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 Li₃YCl_xBr_6-x and Li₃GdCl₃Br₃ and achieved very high room-temperature conductivities of 6.1 and 11 mS cm^-1 for Li₃YCl_4.5Br_1.5 and Li₃GdCl₃Br₃, respectively. These findings open new routes to the design of room-temperature superionic conductors for high-performance solid batteries.