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 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 Li7–2x–3yAlyLa3Zr2–xWxO12 (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 m2 s–1 and 10–12 m2 s–1 at the octahedral and tetrahedral sites, respectively.