Using a materials genome approach on the basis of density functional theory, we have formulated a new class of inorganic electrolytes for the fast diffusion of Li + ions by fine-tuning the lattice chemistry of anti-perovskite structures. Systematic modelling was carried out to determine the structural stability and ion transport characteristics in Li 3 AX-based cubic anti-perovskites by alloying on the chalcogen lattice site (A) and alternative occupancy of the halogen site (X). In addition to identifying effective ways of reducing diffusion barriers for Li + ions in anti-perovskite phases via the suitable designation of lattice occupancy, the current theoretical study has led to the discovery and synthesis of a new phase with a double-anti-perovskite structure, Li 6 OSI 2 (or Li 3 O 0.5 S 0.5 I). This new compound has a fairly low activation barrier for Li + diffusion, together with a wide energy band gap to hinder the conduction of electrons.