Ion Migration Mechanisms in the Sodium Sulfide Solid ElectrolyteNa3-xSb1-xWxS4

The desire to increase the energy density and enhance the safety ofbatteries has motivated interest in solid-state batteries that employ solid electrolytes.Recently, the sodium solid electrolyte, Na3-xSb1-xWxS4, was reported to exhibit anionic conductivity exceeding that of all lithium (and sodium) solid conductors.Notably, this compound's crystal structure contains complex anions???specifically,tetrahedral SbS4and WS4. Prior studies on related solid electrolytes have argued thatreorientations of the complex anions facilitate cation mobility through the so-calledpaddlewheel effect. Here,ab initiomolecular dynamics are used to probe potentialcontributions 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 reorientationsof the SbS4and WS4tetrahedra. This implies that the high conductivity of this phasedoes not result from a paddlewheel effect. Instead, the conductivity is well explainedby a classical vacancy model and a strong overlap of cation vibrational modes with anion librations.

Automated summary

The desire to increase energy density and enhance safety of batteries has led to the development of solid-state batteries with solid electrolytes. The sodium solid electrolyte Na3-xSb1-xWxS4 has been reported to exhibit higher anionic conductivity than lithium solid conductors. This study investigates the contribution of coupled cation-anion dynamics on the mobility of Na-ions using ab initio molecular dynamics. The results suggest that the high conductivity of Na3-xSb1-xWxS4 is not due to the paddlewheel effect but can be well explained by a classical vacancy model and overlap of cation vibrational modes with anion librations.