A comparison of the transport properties of lithium-stuffed garnets and the conventional phases Li3Ln3Te2O12

The structures of new phases Li6CaLa2Sb2O12 and Li6.4Ca1.4La2Sb2O12 have been characterised using neutron powder diffraction. Rietveld analyses show that both compounds crystallise in the space group la3̄d and contain the lithium cations in a complex arrangement with occupational disorder across oxide tetrahedra and distorted oxide octahedra, with considerable positional disorder in the latter. Variable temperature neutron diffraction experiments on Li6.4Ca1.4La2Sb2O12 show the structure is largely invariant with only a small variation in the lithium distribution as a function of temperature. Impedance spectroscopy measurements show that the total conductivity of Li6CaLa2Sb2O12 is several orders of magnitude smaller than related lithium-stuffed garnets with σ=10−7 S cm−1 at 95 °C and an activation energy of 0.82(3) eV. The transport properties of the conventional garnets Li3Gd3Te2O12, Li3Tb3Te2O12, Li3Er3Te2O12 and Li3Lu3Te2O12 have been evaluated and consistently show much lower values of conductivity, σ≤4.4×10−6 S cm−1 at 285 °C and activation energies in the range 0.77(4)≤Ea/eV≤1.21(3).

Graphical abstractThe lithium-stuffed garnets Li6CaLa2Sb2O12 and Li6.4Ca1.4La1.6Sb2O12 accommodate lithium in a complex distribution across oxide tetrahedra and octahedra. The total conductivity of Li6CaLa2Sb2O12 is considerably lower than reported for related fast-ion conducting garnets due to a much larger intra-grain contribution to the resistivity than is commonly found for this family of compounds.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Lithium stuffed garnets have a complex distribution of Li+ across multiple sites. ► They show fast lithium ion conductivity with an activation energy of 0.82 eV. ► This conductivity is much lower than related garnets due to grain boundary effects. ► The stoichiometric garnets show minimal ion mobility.