The thesis presents the research results obtained while studying novel chemical strategies for preparing Si(100)-based electrochemical platforms suitable for aqueous environments. A primary research aim was the preparation of well-passivated Si(100) surfaces amenable to further chemical derivatization. The preparation and functionalization of alkyne-terminated alkyl monolayers on Si(100) surfaces using the Huisgen 1,3-dipolar ???click??? cycloaddition of azides with surface-bound acetylenes is reported and shown to be a versatile, experimentally simple, chemically unambiguous modular approach to modified silicon surfaces. Covalently immobilized, structurally well-defined acetylenyl organic monolayers are prepared from a commercially available ??,??-diyne (1,8-nonadiyne) species using a one-step thermal hydrosilylation procedure. Subsequent derivatization of the alkyne-terminated monolayers in aqueous environments with representative azide species affords disubstituted surface-bound [1,2,3]-triazole species. Neither activation procedures nor protection/deprotection schemes are required, as is the case with more established grafting approaches for silicon surfaces. The described surface modification scheme has been used in preparing modified Si(100) electrode surfaces, where modular components such as ferrocene derivatives or electrochemically ???switchable??? linker molecules are introduced onto the passivated silicon surface. An implementation study to prepare modified light-addressable ???switchable??? Si(100) electrodes is also reported. Negligible oxidation of the substrate was generally observed after exposure to aqueous systems for extended periods (tens of hours), and the electroactive monolayers showed a robust and reversible behaviour. The proposed concept of modular components and high-yielding coupling procedures has been shown on Si(100) surfaces and also extended to illustrate the functionalization of porous silicon rugate filters.

Rechargeable lithium ion and lithium (Li) metal batteries with high energy density and stability are in high demand for the development of electric vehicles and smart grids. Intensive efforts have been devoted to developing high capacity battery electrodes. However, the known high capacity electrode materials experience fast capacity fading and have limited cycle life due to electromechanical degradations, such as fracture of Si-based electrodes and dendrite growth in Li metal electrodes. A fundamental understanding of electromechanical degradation mechanisms of high capacity electrodes will provide insights into strategies for improving their electrochemical performance. Thus, this dissertation focuses on mechanical properties, microstructure changes, and degradation mechanisms of Si composite electrodes and Li metal electrodes. Based on these findings, possible coping strategies are proposed to improve the cycling stability of both electrodes. The poor cycling life of Si-based electrodes is caused by the repeated lithiation/delithiation-induced huge volumetric change in Si particles, which leads to the fracture of particles, excessive formation of solid electrolyte interphase on the newly exposed surface, as well as the loss of electronic conductivity between Si particles and the conductive matrix. The expansion/contraction of Si particles during cycling also causes the changes in the mechanical properties, microstructure, and porosity of Si composite electrodes. Understanding the relationship between mechanical property evolution, microstructure degradation, and capacity fading is essential for the design of Si composite electrodes. Using an environmental nanoindentation system, in situ microscope cell, and electrochemical impedance spectroscopy, I investigated the mechanical properties, cracking behavior, and lithiation/delithiation kinetics of Si composite electrodes made with different polymeric binders, including polyvinylidene fluoride, Nafion, sodium-carboxymethyl cellulose, and sodium-alginate, in their realistic working environment. The mechanical property evolution is determined by the state-of-charge, porosity, irreversible volume change, and mechanical behavior of binders. Periodical crack opening and closing happens in Si composite electrodes prepared with binders that have strong adhesion with Si. Mechanical degradations, e.g., irreversible volume change, cracking, and debonding between binders and Si particles, are correlated with the evolution of lithiation/delithiation kinetics and the capacity fading of Si composite electrodes. Based on these findings, a partial charging approach is proposed and confirmed experimentally to improve the cycling stability of Si composite electrodes. Li metal electrodes suffer from the low Coulombic efficiency, high electrochemical reactivity with the electrolytes, and the safety hazards caused by the uncontrollable dendrite growth during cycling. Mechanical suppression by using solid electrolytes and artificial SEI is a promising strategy to inhibit the formation of Li dendrites. Mechanical properties of bulk and mossy Li are required for designing mechanical inhibitors and improving the stability of the Li | inhibitor interface. Using an environmental nanoindentation system, I studied the mechanical behavior, especially the time-dependent behavior, of bulk Li and porous mossy Li at ambient temperature. By combining finite element (FE) modeling with experiments, a constitutive law was determined for the viscoplastic deformation of Li metal. FE modeling also demonstrates that the elasticity has a negligible influence on the indentation deformation of bulk Li. Flat punch indentation measurements showed that mossy Li has significantly higher deformation and creep resistance than bulk Li despite of its porous microstructure. The mechanical parameters of bulk and mossy Li may be helpful to develop of dendrite-free Li metal electrodes.

Die experimentellen Daten können bei Dr. Sebastian Risse, Helmholtz-Zentrum Berlin, eingesehen werden. / Silizium-Elektroden werden aufgrund ihrer um eine Gröÿenordnung höheren Kapazität als mögliches Elektrodenmaterial in Lithium-Ionen-Batterien betrachtet. Diese Kapazität geht jedoch mit einer Volumenausdehnung von bis zu 310 % einher. Dies begünstigt einen schnellen Kapazitätsabfall und ein kontinuierliches Wachstum der SEI-Schicht. Ziel dieser Arbeit ist es daher, die Morphologie-Änderung der Siliziumelektrode während des Lithiierungs-Prozesses besser zu verstehen unter Nutzung von operando-Methoden Im ersten Teil wurde Neutronenreflektometrie (NR) genutzt, um die Morphologie-Änderung auf der Nanometerskala einer Siliziumelektrode zu untersuchen. Das Wachsen/Schrumpfen der lithiierten Zone im Silizium wurde beobachtet. Auf der Oberfläche der Elektrode wächst im delithiierten Zustand eine Grenzschicht, welche die Lithiierung verhindert. Nachdem diese Schicht aufgelöst ist, kann Lithium eingelagert werden. Im zweiten Teil wurde operando Röntgen- Phasenkontrast-Radiographie genutzt. Ein rechteckiges Riss-Gitter wurde dabei im delithiierten Zustand beobachtet, welches sich während der Lithiierung schließt. Dieses Gitter ist entlang der Kristallachsen des Siliziums orientiert. Im nächsten Zyklus entsteht das Gitter am selben Ort wieder, und breitet sich mit steigender Zyklenzahl über die Elektrode aus. Im dritten Teil wurde der Einfluss einer künstlichen Grenzschicht auf die Lithiierung untersucht. Erneut wurde NR genutzt. Die künstliche Schicht verringert das Wachstum der SEI-Schicht, unterdrückt es jedoch nicht komplett. Nach 2 Zyklen ist die Grenzschicht degradiert, und Seitenreaktionen können beobachtet werden. / Silicon electrodes receive great interest as potential electrode material in lithium-based batteries due to their one order of magnitude higher capacity. This is accompanied by a volume expansion of up to 310 %, leading to an accelerated capacity loss of the electrodes. The volume expansion creates mechanical stress, leading to fracturization of the electrode and the continuous growth of the solid-electrolyte-interphase (SEI) layer under the consumption of active material. The aim of this thesis is to investigate the morphological changes of silicon electrodes during lithiation/ delithiation. Especially operando-techniques are well-suited to investigate these morphological changes since they allow us to precisely link structural data and the electrochemical state. The first project uses operando neutron reflectometry (NR) and in-situ electrochemical impedance spectroscopy (EIS) to analyze the morphology change of the silicon surface on the nanometer-scale. The growth and shrinkage of the lithiated layers within the electrode as well as the lithium concentration was determined with this method. An SEI-layer forms on top of the silicon electrode in the delithiated state, which hinders the lithium uptake in the initial part of the subsequent lithiation. The second project analyzes the morphology-change of the electrode on the µm-scale. Here the fracturization of the silicon electrode is investigated by operando X-ray phase-contrast radiography. A rectangular fracturization pattern was observed during the second half of the delithiation, which vanished again during the lithiation. The third project investigates the influence of an artificial coating layer on the lithiation process. Again operando NR was chosen as analysis tool. The artificial coating decreased the formation of the SEI-layer within the first cycles, but did not suppress it completely. However, this layer degraded already in an early stage of cycling, resulting in the occurrence of side reactions afterward.