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US9,203,084B2 78 aditive(suchascarbonblack)whichareembracedby grapheneshetstoformamoreorlessphericalparticle. Furtheralternatively,theprocesscanfurthercomprisea stepofmixingmultiplepiecesofcathodeactivematerial coatedgraphenematerialshetsandaresinbinderand/ora conductivefilertoformacathodelayeronacathodecurent collector. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1Schematicofaprocessforproducingcathodeactive material-coatedgrapheneshets. FIG.2SEMimageofagraphene-enhancedcathodepar ticulate(secondaryparticle)accordingtoapreferredembodi mentofthepresentinvention FIG.3SEMimageofagraphene-enhancedanodeparticu late(secondaryparticle)forcomparisonpurpose. FIG.4Thecathodeactivematerialcoatingvolumefraction ofcoatedgrapheneshetsplotedasafunctionoftheactive material coating thickness for various graphene platelet thicknessvalues. 10 15 lithiumsecondarybaterycanbecylindrical,square,button like,etc.Thepresentinventionisnotlimitedtoanybatery shapeorconfiguration. Forconvenience,we wiluselithiumironphosphate (LFP),vanadiumoxide(V.O.),sulfur(S),andcoperphtha locyanine(CuPe)asilustrativeexamplesofthecathode activematerial.Thisshouldnotbeconstruedaslimitingthe Scopeoftheinvention. AsilustratedinFIGS.5(A)and5(B),alithium-ionbatery celistypicalycomposedofananodecurentcolector(e.g. Cufoil),ananodeelectrode(anodeactivemateriallayer),a porousseparatorand/oranelectrolytecomponent,acathode electrode(cathodeactivemateriallayer),andacathodecur rentcolector(e.g.Alfoil).Inamorecommonlyusedcel configuration(FIG.5(B),theanodelayeriscomposedof particlesofananodeactivematerial(e.g.graphiteorSi),a conductiveaditive(e.g.carbonblackparticles),andaresin binder(e.g.SBRorPVDF).Thecathodelayeriscomposedof particlesofacathodeactivematerial(e.g.LFPparticles),a conductiveaditive(e.g.carbonblackparticles),andaresin binder(e.g.PVDF).Boththeanodeandthecathodelayersare typicaly100-300umthicktogiverisetoasuficientamount ofcurentperunitelectrodearea.Thisthicknessrangeisan industry-acceptedconstraintunderwhichabaterydesigner mustwork.Thisconstraintisduetoseveralreasons:(a)the existingbateryelectrodecoatingmachinesarenotequipped tocoatexcesivelythinorexcesivelythickelectrodelayers: (b)athinerlayerispreferedbasedontheconsiderationof reducedlithiumiondifusionpathlengths;but,toothina layer(e.g.<100um)doesnotcontainaSuficientamountof anactivelithiumstoragematerial(hence,insuficientcurent output);and(c)alnon-activemateriallayersinabaterycel (e.g.curentcolectorsandseparator)mustbekepttoamini muminordertoobtainaminimumoverheadweightanda maximumlithiumstoragecapabilityand,hence,amaximized energydensity(Wk/kgorWh/Lofcel). FIG.5(A)schematicofapriorartlithium-ionbaterycel composedofananodecurentcolector,ananodeelectrode (e.g.thinSicoatinglayer),aporousseparator,acathode25 electrode(e.g.Sulfurlayer),andacathodecurentcolector, (B)theelectrodelayeriscomposedofdiscreteparticlesofan activematerial(e.g.graphiteortinoxideparticlesinthe anodelayerorLiCoO inthecathodelayer). FIG.6(A)apriorartcathodecontainingathinfilmS30 coatedonashetofAlfoilcurentcolector;(B)schematicof anexampleofaS-coatedgrapheneshetofthepresentinven tion. FIG.7Cyclingbehaviorsofthrelithiumbaterieshaving agraphene-SupportedV2O5coating,agraphene/V2O5nano35 particlemixture(co-precipitatedV2O5withGO),andacar bonnanofiber(CNF)-supportedVOscoating,respectively, Inalescommonlyusedcelconfiguration,asilustratedin asacathodeactivematerial. FIG.8Cyclingbehaviorsofalithium-ionbateryhavinga graphene-SupportedLFPcoatingcathodeofthepresent40 invention,thoseofabaterycontainingaLFP-graphenesheet mixturecathode,andthoseofabaterycontainingacarbon coatedLFPcomposite-basedcathode. FIG.5(A),eithertheanodeactivematerial(e.g.Si)orthe cathodeactivematerial(e.g.S)isdepositedinathinfilmform directlyontoacurentcolector.Suchasasheetofcopperfoil orAlfoil.However,suchathinfilmstructurewithan extremely small thicknes-direction dimension (typicaly muchsmallerthan500nm,oftennecesarilythinerthan100 FIG.9RagoneplotsofthreLiScelshavingaS-coated um)impliesthatonlyasmallamountofactivematerialcanbe graphenebasedcathodewithaScoatingthicknessof20nm,45 65nm,and125nm,respectively. FIG.10Cyclingbehaviorsofalithiummetalbateryhav ingagraphene-supportedCuPeorganiccoating(25nm)cath odeofthepresentinvention,thoseofabaterycontaininga graphene-SupportedCuPecoating(120nm)basedcathode,50 andthoseofabaterycontainingaCuPe-carbonblackcom posite-basedcathode. incorporatedinanelectrode(giventhesameelectrodeor curentcolectorSurfacearea),providingalowtotallithium storagecapacityandlowlithiumstoragecapacityperunit electrodesurfacearea.Suchathinfilmmusthaveathickness lesthan100nmtobemoreresistanttocycling-induced cracking(fortheanode)ortofacilitateafulutilizationofthe cathodeactivematerial.Suchaconstraintfurtherdiminishes thetotallithiumstoragecapacityandthelithiumstorage capacityperunitelectrodesurfacearea.Suchathin-film bateryhasverylimitedscopeofaplication.(Ontheother hand,aSilayerthickerthan100nmhasbeenfoundtoexhibit poorcrackingresistanceduringbaterycharge/discharge cycles.Ittakesbutafewcyclestogetfragmented).ASulfur layerthickerthan100nmdoesnotallowlithiumionstofuly penetrateandreachfulbodyoftheSlayer,resultinginapoor sulfurutilizationrate.A desirableelectrodethicknessisat least100um,withindividualactivematerialcoatingorpar ticlehavingadimensiondesirablylesthan100nm.These thin-filmelectrodes(withathickness<100nm)directly depositedonacurentcolectorfalshortoftherequired thicknessbythre(3)ordersofmagnitude.Asafurtherprob lem,alofthecathodeactivematerialsarenotconductiveto bothelectronsandlithiumions.Alargelayerthickness DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 55 Thisinventionisdirectedatgraphene-enabledcathode activematerials,aprocessforproducingsuchacathode activematerial,acathode(positiveelectrode)containingSuch acathodeactivematerial,andalithiumbaterycontaining60 suchacathode.Thislithiumbaterycanbeaprimarybatery, butispreferablyasecondarybateryselectedfromalithium ionbatery,alithiummetalsecondarybatery(e.g.using lithiummetalasananodeactivematerial),oralithium-sulfur batery.Thebateryisbasedonanon-aqueouselectrolyte,a65 polymergelelectrolyte,anionicliquidelectrolyte,aquasi solidelectrolyte,orasolid-stateelectrolyte.TheshapeofaPDF Image | CATHODE ACTIVE MATERIAL-COATED DISCRETE GRAPHENE SHEETS FOR LITHIUM
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