All-solid-state lithium batteries (ASBs) based on a garnet-type ceramic separator and a thick composite cathodes consisting of a solid-state electrolyte (SSE) and a cathode active material (CAM) are promising candidates for future energy storage systems. They can solve the current challenges of conventional lithium-ion batteries by providing higher energy densities since they enable the use of a Li-metal anode and intrinsic safety due to the substitution of the flammable organic electrolyte by a ceramic like Li7La3Zr2O12 (LLZO) garnet. However, the fabrication of such ceramic batteries is challenging due to the necessary high processing temperatures which lead to material compatibility issues and  during the battery operation electrochemical degradation. Thus, we focus on the further development of ceramic components and innovative manufacturing processes, always with a view to scalability, energy efficiency and cost reduction. Free-standing LLZO separator foils can be obtained by an environmentally friendly, water-based tape casting process. The sintering of dense ceramic composite cathodes by conventional routes requires temperatures up to 1050 °C and therefore only the thermodynamically stable LiCoO2 (LCO) could be used as CAM in garnet-type ASBs. However, it is absolutely necessary to implement high-capacity, cobalt-lean CAMs like LiNixCoyMn1–x–yO2 (NCM) to further enhance the energy density of ASBs, reduce their toxicity and the dependence on critical raw materials. Unfortunately, such CAMs are known for their reactivity with LLZO during co-sintering at elevated temperatures. Thus, the application of advanced sintering technologies like Ultrafast High-temperature Sintering, Field Assisted Sintering Technology / Spark Plasma Sintering, and Rapid Thermal Processing was investigated as strategy to reduce the sintering temperature, time, and energy consumption while enabling new material combinations. Furthermore, the use of a sintering additive like Li3BO3 (LBO) within the composite cathode enables the fabrication of a fully inorganic ASB at already 750 °C, so that it is possible to obtain a dense composite cathode consisting of LLZO, LBO and the high capacity, Ni-rich NCM811 as CAM. The material degradation during the battery fabrication processes as well as the electrochemical degradation during battery operation were precisely analyzed by using experimental approaches such as in situ high temperature XRD, DTA/TG/MS, SEM, TEM, EDS, TOF-SIMS, and nuclear reaction analysis (NRA) in combination with simulation studies.