Charge, Potential, and Phase Stability of Layered

0013-4651/2002/5͑7͒/A145/4/$7.00©TheElectrochemicalSociety,Inc.

A145

Charge,Potential,andPhaseStabilityofLayeredLi„Ni0.5Mn0.5…O2

J.ReedandG.Ceder*,z

DepartmentofMaterialsScienceandEngineering,MassachusettsInstituteofTechnology,Cambridge,Massachusetts02139,USA

Li(Ni,Mn)O2materialshaverecentlyshownpromiseashighcapacitystableelectrodesforadvancedrechargeablelithiumbatter-ies.Usingfirstprinciplesquantummechanicalenergycomputationswedemonstratethatthestabilityofthesematerialsisduetotheparticularvalencedistributiononthetransitionmetalsinthismaterial.SpindensitycalculationsindicatethattheMnionhasoxidationstateϩ4independentlyoftheLicontentinthematerial,whileNiisoxidizedfromNi2ϩtoNi4ϩuponremovingLi.ThehighinsertionvoltagefortheLiNi0.5Mn0.5O2canbepartlyattributedtothechangeinMn-NiinteractionuponLicycling.©2002TheElectrochemicalSociety.͓DOI:10.1149/1.1480135͔Allrightsreserved.

ManuscriptsubmittedNovember17,2001;revisedmanuscriptreceivedMarch7,2002.AvailableelectronicallyApril25,2002.

Layeredlithium-manganeseoxidesareofinterestforuseinre-chargeablelithiumbatteriesbecauseoftheirpotentialforveryhighcapacity,relativesafety,andaffordability.TheirinherentsafetyisderivedfromthefactthatbothMn3ϩ͑presentattheendofdis-charge͒andMn4ϩ͑attheendofcharge͒arequitestablevalencestates.Whilemanganeseoxideswiththespinelstructureandstoichi-ometryLiMn2O4havebeenusedinbatteries,theircapacityislim-ited,andseverestabilityissuesexistwiththematerial.Layeredlithiummanganeseoxides,ontheotherhand,haveatheoreticalcapacityof288mAh/g.Unfortunately,thelayered␣-NaFeO2struc-tureisnotthegroundstateforLiMnO21andonehastoresorttoeithermetastableprocessingroutes2,3startingfromaNaMnO2,ortocompositionalmodificationstoincreasethestabilityofthelayeredphaseovertheotherpossiblepolytopes.4-7Almostallofthepureorlightlydopedlayeredmanganeseoxideshaveshownarapidtrans-formationtoaspineluponcycling.8,9Whilethisspinelcaninsomecasesmaintaincapacity,10ithasalessfavorablevoltageprofileandtheremainingdisorderinthestructurelimitsitscurrentdensity.Usinghigherdopinglevels,ithasbeenpossibletostabilizethelayeredstructureagainsttransformationtospinel.Inparticular,co-dopingofLiandCr5,11hasbeenparticularlysuccessful.Ni-dopedMnmaterialswerefirstsynthesizedseveralyearsago,12,13butthisapproachhasgainedrenewedinterestnowthatgoodcyclingbehav-iorforthesematerialshasbeendemonstrated.5,14

Someuncertaintyexistswithregardstothevalencestatesinthesemixed-metalcompounds.InLi(Li,Cr,Mn)O2theobservedca-pacitycouldonlybeexplainedbythecyclingofCr3ϩtoCr6ϩ,5afactlaterconfirmedwithX-rayabsorptionspectroscopy.15InLiNi0.5Mn0.5O2ithasbeenspeculated5,14thattheNiandMnions,respectively,havevalenceϩ2andϩ4,thoughearlierworkbySpahr13assumedbothNiandMntohavevalenceϩ3inthestartingmaterial.BecausethecapacityofLiNi0.5Mn0.5O2isϾ200mAh/g,theassumptionofNi2ϩandMn4ϩinthestartingmaterialrequiresthatthenickelioncyclesbetweenNi2ϩandNi4ϩ.ThepurposeofthispaperistoclarifythevalencestatesinLiNi0.5Mn0.5O2andcharacterizetheelectronicandstructuralchangesthatoccurupondelithiation.Theoriginforthehighpotentialofthismaterialisalsodiscussed.Theenergies,intercalationpotentials,geometries,andelectronicstructureoftheLixNi0.5Mn0.5O2materialsareobtainedusingfirstprinciplesquantummechanicalcomputationsinthegen-eralizedgradientapproximationtodensityfunctionaltheory.Ultra-softpseudopotentialsandthePerdew-Wangexchangecorrelationfunctionwereused,asimplementedinVASP.16Allcalculationswereperformedwithspinpolarization,previouslydemonstratedtobecrucialinmanganeseoxides.1Reasonableintercalationpotentials

*ElectrochemicalSocietyActiveMember.

z

E-mail:gceder@mit.edu

andgeometricalinformationcanbeobtainedwithfirstprinciplesmethods,ashasbeenamplydemonstrated.17-19TodescribetheLiNi0.5Mn0.5O2system,asupercellwithtwoformulaunitswasused.Becausethiscomputationalapproachrequirestheuseofperiodiccells͑asdomostcomputationalmethods͒,theMnandNiarelong-rangeorderedinrowsonthetriangularlatticeoftransitionmetalsites.Inallcells,thesymmetrywasloweredenough͑lowerthanthatassociatedwithLi/vacancyorNi/Mnorderingalone͒sothatJahn-Tellerdistortionscouldtakeplaceifenergeticallyfavorable.Inprac-tice,thismeansthatthesymmetryisalwaysasubgroupoftheC2/mgroupofthemonocliniclayeredLiMnO2.

Thevalencestateofahigh-spintransitionmetalioncanbestbedeterminedbyintegratingthespin-polarizationdensityinaspherearoundtheion.Integratingspindensityismuchmoreeffectivethanintegratingthechargedensity,astheformerfiltersouttheelectroniccontributionfromtheoxygenp-stateswhichusuallycarryverylittlenetelectronspin.Fortherelevantions,Mn4ϩ,Mn3ϩ,andMn2ϩ,weexpectatotalelectronspincountof,respectively,3,4,and5͑inunitsof1/2␮B͒.ForNi4ϩ,Ni3ϩ,andNi2ϩweexpect0,1,and2,respectively,electronspinsastheNi4ϩhasacoreofnonspinpolar-ized,fullt2glevels.Figure1ashowstheintegratedspinasfunctionofintegrationradiusaroundNiandMnintheLiNi0.5Mn0.5O2struc-ture.Theintegratedmomentincreasessteeplyasweintegratethroughthed-statesofthemetalion,butthenreachesaplateauvaluebecausethechargedensityoftheoxygenionsdoesnotcon-tributetothespindensity.Afterthisplateautheintegratedvalueincreasesagainasspinfromneighboringtransitionmetalsispickedup.TheMnioninLiNi0.5Mn0.5O2clearlycarriesthreeelectrons,correspondingtoavalenceofMn4ϩ.ThemomentaroundNiisslightlybelowwhatisexpectedforNi2ϩ.Theremainderofthemomentisprobablyontheoxygenionsasistypicalfornickeloxides.SomeevidenceforthisliesinthefactthatthepointwheretheintegratedspindensityrisesfromtheplateauvalueisatashorterradiusthanfortheMn4ϩion.ThespinintegrationinFig.1aindi-catesthattheformalvalencestatesareLiNi0.5IIMn0.5IVO2.Furtherevidencecanbefoundfromthechangesinspindensityuponlithiumremoval.Figure1bshowssimilarspinintegrationsforthedelithiatedmaterialNi0.5Mn0.5O2.ThespinonMnisbarelydiffer-entfromwhatitisinthefullylithiatedmaterial,whileNihaslostmostofitsmoment,consistentwiththeelectronconfigurationforNi4ϩinNi0.5Mn0.5O2.Figure1aandbofferstrongevidencethatinLiNi0.5Mn0.5O2thecorrectvalenceassignmentisNi2ϩandMn4ϩ.UponLiremovalNiϩ2isoxidizedtoNi4ϩwhiletheMn4ϩionremainsunchanged.

TheselectiveoxidationofNiinthesematerialsisalsoconsistentwiththemetal-oxygenbondlengthvariationspredictedfromthecomputations͑Fig.2͒.Ni-OandMn-OareseparatelyshowninLiNi0.5Mn0.5O2,Li0.5Ni0.5Mn0.5O2,andNi0.5Mn0.5O2.Theverticalbarsontheresultindicatethevariationinbondlengthinthestruc-

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A146ElectrochemicalandSolid-StateLetters,5͑7͒A145-A148͑2002͒

Figure2.PlotofNi-O͑-•-͒andMn-O͑͒bondlengthsasafunctionoflithiumcontent.CalculationsperformedatcompositionsofLiNi1/2Mn1/2O2,Li1/2Ni1/2Mn1/2O2,andNi1/2Mn1/2O2.Thedatapointscorrespondtotheav-eragebondlengthswhilethebarsateachpointindicatethespreadbetweenthemaximumandminimumbondlengths.Notethatat1/2lithiumcontenttheminimumNi-ObondlengthislessthantheminimumMn-Obondlength,whilethemaximumNi-ObondlengthisgreaterthanthemaximumMn-Obondlength.ThisisduetotheNi3ϩundergoingaJahn-Tellerdistortion.

Figure1.͑a͒Integratedspinasafunctionofintegrationradius͑Å͒aroundNiandMninLiNi1/2Mn1/2O2.͑b͒Integratedspinasafunctionofintegrationradius͑Å͒aroundNiandMninNi1/2Mn1/2O2.

turebecausethesymmetryofoursupercell͑lowerthanR¯3m͒allowsforslightlydifferentbondlengthsinagivenoctahedron.TheMn-ObondlengthhardlyvarieswithLicomposition,confirmingthatlittleornochangetoitsvalencestateoccurs.Incontrast,theNi-ObondlengthchangesdramaticallywithLicomposition.Forthedelithiatedmaterial,theNi-Obondlengthisaround1.90Å,verytypicalofNi4ϩ.20,21TheverylargespreadinNi-OdistancesatxLiϭ0.5isduetotheJahn-TellerdistortionaroundNi3ϩion.TheJahn-TellerdistortionisofthepositiveQ3type,22sothattherearefourshortandtwolongbonds.Inarealmaterial,thespreadinbondlengthsmaybesomewhatlessduetothedisorderoftheNi/Mnions.TableIshowsthecalculatedandexperimentallymeasuredlatticeparameters.Thecalculatednumbersaresomewhatlargerthanthosemeasured,asisoftenthecaseforcomputationsinthegeneralizedgradientapproximation.

WehavealsocalculatedtheaveragedischargepotentialforthesystemLiNi0.5Mn0.5O2/Ni0.5Mn0.5O2.Thesearecomparedtoex-perimentaldatainTableII.Thecalculatedpotentialsarebelowtheexperimentalvalues,asistypicalwithstandardfirstprinciplesen-ergymethods.17Hence,tomakeabetterpredictionpossible,wehaveestimatedacorrectionbasedonthedifferencebetweenmea-suredandcalculatedpotentialsforLiNiO2.Thiscorrection(ϩ0.73V)isaddedtothecalculatedpotentialtogivetheresultinthelastcolumn.Weemphasizethatthisadjustmentispurelyphe-nomenologicalandforthepurposeoffacilitatingthedirectcompari-sonwithexperimentsforLiNi0.5Mn0.5O2.ThedatainthiscolumnagreeswellwiththemeasuredvaluesforLiNi0.5Mn0.5O2.InthelastrowofthetablethepotentialisbrokendownintotheaveragefortheintervalLiNi0.5Mn0.5O2toLi0.5Ni0.5Mn0.5O2,andLi0.5Ni0.5Mn0.5O2toNi0.5Mn0.5O2.Someindicationofthevariationofpotentialuponchargecanbederivedfromthis.

TableIIhighlightsthefactthatthepotentialofLiNi0.5Mn0.5O2isactuallyveryclosetothatofLiNiO2.Thisissurprisingbecauseourresultsindicatethatdifferentredoxcouplesareactiveinbothmate-rials.InLiNiO2onlyNi3ϩ/Ni4ϩisactive,whilebothNi2ϩ/Ni3ϩandNi3ϩ/Ni4ϩoccurinLiNi0.5Mn0.5O2.HencetheaveragepotentialforLiNi0.5Mn0.5O2shouldbelowerthanforLiNiO2.Evenifonebe-

TableI.Comparisonofexperimentalwithcalculatedlatticeparameters.ExperimentalandcalculatedlatticeparametersCompLiMnO2

LiMn1/2Ni1/2O2

Structure͑exporcalc͒C2/mlayered

exp.calc.

rhombohedrallayered

exp.͑1͒exp.͑2͒calc.

rhombohedrallayered

exp.͑1͒exp.͑2͒calc.

Latticeparameters

aϭ5.44Åbϭ2.81Åcϭ5.38Å␤ϭ116°aϭ5.58Åbϭ2.83Åcϭ5.49Å␤ϭ117°

aϭ2.2Åcϭ14.301Åaϭ2.4Åcϭ14.277Åaϭ2.914Åcϭ14.398Åaϭ2.885Åcϭ14.197Åaϭ2.4Åcϭ14.226Åaϭ2.943Åcϭ14.287Å

LiNiO2

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ElectrochemicalandSolid-StateLetters,5͑7͒A145-A148͑2002͒A147

TableII.ComparisonbetweencalculatedandmeasuredaveragedischargepotentialsfortheLiNi0.5Mn0.5O2andLiNiO2systems.ThelastcolumnincludesacorrectiontothecomputedvoltagebasedonthedifferencebetweencalculationandexperimentforpureLiNiO2.Thelasttworowsofthetableshowthepredictedaveragepotentialsinthefirstandlasthalfofthedischarge.

Calculated

LiNiO2

LiNi0.5Mn0.5O2LiNi0.5Mn0.5O2

3.173.22

0.5ϽxliϽ1

2.94

0ϽxliϽ0.5

3.51

Experimental3.9͓28͔3.9͓14͔

0.5ϽxliϽ1

3.67

Adjusted3.93.95

0ϽxliϽ0.5

4.24

lievedthatMnparticipatesintheredoxprocessthehigherpotentialisdifficulttoexplain,sincetheMn3ϩ/Mn4ϩcoupleisbelowthatofNi3ϩ/Ni4ϩ.Theseexperimentalandtheoreticalresultsarefurtherevidencethatstronginteractionsexistbetweentheredoxcouplesofmetalswhentheyaremixed.23Ingeneralalloytheory,24ameasureoftheeffectiveNi-MninteractionscanbeobtainedbycomparingtheenergyofLiNi0.5Mn0.5O2totheaverageenergyofLiNiO2andLiMnO2.If⌬EmixϭE(LiNi0.5Mn0.5O2)Ϫ1/2͓E(LiNiO2)ϩE(LiMnO2)͔isnegative,NiandMnhaveaneffectiveattractiveinteractionandthesystemwillbeeitherrandomlymixedorordered,dependingonthestrengthoftheinteractionandthepreparationtemperature.If⌬Emixispositive,localphaseseparationintoMnandNirichregionsisenergeticallypreferred,thoughrandommixingmaybeachievedifthesynthesistemperatureishighenough.FromcalculatingtherelevantenergynumbersintheaboveequationwefindthatforLiNi0.5Mn0.5O2⌬EmixisϪ216meVperformulaunit,indicatingastrongordering͑attractive͒tendencybetweenNiandMn.Similarly,forthedelithiatedmaterial⌬EmixϭE(Ni0.5Mn0.5O2)Ϫ1/2͓E(NiO2)ϩE(MnO2)͔iscomputedtobeϩ50meV,indicatingrepulsiveNi-Mninteractions.Theseresultsgivesomeinsightastowhythevoltageishigherthanmaybeexpectedforthissystem.Ni-Mninteractionsgofrombeingattrac-tiveinLiNi0.5Mn0.5O2tobeingrepulsiveinNi0.5Mn0.5O2.Hence,toremovelithiumonenotonlyhastosupplythebindingenergyfortheLiionandelectron,butalsothestrongenergyincreaseinthesystemduetotheMn-Nibondsbecomingunfavorable͑asthatinteractionturnsfromattractivetorepulsive͒.Itcanbeeasilydeducedthattheeffectofchangesinthemetal-metalinteractionsupontheaveragedischargepotentialisgivenby

⌬␾ϭ⌬Emix͑XLiϭ1͒Ϫ⌬Emix͑XLiϭ0͒

where␾istheequilibriumpotentialovertherange0ϽxLiϽ1.FortheLiNi0.5Mn0.5O2system,thisresultindicatesthatthepotentialisraisedbyabout266mVoverwhatwouldbeexpectedifnoNi-Mninteractionswerepresent͑e.g.,ifNi2ϩ/Ni4ϩactedinapurehost,withoutthepresenceofMn͒.Thesenumbersarederivedfromcal-culationsonanorderedsupercellofNiandMn.WehaveestimatedthatiftheNiandMnionswerefullyrandomized͑ratherthanor-deredinthesupercellthatweusedforthecalculations͒theincreaseinpotentialwouldbeslightlylessandabout200mV.

TheeffectofLiontheNi-Mninteractionscanbeeasilyunder-stood,usingwhatisknownaboutthemiscibilityofoxides.25Theeffectiveinteraction,forstudyingphasestabilityandmixingisnotthebareionicinteractionbuttheenergydifferencebetweentheav-erageofidenticalpairs͑i.e.,Ni-NiandMn-Mn͒anddifferentpairs͑i.e.,Ni-Mn͒.HencethesimplestwaytosamplethisdifferenceistoconsiderthedifferenceinenergybetweenLiNi0.5Mn0.5O2͑whereNi-Mnbondsarepresent͒andLiNiO2͑withNi-Nibonds͒andLiMnO2͑withMn-Mnbonds͒.Inthedelithiatedstate,NiandMnhavethesameϩ4valenceandthereisnonetelectrostaticinterac-tionforexchangingtheirpositions.Itcanbeshownthatforsuchiso-valentions,thenetinteractionisduetosizeeffects,andisal-waysrepulsive.26Thisagreeswithourresultofapositivemixingenergyinthedelithiatedstate.Ontheotherhand,inthelithiated

materialthedifferentvalenceofNiandMnleadstoastrongeffec-tiveattractiveinteractionandhenceexplainstheorderingormixingtendency.

OurresultsconclusivelyindicatethatinLiNi0.5Mn0.5O2͑andhenceintherelatedsystemsLi͓NixLi(1/3Ϫ2x/3)Mn(2/3Ϫx/3)͔O2͒14NiistheelectrochemicallyactiveionandcyclesbetweenNi2ϩandNi4.Thematerialremainskineticallystableagainsttransformationtospi-nelbecauseMnisnotpresentinoxidationstateslowerthanϩ4.WerecentlyshowedthattheveryrapidtransformationoflayeredLiMnO2tospinelisduetotheeasewithwhichMn3ϩdispropor-tionatestoMn2ϩandMn4ϩ.27ThisallowsMntorapidlymigratethroughtetrahedralsitesasMn2ϩ.Mn4ϩ,ontheotherhand,wasshowntohaveaveryhighactivationbarrierfordiffusionthroughthetetrahedralsite.Hence,layeredoxideswithonlymanganeseinthe4ϩoxidationstateareexpectedtobequitestable.IncyclingbetweenLiNi0.5Mn0.5O2andNi0.5Mn0.5O2theNi-Mnarrangementremainsfixedduetothelackofanytransitionmetalmobilityatroomtemperature,buttheinteractionsbetweentheionschangecon-siderably.Inthissystem,thischangeinNi-MninteractionscausesthevoltagetoincreasefortheNi2ϩ/Ni4coupleoverwhatitwouldbeinanoninteractingmatrix.

TheclassofmaterialsinwhichthevalenceofNiisϩ2andMnisϩ4seemstopossessmanydesirablefeaturesforacathodema-terial.Theyalsopointatnewandinterestingdirectionsforcathoderesearch.Thecombinationofexperimentaldataonhighlydopedsystems,andourunderstandingoftheroleofMn3ϩintheproblemsofmanyMn-oxides,clearlyindicatesthatstablelayeredMnoxides,containingonlyMn4ϩcanbemade.InthesematerialsMnhasgivenupitsroleasanelectrochemicallyactivecenterandispresentonlyas‘‘filler.’’Hence,otherelementsthatcantakeontheϩ4oxidationstateinthelayeredoxideenvironment,couldbeselectedonthebasisofcost,weight,processability,environmentalbehavior,etc.,asasubstituteforMn.

Acknowledgments

TheauthorsacknowledgethesupportoftheNationalScienceFoundation͑MRSECProgram͒undercontractno.DMR98-041.DiscussionswithDaneMorgan,ElenaArroyo,andAntonVanderVenwerehelpfulindevelopingtheideasinthispaper.ComputingresourcesfromNPACItheNationalPartnershipforAdvancedCom-putingInfrastructure͑NSF͒aregratefullyacknowledged.

TheMassachusettsInstituteofTechnologyassistedinmeetingthepubli-cationcostsofthisarticle.

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