All-solid-state lithium-ion batteries are considered a promising alternative to conventional liquid electrolyte-based lithium-ion batteries. The use of solid electrolytes could enable lithium metal as the anode material, which would lead to higher energy densities compared to conventional energy storage systems. At the same time, safety aspects ... could be improved by replacing highly flammable organic liquid electrolytes, making such systems particularly attractive for the mobility sector. Thiophosphate solid electrolytes are considered particularly promising in this context, as this materials class usually exhibits a high ionic partial conductivity and is suitable for conventional industrial manufacturing processes due to their advantageous mechanical properties (i.e., their malleability). However, large-scale application of all-solid-state lithium-ion batteries is currently still hindered by numerous problems. On the positive electrode side, interfacial reactions of the cathode active material with the thiophosphate solid electrolyte are considered to be one of the main reasons for rapid capacity loss of the battery and poor long-term stability. Detailed knowledge on such interfacial phenomena is scarce and studies on this subject are rarely differentiated, currently hindering a fundamental understanding and thus preventing a targeted solution to the problem. In this work, interfacial degradation phenomena in lithium thiophosphate- and LiNi0.6Co0.2Mn0.2O2-based composite cathodes were systematically investigated. It was shown that interfacial degradation occurs at all interfaces within the composite cathode. This includes interfacial reactions of the solid electrolyte against the (i) current collector, (ii) cathode active material, and, if used, (iii) carbon-containing conductive additive, which is often employed to enhance the electronic partial conductivity and to increase cathode active material utilization. By combining spectrometric and spectroscopic studies by means of time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy, it was possible to separate the convoluted degradation processes and provide detailed insights into the reaction processes and the underlying chemistry. In addition, the reaction zones within the composite cathodes could be visualized based on local compositional information with high spatial resolution. Based on the knowledge gained, interfacial protection concepts were developed and investigated in this doctoral thesis. This includes protection concepts for carbon-based conductive additives and for cathode active materials. Analyses of a Li2CO3/LiNbO3-based coating on the cathode active material LiNi0.6Co0.2Mn0.2O2 showed that the protective effect can be attributed to the suppression of the interfacial reaction, in particular, of oxygenated phosphorus and sulfur compounds. Furthermore, it was possible to discuss the influence of the coating on the battery performance and the interfacial phenomena based on its microstructure (i.e., morphology and chemical composition). The results of this work extend the knowledge and understanding of interfacial degradation and corresponding protection concepts in composite cathodes. Such knowledge is essential for developing targeted protection concepts, overcoming problems related to interfacial degradation, and paving the way to long-term stability in all-solid-state lithium-ion batteries. The analytical approaches and workflows established in this doctoral thesis provide the foundation for future investigations on interfacial processes. Corresponding concepts can be transferred to other systems and material combinations, thus enabling the analytical characterization of protection concepts.