Switching on Fast Lithium Ion Conductivity in Garnets: The Structure and Transport Properties of Li_3+xNd₃Te_2−xSb_xO₁₂

Abstract

Polycrystalline samples of the garnets Li 3+x Nd 3 Te 2-x Sb x O 12 have been prepared by high temperature solid state synthesis. X-ray and neutron powder diffraction data show that all compounds crystallize in the space group Ia3 j d with lattice parameters in the range 12.55576(12) Å for x ) 0.05 to 12.6253(2) Å for x ) 1.5. The lithium is distributed over a mixture of oxide tetrahedra and heavily distorted octahedra. Increasing the lithium content in these compounds leads to the introduction of vacancies onto the tetrahedral position and an increasing concentration of lithium found in the octahedra. The latter exhibit considerable positional disorder with two lithium cations positions within each octahedron. Impedance measurements show fast ion conduction with an activation energy of ca. 0.59(6) eV that is largely invariant with composition. Solid-state Li NMR measurements indicate that there is no exchange of lithium between the different coordination environments. These results indicate that lithium conduction in the garnet structure occurs exclusively via a network of edge-linked distorted oxide octahedra and that the tetrahedrally coordination lithium plays no part in the transport properties.

Effects of Sublattice Symmetry and Frustration on Ionic Transport in Garnet Solid Electrolytes

“…In order to investigate the effects of dynamics on this trend we performed ab-initio molecular dynamics (AIMD) at several temperatures, concluding that entropic effects have a minor effect on the the trend. Qualitatively similar results have been observed in several experimental reports [4,5,8,9], but quantitative results vary, likely due to differences in material preparation.…”

“…Other cations occupy the 24c (B) and 16a (C) sites, and oxygen occupies the 96h site. It has been recognized that Li site arrangements vary with Li content x, but there are conflicting reports of the trends coming from experimental measurements [4][5][6]. Li arrangement is difficult to refine, even with neutron scattering, and the nature of ionic conduction is impossible to observe directly.…”

A review of the structural architecture of tellurium oxycompounds

“…However, the C cation Li is now so low in charge that it no longer forms a structural unit with the octahedral cations: the Nd-O bond valence (0.375) is larger than that of . Furthermore, both these compounds have considerably higher Li contents than the 2 atoms per formula unit expected for a normal garnet, due to stuffing of additional Li into normally vacant interstices, and the Li are very mobile in the structure, making the compounds fast-ion conductors (O'Callaghan et al, 2008).…”

Lithium Ion Conduction in Oxides

“…[173,177] A study of the solid solution that bridges the gap between the insulating Li 3 Nd 3 Te 2 O 12 and the fast ion conducing phase Li 5 La 3 Ta 2 O 12 has identified the pathway for lithium conductivity. [185] On introducing as little as 2% lithium into the oxide octahedra, the conductivity increases by two orders of magnitude compared with Li 3 Nd 3 Te 2 O 12 as shown in Figure 3.27. This is accompanied by a step change in the activation energy from ca 1.3 eV to a value, ca 0.6 eV, that is similar to that reported in the fast ion conducting phases.…”