静电纺丝大量制备自支撑WS2碳纤维复合材料及其作为高性能锂电负极材料的

静电纺丝大量制备自支撑静电纺丝大量制备自支撑WS2WS2碳纤维复合材料及其作为高性碳纤维复合材料及其作为高性 能锂电负极材料的能锂电负极材料的 ArticleScalable productionof self supported WS2 Fsby electrospinningas theanode for high performance lithium ionbatteriesShasha Zhou Junnian Chen Lin Gan Qing Zhang Zhi Zheng Huiqiao Li Tianyou ZhaiReceived 11Novemberxx Revised 26Novemberxx Aepted 27Nov emberxx Published online 25Januaryxx Science ChinaPress andSpringer Verlag BerlinHeidelbergxxAbstract WS2 carbon nanof i bers WS2 Fs areobtained bya simpleelectrospinning methodin which few single layer WS2is uniformlyembedded incarbon f i bers When usedas theactive anode material for Li ion cells thesenanof i bers exhibita f i rst cycle discharge charge capacity of941 756mAh g at100mA g andmaintain acapacity of458mAh g after100cycles at1A g The evolutionof sizeand crystallinity of WS2with heating treatment aresystem atically studied which arefound tostrongly inf l uencethef inal electrochemical performance Interestingly the WS2samples of lowest crystallinityshow the highest performanceamongall studiedsamples which couldresult from the largeinterfacialcapacity for Li ionsdue to their large specif i csurface area More interestingly the inherentf l exible attri bute ofelectrospun nanof i bersrenders thema greatpotentialin theutilization ofbinder free anodes Similar highdis charge charge capacity of761 604mAh g with a f i rstcoulombic eff i ciency of79 4 has been achieved in thesebinder free anodes Considering theuniversal ofsuch simpleandscalable preparationstrategy it isvery likelyto extendthis method toother similartwo dimensional layeredmate rials besidesWS2and providesa promising candidate elec trode fordeveloping f l exible battery devices Keywords WS2 Fs Electrospinning Li ionbatteries Anode Self supported Crystallinity1IntroductionLithium ion batteries LIBs a rechargeablepower supplyunit have dominatedthe mercialmarket ofportableelectronic devicesfor manyyears 1 7 Graphite the widelyused anode material delivers atheoreticalcapacity ofonly372mAh g which severelylimits theuse ofLIBsin thelong time serviceelectric appliancessuch aselectromobiles 2 4 8 9 Therefore it isimperative toexplorehigher energydensity alternativesto graphite Sim ilar to the layeredgraphite transition metal dichalcogenides TMDs MX2 M W Mo V X S Se Te a sandwichstructurein whichthe X M X layersare covalentlybondedbetween in plane atomsand coupledwith weakvan derWaalsforces vertically have attractedconsiderable attentioninrecent years 9 13 In contrastto graphite TMDs haveamuch largerinterlayer spacing 0 6nm which providesidealchannels for the diffusionof Li as wellas thetrans portation ofelectrons Moreover the lithiationof MX2notonly involvesthe intercalation of Li into the layers butalsoundergoes subsequentlyconversion reactionwhich willfur ther induceexcess capacityfor Listorage 14 16 There fore TMDs areexpected to be a better alternativeto graphiteas anode materials for Listorage Among reportedTMDs MoS2is oneof the mostlystudied materialsfor theapplication inLIBs 5 14 17 19 Shasha Zhouand JunnianChen contributedequally tothis work S Zhou J Chen L Gan Q Zhang Z Zheng H Li the peaksat 1 370and 1 600cm 1are assignedtotheD band corresponding tothedefects existingin thegraphitic carbonmaterials and Gband related tothesp2carbon atomsin thegraphiticlayers of carbon consistent with the valuedemonstrated inpreviousreports 2 3 35 The resultsconf irm thatoursamples areconstituted by WS2and carbonponent Figure3shows thethermogravimetry TG curve oftheWS2 Fs posite The TGtest wascarriedoutin airatmosphere As seenfromtheTG result the curvehas twoweightloss from300to550 C WS2was oxidizedintoWO3corresponding tothe fi rst step weightloss ofabout6 5 The weightloss of11 4 in thesecond steprep resents thedepositionofthe Fs 15 36 By cal culation the ratioof WS2in WS2 Fs positeis about88 6 TEM imagesoftheWS2 Fs samplesobtained atdifferent calcination temperatureare shownin Fig 4 Thelow magnif ication TEMimage shownin Fig 4a confi rmstheresults ofSEM imagesinwhichthe smoothnanofi bersare interconnectedwithadiameter of 200nm From thehigh resolution TEM HRTEM in Fig 4b we cansee thatsmalland thinnanoplates uniformlydisperse inthe carbonf i bers The selected area electrondiffraction SAED pat tern shownintheinset displaysatypicalphotograph ofpolycrystallineand the 103 100 110 diffraction ringsarecorresponding tothe XRDprofi les precisely To illus trate thedifference betweenthe threeposites weobtained detailed structuresof WS2 Fs 500 WS2 Fs 700and WS2 Fs 900are shownin Fig 4c e respectively For WS2 Fs 500 ultraf ine single layer marked by the whiteframe whosethickness is0 32nmwhich isequal tothe distancebetween twoS layersin asinglelayer or few layer 3 6layers WS2nanoplates areembedded inthecarbonfi bersdisorderly The distancebetweenthe layersis0 66nm as shownin Fig 4c and thevalueis slightlylarger than0 62nm for the bulkWS2inthe directionvertical to 002 plane This canbe explainedasfollows The smalland thincharacters ofthese nano plates inducea weakerinterlayer forceso that the S W Slayers inthe nanocrystalsdeparture fromeachotherbyforces fromthe surroundingcarbon atoms The enlargedinterlayer space willreduce theresistance of Li diffusion In addition the lateral dimension ofthese small units isonly2 5nm which willprovide ashort transportpath forbothelectrons andLi ions As shownin Fig 4d e thecrystalline sizeof WS2grows rapidlyalong with theincrease intemperature In thecase of WS2 Fs 700 smallunitsof WS2aggregate togetherresulting intheincrease inboth layernumber 6layers mostly and lat eraldimensionsize 10nm The distancebetweenlayers reducesto0 64nm This isbecause thegrowth ofFig 2 Color online XRDpatterns a and Ramanspectra b ofWS2 Fs undergonedifferent thermal treatment conditionsFig 3TG curveof WS2 Fs230Sci Bull xx 61 3 227 235WS2crystals inboth verticaland horizontaldirectionswould renderstronger binding forces betweenthelayers As forWS2 Fs 900 theWS2units enlargedfurther withhighcrystallinity The continuousand multilayered 10layers WS2grows alongthe directionof lengthdirectionof the fiber The observedinterlayerspacefurther decreasesto0 62nm which isequal to that ofbulk phase WS2 The electrochemical performances ofdifferent WS2 Fs sampleswere investigatedby GVdischarge andchargeinthewindow of5mV to3V at a current densityof0 1A g andCV between0 01and3V ata scanrate of0 1mV s Figure5a showsthe CVcurves of WS2 Fs 500attheinitial threecycles In thefi rst cycle a smallcathodicpeak at1 75V isobserved which correspondstothe lithiuminsertion intoWS2toformLi xWS2 2 9 24 37 The peak at0 63V canbe attributedtothesubsequentconversion reactionby whichWS2has beenreduced toWatoms embeddedin Li2S matrix 15 34 Meanwhile asolid electrolyteinterface SEI fi lm originatingfrom theelectrolytedeposition isformed duringthe dischargeprocess which shouldbe responsiblefor theirreversiblecapacity loss 1 2 During theanodic scans two smallpeaksaround1 15and1 55V anda sharppeakat2 23Vare assignedtotheextraction ofLi ionsand theconversionof Li2S toS 15 From thesecond cycle the reductionpeakat0 63V disappears while theoriginal reductionpeakat1 75V isreplaced bya newpeak appearingin thepotentialrange from1 65to2 2V The well overlappedcurves withunchanged redoxpeaks afterthefi rstcycleimply thegood reversibilityduring lithiationand delithia tion 38 The CVcurves of WS2 Fs 700and WS2 Fs 900are shownin Fig 5b c respectively Comparedto WS2 Fs 500 the CVplots beethinner and thereduction oxidation peaksbee sharper while thelocationofthepeaks doesnot changetoo much The fi rstthreedischarge charge cyclesofWS2 Fs 500sampleare shownin Fig 5d The sampledelivers aninitial dis charge capacityof941mAh ganda charge capacityof756mAh g correspondingto afi rstcoulombiceffi ciencyof80 3 The coulombiceffi ciency increasesto99 1 after10cycles The nearlyrepeated discharge chargecurves afterthefi rstcycle arewell agreedwiththeCVmeasurements The fi rstdischarge charge capacityare586 482and579 490mAh g forWS2 Fs 700and WS2 Fs 900 respectively as shownin Fig 5e f which arelowerthan thatofWS2 Fs 500 Fig 4Low magnif ication TEMimage a and HRTEMimage b ofWS2 Fs 500 inset SAED patternofWS2 Fs 500 HRTEM imagesofWS2 Fs 500 c WS2 Fs 700 d and WS2 Fs 900 e respectivelySci Bull xx 61 3 227 235231Note thatthecapacity values aboveare basedon thetotalmass ofthe electrodewhich includesthe activematerials conductive additiveand polymerbinder Ifconsidering theactive materialsonly theWS2 Fs 500can deliverafi rstdischarge charge capacityof1 494 1 200mAh g which islarger thanthe valuereported pre viously e g Zhu andco workers 37 mixed WS2andgraphite ina massof1 1showing adischarge chargecapacity of 1 000 820mAh g Kang andco workers 36 prepared WS2 carbon positemicrospheres byspraypyrolysis whichdisplayed aninitial discharge chargecapacity of1 055 714mAh g The goodperformance ofoursamples canbe attributedtothefollowing factors 1 Innumerable poresand largespecif icsurface area broughtbythe3D architecturemade upofinterconnectedcarbonnanof i bers providefast diffusionofLi and electrons 2 Large specificsurfaceareaofthenanosized WS2intro duces extrainterfacialstorage sites forLi ionsand that willinduce excesscapacity 3 As agood conductor carbonfiberswith1D characterplay atremendous rolein theconductionof currents 4 Another important role thecarbonfibers playhere isto relievethe structuralstraincaused bythe volume changeduringLi ion insertionandextraction The differentelectrochemical propertiesofWS2 Ts samplesmay relateto theirdegree ofcrystallinity anddimensionalsize As shownin Fig 6 withtheincrease inheatingtreatment the crystallinityofWS2improvedgradually withexpansion ofboth verticaland horizontaldirection For WS2 Fs 500 the enlargedinterface spacewillbenef ittheinsertion ofLi into thegalleries Mean while the thinand smallWS2nanoplates supplya shortpathwayforthetransportationofLi andelectrons Thetiny e venlydispersiveWS2nanoplateswithhugespecif icsurfacearea offera lotof interfacialstoragesitesbetween Sand CatomsforLi especially forsingle layer WS2where bothsidescan aeptLi ions which inducesthe excesscapacity Thatistosay interfacialstoragemechanismisals oinvolvedapart fromusual intercalationmechanism Carbon evenlydistributingaround WS2plays animportantrolein aom modating thevolumechange For WS2 Fs 700and WS2 Fs 900 the prolongedlateral distanceand narrowedinterlayerspace Fig 3d e enhanced thediff iculty forLiions toinsert Meanwhile the interfacialsitesforLi ionstorage wouldreduce rapidlywiththegrain growth Both ofthem would leadtoadecrease inthe totalnumber ofthestorage sitesforLi The storagemechanism ofthe threesamplesalso givesan explanationforthedifference oftheCV curves In conditionof highercrystallinity the homo geneous degreeofthepound consistingbyWS2andcarbon willreduce andthatwillgreatly affectthe perfor mance of carboninremitting thestrain causedby volumeexpansion shrinkage ofWS2intheprocess ofdischarge charge All ofthemwouldinduce inferiorelectrochemicalperformance inevitably especially thecapacity Fig 5CV ofWS2 Fs 500 a WS2 Fs 700 b and WS2 Fs 900 c over avoltage rangeof0 01 3 00V ata scanningrateof0 1mV s Discharge charge profi lesofWS2 Fs 500 d WS2 Fs 700 e and WS2 Fs 900 f atacurrentdensityof100mA g232Sci Bull xx 61 3 227 235Figure7a showsthe cyclingperformance ofthe threesamples atacurrent of1A g The remarkablecyclingstability andoutstanding rateperformance ofWS2 Fs 500are manifestedin parisonwithWS2 Fs 700and WS2 Fs 900 furtherconfi rmingthe analysisabove For WS2 Fs 500 its firstdischarge capacityisas highas685mAh g After100cycles it canstillmaintain at458mAh g withthecapacity retentionof66 9 As forWS2 Fs 700and WS2 Fs 900 thecapacity ofonly220and135mAh g isretained after100cycles respectively Figure7b showsthe rateperfor mances ofWS2 Fs samplesobtained atdifferentcal cination temperature The capacityofWS2 Fs 500isaround770 635 554and495mAh g whenthe currentdensityincreases from0 1 0 2 0 5to1 0A g respec tively A capacityof735mAh g canbe recoveredwhenthe currentbacks to0 1A g exhibiting agood ratecapabilityofWS2 Fs 500 In contrast WS2 Fs 700and WS2 Fs 900display amuch lowercapacityvalue e g their capacitydecays to260and190mAh gatthecurrent of1A g As showninFig 8a both theas spun NH4 2WS4 PVP nanofibers and thegray blackWS2 Fs fi lmcanbe curledtoacylinder shapewithout anydamage which indicatesthehighflexibility ofthe membraneevenafter theannealing step In viewoftheremarkable flexi bility andthe satiablethickness ofthe sampleWS2 Fs 500 thefi lmcan serveasaself supported anodeelectrodefor celltests Amazingly without anyaddition ofbinderand conductiveagent the self supported electrodestillshows goodelectrochemicalperformance asshowninFig 8b The firstdischarge andchargecapacityis ashighas761and604mAh g respectively And thefirstcoulombic efficiencyis calculatedtobe79 4 which isparabletothatfabricated viatraditional positeelectrodeprocedure Figure7c showsthe cyclingbehaviorof theself supported WS2 Fs 500electrode ata currentdensityof100mA g A capacityof334mAh gcanbemaintained after50cycles The slowdegradation ofcapacityupon cyclingmay attributetothepoor conduc tivity Carbon originatedfromthecarbonization ofPVP at500 C hasweaker conductivitythan C45which isused asconductiveagent In addition the structureof self sup ported electrodecan notbe wellmaintained duringthecharge discharge procedurewithout theadhesive effectofbinder But thissimplif ied methodfor electrodemanufac ture undoubtedlysupplies a promising approachto thefabricationof fl exibledevices Fig 6Schematic diagramshowing thechange ofthe crystallinityofWS2with increasingthermaltreatmentandthelithiation patternsofWS2 Fs 500 I WS2 Fs 700 II and WS2 Fs 900 III respectivelySci Bull xx 61 3 227 2352334ConclusionsIn summary flexible WS2 Fs filmwas preparedby asimpleand facileelectrospinning methodinascalable way Themorphologiesandcrystallinityof nanofiberscan betunedbytheheatingtreatment Higher temperatureinducedlarger sizeand bettercrystallinityofWS2 how ever the Li ion storagecapacityofcorresponding nanofi bers behavedreversely The lowestcrystallinity nanofibers WS2 Fs 500 showed themost excellentperformancefor Li ion batteries resulting fromtheir uniquestructuresin whicha largenumber offew single layer WS2nano plates uniformlydispersedinthe frameofcarbon nanofi bers Such aunique positestructure providesa hugespecificsurfacearea which rendersLi ionsaessible andfasttransport pathsand hencea largerLi ion storagecapacity Further the inherentflexibility ofelectrospunnanof ibers makesa high performance binder free self supported anodepossible In viewofthemerits ofWS2 F electrospunnanofibersanduniversal ofthis simplefabricationstrategy apromisingapplication inLi ionstorage andflexiblebatterydevices canbe expected Acknowledgments Thiswork wassupported bythe NationalBasicResearch Programof China xxCB932600 the NationalNatureScience Foundationof China 21571073and51302099 the ProgramforHUST InterdisciplinaryInnovation Team the FundamentalFig 7a Cyclingperformances ofsamplesat1A g for100cycles b Rateperformances ofsamples andWS2 Fs 900at currentsof0 1 0 2 0 5and1 0A gFig 8a Aphotograph ofthe curledas spun NH4 2WS4 PVPmembrane andWS2 Fs membrane b Thefirst threedischarge charge profilesof self supported WS2 Fs 500anode ata currentdensityof100mA g c Cyclingperformance ofself supported WS2 Fs 500anode at100mA gfor50cycles234Sci Bull xx 61 3 227 235Research FundsfortheCentral University and Programfor NewCenturyExcellent Talentsin University NCET 13 0227 We thanktheAnalytical andTesting Centerof HuazhongUniversityofScienceand Technology Conf lict ofinterest Theauthors declarethat theyhave noconf lictof interest References1 Scrosati B Hassoun J Sun YK xx Lithium ion batteries Alook intothe future Energy EnvironSci4 3287 32952 Chen RJ Zhao T Wu WPet al xx Free standing hierarchi cally sandwich type tungstendisulf ide nanotubes graphene anodefor lithium ion batteries Nano Lett14 5899 59043 Duan ZQ Sun YC Liu YTet al xx Scalable productionoftransition metaldisulphide graphitenanof lakeposites forhighperformance lithiumstorage RSC Adv4 41543 415504 Chang K Chen WX xx Single layer MoS2 graphene dispersedinamorphouscarbon towardshighelectrochemicalperfor mancesinrechargeable lithiumion batteries J MaterChem21 17175 171845 Li HL Yu K Fu Het al xx MoS2 graphene hybridnanofl owerswith enhancedelectrochemicalperformancesasanode for lithium ion batteries J PhysChem C119 7959 79686 Zhang JJ Yu A xx Nanostructured transition metal oxidesasadvanced anodesforlithium ion batteries Sci Bull60 823 8387 Song LX Yang SJ Wei Wet al xx Hierarchical SnO2nanoflowers assembledby atomicthickness nanosheetsas anodematerialforlithiumion battery Sci Bull60 892 8958 Liu Y Wang W Huang HBet al xx The highlyenhancedperformance oflamellar WS2nanosheet electrodesupon inter calationofsingle walled carbonnanotubes forsupercapacitorsand lithiumions batteries Chem Commun50 4485 44889 Chen DY Ji G Ding Bet al xx In situnitrogenated graphene few layer WS2posites forfast andreversible Li storage Nanoscale5 7890 789610 Chhowalla M Shin HS Eda Get al xx The chemistryof two dimensional layeredtransition metaldichalcogenide nanosheets Nat Chem5 263 27511 Rao R Gopalakrishnan K Maitra U xx Comparativestudy ofpotential applicationsof graphene MoS2 and othertwo dimensional materialsin energydevices sensors and relatedareas ACS ApplMater Interfaces7 7809 783212 Wang H Feng HB Li JH xx Graphene andgraphene likelayered transitionmetaldichalcogenidesin energyconversionand storage Small10 2165 218113 Fang WY Zhao HB Xie YP xx Facile hydrothermalsyn thesisofVS2 graphene nanopositeswith superiorhigh ratecapability aslithium ion batterycathodes ACS ApplMaterInterfaces7 13044 1305214 Zhu CB Mu XK van AkenPA et al xx Single layeredultrasmall nanoplatesof MoS2embeddedincarbonnanofi berswithexcellent electrochemicalperformanceforlithium andsodiumstorage Angew ChemInt Ed53 2152 215615 Yu S Jung JW Kim ID xx Single layersofWS2nanoplatesembedded innitrogen doped carbonnanofibers