Ion Migration Mechanisms in the Sodium Sulfide Solid Electrolyte Na_3–xSb_1–xW_xS₄

The desire to increase the energy density and enhance the safety of batteries has motivated interest in solid-state batteries that employ solid electrolytes. Recently, the sodium solid electrolyte, Na3–xSb1–xWxS4, was reported to exhibit an ionic conductivity exceeding that of all lithium (and sodium) solid conductors. Notably, this compound’s crystal structure contains complex anions─specifically, tetrahedral SbS4 and WS4. Prior studies on related solid electrolytes have argued that reorientations of the complex anions facilitate cation mobility through the so-called paddlewheel effect. Here, ab initio molecular dynamics are used to probe potential contributions of coupled cation–anion dynamics on the mobility of Na-ions. Although Na+ migration is observed to involve the concerted motion of multiple Na-ions, limited evidence is found for contributions to Na migration from reorientations of the SbS4 and WS4 tetrahedra. This implies that the high conductivity of this phase does not result from a paddlewheel effect. Instead, the conductivity is well explained by a classical vacancy model and a strong overlap of cation vibrational modes with anion librations.