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US9,203,084B2 1. 2 CATHODE ACTIVE MATERAL-COATED DISCRETE GRAPHENE SHEETS FOR LITHIUM BATTERIESAND PROCESS FOR PRODUCING SAME FIELD OF THE INVENTION The presentinvention relatesgeneralytothefieldof lithiummetalorlithiumionbateriesand,inparticular,toa graphene-enhancedcathodeofalithiummetalbatery,10 lithium-sulfurbatery,orlithium-ionbatery. aconductiveaditiveforthecathodeofalithiumbatery. Theseincludecarbonnano-tubes(CNTs),vapor-growncar bonnano-fibers(VG-CNFs),andsimplecarboncoatingon thesurfaceofcathodeactivematerialparticles.Theresulthas notbeensatisfactoryandhence,asoftoday,carbonblackand artificialgraphiteparticlesarepracticalytheonlytwotypes ofcathodeconductiveaditiveswidelyusedinlithiumion bateryindustry.Thereasonsarebeyondjusttheobvioushigh costsofbothCNTsandVG-CNFs.Thedificultyindisentan glingCNTsandVG-CNFsanduniformlydispersingthemin aliquidorSolidmediumhasbeenanimpedimenttothemore widespreadutilizationoftheseexpensivematerialsasacon ductiveaditive.Aditionaly,theproductionofbothCNTs BACKGROUND Duetoextremelypoorelectricalconductivityofalcath-15andVG-CNFs normallyrequiretheuseofasignificant ode(positiveelectrode)activematerialsinalithium-ion, lithiummetal,orlithium-sulfurcel,aconductiveadditive (e.g.carbonblack,finegraphiteparticles,expandedgraphite particles,ortheircombinations),typicalyintheamountof 5%-20%,mustbeaddedintotheelectrode.Inthecaseofa lithium-sulfurcel,acarbonamountashighas50%byweight isusedasaconductiveSupportforSulfurinthecathode. However,theconductiveadditiveisnotanelectrodeactive material(i.e.itisnotcapableofreversiblystoringlithium ions).Theuseofanon-activematerialmeansthattherelative25 proportionofanelectrodeactivematerial,suchasLiFePO, isreducedordiluted.Forinstance,theincorporationof5%by weightofPVDF asabinderand5% ofcarbonblackasa conductive additive in a cathode would mean thatthe maxi mumamountofthecathodeactivematerial(e.g.,lithium30 cobaltoxide)isonly90%,efectivelyreducingthetotal lithiumionstoragecapacity.Sincethespecificcapacitiesof themorecommonlyusedcathodeactivematerialsarealready verylow(140-170mAh/g),thisproblemisfurtheraggravated ifasignificantamountofnon-activematerialsisusedtodilute 35 theconcentrationoftheactivematerial. State-of-the-artcarbonblack(CB)materials,asaconduc tiveaditive,haveseveraldrawbacks: (1)CBSaretypicalyavailableintheformofaggregatesof multipleprimaryparticlesthataretypicalysphericalin40 shape.Duetothisgeometricfeature(largestdimension-to amountoftransitionmetalnanoparticlesasacatalyst.Itis dificulttoremoveandimpossibletototalyremovethese transitionmetalparticles,whichcanhaveadverseefecton thecyclingstabilityofalithiummetal. Asforthelesexpensivecarboncoating,beingconsidered foruseinlithiumironphosphate,theconductivityofthe carboncoating(typicalyobtainedbyconvertingaprecursor SuchasSugarorresinviapyrolyzation)isrelativelylow.It wouldtakeagraphitizationtreatmenttorenderthecarbon coatingmoreconductive,butthistreatmentrequiresatem peraturehigherthan2,000°C.,whichwoulddegradethe underlyingcathodeactivematerial(e.g.,LiFePO). Asanalternativeapproach,Ding,etalinvestigatedthe electrochemicalbehaviorofLiFePO/graphenecomposites Y.Ding,etal.“Preparationofnano-structuredLiFePO/ graphenecompositesbyco-precipitationmethod.”Electro chemistryCommunications12(2010)10-13.Theco-pre cipitationmethodleadstotheformationofLiFePOnano particlescoatedonbothprimarySurfacesofgraphenenano shets. The cathode is then prepared by stacking these LiFePO-coatedgrapheneshetstogether.Thisapproachhas severalmajordrawbacks: (1)Withthetwoprimarysurfacesofagrapheneshet attachedwithLiFePO nano-particles,theresulting electrodeentailsmany insulator-to-insulatorcontacts Smalestdimensionratiooraspectratio~1)andthenotion betweentwoadjoiningcoatedshetsinastack. thatCBSareaminorityphasedispersedasdiscretepar (2)Onlylesthan30% ofthegraphenesurfaceareais ticlesinanelectricalyinsulatingmatrix(e.g.lithium coveredbyLiFePO particlesoneitherside.Thisisa cobaltoxideandlithiumironphosphate),alargeamountof45 relativelylowproportionofthecathodeactivematerial. CBsisrequiredtoreachapercolationthresholdwherethe (3)The LiFePO particlesareeasilydetachedfrom CBparticlesarecombinedtoforma3-Dnetworkofelec grapheneshetsduringhandlingandelectrodeproduc tron-conductingpaths. tion. (2)CBsthemselveshavearelativelylowelectricalconduc (4)Wehavefoundthatthenanoparticle-atachedgraphene tivityand,hence,theresultingelectroderemainstobeof50 relatively low conductivity even when the percolation thresholdisreached.A relativelyhighproportionofCBS (farbeyondthepercolationthreshold)mustbeincorpo ratedinthecathodetomaketheresultingcompositeelec trodereasonablyconducting. Clearly,anurgentneedexistsforamoreefectiveelectri shetsaspreparedbytheco-precipitationmethodare not amenable to fabrication of cathodes with current electrodecoatingequipment.Inparticular,thesepar ticle-atachedgrapheneshetscouldnotbecompacted intoadensestatewithahighmassperunitelectrode volume.Inotherwords,thecathodetapdensityisrela tivelylow.Thisisaveryseriousisuesincealofthe commonly used cathode active materials, including LiFePO, already have a very low specific capacity (mAh/g),andnotbeingabletopackalargemassofa cathodeactivematerialintoagivenelectrodeVolume wouldmeananexcessivelylowoveralcapacityatthe cathodeside.(Itmaybenotedthatthetypicalspecific capacity(140-170mAh/g)ofacathodeactivematerialis alreadymuchlowerthanthat(330-360mAh/g)ofan anode active material. Such an imbalance has been a calyconductiveaditivematerial.Preferably,thiselectri calyconductiveaditiveisalsoofhighthermalconductivity. Suchathermallyconductiveaditivewouldbecapableof disipatingtheheatgenerated from theelectrochemical operationoftheLi-ionbatery,therebyincreasingthereli abilityofthebateryanddecreasingthelikelihodthatthe baterywilsuferfromthermalrunawayandrupture.Witha highelectricalconductivity,therewouldbenoneedtoadda highproportionofconductiveaditives. 55 60 65 Therehavebeenseveralattemptstouseothercarbonnano majorisueinthedesignandfabricationoflithiumion materialsthancarbonblack(CB)oracetyleneblack(AB)as bateries).PDF Image | CATHODE ACTIVE MATERIAL-COATED DISCRETE GRAPHENE SHEETS FOR LITHIUM
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