Steffen Jeschke

3D laser scanning confocal microscopy of siloxane-based comb and double-comb polymers in PVDF-HFP thin films

Currently, atomic force microscopy is the preferred technique to determine roughness on membrane surfaces. In this paper, a new method to measure surface roughness is presented using a 3D laser scanning confocal microscope for high-resolution topographic analysis and is compared to conventional SEM. For this study, the surfaces of eight samples based on a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) host polymer with different liquid interpenetrating components were analyzed. Polymethylhydrosiloxane, triethylene glycolallylmethylether, (3,3,3-trifluoropropyl)methylcyclotrisiloxane (D3-C2H4CF3), polysiloxane-comb-propyloxymethoxytriglycol (PSx), polysiloxane-comb-propyl-3,3,3-trifluoro (PSx-C2H4CF3), poly[bis(2-(2-methoxyethoxy) ethoxy) phosphazene, or poly[bis(trifluoro)ethoxy] phosphazene was chosen as interpenetrating compound to investigate the impact of comb and double-comb-structured polymer backbones, as well as their dipolar or fluorous residues on the PVDF-HFP-miscibility. Different phases of the constituting ingredients were identified via their thermal properties determined by DSC. Additionally, the COSMO-RS method supported the experimental results, and with regard to computed σ-profiles, new modified structures for polysiloxane and polyphosphazene synthesis were suggested.

Predicting the Solubility of Sulfur: A COSMO-RS-Based Approach to Investigate Electrolytes for Li-S Batteries

Lithium-sulfur (Li-S) batteries are, in theory, considering their basic reactions, very promising from a specific energy density point of view, but have poor power rate capabilities. The dissolution of sulfur from the C/S cathode in the electrolyte is a rate-determining and crucial step for the functionality. To date, time-consuming experimental methods, such as HPLC/UV, have been used to quantify the corresponding solubilities. Here, we use a computational fluid-phase thermodynamics approach, the conductor-like screening model for real solvents (COSMO-RS), to compute the solubilities of sulfur in different binary and ternary electrolytes. By using both explicit and implicit solvation approaches for lithium bistrifluoromethanesulfonimidate (LiTFSI)-containing electrolytes, a deviation of <0.4 log units was achieved with respect to experimental data, within the range of experimental error, thus proving COSMO-RS to be a useful tool for exploring novel Li-S battery electrolytes.

Catching TFSI: A computational-experimental approach to β-cyclodextrin-based host-guest systems as electrolytes for Li-ion batteries

Cyclodextrins (CDs) are pyranoside-based macromolecules with a hydrophobic cavity to encapsulate small molecules. They are used as molecular vehicles, for instance in pharmaceutical drug delivery or as solubility enhancer of monomers for their polymerization in aqueous solution. In this context, it was discovered about 10 years ago that the bis(trifluoromethylsulonyl)imide (TFSI) anion forms host-guest complexes with βCD in aqueous media. This sparked interest in using the TFSI anion in lithium-based battery electrolytes open for its encapsulation by βCD as an attractive approach to increase the contribution of the cation to the total ion conductivity. By using semi-empirical quantum mechanical (SQM) methods and the conductor-like screening model for a real solvent (COSMO-RS), a randomly methylated βCD (RMβCD) is here identified as a suitable host for TFSI when using organic solvents often used in battery technology. By combining molecular dynamics (MD) simulations with different NMR and FTIR experiments, the formation of the corresponding RMβCD-TFSI complex was investigated. Finally, the effects of the addition RMβCD to a set of electrolytes on the ion conductivity are measured and explained using three distinct scenarios.

Transport of Ions in Salt-in-Polymer Membranes

Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. However, the ion transport mechanism in polymers has been one of the main barriers to further improvement in Li-ion batteries and is still not completely clarified. In an effort to gain a better understanding of the conduction phenomena in electrolytes, a comprehensive survey of all transport mechanism including solvation, segmental motion and hopping, is presented here. Included are a survey of the fundamentals of diffusion and conductivity in polymer electrolytes; recent developments in Li salts; and a detailed discussion about ion transport mechanism with representative references.