Scandium Induced Structural Disorder and Vacancy Engineering in Li₃Sb – Superior Ionic Conductivity in Li_3−3xSc_xSb

Solid-state electrolytes are indispensable for all-solid-state batteries. Sulfide-based solid electrolytes, such as Li₁₀MP₂S₁₂ (M = Ge, Sn) and Li₆PS₅X (X = Cl, Br, I), exhibit excellent ionic conductivities, with the fastest Li⁺ ion conductor, Li_9.54[Si_0.6Ge_0.4]_1.74P_1.44S_11.1Br_0.3O_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 Li₃Sb, showing its transformation to a superionic conductor through aliovalent substitution of lithium with scandium. Resulting Li_2.55Sc_0.15Sb 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_3−3xSc_xSb, maintains the β-Li₃Sb structure up to a nominal composition of x(Sc) = 0.15, with Sc³⁺ 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.