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, Na_3–xSb_1–xW_xS₄, 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.