Cycle and calendar life study of a graphite_LiNi1_3Mn1_3Co1_3O2 Li-ion high energy system.

580S.K?bitzetal./JournalofPowerSources239(2013)572e583

1.051Cact/CBOT0.950.90.850.80.75

0

100

200300t/days

400

50% SOC CV 40°C20% SOC CV 40°C50% SOC CV 60°C20% SOC CV 60°CCalculated 50-20

Fig.13.Analysisoflossesat20%SOC.Thedifferenceofactualcapacitybetween50%SOCand20%SOCatT?60??Cisaddedtothe50%SOCatT?40??Ccapacity.The“Calculated50e20”curveistheresult.

test.Takingintoaccountthesmalldependencyofresistancein-creaseonSOC(seeSection3.5),alsoscatteringofvaluesmightbeaproblemforinterpretationofthecurve.Similarto90%SOC,BOTforthealternationtestwassixmonthslaterthantheothertest,whichmightexplainthestrongcapacitydeclineatBOTcomparedtothe50%SOCCVtestintheplotduetoearlierstorageconditions.Howeverjustasthe90%SOCtests,acomparisontoinitialvaluesdidnotshowsigni?cantcapacitydeclineorresistanceincreaseatBOT.Thoughthecomplexbehaviorateachtimestepofthistestcannotbeexplainedbyasimplemodel,amethodisdiscussedinthefollowingtoapproximatetheoverallagingofthealternationtest.Totakeintoaccountthevoltagedependency,thevaluesderivedinSection3.5areusedtodeterminetheaverage(avg)weightedSOCduringthealternationtest.ThisweightedaverageSOCissupposedtocorrespondtotheSOCofacalendarlifeCVtest,showingthesamegradeofaging.TheresultingaverageweightedSOCiscalculatedviaequation(2).FromFig.15(a)itcanbeobserved,thatwhenevergoingfromhighertolowervoltageacapacityrecoverytakesplace,similartothe100%OCtestatT?40??C.Goingfromlowertohighervoltage,capacitydeclineacceleratesagainandthealreadyknowncapacityrecoveryeffectsseemtovanish.TheresistanceinFig.15(b)showsnocleardependencyonSOC.20%SOCseemstobesomehowbene?cialtoresistanceincrease,allotherSOCleadtoresistanceincrease.Nevertheless,theoverallresistanceofthetestcomparedtootherstoragetestsappearsquitelow,especiallyatthebeginningofthePSOCavg;weighted?iaeSOCT$SOCi$tiPaeSOCT$tii(2)1009080SOC/p60504030200100200t/days300400SOC avgSOC avg, weighted capacitySOC avg, weighted resistanceSOC test profileFig.14.Alternationtestpro?leandcalculatedresultingarithmeticaverageSOCandaverageweightedSOCforresistanceincreaseandcapacityloss.

a(SOC)istakenfromTable4atT?40??CandtiistheperiodoftimeatSOCi.ResultingaverageweightedSOCisshowninFig.14forweightingwithcapacityandresistanceparameters.Alsothearithmeticmeanvalueisshown,correspondingtoana(SOC)?1.SincethisrathersimpleapproximationisnotsupposedtotakeintoaccounttheeffectofcapacityincreaseatlowSOC,a(20%SOC/50%SOC)issettoone.Calculationpredictsacapacityfadeat400dayscorrespondingtoatestat80%SOCCV.Fig.15(a)showsthecom-parisonof80%OCcapacity,whichissimilarto80%CVmentionedbefore,andalternationbehavingthesameway.Thoughignoringformercellhistory,after400daysbothtestsshownearlythesamecapacitydecline.SincethecapacitygainatlowSOCdoesnotseemtobepermanentathigherSOC,thecapacitypredictionusingaweightedSOCisquitecorrectatdifferenttimepointswithhigherpreviousSOC.Theobservedcapacitygainphenomenonmightbeanerrorsourceforonlinecalculationofactualcapacitiesinbatteryman-agementsystems,sincethegainachievedat214daysisinpartsstillpersistentevenafter22daysofstorageat100%SOC.Forlong-termlifetimeprediction,withapro?leincludinghigherSOC,thesimpleapproximationderivedinthischaptercouldbesuf?cient.Inanycase,theresistancecurvesbehaviorcouldnotbepredictedwithFig.15.NormalizedactualcapacityCact(a)andresistanceRact(b)vs.timeforcalendarlifescenariosatT?40??C.AlternationtestwithCVatchangingSOC.AppliedSOCpro?levaluesbetweenRPTsareshowninthediagrams.

S.K?bitzetal./JournalofPowerSources239(2013)572e583581

Table4Agingfactorsforcapacityfadeandresistancerise.Forcalculationthelastsetofvalueswasused,whereallvoltageswerestillintest.E.g.atestat100%SOCCVandT?40??Chasa3.8timeshighercapacitylossthanthe50%SOCCVtestatthesametemperatureandtime.TemperatureClossRincreaseClossRincrease40C40??C60??C60??C??RelatedtoSOC50 %SOC?100%CV3.82.09.38SOC?90%CV21.631.2SOC?80%CV1.82131.52SOC?50%CV11.22.01.4SOC?20%CVe111[a]10,950,9[b]50% SOCCVdata 25°Cfit 25°C, sqrtfit 25°C, sqrt+lineardata 40°Cfit 40°C, sqrtfit 40°C, sqrt+lineardata 50°Cfit 50°C, sqrtfit 50°C, sqrt+lineardata 60°Cfit 60°C, sqrtfit 60°C, sqrt+linear2,22Ract/RBOT50% SOCCVdata 25°Cfit 25°C, sqrtfit 25°C, sqrt+lineardata 40°Cfit 40°C, sqrtfit 40°C, sqrt+lineardata 50°Cfit 50°C, sqrtfit 50°C sqrt+lineardata 60°Cfit 60°C, sqrtfit 60°C, sqrt+linearCact/CBOT0,850,80,750,70,650100200300400500600700t/days1,81,61,41,210100200300400500600700t/daysFig.16.ActualcapacityCact(a)andresistanceRact(b)overtimeforcellsstoredat50%SOCatdifferenttemperatures.Thedotsshowthemeasurementdatafortheeachcellintest,thesolidlinesthe?ttingresultusingasinglesquareroottimebehavior(eq.(3))andthedashedlinesdepictthe?ttingresultusingacombinationofsquarerootandlinearbehaviorforthetimedependency(eq.(4)).

thismethod.Takingintoaccountthecurvesat100%SOCfromtheprevioussections,theriseofresistancewasexpectedtobehigh.IncontrasttheresistanceinFig.15(b)duringthe100%SOCperiodsdoesonlyleadtoanatthemostmoderateresistancerise.Onereasonmightbethatsuchasimplemodelignoringtheprevioushistorycannotbeusedinhere.Alsoitcannotbeexcludedthatpartsofthecurvesbehavioranddifferencetotheagingcurvesnexttothealternationtestisduetoscatteringofthevalues.3.7.RegressionfunctionandcalendarlifetimeextrapolationBecausestronghintspointtotheaforementionedprocessesontheanodesidebeingthestrongestagingeffect,thisistakenasworkinghypothesisinthefollowing.Inliteraturedifferenttheoriescanbefoundaboutthe?rstprincipleprocessesofSEIgrowthduringaging.Asmentionedintheprevioussection,Brousselyetal.describeinRef.[4]theSEIformingattheSEIsurface/electrolyteinterfaceandthereforeidentifytheelectronicconductivityoftheSEIastheratelimitingfactorforSEIformation.Ploehnetal.[17]ontheotherhandidentifythesolventdiffusionprocessastheratelimitingfactor.However,mathematicaldescriptionsofboththe-oriesleadtotheresultthatSEIformationhastofollowasquarerootoftimedependency.Thecapacityfadeandresistanceincreasemeasuredinthisworksupportthistheory.SimilartotheworkofEckeretal.[3]inthefollowingafunctionwithsquarerootoftimedependency,aswellasacombinationofasquarerootandlinearfunctionhavebeen?ttedtothemeasuredagingresults:?ttingresultsforcapacityfadeandresistanceincreaseforcellsstoredat50%SOCatdifferenttemperatures.InTable5theresulting?ttingparametersandthecorrespondingcorrelationcoef?cientsR2forcapacityfadearegivenexemplarily.Itcanbeseen,thatasquarerootoftimefunctiondescribestheagingofthecellsquitewell.Itcanalsobeseen,thatanadditionallineartermdoesnotimprovethe?ttingresultssigni?cantly.Valuesforthelinear?ttingparameterb2of10à4to10à14show,thatthelinearcontributiontoagingisquitesmallandcanbeneglectedinordertoreducethenumberoffreeparameters.Similarresultscanbefoundfortheresistance.Thereforethesquarerootfunctionwillbeusedinthefollowing.Basedonthe?ttingresultslifetimeextrapolationscanbecalculatedforthevarioustestconditions.Fig.17showscapacitylifetimein(a),de?nedasthetime,wherecapacityreaches70%ofinitialcapacityandresistivelifetimein(b)de?nedasthetime,wheretheresistancereaches200%ofinitialresistanceovertem-peratureatdifferentSOC.AtamoderatetemperatureofT?40??Cthecapacitylifetimerangesfrom2to17yearsdependingonSOC.Acellstoredatconstantvoltageof4.18VatT?40??Cforexampleisonlyexpectedtolive2years.Resistivelifetimeiscomparabletocapacitylifetimeforthisspecialde?nition.AtT?40??Cresistivelifetimevariesbetween2and18yearsfordifferentSOC.AtverymoderateconditionslikeT?25??Cand50%SOCveryhighlifetimescanbereachedaccordingtothesquarerootextrapolation.Capacityp??AetT?1?a1$tAinitp??AetT?1?a2$t?b2$tAinit(3)Table5Fittingresultforcapacityfadeforcellsstoredat50%SOCanddifferenttemperatures.Forthe?ttingeqs.(3)and(4)wereused.The?ttingparametersaswellasthecorrespondingcorrelationcoef?cientsR2areshown.T[??C]25405060a10.00170.00380.00610.0109R20.97940.98850.99790.9802a20.00160.00380.00570.0080b23.86.72.241.78????10à610à710à510à4R20.98130.98850.99940.9993(4)Adenotescellcapacityorresistance,respectivelyanda1,a2andb2areconstantsdeterminedbythe?ttingprocess.Fig.16showsthe582S.K?bitzetal./JournalofPowerSources239(2013)572e583

[a]

908070605040302010020lifetime/yearsCapacitive lifetime (C= 70%)

[b]

3530lifetime/years252015105020

100% SOC90% SOC80% SOC50% SOC20% SOCResistive lifetime (R= 200%)

100% SOC90% SOC80% SOC50% SOC20% SOC40

60T/°C

8010040

60T/°C

80100

Fig.17.Capacitylifetime(endoflife(EOL)criterion:Cact?70?OT)(a)andresistivelifetime(EOLcriterion:Ract?200%RBOT)vs.temperatureforcalendarlifetestsatdifferentcellvoltagelevels.

lifetimebecomes85years,resistivelifetime32years.Onecanarguethatthesenumbersarenotrealistic,butithastobekeptinmindthatthesquarerootoftimeapproachforthelifetimeextrapolationonlyaccountsfortheinternalagingmechanismsofSEIformation.Allotheragingmechanismslikeleakageorlossofelectrolytee.g.causedbypouchbagrupture,orelectrodeirreversiblelossduetoactivematerialdegradationarenotincludedinthisapproach.Furthermore,usingasquarerootapproach,scatteringinthetestdatain?uencesthepredictedlifetimequitealot.Neglectingthecellwithlowestcapacitydegradationinthetestwithcellsstoredat25??Cand50%SOCatCVleadstoacapacitylifetimeof56years,incomparisonto85years,ifallthreecellsareconsideredinthe?tting.Inadditiontothelimitationsonlifetimepredictionsmentionedbeforeithastobeassured,thattheagingin?uenceissigni?cantlyhigherthanthescatteringofthecellsinonetestset,whichismostlythecaseforhighertemperaturesleadingtohighercapacitylossandresistanceincrease.Usingtheseacceleratedagingtestshasthedisadvantageofpossiblytriggeringagingprocesseswhichareun-likelytohappeninrealapplications.TheseadditionalagingeffectsathighertemperatureswouldalsoinvalidatetheArrheniusextrapolationofthetestdatatolowertemperatures.Ifthementionedlimitationsofthisapproachareconsidered,itcanbeusedforlifetimeestimationofhighqualitycells,wheremoderateSEIevolutionisthemainagingmechanism[13].4.ConclusionAnextensiveagingstudyofagraphitejLiNi1/3Mn1/3Co1/3O2(NMC)Li-ionpouchcellhasbeenpresentedinthisworkbasedonreferenceparameterteststodeterminetheagingbehavior.Calen-darlifetestsatdifferentvoltagesandtemperatureshavebeenconductedaswellascyclingtestsatdifferentaverageSOCandcycledepths.Thetemperaturedependencycouldbeidenti?edasArrheniustypeandactivationenergiescouldbedetermined,whichareingoodaccordancetotheliterature.Thevoltagedependencybetween50%and90%SOCisrathersmall.However,storingthecellsat20%SOCcanbeevenbene?cialforcapacity,whichcanalsobeobservedinatestwithalternatingvoltagelevels.Thiseffectseemstobetemperatureindependent.AcomparisonbetweencalendarlifeagingatOCandCVshowedsigni?cantdifferencesonlyat100%SOCincapacitydecline.Thisiscausedbytherelativesmalldependencyoftheagingoncellvoltagebelow100%SOC.ThereforedeviationsfromthesetlowerSOCintheOCtestsduetoself-dischargeareofminorimportance.SEIformationontheanodeisexpectedtobethemainagingmechanismforcalendarlifetests,causingasquarerootoftimeshapedagingbehavior.Theagingbehaviorofcyclingtestshasbeenidenti?edasdependentontemperatureandcycledepth.Cyclingtestsat10%DODshowacapacitydeclineequaltocalendarlifeagingtests.HigherDOD(50%andfullcycling)ledtoincreasedadditionalcapacityfade.Acompleximpactofcyclingonresistanceincreasehasbeenobserved.AtT?25??CandhighDODtherewasevenasmallerresistanceincreasethaninthecorrespondingcalendarlifetest.Takingresultsfrompost-mortemanalysisfrompaperpartBintoaccount,theresistanceforcyclicagingappearstobedependedontheinteractionofvolumeincreaseanddepositionreactionsontheanodeside.Furtherstudiesincludingpost-mortemanalysisinpaperpartBandEISwillbepresentedinseparatepublicationsforclari?cationoftheoccurringagingef-fects.Moredataandanin-depthanalysisofthecyclingtestswillfollowaswell.AcknowledgmentsThisworkwassponsoredbytheGermanFederalMinistryofEducationandResearch(BMBF)undercontractno.03X4613Gaspartoftheprogram“LIB2015”.TheauthorsaregratefultoConti-nentalandDaimlerfortheirassistancetothisstudyaspartnersofthisproject.SpecialthanksgotoZSWforprovidingresultsfrompost-mortemanalysis.References[1]M.Wachtler,M.K?nig,M.Kasper,M.Fleischhammer,B.Emmermacher,P.Axmann,M.Wohlfahrt-Mehrens,J.PowerSources,submittedforpublication.[2]M.Safari,M.Morcrette,A.Teyssot,C.Delacourt,J.Electrochem.Soc.156(2009)A145eA153.[3]M.Ecker,J.B.Gerschler,J.Vogel,S.K?bitz,F.Hust,P.Dechent,D.U.Sauer,J.PowerSources215(2012)248e257.[4]M.Broussely,S.Herreyre,P.Biensan,P.Kasztejna,K.Nechev,R.Staniewicz,J.PowerSources97e98(2001)13e21.[5]G.Ning,B.Haran,B.N.Popov,J.PowerSources117(2003)160e169.[6]J.Shim,K.A.Striebel,J.PowerSources122(2003)188e194.[7]R.Wright,C.Motloch,J.Belt,J.Christophersen,C.Ho,R.Richardson,I.Bloom,S.Jones,V.Battaglia,G.Henriksen,T.Unkelhaeuser,D.Ingersoll,H.Case,S.Rogers,R.Sutula,J.PowerSources110(2002)445e470.[8]I.Bloom,B.Cole,J.Sohn,S.Jones,E.Polzin,V.Battaglia,G.Henriksen,C.Motloch,R.Richardson,T.Unkelhaeuser,D.Ingersoll,H.Case,J.PowerSources101(2001)238e247.[9]J.Wang,P.Liu,J.Hicks-Garner,E.Sherman,S.Soukiazian,M.Verbrugge,H.Tataria,J.Musser,P.Finamore,J.PowerSources196(2011)3942e3948.[10]M.Kassem,J.Bernard,R.Revel,S.Pélissier,F.Duclaud,C.Delacourt,J.PowerSources208(2012)296e305.[11]I.Bloom,L.K.Walker,J.K.Basco,D.P.Abraham,J.P.Christophersen,C.D.Ho,J.PowerSources195(2010)877e882.S.K?bitzetal./JournalofPowerSources239(2013)572e583

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