.AngewandteReviews
structurebyin-planecationrearrangement),itisproposedthatchargecompensationinthismaterialisfulfilledbypartialremovalofoxygenfromthestructure.ThismaterialshowshighpromiseforNaionbatteryapplicationsbecauseofitshighspecificcapacity,moderatecapacityretention,andalsorelativelyhighsodiumcontentwhichisadvantageousforuseinafullNaioncell.
Bruceandco-workersinvestigatedP2-Na0.67Mn1àyMgyO2(y=0,0.05,0.1,0.2)madeofearth-abundantelements.[39]ElectrochemicalstudiesdemonstratedthatsubstitutionofMnwithMg(upto25%)enhancesthecapacityretention,decreasesthepolarization,andsuppressesthephasetransi-tionsthatpureP2-Na0.67MnO2undergoesduringthecharge/dischargeprocess.Theeffectofthecoolingrateduringsynthesisonthecrystalstructureandelectrochemicalperfor-manceofthecompoundswasalsoexamined.BothMgsubstitutionandslowcoolingsuppresstheorthorhombicdistortionbyincreasingtheaverageoxidationstateofMn—orinotherwords,decreasingtheconcentrationofJahn—Teller-activeMn3+ions—whichleadstoimprovedcyclabilityattheexpenseofaslightlossofcapacity.P2-Na0.67Mn0.8Mg0.2O2deliversaninitialdischargecapacityof150mAhgà1between1.5and4Vwithanexcellentretentionofcapacityof96%over25cycles.Ontheotherhand,Komabaandco-workersreportedasurprisinglyhighdis-chargecapacityforasimilarcomposition,Na0.67Mg0.28Mn0.72O2,whenchargedtohighervoltages.[40]Thismaterialdeliversadischargecapacityof220mAhgà1between1.5and4.5V,beyondthecapacityadaptedfromtheMn3+/Mn4+redoxreaction,althoughfadingoccursoncycling.Alargefractionofthisreversiblecapacityisassociatedwithawell-definedvoltageplateauat4.2V(Figure4).ThisanomalousreversiblecapacityisproposedtooriginatefromactivationoftheredoxreactionoftheoxideionsbyMgions,similartotheeffectoflithiuminLi-richmanganeseoxides,orP2-Na5/6[Li1/4Mn3/4]O2asdiscussedabove.ThisisthefirstreportofactivationbyMg2+.AlthoughtheproblemofvoltagefadingintheLi-andMn-richanaloguesofthesematerialsusedinLiionbatterieshasproventobeunsurmountabletodate,[41]itremainsanopenquestionastowhetherthe
L.F.Nazaretal.
differentstructuralfeaturesofthesodium-based“high-voltage”layeredoxideswillbesufficienttoovercomethisissue.Theconceptshowspromiseforfutureexplorations.
Achievingfullcapacityinsodium-deficientP2phasesinpracticalfull-cellNaionbatteriesrequiressodiumcompen-sationbystrategiessuchaspredischargewithmetallicsodiumortheuseofsacrificialsalts.O3phases,ontheotherhand,havetheadvantageoffullsodiumcontent.Johnsonandco-workershaveinvestigatedtheperformanceofalayeredsodiummetaloxideelectrodeinafullcellbasedonO3-Na-Ni1/3Fe1/3Mn1/3O2/hardcarbon.[42]Thecapacityofthecellislimitedduetoirreversibleprocessesassociatedwiththecarbonnegativeelectrodethatemergefromtheformationofasolidelectrolyteinterphase(SEI)inthefirstcycle.Never-theless,thecelldemonstratesexcellentstabilityandasmoothcharge/dischargeprofilecoupledwithanimpressiveratecapability.Thecelldeliveredaspecificcapacityof100mAhgà1for150cyclesinthevoltagerangeof1.5–4Vatarateof0.5C.ExsituX-raydiffractionanalysisshowedthat
ˉmstructureofthepositiveelectrodematerialwastheR3
preservedafter123cycles.
High-performance,low-costO3layeredoxidesarepartic-ularlydesirable.Yamadaandco-workersinvestigatedtheeffectofNisubstitutionontheelectronicandelectrochemicalpropertiesofO3-NaFeO2.[43]SolidsolutionsofO3-NaFeO2andNaNiO2wereformedoveronlyanarrowcompositionalrange(0.5 y 0.7inNaFe1àyNiyO2).Phaseseparationinothercompositionsisattributedtolargelocallatticestraininducedbysubstitutionofsphericalhigh-spinFe3+byanisotropiclow-spinNi3+ions.Theelectrochemicalperfor-manceofO3-NaFe1àyNiyO2(y=0,0.5,0.7)showedthatsubstitutingironwithnickelionsresultsinanenhanceddischargecapacityandimprovedcyclabilitybutattheexpenseofadecreasedvoltageofthecell(Figure5).ExsituX-raydiffractionrevealedthatsodiumdeintercalationfromNaxFe0.3Ni0.7O2progressesthroughasolid-solutionprocessfor
Figure4.Galvanostaticcharge/dischargecurveofP2-Na2/3Mg0.28-Mn0.72O2inaNacellcycledinthevoltagerangeof1.5–4.4Vatarateof10mAgà1.ReproducedfromRef.[40]withpermission.Copyright2014,TheRoyalSocietyofChemistry.Figure5.Galvanostaticcharge/dischargecurvesofO3-NaFe1àyNiyO2(y=0,0.5,0.7)cycledinthevoltagerangeof2–3.8Vatarateof
30mAgà1.ReproducedfromRef.[43]withpermission.Copyright2014,AmericanChemicalSociety.
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AngewandteChemiemostofthecyclingrange,incontrasttothetwo-phaseprocessWehaverecentlydemonstrated,throughadeliberatestudyinNaxFeO2.ImprovedcyclingstabilityintheNi-substitutedontheuptakeofgasesbyP2sodium-deficientphasessuchaselectrodematerialsisbelievedtobeduetosuppressionoftheNaxMn0.5Fe0.5O2,thatthereactivityismuchmorecomplexphaseseparationinadditiontodecreasingformationofJahn–thanpreviouslythought.[47]
TellerFe4+ions.
Despitetheadvantagesthatlayeredsodiumtransitionmetaloxidesofferforelectrochemicalenergystorageappli-3.2.Sodium“Polyanion”Electrochemistry
cations,theirairsensitivityisachallenge.ThisisanimportantissueintermsofthereproducibilityofresultsfromresearchPolyanionicframeworkcompoundsbasedonphosphates,studiesonthesematerials,inadditiontoconcernsoverfluorophosphates,sulfates,andothernew“polyanion”tetra-storageandhandlingfromalarge-scaleapplicationpointofhedralmotifsofthetypeXO4ortrigonalXO3combinedwithview.ItwasreportedlongagothatwatermoleculescanoctahedralmetalMO6(orothercoordinationcenters)offerintercalateintoP2-Na2/3[CoxNi1/3àxMn2/3](x=1/6,1/3),butvastopportunitiesfordevelopingnovelcathodesystemsfornotintoitslithiatedcounterpart.[44]WhatdifferentiatesNaions.Diverseopen-frameworkstructures,thepresenceofsodiumfromthelithiummetaloxideframeworkarethelow-energyNa+migrationpathways,possibilitiesoftuninglargeemptyprismaticsitesandthelargeinterslabspacetheoperatingvoltagebymodifyingthelocalenvironments,availableintheP2structure.Ontheotherhand,nowaterandfavorablestructuralenergeticsforaflatvoltageresponseintercalationwasobservedforP2-Na2/3[Ni1/3Mn2/3]O2asoffersomecrucialadvantages.Inaddition,theirrobustclaimedinthesamereport.Inamorerecentstudy,Passerinicovalentframeworksrenderthemthermallystableandandco-workersinvestigatedthecorrelationbetweentheensureimpressiveoxidativestabilityathighchargingvol-intercalationofwaterandthesodiumcontentinthemixedtages.Itisnosurprisethatinparalleltooxidematerials,P2/P3structureNaxNi0.22Co0.11Mn0.66O2.[45]ThecellswereaplethoraofpolyanionicsodiumcompoundshavebeenchargedtodifferentvoltagescorrespondingtospecificexploredascathodematerialsforsecondaryNaionbatteries.sodiumcontents,disassembled,andexposedtoairforSomenotableexamplesarediscussedbelow.
analysis.XRDdataofthecathodessuggestthatwaterdoesnotintercalateintothismaterialuntilthesodiumcontent3.2.1.Phosphates,Fluorophosphates,andPyrophosphatesdecreasesbelowathresholdamount.Belowx%0.33,intense(00l)’peaksappear,indicativeofahighlyorderedhydratedNaFePO4,unlikeitscelebratedlithiumanalogueolivine-phasewithlargeinterslabdistances.Inanotherstudy,PrakashtypeLiFePO4,crystallizesinthemorethermodynamicallyandco-workersreportedthestructuralinstabilityofO3-stablemaricitestructure(Figure7a,b),[48]whichhasnofreeNaNi1/3Mn1/3Co1/3O2inanambientatmosphere.[46]XRDpathwaysforthediffusionofNaions.[49,50]Electrochemicallyanalysisshowedthatas-preparedrhombohedralO3-NaNi1/3-activeolivine-typeNaFePO4waspreparedbylow-temper-Mn1/3Co1/3O2undergoesphasetransitionstomonoclinicO1atureLi/NaexchangefromLiFePO4.ThestructuralfeaturesandthenrhombohedralP3phaseswhenagedinair(Figure6).
arepreservedinthisprocess.[51]Itselectrochemicalprofile(Figure8a)exhibitsadistinctintermediatephase,Na0.7FePO4.[52]Despitecalculationsthatsuggestgoodionmobility,[53]itselectrochemicalkineticsareverysluggishcomparedtoLiFePO4.Thisisingreatpartduetoamuchlargervolumechangebetweenthereducedandoxidizedphases.TheexcellentkineticsoftheLiFePO4systemhaverecentlybeenattributedtotheparticipationofasolid-solutionphasewhichisaccessedatrelativelylowoverpoten-tials.[54,55]SuchametastablephaseisunlikelyfortheNaionanalogue.
Themore-promisingNASICONcompoundswithopen3Dframeworks,whicharebuilt-upofcorner-sharingMO6andXO4polyhedra,possesslargetunnelsforthefastconductionofNaions.Thesecompoundswereinitiallyexploredassolidelectrolytes[6]andmorerecentlyasinsertionmaterials.AmongstthevariousNASICONcompounds,Na3V2(PO4)3hasemergedasaninterestingcandidatebecauseofitsimpressiveenergydensity(400Whkgà1)andthermalstabilityinthechargedstate.[56]ItselectrochemicalprofileFigure6.PowderX-raydiffractogramofNaNiexhibitstwovoltageplateausat3.4Vand1.6V,which1/3Mn1/3Co1/3O2showingitsstructuralinstabilityonaginginair.a)Crystalstructureoftheas-correspondtotheV3+/V4+andV2+/V3+redoxcouples,preparedrhombohedralO3phase.b)MonoclinicO1phaseafter
respectively.Onlythehighervoltagecoupleissuitableforexposuretoairfor15days.c)RhombohedralP3phaseafterexposureapositiveelectrode.Owingtoitspoorelectronicconductivity,toairfor30days.ReproducedfromRef.[46]withpermission.Copy-researchershavestruggledtoachievethetheoreticalspecificright2012,AmericanChemicalSociety.
capacityatpracticalcurrentrates,butnanostructureddesign
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L.F.Nazaretal.
hascometotherescue.Impressive
cyclingstabilityandratecapabilityhavebeendemonstratedintwoseparatereportsbyutilizingcarbon-coatednano-particlesembeddedinaporouscarbonmatrix[57]andacarbonnanofiberwithencapsulatednanoparticlesproducedbyelectrospinning.[58]
Theinclusionofhighlyelectroneg-ativefluorineatomsinthecovalentpolyanionicframeworkisknowntoboostthevoltageoftheactiveredoxcouple.Oneexampleisthefluorine-containingNASICONanalogueNa3V2-(PO4)2F3,[59]whichispromisingbecauseofitshighaveragevoltageof3.9Vandtheoreticallypredictedsingle-phasebehaviorwithnegligiblevolumechange(2%).[60]OtherexamplesFigure7.Structuresofa)olivineLiFePO4andb)mariciteNaFePO4inthe101plane.Reproduced
includeNa2FePO4F,[61]andfromRef.[50]withpermission.Copyright2011,AmericanChemicalSociety.c)FeandNalayers
[62]
Na1.5VPO4.8F0.7.TheformerconsistsintheNa2FePO4Fstructure(FeO6octahedraareshowningray,PO4tetrahedraarepink,andthe
blackspheresdenoteNa).AdaptedfromRef.[94]withpermission.Copyright2013,TheRoyalofalayer-liketwo-dimensionalframe-SocietyofChemistry.d)CrystalstructureofNa1.5VPO4.8F0.7withVO5F/VO4F2octahedrashowninworkofFe2O7F2bioctahedraconnectedcyanandyellowspheresrepresentingtheNa+ions.ReproducedfromRef.[62]withpermission.byPO4tetrahedra(Figure7c),whichCopyright2013,AmericanChemicalSociety.
housetwoNa+ionsintheinterlayerspace.Thismaterialwasfirstreportedbyourgroupin2007,[61]andhasbeenthefocusofseveralexcellentstudiesonNaioncellssincethen.[63,64]RetentionofoneoftheNa+ionsinthestructureonoxidationpropsopenthelayers,therebyresultinginasmallvolumechange(3.7%).[61]Theelec-trochemicalprofileofNa2FePO4FisshowninFigure8b.Boththechargeanddischargeprofilesexhibittwotwo-phaseplateaus,cen-teredat2.90Vand3.05VversusNa/Na+,and80%ofthe120mAhgà1theoreticalcapacitywassustainedafter10cycles.AcombinationofM?ssbauerandXRDstudiesrevealedasingleline-phasecomposition—Na1.5FePO4F—thatadoptsFe2+/3+orderingintheformofpaireddimers.[65]Thislocalizestheelectron-chargecarriers,andsodespitetheexcellentmobilityoftheNaions,theelectrochemicalkineticsarenotasfavorableasinNa1.5VPO4.8F0.7,whichalsoexhibitsalayeredstructure.Thelattercompoundisconstructedofcorner-sharingVO5F/VO4F2octahedra(V4+andV3+)andPO4tetrahedra(Figure7d).[62]Parketal.demonstratedtheexceptionalcyclingperformanceofthismaterial,with95%and84êpacityreten-Figure8.a)TypicalelectrochemicalprofileofolivineFePO4withandwithoutintercalatedsodium/ReproducedfromRef.[48].Copyright2010,AmericanChemicalSociety.b)Elec-tionafter100and500cycles,respectively,at
+
trochemicalprofileofNa2FePO4FcycledversusNa/NaatarateofC/15.Adaptedfromarateof1C(Figure8c)andnegligibleRef.[63].Copyright2009,TheElectrochemicalSociety.c)Electrochemicalcyclabilityoftheoverpotentialthroughoutthecharge/dis-Na1.5VPO4.8F0.7electrodeat258C(bottom)and608C(top)at1Cratebetween2.0–4.7V.
chargeprocess.Suchoutstandingelectro-ReproducedfromRef.[62].Copyright2013,AmericanChemicalSociety.d)C/20charge/
chemicalperformanceisattributedtothedischargecurveofNa3TiP3O9NagainstNa/Na+.ThebluedottedlineshowstheDFT
smallvolumechangeonNaioncycling,fastcalculatedvoltageprofile.ReproducedwithpermissionfromRef.[76].Copyright2014,
Naiondiffusion(Ea%0.35eV)intheabAmericanChemicalSociety.
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SodiumIonBatteries
AngewandteChemieplaneoftheP42/mnmstructure,andlackoforderedcompositionswithintheredoxwindow.
InspiredbythesuccessofLi2MP2O7compounds,theYamadaresearchgrouphasdevelopedaseriesofsodium
analogues:triclinic(P1
ˉ)Na2FeP2O7,[66](P1)Na2MnP2O7[67]with3DNaionchannels,andlayeredorthorhombicNa2CoP2O7,[68]with2DchannelsforNa+mobility.AllthreedemonstratereversibleNaionstoragewithacapacityofapproximately80mAhgà1atanaveragevoltageof3VfortheFeandCocompoundsandabout3.6VfortheMncompound.ThisisoneofthehighestreportedMn2+/Mn3+redoxcouplesforasodiumpyrophosphate;theelectrochemicalactivityincomparisontotheinactivemanganesephosphatessuchasLiMnPO4isnoteworthy.3.2.2.FluorosulfatesandSulfates
Studiesonfluorosulfateswithastructurerelatedtotheopenframeworkofthemineraltavorite[69]revealinterestingcomparisonstotheirlithiumanalogues.Theyofferadvan-tagesovertheirphosphatecousinsbecauseofagreaterinductiveeffectthatraisesthepotentialbyasmuchas0.8V.Theirmoisturesensitivitydemandssynthesisinnon-aqueousmedia,whichresultedinthediscoveryofacorner-sharedNaFeSO4Fframework.[70,71]AtomisticmodelingstudiesoftheenergybarriertoNahoppinginthismaterialrevealedthatitiseffectivelya1Dionconductorwithanactivationbarrierof0.6eV,[72]insharpcontrasttoLiFeSO4Fwhichexhibits3DLi+ionhoppingwithabarrieraslowas0.35eVasaconsequenceofitsslightlydifferentstructure.ThelimitedionmobilityofNaFeSO4F,alongwithalargevolumechangeonredox,resultsinmuchpoorerelectrochemicalproperties.
Sulfateshavebeenexploredmorerecently.PolymorphsofFigure9.a)ThestructureofNa2Fe2(SO4)3projectedalongthecaxis.thebloedite-typeNaGreenoctahedra,yellowoctahedra,andbluespheresdenoteFeO6,2M(SO4)2·4H2O(M=Mg,Fe,Co,Ni),[73]SO4,andNa,respectively.b)Galvanostaticdischarge/chargeprofileofandkr?hnkite-typeNa2Fe(SO4)2·2H2O[74]havebeensynthe-Na2àxFe2(SO4)3cycledataC/20rateagainstNaina2–4.5Vwindow.sizedandexploredelectrochemically.Bothbloedite-typeTheinsetshowsthedifferentialgalvanostaticprofilewithtwodistinc-Na2Fe(SO4)2·4H2Oanditsdehydratedderivativeshowelec-tivepeaksduringthefirstcharge.AdaptedfromRef.[75]withtrochemicalactivityatapproximately3.3VversusNa,withpermission.Copyright2014,NaturePublishingGroup.
theformerlosingstructuralwatersimultaneouslywithdesodiation,whichresultsinamorphizationoncharging.Incontrast,kr?hnkite-typeNa2Fe(SO4)2·2H2OdisplaysaFe2+/3.2.3.NewPolyanionMaterials
Fe3+redoxcoupleatabout3.25Vwithgoodstructuralreversibility.DiscoveryofasulfatewithanewcompositionAninterestingnewadditiontothepolyanioniccathodeandstructurebasedonthealluaudite-typeframework,familyisthenitridophosphateNa3TiP3O9N.ThiscompoundisNa2Fe2(SO4)3,representsaleapforward.[75]Ithasregisteredcreatedbysynthesisinflowingammonia,whichresultsinthehighesteverFe3+/Fe2+redoxpotentialat3.8VversusNapartialsubstitutionofoxygenbynitrogeninthephosphatealongwithfastkinetics.Auniquestructureamongirongroups.[76]Thismaterial,effectivelyNa3Ti2(PO4)2(PO3N),sulfatesthatexhibitsedge-sharedFeO6octahedra(Figure9a,displaysahighaveragevoltageof2.7VagainstNa(0.6Vunlikethecorner-sharedNASICONanalogues)accountsforhigherthantheequivalentTi4+/Ti3+redoxcoupleofthetheremarkablyhighredoxpotentialofthiscompound.AparentNASICON-typeNa3Ti2(PO4)3cathode)becauseofthereversiblecapacityof102mAhgà1—85%ofthetheoreticalinductiveeffectoftheN3àioninthe[PO3N]groups.value—isdelivered.AslopingvoltageprofileovertheentireElectrochemicalNade/intercalationoccursthroughasolid-rangeofNacomposition(Figure9b)indicatesasingle-phasesolutionroutewithanextremelylowvolumechangeforreactionmechanismasverifiedbyinsituXRDstudies.AextractionoftheNaions(<1%).Theelectrochemicalstrikinglysmallvolumechangeof1.6%uponde/sodiationvoltageprofileoverlaidwiththatcalculatedfromDFTisaccountsforthehighreversibilityandimpressiverateshowninFigure8d.EventhoughthecapacityoftheTiperformanceofthematerial.
compoundislimited(74mAhgà1),thepresenceoftwocyclableNaionsinthelatticesuggestsasignificantincrease
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inthecapacityofapossibleanalogueinwhichmultipleredoxprocessescanbeaccessed.
Anothernewclassofpolyanioniccompounds,thecarbonophosphates,hasrecentlybeenintroducedascathodematerialsbytheCederresearchgroup.[77]Abinitiocomputa-tionalmodelingrevealedthattheNa3MCO3PO4(M=Mn,Fe)structureissuitableformorethantworedoxprocesses,butatslowrates.ThesynthesisandNaionintercalationpropertiesoftheFeanaloguehavealsobeendemonstratedveryrecently.[78]InsituandexsituX-rayabsorptionnear-edgespectroscopy(XANES)havebeenapplied,therebyuncoveringtheactivityofboththeFe2+/Fe3+andFe3+/Fe4+redoxcouplesduringdischargeandchargeprocesses.
L.F.Nazaretal.
Figure10.FrameworkofPrussianblueanaloguesshowingthedoubleperovskitelatticeofMIIandFeIII,andorderedarrangementoftheCNàlinkers.Reproducedfromref.[80]withpermission.Copyright2012,RoyalSocietyofChemistry.
3.3.PrussianBlueCathodes
Inrecognitionthatlatticevolumeexpansion/contractiononredoxismoreproblematicforthelargerNa+ionthanLi+,so-calledPrussian-blueanalogues(PBAs)havebeeninves-tigatedasintriguingalternativecathodematerialstothelayeredoxideandpolyanionstructures.Alkalimetalde/insertionislesshinderedintherelativelyopenframeworkPBAstructure,whichallowsformoreflexibilityincationaccommodationandextractiononredox.TheprototypeidealPrussianblue,(K[FeIIIFeII(CN)6]y·H2O,y%1–5)hasacubicstructureconsistingofFeIIandFeIIIionssittingonalternatecornersofcorner-sharedironoctahedrabridgedbycyano(C??N)àligands.[79]Thecyanoligandslinktheactivemetalredoxcenterstogethertoformcagelikestructures.Thisopen-frameworkcontainschannels(3.2??)andinterstitialsites(4.6??)thatenablerapidsolid-statediffusionofawidevarietyofions,includingNa+.Althoughitiscalled“soluble”Prussianblue(SPB),itisactuallyhighlyinsoluble.
Oncethedomainofthecoordinationchemistofthelastcentury,PBAshavegeneratedenormousinterestforbothaqueous(wherethevoltageislimitedto1.5V)[13]andnon-aqueousbatteriesinthelastthreeyears.Thelatterarereviewedhere.PBAscanbemoregenerallydescribedasNa2àxMA[MB(CN)6]1ày&y·zH2O.ManydifferenttransitionmetalsMAandMBcanbeaccommodated,andhexacyano-metallatevacancies(&y)inthe[MB(CN)6]4àmoitiesoftenexist.Inso-calledinsolublePrussianblue,Fe4[FeCN)6]3,forexample,one-quarterofthe[FeII(CN)6]4àsitesareoccupiedbywater.Crystal-field-derivedenergeticfactorsdrivedirec-tionalalignmentoftheCNàionswithinthedouble-perovskiteMA[MB(CN)6]cage,andtheformationoflow-spinMBC6andhigh-spinMAN6metaloctahedra.ThecarbonatomoftheCNàisconnectedtoMB,andtheNatomoftheCNàisbondedtoMA.Thesematerialsarecapable—inprinciple—ofrever-sibleextractionoftwoNaionsperformulaunitathighrateswithgoodcyclelife,ifbothmetalcentersareMII.ThestructureofatypicalPBAisshowninFigure10.
Asaconsequenceoftheirlowcostandfacilesynthesis,thehexacyanoferrates(MB=Fe)arethemostpopularPBAs.Goodenoughetal.introducedtheKMFe(CN)6family(MII=Mn,F,Co,Ni,Zn)asnon-aqueoussodiumioncathodesin2012.[80]TheFeIIcompoundexhibitsacapacityofapprox-imately100mAhgà1inaNaioncell,withtwohigh-potential
plateausat3.5Vand2.8V.Thehigherpotentialcorrespondstothehigh-spinFeIII/FeIIcouplebondingtoN,andthelowervoltagetothelow-spinFeIII/FeIIcouplebondingtoC.Acapacityofabout60mAhgà1,closetothetheoreticalvalue,wasreportedforNa2Zn3[FeII(CN)6]2·xH2O.[81]Thinfilmsofhexacyanoferrateshavebeenexploredfortheirhighratecapabilities[82]andmesoporousmodifications.[83]FrameworkswhereMA=MB=FeorMnareparticularlyattractiveforlargeapplicationswherecostisafactor.Sodiummanganesehexacyanoferrate(Na2àxMnFe(CN)6)withahighsodiumconcentrationwasreportedtobeapromisingcathodematerialbecauseofitshighcapacity(134mAhgà1)andvoltageresultingfromtheMnIII/MnIIredoxcouple.[84]How-ever,only120mAhgà1wasmaintainedover30cycles.Thematerial“Berlingreen”[FeIIIFeIII(CN)6]wasreportedtoundergosodiuminsertionwithareversiblecapacityof120mAhgà1and87êpacityretentionover500cycles.[85]ThiswasclaimedtobeachievedbycontrollingthepurityandcrystallinityofthePBAstructure.
AmoredesirabletacticistoaccessareducedvariationofthePBAcompositions—“Prussianwhite”,whichismostlyFeII,counterbalancedbyahighsodiumcontentintheNa2àxFe[Fe(CN)6]framework.ThisallowsforaNa-freeanodetobeusedpractically,similartothelayeredoxideandpolyanionmaterials.AvariablesodiumcompositionNa2àxFe[Fe(CN)6]waspreparedbyafacilesyntheticproce-durebyusingNa4[Fe(CN)6]astheprecursor.[86]Rapidprecipitationfromtheacidicmediumgiveslow-qualitymaterial(seeFigure11)whereahighlevelofverydisordered[Fe(CN)6]vacanciesareproposedtoexist(presumablyfilledwithwater),whereastheslower-growingprocessproduceshigh-quality(HQ)-NaFenanocubeswithamorecrystallinestructure.Thepositiveeffectofthelowwatercontentandsmallnumberof[Fe(CN)6]vacanciesontheelectrochemicalperformanceofHQ-NaFeisshowninFigure11c,d.TheoptimizedmaterialexhibitsaCoulombicefficiencyofabout98%.
TheimportanceofminimizingthefractionofvacancysitesandmaximizingMIIcontentisnicelydemonstratedbytheveryrecentreportofNa2MnII[MnII(CN)6]bytheCuiresearchgroup.[87]Theywereabletocrystallize100nmcubesofalmostperfectlystoichiometricNa1.96Mn[Mn-(CN)6]0.99&0.01·2H2Obyusingalargeexcessofsodium.Asurprisinglyhighreversiblecapacityof209mAhgà1wasachievedinapropylenecarbonateelectrolyte.Theadditional
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