Studyonthechemicalandchemo-mechanicalstabilityofall-solid-statebatteriesusingsulfidebasedsolidelectrolyte

Abstract

최근급격한전기자동차시장의성장으로이차전지의에너지밀도및안정성향상에대한요구가증대되고있다.기존액체전해질을고체전해질로대체한전고체전지는높은에너지밀도와안정성으로가장유망한차세대전지로손꼽히고있다.전고체전지의성능개선을위해서는계면불안정성의극복이필수적이다.전고체전지의계면불안정성은대표적으로화학-기계적(chemo-mechanical),화학적(chemical)불안정성으로나눌수있다.본학위논문에서는전고체전지의계면안정성이전기화학성능에끼치는영향에대해탐구하고계면불안정성을해결할수있는방법에대한연구를진행하였다. LiNi0.8Co0.1Mn0.1O2¬(NCM)은높은이론용량과낮은가격으로전고체전지의상용화에적합한양극소재로고려되고있다.하지만NCM소재는적절한복합체형성과정이없는경우,계면에서의고체전해질과접촉이충분하게형성되지못하며,충·방전과정에서의부피변화로인한crack생성으로인해낮은화학-기계적안정성을보인다.NCM과황화물계고체전해질의충분한초기접촉형성을위해,mechano-fusion장비를도입,강한물리적힘을통해NCM표면에균일하게황화물계고체전해질을코팅하였다.그결과NCM과고체전해질간초기접촉면적이향상되고,균일한미세구조인해구동시부피변화에의한접촉유실이개선되어우수한전기화학성능을확인할수있었다.구동중NCM활물질의crack형성을억제하기위해Tidopant를도입하여입자의경도를향상시켰다.해당입자를적용한전고체전지구동시입자의crack이억제되어극판의화학-기계적안정성이향상되었다.황화계물계고체전해질은NCM또는리튬메탈과접촉시화학적부반응을일으켜계면저항을증가시킨다.이를해결하고자NCM과리튬메탈각각에보호층을도입해화학적불안정성을극복하고자하였다.NCM표면에TiO2물질을코팅한후열처리하여잔존하는불순물과의반응을통해코팅층을형성하였다.이를통해황화물계고체전해질과NCM활물질과의부반응이억제되었고,안정적인전기화학성능을확인할수있었다.리튬메탈의안정성확보를위해ChemicalVaporDeposition방법을이용하여나노구조체의Li2Se¬보호층을형성하였다.반응시간과냉각시간조절시,각각나노파티클,나노로드,나노월의형상을갖는보호층이형성되는것을확인하였다.형성된보호층이리튬메탈과고체전해질의부반응을억제하여,향상된전기화학성능을확인할수있었다. |All-solid-statebatterieshavebeenconsideredthemostpromisingnext-generationenergystoragesystemsduetotheirpotentialtoincreaseenergydensityandultimatesafety.However,thepoorelectrochemicalperformanceoftheall-solid-statebatteries,causedbythehighinterfacialresistancebetweenasulfide-basedsolidelectrolyteandactivematerials,limitedtheirpracticalapplication.Theoriginofhighinterfacialresistancecanbecategorizedaspoormechanicalstabilityandtheundesirablesidereactionbetweentheactivematerialsandsulfide-basedsolidelectrolytes.Inthisthesis,theelectrochemicalperformanceofall-solid-statebatteriesassociatedwithchemo-mechanicalandchemicalinstabilityattheinterfacewasstudied,andtheapproachtohandlingissuesrelatedtotheinterfaceproblemswasalsodiscussed. Securingsufficientphysicalcontactbycoatingthesolidelectrolyteonthecathodeactivematerialsisaneffectivemethodtomaximizethecontactareabetweenthesolidelectrolyteandcathodeactivematerials.Typically,solidelectrolytecoatingoncathodeactivematerialsviaawetcoatingprocesshasbeenstudied.However,therearerestrictionsonselectingasolventduetothesevereundesirablereactionbetweenthesulfide-basedsolidelectrolytesandsolventwithhighpolarity.Theionicconductivityofsolidelectrolytescouldalsodecreaseduringtherecrystallizationprocessduetothegenerationofanamorphousmatrix.Toovercomethislimitationsolvent-freemechano-fusionprocessisintroduced.Thehighshearandcompressionforcesusedinthemechano-fusionprocess,whichareforcedthroughanarrowgap,enablethecoatingofathinandrobustlayer.Inaddition,itispossibletocontrolthecoatingthicknessofcoatingmaterials,maintainingtheoriginalshapeofthehostmaterials.TheconformallycoatedthinsolidelectrolyteslayeronthesurfaceofNCMenablesthehomogeneousdistributionofsolidelectrolytesintheoverallelectrode.Theenhancedmicrostructureoftheelectrodebymechano-fusionmethodenablestheoperationwithharshconditionssuchashighcathodeactivematerialscontentsandlowstackpressurewithoutnoticeableperformancedegradation.TheperformancedegradationparameterswiththehighmassfractionofcathodeactivematerialsintheelectrodewerestudiedbySEMobservationbeforeandaftercyclingandEISanalysis. Evenifthesufficientinitialcontactisachieved,thevolumechangeofthecathodeactivematerialsduringthecyclingcouldresultinfurtherphysicalcontactlossduetothecrackformation.Inthisregard,themechanicalpropertiesofcathodeactivematerialsneedtobeconsideredtoimprovethemechanicalstabilityoftheelectrode.Furthermore,thestackpressureduringtheoperationofall-solid-statebatteriestopreventcontactlossintheelectrodecanintensifythestressinanelectrode,leadingtocracksintheNCMparticleandtheelectrode.Therefore,thehardnessofthecathodeactivematerialsneedstobehighenoughtorepressthegenerationofcracksduringcycling,whichtriggersenhancedresistanceinall-solid-statebatteries.Inthisregard,therelationbetweenthemechanicalpropertyofthecathodeactivematerialsandthechemo-mechanicalbehaviorwasalsodiscussedinthisthesis.TheTi-dopedNCMwasprepared,whichhasenhancedhardnessduetostrongTi-Obonding.TheTi-dopedNCMwascharacterizedbytransmissionelectronmicroscopyandX-raydiffractometry.Thehardnessoftheparticleswasanalyzedusingnanoindentation.Theelectrochemicalperformanceandmicrostructuralbehaviorofelectrodesassociatedwiththehardnessofcathodeactivematerialsisalsoinvestigated. Thechemicalinstabilitybetweentheactivematerialsandsulfide-basedsolidelectrolytesisanotherproblemofall-solid-statebatteries.Theresultingchemicallyunfavorableactivematerialsandsolidelectrolytesdeteriorateelectrochemicalperformanceintermsofinitialcapacity,Coulombicefficiency,andcycleretention.Employingthechemicallystableprotectivelayerisaneffectivewaytosuppresschemicalsidereactions.Theoriginoftheenhancedelectrochemicalpropertiesofelectrodesisstudied,andtheparameterfortheprotectivelayerenablescontrollingthesidereactionisdiscussed. LiNbO3isthemostwidelyemployedprotectivelayerforcathodeduetoitsrelativelyhighionicandlowelectronicconductivity.However,niobiumisnotearth-abundantmaterial,whichcouldincreasefabricationcosts.Alternativecoatingmaterialsareneededforthemassproductionofcathodeactivematerialswithacceptableelectrochemicalperformanceinall-solid-statebatteries.Inthisrespect,theLi4Ti5O12isthepromisingcoatinglayerinsulfide-basedall-solid-statebatteriesasitcaneffectivitycontrolthechemicalinstabilitybetweencathodeactivematerialsandsolidelectrolytes.Recentresearchhasshownthattheresiduallithiumcompoundsonthecathodeactivematerialsacceleratetheoxidationofsulfide-basedsolidelectrolytesduringcharging,resultinginthedegradationofelectrochemicalperformance.Althoughthewashingprocesscanimproveelectrochemicalperformancebyremovingtheresiduallithiumcompounds,additionalprocessingtimeandcostsarenecessary.Apreviousreportfoundthatresiduallithiumcompoundscanbeusedassourcematerialstosynthesizeanionicconductivecoatinglayeraftersuitabletreatment.Usingresiduallithiumcompoundsassourcematerialsforthecoatinglayerisatime-savingandcost-effectivemethodtomakeaprotectivelayerforcathodeactivematerials.TheNCMretainingsurfaceimpuritieswasdirectlycoatedTiO2materials,usingatomiclayerdeposition.ThecoatedTiO2materialsservedasthesourcematerialsfortheLi4Ti5O12coatinglayerandasthedopantfortheNCMparticles.AnLTOcoatingcontrolsthechemicalinstabilitybetweenNCMandsolidelectrolyte. Intheanode,Limetalisconsideredastheultimateanodematerialforall-solid-statebatteriesduetoitshightheoreticalcapacityandlowpotential.However,sulfide-basedsolidelectrolytesreactwithLimetalandformdecompositionproductswithlowionicconductivity.Thesedecompositionproductsattheinterfacebetweenthesulfide-basedsolidelectrolyteandLimetalanodesignificantlyincreasetheinterfacialresistance,whichinducesaccumulatedvoidswiththesluggishLireplenishingrateduringcycling.Inthisregard,theintroductionofaLiprotectivelayerwiththermodynamicalstability,highionicconductivity,andlowelectronicconductivitycouldbeastraightforwardstrategyforhigh-performanceall-solid-statebatteries.Consideringthisparameter,theLi2SewasadoptedfortheLiprotectivelayer.ThedirectdepositionofseleniumonLimetalandthesubsequentformationofLi2Seisafacileandeffectivemethodtobuildalow-resistanceinterfacebetweentheLiandSSE.ThestrategyofintroducingLiselenideisbasedonseveralfactors;1)selenium,theimmediateneighborofsulfur,hassimilarchemicalpropertiesandcompatibilitywithsulfide-basedmaterials,2)seleniumislessreactivetowardLiandmorecontrollable,3)dependingonvaporpressure,asmallnumberofatomsandmoleculesofliquid-stateseleniumeasilyevaporatesfromthesurfaceintherangeof100–500℃abovethemeltingtemperatureandbelowtheboilingtemperature,andgas-phaseseleniumcanreactwithmetals,resultinginvariousmetalcompounds,and4)oneofmetalselenides,Li2Se,haslowelectricalconductivity(bandgap:~2.997eV)andcouldhavehigherionicconductivitythanLi2S(~10−5Scm−1),whichissuitableforaprotectivelayer.Asaresult,afullcellcoupledwithaLiCoO2-basedcathodeshowssignificantlyenhancedelectrochemicalperformanceanddemonstratesthepracticaluseofLianodeswithLi2Selayersforall-solid-statebatteryapplications.Theseexperimentalresultsandtheassociatedmechanismofthechemo-mechanicalandchemicaldegradationeffectontheelectrochemicalpropertiesbehavioroftheall-solid-statebatteriescanbeusedtobenefitindesigninghigh-performanceall-solid-statebatteries.