Inorganic superionic conductors have high ionic conductivity and excellent thermal stability, but their poor interface compatibility with lithium metal electrodes hinders their application in all-solid-state lithium metal batteries 1, 2. Here, we report a lithium superionic conductor based on LaCl 3 with excellent interfacial compatibility with lithium metal electrodes. Compared with the Li 3 MCl 6 (M = Y, In, Sc, and Ho) electrolyte lattices 3, 4, 5, 6, the UCl 3 type LaCl 3 lattice has large 1D channels for fast Li + conduction, and the La vacancies are interconnected by Ta doping to form a 3D Li + mobility network. The optimized Li 0.388 Ta 0.238 La 0.475 Cl 3 electrolyte exhibits a Li + conductivity of 3.02 mS cm -1 and a low activation energy of 0.197 eV at 30 °C. It also generates a gradient interface passivation layer to stabilize the lithium metal electrodes, enabling long-term cycling of Li-Li symmetric batteries (1 mAh cm-2) for more than 5,000 hours. When directly coupled with the uncoated LiNi 0.5Co 0.2Mn 0.3O 2 positive electrode and bare lithium metal negative electrode, Li 0.388Ta 0.238La 0.475Cl 3 electrolyte can make the solid state battery cycle more than 100 times, the cut-off voltage is 4.35V. The area capacity is more than 1 mAh cm -2. We also show that lanthanide metal chlorides (LnCl 3; Ln = La, Ce, Nd, Sm, and Gd) in the fast Li + conduction, which suggests that LnCl 3 solid electrolyte systems can provide further developments in conductivity and practicality.