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COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES SAFEGUARDING LITHIUM-ION BATTERY SEPARATORS THERMAL ANALYSIS OF LITHIUM-ION BATTERIES NEW SCIENCE SUSTAINABLE ENERGY JOURNAL ISSUE IV UL.COM/NEWSCIENCE LITHIUM-ION BATTERIES

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Page 1: LITHIUM-ION€¦ · • THERMAL ANALYSIS OF LITHIUM-ION BATTERIES NEW SCIENCE SUSTAINABLE ENERGY JOURNAL ISSUE IV UL.COM/NEWSCIENCE LITHIUM-ION BATTERIES. 2345678327395NENW SC IBATACARHL

• COMPUTATIONALMODELINGOFLITHIUM-IONBATTERIES• SAFEGUARDINGLITHIUM-IONBATTERYSEPARATORS• THERMALANALYSISOFLITHIUM-IONBATTERIES

NEW SCIENCE SUSTAINABLE ENERGY

JOURNAL ISSUE IV UL.COM/NEWSCIENCE

LITHIUM-ION BATTERIES

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NEW SCIENCE: SUSTAINABLE ENERGY 2

NEW CHALLENGESCALL FOR NEW SCIENCE

Progress is an unstoppable, transformative force. New technologies, product advances and globalization are arriving one on top of another at a dizzying pace. Innovation makes us more efficient, more productive and more connected. But there is a cost, and that cost is risk. To help mitigate the emerging risks, UL is developing New Science. Through fundamental discovery, testing methodologies and equipment, procedures, software and standards, UL is creating new and important ways to make the world a safer place.

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NEW SCIENCE: SUSTAINABLE ENERGY 3

SUSTAINABLE ENERGYLITHIUM-ION BATTERIES OVERVIEW

THERMAL ANALYSIS OF LITHIUM-ION BATTERIES, PG.13

SAFEGUARDING LITHIUM-ION BATTERY SEPARATORS, PG.9

COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES, PG.4

We established a comprehensive thermal analysis capability that enables us to identify and measure exothermic and endothermic reactions within a lithium-ion battery cell.

UL applied materials science to develop a testing approach that establishes a more robust evaluation of lithium-ion battery separators. These tests help ensure that lithium-ion battery design is safe for its intended use.

We developed a unique thermal model of the common 18650 lithium-ion battery cell that enhances our ability to mathematically simulate, explore and understand the causes and severity of internal short circuits.

Our Lithium-ion Batteries journal covers three key subjects that further demonstrate how UL is working to enhance the safety of this sustainable energy source. Computational Modeling is a critical tool we use to complement laboratory experiments and field data. We are able to mathematically reproduce and examine what happens in a lithium-ion battery when an internal short circuit (ISC) occurs. This work is critical because the vast majority of field failures can be attributed to ISCs. Lithium-ion Battery Separators are also a focus of our research because of their central role in facilitating battery performance and in helping to prevent ISCs. We applied materials science to develop a robust set of tests to identify potential safety risks of today’s increasingly thinner and lighter separators. Additionally, we have developed an in-depth Thermal Analysis capability that allows us to exhaustively study heat generation, which is related to the most significant safety issues in lithium-ion batteries.

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NEW SCIENCE: SUSTAINABLE ENERGY 4

COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES

SAFEGUARDINGLITHIUM-IONBATTERYSEPARATORS

THERMALANALYSISOFLITHIUM-IONBATTERIES

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WHY COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES MATTERS Lithium-ionbatteriesareavitalsourceofsustainableenergyandarebeingusedacrossanexpandingarrayofapplications—fromportableelectronicdevicestoelectricvehiclestostationaryenergystorage.Importantly,thehighenergydensitythatcontributestotheperformanceandpopularityofthesebatteriesalsomakesasmallpercentageofthemvulnerabletointernalshortcircuits(ISCs)thatcanresultinfiresorexplosions.ComputationalmodelingisamethodULisusingtobetterunderstandISCs,alongwiththemechanismsofheatgenerationthatoccurduetotheelectrochemicalreactionswithinalithium-ionbatteryandtheprimarycausesofthermalrunaway,whichrepresentstheworst-case—andmosthazardous—scenarioofpossiblelithium-ionbatteryfailuremodes.

CONTEXT Theenergydensityoflithium-ionbatterieshastripledsincetheircommercialintroductionin1991.1Energydensity—orabattery’senergydeliverycapabilityrelativetoitsweightorvolume—isoneoftheadvantagesoflithium-ionbatteries,whichhavethehighestenergydensityandthehighestvoltageofanycommerciallyavailablerechargeablebattery.2 Thiswaslargelyaccomplishedbypackingmoreactivematerialintothecellandmakingtheelectrodesandseparatorsthinner.3Ultimately,thismeansthatlithium-ionbatteriesaresmallerandlighterthanotherbatteries;thisisoneoftheprimaryreasonsfortheirgrowingpopularity.

Thepowerthatalithium-ionbatterypacksintoitsvolumeislargelyachievedbytheuseofanelectrolytecomposedoflithiumsaltsinflammableorganicsolventssuchasethylenecarbonateandethylmethylcarbonate.Thisdiffersfromtheelectrolytesinothercommontypesofbatteries,whicharecomposedofacidorbaseaqueoussolutionsthatarenonflammable.4Theflammableelectrolyte,coupledwithincreasinglythinandlightseparators,increasestheriskoffailure.Currently,lithium-ionbatterieshaveafailurerateofapproximatelyonein10millioncells.5Althoughthismayseemrelativelylow,inthecontextofthe4.4billionlithium-ionbatterycellsmanufacturedin2012,6thefailurerateequatesto440potentialbatteryfailuresinoneyear,afigurethatcouldincreaseasthelithium-ionbatterymarketdoublesoverthenextfouryears.7

Lithium-ionbatterieshavebeeninvolvedinseveralhighlypublicizedincidentsoverthepastseveralyears.8InmanycasesthebatteryfailureswerelinkedtoISCsthatledtothermalrunaway,resultingintheexplosivereleaseofenergyalongwithfire.9Thus,itbecameimperativeforULtogainabetterunderstandingofISCstoidentifythefactorsthatcausethemostheatgenerationwithintheshortesttime,leadingtocatastrophicbatteryfailure.

Thermal modeling is a method UL is using to better understand ISCs, along with the mechanisms of heat generation that occur due to the electrochemical reactions within a lithium-ion battery and the primary causes of thermal runaway.

SUSTAINABLE ENERGY JOURNAL/COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES

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WHAT DID UL DO? Wedevelopedauniquecomputationalthermalmodelingcapabilitytoenableustomathematicallysimulate,exploreandunderstandthecausesandseverityofISCs.OurthermalmodelwasdevelopedusingSC/Tetracomputationalfluiddynamic(CFD)softwareandbuiltonacomplexgeometricrepresentationofthe18650typeorothercylindricallithium-ionbattery,makingitespeciallysuitableforthesecells.Themodelprovidesarobustsimulationoflithium-ionbatterycharacteristicsbecauseallofthethermalandphysicalpropertiesofeachcomponentarebasedoninformationfrominvestigatedliteratureorrealexperimentaldata.10ThisgivesustheabilitytoinvestigateallthefactorsinvolvedwithanISC.

UL’sthermalmodeltakesintoaccountdifferentsourcesofheatgeneration—electrical,electrochemicalandchemical—andencompassesthefactorsrelatedtomaterialandconstructiondesignthatarethedominantelementsindeterminingthecapabilityofacelltodissipateexcessheat.ThisenablesULscientistsandengineerstoinvestigatethecomponentsand/orISCconditionsthatarethecriticalfactorsindeterminingthetemperaturerisedistributionwithinthewholecellaswellasoutcomesinvolvedwithanISCundersteadyandpseudo-steadystateconditions.11Ourkeyfindingsinclude:

KEYCHARACTERISTICSOFANISCEVENT

Ourmodelshowedthatotherthaninacasecausedbymechanicalabuse, mostISCeventsbeginonasmallscaleinalocalizedareawithinasingle lithium-ionbatterycell,whichresultsinlocalizedheatingatthevery beginningoftheevent.Further,oncetheISCistriggered,allsafetydesigns outsideofcellshavenoeffect,andmostofthesafetydeviceswithinthe cellswillalsobebypassed(e.g.,thefusethatisdesignedtoopentovent excessvoltageorheat).12

ISCBEHAVIORS

ThreeprincipalfindingsrelativetoISCbehaviorswereobservedfromour IndentationInducedISCTest(formoreinformation,refertothearticle “IndentationInducedISCTest”).Theworst-casescenarioofanISCcanoccur whentheresistanceatthepointoftheISCissimilartotheresistancelevel ofthewholecell.ThisleadstothemaximumgenerationofJouleheatfrom theISCpointtothesurroundingareainthebattery.Inaddition,ourmodel showedthatthelocationofanISCisdirectlyrelatedtoitsseverity.AnISC

UL developed a unique computational thermal modeling capability to enable us to mathematically simulate, explore and understand the causes and severity of ISCs.

SUSTAINABLE ENERGY JOURNAL/COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES

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locatedinthecenterofthebottomsideofacylindricallithium-ionbattery exhibitsthehighestriseintemperature,reaching1,066°F.

OurthirdkeyfindingaboutISCbehaviorsiscounterintuitive.Assuming thesameelectrochemistryenergyandconstantresistanceattheISC pointandwithinthewholecell,wefoundthatthesmallertheinitial ISC,thegreateritsseverityandthemorelikelyitistoleadtothermal runaway(i.e.,therapidbuildupofheatinsidethebatterythatcanlead tofireorexplosion).13

FAILUREMECHANISMSINANISC

Usingourthermalmodel,wewereabletoassesstheimpactofmaterials, productdesign,batteryconstructionandelectrochemistryonISCs.We identifiedtheheat-generationprofilesthroughthreedifferentheating sources.Onecommonheatsourceinalithium-ionbatteryistheresultof localizedoverheatingbytheJouleeffect,whichmeansthecurrentflows throughtheISCpointinthebattery,generatinglocalizedoverheatingdue totheimpedanceattheISCbridgingpoint.Thesecondheatsourceiswhen anISCtriggersanelectrochemicalreactioninthebatterymaterialvia electrontransferthatleadsto“globalized”overheatingofthebattery. ThefinalheatsourcewedetectedinvolvesanISCthatcreatesalocalized overheatingconditionthatconsequentlycausesmaterialdecomposition aroundthepointoftheISC.

Theprofilesofallthreeheatsourcescanbesimulatedusingdifferent materialproperties,whichcanbeflexiblysetbythesimulationtool.We alsodevelopedageometrymodel,composedofadetailedspiral“jellyroll” structurethatincludesallthecomponentswithinthecellwithappropriate thermalpropertysettings.Thisenablesustocreateavirtualheat- conductingmediumthatcansimulatetheheatdissipationcapability ofarealbattery.Thissimulationisimportantbecausetheheat-generation rateandabattery’sabilitytodissipateheatarethetwocriticalfactors involvedwiththermalrunawayandpotentiallysignificantfireand explosionhazards.Weobservedthatthefastertheheatbuildswithin thebattery,thelesslikelythebattery’sfail-safedeviceswillbeableto effectivelyventtheexcessheatquicklyenough,andthemorelikelythe ISCwillleadtothermalrunaway.14

Using our thermal model, we were able to assess the impact of materials, product design, battery construction and electrochemistry on ISCs.

SUSTAINABLE ENERGY JOURNAL/COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES

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SUSTAINABLE ENERGY JOURNAL/COMPUTATIONAL MODELING OF LITHIUM-ION BATTERIES 8

IMPACT Computationalmodelingisanimportanttoolweusetogaininsightintohowaproductfunctions,howitmalfunctionsandtherisksinvolved.Forlithium-ionbatteries,ULappliedthistooltodevelopathermalmodeltohelpusproactivelygainabetterunderstandingofhowISCsworkandwhysomelithium-ionbatterycellsfailwhileothersdonot.WithnewinsightsaboutthecharacteristicsandbehaviorsofISCsaswellastheirfailuremechanisms,wearebetterabletohelpmanufacturerssafeguardthebatteriestheymakeacrossthedesign,materialsspecification,manufacturingandqualityassuranceprocesses.

SinceyouwereinterestedinreadingComputationalModelingofLithium-IonBatteries,wethoughtyoumightfindthefollowingrelatedarticleofinterest.

RELATED ARTICLE

INDENTATION INDUCED ISC TESTING

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NEW SCIENCE: SUSTAINABLE ENERGY 9

COMPUTATIONALMODELINGOFLITHIUM-IONBATTERIES

SAFEGUARDING LITHIUM-ION BATTERY SEPARATORS

THERMALANALYSISOFLITHIUM-IONBATTERIES

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WHY THE LITHIUM-ION BATTERY SEPARATOR RECOGNITION PROGRAM MATTERS

Theseparatorisacriticalcomponentwithinalithium-ionbatterythatsupportsitsoverallperformancewhilefacilitatingitssafeoperation.Asdemandforlithium-ionbatterieshasgrownacrossabroadarrayofconsumer,industrialandinfrastructureapplications,sohasthepushtoimprovetheirenergydensityandbatterylife.Tohelpimprovebatteryperformance,separatorshavebeenmadeprogressivelythinnerandlighter,whichhasdiminishedthecapabilityofsometoperformcriticalfunctionssuchaspreventingelectrodecontactorshuttingthebatterydownwhenoverheatingoccurs.Theresultantsafetyriskscouldhinderthecontinuedadvancementoflithium-ionbatteryperformanceaswellastheexpansionofthissustainableenergysource.

CONTEXT Researchhasshownthatathinnerseparatoriscriticaltoachievinghigherenergyandpowerdensityinalithium-ionbattery.1Overthepasttwodecades,theongoingreductionofseparatorthicknessandadditionofactivematerialshavecontributedtoathreefoldincreaseintheenergydensityoflithium-ionbatteries.2Thesebatteriesnowofferunmatchedperformance,whichhashelpedlithium-ionbecomethebatterychemistryofchoiceforarangeofapplications—firstforconsumerelectronics,butmoreandmoreforautomobilesandtheelectricpowergrid.3Evidenceofthegrowingroleoflithium-ionbatteriescanbeseeninthesizeofthemarket,whichreached$11.7billionin2012andisprojectedbyFrost&Sullivantodoubleby2016.4

Oneoftheprimarycontributorstothesafetyrisksoflithium-ionbatteriesistheiruseofflammableelectrolytesolutions.5Thisunderscoresthecriticalroletheseparatorplaysinpreventingelectroniccontactoftheelectrodeswhileenablingfreeionictransportandisolatingtheelectronicflowwithinthebattery.6Thereductionoftheseparatorthicknessto18µmorlessmeansthatsomelithium-ionbatteriesarenowmoresusceptibletomanufacturingdefectsandexternaldamage.7Specifically,thistranslatestoapotentiallyincreasedriskofinternalshortcircuits,whichcanleadtothermalrunaway—therapidbuildupofheatinsidethebattery—thatignitestheflammableelectrolyteorraisesthevaporpressureuntilthecellbursts.8

The reduction of the separator thickness to 18μm or less means that some lithium-ion batteries may now be more susceptible to manufacturing defects and external damage.

SUSTAINABLE ENERGY JOURNAL/ SAFEGUARDING LITHIUM-ION BATTERY SEPARATORS

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WHAT DID UL DO? ULappliedmaterialssciencetodevelopacomprehensivetestingapproachthatestablishesamorerobustevaluationoflithium-ionbatteryseparators.Thekeycharacteristicsoftheseparatorasitrelatestopotentialsafetyrisksandperformancewereidentified.WethendevelopedaSeparatorRecognitionProgramcomprisingasuiteoftests—someofwhichalreadyexistedwhileothersweredevelopedspecificallyfortheprogram—includingpermeability,thickness,materialcomposition,tensilestrength,puncturestrength,dimensionalstability,shutdowntemperatureandmeltingtemperature.9ComponentRecognitionisaserviceULoffersthatcoverstheevaluationofcomponentsormaterialsintendedforincorporationanduseinotherend-useproductsorsystems.10

UL’sSeparatorRecognitionProgramwasdesignedtoidentifythekeypropertiesofaseparatortohelpensurethatitisappropriateandsafeforthescopeofitsintendeduse.Wedevelopedtheprogramtohelpdrivesomeconsistencyintheevaluationofalltheimportantseparatorparameters.11Ourrecognitionprogramoutlineseightspecificparametersofalithium-ionbatteryseparatorthatourresearchandexperienceshowtobecriticaltobothbatterysafetyandperformance.12Forfiveoftheparameters,existingtestprotocolsareideal.Forseparatorthickness,dimensionalstabilityandshutdowntemperature,weneededtocreatenewtestprotocols.13 KEYPARAMETERSANDTESTSINTHEULSEPARATORRECOGNITIONPROGRAM14

TYPE OF TEST CRITICAL PARAMETER TEST PROTOCOL

IdentificationTests Permeability ASTMD726

Thickness modified

MaterialID UL746A

MechanicalStrengthTests TensileStrength ASTMD882

PunctureStrength ASTMD3763

ThermalTests DimensionalStability modified

ShutdownTemperature modified

MeltTemperature UL746A

Withthiscomprehensivesetofparametersandcorrespondingtestprotocols, ourSeparatorRecognitionProgramisintendedtohelpmanufacturersestablishtightercontrolsoncriticalbatterysafetycomponents.

UL applied materials science to develop a comprehensive testing approach that establishes a more robust evaluation of lithium-ion battery separators.

SUSTAINABLE ENERGY JOURNAL/ SAFEGUARDING LITHIUM-ION BATTERY SEPARATORS

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SUSTAINABLE ENERGY JOURNAL/ SAFEGUARDING LITHIUM-ION BATTERY SEPARATORS 12

IMPACT TheSeparatorRecognitionProgramextendsUL’scommitmenttoensurethesafetyoflithium-ionbatteries,focusingononespecificandvitalcomponent.Toimplementtheprogram,weareconductingandwitnessingtests,andwealsopublishparametersintheULonlinedirectory.Stillinitsearlystages,theprogramisnotyetfullyutilizedbytheindustry.However,today,lithium-ionbatteryandseparatormanufacturerscansubmitseparatorstoULforsafetytestingandcertification.Importantly,therearenopass/failcriteria;instead,ourprogramisfocusedonidentifyingthepropertiesoftestedseparators.Inthisway,wecanhelpmanufacturersadvancetheperformanceoflithium-ionbatterieswhileenhancingtheirsafety.

SinceyouwereinterestedinreadingSafeguardingLithium-IonBatterySeparators,wethoughtyoumightfindthefollowingrelatedarticlesofinterest.

RELATED ARTICLES

INDENTATION INDUCED ISC TESTING

AGING EFFECTS OF LITHIUM-ION BATTERIES

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NEW SCIENCE: SUSTAINABLE ENERGY 13

COMPUTATIONALMODELINGOFLITHIUM-IONBATTERIES

SAFEGUARDINGLITHIUM-IONBATTERYSEPARATORS

THERMAL ANALYSIS OF LITHIUM-ION BATTERIES

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WHY THERMAL ANALYSIS OF LITHIUM-ION BATTERIES MATTERS

Theuseoflithium-ionbatteriesisundergoingdramaticgrowth,primarilyduetotheirindustry-leadingenergydensityandcomparativelylongbatterylife.1Astheleadingrechargeablebatteryforconsumerelectronicsandelectricvehicles(EVs),andarapidlygrowingsourceforenergystorage,lithium-ionbatteriesareplayinganincreasinglycriticalroleinfacilitatingenergysustainability.2Onecriticalconcern,however,isthatasmallpercentageoflithium-ionbatteriesexperienceinternalshortcircuits(ISCs)thatresultinthermalrunaway,therapidbuildupofheatwithinthebatterythatleadstotheexplosivereleaseofenergyorfire.3Becausethemostsignificantsafetyissuesforlithium-ionbatteriesarerelatedtoheatgeneration,thermalanalysis—examiningtheimpactoftemperatureonlithium-ionbatteryperformanceandsafety—providesacriticallyimportantwaytopinpoint,understandandmitigatetherisks.4

CONTEXT

Therearethreepotentialcausesofheatgenerationthatcanmakelithium-ionbatteriesunsafe:

1. Animproperload(i.e.,flowofelectricenergy)betweenthepositiveandnegativepolesinabatterycangenerateheat.Thiscanbecaused,forexample,byanexternalshortcircuit(e.g.,aDCchargerisconnectedwiththepolarityreversed)orbyhigh-ratecharging/discharging.Inaddition,moreheatcanbegeneratedbyoverchargingorover-dischargingthebattery.Improperloadwillusuallyheattheentirecelluniformly,astheelectrochemicalreactionoccursalongboththeelectrodes.

2. HeatcanalsobegeneratedfrominsidethebatterycellbyanISCcausedbycontaminationthatoccurredontheproductionline(e.g.,introducingmetalparticlesintothecell)orbydendriteformation(i.e.,lithiummetaldepositsthatoccurduetoapolarizationeffectorafteroverchargeconditioning).AnISCcangenerateasubstantialamountofheatthatwillaccumulatelocallyinsidethecellwithinseconds.

3. Unsafeheatgenerationinalithium-ionbatterycanalsooccurwhentheambient(orsurrounding)environmentoverheatsthebattery.Thisexternaloverheatingofalithium-ionbatterywillsometimestriggerexothermic(i.e.,heat-releasing)reactionsinsidethebattery.Thegeneratedheatmaytheninduceadditionalheat-generatingreactions,eventuallyresultinginthermalrunaway.5Eveniftheexternaloverheatingisminorandcausesonlyavery

As the leading rechargeable battery for consumer electronics and EVs, and a rapidly growing source for energy storage, lithium-ion batteries are playing an increasingly critical role in facilitating energy sustainability.

SUSTAINABLE ENERGY JOURNAL/THERMAL ANALYSIS OF LITHIUM-ION BATTERIES

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preliminaryself-heatinginthebattery(i.e.,theinitialself-heatingcanbe triggeredbetween60°-70°C),theremaybesafetyissuesinfurtheruseofthe batterybecausetheprotectivelayer(i.e.,solidelectrolyteinterfaceorSEI) betweenelectrodeandelectrolytemayhavebeendestroyed.

Inallthreecases,whentheheatgenerationinsidethebatteryexceedsthebattery’sdissipationcapacity,theresultwillbethermalrunaway,inwhichaninitialheateventinthebatterytriggersadditionalexothermicreactionsveryrapidlyinachainreaction.Whenthishappens,thebatterywilltypicallycatchfire,ruptureorexplode.6WHAT DID UL DO?

Weestablishedacomprehensivethermalanalysiscapabilitythatincludesallexistingtechniques,whichenablesustomeasureexothermic(heat-emitting)andendothermic(heat-absorbing)reactionswithinalithium-ionbatteryonaconstituentmateriallevelortomeasurethereactionsinacellasawhole.7Thisallowsustoexamineandbetterunderstandthepotentialsafetyrisksofdifferentmaterialsandcomponentsinabattery,aswellastheircomplexelectrochemicalandfunctionalinteractionsandhowtheseareaffectedbyheatingconditions.8Weusefivecoremethodstoconductthermalpropertyanalysisoflithium-ionbatteries:9

TEST METHODOLOGY LEVEL OF TEST SAMPLE PURPOSE

Differential Scanning Calorimetry (DSC)

Material/Component To study interaction effects between different battery materials and components and to examine the thermal stability of electrode materials under different state-of-charge conditions.

Accelerating Rate Calorimetry (ARC)

Component/Cell/Module To provide an adiabatic (i.e., isolated) environment that eliminates measurement inaccuracies, enabling us to demonstrate chemical reactions and thermodynamic models within the battery, and helping us identify the point at which a battery becomes self-heating.

Thermogravimetric Analysis-Mass (TGA-Mass)

Material/Component To characterize materials based on how their mass changes as a function of temperature or time. The outcome of a TGA-Mass analysis is often compared with the results of DSC to determine the thermal decomposition of chemical materials in a lithium-ion battery.

Thermal Ramp Cell To study the behavior of lithium-ion batteries — primarily heat generation and gas buildup — under thermal abuse conditions.

Thermal Imaging Cell/Module/Battery To identify the uniformity of heat distribution in a lithium-ion battery while it is charging and discharging and to help predict the failure modes of batteries typically used under rapid charge/discharge conditions.

SUSTAINABLE ENERGY JOURNAL/THERMAL ANALYSIS OF LITHIUM-ION BATTERIES

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Ourresearchhasproducedseveralkeyfindingsaboutthetypesoffundamentalthermalreactionscausedbytheoverheatingoflithium-ionbatteries(withastandardLiCoO

2chemistry).

• SeparatorMelting—Lithium-ionbatteryseparatorshaveaporousstructureandaremadeofpolyethylene(PE)orpolypropylene(PP).Undernormalconditions,PEwillmeltatapproximately130°CandPPataround160°Cinendothermicreactions,helpingblockiontransferandshutdowntheinnercircuitofanoverheatingbattery.

• DecompositionofElectrolyte—Lithium-ionbatteryelectrolytesareorganicsolventsthatdecomposeunderoverheatingconditions.Thedecompositionofthesolventwillsometimesproduceactiveproductsaswellasgaseoussubstancesthatpressurizethecell.Generally,electrolytedecompositiondoesnotresultinsignificantheatgenerationbyitself,butthesideproductswillsometimesreactwiththebatteryelectrodesathighertemperatures.

• ReductionReactionoftheAnode(i.e.,negativeelectrode)withtheElectrolyte—theSEI,athinfilmthathasthesamechemistryasaliquidelectrolytebutisinadifferentform,iseasilyformedontheanode-electrolyteinterface.Evenmildheatingofalithium-ionbatterywillstimulateSEIformation,buttheSEIfilmwillmeltifthebatteryisfurtherheatedto80°-120°C,anditwillreleaseasmallamountofheat.UnderanARCtest,theinitialSEIdecompositionwithinacellcanbetriggeredasearlyas60°-70°Cunderthesimulatedadiabaticcondition.

• TheOxidationReactionoftheCathode(i.e.,positiveelectrode)withtheElectrolyte—Whenthetemperatureofalithium-ionbatterygoesbeyond180°C,thecathodeandtheelectrolytewillinteractinawaythatreleasessignificantheat.Ifthisheatcannotbedissipatedeffectively,itwillfinallyleadtomaterialdecompositionofthecathode,releasingmoreheatandcausingthermalrunaway.

• DecompositionofElectrodeMaterials—Athighertemperatures,typically150°-300°C,theelectrodematerialwilldecompose.Oncethisreactionisinitiated,asubstantialamountofheatisgenerallyreleased,causingthebatterytoexplodeorcatchfire.Inaddition,thedecompositionofthecathodewillproduceoxygengas,whichfurtherincreasesthepotentialforthermalrunaway.10

UL’s research has produced several key findings about the types of fundamental thermal reactions caused by the overheating of lithium-ion batteries (with a standard LiCoO2 chemistry).

SUSTAINABLE ENERGY JOURNAL/THERMAL ANALYSIS OF LITHIUM-ION BATTERIES

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17SUSTAINABLE ENERGY JOURNAL/THERMAL ANALYSIS OF LITHIUM-ION BATTERIES 17

IMPACT

Safeguardinglithium-ionbatteriesismademoredifficultbythecomplexinteractionsofmorethan10typesofmaterialsandcomponents.Withacomprehensivesetofthermalanalysistoolsandmethods,ULisabletoholisticallyexamineandbetterunderstandhowthesebatteriesreacttoheatconditions,whichisimportantbecausealllithium-ionbatterysafetyissuesinvolveheatgeneration.Ourthermalanalysisisespeciallycriticaltoday,astheindustrypushestoadvancelithium-ionbatteryperformancethroughtheuseofnewmaterialsandbatterydesigns.Withthermalanalysis,wearecontinuingtohelppinpointsafetyrisks,identifysafeperformanceenvelopesanddevelopeffectiverisk-mitigationapproaches.11

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18SUSTAINABLE ENERGY JOURNAL/SOURCES

SOURCES

COMPUTATIONALMODELINGOFLITHIUM-IONBATTERIES

SAFEGUARDINGLITHIUM-IONBATTERYSEPARATORS

THERMALANALYSISOFLITHIUM-IONBATTERIES

1 Savitz, E., “Apple: What Lithium-Ion Battery Limits Could Mean for iPhone 5,” Forbes, 18 July 2012. Web: 20 July 2013. http://www.forbes.com/sites/ericsavitz/2012/07/18/apple-what-lithium-ion-battery-limits-could-mean-for-iphone-5/.

2 “About Rechargeable Batteries,” AA Portable Power Corp., 2013. Web: 18 Nov. 2013. http://www.batteryspace.com/batteryknowledge.aspx.

3 Orendorff, C.J., “The Role of Separators in Lithium-Ion Cell Safety,” The Electrochemical Society Interface, Summer 2012. Web: 29 July 2013. http://www.electrochem.org/dl/interface/sum/sum12/sum12_p061_065.pdf.

4 Jacoby, M., “Assessing the Safety of Lithium-Ion Batteries,” Chemical & Engineering News, Volume 91, Issue 6, 11 Feb. 2013. Web: 10 July 2013. http://cen.acs.org/articles/91/i6/Assessing-Safety-Lithium-Ion-Batteries.html.

5 Ibid.

6 “Standards Are Tightened for Lithium-Ion Batteries,” New York Times, 12 June 2013. Web: 19 Nov. 2013. http://www.nytimes.com/2013/06/13/business/lithium-ion-battery-standards-are-tightened.html?_r=0.

7 “Global Lithium-Ion Battery Market to Double Despite Recent Issues,” Frost & Sullivan, 21 Feb. 2013. Web: 18 Nov. 2013. http://www.thefreelibrary.com/Frost+%26+Sullivan,+Global+Lithium-ion+Market+to+Double+Despite+Recent...-a0319660807.

8 Wu, A., et al., “Simulation of Internal Short Circuits in Lithium-Ion Cells,” UL Corporate Research and NASA Johnson Space Center, 2012. White paper, 29 May 2013.

9 Ibid.

10 Wu, A., “Brief Introduction of Thermal Modeling of ISC in LIB,” UL, 2011. Presentation, 4 Sept. 2013.

11 Ibid.

12 Ibid.

13 Wu, A., Personal Interview, 30 Sept. 2013.

14 Wu, A., “Brief Introduction of Thermal Modeling of ISC in LIB,” UL, 2011. Presentation, 4 Sept. 2013.

1 “Li-ion Battery Separators,” UL, 2007. Presentation, 3 Sept. 2013.

2 Savitz, E., “Apple: What Lithium-Ion Battery Limits Could Mean for iPhone 5,” Forbes, 18 July 2012. Web: 20 July 2013. http://www.forbes.com/sites/ericsavitz/2012/07/18/apple-what-lithium-ion-battery-limits-could-mean-for-iphone-5/.

3 Jaffe, S., “The Lithium Ion Inflection Point,” Battery Power, 9 Oct. 2013. Web: 21 Nov. 2013. http://www.batterypoweronline.com/main/articles/the-lithium-ion-inflection-point/.

4 “Global Lithium-ion Battery Market to Double Despite Recent Issues,” Frost & Sullivan, 21 Feb. 2013. Web: 18 Nov. 2013. http://www.thefreelibrary.com/Frost+%26+Sullivan,+Global+Lithium-ion+Market+to+Double+Despite+Recent...-a0319660807.

5 Jacoby, M., “Assessing the Safety of Lithium-Ion Batteries,” Chemical & Engineering News, Volume 91, Issue 6, 11 Feb. 2013. Web: 10 July 2013. http://cen.acs.org/articles/91/i6/Assessing-Safety-Lithium-Ion-Batteries.html.

6 “Li-ion Battery Separators,” UL, 2007. Presentation, 3 Sept. 2013.

7 Orendorff, C.J., “The Role of Separators in Lithium-Ion Cell Safety,” The Electrochemical Society Interface, Summer 2012. Web: 29 July 2013. http://www.electrochem.org/dl/interface/sum/sum12/sum12_p061_065.pdf.

8 Jacoby, M., “Assessing the Safety of Lithium-Ion Batteries,” Chemical & Engineering News, Volume 91, Issue 6, 11 Feb. 2013. Web: 10 July 2013. http://cen.acs.org/articles/91/i6/Assessing-Safety-Lithium-Ion-Batteries.html.

9 Wang, E., Personal Interview, 25 Sept. 2013.

10 “UL Component Recognition,” UL, 2013. Web: 2 Dec. 2013. http://www.ul.com/global/eng/pages/corporate/aboutul/ulmarks/promotional/mark3/#comp_recog.

11 “Li-ion Battery Separators,” UL, 2007. Presentation, 3 Sept. 2013.

12 Ibid.

13 Ibid.

14 Ibid.

1 Wu, A., et al., “Polarization and Thermal Stability Effects on Aged Lithium-ion Batteries,” UL, 2012. Presentation: 30 May 2013.

2 Jaffe, S., “The Lithium Ion Inflection Point,” Battery Power, 9 Oct. 2013. Web: 21 Nov. 2013. http://www.batterypoweronline.com/main/articles/the-lithium-ion-inflection-point/.

3 Jacoby, M., “Assessing the Safety of Lithium-Ion Batteries,” Chemical & Engineering News, Volume 91, Issue 6, 11 Feb. 2013. Web: 10 July 2013. http://cen.acs.org/articles/91/i6/Assessing-Safety-Lithium-Ion-Batteries.html.

4 Wu, A., “Thermal Runaway Mechanism and Thermal Analysis Technologies for Lithium-ion Batteries, UL, Aug. 2008. Report (draft): 8 Oct. 2013.

5 Ibid.

6 Ibid.

7 Wu, A., Personal Interview, 4 Dec. 2013.

8 Wu, A., “Thermal Runaway Mechanism and Thermal Analysis Technologies for Lithium-ion Batteries, UL, Aug. 2008. Report (draft): 8 Oct. 2013.

9 Ibid.

10 Ibid.

11 Wu, A., Personal Interview, 4 Dec. 2013.

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