C12-Optimization-of-MCMB-based-laminates-for-lithi

189OptimizationofMCMB-basedlaminatesforlithiumionbatteriesforPHEVsIntroductionTodevelopelectrodestomeettheaggressivePHEVgoals,oneimportantwayistooptimizetheinactivematerialcontent.Poly(vinylidenedifluoride)(PVDF)binderandacetyleneblackarewidelyadoptedinactivematerialsintheproductionoflithiumionbatteryelectrodes.Theseinactivematerialsprovideadhesionandanelectronicconductivepathbetweenelectrodeandcurrentcollectorandbetweenactiveparticles.However,introductionofexcessivelevelsofinactivematerialwilldecreasethespecificcapacityandenergydensityoftheelectrode.ThepurposeofthisstudyistoinvestigatetheeffectsofdifferentlevelsofcarbonandbinderontheelectrochemicalbehaviorofMCMBanodeandontheprolongedcyclingperformanceofthefullcellwhencombinedwithLi[Ni1/3Mn1/3Co1/3]O2cathodewithagoaltooptimizetheinactivelevelsinanMCMB-basedelectrode.ExperimentTheMCMBanodematerialwassuppliedbyOsakaGas,Japan.AslurryconsistingofdifferentamountsofMCMB,PVdF,acetlyleneblackwerepreparedbymixinginN-methyl-2-pyrrolidone(NMP).Coatedfilmsoncopperfoilwerepreparedbythedoctorblademethod.Allofthefilmswerecontrolledtohaveapproximatelythesameloadingofactivematerial(30mg/cm2)tothebestofourability.Allofthefilmswerecompressedto35%porosityusingacalenderingmachine.Filmconductivitywasdeterminedbyusinga4-pointprobe.Coincellswereassembledinanargon-filledglovebox.LithiumfoilandLi[Ni1/3Mn1/3Co1/3]O2laminateswereusedasthecounterelectrodesforhalfcellsandfullcells,respectively.TheseparatoremployedwasCelgard2400.1MLiPF6/EC+DEC(1:2)wasusedastheelectrolyte.ElectrochemicalmeasurementswereperformedbyusingaMaccorbatterycycler.01020304050607080Electrodethickness(micron)PVDF:carbon=5:1PVDF:carbon=5:2PVDF:carbon=5:42%PVDF5%PVDF8%PVDF12%PVDF23456789101112051015PVDFcontent(%)Conductivity(S/cm)PVDF:carbon=5:4PVDF:carbon=5:2PVDF:carbon=5:1Fig.1showsthevariationoftheelectrodethicknesswithdifferentinactivematerialratios(thethicknessofCufoilisnotincluded.)Thisresultclearlyshowsthatremovalofinactivematerialreducestheelectrodethicknesswiththesameactivematerialloading.Reductionofelectrodethicknesswillinevitablyhelptoincreasetheelectrode’senergydensity.Fig.2showsthevariationofthefilmconductivitywithdifferentinactivematerialratiosandamountofbinder.ItwasseenthatthefilmconductivityincreasedforagivenPVDFcontentwithdecreaseofPVDF/carbonratiofrom5:1to5:4.AtthesamePVDF/carbonratio,amaximumconductivitywasobtainedataround8%PVDFcontent.Fortheelectrochemicalstudies,wefixedthePVDF:carbonratioat5:2andinvestigatedtheeffectsoftotalinactivecontentontheelectrochemicalbehavioroftheMCMBanode.Fig.3showsthevariationsofthespecificcapacityandfirstcoulombicefficiencyoftheMCMBanodeswiththetotalamountsofinactivematerial.ItisseenthatthespecificcapacityoftheMCMBanodeslightlyincreasedwithincreasinginactiveamount.Thisimpliesthattheutilityoftheactivematerialisimprovedduetobetteradhesionbetweenparticles.However,thisisaccompaniedbyadecreaseinthefist-cyclecoulombicefficiency.TomakeagoodMCMBlaminate,acompromisemustbemadebetweenobtainingahighspecificcapacityandahighfirst-cyclecoulombicefficiency.Fig.4showsthecyclingbehavioroftheMCMBanodeswhencyclewithaLi[Ni1/3Mn1/3Co1/3]O2cathode.Thesefullcellswerecycledbetween3and4.4VatC/2chargeandCdischarge.Cycleablityofthesefullcellsisfoundtobestronglydependentonthecompositionoftheanode.ThecyclingperformanceissignificantlyimprovedwhentheamountofPVDFisincreasefrom2%to8%.AfurtherincreaseoftheinactivematerialdecreasesthelongtermcyclingFig.1VariationofelectrodethicknesswithinactivematerialratiosandamountsFig.2Variationoffilmconductivitywithinactivematerialratiosandamounts.190performance.27027528028529029530030505101520Inactivematerialamount(%)Specificcapacity(mAh/g)8889909192939495Firstefficiency(%)SpecificcapacityCoulombicefficiency020406080100120050100150200CyclenumberCapacityretention(%)2%PVDF5%PVDF8%PVDF12%PVDFAcknowledgmentsTheauthorswouldlikethankU.S.D.O.EundertheOfficeofFreedomCARandtheNSFC(21073129)Fig.3VariationsofthespecificcapacityandfirstefficiencywiththetotalamountsofinactivematerialFig.4CyclingcapabilityofthefullcellswhencombinedwithLi[Ni1/3Mn1/3Co1/3]O2cathode.