In order to increase the industrial appeal of solid-state batteries (SSBs) based on garnet Li₇La₃Zr₂O₁₂ (LLZO), the possibility of manufacturing them using scalable production processes such as tape casting must be demonstrated. In particular, the scalable production of thick, high-capacity oxide-ceramic composite cathodes based on Ni-rich LiNi_xMn_yCo_zO₂ (NMC) remains a key challenge on the path to realizing ceramic SSBs with competitive energy densities. The limited thermal compatibility between NMC and LLZO during sintering requires the use of a sintering aid such as Li₃BO₃ (LBO), which, however, impairs the rheological stability of tape casting slurries. In this work, we have developed a new tape-casting process for thick oxide-ceramic composite cathodes by establishing a slurry formulation that is compatible with multiphase cathode compositions and enables the reproducible fabrication of composite NMC cathode tapes with high active material loading. Two cathode configurations are investigated: NMC-LLZO-LBO composite cathodes, where LLZO serves as the catholyte, and LLZO-free NMC-LBO cathodes. After cosintering, the residual porosity of the ceramic cathodes is infiltrated with a polymer electrolyte with and without conductive carbon additives to form hybrid polymer-ceramic SSBs. By systematically correlating cathode chemistry, secondary phase formation, and electrochemical performance, this study demonstrates how cathode chemistry and the balance between ionic and electronic transport pathways influence capacity utilization and cycle stability in thick, tape-cast NMC-based composite cathodes.

Ceramic membranes made of garnet Li7Zr3La2O12 (LLZO) are promising separators for lithium metal batteries because they are chemically stable to lithium metal and can resist the growth of lithium dendrites. Free-standing garnet separators can be produced on a large scale using tape casting and sintering slurries containing LLZO powder, but the quality of the separators is severely compromized by the protonation of the moisture-sensitive LLZO during processing and the irreversible loss of lithium during sintering. In this work, an approach is presented to mitigate the degradation of the LLZO and produce high quality separators using Li2CO3 as a source of excess lithium. By systematically investigating the effects of Li2CO3 addition during the different steps of the tape casting process and the intricate relationship between the protonation and relithiation of LLZO phase, the formation of highly protonated LLZO during ball milling was identified as the most critical step. It was shown that the addition of minimal amounts of Li2CO3 during wet milling effectively suppresses LLZO protonation and ensure the effectiveness of relithiation during subsequent sintering. Using this modified method, flat LLZO separators with a relative density of 95.3 % were prepared in a simplified process with a significantly reduced excess lithium of only 5 mol % with respect to the stoichiometric LLZO, exhibiting an ionic conductivity of 0.18 mS cm−1 at room temperature and a critical current density of 1 mA cm−2 at 60 °C for lithium stripping/plating.