A LaCl₃-based lithium superionic conductor compatible with lithium metal

Inorganic superionic conductors possess high ionic conductivity and excellent thermal stability but their poor interfacial compatibility with lithium metal electrodes precludes application in all-solid-state lithium metal batteries^1,2. Here we report a LaCl₃-based lithium superionic conductor possessing excellent interfacial compatibility with lithium metal electrodes. In contrast to a Li₃MCl₆ (M = Y, In, Sc and Ho) electrolyte lattice^3-6, the UCl₃-type LaCl₃ lattice has large, one-dimensional channels for rapid Li⁺ conduction, interconnected by La vacancies via Ta doping and resulting in a three-dimensional Li⁺ migration network. The optimized Li_0.388Ta_0.238La_0.475Cl₃ electrolyte exhibits Li⁺ conductivity of 3.02 mS cm^-1 at 30 °C and a low activation energy of 0.197 eV. It also generates a gradient interfacial passivation layer to stabilize the Li metal electrode for long-term cycling of a Li-Li symmetric cell (1 mAh cm^-2) for more than 5,000 h. When directly coupled with an uncoated LiNi_0.5Co_0.2Mn_0.3O₂ cathode and bare Li metal anode, the Li_0.388Ta_0.238La_0.475Cl₃ electrolyte enables a solid battery to run for more than 100 cycles with a cutoff voltage of 4.35 V and areal capacity of more than 1 mAh cm^-2. We also demonstrate rapid Li⁺ conduction in lanthanide metal chlorides (LnCl₃; Ln = La, Ce, Nd, Sm and Gd), suggesting that the LnCl₃ solid electrolyte system could provide further developments in conductivity and utility.