All solid-state batteries (ASSB) are the next generation of safe and high-energy-density energy storage technology. Their development is currently impeded by the stability issues of solid-state electrolyte (SSE) limiting the creation of interfaces of quality high enough to ensure efficient transfer of charges. The garnet Li7−xLa3Zr2−xTaxO12 (LLZO:Ta) is one of the most appealing oxide-based SSE but its high sensitivity to moist air engenders difficulties in designing process to densify and assemble components of the ASSB. Based on a careful investigation of LLZO:Ta thermal and chemical stability, a heat treatment is designed and shown to fully reverse the protonation process. The investigation of spark plasma sintering protocols confirms that pretreated LLZO:Ta can be densified at a temperature as low as 850 °C or with a short duration (few seconds) at 900 °C. The characterization of obtained ceramics shows conductivities close to the bulk properties and confirms the absence of influence of grain boundaries. The study of composite electrodes shows that LLZO:Ta is suitable to act both as SSE separator and ionic percolator in positive electrode, the selection of active material remains the main issue to target self-standing Li-based ASSB.