An Entropically Stabilized Fast-Ion Conductor: Li3.25[Si0.25P0.75]S4

We report on a family of lithium fast ion conductors, Li3+x[SixP1-x]S-4, that exhibit an entropically stabilized structure type in a solid solution regime (0.15 < x < 0.33) with superionic conductivity above 1 mS.cm(-1). Exploration of the influence of aliovalent substitution in the thermodynamically unstable beta-Li3PS4 lattice using a combination of single crystal X-ray and powder neutron diffraction, the maximum entropy method, and impedance spectroscopy reveals that substitution induces structural splitting of the localized Li sites, effectively stabilizing bulk beta-Li3PS4 at room temperature and delocalizing lithium ion density. The optimal material, Li-3.25[Si0.25P0.75]S-4, exhibits inherent entropic site disorder and a frustrated energy landscape, resulting in a high conductivity of 1.22 mS.cm(-1) that represents an increase of three orders of magnitude compared to bulk beta-Li3PS4 and one order of magnitude higher than the nanoporous form. The enhanced ion conduction and lowered activation barrier with increasing site disorder as a result of aliovalent tuning reveals an important strategy toward the design of fast ion conductors that are vital as solid state electrolytes.