Impact of composite structure and morphology on electronic and ionic conductivity of carbon contained LiCoO2 cathode

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Impact

of

composite

structure

and

morphology

on

electronic

and

ionic

conductivity

of

carbon

contained

LiCoO

2 cathode

Nam

Hee

Kwon

a,∗

_,

_Hui

_Yin

a

_,

_Pierre

_Brodard

b

_,

_Claudia

_Sugnaux

a

_,

_Katharina

_M.

_Fromm

a

a_University_of_Fribourg,_Department_of_Chemistry,_Chemin_du_Musée_9,_CH-1700_Fribourg,_Switzerland

b_University_of_Applied_Sciences_of_Western_Switzerland,_College_of_Engineering_and_Architecture_of_Fribourg,_Boulevard_de_Pérolles_80,_CH-1705_Fribourg,

Switzerland

Cathodesinlithiumionbatteriesconsistofanionicconductor,anelectronicconductorandabinderin ordertomakeacompositethatisbothelectronicallyandionicallyconductive.Thecarboncoatingon thecathodematerialplaysacriticalrolefortheelectrochemicalpropertiesoflithiumionbatteriesdue totheincreasedelectronicconductivity.Weexplaintherelationshipbetweentheelectrochemical prop-ertiesandthecharacteristicsofcompositespreparedusingtheball-millingprocessinthisreport.We investigatedtwotypesofcarbonaceousmaterials(graphiteandcarbonblack)inLiCoO2electrodes.These

selectedcarbonmaterialshavedifferentcharacteristicsandstructureuponball-millingwithLiCoO2.The

compositepreparedbyball-millingfor5minleadstobettermixingofcarbonandLiCoO2,anintimate

con-tactofcarbononLiCoO2,ahigherlithiumiondiffusion(DLi)thannonball-milledandlongerball-milled

composites.Ontheotherhand,alongertimeofball-milling(30and60min)decreasestheelectronic andionicconductivityduetoanincreaseofdisorderedstructureofcarbonandathickanddensecarbon coatinglayeronLiCoO2particles,preventingthediffusionoflithiumions,respectively.

1. Introduction

High-performancecathodesoflithiumionbatteriesrequireto beelectronicallyandionicallyconductiveinordertofacilitatethe fasttransport ofelectronsand lithiumionsthatare insertedor extractedfromthetransitionmetaloxideparticles(LiCoO2,[1–5]

LiMn2O4,[6]LiMO2(M=mixedtransitionmetals),[7,8]LiFePO4,

LiMnPO4 [9,10]etc.).Ingeneral,theionicconductorsofcathode

materialsneedanelectronicconductortoimprovetheelectronic conductivityoftheelectrode.Carbonmaterialiscommonlyused asaconductiveadditivetofacilitatethetransportofelectronsin thecathode.Theelectronicconductivityofcarbondependsonits purity,itsorderedand/ordisorderedstructure,themanufacturing processesandthecarbonaceoussourcesetc.[11–13].

Moreimportantly,theelectronicconductivityofcompositeis notonlyrelatedtothecarbonmaterialitselfbutalsotheprocessof assemblingthecompositetogetherwiththeactivematerial,which strongly inﬂuences thecomposite structure [14–18].Therefore, the preparation ofcomposite is a crucial step tomaximizethe

∗ Correspondingauthor.Tel.:+41796780008;fax:+41263009738.

E-mailaddresses:namhee.kwon@unifr.ch,nhkwon2011@hotmail.com

(N.H.Kwon).

electrochemical properties of the electrode. Recently, Kwon reported that the morphology of composite is affected by the preparationconditionsofmakingacompositeviaball-milling[18]. Amechanical mixing improvesthehomogeneityof carbon and theactivematerial,enhancingtheelectrochemicalpropertiesof cathode.However,therearestillscientiﬁcquestionsabouta ball-millingprocessremainingasfollows;i)doestheball-millingaffect thelocalstructureofcarbon?ii)doestheelectronicconductivity ofthecompositechangeuponball-milling?,andiii)knowingthat mechanicalmillingbreakslargeparticles,howwouldthelithium iondiffusionoftheactivematerialbeaffecteduponmilling?To answerthose questions, weevaluated theeffectof ball-milling intermsoftheelectronicandionicconductivityofcompositeas wellasthestructureofcarbonandthemorphologyofcomposite preparedbyball-milling.

2. Experimental

Graphite(SFG,Timcal,Belgium)andcarbonblack(Ketjenblack 600, Akzo Nobel, Germany) wereused to preparea composite materialwithcommercialLiCoO2(Aldrich).Thesecarbon

materi-alsarereferredtoas“SFG”and“Ket”,respectively,inthefollowing. Thesematerialswereball-milledinahorizontalset-up(RetchMM 400)for 5, 30 or60minwith30/s ofthe frequencyin a jar of

Published in (OHFWURFKLPLFD$FWD±

which should be cited to refer to this work.

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Fig.1.Thecyclicvoltammogramsofcarbon/LiCoO2compositeelectrodes(a)with10wt%ofSFGand(b)with10wt%ofKetpreparedbyball-millingfor0,5,30and60min.

Thesweepscanratewas0.1mV/s.TheelectrolytewasEC:DMCwith1MLiPF6salt.

10mlwithtwostainlesssteelballsof10mmindiameter.After creatinga composite of carbon and LiCoO2 viaball-milling, an

electrodewaspreparedonaluminumfoilwiththecompositeof carbon/LiCoO2(C-LiCoO2)andpolyvinylideneﬂuoride(PVDF)in

N-methyl-2-pyrrolidone(NMP).TheweightratiooftheLiCoO2active

material,carbon and binder was80:10:10.Thetested cathodes had5×5mmofsurfaceareaandcontainedtypicallyabout1mg ofactivematerialafterdrying. Theelectrodesweredriedunder vacuumat120◦Covernight.Lithiummetalandethylene carbon-ate(EC)anddimethylcarbonate(DMC)mixture(1:1v/v)with1M LiPF6 wereusedasa counter electrode andelectrolyte,

respec-tively.Apotentiostat(PrincetonAppliedResearch 273A)andan Arbinbatterytestinstrument(version4.27)wereusedto exam-inetheelectrochemicalpropertiesofthecarbon-LiCoO2composite

electrodes.

Ramanspectroscopywasappliedtostudythestructureof car-bonbeforeandafterball-milling.Theexperimentswerecarried outwithaconfocalmicro-Ramanspectrometer(HORIBALabRAM HR800,combinedwithanopticalmicroscopeOlympusBX41).We usedalaserat531.8nmforexcitation,attenuatedwithneutral den-sityﬁlters(ND0.4andND2,60%and99%attenuation,respectively), resultinginlaserpowersof16mWand0.4mWonsamplewiththe 50Xobjectiveemployed,respectively.Becauseoftheconcentration ofthelightontoaverysmallspot(diameterapprox.2␮m),the attenuationofthelaserpowerwithﬁlterswasnecessarytoavoid thermaldegradationofthesample.Theacquisitiontimewassetto 2×0.5s.Themorphologyandthesizeoftheparticleswere deter-minedusingscanningelectronmicroscopy(SEM,PhilipsXL30)and

transmissionelectronmicroscopy(TEM,FEI/PhilipsCM100).The Brunauer-Emmett-Teller(BET)nitrogenadsorption methodwas usedtomeasurethespeciﬁcsurfaceareaofLiCoO2,carbon

mate-rialsand C-LiCoO2 compositesbeforeandafterball-milling.The

electronicconductivitywasmeasuredwithdensepelletsof com-posites and LiCoO2 material usingdirect current (DC)with the

4-pointprobe method.The pellets in 13mm of diameterwere pressedby5tonsinordertomakeadensebody.Thelithiumion dif-fusioncoefﬁcients(DLi)oftheelectrodesweredeterminedbycyclic

voltammetrytechniqueusingthelinearrelationshipbetweenpeak current,thelinearsweeprateandiondiffusionconstantfromthe Randles-Sevcikequation[19].Eachiondiffusioncoefﬁcientisthe meanvalueofthreeelectrodestestedunderthesamemeasurement conditions.

3. Resultsanddiscussion

ThedifferentelectrodecompositematerialswithSFGandKet wereinvestigatedfortheirelectrochemicalproperties.Fig.1shows thecyclicvoltammogramcurvesofSFGandKetcontainingLiCoO2

compositeelectrodes.Thecomposite ofSFG/LiCoO2 preparedby

ball-millingfor0 and5minshowedsharpredoxcurrent peaks, whilethe30 and60minof ball-milledcompositeshada wider potentialdistancebetweenthereduction andoxidation current peaksandbroaderpeaksatbothredoxevents(Fig.1(a)).Withthe Ket/LiCoO2composite(Fig.1(b)),theredoxcurrentdensities

with-outballmillingareweaker,andfor30and60minevenirreversible, while onlythe 5minball-milledsample shows good reversible

Fig.2. Theratecapacities(C/10,C/5and1C)ofthecompositeselectrodesSFG/LiCoO2(a)andKet/LiCoO2(b).Thecompositeswerepreparedbyball-millingfor0,5,30and

60min.Thevoltagewindowwasbetween2.7and4.4Vvs.Li+_/Li._Li_metal_and_EC:DMC_with₁_M_LiPF

6wereusedasanodeandelectrolyte,respectively.

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Fig.3. Ramanspectrumof(a)SFG/LiCoO2and(b)Ket/LiCoO2compositepreparedbyball-millingforvarioustime.

redoxbehavior.Thus,ashortball-millingprovidedsuperior elec-trochemicalpropertiescomparedtonoorlongerball-milling.

Fig.2(a)and(b)showthedischargecapacitiesofthe compos-ites,SFG/LiCoO2 andKet/LiCoO2atC/10,C/5and1C.Thevoltage

window wasbetween2.7 and 4.4V vs.Li+_/Li. ₅_min_ball-milled

SFG/LiCoO2 composite electrode providedthehighest discharge

capacitiesof220,196and106mAh/gatC/10,C/5and1C, respec-tively,amongalldifferentball-millingprocessesshowninFig.2(a). Theﬁrstcapacityofnon-ball-milledLiCoO2compositeelectrode

reachedover150mAh/gatC/10,whichisthemaximumpractical capacity.However,thefollowingcapacitiesdecreasedrapidly prob-ablyduetothehighuppervoltagelimit(4.4Vvs.Li/Li+_)._Fig.₂_(b)

shows thesametrend oftheball-millingeffect,sincethe5min ball-milled Ket/LiCoO2 composite electrode reachedthehighest

dischargecapacitiesof156,142and109mAh/gatC/10,C/5and1C, respectivelyamongtheseriesofKet/LiCoO2compositeelectrodes.

Thedischargecapacitiesof5minball-milledSFG/LiCoO2

compos-iteelectrodearehigherthanthoseof5minball-milledKet/LiCoO2

atallstudiedcurrentdensities.

In order to evaluate the effect of a short and long time of ball-millingontheelectrochemicalproperties,theC-Cbond struc-turesofbothcarbonmaterialswereﬁrstcharacterizedbyRaman spectroscopy.Fig.3(a)SFGand(b)Ketshowtwomainbandsat 1350and1580cm−1,correspondingtodisorderedDandorderedG band,respectively[20].IncaseofFig.3(a),theratioofintensities I1350/I1580increasesuponthemillingtime.Itmeansthatthe

disor-deredDbandbecomesmoredominantinthestructureofSFGupon milling.Fig.3(b)showstheRamanspectrumofKetinthe compos-ite,indicatingnotsurprisinglythattheinitialKetleadstoahigher intensityofdisorderedDbandat1350cm−1thantheonefortheG bandat1580cm−1.BothDandGbandsat1350and1580cm−1get broaderandlowerinintensitiesuponball-milling.Theball-milling processincreasesthusthedensityofdefectsand/orreducesthe crystallite size[21].In addition,thebandat1200cm−1 in ball-milled SFGand Ketis indicative of thepresence of amorphous sp3_-hybridized_carbon_atoms._Therefore,_milling_of_SFG_seems_to

decreasethecrystallitesize(D/Gratioincreasing)andgenerates amorphoussp3_carbon_atoms_(band_at₁₂₀₀_cm−1_appearing),_while

incarbonblack,themillingleadstoonlyslightgenerationof amor-phoussp3_-hybridized_carbon_atoms_(low_band_at₁₂₀₀_cm−1₎_and

increasesthedisorder/numberofdefects(D/Grationotchanging much,butbandsgetsbroaderandweaker).

Next, theelectronicconductivities(Fig.4)of thecomposites SFG/LiCoO2, Ket/LiCoO2 and LiCoO2 were measured by

four-point probe method.We foundout twomain resultsfromthis

measurement.Oneisthattheelectronicconductivitiesofboth com-positesdecreaseuponball-milling.Secondisthatthecomposite ofKet/LiCoO2hasalowerelectronicconductivitythantheoneof

SFG/LiCoO2.Thisresultsupportsthedataofelectrochemistry(Fig.1

and2(a))andRamanspectrum(Fig.3);higherelectronic conduc-tivitiescomefromhigherintensitiesoforderedGband(I1580)than

disorderedDband(I1350)ofthecompositespreparedfor0 and

5minball-milling.Intheend,thecompositehavinghigher elec-tronicconductivityprovidessuperiorelectrochemicalproperties showninFig.1and2(a).Increasingtheball-millingtimeprovides more defectstructure inboth SFGand Ket. Astheintensity of orderedGband(I1580)indicatesthatSFGhasahigherintensitythan

KetinRamanspectrumshowninFig.3,thepristinestructureofSFG ismoreorderedthantheoneofKet.ThisRamandatasupportsthe higherelectronicconductivityofSFG/LiCoO2 compositethanthe

oneofKet/LiCoO2beforeandafter5minofball-millingshownin

Fig.4.However,thelongertheball-milling,themoredisorderedis thestructureincarbonandthelesselectronicallyconductiveisthe composite.

TheelectronicconductivityofinitialLiCoO2withoutball-milling

andcarbonwasmeasuredby4-pointprobemethodtobe3×10−3 S/m.Afterball-millingfor5, 30and 60min,theelectronic con-ductivitiesofLiCoO2 withoutcarbonwere2×10−3,6×10−3and

5×10−3 S/m, respectively. Thus, the electronic conductivity of LiCoO2didnotchangesigniﬁcantlyuponball-millingcomparedto

thecomposites.

Fig.4.Theelectronicconductivitiesofthecomposites,SFG/LiCoO2,Ket/LiCoO2and

LiCoO2withoutcarbonversusball-millingtime.

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Fig.5. Theimagesofscanningelectronmicroscopyofthecompositematerials:(a)and(c)SFG/LiCoO2preparedbyball-millingfor0and5min,respectivelyand(b)and(d)

Ket/LiCoO2preparedbyball-millingfor0and5min,respectively.

Electrodesrequiretobeelectronicallyconductivetoproduce theelectrical energy while theredoxreaction involving transi-tionmetaloccurs.Alongball-milling,whichdecreasestheordered structureofcarbon,islessdesiredduetothedecreasingofthe elec-tronicconductivity.However,ball-millinginducesabettermixing ofthecompositeandimprovestheintimatecontactbetween car-bonandtheactivematerialasourpreviousstudiesshowed[22,23]. Therefore,weinvestigatedthemorphologiesofthosecomposites withelectronmicroscopies,usingSEMandTEM.

TheinitialshapeandsizeofSFG/LiCoO2andKet/LiCoO2before

ball-millingcanbeeasilydifferentiatedbySEM(Fig.5(a)and(b)). TheparticlesofKet(Fig.5(b))areratheragglomeratedatthe cor-nersoflargeLiCoO2particles.Afterball-millingofthecomposite,it

ishomogeneouslymixedandtheshapesofLiCoO2andcarbon

par-ticlesbecameirregularandlargeLiCoO2particlesarebrokendown

tosmallersizes(Fig.5(c)and(d)).Thus,theball-millingprocess brokethemicron-sizedlargeparticlesofLiCoO2andSFG.

Thedifferenceofshortandlongball-millingofLiCoO2without

carbonwasveriﬁedbySEM(Fig.S1).Alongerball-millingof30and 60mincreatesthesecondaryagglomerations(>10␮m)ofparticles, likelyduetolocalheating(theinsetimageofFig.S1(c)and(d))The SSAofLiCoO2andthecompositesofSFG/LiCoO2andKet/LiCoO2are

discussedbelow.WehaveveriﬁedthephaseofLiCoO2beforeand

afterballmillingbyXRD.Therewasnophasetransformationof LiCoO2andnoextraimpuritiescreatedbyballmilling(Fig.S2).

Tostudyinmoredetailthemorphologyofcarbonandthe sur-faceofcompositebeforeandafterball-milling,TEMimagesofthe compositesweretaken(Fig.6).The0minball-milledcomposites, Fig.6(a)hasratherseparatedparticlesofcarbonandLiCoO2.Fig.6

(b)showsthesurfaceofLiCoO2whichispartiallycoatedwithKet.

TheinterfacebetweenKetandLiCoO2particlesinFig.6(b)showsa

signiﬁcantdifferencefromthehomogeneouslycoatedLiCoO2

par-ticlesinFig.6(d).The5minball-milledcomposites(Fig.6(c)and (d))haveahomogeneousdistributionofcarbonandLiCoO2

parti-cles.ThecarbonmaterialsofbothSFGandKetareattachedonthe particlesofLiCoO2.Particularly,Fig.6(d)showsahomogeneously

coatedthinlayerofcarbon(greyintheimage)onthesurfaceof LiCoO2(blackintheimage).The5-minball-millingprovidesthusa

closeandmorehomogenouscontactofcarbonwithLiCoO2.

How-ever,alongermillingleadstoadenseandthicklayerofcarbonon LiCoO2 particles,asshowninFig.6(e)and(f).Thiscarbonlayer

canpreventthediffusionoflithiumions.Inaddition,itprovided adecreaseofelectronicconductivitybycreatingadefectstructure ofcarbon.Asaresult,theelectrochemicalpropertiesofthe com-positespreparedfor30and60minball-millingwereinferiortothe onesofthecompositesat5minofball-millingasshowninFig.2.

Theelectrodesforlithiumionbatteriesmustbealsoionically conductivetoobtaintheinsertion/extractionreactionoflithium ions.Wethereforeinvestigatedthediffusionoflithiumionsinthe compositeelectrodes.Thelithiumiondiffusioncoefﬁcients(DLi)

ofthecomposite electrodesweredetermined bycyclic voltam-mograms at various scan rates with using the Randles–Sevcik equationdescribedbelow:[19]

Ip=(2.69×105)n3/2AD1/2_Li Cv1/2

whereIpisthepeakcurrent;nisthenumberoftransferelectrons; Aisthesurfaceareaoftheelectrode;Cistheconcentrationof reac-tants;andvisthescanrate.Fig.7showsoneexampleofthecyclic voltammogramsofSFG/LiCoO2compositepreparedbyball-milling

for5minatvariousscanratesinordertodeterminetheanodic cur-rentpeaks.Then,thegraphsofthesquarerootofthescanrate,mV

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Fig.6. TheimagesoftransmissionelectronmicroscopyofSFG/LiCoO2(left-handsideimages)andKet/LiCoO2(right-handsideimages)compositepreparedbyball-milled

for0min(a)and(b),5min(c)and(d),and60min(e)and(f).

½_S−1/2_,_versus_the_anodic_peak_give_the_slopes_of_each_electrode,_as

shownintheinsetofFig.7.Theseslopesrepresentthesquareroot ofthelithiumiondiffusioncoefﬁcientvalue,DLi.

Basedonthecalculationabove,Fig.8showsthelithiumion dif-fusioncoefﬁcientsofthecomposites’electrodesversusthetimeof ball-milling.Thelithiumiondiffusioncoefﬁcientsdecreasedwith theincrease ofball-millingtimeinboth compositematerialsof SFG/LiCoO2andKet/LiCoO2.Themainreasonofthismayberelated

tothedenseandthicklayerofcarbononLiCoO2preparedby

ball-millingfor30and60min,whichblocksthepenetrationoflithium ionsintotheparticleofLiCoO2asTEMimagesindicateinFig.6.On

theotherhand,the5minball-milledSFG/LiCoO2 composite

pro-videdhighestDLi among allthecomposites.Thisis becausethe

smallerparticlesizeofLiCoO2shortenedthediffusionpathlength

oflithiumionscomparedtotheinitialparticlesizeofLiCoO2.This

isconﬁrmedbythemeasurementofthespeciﬁcsurfacearea(SSA) ofLiCoO2,pristineandball-milledfor5min.TheSSAsofLiCoO2,

thecompositesofSFG/LiCoO2andKet/LiCoO2areshowninFig.9.

ItrevealedthattheSSAofLiCoO2increasedfrom0.52m2/g

(cor-respondingtoaparticlesizeof2.28␮m)ofthepristinematerialto 4.57m2_/g_{(corresponding}_to_a_particle_size_of_0.26_␮m)_after₅_min

ball-milling.Furtherball-milling(30min)stillincreasedtheSSAof LiCoO2,thusleadingtoyetsmallerparticlesizeofLiCoO2.However,

30and60minofball-millingproducedalsomoreagglomerated particlesof<10␮masshownintheinsertofFig.6(c)and(d). There-fore,theelectrochemicalpropertiesoflongerball-milledcomposite electrodeswereinferiortotheshortermillingbecauseofthedense carbonlayerandthedecreasedtheionicconductivityofthe com-posite.

According to Fick’s law, ion transport is governed by the distance;␶=L2_/D,_where_D_is_the_ion_diffusion_coefﬁcient_(the

diffu-sivity),Listhedistanceofdiffusionlength,and␶isthetimeforthe lithiumionsneededforthediffusiondistance.Thus,itisclearthat thesmallerparticlesizedLiCoO2hastheshorterdiffusionlength

forlithiumionscomparedtothelargerparticlesofLiCoO2within

acompositeelectrode.TheDLiofotherLiCoO2-carboncomposite

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Fig.7.Cyclicvoltammogramcurvesatthescanratesfrom0.1to1mV/softheSFG/LiCoO2compositeelectrode.Thecompositeispreparedbyball-millingfor5min.The

insetﬁgure:TherelationshipbetweentheanodicpeakcurrentsandthesquarerootofthescanrateofSFG/LiCoO2withvariousball-millingtimeofthecomposite.

electrodesisreportedbetween10−13to10−7cm2_S−1_[24–29]_._The

largedifferenceofDLiisduetothedifferentformulausedfor

deter-miningthediffusioncoefﬁcientsbydifferenttechniquesapplied. Insummary,weshowedthatthecharacteristicsofthe com-positeandthestructureofcarbonwerechangedviaball-milling.

Fig.8.ThediffusioncoefﬁcientsoflithiumioninLiCoO2withSFGorKetdecrease

withincreasingthetimeofball-milling.

Fig.9.SSAsofthecomposites,SFG/LiCoO2andKet/LiCoO2andLiCoO2preparedby

ball-millingforvarioustime.

Thosecharacteristicsaffecttheelectrochemicalpropertiesofthe electrodes.Ashortball-millingimprovedtheelectrochemical prop-ertiesbasedontheshorterdiffusionpathlengthoflithiumions bybreakinglargeparticles,onthehomogeneouscarboncoating andontheintimatemixing,whileatthesametimekeepinghigh electronicconductivity.However,alongmillingtimeincreasesthe defectstructureofcarbon,decreasestheelectronicconductivity ofcomposite,leadstoathickcarbonlayerinthecompositeand degradesthustheelectrochemicalproperties.

4. Conclusions

Theelectronicandionicconductivitiesofthecathodedepend onthemorphologyofthecompositeinﬂuencedbythepreparation methodandtheinitialcharacteristicsofcarbon.Weapplieda ball-millingtopreparecompositesofdifferentcarbonsourcesandthe activematerialLiCoO2.Theball-millingchangesthecharacteristics

ofthecompositeasfollows;i)ashortball-milling(5min)improved thehomogeneityofthecompositeandprovidesintimatecontact ofcarbonandLiCoO2,whilealongball-millingleadstoathickand

densecarbonlayeronLiCoO2 particles;ii)theionicconductivity

improvedafterashortball-millingtimeduetothesmaller parti-clesizeofLiCoO2andimprovedtheelectrochemicalproperties,as

shownbythelithiumiondiffusioncoefﬁcient;iii)thedisordered structureofSFGandKetincreaseduponball-milling,asobserved byRamanspectroscopy;andiv)alongtimeofball-milling(30and 60min)decreasedtheelectronicconductivityoftheformed com-posite,Therefore,theelectrochemicalpropertiesofthecathodeare affectedbythecharacteristicsofcathodecompositerelatedwith thetransportofelectronsandions.

Acknowledgement

ThisstudywassupportedbytheSwissNationalScience Founda-tion(NationalResearchProgram64),EKZ(Elektrizitätswerkedes KantonsZürich),theFriMat(theFribourgCenterforNanomaterials) andtheUniversityofFribourg.TheauthorsthankTimcal(Belgium) andAkzoNobel(Germany)forkindlyprovidingcarbonmaterials.

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AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,

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