Lithium conducting solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 obtained via solution chemistry

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DOI: 10.1016/j.jeurceramsoc.2012.08.005

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Duluard, Sandrine and Paillassa, Aude and Puech, Laurent and Vinatier,

Philippe and Turq, Viviane and Rozier, Patrick and Lenormand, Pascal and

Taberna, Pierre-Louis and Simon, Patrice and Ansart, Florence Lithium

conducting solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 obtained via solution

chemistry. (2013) Journal of the European Ceramic Society, vol. 33 (n° 6). pp.

1145-1153. ISSN 0955-2219

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rchive

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Lithium

conducting

solid

electrolyte

Li

1.3

Al

0.3

Ti

1.7

(PO

4

)

3

obtained

via

solution

chemistry

Sandrine

Duluard

,

Aude

Paillassa

,

Laurent

Puech

,

Philippe

Vinatier

,

Viviane

Turq

,

Patrick

Rozier

,

Pascal

Lenormand

,

Pierre-Louis

Taberna

,

Patrice

Simon

,

Florence

Ansart

a_{InstitutCarnotCIRIMAT(CNRS/UniversitéPaulSabatier-UMR5085),118routedeNarbonne,F-31602ToulouseCedex9,France} b_{ICMCB,CNRS,UniversitédeBordeaux,sitedel’ENSCBP-IPB(CNRS-UPR9048),87avenueduDr.Schweitzer,F-33608Pessac,France}

Abstract

NaSICON-typelithiumconductorLi1.3Al0.3Ti1.7(PO4)3(LATP)issynthesizedwithcontrolledgrainsizeandcompositionusingsolutionchemistry.

Afterthermaltreatmentat850◦_{C,sub-microniccrystallizedpowderswithhighpurityareobtained.TheyareconvertedintoceramicthroughSpark}

PlasmaSinteringat850–1000◦_{C.Byvaryingtheprocessingparameters,pelletwithconductivitiesupto1.6×10}−4_{S/cmwithdensityof97%of}

thetheoreticaldensityhavebeenobtained.XRD,FEG-SEM,ac-impedanceandVickersindentationwereusedtocharacterizetheproducts.The

influenceofsinteringparametersonpelletcomposition,microstructureandconductivityisdiscussedinadditiontotheanalysisofthemechanical

behaviorofthegrainsinterfaces.

Keywords:Powders-chemicalpreparation;Sintering;Microstructure;Ionicconductivity;Batteries

1. Introduction

Batteriesarekeysystemsforthedevelopmentof technolo-giessuchasportabledevicesandtransportationsystems.One of themost appealingchallenges isthe increaseof their spe-cificmassenergyforapplicationsrequiringlargeenergystorage capacitye.g.electricvehicleorgridstorage.1,2 Inthisrespect, reversible lithium-air batterieswith an expected energy den-sity of more than 500Wh/kg i.e. a potential range of more than800kmhaveshownpromisingresults.3–5Sofar,twomain solvent-basedtechnologieshavebeen studied:aqueous6,7 and non aqueous lithium-air devices.8,9 Other concepts are also beinginvestigated,e.g.anall-solidstatedevicewasdescribed recently by Kumar et al.10 In water-based devices, the main electrolyteisaqueous-saturatedlithia.Inordertoprevent reac-tions of the lithium anode with the aqueous electrolyte, an ionicconductingseparatorisnecessary.Accordingly,a lithium-conducting solid electrolyte is necessary for both all-solid state andwater-basedtechnologies. Solidelectrolytessuch as

∗_{Correspondingauthor.Tel.:+33561557292;fax:+33561556163.}

E-mailaddress:duluard@chimie.ups-tlse.fr(S.Duluard).

Li3Ncompoundsandsulfide-basedglassesaregoodcandidates

withconductivityashighas6×10−3_S/cm.11_{However,being}

hygroscopic, their fabrication in inert atmosphere is manda-tory.OxideelectrolytessuchasLiSICONelectrolytearemore suited due to their ease of preparation. Even though LiSI-CON typecompound Li2+2xZn1−xGeO4 (0.45<x<0.55) was

reportedasbeinghighlyconductive,itsconductivityisstilltoo low atroomtemperature (10−6S/cm).12 With conductivityin the rangeof 10−4S/cm, NaSICONtypeLi1+xAlxTi2−x(PO4)3

(LATP) compounds with x=0.3–0.4 benefitfrom oneof the best lithium conductivity amongst air and moisture stable compounds.13 In stronglyreducing environment (e.g.lithium metalelectrode),LATPmaterialsarecombinedwitha protec-tivelayer(e.g.LiPON)toavoidTi4+reduction.14Thesynthesis of LATP via solid-state chemistry is much documented.15,16 Thanks to the large choice in the raw material nature and synthesis parameters, solution chemistry favors the prepara-tionofpowderswithcontrolledcompositionandmorphology. In this work, we propose a method for the preparation of high-purity LATP powder by co-precipitation. By optimiza-tion of the sintering parameters, pellets with conductivity as high as 1.6×10−4S/cm at room temperature were pre-pared.

Fig.1.ProjectionofthestructureofLi1+xAlxTi2−x(PO4)3.TheMIandMIIintercalationsitescorrespondrespectivelytothemainoccupiedandexcess(x)Li+sites.

2. Experimental

Crystallographicphasesanalysis wascarried out byX-ray diffractionwithaBruckerD4Endeavordiffractometerusinga Cu Karadiationsource(Ka=0.15418nm).Elementary anal-yses were performed by SCA – CNRS. Inductively couple plasma (ICP) analyses were used for quantity determination oflithium,aluminium,titaniumandphosphorus. Thermogravi-metric analyses were performed on a Setaram TG-DTA 92 microbalance with 30mg of sample and alumina as a refer-ence.ScanningelectronmicroscopewithFieldEmissionGun (FEG-SEM)JEOLJSM6700Fwitha5kVacceleratingvoltage wasusedformorphologicalandmicrostructuralinvestigations. Agglomeratessizesweredeterminedbylasergranulometry.The densityofthepelletswasdeterminedviaArchimede’smethod on a KERN ARJ 220-4M balance. A density of 2.92g/cm3 is taken as the theoreticalvalue of bulk LATP(x=0.3). Sur-face roughness of the pellets was studied by optical surface profilometry with an optical interferometer ZygoNew View 100withMetroProTMsoftware.Impedancespectroscopywas used to determine the conductivity of the pellets. Gold elec-trodesweredepositedonthetwofacesofthepelletsbyagold sputter-coater. Impedance measurements at frequencies from 1Hzto107Hzwithvaryingtemperaturewereperformedusing aSolartron1260impedanceanalyserandafurnaceequipped withaEurothermtemperaturecontroller.Microhardnesstests wereperformedonapolishedsurfacewithaVickersindenter HMV-2000Shimadzu.

3. LATPpowderssynthesisandcharacterization

The powder synthesis is based on the co-precipitation of alkoxideandmetallicsaltsadaptedfromaprocedureofCretin etal.17Thewater/ethanolratio,hydrolysisratioandannealing temperaturewereoptimized.

3.1. LATPpowdersynthesis

Liacetatedihydrate(CH3CO2Li·2H2O,≥99%),ammonium

phosphatemonobasic(NH4H2PO4,99.999%pure),aluminium

tri-sec butoxide (Al(tri-sec-OBu), ≥97%) and titanium iso-propoxide (Ti(OiPr)4, ≥97%) were used as raw precursors

and purchased from Sigma–Aldrich. A solution v/v 80/20 water/ethanol with a concentration of 0.4mol/L of metallic saltsandstoichiometricproportionsof precursorsis obtained byadditionofawaterbasedsolutionwithappropriateamounts of lithium acetate and ammonium phosphate monobasic in theethanolsolutioncontainingtitaniumandaluminium alkox-ides.Thehydrolysisratio(h=[H2O]/([Ti(OiPr)4]+

[Al(tri-sec-OBu)])washigh(h=200),sothatpolycondensationoccurred andaprecipitatewasformed.Solutionwasstirredfor2hatroom temperature.Aftersolventevaporationat80◦Cfor48hawhite powder was obtained. The resulting powder wasannealed at temperaturefrom450◦Cto1100◦Cfor150minwitha100◦C/h temperaturerampinheatingand300◦C/hincooling.

3.2. Structuredescriptionandphaseanalysis

Li1+xAlxTi2−x(PO4)3 (LATP, x=0–0.5) belongs to the

NaSICON-type (Na1+xZr2SixP3−xO12, 0<x<3) structure. It

crystallizesintherhombohedralsystem(spacegroupR3¯c)with cellparametersa=8.48(2) ˚A,c=20.76(2) ˚A.16Thestructureis builtupwithTiO6octahedra andPO4 tetrahedrasharing

cor-nerstoforma3-DopenframeworkasschematizedinFig.1.Li cationsarelocatedintotwositeslabeledMIandMII.Themain

one(MI),whichisidenticaltothatofun-substitutedLTP

par-entstructure,correspondstoadistortedoctahedraloxygenated environment.ThesubstitutionofAl3+toTi4+leads,forcharge compensation,toextra lithium ionslocated inMII sites with

irregulareightcoordinatedsites.18,19

Byanalogy withNaSICON,themigrationpathway of Li+ ionsinLTPisspeculatedtooccurviabottlenecksalongapath

Fig.2.X-raydiffractiondiagramsmeasuredatroomtemperatureforLATP powderannealedattemperaturefrom450◦_Cto1100◦_C.

MI-MII-MI.19,20Itwasalsoshownthatthenarrowestplacesof

thebottleneckshapedchannelare largeenoughforLi+ ionto migratewithoutanydistortionoftheLTPnetwork.TiIV substitu-tionbyAlIIIcontributestoopenthebottlenecksizebyincreasing theoccupationoftheMIIsites.21Thiseffectaddedtotheincrease

inLi+concentrationfavorsLi+diffusioninLATPascompared to LTP. This results in conductivity at room temperature of 10−4S/cmfor LATPfor x=0.3–0.4versus2×10−6S/cmfor LTP.22

X-raydiffractionpatternsofsamplesannealedattemperature rangingfrom450◦Cto1100◦CarereportedinFig.2.The crys-tallizationofLATPoccursat700◦C.PureLATPisobtainedfor annealingtemperaturesfrom750◦Cto850◦C.Above850◦C, decomposition occurs leading to the formation of secondary phasesAlPO4,TiO2andLi4P2O7.Forfurtherexperiments,the

annealingtemperatureisthensetat850◦_C.

The composition of this powder was checked by induc-tivelycoupledplasma(ICP)elementaryanalyses.Acomposition of Li1.21±0.01Al0.32Ti1.72±0.02P3O12 was then obtained which

impliesanevaporationof0.2%w/woflithiumascomparedto thetheoreticalstoichiometryLi1+xAlxTi2−x(PO4)3withx=0.3.

Thiscorrespondstoamaximumof 2–4w/w% of impurities (AlPO4orLi4P2O7by-productconsidered)presentinthe

pow-der.

3.3. Powdermorphology

Scanning electron micrographs of LATP powder are pre-sentedinFig.3.Thenonannealedpowderismostlycomposed of particlesof 50–100nm insize withadditionallarger parti-clesof200–400nm.Theannealingat850◦Cleadstoparticles largerthan100nmwithalargedistributioninshapes.Theresults of the 850◦C annealed powder are confirmed by laser gran-ulometry, as presented in Fig. 3(b), with a calculated mean

Fig.3.ImagesofLATPpowderobtainedbyscanningelectronmicroscopyand distributioninsizeofparticlesobtainedbylasergranulometry:(a)powderbefore thermaltreatment,(b)powderannealedat850◦_C.

diameterof520nmand90%oftheagglomeratesintherange of400–1000nm.

3.4. Powderconductivity

Toourknowledge,whereasLATPstructurewithx=0.3has been demonstrated as convenient for lithium conduction, no mentionof thepercolationbehaviorof suchpowder hasbeen reportedintheliterature.Inordertodeterminewhethera perco-lationnetworkcouldbeformedatroomtemperaturebetweenthe LATPgrains,theconductivityofapressedpellethasbeen mea-sured by impedancespectroscopy.The chosen LATP powder (annealed850◦_{C,2h30)waspressedinbetweentwobrassdisc}

electrodes. The maximum compaction, whichcorresponds to theminimuminsamplethickness,isreachedatanapplied pres-sureof110MPa.Nyquistimpedanceplotsforseveralapplied pressurepresentthesamebehavior.Onesemicircleisobserved at highfrequencyandan inclinedtail isobservedatlow fre-quencies.Theresistancewasmeasuredbyfittingthesemi-circle

Fig.4.Impedancespectraofthepressedpowderdependingonthepressure appliedandcorrespondingconductivity.

with R1 the electrical resistance of the measurement system,

R2 the resistance of the electrolyte andCPE the

correspond-ing constant phase element.23 The Nyquist plots normalized insurfaceandthicknessaregiveninFig.4(a).No difference of resistivityis measured as compactionincreases and thick-nessdecreasessothatthemeasuredphenomenonarisesdirectly fromthe resistancebeingproportionaltothepelletthickness. Whateverthecompactionratio,alowconductivity(intherange of 3.1–3.6×10−7_{S/cm)isobtained. Thecontactbetweenthe}

grainsisnot efficientenoughtocreateapercolation network thatwouldenhancelithiumconductivity.

4. Ceramicsintering

Sincephysicalbindingisnotsufficientenoughforallowing lithiumconduction withinthepellets,sintering ofthepowder wasperformedtopromotechemicalbondingbetweenthegrains. LATP powders annealed at 850◦C were pressed into pellets betweentwosheetsofgraphitepapers(Papyex®)andsintered under vacuum using the SparkPlasma Sintering (SPS) tech-nique inaSPS 2080Sumitomo apparatus. Apulsesequence (12pulses,2idletime,3.3ms/cycle)withadjustedcurrentwas applied.Thediewasheateduptothesinteringtemperaturewitha 100◦C/minramp.Anisostaticpressureof100MPawasapplied beforeheating.Thebeginningofthesinteringoccursat750◦_C

withamaximumintheshrinkagerateat800–850◦_Casobserved

bydilatometricstudies.

4.1. Sinteredpelletsandprocessingparameters

Asummaryofthe varioussinteringconditionsisindicated inTable1.Temperature from850◦Cto1000◦Candduration

Table1

Sinteringconditionsforthepreparationofpellets.

Ceramicpellets Sinteringtemperature(◦_{C) Sinteringduration(min)}

A1-850-5 850 5 B1-900-5 900 5 C1-950-5 950 5 D1-1000-5 1000 5 A2-850-10 850 10 A3-850-15 850 15 A4-850-20 850 20

Fig.5.X-raydiffractiondiagramsatroomtemperatureforpelletssinteredby SPSunder100MPaappliedpressure,vacuumatmosphere.Sinteringduration: 5min.Sinteringtemperatures:850◦_C,900◦_C,950◦_C,1000◦_C.

from5min to20min wereused.Aftersintering, thesamples werepolishedinordertoremovethegraphitepaper.

4.2. Crystallographicphasesanalysisofthepellets

XRD patterns for the sintered pellets versus the sin-tering temperature and sintering duration are presented in Figs. 5 and6.All the majorpeaks areindexed inthecrystal systemof the LATPpowder. Nochange incellparametersis observed(a=8.48±0.02 ˚Aandc=20.76±0.03 ˚A).Side prod-uctsAlPO4appearinallthepellets,exceptforthepelletsintered

atthelowesttemperatureforshorttime(850◦_{C,5min)inwhich}

onlyasmallamountofLi4P2O7ispresent.Li4P2O7couldbe

presentinallthesamples,howeverinsomecasesonlythemajor peak(2θ=20.5◦)thatisacommonpeakwithAlPO4phasesis

visible.Inaddition,aLi2C2phase isdetectedfor pellets

sin-teredfor morethan 5min.A sharpeningof the peakson the XRDpatternsisobservedforsinteringtemperaturefrom850◦C to950◦C, so that the primary particles increasein size with increasingsinteringtemperature.

Fig.6.X-raydiffractiondiagramsatroomtemperatureforpelletssinteredby SPSunder100MPaappliedpressure,vacuumatmosphere.Sintering tempera-ture:850◦_{C.Sinteringduration:5,10,15,20min.}

Fig.7.DensitymeasuredbyArchimede’smethodandconductivitymeasured byelectrochemicalimpedancespectroscopy.PelletsA1-850-5,B1-900-5, C1-950-5,D1-1000-5.Durationofsintering:5min.

5. Resultsanddiscussions

5.1. Densityandconductivity

Figs.7 and8show theevolutionof thedensity(measured atroomtemperature)oftheSPS-preparedpelletsversus sinter-ingtemperatureandduration.Whateverthesinteringconditions, pelletswithdensityrangingfrom2.82g/cm3to2.84g/cm3(i.e. 97%ofthetheoreticaldensity)wereobtained.Itshouldbenoted thatsinceallthesampleshavethesamedensity,thedirect com-parisonoftheconductivityoftheLATPpelletsisaccurate.

Roomtemperatureconductivitiesofthepelletsexhibita sim-ilarbehaviortotheonedescribedforLATPpowderinSection 3.4. Only one semi-circle, representative of the total resis-tanceoftheelectrolyte,isobserved.Valuesfrom1×10−4_S/cm

to 1.6×10−4_{S/cm are obtained for pellets sintered between}

850◦Cand950◦C.Thesevalues,similartothosereportedin the literature,1,3,24 indicate in addition that the conductivity increaseswithincreasingtemperatures.However,while sinter-ingattemperature above1000◦Cleadstoalmostfullydense pellets,theirconductivitiesdropdownto1×10−5S/cmwhich fullycontradicttheexpectedevolution.

Theeffectofsinteringdurationhasalsobeenstudiedand plot-tedinFig.8forasinteringtemperatureof850◦C.Thedensities

Fig.8.DensitymeasuredbyArchimede’smethodandconductivitymeasured byelectrochemicalimpedancespectroscopy.PelletsA1-850-5,A2-850-10, A3-850-15,A4-850-20.Temperatureofsintering:850◦_C.

andconductivitiesremainthesameindependentoftheduration ofsinteringfrom5minto20min.Then,thesecondaryphase for-mationdetectedbyXRD(cf.Section4.2)onthesamplesintered at10–20minisnotdetrimentaltodensityandconductivity.

5.2. Microstructuralanalysisofthepellets

Inordertodeterminewhetherthedifferenceinconductivity couldberelatedtoachangeinmicrostructure,SEMmicrographs analysisofthecross-sectionofthesampleshasbeenperformed. Themicrostructureofthepelletsstronglydependsonthe sinter-ingtemperature.Whereasthepelletsinteredat1000◦Cpresenta uniformmicrostructurewithlargegrains(6–30mmindiameter),

forpelletssinteredat850◦Candupto950◦C,aheterogeneous microstructureappears.AspresentedinFig.9,partswithonly small grainsof200–300nmindiameterwerepresentcloseto zoneswithlargegrainsoftensofmicrometres.Astemperature increases,thesurfaceareacoveredbylargegrainsbecomes pre-dominant. On thecontrary,the durationof sinteringdoes not mainlyinfluence themicrostructure. Thesame structurewith largeandsmallgrainsisobtainedforpelletssinteredat850◦C for 5, 10, 15 and20min (micrographsnot shown here). For LATPmaterials(x=0.3–0.4),bulkconductivityisreportedto be higherthan grainboundariesconductivity.17 The presence oflargergrainsisthenexpectedtobebeneficialto conductiv-itywhichfully agreeswiththeobservedevolutionfor pellets sinteredbetween850◦Cand950◦C.However,thedropdown inconductivityobservedat1000◦C,whereasonlyapopulation oflargegrainsisobserved,contradictsthisexpectedevolution. TheaggregationofanonLi+conductivesecondaryphaseatthe grainboundariescouldexplainthisphenomenon.Howeverno significantdifferenceingrainboundariescompositionshasbeen observedbyEDXanalysesonconductiveandnonconductive samples.Acarefulobservationofpelletssinteredabove1000◦C indicatesthatthecohesionbetweenlargegrainsisweakasshown inFig.10.Thenthelackofcohesionbetweenthegrainscanbe suspectedtobeattheoriginofthedropinconductivity.

5.3. Mechanicalproperties

Inordertostudythemechanicalbehaviorofthezoneswith small grainsandthe zones withlarge grains, Vickers micro-hardnessmeasurementsundervariousnormalloads(2N,2.9N, 4.9Nand9.8N)wereperformed.OpticalmicroscopyandSEM analysesoftheindentedsurfaceshavebeencarriedout.

AVickershardnessdecreasefrom700–800HVforthezones withsmallgrainstoonly300–400HVinthezonesconsisting oflargegrainsisobserved.Thedecreaseofplasticyieldstress withincreasinggrainsizehasbeendescribedbytheHall–Petch law:σy=σ∞+kd−1/2,withdthegrainsize,kaconstant,σ∞

yieldstressof thesinglecrystal,σyplasticyieldstressthatis

consideredasproportionaltothehardnessforsuchceramics.25 Thislawisrelevantforgrainsfromseveraltensof nanome-ters to several tens of microns such as the size of grainsin the materials studied in thiswork. Moreover,fractures surg-ingfromthesidesoftheVickersindentationareonlyvisiblein

Fig.9.SEMimagesofthefracturesurfaceofpelletssinteredat(a)850◦_C,(b)900◦_Cand(c)950◦_C.

Fig.11.OpticalmicrographsofVickershardnessindentationmarksinsinteredpelletsforseveralappliedloads.(a)Zoneswithsmallgrainsand(b)zoneswithlarge grains.

thezoneswithsmallgrainsduetothehigherhardnessofthis zone.

Optical micrographsafter indentation on zones withlarge grains andzones with small grainsare shownin Fig. 11. A

deformationaroundtheindentationisvisibleinbothcaseseven forthelowestload(2N).Thezoneswithlargegrainsaremore subject to largedeformations at the edge of the indentation. Underaloadfrom2Nto9.8N,themarkleftinthiszoneis1.5

Fig.12.MicrographsobtainedbySEMonsamplesurfacesafterindentationat9.8Nload(a)inthezoneswithsmallgrains(b)inthezoneswithlargegrains.

largerindiagonalthaninthezonewithsmallgrains.Moreover, broken partsoflargegrainsareobservedinFig.12,bySEM micrographicanalysisattheedgeoftheindentation.

Thistendencyof largegrainstobetaken offthe matrixis alsosupportedbysurfaceroughnessmeasurementsafter polish-ingofthesamples.Whereaspartswithsmallgrainsaresmooth with apeak-to-valley height of 0.4mm, aresidual roughness

afterpolishingismeasuredonthezoneswithlargegrainswitha peak-to-valleyvalueof10mm(whichistheaveragesizeoflarge

grains).Moreover,theskewnessoftheroughnessprofileis neg-ative withvaluesfrom−1to−3.Theselargenegativevalues aretypicalofasymmetricalroughnessprofileswith preponder-anceofholesascomparedtohills(thebulkofthematerialbeing abovethemeanline).

Hence,lackofcohesionbetweenthelargegrainscouldbe thelimitingparametertolithiummigrationandtheexplanation of the lowertotalionicconductivity ofthe pelletssintered at 1000◦C.Fortheotherpellets,thepresenceofzoneswithlarge grainsisnotdetrimentaltotheglobalconductivity.Wecanthen inferthatinthesecasesthezoneswithsmallgrainspromotethe mechanicalcohesionbetweenthelargeparticles.

6. Conclusion

Powders with high purity have been prepared by co-precipitationmethod.LATPpelletssintered bySpark Plasma Sinteringwithconductivitiesamongstthehighestpublishedare obtainedwith1.6×10−4S/cmatroomtemperature.Veryhigh densitiesarealsoobtained(97% ofthetheoreticaldensity)so thatthesemembranescanbereadilyusedasseparatorfor aque-ouslithium-airbatteries.Moreover,thesedenseandconductive pelletsweresynthesizedattemperaturesaslowas850◦Candfor treatmentdurationof5minwhereasbyclassicalsinteringat tem-peratureof1000◦Cfor1hwouldbenecessary.Anewfeature

thatcouldbebroughttotheknowledgeoftheresearchfieldisthe correlationofthedropinconductivitywiththelossofcohesion ofgrainsasSPSsinteringtemperaturereaches1000◦C. Investi-gationsaregoingontodeterminetheoriginofthisphenomenon, wherethepresenceofamorphousphasesatthegrainboundaries isbeinghypothesized.

Acknowledgements

ThisworkwassupportedbytheFrenchNationalResearch Agency(projectANRLiO2).TheSPSsinteringwasperformed

attheSPSFrenchNationalPlatform(PNF2/CNRS)atToulouse. WeespeciallythankGeoffroyChevallierandClaudeEstournes fortheirtechnicalsupport.

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