eth zurich
TRANSCRIPT
-
Research Collection
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
Rights / License: In Copyright - Non-Commercial Use Permitted
This page was generated automatically upon download from the ETH Zurich Research Collection. For moreinformation please consult the Terms of use.
ETH Library
https://doi.org/10.3929/ethz-a-007620352http://rightsstatements.org/page/InC-NC/1.0/https://www.research-collection.ethz.chhttps://www.research-collection.ethz.ch/terms-of-use
-
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.
-
iv
-
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
-
x
-
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
-
xiv
-
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.
-
1.4RelevantDefinitionsandConcepts
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
-
1.4RelevantDefinitionsandConcepts
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.
-
1.4RelevantDefinitionsandConcepts
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
-
1.6TheLithiumIonBattery
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.
-
1.6TheLithiumIonBattery
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
-
1.6TheLithiumIonBattery
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.
-
1.6TheLithiumIonBattery
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
-
1.6TheLithiumIonBattery
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
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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.
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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.
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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.
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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)
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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
-
2VitreousandCrystalizedMaterialsintheLi2OV2O5P2O5System
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