Preparing method of lithum-lanthanum-zirconium-based solid electrolyte

The present invention relates to a method for preparing a lithium (Li)-lanthanum (La)-zirconium (Zr)-based solid electrolyte, comprising the following steps: adding a first lithium source to a La-Zr precursor and primarily thermally treating the same, and preparing a first Li-La-Zr-based precursor; adding a second lithium source to the Li-La-Zr-based precursor, secondarily thermally treating the same, and preparing a second Li-La-Zr-based precursor; and sintering the second Li-La-Zr-based precursor and producing a solid electrolyte including a Li-La-Zr-based composite oxide. According to the present invention, physical processability and electrical properties (especially, ion conductivity) can be improved.