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Lithium-IonBatteries

锂离子电池容量衰减机机理和副反应

PankajAroratandRalphE.White*

作者：

CenterForElectrochemicalEngineering,DepartmentofChemicalEngineering,UniversityofSouthCarolina,Columbia,SouthCarolina29208,USA

美国，南卡罗来纳，年哥伦比亚29208，南卡罗来纳大学，化学工程系，中心电化学工程

ABSTRACT

Thecapacityofalithium-ionbatterydecreasesduringcycling.Thiscapacitylossorfadeoccursduetoseveraldifferentmechanismswhichareduetoorareassociatedwithunwantedsidereactionsthatoccurinthesebatteries.Thesereactionsoccurduringoverchargeoroverdischargeandcauseelectrolytedecomposition,passivefilmformation,activematerialdissolution,andotherphenomena.Thesecapacitylossmechanismsarenotincludedinthepresentlithium-ionbatterymathematicalmodelsavailableintheopenliterature.Consequently,thesemodelscannotbeusedtopredictcellperformanceduringcyclingandunderabuseconditions.Thisarticlepresentsareviewofthecurrentliteratureoncapacityfademechanismsandattemptstodescribetheinformationneededandthedirectionsthatmaybetakentoincludethesemechanismsinadvancedlithium-ionbatterymodels.

Introduction

Thetypicallithium-ioncell（Fig.1）ismadeupofacokeorgraphitenegativeelectrode,anelectrolytewhichservesasanionicpathbetweenelectrodesandseparatesthetwomaterials,andametaloxide（suchasLiCoO2,LiMn2O4,orLiNiO2）positiveelectrode.Thissecondary（rechargeable）lithium-ioncellhasbeencommercializedonlyrecently.Batteriesbasedonthisconcepthavereachedtheconsumermarket,andlithium-ionelectricvehiclebatteriesareunderstudyinindustry.Thelithium-ionbatterymarkethasbeeninaperiodoftremendousgrowtheversinceSonyintroducedthefirstcommercialcellin1990.Withenergydensityexceeding130Wh/kg（e.g.,MatsushitaCGR17500）andcyclelifeofmorethan1000cycles（e.g.,Sony18650）inmanycases,thelithium-ionbatterysystemhasbecomeincreasinglypopularinapplicationssuchascellularphones,portablecomputers,andcamcorders.Asmorelithium-ionbatterymanufacturersenterthemarketandnewmaterialsaredeveloped,costreductionshouldspurgrowthinnewapplications.SeveralmanufacturerssuchasSonyCorporation,SanyoElectricCompany,MatsushitaElectricIndustrialCompany,MoliEnergyLimited,andA&

TBatteryCorporationhavestartedmanufacturinglithium-ionbatteriesforcellularphonesandlaptopcomputers.Yoda1hasconsideredthisadvancementanddescribedafuturebatterysocietyinwhichthelithium-ionbatteryplaysadominantrole.

Severalmathematicalmodelsoftheselithium-ioncellshavebeenpublished.Unfortunately,noneofthesemodelsincludecapacityfadeprocessesexplicitlyintheirmathematicaldescriptionofbatterybehavior.Theobjectiveofthepresentworkistoreviewthecurrentunderstandingofthemechanismsofcapacityfadeinlithium-ionbatteries.Advancesinmodelinglithium-ioncellsmustresultfromimprovementsinthefundamentalunderstandingoftheseprocessesandthecollectionofrelevantexperimentaldata.

Someoftheprocessesthatareknowntoleadtocapacityfadeinlithium-ioncellsarelithiumdeposition（overchargeconditions）,electrolytedecomposition,activematerialdissolution,phasechangesintheinsertionelectrodematerials,andpassivefilmformationovertheelectrodeandcurrentcollectorsurfaces.Quantifyingthesedegradationprocesseswillimprovethepredictivecapabilityofbatterymodelsultimatelyleadingtolessexpensiveandhigherqualitybatteries.Significantimprovementsarerequiredinperformancestandardssuchasenergydensityandcyclelife,whilemaintaininghighenvironmental,safety,andcoststandards.Suchprogresswillrequireconsiderableadvancesinourunderstandingofelectrodeandelectrolytematerials,andthefundamentalphysicalandchemicalprocessesthatleadtocapacitylossandresistanceincreaseincommerciallithium-ionbatteries.Theprocessofdevelopingmathematicalmodelsforlithiumioncellsthatcontainthesecapacityfadeprocessesnotonlyprovidesatoolforbatterydesignbutalsoprovidesameansofunderstandingbetterhowthoseprocessesoccur.

PresentLithium-IonBatteryModels

Thedevelopmentofadetailedmathematicalmodelisimportanttothedesignandoptimizationoflithiumsecondarycellsandcriticalintheirscale-up.Westdevelopedapseudotwo-dimensionalmodelofasingleporousinsertionelectrodeaccountingfortransportinthesolutionphaseforabinaryelectrolytewithconstantphysicalpropertiesanddiffusionoflithiumionsintothecylindricalelectrodeparticles.Theinsertionprocesswasassumedtobediffusionlimited,andhencecharge-transferresistanceattheinterfacebetweenelectrolyteandactivematerialwasneglected.LaterMaoandWhitedevelopedasimilarmodelwiththeadditionofaseparatoradjacenttotheporousinsertionelectrode.Thesemodelscoveronlyasingleporouselectrode;

thus,theydonothavetheadvantagesofafull-cell-sandwichmodelforthetreatmentofcomplex,interactingphenomenabetweenthecelllayers.ThesemodelsconfinethemselvestotreatinginsertionintoTiS2withthekineticsfortheinsertionprocessassumedtobeinfinitelyfast.SpotnitzaccountedforelectrodekineticsintheirmodelfordischargeoftheTiS2,intercalationcathode.

Thegalvanostaticchargeanddischargeofalithiummetal/solidpolymerseparator/insertionpositiveelectrodecellwasmodeledusingconcentrated-solutiontheorybyDoyle.Themodelisgeneralenoughtoincludeawiderangeofseparatormaterials,lithiumsalts,andcompositeinsertionelectrodes.Concentrated-solutiontheoryisusedtodescribethetransportprocesses,asithasbeenconcludedthationpairingandionassociationareveryimportantinsolidpolymerelectrolytes.Thisapproachalsoprovidesadvantagesoverdilutesolutiontheorytoaccountforvolumechanges.Butler-Volmer-typekineticexpressionswereusedinthismodeltoaccountforthekineticsofthecharge-transferprocessesateachelectrode.ThepositiveelectrodeinsertionprocesswasdescribedusingPick'

slawwithaconstantlithiumdiffusioncoefficientintheactivematerial.Thevolumechangesinthesystemandfilmformationatthelithium/polymerinterfacewereneglectedandaverysimplisticcaseofconstantelectrodefilmresistanceswasconsidered.Long-termdegradationofthecellduetoirreversiblereactions（sidereactions）orlossofinterfacialcontactisnotpredictableusingthismodel.

Fullerdevelopedageneralmodelforlithiumioninsertioncellsthatcanbeappliedtoanypairoflithium-ioninsertionelectrodesandanybinaryelectrolytesystemgiventherequisitephysicalpropertydata.Fullerworkdemonstratedtheimportanceofknowingthedependenceoftheopen-circuitpotentialonthestateofchargefortheinsertionmaterialsusedinlithium-ioncells.Theslopesofthesecurvescontrolthecurrentdistributioninsidetheporouselectrodes,withmoreslopedopen-circuitpotentialfunctionsleadingtomoreuniformcurrentdistributionsandhencebetterutilizationofactivematerial.OptimizationstudieswerecarriedoutfortheBellcoreplasticlithium-ionsystem.Themodelwasalsousedtopredicttheeffectsofrelaxationtimeonmultiplecharge-dischargecyclesandonpeakpower.

Doylemodifiedtheduallithium-ionmodeltoincludefilmresistancesonbothelectrodesandmadedirectcomparisonswithexperimentalcelldatafortheLiC6-LiPF6,ethylenecarbonate/dimethylcarbonate（EC/DMC）,KynarFLEX-ILiyMn2O4system.Comparisonsbetweendataandthenumericalsimulationssuggestedthatthereisadditionalresistancepresentinthesystemnotpredictedbypresentmodels.Thedischargeperformanceofthecellswasdescribedsatisfactorilybyincludingeitherafilmresistanceontheelectrodeparticlesorbycontactresistancesbetweenthecelllayersorcurrent-collectorinterfaces.OneemphasisofthisworkwasintheuseofthebatterymodelforthedesignandoptimizationofthecellforparticularapplicationsusingsimulatedRagoneplots.

Thermalmodelingisveryimportantforlithiumbatteriesbecauseheatproducedduringdischargemaycauseeitherirreversiblesidereactionsormeltingofmetalliclithium,ChenandEvanscarriedoutathermalanalystsoflithiumionbatteriesduringcharge-dischargeandthermalrunawayusinganenergybalanceandasimplifieddescriptionoftheelectrochemicalbehaviorofthesystem.Theiranalysisofheattransportandtheexistenceofhighlylocalizedheatsourcesduetobatteryabuseindicatedthatlocalizedheatingmayraisethebatterytemperatureveryquicklytothethermalrunawayonsettemperature,abovewhichitmaykeepincreasingrapidlyduetoexothermicsidereactionstriggeredathightemperature.PalsandNewmandevelopedamodeltopredictthethermalbehavioroflithiummetal-solidpolymerelectrolytecellsandcellstacks.Thismodelcoupledanintegratedenergybalancetoafullcell-sandwichmodeloftheelectrochemicalbehaviorofthecells.Bothofthesemodelsemphasizedtheimportanceofconsiderationsofheatremovalandthermalcontrolinlithiumpolymerbatterysystems.

VerbruggeandKochdevelopedamathematicalmodelforlithiumintercalationprocessesassociatedwithacylindricalcarbonmicrofiber.Theycharacterizedandmodeledthelithiumintercalationprocessinsingle-fibercarbonmicroelectrodesincludingtransportprocessesinbothphasesandthekineticsofchargetransferattheinterface.Theprimarypurposeofthemodelwastopredictthepotentialasafunctionoffractionaloccupancyofintercalatedlithium.Theoverchargeprotectionfo