New Halide-Based Sodium-Ion Conductors Na₃Y₂Cl₉ Inversely Designed by Building Block Construction

Due to the excellent ionic conductivity and compatibility with high-voltage cathodes, halide-based superionic conductors as promising electrolytes have received widespread attention. A series of halide-based conductors, including Na₃YCl₆, are investigated aiming to find new solid electrolytes for sodium-ion batteries. However, Na₃YCl₆ with high ionic conductivity is meta-stable in thermostability while the stable phase exhibits poor ionic transport properties. In this work, we find that the coplanar formed anionic group (Y₂Cl₉)^3- is the result of a combination of the structural features of the fast ion phase and stable phase of Na₃YCl₆ by systematic analysis of crystal structures. Aiming to find fast sodium-ion conductors, the three-step structure construction method using functional (Y₂Cl₉)^3- groups as building blocks is proposed, and three new crystal structures in the composition of Na₃Y₂Cl₉ with the space group of P6₃, Cc, and R32 are obtained. Na⁺ transport properties, thermostability, and electrochemical window of these structures with various symmetries are investigated by first-principles calculation methods. The results show that the principle to inverse design crystal structures of halides by basic blocks, e.g., anion groups and mobile cations, is proven to be effective and successful. For P6₃-Na₃Y₂Cl₉ with outstanding transport properties, the simulation results indicate that its superionic behavior is attributed to the coherent diffusion connecting two directions. The synchronization of the migration pathways along the ab plane and the migration pathways along the c direction promotes the Na ion conductivity in Na₃Y₂Cl₉. Our research will promote the understanding of the transport mechanism in halide-based electrolytes, and the structure construction method based on functional basic building blocks and special stacking modes will accelerate the inverse design of inorganic crystal structures.