Engineering Lithium Vacancies Through Group 13 Metal Incorporation in Cubic Li 3 Sb

The development of novel solid electrolyte classes is crucial for advancing future energy storage systems. Understanding structure–property relationships enables targeted material design to achieve improved performance. Herein, we present two new cubic solid solution series, Li 12−3 x Ga x Sb 4 and Li 12−3 x In x Sb 4 , which exhibit structural stabilization of a cubic phase derived from β‐Li 3 Sb under ambient conditions through crystallization in the space group Fmm (no. 225). The series demonstrates the ability to tailor void occupancy and vacancy formation via metal insertion. The metal content can be varied over a broad range of x = 0.25–0.75 for gallium and 0.25–1.33 for indium, as confirmed by powder and single‐crystal X‐ray diffraction and powder synchrotron diffraction. Furthermore, increasing the gallium concentration, combined with an adapted synthesis route, yields a tetragonal gallium‐rich phase. Electrochemical studies reveal a decrease in electronic conductivity with increasing group 13 metal content and an ion mobility of ≈ 10 −3 S cm −1 at high indium contents of Li 8 In 1.33 Sb 4 .