Ultracentrifugation: An effective novel route to ultrafast nanomaterials for hybrid supercapacitors

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Ultracentrifugation: An effective novel route to ultrafast

nanomaterials for hybrid supercapacitors

Etsuro Iwama, Patrice Simon, Katsuhiko Naoi

To cite this version:

Etsuro Iwama, Patrice Simon, Katsuhiko Naoi. Ultracentrifugation: An effective novel route to

ultra-fast nanomaterials for hybrid supercapacitors. Current Opinion in Electrochemistry, Elsevier, 2017, 6

(1), pp.120-126. �10.1016/j.coelec.2017.10.011�. �hal-02045881�

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To cite this version:

Iwama, Etsuro and Simon, Patrice

and Naoi, Katsuhiko Ultracentrifugation:

An effective novel route to ultrafast nanomaterials for hybrid supercapacitors.

(2017) Current Opinion in Electrochemistry, 6 (1). 120-126. ISSN 2451-9103

Ultracentrifugation:

An

effective

novel

route

to

ultrafast

nanomaterials

for

hybrid

supercapacitors

Etsuro

Iwama

,

Patrice

Simon

and

Katsuhiko

Naoi

Energystoragedevicesaresomeofthemostimportant environmentaltechnologiesthatarehighlyinfluentialin advancingourlifeinthefuturesociety.Specifically,

electrochemicalcapacitorisanenergyfacilitatorthatexhibits anefficient/economicalcharginganddischarging

characteristicswithlonglifespan.Thus,thecapacitor technologyisregardedaspromisingduetoanincreasing effectivenesswhencombinedwithrenewable(solar/wind/micro hydraulic)energysources.Inthisconnection,Li–ion-based hybridsupercapacitorsandtheirfunctionalmaterialsarebeing vigorouslyresearchedinhopestoimprovetheir

capacity/voltageandthereforetheirenergydensity.Transition metaloxidesareamongthemostpopularmaterialsutilizedin thispurpose.Thankstohighvoltageandassociatedhigh energydensity,theyaretunedasbothhighenergyandhigh powermaterials.Inrecentyears,thestructural/textural propertiesofoxides,includingparticlesize,crystallinity, defects,andporosity,weresuccessfullyfine-tunedtoachieve highrateperformanceover300C.Thepresentreviewwill describepseudo-capacitivenanosizedoxidespreparedwithin situsynthesistechniquecalled“ultracentrifugation”,showing ultrafastelectrochemicalresponseevenmorethanEDLC.

Addresses

1_{DepartmentofAppliedChemistry,TokyoUniversityofAgriculture&} Technology,2-24-16Naka-cho,Koganei,Tokyo184-8588,Japan 2_{InstituteofGlobalInnovationResearch,TokyoUniversityofAgriculture} andTechnology,2-24-16Naka-cho,Koganei,Tokyo184-8588,Japan 3_{CIRIMAT,Université deToulouse,CNRS,INPT,UPS,118routede} Narbonne,31062Toulousecedex9,France

4_{RéseausurleStockageElectrochimiquedel’Energie,RS2EFRCNRS} 3459,France

5_{AdvancedCapacitorResearchCenter,TokyoUniversityofAgriculture} &Technology,2-24-16Naka-cho,Koganei,Tokyo184-8588,Japan ∗_{Correspondingauthor:Naoi,Katsuhiko (k-naoi@cc.tuat.ac.jp)}

https://doi.org/10.1016/j.coelec.2017.10.011

Introduction

Highly efficient and stable energy storage devices are neededtodevelopefficientsmartmobilitysystemfor ur-ban transportation or stationary applications large-scale storage of energy produced from renewable sources. Amongsuchdevices,supercapacitors,alsocalledelectric doublelayercapacitors (EDLCs),showsuperior perfor-mancessuchashighpowerelectrochemicaldeviceswith fastcharging–dischargingcapability,remarkablestability andcyclelifecomparedtoLi–ionbatteries[1••,2].The energydensityofsupercapacitorsislowandneedstobe enhanced to furtherexpand their rangeof applications. Designinghybridsupercapacitorisapromisingrouteto furtherimprove the performanceof supercapacitors,by combiningbothanactivatedcarbonelectrodeandalarge capacity faradic (pseudocapacitive or battery-like) elec-trode [3].Transition metal oxides are among the most popularmaterialsutilizedinthispurpose[4–11•].Thanks tohighvoltageandassociatedhighenergydensity,they aretunedasbothhighenergyandhighpowermaterials. Inrecentwork,thesynthesisofactivematerialsbymeans ofthe“ultracentrifugation” process,calledUCtreatment, has been explored to further extend the performance of lithium metaloxides such as Li4Ti5O12 (LTO) [12],

bronze-typeTiO2 (TiO2(B))[13],Li3VO4 (LVO)[14••],

and phosphate compounds such as LiFePO4 (LFP)

[15••]andLi3V2(PO4)3[16].TheUCtreatmentenables

the preparation of nanosized and dimension-controlled (1D or 2D) materials directly bonded on high-surface areaconductingnanocarbons such as carbon nanotubes (CNT) (seethe example of nano-LTO/CNT shownin

Figure 1) [3,12]. Using UC treatment and depending on the nature of nanocarbons, structural/textural prop-erties of such active materials—including particle size, crystallinity,defects,andporosity—canbefine-tunedto achieveultrafastelectrochemicalperformancewithhigh stability.This review presentsthree examples of metal oxides(TiO2(B),LVO,LFP)/nanocarboncomposites

pre-paredviaUCtreatment,showingpromising electrochem-icalperformanceswhichareneededtodevelopthenext generationofhybridsupercapacitors.

TiO

(B):

dimension

control

and

hyper-dispersion

of

nano

metal

oxides

within

a

nanocarbon

matrix

Bronze-type TiO2 (TiO2(B)) has a good potential to

enhance the electric conductivity (≈10−2 −1 cm−1) compared to other Ti-based oxides, like LTO (≈10−13 −1 cm−1) [12] and TiO2 polymorphs such

Figure1

SchematicillustrationofsynthesisofnanocrystallineLi4Ti5O12[3,12,17]whichwerepreparedwiththenanocarbons(MWCNT)co-existencethrough ultracentrifugation(∼75,000g)alongwithhigh-resolutiontransmissionelectron(HRTEM)imagesofnanocrystallineLi4Ti5O12/MWCNTcomposites.

as anatase and rutile (10−14–10−13 −1 cm−1) [18]. TiO2(B)hasatheoreticalcapacityof335mAhg−1which

istwiceofLTO.ItsLi+diffusionproceedsnormallyalong the b-axistunnel resultingin relatively poor Li+ diffu-sion coefficient of 10−14–10−16 cm2 _s−1 _{[19–21], while}

the LTO shows 3–6 orders of magnitude (10−8−10−13 cm2 _s−1_{) [12,22]. Conventional TiO}

2(B) synthesis via

hydrothermal treatment fromalkaline titanatesleads to cylindricalmorphologywithlongb-axis,resultinginpoor

C-ratecapability [23].Downsizingtheparticlesizeisan effective way to shorten the b-axis diffusion length in TiO2(B) crystal [24,25].Operation over 100C, however,

hasbeenhamperedbecauseoftheinevitable agglomera-tionofTiO2(B)nanoparticlesbelow10nm[26–28],that

limitstheaccessibilityofLi+fromthebulkelectrolyte. Hyper-dispersed single-nano TiO2(B) crystals were

uniformly formed in a MWCNT matrix using UC treatmentcombinedwithafollow-uphydrothermal treat-ment. These TiO2(B)/MWCNT composites have

size-controlledcrystallineTiO2(B)particles(5nminaverage)

and anisotropic crystal growth (ultrashort along b-axis) limitingtheagglomerationoftheTiO2(B)nanoparticles

(seeFigure2a),describedas short-TiO2(B)(S-TiO2(B))

[13]. For comparison purpose, we prepared rod-type TiO2(B)crystalswithlongb-axis(eighttimeslongerthan

S-TiO2(B)inaverage[13])whichcontainssameamount

ofMWCNT(30wt%) astheS-TiO2(B).Cyclic

voltam-mogramsforS-andL-TiO2(B)at10mVs−1(Figure2b)

shows thattheshape andthe numberof the peaksare differentbetweenthetwosamples;theS-TiO2(B)shows

twosharppeaksat1.55and1.65Vvs.Li/Li+,whilethe L-TiO2(B)possessessingle broadpeakwhichissimilar

to thereported CVshapeof TiO2(B)nanowire [29,30].

Thesharpeningofthepeaksfor S-TiO2(B)comesfrom

theshorteningofb-axislengthandtheincreasednumber of diffusion paths, which enable a fast Li+ access and intercalationinto TiO2(B) A1 and A2 sitesalong theb

-axisdiffusionchannelintheTiO2(B)crystals(Figure2c).

Such ultrashort b-axis length and hyper dispersion of TiO2(B)withintheMWCNTmatriximprovesthepower

capabilityof TiO2(B)byenablingultrafast Li+

deinter-calation(235mAhg−1at300C,1C=335mAg−1),which isfarsuperiorto theL-TiO2(B)[13].TheseUC-treated

TiO2(B)/MWCNT nanocomposites with controlled

(ul-trashort) b-axis length can be used to prepare hybrid supercapacitorwithhigherenergydensity.

Li

VO

:

electrochemical

activation;

control

of

crystal

structure

of

nano

metal

oxides

for

Li

diffusion

enhancement

via

electrochemical

method

To further increase the energy density, one way is to replacemore positiveTiO2 (B)electrode (1.2–1.6Vvs.

Figure2

(a) HRTEMimagefortheshortb-axisTiO2(B)(S-TiO2(B),b-axislength=3–5nm))nanoparticlesdispersedwithinMWCNTmatrixalongwitha representativeHRTEMimageanditsschematicillustration.(b)CyclicvoltammogramsforS-TiO2(B)andlongb-axisTiO2(B)(L-TiO2(B),b-axis length=40–60nm)[13]atascanrateof10mVs−1.(c)SchematicillustrationofLiinsertionintoS-TiO2(B).

Li/Li+)withothernegativeelectrodesoperatingatlower redox potential [31]. Li3VO4 (LVO) has been recently

reported to reversibly intercalate up to 2 Li per LVO atlowpotential (0.1−1.0Vvs.Li),leading toacapacity of 394mAhg−1 [32–37].However,LVOexhibitsavery low electronic conductivity (<10−10 −1 cm−1), which isdetrimentaltoachievinghigh-powerperformance[38– 40].Itshowsas wellalarge voltagehysteresisbetween charge and discharge (<500mV) that limitsits useas a negativeelectrodeinelectrochemicalenergystorage de-vices[41,42].

Using the UC treatment process,nanoparticles of LVO weresuccessfullyentangledonthesurfaceofMWCNT (40wt%)and uniformlydispersedwithintheMWCNT matrix[14••].MagnifiedTEMimagesshowthatthe com-positeismadeofnanoparticles(sizebelow50nm)clearly identified as LVO by the lattice fringes of (0 1 0) and (0 02)phases (Figure3a).The capacityof the compos-itereached330mAhg−1whencycledinapotentialrange of2.5Vdownto0.1Vvs.Li.Itshowshighratecapability, with50% ofcapacity retentionat20Ag−1, correspond-ing toabout 50Crate for LVOand 500Cfor AC. More-over,after removalofthecontributionof theMWCNT matrix,LVOshowsafaradicefficiencyof 95%at1st cy-cle,constantover1000cycles.Suchhighreversibilityof LVOenabledtheinvestigationoftheLiinsertion mech-anismintoLVOcrystal.Acarefulinvestigationbymeans of inoperandoXRDand X-rayabsorptionfine structure (XAFS)measurementsrevealedtheexistenceofan

irre-versiblestructuretransformationduringthefirstlithiation reaction,assimilatedasanactivationprocess.This activa-tionswitchesthereactionmechanismfromaslow “two-phase” toafast“solid-solution” processinalimited po-tentialwindow(2.5Vdownto0.76Vvs.Li),asshownin

Figure3bandc.Inthispotentialrange,theLi+ intercala-tionisacceleratedthankstoafastsolid-solutioninsertion mechanismwithasmallhysteresis,leadingtohigher en-ergyefficiencythatisrequiredforhybridsupercapacitors.

LiFePO

(LFP):

defective

(crystalline/amorphous)

control

of

nano

metal

oxides

within

the

peculiar

core-shell

LFP/graphitic

carbon

structure

LiFePO4 (lithium ironphosphate, LFP) haslong been

investigated as a cathode material in Li–ion batteries because of its relatively high theoretical capacity of 170 mAh g−1, low cost and high electrochemical and thermal stabilities [43]. The Li intercalation electro-chemicalreactionofLFPproceedsthroughatwo-phase reactionbetweenLi-richLi1−aFePO4(LFP)andLi-poor

LibFePO4(FP)[44]withLiinsertion/deinsertion

occur-ringalongthebaxis[45].However,thelimiteddiffusion kinetics of Li ions at the LFP/FP interface together with the poor electronic conductivity of the pristine olivine-LFP(10−10–10−7−1cm−1)[46]limitthepower capability of thematerial. Downsizing theparticle size (5–100nm)failedtoenhancethepowerperformancedue to the re-aggregation of particles and the difficulty in creatingefficientelectron pathways[47].The synthesis

Figure3

(a) HRTEMimageofUC-derivedLi3VO4(LVO)nanocrystals(b)Capacityplotswithdifferentdischarge(lithiation)currentdensitiesforactivatedand non-activatedUC-LVO.Inset:Chargedischargecurvesforactivatedandnon-activatedUC-LVObetween0.76–2.5Vvs.Li.(c)ComparisonofXRD patternsof2θ =35−37° showingdifferentreactionmechanismforactivatedandnon-activatedUC-LVO.

ofcarboncoatingsontoLFPparticleshasalsobeen pro-posed,byaddingcarbonprecursorsduringthesynthesis of LFP [48,49]. However, non-conformal amorphous carbon coatings did not show enough improvement in electrical conductivity when nanosized LFP particles wereprepared[15••].

Single nanosized LFP crystals encapsulated within hollow-structuredgraphiticcarbonsweresynthesizedvia UCtreatment.Evidencedbythecombinationof spectro-scopic and X-ray diffraction characterization [15••], the LFP/graphiticcarboncompositematerialhasacoreLFP (crystalline(core1)/amorphous(core2))/graphiticcarbon shellstructureasillustratedinFigure4ainsettop. Com-parisonofthescanningelectronmicroscope(SEM,Figure 4b)anddark-fieldimagesshowtheencapsulationofLFP particles core (white spots in Figure 4c) with the size of 10–20nmwithinacarbon shell.Unlike conventional LFPwhereLi+intercalationisachievedatconstant po-tentialthroughatwo-phasereactionmechanism[44],the galvanostatic charge/discharge profile of the composite showed different electrochemical signatures both with plateau and slopingregion(Figure4ainsetbelow).The sloping profiles below 3.4V corresponds to amorphous LFPcontaining Fe3+ _defects[50•_{,51],and theLi+}

dif-fusioncoefficientoftheamorphousLFP(10−11cm2s−1) was found to be two orders of magnitude higher than that of crystalline LFP core phase (10−13 cm2 _s−1_{) in}

theplateauregion.TheLFP/graphiticcarboncomposites haveanextremelyhighratecapabilitybothinchargeand discharge;89,60,36,and24mAhg−1at1,100,300,and 480C,respectively(Figure4a).Suchalinearrelationship

meansthatthecompositescanofferahigh-power capabil-ityofthematerialindischargeaswellasincharge,such asexpectedforthepracticaluseofhybridsupercapacitors. Suchresultspavethewayfordesigninghighenergyand highpowermaterialstobeusedinhybridsupercapacitors.

Conclusions

and

remarks

The ultracentrifugation (UC)-treated transition metal oxides/nanocarbon composites described in this re-view are newly synthesized materials, which may be excellent candidates as electrode active materials for the next generation hybrid supercapacitors. Nanosized and dimension-controlled materials directly bonded on high-surface area conducting carbons through the UC treatment showed ultrafast electrochemical perfor-mance. Hyper-dispersed single nano TiO2(B) crystals

with anisotropic crystal growth (ultrashort along b-axis) wereuniformlyformedinaMWCNTmatrixusingUC treatment combined with a follow-up hydrothermal treatment. The ultrashort b-axis length and hyper dis-persionof UC-TiO2(B)overcomeitspoor Li+ diffusion

coefficientsandimprovethepowercapabilityofTiO2(B)

byenablingultrafast Li+ deintercalation(235 mAhg−1 at300C).To furtherincrease theenergy densityof the more positive TiO2(B),LVO whose operationpotential

is below 1.0V (down to 0.1V vs. Li/Li+) was chosen. The combination of the activation process of UC-LVO/MWCNT composites and limited-voltage-range operation(2.5Vdownto0.76Vvs.Li)improvedtheLi+ intercalation and deintercalation with a small voltage hysteresis(below0.1V),thankstothefastsolid-solution process of the LVO after activation. Highly dispersed

Figure4

(a) Plotsofdischargecapacityvs.chargecapacityofahalf-cellconsistingofLi/1MLiPF6inEC+DEC/(LFP/graphiticcarboncomposites)asa functionofC-rate:Insettop:Schematicillustrationofthecore-shellnanostructureoftheLFP/graphiticcarboncomposite,representingaminute structureconsistingofanamorphousoutersphereofaLFPcontainingFe3+_{defectsandaninnersphereofcrystallineLFP.Insetbottom:}

Charge-dischargeprofilesofcompositesatdifferentchargeC-ratesfrom1to480C.(b)Scanningelectronmicroscope(SEM),(c)dark-fieldimagesof theUC-derivedLFP/graphiticcarboncomposites,wherebyeachsphereLFPcoreisaccommodated/encapsulatedwithinthehollowstructured graphiticcarbonshells.

defective (crystalline/amorphous) LFP nanoparticles encapsulated within hollow-structured graphitic car-bon enabled ultrafast discharge rates (60 mAh g−1 at 100C, 36 mAh g−1 at 300C) and ultrafast charge rates (60 mAh g−1 at 100C, 36 mAh g−1 at 300C), showing promising characteristics as a positiveelectrode for the next generation hybrid supercapacitors. The specific structures of the composites prepared by the UC-treatmentcontributetoachieveultrafastelectrochemical performance which are needed to develop the next generationofsupercapacitors.

Acknowledgments

ThisstudywassupportedbytheGlobalInnovationResearchOrganization inTUAT.ThisstudywasalsosupportedbyJSPSGrant-inAidforScientific Research(KAKENHI)Aundergrantno.JP25249140,andKAKENHI Grant-in-AidforYoungScientistsBgrantno.JP16K17970.

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