Enhancing Lithium\nConductivity Using High-Valence\nCations in Cubic Spinel Halide Solid Electrolytes

Halide solid electrolytes for all-solid-state batteries\nhave recently\nemerged as competitors to oxide and sulfide solid electrolytes due\nto their excellent electrochemical properties. This ab initio study\nunveils the dynamic nature of Li<sub>2</sub>Sc<sub>2/3</sub>Cl<sub>4</sub>, a rare superionic conductor among cubic spinel halide materials.\nLi ions in Li<sub>2</sub>Sc<sub>2/3</sub>Cl<sub>4</sub> prefer to\noccupy some of the tetrahedral 8a, octahedral 16c, and octahedral\n16d sites, leading to disordered Li distribution. Li ions in Li<sub>2</sub>Sc<sub>2/3</sub>Cl<sub>4</sub> diffuse through the single-ion\ndiffusion mechanism rather than the concerted diffusion mechanism,\nproviding a high conductivity of 1.36 mS cm<sup>–1</sup>. Li\nions at the 16d site diffuse as actively as those at the 8a/16c site,\nan unexpected result that runs counter to the conventional view. In\nLi<sub>2</sub>MgCl<sub>4</sub>, the same cubic spinel as Li<sub>2</sub>Sc<sub>2/3</sub>Cl<sub>4</sub>, Li ions at the 8a/16c site diffuse\nactively, but those at the 16d site are almost immobile, resulting\nin a very low conductivity of 5.3 × 10<sup>–4</sup> mS\ncm<sup>–1</sup>. The extremely higher conductivity in Li<sub>2</sub>Sc<sub>2/3</sub>Cl<sub>4</sub> than in Li<sub>2</sub>MgCl<sub>4</sub> is because the concentration of Sc<sup>3+</sup>/Mg<sup>2+</sup> cations blocking the movement of Li ions at the 16d site is lower\nin Li<sub>2</sub>Sc<sub>2/3</sub>Cl<sub>4</sub> than in Li<sub>2</sub>MgCl<sub>4</sub>. Designing cubic spinel materials containing high-valence\ncations is proposed as a way to increase conductivity by reducing\nthe concentration of multivalent cations that impede Li diffusion.\nThis study sheds new light on how to control conductivity using site-dependent\nLi mobility in solid electrolytes.