Ionogel-based solid-state supercapacitor operating over a wide range of temperature

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OULOUSE

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To link to this article : DOI:10.1016/j.electacta.2016.02.013

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To cite this version :

Nègre, Léo and Daffos, Barbara and Turq,

Viviane and Taberna, Pierre-Louis and Simon, Patrice

Ionogel-based solid-state supercapacitor operating over a wide range of

temperature. (2016) Electrochimica Acta, vol. 206. pp. 490-495.

ISSN 0013-4686

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Ionogel-based

solid-state

supercapacitor

operating

over

a

wide

range

of

temperature

L.

Negre

a,b

_,

_B.

_Daffos

a,b

_,

_V.

_Turq

a

_,

_P.L.

_Taberna

a,b

_,

_P.

_Simon

a,b,

_*

a_{CIRIMAT,UniversitédeToulouse,CNRS,UT3,118routedeNarbonne,31062ToulouseCedex,France} b_{RéseausurleStockageElectrochimiquedel’Energie(RS2E),FRCNRS3459,France}

ABSTRACT

Aninorganicgelpolymerelectrolytebasedontheconfinementofanionicliquidmixture(1:1byweight ormolarratio)ofN-methyl-N-propylpiperidiniumbis(fluorosulfonyl)imide(PIP13FSI)and N-butyl-N-methylpyrrolidiniumbis(fluorosulfonyl)imide(PYR14FSI)intoaSiO2matrixpreparedfromasol-gel

methodwaspreparedandfurtherusedaselectrolyteinanallsolid-statesupercapacitor.Thesynthetized ionogelexhibitsahighionicconductivityoverawidetemperaturerange(from0.2mScm!1_at!40"_Cup

to10mScm!1_at60"_{C).Theionogel-basedsupercapacitorusingtwoactivatedcarbonelectrodescanbe}

operatedover3Vcellvoltagewindow.Moreoverthisallsolid-supercapacitorshowsacapacitanceupto 90Fg!1_{atroomtemperature.Theseencouragingresultsshowtheinterestofdevelopingsuchdevices,}

includingnon-toxicandsaferelectrolytes,packagingissuesandflexibledevicesdevelopment.

1.Introduction

Electrochemicaldoublelayercapacitors(EDLCs),alsoknownas supercapacitors, storeenergy by ion electrosorption onporous electrodes,mostlyactivatedcarbons[1].ThekeyfeatureofEDLCs compared to Li-ion batteries is their highpower delivery (full chargeordischargedcanbeachievedwithinfewsecondsortensof seconds) and high cyclability. Despite abounding research on pseudocapacitive materials with promising performance [2–4], conventionalEDLCsusinghighsurfaceareacarbonelectrodesand non-aqueousliquidelectrolytesrepresentthestateoftheart[5]. However, thecurrent liquid electrolytesusedin supercapacitor devices sufferfromseveral drawbacks.Conventional supercapa-citorsemploytheuseofsolvent-basedelectrolyteswhichallowion conduction at sub-zerotemperatures, e.g NEt4BF4 in propylene

carbonate (PC)or acetonitrile (AN). Nevertheless,theseorganic solventsreducethevoltagewindowofthedeviceandANcanpose securityissuesduetoitsflammabilityandlowflashpointof2"_C.

Electrolyteleaks,whichcanoccurbecauseofinternalgasrelease,is also animportantconcern.Since themaximumspecific energy increases with squared of the voltage window (EmaxðinWhÞ¼1

2&CðinFÞ&DV2maxðinVÞ

.

[1]),thetypeofelectrolyteused is important indetermining theenergydensity andsafety ofa device.Astrategytotackletheseissuesistoreplaceliquidbysolid

–orgelified–electrolytes.Severalmaterialshavebeenutilizedas solidpolymerelectrolytes,suchasNafion[6], poly(etheretherke-tone)[7],polybenzimidazole[8],poly(methylmethacrylate) [9]

andpoly(ethyleneoxide)[10]arepromisingcandidatesofferingas welleasyassembly,lowcost,andflexiblepackaging.However,the operatingvoltageislimitedtolessthan2Vbecauseoftheuseof protonor proton-like conductingpolymers[11].More recently, ionogels,whicharesolidorquasi-solidelectrolytesbasedonthe trappingofroomtemperatureionicliquids(RTILs)inasilicamatrix havebeenproposed[12–14]becausetheycanreachhighvoltage, up to 4V [15]. However, based on ionic liquid salt, the ionic conductivityat low temperatureis still anissuefor developing solid-state supercapacitors operating with a wide range of temperature.

Inthispaper,weproposetodevelopasolid-stateelectrolyte basedonaionicliquidmixturetrappedintoaninorganicsilica network. Following previous work [16], we used an mixture composed of (1:1 by weight or molar ratio) N-methyl-N-propylpiperidinium bis(fluorosulfonyl) imide(PIP13-FSI) and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (PYR14-FSI).Inthesemixtures,cationshavethesamemolecularweight andthesamenumberofatomsofthesamenature,withtheonly differencebeingtheircationmolecularstructure:afive-member (piperidinium)orsix-member(pyrrolidinium) heterocycle.Asa result,theincreasingdisorderandasymmetryamongthecations hinderslatticeformation,thustoloweringthemeltingpointwhile maintaininggoodmiscibilityandconductivity[17].Theliquidstate ofthisILmixturecanbemaintainedattemperaturesseveraltens

*Correspondingauthor.

E-mailaddress:[email protected](P.Simon).

ofdegreeslowerthantheindividualILs.Ionicliquidmixture-based ionogelhavebeendesignedforenlargingtheoperating tempera-ture range of these solid state electrolytes, thus offering new opportunitiesforsolid-statesupercapacitors.

2.Experimental

Ionogel synthesis procedure was described elsewhere [14]. Basically,asolidionogel-basedelectrolytewassynthetizedusing two different sol-gel agents: TMOS (TetraMethyl Ortho Silicate 98%,fromSigma–Aldrich)andTEOS(TetraEthylOrthoSilicate98%, fromSigma–Aldrich)underacidicconditions(Methanoicacid98%, fromVWR).TheTEOS:TMOS:MAvolumetric ratiowas 2:2:5.All reactantsweremixedtogetherundermoderatestirring(300rpm) at40"_{Cduring18min.Thesecondstepwastheadditionof60%by}

volumeof ionic liquidmixture(PIP13-FSI):(PYR14-FSI):50:50 in weight.Comparedtoourpreviouswork using1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonylimide) EMITFSI-based ionogel[14],ILcontentwasdecreasedfrom70%downto60%vol toimprovethemechanicalpropertiesofthegel.Then,theresulting solutionwascastedinsealedcylindricalcontainersandkeptfor 24hoursatroomtemperatureforgelationandthendriedat50"_C

during4 days.Allthecells used forconductivitymeasurement wereassembledina

MBraüngloveboxoperatingunderargonatmosphere(O2and

H2Ocontents lower than 0.1ppm) toavoid any contamination.

Samplespreparedforfurthermechanicalcharacterizationswere injectedintostainlesssteelmolds,whichensuredgood reproduc-ibilityofmeasurements.

2.1.Supercapacitorcellassembly

Activematerialwasmadeof95wt%ofPICACTIFSC,astandard highspecificsurfaceareaactivatedcarbon(2300m2_g!1_)[18],and

5wt%ofPTFE(60wt%PTFEinwaterDuPontNemours,France)[19]. Carbonelectrodesweredriedoutinavacuumovenduring2hours then 250

m

L of silane-ILmixture was spread ontoan electrode whichwaslaidinanappropriatemould,thenanotherelectrode

Fig.1.ConductivityvariationversustemperatureofneatIlsmixture(PIP13-PYR14/ FSI)andionogelbetween!50"_Cand80"_{C.Conductivitieswerecalculatedfrom}

high frequency intercept with real axis of the Nyquist plotsobtained from ElectrochemicalImpedanceSpectroscopy(EIS)measurements.

wasdepositedontothesurfaceoftheliquidSilane-ILsmixture.The gel formation was obtained following procedure previously described. The resulting Carbon-Ionogel-Carbon sandwich was placed between two gold current collectors in a RHD cell for furtherelectrochemicaltests.Thistechniqueoffersawaytowork

with a large amount of active material (from few mg up to 15mgcm!2_).

Electrochemicaltestswerecarried outusingan Autolab PG-STAT128N(Metrohm,Switzerland).ARHDCell(RHD,Germany) wasusedforprecisecontrolofthecelltemperaturefrom!40"_Cto

+60"_{C.Prioranymeasurement,thecellwasmaintainedattheset}

temperature for 2h until equilibration. After sandwiching the samplesbetweentwogoldcurrentcollectors,ionicconductivity measurementsoftheionogelwereperformedbyElectrochemical ImpedanceSpectroscopy(EIS)for each temperature;a biascell voltageof0Vwithafrequencyrangefrom25kHzdownto0.01Hz wereapplied.Conductivitieswerecalculatedfromhighfrequency interceptwiththerealaxisoftheNyquistPlots.Allelectrochemical testsof the supercapacitors wereperformed withan electrode massloadingof5mgcm!2_.

Mechanical characterizations of theionogel wereconducted using an UltraNanoHardness Tester from CSM Instruments (Switzerland).Young’smodulusEandhardnessHandpercentage of elastic recovery were measured with a modified Berkovich three-sidedpyramiddiamondindenter.Themaximalnormalforce was 200

m

N; the maximum force was keptduring 60s before discharge.Theloadingratewas400

m

Nmin!1_{andthemaximum}

penetrationdepthwas12

m

m(valuelowerthanthetenthofthe sample thickness (1.5mm) thus the mechanical answer of the substratewasneglected).ThePoissonratioofthehybridcoatings wastakenatamediumvalueof

y

=0.3(valuebetweenthePoisson ratioofamorphoussilica(0.17)andthatofconventionalpolymeric materials(0.5)).Experimentswereperformedatambient temper-ature.InallCSMnano-indentationtests,atotalofthreeindents wereaveragedtodeterminethemeanYoung'smodulusand nano-hardness.Theanalysis proceduresuggestedbyOliver andPharr

[20]wereusedtocalculatethehardnessandelasticmodulus. 3.Resultsanddiscussions

3.1.Ionogelconductivity

Fig.1shows thechangeof theionogelandneat ILsmixture conductivitieswithtemperature(Arrheniusplot) between80"_C

and!50"_{C.AscanbeseeninFig. 1,theionogelconductivityisclose}

tothatof theneatIL mixturein the!20"_{C +60}"_{C temperature}

Fig.3. CyclicVoltammetryofan ionogel-basedsupercapacitorusingactivated carbonelectrodes,at5mVs!1_{atvarioustemperatures(from}

!40to60"_C).

Fig.4.EISplotof(A)supercapacitorusingtwocarbonglasselectrodesandanionogelsoakedseparatorbetween1MHzand200Hz(B)supercapacitorusingtwoactivated carbonelectrodesandanionogelsoakedseparatorbetween1MHzand0.1Hz.BiasVoltage:0V.Signalamplitude:5mVRMS.

Table1

Gravimetricandarealcapacitancecalculatedfromthe5mVs!1_CVsduringthe

dischargeatdifferenttemperatures.

Temperature !40"_{C !20}"_{C 0}"_{C 20}"_{C 40}"_{C 60}"_C

Gravimetriccapacitance(Fg!1_{) 34 68 89 91 94 96}

range,whichevidencesasmallimpactofthepresenceofthesilica matrix.Theroomtemperatureconductivity(20"_{C)wasmeasured}

at5.5mScm!1_{forboththeneatILmixtureandtheionogel,which}

iscomparabletoconventionalpropylenecarbonate-basedliquid electrolytes[21]andmatcheswiththestandardsneededforuseas electrolytein supercapacitordevices. However,for temperature below!20"_{C,thelimitedmobilityoftheelectrolyteduetothe}

confinementeffectinsidethesilicamatrixisassumedtolimitthe conductivity.A conductivity upto0.2 mS.cm!1_{was still}

main-tained at !40"_{C. Such high conductivity values at such low}

temperature,toourknowledge,haveneverbeenreportedinthe literature for ionogel electrolyte. Nevertheless, these kind of electrolytes have been intended to suit to mechanical stress experiencesinceagoodmechanicalstabilityhastobe.Mechanical

tests were carried out using a nano-indentation technique to assessthemechanicalpropertiesofasmadeionogels.

3.2.Mechanicalcharacterizations

Mechanical properties of ionogels were measured by nano-indentation.Fig.2showstheloadasafunctionofthepenetration depth plotfor a 1.5mm-thick ionogelpellet usinga Berkovitch indenter.IonogelsexhibitratherlowvaluesofbothVickersHardness (0.11' 0.02MPa) and elastic modulus (0.935'0.114 MPa) as comparedto other gelelectrolytes: 26MPafor PMMA [22] and 35–50MPaforPVdF-basedpolymer[23].Theelasticcontributionto theloadingcanbeobtainedfromFig.2,bysubtractingtheplastic displacement to the total displacement normalized to the total

Fig.5.EISPlotofionogel-basedsupercapacitoratdifferenttemperature(a)!40"_C,!20"_Cand0"_C(b)20"_C,40"_Cand60"_{C.Biasvoltage:0V.Signalamplitude:150mVRMS.}

Fig.6.Evolutionofcapacitanceversuspotentialscanrateofionogel-basedsupercapacitorwithin3Vatdifferenttemperatures(A)!40"_C,!20"_Cand0"_C(B)20"_C,40"_Cand

displacement.Anelasticrecoveryof45%wasmeasured,thathasto becomparedto14%forpureTEOSandTMOSsilicagel.Thetrapped ionicliquidisactingasaplasticizerintheresultingionogel. 3.3.Ionogelassolid-stateelectrolyteforsupercapacitor

An all-solid supercapacitor was assembled using porous PICACTIF activated carbon based electrodes stacked with the ionogelaselectrolyte(seeexperimental).InFig.3arepresented CyclicVoltammogramsobtained(CV)at5mVs!1_between0and

3Vatvarioustemperatures(from!40to60"_C).Downto!20"_C

theelectrochemicalsignaturesarequietdecentsincea rectangu-lar-shape characteristicwas achievedwhich was expectedfora capacitive behaviour based on charging/discharging of the electricaldoublelayer.

Aspecificcapacitanceof91Fg!1_{(pergramofactivatedcarbon)}

havebeenobtainedat20"_{Cwithacellvoltageof3V,whichisvery}

similar to that was measured in conventional organic liquid electrolyte [19]. Table 1 summarizes gravimetric and areal capacitancescalculatedfromtheCVsatapotential scanrateof 5mVs!1_{between0and3Vfrom!40to60}"_C.

Asobservedinthistable,thecapacitivebehaviouriskeptdown totemperatureaslowas!20"_{C;withonly25%capacitancelossat}

!20"_{C.TheseresultsareconsistentwithFig.1thatshowedthat}

ionogelionicconductivitywasstilldecent(0.2mScm!1_)atsuch

lowtemperature.At!40"_{C,thecrystallizationoftheILconfinedin}

pores of thecarbon structure is assumed to occur. Despite an electrochemicalkineticsmainlycontrolledbyohmicdrop,40%of thecapacitanceobtainedat20"_{Cwaspreserved.Interestinglythe}

electrochemicalbehaviourofall-solidsupercapacitorwasfoundto beveryclosetoliquid-basedsupercapacitors.Suchperformance couldbeexplainbyawettingcarbonmicroporebytheILthatcan movefreelywiththesiliciumnetwork.Tocheckthehypothese,we carriedouttodesignanadditionalexperimentusinganon-porous carbon(carbonglass)forcomparisonprupose.

Intheexperiment,an(EMITFSI)-basedionogelsoakedseparator wasusedtoobservethecarbon/ionogelinterface;EISmeasurements were madeeach30minbetween 1MHz–200Hz to measurethe change of theimpedance with time. Fig. 4A shows thesoaked separatorbetweentwocarbonglasselectrodes.Sameexperiment wasconductedwithtwoactivatedcarbonelectrodeplacedbetween thesameseparatorsoakedwithionogel(Fig.4B).

While the impedance is kept constant for the non porous electrode(Fig.4B),therealpartoftheimpedanceincreaseswith timewhenusingporouscarbonelectrode.Thisisassumedtobe linked tothehydrophobicityof thecarbonwhichattracts ionic liquid (hydrophobic) from the silica host network (which is hydrophilic)intothecarbonpores;besides,capillarityforcecan alsocontributetoattractILinthepores.Thisbehaviourallowsa (partial) filling of the carbon microporous structure with ions leadingtosuchhighcapacitance.

In Fig.5 arepresentedtheNyquist plotsof acarbon-carbon ((PIP13-FSI)0.5(PYR14-FSI)0.5) ionogelbased supercapacitorata biascellvoltageof0V.

In the 20"_{C ! 60}"_{C temperature range (Fig. 5A), the high}

frequencyseriesresistance,whichistheionicbulkresistance,was measured at 20

V

cm2 _{for thewholecell at roomtemperature}

(20"_{C), which is comparable to our previous work [14]. As}

expected,whenthetemperatureincreasesfrom0"_Cupto60"_C,

thehighfrequencyresistancedecreasesfrom52to9.2

V

cm2_.The

sharp increase of the imaginary partof theimpedance at low frequencies ischaracteristic ofthesystem capacitivebehaviour. Evenfor sub-zerotemperatures,EIS measurementsstill showa capacitivebehaviour,despitethepresenceofagrowingsemi-circle athighfrequency,attributedtotheonsetofgelationthatisrelated

to decreased mobility of the electrolyte ions at such low temperatures.

Fig.6showsthechangeofthearealandgravimetriccapacitance (derivedfromtheslopeoftheQ-Vcurveduringcelldischarge)of theactivatedcarbonelectrodesversusthepotentialscanrateat various temperatures. The capacitance of the ionogel-based supercapacitor decreases with increasing scan rate, since the ohmicdropincreaseswiththescan rate. Temperatureplaysan importantrole,sincealreadymentionedseriesresistanceincrease at low temperaturewhile a fastdecrease of thecapacitance is observedathigherscanrate.

Inhere,itwasexpectedsincecapacitancelimitationispurely due to ohmicdrop. The effective accessible surface is strongly dependentonthescanrate[17].

4.Conclusion

Thepresentpaperpresentssomeresultsobtainedwitha carbon-carbonsolid-statesupercapacitorusing((PIP13-FSI)0.5(PYR14-FSI) 0.5)-basedionogelasbothelectrolyteandseparator.Thankstohigh ionicconductivityoftheionogel,ionsfromthegelareabletofulfil porosityofthecarbonelectrodesleadingtosuchhighcapacitance (95Fg!1_{at room temperature). Anall solid supercapacitor with}

attractiveareal capacitance up to 100mFcm!2 _{per electrodeat}

!40"_Cfor5mVs!1_{scanrateupto 3Vcellvoltagecouldbewas}

assembled.Suchdevicescouldefficientlymatchmultiplecriteria (high capacitance, good power density and high cell potential window)neededforsolidsupercapacitorapplications.

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