The study demonstrates the creation of lithium (Li) superionic conductivity in face-centred cubic (fcc) oxides through the introduction of face-sharing Li configurations via cation over-stoichiometry in rocksalt-type lattices. The researchers synthesized a Li–In–Sn–O compound (o-LISO) with excess Li, which exhibits a total Li superionic conductivity of 3.38 × 10^4 S cm^-1 at room temperature and a low migration barrier of 255 meV. This is achieved by forming a novel spinel-like structure with unconventional stoichiometry, which raises the energy of Li ions and promotes fast Li-ion conduction. The work opens up new possibilities for designing Li superionic conductors within the fcc structural framework, providing a fertile ground for discovering new solid-state electrolytes. The study also explores the thermodynamic stability of over-stoichiometric rocksalt-type oxides composed of various metal species, offering guidelines for future research in this area.The study demonstrates the creation of lithium (Li) superionic conductivity in face-centred cubic (fcc) oxides through the introduction of face-sharing Li configurations via cation over-stoichiometry in rocksalt-type lattices. The researchers synthesized a Li–In–Sn–O compound (o-LISO) with excess Li, which exhibits a total Li superionic conductivity of 3.38 × 10^4 S cm^-1 at room temperature and a low migration barrier of 255 meV. This is achieved by forming a novel spinel-like structure with unconventional stoichiometry, which raises the energy of Li ions and promotes fast Li-ion conduction. The work opens up new possibilities for designing Li superionic conductors within the fcc structural framework, providing a fertile ground for discovering new solid-state electrolytes. The study also explores the thermodynamic stability of over-stoichiometric rocksalt-type oxides composed of various metal species, offering guidelines for future research in this area.