Here, a predominating crystal structure of the solid electrolyte powder is changed to a cubic structure or tetragonal structure depending on the heat treatment temperature. When the heat treatment temperature is 600 to 1200° C., the solid electrolyte powder has a structure in which a cubic structure and a tetragonal structure coexist. Further, physical properties may be improved using a characteristic that the cubic structure or the tetragonal structure is changed at a calcination temperature the same as or higher than the heat treatment temperature.

For example, the heat treatment temperature may be 700 to 800° C., and the solid electrolyte powder may be changed to have a complete cubic structure and a 90% or higher high-density fine structure by calcination at 1200° C. for about 5 hours. The heat treatment temperature may be approximately 900° C., and the solid electrolyte powder may maintain a complete tetragonal structure and a high-density fine structure with a relative density of 60% or higher by calcination at approximately 900° C. for about 5 hours.

In the manufacturing method, the complex agent may be 5 N ammonia solution, and the complex agent may be added such that the aqueous solution has a pH of 10 to 11. For example, the mixing of the starting material with the complex agent may be performed by titration with the starting material at 4 ml/min simultaneously with titration with the complex agent at 4 ml/min. In the forming of the precipitate, a sodium hydroxide (NaOH) solution may be added for pH adjustment in, and the NaOH solution may be 1 M and be automatically titrated based on a pH change in a co-precipitation reactor with co-precipitation proceeding.