Sodium ion conductivity of hexagonal layered P2-type phases with multiple cationic substitutions

Abstract

It is now widely assumed that conductivity of solid electrolytes may be markedly enhanced by the “high-entropy” (HE) effect. However, HE electrolytes usually differ from their simpler isomorphs by other factors affecting conductivity: lattice expansion/contraction, mobile ion content, bond ionicity, quality of samples. To attribute enhanced conductivity to the HE effect, all other factors should be identical. This work compares “simple” and “HE” ceramic sodium-ion conductors of two related structure types: P2 Na_x(M_yTi_1-y)O₂ and honeycomb-ordered P2S Na₂M₂TeO₆, with similar unit cell data, x values, density, and texture and does not reveal any considerable HE effect. In particular, new Na₂M₂TeO₆ conductors combining three to five divalent M (from the list Mg, Co, Ni, Cu, Zn) show similar conductivities with their monocation analogs. Higher conductivity of the P2-type titanates (2–4 mS/cm at 373 K) is explained by the geometrical effect due to smaller size of octahedral cations and possibility of decreased x values.