This master's thesis deals with a topic of determination of the most suitable ratio of cathode materials for the lithium-sulfur systems. The first two chapters provide a general introduction to the topic of electrochemical energy sources and present the commonly used primary and secondary battery systems with emphasis on their characteristics and applications. The core of the theoretical part is dedicated to lithium-ion and lithium-sulfur batteries, their working principles along with the benefits or drawbacks related to the particular systems, and widely used materials. The experimental part briefly comments on determining the suitable electrode paste preparation method, the subsequent main part is focused on evaluation of electrochemical performance of cells using different ratios of cathode materials. Five samples of cathode materials were prepared, where the sulfur ratio is in range from 64 to 88 wt. %. Finally, the comparison of all prepared ratios in terms of their electrochemical properties is provided.

Polymer-based carbide-derived carbons (CDCs) with combined micro- and mesopores are prepared by an advantageous sacrificial templating approach using poly(methylmethacrylate) (PMMA) spheres as the pore forming material. Resulting CDCs reveal uniform pore size and pore shape with a specific surface area of 2434 m2 g−1 and a total pore volume as high as 2.64 cm3 g−1. The bimodal CDC material is a highly attractive host structure for the active material in lithium–sulfur (Li–S) battery cathodes. It facilitates the utilization of high molarity electrolytes and therefore the cells exhibit good rate performance and stability. The cathodes in the 5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte show the highest discharge capacities (up to 1404 mA h gs−1) and capacity retention (72% after 50 cycles at C/5). The unique network structure of the carbon host enables uniform distribution of sulfur through the conductive media and at the same time it facilitates rapid access for the electrolyte to the active material.

Adsorption is a versatile purification technique to selectively separate different peptide fractions from a mixture using mild operation conditions. Porous carbons are ideally suited to separate ACE-inhibiting dipeptides by combining tailored size exclusion and polarity selectivity. The desired peptide fraction is mostly hydrophobic and very small and should adsorb inside hydrophobic micropores. The second topic of this thesis is linked to energy storage. The lithium-sulfur battery is a promising alternative to common lithium-ion batteries with theoretical capacities of up to 1672 mAh g−1 sulfur. The second aim of this thesis is to conduct an in-depth investigation of polysulfides interacting with selected carbon materials in a simplified battery electrolyte environment. The focus of this study is laid on the impact of surface polarity and pore size distribution of the carbon to develop a quantitative correlation between polysulfide retention and porosity metrics. Both, the enrichment of ACE-inhibitors and the retention of polysulfides rely on liquid phase adsorption in porous materials, linking the above mentioned topics. This thesis not only aims to develop an enrichment process or to find a superior battery cathode but also strives to explore structure-property relationships that are universally valid. Understanding the complex interplay of pore size and polarity leading to selective interactions between pore wall and the adsorbed species is given a high priority.

<p>As the future of energy looks increasingly electrified, the development of safe and sustainable battery technologies has never been more relevant. This is particularly critical for applications in stationary energy storage and transportation, where batteries must be produced and stored at large scale. Sustainability is necessary to meet the volume of demand at reasonable cost without straining resources. Safety is also paramount since fires can easily spread from one cell to the next and result in catastrophe when batteries are stored in proximity for large power banks or EVs. The focus of this thesis is thus to design and engineer materials for rechargeable batteries, which improve safety and sustainability while still enhancing the electrochemical performance. Towards this end, polymers play a central role throughout this thesis work due to their tunable chemical and physical properties.</p>

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Gegenstand der vorliegenden Arbeit ist die Herstellung von nanostrukturierten Kohlenstoffen. Die Synthese erfolgt dabei durch die Zwillingspolymerisation der siliziumhaltigen Zwillingsmonomere 2,2’Spirobi[4H-1,3,2-benzodioxasilin] sowie Tetrafurfuryloxysilan. Die entstehenden Nanokomposite werden anschließend carbonisiert und das SiO2-Netzwerk herausgelöst. Die Zwillingsmonomere wurden dabei zunächst templatfrei umgesetzt, um Einflüsse verschiedener Reaktionsparameter auf die Eigenschaften der erhaltenen Kohlenstoffe zu evaluieren. Des Weiteren wurde studiert, wie sich die Zugabe von Hart-Templaten auf das Polymerisationsverhalten der Zwillingsmonomere, sowie die Porosität und Morphologie der daraus resultierenden Kohlenstoffe auswirkt. Für die Charakterisierung der nanostrukturierten Kohlenstoffe wurde vorwiegend auf Elektronenmikroskopie und Stickstoffsorptions-Experimente zurückgegriffen. Mit Hilfe der Zwillingspolymerisation an Hart-Templaten, wie SiO2-Partikeln, Glasfasern und ORMOCER®en konnte die Morphologie, Geometrie, Größe und Porentextur der Kohlenstoffe eingestellt und ein modulares Synthesekonzept für poröse, nanostrukturierte Kohlenstoffe entwickelt werden. Ferner wurden ausgewählte Kohlenstoffe auf Anwendung als Wasserstoffspeicher und Elektrodenmaterial in Lithium-Schwefel-Zellen getestet. In diesem Zusammenhang wurden die Thermogravimetrie, die Differenzkalorimetrie und Stickstoff-Sorptionsmessungen eingesetzt, um die Batterieeigenschaften in Zukunft ohne das Durchführen aufwendiger Zelltests zu prognostizieren.

Ces travaux ont permis d’approfondir les connaissances du mécanisme de déchargepeu conventionnel de l’accumulateur Li/S et de ses limitations. L’ensemble desrésultats a convergé vers une unique conclusion, à savoir que le système Li/S estprincipalement limité par le phénomène de passivation de l’électrode positive en finde décharge. Les polysulfures de lithium à chaines courtes précipitent à la surface del’électrode positive de soufre. Isolants électroniques, ils sont responsables de la perteprogressive de surface active de l’électrode et de la fin prématurée de la décharge.Ainsi, les performances électrochimiques ont pu être significativement améliorées entravaillant sur la morphologie de l’électrode positive, et sur la composition del’électrolyte. En augmentant la surface spécifique de l’électrode, la quantité depolysulfures de lithium qui peut précipiter en fin de décharge est augmentée, et lapassivation totale de l’électrode est retardée. En augmentant la solubilité despolysulfures de lithium dans l’électrolyte, la précipitation des espèces est retardée etla décharge prolongée. Dans cette optique, les solvants de type PEGDME semblentêtre les plus prometteurs à ce jour. Enfin, un mécanisme possible de réduction dusoufre en électrolyte de type éther a pu être proposé. / This work aimed at better understanding the Li/S cell discharge mechanism and itslimiting parameters. A general conclusion was following from these data: the Li/Ssystem is mainly limited by the passivation process of the sulfur positive electrode,occurring at the end of discharge. Insulating lithium polysulfides precipitate on thepositive electrode surface, thus leading to a gradual loss of the electrode activesurface and to the early end of discharge. As a consequence, the electrochemicalperformances can be significantly improved by working either on the positiveelectrode morphology or on the organic electrolyte composition. Increasing thespecific surface of the positive electrode enables to increase the amount ofpolysulfide compounds that can precipitate on the electrode, thus delaying the fullpassivation of the sulfur electrode and the end of discharge. Working on the organicelectrolyte composition enables to increase the polysulfide solubility and to preventthem from quickly precipitating, thus delaying the end of discharge too. To thispurpose, PEGDME solvents seem to be quite promising. Finally, a possiblemechanism for sulfur reduction in ether-based electrolytes could be proposed.

Batteries based on lithium-sulfur technology have the potential to increase the amount of stored energy in comparison with current lithium-ion systems while maintaining the same weight. Achieving this goal is possible due to the high theoretical energy density of sulfur, which exceeds the values of other materials used in batteries. However, these batteries suffer from several failing, which still hinder the practical use. The use of amorphous sulfur instead of its crystalline form might lead to increasing of durability and charge efficiency. Inverse vulcanization of sulfur seems to be the perspective method for the preparation of polymer sulfur because it prevents its recrystallization and stabilizes amorphous sulfur. The aim of this thesis is the preparation and study of properties of materials made of inverse vulcanized sulfur.

Lithium–sulfur batteries are highly attractive energy storage systems, but suffer from structural anode and cathode degradation, capacity fade and fast cell failure (dry out). To address these issues, a carbide-derived carbon (DUT-107) featuring a high surface area (2088 m² g⁻¹), high total pore volume (3.17 cm³ g⁻¹) and hierarchical micro-, meso- and macropore structure is applied as a rigid scaffold for sulfur infiltration. The DUT-107/S cathodes combine excellent mechanical stability and high initial capacities (1098–1208 mA h gs ⁻¹) with high sulfur content (69.7 wt% per total electrode) and loading (2.3–2.9 mgs cm⁻²). Derived from the effect of the electrolyte-to-sulfur ratio on capacity retention and cyclability, conducting salt is substituted by polysulfide additive for reduced polysulfide leakage and capacity stabilization. Moreover, in a full cell model system using a prelithiated hard carbon anode, the performance of DUT-107/S cathodes is demonstrated over 4100 cycles (final capacity of 422 mA h gs ⁻¹), with a very low capacity decay of 0.0118% per cycle. Application of PS additive further boosts the performance (final capacity of 554 mA h gs ⁻¹), although a slightly higher decay of 0.0125% per cycle is observed.