Toward Understanding the Lithium Transport Mechanism
in Garnet-type Solid Electrolytes: Li<sup>+</sup> Ion Exchanges and
Their Mobility at Octahedral/Tetrahedral Sites
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Abstract
The
cubic garnet-type solid electrolyte Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> with aliovalent doping exhibits a high
ionic conductivity, reaching up to ∼10<sup>–3</sup> S/cm
at room temperature. Fully understanding the Li<sup>+</sup> transport
mechanism including Li<sup>+</sup> mobility at different sites is
a key topic in this field, and Li<sub>7–2<i>x</i>–3<i>y</i></sub>Al<sub><i>y</i></sub>La<sub>3</sub>Zr<sub>2–<i>x</i></sub>W<sub><i>x</i></sub>O<sub>12</sub> (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<sup>+</sup> ion jumping, but it does
noticeably vary the distribution of Li<sup>+</sup> ions, electrostatic
attraction/repulsion, and crystal defects, which increase the lithium
jump rate and the creation energy of mobile Li<sup>+</sup> 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<sup>–9</sup> m<sup>2</sup> s<sup>–1</sup> and 10<sup>–12</sup> m<sup>2</sup> s<sup>–1</sup> at the octahedral
and tetrahedral sites, respectively