Scandium Induced Structural Disorder and Vacancy Engineering in Li₃Sb – Superior Ionic Conductivity in Li₃−₃xScxSb

Solid‐state electrolytes are indispensable for all‐solid‐state batteries. Sulfide‐based solid electrolytes, such as Li 10 M P 2 S 12 ( M = Ge, Sn) and Li 6 PS 5 X ( X = Cl, Br, I), exhibit excellent ionic conductivities, with the fastest Li + ion conductor, Li 9.54 [Si 0.6 Ge 0.4 ] 1.74 P 1.44 S 11.1 Br 0.3 O 0.6 , achieving 32 mS cm −1 at room temperature. Phosphide‐based solid electrolytes have recently shown great potential with diverse structures and variable ionic conductivities. This compound class is expanded to the heavier homolog Li 3 Sb, showing its transformation to a superionic conductor through aliovalent substitution of lithium with scandium. Resulting Li 2.55 Sc 0.15 Sb shows an unexpected high ionic conductivity of 42(6) mS cm −1 at 298 K under electron‐blocking conditions in line with a very low activation energy of 17.6(8) kJ mol −1 , representing the highest and lowest reported values, respectively, for a solid Li‐ion conductor so far. Additionally, the compound exhibits a significant, but two orders of magnitude lower electronic conductivity making it a promising candidate for mixed ionic‐electronic conductor (MIEC). The series of new compounds Li 3−3 x Sc x Sb, maintains the β‐Li 3 Sb structure up to a nominal composition of x (Sc) = 0.15, with Sc 3+ ions occupying the tetrahedral voids of the face‐centered cubic Sb anion arrangement and creating vacancies that facilitate efficient Li + ion diffusion pathways. This work proposes a general design strategy for vacancy engineering in which replacement of Li with Sc in simple binary compounds has a direct impact on the ion mobility.