1、Cite this:NewCarbonMaterials,2023,38(3):510-521 DOI:10.1016/S1872-5805(23)60731-0A bimetallic sulfide Co9S8/MoS2/C heterojunction in a three-dimen-sional carbon structure for increasing sodium ion storageCHENHong,MUJian-jia,BIANYu-hua,GAOXuan-wen*,WANGDa,LIUZhao-meng,LUOWen-bin*（Institute for Energy
2、 Electrochemistry and Urban Mines Metallurgy,School of Metallurgy,Northeastern University,Shenyang 110819,China）Abstract：Thesynthesisofhigh-rateandlong-lifeanodematerialsforsodiumionbatteries(SIBs)hasattractedmuchattention.However,theslowkineticsandlargeincreaseinvolumeofthebatteriesremainmajorprobl
3、ems.Bothmetal-organicframeworksandMoS2haveshownpropertiessuitableforSIBs,makingresearchontheircompositesystemsanattractiveareaofresearch.Wereporttheformationofflower-likeCo9S8/MoS2/Ccompositesbyasimultaneousvulcanization-carbonizationprocessusingMoCl5astheMosourceanda2-methylimidazolecobaltsaltasthe
4、CoandCprecursoratdifferenttemperatures（600,700and800C）insub-limedsulfur.Theeffectoftheheterojunctiononthediffusionkineticswasanalyzedusingdensityfunctionaltheory.Theresultsin-dicatethattheelectronicstructureisdifferentattheinterfaceintheheterogeneousstructure,exhibitingtypicalmetallicpropertiesandbe
5、tterelectronicconductivity.Inaddition,theanodematerialCo9S8/MoS2/Csynthesizedat700Chadthemoststablestructureandbestelectrochemicalperformanceofthethreesamples.Notably,thedischargecapacityofCo9S8/MoS2/C-700fullyrecoveredfrom368to571mAhg1andthenstabilizedat543mAhg1whenthecurrentdensitywasrestoredfrom4
6、000to40mAg1.Thisworkdemonstratesthepreparationofheterojunctionmaterialsforcompositeanodematerialsasasteptoproducinghigh-performancemet-alSIBs.Key words:Sodiumionbatteries；Anode；Metal-organicframe；MoS2；Co9S81IntroductionLithium-ionbatteries are now commonly em-ployed in portable consumer electronics
7、gadgets aswell as emerging electric vehicles12.However,themassiveriseoflithium-ionbatteries(LIBs)impliesajump in lithium resource prices,as well as a limitedandunevendistributionoflithiumreservesaroundtheglobe,limiting the development of LIBs35.Due totheirabundantmaterialsandsimilaritywithlithiumsch
8、emistry,sodium-ion batteries(SIBs)have drawn agreat deal of interest and are considered capable ofpartiallyreplacingLIBs67.Transitionmetalsulfides(TMS)havereceivedagreatdealofattentionasan-odematerialsforSIBsduetotheirenormoustheoret-ical capacity and high safety,as well as their higherelectricalcon
9、ductivityandfastercharge-dischargere-actionkineticsthanoxides.Amongrecentlyproducedanode materials,MoS2 is a classic example of TMS,withsimplepreparation,abundantrawmaterials,highinterlayerspacing(0.62nm)andweakinterlayervanderWaalsforces811.TMSdoes,however,havesev-eral flaws.On one hand,TMS experie
10、nces severevolumeexpansionduringchargeanddischarge,caus-ingcyclestabilitytobepoor12.Ontheotherhand,al-thoughthan oxides,TMS have generally lower con-ductivitywhichcausesdelayedelectrochemicalreac-tionkinetics and results in inadequate rate perform-ance1314.Theconstruction of heterogeneous metal sulf
11、-idesis an effective method for addressing the draw-back of low electrical conductivity1516.When com-pared to single-phase metal sulfides,heterogeneousmetalsulfidescannotonlypromotethecreationofaninternalelectricfield,butalsoenhancetheactivityofelectrochemicalreactions at the heterointerface,in-Rece
12、ived date：2023-01-05；Revised date：2023-02-28Corresponding author：GAOXuan-wen,AssociateProfessor.E-mail:;LUOWen-Bin,Professor.E-mail:Author introduction：CHENHong,Ph.Dcandidate.E-mail:Supplementarydataassociatedwiththisarticlecanbefoundintheonlineversion.第38卷第3期 新型炭材料（中英文）Vol.38No.32023 年6月 NEWCARBONM
13、ATERIALS Jun.2023creasing electrical conductivity1719.It can also im-provecrystallinityattheheterointerface,aswellasin-ducelattice mismatches,distortions and defects,al-lowing the reaction kinetics of electrode materials tobe tuned2022.Due to the continuous insertion andconversion/alloying reaction
14、in SIBs,there is rapidvolume change of electrode materials.To solve theaboveproblems,porousnanostructureengineeringcanbeconstructedtoimprovethestabilityofsodiumstor-ageperformanceofmixedmetalsulfides.Metal-organicframeworks(MOFs),asporousin-organic-organic hybrid materials with high specificsurface
15、area,controllable structure,and tunable poresize,arewidelyusedasprecursorsforthepreparationof carbon-metal or metal sulfide composites2325.Ingeneral,MOFsexhibitimpressiveperformanceasact-ivematerialsforenergystorageduetotheircomplexand varied structural advantages2628.Moreover,thenanostructureofMOFs
16、cannotonlyshortenthediffu-sion path of Na ions,but also withstand the volumeexpansioncausedbytheinsertionofNaions,provid-ingastablesupportfortheoverallstructure2829.With the aim of overcoming the weaknesses ofany single component,in this work,a flower-likeCo9S8/MoS2/Ccompositewasdesignedandconstruc-
17、ted by a simultaneous vulcanization-carbonizationmethodatdifferenttemperaturesusingCo-ZIFaspre-cursorand adding Mo source.By systematically in-vestigatingtheeffectofsulfurvacanciesandmicroto-pography on sodium storage behaviour,the flower-likeCo9S8/MoS2/Cmicrospheresformedbyintercon-nected nanosheet
18、 arrays(Co9S8/MoS2/C-700)possessthebestelectrochemicalperformance.Heterogeneousmetalsulfidescannotonlyenhancetheelectricalcon-ductivityof the electrode material due to the hetero-structure,but also facilitate electron/ion transport.Most importantly,the spherical structure provides astablesupportandt
19、heporousstructureprovidesabuf-ferspaceforthevolumedeformationcausedbysub-sequentredoxreactions.Itisworthnotingthatthedis-chargecapacityofCo9S8/MoS2/C-700canfullyrecov-er from 368 to 571 mAh g1 and then stabilize at543mAhg1whenthecurrentdensityisrestoredfrom4000to40mAg1.2Experimental 2.1 Materials sy
20、nthesisZIF-67 was synthesized by a room temperatureprecipitationmethod that has been reported previ-ously30.Typically,Co(NO3)26H2O(5.82g,0.02mol)and 2-MeIm(6.16 g,0.075 mol)were dissolved in150 mL of methanol to form clarified liquor.Then,the solution of 2-MeIm mixture was poured into thesolutionofC
21、o(NO3)26H2Oandstirredevenlyatroomtemperaturefor24h.Finally,thepurpleZIF-67wasobtainedbycentrifugingthemixtureandwashingsev-eral times with methanol and drying at 60 Covernight.20mgoftheaboveZIF-67templatewasdisper-sed in 40 mL CH3OH to form a uniform solution.Then 1.5 mL MoCl5(0.2 mol L1)solution an
22、d0.1664gofNH4HCO3weresuccessivelyaddedtothesuspension.Aftermagneticstirringfor7h,thebrownprecipitate was collected by centrifugation,washedthree times with deionised(DI)water,and dried at60Covernight.Co9S8/MoS2/C-700wasfabricatedbyasimultan-eous vulcanization-carbonization method.0.02 g ofas-prepare
23、dMo-based precursor and 0.05 g of sub-limedsulphurwereuniformlymixedandplacedinthecentreofaquartztube.Afterrinsingwithhigh-purityargon,thefurnacewasheatedto700Cataheatingrate of 5 C min1 and maintained for 1 h under aH2/Ar(90%v/v)mixedatmosphere.For comparison,the samples of Co9S8/MoS2/C-600 and Co9
24、S8/MoS2/C-800 were also synthesized inthesamemannerunderthesameconditionsexceptthecalcinationtemperature was 600 C and 800 C,re-spectively.2.2 Materials characterizationThe phase compositions of the ZIF-67 templateandCo9S8/MoS2/C-700,600,800wereanalysedbyX-ray diffraction(XRD)(Deutschland BRUKER D8)
25、usingCu-Kradiation.Scanningelectronmicroscopy(SEM SU8220,Hitachi High-Tech Company),high-resolutiontransmissionelectronmicroscopy(HRTEM)第3期 CHENHongetal:AbimetallicsulfideCo9S8/MoS2/Cheterojunctioninathree-dimensionalcarbon 511andasuper-XEDSdetectorsystem(Bruker,Super-X,USA)was used to examine the m
26、orphology,micro-structureof the samples and energy dispersive spec-trometry(EDS)elementalmaps.X-rayphotoelectronspectroscopy(XPS,ThermoESCALAB250XIAlKX-raysource)wereusedtoanalyzeelementalchemic-alcompositionsofmaterial.2.3 Electrochemical measurementsThe as-obtained active material,carbon black,and
27、 sodium carboxymethyl cellulose(CMC)weremixedwithDIwaterinaweightratioof721toformaslurry,andcoatedonCufoil.Thecoatedfoilwasdriedat80Cinvacuumovernightandcutintodiscswithadiameterof12mm.Glassfiberwasusedas the separator and metallic sodium was used ascounter electrode for SIBs.1.0 mol L1 NaClO4 dis-s
28、olved in EC/DMC(11,volume ratio)with FEC(5%,mass fraction)was used as electrolyte.Theabove working electrode,glass fiber,electrolyte andcounterelectrodeswereassembledinagloveboxofAratmospheretoobtainthehalf-cells.Thechargeanddischarge test measurement was carried out in thevoltage range of 3.0-0.01
29、V versus Na/Na+at roomtemperature(25 C)using LAND CT2001A testingsystem.Cyclicvoltammetry(CV)testswereconduc-tedonanLANHEM340Ainstrumentelectrochemic-al workstation between 0.01 and 3.0 V vs.Na+/Nawiththestatedscanrates.Electrochemicalimpedancespectroscopy(EIS)was carried out on the Princeton2273atf
30、requenciesof100kHzto10MHz.2.4 Density functional theory calculationThedensityfunctionaltheory(DFT)calculationswereperformed using the Vienna Ab initio Simula-tion Package(VASP)3132.The Perdew-Burke-Ernzerhof(PBE)functional generalized gradient ap-proximation(GGA)was considered to describe theexchang
31、e-correlation.The kinetic energy cutoff fortheplanewavewasabout520eV,whichwasappliedforthewavefunctionexpansionofS,MoandCo.Inaddition,Brillouinzoneintegrationonthegridwitha333and12121kgridmeshwascarriedoutto achieve geometry optimization and calculation ofthedensityofstates(DOS),respectively.TheMoan
32、dCo atoms use the DFT+U technique,where the UJparametersforMoandCostatesaresettobe5.5and3.32eV,respectively.Thequasi-Newtonmethodwithenergyand force convergence criteria,and the New-tonmethodwithenergyandforceconvergencecriter-iaof1.0106eVperatomand0.01eV1inthestructuraloptimizationwereperformedforM
33、oS2andCo9S8toobtainhighaccuracy.ThevanderWaalsin-teraction force was analyzed with a semi-empiricalDFT-D3 method.Virtual interaction was avoided byapplyinga15vacuumlayerthickness.3Resultsanddiscussion 3.1 Structure and compositionThe synthesis process of the flower-likeCo9S8/MoS2/Cnanocomposite is s
34、chematically illus-tratedinFig.1a.TheuniformZIF-67nanocrystalpre-cursorwasgeneratedbyasolvothermaltechnique,asevidenced by scanning electron microscopy(SEM)image(Fig.S1a).Then,molybdenum pentachloridewasaddedtothesolutioncontainingZIF-67precurs-orasamolybdenumsourcetoobtainauniformmo-lybdenumprecurs
35、or.Finally,because the electroneg-ativityofCo(1.9)isslightlyhigherthanthatofMo(1.8),CoandMoionswillreactwithsulfurpowderathigh temperature to form Co9S8,and then cause theheterogeneous nucleation and subsequent growth ofMoS2nanoparticlesonthesurfaceofCo9S8.The crystal structure of as-synthesized sam
36、plewas studied by X-ray diffraction(XRD).The XRDpattern of the Co9S8/MoS2/C composite prepared atdifferent temperatures(600,700,800 C)are showninFig.1b.Asexpected,allsamplesshowsimilardif-fraction patterns.There are a series of diffractionpeaks around 14.1,32.5,58.1 and 60.1,whichshould be indexed t
37、o the(002),(101),(103),(110)and(008)planes of the MoS2(JCPDS No.75-1539).Andother peaks at 29.6 and 51.2 can also be ob-served,which match well with the(311)and(440)planes of cubic Co9S8(JCPDS No.86-2273).TheseprovetheformationoftheMoS2andCo9S8phasesinthepolyhedralstructures.Additionally,thereisnodi
38、f-fraction peak of S in the image.The XRD resultsshowthehighpurityofthesynthesizedsample.512 新型炭材料（中英文）第38 卷SEMandTEMwereusedtoinvestigatethemor-phologyandmicrostructureofthematerial.ItcanbeclearlyseenfromFig.S1athattheZIF-67precursorisaregularpolyhedron.Inaddition,thepreparedZIF-67is a uniform poly
39、hedron of similar size as shown inFig.S1b.In Fig.S1c-d and Fig.1c,the smooth sur-faceofZIF-67iscompletelycoveredbyalargenum-ber of folds,which are composed of vertical andcross-linked arrays of MoS2 nanosheets.To increasethedegreeofcrystallinityintheproduct,thecalcina-tiontemperatureis600-800C.Inadd
40、ition,thesepre-pared nanosheets are also connected to each other,providing abundant open spaces and active sites,formingatypicallooseandporousarchitecturalstruc-ture.AscanbeseenfromFig.S2,withthegradualin-creaseoftemperature,thecrystallinityofthematerialincreasesandthereispartialcollapseonthesurface
41、.When the temperature was increased to 800 C,thedegree of crystallization increased and the shape ofthe resulting Co9S8/MoS2/C-800 microspheres wasdistorted,andduetothecollapseoftheactivemateri-alat higher temperatures,many nanosheets were ir-regularly aggregated.This would greatly reduce theprobabi
42、lityof ion insertion and extraction.Consider-ing the influence of temperature on crystallinity andelectrochemical properties,Co9S8/MoS2-700 couldachievefarsuperiorperformancethanothersamples,andwillbediscussedinthefollowingsection.Togainfurtherinsight into the composition and microstruc-ture of the
43、Co9S8/MoS2/C-700,the element mappingspectraandTEManalyseswereperformed.TheTEMimage(Fig.1d-e)revealed the intimate contactbetweenMoS2nanoparticlesandtheCo9S8substrate.Theabundanthetero-interfacecreatedmayactasact-ivesitesforfastNa+storage33.Fig.1dshowsMoS2nanosheets with approximately 7 atomic layers
44、formed on a porous skeleton substrate.In Fig.S3(a-b)，the lattice spacing of MoS2 and were 0.678 nm(a)(b)(c)(f)(g)(h)(i)(j)(k)Co Mo S N C10 nm200 nm10 nm10 nmd=0.678 nmd=0.275 nmd=0.294 nm(0 0 2)MoS2(1 0 3)Co9S8(3 1 1)d=0.191 nmCo9S8(5 1 1)7 layer6 layer(d)(e)MoCl5StirringZIF-67Mo S CoMo-precursor Co
45、9S8/MoS2CalcinationS sourceTop Side10 20 30 40 50 60 70 80 90Intensity/a.u.2/()2H-MoS 2(JCPDS NO.37-1492)Co 9S 8/MoS 2-800Co 9S 8/MoS 2-700Co 9S 8/MoS 2-600(002)(101)(103)(006)(105)(311)(440)(110)Co 9S 8(JCPDS NO.86-2273)Fig.1(a)Schematicillustrationoftheflower-likeCo9S8/MoS2/Cheteroball.(b)XRDpatte
46、rnsZIF-67precursorandCo9S8/MoS2/Csynthesizedat600C,700C,800C.(c)SEMimageand(d,e)HRTEMimageofCo9S8/MoS2/C-700.(f-k)EDSmappingimagesoftheflower-likeCo9S8/MoS2/C-700hetero-ball 第3期 CHENHongetal:AbimetallicsulfideCo9S8/MoS2/Cheterojunctioninathree-dimensionalcarbon 513(002)and 0.275 nm(103),and Fig.S3c
47、shows the(311)plane of rutile Co9S8 correspondingto the lat-ticefringeof0.294nm.FromFig.1(f-k),theS,MoandCoelementsareuniformlydistributedthroughoutthewholemicrosphere.X-rayphotoelectronspectroscopy(XPS)analys-iswasthenconductedtoinvestigatethesamplesandbinding states of each element.Fig.S4a shows t
48、hesurveyspectrumoftheCo9S8/MoS2/C-700hetero-ball.ThesurveyXPSspectrumtestifiestheexistenceofC,N,Mo,Co,OandSelements.Fig.S4bdepictstheC1sspectrumfittedinto3peaksatbindingenergiesof284.8,286.4and288.8eV,whichshouldberelatedtotheCC,CN/OandCObonds,respectively34.IntheMo3dspectrum(Fig.2a),thetwomajorpeak
49、sat232.0and229.1eVareingoodagreementwithMo3d3/2 and Mo 3d5/2 of Mo4+,further confirming theformationofMoS2inthesimple.Thepeakat226.3isassigned to S 2s35.Meanwhile,the two weak peakslocatedat236.0eVareattributedtotheMo6+becauseofslightsurfaceoxidation36.Thehigh-resolutionCo2pspectrumexhibitssixmainpe
50、aksat778.9,782.6,786.9,794.1,798.7 and 804.6 eV(Fig.2b).Amongthem,thebindingenergiesat778.9and794.1eVcor-respond to Co 2p3/2 and Co 2p1/2,and the 2 satellitepeaksat804.6and786.9eVcanbeattributedtothetypical peaks for Co3+.Strong peaks at 782.6 and798.7eVcouldbeattributedtoCo2p3/2andCo2p1/2ofCo2+37.F
