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N dordre 208-2014 Anne 2014
Thse prsente devant
LUNIVERSITE CLAUDE BERNARD LYON 1
ECOLE DOCTORALE DE CHIMIE 206
Spcialit Chimie
soutenue publiquement le 24 Octobre 2014
pour lobtention du
DIPLOME DE DOCTORAT (arrt du 7 aot 2006)
Par
La CHANCELIER
DEVELOPPEMENT DE SOLUTIONS INNOVANTES DELECTROLYTES
POUR SECURISER LES ACCUMULATEURS LITHIUM-ION
Directrice de thse Dr Catherine C. SANTINI CNRS
Co-encadrants Dr Sophie MAILLEY CEA, LITEN Dr Thibaut GUTEL CEA,
LITEN
JURY Pr Giovanni B. APPETECCHI ENEA, Rome, Italie Rapporteur Dr
Corinne LAGROST Universit de Rennes Rapporteur Dr Ccile TESSIER
SAFT, Bordeaux Examinateur Pr Bruno ANDRIOLETTI Universit de Lyon 1
Examinateur Dr Guy MARLAIR INERIS Examinateur
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UNIVERSITE CLAUDE BERNARD - LYON 1
Prsident de lUniversit
Vice-prsident du Conseil dAdministration
Vice-prsident du Conseil des Etudes et de la Vie
Universitaire
Vice-prsident du Conseil Scientifique
Directeur Gnral des Services
M. Franois-Nol GILLY
M. le Professeur Hamda BEN HADID
M. le Professeur Philippe LALLE
M. le Professeur Germain GILLET
M. Alain HELLEU
COMPOSANTES SANTE Facult de Mdecine Lyon Est Claude Bernard
Facult de Mdecine et de Maeutique Lyon Sud Charles Mrieux
Facult dOdontologie
Institut des Sciences Pharmaceutiques et Biologiques
Institut des Sciences et Techniques de la Radaptation
Dpartement de formation et Centre de Recherche en Biologie
Humaine
Directeur: M. le Professeur J. ETIENNE
Directeur: Mme la Professeure C. BURILLON Directeur: M. le
Professeur D. BOURGEOIS
Directeur: Mme la Professeure C. VINCIGUERRA
Directeur: M. le Professeur Y. MATILLON
Directeur: M. le Professeur P. FARGE
COMPOSANTES ET DEPARTEMENTS DE SCIENCES ET TECHNOLOGIE
Facult des Sciences et Technologies Dpartement Biologie
Dpartement Chimie Biochimie Dpartement GEP Dpartement Informatique
Dpartement Mathmatiques Dpartement Mcanique Dpartement Physique
Dpartement Sciences de la Terre
UFR Sciences et Techniques des Activits Physiques et
Sportives
Observatoire des Sciences de lUnivers de Lyon
Polytech Lyon
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Institut Universitaire de Technologie de Lyon 1
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Directeur: M. le Professeur F. DE MARCHI Directeur: M. le
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H. BEN HADID Directeur: Mme S. FLECK Directeur: Mme la Professeure
I. DANIEL
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Directeur: M. G. PIGNAULT
Directeur: M. C. VITON
Directeur: M. A. MOUGNIOTTE
Administrateur provisoire: M. N. LEBOISNE
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Thse prpare au sein de:
Laboratoire des Composants pour Batteries (LCB)
CEA, LITEN
(Laboratoire dInnovation pour les Technologies des Energies
Nouvelles)
17 rue des martyrs 38054 Grenoble
France
et
Laboratoire de Chimie OrganoMtallique de Surface (LCOMS)
C2P2
(Chimie, Catalyse, Polymres et Procds)
UMR 5265 CNRS-Universit Lyon 1-ESCPE
43 Bd du 11 Novembre 1918 69616 Villeurbanne
France
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N'allez pas l o le chemin peut mener ; Allez l o il n'y a pas de
chemin et laissez une trace.
Ralph Waldo Emerson
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Acknowledgments
First I would like to thank Giovanni Appetecchi and Corinne
Lagrost, who accepted to review this work, for their fruitful
questions and remarks. Thank you for helping me to improve this
manuscript. Thanks to Ccile Tessier, Guy Marlair and Bruno
Andrioletti for their interest in this work, their presence to my
defense, and the sincerely interesting discussion. Merci mes
encadrants, grce qui je suis heureuse du travail accompli :
Catherine, merci de mavoir pousse dans les bonnes directions
scientifiques, davoir t lcoute professionnellement et humainement.
Merci pour votre aide, votre patience, votre positivisme, votre
confiance, et votre suivi. Thibaut, merci pour ta comprhension, ta
disponibilit, pour les discussions et conseils. Merci pour ton
soutien et tes encouragements tout au long de la thse. Sophie,
merci pour ton encadrement, la confiance et la libert accordes.
Merci Bernadette Charleux puis Timothy Mckenna pour mavoir
accueillie au sein du laboratoire C2P2. Merci Sbastien Patoux et
Marlne Rey pour leur accueil chaleureux au sein du LMB, LCB et
SCGE. Merci de mavoir permis deffectuer de lenseignement durant ces
trois ans, et merci de mavoir offert lopportunit de participer des
congrs, expriences combien enrichissantes. Merci maintenant tous
les collgues qui sont devenus bien plus :
Tout dabord, ceux avec qui jai commenc mais qui sont partis,
plus ou moins loin: Hassan, merci pour ton aide, ton efficacit et
tes multiples rponses mes multiples questions en synthse. Merci
pour les footings, les discussions, les dlicieux repas et ths
libanais. Merci pour le petit dej au Hilton et les selfies Miami !
Inga, merci davoir tant couru et discut avec moi. Merci pour ton
coute, ta culture, merci de mavoir fait dcouvrir ton pays, le caf
Juliette, et pour ton art de faire de belles soires! Laurent, merci
pour les dfoulements au squash, les dcouvertes de bonnes adresses
et de mavoir tant fait rire. Merci pour tes mails adorables mme de
si loin.
Ensuite, ceux qui ont toujours t l, que jai ador savoir dans les
parages: Philippe, merci pour tes conseils vgtariens, merci dtre
absolument toujours prt aider. Merci pour les ides de balade jamais
ralises et les autres sorties ! Anthony, merci pour les randos, les
conseils sportifs, littraires, cinmatographiques... Merci pour les
rservations de notre table pour les quiz du Bryans ! Walid, Cherif,
merci davoir t toujours dispo pour un coup de main et pour
discuter. Sylwia, ma Sylwia Merci ! Pour nos fous rires, nos
discussions passionnes, notre persverance, nos sorties Thank you
for awesome moments, for example with Hulk and megarich !
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10
Enfin les collgues qui ont t suffisamment l pour que a compte
Lyon:
Fred, merci pour une belle collaboration cruciverbiste, pour tre
parmi ceux qui se bougent au labo et hors labo, ce qui a cr de
beaux souvenirs. Merci. Teresa, merci pour ton soutien les jours
moyens. Merci pour ta bonne humeur, ta ractivit, ton dynamisme, ton
adorabilit La motivation mutuelle fut splendide, mme en Aot Je ne
serai jamais bien loin ! Thibault, Ewelina, Phillip, Giuliana,
Andreia, Walid, merci pour votre joie de vivre, pour les
discussions plus ou moins srieuses, votre enthousiasme pour sortir,
pour lambiance multiculturelle que jadore Oui, vous mritez chacun
plus que cette phrase, a lot more ! ! ! By the way, special thanks
to our lovely Anoocoms team ! Merci Nico pour les BBQ, David,
Delphine, Fadila, Guilhem, Iuliia, Stphane, Popoff, Henri, Leila
Merci aux stagiaires avec qui jai eu loccasion de travailler :
Piotr, Elodie et Cristina. Pierre et Arthur, merci pour la bonne
ambiance pendant ma priode dans lopen space. Merci Emmanuelle pour
ton aide et ta disponibilit. Mos, Aimery, Emile, Laurent,
Alessandra, et le reste de lquipe, merci tous pour lambiance
richissime.
Et Grenoble: Greg, merci pour tes conseils en salle anhydre,
merci pour les coups de main successifs, merci pour ta bonne
humeur, ton coute, tes blagues, ta sincrit ! Djamel, merci pour ton
aide, indispensable (Arbin, lectrodes). Merci pour ton humour et
pour les sorties au top ! Kim, merci davoir simplifi toutes les
complications administratives, avec le sourire ! Et, aussi, merci
pour lorganisation des vnements non professionnels hyper sympas !
Merci Charles pour ton assistance rgulire (informatique, technique,
hbergement). Merci Isabel (aprs tavoir perdue de vue cest vraiment
gnial de te retrouver pour discuter, nager, boire un verre) ! Merci
Adriana, Melody, Hanaa, JB, Mathieu, Elise (pour les nombreuses
requtes sur mes lectrodes), Cline, Aurlien, Justin, Vincent, Lise,
Graldine (et ta grand-mre). Merci Sandra (linattendu) et Marco
(htel Suisse et Bordeaux) pour avoir agrment de discussions sympas
mes soires Grenoble en solo! Cette thse ma permis de travailler
avec beaucoup de personnes que je remercie vivement, et en
particulier : En XPS, merci Jean-Luc et Laurence pour votre courage
avec mes lectrodes. Merci Anass pour ta patience, ta gentillesse,
ta pdagogie. En ATG, merci Olivier pour la formation, et la
confiance accorde. En DRIFT-GC-MS, merci Kai pour ton aide, ta
disponibilit. En RMN, merci Christine et Frdric. En MS, merci
Philippe et Johann pour laide avec le spectro, malgr les fuites En
GC-IR, merci Isabel. Hamed, merci infiniment pour ton coute, ta
ractivit, ton enthousiasme. Ce fut un rel plaisir de minvestir dans
lenseignement sous ta tutelle. Arnaud, merci pour mavoir forme de A
Z sur les TPs, merci pour ta patience et ton enthousiasme.
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I would like also to thank the great people I met in congresses
around the world, for their participation to my very very very good
time there ! In particular I would like to name Claudiu (ECS), Mei
(COIL and London or Berlin soon and for the proofreading mon chou
!), Marco (IMLB), Edgard (ECS and Miami), Giuseppe (IMLB), Tom
(IMLB), Andreas (XPS in Roscoff), Paul (COIL, Netherlands), Max
(COIL), Jakub (IMLB), Sigelinde (XPS in Roscoff), Carmen (EUCHEM,
COIL) I really hope to keep in touch with you all !
Claire et Mlissa, merci davoir emmnag Lyon, merci pour les
sessions piscine, les longues discussions, profondes ou lgres, et
les conseils, ponctus de fous rires. Sur la vie, les mecs, les
vernis, la piscine, les cheveux, la vie quoi Merci Margaux, parce
que je sais que je peux compter sur toi depuis toujours ! Merci
Camille et Kelly retrouves ! Et merci Ebru Parce que 10 ans damiti,
cest beau, cest bon, cest drle, cest fort, cest grand, cest
magique, cest magnifique ! ! ! Merci Rom, Alex, Denis, Seb, Fred,
Tomtom, Marina, Simon Merci dtre toujours motivs pour que lon reste
en contact. Merci pour les sorties (ski, soutenances 2014 ne
perdons pas le rythme !) et les discussions ! Merci Kevin, pour ton
soutien, tes blagues, tes rflexions et tes textos philosophiques
mrement rflchis, et merci pour la totale disponibilit de ton
luxueux appart. Merci Juliette, Benji, Ben, Bastien, Brice, lautre
Brice, Camille, Sami, pour des soires et des vacances au top !
Pierre, merci de mavoir soutenue, voire pousse. Merci pour le succs
international des animations powerpoint. Un immense merci pour tant
dautres choses. Merci la famille ! Merci au soutien de tous les
oncles, tantes, cousines, cousins. De prs, de loin, cest tellement
important de vous savoir mes cts ! Merci mes Mamies pour leur
force, leur gnrosit, leur coute. Entre autres qualits. Maman, Papa,
merci pour votre coute inlassable, votre soutien
incommensurablement indfectible, votre confiance inbranlable. Bro,
merci pour tout. On parle moins mais ce nen est pas moins fort
Merci toutes les personnes qui sont venues assister ma soutenance,
et celles qui mont envoy leurs encouragements. Merci tous ceux et
toutes celles que je ne citerai pas mais avec qui jai eu plaisir
passer ou repasser du temps, avec qui jai pu discuter, rire,
danser, pleurer, courir, dlirer, jouer, vivre... Bref, merci toutes
les personnes qui ont contribu crer de beaux moments. Merci toutes
les personnes qui mont aide me construire jusquici, grce qui je me
sens heureuse et panouie, et merci par avance tous ceux et celles
qui feront que a dure !
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Table of contents
13
Abstracts
Summary in french
Introduction
Chapter 1 State of the art General context Lithium-ion batteries
Strategy of the work References
Chapter 2 Decomposition temperatures of ionic liquids
Introduction General synthesis procedure Experimental parameters
for TGA Gathering of decomposition temperatures Stability trends
Conclusion Experimental part References
Chapter 3 Thermal stability Introduction Decomposition
temperatures Isothermal experiments Maximum operating temperatures
Effect of lithium salt concentration Effect of atmosphere Thermal
treatment of IL-based electrolytes Combustion behaviour Thermal
stability of electrodes Conclusion Experimental part References
Chapter 4 Electrochemical stability Introduction Electrochemical
windows Cycling tests in coin cells Lithium diffusion coefficients
Lithium insertion Cycling tests in pouch cells Overcharge behaviour
Conclusion Experimental part References
Conclusion and outlooks
Appendixes
15
19
31
37 40 41 55 57
65 68 69 70 75 78 91 92 94
101 104 104 106 110 112 115 117 124 128 132 133 135
139 142 143 145 148 151 153 155 166 167 171
175
183
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Dveloppement de solutions innovantes dlectrolytes pour scuriser
les accumulateurs lithium-ion
Mots-clefs batterie lithium-ion; lectrolyte; scurit; liquide
ionique;
stabilits thermique et lectrochimique; combustion; surcharge;
cyclage
Rsum
Les batteries lithium-ion dominent le march des appareils
nomades et celui des vhicules
lectriques. Nanmoins elles posent des problmes de scurit lis
leur lectrolyte,
contenant des carbonates inflammables et volatils. Pour scuriser
ces systmes, les liquides
ioniques (LI) sont tudis comme lectrolytes alternatifs. Ce sont
des sels liquides
temprature ambiante, rputs stables thermiquement et non
inflammables. Ce caractre
scuritaire des LI, souvent avanc, est pourtant peu tay par des
expriences probantes. Les
travaux de cette thse visent comprendre le comportement de ces
LI en situations abusives,
telles quun chauffement de la batterie, un feu ou une surcharge.
Les tempratures de
dcomposition de LI contenant les cations imidazolium ou
pyrrolidinium diffremment
substitus et lanion bis(trifluoromethanesulfonyl)imide ont t
dtermines par analyse
thermogravimtrique (ATG). Une analyse critique des donnes (de la
littrature et de nos
mesures) a permis de dfinir une procdure optimise, pour obtenir
des rsultats
reproductibles et comparables. Des lectrolytes constitus de
mlanges de carbonates ou de LI
et de sels de lithium ont t analyss par ATG dynamique et
isotherme, et leurs produits de
dcomposition ont t identifis. Leur comportement au feu a t test
par la mesure des
chaleurs de combustion, des dlais dinflammation et
lidentification des gaz gnrs. Des
tests de cyclage lectrochimique ont t mens avec ces mmes
lectrolytes dans des systmes
lithium-ion constitus des lectrodes Li4Ti5O12 et
LiNi1/3Mn1/3Co1/3O2. Lvolution des
lectrolytes et des surfaces des lectrodes en situation de
surcharge a t examine.
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Development of innovative electrolytes for safer lithium-ion
batteries
Keywords lithium-ion; battery; electrolyte; safety; ionic
liquid;
thermal and electrochemical stabilities; combustion; overcharge;
cycling
Abstract
Lithium-ion batteries are dominating both the nomad device and
electric vehicle markets.
However they raise safety concerns related to their electrolyte,
which consists of flammable
and volatile carbonate mixtures and toxic salts. The replacement
of the latter by ionic liquids
(IL), liquid salts claimed to be thermally stable and
non-flammable, could provide a safer
alternative. Yet this often claimed feature has been poorly
examined by experiments. The
work of this thesis investigates IL behaviour under abuse
conditions such as overheating, fire
or overcharge. Decomposition temperatures of IL based on
differently substituted
imidazolium or pyrrolidinium cations and the
bis(trifluoromethanesulfonyl)imide anion were
determined by thermogravimetric analysis (TGA). A critical study
of gathered data (from
literature and our work) led to the determination of an
optimised procedure to obtain
reproducible and comparable results. Electrolytes based on
carbonates mixtures or IL and
containing lithium salt were studied by dynamic and isothermal
TGA, and their
decomposition products were identified. Their combustion
behaviour was also tested by
measuring heats of combustion and ignition delays. Emitted gases
were analysed and
quantified. Electrochemical cycling tests were carried out with
these electrolytes in
lithium-ion systems based on Li4Ti5O12 and LiNi1/3Mn1/3Co1/3O2
electrodes. The evolution of
the electrolytes and electrodes surface was also examined under
overcharge.
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18
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RSUM SUBSTANTIEL
EN FRANAIS
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French summary 20
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French summary 21
1. Introduction gnrale
Le stockage de lnergie est au cur des enjeux de notre socit,
notamment avec lessor des
nergies renouvelables et des vhicules lectriques. Du fait de
leurs performances, les
batteries de technologie lithium-ion sont actuellement les plus
utilises, notamment pour les
appareils nomades (63% du march mondial). Si leur dangerosit
reste limite pour des
appareils de petite taille, elles peuvent poser des problmes de
scurit pour des applications
telles que les vhicules lectriques. Ces accumulateurs doivent en
effet pouvoir rsister des
situations de surchauffe, surcharge, surdcharge, court-circuit
ou choc.
Les batteries lithium-ion stockent de llectricit par
insertions-dsinsertions successives des
ions lithium dans chaque matriau dlectrode. Llectrode positive
est gnralement un
oxyde mtallique base de mtaux de transition tels que le cobalt,
le fer, le nickel ou le
manganse. Llectrode ngative est constitue de graphite. Ces deux
lectrodes sont spares
par un isolant lectronique, imbib dune solution conductrice
ionique, appele lectrolyte.
Pendant lutilisation (dcharge), les ions Li+ sinsrent dans
llectrode positive, gnrant un
flux dlectrons dans le circuit extrieur, qui alimente lappareil
connect, Figure 1.[1] Lors de
la recharge, un courant est impos pour forcer la migration des
ions Li+ vers llectrode
ngative. Lalternance de charges et dcharges est appele cyclage
lectrochimique.
Figure 1: De gauche droite, Schma dune batterie en dcharge,
carbonate dthylne (EC),
carbonate de dithyle (DEC) et hexafluorophosphate de lithium
(LiPF6)
Llectrolyte est constitu de mlanges de carbonates, Figure 1, qui
solubilisent bien le sel de
lithium et fournissent de bonnes performances lectrochimiques.
Dans notre cas llectrolyte
utilis, not [EC:DEC][LiPF6], est un mlange qui-volumique de EC
et DEC contenant
1 mol.L-1 de LiPF6. Cependant ces liquides volatils et
inflammables peuvent mener des
problmes de scurit (incendie, explosion). De plus le sel LiPF6
mne la formation de
composs toxiques comme lacide fluorhydrique (HF). Pour les
remplacer, certains sels
fondus appels liquides ioniques (LI) (sels frquemment liquides
temprature ambiante)
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French summary 22
sont des candidats potentiels, pouvant prsenter de bonnes
performances.[2-5] Ils sont
composs dun cation souvent issu dune amine et dun anion
gnralement fluor, et
prsentent une bonne conductivit ionique. Les LI sont liquides
sur une large gamme de
temprature,[6] dont la limite suprieure est leur temprature de
dcomposition (et non leur
bullition) gnralement assez leve. De plus, ils possdent une
pression de vapeur saturante
ngligeable, ce qui leur confre une faible inflammabilit[7] et
les rend plus scuritaires.
Cet aspect de suret des LI est un argument souvent avanc,[8]
mais peu tay par des
expriences probantes. Les travaux mens dans le cadre de cette
thse visent comprendre le
comportement des LI lorsquils sont soumis des conditions dites
abusives, telles quun
chauffement de la cellule, un feu, une surcharge etc.
2. Stabilit thermique des lectrolytes
Parmi les plus utiliss, les cations imidazolium et pyrrolidinium
combins lanion fluor
bis(trifluoromethanesulfonyl)imide [NTf2] ont t slectionns,
Figure 2. Les lectrolytes
correspondants, contenant 1 mol.L-1 de sel de lithium LiNTf2,
seront nots [cation][Li][NTf2].
Ces LI, dont la synthse et la purification sont maitrises,
prsentent une haute stabilit
thermique et des proprits physicochimiques adaptes leur
utilisation en batteries
(viscosit, conductivit).
Figure 2: de gauche droite, les cations
1-butyl-3-methylimidazolium [C1C4Im],
N-butyl-N-methylpyrrolidinium [PYR14], et lanion [NTf2]
La dtermination de la temprature de dcomposition (Td) par
Analyse ThermoGravimtrique
(ATG) est couramment utilise pour dfinir la stabilit thermique
des LI. Il sagit de suivre la
dcomposition de lchantillon (rvle par une perte de masse)
pendant une monte en
temprature dans des conditions contrles (atmosphre, rampe de
chauffe). Suivant les
paramtres exprimentaux utiliss, les valeurs de Td pour un mme
produit varient de plus de
100 C. Une analyse critique des donnes de la littrature et de
nos rsultats nous a mens
dfinir une procdure optimise, permettant dobtenir des rsultats
reproductibles et
comparables. Ces lectrolytes ont des tempratures de dcomposition
suprieures de plus de
200 C celle des carbonates, Figure 3.
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French summary 23
Figure 3: Profils de stabilit thermique tablis par ATG entre 30
et 500 C pour
[C1C4Im][Li][NTf2] (Td: 357 C), [PYR14][Li][NTf2] (Td: 339 C),
[C1C1C4Im][Li][NTf2] (Td: 339 C) and [EC:DEC][LiPF6] (Td: 50 C).
Echantillons de 10 mg; vitesse de chauffe de 5 C.min-1 sous
argon;
creusets en aluminium scells
Nanmoins lATG ne permet pas didentifier les produits de
dcomposition. Les deux
lectrolytes [C1C4Im][Li][NTf2] et [PYR14][Li][NTf2] ont t traits
sous vide deux heures
350 C et analyss. Pour les deux solutions, des hydrocarbures
gazeux inflammables
(typiquement des butnes) issus de llimination des chaines
alkyles cationiques ont t
identifis par spectromtrie de masse, rsonance magntique nuclaire
et chromatographie en
phase gazeuse, Figure 4. La dcomposition de lanion, contenant du
fluor et du soufre, a men
la formation despces toxiques telles que de lacide fluorhydrique
et le dioxyde de soufre.[9]
Figure 4: Analyse par chromatographie en phase gaz des
constituants de la phase gaz issue de la
dcomposition thermique des lectrolytes (temps de rtention des
butnes: 5.85 min)
100 200 300 400 5000
20
40
60
80
100
Mas
se (%
)
Temprature (C)
[C1C
4Im][Li][NTf
2]
[PYR14
][Li][NTf2]
[C1C
1C
4Im][Li][NTf
2]
[EC:DEC][LiPF6]
Td
< 100 C > 330 C
0 1 2 3 4 5 6 7 8 9 10
2,147,64
1,31
Inte
nsit
Temps de rtention (min)
[C1C4Im][Li][NTf2] [PYR14][Li][NTf2]
1,77
3,70
5,70
5,85
6,01
7,48
7,96
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French summary 24
3. Comportement en combustion des lectrolytes
Le comportement au feu de ces lectrolytes a t galement test. Les
chaleurs de combustion
et les dlais dinflammation de chaque lectrolyte ont t dtermins
par calorimtrie incendie
(norme ISO:12136), confirmant que ces LI ont une faible
inflammabilit, en particulier celui
bas sur limidazolium. Le dlai dinflammation est denviron cinq
minutes pour les
lectrolytes bass sur les LI, alors quil est de trente secondes
pour les carbonates, Tableau 2.
Une fois leur combustion amorce, les lectrolytes base de LI
dgagent presque deux fois
moins de chaleur (~ 8 vs 14 MJ.kg-1) que les carbonates. A titre
de comparaison, la chaleur de
combustion du bois est de 15 MJ.kg-1.
[C1C4Im][Li][NTf2] [PYR14][Li][NTf2] Carbonates[10]
Dlai dinflammation (min) 5 5.5 0.5 Chaleur de combustion
(MJ.kg-1) 7.7 8.2 14
Tableau 1: Comportement au feu des diffrents lectrolytes Afin
dtablir la toxicit en cas de feu, les produits asphyxiants (HCN,
CO) ou irritants (NOx,
SO2, et HF) mis lors de la combustion ont t quantifis. Les
espces mises sont les mme
pour les lectrolytes [C1C4Im][Li][NTf2] et [PYR14][Li][NTf2],
avec une mission de gaz
toxiques provenant de la dcomposition de lanion NTf2. La
dcomposition des cations
produit des espces inflammables.
Facteurs dmission
(mg.g-1) [C1C4Im][Li][NTf2] [PYR14][Li][NTf2]
CO2 552 531 CO 21.6 25.1
Suies 11 37.7 Hydrocarbures 4.8 0.6
SO2 353 317 NO 4.9 3.0 HF 294.8 216.6
HCN 6.9 8.3 Tableau 2: Facteurs dmission de diffrents gaz forms
pendant la combustion des lectrolytes
[C1C4Im][Li][NTf2] et [PYR14][Li][NTf2]
-
French summary 25
4. Caractrisations electrochimiques Les lectrolytes
[EC:DEC][LiPF6], [C1C4Im][Li][NTf2], [PYR14][Li][NTf2] et
[C1C1C4Im][Li][NTf2] ont t utiliss en tant qulectrolytes dans
des batteries (de formats
pile bouton et sachet souple) avec Li4Ti5O12 (LTO) et
LiNi1/3Co1/3Mn1/3O2 (NMC) en tant que
matriaux dlectrodes ngative et positive. Les cyclages
galvanostatiques ont t effectus
25 et 60 C, un rgime de C/10 (charges et dcharges en 10 h),
Figure 5.
Figure 5: Performances en cyclage des quatre lectrolytes 25 C
(gauche) et 60 C (droite);
5 cycles C/20 suivis de 95 cycles C/10 entre 1 et 3.5 V A 25 C,
les cellules dmontrent un cyclage rversible et trs stable.
Llectrolyte
[EC:DEC][LiPF6] donne la plus haute capacit (181 mAh.g-1)
compare celle de
[C1C4Im][Li][NTf2] (110 mAh.g-1) et de [PYR14][Li][NTf2] (33
mAh.g-1).
[C1C1C4Im][Li][NTf2] na pas permis de cycler les cellules. A 60
C, les capacits initiales
sont plus leves qu 25 C, probablement grce une diminution de la
viscosit. Cependant
les performances chutent de faon continue pour chaque
lectrolyte. En particulier aprs
environ 60 cycles, une diminution brutale est observe pour les
lectrolytes
[C1C4Im][Li][NTf2] et [EC:DEC][LiPF6].
Les performances des cellules contenant des liquides ioniques
sont infrieures celles des
carbonates pour les deux tempratures et peuvent tre attribues la
viscosit plus leve des
liquides ioniques. Pour vrifier ceci, les cintiques de diffusion
des ions lithium au sein des
lectrodes et de llectrolyte ont t tudies par voltammtrie
cyclique et par RMN en phase
liquide. La diffusion du lithium sest avre dix fois plus rapide
au sein des carbonates que
dans les LI, Tableau 3. Pour tous les lectrolytes, la diffusion
dans la phase liquide sest
rvle 10 000 fois plus rapide que dans llectrode.
0 20 40 60 80 1000
50
100
150
200
[EC:DEC][LiPF6]
[C1C
4Im][Li][NTf
2]
[PYR14
][Li][NTf2]
[C1C
1C
4Im][Li][NTf
2]
Cap
acit
dc
harg
e (m
Ah.
g-1 )
Numro de cycle
0 20 40 60 80 1000
50
100
150
200
Cap
acit
dc
harg
e (m
Ah.
g-1 )
Numro de cycle
-
French summary 26
[EC:DEC][LiPF6] [C1C4Im][Li][NTf2] [C1C1C4Im][Li][NTf2]
[PYR14][Li][NTf2]
DLi par VC 3.53.10-10 2.68.10-11 1.40.10-11 1.60.10-12 DLi par
RMN 3.08.10-6 1.69.10-7 2.24.10-7 1.44.10-7
Tableau 3: Coefficients de diffusion de Li+ (cm.s-1) dtermins 60
C, dans les lectrodes par voltammtrie cyclique (VC) et en solution
par RMN du 7Li
Cependant, ces rsultats montrant de meilleures performances des
carbonates 60 C
diffrent de la littrature. Gnralement les liquides ioniques sont
plus performants que les
carbonates haute temprature, ces derniers ntant pas stables
thermiquement.[9, 11-13] Dautre
part, [PYR14][Li][NTf2] sest rvl moins performant que
[C1C4Im][Li][NTf2] alors quil
fournit de plus hautes capacits en demi-piles Li //
graphite.[14] Enfin, llectrolyte
[C1C1C4Im][Li][NTf2] na pas permis de cycler les batteries bases
sur LTO // NMC (capacit
de dcharge nulle aprs 10 cycles) alors quil donne de meilleures
performances dans le cas
de batteries graphite // LiFePO4.[15] Ces rsultats laissent
penser que llectrode positive NMC
joue un rle dstabilisant. Pour vrifier cette hypothse des tests
ont t mens par
voltammtrie cyclique (VC) pour analyser les mcanismes dinsertion
du lithium dans les
lectrodes.
Figure 6: Comparaison des seconds cycles par VC des quatre
lectrolytes 60C
dans des cellules LTO // NMC Les processus dinsertion et de
dsinsertion du lithium sont observs pour tous les lectrolytes
tudis, Figure 6. Les intensits de courant sont plus faibles dans
le cas des liquides ioniques
que pour [EC:DEC][LiPF6], donnant lieu un aplatissement des
pics. Le dcalage entre le
potentiel dinsertion et de dsinsertion est plus important dans
le cas des LI (~ 0.45 V contre
0.14 V). De plus, les pics sont largis dans le cas des LI, ce
qui permet de visualiser plusieurs
processus doxydation et de rduction des mtaux de llectrode
positive. Daprs la
1,0 1,5 2,0 2,5 3,0-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
Den
sit
de
cour
ant (
mA
.cm
-2)
Tension (V)
[EC:DEC][LiPF6]
[C1C
4Im][Li][NTf
2]
[C1C
1C
4Im][Li][NTf
2]
[PYR14
][Li][NTf2]
-
French summary 27
littrature le manganse ne joue pas de rle dans ces procds et les
ractions redox se font
dans lordre Ni2+/Ni3+, Ni3+/Ni4+ puis Co3+/Co4+.[16, 17] Il est
possible que la prsence dions
mtalliques au degr doxydation (+II) mne la formation dagrgats de
type [M(NTf2)n]x-
(M = Ni, Co ou Mn), qui favoriserait la dissolution de la matire
active et donc diminuerait la
capacit disponible lors du cyclage.[18]
5. Analyses en surcharge Des cellules en sachets souples LTO //
NMC de plus grande capacit (~10 mAh) ont t
assembles avec les lectrolytes [EC:DEC][LiPF6],
[C1C4Im][Li][NTf2] et [PYR14][Li][NTf2]
afin didentifier les espces volatiles formes lors de la
dcomposition des lectrolytes
pendant une surcharge. Une charge jusqu 4.5 V a t applique (6 V
vs Li+/Li) un rgime
de charge de C/10, et cette tension a t maintenue pendant 20 h
60 C. Pendant la phase
tension constante, le courant utilis pour maintenir cette
tension tait plus important dans le
cas de [C1C4Im][Li][NTf2] (0.4 vs 0.1 mA, Tableau 4), rvlant une
moins bonne stabilit de
cet lectrolyte dans ces conditions. Cela a t confirm par les
mesures des volumes des
cellules, qui ont augment de 2.81 mL pour [C1C4Im][Li][NTf2], de
0.30 mL pour
[EC:DEC][LiPF6], et de 0.12 mL pour [PYR14][Li][NTf2].
Courant de fuite (mA)
Augmentation de volume (mL)
[EC:DEC][LiPF6] 0.13 0.30 [C1C4Im][Li][NTf2] 0.40 2.81
[PYR14][Li][NTf2] 0.05 0.12
Tableau 4: Courants de fuite et augmentations de volume des
cellules la fin de la surcharge Pour comprendre cette instabilit du
liquide ionique contenant le cation imidazolium, les gaz
gnrs dans le sachet souple ont t analyss par chromatographie en
phase gaz couple avec
un spectromtre infrarouge (GC-IR). Les gaz forms taient
notamment du dioxyde de
carbone, des fragments issus de lanion et des alcanes issus du
cation, Figure 7. Ces produits
sont donc diffrents de ceux observs lors de la dcomposition
thermique de cet lectrolyte.
-
French summary 28
Figure 7: Chromatogramme des gaz gnrs dans la cellule contenant
llectrolyte
[C1C4Im][Li][NTf2] aprs une surcharge 60 C Des analyses de
diffraction des rayons X et microscopie lectronique ont t ralises
sur les
lectrodes, aprs leur utilisation en cellules surcharges. La
morphologie des deux lectrodes
na pas t modifie, et la structure de LTO est stable. Pour la
NMC, la structure
cristallographique a volu vers une phase dlithie, Figure 8.
Figure 8: Diffractogrammes (DRX) des lectrodes ngatives (LTO,
gauche) et positive (NMC, droite)
avant utilisation et aprs surcharge; #: dome en polymre utilis
pour garder lchantillon sous atmosphere inerte; *: collecteur de
courant en aluminium
Une tude dtaille des lectrodes aprs surcharge par Spectroscopie
des Photolectrons X
(XPS) a dmontr que les surfaces des lectrodes taient masques.
Elles taient recouvertes
de liquide ionique dans le cas de la positive, et de ses
produits de dcomposition dans le cas
de llectrode ngative.
10 20 30 40 50 60 70
u.a.
2
(111
)
#
(220
)
(311
)
(400
)
(222
)
(331
)
(333
)
(440
)
*
(531
)
LTO originel (bas) LTO aprs surcharge (haut)
10 20 30 40 50 60 70
#
u.a.
2 ()
*
(003
)
(101
)
(006
)(1
02) (1
04)
(105
)
(107
)
(110
)(1
08) (
113)
NMC originel (bas) NMC aprs surcharge (haut)
-
French summary 29
6. Conclusion
Les lectrolytes forms par dissolution de 1 mol.L-1 de LiNTf2 au
sein de [C1C4Im][NTf2] et
[PYR14][NTf2] prsentent une grande stabilit thermique compars
aux carbonates
[EC:DEC][LiPF6], avec des tempratures de dcomposition suprieures
300 C. Les
produits drivs de limidazolium sont plus stables thermiquement
que les pyrrolidinium. Ces
deux LI sont des espces trs peu combustibles, avec des dlais
dinflammation suprieurs
cinq minutes (augments dun facteur 10 par rapport aux
lectrolytes classiques). La
formation de gaz toxiques ou inflammables lors de la combustion
est nanmoins prendre en
compte selon les applications vises.
En ce qui concerne les performances lectrochimiques,
lutilisation de carbonates conduit de
meilleures capacits, y compris 60 C. La meilleure stabilit
lectrochimique dans les
batteries LTO // NMC est observe pour [EC:DEC][LiPF6], suivi de
[PYR14][Li][NTf2] puis
[C1C4Im][Li][NTf2], ce qui est un ordre inverse celui de la
stabilit thermique. En situation
de surcharge, llectrolyte [C1C4Im][Li][NTf2] sest rvl le moins
stable, gnrant 10 fois
plus de gaz que [EC:DEC][LiPF6] et [PYR14][Li][NTf2].
-
French summary 30
7. Rfrences
[1] J. M. Tarascon, "Vers des accumulateurs plus performants",
L'actualit Chimique, 2002, 3(251), p130 [2] A. Lewandowski and A.
Swiderska-Mocek, "Ionic liquids as electrolytes for Li-ion
batteries-An
overview of electrochemical studies", J. Power Sources, 2009,
194(2), p601 [3] M. Armand, F. Endres, D. R. MacFarlane, H. Ohno
and B. Scrosati, "Ionic-liquid materials for the
electrochemical challenges of the future", Nat. Mater., 2009,
8(8), p621 [4] D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M.
Pringle, P. C. Howlett, G. D. Elliott, J. H. Davis,
M. Watanabe, P. Simon and C. A. Angell, "Energy applications of
ionic liquids", Energy Environ. Sci., 2014, 7(1), p232
[5] A. Balducci, S. S. Jeong, G. T. Kim, S. Passerini, M.
Winter, M. Schmuck, G. B. Appetecchi, R. Marcilla, D. Mecerreyes,
V. Barsukov, V. Khomenko, I. Cantero, I. De Meatza, M. Holzapfel
and N. Tran, "Development of safe, green and high performance ionic
liquids-based batteries (ILLIBATT project)", J. Power Sources,
2011, 196(22), p9719
[6] H. Ohno, Electrochemical aspects of ionic liquid, 2nd ed.,
Wiley, N. Y., 2011 [7] A. O. Diallo, C. Len, A. B. Morgan and G.
Marlair, "Revisiting physico-chemical hazards of ionic
liquids", Sep. Purif. Technol., 2012, 97, p228 [8] M. Galinski,
A. Lewandowski and I. Stepniak, "Ionic liquids as electrolytes",
Electrochim. Acta, 2006,
51(26), p5567 [9] L. Chancelier, A. O. Diallo, C. C. Santini, G.
Marlair, T. Gutel, S. Mailley and C. Len, "Targeting
adequate thermal stability and fire safety in selecting ionic
liquid-based electrolytes for energy storage", Phys. Chem. Chem.
Phys., 2014, 16(5), p1967
[10] G. G. Eshetu, S. Grugeon, S. Laruelle, S. Boyanov, A.
Lecocq, J. P. Bertrand and G. Marlair, "In-depth safety-focused
analysis of solvents used in electrolytes for large scale lithium
ion batteries", Phys. Chem. Chem. Phys., 2013, 15(23), p9145
[11] S. Menne, R. S. Khnel and A. Balducci, "The influence of
the electrochemical and thermal stability of mixtures of ionic
liquid and organic carbonate on the performance of high power
lithium-ion batteries", Electrochim. Acta, 2013, 90, p641
[12] J. Li, S. Jeong, R. Kloepsch, M. Winter and S. Passerini,
"Improved electrochemical performance of LiMO2 (M=Mn, Ni,
Co)-Li2MnO3 cathode materials in ionic liquid-based electrolyte",
J. Power Sources, 2013, 239(0), p490
[13] N. Wongittharom, T.-C. Lee, C.-H. Hsu, G. Ting-Kuo Fey,
K.-P. Huang and J.-K. Chang, "Electrochemical performance of
rechargeable Li/LiFePO4 cells with ionic liquid electrolyte:
Effects of Li salt at 25 C and 50 C", J. Power Sources, 2013, 240,
p676
[14] A. Guerfi, S. Duchesne, Y. Kobayashi, A. Vijh and K.
Zaghib, "LiFePO4 and graphite electrodes with ionic liquids based
on bis(fluorosulfonyl)imide (FSI)(-) for Li-ion batteries", J.
Power Sources, 2008, 175(2), p866
[15] H. Srour, "Dveloppement d'un lectrolyte base de liquide
ionique pour accumulateur au Lithium", Thesis from Universit Claude
Bernard Lyon 1 (Lyon), 2013
[16] K. M. Shaju, G. V. S. Rao and B. V. R. Chowdari,
"Performance of layered Li(Ni1/3Co1/3Mn1/3)O-2 as cathode for
Li-ion batteries", Electrochim. Acta, 2002, 48(2), p145
[17] A. Deb, U. Bergmann, S. P. Cramer and E. J. Cairns, "In
situ x-ray absorption spectroscopic study of the
Li[Ni1/3Co1/3Mn1/3]O2 cathode material", J. Appl. Phys., 2005,
97(11), p113523
[18] H. Zheng, Q. Sun, G. Liu, X. Song and V. S. Battaglia,
"Correlation between dissolution behavior and electrochemical
cycling performance for LiNi1/3Co1/3Mn1/3O2-based cells", J. Power
Sources, 2012, 207(0), p134
-
GENERAL
INTRODUCTION
-
General introduction 32
-
General introduction 33
Both the nomad device and electric vehicle markets are growing,
requiring a fast development
of energy storage systems (ESS). These systems must be capable
of addressing a number of
technical challenges for the sustainable development of
electromobility (terrestrial, maritime
and aerial to some extent). They should efficiently store
intermittent renewable sources of
energy (wind, solar, water) and be part of smart grids
applications.[1] ESS previously had to
rely on lead-acid, nickel cadmium and nickel metal hydride
technologies. Commercialized
since the 90s for the consumer market, lithium-ion (Li-ion)
technology and its variants have
taken the lead regarding those emerging applications, as they
offer significantly higher power
and energy densities (up to 2 000 W.kg-1 and 120
Wh.kg-1).[2]
However safety has been identified in a number of studies as a
potential market restraint.[3, 4]
In the aim of improving the safety of these systems, all
components of the cell have to be
considered. Major improvements were already made in the domains
of battery management
systems, cell designs and separators. The electrodes were also
developed in order to provide
safer batteries, as for the negative one, metallic lithium can
be replaced by graphite (Cgr) or a
metallic oxide such as Li4Ti5O12 (LTO). These electrodes could
reduce the risk of shortcircuit
due to the growth of lithium dendrites on the surface.[5, 6] In
the case of the positive electrode
material, the state of the art Li-ion batteries use mostly
LiCoO2 (LCO), but this oxide has a
low thermal stability. Mixed metallic oxides were widely
examined with varying
compositions of cobalt, manganese, nickel and aluminium metals.
In particular
LiNi1/3Mn1/3Co1/3O2 (NMC) showed good cycling stability and rate
capability, higher thermal
stability in the charged state, lower cost and reduced
toxicity.[7, 8]
The electrolyte is commonly a mixture of organic solvents such
as propylene carbonate (PC),
ethylene carbonate (EC), dimethyl carbonate (DMC) or diethyl
carbonate (DEC), containing a
dissolved inorganic lithium salt, typically lithium
hexafluorophosphate (LiPF6). These
solvents are very flammable, possess low flash points and are
highly volatile.[9] Their
decomposition under moderate thermal stress can lead fast to
thermal runaway of the cell,
which can cause the electrolyte ignition or even explosion.
-
General introduction 34
In order to solve these safety issues, other types of
electrolytes are studied, in particular ionic
liquids (IL), widely considered both in literature[10-12] and in
industrial projects.[13-15] IL are
defined as salts liquid below 100 C and result from the
association of an organic cation with
an organic or inorganic anion. Consequently, they can be
designed for a chosen application by
tuning the nature of the anion or/and of the cation. IL exhibit
negligible vapour pressure,
similar to solid salts, and unlike most organic solvents, they
do not vaporize unless heated to
the point of thermal decomposition, typically 200 C to 300
C.[16] They have flash points
higher than 200 C[17] and are considered as non-flammable.[18,
19]
Contrarily to the current carbonate-based electrolytes,[20]
thermal stability of IL for use as
electrolytes in case of abuse conditions (fire, shortcircuit,
impact, overcharge or
overdischarge) has been poorly examined by experiments.[19] In
the vast domain of IL,
imidazolium and pyrrolidinium cations associated to [NTf2] anion
were selected for their high
decomposition temperatures.[21, 22]
The aim of this thesis is to investigate thermal stability up to
combustion and electrochemical
behaviour up to overcharge of these IL and their corresponding
electrolytes (defined as
solution of lithium salt in IL) for Li-ion cells. The possible
routes of degradation of IL and
electrolytes during thermal and electrochemical abuse tests will
be investigated under
different experimental conditions. In the first chapter, the
state of the art of lithium-ion
batteries will be described. The choice of positive and negative
electrodes and electrolytes
will be discussed and the objectives of the work will be
described.
In the second chapter, the thermal stability of selected IL will
be evaluated. A critical study of
gathered data (from literature and our work) will lead to the
determination of an optimised
procedure to obtain reproducible and comparable results. The
stability of imidazolium IL
associated to bis(trifluoromethanesulfonyl)imide anion NTf2 will
be investigated according to
several modifications of the alkyl chains.
-
General introduction 35
The third chapter will focus on the thermal stability of the
cell components by different
techniques. Decomposition temperatures will be determined by TGA
technique. Combustion
behaviours will be investigated by measuring heats of
combustion, ignition delays and
analysing emitted gases.
The fourth chapter will be devoted to the study of full Li-ion
cells constituted of Li4Ti5O12
and LiNi1/3Mn1/3Co1/3O2 electrodes using electrochemical
techniques. The stability of the
systems will be studied under cycling and overcharge. The
evolution of the system will be
analysed by volume measurements and surface techniques such as
SEM, XRD or XPS.
Finally conclusions of this work will be presented, and some
perspectives will be given for
future work.
-
General introduction 36
References
[1] C. J. Barnhart and S. M. Benson, "On the importance of
reducing the energetic and material demands of electrical energy
storage", Energy Environ. Sci., 2013, 6(4), p1083
[2] "History of battery invention and development",
http://blog.genport.it/?p=133, accessed August 2014 [3] R. S.
Khnel, N. Bckenfeld, S. Passerini, M. Winter and A. Balducci,
"Mixtures of ionic liquid and
organic carbonate as electrolyte with improved safety and
performance for rechargeable lithium batteries", Electrochim. Acta,
2011, 56(11), p4092
[4] "World Hybrid Electric and Electric Vehicle Lithium-ion
Battery Market", 2009, report from in Frost & Sullivan,
http://www.frost.com (accessed September 24, 2013)
[5] X.-W. Zhang, Y. Li, S. A. Khan and P. S. Fedkiw, "Inhibition
of Lithium Dendrites by Fumed Silica-Based Composite Electrolytes",
J. Electrochem. Soc., 2004, 151(8), pA1257
[6] C. Brissot, M. Rosso, J. N. Chazalviel and S. Lascaud,
"Dendritic growth mechanisms in lithium/polymer cells", J. Power
Sources, 1999, 81, p925
[7] A. Deb, U. Bergmann, S. P. Cramer and E. J. Cairns, "In situ
x-ray absorption spectroscopic study of the Li[Ni1/3Co1/3Mn1/3]O2
cathode material", J. Appl. Phys., 2005, 97(11), p113523
[8] H. Zheng, Q. Sun, G. Liu, X. Song and V. S. Battaglia,
"Correlation between dissolution behavior and electrochemical
cycling performance for LiNi1/3Co1/3Mn1/3O2-based cells", J. Power
Sources, 2012, 207(0), p134
[9] G. G. Eshetu, S. Grugeon, S. Laruelle, S. Boyanov, A.
Lecocq, J. P. Bertrand and G. Marlair, "In-depth safety-focused
analysis of solvents used in electrolytes for large scale lithium
ion batteries", Phys. Chem. Chem. Phys., 2013, 15(23), p9145
[10] D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M. Pringle,
P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe, P. Simon
and C. A. Angell, "Energy applications of ionic liquids", Energy
Environ. Sci., 2014, 7(1), p232
[11] B. Scrosati and J. Garche, "Lithium batteries: Status,
prospects and future", J. Power Sources, 2010, 195(9), p2419
[12] M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B.
Scrosati, "Ionic-liquid materials for the electrochemical
challenges of the future", Nat. Mater., 2009, 8(8), p621
[13] Z. Zheng, B. Gu, H. Wang, L. Ke and Y. Nie, "Lithium ion
secondary battery including ionic liquid electrolyte", Microvast
New Materials patent, China, US 2013/0029232 A1, 2013
[14] A. Balducci, S. S. Jeong, G. T. Kim, S. Passerini, M.
Winter, M. Schmuck, G. B. Appetecchi, R. Marcilla, D. Mecerreyes,
V. Barsukov, V. Khomenko, I. Cantero, I. De Meatza, M. Holzapfel
and N. Tran, "Development of safe, green and high performance ionic
liquids-based batteries (ILLIBATT project)", J. Power Sources,
2011, 196(22), p9719
[15] C. Siret, L. Caratero and P. Biensan, "Lithium-ion battery
containing an electrolyte that comprises an ionic liquid", SAFT
patent, France, WO 2009/007540, 2009
[16] P. Wasserscheid and T. Welton, Ionic liquids in synthesis,
2nd Ed., Wiley-VCH, Weinheim, 2008 [17] D. M. Fox, J. W. Gilman, A.
B. Morgan, J. R. Shields, P. H. Maupin, R. E. Lyon, H. C. De Long
and P.
C. Trulove, "Flammability and thermal analysis characterization
of imidazolium-based ionic liquids", Ind. Eng. Chem. Res., 2008,
47(16), p6327
[18] A. O. Diallo, C. Len, A. B. Morgan and G. Marlair,
"Revisiting physico-chemical hazards of ionic liquids", Sep. Purif.
Technol., 2012, 97, p228
[19] C. S. Stefan, D. Lemordant, P. Biensan, C. Siret and B.
Claude-Montigny, "Thermal stability and crystallization of
N-alkyl-N-alkyl'-pyrrolidinium imides", J. Therm. Anal. Calorim.,
2010, 102(2), p685
[20] P. Andersson, P. Blomqvist, A. Lorn and F. Larsson,
"Investigation of fire emissions from Li-ion batteries", 2013,
report from SP Technical Research Institute of Sweden
[21] C. Maton, N. De Vos and C. V. Stevens, "Ionic liquid
thermal stabilities: decomposition mechanisms and analysis tools",
Chem. Soc. Rev., 2013, 42(13), p5963
[22] S. A. Forsyth, S. R. Batten, Q. Dai and D. R. MacFarlane,
"Ionic Liquids Based on Imidazolium and Pyrrolidinium Salts of the
Tricyanomethanide Anion", Aust. J. Chem., 2004, 57(2), p121
-
CHAPTER 1
STATE OF THE ART
-
Chapter 1 38
-
Chapter 1 39
Table of contents
1. General context
.............................................................................................................
40
2. Lithium-ion batteries
.....................................................................................................
41
2.1. Safety issues
..............................................................................................................
43
2.2. Electrolytes
................................................................................................................
45
2.3. Ionic liquids
...............................................................................................................
47
2.4. Electrodes
..................................................................................................................
51
3. Strategy of this work
.....................................................................................................
55
4. References
.....................................................................................................................
57
-
Chapter 1 40
1. General context
Energy storage is a crucial driving force to develop major
markets. The nomad device one is
growing, in particular for smartphones and laptops, requiring a
fast development of energy
storage systems (ESS), Figure 1, left. Indeed lots of
functionalities are developed, calling for
more energy and power. Furthermore, pushed by the global warming
and in order to limit CO2
emissions, the part of renewable energies (related to wind or
sun) in the world is strongly
increasing. But their development is refrained by the need to
store their intermittent produced
energy. ESS have to be further developed, to allow the storage
of these energy sources, and to
be part of smart grids applications, Table 1.[1] In the same
context, electric vehicle market is
also expanding, asking for reliable, safe and long range cars,
Figure 1, right. Different types of
ESS are required for these applications, as electric vehicles
(EV), hybrid electric vehicles
(HEV) and plug-in electric vehicle (PEV) do not have the same
specifications.
Electrochemical energy storage turned out to be very attractive
for this kind of applications, as
they convert chemical energy into electric one with high
efficiencies. They include various
devices such as batteries, supercapacitors, fuel cells combined
with electrolyser etc, batteries
being the most popular among them.
Figure 1: Left: Battery sales worldwide for the phones and
laptops between 2000 and 2011[2]
Right: EV, HEV and P-HEV battery needs estimated between 2005
& 2020[2]
Capacities (GW) 2010 2011 2012
Total renewable power 1250 1355 1470
Solar photovoltaic 40 71 100
Wind power 198 238 283 Table 1: Indicators of renewable energy
development worldwide[3]
-
Chapter 1 41
2. Lithium-ion batteries In this context requiring large
capacities and stationary setups, secondary batteries are the
most relevant, as they can be charged and discharged several
times. Contrarily, primary
batteries mainly based on alkaline cells are used in toys,
remote controls, clocks etc since their
price is low and they provide one-time use.[4] Secondary
batteries previously relied on lead-
acid (Pb), nickel cadmium (NiCd) and nickel metal hydride (NiMH)
technologies. Since
the 90s, lithium-based chemistries were commercialised for the
consumer market, and they
took the lead regarding emerging applications, Figure 2.
Figure 2: Estimation of total lithium-ion transportation battery
revenue by regions, world markets[5]
The enthusiasm shown for this technology is due to the
significantly higher power and energy
densities of Li-ion batteries compared to Ni-MH chemistry
(250-360 Wh.L-1 vs
140-300 Wh.L-1 and 100-160 Wh.kg-1 vs 30-80 Wh.kg-1). Also, the
specific energy density
(in Wh.kg-1) of a lithium-ion cell is three times the one of a
Ni-Cd one, Figure 3.[6, 7] In
addition to this significant increase in energy density, Li-ion
cells present several other
advantages, such as reliability, good cycle life and no memory
effect, i.e. they do not self-
discharge, Table 2. Li-ion batteries also permit wide design
flexibility, they can be designed
for high power or high energy density, and they can be
commercialised in various formats
such as cylindrical, coin, flat, and prismatic.[8]
-
Chapter 1 42
Figure 3: Ragone plot presenting energy and power densities of
different battery technologies[9]
Ni-Cd Ni-MH Li-ion Power capability + + + + Energy density - + +
+ Specific energy o + + +
Cycle life + + o o Calendar life + + ++ +
Price + + + - Self discharge - - + +
Temperature behaviour + + o o Reliability + + + o
Fast charging + + + o Table 2: Comparison of performance
parameters by chemistries for use in power tools;
++: very good, +: good, o: neutral, -: disadvantage[10]
Their development is further favoured by the price decrease,
from 1250 $ per kWh in 2009, to
210 $ per kWh in 2014 and which is expected to fall to 160 $ per
kWh in 2025.[11-13] Still a lot
of research is devoted to increase the energy density delivered
by Li-ion batteries, e.g. in the
context of electric vehicles (EV) for extended autonomy, ideally
achievable with one fast
charge. For these applications, higher power densities are also
sought for initiation,
acceleration and breaking of the vehicle, requiring innovative
chemistries for both the
electrode and electrolyte components.
A lithium-ion (Li-ion) battery is the gathering of several cells
in parallel or series circuits.
Each cell is constituted of three main components which are the
negative electrode, the
positive electrode and the electrolyte. The electrodes are set
apart via a separator, soaked with
the electrolyte. The role of the separator is to insulate
electronically the positive from the
negative electrode, while conducting lithium ions (Li+). During
discharge (when the cell is
used as energy supply), Li+ cations move from the negative
electrode to the positive one,
-
Chapter 1 43
through the electrolyte and the separator, Figure 4. It
generates an electron flow in the external
circuit, from the negative electrode to the positive electrode.
During the charge of the cell, an
external electrical power source forces an electron flow in the
opposite direction. To
compensate these negative charges, the Li+ ions migrate from the
positive electrode to get
intercalated into the porous negative electrode, in three steps.
First, the solvated Li+ cations
migrate through the liquid electrolyte and separator. Then, they
separate from the solvation
shell to penetrate the electrode material, while an electron
from the external circuit balances
the charge by reducing metallic elements of the electrode.
Finally, Li+ can diffuse into the host
electrode.
Figure 4: Schema of lithium-ion cell during discharge (in
use)[14]
2.1. Safety issues
Despite all these advantages, the deployment of this technology
for electric and hybrid cars is
restrained by a number of accidents caused by Li-ion batteries,
Table 3.
Date Device catching fire Place Fire causes February 2014 Tesla
car Canada Unknown
November 2013 Tesla car USA Impact January 2013 Boeing 787
Dreamliner battery Japan Overheated lithium battery
July 2011 EV bus China Overheated LiFePO4 batteries April 2011
EV taxi China 16 Ah LiFePO4 batteries
September 2010 Boeing B747 cargoplace Dubai Overheated lithium
battery March 2010 IPod Nano Japan Overheated lithium battery
January 2010 EV buses China Overheated LiFePO4 batteries July
2009 Cargo plane China Spontaneous combustion June 2008 Laptop
Japan Overheated battery June 2008 Honda HEV Japan Overheated
LiFePO4 batteries
2006 up to now Mobile phone Short-circtuit, overheating Table 3:
Examples of lithium ion battery accidents in the past few years
inspired from reference[15]
-
Chapter 1 44
The rate of charge or discharge and the engineering of the
battery pack also influence its
safety. When one cell undergoes thermal runaway, the adjacent
ones may also heat up and
fail, causing the entire battery to ignite or stop, Figure
5.[15] Thermal management is thus
crucial to limit the propagation of such thermal event, focusing
on cell-to-cell thermal
conduction. To overcome this aggravation, many external safety
mechanisms exist. At the cell level, a pressure vent can reduce the
risk of explosion by lowering the internal pressure, or an
interrupt can stop current to avoid overcharge. Also shut-down
separators are developed to
melt at a critical temperature, isolating the cell from further
damage and preventing from
thermal runaway.[16, 17] At the battery level, Battery
Management System (BMS) can
supervise each cell voltage and temperature, balance the cells,
warn the operator or stop the
battery if required.[18]
Figure 5: Thermal management between battery cells to limit
thermal runaway[18]
In order to enhance the safety of lithium-ion batteries, active
research is carried out on a
variety of approaches, from material design to packaging. In
particular, electrolytes are under
strong development.
-
Chapter 1 45
2.2. Electrolytes
Electrolytes are constituted of a lithium salt dissolved in a
solvent or a mixture of solvents. Its
role is to provide a good conduction of lithium ions in the
potential range used to cycle the
cell. The required properties are electrochemical stability,
embodied by electrochemical
window (EW), thermal stability, represented by decomposition
temperature and flash point,
and ionic conductivity, Figure 6. These properties are desired
as high as possible in the
application temperature range, e.g. in the case of electric
vehicles, from - 40 C to 70 C.
Furthermore electrolytes must be chemically compatible (i.e.
inert) with both negative and
positive electrode materials, in the voltage range of the
cell.
Figure 6: Electrochemical window and conductivities of several
electrolytes families[19]
Electrochemical stability of electrolytes can be measured by
linear sweep voltammetry or
cyclic voltammetry. It reveals the limits in oxidation and
reduction of the solution. Thermal
stability must be considered, in order to ensure the stability
of the system, and can be
represented by decomposition temperature. It is measured by
thermogravimetric analyses,
which shows the decomposition observed by mass loss. The
flashpoint is the temperature at
which the electrolyte forms an ignitable vapour mixture, i.e. it
requires an ignition source. The
-
Chapter 1 46
autoignition temperature can also be considered, representing
the minimum temperature
required to ignite a gas or vapour in air without any ignition
source. Ionic conductivity can be
measured by impedance spectroscopy, and it translates the
capacity of the lithium cations to
move through the solvent. This property is linked to the
viscosity, which influences diffusion
coefficients.[20]
The electrolyte is usually a mixture of carbonates, to which a
lithium salt is added. The
mixture combines low viscosity solvent such as diethylcarbonate
(DEC, < 1 cP)[21] or
dimethyl carbonate (DMC), and a high dielectric constant solvent
such as ethylene carbonate
(EC, > 80)[22] or propylene carbonate (PC), Figure 7.[23, 24]
These non-aqueous electrolytes
generally use lithium salts with non-coordinating anions such as
hexafluorophosphate (PF6),
perchlorate (ClO4), tetrafluoroborate (BF4) or triflate
(CF3SO3). In this work a binary and
equi-volumic mixture of EC and DEC was chosen as a reference,
the electrolyte was formed
by adding 1 mol.L-1 of LiPF6, and this solution will be referred
to as [EC:DEC][LiPF6].
Figure 7: From left to right, dimethyl carbonate (DMC), diethyl
carbonate (DEC),
ethylene carbonate (EC), propylene carbonate (PC) and lithium
hexafluorophosphate (LiPF6)
These solvents present inherent drawbacks rising safety
concerns, associated to their low
thermal stability, little electrochemical stability at low
potentials, and high flammability and
volatility. Thermal runaway is the major malfunction of this
type of battery, occurring when
an exothermic reaction goes out of control. It typically happens
after exposition to high
temperature or after a short-circuit. First the solid
electrolyte interphase (SEI) on the negative
electrode can break from relatively low temperature (~ 70 C),
allowing the electrolyte to
react with the carbon electrode and intercalated lithium,
generating heat. This energy release
accelerates the reaction, which causes further temperature rise.
Gases such as hydrocarbons
from the electrolyte and hydrogen, oxygen, carbon dioxide or
carbon monoxide from the
electrodes can be generated, leading to a pressure increase
inside the cell.[25, 26] When both
pressure and temperature rise too much, it can cause ignition or
even explosion of the cell.
These electrolytes based on carbonates also suffer from low
thermal stability, as in particular
-
Chapter 1 47
their flashpoints are lower than 40 C.[27] The electrolyte can
irreversibly deteriorate from
60 C, and performances are diminished below -20 C, close to
their freezing point.
To improve the safety of the electrolytes, considerable effort
is underway. A wide variety of
additives are designed for specific roles, including flame
retardant, overcharge protector, SEI
or lithium salt stabilizer etc.[21, 28-31] Novel lithium salts
are also synthesised and tested as
alternatives to LiPF6, to reduce toxicity related to HF
formation by hydrolysis.[32-35] Another
approach is the replacement of conventional liquid electrolyte
solvents. In this approach
polymers are intensively studied as they provide solid-state
safer properties with good
conductivities.[36-40] A new category of solvents is also widely
considered and studied, namely
ionic liquids.
2.3. Ionic liquids Ionic liquids (IL) are defined as salts,
liquid below 100 C. They result from the association of
an organic cation, Figure 8, with an organic or inorganic anion,
Figure 9. Consequently, they
can be theoretically designed for a chosen application by tuning
the nature of the anion or/and
of the cation. However this was found to be difficult to predict
properties of novel IL.
Figure 8: Common IL cations, from left to right: imidazolium
[Im], pyrrolidinium [PYR],
piperidinium, ammonium and phosphonium; R represent alkyl
chains
Figure 9: Common IL anions, from left to right:
tetrafluoroborate (BF4),
hexafluorophosphate (PF6), bis(trifluoromethanesulfonyl)imide
(NTf2), dicyanamide (dca) and acetate (OAc)
One of the first reported IL, ethylammonium nitrate,
[N(C2H5)H3][NO3], was synthesised and
reported by Walden in 1914.[41] The first generations of IL were
mainly based on
chloroaluminate anions (AlCl4 or Al2Cl7),[42, 43] affording
water sensitive, toxic and corrosive
-
Chapter 1 48
solutions. Then in the 1990s, an important breakthrough was the
use of water stable anions
such as BF4 and NO3.[44] Since then, IL were used in a large
range of applications such as
catalysts solvents,[45-49] separation media,[50-52]
electrolytes,[53-62] or heat transfer fluids.[63-66]
They are generating high interest from the scientific community
as shown by an increasing
number of publications, Figure 10.[67, 68]
Figure 10: Evolution of the number of publications containing
the term ionic liquid or ionic
liquids in the topic from Web of knowledge from 1988 to 2014
These salts possess low melting points thanks to the large
volume and the asymmetry of their
constituting ions. It sterically prevents the formation of a
regular network like e.g. in the case
of sodium chloride (NaCl), Figure 11.[69] IL exhibit negligible
vapour pressure, similarly to
solid salts,[70] hence they do not vaporize unless heated to the
point of thermal decomposition,
typically 200 to 300 C.[53] Their viscosity can be of 30-60
cP,[53, 71-73] they have flash points
higher than 200 C,[74, 75] and are hardly flammable.[76-79]
Figure 11: Structure of NaCl (left) and [Im][PF6] (right)
salts;
For the IL, red zones represent ionic parts and green ones
represent nonpolar side chains[69]
These properties, associated to a high electrochemical stability
(often superior to 4 V) and a
good ionic conductivity (1 to 10 mS.cm-1) highlight their
possible use as safe electrolytes.[20,
-
Chapter 1 49
57, 72, 80-84] This is confirmed by the wide interest for these
solvents, showed both in
literature[20, 61, 68, 81, 85-90] and in industrial
projects,[91-95] in particular as electrolytes for Li-ion
batteries.
In the case of IL-based electrolytes, many families have been
studied, with various electrode
couples.[67, 96] Usually 10 to 100 cycles are reported, and the
major part of published cycling
results is carried in half cells. The temperature has a strong
effect on the performances of the
batteries containing IL, as the viscosity is a crucial
limitation.[57] In the following tables are
listed some of the published results, with IL based on
imidazolium (Table 4), pyrrolidinium
(Table 5) and other cations (Table 6). They are classified by
increasing temperature and
decreasing capacity, and parameters such as cycling rate and
electrode nature are indicated.
Electrolyte solvent based on imidazolium Electrodes
T (C)
Cycle number
Capacity
(mAh.g-1
) C-rate Ref.
[C1C
2Im][NTf
2] LTO / LCO 25 200 106 C [61]
[C1C
nIm][NTf
2],
n=4, 6, 8 Li / LCO 25 120 100 C/8 [56]
[C1C
2Im][BF
4] LTO / LCO 25 50 120 C/5 [97]
[C1C
2Im][FSI] Li / Cgr 25 30 360 C/5 [98]
[C1C
1C
3Im][NTf
2] Li / LMO 30 50 105 C/8 [99]
[C1C
1C
3Im][NTf
2] Li / LCO 30 50 120 C/8 [88]
[C1C
4Im][NTf
2] LTO / LFP 60 100 120 C/10 [100]
[C1C
6Im][NTf
2] LTO / LFP 60 40 130 C/10 [101]
Table 4: Cycling performances of batteries containing ionic
liquids based on imidazolium cations as electrolytes in different
conditions (cycling rate, temperature, electrodes)
-
Chapter 1 50
Electrolyte solvent based on pyrrolidinium Electrodes
T (C)
Cycle number
Capacity (mAh.g
-1)
C-rate Ref.
[PYR14
][FSI] Li / LFP 20 220 165 C/10 [93]
[PYR13
][FSI] Li / LFP 25 90 149 C/4 [96]
[PYR14
][NTf2] Li/LiNi0.5Mn1.5O4 25 30 95 C/10 [102]
[PYR13
][FSI] Li / Cgr 30 150 350 C/15 [103]
[PYR14
][PIP13
][FSI] LTO / LFP 60 20 90 C/2 [104]
[PYR14
][NTf2] Cgr / LFP 60 100 80 C/10 [105]
Table 5: Cycling performances of batteries containing ionic
liquids based on pyrrolidinion cations as electrolytes in different
conditions (cycling rate, temperature, electrodes)
Electrolyte solvent Electrodes T (C) Cycle
number Capacity (mAh.g
-1)
C-rate Ref.
Polymer electrolyte based on [PYR
14][NTf
2] LTO / LFP 20 800 550 C/10
[106]
[PIP13][NTf2] Li / LMNO 20 50 140 C/16 [107]
[ether-functionalised ammonium][NTf
2] Li / LFP 25 50 150 C/10
[108]
Gelled electrolyte based on [C1C2Im][BF4]
LTO / LCO 25 50 120 C/5 [97]
[PIP13][NTf2] Li / LCO 25 28 115 C/10 [109]
[N5555][NTf2] Li / LCO 25 50 120 C/10 [110]
[PIP13][NTf2] Li /LCO 30 then 60 50 125 C/10 [111]
Polymer electrolyte based on [PYR
14][NTf
2] Li / LFP 40 600 450 C/10
[106]
Polymer electrolyte based on [PYR
13][NTf
2] Li / LFP 40 80 120 C/10
[112]
Polymer electrolyte based on [Im][NTf2]
Li / LFP 60 80 160 C/10 [37]
Table 6: Cycling performances of batteries containing ionic
liquids as electrolyte components in different conditions (cycling
rate, temperature, electrodes)
From this literature survey, we observed that the most used and
efficient anion was NTf2. It
can be explained by its high thermal stability, adapted
physicochemical properties, i.e. low
viscosity. For the cations, imidazolium and pyrrolidinium are
the most used ones. Indeed they
allow easy chemical modification by changing the length, the
number and the functionality of
the alkyl chains. Thus one can tune the electrochemical and
physicochemical properties, such
-
Chapter 1 51
as oxidation limits or viscosity. The synthesis of IL combining
NTf2 anion with these two
cations is quite easy and provides high yields and purity.
2.4. Electrodes
Lithium-ion batteries can be designed for specific applications
by selecting adequate
components, especially intercalation materials. Thick electrodes
afford high energy density
cells, while thin electrodes provide powerful ones. Commonly
used negative electrode
materials are metallic lithium (Li), graphite (Cgr) or lithium
titanate (Li4Ti5O12, LTO). The
positive electrode can be a layered oxide (such as lithium
cobalt oxide, LiCoO2, LCO), a
polyanionic framework (such as lithium iron phosphate, LiFePO4,
LFP), or a spinel (such as
lithium manganese oxide, LiMn2O4, LMO). The most commonly used
positive electrodes are
oxides comprising one or up to three metals, including cobalt,
nickel, aluminium or
manganese.[113-117] The choice of the electrode couple
determines the nominal voltage of the
cell, Figure 12, which sets its energy density. For example the
nominal voltage of cells
constituted of Cgr // LFP or LTO // LCO electrodes is 3.2 V or
2.5 V respectively. A brief
presentation of the most common electrode materials is given
below.
Figure 12: Voltage profiles of different Li-ion battery
electrodes
Lithium metal is attractive for high capacity batteries (3.86
Ah.g-1), as its potential is very low
(-3.04 V vs SHE) and it is the lightest metal (M = 6.94 g.mol-1
and = 0.53 g.cm-3).[118] In this
case the battery is called half-cell, not Li-ion, referring to
the absence of metallic lithium. The
successive lithium depositions induce morphological changes on
the electrode surface. They
can generate cracks, which break the passivation layer and
promote development of lithium
dendrites, as observed by in situ scanning electron microscopy
(SEM), Figure 13.[7, 119, 120]
-
Chapter 1 52
Figure 13: Left: Dendrite growth mechanism via lithium
deposition underneath the surface film,
volume change, surface film crack and dendrite formation[119]
Right: SEM picture of lithium dendrite formed on the
surface[120]
Common negative electrodes are based on graphite, which
theoretical capacity is
372 mAh.g-1. This electrode overcomes the problem of dendrites,
as lithium ions get inserted
in a reversible way in the structure. However the major failure
mechanism is caused by the
co-intercalation of solvent molecules, pushed together with Li+
cations between the graphene
planes.[121] It makes the graphite exfoliate and decompose into
a dust of graphene sheets.[119]
To prevent this process a protective layer is required, called
Solid Electrolyte Interphase
(SEI).[122] This organic coating, formed by electrolyte
decomposition products, should allow
easy transport of lithium ions, should present low resistance,
and should be homogeneous and
stable upon cycling. In the case of electrolytes based on ionic
liquids, this layer is not
efficiently formed and cycling is inhibited.[123, 124] Hence
additives such as vinylene carbonate
(VC) or vinyl ethylene carbonate (VEC), known to create a good
protection layer, can be
added, Figure 14.[125-127] These additives allow reaching high
capacities and good
reversibility. However, the complexity of the system is
increased, and its safety is depleted as
these additives are flammable and volatile.
Figure 14: Left: Vinylene carbonate; Right: Vinyl ethylene
carbonate
Lithium titanium oxide Li4Ti5O12 (LTO) has a theoretical
capacity of 175 mAh.g-1, and
approximately 165 mAh.g-1 in practice. LTO electrodes exhibit
reduced energy density, about
half that of Cgr, because they operate at higher potential (1.5
V instead of less than
0.2 V vs Li+/Li). The interest of such material is that thanks
to its high potential, there is no
need for SEI layer so they do not require electrolyte additives,
and lithium plating is avoided.
-
Chapter 1 53
Contrarily to Cgr, LTO materials allow the use of aluminium
current collector, cheaper than
copper. Moreover they can achieve high power (high cycling
rates) and improve the cell
safety since they present almost zero strain insertion.[61, 93,
128-130] They are also thermally
stable, generating approximately ten times less heat than
graphite during decomposition,
Figure 15, left.[131]
State of the art Li-ion batteries use mostly LiCoO2 (LCO) as
positive electrode material. But
the thermal instability of this oxide, its toxicity and the high
cost of cobalt may limit its
further development.[132, 133] LFP is a safer alternative, but
associated to LTO the cell voltage
is quite low, 2 V, Figure 12, vide supra. To increase this
value, high voltage materials such as
mixed metallic oxides of cobalt, manganese, nickel and aluminium
were widely examined,
with varying compositions. In particular LiNi1/3Mn1/3Co1/3O2
(NMC) showed good cycling
stability and rate capability, lower cost, reduced toxicity and
better thermal stability, Figure
15, right.[134-136]
Figure 15: DSC profiles of graphite and LTO electrode materials
(left)[131]
LCO and NCM (NMC) electrode materials (right)[137] Taking into
account the characteristics of each electrode material, in this
work the Li-ion
system will be based on LTO // NMC electrodes, Figure 16. In
order to have a good
reproducibility, a single roll of each electrode will be used in
all this work, which features are
described in Table 7.
-
Chapter 1 54
Figure 16: Comparison between six important characteristics of
positive and negative electrodes[138]
Feature LTO NMC
Synthesis solvent water N-methyl pyrrolidone
Binder carboxymethylcellulose (CMC) polyvinylidene fluoride
(PVDF)
Capacity (mAh.cm-2) 1.27 1.10
Loading (mg.cm-2) 7.56 6.8
Nominal voltage (V vs Li+/Li) 1.5 4.3
Thickness (m) 60 60
Porosity (%) 35 35
Current collector Aluminium, 20 m Table 7: Characteristics of
LTO and NMC electrodes used in this work
-
Chapter 1 55
3. Strategy of this work In this first chapter, the state of the
art of lithium-ion batteries was described. Lithium-ion
batteries dominate both the nomad device and electric vehicle
markets, as they provide more
energy per unit of weight than other chemistries. However they
raise safety concerns, leading
to a number of accidents. To enhance safety, the positive and
negative electrodes
LiNi1/3Mn1/3Co1/3O2 (NMC) and Li4Ti5O12 (LTO) were chosen. The
electrolyte, consisting of
flammable and volatile carbonate mixtures, is the most hazardous
component. The
replacement of the latter by ionic liquids (IL), liquid salts
claimed to be thermally stable and
non-flammable, could provide a safer alternative. A large number
of research groups report
their high performances,[56, 61, 101] yet their often claimed
safety has been poorly examined by
experiments. In particular studies of IL for use as electrolytes
were scarce in case of abuse
conditions such as fire, shortcircuit, or overcharge.
The work of this thesis will investigate IL behaviour under
abuse conditions. It will include
thermal stability up to combustion and electrochemical behaviour
up to overcharge. It targets
to help defining the safety of IL-containing cells, in
particular the thermal stability of
electrolytes. Imidazolium and pyrrolidinium cations associated
to
bis(trifluoromethylsulfonyl)imide anion will be selected from
the vast domain of IL for their high decomposition temperatures and
adapted physicochemical properties (low viscosity, high
ionic conductivity, wide electrochemical window).[80,
139-143]
In the second chapter, the thermal stability of
imidazolium-based IL will be evaluated
according to several trends. A critical study of gathered data
(from literature and our work)
will lead to determine an optimised procedure to obtain
reproducible and comparable results
of decomposition temperatures. The influence of several
structures of alkyl chains (such as
length, branching or functionalization) will be studied in order
to deduce the most stable
imidazolium IL.
The third chapter will analyse the possible routes of
degradation of the selected IL and their
corresponding electrolytes, during thermal abuse tests and under
different experimental
conditions. In particular the combustion behaviours will be
investigated by measuring heats of
combustion and ignition delays, and analysing emitted gases.
-
Chapter 1 56
The last chapter will be devoted to the study of the
electrochemical stability of full lithium-ion
cells constituted of Li4Ti5O12 and LiNi1/3Mn1/3Co1/3O2 electrode
materials and different
electrolytes using electrochemical techniques. The stability of
the systems will be studied by
cycling tests and cyclic voltammetry experiments. To provide a
good understanding of the
influence of overcharge on the electrochemical behaviour of the
cell, the evolution of the
electrolytes and electrodes surface will also be examined under
overcharge.
-
Chapter 1 57
4. References
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reducing the energetic and material demands of electrical energy
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[2] "World Hybrid Electric and Electric Vehicle Lithium-ion
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http://www.frost.com (accessed September 24, 2013)
[3] E. Martinot, "Renewables Global Futures Report", 2013,
report from REN21 [4] "How do alkaline batteries work ?"
http://www.energizer.com/learning-center/Pages/how-batteries-
work.aspx, accessed August 2014 [5] "Electric Vehicle Batteries
Report", 2012, report from Pike Research,
http://www.navigantresearch.com/ [6] C. Glaize and S. Genies,
Lithium Batteries and Other Electrochemical Storage Systems,
Wiley-ISTE
ed., 2013 [7] B. J. Landi, M. J. Ganter, C. D. Cress, R. A.
DiLeo and R. P. Raffaelle, "Carbon nanotubes for lithium
ion batteries", Energ. Environ. Sci., 2009, 2(6), p638 [8] T. B.
Reddy and D. Linden, Linden's handbook of batteries, 4th ed.,
McGraw-Hill, New York, 2011 [9] "History of battery invention and
development", http://blog.genport.it/?p=133, accessed August 2014
[10] W. Weydanz, in Encyclopedia of Electrochemical Power Sources,
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strategy: Cell or System ?" Navigant Research, during Battery
Safety congress,
in San Diego, CA, USA, 2013 [12] "Electric vehicles in Europe:
gearing up for a new phase?" 2014, report from McKinsey&Co [13]
"Electric and plug-in hybrid vehicle road map", 2010, report from
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