Phase-structure-dependent Na ion transport in yttrium-iodide sodium superionic conductor Na3YI6

Abstract

All-solid-state sodium-ion batteries that utilize inorganic superionic conductors show great potential in energy storage applications. However, suitable Na superionic conductors with desirable high ionic conductivities and cross-linked diffusion networks are still in their infancy. In this work, we systematically studied the structural stabilities and Na ion transport mechanisms of the halide-based Na superionic conductors with different phase structures using first-principle calculations and data mining techniques. The iodide-based Na₃YI₆ with C2/m, Pm1, and P1c space groups possess fcc and hcp anion sublattices with stable octahedral Na occupations, differing from the reported fast-ion transport mechanism in the sulfide bcc anion ones with tetrahedral Na occupations. The Oct–Tet–Oct diffusion pathway and Na-site weighted phonon vibrational frequencies play crucial roles in the synergistic Na-ion transport capabilities, leading to Na ionic conductivities of 0.35, 0.18, and 9.1 × 10⁻³ mS cm⁻¹ at room temperature and corresponding activation energies of 315.5, 351.1, and 454.0 meV for C2/m-Na₃YI₆, Pm1-Na₃YI₆, and P1c-Na₃YI₆, respectively. Restricted by the decisive diffusion triangle, P1c-Na₃YI₆ shows deficient Na-kinetic performance with one-dimensional blocked ion transport channels. The phase-structure-dependent ion transport networks involving Oct–Tet and Oct–Oct pathways broaden the diffusion channels and provide rational guidance for the experimental design of halide-based Na superionic conductors.