Low temperature synthesis of garnet solid state electrolytes: implications on aluminium incorporation in Li₇La₃Zr₂O₁₂

Li ion conducting garnet electrolytes are attracting considerable interest for potential use in all solid state batteries. Nevertheless, their synthesis can be challenging due to the high temperatures required leading to significant Li loss, and consequently the need to add excess Li to counteract this. In this work, we report a low temperature biopolymer sol-gel route to synthesise these garnet materials using Agar (to ensure homogeneous mixing and nucleation through this biotemplating matrix), with the formation of the garnet phase starting at temperatures as low as 600 °C, with single phase samples of tetragonal Li ₇La ₃Zr ₂O ₁₂, and cubic Li _6.4Al _0.2La ₃Zr ₂O ₁₂ prepared at 700 °C (~ 400 °C lower than the conventional solid state routes). Significantly, this route also allowed the synthesis of these garnets without the need for Li excess for the first time, due to the low temperature limiting Li loss. Moreover, if Li excess was used in the synthesis of cubic Li _6.4Al _0.2La ₃Zr ₂O ₁₂, Al incorporation was not observed at this temperature, and rather tetragonal Li ₇La ₃Zr ₂O ₁₂ was obtained. Contrary to previous assumptions, this indicates that the Li is more stable in the structure than Al at low temperature. Thus, Al incorporation only occurs if there is a deficiency of Li in the starting material, or if the sample is heated to elevated temperatures to induce Li volatility, so as to drive the incorporation of Al to charge balance the resultant Li loss.