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23 24 US 10,991,944B2 phasesulfuronexternalcarbonsurfacesmakeitposibleto duringdischarge-chargecycles.Nevertheles,gamma providecomercialy-relevantsulfurloadingsincarbonate monoclinicphasesulfurstilremainsexposedanduncon electrolyte. finedonthesurfaceofCNFs Ahandfulofreportsintheliteraturehavereportedsingle FIG.12CshowsaTEMimagetakenafterlithiationina plateaudischargeincarbonateelectrolyteincathodeswhere 5 completelydischargedsamplepost10cyclesshowingthe sulfurwasshowntobefulyconfinedinsub-nanopores. adherenceofsulfurtocarbonnanofibers.Furthermore,the Suchdischargebehaviorhasbenexplainedviaconfine- preservedfre-standinggammasulfurdeposited-CNFs ment-drivenhypotheses,oneexamplebeingthatthesub- architectureinthisTEMimagecoroborateswiththeexcel nanometerporesizeslimitthesulfurchainlengthduring lentcyclingperformanceimplyingthattheelectrodestruc synthesisresultingintheformationofsmalersulfuralo-10turecanafordtoacomodatethevolumeexpansionof tropes(S2-S4),whicharethendirectlyconvertedtoLi2S sulfurduringcyclingandalsofacilitatebeterelectrode duringdischargeeliminatinglong-chainpolysulfides. electrolytecontactthroughoutthecyclelife. Anotherhypothesisthathasbeenputforthintheliterature PostmortemXPSandXRDAnalysis isthattheconfinementofS,moleculesinsub-nanometer FIGS.13A-131providethepostmortemXPSandXRD poresforcesdesolvationoftheLi-ionsandleadstosolid-15databothafterdischargeandaftercharge.FIG.13Dshows statelithiation/de-lithiationpreventingadversereactions typicalcharge-dischargevoltageprofilesobtainedforthe betweenpolysulfidesandcarbonatespecies[FuandXin]. samplesandthespecificpointswherespectroscopydatawas Theseworksreliedoncompletenano-confinementofsulfur. colectedafterdischargeandaftercharge.PriortoXPS Incontrast,inthecathodesofthepresentinvention,sulfur analysis,thecycledsampleswerethoroughlyrinsedwiththe islargelypresentontheexternalcarbonsurface.Exposed20electrolytesolventanddriedoutunderAratmosphereand unconfinedsulfurofthistypehasbenpreviouslyasociated laterunderdynamicvacuumfor48hours.Thesampleswere withireversiblereactionswithcarbonateelectrolyteand thenloadedinanXPStransferasemblyintheglovebox bateryshutdownafterthefirstcycleasshownbyKimetal. andtransferedtoanXPSvacuumchamberavoidingany Onestrikingdiferenceofthepresentinventionisthecrystal contactwithambientatmosphere.Apasenergyof23.5eV, structureofthesulfurinthecathodes.MostLi–Sliterature,25withastepsizeof0.5eVwasusedtogatherthehigh regardlesoftheelectrolyte,usesa-orthorhombicsulfur, resolutionspectra.CasaXPSVersion2.3.19PR1.0software whichisthemoststablesulfuralotropeatroomtempera- wasusedforpeakfiting.TheXPSspectrawerecalibrated ture.Itisthereforelikelythatthesingleplateaubehavior bysetingthevalenceedgetozero,whichwascalculatedby senreversiblyandconsistentlyfor20cyclesinthe fitingthevalenceedgewithastep-downfunctionand presentinventionisdirectlylinkedtotheroleofgamma- 30setingtheintersectiontoOeV.Theconductiveelements monoclinicsulfurphase. werefitedusinganasymetricLorentzianlineshape.The Aposiblereasonforsuchasignificantefectofsulfur non-conductivepeaks,ontheotherhand,werefitedusing crystalstructurecouldbethediferenceinphasedensity. anasymetricGausian/Lorentzianlineshape.Theback Whiletherearediscrepanciesinthereportsondensitiesof groundwasdeterminedusingtheShirleyalgorithm,which varioussulfuralotropesassynthesizingametastablealo-35isabuilt-infunctionintheCasaXPSsoftware. tropeisnon-trivial,Meyeretal.didgroundbreakingwork Asdiscusedearlier,inthepristinesamplewithvapor onsulfuralotropesintheearly1960's.Hereportedthe depositedgammamonoclinicphasesulfur,wesethepres densityforgamma-monoclinic-sulfurasbeinghigherthan enceofadventitiouscarbon,Cat284.6eVfromtheCNF itsa-counterpart(2.19g/cm2vs2.069g/cm”).Theclose surface.TheS2Pspectrashowsthepresenceofsulfur compactneswithinthegamma-monocliniccrystalstructure40doubletpeaks(S2P3/2andS2P1/2)positionedat163.7and posiblyprovidesgreaterstabilityandeasylithiationintothe 164.9eVwithapeakseparationof1.18eV.Inadition,we gammamonocliniccrystalstructureincarbonateelectrolyte. seapeakathigherbindingenergy(168.94eV)asociated Intheetherelectrolyte,itisbelievedthatgammamonoclinic withtheformationofsurfaceoxides(S—0)duringhigh phasesulfurconvertstoamorefavorablephasetoyielda temperaturedeposition.Similarbondscanbeseninthe two-plateaudischarge.Studyonstabilityofthisunique45Olsspectra,whereinthepeakat531.86eVcanbeatributed sulfurcrystalstructureinvariouselectrolytesisunderway. tothesurfaceoxide.Extremecarewastakenduringtransfer PostmortemstudieswereconductedusingXRDandXPS ofthesamples,henceanotherpeakat532.34eVwas tounderstandtheredoxproductsafterchargeanddischarge ascribedtoasurfaceoxidizedSOxgroup. cyclesandtoprovideevidencethatthestablecapacityis Aftercompletedischarge,theS2pspectrashowsthe indedlargelyaresultofthedesiredsulfurtoLi2Sreactions50apearanceofanewstrongpeakatalowerbindingenergy (andnotanyunwanteddegradationreactions).Thisisalso of161.8eVasociatedwithlithiumsulfide(Li2S)deposi particularlyimportantasmostpapersreportingsinglepla- tion.Interestingly,thepresenceofanewpeakat685.5eV teaudischargeprofileinLi–Sbateriesdonotprovide isatributedtoLiFintheF1sspectra.Inadition,wesea reactantand/orproductcharacterizationfordeperunder- diminishedpeakcontributionfromsurfacedepositedelec standingofthechargestoragemechanismandtoevaluate5trolytesalts(LiFxNy)at68eVcomparedtothecharged electrolytedecomposition(ifany).Belowwediscusboth samplespectra.ThesignatureofLifspecieswasnotsenin postmortemspectroscopyandmicroscopydata. thepostmortemXRD spectrum(tobediscusedbelow) PostmortemSEMandTEMAnalysis denotingitsextremelylowcontribution.PostmortemSEM Tounderstandchemistryandsurfacemorphologyafter orTEMimagesofchargedanddischargedsamplesshown harsh cycling conditions, we conducted post mortem 60 above did not demonstrate formation of spherical agglom microscopyofcycledcels.Thesurfacemorphologygamma eratesonthesurfaceofCNFsoftenasociatedwithLiF sulfurdeposited-CNFsafter20chargeanddischargecycles formation,furthersugestinglowLiFdepositionoriginating atC/20isshowninFIGS.12A-12B.Comparedtopristine fromspecificsites. samples,thechargedanddischargedsamplesstilretain Thecarbonateelectrolyteisexpectedtobestableandnot theirfrestandingarchitecture.However,thesurfacedepos-65decomposeinthe1-4.2VvsLi/Li+rangeandtheformation itedgammamonoclinicphasesulfurredistributesitselfon ofLiFcanbeatributedtodecompositionofthesaltandnot thesurfaceposiblyduetovolumeexpansion-contraction theorganicelectrolyteasnoorganicspeciescanbeseninPDF Image | SYNTHESIS OF GAMMA MONOCLINIC SULFUR
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