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13 ventSulfurfromgetingdissolvedinelectrolyte(amajor causeforrapidcapacitydecayinaLi-Scel).Thenotion thattheexteriorsurfaceisembracedwithhighlyconductive grapheneshetsimpliesthattheseshetscannaturalyforma 3-Dnetworkofelectron-conductingpathswhenmultiplepar ticulatesarepackedtogetherinananode.FIG.3showsa correspondingparticulatefromSi-coatedgrapheneshetsfor useinananodeofalithium-ionbatery. 14 cycloaliphaticcarboxylicacid,straightchainorbranched chaincarboxylicacid,saturatedandunsaturatedmonocar boxylicacids,dicarboxylicacidsandpolycarboxylicacids thathave1-10carbonatoms,alkylestersthereof,andcom binationsthereof.Preferably,thecarboxylicacidisselected fromthegroupconsistingofSaturatedaliphaticcarboxylic acidsoftheformulaH(CH),COOH,whereinnisanumber offrom0to5,includingformic,acetic,propionic,butyric, pentanoic,andhexanoicacids,anydridesthereof,reactive carboxylicacidderivativesthereof,andcombinations thereof.Themostpreferedcarboxylicacidsareformicacid andaceticacid. US9,203,084B2 Thesemoreorlessphericalsecondaryparticlescanbe easilyhandledandmadeintoelectrodesusingexistingbat10 teryelectrodecoatingmachines.Theseparticulateswere foundtoleadtoelectrodesthathaveahighertapdensity (weightpervolumeoftheelectrode),whichisaveryimpor Instep(e),optionaly,particlesofacarbonorgraphite tantparameterforanelectrode. materialmaybeaddedalongwiththecathodeactivematerial Asapreferredembodiment,theprocessofproducing15 coatedgrapheneshets. graphene-enhancedparticulatesfromactivematerial-coated Thestepofdryingthemulti-componentSuspensionto grapheneshetscomprises(i)preparingaprecursormixture ofgrapheneorgrapheneprecursorwithacathodeactive material-coatedgrapheneshets;and(i)thermallyand/or chemicallyconvertingtheprecursormixturetothegraphene enhancedanodeparticulate.Describedinmoredetail,the processentails: (a)dispersingorimmersingalaminargraphitematerial(e.g., graphitepowder)inamixtureofanintercalantandan oxidant(e.g.,concentratedSulfuricacidandnitricacid,25 respectively)toobtainagraphiteintercalationcompound (GIC)orgraphiteoxide(GO); formtheprecursormixturemaybeconductedusingaspray drying,spray-pyrolysis,fluidized-beddryingprocedure,or anystepthatinvolvesatomizingoraerosolizingtheSuspen Sion.Thestepofconvertingmaycompriseasintering,heat treatment,spray-pyrolysis,orfluidizedbeddryingorheating procedure.Thestepofconvertingmaycompriseaprocedure ofchemicallyorthermallyreducingthegrapheneprecursor toreduceoreliminateoxygenorfluorinecontentandother non-carbonelementsofthegrapheneprecursor,which grapheneprecursormaycontaingrapheneoxideorgraphene fluoride.Uponconversion,thegrapheneintheparticulatehas anoxygencontenttypicalylesthan5%byweight. (b)exposingtheresultingGICorGOtoathermalshock, preferablyinatemperaturerangeof600-1,100°C.fora Asanotherpreferedembodiment,theprocessmaybegin shortperiodoftime(typicaly15to60seconds),toobtain30 exfoliatedgraphiteorgraphiteworms;and (c)dispersingexfoliatedgraphiteinaliquid(e.g.water)and mechanicallyseparatingindividualnanographeneplate letsorshetsfromgraphitewormsusing,forinstance,a high-shearmixeroranultrasonicatortoobtainagraphene35 withtheproductionofaprecursorSolutionorSuspensionof pristine graphene (non-oxidized graphene) directly from graphiteparticles,whichisfollowedbytheaditionofa cathodeactivematerial-coatedgrapheneshetstothissolu tionorSuspensiontoobtainaprecursormixture.Theproduc tionofaprecursorSolutionorSuspensionmayincludethe followingsteps: (a) Preparing a suspension containing pristine nano grapheneplatelets(NGPs)dispersedinaliquidmedium using,forinstance,directultrasonication(e.g.,aprocess disclosedby usinU.S.patentaplicationSer.No. 11/800,728(May8,2007)); (b)Optionallyremovingsomeoftheliquidfromthesus pension; (c)Addingadesiredamountofcathodeactivematerial coatedgrapheneshetstoobtainaprecursormixture SuspensionorSolution; (d)RemovingtheliquidfromtheSuspensiontoobtaina precursormixturesolid;and (e)Thermallyand/orchemicallyconvertingtheprecursor mixturesolidtothegraphene-enhancedcathodeparticu late. Forthepreparationofacathode,multiplegraphene-en hancedparticulatesaremixedwithabinderSolution(e.g., PVDFinNMP)toobtainasluryorpaste.Adesiredamount ofthesluryorpasteisthencoatedontoacurentcolector, allowingtheliquidtoevaporateandleavingbehindanelec trode bonded to a surface of a current electrode. For examples,particulatescontainingS-coatedgrapheneshets andgraphiteparticlesembracedbyseparategrapheneshets maybeaddedtoasolutioncontainingasolvent(NMP).The resultingpastemaybecoatedontoanaluminumfoilasa curentcolectortoformacoatinglayerof50-500Lumthick (preferably100-250um).Byallowingthesolventtovaporize oneobtainsapositiveelectrode(cathode)foralithiumbat tery. orgrapheneprecursorSuspension;or,alternatively, (d)re-dispersingtheexfoliatedgraphitetoaliquidmedium containinganacid(e.g.,Sulfuricacid),anoxidizingagent (e.g.nitricacid),oranorganicsolvent(e.g.,NMP)ata desiredtemperatureforadurationoftimeuntiltheexfoli atedgraphiteisconvertedintographeneoxideorgraphene dissolvedintheliquidmedium.Theacidispreferablya weakacid(suchasdilutedSulfuricacid)oramoreenvi ronmentallybenignacid,suchasformicacid,aceticacid, citricacid,carboxylicacid,andcombinationsthereof.The exfoliatedgraphite,whendispersedintheseacids,was graduallydispersedandesentialydissolvedtoforma graphene or graphene oxide solution or Suspension. Althoughnotarequiredoperation,stiring,mechanical shearing,orultrasonicationcanbeusedtoaceleratethe dispersionanddisolutionstep; (e)dispersingcathodeactivematerial-coatedgrapheneshets tothegrapheneorgrapheneprecursorSolutionorSuspen sionpreparedinstep(c)orstep(d)toobtainaprecursor mixtureSuspension;and (f)thermallyand/orchemicallyconvertingtheprecursormix turetothegraphene-enhancedanodeparticulate. Anoptional,butdesirableintermediatestepbetween(e) 40 45 50 55 and(f)involvesdryingtheSuspensiontoformtheprecursor mixtureinasolidstate.Iftheprecursormixturecontainsa60 precursortographene(e.g.grapheneoxide),thentheprecur Sormaybesubjectedtoachemicalorthermalreduction treatment.Aheattreatmentatatemperatureofpreferably 500-1,000°C.for1-2hourswouldservetoeliminateamajor ityoftheoxygencontentfromthegrapheneshets. 65 Thecarboxylicacidusedinstep(d)maybeselectedfrom Intheaforementionedexamples,thestartingmaterialfor thegroupconsistingofaromaticcarboxylicacid,aliphaticor thepreparationofNGPsisagraphiticmaterialthatmaybePDF Image | CATHODE ACTIVE MATERIAL-COATED DISCRETE GRAPHENE SHEETS FOR LITHIUM
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