Lattice Dynamical Approach for Finding the Lithium Superionic Conductor Li₃ErI₆

Driven by the increasing attention that the superionic conductors Li₃MX₆ (M = Y, Er, In, La; X = Cl, Br, I) have gained recently for the use of solid-state batteries, and the idea that a softer, more polarizable anion sublattice is beneficial for ionic transport, here we report Li₃ErI₆, the first experimentally obtained iodine-based compound within this material system of ionic conductors. Using a combination of synchrotron and neutron diffraction, we elucidate the structure, the lithium positions, and possible diffusion pathways of Li₃ErI₆. Temperature-dependent impedance spectroscopy shows low activation energies of 0.37 and 0.38 eV alongside promising ionic conductivities of 0.65 and 0.39 mS·cm^–1 directly after ball milling and the subsequently annealed Li₃ErI₆, respectively. Speed of sound measurements are used to determine the Debye frequency of the lattice as a descriptor of the lattice dynamics and overall lattice softness, and Li₃ErI₆ is compared to the known material Li₃ErCl₆. The softer, more polarizable framework from the iodide anion leads to improved ionic transport, showing that the idea of softer lattices holds up in this class of materials. This work provides Li₃ErI₆ as an interesting framework for optimization in the class of halide-based ionic conductors.