InsitudilatometeranalysisofLi[NixCoyMn1–x–y]O2electrodesasafunctionofworkingvoltageforlithiumionbattery

Abstract

Lithiumionbatteries(LIBs)fieldhavebeencontinuouslyexpandedforthediversificationofportableelectronicdevices,electricvehicle(EV)duetotheinternationalclimateagreementandESSmarket.Li[NixCoyMn1–x–y]O2(NCM)cathodematerialsarehavebeenthemostwidelyemployedlayeredstructurecathodematerialduetotheirhighreversiblecapacityandhighenergydensityinlongtermcyclingforgoodoperatingmileageandhighpowervehicle.However,NCMcathodematerialslackingmechanicaldurabilitystillhaveproblemsofincreasinginternalresistanceandreducingcapacityretention.NCMcathodeactivematerialisknowntocollapseasamaterialischargedanddischarged,andthisisbecausethecrystalc-axisexpandsandcontractsrepeatedlywhilethetransitionmetalismigratedwithLivacancygeneratedduringdelithiation,andthethicknessvariationadverselyaffectsthelonglifecharacteristicsofthematerial. Thisexpansionbehaviorhasbeenmeasuredforeachvariableattheactivemateriallevel,butthereisnoevaluationoftheexpansionbehavioracrosstheserialcyclesattheelectrodelevel,andthereasonwhytheexpansionbehaviorofthecathodegraduallydecreasesasthecycleprogressesissaidtobebecausea‘steadystat’hasnotbeenanalyzed.Herein,thispaperexaminedhowtheexpansionbehaviorismanifestedbyvoltagewindowsasacycleprogressesatthelevelsofNCM622andNCM880606throughhalf-celltypein-situelectrochemicaldilatometer.Inaddition,XRDRietveldrefinement,depthXPS,andTEMwereconductedtoidentifyfactorsaffectingtheexpansionbehavior,andcycleperformanceandratecapabilitywereconductedtodeterminehowthedifferenceinexpansionbehaviorismanifestedinelectrochemicalcharacteristics. |리튬이온전지(LIB)분야는국제기후협약및ESS시장확장에따른휴대용전자기기,전기자동차(EV)의다양화를위해지속적으로확대되고있는가운데많은배터리회사들이층상구조양극활물질소재를채용하고있다.Li[NixCoyMn1–x–y]O2(NCM)양극재는우수한작동주행거리와고출력자동차를위한장기수명평가에서높은가역용량과높은에너지밀도로인해가장널리사용되고있다.그러나,기계적내구성이결여된NCM양극활물질은내부저항이증가하고용량유지율이감소하는문제점이여전히존재한다.NCM양극활물질은충방전이진행됨에따라붕괴하는것으로알려져있는데,이는탈리튬화과정에서발생한Li공백과함께전이금속이이동하면서결정상의c축이팽창과수축을반복하게되고두께변화가활물질의장수명특성에악영향을미치기때문이다.이러한팽창거동은활물질수준에서변수별로연구되어왔으나,전극수준에서연속된사이클에걸친팽창거동에대한평가가부족하고,사이클이진행됨에따라양극의팽창거동이점차감소하는이유로알려진‘안정된상태’에대한충분한분석과연구는진행되지않았다.본논문에서는Half-celltypein-situelectrochemicaldilatometer를통해연속된cycle의팽창량과전기화학적분석을통해NCM622및NCM880606양극에서구동전압구간에따라팽창거동이어떻게발현되는지를살펴보았다.본연구에서이루어진구조적특성과전기화학특성을분석한자세한내용은본문에서다루고자한다.