Low-temperature densification of Al-doped Li7La3Zr2O12: a reliable and controllable synthesis of fast-ion conducting garnets

Abstract

The application of Li₇La₃Zr₂O₁₂ as a Li⁺ solid electrolyte is hampered by the lack of a reliable procedure to obtain and densify the fast-ion conducting cubic garnet polymorph. Dense cubic Li₇La₃Zr₂O₁₂-type phases are typically formed as a result of Al-incorporation in an unreliable reaction with the alumina crucible at elevated temperatures of up to 1230 °C. High Al³⁺-incorporation levels are also believed to hinder the three-dimensional movement of Li⁺ in these materials. Here, a new, facile hybrid sol–gel solid-state approach has been developed in order to accomplish reliable and controllable synthesis of these phases with low Al-incorporation levels. In this procedure, sol–gel processed solid precursors of Li₇La₃Zr₂O₁₂ and Al₂O₃ nanosheets are simply mixed using a pestle and mortar and allowed to react at 1100 °C for 3 h to produce dense cubic phases. Fast-ion conducting Al-doped Li₇La₃Zr₂O₁₂ phases with the lowest reported Al³⁺-content (∼0.12 mol per formula unit), total conductivities of ∼3 × 10⁻⁴ S cm⁻¹, bulk conductivities up to 0.6 mS and ion conduction activation energies as low as 0.27 eV, have been successfully achieved. The ease of lithium diffusion in these materials is attributed to the formation of dense cubic phases with low Al³⁺ dopant ratios. This approach is applicable to Li_7−xLa₃Zr_2−xTa_xO₁₂ phases and opens up a new synthetic avenue to Li₇La₃Zr₂O₁₂-type materials with greater control over resulting characteristics for energy storage applications.