CATHODE ACTIVE MATERIAL-COATED DISCRETE GRAPHENE SHEETS FOR LITHIUM

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CATHODE ACTIVE MATERIAL-COATED DISCRETE GRAPHENE SHEETS FOR LITHIUM ( cathode-active-material-coated-discrete-graphene-sheets-for- )

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impliesanexcessivelyhighinternalresistanceandapoor activematerialutilizationrate. Inotherwords,thereareseveralconflictingfactorsthat mustbeconsideredconcurrentlywhenitcomestothedesign andselectionofacathodeoranodeactivematerialintermsof 5 materialtype,shape,size,porosity,andelectrodelayerthick nes.Thusfar,therehasbeennoefectivesolutionofferedby anypriorartteachingtotheseoftenconflictingproblems.We havesolvedthesechallengingisuesthathavetroubledbat terydesignersandelectrochemistsalikeformorethan3010 yearsbydevelopingthecathodeactivematerial-coated grapheneshetsashereindisclosed. Anembodimentofthepresentinventionisagraphene enhancedcathodeactivematerial,whichisacathodeactive15 material-coatedgrapheneshet.Insuchacoatedshet,the graphenematerialisinanamountfrom0.1%to99.5%by weightandthecathodeactivematerialisinanamountofat least0.5%byweight(typicalyandpreferably>60%,more typicalyandpreferably>80%,andmosttypicalyandpref erably>90%),albasedonthetotalweightofthegraphene material and the cathode active material combined. These piecesofcoatedshetshaveatypicallength/widthfrom100 nmto10um,moretypicaly500nmto5um(butmost preferably<3um).Typicaly,atleast60%ofthesurfacearea25 ofaprimarySurfaceoftheSupportinggrapheneshetis coveredbyathinlayerofthedesiredactivematerial(e.g.Sor MoS).Thissurfacecoverageispreferably>80%,morepref erably>90%,andmostpreferably>99%.Thethicknesofthe cathodeactivematerialcoatinglayerispreferablyfrom2nm30 to100nm,andmorepreferablyfrom5nmto20nm.Thereare significantandnon-trivialreasonsforspecifyingthesepre fereddimensions,furtherexplainedbelow: AsilustratedinFIG.6(A),apriorartcathodeisathinfilm35 sulfurorlithiumpolysulfidecoatedonashetofAlfoil curentcolector.ThethinestavailableAlfoilis10-20um thickandthemaximumScoatingfilmthicknessis100nm, beyondwhichtheSlayerwouldnotallowforahighsulfur utilizationrate.ThisproblemisnotjustlimitedtoS;rather,it40 isobservedforallactivematerialsinathinfilmform.With Suchaconfiguration(e.g.10umthickAlfoiland100nm 10 baterycelleadingtoahighratecapability.Thesefeatures havenotbeenposiblewithanypriorartcathodeactive materialsandelectrodes. FIG. 4 shows the volume fraction of the S coating in S-coatedgrapheneshetsplotedasafunctionoftheScoat ingthicknessforvariousgrapheneplateletthicknessvalues. Thischartindicatesthat,withasingle-layergraphenesheet (<0.5nmthick),aScoatingthicknessasthinas1nmimplies aSvolumefraction>66%.Onecanimaginethatsuchan ultra-thinScoatingwouldenableextremelyeasytransportof boththeelectronsandlithiumionswithultra-lowresistance, enablingultra-fastbaterychargeanddischarge.A Sulfur coatingthicknessof20nmwouldmeanaSVolumeorweight fraction>96%,leavingbehindverylowpercentageofnon activematerial.Withamulti-layergrapheneplateletasa SupportingSubstrate(e.g.5nm),aScoatingthicknessof8nm issuficienttoachieveanactivematerialpercentage>60%. OnemightarguethatonecouldeasilyachieveaSulfurloading higherthan70%,80%,oreven90%bysimplymixingsulfur withacarbonmateriallesthan30%,20%,or10%.However, inSuchamixture,asignificantproportionoftheSulfuris presentinlargeZonesordomainssignificantlylargerthan100 nminsize.Assuch,ahighpercentageofsulfurwouldnotbe readilyacesibletolithiumionsandthesulfurutilization rateislow.InaconventionalLi-Scel,acarbonloadingas highas50%isverycommon.Evenwithsuchahighpercent ageofinactivematerial,theSulfurutilizationrateremains verylow.Thisiswhyoneoftenobservesasulfurcathode specificcapacityof<600mAh/geventhoughthetheoretical capacityofsulfuris1,675mAh/g.(With50%carbonand 50%sulfur,thecompositesulfurisexpectedtogiverisetoa cathodespecificcapacityof1,675x0.5–837.5mAh/g,based onthecompositeweight,providedthat100%ofthesulfuris fulyutilized.But,theactualcapacityisnormallysignifi cantlylowerthanthis.Anactualcapacityof600mAh/g would mean aSulfurutilizationrateof600/837.5=71.6%. Thislowsulfurutilizationratehasbeenamostseriousprob lemassociatedwithLi Sbateries). Wehavefurtherobservedthatthepresentlyinventedcath odeactivematerial-coatedgrapheneshetscanbecombined, withorwithoutaditionalneatgrapheneshetsand/orother conductiveaditive,intosecondaryparticles,typicaly1-20 US9,203,084B2 thickSfilm),thevolumefractionofS(theactivematerial umindiameter.Withthelength/widthofthesupporting responsibleforstoringordischarginglithium)isonly100/ 100+10,000=1/101<1%.WithanAlphysicaldensityof2.745 g/cmandSdensityof2.0g/cm,thisimpliesaSweight fractionlesthan1%.Inotherwords,morethan99%by weightorbyVolumeofthisconfigurationisaluminum,which isnotacathodeactivematerialincapableofreversiblystoring lithiumionsinabaterycel.Thisisanextremelyundesirable50 andnon-eficientbaterydesign. Bycontrast,referingtoFIG.6(B)asanilustrative example, the Sulfur coating layer is 20 nm thick and the Supportinggrapheneshetis1nmthick,implyingacathode activematerialpercentageof20/21=95%byvolume.Witha55 graphenephysicaldensityof2.2g/cmandSdensityof2.0 g/cm,thisimpliesasulfurweightfractionofaproximately 94.7%,havingaveryminimalnon-activematerial.TheSup portinggrapheneshetplaystheroleofaconductingfiler, andeverypieceofsulfurisguaranteedtobeindirectphysical60 contactwithanunderlying,highlyconductinggraphene shet.Graphenehasanelectricalconductivityhigherthanthe conductivityofcarbonblackbyatleast3ordersofmagni tude.Duringbateryoperations,bothelectronsandlithium ionsonlyhavetotraveltoamaximumpathlengthof20nm,65 resultinginverylowinternalresistance,andSuchashort transportpathalsoenablesfastchargeandrechargeofa grapheneshetsbeingintherangeof300nmto5um,the secondaryparticletypicalyhasadiameterlesthan10um. WithaSupportinggrapheneshetlength<3um,theresulting secondaryparticlehasatypicaldiameter<5um,whichis idealforuseasacathodeactivematerialintermsofelectrode productionease,electrodetapdensity,ratecapability,and cyclingstabilityofresultinglithiumbateries.Theseobser Vationsmeanthatthepresentlyinventedcathodeactivemate rial-coatedgrapheneshetsareidealcathodestructures. Inanembodimentofthepresentinvention,theprocessfor producingSuchacathode-coatedshetcomprises(a)provid ingacontinuousfilmofagraphenematerialintoadeposition Zone;(b)introducingvapororatomsofaprecursorcathode activematerialintothedepositionZone,allowingthevaporor atomstodepositontoaSurfaceofthegraphenematerialfilm toformacoatedfilmofacathodeactivematerial-coated graphenematerial;and(c)mechanicallybreakingthisfilm intomultiplepiecesofcathodeactivematerial-coated grapheneshets.Thisprocessmayfurtherincludeastep(d) ofcombiningmultiplepiecesofcathodeactivematerial coatedgrapheneshets,alongwithanoptionalbinderand/or anoptionalcarbon/graphitematerial,toformmultiplesec ondaryparticlesthatarethencombinedtoformacathode electrode.Step(d)mayentailcombiningmultiplepiecesof

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