Functional Materials for LIB 锂离子电池材料.docx

Functional Materials for LIB 锂离子电池材料.docx

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FunctionalMaterialsforLIB锂离子电池材料

Report

FunctionalMaterialsforLithium-IonBatteries

1.Introduction

Energyisoneofthemostimportanttopicsinthe21stcentury.Withtherapiddepletionoffossilfuelsandincreasinglyworsenedenvironmentalpollutioncausedbyvastfossil-fuelconsumption,thereishighdemandtomakeefficientuseofenergyandtoseekrenewableandcleanenergysourcesthatcansubstitutefossilfuelstoenablethesustainabledevelopmentofoureconomyandsociety.Energystorage,anintermediatesteptotheversatile,clean,andefficientuseofenergy,hasreceivedworldwideconcernandincreasingresearchinterest.

Toreplacethetraditionalfossilfuels,thereisastrongandever-growingdemandforrenewableenergyresourcesandreliableenergystorageandconversiondevices.Rechargeable（orsecondary）batteriesareamongthemostsuccessfultriedandtruetechnologiesthatcanrepeatedlygeneratecleanelectricityfromstoredmaterialsandconvertreverselyelectricenergyintochemicalenergy.Hundredsofelectrochemicalredoxcoupleshavebeenproposedtoconstructrechargeablebatteries.Currently,themostcommonrechargeablebatteriesincludethelithium-ion,nickel-metalhydride（Ni-MH）,leadacid,redoxflow,andsodium–sulfur（Na-S）systems.Figure1showsthebasicelectrodereactionsinvolvedinthesebatteries,whichexhibitdifferentvoltageandcapacity.

Figure1.Materialchemistryofrepresentativerechargeablebatteries.ΦoandEoarethestandardreductionpotentialofthehalfcellandtheoverallcellat25℃,respectively.FortheNa-Ssystem,theelectrodepotentialvarieswithoperatingtemperature.

Undoubtedly,lithium-ionbatteriesarethemostpopularrechargeablebatteries.Duringthepastthreedecades,tremendousresearchefforthasbeendevotedtoinvestigatingtheelectrochemicalperformanceofawidevarietyofactivematerialsandelectrolytes,asoutlinedinFigure2.

Figure2.Representativeapplicationsforrechargeablebatteries.

SinceresearchersatSonyEnergytechdevelopedthefirstcommercialLi-ionbatteriesinthelate1980s,avarietyofeffortshavebeenundertakentoimprovethebatterymaterials.ALIBismainlycomposedofananode（negative）,acathode（positive）,anelectrolyte,andaseparator.ThepositiveelectrodematerialsaretypicallyLi-containingmetaloxideswithalayeredstructure（suchaslithiumcobaltoxide）ortunnel-structuredmaterials（suchaslithiummanganeseoxide）.Thenegativeelectrodematerialsincludeinsertion-typematerials（suchascarbon,Li4Ti5O12,TiO2,etc.）,conversion-typematerials（suchasironoxides,nickeloxides,cobaltoxides,etc.）,andalloying-typematerials（suchasSi,Sn,etc.）.Theelectrolyteshouldbeagoodionicconductorandelectronicinsulator.Mostoftheelectrolytesarebasedonthesolutionofinorganiclithiumsaltsdissolvedinamixtureoftwoormoreorganicsolvents.ThefunctionoftheseparatoristopreventshortcircuitingbetweenthenegativeandpositiveelectrodesandtoprovideabundantchannelsfortransportationofLiionduringcharging/discharging.Whenabatteryiscycled,Liionsexchangebetweenthepositiveandnegativeelectrodes.Hereinthisreport,areviewofthelatestprogressisprovided,withamainfocusonthenewcathodeandanodematerialsandnovelelectrodestructures,suchasnanostructuresandnanocomposites.

2.AnodeMaterials

2.1Carbonaceousmaterials

CommercialLIBsareusuallybasedoncarbonaceousanodematerials,inwhichLiisinsertedduringcharging.TheresultingLi-interactedcarbonsshowalowpotentialclosetothatofametallicLielectrode.Byusingacarbonaceousanode,theknottyproblemofdendriteformationintheinitiallyemployedLimetalanodecanbeavoided,andthesafetyandreliabilityofLIBsareimproved.Carbonmaterialscanbetypicallyspecifiedintothreegroups,namely,graphiteandgraphitizedmaterials,ungraphitizedsoftcarbon,andhardcarbon.Graphiteismostwidelyusedduetoitsstablespecificcapacity,smallirreversiblecapacity,andgoodcyclingperformance.Intercalation/deintercalationofLi+inandoutofthegraphiteuponcharge/dischargeformsthebasisofgraphiteastheanodeofthebattery（Figure3）.Mesocarbonmicrobeads（MCMBs）derivedfrompetroleumorcoalproductsexhibitdesirableoverallperformancewithmoderatehighcapacityandexcellentcyclability.Theelectrochemicalpropertiesofsoftcarbonsheat-treatedbelow1000℃andhardcarbonsarelargelydeterminedbytheirsurfacecharacteristicsandporestructures.Hardcarbondisplaysahighlithiumstoragecapacityandgoodpowercapability,butpoorelectricalconductivityandalargeirreversiblecapacity.Althoughcarbonousanodematerialsreceivewide-rangeapplicationsincommercializedLIBs,itisrecognizedthatgraphiticcarbonssufferfromsolventco-intercalationinpropylene-carbonate-basedelectrolytes,whichresultsinlargeinterlayerexpansionandsubsequentdegradationofthegraphiticstructure.Moreimportantly,thegravimetricandvolumetriccapacityofcarbonmaterialsislimited.TherapiddevelopmentofelectronicdevicesandEVsdemandsamuchhigherenergydensityaswellasahigherpowerdensityandasmallerirreversiblecapacityforanodes.

Figure3.SchematicrepresentationoftheworkingprincipleaLi-ionbattery.

2.2Lithiumalloys

Somemain-groupelements（e.g.,Si,Ge,Sn,Al,Bi,Zn,andSb）canalloywithlithiumatalowpotential.Representatively,SiandSncangeneratelithiumalloyswithLicompositionsuptoLi4.4M,givingtheoreticalspecificcapacitiesashighas4200and993mAh/g,respectively.Theirvolumetriccapacitiesalsoexceedthatofgraphite（1750mAh/cm3forSiand1000mAh/cm3forSnversus855mAh/cm3forgraphite）.Figure4comparesthetheoreticalgravimetricandvolumetriccapacitiesofLixMalloysatfulllithiationandthecapacityfortheintercalationoflithiumintocarbon.Unfortunately,hugevolumechangesoccurduringtheelectrochemicallithiation/delithiationprocess.Forexample,avolumeexpansionontheorderof400%occurredduringtheformationofLi4.4Sialloy,whichcausescrackingandeventualpulverizationoftheelectrodeandresultsinrapidcapacitydecline.Anotherdisadvantageassociatedwiththesiliconanodeisthepoorelectronicconductivity,whichimpedesfastlithiation.Toovercomethesehurdles,substantialefforthasbeenfocusedonusingnanoscalematerialsand/ordispersingtheactivecomponentinarigidmatrixtoformcompositestructures.

Figure4.Thetheoreticalspecificandvolumetriccapacitiesofvariousfullylithiatedphasesofelectrochemicallyactivemetalelements.Thevolumetriccapacityiscalculatedusingthefullylithiatedvolume.

Nanostructuredalloysaremoreflexibletoaccommodatethestraininducedbyvolumevariationsbecausethetoughnessandadhesioneffectwithingrainboundariesmayincreaseatthenanoscale.Thedurabilityofnanoparticlesstemsfromtwoaspects.First,thesmallsizelimitsthemaximumpossiblelithiumconcentrationgradient.Moreover,thesurfaceisunconstrainedinthenormaldirectionandthusprovidesstressreductioninthesurfacevicinity,whichencompassessmostofthegrainsfornanoparticles.

Somepeculiarlydesignednanoscaleelectrodes,suchashollownanostructures,havemorefreespaceandcanendurelargerstresses.Forexample,nest-likeSihollownanospheresallowedfastionicdiffusionandfacilestrainrelaxation,yieldingasignificantlyimprovedreversiblecapacityandrateperformanceincomparisonwiththeirbulkcounterparts.Inaddition,aSinanowirearrayfirmlygrownonacurrentcollectorenabledefficient1Delectrontransportalongeachwirewhileundergoingsuperplasticdeformationuponalloyinganddealloying.Inthecaseoftin-basedmaterials,porousstructuresincludingSn@Cnanoparticlesencapsulatedinbamboo-likehollowcarbonnanofibers,Snnanoparticlesconfinedinelastichollowcarbonspheres,SnO2nanoparticlesloadingoncross-stackedcarbonnanotubesheets,andcoaxialSnO2@Chollowspheresprovedtobeinterestinganodecandidatesforhighlyreversiblelithiumstorage.

Anotherinnovativeapproachisemployingintermetallicphases,MM′,whereMistheactiveelementthatcanelectrochemicallyalloywithLiandM′isaninactivemetal（e.g.,Cu,Ni,Co,andFe）thatsuppressesthevolumechange.Asanexample,Ni3Sn4nanoparticlesdepositedoncoppernanorodscouldmaintainhighcapacityforhundredsofcycles.Interestingly,theelectrodemorphologywasessentiallypreservedaftercycling,incontrasttotheformationofcracksonthesurfaceofcompactelectrodes.Theexcellentcapacityretentionwaslargelyattributedtothelargefreespacebetweenthealloypillarsthateffectivelyaccommodatedthevolumevariation.

2.3Transitionmetaloxides

In2000,Poizotetal.reportedforthefirsttimethatlithiumcanbestoredreversiblyintransitionmetal（TM）oxidesthroughaheterogeneousconversionreaction:

Li+TMO→Li2O+TM

whereTMisCo,Fe,Ni,andCu.Later,reversiblelithiumstoragewasalsoobservedinTMfluorides,sulfides,nitrides,andphosphides.ItisveryinterestinginviewoffundamentalresearchthatveryinertLiForLi2OcanreactwithaTMatroomtemperature.Thiswasunexpectedpreviously.

Thereversibleelectrochemicallithiumstorageproceedsmoreeasilywiththenanoscaleelectrode.Therefore,researchontransition-metaloxideshasfocusedonnanostructures.SimilartotheLi-alloyingprocess,theconversionreactionleadstovolumevariationupontheelectrochemicalcycling.Conceptually,efficientapproachessuchasreducingparticlesizeorusingnanocompositesareapplicabletometal-oxidebasedanodematerialsaswell.

Apartfromtheprom