Enhancing Lithium Conductivity Using High-Valence Cations in Cubic Spinel Halide Solid Electrolytes

Halide solid electrolytes for all-solid-state batteries have recently emerged as competitors to oxide and sulfide solid electrolytes due to their excellent electrochemical properties. This ab initio study unveils the dynamic nature of Li₂Sc_2/3Cl₄, a rare superionic conductor among cubic spinel halide materials. Li ions in Li₂Sc_2/3Cl₄ prefer to occupy some of the tetrahedral 8a, octahedral 16c, and octahedral 16d sites, leading to disordered Li distribution. Li ions in Li₂Sc_2/3Cl₄ diffuse through the single-ion diffusion mechanism rather than the concerted diffusion mechanism, providing a high conductivity of 1.36 mS cm^-1. Li ions at the 16d site diffuse as actively as those at the 8a/16c site, an unexpected result that runs counter to the conventional view. In Li₂MgCl₄, the same cubic spinel as Li₂Sc_2/3Cl₄, Li ions at the 8a/16c site diffuse actively, but those at the 16d site are almost immobile, resulting in a very low conductivity of 5.3 × 10^-4 mS cm^-1. The extremely higher conductivity in Li₂Sc_2/3Cl₄ than in Li₂MgCl₄ is because the concentration of Sc³⁺/Mg²⁺ cations blocking the movement of Li ions at the 16d site is lower in Li₂Sc_2/3Cl₄ than in Li₂MgCl₄. Designing cubic spinel materials containing high-valence cations is proposed as a way to increase conductivity by reducing the concentration of multivalent cations that impede Li diffusion. This study sheds new light on how to control conductivity using site-dependent Li mobility in solid electrolytes.