Challenges for Rechargeable Li Batteries

Chem.Mater.2010,22,587–603587

DOI:10.1021/cm901452z

ChallengesforRechargeableLiBatteries

†

JohnB.Goodenough*andYoungsikKim

TexasMaterialsInstitute,UniversityofTexasatAustin,Austin,Texas78712

ReceivedMay27,2009.RevidManuscriptReceivedJuly9,2009

ThechallengesforfurtherdevelopmentofLirechargeablebatteriesforelectricvehiclesare

reviewed.Mostimportantissafety,whichrequiresdevelopmentofanonflammableelectrolytewith

eitheralargerwindowbetweenitslowestunoccupiedmolecularorbital(LUMO)andhighest

occupiedmolecularorbital(HOMO)oraconstituent(oradditive)thatcandeveloprapidlyasolid/

electrolyte-interface(SEI)layertopreventplatingofLionacarbonanodeduringafastchargeofthe

battery.AhighLi

þ-4

-ionconductivity(σ>10S/cm)intheelectrolyteandacrosstheelectrode/

Li

electrolyteinterfaceisneededforapowerbattery.Importantalsoisanincreainthedensityofthe

storedenergy,whichistheproductofthevoltageandcapacityofreversibleLiinrtion/extraction

into/fromtheelectrodes.Itwillbedifficulttodesignabetteranodethancarbon,butcarbonrequires

formationofanSEIlayer,whichinvolvesanirreversiblecapacityloss.Thedesignofacathode

compodofenvironmentallybenign,low-costmaterialsthathasitselectrochemicalpotentialμ

C

well-matchedtotheHOMOoftheelectrolyteandallowsaccesstotwoLiatomspertransition-metal

cationwouldincreatheenergydensity,butitisadauntingchallenge.Tworedoxcouplescanbe

accesdwherethecationredoxcouplesare“pinned”atthetopoftheO2pbands,buttotake

advantageofthispossibility,itmustberealizedinaframeworkstructurethatcanacceptmorethan

oneLiatompertransition-metalcation.Moreover,suchasituationreprentsanintrinsicvoltage

limitofthecathode,andmatchingthislimittotheHOMOoftheelectrolyterequirestheabilityto

tunetheintrinsicvoltagelimit.Finally,thechemicalcompatibilityinthebatterymustallowalong

rvicelife.

Introduction

Itisnowalmostuniversallyrecognizedthatgaous

emissionsfromtheburningoffossilfuelsandbiomassare

notonlypollutingtheairoflarge,moderncitiesbutare

alsocreatingaglobalwarmingwithalarmingcon-

quences.Moreover,adependenceonforeignoiland/or

gascreatesnationalvulnerabilitiesthatendangersocial

stability.Theconcernsareconcentratingattentiononce

againonnationalinitiativestoreevaluateutilizationof

alternativeenergysourcesandreplacementoftheinternal

combustionenginewithawirelesselectricmotor.

Solarradiation,wind,andwavesreprentenergy

sourcesthatarevariableintimeanddiffuinspace.

1

Thesourcesrequireenergystorage.Nuclearreactors

provideaconstantenergysourcewithassociatedpro-

blemsofradioactivewastedisposal.Geothermalenergyis

restrictedinlocation.Theenergysourcesalsobenefit

fromelectricalenergystorage.Theenergycarriersarethe

electricitygrid,electromagneticwaves,andchemicalen-

ergy.Themostconvenientformofenergystorageis

portablechemicalenergy,whichisthereasonforour

addictiontofossilfuelsforheat,propulsion,lighting,and

communication.Thebatteryprovidestheportabilityof

Acceptedaspartofthe2010“MaterialsChemistryofEnergyConversion

SpecialIssue”.

*Authortowhomcorrespondenceshouldbedirected.E-mail:

jgoodenough@.

r

2009AmericanChemicalSociety

†

storedchemicalenergywiththeabilitytodeliverthis

energyaselectricalenergywithahighconversioneffi-

ciencyandnogaousexhaust.Moreover,thealternative

energysourcesarepreferablyconvertedtod.c.electrical

energywell-matchedtostorageaschemicalenergyina

battery.Whereasalternativeenergysourcesarestation-

ary,whichallowsothermeansofenergystoragetobe

competitivewithabattery,electricvehiclesrequirethe

portablestoredenergyofafuelfedtoafuelcellorofa

battery.Therefore,ofparticularinterestisalow-cost,

safe,rechargeable(condary)batteryofhighvoltage,

capacity,andratecapability.

Thehigherstoredvolumeandgravimetricenergy

densityofaLibatteryhasenabledrealizationofthe

cellulartelephoneandlap-topcomputer.However,cost,

safety,storedenergydensity,charge/dischargerates,and

rvicelifeareissuesthatcontinuetoplaguethedevelop-

mentoftheLibatteryforthepotentialmassmarketof

electricvehiclestoalleviatedistributedCO

2

emissions

andnoipollution.

2

Abatteryconsistsofagroupofinterconnectedelectro-

chemicalcells.Here,wefocusonbatteriesforelectric

vehicleswherecost,gravimetricenergydensity,andthe

performanceuniformityofindividualcellsinalarge,

multicellbatteryareofmoreconcernthanthevolume

energydensityconsideredcriticalforhand-heldappli-

ances.Moreover,weconsideronlythechoiceofactive

materialsintheindividualcellsofacondarybattery,

/cmPublishedonWeb08/28/2009

588Chem.Mater.,Vol.22,No.3,2010GoodenoughandKim

viz.theanode(negativeelectrode),thecathode(positive

electrode),andtheelectrolytebetweentheelectrodes.

PreliminaryConsiderations

intheelectrodesandtheelectrolyteofathermodynami-

Figure1isaschematicoftherelativeelectronenergies

callystablebatterycellhavinganaqueouselectrolyte.

Theanodeisthereductant,thecathodeistheoxidant,

andtheenergyparationE

molecularorbital(LUMO)andthehighestoccupied

molecularorbital(HOMO)oftheelectrolyteisthe“win-

g

ofthelowestunoccupied

dow”oftheelectrolyte.Thetwoelectrodesareelectronic

conductorswithanodeandcathodeelectrochemical

potentialsμ

withaμabovetheLUMOwillreducetheelectrolyte

unlessapassivationlayercreatesabarriertoelectron

energiesandμ(theirFermiε).Ananode

ACF

A

transferfromtheanodetotheelectrolyteLUMO;a

cathodewithaμ

electrolyteunlessapassivationlayerblockselectron

C

belowtheHOMOwilloxidizethe

transferfromtheelectrolyteHOMOtothecathode.

Therefore,thermodynamicstabilityrequireslocating

theelectrodeelectrochemicalpotentialsμandμwithin

thewindowoftheelectrolyte,whichconstrainstheopen-

circuitvoltageVofabatterycellto

AC

oc

eV¼μ-μð1Þ

ocg

AC

eE

whereeisthemagnitudeoftheelectroncharge.A

passivatingsolid/electrolyte-interface(SEI)layeratthe

electrode/electrolyteboundarycangiveakineticstability

toalargerVprovidedthateV-Eisnottoolarge.

ococg

circuitwheretheydoufulworkbeforeenteringthe

Ondischarge,electronsleavetheanodeviaanexternal

cathode.Toretainchargeneutralityintheelectrodes,

cationsarereleadfromtheanodetotheelectrolyteand

theworkingcationoftheelectrolyte,theH

aqueouselectrolyte,carriespositivechargetothecathode

toprovidechargeneutralityinthecathode.Theprocessis

reverdonchargeinarechargeable(condary)battery.

þ

ioninan

capacityofreversiblechargetransferperunitweight(Ah/

TheenergydensityofabatterycellisΛV

g)betweentheanodeandcathode.Λdecreaswiththe

oc

;Λisthe

rateofchargeordischarge,i.e.themagnitudeofthe

electroniccurrentintheexternalcircuit,whichmustbe

matchedbytheinternalioniccurrentwithinthebattery.

Sincetheioniccurrentdensityoftheelectrolyteand

electrodes,includingtherateofiontransferacrossthe

electrode/electrolyteinterface,ismuchsmallerthan

theelectroniccurrentdensity,theelectrodesandelectro-

lytehavealargesurfaceareaandasmallthickness.

Nevertheless,athighcurrentdensities,theionicmotion

withinanelectrodeand/oracrossanelectrode/electrolyte

interfaceistooslowforthechargedistributiontoreach

equilibrium,whichiswhythereversiblecapacityde-

creaswithincreasingcurrentdensityinthebattery

andwhythiscapacitylossisrecoveredonreducingthe

rateofchargeand/ordischarge.

electrolyteoverthepracticalambient-temperaturerange

ThehighH

þ

-ionconductivityrequiredofanaqueous

Figure1.Schematicopen-circuitenergydiagramofanaqueouselectro-

lyte.ΦandΦaretheanodeandcathodeworkfunctions.Eisthe

windowoftheelectrolyteforthermodynamicstability.Aμ>LUMO

ACg

and/oraμ

SEIlayer.

HOMOrequiresakineticstabilitybytheformation<ofan

A

C

isonlyfoundinliquidorimmobilized-liquidwater,and

anE

ordertoobtainacellwithahigherVandthereforea

foranaqueouselectrolytelimitsV≈1.3eV.In

goc

higherenergydensityΛV,itisnecessarytoturntoa

nonaqueouselectrolytewithalargerE.Thisobrva-

oc

oc

tion,inturn,hasledtotheLi-ionbatterysincelithium

saltsaresolubleinsomenonaqueousliquidsandpoly-

g

mers.However,inthisca,theHOMOofthesaltaswell

þ

asthatofthesolventmaydeterminethelimitingμofthe

cathode.

C

determined,itisnecessarytodesignelectrodesofhigh

OncethewindowoftheLi-ionelectrolytehasbeen

þ

capacitythathavetheirμ

andHOMOoftheelectrolyte.ElementalLiwouldbethe

idealanode,buttheε=μofLiliesabovetheLUMO

ofpractical,knownnonaqueouselectrolytes.Therefore,

AC

andμmatchedtotheLUMO

0

FA

uofLi

ingSEIlayerisformed.TheSEIlayerallowsuofLias

0

asananodeisonlypossiblebecauapassivat-

0

ananodeinhalf-cellsudtoobtaintheμorμofa

practicalelectroderelativetotheLi

0

AC

onrepeatedcharge/dischargecycles,breakingoftheSEI

þ0

/Lienergylevel;but

layerinlectedareasresultsintheformationofdendrites

thatcangrowacrosstheelectrolytetoshort-circuitacell

ofthebatterywithdangerousconquences.Therefore,

wemustdesigneither(1)ananodewithaμ

theLUMOoftheelectrolyteaswellasacathodewithaμ

matchedtotheHOMOoftheelectrolyteor(2)astable

passivatingSEIlayerthatlf-healsrapidlywhenbroken

A

matchedto

C

bythechangesinelectrodevolumethatoccurinacharge/

dischargecycle;theSEIlayermustalsopermitafastLi

iontransferbetweentheelectrodeandtheelectrolyte

withoutblockingelectrontransferbetweentheactive

þ

-

particleandthecurrentcollector.

operofarechargeableLibatteryforthepotential

Insummary,theformidablechallengesforthedevel-

massmarketofelectricvehiclesarethree-fold:toidentify

ReviewChem.Mater.,Vol.22,No.3,2010589

Table1.NonaqueousElectrolytesforLi-IonBatteries

ElectrolytesExampleofclassicalelectrolytes

Liquidorganic1MLiPF

1MLiPFEC:DMCin(1:1)101.3>5.0

6

inEC:DEC(1:1)71.34.5Flammable

6

Ionicliquids1MLiTFSIinEMI-TFSI2.01.05.3Non-flammable

1MLiBFinEMI-BF8.00.95.3

44

PolymerLiTFSI-P(EO/MEEGE)0.1<0.04.7Flammable

LiClO

482

-PEOþ10wt%TiO0.02<0.05.0

InorganicsolidLiGePS(x=0.75)2.2<0.0>5.0Non-flammable

0.05LiSiOþ0.57LiSþ0.38SiS1.0<0.0>8.0

4-x1-xx4

4422

InorganicliquidLiAlClþSO70-4.4Non-flammable

42

Liquidorganicþ0.04LiPF

Polymer0.14PAN

6

þ0.2ECþ0.62DMCþ4.2-4.4Flammable

LiClOþECþPCþPVdF3.0-5.0

4

Ionicliquidþ1MLiTFSIþPTFSIþ0.18<0.05.8Lessflammable

PolymerPVdF-HFP

13

IonicliquidþPolymer56wt%LiTFSI-Py

þLiquidorganic30wt%PVdF-HFPþ

14wt%EC/PC

24

TFSIþ1.54.2Lessflammable

Polymer2vol%LiClO-TEC-19þ98vol%0.03<0.0>4.5Non-flammable

þInorganicsolid95(0.6Liþ0.4LiSS)þ5LiSiO

4

2244

IonicliquidþLiquidorganic

19

low-cost,environmentallybenignmaterialsforthekineticstabilityisrequiredbecautheelectrode

threeactivecomponentsofabatterycell,viz.(1)a

nonaqueouselectrolyteofhighLi-ionconductivity

(σ>10S/cm)overthepracticalambient-temperature

3-

þ

range-40<T<60°Cthathasawindowallowing8)Lowtoxicityandlowcost.

Li

athermodynamicallystableVMeetingalltherequirementsprovestobea

oc

g4Vand(2)ananode

and(3)acathodewiththeirμandμvalueswell-formidablechallenge.

matchedtothewindowoftheelectrolyteaswellaseach

allowingafastcharge/dischargecycleoflargereversible

AC

capacity.

Electrolytes

electrolytemustsatisfyveraladditionalrequirements

InadditiontoalargeelectrolytewindowE

suchas:

g

,the

1)Retentionoftheelectrode/electrolyteinterface

duringcyclingwhentheelectrodeparticlesare

2)ALi

changingtheirvolume.

þ-4

-ionconductivityσ>10S/cmoverthe

Li

3)Anelectronicconductivityσ<10S/cm.

temperaturerangeofbatteryoperation.

4)Atransferencenumberσ

includesconductivitiesbyotherionsintheelec-

e

-10

Litotaltotal

/σ≈1,whereσ

5)Chemicalstabilityoverambienttemperatureranges

trolyteaswellasσþσ.

Lie

6)Chemicalstabilitywithrespecttotheelectrodes,

andtemperaturesinthebatteryunderhighpower.

includingtheabilitytoformrapidlyapassiva-carbonate,thesolventsinclude,inmostcas,ethylene

tingsolid/electrolyte-interface(SEI)layerwherecarbonate(EC)becautheECprovidesapassivating

Ionicconductivity

(Â10s/cm)

-3

þ0

atroomtempRemark

window(V)vsLi

Electrochemical

/Li

ReductionOxidation

376

373

151515

151616

242424

262626

282828

303030

2020

3838

3939

434343

0.81

444444

464646

---Non-flammable

potentialliesoutsidetheelectrolytewindow.

7)Safematerials,i.e.,preferablynonflammableand

nonexplosiveifshort-circuited.

specificallydesignedforaparticularbatteryapplication.

TypesofElectrolytes.Ingeneral,theelectrolyteis

Table1showsveraldifferentmaterialsthathavebeen

udaselectrolytesforLibatteries.

nicliquidsthatarereasonablygoodsolventsforLi

OrganicLiquidElectrolytes.Carbonatesareorga-

salts.

3,4

ca.4.7Vandareductionpotential(LUMO)near1.0

Theyhaveanoxidationpotential(HOMO)at

3,5,6

V.

7þ0

potential.)Moreover,theyhavearelativelylowviscos-

(AllvoltagesinthispaperarereferredtotheLi/Li

ity,whichresultsinalowactivationenergyforLi-ion

diffusion.Therefore,themostcommonlyudelectro-

þ

lytesarecarbonatesorcarbonateblendsconsistingof

oneormoreofthefollowing:propylenecarbonate(PC),

ethylenecarbonate(EC),diethylcarbonate(DEC),

dimethylcarbonate(DMC),orethylmethylcarbonate

(EMC).Inordertobeabletoucarbonastheanode,

whichhasitselectrochemicalpotentialabove(ata

lowervoltageversusLi

þ0

/Li)thatoftheLUMOofa

590Chem.Mater.,Vol.22,No.3,2010GoodenoughandKim

solid/electrolyte-interface(SEI)layeronthesurfaceofa

carbonanodethatprotectstheelectrolytesfromfurther

decompositionafterSEIformation.However,carbo-chemicalwindowandadditionallymeettheelectrolyte

nate-badsolventsarehighlyflammablewithflashrequirementsfrom2to7,not1.Forthereasons,

pointsbelow30°C.Inaddition,thepreferredsalt,laboratory-sizeall-solid-stateLi-ionbatterieshavebeen

10

8,9

LiPF,canundergoanautocatalyticdecompositionintoHowever,thefirstoftheadditional

LiFandPF;thePFreactsirreversiblywithanywaterelectrolyterequirementshasexcludedinorganicsolid

prent(PFþHO=PFOþ2HF)and,above60°C,Li-ionelectrolytesfromconsiderationforlarge-scale

6

55

withacarbonateelectrolyte.Thereactionsdegradebatterieshavingsolidelectrodes.Theyhaveonlybeen

523

11

thebatteryandleadtosafetyhazards.However,addi-udinthin-filmbatteryapplications.

tivesudtolowertheoperatingtemperaturehavebeenHybridElectrolyteSystem.Hybridelectrolytesare

showntopreventtheautocatalyticdecompositionofblendsoforganicliquidelectrolytes,ionicliquids,poly-

LiPF

12

6

salt.merelectrolytes,and/orinorganicsolidelectrolytes:

(RTILs)havebeenrecentlyconsideredasalternativeIonicliquidþpolymerelectrolyte(ionicliquidpoly-

IonicLiquids.Room-temperatureionicliquidsPolymerþorganicliquid(polymergel)

13-17

electrolytesforLi-ionbatteriesbecautheyofferveralmergel)

advantagesovercarbonate-badelectrolytes:ahighIonicliquidþpolymerelectrolyteþliquidorganic

oxidationpotential(∼5.3VvsLi/Li),nonflammabil-electrolyte

ity,alowvaporpressure,betterthermalstability,low

þ043,44

toxicity,highboilingpoints,andahighLi-saltsolubility.

Unfortunately,theyhaveahigherviscosity,whichre-

ducestheirLi-ionconductivity.Ionicliquidsbadon

imidazolium-badcationswouldappeartobethemost

þ

appropriatecandidatesforLibatteriesduetotheirlower

viscosityandahighLi-saltsolubilityatroomtempera-

ture.However,theionicliquidshavepoorstabilityat

voltagesbelow1.1V,

mustbeaddedtointroduceastableSEIlayeronacarbon

18

sothatadditivessuchasECorVC

anode.Analternativeapproachistoincreaσ

addingaliquidcarbonatetoanionicliquid,butata

concentrationthatretainsthenonflammabilityofthe

Li

by

ionicliquid.

increatheoxidationvoltage(lowerHOMOenergy)of

19

Withthisstrategy,itisalsopossibleto

thehybridelectrolytefromthatofthecarbonate.Inspite

ofextensiverearch,noRTILshaveyetbeenintroduced

intolargepowerbatteries.

electrolytebadonLiAlCl

InorganicLiquidElectrolytes.Theinorganicliquid

nandHambitzer

20,21

andSOpropodbyStas-

42

σÂ10S/cmandisnonflammable,=7butitselectro-

lytewindowappearstobetoosmalltobecompetitive.

-2

hasagoodroom-temperature

Li

astheparatoroftheelectrodes,andasolidpolymer

SolidPolymerElectrolytes.Asolidelectrolytecanact

electrolytecanalsoretaincontactoveranelectrode/

electrolyteinterfaceduringmodestchangesoftheelec-

trodevolumewiththestateofchargeofthebattery.

Polyethyleneoxides(PEOs)containingalithiumsalt

(LiPF

LiAsFor)arelow-cost,nontoxic,Li-ion

2422-þ

polymerelectrolyteswithgoodchemicalstability,butthe

66

Li-ionconductivity,σ<10S/cmatroomtempera-

þ-5

ture,istoolowforapower-batterysystem.Theintroduc-

tionofoxideparticles(e.g.,Al

ZrO)createsamoreamorphouspolymermatrix

Li

2322

O,TiO,SiO,or

byinhibitingchaincrystallizationandattractingLi

25-27

2

þ

fromitssalt.TheresultisanenhancedσandLi-ion

transferencenumber,butσ

Li

thatofthecarbonateelectrolytes.

Li

isstillnotcomparableto

conductingmaterialshavingaσ>10S/cmhave

InorganicSolidElectrolytes.InorganicsolidLi-ion

þ

Li

-428-31

beenconsideredforLi-badelectrolytes,ashasbeen

extensivelyreviewed,

32

becautheyhaveawideelectro-

investigated.

33-35

þ

36

41-45

37-40

Ionicliquidþliquidorganicelectrolyte

19

Polymerelectrolyteþinorganicsolidelectrolyte

46-48

gatedinattemptstoexploittheadvantagesofeach

Themixturesoftwoormoreelectrolytesareinvesti-

constituent,butthedisadvantagesofeachalsoappear.

Forexample,theionicconductivityisincreadinthe

polymergelelectrolytes,buttheyarestillflammableand

havetheirreversiblecapacitylossbelow1Vassociated

withformationofapassivationlayer.

40

modynamicstabilityoftheelectrolytevisavistheelec-

Electrode-ElectrolyteCompatibility.Althoughther-

trodesispossiblewheretheμ

withinthewindowoftheelectrolyte,neverthelesschemi-

󰀁

calreactionsbetweentheelectrodeandtheelectrolyte

AC

andμoftheelectrodeslie

mayoccur.Forexample,thereversibleelectrochemical

intercalationofLiintoLi

byuoftheelectrolyteLiClOinPC,buttheelectro-

lyteLiPFinEC/DECallowsfullelectrochemicalcycling

betweenLiVSandVS.

x2

VSwasoriginallyfrustrated

4

49

6

typeofelectrode-electrolytereaction;anelectrodesur-

ThecathodespinelLi[Mn]Oprovidesanother

22

50

1-x24

facedisproportionationreaction2Mn=MnþMn

resultsindissolutionoftheMnfromtheelectrodeinto

theelectrolyte.Thisreaction,unlesssuppresd,gives

2þ

3þ2þ4þ

anirreversiblecapacitylossofthecathodeandmigration

51

oftheMnacrosstheelectrolytetotheanodeduring

chargetoblockLi-ioninrtionintotheanode.The

2þ

þ

resultisanintolerablelimitationofthervicelifeof

thecell.

Inadditiontochemicalstabilityvisavistheelectrodes

󰀁

andhighertemperatures,theelectrolyteshouldnotbe

decompodbyananodeμ

electrolyteLUMOoracathodeμ

theHOMO.However,ifμorμlieoutsidethewindow

oftheelectrolyte,kineticstabilitymaybeachievedby

A

atahigherenergythanthe

atalowerenergythan

C

AC

formationofapassivatingSEIlayeronthesurfaceofthe

electrode,butattheexpenofthelossincapacitytoform

thelayer.Moreover,duringafastcharge,theconcentra-

tionofLi

layer,andwhereachangeinvolumeoftheelectrode

þ

ionsmaybuilduponthesurfaceoftheSEI

ReviewChem.Mater.,Vol.22,No.3,2010591

Figure2.(a)VoltageprofilesversusLi/LiofthedischargecurvesofLiC,LiTiSandLi[Ti]S,LiCoO,andLiCoPO.(b)Schematicoftheir

þ0

x6x2x24x2x4

correspondingenergyvsdensityofstatesshowingtherelativepositionsoftheFermienergyinanitinerantelectronbandforLiC,theTi/Tiredox

x6

4þ3þ

coupleforLiTiSandLi[Ti]S,theCo/CoredoxcoupleforLiCoO,andtheCo/CoredoxcoupleforLiCoPO.

x2x24x2x4

4þ3þ3þ2þ

breakstheSEIlayer,Limaybeplatedoutbeforethe

0

breakishealed.Liplatingcanresultindendritesthat

growacrosstheelectrolyte.Thisproblemcreatesasafety

issuethathashauntedtheuofacarbonanodeinlarge-

scalepowerbatteries.Theproblemsneedtobemana-

gedifsafetystandardsaretobemetwithanyanode,

includingcarbon,thathasitsμ

A

abovetheLUMOofthe

electrolyte.

Acathodeμatalowerenergythantheelectrolyte

C

HOMOmustbedistinguishedfromanintrinsicvoltage

limitofthecathode,asisdiscusdbelow.Aswith

anodes,passivatinglayersoncathodesarebestformed

insitusothatelectroniccontactwiththecathodecurrent

collectorisnotbroken.Preliminarywork

52-54

onpas-

sivatingSEIlayersonoxidecathodeshasfoundthem

tobeunstable.Thisfieldhasyettobeadequately

rearched.

Electrodes

Thedesignofanelectrodeinvolvestailoringoftheμ

A

ofananodeorμofacathodetotheLUMOorHOMO

C

oftheLi-ionelectrolytetobeud;theelectrodemust

þ

alsobechemicallystableintheelectrolyte.Todate,

practicalelectrodeshaveallhadhoststructuresinto/from

whichguestLiatomscanbeinrted/extractedreversibly.

FactorsDeterminingμandμ.Theenergyofagiven

AC

μorμmaycorrespondtotheFermienergyinan

AC

itinerant-electronband,asisthecaforcarbon,orthe

energyofaredoxcoupleofatransition-metalcation.

Tailoringoftheenergyofaredoxcoupledependsnot

onlyontheformalvalencestateofthecation,butalsoon

thecovalentcomponentofitsnearest-neighborbonding,

whichisinfluencedbytheplacementandcharacterofany

countercationsandbytheMadelungenergyoftheionic

componentofthebonding,whichisinfluencedbythe

structure.Inaddition,thepositionofaredoxcouple

relativetothebottomofabroadconductionbandorto

thetopofananionpbandmaydeterminetheintrinsic

voltagelimitversusLi

þ0

/Liofagivenelectrode.This

problemarisforaμwheretheactiveredoxcoupleis

C

“pinned”atthetopoftheanionpbands.Pinningofredox

couplesandtheintrinsicvoltagelimitareconceptsde-

scribedbelow.However,wefirstdemonstrateinFigure2

therangeofvoltagesthatareexhibitedbyhoststructures

592Chem.Mater.,Vol.22,No.3,2010GoodenoughandKim

into/fromwhichLiionshavebeeninrtedreversibly.

Carbon,Li

pounds,the[Ti]SspinelhostofLi[Ti]Sisstrongly

bondedin3D,andLiCoPOshowstheinfluenceofthe

TiS,andLiCoOarealllayeredcom-

þ

x2x2

24x24

countercationofthe(PO)polyanionontheCo/

x4

CocouplerelativetotheCo/Cocoupleinthe

2þ4þ3þ

layeredLiCoO(chargesonionsreprentformal

4

3-3þ

valencestates,notactualcharges).Theuppervoltage

x2

limitsinthesulfidesaremuchlowerthanthointhe

oxides.InFigure3,wealsoshowhowtheMn/Mn

morethan1eVwheretheLi

Licoupleof[Mn]Oinaspinelframeworkisshiftedby

4þ3þ

x22

fromoctahedraltotetrahedralsitesas2>x>1

decreasto1>x>0.Theinfluenceofstructureis

þ

ionschangetheirposition

exemplifiedbythecomparisoninFigure4ofthevol-

tagesfromtheFe

PONASICONstructureofLi),theFe(PO,and

3þ2þ

/FecoupleintheolivineLiFe-

somediphosphates.

x

43þx243

packedhexagonalarraywithTioccupyingalternate(001)

HostStructures.ThesulfuratomsofTiSformaclo-

55

planesofoctahedralsites.TheTiSsheetsofedge-

2

sharedoctahedraareheldtogetherbyvanderWaals

forces.Steeleoriginallysuggestedthatintercalationof

6/3

LiintotheemptyoctahedralsitesbetweentheTiS

sheetswouldbereversible,whichmadeTiSapotential

56

6/3

cathodeforarechargeableLibattery.Whittinghamwas

2

thefirsttodemonstratefast,reversibleLiinrtioninto

TiSthesolid-solutionrange0exe1ofLioverTiS.

57

However,attemptstomakeaTiS/Libatteryfailed

2x2

becaudendriteformationontheLianodecaud

2

0

0

explosivefailure.Nevertheless,theearlyexperiments

demonstratedthatcompoundsintowhichLicanbe

inrted/extractedreversiblyarecandidateelectrodes

fortherechargeableLibattery.

relativetotheLi

Thehorizontal,dashedlinesofFigure5aretheenergies

HOMOoftheEC/DECsolventcontainingthemore

þ0

benignLiPFastheLi-ionsalt.Thisfigureshowsthat

/LipotentialoftheLUMOand

þ

theenergyμ

6

notwell-matchedtotheHOMOofthiselectrolyte.Rea-

Cx2

oftheTi/TiredoxcoupleofLiTiSis

4þ3þ

lizationthatTiSthevoltagelimitversusapproachesLi/

2

þ

Li

0

inrtionintolayeredoxides.However,layeredoxides

ofalayeredsulfidesuggestedexplorationofLi

58

areonlyfoundwhereatransition-metalcationformsan

MdObondaswiththevanadylVdOandmolybdyl

ModOcationsofV

hand,LiMOoxidesforminganorderedrock-saltstruc-

253

OandMoO.Ontheother

59,60

turewithLiandtransition-metalMatomsonalternate

2

(111)octahedral-siteplanesinvitedinvestigationof

reversibleLiextraction.RemovalofLifromordered

LiMOallowsoperatingonanM/Mcoupleoflower

58,61

4þ3þ

energythantheTi

2

removalofLileavesametastablecompound,andM

4þ3þ

/TicoupleofTiS.However,

2

cationsstableintetrahedralsiteseithermoveintothe

partiallyoccupiedLilayerortransformthestructureto

spinelonremovalofhalfoftheLi.Moreover,goodorder

oftheLiandMatomsintheinitialLiMO

Nevertheless,removalofhalfoftheLifromwell-ordered

2

isrequired.

LiCoO

2C

ataμ≈4.0VversusLi/LifortheCo/Co

þ04þ3þ

Figure3.VoltageprofileversusLi/LiofthespinelLiMnO

(2gxg0).TheLiionsareshiftedcooperativelyfromthetetrahedral

totheoctahedralsitesasxincreasthoughx=1.Adaptedfrom

þ

þ0

x24

ref67.

Figure4.PositionsoftheFe/FeredoxcouplesrelativetotheFermi

energyoflithiumindifferentphosphates.Reprintedwithpermissionfrom

3þ2þ

ref55.Copyright1997TheUniversityofTexasatAustin.

coupleprovedstable.Thiscouplehasagoodmatchtothe

HOMOoftheLiPFinEC/DECelectrolyte,butonly

oneLifortwocobaltreprentsareducedcapacity;and

Coistooexpensiveandtoxicforalarge-batterymass

6

market.

intercalationanodetoreplaceLi

Graphitehasalayeredstructurethatemedtoofferan

ugraphitewerefrustratedbyreductionoftheelectro-

0

,butearlyattemptsto

lyteonLiinrtion.AsisshowninFigure5,theμof

carbonlieswellabovetheLUMOofacarbonateelectro-

lyte,whichiswhyidentificationofaLiintercalation

62

A

compoundisnotasufficientconditionforaviable

electrode.Ontheotherhand,incorporationofethylene

carbonate(EC)intothecarbonateelectrolytepromotes

formationofanSEIlayeronthecarbonthatprovidesa

kineticstability.

8

Anirreversiblecapacitylossonthe

ReviewChem.Mater.,Vol.22,No.3,2010593

Figure5.Voltageversuscapacityofveralelectrodematerialsrelativeto

thewindowoftheelectrolyte1MLiPF

6

inEC/DEC(1:1).

initialchargeofthecarbonanodeisassociatedwiththe

formationofathin,amorphousSEIlayeronthe

carbonthatstabilizesreversibleLiinrtion/extraction

onsubquentcharge/dischargecycles,eFigure6,with

areversiblecapacityof370mAh/g.Disorderedcarbon

ratherthangraphiticcarbonprovidesabettercapacity.

WithapassivatedcarbonanodeandLiCoOasthe

cathode,membersoftheSonycorporationlaunchedthe

63

hand-heldwirelessrevolutionwiththeirintroductionof

2

thewirelesstelephone.

64

turesofferstrong3Dbondingaswellasinterstitialspace

Thenextstepwastorecognizethatframeworkstruc-

fortheinrtionofLiions.Forexample,theA[B]X

spinelscontainBcationsinoctahedralsitesandAcations

intetrahedralsitesofaclo-packed-cubicX-atomarray.

þ

24

TheBcationsareorderedtogivea3D-bonded[B]X

frameworkinwhichtheinterstitialspaceisintercon-

nectedbyedge-sharingoctahedralsitesthatsharefaces

24

withthetetrahedralAsites;eFigure7.Murphyand

colleagues

65

rtedLiintothe[Ti]Sspinelframework.Inthissulfide,

removedCufromCu[Ti]Sandthenin-

24

Liionsinitiallyentertheoctahedralsitesoftheinter-

þ

stitialspaceratherthanthetetrahedralsites,sothe

24

voltageversusxprofileofLi

esntiallyidenticaltothatofthelayeredLiTiS.

[Ti]S,0exe1,was

x24

x2

57

IndependentworkatOxfordshowedthatLicanbe

inrtedintotheoxospinels;butinoxidestheLispecies

occupythetetrahedralsitesinLi]O[B.Oninrtion

66

ofLiintoLi[B]O,Coulombinteractionsbetweenthe

LiionsdisplacealltheLiionstotheoctahedralsites.In

þþ

1-x24

24

thespinelLi[Mn]O,thehigh-spinMnionsareJahn-

Tellerions,andcooperativeorbitalorderingforaratio

24

3þ

Mn/Mn>0.5distortsthecubicstructuretotetra-

3þ4þ

gonaltogiveacoexistenceoftwophasratherthana

solidsolutionandthereforeaflatV≈3.0VforLi-

[MnSubquently,Thackerayetal.showedthaton]O.

removaloftheLifromthetetrahedralsitesofLi

oc1þx

67

24

[Mn]O,theVversusxprofilewasat4.0VversusLi/

LiforthesameMn/Mnredoxcouple.Theobrva-

04þ3þ

1-x

-

2

4oc

þ

tions,summarizedinFigure3,showedthatshiftingtheLi

stepintheMn/Mnredoxcoupleasaresultofthe

fromoctahedraltotetrahedralionssitesproducesa1eV

þ

inductiveeffectoftheLiions.However,italsoshows

4þ3þ

þ

Figure6.(a)Voltagecurvesofgraphitetestedin1MLiClOinPCand1

MLiAsFinPC:EC(1:1)electrolytes.TheelectrolyteisreducedatV≈0.7

Vin1MLiClOinPC.AnSEIlayerisformedintheEC-badelectrolyte

between0.8and0.4VversusLi/Li,whichallowsfurtherintercalationof

4

6

4

Li

þ

SchematicprentationoftheformationoftheSEIlayerbydecomposi-

ionsafteraninitialcapacityloss.Adaptedfromrefs62and8.(b)

þ0

tionofEC-badelectrolytes.Adaptedfromref9.

Figure7.TwoquadrantsofthecubicspinelA[B]Xshowing

theoccupiedtetrahedralsites(8a),occupiedoctahedralsites(16d),

andunoccupiedoctahedralsites(16c).TheLispeciesofLi

occupy8atetrahedralsites,andthoofLi[B]Ooccupyonly

24

1-x24

unoccupiedoctahedralsites(16c).TheLispeciesofLi

onlyunoccupiedoctahedralsites(16c)forallxof0exe2.Adapted

[B]O

1þx24

fromref77.

x24

[Ti]Soccupy

thatuoftheoxospinelslimitstheoperativecapacity

tooneLipertwoB-sitecationsasinlayeredLiCoO

AlthoughMnischeaperandenvironmentallymore

2

.

594Chem.Mater.,Vol.22,No.3,2010GoodenoughandKim

benignthanCo,ithasprovennecessarytosubstitutesome)

LiandNiforMntosuppressLiorderatLipolyanionforoxygenopensthehostframework.More-

0.524

[Mn]Oand

dissolutionofMntotheelectrolyteonrepeatedcharge/over,theobrvationofa0.6VincreaintheVfrom

dischargecycling;theresultingfurtherlossofcapacityisLiFe(MoO)toLiFe(SO),eachoperatingon

onlypartiallyregainedbytheabilitytoreplacesometheFe/Fecouple,demonstratedthatsignificanttun-

oxygenwithfluorine.

68,69

Another3DframeworkistheM(XO)structureofthroughtheinductiveeffectbychangingthecounter-

243

hexagonalFe(SO),whichconsistsofclustersoftwocationinthepolyanion.Withthisframework,itwas

243

MOoctahedrabridgedbythreecorner-sharing(XO)

64

tetrahedra;theoctahedraoftheclusterssharecorners

withthetetrahedraofneighboringclusterstocreatean)with(SO).

open3Dhoststructurecapableofacceptingupto5Li

atomsperformulaunitintoitsinterstitialspace;e

Figure8.ThisframeworkisreferredtoastheNASICON

structuresinceNa

1þ3x21-xx43x4

Zr(PSiO)wasshowntoLiforLiFePO,0exe1,asaresultofasmall

700

supportfastNa-ionconduction,i.e.tobeaNA-ion

þ

SuperiorIonicCONductor.Substitutionofan(XO

4

71

oc

3þx2433þx243

3þ2þ

ingoftheenergyofaredoxcouplecanalsobeachieved

possibletodeterminetherelativeenergiesofveralredox

couplesinanoxide,Figure9,andhowtheshifttogether