京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20581号 / 工博第4361号 / 新制||工||1678(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 安部 武志, 教授 作花 哲夫, 教授 阿部 竜 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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In this thesis, a Sb/C composite electrode for sodium-ion batteries isprepared by a simple high energy ball milling and calenderingmethod. The prepared Sb/C composite electrode was assembled in ahalf-cell and symmetrical cell setups in order to perform avariety of electrochemical measurements.The composite electrode showed a reversible specific capacity of595 mAh/g, at a discharge/charge current rate of 15 mA/g. Theelectrode also showed a relatively good performance (compared toprevious studies) of 95% capacity retention after more than 100cycles, at a higher discharge/charge current rate of 60 mA/g. Theelectrode furthermore showed excellent self-dischargecharacteristics, in pause tests implemented over 200 hours (overeight days), which underlined the electrode materials good shelflife properties. A series of Sb/C symmetrical cells assembledthrough-out the project, furthermore, highlighted the stability ofthe solid electrolyte interface (SEI) layer formed on the Sb/Ccomposite electrode during cycling. Scanning electron microscopy(SEM) and Energy-dispersive X-ray spectroscopy (EDS) were used tocharacterize the surface morphology and composition of the Sb/Celectrode, respectively.A non-milled and milled (12 hours) graphite electrodes were alsoprepared for reference and comparison. The milled graphite matrixelectrode provided a reversible capacity of 95 mAhg-1 and acoulombic efficiency (CE) of 99% in over 250 cycles, at a currentrate of 30 mA/g. Milled and non-milled graphite were characterizedwith SEM and Raman spectroscopy, to help have a fundamentalunderstanding of the particle size and material phase,respectively.

The performance and lifetime of lithium-ion batteries are strongly influenced by their composition. One category of critical components are electrolyte additives, which are included primarily to stabilize electrode/electrolyte interfaces in the battery cells by forming passivation layers. The presented study aimed to identify and study such an additive that could form a hydrogen-evolution-suppressing solid electrolyte interphase (SEI) in lithium-ion batteries based on aqueous electrolytes. A promising molecular additive, ethyl 2,2-difluoroacetate (EDFA), was found to hold the qualities required for an SEI former and was herein further analyzed electrochemically. Analysis of the battery cells were performed with linear sweep voltammetry and cyclic voltammetry with varying scan rate and EDFA concentrations. Results show that both 1 and 10 w-% EDFA in the electrolyte produced hydrogen-evolution-suppressing SEI:s, although the higher concentration provided no apparent benefit. Lithium-ion full-cells based on LiMn2O4 vs. Li4Ti5O12 active materials displayed poor, though partly reversible, dis-/charge cycling despite the operation of the electrode far outside the electrochemical stability window of the electrolyte. Inclusion of reference electrodes in the lithium-ion cells proved to be immensely challenging with unpredictable drifts in their electrode potentials during operation. To summarize, HER-suppressing electrolyte additives are demonstrated to be a promising approach to stabilize high-voltage operation of aqueous lithium-ion cells although further studies are necessary before any practical application thereof can be realized. Electrochemical evaluation of the reaction mechanism and efficiency of the electrolyte additives relies however heavily on the use of reference electrodes and further development thereof is necessary.

Cette thèse se positionne dans le domaine des batteries lithium-ion. Elle a pourobjectif de mieux comprendre le fonctionnement de l’électrode négative de graphiteen étudiant le processus de formation du film de passivation, couramment appeléSEI (Solid Electrolyte Interface) créé à l’interface avec l’électrolyte. Ce travail nousa conduit à proposer des modèles pouvant expliquer comment se forme la SEI et àidentifier les phénomènes qui entrent en jeu dans le fonctionnement de la batterie.La SEI résulte de la réaction entre l’électrode de graphite, les ions lithium et les moléculesorganiques de l’électrolyte qui survient lors du premier processus d’insertion.Elle est principalement composée des produits de décomposition de l’électrolyte etles ions lithium consommés ne sont plus échangeables. Elle est donc responsable dela capacité irréversible observée lors du premier cycle de formation, correspondantà la différence de capacité entre le processus d’insertion et le processus de désinsertion.Il est donc essentiel de mieux comprendre les paramètres qui l’influencentpour pouvoir ainsi la contrôler et limiter la perte irréversible de capacité. Les performancesen capacité de l’élément lithium-ion sont directement liées à cette valeurde capacité irréversible, elle doit être limitée afin de maximiser la quantité d’ionslithium échangée entre l’électrode négative et l’électrode positive. La stabilité dela SEI conditionne ensuite le comportement en cyclage de l’électrode au cours dutemps.Dans ce mémoire de thèse, nous avons choisi de caractériser le comportement del’électrode de graphite en faisant varier la nature de l’électrolyte et la taille desparticules de graphite tout en restant le plus proche possible du fonctionnementd’une vraie batterie. Au travers des techniques de caractérisations électrochimiques(cyclage galvanostatique, spectroscopie d’impédance) associées à des techniques decaractérisation de surface (spectroscopie de photoélectrons X, microscopie électroniqueà balayage), les résultats obtenus ont permis de proposer un nouveau modèlede formation de la SEI.Pour l’électrolyte, nous avons choisi de ne regarder que l’effet du solvant (le carbonatede propylène) et de l’additif (le carbonate de vinylène). Ces deux composésentrent dans la composition des électrolytes utilisés dans les éléments lithium-ioncommerciaux. Pour l’électrode de graphite, le choix des particules s’avère primordialpuisque chaque type de particules possède une chimie de surface spécifique (plans223basaux ou plans prismatiques) susceptible de réagir différemment vis-à-vis de l’électrolyte.Deux particules de graphite, de taille et de morphologie différentes, ont étéétudiées. Elles sont utilisées séparément en tant que matière active dans les électrodesnégatives des batteries lithium-ion. Notre spécificité est d’avoir préparé desélectrodes constituées par un mélange de ces deux particules et de les avoir ensuitecaractérisées en formation. L’application de conditions de fonctionnement différentescomme le régime de cyclage et la température d’essai ont mis en évidence les valeursidéales conduisant à minimiser la dégradation de l’électrolyte et à optimiser laqualité du film.Nous avons abouti, au travers de l’ensemble des méthodes de caractérisations misesen oeuvre, à une meilleure compréhension des mécanismes de formation du film depassivation permettant ainsi d’améliorer cette étape essentielle à la pérennité desperformances de l’électrode dans le temps. Ce travail a donc un réel impact auniveau industriel. Le modèle de formation proposé apporte un éclairage nouveau auprocessus de formation et peut permettre également d’aider en amont à la fabricationdes particules de graphite. / This work relates to the lithium ion battery field. The purpose of this study is tobetter understand the behavior of graphite electrodes by focusing on the formationof a passive layer named Solid Electolyte Interface (SEI) which is formed at thegraphite/electrolyte interface. This work has led us to put forward models whichcan explain the SEI formation and identify the reactions which take place in alithium ion battery.The SEI results from reactions between graphite electrode, lithium ions and organicmolecules from the electrolyte during the first charge of the lithium ion battery. It ismainly composed of decomposition products from the electrolyte. Consumed lithiumions can no longer be used in the next cycle. The SEI is therefore responsible for theirreversible capacity during the first formation cycle which is the charge loss betweenthe intercalation process and the deintercalation process. It is necessary to betterunderstand the impact of the formation conditions and other parameters in orderto control and limit the irreversible charge loss. Lithium ion battery performancesdepend on this irreversible capacity, this value has to be reduced in order to maximizethe amount of exchanged lithium ions between negative and positive electrodes. TheSEI stability will determine the electrode behavior upon cycling.In this thesis, we chose to study the graphite behavior by testing several electrolytecompositions and graphite particle sizes in electrochemical cells similar to areal battery. Electrochemical techniques (galvanostatic cycling and electrochemicalimpedance spectroscopy) and surface analyses (X-ray photoelectron spectroscopy,scanning electron microscopy) will be combined. These results helped us to developa new model of the SEI formation.For the electrolyte, we chose to study the effect of the solvent (propylene carbonate)and the additive (vinylene carbonate). Both components are commonly used inthe electrolyte for commercial lithium ion batteries. For the graphite electrode, thechoice of graphite particles is essential because each graphite family has its ownsurface chemistry (basal and prismatic surfaces) which can react in many wayswith the electrolyte. Two graphite particles, with specific sizes and morphologiesare studied. They are separately used as active materials for negative electrodes inlithium ion batteries. Our unique approach is to prepare graphite electrodes basedon a mix of both particles with various compositions and then test the electrode225performances. After testing several formation conditions such as the cycling rateand the temperature, we found the ideal formation conditions for minimizing theelectrolyte decomposition and optimizing the film quality.Finally, based on all the characterization methods, we came to a better understandingof the film formation process. In this way, we have improved this essentialpreliminary step which can now lead to more durable cycling performances overtime. This study can have a major impact on the industrial level. The formationmodel cast a new light on the formation process and can therefore help to makeefficient graphite electrodes.