Garnet-related Li6.25Al0.25La3Zr2O12 (Al-LLZO) powder was prepared by calcination the precursor synthesized a Couette.Taylor reactor. The powder comprised regular blocks (1-2 mm) and consisted of fine particles (average particle size: D50 = 600 nm, specific surface area: 2.42 m2 g-1) that adopted a highly crystalline nanoscale cubic structure (lattice parameter: a = 13.07769 Å; crystallite size: 41.4 nm). A composite electrolyte (CE) sheet in which the Al-LLZO nanopowder was finely dispersed was prepared by casting a slurry comprising 70 wt% Al-LLZO, polyethylene oxide (PEO), and lithium salt (LiClO4). The total ionic conductivity of the CE sheet was 7.59 x 10-6 S cm-1 at 25 °C and 1.93 x 10-3 S cm-1 at 70 °C, compared with 3.03 x 10-4 S cm-1 at 25 °C for the Al-LLZO pellet. Cyclic voltammetry showed that the electrochemical potential of the CE sheet was superior to that of PEO sheet. For application of the CE in all-solid lithium batteries (ASLBs), a composite NCM-based cathode composed of Al LLZO, PEO, Super-P, and VGCF was also prepared. The discharge capacity of the cell was ~122 mAh g-1 at 0.1C & 70 °C in voltages of 3.0-4.1 V, and 80% of the initial capacity was maintained over 100 cycles with high rate characteristics.