Dominik Voigt

The use of localized high‐concentration electrolytes (LHCEs) has emerged as a promising strategy to improve the stability of Li metal electrodes while overcoming the high viscosity and cost associated with high‐concentration electrolytes (HCEs). In this work, we investigate how electrolyte solvation structure and further physicochemical properties, like Li electrodeposition and SEI formation, evolve across a wide concentration range—from diluted electrolytes to HCEs and LHCEs—and how these changes influence Li reactivity and solid electrolyte interphase (SEI) formation in NCM ‖ Li cells. Ether‐based electrolytes composed of LiTFSI as conducting salt and 1,2‐dimethoxyethane (DME) as solvent are systematically modified using 1,1,2,2‐tetrafluoroethyl 2,2,3,3‐tetrafluoropropylether (TTE) as a non‐ion‐coordinating diluent to form LHCEs. Surface‐sensitive characterization, including time‐of‐flight secondary ion mass spectrometry, is employed to analyze the SEI composition. By correlating electrolyte concentration, solvation structure, and interfacial chemistry, this study focuses on the investigation of the Li metal surface after spontaneous formation of the SEI with the electrolytes and after electrodeposition/‐dissolution in Cu ‖ Li as well as high‐energy NCM ‖ Li cells.

The specific energy of Li ion batteries with layered‐oxide‐based cathodes can be enhanced by enhancing charge cut‐off voltage, which increases both, mean discharge voltage and capacity. However, the increased delithiation capacity facilitates thermodynamically‐driven phase changes and increases transition metal (TM) dissolution, finally initiate a failure cascade, known as “electrode crosstalk”. This can trigger formation of high surface area lithium (HSAL), active Li/capacity loss, and consequently decrease the cycle life. Here, after screening of bio‐based additives, the maleimide (MI) is shown to be even superior compared to the effective and literature‐known TM‐scavenging additive lithium difluorophosphate (LiDFP). Although, the electrode crosstalk and the TM amount on the anode are not suppressed with MI, the anode surface remains intact, and is even free from HSAL deposits. The solid electrolyte interphase (SEI) is shown to consist from oligo and polymer‐based MI decomposition products and is concluded to scavenge TMs on the outer SEI layer via its characteristic coordinating chemistry. This way the TM diffusion toward the inner SEI layer is suppressed, where it otherwise would block the Li ⁺ pathway and initiate the well‐known failure cascades, that is, locally increase resistance, trigger HSAL formation and capacity fade, in worst case rollover failure.

A general procedure based on electrostatic self-assembly for preparing nanocomposites based on carbon nanotubes (CNTs) and ternary chalcogenide semiconductor nanoparticles is shown. This was achieved by surface functionalization of the single components through well-established protocols, for CNTs, and a transferable general strategy for the nanoparticles. Heterostructures were then synthesized through electrostatic interaction between oppositely charged components. Structural, colloidal, and optical properties were characterized by transmission electron microscopy, X-ray diffraction, infrared spectroscopy, dynamic light scattering, ζ-potential, and absorption- and (time-resolved) photoluminescence measurements. Interestingly, the nanocomposites showed a blue shift in their excitation and emission spectra when compared to the pure nanoparticles but only when analyzed in powder form. Further investigations in the form of density functional theory (DFT) calculations were performed to evaluate the origin of the change in the optical properties.

The linear photochemical response of materials depends on two critical parameters: the size of the optical band gap determines the onset of optical excitation, whereas the absolute energetic positions of the band edges define the reductive or oxidative character of photo-generated electrons and holes. Tuning these characteristics is necessary for many potential applications and can be achieved through changes in the bulk composition or particle size, adjustment of the surface chemistry or the application of electrostatic fields. In this contribution the influence of surface chemistry and fields is investigated systematically with the help of standard DFT calculations for a typical case, namely composites prepared from ZnS quantum dots and functionalized carbon nanotubes. After comparing results with existing qualitative and quantitative experimental data, it is shown conclusively, that the details of the surface chemistry (especially defects) in combination with electrostatic fields have the largest influence. In conclusion, the development of novel or improved photoresponsive materials therefore will have to integrate a careful analysis of the interplay between surface chemistry, surface charges and interaction with the material environment or substrate.