Local and Global Structural Effects of Doping on Ionic Conductivity in Na3SbS4 Solid Electrolyte

Among Na-ion solid electrolytes, Na3SbS4 has achieved high ionic conductivity (s_ion) exceeding 10 mS/cm through aliovalent doping. s_ion enhancement due to aliovalent doping is qualitatively explained by the increase in the concentration of defects that mediate ion diffusion. However, a rigorous atomic-scale mechanistic explanation is needed. Doping also affects s_ion by modifying ion mobility - an effect that is not well understood and often overlooked. We use first-principles defect calculations to mechanistically explain and quantify the increase/decrease in Na vacancy concentration due to aliovalent doping of Na3SbS4. By focusing on isovalent doping, we reveal local and global structural effects of doping on the migration barrier, and therefore, ion mobility. In conjunction with experiments, we demonstrate the interplay between the local and global effects. Doping with heavier anions to achieve more polarizable frameworks is a common approach to enhancing s_ion. Our findings present a unique approach to enhancing s_ion by doping with smaller and lighter cations that form stiffer bonds with anions, which in turn soften the parent framework.