfacechemicalcompositionisidenticalfora smoothandaroughsurface[53].Whenthesurfaceisroughened, theapparentcontactangle(app)canbealteredbyageometric

factor,givenbyWenzel’sequation[54]:

cos app=_r_cos ₍₄₎

whererisaratiooftruesurfaceareatothehorizontalprojection ofsurfacearea.

FromTable1,theseparatorswithorwithoutcoatinghave

sim-ilarporesizeandspecificsurfacearea.Theseresultsindicate a similar roughnessonthesurfaceofseparators beforeandafter coating.Assuch,thervaluesinEq.(4)aresimilarfordifferent sepa-rators,andwecanthereforedirectlyusetheapparentcontactangle (app)insteadofcontactangle()forrelativecomparisonof

wet-tabilityoftheseparators.AsshowninTable2,thecontactangle decreasesfrom38◦ _to₃₀◦_for_uncoated_surface_and_a_single_dip

coatedsurface,respectivelyduetotheformationofhydrophilic SiO2 particles. With further increase of coating thickness, the

(7)

D.Fuetal./JournalofPowerSources206 (2012) 325–333 329

Fig.4.SEMimagesofseparatorsurfacewithandwithoutSiO2particlescoating.(a)Withoutcoating,(b)8%,(c)20%,(d)27%,(e)45%,(f)55%,and(g)63%coatinggain.

contactangledrasticallyreducedtobelowthemeasurementlimit oftheKencosystemwhichisbelow10◦_._Such_a_low_contact_angle

representsanexcellentwettabilityofthecoatedsurface.The rel-ativelyhighercontactangleofthesingledipcoatedsurfacecould indicatethattheseparatorsurfacewasnotentirelycovered.In sum-mary,allthecontactanglesofthecoatedseparatorsascompared totheuncoatedseparatorsurfaceareconsistentlyreducedwhich clearlydemonstratesthatcoatingimprovedthewettabilityofthe separatorsurface.

Fig.5 showstheelectrolyteuptake asa functionofcoating gain.Aswasaforementioned,thehydrophilicSiO2particlesinthe

coatingimprovethewettabilitytherebyenhancingtheelectrolyte retentionintheseparators.FortheuncoatedPPmembrane,the electrolytefilled intotheporesandtheelectrolyteuptakewas proportionaltotheseparatorporosity,i.e.theporosityofthe sep-aratoractsastheonlyavenueforelectrolyteuptake.Inthecase ofSiO2particlescoatedseparators,however,liquidelectrolytewas

(8)

Author's personal copy

Table2

Contactanglesof1MLiPF6inDMC/EC(1:1involume)onuncoatedseparatorand

separatorscoatedwithvariousweightgain.

Coatinggain(%) Contactangles(◦₎

0 38 8 30 20 <10a 27 <10a 45 <10a 55 <10a 63 <10a

a _The_contact_angle_meter_from_Kernco_Instruments_has_a_measurement_range_of

10–120◦_with_a_low_limit_of₁₀◦_.

0 10 20 30 40 50 60 70 146 148 150 152 154 156 158 160 162 164 166 Electrolyte uptake (%) Coating gain (%)

Fig.5.Electrolyteuptake(%)ofseparatorswithvariouscoatinggain.

coatingcomprisedofthehydrophilicSiO2particlesandthe

PVdF-HFPnetwork.

Theseparatormustbemechanicallystrongtowithstandhigh tensionduringthebatteryassemblyandtoresisttheprotrusion ofthedendriticcrystalsformedduringcellcycling.Fig.6shows theload (maximum)of theseparators withand without coat-ing.Despitenon-negligiblestandarddeviations,ageneraltrendof increasingstrengthoftheseparatorswithincreasingcoatinggain couldbeobserved.Comparedtotheuncoatedseparator,theload (maximum)increasedby∼10%at8%coatinggainandcontinues

150 140 130 120 110 100 0 10 20 30 40 50 0% 8% 20% 27% 45% 55% 63% Shrinkage (%) Temperature (ºC) Coating gain

Fig.7.Thermalshrinkage(%)atvarioustemperaturesandwithdifferentcoating gain.

withfurtherincreaseofcoating.ItisbelievedthatSiO2particles

andpolymerchainsinPVdF-HFPinteractwitheachotherbyvan derWaalsforce,etc.,thusformingcross-linkedporousstructures andrestrictingthemotionofmolecularchains[55].

Thermalshrinkageofseparatorsisanotherimportantissue per-tainingtonotonlybatteryperformancebutalsosafety.Aseverely shrunkseparatorcausedbytheheatgeneratedduringcellcycling, particularlyunderhighpoweroutputconditionssuchasfor elec-tricvehiclesapplications,couldresultinshortingoftheelectrodes alongtheperimeteroftheseparator.Theceramiccoatinglayersare expectedtopreventtheseparatorsfromthermalshrinkage,dueto theexistenceoftheheat-resistantSiO2nanoparticles.Accordingly,

thethermalshrinkageofthecoatedanduncoatedseparatorswas observedbymeasuringthedimensionalchange(area-based)after theseparatorwassubjectedtoheattreatmentatvarious tempera-turesfor0.5h.Fig.7showsthatalltheSiO2coatedseparatorshave

areducedthermalshrinkagethantheuncoatedPPseparatorovera widerrangeoftemperatures,whichverifiesthattheintroductionof ceramiccoatinglayersiseffectiveinimprovingthethermal perfor-manceofseparators[21].Inaddition,thethermalshrinkageofthe coatedseparatorswasfurtherexaminedasafunctionofcoating thickness.Atrelativelylowtemperatures,thethermalshrinkage

70 60 50 40 30 20 10 0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 Load at Max (N) Coating gain (%)

MD

70 60 50 40 30 20 10 0 1.14 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30 1.32 1.34 Load at Max (N) Coating gain (%)

TD

(9)

D.Fuetal./JournalofPowerSources206 (2012) 325–333 331 1000 900 800 700 600 500 400 300 200 100 5 80 90 100 110 120 130 140

Discharge current density (mA·g-1)

Capacity (mAh·g -1 ) 0% 8% 20% 27% 45% 55% 63%

Fig.8.Dischargecapacityofseparatorswithdifferentcoatinggainatvarious dis-chargerates.Thechargingwasconductedat5mAg−1_.

forsampleswithdifferentcoatingthicknessdoesnotexhibit sig-nificantchange.However,at furtherelevatedtemperatures,the benefitofcoatingonthermalshrinkagebecomesmorepronounced withtheincreaseofcoatingthickness.Thisimprovementinthe thermalshrinkageofthecoatedseparatorsisattributedtothe pres-enceofalargeamountofheatresistantSiO2nanoparticlesinthe

ceramiccoatinglayer[26](coefficientofthermalexpansion:PP −_8–10×₁₀−4◦_C−1_[56]_,_SiO

25×₁₀−7◦_C−1_[57]_).

Inaddition,sufficient adhesionisalways animportant con-siderationincoatingdevelopment.Unfortunately,theseparator isa porous membrane ofonly25␮mthick. It issoft, flexible, andstretchy,whichmakesareliablequantitativemeasurementof bondingstrengthextremelychallenging,ifatallpossible. How-ever,tohavesomeunderstandingaboutthecoatingadhesion,we conductedabasicscotchtapetesttoseeifanyparticlescanbe peeledofffromtheseparator.Repeatedtestswerecarriedoutand noseparationofthecoatingwasobserved.Inaddition,particles couldfalloffwhensubjectedtoheatingandcooling,butno parti-cleswereobservedtoseparatefromthesubstratewhensubjected tothethermalshrinkagetestdescribedabovewithina tempera-turerangefromroomtemperatureto150◦_C._This_temperature_is

significantlyhigherthanthenormaloperatingtemperatureofa Libattery.Inaddition,adhesionismostlyaconcernwhentensile forceisappliedtoacoating.Thatiswhyacompressiveinternal stressispreferredandatensileinternalstressisavoidedincoating developmentwhenattemptingtoachieveahighadhesion.Inthe caseofLibatteryassembling,theseparatorissandwichedbetween theelectrodeswithstainlesssteelspringwashersoutsideofeach electrode.Thisputstheseparatorinacompressedstate,namely, acompressiveforceisappliedasopposedtoatensileforce.The ScotchTapeandthethermalcyclingtestdescribedabovealong withthecompressedstatetheseparatorexistsinabattery assem-blysuggestthatadhesionisnotexpectedtobeaconcerninour work.Thisprovesthatthebinder(PVdF-HFP)effectivelyformsa gelnetworkduringthedepositionandfirmlybindstheparticles insidethegelmatrixandontothesubstratesurface.

Fig.8showsthedischargecapacityatdifferentdischargerates forbatteriesfabricatedusinguncoatedseparatorandseparators coatedofvariousthicknesses.Allthechargingwasconductedat 5mAg−1_._As_can_be_clearly_seen,_all_the_coated_separators

con-sistentlyresultedinanincreaseinbatterycapacityascompared tothebatteryusinguncoatedseparator.Inaddition,thisincrease becomesmoresignificantwithanincreasingrateofdischarge.This

10 20 30 40 50 60 70 80 90 0.95 0.96 0.97 0.98 0.99 1.00 0% 8% 20% 27% 45% 55% 63% Coulombic ef fi ciency Cycle numbers Coating gain

Fig. 9.Coulombic efficiency of separators with different coating gain at a charge/dischargerateof50/50mAg−1_.

clearlyshowsamuchimprovedratecapabilityofbatteriesusing SiO2coatedseparators.AnimprovedratecapabilityafterAl2O3

coatingwasreportedbyChoietal.[24].Theimprovedrate perfor-mancewasascribedtothefavourableinterfacialchargetransport betweentheelectrodesandtheelectrolytesinthecell,because thecoatinglayeronbothsidesoftheseparatorwasabletoassist intheadheringofseparatortotheelectrodesaftersoakinginthe electrolytesolution.Itshouldbestressedthatthisobservationis notwellunderstoodbecausecoatingisexpectedtoblockthe orig-inalporesintheseparator.WhileChoi’sexplanationmightalso betrueinourcase,wecannotruleoutthepossibilitiesofother effects.Anexampleofthesepossibilitiesisrelatedtothewetting mechanismoftheseparators.Theuncoatedseparatorhaspoor wet-tabilitywithacontactangleof38◦_while_all_the_coatings_with_more

thanonedipshowamuchimprovedwettabilityrepresentedby theverylowcontactanglesevenbelowthedetectionlimitof10◦_.

Eventhoughcoatingreducedtheporosity,thedrasticallyimproved wettabilityindicatesasignificantlyincreasedadhesiveforceatthe electrolyte/separatorinterface,includingtheinnersurfaceofthe pores.Thisincreasedadhesiveforceofthecoatedseparatorsvsthe cohesiveforcewithintheelectrolytespecies(moleculesandions) thereforeenhancesthetransportofthesespecies.Inaddition,all thespecieshavemuchsmallerradius(Li+_of_0.09–0.109_nm,_PF

6−of

0.16nm,ECmolecularof0.25nm,andDECmolecularof0.30nm) ascomparedtotheporesizeoftheseparatoronamicronmeter scale.Thisenhancedtransportofspeciesintheelectrolyte sug-gestsareductionofion/moleculetransportimpedanceasreported byothers[24,26].Thisreducedimpedancecouldthenfacilitatethe kineticsofcharge/dischargereactions,resultinginan improved rateperformanceparticularlyathigherrates.Whilethebeneficial effectofSiO2coatingontheratecapabilityofbatteryisshown

basedonour deposition methodandour experimental results, moresystematicstudieswillbesystematicallycarriedoutfor fur-therunderstanding.

Fig.9showstheCoulombicefficiencyofseparatorswithvarious coatinggainsasafunctionofcharge/dischargecyclenumbers.The cellswerecycledat10mAg−1_for₁₀_cycles_followed_by₉₀_cycles

at50mAg−1_between_the_voltages_of_2.5_and_4.2._The

Coulom-bicefficiencyisdefinedastheratioofthedischargecapacityto chargecapacity.Duringthefirstcycle,theCoulombicefficiencyis relativelylow.OneofthemainreasonsisthattheCoulombic effi-ciencyisassociatedwiththeirreversiblecapacitylossduringthe firstcyclethatinvolveselectrolytedecompositionandsubsequent

(10)

Author's personal copy

Fig.10.SchematicofSEIformationonuncoatedandcoatedPPseparator.

formationofasurfacefilm(solidelectrolyteinterphaseor inter-face:SEI)ontheelectrodes[58,59].Thesurfacefilmisconductive forlithiumionsbutisinsulatingelectronically,whichpreventsthe electrodesinteractionwithnon-aqueousorganicsolutions.Some lithiumionsareconsumedbytheformationofSEIandtherefore notcontributing tothe charge/dischargecycling,resulting ina lossofCoulombicefficiencyduringthefirstcycle[60].However, ascanbeseenfromFig.9,theCoulombicefficiencyofallthe6 coatedseparatorsishigherthantheplainseparator.Thismight berelatedtoamoreefficientSEIformation.Forexample,when thecoatedseparatorisinclose contactwiththeelectrode sur-face,theSEIfilmcouldnowbeformedduringthefirstcycleas acompositefilmconsistingoflithiumsalt(s)thatfillintheporous structureofSiO2coating,asopposedtotheconventionalSEIfilm

ofonlylithiumsalt(s),seetheschematicillustrationinFig.10.This insulatingfilmstartswithanalreadyinsulatinglayerofSiO2

par-ticles,thusrequiringonlyaportionofthelithiumionsconsumed toformanSEIwithoutSiO2particles.Assuch,morelithiumions

arereservedforcharge/dischargecyclingtherebyincreasingthe cellCoulombicefficiency.Afterthefirstcycleinourwork,orthe firstseveralcyclesinotherreportedcases,thereservedamount oflithiumionscontinuetobeavailableformaintainingahigher efficiencyofthecellsusingcoatedseparatorsforextendedcycles. Whilethisisanewmodeltryingtoexplainthephenomenonofcell ‘activation’duringthefirstorearlycyclesoflithium-ioncells,more researchisrequiredtofurtherverifyorconfirmthemodel.Other possibilitiesfortheCoulombicefficiencyincreasecouldinclude improvedelectrochemicalkineticsbecauseoftheenhanced elec-trolyteupdateandsurfacewettabilityoftheseparatorswithSiO2

coating.

4. Conclusions

A new separator was successfully developed by depositing SiO2 ceramiclayers ofnanoparticlesontothePPseparators for

lithium-ionbatteryusing asimpleyet effectiveSiO2 formation

anddepositionprocess. Theexistenceoftheheat-resistant and hydrophilicSiO2 coatinglayers resulted innot onlya

substan-tial reduction inthethermal shrinkage,but also enhancement intensilestrengthandimprovementinsurfacewettability, elec-trolyteuptakeandcellperformancesascomparedtotheplainPP separator.Themuchimprovedbatteryratecapabilityandhigher Coulombicefficiency representbetterelectrochemical cell per-formancesuitableparticularlyforhighpowerapplications,while theincreasedmechanicalstrengthandreducedthermalshrinkage translateintoenhancedbatterysafety.Thisnewtechnologyis sim-ple,costeffective,andcanbeeasilycommercializedasaroll-to-roll productiononalargescale.Inaddition,theformationofapossible compositeSEIfilmcouldexplainthehigherCoulombicefficiency, thoughfurtherinvestigationwillneedtobeconducted.

Acknowledgement

Theauthorsaregratefultothefinancialsupportprovidedbythe ElectricMobilityProgramofPERD,NaturalResourcesCanada.

References

[1]M.Armand,J.M.Tarascon,Nature451(2008)652–657.

[2]D.Linden,T.B.Reddy,HandbookofBatteries,3rded.,McGraw-Hill,NewYork, 2002.

[3]F.Croce,G.B.Appetecchi,L.Persi,B.Scrosati,Nature394(1998)456–458. [4]G.Venugopal,J.Moore,J.Howard,S.Pendalwar,JournalofPowerSources77

(1999)34–41.

[5]B.L.Luan,G.Campbell,M.Gauthier,X.Y.Liu,I.Davidson,J.Nagata,M.Lepinay, F.Bernier,S.Argue,ECSTransactions25(2010)59–71.

[6]R.J.Brodd,H.M.Friend,J.C.Nardi,LithiumionBatteryTechnology,ITE-JECPress, 1995.

[7] S.S.Zhang,JournalofPowerSources164(2007)351–364. [8]P.Arora,Z.M.Zhang,ChemicalReviews104(2004)4419–4462. [9]C.T.Love,JournalofPowerSources196(2011)2905–2912.

[10]C.M.López,J.T.Vaughey,D.W.Dees,JournaloftheElectrochemicalSociety156 (2009)A726–A729.

[11]X.W.Zhang,Y.Li,S.A.Khan,P.S.Fedkiw,JournaloftheElectrochemicalSociety 151(2004)A1257–A1263.

[12]E.Eweka,J.R.Owen,A.Ritchie,JournalofPowerSources65(1997)247–251. [13]H.S. Jeong,J.H. Kim, S.Y.Lee, Journalof Materials Chemistry 20(2010)

9180–9186.

[14]T.H.Cho,M.Tanaka,H.Onishi,Y.Kondo,T.Nakamura,H.Yamazaki,S.Tanase, T.Sakai,JournalofPowerSources181(2008)155–160.

[15]Y.LeeMin,J.W.Kim,N.S.Choi,J.LeeAn,W.H.Seol,J.K.Park,JournalofPower Sources139(2005)235–241.

[16]M.Tanaka,T.H.Cho,T.Nakamura,T.Tarao,M.Kawabe,T.Sakai, Electrochem-istry78(2010)982–987.

[17]Y.Liang,L.Ji,B.Guo,Z.Lin,Y.Yao,Y.Li,M.Alcoutlabi,Y.Qiu,X.Zhang,Journal ofPowerSources196(2011)436–441.

[18]T.H.Cho,M.Tanaka,H.Ohnishi,Y.Kondo,M.Yoshikazu,T.Nakamura,T.Sakai, JournalofPowerSources195(2010)4272–4277.

[19] J.H.Park,J.H.Cho,W.Park,D.Ryoo,S.J.Yoon,J.H.Kim,Y.U.Jeong,S.Y.Lee, JournalofPowerSources195(2010)8306–8310.

[20]H.S.Jeong,S.C.Hong, S.Y.Lee,JournalofMembraneScience 364(2010) 177–182.

[21]H.S.Jeong,D.W.Kim,Y.U.Jeong,S.Y.Lee,JournalofPowerSources195(2010) 6116–6121.

[22]H.S.Jeong,J.H.Noh,C.G.Hwang,S.H.Kim,S.Y.Lee,MacromolecularChemistry andPhysics211(2010)420–425.

[23]Y.S.Chung,S.H.Yoo,C.K.Kim,IndustrialandEngineeringChemistryResearch 48(2009)4346–4351.

[24]J.A.Choi,S.H.Kim,D.W.Kim,JournalofPowerSources195(2010)6192–6196. [25]M.Kim,G.Y.Han,K.J.Yoon,J.H.Park,JournalofPowerSources195(2010)

8302–8305.

[26]H.S.Jeong,S.Y.Lee,JournalofPowerSources196(2011)6716–6722. [27]J.C.Su,S.Y.Liang,W.L.Liu,T.C.Jan,JournalofManufacturingScienceand

Engineering-TransactionsoftheASME126(2004)779–786.

[28]W.Stöber,A.Fink,E.Bohn,JournalofColloidandInterfaceScience26(1968) 62–69.

[29]H.A.Ketelson,R.Pelton,M.A.Brook,Langmuir12(1996)1134–1140. [30]D.L.Green,J.S.Lin,Y.F.Lam,M.Z.C.Hu,D.W.Schaefer,M.T.Harris,Journalof

ColloidandInterfaceScience266(2003)346–358.

[31]G.H.Bogush,C.F.ZukoskiIv,JournalofColloidandInterfaceScience142(1991) 19–34.

[32]G.H.Bogush,C.F.ZukoskiIv,JournalofColloidandInterfaceScience142(1991) 1–18.

[33] T.Matsoukas,E.Gulari,JournalofColloidandInterfaceScience132(1989) 13–21.

(11)

D.Fuetal./JournalofPowerSources206 (2012) 325–333 333 [34]T.Matsoukas,E.Gulari,JournalofColloidandInterfaceScience124(1988)

252–261.

[35]D.L.Green,S.Jayasundara,Y.F.Lam,M.T.Harris,JournalofNon-Crystalline Solids315(2003)166–179.

[36]S.Sadasivan,A.K.Dubey,Y.Li,D.H.Rasmussen,JournalofSol–GelScienceand Technology12(1998)5–14.

[37]S.L.Chen,P.Dong,G.H.Yang,J.J.Yang,IndustrialandEngineeringChemistry Research35(1996)4487–4493.

[38]C.J.Brinker,G.W.Scherer,Sol–GelScience,AcademicPress,Boston,1990. [39] S.L.Chen,P.Dong,G.H.Yang,JournalofColloidandInterfaceScience189(1997)

268–272.

[40] A.vanBlaaderen,A.P.M.Kentgens,JournalofNon-CrystallineSolids149(1992) 161–178.

[41] A.VanBlaaderen,J.VanGeest,A.Vrij,JournalofColloidandInterfaceScience 154(1992)481–501.

[42] X.D.Wang,Z.X.Shen,T.Sang,X.B.Cheng,M.F.Li,L.Y.Chen,Z.S.Wang,Journal ofColloidandInterfaceScience341(2010)23–29.

[43] B.V.Crist,HandbookofMonochromaticXPSSpectra:Semiconductors,Wiley, 2000.

[44] N.Fairley,CasaXPSManual2.3.15Rev1.2,CasaSoftwareLtd.,2009. [45] G.Friedbacher,H.Bubert,SurfaceandThinFilmAnalysis:ACompendiumof

Principles,Instrumentation,andApplications,JohnWiley&Sons,2011.

[46]G.P.Lopez,D.G.Castner,B.D.Ratner,SurfaceandInterfaceAnalysis17(1991) 267–272.

[47]J.M.Kim,S.M.Chang,S.M.Kong,K.-S.Kim,J.Kim,W.-S.Kim,Ceramics Interna-tional35(2009)1015–1019.

[48]F.Tran-Van,M.Provencher,Y.Choquette,D.Delabouglise,ElectrochimicaActa 44(1999)2789–2792.

[49]C.J.Brinker,G.C.Frye,A.J. Hurd,C.S.Ashley,ThinSolidFilms201(1991) 97–108.

[50]C.J.Brinker,Y.Lu,A.Sellinger,H.Fan,AdvancedMaterials11(1999)579–585. [51] Y.Masuda,T.Itoh,K.Koumoto,Langmuir21(2005)4478–4481.

[52]Y.Masuda,M.Itoh,T.Yonezawa,K.Koumoto,Langmuir18(2002)4155–4159. [53] B.Luan,M.Yeung,W.Wells,X.Liu,AppliedSurfaceScience156(2000)

26–38.

[54] R.N.Wenzel,Industrial&EngineeringChemistry28(1936)988–994. [55]Y.Zhang,G.Zhang,T.Du,L.Zhang,ElectrochimicaActa55(2010)5793–5797. [56] F.Rodriguez(Ed.),PrinciplesofPolymerSystems,3rded.,HemispherePub.

Corp.,NewYork,1989.

[57] S.Franssila(Ed.),IntroductiontoMicrofabrication,2nded.,JohnWiley&Sons, 2010.

[58] J.O.Besenhard,HandbookofBatteryMaterials,Wiley-VCH,1999. [59] T.Nakajima,JournalofFluorineChemistry128(2007)277–284. [60]Z.Ogumi,S.K.Jeong,Electrochemistry71(2003)1011–1017.