Sven Uhlenbruck

Silicon is one of the most promising anode materials in lithium batteries due to its high storage capacity; however, it suffers from extensive volume changes during charge and discharge in a battery cell. Mixtures of silicon with transition metals potentially forming transition metal silicides are under intense investigation as mechanically stable and electronically conductive frameworks with additional silicon in between. The overall capacity for lithium storage in such mixtures in general decreases with increasing transition metal content, and the interpretation of the data in the literature is controversial. In this work, it is shown how approaches toward high-throughput experimentation allow a deeper insight into the details of phase formation in transition metal silicides and thus their impact on the electrochemical performance of silicon-silicide frameworks as electrodes for advanced lithium batteries. Silicides appear to evolve solely in narrow compositional ranges around commensurate ratios of cobalt and silicon, while the observed capacity reduction is attributed to an impeded and finally blocked motion of lithium ions.

The phosphate lithium-ion conductor Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) is an economically attractive solid electrolyte for the fabrication of safe and robust solid-state batteries, but high sintering temperatures pose a material engineering challenge for the fabrication of cell components. In particular, the high surface roughness of composite cathodes resulting from enhanced crystal growth is detrimental to their integration into cells with practical energy density. In this work, we demonstrate that efficient free-standing ceramic cathodes of LATP and LiFePO₄ (LFP) can be produced by using a scalable tape casting process. This is achieved by adding 5 wt % of Li₂WO₄ (LWO) to the casting slurry and optimizing the fabrication process. LWO lowers the sintering temperature without affecting the phase composition of the materials, resulting in mechanically stable, electronically conductive, and free-standing cathodes with a smooth, homogeneous surface. The optimized cathode microstructure enables the deposition of a thin polymer separator attached to the Li metal anode to produce a cell with good volumetric and gravimetric energy densities of 289 Wh dm^-3 and 180 Wh kg^-1, respectively, on the cell level and Coulombic efficiency above 99% after 30 cycles at 30 °C.

Columnar structured thermal barrier coatings (TBCs) have been intensively investigated due to their potential to enhance the durability and reliability of gas turbine engine components. These coatings consist of vertically aligned columns that provide excellent resistance to thermal cycling. In this study, the lifetime of columnar suspension-plasma-sprayed (SPS) TBCs was evaluated using burner rig tests. The tests were carried out under high-temperature conditions. Significantly, the pre-oxidation of the bondcoat during diffusion bonding treatment was found to have a substantial impact on the performance of the SPS TBCs. The optimized treatment resulted in columnar SPS TBCs demonstrating excellent thermal stability and resistance under the test conditions. The lifetime of the coatings was significantly extended compared to conventional TBCs by pre-oxidation of the CoNiCrAlY bondcoat in argon, which suggests that columnar SPS TBCs have great potential for use in gas turbine engines.

Solid‐state batteries (SSBs) with a Li₇La₃Zr₂O₁₂ (LLZO) garnet electrolyte are attracting much attention as robust and safe alternative to conventional lithium‐ion batteries. Technical challenges in the practical implementation of garnet SSBs are related to the need for high‐temperature sintering, which often leads to undesirable chemical reactions with the cathode material. While these reactions are well understood for composite cathodes, very little is known about similar processes between cathode and separator during battery fabrication. This work focuses on understanding the processes between the composite LiCoO₂‐LLZO cathode and the LLZO separator and how they affect the battery performance. The extensive diffusion of Co‐ions within LLZO, which leads to the often‐observed LLZO darkening, is shown to have a significant impact on ionic conductivity, electronic conductivity, and dendrite stability of the separator. Experimental data coupled with large‐scale molecular dynamics simulations uncover the diffusion mechanism for Co‐ions and identify secondary phases that form during these interactions. In addition to extensive Co‐ion diffusion within the grains, a non‐uniform segregation of Co‐ions at grain boundaries is found leading to the formation of three distinct Co‐containing phases. This work offers a general approach to studying the fundamental ion diffusion processes that occur during the fabrication of oxide SSBs.

The brick layer model in electrochemical impedance spectroscopy has been successfully used for decades to describe the behavior of polycrystalline materials. Equivalent circuits were created to obtain information about grain and grain boundary properties from electrochemical impedance spectroscopy measurements. Previous publications have expanded on initial interpretations and described grains and grain boundaries as plate capacitors filled with the grain and grain boundary material, respectively. However, this approach poses a number of significant issues, since key assumptions for the equivalent circuits do not match with the actual experimental situation of the material when exposed to an outer electric field, thus calling into question the entire approach for the interpretation.