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

The application of Li7La3Zr2O12 as a Li+ battery 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. Here, a new, facile hybrid sol-gel solid-state approach has been developed in order to accomplish reliable and controllable synthesis of these important phases. In this procedure, sol-gel processed solid precursors of Li7La3Zr2O12 and amorphous Al2O3 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 fu), 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 Al-incorporation levels. The introduction of such a reliable solution-based approach to materials with desirable properties opens up a new synthetic avenue to advanced functional materials with greater control over resulting characteristics for energy storage applications.