This study demonstrates that face-sharing lithium (Li) configurations in face-centred cubic (fcc) oxides can enable Li superionic conductivity. By introducing excess Li into a rocksalt-type lattice, the researchers created a novel spinel-like structure with unconventional stoichiometry, which lowers the energy barrier for Li ion migration. The resulting Li–In–Sn–O compound, o-LISO, exhibits a Li superionic conductivity of 3.38 × 10⁻⁴ S cm⁻¹ at room temperature with a low migration barrier of 255 meV, significantly higher than that of stoichiometric rocksalt-type compounds. The face-sharing Li configurations enhance Li-ion conduction by promoting fast ion migration and creating a 3D connected network of Li ions. The study also shows that over-stoichiometric Li in rocksalt-type oxides can lead to spinel-like ordering, which further improves ionic conductivity. The findings suggest that fcc-type oxides can be used to design Li superionic conductors with high chemical flexibility, opening new avenues for solid-state electrolytes in all-solid-state batteries. The research provides guidelines for designing over-stoichiometric rocksalt-type compounds with high Li-ion conductivity, emphasizing the importance of large redox-inactive metal cations and high Li-excess levels in stabilizing face-sharing Li configurations. The study highlights the potential of fcc-type oxides as promising candidates for Li superionic conductors, offering a new design space for future solid-state electrolytes.This study demonstrates that face-sharing lithium (Li) configurations in face-centred cubic (fcc) oxides can enable Li superionic conductivity. By introducing excess Li into a rocksalt-type lattice, the researchers created a novel spinel-like structure with unconventional stoichiometry, which lowers the energy barrier for Li ion migration. The resulting Li–In–Sn–O compound, o-LISO, exhibits a Li superionic conductivity of 3.38 × 10⁻⁴ S cm⁻¹ at room temperature with a low migration barrier of 255 meV, significantly higher than that of stoichiometric rocksalt-type compounds. The face-sharing Li configurations enhance Li-ion conduction by promoting fast ion migration and creating a 3D connected network of Li ions. The study also shows that over-stoichiometric Li in rocksalt-type oxides can lead to spinel-like ordering, which further improves ionic conductivity. The findings suggest that fcc-type oxides can be used to design Li superionic conductors with high chemical flexibility, opening new avenues for solid-state electrolytes in all-solid-state batteries. The research provides guidelines for designing over-stoichiometric rocksalt-type compounds with high Li-ion conductivity, emphasizing the importance of large redox-inactive metal cations and high Li-excess levels in stabilizing face-sharing Li configurations. The study highlights the potential of fcc-type oxides as promising candidates for Li superionic conductors, offering a new design space for future solid-state electrolytes.