This thesis is focused on the use of light and LEDs for communication between the light and the user. In the introduction, the principles of VLC, which Li-Fi is part of, are described. The work also contains issues on the topics of Li-Fi, such as methods of modulation of carrier signals for data transmission, comparisons between other wireless communications, individual elements of optical frontends, current versions of Li-Fi and other possible uses of this technology. The work also contains the designing of a system working on the principle of Li-Fi.

This thesis deals with the development and current issues of Li-ion and Li-S accumulators, especially the separators. In the theoretical part is described history of Li-ion batteries, their properties and materials for the positive electrode. Li-S batteries and their problems are also described in this diploma thesis. In the practical part, electrochemical methods were described, and several separator samples with various modifications were created. These samples were then photographed using an SEM electron microscope and evaluated using electrochemical methods.

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Current commercial intercalation cathodes are approaching their theoretical capacity edges, which limits further improvement of the energy density in Li-ion batteries. To overcome this limitation, Li-rich antiperovskite cathodes were developed, utilizing both cationic and anionic redox activities. This class of materials has the general formula (Li2TM)ChO, where TM and Ch represent a transition metal (Fe, Mn, Co) and chalcogen ion (S, Se), respectively. This work focuses on understanding the reaction mechanism, exploring novel approaches for optimizing the electrochemical performance, and developing a scalable synthesis method for the antiperovskite cathodes. Firstly, the effect of substituting S by Se in the solid-state synthesized (Li2Fe)SO on the structural and electrochemical performance is thoroughly investigated. The anionic substitution was found to improve the structural and thermal stabilities of (Li2Fe)SO material. The cyclic voltammetry data confirmed both the cationic (Fe) and anionic (S/Se) redox activities, with possibility of controlling the anionic redox potential through the anionic substitution. It was observed that the electrochemical performance exhibits a non-linear dependence on the degree of anionic substitution. Among the prepared (Li2Fe)S1-xSexO (x = 0.1-0.9) compositions, (Li2Fe)S0.7Se0.3O exhibited the best electrochemical performance with a specific capacity 245 mAhg-1 and good cycling stability at low current rate. Ex-situ and in-situ measurements suggested an enhanced structural stability of (Li2Fe)S0.7Se0.3O during electrochemical cycling compared to (Li2Fe)SO, which could be one of the reasons for its superior performance at low current rates. The second part of this work focuses on understanding the reaction mechanism of (Li2Fe)SeO prepared by solid-state reaction (SSR) method and exploring the impact of cationic substitution of Fe by Mn on its structural and electrochemical properties. Electrochemical investigations showed that the cationic redox activity leads to a reversible cycling behaviour, indicating its role in the stable performance. Whereas, the anionic redox activity leads to partial decomposition of the (Li2Fe)SeO cathode to an electrochemically active phase. In general, although the electrochemical activity of the phase resulting from the partial decomposition of any antiperovskite composition can compensate the initial capacity loss, it opens a channel for capacity fading over long term of cycling. The (Li2Fe)SeO cathode could deliver an initial specific discharge capacity of 165 mAhg-1, which declined to only 140 mAhg-1 after 100 cycles, indicating a good cycling performance. Even at high current rate (1C), the (Li2Fe)SeO could provide a reasonable specific capacity of 100 mAhg-1. Replacing Fe with Mn reduced the overall redox activity of the cationic and anionic redox processes, when using active material: carbon: binder weight ratio of 70:15:15 wt. %. This may result from impeded kinetics and the Jahn-Teller effect associated with Mn2+/3+ redox. However, low substitution levels can be beneficial in optimizing the performance while minimizing the negative effects associated with Mn2+/3+ redox pair. Modifying the electrode ratio to 85:10:5 wt. % improved the specific capacity for (Li2Fe0.9Mn0.1)SeO, surpassing that of the unsubstituted (Li2Fe)SeO cathode. These findings highlight the role of controlled substitution and electrode ratio in optimizing the electrochemical performance of antiperovskite cathodes. The third part of this work focuses on developing scalable, controllable, and sustainable synthesis of antiperovskite cathodes using mechanochemical (MC) method based on ball milling (BM), which is crucial for practical application. Both (Li2Fe)SeO and (Li2Fe)SO antiperovskite cathodes have been successfully prepared by direct MC without the need for external heating, which is advantageous for energy saving. Post-heat treatment after the milling was found to be an effective strategy for controlling the morphological and electrochemical properties of both materials. Both ball-milled materials revealed similar reaction mechanism to the (Li2Fe)SeO prepared by SSR method, involving both cationic and anionic redox activities. The ball-milled (Li2Fe)SeO displayed an average specific discharge capacity of 255 mAhg-1 at 0.1C and 138 mAhg-1 at 1C in the potential range 1-3 V. Transmission electron microscopy and magnetic investigations revealed a partial conversion of the mechanochemically synthesized (Li2Fe)SeO into an electrochemically active Fe1-xSex phase during the anionic redox process. On the other hand, mechanochemically synthesized (Li2Fe)SO exhibited an average specific discharge capacity of 340 and 133 mAhg-1 at 0.1C and 1C, respectively, in the potential range 1-3 V. Excluding the anionic redox activity of both materials by restricting the potential scanning range was found to be advantageous for enhancing the cycling stability over long range of cycling. This highlights the critical role of controlling the potential range on the electrochemical performance and cyclability of these materials.

The expanding constant is a measure of graph connectivity that is important for certain applications. This paper discusses the mathematical foundations for the construction of Winnie Li's graphs and for the proof that Winnie Li's graphs are Ramanujan. The paper also establishes the implications of the Ramanujan property for the expanding constant. / Master of Science

Le travail de thèse présenté dans ce mémoire, est consacré à l'étude des interactions interfaciales entre une espèce active électrochimiquement (l'étain) et une matrice (le borophosphate) capable d'absorber les variations volumiques dues à la formation électrochimique des diverses compositions Li-Sn (« buffer »). L'objectif de cette étude est de comprendre la nature des réactions ayant lieu avec l'introduction du Li dans le matériau composite. Pour cela, nous avons réalisé une étude détaillée d'un composite de référence mis au point dans des études précédentes Sn-0,4 BPO4 ; nous avons évalué l'influence du type de matrice et de la voie de synthèse sur son comportement global. Le matériau composite a pu être décrit comme possédant une interface vitreuse contenant de l'étain oxydé (SnII) qui lui donne la structuration suivante : Elément actif Sn0(1-w)/SnIIwBxPyOz/BPO4 Phase support Interphase. Des études in situ operando complémentaires en diffraction des rayons X et spectrométrie Mößbauer ont permis d'analyser le comportement électrochimique du matériau composite : un premier processus correspond à l'extrusion d'une petite partie d'étain métallique de la zone interfaciale qui augmente la conductivité électronique du composite ; il est suivi par une réorganisation de l'interface avec extrusion de tout le contenu en étain et la formation des premières compositions Li-Sn. Enfin, le cyclage galvanostatique se poursuit grâce à la formation de plusieurs compositions Li-Sn riches en étain (Li2Sn5 et LiSn) et puis enrichies en lithium (Li13Sn5 et Li7Sn2). / The Phd work, presented in this manuscript, is devoted to the study of the interface interactions between an electroactive species (tin) and a matrix (borophosphate). The latter has a buffer role and is thus able to absorb the volume variations taking place during the Li-Sn electrochemical reaction.The aim of this study is to understand the nature of the reactions occurring during lithium introduction in the composite. In order to do that, a detailed study of a previously studied reference composite (Sn-0,4 BPO4) has been undertaken. The effect of some modified matrixes as well as the synthesis route has also been evaluated. The composite material can be described as having a glassy interface containing some oxidized tin (SnII) which leads to the following global structure: Active element Sn0(1-w)/SnIIwBxPyOz/BPO4 Buffering phase Interphase. A complementary in situ operando study (X-ray diffraction and Mößbauer spectroscopy) gave the possibility to analyze the electrochemical behavior of the material. A first process corresponds to a small tin extrusion from the interfacial zone. This contributes to the increase of the electrical conductivity of the composite material which is followed by the interphase reorganization with the extrusion of the whole tin content. Li-Sn reactions take place then, with the galvanostatic cycling going on between the tin rich compositions (Li2Sn5 and LiSn) and the lithium rich ones (Li13Sn5 and Li7Sn2).

Night of no Exile is a collection of poems preceded by a critical article entitled "‘Exile seems both a blessing and a curse': A Blissful Reading of Li-Young Lee's Poetry." That article discusses Lee's quest to achieve communication, truth, and transcendence through poetic language and concludes that he finally reaches his goal through a leap from narrative poetry to lyricism. The "exile" alluded to in the title of the article is not only geographic, but also interioran exile due to the natural limitations of all languages, and which can be bridged only in linguistic ways. Lee's solution to that problem (lyricism) turns his poetry into what Roland Barthes would call "a text of bliss," a text that manages to deeply destabilize language, while simultaneously achieving a new kind of meaning. In the main body of the manuscript, the first section contains short love lyrics. The second section, "Night of no Exile," is an attempt at the demanding genre of the longer lyric poem. The third section uses short lyrics to explore various topics, such as discovering one's identity, friendship and solidarity between women, family history, and childhood memories. Finally, the last section includes poems, four of them longer, attempting to combine narrative and lyric impulses in a way not unlike Li-Young Lee's experimentation with those two genres.

Lors du fonctionnement des batteries Li-ion, la dégradation progressive de l’électrolyte engendre la génération des gaz qui sont à l’origine du phénomène des surpressions dans ces dispositifs, et a pour conséquence des problèmes de sécurité. Cette thèse aborde l’étude de la solubilité des gaz issus des réactions de dégradation des électrolytes tels que le CO2, CH4, ou encore C2H4 dans plusieurs systèmes simples (solvants purs) ou complexes (mélanges binaires, ternaires et quaternaires avec sel de lithium), en fonction de la température, de la structure des solvants et des sels, ainsi que de leurs concentrations en solution. A cet effet, nous avons mesuré préalablement les propriétés volumétriques, de transport, ainsi que les pressions de vapeur des électrolytes formulés en fonction de la composition et de la température. / The performance and the safety of a lithium-ion battery depend to a great extent on the stability of the electrolyte solution, because the high voltage of the battery may cause the decomposition of lithium salt or organic solvents, which limits then the battery lifetime. During these degradations, several gases are, generally, generated like the CO2, CO, CH4 and C2H4, which induce in fact several problems related to the pressure increase inside the sealed cell. The main objective of this PhD thesis is to understand the key thermodynamic parameters which drive the gas dissolution in classical solvents and electrolytes. For that, several pure solvents and electrolytes have been firstly investigated to determine their volumetric and transport properties, as well as, their vapour pressure as the function of temperature and composition.

L’accroissement des performances des accumulateurs Li-ion sur les 25 dernières années découle principalement de l’optimisation de leurs composants inactifs. Aujourd’hui, l’urgence environnementale impose de développer de nouveaux matériaux actifs d’électrode pour proposer la prochaine génération d’accumulateur qui participera à la transition énergétique. A cet effet, le silicium pourrait avantageusement remplacer le graphite des électrodes négatives à moyen terme. Cependant la rétention de capacité des électrodes de silicium est mise à mal par l’expansion volumique que le matériau subit lors sa réaction d’alliage avec le lithium, qui mène à la déconnexion des particules de Si et à une réduction continue de l’électrolyte. Une compréhension de ces phénomènes de vieillissement à l’échelle de la nanoparticule est nécessaire à la conception d’électrodes de silicium viables. Pour ce faire, la technique STEM-EELS a été optimisée de manière à s’affranchir des problèmes d’irradiation qui empêchent l’analyse des matériaux légers d’électrode négative et de la Solid electrolyte interface (SEI), grâce à l’analyse des pertes faibles EELS. Un puissant outil de cartographie de phase est obtenu et utilisé pour mettre en lumière la lithiation cœur-coquille initiale des nanoparticules de silicium cristallin, la morphologie hétérogène et la composition de la SEI, ainsi que la dégradation du silicium à l’issue de cyclages prolongés. Enfin, un modèle de vieillissement original est proposé, en s’appuyant notamment sur un effort de quantification des mesures STEM-EELS sur un grand nombre de nanoparticules. / Over the last 25 years, the performance increase of lithium-ion batteries has been largely driven by the optimization of inactive components. With today’s environmental concerns, the pressure for more cost-effective and energy-dense batteries is enormous and new active materials should be developed to meet those challenges. Silicon’s great theoretical capacity makes it a promising candidate to replace graphite in negative electrodes in the mid-term. So far, Si-based electrodes have however suffered from the colossal volume changes silicon undergoes through its alloying reaction with Li. Si particles will be disconnected from the electrode’s percolating network and the solid electrolyte interface (SEI) continuously grows, causing poor capacity retention. A thorough understanding of both these phenomena, down to the scale of a single silicon nanoparticle (SiNP), is critical to the rational engineering of efficient Si-based electrodes. To this effect, we have developed STEM-EELS into a powerful and versatile toolbox for the study of sensitive materials and heterogeneous systems. Using the low-loss part of the EEL spectrum allows us to overcome the classical limitations of the technique.This is put to use to elucidate the first lithiation mechanism of crystalline SiNPs, revealing Li1.5Si @ Si core-shells which greatly differs from that of microparticles, and propose a comprehensive model to explain this size effect. The implications of that model regarding the stress that develops in the crystalline core of SiNPs are then challenged via stress measurements at the particle scale (nanobeam precession electron diffraction) for the first time, and reveal enormous compressions in excess of 4±2 GPa. Regarding the SEI, the phase-mapping capabilities of STEM-EELS are leveraged to outline the morphology of inorganic and organic components. We show that the latter contracts during electrode discharge in what is referred to as SEI breathing. As electrodes age, disconnection causes a diminishing number of SiNPs to bear the full capacity of the electrode. Overlithiated particles will in turn suffer from larger volumes changes and cause further disconnection in a self-reinforcing detrimental effect. Under extreme conditions, we show that SiNPs even spontaneously turn into a network of thin silicon filaments. Thus an increased active surface will compound the reduction of the electrolyte and the accumulation of the SEI. This can be quantified by summing and averaging STEM-EELS data on 1104 particles. In half-cells, the SEI volume is shown to increase 4-fold after 100 cycles without significant changes in its composition, whereas in full cells the limited lithiation performance understandably leads to a mere 2-fold growth. In addition, as the operating potential of the silicon electrodes increases in full cells – potential slippage – organic products in the SEI switch from being carbonate-rich to oligomer-rich. Finally, we regroup these findings into an extensive aging model of our own, based on both local STEM-EELS analyses and the macro-scale gradients we derived from them as a whole.