Elektrochemische Untersuchung von Energiespeichermaterialien für Lithium-Ionen-Batterien und Superkondensatoren

In the present work, electrode materials for lithium-ion batteries and supercapacitors were studied. The first part of this work is focused on titanium oxide-based anode materials for lithium-ion batteries (TiO2 and Li4Ti5O12). Herein, it was discussed, whether lithium can be stored not only by intercalation into the bulk material (so called faradaic processes), but also in the layers near the surface (so called pseudocapacitive processes). Pseudocapacitive processes are faster than the diffusion-controlled intercalation into the bulk and could therefore improve the power density of the cell. To study this effect, scan rate-dependent cyclic voltammetry is suitable because processes of different time scales are distinguishable. By determining the current flow during intercalation with respect to the scan rate, it could be shown that no diffusion limitation occurs at low scan rates, because the whole material is accessible for lithium ions. For higher scan rates, the diffusion time is shorter and therefore, only a small amount of the material is accessible. The relation between current flow and scan rate indicates a diffusion-limited process without pseudocapacitive effects. In the literature, a method is described to distinguish between pseudocapacitive and faradaic currents. However, by measuring and simulating systems only based on faradaic processes, this method generates artifacts, which can be misinterpreted as pseudocapacitive currents. These artifacts are caused by quasi-reversible electron transfer within the electrode due to poor electronic conductivity. The second and third part of the present work discuss carbon-based electrode materials for supercapacitor applications, where electrostatic energy is stored by double-layer formation. Electrodes for supercapacitor applications exhibit complex morphologies to increase their surface area. However, this influences the electronic conductivity of the material. In order to study the interplay between material morphology and its electronic conductivity, materials based on polyacrylonitrile fibers spun to thin mats and carbonized were analyzed. Within the materials, two different directions of the electronic transport pathway are possible, along the fibers and perpendicular to them. Therefore, three different measurement setups were analyzed, which differ in the direction of transport within the material. By combining results from experiment and simulations of model structures, the relationship between morphology and conductivity was revealed. In the bar-type method, the electron transport in the direction of the fibers is addressed. This transport is limited only by the volume fraction of the conductive phase. Therefore, the intrinsic conductivity of the material could be determined. However, the transport perpendicular to the fiber direction is more relevant for supercapacitor applications. This direction is studied by the parallel-plate method. The conductivity measured with this method is lower due to longer conduction paths and constrictions within the morphology. The conductivity could be increased by improving the fiber-fiber contact. In case of the van-der-Pauw method, no clear relation between conductivity and morphology of the studied material were found. However, the bartype and the parallel-plate method are suitable to determine the intrinsic conductivity and the conduction in the direction most relevant for electrochemical applications. The third part of the work is focused on the analysis of supercapacitor cells with carbonbased electrode materials. Two different carbonization methods were studied – the conventional thermal carbonization in an oven and the carbonization using an infrared laser. Whereas the thermally carbonized material exhibit low surface areas, the laser carbonization yields materials with larger surface areas. However, the electrochemical characterization of the laser carbonized material revealed that only a small amount of the surface area is accessible to the electrolyte. The small surface area of the thermal carbonized material, on the other hand, is almost completely accessible. By activating the material with KOH, it is possible to widen the pores and increase the surface area. According to the area-specific capacity, the entire surface area of both materials is accessible for the electrolyte after activation. Interestingly, both materials exhibit comparable properties after activation in terms of surface area and porosity. Therefore, the capacities of both materials are comparable. However, the capacity of the laser carbonized electrode is slightly larger than for the thermally carbonized material.