ElucidatingtheDilationandContractionBehaviorsofPrelithiatedSiOAnodesforLithium-IonBatteriesUsingInsituDilatometry

Abstract

Astheconcernforglobalclimatechangecontinuestorise,initiativesarebeingmadetodownsizegreenhousegasemissionsandshifttowardsusingrenewablesourcesofenergy.Lithium-ionbatteries(LIBs)arevitalforthistransition,andarecurrentlybeingwidelyusedinvariousapplications,withusesinelectricvehicles(EVs)beingofsignificantinterest.However,priortothecompletereplacementoftheinternalcombustionenginetotakeplace,aspectsofhighenergydensities,longcyclelife,fastchargingcapability,lowcost,andsafetymustbefulfilledfortheLIBs.Forthisreason,siliconanodematerialsareofinterestbecauseoftheirhightheoreticalcapacitycomparedtocommercializedgraphite.However,siliconanodematerialssufferseverevolumechangesthatarisesduringlithiuminsertionandextraction,whichresultsininferiorcyclingperformance.Inthissense,silliconmonoxide(SiO)basedanodematerialsarepromisingcandidatesbecausetheSiOxmatrixcanbufferthevolumeexpansion,thusshowingrelativelyenhancedcyclingperformance.However,theSiOxmatrixreactswithlithiumduringlithiuminsertion,formingirreversiblephaseswhichresultsinlowinitialCoulombicefficiency(ICE).ThelowinitialCoulombicefficiency(ICE)hampersthecompleteutilizationofthematerialincommercialLIBs.Variousprelithiationmethodshavebeenintroducedfrommaterialleveltoelectrodeleveltosolvethisissue.Dehydrogenationdrivenprelithiationusinglithiumhydride(LiH)hasmaderecentprogress,enhancingtheICEover90%throughalithiummetalfree,safe,andpotentiallyscalableprocess.Despitethisimprovement,thecyclestabilitywasobservedtoreducewithincreasingLi/SiratiosduetothecoarseningofSidomains.Inordertoidentifythisbehaviormoreindepth,calculationsofexpansionbasedonquantitativeresults,insituelectrochemicaldilatometry,andexsitucross-sectionalscanningelectronmicroscopy(SEM)wereperformed.TheprelithiatedSiOelectrodesshowedmitigatedexpansionbehaviorscomparedtothepristineSiOelectrode.Duringtheinitialcycles,thesamplewiththehighestLi/SiratiodepictedthelargestexpansionamongtheprelithiatedSiOelectrodes.Long-termdilationresultsafter300cyclesalsoportrayedlargerexpansionwithincreasingLi/Siratios,showingcoherencewiththecycleperformanceresults.TheseresultsdemonstratethattheinitialSidomainsizeofSi-basedanodematerialsisanessentialparameterthataffectstheinitialandlong-termdilationandcontractionbehaviorsofelectrodesthatariseduringcycling.TheexpansionandcontractionbehaviorsoftheprelithiatedSiOelectrodeswillbediscussedmorethoroughlyinthiswork.|지구온난화에대한인식이전세계적으로부각됨에따라,온실가스규제에대한조치와재생가능한에너지원을사용하고자하는전환이활발하게이루어지고있다.그중,리튬이온전지는이러한전환에핵심적인역할은하는기술로,현재다양한어플리케이션중에서도전기자동차의(EVs)적용에대한관심도가높다.하지만,기존의내연기관을완전히대체하기위해서리튬이온전지는높은에너지밀도,장수명,고속충전,저렴한가격과안전성측면에서향상안이필요하다.실리콘은기존의상용화소재인흑연과대비하여높은이론용량을가지기때문에관심을모으고있다.하지만,실리콘은리튬의삽입과탈리과정에서극심한부피변화로인하여수명특성저하문제가존재한다.실리콘산화물계(SiO)음극소재는SiOx매트릭스가부피팽창을완화시킬수있기때문에실리콘계음극소재중비교적우수한수명특성을갖는다.하지만,실리콘산화물계음극소재에서리튬삽입시,SiOx매트릭스와반응하면서비가역상의형성으로인하여리튬소모가일어나면서낮은초기효율을갖는한계점이있다.실리콘산화물계음극소재의낮은초기효율문제를해결하기위하여prelithiation기술에관한다양한연구가진행되고있다.최근수소화리튬을사용하여탈수소화반응에기반한prelithiation을통해리튬금속이불필요한,안전하고대규모화를고려한합성공정으로90%이상의초기효율향상이확인된연구가진행된바가있다.하지만,첨가되는리튬/실리콘비율이증가할수록소재내실리콘도메인의조대화로인하여수명특성저하가확인되었다.본연구는이러한수명특성저하에대해더욱면밀한분석의필요성을확인하여prelithiated전극의실시간팽창및수축거동분석에관한연구를하였다.다양한팽창관련분석을통해prelithiatedSiO전극은기존SiO전극보다감소된팽창거동이확인되었다.초기사이클동안prelithiatedSiO전극중,리튬/실리콘비율이가장높은L67전극에서가장높은팽창거동이관측되었다.장기적인팽창거동분석에서도리튬/실리콘비율이증가함에따라팽창률도증가하였고,이것은수명특성결과와일치하는경향성을보였다.따라서,실리콘계음극소재내실리콘도메인이셀구동상에서전극의초기및장기적인팽창거동과수명특성을결정하는중요한인자인것을확인할수있었다.PrelithiatedSiO전극의팽창및수축거동분석에관한자세한내용은본문에서다루고자한다.