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.