Tuning collective anion motion enables superionic conductivity in solid-state halide electrolytes

Halides of the family Li3MX6 (M = Y, In, Sc and so on, X = halogen) are emerging solid electrolyte materials for all-solid-state Li-ion batteries. They show greater chemical stability and wider electrochemical stability windows than existing sulfide solid electrolytes, but have lower room-temperature ionic conductivities. Here we report the discovery that the superionic transition in Li3YCl6 is triggered by the collective motion of anions, as evidenced by synchrotron X-ray and neutron scattering characterizations and ab initio molecular dynamics simulations. Based on this finding, we used a rational design strategy to lower the transition temperature and thus improve the room-temperature ionic conductivity of this family of compounds. We accordingly synthesized Li3YClxBr6−x and Li3GdCl3Br3 and achieved very high room-temperature conductivities of 6.1 and 11 mS cm−1 for Li3YCl4.5Br1.5 and Li3GdCl3Br3, respectively. These findings open new routes to the design of room-temperature superionic conductors for high-performance solid batteries. While solid-state lithium-ion batteries offer promising energy densities for safe energy storage, typical solid electrolytes show poor room-temperature ionic conduction. Now the origin of the superionic transition observed in Li3YCl6-type Li-ion conductors is revealed by in-depth crystal structure characterizations and improved ionic conductivities achieved by lowering the transition temperature.