氧化硅基电解液用于稳定水系锌锰电池

时间：TIME\@"yyyy'年'M'月'd'日'"2022年3月29日学海无涯页码：第1-页共1页氧化硅基电解液用于稳定水系锌锰电池1Introduction

Aqueouszinc-manganesebatterieswiththeadvantagesofhighsafety,abundantresourcesandhighenergydensity,areconsideredasoneofthemostpromisinglarge-scaleenergystoragebatteries[1-3].Nevertheless,theirpracticalapplicationstillfaceschallenges,whichismainlyduetothedeteriorationofelectrochemicalperformanceofmanganese-basedcathodescausedbyaqueouselectrolyte.Fundamentally,theelectrochemicalstabilitywindow(ESW)ofaqueouselectrolyteislimitedbythethermodynamicpotentialdifferencebetweenoxygenevolutionreaction(OER)andhydrogenevolutionreaction(HER)ofwater.AlthoughtheESWofthesaltsolutioncanbeexpandedrelativetothepurewater(1.23V),itstillcannotmeettherequirementsofthebattery.Ontheonehand,thenarrowESWwillgreatlyrestraintheoperatingoutputvoltageofaqueouszinc-manganesebatteriesandleadtoinsufficientenergydensity.Ontheotherhand,thenarrowESWtendstoleadtogasgeneration(e.g.,O2andH2)orelectrolyteconsumption,whichwilldestroythestructureoftheelectrodesandresultininferiorcyclingstability.

Toovercometheseissuesandachievebetterelectrochemicalperformance,itiscrucialtosuppressthewatersplittingandextendtheESW.Atpresent,alotofresearchesaredevotedtoelectrolyteoptimization,suchasgelelectrolyte[4],ionicliquidelectrolyte[5],organicelectrolyte[6-7],electrolyteadditive[8],etc.Sincemanyissuesinaqueouselectrolytearecausedbyexcessactivewatercontent,manyofthesestrategiesfocusonreducingthecontentofwater.Itwasreportedthatthegelelectrolyte,ionicliquidelectrolyteandorganicelectrolytegreatlyinhibitthedendriteandcorrosionofthezincanodeduetothedecreaseofactivewaterandthezincionsolvationstructuredominatedbyanionicgroups,thenthecyclicstabilityandreversibilityofthezincanodeareimproved[9-10].However,accordingtoZHANGetal’sreport[11],thelesswatercontentinelectrolyteisnotconducivetothereleaseofcapacityandenergyofMn-basedmaterials.Therefore,itisexpectedfromtheperspectiveofelectrolytebyadjustingtheactivewatercontenttorealizethelong-cycle-lifeandhigh-specific-capacityaqueouszinc-manganesebatteries.

Withthisideainmind,apromisingstrategyistodevelopamixedelectrolytethatbasedoninertinorganicandzincsaltsolution,inwhichtheaddedinorganiccomponentsshouldbeconsideredtoinhibitthedecompositionofwater.Thismixedelectrolytemayinheritthehighconductivityofwatersystemelectrolyteandalsocanovercomevariousadversefactorscausedbytheactivewater.Moreover,manyinorganicsubstances,suchassilicateandinsolublesulfates,areeasytoformfunctionalizedgroupsonthesurfaceinaqueoussolutions,whichcangivetheelectrolytespecialelectrochemicalproperties.Inthiswork,wehavedevelopedanewelectrolyte,whichismadeupofsilica(SiOx)fibersandaZnSO4+MnSO4solution(namedasSi-ZMSOelectrolyte).TheSi-ZMSOelectrolytecanwidentheESWofelectrolyteandrestrainthesidereactionofzincanode,whichcanstabilizethecyclicperformanceofZn-MnO2batteriescomparedtopureZnSO4electrolyte.

2Experimental

SynthesisofSiOx/ZnSO4+MnSO4electrolyte:ThesynthesisofSiOxfiberswasreferredtothereportofLIU’sgroup[12].Inatypicalsynthesis,appropriateamountofSiOxfiberswereaddedto10mL2mol/LZnSO4+0.1mol/LMnSO4aqueoussolution(definedasZMSOelectrolyte),thenthesolutionwasplacedunderultrasoundfor30mintoobtaintheviscousmixedelectrolyte.Theelectrolytecontaining0.5gand1gofSiOxfiberswerenamedasSi-ZMSOandSi-ZMSO-10,respectively.Electrolytepreparationcanbescaledupbyadjustingtheamountofrawmaterials.

SynthesisofMnO2nanorods:Thesynthesismethodissimilartothepreviousreport[13].The15mL0.15mol/LMnSO4solution(solutionA)and15mL0.10mol/LKMnO4solution(solutionB)werefirstlyprepared.SolutionBwasaddedtosolutionAdropbydropandstirredfor10min.ThesolutionwasfurthertransferredtoaTeflon-linedautoclaveandheatedat160℃for12htoobtainMnO2nanorods.

Materialcharacterization:X-raydiffraction(XRD)datawerecollectedusingaRigakuD/max2500powderdiffractometer(CuKα,λ=0.15405nm).Thescanningelectronmicroscope(SEM)imageswerecollectedonaSirion200operatingat10kV.Thetransmissionelectronmicroscopy(TEM)imageswerecollectedusingaTitanG260-300transmissionelectronmicroscopy.

Electrochemicalmeasurement:Topreparethecathodeelectrode,aslurrymixedwith70%MnO2,20%acetyleneblack,and10%polyvinylidenefluoridewascoatedontoastainlesssteelwiremeshdiskanddriedat80℃invacuumovernight.Thezincmetalfoilwasusedasanode.Glassfiberfilterpaperwasusedasaseparator.ZMSOorSi-ZMSOelectrolytewasusedaselectrolyte.Thecyclingstabilityandratecapabilitywerestudiedusingamultichannelbatterytestingsystem(LandCT2022A).

3Resultsanddiscussion

Figure1(a)showsthatthemorphologyofSiOxsampleisinterwovennanofibers.ItcanbeclearlyseenintheenlargedSEMimage(Figure1(b))thattherearealotofinterspacesbetweenthenanofibers.Thisfeaturemakesiteasytomixevenlywithaqueouselectrolyte.TheTEMimage(Figure1(c))furtherconfirmsthecharacteristicofalternatingintervalsofSiOxsamples.Eachnanofiberisuniformandhasadiameterof50nm,asshowninFigure1(d).Si-ZMSOelectrolytewaspreparedviaadding0.5gofSiOxnanofibersinto10mL2mol/LZMSOelectrolyte,whichformsamushymixedelectrolyte.OpticalphotographofSi-ZMSOelectrolyteinaninvertedtransparentbottle(Figure2(a))demonstratesthatthiselectrolyteisviscousandhasweakfluidity.Weallknowthathighionicconductivityistheuniqueadvantageofaqueouselectrolyte,andtheinfluenceoffilleronionicconductivityneedstobeconsidered.ItisgratifyingthattheionicconductivityofSi-ZMSOelectrolyteis6.0mS/cm,whichiscomparabletothatofaqueousZMSOelectrolyte(6.4mS/cm)inourtests,asshowninFigure2(b).IntheSi-ZMSOelectrolyte,theinterwovenSiOxnanofiberscanconstructanetworkskeletonforliquidelectrolyteinwhichionscanmovefreely.Thismaximizestheionicconductivityoftheelectrolyte.Comparedtotheaqueouscounterpart,Si-ZMSOelectrolyteshowsawiderstablevoltagewindow(Figure2(c)).TheadditionofSiOxnanofiberscanreducethecontentofactivewaterinelectrolyte.Inaddition,previousreportshavedemonstratedthatthesurfaceofsilicananowireshasmanyhydrophilicfunctionalgroups[14],whichcanfurtherlimittheactivityofwater.Suchhybridelectrolyteissuitableforuseinzinc-manganesebatteriesbecausethereducedwateractivityisconducivetothestabilityandreversibilityofthezincanode(Figure2(d)).Atthesametime,theelectrolyteretainstheappropriatewatercontent,whichcanmatchthemanganese-basedcathodematerialstoplayahighcapacityandstableperformance.

Figure1CharacterizationofSiOxnanofibers:(a,b)SEMimages;(c,d)TEMimages

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Figure2CharacterizationofSi-ZMSOandZnSO4+0.1mol/LMnSO4electrolyte:(a)OpticalphotographofSi-ZMSOelectrolyte;(b)Ionicconductivity;(c)LSVcurvesofSi-ZMSOandZnSO4+0.1mol/LMnSO4electrolyte;(d)DiagramofZn/MnO2batterywithSi-ZMSOelectrolyte

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InordertodemonstratetheadvantagesofSi-ZMSOelectrolyte,cyclingstabilityofZn||Znsymmetricalbatteriesinvariouselectrolyteswasconducted.Inthe2mol/LZnSO4electrolyte,theZn||Znsymmetricalbatteryquicklyshortedoutaftera70-hourcycleatthecurrentdensityof1mA/cm2andareacapacityof1mA·h/cm2,asshowninFigure3(a),whichisconsistentwithmostreportedresults[15-17].AlthoughtheZMSOelectrolytewithMnSO4additive,whichisatypicalelectrolyteforzinc-manganesebatteries,canreducetheelectrochemicalpolarization,thecyclelifeofZn||Znsymmetricalbatteryisonlyextendedto110h(Figure3(b)).HUANGetal[18]andLIetal[19]provedthattheMn(OH)2sphereparticlesformedintheelectrolyteaggregated,andtheZn2+flowwasmechanicallyadjustedtomaketheionfielduniform.Inaddition,thenucleationoverpotentialoftheZnSO4/MnSO4mixtureelectrolyteissmallerthanthatoftheZnSO4electrolyte,whichisalsobeneficialtoretardthegrowthofzincdendritesandtheuniformdepositionofzinc.Therefore,Mn2+additiveinZnSO4electrolytedoessuppresstheformationofdendrite-likezinc,andimproveselectrochemicalstabilityofZn||ZnsymmetricalbatterywithZnSO4/MnSO4mixtureelectrolyte[18].Moreover,Mn2+intheformofMnSO4wasoftenaddedintoZnSO4aqueouselectrolytetosuppressthedissolutionofMnO2cathodeandprovideextracapacitybydepositiononthecathode[19].ComparedtotheZMSOelectrolyte,electrochemicalpolarizationincreasesintheSi-ZMSOelectrolyte.Generally,theincreasedelectrochemicalpolarizationwasnotadvantageousforthelong-termstablecycle.However,itwasfoundthattheZn||ZnsymmetricalbatterywithSi-ZMSOelectrolyteexhibitedmoreexcellentcyclestabilityover400hatthecurrentdensityof1mA/cm2withanareacapacityof1mA·h/cm2(Figure3(c)).Thesmoothdepositionattheanodeinterfacemaybethekeyfactorforthestablecycleofzincanode.WhenthecontentofSiOxnanofiberswasincreased,suchasinSi-ZMSO-10electrolyte,theZn||ZnsymmetricalbatteryshowedasimilarcyclicbehaviorwiththatofSi-ZMSOelectrolyteat200hbefore.Butafterthat,theoverpotentialinSi-ZMSO-10electrolyterapidlyincreased,andthenitexpiredat300h.Thismaybeduetothefactthattheincreasedinterfacialimpedanceresultedfromtheadditionofexcessivesolids.Asthecurrentdensityincreasesto5mA/cm2,Zn||ZnsymmetricalbatterywithSi-ZMSOelectrolytealsodemonstratedagoodcyclicstabilityover400h,whilethefailureismorepronouncedinSi-ZMSO-10electrolyte(Figure3(d)).TheresultsindicatethattheappropriateamountofSiOxisbeneficialtotheimprovementofcyclestability.

Figure3CyclingstabilityofZn||Znsymmetricalbatteries:(a)2mol/LZnSO4at1mA/cm2;(b)2mol/LZnSO4+0.1mol/LMnSO4at1mA/cm2;(c)Si-ZMSOandSi-ZMSO-10at1mA/cm2;(d)Si-ZMSOandSi-ZMSO-10at5mA/cm2

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TheexcellentcyclicstabilityofZnanodeisascribedtotheuniformdeposition,whichisalsoreflectedinothermixedstateelectrolytes[20-22].TounderstandthedistinctionofZndepositingbehaviorbetweenSi-ZMSOandZMSOelectrolytes,ex-situanalysisofZnanodeafter1stdischargedstatewasconducted.AsshowninFigure4(a),afterdischarginginSi-ZMSOelectrolyte,SEMimageshowsasmoothflatsurfaceofZnanode.Thehigh-magnificationSEMimage(Figure4(b))indicatesthatthemorphologyofdepositingproductionisconnectedparticles.InZMSOelectrolyte,however,itisclearthatthesurfaceofthezincanodehasaccumulatedby-products(Figure4(c)).SEMimageathigh-magnificationshowsthattheseby-productsexhibittheshapeofprominences,asshowninFigure4(d).Alargenumberofby-productswillcauseotherproblemssuchascorrosion,hydrogenevolution,[23]andwillnotbeconducivetothestabilityofthezincanode.Suchsidereactionsinaqueouselectrolytewouldbedetrimentalforlarge-scalepracticalapplicationofZIBs[24-25].TheXRDpatternsofzincanodeafterdischargedstateinZMSOelectrolytealsoconfirmedtheaggregationofby-products.Afterdeposition,thezincanodeinZMSOelectrolyteshowsseveralobviousnewpeaks,exceptforthediffractionpeakofZnphase(Figure4(e)).ThesenewpeaksmatchwellwiththeZn4SO4(OH)6·xH2O(ZSH)phase(e.g.,PDF#44-0673andPDF#39-0689).Onthecontrary,zincanodeinSi-ZMSOelectrolyteshowsweakerdiffractionpeakoftheby-productcomparedtothatinZMSOelectrolyte,whichiscalibratedbytheintensityofthediffractionpeakofZnphase.Theseresultsshowthattheadditionofsilicananofibers(denotedasSi)additivetoZMSOelectrolytecaneffectivelyalleviatethesidereactionbetweenZMSOandZnelectrode,thusmaintainingthestructuralstabilityoftheelectrode.

Figure4Ex-situanalysisofZnanodeinZMSOandSi-ZMSOelectrolytes:(a)Low-magnificationand(b)high-magnificationSEMimagesafterdischargedstateinSi-ZMSOelectrolyte;(c)Low-magnificationand(d)high-magnificationSEMimagesafterdischargedstateinZMSOelectrolyte;(e)XRDpatternsofZnanodeafterdischargedstateinZMSOandSi-ZMSOelectrolytes

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Manganese-basedmaterialsareconsideredasthemostpromisingcathodematerialsforaqueouszincmetalbatteries[26-28].However,mostmanganese-basedmaterialsexhibitpoorcycliclife,speciallyunderthelowcurrentdensity.ToexplorethebenefitsoftheSi-ZMSOelectrolyte,theelectrochemicalstoragecapabilityofZn-MnO2batterieswastestedatalowcurrentdensityof100mA·h/g.Figure5(a)showsthedifferentdischarge/chargeprofilesoftheZn-MnO2batterieswithSi-ZMSOandZMSOelectrolytes.Bothsamplesexhibittwodistinctdischargevoltageplatformsbetween1.2Vand1.4V.WenotethattheZn-MnO2batterydeliversahighsecondspecificcapacityofmorethan350mA·h/gintheZMSOelectrolyte,butunfortunatelyitdecayedby62%whenitreached60cycles.Remarkablely,thedischargeplateausofMnO2cathodeexhibitedagradualdisappearanceduringcycling,asshowninpurpledottedline.IntheZMSOelectrolyte,thehighcapacityismainlyduetothelargeamountofactivewaterintheelectrolyte,whichstimulatesthecapacityofmanganese-basedmaterials[11].Buttheproblemscausedbytheaqueouselectrolyteleadtoitspoorcyclingstability,andevendestroytheoriginalmaterialsystem.WhileinSi-ZMSOelectrolyte,Zn-MnO2batterydeliversonlyaspecificcapacityof249mA·h/gatsecondcycles,butitcanmaintainaconsiderablecapacityof209mA·h/gafter60cycles.Moreimportantly,thedischarg