Oxide-based all solid state battery (ASSB) systems require highly enhanced cell assemblies technology in the light of the sintering issue, although various inorganic-based lithium ion conductors have been proposed and attempted to be adopted in battery systems. Here, the compatibility of Li–La–(Zr–Ta)–O garnet related lithium ion conductors with ASSB systems processed by spark plasma sintering (SPS) is reported. Further, the challenges with respect to the material and processing are also discussed. Li7−x La3Zr1.5Ta0.5O12−d processed by SPS shows a high ionic conductivity of 1.35 × 10−3 S cm−1 and 1.23 × 10−2 S cm−1 at 25 °C and 80 °C, respectively. In addition, the material also shows good structural stability, which is a critical challenge encountered in garnet structured materials treated under severe SPS process conditions. An ASSB assembled by SPS using the Li4Ti5O12–Li7−x La3Zr1.5Ta0.5O12−d composite electrode shows a surface capacity of 0.2 mAh cm−2 at 80 °C. The battery shows a well-organized structure with a good lithium diffusion percolation path and the low solid contact resistance without any defects and undesirable reactions arising from the sintering method.