Fast Ion Transport Mechanism and Electrochemical Stability of Trivalent Metal Iodide-based Na Superionic Conductors Na₃XI₆ (X = Sc, Y, La, and In)

The exploration of solid-state sodium superionic conductors with high sodium-ion conductivity, structural and electrochemical stability, and grand interface compatibility has become the key to the next-generation energy storage applications with high energy density and long cycling life. Among them, halide-based compounds exhibit great potential with the higher electronegativity of halogens than that of the sulfur element. In this work, combined with first-principles calculation and ab initio molecular dynamic simulation, the investigation of trivalent metal iodide-based Na superionic conductors C2/m-Na₃XI₆ (X = Sc, Y, La, and In) was conducted, including the fast ion transport mechanism, structural stability, and interface electrochemical compatibility with electrode materials. Along with the tetrahedral-center saddle site-predominant three-dimensional octahedral–tetrahedral–octahedral diffusion network, C2/m-Na₃XI₆ possesses the merits of high Na ionic conductivities of 0.36, 0.35, and 0.20 mS cm^–1 for Na₃ScI₆, Na₃YI₆, and Na₃LaI₆, respectively. Benefiting from its structural stabilities, C2/m-Na₃XI₆ exhibits lower interface reaction energy and better electrochemical compatibility in contact with both Na metal and high-voltage poly-anion (fluoro)­phosphate cathode materials than those of sulfide-based ones. Our theoretical work provides rational design principles for screening and guiding iodide-based C2/m-Na₃XI₆ (X = Sc, Y, La, and In) as promising Na superionic conductor candidates used in all-solid-state energy storage applications.