Plastic-Crystalline Solid-State Electrolytes

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Electrolyte materials with high ionic conductivity are essential for various energy-storage and -conversion devices, e.g., batteries, fuel cells or supercapacitors. The non-continuous nature of solar and wind energy and the future success of electromobility require significant advances in energy-storage technologies. Thus, finding better electrolytes is one key factor for ensuring a sustainable energy supply of tomorrow. The latest development in this field, to be pursued in this project, are so-called plastic crystals, doped with salts to introduce ionic charge carriers. The molecules in plastic crystals can freely reorient which, via a "revolving door" like mechanism, is believed to be the reason for the surprisingly high ionic conductivity of some members of this material class. Plastic crystals are solid-state electrolytes and thus have invaluable advantages compared to the ubiquitous liquid electrolytes used in current battery technology. However, further optimisation of their properties is needed to make them ready for application. Recently our group found a tremendous increase of the conductivity in the best plastic-crystal electrolyte known so far, when mixing this material with another, related compound consisting of larger molecules. We believe that this effect, which can be well understood within the "revolving door" framework, is the key to make plastic crystals suitable for applications in energy devices. Within this project, we want to further explore this new path towards better solid-state electrolyte materials. For this purpose, we plan to investigate the properties of various mixed plastic-crystalline systems belonging to different material classes with different concentrations and types of admixed salts. We will use dielectric spectroscopy, supplemented by differential scanning calorimetry (DSC) and cyclic voltammetry to characterize the samples. Aside of revealing precise information on the intrinsic electrical conductivity, dielectric spectroscopy is ideally suited to investigate the microscopic mechanisms of ionic charge transport in plastic crystals as it is sensitive to both the reorientational motions of the molecules and the translational motions of the ions, which are believed to be closely coupled. One goal of this project is achieving a better understanding of the involved microscopic charge-transport mechanisms in plastic crystals. Moreover, we want to find ways to optimize their ionic conductivity aiming at conductivities sufficiently high for application.