A solid electrolyte for a lithium secondary battery having a novel crystal structure and a method for producing the same is provided. The solid electrolyte has excellent lithium-ion conductivity and is chemically stable. The solid electrolyte comprises lithium (Li), germanium (Ge), and sulfur (S), forming a ternary compound with Ge.sub.2S.sub.7 polyhedra and GeS.sub.4 tetrahedra in a unit cell. The electrolyte may include compounds such as Li.sub.16Ge.sub.5S.sub.18 and demonstrates lithium-ion conductivity of at least 8.710.sup.6 S.Math.cm.sup.1 at 25 C. Production methods involve either milling lithium and germanium sulfides to form an amorphous intermediate material, followed by heat treatment, or using crystalline raw materials like Li.sub.2GeS.sub.3 and Li.sub.4GeS.sub.4 directly. Both methods produce a stable, high-conductivity solid electrolyte suitable for all-solid-state lithium batteries, addressing safety and performance limitations of conventional electrolytes.