Synthese, Charakterisierung und Optimierung neuer Materialien für die Anwendung in leistungsstarken Lithiumionen- und Lithiumfestkörperbatterien

The results of my PhD can be divided into the research fields of LIBs and ASSBs. While the LIB is already widely developed and used, the ASSB is still in its infancy. However, it represents an attractive alternative to outperform the energy density of LIBs and thus enable the broad application of EVs. Even though relatively acceptable ranges have already been achieved with LIBs, especially in the luxury car segment, SSBs, provided they use a lithium anode or alloy, would increase them significantly. Good ranges would then also be possible for small cars. However, it should be mentioned at this point that there are also efforts to establish the lithium anode or larger amounts of silicon in the anode in LIBs. This would also provide a massive increase in energy density. So, the advantage of higher energy density in ASSBs is anything but carved in stone, unlike the increased safety aspect. However, it is very likely that different battery concepts of LIBs and ASSBs will be commercialized in the next few years, and they will be used depending on the application, which is why research on both systems continues to be tremendously important. In my dissertation, the first two publications (sections 4.1 and 4.2) dealt with the application of alternative electrolyte systems in LIBs and the possibility of increasing the energy density in these by using thicker cathode layers. In addition to the use of lithium and the use of new active materials on the cathode side, this concept represents a further possibility to increase the energy density of the battery. However, it has been observed in the literature that at low current densities, the thick electrodes also perform well, and only at high current densities, which are necessary for a practical C-rate, they suffer from poor discharge capacities. In the first study, it was shown that the total impedance increases when using different electrolytes in the order 1 M LiPF6 in EC/EMC 50:50 < Pyrr13FSI/LiFSI < G4/LiTFSI 50:50 due to lower ionic conductivity and transfer number. More importantly, however, it was shown that the total resistance of the cell does not increase with increasing thickness, which is why the principal application of thick electrodes is conceivable. In the paper based on this, the limiting factor for thick cathodes was elucidated in detail by means of impedance spectroscopy. It was shown that, on the cathode side, solid-phase diffusion in particular has a major influence on the resistance and thus the overvoltage of the cell. Simulations showed the influence of the individual parameters on the overall resistance of the cell and the extent to which this can be reduced. For example, the impedance could be significantly reduced by using smaller active material particles and materials with higher solid-phase diffusion coefficients, which should allow higher current densities, at least on the cathode side. This can be used either to use thicker cathodes and thereby further increase the energy density of the cell, or to charge the battery cells faster. However, the anode must not be ignored in either case. High current densities during charging can lead to the deposition of elemental lithium on the anode side, so that the anode must also be adapted to the high current densities.[380] Here, too, the solid-state battery could ultimately prove to be superior, since there are solid electrolytes that have more than twice the ionic conductivity and have a lithium transference number of 1. However, research in the field of solid-state batteries is still much more fundamental. The three publications produced in the course of this work, therefore, also deal with more fundamental issues. For example, the first study showed that the morphology of solid electrolytes has a fundamental effect on the ionic conductivity. While amorphous and glass-ceramic sulfidic solid electrolytes are softer due to the lack of crystalline surface structure, the surfaces of the particles can sinter together more easily, in the case of the latter it is primarily the precompression pressure that is decisive for the measured ionic conductivity. The stack pressure itself only has to be high enough to compensate for the poor contacting of the interfaces to the current collector. Microcrystalline systems, on the other hand, show a different behavior. These are more dependent on the stack pressure, because the grain boundary interfaces of the particles are retained and thus the contacting subsequently deteriorates again. However, this behavior can also be improved with the microcrystalline sulfidic solid electrolytes by heating the pellet up to the crystallization temperature. This causes the grains to melt together. However, this is not a process, which can be used in composite cathodes. In general, this publication has shown, which measurement parameters are important for the determination of high ionic conductivities and what should be taken into account when measuring conductivities in sulfidic electrolytes. Much more important, however, is the fact that crystalline sulfide solid electrolytes reach their maximum ionic conductivity only at measuring pressures of over 250 MPa, while amorphous and glass-ceramic sulfide solid electrolytes already reach their maximum ionic conductivity at below 50 MPa. This could make them interesting candidates for application despite their lower ionic conductivity. Therefore, glass-ceramic solid electrolytes could be interesting candidates for application in ASSBs. To better assess this, it is important to take a closer look at this issue in the composite cathode as well. This project has been started and will be continued in the working group. In publication four, a new synthesis method for a highly ionic conductive solid electrolyte known from literature was presented. This method is characterized by its simplicity since only one synthesis step is required. In this process, the reactants are ground in the ball mill at high revolutions. This results in the solid electrolyte in glass ceramic form with an ionic conductivity of 5.2 mS ∙ cm-1, which makes it the most conductive solid electrolyte currently known in literature that does not require any heating steps. It is not clear to what extent this synthesis method could be of interest to the industry. The high rotational speeds of the ball mill could possibly be a problem, but these could be reduced by using smaller balls. However, in most of the synthesis routes described so far, a ball mill is used first, with which the material is pretreated and then annealed at high temperatures. This is obviously more costly and, secondly, the resulting solid electrolyte is microcrystalline, which can be a disadvantage at low pressures, as shown in Publication [III]. However, its ionic conductivity is again significantly higher at high pressures (up to 24 mS ∙ cm-1 for an annealed pellet).[247] In publication five, the particle size of such a nanocrystalline solid electrolyte was reduced to achieve better mixing in the cathode. It was observed, that as the milling time increases, the particle size decreases, but with it the ionic conductivity also decreases. Therefore, the optimum milling time was determined at which the particles are small enough to ensure good mixing, and at the same time the ionic conductivity is still as high as possible. Batteries were cycled at 0.6 C and they could still be charged and discharged almost completely. In summary, fundamental knowledge was gained in this work on both the LIB and ASSB sides. Impedance spectroscopy and simulations based on a TLM were used to find limiting factors in the cathode of the LIBs. For the ASSBs on the other hand, it could be shown, that in the identification of potential sulfide solid electrolytes SSBs, the morphology plays an important, but so far completely neglected, role. In addition, it was shown that a one-step synthesis could be used to prepare a highly conductive glass-ceramic solid electrolyte. By particle size optimization in the composite cathode of such a solid electrolyte, it was also possible to optimize the capacity of the battery at relatively high C-rates. All this shows that the glass ceramic solid electrolytes are a good alternative to the more common microcrystalline solid electrolytes, as they reach their maximum ionic conductivity at low pressure, reach ionic conductivities comparable to the common carbonate based liquid electrolytes, and can be used in the composite cathodes of ASSBs if the particle size is optimized.