Local and Global Structural Effects of Doping on Ionic Conductivity in the Na ₃ SbS ₄ Solid Electrolyte

The Na 3 SbS 4 solid electrolyte has achieved high ionic conductivity (σ ion ) exceeding 10 mS/cm through aliovalent doping. While σ ion is enhanced through aliovalent doping, a rigorous atomic-scale mechanistic explanation is needed. Doping also affects σ 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 Na 3 SbS 4 . 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 σ ion . Our findings present a unique approach to enhancing σ ion by doping with smaller and lighter cations that soften the parent framework.