eth zurich

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Doctoral Thesis

From amorphous to crystalline via vitreous cathode materials forrechargeable lithium ion-batteries

Author(s): Wchter, Florian L.

Publication Date: 2012

Permanent Link: https://doi.org/10.3929/ethz-a-007620352

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DISS.ETHNo.20863

FromAmorphoustoCrystallineviaVitreousCathodeMaterials

forRechargeableLithiumIonBatteries

Adissertationsubmittedtothe

ETHZURICH

forthedegreeof

DOCTOROFSCIENCES

presentedby

FlorianLeonardtWchter

Dipl.Chem.,RheinischeFriedrichWilhelmUniversittBonn

bornAugust18,1983

inDsseldorf,Germany

CitizenofGermany

Acceptedontherecommendationof

Prof.Dr.ReinhardNesper,examiner

Prof.Dr.HansjrgGrtzmacher,coexaminer

2012

ii

ChemikerdrfennichtnurneueMaterialherstellenundderenStrukturenaufklren,sondernsiemssenauchlernen,derenEigenschaftenquantitativzubestimmen.

RoaldHoffmann

iii

Acknowledgements

Iwouldliketoexpressmysinceregratitudeto:

Prof.ReinhardNesper,forgivingmetheopportunitytograduateunderhissupervision.I

haveappreciatedthefreedomandtheverychallengingtopics.Thedeepholes,webump

in,wereverygoodexperiencesforbecomingabetterresearcher.Furthermore,hehasa

greatpersonalityandIlearnedmanythings,whicharenotchemistryrelated.Thankyou,

Reinhard.

Prof.HansjrgGrtzmacher,fortakingonthetaskofcoexaminer.

Dr.FrankKrumeich,fordoingallSEMandTEManalysis,Iusedinthepresentedwork.

Dr.MichaelWrle,forhishelprelatedtoallkindsofXRDexperiments.

Dr.MihaiViciu, for the fruitfuldiscussionsabout chemistryandnot chemistry related

topicsandforteachingmeEnglish.

Dr.SerhiyBudnyk,forwelcomingmefriendlyandteachingmeinallsynthesismethods

ofsolidstatechemistryandintheEastEuropeanlabvelocity.

Christian Mensing, for the technical assistance and teaching me in property

characterizationmethods.

AdamSlabonandMartinKotyrba,guysitisnicetodiscussandsometimesfightagainst

you.Ithinkweareagreatchemicalteam.Highspeed,me,realscientist.

DominikusHeift forproviding theNaOCP and the great time.A littlebitof Klscher

Klngelshouldbeeverywhere.

Allotherguys fromtheNespergroupforthegreattime:Dr.XiaoJunWang,Dr.Dipan

Kundu, Dr. Liliana Viciu, Dr. YoannMettan, Philipp Reibisch, Frederic Hilty,Matthias

Herrmann, Dr. Barbara Hellermann,Michele Furlotti, Dr. Eduardo Cuervo Reyes, Dr.

RiccardaCaputoandSemihAyfon.

Lastly Iwant to thankBelenosCleanPowerAG, for financing this interesting research

project.

v

Abstract

One of the biggest problems of the modern society is the climate change. Electro

mobility isconsideredtobethekeytechnologytoreducetheemissionofgreenhouse

gases.LithiumIonBatteriesseemtobethemostpromisingenergystoragesystem for

electric vehicle applications. In this work, I am going to present various new and

modified cathode materials, as well as a synthesis method, which allows chemical

lithiationandsurfacecoatingsoftransitionmetalcompoundsinhighoxidationstatesat

thesametime.

Inthefirstpartofthedoctoralthesis,synthesisandelectrochemicalcharacterizationof

variousvitreousandcrystallizedmaterialsoftheLi2O*V2O5*P2O5systemarepresented.

PureV2O5*P2O5glassesexhibitlargeirreversiblespecificcharge,correspondingtothree

LiperPatom, in the initial insertion.These threeLiequivalentspoint to formationof

orthophosphategroups inside theglass.The reaction irreversibly reduces theaverage

oxidation state of the glass, and therefore the cell voltage is decreased.When using

Li3PO4asnetworkformerandmodifier,noirreversibleprocessesoccurwithintheinitial

cycle.Fullspecificchargeandhigh redoxpotentialarepreserved.Thecyclabilityofall

synthesizedmaterialsisbettercomparedtopureV2O5electrodes.

The second part of the thesis dealswith a synthesismethodwhich allows chemical

lithiation and coatings of oxidants at the same time. Especially carbon coatings can

stronglyimprovetheelectrochemicalpropertiesofcathodematerials.WhenusingLi2C2

ascarbonandlithiumsource,thecarbideanionC22isoxidizedtocarbononthesurface

of the oxidant, while lithium inserts the reduced oxidant. This new carbon coating

method isshownonthebasisofcrystallineandamorphousLixV2O5aswellasLixMoO3,

H2V3O8,LiFePO4,Li3PO4*V2O5glasses.

ThisconceptisextendedtoNaOCPascoatingandinsertionreagent.Duringthereaction

of NaOCP and V2O5 and MoO3 respectively, Na+ions insert into the oxidant and a

polymeric shell is formedaround the reducedandmetalintercalated transitionmetal

oxidecore.

vi

In the third part permanganate based cathode materials are presented. The

decompositionofLiMnO4wasstudiedunderthesynthesisconditionofLi3MnO4(220C

under oxygen flow). The product is a mixture of nano crystalline Li2MnO3 and an

amorphousphase.Electrochemicalbehaviorandstoichiometrypointtotheformationof

anamorphousMnO2material.

Themainproblemofusingpermanganatebasedcathodematerialsisthehighsolubility

of LiMnO4 in common electrolytes. Therefore,mixed compounds ofMnO43 and the

isolobal PO43were targeted inwhich the phosphate groupswould act as a kind of

insoluble network former. The synthesized new mixed materials

Mg3Li12(MnO4)3(MnO3)3(PO4)2 and Li9(MnO4)3(PO4) are amorphous andmost probably

bivalent, i.e.containingboth,Mn4+andMn6+.Thematerialswereclassifiedaselectric

isolators due to the measured conductivity of approximately 1023 S/cm2. This low

electricconductivityleadstoapoorelectrochemicalactivityofQ

vii

Kurzfassung

DiezuknftigeEnergieversorgungundderKlimawandelsindmitdiegrsstenProbleme

der heutigen globalen Gesellschaft. Elektromobilitt gilt als Schlsseltechnologie fr

einen besseren Umgang mit den begrenzten fossilen Rohstoffen und somit fr

verringerteTreibhausgasemissionen.LithiumIonenBatterienscheinendiebesteLsung

fr die Energiespeicherung in Fahrzeugen, unter Umstnden auch in stationren

Anwendungen, zu sein. In dieser Dissertation werden verschiedene neuartige

Kathodenmaterialien und eine Methode zur gleichzeitigen chemischen Beschichtung

undLithiierungvonKathodenmaterialenvorgestellt.

Im ersten Teil dieser Arbeit werden verschiedene Glser und Glaskeramiken des

Li2O*V2O5*P2O5 Systems auf ihre elektrochemischen Eigenschaften hin untersucht.

Dabeizeigtesich,dassreineV2O5*P2O5Glser3LithiumquivalenteproPhosphoratom

irreversibel in das Glasnetzwerk einbauen knnen. Dies deutet auf die Bildung von

Orthophosphatgruppen imGlashin.Der irreversibleLithiumeinbau fhrtaufdereinen

SeitezuirreversiblerspezifischerLadungundaufderanderenSeitezueinerAbsenkung

der durchschnittlichen Zellspannung.DieVerwendung von Li3PO4 alsNetzwerkbildner

undNetzwerkwandlerverhindertdiesen irreversiblenVorgang.Danachverndern sich

weder Spannung noch spezifische Ladung nach der ersten Entladung. Die

Zyklenstabilitten der untersuchten Glser sind deutlich erhht im Vergleich z.B. zu

V2O5Elektroden.

Im zweiten Teil wird eine Methode prsentiert, die chemische Lithiierung und

gleichzeitigeKohlenstoffbeschichtungvonKathodenmaterialmitLithiumcarbiderlaubt.

DieCarbidanionenC22werdenaufderOberflchedesOxidationsmittelszuKohlenstoff

oxidiert, whrend die Lithiumkationen eingelagert werden. Auf dieseWeise knnen

bergangsmetalleinhohenOxidationsstufenmitKohlenstoffbeschichtetwerden,wobei

die Oxidationsstufe fr die weitere Batterieanwendung nicht verloren geht. Dieses

Verfahren wurde anhand folgender kristalliner und nicht kristalliner Verbindungen

erfolgreichdemonstriert:LixV2O5,LixMoO3,H2V3O8,Li3PO4*V2O5Glser.

Das Konzept konnte aufdieBeschichtungsund EinlagerungssquelleNaOCP erweitert

werden.Eswirdgezeigt,dassNa+sowohl inV2O5alsauch inMoO3eingelagertwerden

viii

kann und sich gleichzeitig eine polymere Beschichtung auf den reduzierten Partikeln

ausbildet.

Im letzten Teil dieser Arbeit werden Kathodenmaterialen untersucht, die auf dem

Permanganatanionaufbauen.ZuerstwurdedasZersetzungsproduktvon LiMnO4unter

den Synthesebedingungen fr Li3MnO4 (220C unter Sauerstofffluss) untersucht. Es

zeigtesich,dasssichalseinzigeskristallinesProduktLi2MnO3bildet.DieStchiometrie

der Zersetzungsreaktion und die Analyse des elektrochemischen Verhaltens deuten

daraufhin,dasseineamorpheMnO2VerbindungalszweiteHauptkomponenteentsteht.

Das Hauptproblem bei der Verwendung von permanganatbasierenden Elektroden

materialen, istdie sehrgute Lslichkeit von LiMnO4 inden verwendetenElektrolyten.

Aus diesem Grund solltenMischverbindungen hergestelltwerden, die die isolobalen

BaueinheitenMnO43undPO43enthalten.Dieneu synthetisiertenMischverbindungen

Mg3Li12(MnO4)3(MnO3)3(PO4)2undLi9(MnO4)3(PO4)sindamorphundsehrwahrscheinlich

bivalent,d.h.Mn4+ liegtnebenMn6+vor.DieelektrischeLeitfhigkeitdieserMaterialen

betrgt aber nur ca. 1023 S/cm. Vermutlich deshalb sind die erzielten spezifischen

LadungenmitQ

ix

Abbreviations

LIBs LithiumIonBatteries

PV Photovoltaic

XRD XrayDiffraction

DTA DifferenceThermoAnalyses

SEM ScanningElectronMicroscopy

TEM TransitionElectronMicroscopy

CV CyclicVoltammetry

EAM ElectroActiveMaterial

SEI Solidelectrolyteinterface

NiMH Nickelmetalhydride

HOMO HighestOccupiedMoleculeOrbital

LUMO LowestUnoccupiedMoleculeOrbital

PVC PolyvinylideneChloride

PP Polypropylene

OCV OpenCircuitVoltage

Si/C Siliconcarboncomposite

CVD Chemicalvapordeposition

NMC Li(Ni1xzMnxCoz)O2(with:1xz=1)

LFS Ligandfieldsplitting

xi

1 Introduction...........................................................................................................................1

1.1 ModernEnergyIssues.........................................................................................1

1.2 RequirementsforLithiumIonBatteriesinEV.....................................................3

1.3 ElectrochemicalStorage.....................................................................................4

1.4 RelevantDefinitionsandConcepts.....................................................................6

1.5 ElectrochemicalMeasurements.........................................................................9

1.6 TheLithiumIonBattery....................................................................................10

1.6.1 Carboncoating.....................................................................................................13

1.6.2 AnodeMaterials...................................................................................................15

1.6.3 CathodeMaterials................................................................................................17

1.7 InfluencesoftheLigandFieldSplittingonCathodicMaterials.........................22

1.8 Glasses..............................................................................................................24

1.8.1 Introduction.........................................................................................................24

1.8.2 Structuralcomposition.........................................................................................25

1.8.3 Properties.............................................................................................................26

1.9 GlassCeramics..................................................................................................27

2 VitreousundCrystalizedMaterialsintheLi2OV2O5P2O5System........................................29

2.1 V2O5*P2O5System.............................................................................................29

2.1.1 ExperimentalV2O5*P2O5glasses...........................................................................32

2.1.2 StructuralCharacterizationofV2O5*P2O5glasses.................................................33

2.1.3 ElectrochemicalCharacterizationofV2O5*P2O5glasses........................................36

2.2 V2O5P2O5GlassCeramics.................................................................................42

2.2.1 ElectrochemicalCharacterizationoftheGlassCeramic........................................43

2.3 GlassformationintheLi2OV2O5P2O5System..................................................45

2.3.1 ExperimentalLi2OV2O5P2O5System....................................................................45

2.3.2 StructuralCharacterizationofMaterialscontainingLiVO3....................................45

2.3.3 ElectrochemicalCharacterizationofMaterialscontainingLiVO3..........................47

2.3.4 StructuralCharacterizationofGlassescontainingLiPO3.......................................49

2.3.5 ElectrochemicalCharacterizationofGlassescontainingLiPO3..............................52

2.4 ExperimentalLi3PO4V2O5System.....................................................................57

2.4.1 StructuralCharacterizationofV2O5*Li3PO4...........................................................57

2.4.2 ElectrochemicalCharacterizationofV2O5*Li3PO4.................................................59

2.5 SubstitutionofV2O5byMxOy(CeO2andCo2O3)................................................63

xii

2.5.1 ExperimentalV2O5Li3PO4MxOySystem................................................................63

2.5.2 StructuralCharacterizationofV2O5Li3PO4MxOy..................................................64

2.5.3 Electrochemicalcharacterization..........................................................................67

2.6 ConclusionsandOutlook...................................................................................70

3 RedoxLi2C2Coatings.............................................................................................................71

3.1 Introduction.......................................................................................................71

3.2 SynthesisofLi2C2...............................................................................................73

3.3 CarboncoatedLixV2O5.......................................................................................74

3.3.1 SynthesisofLixV2O5...............................................................................................75

3.3.2 StructuralCharacterizationoftheHeatCoatedSamples......................................76

3.3.3 StructuralCharacterizationoftheTribochemicalProduct....................................83

3.3.4 ElectrochemicalCharacterizationofLixV2O5(Pmmn).........................................85

3.3.5 ElectrochemicalCharacterizationofLi0.3V2O5andLiV2O5................................87

3.3.6 ElectrochemicalCharacterizationofLiV2O5PreparedbytheTribochemicalMethod 91

3.4 CarboncoatedLiFePO4......................................................................................93

3.4.1 Experimental.........................................................................................................93

3.4.2 CharacterizationofCarbonCoatedLiFePO4..........................................................93

3.4.3 ElectrochemicalCharacterizationofcarboncoatedLiFePO4................................97

3.5 CarbonCoatedLixMoO3.....................................................................................98

3.5.1 Experimental.........................................................................................................98

3.5.2 StructuralCharacterizationofLixMoO3.................................................................98

3.5.3 ElectrochemicalCharacterizationofLixMoO3......................................................101

3.6 CarbonCoatingofH2V3O8................................................................................102

3.6.1 Experimental.......................................................................................................102

3.6.2 StructuralCharacterizationofcarboncoatedLiH2V3O8......................................102

3.6.3 ElectrochemicalCharacterizationofcarboncoatedLiH2V3O8.............................105

3.7 CarbonCoatingofV2O5*P2O5andV2O5*Li3PO4Glasses...................................106

3.7.1 Experimental.......................................................................................................106

3.7.2 StructuralCharacterizationoftheglasscarboncomposites...............................106

3.7.3 ElectrochemicalCharacterizationoftheGlassCarboncomposites....................109

3.8 ConclusionandOutlook...................................................................................111

4 RedoxCoatingsNaOCP.......................................................................................................113

4.1 Introduction.....................................................................................................113

xiii

4.2 Experimental...................................................................................................113

4.3 ResultsandDiscussionV2O5............................................................................114

4.4 ResultsandDiscussionMoO3..........................................................................117

4.5 ConclusionandOutlook..................................................................................119

5 PermanganateBasedMaterialsasCathodeMaterialsinLIBs............................................121

5.1 Introduction....................................................................................................121

5.2 Experimental...................................................................................................123

5.3 CharacterizationoftheDecompositionProductofLiMnO4............................128

5.4 Li3MnO4andRelatedCompounds...................................................................131

5.5 CrystalStructureDeterminationof[Ca(H2O)4(MnO4)4/2]1.............................139

5.5.1 Experimental......................................................................................................139

5.5.2 Discussionofthestructure.................................................................................139

5.6 Conclusions.....................................................................................................142

6 SummaryandOutlook.......................................................................................................143

7 Literature...........................................................................................................................145

8 Appendix............................................................................................................................149

8.1 ExperimentalInfrastructure............................................................................149

8.2 RietveldrefinementofLi0.3V2O5...................................................................151

8.3 RietveldrefinementofLi0.5V2O5...................................................................153

8.4 CrystallographicDataofCa(MnO4)2*4H2O.....................................................155

9 Curriculum Vitae..............................................................................................................159

1Introduction

1

1 Introduction

1.1 ModernEnergyIssuesOneofthemostdebatedquestionsinthemodernworldis:Howwillalltheenergywe

areconsumingbesustainablyproducedandstored?Attemptsfor integratingdifferent

renewableenergysources likephotovoltaic (PV),wind,biogas,water,andgeothermal

energybroughtnewchallengestotheelectricgridmanagement.The incorporationof

inconstantorsporadicenergyproductionnecessarilyasksfornewshortand longterm

energystoragemeans.Furthermore,theCO2emissionreductionispresumedtobethe

key towards decelerating the climate change. One of themain producers of CO2 is

transportation in using extensively fossil fuels. That iswhy, strong efforts are being

undertakentoreplacethecombustionenginevehicles(CEV)byfullelectricvehicles(EV)

orbyHybridvehicles(HV).Almostallcarcompanieshavebroughthybridvehiclestothe

market sinceToyota launched thePrius in1997 [1]. Inhybrid vehicles, relative small

batteries are employed (300kminastandardfamilycarand2.thenecessarylifetime.

AnotherpotentialapplicationofhighenergyLIBs is the supportof theelectricpower

grid.Wind turbinesandphotovoltaic (PV)produceenergy inconstantly.Thus thegrid

management has to equalize the up and downturns by switch on/off conventional

powersources.Theseconventionalpowersourcesneedtimetorunup.Butthevoltage

1.4RelevantDefinitionsandConcepts

2

and the frequencyof thegridhave tobe stable in the runupphase.For these short

timescales (t

1Introduction

3

1.2 RequirementsforLithiumIonBatteriesinEVToreachthegoalofadriverfriendlycar,thebatterypackofanEVhastostoreatleast

50kWh(better100kWh).Thiscorrespondstoabatterypackofaround416kg(832kg),

calculated for the specific energy of the at present, most reliable system

LiFePO4/Graphite(120Wh/kg)[4].Reliabilityisaverychallengingpoint,becauseideally

the life timeof thebatterypackshouldbe intherangeofthe life timeof thecar for

severalreasonsbutmostly intermsofcostrequirements. In2010theaveragecarage

was8.5yearsbyanyearlyaverageof18693kmtraveledforGermany[5].Therefore,an

EVbatterypackhastobeabletopowerafamilycarover158890km,correspondingto

530 fulldischarge/charge cycles (300 km/fulldischarge).AnEVwillbe chargedmore

oftenthanthese530times,becauseofthesmallermeantraveldistances.However,itis

yet difficult to judge between lower number of deep discharge and multiple half

charge/discharges for life time expectation. Thus thebenchmarkof 2000 cycleswith

capacity retentionof80% isbasedon200 fullbattery charges (corresponding to200

workingdays)over10years.

Another issuetotarget isthepriceofanEVbattery.ThecostgoalofSAFT,oneofthe

biggestbatteryproducers inEurope, isUS$200/kWh [6],which results inUS$10000

perbatterypack(50kWh).Atpresentcathodematerialisinvolvedby40%concerning

weightand45%concerningthecoststothewholesystem.Accordingtothat,theupper

pricethresholdofafabricatedcathodematerial isaboutUS$36perkg.Areductionof

theweightofthecathode(40%)wouldclearlyreducetheoverallweightofthebattery.

The present target for cathodematerials is a specific energy of at least 800Wh/kg

combinedwith theabovementioned stability.Such cathodematerials can reduce the

overallweightbyapproximately12%,correspondingto60kgfora50kWhbattery.


4

1.3 ElectrochemicalStorageThebasis foranelectrochemicalcell isa redox reaction.Contrary toa redox reaction

done inabeaker,theoxidationandthereductionprocessesarespatiallyseparated in

anelectrochemicalcell.Thesesocalledhalfcellsareconnectedbyan ionicconductor

(electrolyte)andanelectricconductorequippedwithacostumerorapotentiostat.The

electronand the charge carrier flow aredisjoined. The cathode and anode assembly

have to be electric and ionic conductive to achieve a recombination of the charge

carriers and the electrons in the electrodes. One of the oldest and best known

electrochemical cell is the Daniel Element. It is based on the observation of copper

plating on a zinc rodwhen it is inserted into a copper sulfate solution. In this very

simplifieddescription theelectrolyteandall interfacesareneglectedand thehalfcell

reactionsare:

: 2 11: 2 12

Asmentionedabove,thetwohalfcellshavetobeseparated.Thesetupconsistsofan

anodichalfcell (Zn/Zn2+)andacathodichalfcell (Cu2+/Cu). Interfacesarerepresented

byaverticalbarintheelectrochemicalwriting(13).

| | | | 13

ThespecifichalfcellpotentialE0canbecalculatedfromthethermodynamicdataofthe

halfcell reaction. In thermodynamic equilibrium the halfcell potential is given by

equation14.

14

with: G0:StandardGibbsfreeenergy;z:numberofelectrons

F:Faradayconstant;E0:Standardelectrodepotential.

1Introduction

5

Halfcellpotentials cannotbemeasured in an absolute sense. Therefore,a reference

system isneeded.The standardhydrogenelectrode (SHE) isused for thispurpose:A

platinatedplatinumelectrode isflushedwithhydrogen ina1mol/lHClwatersolution

(T=25C,p=1bar,allactivespeciesatunityactivity).

FornonstandardconditionstheNernstequationisusedtodeterminethepotentialof

thehalfcellatequilibrium:

15

with: R:gasconstant;T:absolutetemperature; i:stoichiometriccoefficient;ai:activity

The cell voltageofanelectrochemical cell is calculated from theelectrodepotentials

(reductionpotentials)ofthehalfreactions.TheoveralltheoreticalcellvoltageE0ofa

galvaniccell isobtainedbysubtracting thehalfcellpotentialof theoxidation (anode)

fromthereductionhalfcellpotential(cathode).

16


6

1.4 RelevantDefinitionsandConcepts

In this chapter some general definitions of terms used in electrochemistry are

summarized.

ThecellvoltagecanbetheoreticallycalculatedfromtheGibbsfreeenergyofthe

correspondingchemicalreaction (14).Thecorrespondingthermodynamicdata

ofcomplexsolidsareverydifficulttodeterminebecausetheydependonmany

parameters,ascoordinationsphere,oxidationstateandchemicalenvironment.

Thuscalculatedcellvoltagesarebasedonmanyapproximations.

Theoverpotentialiscalculatedbydividingthedifferencesofenergiesrequired

forchargingandgainedduringdischarging, respectively,bythespecificcharge

ofthelithiuminsertion.

17

The requiredchargingpotentialhas tobehigher than thedischargepotential.

Gabersceketal.explainedthethermodynamicoriginofthishysteresis [7].The

intrinsicoverpotentialcanbeduetodifferentfactors:

o Theelectroactivematerial(EAM)hasahighelectricresistivity.o TheEAMhasahighionicresistivity.o A strongSEIformation (solidelectrolyte interface) increases theoverall

resistivity.

o A high activation barrier has to be overcome (for instance: covalentbondshavetobebrokenup).

Thecurrentdensity j(t) isdefinedby theamountofcurrent flowing througha

givensurface.

/ 18

1Introduction

7

ThecapacityQistheamountofchargeobtainablebyaparticularcell.

19

The theoretical specific chargeqth, respectively, the theoretical chargedensity

QV,th,describestheamountofchargepermassunitm,respectively,pervolume

unitV, of the electro activematerial. Each can be calculated by applying the

stoichiometricreaction.

110

, 111

The practical charge q or the practical charge density QV is the total charge

obtained fromapractical cell,dividedby the totalmassor thevolumeof the

complete system leading to specific gravimetric or volumetric charge,

respectively.A system in thissensecanbeacathode (EAM+binder+carbon

additives)orananode (EAM+binder+carbonadditives)or thecompletecell

includingelectrolyte,packaging,etc.

1 / 112

1 113

Inanidealbattery,thespecificchargeforchargingisequaltothespecificcharge

ofdischargingthecell.Inapracticalcell,oftenirreversibleprocessestakeplace.

A wellknown and much investigated example is the SEIformation on the

graphite anode through electrolyte decomposition under highly reducing

conditions. The resulting specific charge differences are called irreversible

specificchargelosses.Theycanbedefinedinabsolutevaluesorinpercentage.

100 % 114

Ifq(discharge)>q(charge)irreversibleprocessesproceedontheanode.

Ifq(charge)>q(discharge)irreversibleprocessesproceedonthecathode.


8

The theoretical specificenergyth,or the theoreticalenergydensityWV,thare

calculatedbytheproductofaverageinsertion/extractionpotentialEandtheqth.

115, 116

Thepractical specific energyor thepractical energydensityWV is the total

electricenergyobtainablefromasystemdividedbythemassorthevolume.

1 117

1 118

Theenergyefficiency(Faradayefficiency)ofabatteryisgivenbythequotient

ofgainedenergyduringdischargeandspentenergyduringthecharge.

1 % 119

The specificpowerp is the capability todeliverpowerpermass. The specific

poweristheproductofdischargecurrentandvoltageofthecell.Thevoltageof

acellisdependingonthecharginglevelandonthedischargecurrent.Thusthe

specific power decreases during the discharge by a constant current which

movesthesystemoffthermodynamicequilibrium.

TheCrate isameasure for thechargingordischarging time, respectively.The

meaning of charged/discharged at 1C means that the system is charged or

dischargedwithinanhour,respectively.Theappliedcurrentiscalculatedbythe

productofqthandthemassoftheactivematerialmdividedbythetime(1h).For

instance,169Aarerequiredtocharge1kgLiFePO4inonehour:

1kg LiFePO 169Ah/kg / 1h 169 A 120

1Introduction

9

1.5 ElectrochemicalMeasurementsThe electrochemical measurements were carried out in homemade test cells. The

technicaldrawingofthetestcellsisdisplayedinFig.12.Thedescriptionofallpartsand

the used materials are given Table 11. All parts that come in contact with the

electrolytearemadefromtitaniumorpolypropylene(PP).Theworkingelectrodeswere

preparedonthecurrentcollector3whiletheanode,madeoflithiumfoil,issupported

bytheLisupport8.Twoseparatorswereplacedbetweentheelectrodes:

aPP separator (Celagard)directlyon the cathode,whichpreventsan internal

shortcutbylithiumdendrites.

asilicaseparatorwhichsucksuptheelectrolyteandpreventsthedryingoutof

thecell.

Theworking electrodewas prepared as described in the corresponding subchapters.

Thebattery testcellswereassembled insideanargon filleddrygloveboxunder inert

conditions.

Fig.12:Technicaldrawingoftheinhousetestcelldesign.

1.6TheLithiumIonBattery

10

Table11:Descriptionofthetestcellparts.Polyvinylchloride(PVC).

Number Ele2ment Material1 ContainerTop Steel2 ContainerDown Steel3 Ballbearing;Ball Steel4 Insulation PVC5 Inwardcontainer Titanium6 CurrentCollector Titanium7 Tube PP8 LithiumSupport Titanium9 PPSealing PP

Galvanostatic and potentiodynamic measurements were monitored by Astrol, a

program from Astrol Electronic AG. A potentiostat (BATSMAL battery cycler) was

connectedusingaserialcabletoapersonalcomputerviaaserial/analogconverter.All

potentials,mentionedisthiswork,arerelatedtoLi/Li+asanode.

1.6 TheLithiumIonBatteryLithiumisoneofthelightestelementsandhasthelowestredoxpotential.Thusitsuse

inbatteries isobvious,andtherefore,thehistoryofLiBs is longandhasstartedabout

1950 [8]. One of themain observationswas that Limetal is stable in nonaqueous

electrolytesasmolten saltsororganic solvents suchaspropylene carbonate [9].This

stability is brought about by an SEIformationwhich allows Li+ to pass and go into

solution.Alreadyinthe late1960sandearly1970sprimaryLIBswererelativelyquickly

brought to the market. Some of these systems are still in use and/or under

investigation, such as the Lithium/manganese oxide (Li/MnO2) or lithium polycarbon

monoflouride (Li/CFx) [10] systems.The commercializationof the secondaryLIBs took

much more time. The main problems were strong SEI and dendrite formation,

respectively,ontheLimetalanode.Inaddition,theLimetalanodesposedaninherent

riskofathermalrunawayreaction,whichentersintoaseveresafetyproblem.Thenext

step forwardwas introduced byA. Yoshino, assembling the so called Rocking chair

batteryand filingapatent [3].Allmaterials,Yoshinoused,wereknownatthattime,

butA. Yoshinowas the firstwho combined the known parts to aworking and cost

efficientsystem:

1Introduction

11

cathode:LiCoO2,inventedbyGoodenoughetal.in1979[11]

Electrolyte:Propylenecarbonateassolvent,knownsincethe1960th

Anode:carbonaceousmaterials,suchasgraphite,inventedbyYazamiin1983[12]

Safety tests of the assembled cell were the last very important step, before the

commercialization began. Yoshino proved the safety of the assembled battery via

dropping an iron lump on a LIB. The battery did not ignite, and the Rockingchair

batterywasfirstlybroughttomarketbySONYCorp.in1991,andbyajointventureof

A.KaseiandToshibain1992.

ThisLithiumIonBatterysetupiscalledrockingchairsystembecausethelithiumions

are transferred back and forth between the cathodic and anodic intercalation hosts,

theirchairs[13](Fig.13).

Fig.13:Schematicillustrationoftherockingchairbatteryinthedischargestate.

TheRockingchairbatteryisillustratedinFig.13inthedischargestate.Thesetupcan

be easily assembled because LiCoO2 and Graphite are stable in air at ambient

conditions.When a charge current is applied, the LiCoO2 is oxidized (121) and the

Graphite is reduced (122). The Liions aremoving through the electrolytewhile the

electrons are transported through the outer electric connection until the battery is


12

chargedtoLiC6andLi0.5CoO2.Duringthedischarge,thereactionproceeds intheother

direction(fromrightto left)anddelivers itsenergy.Allprocesses,thedelithiationand

the lithiationoftheanodeandcathodematerialhavetobereversibleprocesseswith

reasonablekineticandthermodynamicparameters.

: 121: 122

For both, charging and discharging lithium cations and electrons have to have

reasonable mobility, otherwise internal resistances result and polarization occurs.

Unfortunately,mostcathodematerialsaresemiconductors,suchasLiCoO2,LiMn2O4,or

insulators as LiFePO4. In that case an electrode compositehas tobedesignedwhich

exhibitselectronicaswellasionicconductivitiesinsufficientmanner.Ifthecompositeis

preparedfromsphericalparticles ithastoconsistofat least16vol.%ofelectronically

conductiveadditives,aspredictedbypercolationtheory[14].Graphiteandnanosized

amorphouscarbon(SuperP) ina1:3ratioarethemostcommonconductiveadditives,

usedinLIB.Theoptimalcarbonwt.%foraLiFePO4compositeelectrodewasdetermined

tobe1011wt.%by severalgroups.Thisamount is inaccordancewith thepredicted

16vol.%,asdemonstratedinEquation123.

16 .% 2.116 .% 2.1 84 .% 3.52 /100 10.2 %

123

Anotherprocessthathastobecontrolledisthevolumeexpansions/contractionsofthe

electrodesduringtheinsertion/desertionprocesses.Thisvolumeworkshouldideallybe

assmallaspossibleto inhibitcracksandcontact losses insidetheelectrodes.Abinder

(forinstancepolyvinylidenefluoride(PVDF))isneededtoformaflexiblenetwork,which

keeps the electrode composite together during the volumetric expansion and

contractionoftheactivematerial.SEMpicturesofanelectrodecompositeconsistingof

theactivematerials,SuperP,Graphite,andPVDFareshowninFig.14.

1Introduction

13

Fig.14:SEMimageofanelectrodecomposite.

Inaddition,theLiIonconductivityandtheelectronicconductivityofanactivematerial

are important. These are intrinsic properties of thematerials. The reduction of the

particlessizetotherangeofthehoppinglengthofanelectronistheonlypossibilityto

change these intrinsicproperties.Thestrongdevelopmentof thenanotechnologyhas

made applicable even insulators as possible cathode materials, such as LiFePO4 or

LiFeSO4F.

Further improvementcanbeachievedby coating theelectroactiveparticleswithan

electronicand/orionicconductiveshell,suchasamorphouscarbon.

1.6.1 CarboncoatingDuring thedevelopmentof thecathodematerialLiFePO4, thestrongenhancementof

theelectrochemicalbehaviorofnanoscopicLiFePO4andonlyofthenanoscopicform

throughacarboncoatingwasdiscoveredin2001[15].

InFig.15 theelectrochemicalbehaviorofuncoated LiFePO4 (red curves)and carbon

coated LiFePO4 (blue curves) are compared. Both electrodes are containing LiFePO4

from the same batch of a LiFePO4 synthesis. Carbon coating was done via lactose

decomposition, according to the procedure described by Fotedar [16]. After proper

coating the capacityof thematerialmassively increases from60Ah/kg to148Ah/kg,

correspondingto88%ofqth.Inaddition,theoverpotentialdecreasessubstantially.


14

Fig.15:Electrochemicalcyclingofpure (redcurves)andcarboncoatedLiFePO4 (bluecurves);theoreticalcapacity=170Ah/kg.

Fourmain effects of the carbon coating have been elucidated: 1. reduction of Fe3+

impurities to Fe2+ during synthesis, 2. prevention ofOstwald ripening, 3. increase of

electricconductivitybetweenLiFePO4particlesandthecurrentcollector,aswellas4.

enhancementofLiionmobility (4) [15,1721].Variouscarboncoatingmethodswere

investigatedusingdifferentcarbonsources.Allofthemarebasedonorganicprecursor

decomposition,suchassugars[22],polymers[23],orglycols[24]ininertatmosphere.

Whensuchcarboncoatingmethodsareappliedtotransitionmetaloxides,theoxides

are reduced eventually down to the elementalmetals. For example, themixture of

Fe3O4 and PVC (1:1 by weight) reacts to carbon coated Fe above 580C [25].

Therefore,suchreducingcarboncoatingmethodsarenotfeasibleforcathodematerials

inhighoxidationstates.Theonlypossibleprocedureofcoatingtransitionmetaloxides

inhighoxidationstateswasreportedbyChenetal.[26].Theyusedmesoporouscarbon

asa template for carbon coatedV2O5nanoparticles.TheV2O5wasmelted inside the

carbontemplate,followedbypartialremovalofthecarboninairat600C.

Anotherapplication fieldof carbon coatedmaterials isphotocatalysis.A carbon shell

leadstoastrongincreaseofcatalyticallyactivesurfaceareaoftheparticles.Thishigher

1Introduction

15

surfaceareabringsaboutabetterabsorbanceofthepollutants.Asanexample,carbon

coatedTiO2showsastronglyimprovedactivitycomparedtopureTiO2particles[2729].

1.6.2 AnodeMaterials

Graphite

SincethefirstLiBhasbeenbroughttomarket,graphitewasusedastheanodematerial.

At themoment graphite shows the best combination of the relevant properties for

anode materials due to its low price, easy handling, reasonable specific charge

(372Ah/kg)combinedwitha low lithiumextractionpotential (0.3V)andaverygood

cyclingperformance(>1000cycles,[30]).However,therelativelylowspecificchargeof

graphite compared to other possible anode materials, for instance silicon with

4200Ah/kg, leaves enough space for improvements. Thus many alternative anode

materialsareunderinvestigation.

Metaloxides

Titanium dioxide inserts up to one lithium equivalent reversibly (335 Ah/kg). The

capacity retention (80% over 100 cycles) is good. The main drawback is the high

average lithium extractionpotentialof 1.7V vs. Li. Thus the cell averagepotential is

1.4V lower than in a cellwith a graphitic anode. Themain advantage of this high

extraction potential is the possible replacement of the copper current collector by

aluminumfoil,whichwouldlowerthewholemassofthebatterysubstantially.

Snbasedanodematerialshavebeenintensivelystudiedinthelastyears[31,32].Inone

route,theactiveSnmaterialcanbeformedinsitubyelectrochemicalreductionofSnO2

(Eq.124). The thus formed Snspecies intercalates up to 4.2 lithium equivalents,

corresponding to 900 Ah/kg (Eq.125). Carbon coated SnO2 electrodes exhibit good

cycling properties. The capacity retention reaches 450 Ah/kg over 100 cycles (50%

retention)[33].

4 4 124 0 4.4 125


16

Nanoscopictransitionmetaloxides(MxOy;M=Fe,Co,Mo,etc.)canundergoconversion

reactions (126). The reversibility of these reactions is provided due to the high

reactivityoftheinsituformedmetalandLi2Onanoparticles.Hematite(Fe2O3)shows

a reversible insertion capacity of up to 1000Ah/kg in the 2.5 to 0.5 V range [34].

AnotherrepresentativeoftheseconversionmaterialsisCo3O4thatexhibits1000Ah/kg

over50cycles[35].

2 126

Siliconbasedanodes

Siliconhasthehighestknowntheoreticalspecificcharge(4200Ah/kg)duetoitsability

of forming a series of alloysup to Li21Si5. Thus intensive researchhasbeendoneon

siliconbasedanodematerialsinthelastthirtyyears[36].Butstrongcapacityfadinghas

prevented thecommercialization, so far.One reason is thehighvolumeexpansionof

400%during lithium insertion.Siliconpowderanodeswithmicrometersizedparticles

showaspecific insertionchargeclose tothe theoreticalvalue,but the firstextraction

yieldsbackonly1/3ofthespecificchargeoftheloadinghalfcycle[37].Usageofnano

scalesiliconpowdersreducesthe irreversiblespecificchargestrongly,butthecapacity

fadingwasnotimproved[38].Inactivematrixmaterialswithahighmechanicalstrength,

suchasTiN,TiB2,SiC,couldnotresistthevolumeexpansion forceofLixSi,theanodes

werepulverizedandfadingremainedsimilartothatofpuresiliconelectrodes[39].SiOx

compoundsshowamorestablecyclingbehavior(800Ah/kgover25cycles),butmostof

the specific charge is gained only above 1.5V against lithiummetal [40]. The actual

focus liesonSilicon/carboncomposites (Si/C) [41].Manydifferentsynthesis routesof

Si/Ccompositeshavebeendescribed:pyrolysis [42],solgelsynthesis[43],mechanical

milling[44]andchemicalvapordeposition[45].A.Magasinskidemonstratesthatsilicon

anodescanactasthenextgenerationofanodicmaterials.HetestedaSi/Ccomposite

electrodewith1600Ah/kgover100cycles.Theactivematerialwasprocessedinatwo

stepCVDprocess:(1)Sidepositedonannealedcarbonblack,(2)carbonCVDonthefirst

composite[46].

1Introduction

17

1.6.3 CathodeMaterialsNaFeO2

LiCoO2 is themost employed compound of the layered NaFeO2 structure type in

batteryrelatedapplications.Furthermore, itstill isthemostusedcathodematerials in

commercialLIBs.Thelithiuminsertion/desertionreactioniswellcharacterized.Thehigh

reversibility isbasedon its layeredstructure(spacegroupR3m),because lithium ions

can intercalate/deintercalate between/from the interlayer spaces. Its high discharge

voltage(4.2V)incombinationwithaspecificchargeof140Ah/kgleadstoaquitehigh

specificenergyof590Wh/kg.But,thehighvoltagecausessafetyconcerns,becausean

overchargecan leadtoan ignitionofthebattery [47,48]. Inaddition,LiCoO2 iscostly

whichmakesbigbatterypackshighlyexpensive.Furthermore,thecyclingperformances

of LiCoO2 cathodes are not good enough for car applications (2000 cycleswith 80%

energy retention areby farnot reached). The electrode degradationwas thoroughly

investigatedandhasmanyreasons.Twomainpointsare:(1)duringdeepdischargeCo2+

cations can dissolve in the electrolyte [49]; (2) LiCoO2 shows a strong volumework

during thedesertion/insertionprocess.Thisexpansionappliesaphysicalstress to the

cathode,whichcausescracksandcontactlossesintheelectrodecomposite[50].

LiNiO2alsocrystallizeswiththeNaFeO2structure.Itselectrochemicalperformanceis

worsethanthatoftheCobaltoxide.Itexhibitsstrongcapacityfadingwhichoriginates

fromadisorderingoftheNications intothe interlayerspace[51].However,the lower

costs and the higher specific capacity of the material make it attractive for

commercialization.

ThemostpromisingcandidatesoftheNaFeO2structurefamilyaretheNMCmaterials

(Li(Ni1xzMnxCoz)O2 (with: 1xz=1)with energy densities up to 850Wh/kg [52]. These

compoundsclasswasinventedbyM.ColucciaatETHZrichin2000[53].Manygroups

work on the improvement of their cyclability. Surface treatment like coatings with

CoPO4[54]orpartialreplacementofthetransitionmetalsbymaingroupelements(Al)

[55]resultinamorestablecathodematerial.Furtherimprovementcanbeachievedby

using themixed cathode0.4Li2MnO3*0.6LiMn0.4Ni0.2Co0.2O2 [34].The specific charge

increasesto200250Ah/kgwithacapacityretentionof100%after50cycles.


18

Spinel

The spinel LiMn2O4 is another well investigated cathode material. The lithium ions

occupythetetrahedralpositionsandthusthestructureremainsstableduringtheredox

process.Themainadvantageisthe lowpriceofthematerial.Onthenegativesiteare,

thelowenergydensity(300Wh/kg)andtheoccurrenceofirreversiblephasetransitions

duringcycling[56],whichcausesirreversiblecapacitylosses.Carboncoatingviasucrose

decomposition improves the rate capability strongly so that chargingwithinminutes

becomespossible [57].Replacementofmanganesebynickel [58]and chromium [59]

increasesthecyclingstability,too.Inaddition,chromiumsubstitutionraisestheaverage

voltageto4.5VagainstLimetal(590Wh/kg).

Phosphates

At present, LiFePO4 exhibits promising characteristics for large scale battery

applications, such as grid support or powering electric vehicles, but suffers from

relatively lowspecificcapacity.SincecarboncoatingmethodsofLiFePO4areavailable,

itselectrochemicalperformance(170Ah/kgat3.4V,Fig.15) iscomparabletothatof

LiCoO2.(Thestronginfluenceofthecarboncoatinganddifferentpreparationmethods

are discussed in Chapter 1.6.1) In addition, its low production costs (25 US$ per

kilogram), nontoxicity, environmental friendliness and very good electrochemical

cyclabilitycombinedwithitsreasonablespecificenergy(560Wh/kg),pusheditintothe

focus ofmany scientists and battery companies [60]. The described syntheses vary

from: (1) classical solidstatemethods (cheap, larger particles, lower electrochemical

activity)[61];(2)solgelroutes(highquality,verypurematerial,difficultscaleup)[62];

(3) microwave assisted syntheses (costefficient, insitu carbon coating via glucose

decomposition,goodqualitymaterial)[63];(4)hydrothermalmethods(timeconsuming,

less output, high quality) [64]; (5) carbothermal reduction strategies (cheap iron

precursorsFe2O3, insitucarboncoatingviaglucosedecomposition, lowspecificcharge

133 Ah/kg, due to Fe3+ impurities)[65]; and (6) Spray pyrolysis approaches (costly,

difficultconditionadjustment,highquality)[66].

LiCoPO4 and LiNiPO4 exhibit higher discharge voltages at 4.8V respectively 5.1V

compared to LiFePO4.Thus theymaybecome interesting cathodesmaterials forhigh

1Introduction

19

power applications [67].Many pyrophosphates andmixed phosphate/pyrophosphate

compounds of V,Mn, Fe, Co and Ni have been characterized and electrochemically

tested.Inthepyrophosphatecompounds(P2O74),thelithiuminsertionvoltageisclearly

increased [68],but theworkingpotential is touching the thresholdof theelectrolyte

stabilitywindow(upto5.2V).

Vanadiumoxides

Vanadiumoxides,especiallyV2O5and LiV3O8,arewell characterizedpossible cathode

materials[69].In1992Westetal.reportedthatelectrochemicallithiumintercalationof

up to three Li equivalentsper formulaunit ispossible in thehostV2O5(space group

Pmmn)[70]. Inaddition, several intercalated lithiumvanadiumoxides canbeusedas

cathodematerials: LixV2O5 (0.3


20

Fig.16:Discharging/chargingpotentialcurvesofV2O5.

LiV2O5 (space group A12/m1) cycles reversibly in the x = 0.15 to 2.0 range. Two

distinctplateausoccurat3.6and2.4V.Thespecificchargeretentionishigh(97%)over

thefirst15cycles.Onfurthercycling,degradationoftheelectrodetakesplace[72].

Li0.3V2O5 (space group Pnma) exhibits five different lithium insertion processes,

correspondingtoanequalnumberofphasetransitions,inthe4.5to1.5voltagerange.

Theobservedcapacityof320Ah/kgcorresponds toanexchangeofabout2.5 lithium

equivalentsperformulaunit[73].

The electrochemical behavior of LiV3O8 has very thoroughly been investigated. It

exhibitsaspecificchargewith280Ah/kgatanaverageinsertionvoltageof2.8V.Four

very characteristic reductionprocessesareobservable in the small2.9 to2.5 voltage

window[74].

Allvanadiumbasedcathodematerialsdoesnotshowhighcapacity retentionsmainly

duetoastrongSEIformationonthecathode`ssurface.ThiscapacityfadingduetoSEI

growth,wasnicelyillustratedonthebasisofthecathodematerialLi1.1V3O8byTanguyet

al[75].

1Introduction

21

Othercathodematerials

Transitionmetalfluorosulfatesarebeing intensively investigatedascathodematerials.

Thesematerialsshowhigherdischargepotentialsthanthecorrespondingphosphatesor

oxides. But the specific charges are significantly lower due to the largermolecular

weight[76].Transitionmetalsilicateshavebeenattractedsignificantattentioninrecent

years.ThehighlyLichargedsilicateopensthepossibilitytousemorethanoneoxidation

state,asitistheoreticallypredictedforLi2FeSiO4[77].

1.8Glasses

22

1.7 InfluencesoftheLigandFieldSplittingonCathodicMaterialsThe ligand field splitting (LFS)hasa strong influenceofmaterials`properties, suchas

crystalstructures,colorsandredoxpotentials.Itiswellknownthat,theholeoccupation

in a transition metal spinel can be explained by the ligand field theory, and the

formation of an inverse or a normal spinel can be predicted [78]. Furthermore, the

redoxpotentialsof complexes are affected stronglybydifferent ligand environments

[79].

The ligand fieldsplittinghas thestrongesteffectontheHOMOLUMOgap.Theredox

potentialofanycompound isgivenbytherequiredenergy forremovingoneelectron

outoftheHOMOofthereducedspecies.Duringthereductionprocess,theHOMOof

thereducedspeciesisconvertedtotheLUMOoftheoxidizedspeciesandconsecutively

filledduringthereductionprocess.Theexplanationoftheredoxpotentialsofdissolved

complexes by LFS is established, but the influence on solid materials is not often

mentioned. In Fig. 17 the LFS of the redox processes Mn3+ to Mn5+ octahedral

coordinated(left)andMn5+toMn7+tetrahedralcoordinated(right)arecompared.The

oxidationofoctahedrallycoordinatedMn3+toMn4+ leads toa removalofanelectron

from the antibonding eg orbitals. The required potential has experimentally been

determinedtobeabout4VagainstLimetalinmanycompounds,suchasLiMn2O4[56].

Thenextoxidation step (Mn4+/Mn5+,octahedral) requires the removalofan electron

outofthedeeper lyingt2gorbital.That iswhytheexpectedpotentialofthisoxidation

step(Mn4+/Mn5+,octahedral)hastobemuchhigherthanthe4Vcorrespondingtothe

Mn3+/Mn4+pair.Thisoxidation isnot feasible inhithertoknownelectrolytes,because

the oxidation potential,Mn4+/Mn5+ in octahedral coordinated, has to be above the

stabilitywindowofsuchcommonelectrolytes.

A tetrahedral ligand field originates a lower splitting. The HOMO of Mn5+,6+,7+

(tetrahedral) has a higher energy compared to the HOMO ofMn4+ (octahedral), as

shown inFig.17right.TheredoxpotentialofMnO4insolution(3.85V)iswellknown

[79]. This value arises to the removalof the last electronoutof the egorbitals. The

redoxpotentialofMn5+/Mn6+isexpectedtobesimilarbecausetheelectronisremoved

from the identicalorbital set. For comparison, the solution redoxpotential is 3.75V

[79]. Consequently, the oxidation of Mn5+ tetrahedrally coordinated to Mn6+

1Introduction

23

tetrahedrallycoordinatedhasaslightly lowerpotentialthantheoxidationMn+6/Mn7+.

ThesepotentialsarefeasibleinthestandardLIBelectrolytes.Insummary,tetrahedrally

coordinatedmanganese compoundshave tobe considered, ifhigheroxidation states

thanMn4+,shouldbeemployed.TheseconsiderationscanbeappliedtotheCr3+/4+to6+

redoxcouples,too.

Thesocalledfloatingvoltage,thealmost linearvoltagedropduringelectrochemical

lithium insertionofnoncrystallinematerials, canbealso explained via LFS theory.A

slightshiftofthecoordinationgeometryofthecation leadstoadistortionandthusa

changeofbindingenergyoftheelectronicvalencestates.Astrongerdistortioncausesa

lower LUMO of the orbital set, which brings about a slightly reduced reduction

potential.

Fig. 17: Ligand field splitting and redox processes: leftMn3+ toMn5+ in octahedralcoordination; right: transition Mn5+ to Mn7+ in tetrahedral coordination. Forconvenience,theenergybarycenterisdepictedasthesame,althoughitmaynotbeforthe two typesof coordination.Thehorizontaldotted linedisplays thehypothetical Lipotential.

1.8Glasses

24

1.8 Glasses

1.8.1 IntroductionTheborderbetween vitreousandamorphousmaterials isnot consistentlydefined in

literature. For clarification of the glossary, I present the mostly used definition in

literatureinthissubchapter.

One of the pioneers of glass researchwasG. Tammanwho studied this field in the

beginningofthe20thcentury[80].Hestartshisbookwiththefollowingdefinition:

ImGlaszustandbefindendiefesten,nichtkristallisiertenStoffe

Theglassystateconcernssolidbutnotcrystallizedmaterials.

Obviously,thediscriminationbetweenvitreousandamorphousmaterialsisnotpossible

by thisdefinition. In the followingyears thebehaviorof theviscosityvs. temperature

was included in some definitions. But theywere so difficultly constructed that they

couldnotsucceedasageneraldefinition.Untilthisdaythevaliddefinition isgivenby

theAmericanSocietyforTestingMaterials:

Aglassisaninorganicproductwhichmeltsandresolidifiesmainlywithout

crystallization.

The Deutsches Intstitut fr Normen (DIN) adopted this definition, too. But this

definitionexcludesorganicglassesandthose inorganicglassessynthesizedviathesol

gelmethod,becausetheyareneither inorganicnorcooledfromamelt.Todistinguish

betweenthevitreousmaterials,presentedinChapter2,andtheamorphousmaterials,

presentedinChapter5,Iwouldliketorefertothisofficialdefinitionbutmodifyitto:

Avitreousmaterialisanamorphoussolidthatexhibitsaglasstransitiontemperature.It

isfurthermorecharacterizedbyacollectivesurfacetension.

This definition also includes all vitreous materials, which are synthesized by low

temperaturemethods,but itexcludesamorphousmaterials, forexample thosewhich

decomposebeforetheymelt.Thisgeneraldefinitionallowsthediscriminationbetween

vitreous(presentedinChapter2)andamorphouscompounds(discussedinChapter5).

1Introduction

25

1.8.2 StructuralCompositionThe network theory given by W. J. Zachariasen and corroborated by B.E. Warren

discriminatesbetweenthreedifferentbuildingunits[81].

Networkformers,suchasSi,B,Ge,As,(aschalkogenides)andBe(asBeF2)etc.

Theirtypicalcoordinationnumbersare3or4.Thesematerialsshouldbeableto

form polyhedral building unitswhich do not allow for denser packing in the

crystallinestate.

Networkmodifiers:Na,K,Ca,Baetc.TypicalcoordinationnumbersareZ6,but

coordination can change easily. These cations loosen the network up and

saturatetheterminaloxygens.

Metaloxides,suchasMxOyM=Al,V,Mg,Zn,Pb,Be,Nb,Ta.Theirtypicalmetal

coordinationnumbersare46.Theseoxidescanacteitherasnetworkformeror

network modifier, but they cannot form a glass only by themselves, as the

networkformerscando.

In Fig. 18 crystalline SiO2, amorphous SiO2 glass and a sodium silicate glass are

compared.Socalledhigh (spacegroupP6222)and lowquartz (spacegroupP3221)

arecrystallineorderedphaseswithtetrahedraloxygencoordinationofsilicon.The

change from the crystalline to the glassy state is due to a denser packing in the

glassy stateaccompaniedby introductionof irregularitiesof theSiOring systems

eitherbydistortionorby ringextensionorsizereduction. Incontrast,thesodium

silicate glasses do not contain ring arrangements but their local structures are

dominatedby silicate chainsand their terminaloxygenatomsare coordinatedby

sodium cations (networkmodifier). As alreadymentioned, translation symmetry

doesnotexistinvitreousmaterials.Theabsenceoftranslationsymmetryaffectsthe

habitusofaglassstrongly,because itcanadaptanygeometry.Consequentlythey

aremissinglongrangeorderandabsenceofBraggreflections.

1.8Glasses

26

Fig. 18: Comparison of the interlayer structure of crystalline quartz, SiO2 glass andsodiumsilicateglass.

1.8.3 PropertiesTwopropertiesofglassesareverycharacteristic:thespecificheatandtheviscosity.The

specificheatcpisgivenby

127

The specific heat vs. temperature relations of a glass (continuous line) and its

correspondingcrystalphase(dottedline)areshowninFig.19.Abovethemeltingpoint

an identicalmeltformsfromboth,glassandcrystalphase.Thetwocpcurvesare lying

on topofeachother.At themeltingpointFp, the curvesdiffer strongly.The specific

heatofthecrystalphaseshowsastep,asit istypicalforafirstorderphasetransition.

Unlike,thespecificheatoftheglasscontinuesthelineardecrease,anundercooledmelt

is formed (FpTb). In the softening interval (TbTa) theundercooledmelt freezesunder

continuousdecreaseof the specificheat.The inflectionpoint`s temperature is called

glasstransitiontemperatureTg.Notuntiltemperatureclosetotheabsolutezeropoint,

thespecificheatsoftheglassandcrystallinephasereach thesamevalueandcomply

with theDebyeT3 law.Thehigher specificheatof the glass causes ahigher intrinsic

energy and therefore, the glass is always ametastable compound compared to the

correspondingcrystalphase.

1Introduction

27

Fig.19:Comparisonofthespecificheatvs.temperaturerelationsofaglass(continuousblack line) and the corresponding crystalphasewith a sharpphase transition (x). Ta:Startofthesofteninginterval,Tg:glasstransitiontemperature,Tb:endofthesofteninginterval,Fp:meltingpointofthecrystallinephase.

Onheating,inthesofteningintervalbetweenTaandTbtheviscosityofaglassdecreases

with increasing temperature. That iswhy one can form glasses already below their

meltingpoints.Thedecreaseofviscositycanbeexplainedbyacontinuousmaceration

of the network, because fixed atom positions do not exist (contrary to the crystal

phase).Thesedifferentviscositiesoftheglassescanbefrozen inbyfastquenching,so

that the same glass can show different densities, depending on the conditions of

quenching. This property is called influence of the glass history and it is very

characteristicofvitreousmaterials.

1.9 GlassCeramicsGlass ceramics consistofat leastoneormore crystallinephasesandat leastoneor

moreglassphases.Thecompositeresultsfromacontrolledcoolingprocesswithpartial

crystallizationinmeltorglassmatrix[82].Theaimofthecontrolledcrystallizationisthe

segregation of crystals out of the noncrystalline matrix. Thus an arrangement of

crystallites and vitreous particles can be achieved that shows unique properties

differentfromjustamixtureofthesamecrystallitesandthesameglassyparticles.The

1.8Glasses

28

keyvariablesarecrystallitesizes,theirhabitusandthetypeofthecrystallitesplusthe

interconnectednessbetweenglassyandcrystallineareas.Thecrystallizationofaglass

startsattheglasstransitiontemperatureTg(Fig.110).Themaximumofthenucleation

rate Ioccursata temperature lower than themaximumof thecrystalgrowth rateV.

Thenucleation is impossiblewithintheOswaldMiersarea,becausenonucleuscanbe

formed here.With the help of this information an optimized furnace profile can be

planed.For Instance: ifmanynanometercrystallitesarerequired,atemperatureclose

toTgandashortreactiontimeshouldbechosentoyieldahighnucleationrateatalow

crystalgrowthrate.

Fig.110:CrystalgrowthrateVandnucleationrateIasafunctionofthetemperature.

2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System

29

2 VitreousundCrystalizedMaterialsintheLi2OV2O5P2O5System

2.1 V2O5*P2O5SystemThemain challenge in using V2O5 as positive electrodematerial for LIBs is the poor

cyclability,asmentionedinChapter1.6.3.Manyresearcherswerefocusedoncrystalline

materials, but the approach of embedding V2O5 into a glass were not studied

intensively, yet. E.Roscoe described the glass formation in the V2O5*P2O5 system in

1868forthe firsttime.Hereportedglassformation inmeltswithat least1wt.%P2O5

[83,84].

G. Tamman and E. Jenckel investigated this system intensively and observed glass

formationwithatleast5wt.%P2O5.Inaddition,theydiscussedthattheglassformation

isaccompaniedbyoxygen lossand consequentlybyapartial reductionofV5+ toV4+.

Furthermore, they described a positive dependency between the applied pressure

duringquenchingandtheresultingdensityoftheglass[85].Aroundthirtyyears later,

P.L.Bayntonetal.determinedtheelectricpropertiesandobservedsemiconductivity,

as it was shown for molten V2O5 by Yurkov before [86]. The measured electric

conductivitieswereinthe4.8*104to5.6*105Scm1rangeforcomposition7090mole

percentage V2O5 [87, 88]. A common condition for an increase of conductivity and

occurrenceof semi conductingbehavior is the coexistenceofmore thanone valence

statesofthetransitionmetalionsinsuchaglass.Suchdifferentoxidationsstatescanbe

causedbyoxygen lossduring theannealingprocess,asdescribedbyTamman.These

early findings of semi conductivity were the basis of intensive investigation of the

thermopowerofglassesbymanyresearchgroupsinthe50sand60s[8991].

In 1985, Sakurai et al. published a short communication about the electrochemical

behavioroftheV2O5*P2O5glassesinLIBs[92]andpatentedthesevitreouscompounds

in1987 [93].Theyalsopublisheda reportabout thexV2O5*yP2O5glasses inLIBs,and

explained the almost linear voltagedevelopmentduringdischarges and charges.This

behaviorisverycharacteristicofnoncrystallinematerialsandarisesfromthestructural

randomness (lack of long range order, [94]). Also ligand field splitting helps to

understand this phenomenon of gradual voltage change. Any change of the

coordinationspherewillcauseacorrespondingchangeoftheelectrochemicalpotential.

2.1V2O5*P2O5System

30

Suchchangesasfoundinglassesmaybeadvantageousforelectrochemicalapplications,

because there are no defined lattice sites, the host network can adjustmore easily

during the lithium insertion/desertion.Consequently,potential changes appear tobe

almostlinearwiththeongoinginsertion.

Sakuraietal.describedanirreversiblecathodicspecificchargeduringthefirstcycledue

to trapped lithium ions in the vitreous network. They claim that these ions act as

additional networkmodifiers [5] and reported a specific charge of 500Ah/kg in the

rangeof1.04.0VversusLi/Li+forthefirstdischarge.Thespecificchargeofthesecond

cyclereachedonly350Ah/kgandthecapacityretentionwasverypoorinthisextended

voltage range. However, the same glassy electrodematerial tested in the 2.0 3.5

voltagerangedisplayed100%specificcharge retentionof150Ah/kg fromthe10thto

the600thcycle.

Inthefollowingyears,manyresearcherswereagainfocusedontheexactdetermination

oftheglassformingregionaswellastheelectronicandionicconductivitiesofthepure

xV2O5*yP2O5,relatedglassescontainingnetworkmodifiersuchasLi2O,Na2O,Ba2Oetc.

[95]andnanocrystallizedglassceramics[96,97].

Takahashietal.exploredtheglassformingregionandtheelectricalconductivityinthe

vitreousandcrystallizedLi2OV2O5P2O5system.Theyreportedonlyasmallinfluenceon

theelectricalconductivitybythelithiumcontent,iftheV2O5/P2O5ratioiskeptconstant.

Theymeasuredconductivitiesof5*103S/cm(10mol%Li2O)and9*103S/cm(20mol%

Li2O)ataV2O5/P2O5ratio9:1[96,98].

Vanadium pentoxide and vanadiumoxiderich vanadophosphates have been often

testedascathodematerials inLIBsdue to theirhigh theoreticalenergydensities [99]

butwith lowcyclingstabilities.TheembeddingofV2O5 inphosphateglassescouldbe

thekeytostabilizetheV2O5electrode.ThetheoreticalspecificchargeofV2O5basedon

the insertion of three equivalents lithium per formula unit V2O5 is 441Ah/kg.

Accordingly,thetheoreticalspecificchargesoftheglassesarecalculatedviatheproduct

ofwt.%V2O5oftheglassandthetheoreticalspecificchargeofpureV2O5.Forexample,

the theoretical specific charge of a glass with 80 wt.% V2O5 is calculated to be

350Ah/kg. The average discharge voltage is expected to be similar to the average


31

dischargevoltage2.7VofpureV2O5.Consequently,thetheoreticalspecificenergy(945

Wh/kg)ofsuchglassesdoesalmostdoublethespecificenergyofLiFePO4(560Wh/kg).

This iswhy, theycould serveas thenextgenerationofcathodematerials inLIBsand

consequentlyhavebeeninvestigatedinthisworkwhichreportsonthemicrostructure

aswellasontheelectrochemicalbehaviorofglassesandglassceramicsofthenominal

systemxV2O5*yP2O5*zLi2O. It isshown that the irreversiblespecificcharge in the first

cycle,reportedbySakurai,canberelatedtotheformationofanorthophosphatephase.

Intheendof thischapter, itwillbeshownthat theuseofLi3PO4asnetwork forming

agentpreventsthisirreversibleprocess.

2.1V2O5*P2O5System

32

2.1.1 ExperimentalV2O5*P2O5glassesSeveralV2O5P2O5glassesinthecompositionrangeof79to91mol%(correspondingto

83to96wt.%)V2O5weresynthesized.Thethoroughlymixedrawoxides(V2O5,99.2%

AlfaAesar;P2O5,AcrosOrganics98%)weremeltedinquartzcruciblesforfourhoursat

700Cinair.

4 13 21

The lowreactiontemperature isnecessarytopreventthereductionofV5+cationsdue

to oxygen loss during annealing at high temperatures. The vitreous samples were

quenchedinsidethecruciblesinwater.Thecolorsoftheglassesarebrownredbutthe

formationofpolyvanadatecationsonthesurface leadstoadarkvioletsurfacecolor.

Unfortunately, the quenching ofmelts inside the crucibles generates compact glass

fragments. Thus all materials had to be crashed firstly, followed by grinding in a

planetaryballmilltwotimes1hat550rpminreversedirection(FritschPulverisette4).

TheamorphousstateofallsampleswasprovenbypowderXRaydiffraction(XRD).The

XRDdatawerecollectedinthe2rangebetween10and90.Thedifferentialthermal

analysis(DTA)measurementsoftheamorphouscompoundswereperformedbetween

25Cto700Cataheatingrampof10C/minundernitrogenflow.Themicrostructures

ofthesampleswereinvestigatedbyscanningelectronmicroscopy(SEM).

Electrodes forelectrochemical testingconsistof82wt.%activematerial,2wt.%PVDF

(Polvniylidene fluoride, Sigma Aldrich M.W. 534.000), 12 wt.% amorphous carbon

(SuperP,Timcal)and4wt.%graphite (Timcal). Ina firststep,200mgactivematerial,

29.3mgSuperPand9.7mggraphitewereground intheballmill in3mltoluenetwo

times30minat530rpm in reversedirection.Thissuspensionwasdried invacuumat

roomtemperatureforthreehours.4.9mgPVDFand2mlsolvent(tolune:THF;4:1)were

added and ultrasonicated for 15min (a longer ultrasonication in THF leads to a

reductionof the vanadium).The resulting slurrywasdispersedona titanium current

collector,driedat room temperature (approx.10min) followedbydryingat80C for

another 12 hours in vacuum. Drying at temperatures above 100C under reduced

atmosphere leads to an oxygen loss and consequently to a lower electrode

700C;4hQuenching


33

performance.Thetwoelectrodecellswereassembled inanargonfilleddryglovebox.

Limetal foil (SigmaAldrich,0.5mm)servedascounterelectrodeand1molarLiPF6 in

ethylencarbonat/dimethylcarbonat (1:1) (Merck LP30) functioned as electrolyte. The

cellsweretestedinthe1.5to4.2resp.4.3voltagerangeatdifferentcurrents.

2.1.2 StructuralCharacterizationofV2O5*P2O5glassesIn Fig. 21, the XRDpattern of the vitreous sample with a nominal composition

8V2O5*2P2O5are representatively shown forallXRDpatternsandDTAdiagrams (Fig.

22)measuredofvitreoussamples.Asforanamorphousmaterialexpected,reflections

could not bemeasured in the XRD powder experiment. The background of the XRD

powder pattern is extremely high due to the strong absorption of the amorphous

compound. Since the amorphousmaterial is synthesized from amelt, it has to be a

glass.ThusadditionalDTAanalysesarenotnecessarytoconfirmthevitrification.

Fig.21:XRDpowderpatternofassynthesizedvitreous8V2O5*2P2O5.

InFig.22, theDTA curveof8V2O5*2P2O5 ispresented.The first thermaleffect isan

endothermic baseline shift belonging to the glass transformation point Tg at 229.5C

(inflectionofthecurve).Theonsetoftheglasstransitionindicatesthebeginningofthe

softening interval between 216.9 and 229.5C. The next following strong exothermic

STOE Powder Diffraction System

2Theta10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.00.0

20.0

40.0

60.0

80.0

100.0

Rel

ativ

e In

tens

ity (%

)

8 V2O5 * 2 P2O5

2.1V2O5*P2O5System

34

complexpeakat254.5CarisesfromthecrystallizationheatofV2O5orV2xPxO5crystals.

Coincidentally, the endothermic baseline shift of the glass transition is exactly

compensatedbytheexothermicbaselineshiftofthecrystallization,sothatthebaseline

reaches the same level after the crystallization peak as it had before the softening.

Finally,thelastendothermicpeakat655.3Cillustratesthemeltingpointofthenewly

formedglassceramic.Consequently,theDTAanalysisconfirmsthevitrificationdueto

the characteristic glass behavior, including softening interval, glass transition point,

crystallizationheatandmeltingpointofthenewlyformedcrystals.

Fig.22:DTAcurveofassynthesizedvitreous8V2O5*2P2O5.

Thegrindingof theglass led toa resultingaverageparticle sizearound2m (cf.Fig.

23).Alongermillingtime(12h)didnotfurtherreducetheparticlesize.Unfortunately,

thissize isstillattheupper limitofactivematerialswith lesserelectronicconductivity

forLIBs.IncaseofyV2O5*xP2O5glassesthegrindingresultisindependentofthesample

composition.Incontrary,themicrostructuresdifferstronglywithcomposition.

100 200 300 400 500 600Temperature /C

-0.1

0.0

0.1

0.2

0.3

DTA /(uV/mg)

Complex Peak: Area:Peak:Onset:End:Width:

-31.45 Vs/mg655.3 C636.7 C663.7 C

23.4 C(37.000 %)

Complex Peak: Area:Peak:Onset:End:Width:

35.55 Vs/mg254.5 C252.5 C269.7 C

15.3 C(37.000 %)

Glass Transition: Onset:Mid:Inflection:End:Delta Cp*:

216.9 C232.1 C229.5 C247.2 C

0.371979 mVs/(gK)[1]

exo


35

Fig.23:SEMimageofthesynthesized8V2O5*2P2O5.

In Fig. 24, the microstructures of two samples with different compositions are

compared:left86V2O5*14P2O5andright89V2O5*11P2O5.AllsampleswithP2O5content

larger than 14mol% have a similar texture, as the shown glass with 14mol%. The

sampleof11mol%P2O5 is representative forallglassescontaining less than11mol%

P2O5.

Fig.24:SEMimagesofsamples86V2O5*14P2O5(left)and89V2O5*11P2O5(right),respectively.

2.1V2O5*P2O5System

36

Inbothsamples,thegrainsdonothaveanypreferredshape,asexpectedforvitreous

particles. Inallsamples,the2mparticlesconsistofgrains inthe100300nmrange.

Nevertheless,thetexturesofglasseswithdifferentcompositionarestronglydissimilar.

In the P2O5 richer sample, the grains are covered with a shell, so that the grain

boundariescannotbeseenclearlyand theparticlessurfaceappearstobedense.The

P2O5 poorer glass obviously exhibits grain boundaries and accordingly the particle

surfacesaremoreporous.

Densestructurescauselongerdiffusionpathwaysbecausethediffusionpathwaysinside

the electro activematerial are long and the electron transport is, at least partially

blockedduetothelowelectronicconductivity.Theresultinglongerdiffusionpathways

for the phosphorus richer samples are expected to enter intohigherover potentials

combinedwithlowerelectrochemicalactivities.

2.1.3 ElectrochemicalCharacterizationofV2O5*P2O5glassesAredoxprocessof theactivematerial isanessential featureduring lithium insertion

desertion reactions tosustain thechargeneutrality (asexplained inChapter2.1).The

phosphategroupsareactingonlyasnetworkformersandtheyarenotparticipating in

the redox process. Consequently, the theoretical specific charges of these glasses

depend only on the vanadium concentrations of the electro active glasses. The qth

valuesaregiveninTable21.Allsamplesweretestedundergalvanostaticconditionsat

aconstantcurrentof20A/kg(C/20)inthe1.5to4.2Vrange.

Theopencircuitvoltages(OCV)ofallglassesreachedvaluesbetween3.68Vand3.7V

and accordingly the OCV is independent of the vanadium concentration for the

investigatedconcentrationrange (Table21). Inthepreviouschapter,thedependency

ofthecompositiononthemicrostructurewasdiscussed.Thequestion,whetherthese

different microstructures are reflected in the electrochemical behavior, will be

discussedinthenextparagraph.


37

Table21:SelectedelectrochemicaldataofV2O5*P2O5glasses:theoreticalspecificchargesqth;practicalspecificchargesQandcorrespondingpercentagesofthetheoreticalvalues;overpotentials;opencircuitvoltagesOCV.

InTable21,selectedelectrochemicaldataoftheV2O5*P2O5glassesaresummarized.A

cleartendencycanbeidentified:ahigherpercentageofthetheoreticalspecificcharge

isreachedas lowertheP2O5content.Theglasscontaining79mol%V2O5reachesonly

76%of itsqthcomparedtothetwosamplescontaining lessthan11mol%,whichgain

91% respectively96%ofqth. Furthermore,ahigherP2O5 content increases theover

potentialmarkedlyfrom0.146V(9mol%P2O5)to0.383V(21mol%P2O5).Thesmall

differenceoftheelectronicconductivity,describedbyTakahashi (Chapter2.1),canbe

neglected. Consequently, the poorer electrochemical performance of the P2O5 richer

samplescanbeexplainedbythedenserglassmicrostructures,showninFig.23.

Representatively,theelectrochemicalbehavioroftheglasscontaining9mol%P2O5will

be discussed based on its differential specific charge plot and its dischargecharge

profileindetail(Fig.25).

The first electrical discharge reaches 396Ah/kg, which corresponds to 96% of the

theoretical capacity (411 Ah/kg). The unstructured voltage change of the electrode

confirms the amorphous state giving rise to quasi straight lineswhich occur as very

broadpeaks in thedifferentialspecificchargeplotbetween1.6Vand3.5V (Fig.25).

Themaxima of the anodic 2.6V and cathodic 2.4V curves are shifted against each

other. Another indication for the over potential is the intersection of the

insertion/desertion,whichshould ideallybeathalfof the totalspecificcharge. In the

discussedsample,the intersection isat1/3ofthetotalcapacity.Oneshouldexpecta

high over potential from these indicators. The overpotentialof =0.4V, calculated

accordingtoequation17,confirmsthesefindings.

mol%V2O5

wt.%V2O5

qth[Ah/kg]

Q(1stcycle)[Ah/kg]

(2ndcycle)[V]

OCV[V]

79 83 367 281(76%) 0.383 3.6986 89 393 280(71%) 0.361 3.6889 91 402 363(91%) 0.315 3.6891 93 411 396(96%) 0.146 3.70

2.1V2O5*P2O5System

38

A second characteristicof these glasses is thedifferencebetween the initialand the

followingcycles.Thetworeductionpeaksofthe initial insertion,thefirstbetween3.5

to2.7Vandthesecondinthe2.4to1.5Vvoltagerange,areirreversible.Thereduction

peakathighvoltagedidnotarise inthesecondcycleagain.The irreversiblereduction

peak at lower voltage disappeared during the first three cycles. The combination of

these two irreversible reduction processes gains an irreversible specific charge of

131Ah/kg.

Fig.25:Left:differentialspecificchargeplot;Right:discharges/chargesof91V2O5*9P2O5.

Asmentioned in the introduction,SakuraiandYamakiobserved thesame irreversible

processes[94].Theyexplainedthisphenomenonbyanirreversiblelithiumintercalation

into the vitreous network. I would like to point out another relation between this

irreversiblecapacityandthe irreversiblereductionpeak inthevoltagerange3.52.7V

andIliketogiveachemicalexplanationfortheobservedprocess.Underconsideration

of the composition and the irreversible specific charge, the following assumption is

taken:

Thelithiuminsertioninthevoltagerange2.51.5Vleadstotheformationof

orthophosphategroupsandpartiallyreducedV2O5x.


39

Tosimplifythecorrespondingreactionweareneglectingthereductionofvanadiumby

furtherlithiuminsertion:

91 9 54 54 91 . 18 22

Thisreactionwouldleadontheonehandtoanirreversiblespecificchargeof86Ah/kg

(correspondingto0.6LiperV2O5)andontheotherhandtoanirreversiblereductionof

vanadium.Especially,thehighpotentialarea isaffectedbya lowermaximumaverage

oxidation state of V+4.7, as it is observed in the differential specific charge plot

(disappearance of the reduction peakbetween 3.5 2.7V after the first discharge).

Anotherconsequencewouldbetheformationofnaked(notembeddedintotheglass)

V2O4.7,whichshouldbehavesimilartoV2O5.Thecolumbicefficiencyofthefirstcycleof

pure V2O5 is around 86% (corresponding to an irreversible specific charge of

Qir=60Ah/kg;Chapter1.6.3).All theoreticalvalues inTable42are correctedby the

percentage portion of the theoretical specific charge yielded (corresponding to the

amountofactiveelectrodematerial).Foran illustration,thefollowingexampleforthe

calculationofqth,ir(V2O5)oftheglasscontaining79wt.%V2O5isgiven:

, 60 .% 60 0.79 281 /367 / 37 /23

The sum of these two effects the known irreversible specific charge of V2O5

(qth,ir(V2O5);53Ah/kg,correlatedtoitswt.%)andtheirreversiblespecificchargedueto

thereaction(qth,ir(reaction);78Ah/kg)yieldsintheobservedvalueof131Ah/kg.Thus

theorthophosphatemodelnicelyfitstothediscussedglasscontaining9mol%P2O5.

Vitreous compounds with higher phosphorus contents do not comply with the

orthophosphatemodel(Table22).However,theyallshowclearlyahigher irreversible

specificchargethanexpectedforpureV2O5.Inaddition,theaveragedischargevoltage

is decreasing with increasing phosphorus content which is caused by a stronger

reduction of the maximum vanadium valence state. Furthermore, the average cell

voltage is significantly lower than the 2.7V, reported for pure V2O5. All these facts

supportachemicalreactionsimilartotheorthophosphatemodel.

2.1V2O5*P2O5System

40

Table 22: Comparison of electrochemical data of V2O5*P2O5 glasses: irreversiblepractical chargeQir(obs.); theoretical irreversible specific charge due to the reactionqth,ir(reaction),theoreticalirreversiblespecificchargeoriginatedbyV2O5qth,ir(V2O5);thetheoreticalvaluesarecorrectedtothepracticalspecificcharge.

mol%V2O5

QIr(obs.)[Ah/kg]

qth,ir(reaction)[Ah/kg]

qth,ir(V2O5)[Ah/kg]

qth,ir(total)[Ah/kg]

aver.Voltage2ndDischarge[V]

79 100 149 37 186 2.23886 75 91 37 128 2.25189 93 90 50 140 2.30991 131 78 53 131 2.327

Thelongercyclingdataacquiredbyglavanostaticchargedischargecyclingispresented

inFig.26.Theelectrochemicalcyclabilitiesofglasseswith89mol%and79mol%V2O5

are compared in this display. The specific charge of both glasses increased by 7.2%

(89mol%) and 5.7% (79mol%) during the first 10 cycles, respectively. This effect is

known foractivematerialswithparticles sizes in the m range [13].During theearly

insertions/desertions, the particles or the grain boundaries break down due to the

volume changes causedby the lithiation/delithiationprocesses. The resulting smaller

particlesizesshortenthediffusiontimes,andconsequentlyleadtoahigheramountof

active electrodematerial. This higher accessibility evokes an increase in the specific

charge, accordingly. Both compounds exhibited good cyclabilities, as indicated by a

capacity lossofonly13% (79mol%), respectively5% (89mol%)after50cycles.Both

curves can be divided into three parts. The anodic irreversible process of lithium

integration into the vitreous network takes place within the first eight (79mol%)

respectively eleven (89mol%) cycles. Afterwards both electrodes cycle stablywith a

coulombicefficiencyof>99%.However, in the25th cycle, the coulombicefficiencyof

both samples dropped significantly. The efficiency of the vanadium poorer sample

decreasesonlyby1.5% compared to the strongerdecreaseof4.5%of the vanadium

richer sample.Most probably, these efficiency drops occurred due to an irreversible

cathodicSEIformationwhichisaknownproblemofV2O5basedcathodematerials.


41

Fig.26:Comparisonoftheelectrochemicalcyclingbehaviorsoftwodifferentphosphateglasses.

89mol %V2O5

79mol %V2O5

anodiccathodic

2.2V2O5*P2O5GlassCeramics

42

2.2 V2O5P2O5GlassCeramicsUnder theabovementioneddescribedconditions,glassceramicswere formed, if the

melts contain less than 7mol% P2O5. The XRD powder pattern of a glass ceramic

containing5mol%P2O5isshowninFig.27.Theceramic`shighvitreouscontentcauses

the lowsignaltonoiseratio.That iswhyaRietveldrefinement isnotaccomplishable,

andtherefore,theXRDpowderpatternisonlyqualitativelydiscussed.Allreflectionsof

themeasuredglass ceramicmatch the literaturepatternofV2O5[100].Nevertheless,

there isa small shift in reflectionpositionsaswellas in the intensitydistribution.All

reflectionswithakcomponentareshiftedtohigherdiffractionangles.Intheinset,one

clearly can see the shift of the (020) from 51.18 to 51.78 2. In addition, the

intensitiesdidnotmatch,because theobservedmain reflection is the (110) (slightly

shifted)comparedtothe (001)mainreflection found in literature.Thesubstitutionof

V5+ by the smaller P5+ leads to a decrease of the cell volume. Thus the observed

reflectionsshiftsindicateasolidsolutionformationofV2xPxO5,asmentionedbySakurai

etal.[92].

Fig.27:XRDpowderpatternofsynthesizedglassceramicwiththenominalcomposition95V2O5*5P2O5.

STOE Powder Diffraction System

2Theta10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.00.0

20.0

40.0

60.0

80.0

100.0

Rel

ativ

e In

tens

ity (%

)

96 V2O5 * 4 P2O5 Vanadium pentoxide_99808

(020)50.0

(020)(110)


43

2.2.1 ElectrochemicalCharacterizationoftheGlassCeramicGalvanostaticchargedischargecyclinghasbeenperformed inthe1.5to4.2voltrange

at constant current of 50A/kg. The cycling behavior and the calculated differential

specificchargeplotaregiveninFig.28.Thefirstreductionisdominatedbythreebroad

peaks (3.4V, 3.2V, 2.3V) and one sharp effect (1.9V), as clearly observable in the

differential specific charge plot (plateaus in the discharge curve). Contrary, the first

chargeisanalmoststraightline.Themaximaofthebroadpeaksandtheamorphization

aftertheinitialdeepdischargeexactlymatchtheelectrochemicalbehaviorofpureV2O5,

though,theinitialreductionprocessesoftheglassceramicsarediffuserthanobserved

inV2O5electrodes.Forexample,thetworeductionpeaksat3.4and3.2Vareobserved

astwoseparatedsharppeaks forV2O5.Theaccordingreductionsof theglassceramic

arealmostmergedintoonebroadpeakwithtwomaximabetween3.0to3.5V.

Fig.28:95V2O5*5P2O5:leftdifferentialspecificchargeplot;rightdischarging/chargingpotentialcurves.

The glass ceramic reached its theoretical capacity of 425 Ah/kg exactly,which is in

agreementwiththeargumentsdiscussedfortheglassmicrostructures(butonlyvalidif

Crateandelectrodepreparationarekeptconstant).Theexpected irreversiblespecific

charge, due to the orthophosphate formation during the first reduction, should be

45Ah/kgdueto5mol%P2O5.Thesumofthiseffectandtheirreversiblespecificcharge,

known forV2O50 (57Ah/kg), lead toanexpected valueof102Ah/kg.Therefore, the

2.2V2O5*P2O5GlassCeramics

44

observedvalueof102Ah/kgmatchestheexpectedvalueperfectly,andconsequently,

thediscussedglassceramicsupportsthedevelopedorthophosphatemodelstrongly.

Theaveragedischargevoltageof2.52Visevidentlyhigherthantheobservedvaluesof

anyvanadateglassduethelowerreductionofthemaximumvalencestate(V2O4.82).The

overpotential=0.353V isunexpectedly largeand it is located inthesamerangeas

observedfortheglassescontainingmorethan10mol%phosphate.Theelectrochemical

cyclability of the glass ceramic is demonstrated in Fig. 29. The specific charge is

decreasingalmostlinearlywiththecyclenumber(approx.0.33%percycle).Hence,the

capacity drops to 67% within 100 cycles. The coulombic efficiency of 96% is an

indicationofan irreversible cathodicSEI formationon the ceramic.Nevertheless, the

phosphateenvironmentstronglyenhancesthecyclabilityoftheglassceramiccompared

topureV2O5.

Fig.29:Electrochemicalcyclabilityofglassceramicwith5mol%P2O5.Thespikesintheanodic curvedisplay an internal short cutdue tophysical contactbetween a lithiumdendriteandthecathode.

anodiccathodic


45

2.3 GlassformationintheLi2OV2O5P2O5System

2.3.1 ExperimentalLi2OV2O5P2O5SystemTheglass forming regionand theelectrical conductivityof the system Li2OV2O5P2O5

wererecently investigatedbythegroupsofH.TakahashiandJ.E.Garbarczyk [15,17].

They used Li2CO3,NH4H2PO4 and V2O5 as startingmaterials and heated themelt to

1000C. I changed thisprocedure to avoid thehigh temperature,which causesmost

likelyapartialreductionofvanadium.Inafirststep,LiPO3wassynthesizedfromLi2CO3

(Fluka,98%)and(NH4)2HPO4(Fluka,99%)at500Cduring5h.TheassynthesizedLiPO3,

respectively LiVO3 (Alfa Aesar, 99.9%) served as lithium source during the glass

formation with V2O5 and/or P2O5 at 700C during 2h followed by quenching. The

vitreoussampleswerequenchedinsidethequartzcruciblesinwater.Theformationof

polyvanadatecationsisevidencedbythevioletcoloronthesurface,asopposedtothe

brownredbulkcolor.Allmaterialshadtobecrashedtoat leastmillimetersizebefore

theyweregroundintheballmillduringtwotimes1hat550rpminreversedirections.

The amorphous stateof all sampleswasprovenby XRDpowdermeasurements. The

XRDdatawerecollectedunderthesameconditionsasdescribed inChapter2.1.1.The

micro structures were investigated using SEM. Electrodes for testing the

electrochemicalbehaviorswerepreparedasdescribed inchapter2.1.1.Galvanostatic

measurementswerecarriedoutinthe1.54.2voltagewindowataconstantcurrentof

100A/kg(approx.C/4).All

eth zurich

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