In addition, the solid electrolyte material with the cubic structure obtained via calcination at 1200° C. according to the present invention had ion conductivity increasing with a rise in measurement temperature, which was a high value of about 10^?2 S/cm at about 300° C. Here, bulk ion conductivity (σ_b=6.122×10^?2 S/cm) was almost the same as total ion conductivity (σ_t=6.016×10^?2 S/cm) including resistance on an interface between particles. Although not shown in the drawings, a result of measuring ion conductivities of pellets calcinated at 1200° C. for 2 hours, 6 hours and 8 hours by the same method showed that the ion conductivities had values between ion conductivities of the pellets calcinated for 5 hours and 10 hours and a pallet sample calcinated for 5 hours had a highest ion conductivity. This result substantially corresponded to the XRD structure analysis, identifying that the ion conductivity of the solid electrolyte with the garnet structure significantly relies on a crystal structure, amount of impurities, and relative density and fine structure of pellets.

Moreover, although not shown in the drawings of the present invention, the co-precipitation synthesized powder may be subjected to first heat treatment at 600 to 800° C., thereby forming powder with a cubic or tetragonal structure. However, in the present invention, when a sheet in pellets manufactured using powder in a random crystal structure obtained from first heat treatment of the powder is subjected to additional second heat treatment at 1200° C. for 5 hours, a complete cubic crystal structure and high-density fine structure are formed to achieve highest ion conductivity at room temperature.