First-Principles DFT Study on Inverse Ruddlesden–Popper Tetragonal Compounds as Solid Electrolytes for All-Solid-State Li⁺-Ion Batteries

Li-rich inverse perovskites have recently attracted great interest as solid electrolytes for all-solid-state batteries. Although so far, there are only relatively few candidate solid electrolytes that were reported with an inverse perovskite structure, this is despite the large variety of crystal systems and structure derivatives that can exist in such materials. In this work, we studied by density functional theory calculations the material space of more than 500 inverse-perovksite-type in silico compounds with n = 1 inverse Ruddlesden–Popper tetragonal (iRPt) structure in the general formula Li4(X1–aXa′)(Z1–bZb′)2 (X, X′ ∈ {O2–, S2–, Se2–, Te2–}; Z, Z′ ∈ {F–, Cl–, Br–, I–}; 0 ≤ a, b ≤ 1). We aimed to identify candidate novel compounds for solid electrolyte use, clarify useful descriptors for solid electrolyte design, and determine the characteristic Li+-ion transport mechanism in this system. About 167 compounds were predicted to be thermodynamically (meta)stable with a decomposition energy below 0.1 eV/atom, and we highlight at least 20 novel compounds belonging to the Li4O(Cl1–bBrb)2 series, O/I-bearing compositions, and O/S-bearing compositions. A modified formulation of the Goldschmidt tolerance factor was found to be a good descriptor for thermodynamic stability and electronic band gap energy of iRPt compounds. Meanwhile, geometric features extracted from the void space map of mobile ion pathways were identified as useful descriptors for ion transport properties. Two representative compounds, I4/mmm Li4OBr2 and Cmcm Li16O3SI8, were determined to have bulk Li+-ion activation energies of 0.29 and 0.46 eV, respectively, based on first-principles molecular dynamics calculations. It was determined by surface calculations that iRPt compounds are easily bulk-cleavable, suggesting a high concentration of grain boundaries during typical synthesis and thus the dependence of overall ionic conductivity toward particle morphology.