We have developed a high performance lithium-ion conducting glass-ceramics. This glass-ceramics has the crystalline form of Li1+x+y Al x Ti2−x Si y P3−y O12 with a NASICON-type structure, and it exhibits a high lithium-ion conductivity of 10−3 Scm−1 or above at room temperature. Moreover, since this material is stable in the open atmosphere and even to exposure to moist air, it is expected to be applied for various uses. One of applications of this material is as a solid electrolyte for a lithium-ion battery. Batteries were developed by combining a LiCoO2 positive electrode, a Li4Ti5O12 negative electrode, and a composite electrolyte. The battery using the composite electrolyte with a higher conductivity exhibited a good charge–discharge characteristic.