Na ion battery-Review-Angew Chem(3)

SodiumIonBatteries

AngewandteChemiethatNa+migrationbarriersmaybelowerthanthecorrespondingLi+migrationbarriersinlayeredoxides.DFTstudieson

Na0.44MnO2,[90]

P2-Na2/3[Ni1/3Mn2/3]-O2,[91]andNaxCoO2[92]illustratetheimportanceofstructure.ThestudybyLeeetal.predictedhigherNa+diffusivityfortheP2-typecom-poundthanthecorrespondingO3-typeLicompound.[91]InaveryrecentarticleinvolvinganabinitiocomputationalbasedstudyofP2-andO3-NaxCoO2,Moetal.gainedcomprehensiveinsightintoconcen-tration-dependentNa+diffusivityinthosestructures(Figure12).[93]BoththeP2andO3structuresshowgoodNa+mobilityoverawiderangeofNaconcentrations,althoughP2outperformsO3exceptatthehighestconcentration.

Tripathietal.usedatomisticFigure11.SEMimagesofa)high-quality(HQ)nanocubesofNapotentialbasedmethodstoexplore

0.61Fe[Fe(CN)6]0.94&0.06andb)low-profilesofHQ-NaFeover

Na+quality(LQ)Na0.13Fe[Fe(CN)6]0.68&0.32.c)Galvanostaticdischarge/chargemigrationinthepolyanion20cycles.d)ComparisonofthecyclingperformanceofHQ-NaFeandLQ-NaFe.Adaptedfrom

materialsolivineNaMPO4(M=Ref.[86]withpermission.Copyright2014,RoyalSocietyofChemistry.

Mn,Fe)andNa2FePO4F,therebyconfirmingthe1DnatureofNa+mobilityintheolivineandhighNa+

capacityabovethatexpectedforextractionoftwoNa+ionsmobilitythrougha2Dnetworkintheacplaneoflayered

fromavacancy-freematerial(170mAhgà1

)arisesfromNa2FePO4F.[94]AtomicpotentialbasedsimulationstudiesreductiontoNa3MnII[MnI(CN)6]oninitialdischarge,whichrevealedthestabilityoftheNa2MP2O7(M=Mn,Fe)frame-allowsforfullcyclingof3Na+

ions.worktowardsoxygenevolution,alongwithacurvedpathfor

4.ComputationalStudiesofNaIonMobility

Synergisticexperimentalandcomputationalapproachestomaterialsdevelopmentarerapidlybecomingthenormtoacceleratematerialsdiscoveryandcharacterization,andtogainfundamentalinsightintostructure–propertyrelation-ships.

Arecentcomprehensivereviewhasaptlysummarizedthepowerofcomputationalapproachesingainingafundamentalunderstandingoftheatomic-scalepropertiesofpositiveelectrodematerialsforLIBsandNIBs.[88]Twomaintypesofcomputationaltechniquesaretypicallyadopted:electronicstructureinvestigationsbasedonDFTandHartree–Fockmethods,andmethodsbasedoninteratomicpotentials.DFTmethodshavebeenappliedtooxideinsertionmaterials.ThedifferencebetweenNaandLiionsinavarietyofcompoundsishighlightedwithrespecttovoltage,phasestability,andactivationenergyforionmobility.[89]TheauthorsdemonstratethatthegenerallylowercalculatedvoltagesforNacom-poundsareduetothesmallerenergygainobtainedfrominsertingNaintoahoststructurerelativetothatofLi.TheFigure12.a)ArrheniusplotofNadiffusivityinP2andO3NaxCoO2,differences,typicallybetween0.18and0.57V,maybeandb)Activationenergy(Ea)anddiffusivityat720KasafunctionofespeciallyadvantageousforthedesignofnegativeelectrodeNaconcentrationxinP2-andO3-NaxCoO2obtainedbyfirstprinciplescalculations.AdaptedfromRef.[93].Copyright2014,AmericanChem-materialsforsodiumbatteries.Theircalculationsalsoshow

icalSociety.

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3DNa+diffusionparalleltothea,b,andccrystallographicdirectionswithalowactivationenergy(%0.5eV).[95]Abini-tiocalculationsonthereversibleandhighratecathodehostNa2Fe2(SO4)3(seeSection3.2)gavequantitativeinsightsintothefastNa+dynamicsinitsopenframeworkstructure.[75]Aliquid-likeactivationenergyvalueof0.14eVwascalculatedfordefectdiffusionalongtheNa3channelsoftheC2/cstructure.Basedonthisstudy,1DNa+conductionwaspostulatedalongthecaxisforboththeNa2andNa3sites,whereasNa1canbeextractedthroughtheNa3site.Thus,alltheNaionsareextractable,withnolimitonachievingthetheoreticalspecificcapacity.

ComputationalapproachesunderlinetheimportanceofthreecrucialconsiderationsinNIBcathodematerialsdesign:1)asmallvolumedifference(<5%)betweentheendmembersforNa+de/intercalation,2)alowactivationenergyforNa+transport,and3)preferablycompleteabsenceofantisitedefectsforgoodcapacityretentionandratecapability.ThedifferentinteractionofthelargerNa+ions(comparedtoLi+)withliquidelectrolytes—includingionmobilityintheelectrolyteandthethermodynamicsofdesolvationatthecathodeinterface—isalsoofimportanceandrelevanttocomputationalstudies,althoughthistopicisbeyondthescopeofthisReview.

L.F.Nazaretal.

representsasafetyhazard,however,sincetheinsertionpotentialisveryclosetothesodiumplatingpotential.AporoushardcarbonpreparedbasedonasilicatemplatingapproachreportedbyWenzeletal.showsapossiblesolu-tion.[103]Thehighporosityandadvancedmicrostructureenhancedthehighratecapability,therebyresultinginacapacityof180mAhgà1forthefirstcycleatarateofC/5.Thiscarbonoutperformedseveralotherindustrialcarbons.

Veryrecently,theinsertionofsodiuminexpandedgraphitewasreported.[104,105]Thespacingofthegraphenelayersstronglyinfluencesthereversiblecapacity.Itincreasesfrom25mAhgà1ingraphite(d=3.4??)to136mAhgà1[104]or174mAhgà1[105]inhighlyreducedgrapheneoxide(d=3.7??),reaching280mAhgà1attheoptimalspacingof4.3??.[104]Theoptimizedmaterialexhibitsastablereversiblecapacityofabout280mAhgà1atcurrentdensitiesof20mAgà1(carbon),droppingto180mAhgà1whentherateisincreasedto100mAgà1(Figure13).Thecapacityretentionismorethan

5.AnodeMaterials

Sodiummetalanodesarecommonlyusedonthelabo-ratoryscaletoevaluatetheperformanceofcathodematerials.However,theformationofdendritesandthesafetyissuesrelatedtosodiummetalcurrentlypreventitsuseasanegativeelectrodeforcommercialapplications.Thus,thesuccessofNaionbatteriesisstronglydependentonthedevelopmentofsafeandefficientanodematerials.

Figure13.Short-termcyclingstabilitytestforgraphite(PG,d=3.4??),grapheneoxide(GO,d=6.1??),reducedgrapheneoxide(EG-1h,

d=4.3??),andreducedgrapheneoxide(EG-5h,d=3.7??)atacurrentdensityof20mAgà1.ReproducedfromRef.[104]withpermission.Copyright2014,NaturePublishingGroup.

5.1.Carbons

Sodiumatomsdonotintercalatesignificantlyingraphiticcarbons,whicharethebasisofthemostcommonnegativeelectrodeinLiionbatteries.[96–98]Asaresult,hard(disor-dered)carbons—forexample,preparedbythepyrolysisofsucrose—arethemoststudiednegativeelectrodematerialsforNaionbatteries.Theyachievereversiblecapacitiesupto300mAhgà1.[99–101]TheelectrochemicalpotentialofNainser-tionintohardcarbonisclosetothatofthemetalitself,whichindicatesthatthereisverylittlecarbon-to-sodiumchargetransfer.ThisisasignatureofamechanismwherebyNaionsfilltheporositygeneratedbythedisorderedhardcarbonlayersratherthanintercalatebetweengraphiticsheets.[98]ThegoodperformanceofhardcarbonasanegativematerialforNaionbatterieswasrecentlydemonstratedinfullcellswithO3-NaMn0.5Ni0.5O2asthepositiveelectrode.[99]Despitesomeelectrolytedecomposition,over70%oftheinitialcapacityismaintainedafter50cyclesatmoderatecurrentdensity,andsimilarresultsareobtainedathighcurrentdensity(300mAgà1(carbon)).[102]Operatinghardcarbonanodesatahighcurrentdensityand/orhighdepthofdischarge

70ˉter2000cycles,whichsuggeststhattheexpandedgraphitenegativeelectrodeshaveveryencouragingagingproperties.Theinsertionpotentialofthesematerialsisaslopingcurvefrom1.5Vto0Vwithmorethan80%ofthecapacityunder1V.Thehighervoltagecomparedtohardcarbonincreasesthesafetyattheexpenseoftheenergydensity.

5.2.LowPotentialTransition-MetalOxidesandPhosphatesAnodesbasedontheinsertionofsodiumintotransition-metaloxidesareofparticularinterest,wherebenefitsarisefromgoodsafetyfeaturesandhighvolumetricenergydensities.SodiuminsertionintoNASICON-typeNaTi2(PO4)3at2.1VwasreportedbyParketal.toundergoatwo-phasemechanism.[106]TheobservedcapacitiesforNaTi2(PO4)3in

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AngewandteChemiebothnon-aqueousandaqueouselectrolytescorrespondto75mAhgà1at5C.Inbothcasesthecapacityretentionisover90%ofthetheoreticalcapacityof133mAhgà1.Theimprovedathigherrates,thussuggestingparasiticreactionspolarizationobservedintheaqueouselectrolyteoncyclingbecauseofthelargesurfacearea.Approachestocreatewassubstantiallysmallerasaresultofitslowerimpedanceimprovedarchitectureswhichhavebeenreportedfortheandviscosity.TheNa+insertion/extractionpotentialislithium-insertionanodematerial,TiNb2O7,[113]mayprovelocatedatthelowerlimitoftheelectrochemicalstabilitybeneficialforitsapplicationinNaionbatteries.

windowoftheaqueousNa2SO4electrolyte,whichmakesThesuperiorcapabilitiesofcathodesbasedontheP2NaTi2(PO4)3anattractivenegativeelectrodeforaqueouslayeredoxideledtotheinvestigationofthesamestructureforsodiumionbatteries.[13]

negativeelectrodeapplications.Thus,Na0.66Li0.22Ti0.78O2,[114]Titanatesandvanadatesusuallyexhibitgoodstabilityatwithlithiuminthetransition-metalsite,showsareversiblelowvoltage,thatis,resistanceagainstaconversionmecha-capacityofapproximately120mAhgà1atC/10withannism.VanadiumlayeredoxidessuchasO3-NaVO2,[4,107]andaveragevoltageof0.7VversusNa/Na+.ThismaterialexhibitsP2-Na0.7VO2,[108]showhighlyreversiblesodiumintercalationgoodratecapabilitywith75mAhgà1at1Cand75êpacityassociatedwithcomplexstructuralevolutionaselucidatedbyretentionafter1200cycles.Thegoodperformanceisattrib-Delmasandco-workers,butthevoltagesrangingfrom2.5Vutedtotheverysmallvolumechange(0.8%)uponsodiumto1.5Varetoohighforapracticalanodematerial.Mostofinsertion.Toincreasetheelectronicconductivityofthethecurrentresearchinterestfocusesontitanates,whichlayeredtitaniumoxidesandachieveefficientbulkstoragegenerallypresentacceptablevoltagesbutpoorelectronicofsodium,FieldenandObrovacusedNi2+ratherthanLi+toconductivitythathindersbulkstorageofsodium.Sodiumsubstitutefortitaniuminthestructure.[115]ByusingthisintercalationintoamorphousTiO2nanotubesshowsthattheirstrategytheyreportedareversiblecapacityof110mAhgà1atsizeiscriticaltoobtainingsignificantNaintercalation.[109]TheacurrentrateofC/10witharelativelyflatpotentialaroundimprovedpropertiesoflargertubes(innerdiameter>80nm,0.7VversusNabyusingacompositeelectrodebasedonwallthickness>15nm)wasattributedtoalargernumberofNa0.6Ni0.3Ti0.7O2withonly10êrbon,whichsignificantlyNa+chargecarriersbecauseofthegreatervolumeofelectro-improvedtheenergydensitycomparedtoothertitanates.Thelytecontainedwithinthetube.Na2Ti3O7exhibitsaparticularlyratecapabilityispoor,however,probablyduetothelargelowdesirablepotential.[110]Theinsertionoftwoadditionalcrystallitesize(10mm).sodiumatoms(180mAhgà1)occursatareversibleplateauaround0.3VversusNa/Na+(Figure14).However,toachieve5.3.Alloys

thiscapacity,aslowcyclingrate(C/25)andacompositeelectrodewith30êrbonblackarenecessary.InadditiontoAsnotedbyChevrierandCeder,[116]muchlessresearchdecreasingtheenergydensity,carbonblackisresponsibleforhasbeencarriedoutonsodiumalloymaterialsasnegativethelargeirreversiblecapacitywhichisofthesameorderofelectrodesforsodiumionbatteriesthanfortheirlithiummagnitudeasthereversiblecapacityobservedonthefirstcounterparts.Alloyshaveveryhighenergydensitiesandlowcycle.Theratecapabilityhasbeensignificantlyimprovedbyredoxpotentials,twohighlydesirableproperties.Thepoten-reductionoftheparticlesizebyeitherhydrothermalsyn-tialsforthesereactionshavebeenpredicted,[116]andshowthesis[111]toobtain8nmparticlescapableofreversiblepromise.Siliconandgermanium,thetwomostpromisingcapacitiesupto110mAhgà1at4C,orhigh-energyballanodematerialsforLiionbatteries,donotintercalatesodiummilling[112]toreducetheparticlesdownto100nmandachieve

atroomtemperature;leadandbismuthachieveinitialsodiationclosetothetheoreticalvaluesbutpresentverypoorcyclability[117,118]asexpectedbecauseoftheirvolumeexpansionof250%and365%,respectively.Thestraincausedbythelargevolumeexpansionuponinsertionisproblematic,evenwithlessthanfulldischarge.Carefuldesignofthemicrostructureoftheelectrodeisthusrequired,whichhasanimpactonboththeenergydensityandcostofmanufacture.Theseissueshavethusfarinhibitedthedevelopmentofsiliconanodesforlithiumbatteries,andthelargersizeofsodiumwithrespecttolithiummakesthedesignofsuchanodesforsodiumionbatteriesevenmorechallenging.[119]

Withanaveragevoltageof0.3VversusNaandatheo-reticalcapacityof790mAhgà1,tinisamorepromisingcandidate.Despiteavolumeexpansionof420%,compositeelectrodesoftinpowderwithapolyacrylatebindershowFigure14.Voltageversuscompositionprofilefortheelectrochemicalareversiblecapacityof500mAhgà1over20cyclesataslowreductionofablankelectrodecontainingonlycarbonblack(bluecyclingrate.[117]Wangetal.usedinsitutransmissionelectroncurve)andacompositeelectrodecontainingNamicroscopytoverifythattheSnparticleswereableto2Ti3O7and30%

carbonblack(redcurve),wherethereversibleinsertionofca.2molofaccommodatethevolumeincreasewithoutcracking.[120]sodiumionspermolNa2Ti3O7isobserved.Reproducedfrom

CarefulanalysisbyinsitutechniquesrevealedthattheRef.[110]withpermission.Copyright2011,AmericanChemicalSociety.

intercalationmechanismisdependentonthephysicalform

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oftheanode:amorphous/nanocrystallinephasesarereportedinthecaseofpowders,[120,121]whereasfoilstendtoshowmuchhighercrystallinity.[122]Inbothcase,kineticeffectsarepredominantandpreventtheformationofthethermody-namicallystablephasespresentintheNa-Snphasediagram.

Onapositivenote,alargereversiblecapacityof600mAhgà1over160cyclesatarateofC/10withanaveragevoltageof0.8VversusNawasreportedformicrocrystallineantimony.[123]Theratecapabilityofthismaterialisexcellent,showingareversiblecapacityof500mAhgà1atarateof4C.Thecompositeelectrodeformulation—amixtureofcarbonblackandvapor-groundcarbonfibersastheconductiveadditive(15%)andcarboxymethylcelluloseasthebinder(15%)—ispartlyresponsible.Itisinterestingtonotethatthesodiumcellsperformbetterthantheirlithiumcounterparts.Apossibleexplanationresidesinthefactthat,incontrasttolithium,sodiuminsertioninantimonygeneratesanamor-phousintermediatephasethatisbelievedtoreleasepartofthestrainassociatedwiththeintercalationprocess.

Theinsertionofsodiumintoamorphousphosphorus[124]achievesaremarkablecapacityof1500mAhgà1duringfirstcyclesatC/10,withanaveragepotentialof0.6VversusNa.About1000mAhgà1isretainedafter80cycles.Adecreaseinthereversiblecapacityisobservedathigherrates,with“only”1000mAhgà1availableat1C.Thepromisingelectrochemicalpropertiescanbeattributedinlargeparttothehigh-energymillingofredphosphorus(70%)andsuperPcarbon(30%)thatresultsinanamorphouscomposite.Theinsertionofsodiumintocrystallinephosphorusyieldsamuchsmallercapacity,thusemphasizingtheimportanceoftheamorphousphasetoreleasethestrainarisingfromintercalation.

Inallthestudiesshowinggoodcyclability,[117,123,124]theadditionoffluorinatedethylenecarbonate(FEC)wasthekeytogreatlyenhancedcapacityretention.Inthesesystems,where“fresh”surfaceisconstantlyexposedtotheelectrolytebyelectrochemicalgrinding,thepassivationoftheanode–electrolyteinterphaseisoftheupmostimportance.ItisfortunatethatFEC,initiallyreportedasanadditiveforhardcarbon,workswellwithsomanydifferentsystems.

L.F.Nazaretal.

themedia),andtheionicdissociationofthesalt(dielectricconstantofthemedia).

Theelectrolytesofinterestforroom-temperatureNaioninsertionbatteriescanbeclassifiedinto1)non-aqueouselectrolytesconsistingofasodiumsaltsolubilizedinanorganicsolventand2)ionicliquids(ILs)consistingofanorganicsalt(R+Xà)dopedwithafractionofthesodiumsaltequivalent(Na+Xà).DespitesignificantprogressinNa+solidelectrolytes,whichshowpossibleapplicationsatroomtemperature,[125]theirconductivitiesarestill1–2ordersofmagnitudeloweratroomtemperaturethanthemoretradi-tionalliquidelectrolytesatpresent,thusleavingmuchscopeforfurtherdevelopment.

6.1.Non-AqueousElectrolytes

Allcurrentnon-aqueouselectrolytesforNaionbatteriesarebasedoncarbonatesolvents,suchasethylenecarbonate(EC)andpropylenecarbonate(PC)becauseoftheirveryhighdielectricconstants,largeelectrochemicalwindows,andlowvolatilities.Ofthesodiumsaltsstudied—sodiumbis(tri-fluoromethane)sulfonimide(NaTFSI),sodiumtriflate(NaOTf),sodiumperchlorate(NaClO4),andsodiumhexa-fluorophosphate(NaPF6)—thelattertwoareoptimum.Asexpectedfromcarbonatesolvents,theiroxidativestabilityisgood,andcompatibilitywiththepositiveelectrodeisnotamajorconcern.ThisisillustratedinthecaseofNaCrO2,whereCoulombicefficienciesgreaterthan99.70%weremeasuredafter50cyclesinsymmetriccells(NarichCrO2/NapoorCrO2)witheitherNaTFSIorNaPF6inallofthecombinationsofPCanddiethylcarbonate(DEC)tested.[126]Manyotherstudiesreportsimilarresultswithbothlayeredoxideandpolyanionpositiveelectrodes.[127–130]Thecorrosionofaluminum,whichstartsat3.3to3.5VversusNa/Na+,isaproblemforNaOTfandNaTFSIsalts(asfortheirLicounterparts).Sodiumperchlorateorhexafluorophosphateallowanincreaseofthevoltagewindowupto5V;[126,130,131]evensmallquantitiesofNaPF6efficientlypassivatealuminumsurfaces,therebyallowingtheuseoftheTFSIanion.[126,131]Forthesamesolventandsaltconcentration,theionicconductiv-ityisslightlyhigherforNaPF6-basedelectrolytesthanNaTFSIelectrolytes,withNaClO4beingintermediate.[130,131]Thisisexplainedbythelowerpolarizingnatureofthehexafluorophosphateanion.[132]Optimizationofthesolventcompositiontolowerviscosityresultsinconductivitiesofabout12mScmà1,whichiscomparablewiththebestLielectrolytes.[131]

Hardcarbonistheonlyanodematerialthathasbeensystematicallystudiedagainstdifferentelectrolytes,buttheresultsarecontradictory.Ponrouchetal.reportverylittleinfluenceofthesalt(NaClO4orNaPF6)onboththecyclingperformanceofhardcarbonelectrodesandthenatureoftheSEIinPC,EC/PC,andEC/PC/DMCmixtures,[127]andaparticularlyhighirreversiblecapacitywasfoundinthecaseof1mNaClO4inEC/DMC(50:50wt%).[131]AnotherreportshowsgoodretentionofcapacityinEC/DMC(50:50vol%)withbothNaPF6andNaClO4,andwithamuchsuperiorbehaviorofthelattersaltinPCsolutions.[133]

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6.Electrolytes

Theinterfaceoftheelectrolytewithboththepositiveandthenegativeelectrodesmeansitschemicalstabilityrequire-mentsconsiderablylimittheusablescopeofmaterial.Chemicalcompatibilityisensuredthroughtheformationofpassivationlayers,referredtoasthesolidelectrolyteinter-phase(SEI).Theformationandphysicalpropertiesofthoseprotectivelayersdependonthenatureoftheelectrode(especiallythenegative),thusimplyingthatthestudyofelectrolytesisnoteasilydecoupledfromtheelectrodes.Forsodiumionbatteries,sincethereisnowidelyacceptednegativeelectrode,systematicelectrolytestudiesarerela-tivelyscarce.Inadditiontoprovidingastableinterface,anelectrolytemustachievegoodionicconductivity.Thisparam-eterismostlyaffectedbytheconcentrationofthechargecarrier(solubilityofthesalt),theionicmobility(viscosityof

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SodiumIonBatteries

AngewandteChemieThediscrepanciesshowthehighdegreeofsensitivitytotheretentionof87ˉter100cycleswasreported,comparedtonatureoftheelectrodeandhighlightthenecessityofmoreonly62%fortheliquidelectrolyte.

worktofullycharacterizethesesystems.

SubstitutionoftheTFSIanionbytheFSI(bis(fluorosul-AstudyonadditivesforNaClO4inPChasshownthatfonyl)imide)anionallowsafurtherdecreaseofthetemper-vinylenecarbonate(VC)—animportantadditiveinLiature.Half-cellsassembledwithNa0.45Ni0.22Co0.11Mn0.66O2aselectrolytes—doesnotefficientlypassivatesodiatedhardthepositiveelectrodeandeitheraNaTSFI-doped[C4mpyr]-carbonelectrodes.[129]ThesamestudyidentifiedFECas[FSI]ionicliquidelectrolyteor0.5mNaPF6inaPC-basedavery-efficientfilm-formingadditive.Mostinterestingly,thenon-aqueouselectrolytewerecompared.ThereversiblesameadditivewassuccessfullyusedwithavarietyofdifferentcapacityatarateofC/10isslightlyhigherintheILcase,[142]anodematerials,thusshowingitsgoodversatility.[134–136]

withagoodretentionof80%ofthecapacityafter100cycles,muchhigherthanthenon-aqueouscell.

Otherpromisingionicliquidshaveyettobetestedfor6.2.IonicLiquids

batteryapplications.[143]Thepublishedresultsshowthatwhensafetyisamoreimportantparameterthanratecapability,ionicToimprovethesafetyofbatteries,muchefforthasbeenliquidelectrolytesareasuperiorchoicetotraditionalnon-devotedtoreducingthevolatilityandflammabilityoforganicaqueouselectrolytes,evenforroom-temperatureapplications.

liquidelectrolytes.Inthisregard,ionicliquids(ILs)mayhelptoaddressthesafetyproblemastheyarepracticallynon-flammable(Figure15).

7.SummaryandOutlook

Asaconsequenceofconcernsoverthefuturecostoflithiumandthesustainabilityoftheresources,thereisnowaglobaltrendtodeveloplow-costbatterieswithhighenergydensitiesthatcanmeetavarietyofemergingapplications.Sodiumionbatterieshavere-emergedaspromisingcandi-dates,especiallyforstationaryenergystorage.NewdirectionshaveevolvedbeyondmoltenNacellsbasedonNa-SandNa-NiCl2thatweredevelopedinthelastdecade.TheyincludeFigure15.Flammabilitytestsofa)conventionalorganicelectrolyteandaqueousNaioncells,andthefast-developingfieldofnon-b)BMP-TFSIILelectrolytewith1mNaTFSI.ReproducedfromaqueousbatteriesthatcouldemployaplethoraofnewlyRef.[138]withpermission,Copyright2014,TheRoyalSocietyofdiscoveredmaterialsforthepositiveandnegativeelectrodesChemistry.

aswellastheelectrolyte.Theseoffernotonlyadvantagesofelementsustainability,butpresentstructuralandelectro-However,ILsusuallyrequirehigheroperatingtemper-chemicaldifferencescomparedtotheirLianaloguesthatcanaturesthanliquidelectrolytesbecauseoftheirhighviscosity,behighlybeneficial.Thisopensupafascinatingareaofsolid-andmeltingpointswhichareoftenjustbarelyaboveroomstateelectrochemistry,whichisripeforexploration.

temperature.TheonlyreportofusinganILelectrolytewithIntermsofambient-temperaturecellsthatoperateontheapracticalanodematerialsuchashardcarbonshowsgoodbasisofintercalationchemistry,thelasttwoyearshavecompatibilitywithaNaTFSI-dopedN-methyl-N-propylpyr-witnessedseveralbreakthroughsinthedevelopmentofrolidiniumbis(fluorosulfonyl)imide(NaFSI-C1C3pyrFSI)layeredsodiumtransition-metaloxidesaspositiveelectrodeelectrolyteat908C.[137]Apartfromthis,moststudiesinvolvematerials,suchasP2-Na2/3[Fe0.5Mn0.5]O2.Theseresultssug-Nametalanodes.ILssuchas1-butyl(propyl)-1-methylpyrro-gestthesesodiumcompoundscancompetewiththeirlithiumlidiniumbis(trifluoromethylsulfonyl)imide([C4(C3)mpyr]-analoguesintermsofenergydensity.Theseexcitingadvances[TFSI])dopedwithNaTFSIareabletoreversiblystripandsuggestthepromiseoflarge-scalecellsevenforvehicledepositsodiumatroomtemperature.[138–140]Amoderateelectrification.Importantly,sodiumandlithiumcompoundsoverpotentialofonly0.2Vforsodiumelectrodepositionarenotstrictlyortypicallyisostructural.Differentelectro-wasmeasuredatroomtemperature;tracesofwaterinthechemicalpropertiesresultandintriguingdifferenceshaveelectrolyteweredetrimental.[138,139]Avoltagewindowof5.2VemergedbetweentheLi+andNa+layeredmetaloxides.versusNa/Na+wasreportedtogetherwithgoodcompatibilityThesearetheconsequenceofthelargerNa+ion,whichresultswiththelayeredoxidecathodematerialNaCrO2.[140]

inmorehighlyorderedsystemsandlessalkali-metal/tran-Compatibilityofthepolyanioncathodematerialsition-metalmixingoncycling,whichisbeneficial.Adraw-NaFePO4withtheNaTFSI-doped[C4mpyr][TFSI]electrolytebackisthatmanyofthelayeredsodiummetaloxidesexhibitwasalsodemonstratedinhalfcells.[141]Comparisonwithonlymoderatecyclingstabilityatpresent,attributedtolargeasimilarcelloperatedina1mNaClO4inanEC/DECvolumechangesonredoxandaproclivitytoP(n)–O(n)phaseelectrolyteshowsthatalthoughthenon-aqueouselectrolytetransitionsinducedbyglidingofthetransition-metalMO2performsmuchbetteratroomtemperature,theILelectrolytelayers.ThisresultsfromtheabilityofNa+toadoptbothdisplaysverysimilarcapacityandratecapabilityattemper-octahedralandprismaticcoordination,whichisnotthecaseaturesaslowas508C.InadditiontotheimprovedthermalforLi+.AlthoughtheseglidingtransitionsarenotfullystabilityoftheILelectrolyte(>4008C),amuchbettercycle

understoodatpresent,theydorepresentanon-topotactic

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