Self-supported sulphurized TiO2 nanotube layers as positive electrodes for lithium microbatteries

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Self-supported sulphurized TiO2 nanotube layers as

positive electrodes for lithium microbatteries

Girish Salian, Milos Krbal, Hanna Sopha, Chrystelle Lebouin, Marie-Vanessa

Coulet, Jan Michalicka, Ludek Hromadko, Alexander Tesfaye, Jan Macak,

Thierry Djenizian

To cite this version:

Girish Salian, Milos Krbal, Hanna Sopha, Chrystelle Lebouin, Marie-Vanessa Coulet, et al..

Self-supported sulphurized TiO2 nanotube layers as positive electrodes for lithium microbatteries. Applied

Materials Today, Elsevier, 2019, 16, pp.257-264. �10.1016/j.apmt.2019.05.015�. �hal-02323068�

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ContentslistsavailableatScienceDirect

Applied

Materials

Today

jo u r n al hom e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p m t

Short

communication

Self-supported

sulphurized

TiO

2 nanotube

layers

as

positive

electrodes

for

lithium

microbatteries

Girish

D. Salian

a

_,

_Milos

_Krbal

b

_,

_Hanna

_Sopha

b,c

_,

_Chrystelle

_Lebouin

d

_,

_{Marie-Vanessa}

_Coulet

d

_,

Jan

Michalicka

c

_,

_Ludek

_Hromadko

b

_,

_Alexander

_T.

_Tesfaye

a

_,

_Jan

_M.

_Macak

b,c

_,

_Thierry

_Djenizian

a,∗ a_Mines_{Saint-Etienne,}_Center_of_{Microelectronics}_in_Provence,_Department_of_Flexible_Electronics,_F-13541,_Gardanne,_France

b_Center_of_Materials_and_{Nanotechnologies,}_Faculty_of_Chemical_Technology,_University_of_Pardubice,_Nam._Cs._Legii_565,_53002,_Pardubice,_Czech_Republic c_Central_European_Institute_of_Technology,_Brno_University_of_Technology,_{Purky ˇnova}_123,₆₁₂_00,_Brno,_Czech_Republic

d_{Aix-Marseille}_University,_CNRS,_MADIREL,_UMR_7246,_Marseille,_France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received4March2019

Receivedinrevisedform22May2019 Accepted30May2019 Keywords: Titaniananotubes Sulphurization Self-supported Cathode Li-ionmicrobatteries

a

b

s

t

r

a

c

t

Wereportthesynthesisandcharacterizationofself-supportedsulphurizedTiO2nanotubelayersasa

cathodematerialforLimicrobatteries.SulphurizedTiO2nanotubeswereobtainedbyannealingof

self-supportedTiO2nanotubesinsulphuratmosphere.Themorphology,structure,compositionandthermal

stabilityoftheTixOySznanotubelayerswerestudiedbyscanningelectronmicroscopy,transmission

electronmicroscopy,X-raydiffraction,X-rayphotoelectronspectroscopyandthermogravimetric anal-ysis.Theelectrochemicalbehaviorsofthechemicallymodiﬁednanotubeswereinvestigatedbycyclic voltammetryandchronopotentiometrytechniques.Thisnanostructuredelectrodeusedasacathode materialshowedhighratecapabilitiesevenatveryfastkinetics.Remarkably,ahighdischargecapacity (340␮Ahcm−2)hasbeenretrievedafter100cycleswith100%coulombicefﬁciencyattestingtheexcellent stabilityoftheelectrode.

1. Introduction

The development of modern microelectronic devices like microelectromechanicalsystems(MEMS),RadioFrequency Iden-tification (RFID) tags, medical implants, or smart cards, have necessitatedtheneedofmicroscalepowersources [1–5].These microscale power sources must not only be miniaturized but also fulfill the energy and the power density requirements of thedevices.Lithiumbatterieshavebeencharacterizedbyhigher energyperunitweightoftheknownenergystoragesystemsand hence theycan be miniaturized tobe integrated in microelec-tronicdevices[2].However,theelectrochemicalperformanceof2D microbatteriescomposedofplanarthin-filmscannotfeedavariety oflowpowerdevices.Inthiscontext,self-supported3D nanoarchi-tecturesareexpectedtoofferseveraladvantagesformicrobatteries suchasshorttransportlengthofcharges,interpenetrationbetween activecomponents,andsmallarealfootprints[6–9].Pristineand chemicallymodified self-supportedTiO2nanotube layers(TNTs)

havebeenextensivelyexploredasanodesforLi-ionmicrobatteries

[2,9–24].

∗ Correspondingauthor.

E-mailaddress:[email protected](T.Djenizian).

TNTshavealsobeenreportedtobegoodintercalationhosts for sulphur [25,26]. Sulphur as a cathode material possesses a hightheoreticalcapacityof1675mAhg−1andhenceahighenergy densitymakingaverygoodcandidateforlithium-sulphur(Li-S) batteries[27–30].However,therearesomelimitationsassociated withLi-Sbatterieslikepoorconductivityofsulphur,shortcycle life,low cyclingefﬁciency,or hugecapacity fademainlydue to thedissolution oftheactivematerialin theelectrolyte[27].To overcometheseproblems,transitionmetalsulphidessuchasTiS2

andTiS3havebeenstudiedaspositiveelectrodesforLibatteries

[31–35].Ingeneral,transitionmetalsulphidesexhibithigh electri-calconductivityandlowsolubilityinelectrolytes[36–38].These properties represent a clearadvantage over elemental sulphur. ThetheoreticalcapacityofcrystallineTiS3(556mAhg−1)ishigher

thanthat ofTiS2 (240mAhg−1)[39,40].Lindicetal.foundthat

theTiS3containingTiOxSythin-ﬁlmsexhibitedhigherareal

capac-itiesfortheinitialfewcyclesbutsufferedfromhighcapacityfade

[35].Hayashietal.reportedthatamorphousTiS3particleshavea

highreversiblecapacityof400mAhg−1inall-solid-statebatteries

[41].Self-supportedTNTscontainingTiS3 phasewasreportedby

Kyeremetangetal.[34].Theelectrodedeliveredgoodinitialareal capacitiesbutahugecontinuouscapacityfadehasbeenobserved duringcyclingtests.Actually,thestabilityofthedischarge capac-itiesdeliveredbythecathodematerialwaspoorwithacapacity

https://doi.org/10.1016/j.apmt.2019.05.015

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258 G.D.Salianetal./AppliedMaterialsToday16(2019)257–264

fadeinamatterof50cycles.Theoriginofthedegradationwas attributedtotheeventualstructuraldistortionofthematerialas thecatholyteformationhasnotbeenevidenced.

Inthepresentstudy,wereportthatremarkable electrochemi-calperformanceofsulphurizedTNTscanbeobtained.Weovercome thecapacityfadingduetostructuraldistortionproblemby control-lingthesynthesisparametersduringthesulphurizationtreatment. Theresultantsulphurizednanotubesdeliverexcellentareal capac-itieswithgoodstabilityofthedischargecapacitiesupto100cycles evenatfastkineticsmakingsulphurizedTNTsapotentialcandidate asapositiveelectrodeforLimicrobatteries.

2. Materialsandmethods

2.1. SynthesisofTNTsandsulphurizedTNTs

The TNTs were obtained by anodizing Ti substrates. Prior toanodization,the Tifoils (127␮m thick,Sigma-Aldrich) were degreasedbysonicationinisopropanolandacetonefor1mineach, thenrinsedinisopropanolanddriedinair.Theelectrochemical setupconsistedofatwo-electrodeconﬁgurationusingaplatinum foilasthecounterelectrode,whiletheTifoils(workingelectrodes) werepressedagainstanO-ringoftheelectrochemicalcell, leav-ing1cm2_open_to_the_electrolyte_that_consisted_of_ethylene_glycol

containing176mMNH4Fand 1.5vol%H2O[42].Theelectrolyte

waspreparedfromreagentgradechemicals.Beforeuse,the elec-trolytewasagedfor9hbyanodizationofblankTisubstratesat60V underthesameconditionsasforthemainanodizationexperiments

[43].Electrochemicalexperimentswerecarriedoutatroom tem-peratureemployingahigh-voltagepotentiostat(PGU–200V,IPS ElektroniklaborGmbH).Theanodizationwascarriedoutat60Vfor 4hwithaninitialsweepingrateof1Vs−1.Aftertheanodizationthe TNTswererinsedandsonicatedinisopropanolfor5minanddried inair.Theas-obtainedpristineTNTsbeingamorphous,theywere afterwardannealedinamufﬂeoven(400◦C,1h)toobtainanatase structure.Inthenextstep,TNTsalongwith0.2gofpuresulphur wereplacedintopre-cleanedquartzampouleswhichwere subse-quentlyevacuatedto10−3Paandsealed.Theevacuatedampoules wereheatedat400◦Cfor3h,andthennaturallycooledtoroom temperatureinsidethefurnace.Thetemperaturewasincreased witharateof5◦Cmin−1.ThesulphurizedTNTlayersaredesignated asS-TNTs.

2.2. Materialcharacterizations

ThemorphologyofthepristineTNTsand S-TNTswere char-acterizedbyaﬁeld-emissionSEM(FESEMJEOLJSM7500F)and a high-resolution transmission electron microscope (FEI Titan Themis 60-300, operated at 300keV) equipped with a high angleannular darkﬁeld (HAADF)detector(Fischione) for scan-ningtransmissionelectronmicroscopy(STEM)imagingandwith energydispersive X-ray(EDX) spectroscope(FEISUPER-X)with 4×30mm2_windowless_silicon_drift_detectors._The_{cross-sectional}

viewsweretakenfrommechanicallybentsamplesorscratched samplesbyscalpel.Thebendingprocessledtotheruptureofthe nanotubelayers,enablingtovisualizeevenindividualnanotubes invariousdetails.DiffractionanalysesoftheS-TNTswerecarried outusing X-ray diffractometer(XRD, D8 Advance, BrukerAXE) todeterminethedifferentphasespresentin thematerialusing CuK␣(=1.54 ˚A)radiationequippedwithasecondarygraphite monochromatorandNa(Tl)I scintillationdetector.The chemi-calcompositionofS-TNTswasdeterminedbyultra-highvacuum (UHV, basepressure below 1_×10−9mbar)X-ray photoelectron spectroscopy(XPS,ESCA2SR,Scienta-Omicron)using monochro-maticAlK␣source(1486.6eV). TheXPSwasperformedonthe

S-TNTpowderscratchedoutfromtheS-TNTlayerinorderto aver-agesignaloverentiredepthofS-TNTlayer.Thestabilityofthe S-TNTswas monitoredusingthermogravimetricanalysis(TGA). ThemeasurementsweredoneusingaQ500 apparatusfromTA Instrument. Inorder toavoid anyoxidization, theS-TNTswere heatedunderAratmosphereandaheatingrateof5◦Cmin−1was appliedfromambienttemperatureupto600◦C.

2.3. Electrochemicalmeasurements

TheelectrochemicalperformanceofthepristineTNTsand S-TNTswerestudiedusingSwagelok-typecells.Forthegalvanostatic charge/discharge cycling tests, pristine TNTs and S-TNTs were assembledinatwo-electrodeconﬁgurationwithametallicLifoil (Aldrich)as thecounter electrode anda glass ﬁber(Whatman) soakedintheelectrolyteusedastheseparator.Theelectrolyte con-sistedofasolutionof1MLiPF6inethylenecarbonate(EC)/diethyl

carbonate (DEC) (1:1, in w/w) (Sigma-Aldrich). For the cyclic voltammetrytests,pristineTNTsandS-TNTswereassembledina three-electrodesystemwithtwoLifoilsactingrespectivelyas ref-erenceandcounterelectrodesandseparatedbyaWhatmanpaper soakedinEC:DECwith1MLiPF6.Thecellswereassembledinan

argon-ﬁlledglovebox(MBraun,Germany),with<0.5ppmH2Oand

<0.5ppmO2atmosphere.Thecyclicvoltammetryandgalvanostatic

charge/dischargetestswerecarriedoutusingaVMP3potentiostat (BioLogic,France).TheCVcurveswererecordedinapotential win-dowof1–3Vvs.Li/Li+_at_a_scan_rate_of_0.1_mV_s−1_,_both_for_pristine

TNTsandS-TNTsGalvanostaticcharge/dischargetestswere per-formedatmultipleC-ratesinthepotentialwindowof1–3Vvs. Li/Li+_.

3. Resultsanddiscussions

Fig. 1a and b shows the SEM images of the top and cross-sectional viewsof thepristine TNTs. Thenanotubeswere characterizedbyanouterdiameterof∼145nm,aninner diame-terof∼135nmandthewallthicknessof∼5nmatthetopofthe nanotubelayers.Duetotheconicalshapeofthenanotubes, how-ever,thetubewallgetsthickerandtheinnernanotubediameter getssmallerindeeperpartsofthenanotubelayer,asshownin ourpreviouswork[42].Thethicknessofthenanotubelayerswas ∼15␮m.Fig.1cshowsthetopviewofS-TNTs.Asobservedfrom theSEMimage,thesulphurdepositionformsathicklayeronthe nanotubesalongtherimofthenanotubewalls.Thethicknessof thislayeris∼35nm.However,thenanotubularmorphologyis pre-servedaftertheheattreatmentinthesulphuratmosphere,whichis apparentfromSEMimagesinFig.1candd,andinadetailed STEM-HAADFimageofafragmentofS-TNTsshowninFig.1e.Inaddition, asapparentfromtheSTEM-EDXelementalmap,thecomposition oftheS-TNTisveryuniformalongthewalls,withSspeciesevenly distributedwithintheTNTwalls(seeFig.S1formoredetails).

Thestructure ofS-TNTshasbeeninvestigatedbyXRD analy-sis.TheXRDpatterngiveninFig.2arevealsthepresenceoftwo phases,i.e.anataseTiO2andTiS3(formulaTi4+S22−S2−).However,

thepresenceofTiS2inthematerialcannotbeexcludedasitcould

bepresentintheamorphousformatthistemperature[35,39,40]. TheTipeaksstemfromtheunderlyingTisubstrate.Inordertohave abetterinsightintothechemicalcompositionofthematerial,the chemicalstateofS-TNTsurfacewasexaminedbyXPS.TheS2p andTi2pspectraareshowninFig.2b.S2pspectrumconsistedof twodoubletsat160.2eVand161.5eVassignedtosulﬁdeS2−_and

disulﬁdepairsS22−,respectively.TheratioofS22−toS2−doublet

areaswasfoundtobe1.2,whichcorrespondedto3:2ratioofTiS3

toTiS2,respectively[44].TheformationofTiSxwasalsoreﬂected

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Fig.1.SEMimagesofthepristineTNTs(a)topviewand(b)cross-sectionalview;theS-TNTs(c)topviewand(d)cross-sectionalview;(e)STEM-HAADFimageandSTEM-EDX elementalmapsofafragmentoftheS-TNTs.

Thepresenceofelementalsulphurcouldnotbeverifiedbecauseit wasnotstableunderUHVconditions.Nohigheroxidationstatesof sulphurweredetected.ThestabilityoftheS-TNTisillustratedon theTGAthermogrammgiveninFig.2c.Ithastobenotedthatthe weightlossesarenormalizedinrespectwiththeestimatedmass ofnanotubes.Theanalysisrevealstwoweightlossstages.Thefirst oneisabout2.4%andoccursbetween150and200◦C.Itisattributed totheevaporationofasmallamountsulphurasconfirmedbythe TGAcurveforpuresulphur[45].Thisresultsuggeststhat unre-actedsulphurwaspresentinthesample,eithertrappedinsidethe nanotubesoradsorbedatthesurface.Thefactthatsulphurwasnot observedinXRDpatternmaybeattributedtoitsamorphousnature resultingfromthesulphurizationprocess.Thesecondweightloss stagestartsaround300◦Candcorrespondstoaweightlossequal to16.9%.ItcanbeattributedtothedecompositionofTiS3 phase

intoTiS2[46,47].

TheelectrochemicalbehaviorofpristineTNTsandS-TNTs sam-ple were studied by cyclic voltammetry (Fig. 3a and c). The reduction peak at 1.75V vs. Li/Li+ _and _the _oxidation _peak _at

2.1Vvs.Li/Li+_correspond_to_the_Li+_{insertion/extraction}_into/from

theanatasephase presentinbothsamples,accordingtoEq.(1) [8,11–13].AdditionallytheareaundertheCVcurvefortheS-TNTs ishigherthanthepristineTNTssuggestinghigherelectron trans-fers.ExaminationoftheCVsuptothe10th_cycle_for_S-TNTs_reveals

thepresenceofpeakstypicalforseveralreactionswithLiionsfor

S-TNTs.Thepresenceofgoodreversibilityisachievedby demon-stratinganoverlapinsweeps.

TiO2+xLi++xe−↔ LixTiO2 for 0 ≤x≤1 (1)

Then,otherreversiblebroadanodic/cathodicpeaksobservedat potentialshigher than2.1Varedue tothepresence ofSin the formofsulphidesasithasbeenreportedforTixOySzthinﬁlms[35].

Hence,TixOySzcanreactwithLiionsaccordingtoEq.(2).

Ti_xO_yS_z+nLi+₊_ne−_↔ _Li

nTixOySz for 0 ≤n≤1 (2)

However,twocathodicpeaksat1.9Vand2.1Vappearingonly intheﬁrstcyclecanbeattributedtotheirreversiblereactionofTiS3

withLi+_._Actually,_TiS

3reactswithupto3Li+performulaunit(2Li+

reactirreversibly)inatwo-stepintercalationmechanismaccording toEqs.(3)and(4)[33,34].

TiS3+2Li++2e−→ Li2TiS3 (3)

Li2TiS3+xLi++xe−↔Li2+xTiS3 for0 ≤x≤ 1 (4)

InEq.(3),the[S22−]bondspresentinTiS3arebrokenduring

theinsertionofLi+_into_the_structure_resulting_in_the_reduction_of

S22−.Thisirreversiblereactionleadstoachangeinthetitanium

coordinationpolyhedronfromtrigonalprismatictothemore sta-bleoctahedrongeometry[33].Thenextreaction(Eq.(4))whichis reversible,takesplaceduringtheinsertionofthethirdLi+_in_Li

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Fig.2. (a)XRDpatternofS-TNTssample;(b)XPSS2pandTi2p(inset)spectraofS-TNTssample;(c)TGAcurvesforS-TNTssampleandpureSulphurrecordedinAr atmospherewithaheatingrateof5◦Cmin−1.

atapotentialofaround2.3Vasithasbeenconﬁrmedbythelarge cathodicpeakvisibleaftertheﬁrstcycle.DisorderedTiS2couldalso

reactwithupto1Li+_per_formula_unit_reversibly_according_to_Eq.

(5)[35,40].

TiS2+Li++e−↔LiTiS2 (5)

Itcanbenoticedthatthepresenceofacathodicplateaubetween 2.2and1.8Vcanbeascribedtoapseudocapacitiveeffect,whichis typicalfornanodomains[48].Gonzálezetal.showedthatshorter nanotubescontributedtotheadditionalcapacityduetolargearea exposedtotheelectrolyte[49].Whiteleyetal.reportedthat dis-ordered LiTiS2 composed of nanocristallites showed additional

chargestoragederivedbyreducingsurfaceTi3+_to_Ti2+_at_higher

voltagesandmorereversiblythantraditionallyshown[50]. Galvanostaticcharge/dischargetests wereperformed onthe pristineTNTsandS-TNTssamplesinhalf-cell(vs.Li/Li+₎

conﬁgu-ration.ThecurrentwasappliedbasedonTNTsassumingaporosity of78%.TheporositycalculationisbasedontheamountoftheTiO2

nanotubespercm2_._The_volume_of_the_TNTs_was_calculated_by

sub-tractingtheinnertubeareafromoutertubearea. Fig.3b andd showsthegalvanostaticcharge/dischargecurvesoftheelectrodes atacurrentdensityof34␮Acm−2(C/10)forthepristineTNTsand S-TNTsrespectively.Eachcycleshowsthepresenceof characteris-ticplateausthatareconsistentwiththepeaksobtainedfromtheCV curves.Theﬁrstcycledeliversadischargecapacityof342␮Ahcm−2 (70mAhg−1) and 748␮Ahcm−2 (197mAhg−1) for the pristine TNTsandS-TNTs,respectively.Theﬁrstirreversiblecapacityisa characteristicoftheTiO2basedelectrodesduetothereactionofLi

ionswithstructuraldefectsandtheresidualwater[9,51,52]. Possi-blewaystomitigatethisirreversiblecapacityhasbeensuggested byBruttietal.,whereinTiO2hasbeenpre-lithiatedbyadding

nano-lithiumpowder[53].The10thcyclegivesadischargecapacityof 285␮Ahcm−2(51mAhg−1)and578␮Ahcm−2(152mAhg−1)for thepristineTNTsandS-TNTsrespectively.Clearlythedischarge capacitiesfortheS-TNTsarehigherthantheirpristine counter-partssuggestingthatthecapacitiesinS-TNTsarecontributedboth

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Fig.3. (a)CyclicvoltammetrycurvesofthepristineTNTssampleatascanrateof0.1mVs−1_;_(b)_{galvanostatic}_{charge/discharge}_curves_of_the_pristine_TNTs_at_C/10_rate;_(c)

cyclicvoltammetrycurvesoftheS-TNTssampleatascanrateof0.1mVs−1;(d)galvanostaticcharge/dischargecurvesoftheS-TNTsatC/10rate.Thepotentialrangeforboth CVandGalvanostatictestswere1–3Vvs.Li/Li+_.

byanataseandthetitaniumsulphides.Theoperatingvoltageof theS-TNTssampleis2.3Vsuggestingthatitisapotentialpositive electrodeforlithiummetalbatteries.

Fig.4a shows thelong-termcycling behaviorof thepristine TNTsand S-TNTssample for 100cyclesat acurrent density of 340␮Acm−2 (1C). The discharge capacities for the S-TNTs are remarkably higher than the pristine TNTs suggesting that the enhanceddischargecapacityofS-TNTsisduetothecontribution oftheanataseandthetitaniumsulphidephases.

The cycle behavior for the S-TNTs shows stable capacities upto 100cycleswith 100% coulombicefﬁciency (Fig. 4b).The 100th cycle delivered a discharge capacity of 400␮Ah cm−2. The capacity retention between the ﬁrst reversible cycle (2nd cycle) and the 100th cycle is 93%. These results show the excellent electrochemical behavior of the S-TNTs. To investi-gate further on the rate performance of the S-TNTs, the cell was cycled at higher current densities (Fig. 4a). The elec-trode delivers discharge capacity values of ∼543␮Ahcm−2 (145mAhg−1)at68␮Acm−2(C/5),∼430␮Ahcm−2(110mAhg−1) at340␮Acm−2(1C),∼300␮Ahcm−2(78mAhg−1)at680␮Acm−2 (2C),∼170␮Ahcm−2(46mAhg−1)at1.7mAcm−2(5C).The elec-trode shows high rate capabilities at very fast kinetics. More remarkably,thedischargecapacitiesareretrievedaftercyclingat veryhighcurrentdensitiesattestingtheexcellentstabilityofthe materialcomparedtopreviouswork[34].

Origins of the high discharge capacity and the outstanding stabilityduringcyclingtestsmayhavetwoexplanations.Firstly, accordingtotheﬁrstcycle,sulphurionsmainlycontributetothe electrochemicalprocesses.Lindicetal.suggestedthattheTiOxSy

ﬁlmsdeliveringgoodcyclestabilityshowedthereductionofthe disulphidebondsweremorereversiblethantheﬁlmswhichgave poorcyclestability[33].Thesamereasoncouldbeattributedinthe presentstudy.Secondly,thesameresearchgroupsuggestedthat thecapacityfadingismainlyattributedtothestructuraldistortions

duringthereactionwithlithium.TheS-TNTssampleinthepresent studyportraysgoodcyclingstability,whichindicatesthat struc-turaldistortionsarenotpresentandthatthestructureistherefore morestable.Whenwecomparetheresultsofthepreviousreports fortheS-TNTsbyKyeremetangetal.[34],thearealcapacitiesherein arehigherandadditionallythedischargecapacitiesupto100cycles isstable.ProbablythestructuraldistortionintheTiOxSynanotubes

fromthepreviousstudy,comesfromthefactthattheformation ofthisphasewasfasterduringtheheattreatment(200◦Cmin−1), whichcouldrenderthephaselesscrystallinewithmorestructural distortionsand defects.Whereasin thepresentstudy,theheat treatmentinthesulphuratmosphere wasdoneat5◦Cmin−1,a muchslowerrateforwhichthematerialhasampletimetoform stablephaseswithlessstructuraldistortions.

Post-mortemanalysiswasperformedafter100cycles.Thecell afterthecyclingtestwasdisassembledtoretrievetheelectrode.

Fig.5aandbshowstheSEMimagesofthecycledsample.In com-parisonwithSEMimagesoftheS-TNTsbeforecycling(Fig.1cand d),thereisnomajordegradationandthenanotubularmorphology ispreserved.Fig.5canddshowstheXRDdiffractogramrecorded ontheS-TNTsampletestedafter100cycles.Inagreementwith theelectrochemicaltests,theTiS3peakactuallydisappearedwhile

anadditionalsmallpeakconﬁrmstheformationoftheLi2TiS3and

LiTiS2phases(Fig.5d).Evenremarkably,anotherpeak

correspond-ingtotheappearanceofLiTiO2isobservedinthediffractogram.

SuchinactiveLTOfromanelectrochemicalpointofviewmay con-tributetothestabilizationoftheelectrodeasithasbeenreported elsewhere[53].Inordertoclarifytheelectrochemicalreactionof theS-TNTelectrodewithLiions,XPSmeasurementshavebeen per-formedafterthe2ndchargeandthe3rddischarge.TheresultingS 2pXPSspectraofS-TNTssamplesareshowninFig.5e.Thecharge anddischargecycleswereaccompaniedbythedeacreaseofTiS3

contentrelativetoTiS2 andbyapartialsurfaceoxidationof

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Fig.4. (a)LongtermcyclingtestsofpristineTNTsandS-TNTssamplefor100cyclesatacurrentdensityof340␮Acm−2_(1C_rate);_(b)_coulombic_efﬁciency_of_the_S-TNTs

samplefor100cycles;(c)electrochemicalperformanceofS-TNTsatvariouscurrentdensities(Crates).

Fig.5. SEMimagesoftheS-TNTssamplepostlong-termcyclingtestsfor100cycles(a)topview;(b)cross-sectionalview;(c)XRDdiffractogramofthecycledsample;(d) zoomedimageoftheXRDdiffractogram;(e)S2pXPSspectraofS-TNTsafter2ndchargeand3rddischarge;(f)EDXspectrumanalysisoftheelectrolyteretrievedfromthe cell.

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featuringthe1:7TiS3toTiS2ratioconﬁrmingthereactions

men-tionedabove.TheoxidationstatesofS−,S4+_,_and_S6+_were_observed

inS2pspectraasdoubletsat163.2,166.7,and168.8eVinboth chargedandthedischargedstateoftheelectrode.Thisresults sug-geststhe formationof a SEI-like layerprobably composedof a mixtureofLiS,Li2SO3,andLi2SO4 thatcouldberesponsiblefor

theinitialcapacityfading.Apost-mortemEDXspectrumanalysis (Fig.5f)oftheelectrolyteretrievedfromthecellafterthe100cycles revealedonlythepresenceofP,C,FandOelementsindicatingthat therewasnosulphurdissolutionintotheelectrolyte(Sisignalis detectedbecausetheelectrolytewasdriedatroomtemperature onaSisubstrate).

4. Conclusions

Self-supportedsulphurizedTiO2nanotube layerswere

inves-tigatedaspotentialpositiveelectrodesforLimicrobatteries.The morphologyofthesulphurizedTNTsrevealedthatthechemical treatmenthasnot altered the morphologyof the titania nano-tubes.Additionally,theSTEM-EDXanalysisconﬁrmedtheuniform distributionofsulphurwithinthenanotubes.Thechemical char-acterizationexperiments revealedthepresence ofanataseTiO2

and TiS3 phases. According to the galvanostatic cycling tests,

the TixOySz nanotubes delivered an average areal capacity of

600␮Ahcm−2atC/10ratewithcoulombicefﬁcienciesof100%and acapacityretentionbetweenthe2ndandthe100thcycleof93% comparedtotheirpristinecounterparts.Therateperformanceof theelectrodesdeliveredstablecapacityvaluesevenatveryfast kinetics(170_␮Ahcm−2at5Crate)andthelong-termcyclingtests upto 100cyclesshowedremarkablestability compared tothe previousreports.Thepost-mortemanalysisrevealedthatthe nan-otubularmorphologyisnotaffectedandthephasesofTiS3andTiS2

areretainedaftercycling.TheEDXanalysisshowsnodissolution ofSulphuraftercyclingandXPSanalysissuggesttheformationof aSEI-likelayercomposedoflithiumsulﬁdesthatcouldbe respon-siblefortheinitialcapacityfading.Thereasonsfortheexcellent electrochemicalbehaviorscanbeattributedtotheoptimized ther-malparametersforthefabricationofTixOySznanotubesandthe

formationofinactiveLiTiO2improvingthestructuralstabilityof

theelectrode.

Acknowledgments

The authors gratefully acknowledge the European Research Council (project No. 638857) and the Ministry of Education, Youth and Sports ofthe CzechRepublic(projects LM2015041, LM2015082,LQ1601andCZ.02.1.01/0.0/0.0/16013/0001829)for ﬁnancialsupportofthiswork,whosepartwascarriedoutwiththe supportofCEITECNanoResearchInfrastructure.Thisworkhasbeen carriedoutthankstothesupportoftheA*MIDEXproject(n◦ ANR-11-IDEX-0001-02)fundedbythe ¨Investissementsd’Avenir”French governmentprogram,managedbytheFrenchNationalResearch Agency(ANR).

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.apmt.2019.05.015.

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