Toward Understanding the Lithium Transport Mechanism in Garnet-type Solid Electrolytes: Li⁺ Ion Exchanges and Their Mobility at Octahedral/Tetrahedral Sites

Abstract

The cubic garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ with aliovalent doping exhibits a high ionic conductivity, reaching up to ∼10^–3 S/cm at room temperature. Fully understanding the Li⁺ transport mechanism including Li⁺ mobility at different sites is a key topic in this field, and Li_{7–2<i>x</i>–3<i>y</i>}Al_<i>y</i>La₃Zr_2–<i>x</i>W_<i>x</i>O₁₂ (0 ≤ <i>x</i> ≤ 1) are selected as target electrolytes. X-ray and neutron diffraction as well as ac impedance results show that a low amount of aliovalent substitution of Zr with W does not obviously affect the crystal structure and the activation energy of Li⁺ ion jumping, but it does noticeably vary the distribution of Li⁺ ions, electrostatic attraction/repulsion, and crystal defects, which increase the lithium jump rate and the creation energy of mobile Li⁺ ions. For the first time, high-resolution NMR results show evidence that the 24d, 96h, and 48g sites can be well-resolved. In addition, ionic exchange between the 24d and 96h sites is clearly observed, demonstrating a lithium transport route of 24d–96h–48g–96h–24d. The lithium mobility at the 24d sites is found to dominate the total ionic conductivity of the samples, with diffusion coefficients of 10^–9 m² s^–1 and 10^–12 m² s^–1 at the octahedral and tetrahedral sites, respectively