Challenges-for-Rechargeable-Li-Batteries

pubs.acs.org/cmPublishedonWeb08/28/2009r2009AmericanChemicalSocietyChem.Mater.2010,22,587–603587DOI:10.1021/cm901452zChallengesforRechargeableLiBatteries†JohnB.Goodenough*andYoungsikKimTexasMaterialsInstitute,UniversityofTexasatAustin,Austin,Texas78712ReceivedMay27,2009.RevisedManuscriptReceivedJuly9,2009ThechallengesforfurtherdevelopmentofLirechargeablebatteriesforelectricvehiclesarereviewed.Mostimportantissafety,whichrequiresdevelopmentofanonflammableelectrolytewitheitheralargerwindowbetweenitslowestunoccupiedmolecularorbital(LUMO)andhighestoccupiedmolecularorbital(HOMO)oraconstituent(oradditive)thatcandeveloprapidlyasolid/electrolyte-interface(SEI)layertopreventplatingofLionacarbonanodeduringafastchargeofthebattery.AhighLiþ-ionconductivity(σLi10-4S/cm)intheelectrolyteandacrosstheelectrode/electrolyteinterfaceisneededforapowerbattery.Importantalsoisanincreaseinthedensityofthestoredenergy,whichistheproductofthevoltageandcapacityofreversibleLiinsertion/extractioninto/fromtheelectrodes.Itwillbedifficulttodesignabetteranodethancarbon,butcarbonrequiresformationofanSEIlayer,whichinvolvesanirreversiblecapacityloss.Thedesignofacathodecomposedofenvironmentallybenign,low-costmaterialsthathasitselectrochemicalpotentialμCwell-matchedtotheHOMOoftheelectrolyteandallowsaccesstotwoLiatomspertransition-metalcationwouldincreasetheenergydensity,butitisadauntingchallenge.Tworedoxcouplescanbeaccessedwherethecationredoxcouplesare“pinned”atthetopoftheO2pbands,buttotakeadvantageofthispossibility,itmustberealizedinaframeworkstructurethatcanacceptmorethanoneLiatompertransition-metalcation.Moreover,suchasituationrepresentsanintrinsicvoltagelimitofthecathode,andmatchingthislimittotheHOMOoftheelectrolyterequirestheabilitytotunetheintrinsicvoltagelimit.Finally,thechemicalcompatibilityinthebatterymustallowalongservicelife.IntroductionItisnowalmostuniversallyrecognizedthatgaseousemissionsfromtheburningoffossilfuelsandbiomassarenotonlypollutingtheairoflarge,moderncitiesbutarealsocreatingaglobalwarmingwithalarmingconse-quences.Moreover,adependenceonforeignoiland/orgascreatesnationalvulnerabilitiesthatendangersocialstability.Theseconcernsareconcentratingattentiononceagainonnationalinitiativestoreevaluateutilizationofalternativeenergysourcesandreplacementoftheinternalcombustionenginewithawirelesselectricmotor.Solarradiation,wind,andwavesrepresentenergysourcesthatarevariableintimeanddiffuseinspace.1Thesesourcesrequireenergystorage.Nuclearreactorsprovideaconstantenergysourcewithassociatedpro-blemsofradioactivewastedisposal.Geothermalenergyisrestrictedinlocation.Theseenergysourcesalsobenefitfromelectricalenergystorage.Theenergycarriersaretheelectricitygrid,electromagneticwaves,andchemicalen-ergy.Themostconvenientformofenergystorageisportablechemicalenergy,whichisthereasonforouraddictiontofossilfuelsforheat,propulsion,lighting,andcommunication.Thebatteryprovidestheportabilityofstoredchemicalenergywiththeabilitytodeliverthisenergyaselectricalenergywithahighconversioneffi-ciencyandnogaseousexhaust.Moreover,thealternativeenergysourcesarepreferablyconvertedtod.c.electricalenergywell-matchedtostorageaschemicalenergyinabattery.Whereasalternativeenergysourcesarestation-ary,whichallowsothermeansofenergystoragetobecompetitivewithabattery,electricvehiclesrequiretheportablestoredenergyofafuelfedtoafuelcellorofabattery.Therefore,ofparticularinterestisalow-cost,safe,rechargeable(secondary)batteryofhighvoltage,capacity,andratecapability.ThehigherstoredvolumeandgravimetricenergydensityofaLibatteryhasenabledrealizationofthecellulartelephoneandlap-topcomputer.However,cost,safety,storedenergydensity,charge/dischargerates,andservicelifeareissuesthatcontinuetoplaguethedevelop-mentoftheLibatteryforthepotentialmassmarketofelectricvehiclestoalleviatedistributedCO2emissionsandnoisepollution.2Abatteryconsistsofagroupofinterconnectedelectro-chemicalcells.Here,wefocusonbatteriesforelectricvehicleswherecost,gravimetricenergydensity,andtheperformanceuniformityofindividualcellsinalarge,multicellbatteryareofmoreconcernthanthevolumeenergydensityconsideredcriticalforhand-heldappli-ances.Moreover,weconsideronlythechoiceofactivematerialsintheindividualcellsofasecondarybattery,†Acceptedaspartofthe2010“MaterialsChemistryofEnergyConversionSpecialIssue”.*Authortowhomcorrespondenceshouldbedirected.E-mail:[e-mail].,Vol.22,No.3,2010GoodenoughandKimviz.theanode(negativeelectrode),thecathode(positiveelectrode),andtheelectrolytebetweentheelectrodes.PreliminaryConsiderationsFigure1isaschematicoftherelativeelectronenergiesintheelectrodesandtheelectrolyteofathermodynami-callystablebatterycellhavinganaqueouselectrolyte.Theanodeisthereductant,thecathodeistheoxidant,andtheenergyseparationEgofthelowestunoccupiedmolecularorbital(LUMO)andthehighestoccupiedmolecularorbital(HOMO)oftheelectrolyteisthe“win-dow”oftheelectrolyte.ThetwoelectrodesareelectronicconductorswithanodeandcathodeelectrochemicalpotentialsμAandμC(theirFermienergiesεF).AnanodewithaμAabovetheLUMOwill