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21 treatment.ThesmothLorentzianasymetricpeakofcar- bonfurtherconfirmsthatsulfurdoesnotreactwiththebare carbonsurface. ElectrochemicalCharacterization 22 lyteandinferinformationaboutthereactionmechanism, diferentialcapacity(dQ/dV)analysisandEISasafunction ofvoltagewereconducted.Aconsistentsinglepeakissen throughoutthe2000cyclesfurthersuportingasingle-phase US 10,991,944B2 FIG.10showstheelectrochemicalperformanceevalua- 5 conversion.Thepeaksminimalyshiftduringthecycling tionofgammasulfurdepositedCNFsusedasfre-standing cathodesinCR2032typecoincelswithreference/counter aslithium.Bothether-andcarbonate-basedelectrolytes wereemployedtounderstandtheelectrochemicalphenom sugestinggoodmaterialintegrityandonlyaminimal increaseinresistanceduringcycling.Asanextstep,the electrochemicalimpedancespectroscopy(EIS)measure mentsofthelithiumhalf-celswithgammasulfurdeposited enonineachsystem.Theelectrodedemonstratesarevers-10CNFsascompositecathodeswerecariedoutatvarious ibleelectrochemicalredoxbehaviorinbothetherandcar bonate-basedelectrolytes.However,thecharge-discharge profilesaredrasticalydiferent.(FIG.4a).Theether-based charge-dischargeprofileexhibitsastandardtwo-plateau behaviorasreportedinmostpriorliteraturereports.Thefirst15voltage.Asseninthisfigure,atypicalNyquistplotconsists plateauat2.3Visatributedtotheconversionofsulfurto long-chainpolysulfidesandthesecondplateauat2.1V representstheconversionoflong-chainpolysulfidestoLi2S2 andLi2S(2.1V).However,thesamegammasulfurdepos itedCNFcathodesincarbonateelectrolytesdemonstratea20lowerpotentialfortheentiredischargecycle.Thetrendis singleplateauat2.0Vinthefirstandalconsecutivecycles reversedwhenthebateryischargedbacktoahigher duringdischargeand2.2Vinchargeprofilessugestingthe potential.Thisobservationcontrastswiththeliterature, posibilityofapolysulfidedigresionroutetodirectlyform whereintheRctisshowntofirstdecreaseandthenincrease lithiumsulfideincarbonateelectrolyte.Thissolid-to-solid backagaininthesamedischargecyclesugestingthe conversionposiblyalsoleadstoahigheroverpotential25formationofsolublepolysulfidesatintermediatevoltages. explainingthelowerplateauvoltageobservedinthecar- TheseintermediatepolysulfidessignificantlylowertheRet bonateelectrolyte.Theelectrochemicalbehaviorisconsis- duetodisapearanceofbothofthesolidinsulatingmate tentwithCVprofiles,whereinthecelswithetherelectrolyte rials—theinitialreactant,sulfurandfinalproduct,LiS.In showtwopeaks,whilethecelswithcarbonateelectrolyte theliterature,theRctofthefinaldischargedcelstilremains onlyshowasinglepeak. 30lowerthantheinitialRct(atOCV)duetoreducedresistance FIG.10Cshowsthelong-termcyclingperformancetested ofLi2Scomparedtopuresulfur.Amonotonicdecreasein undervariousgalvanostaticmodes.Thedischargecapacities Rcduringdischargeinthepresentexperimentsprovides werecalculatedatbothlowandhighcur ratesof0.1C furtherevidencethatweareeliminatingtheformationof and0.5Candthecapacitiesretainedafter10cycleswere polysulfides. 50mAh/gand70mAh/g,respectively.Theresultsforthe35 Toevaluatethepracticalaplicationofthecarbonate dischargecapacitiesatbothlowcurentrate(0.1C)andhigh basedLi–Ssystem,thecelswerecycledwithgamma curentrates(0.5C),areshowninFIGS.18Aand18B, sulfurdeposited-CNFscathodesatvariousCratesand respectively.FIG.10Cshowsthelong-termcyclingdatafor loadings.Asshownearlier,thesebateriesdemonstrate celsmadeusingcarbonateelectrolyte.Thesecelsdemon- stablecapacityata0.5Crateforover2000cycles.To strateultra-stableandprolongedcyclingat0.5Cratefor40demonstratebateryoperationatharshconditions,thebat 2000cycleswithonly0.13%decayaftertheinitial200 teriesweretestedforlongtermcyclingat0.1C.The cycles.Thecelsretainedacapacityof704mAh/gevenafter bateriesprovidedstable~60mAh/gcapacityforover10 200charge-dischargecycles.Theinitialdropincapacity cycleswithasmal0015%decayandacoulombic maybeatributedtothelosofcontactduetovolume eficiency>=9%.Inadition,thesebateriesshowexcelent expansionduringcyclingwhichstabilizesasthecycling45rateperformancewithcapacitiesof170,1080,980,90, proceds.AsshowninFIG.10D,thedischargeprofile continuestoexhibitasingleplateauthroughtheentirecycle lifeof20cycles. 750,600and410mAh/gat1C,2C,5C,10C,15C,30 Cand40C,respectively.Itisinterestingtosethesecels exhibitingacapacityof40mAh/gevenat40Ccore IthasbenestablishedinarecentreportbyKimetal.that spondingtoadischargeandchargetimeofonly~30 polysulfides,ifgenerated,atackcarbonatespeciesvia50seconds.Thetraditionalether-basedbateriesperformonly nucleophilicsubstitutiontoformtheireversibleproducts, upto2Catwhichtheperformancedeterioratessignificantly. thiocarbonateandethyleneglycol,andshutdownfurther FIG.11Bshowsthatthecelsexhibitasimilarsingleplateau electrochemicalactivityafterthefirstcycle.Therefore,our dischargeatalCrates.Suchratecapabilitysugestsefi datasugeststhatthesegammasulfurdepositedCNF-based cientnanoscalecontactbetwenthegamma-monoclinic celscontinuetofolowapolysulfidedigresionroute5sulfurandthehostCNFsandgodinterfacialelectrode throughtheentirecyclingincarbonateelectrolyte,which electrolytecontactowingtothe3Dinter-fiberporousarchi explainsnotonlycontinuedbateryoperationdespitethe tecture.Furthermore,thebinder-frefrestandingformatof presenceofcarbonatespeciesbutalsotheexcelentcycle theCNFhostapearstoprovideuninteruptedelectron stabilityof2000+cycles.Forcomparison,thecyclingdata pathwaysdespitethepresenceofinsulatingsulfur.Thisis inetherelectrolyteisshownwhereinthesematerialsfolow60uniquecomparedtotraditionalslury-basedcathodeswhere astandardroutewithpolysulfidesastheintermediateprod- carbonandsulfurpowdersaremixedtogetherwithlimited ucts.Thecyclingdatainetherelectrolytemaybefoundin tonocontroloverspatialmorphologywhichtherebydete FIG.19.Hereweseagradualdeclineincapacityduetothe rioratesoveralcompositeconductivity.FIG.11Bshowsthe expectedpolysulfideshutlingandsubsequentlosofactive cyclingdataforhighercomercialy-relevantsulfurload material. 65ings.Celswith5mg/cm2ofsulfurdemonstratestable Tofurthercoroboratethisuniqueelectrochemicalbehav- cyclingfor300+cyclesat0.1C.Thisfindingdemonstrates iorofgammamonoclinicphasesulfurincarbonateelectro- thatunconfinedsulfurdepositionusinggammamonoclinic potentialsduringcharge-dischargecycles.FIG.10Fpresents thetypicalNyquistplotsfortheLi–Sbateriesofthe inventionilustratingtheirimpedancetrendsasafunctionof ofasemicircleinthehighfrequencytomediumfrequency range,whichisatributedtotheinterfacialchargetransfer resistance.Thechargetransferresistance(Ret)monotoni calydecreasesasthecathodiccurveprogresestowardsaPDF Image | SYNTHESIS OF GAMMA MONOCLINIC SULFUR
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