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

The cubic garnet-type solid electrolyte Li 7 La 3 Zr 2 O 12 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 x –3 y Al y La 3 Zr 2– x W x O 12 (0 ≤ x ≤ 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 m 2 s –1 and 10 –12 m 2 s –1 at the octahedral and tetrahedral sites, respectively.