Li–O2-and-Li–S-batteries-with-high-energy-storage

NATUREMATERIALS|VOL11|JANUARY2012|(energyperunitweight)andenergydensity(energyperunitvolume).Sincetheirintroductionin1991,Li-ionbatteries(Fig. 1)havetransformedportableelectronicdevices1–4.Newgenerationsofsuchbatterieswillelectrifytransportandfinduseinstationaryelectricitystorage.However,evenwhenfullydeveloped,thehighestenergystoragethatLi-ionbatteriescandeliveristoolowtomeetthedemandsofkeymarkets,suchastrans-port,inthelongterm.ReachingbeyondthehorizonofLi-ionbat-teriesisaformidablechallenge;itrequirestheexplorationofnewchemistry,especiallyelectrochemistry,andnewmaterials2,5,6.Therearefewoptions.Two,basedonlithium,arereceivingintenseinterestatthepresenttimeandwillbediscussedinthisReview:rechargeableLi–air(hereafterreferredtoasLi–O2asO2isthefuel)andLi–Sbat-teries7.Otheroptions,especiallyZn–air,havebeenreviewedindetailrecentlyelsewhere8–14.AlthoughLi–O2andLi–Ssharethesameanode,andhaveactivecathodecomponents(O2andS)thatarenear-estneighboursingroup 16oftheperiodictable,thereareimportantdifferencesrelatedtothedifferentchemistryofOandSandthediffer-entstatesofmatteroftheircathodes.Li–Shasbeeninvestigatedsincethe1940s;theproblemsareformidableandextensiveeffortshavebeenmadetoaddressthemovertheintervening70 years.Importantadvanceshavemaderecently,butsignificantchallengesremain7,15–24.Incomparison,Li–O2,especiallywithanon-aqueouselectrolyte,hasreceivedmuchlessattentionuntilrecently7,8,25–30.AsinthecaseofLi–S,majorchallengeswillhavetobesolvedifLi–O2batteriesaretosucceed.TherenaissanceofinterestinLi–SandtheupsurgeofinterestinLi–O2,basedonaqueousandnon-aqueouselectrolytes,reflectstheneedforelectrochemicalenergy-storagedevicesthatcanofferaleapforward;forexample,deliveringelectricvehicleswithadrivingrangeapproachingthegoalof~500 kmbetweencharging.Inthelimitedspaceavailable,wecannothopetoreviewalltheexcellentworkthathastakenplaceonthesetwobatterytechnologies.Instead,weshallbeginbyconsideringtheenergythatcanbestoredinLi–O2andLi–Scells,andthenexamineeachsystem,howitoperates,andthechallengesfacingresearchthatattemptstoadvanceLi–O2andLi–Sbatteries.Paramountamongthepresentchallengesisafunda-mentalunderstandingofthechemistrytakingplaceinthecellsandthediscoveryofnewmaterials.Li–O2andLi–SbatterieswithhighenergystoragePeterG.Bruce1*,StefanA.Freunberger1,LaurenceJ.Hardwick1†andJean-MarieTarascon2Li-ionbatterieshavetransformedportableelectronicsandwillplayakeyroleintheelectrificationoftransport.However,thehighestenergystoragepossibleforLi-ionbatteriesisinsufficientforthelong-termneedsofsociety,forexample,extended-rangeelectricvehicles.TogobeyondthehorizonofLi-ionbatteriesisaformidablechallenge;therearefewoptions.Hereweconsidertwo:Li–air(O2)andLi–S.TheenergythatcanbestoredinLi–air(basedonaqueousornon-aqueouselectrolytes)andLi–ScellsiscomparedwithLi-ion;theoperationofthecellsisdiscussed,asarethesignificanthurdlesthatwillhavetobeovercomeifsuchbatteriesaretosucceed.Fundamentalscientificadvancesinunderstandingthereactionsoccurringinthecellsaswellasnewmaterialsarekeytoovercomingtheseobstacles.ThepotentialbenefitsofLi–airandLi–Sjustifythecontinuedresearcheffortthatwillbeneeded.EnergystoragefromtheorytopracticeThetheoreticalspecificenergies(gravimetricenergydensities)andenergydensities(volumetricenergydensities)forLi–SandLi–O2aregiveninTable 1,wheretheyarecomparedwiththoseforLi-ionandZn–air.Thevaluesarebasedonthecellreactionsincolumn1,thatis,theenergyobtainedperunitmassorperunitvolumeoftheactivecomponentsoftheanodeandcathode.Oftenavalueof11,586 Wh kg−1isquotedfortheLi–O2cell;howeverthisisbasedonthemassofLialone.Allmetal–aircellsgainmass(O2)astheydis-charge,sothemassofO2shouldbeincluded,asitisinTable 1.TheleapforwardintheoreticalspecificenergyonmigratingfromLi-iontoLi–SandthenLi–O2isclear.ItarisesbecauseLi2S,Li2O2andLiOHinthecathodestoremoreLi,andhencecharge,thanLiCoO2perunitmass,andLimetalstoresmorechargeperunitmassthanagraphite(C6Li)anode.ThetheoreticalenergydensityisalsogreaterforLi–O2andLi–SthanLi-ionbutthegainisnotasgreatasforspecificenergy.Ofcoursethereisalwaysasignificantreductionintheenergystoredinabatteryonmovingfromtheorytopractice.Acompari-sonofpracticalspecificenergiesforseveralrechargeablebatteriesispresentedinFig. 2.ThevaluesforestablishedtechnologiesarewellattestedbutforLi–O2areatbestveryroughestimates,becausesofartherearefewrealisticprototypesonwhichtobasesuchfig-ures.However,thevaluesquotedareinlinewiththosereportedbyothers16,26,28.SeveralfactorsconspiretolowertheenergystorageofpracticalLi–SandLi–O2batteries.Thecathodeineachcasecon-sistsofaporousconductingmatrix(usuallycarbon)inwhichthedischargeproductsform,thusaddingmassandvolumetothecell.MoreLimetalthanisrequiredforthestoichiometricreactionhastobeincluded,tocompensatefortheinefficiencyofLi-metalcycling.TheeffectofthesefactorsisillustratedinBox 1forthecaseo