Al-doped Li7La3Zr2O12 garnet-type solid electrolytes for solid-state Li-Ion batteries

Cubic phase Li7La3Zr2O12 (LLZO) is a promising solid electrolyte for next-generation Li-ion batteries. In this work, the combustion sol–gel technique is used to prepare an Al-doped LLZO solid electrolyte. The crystal structure is investigated, and the cubic phase is confirmed. Densification properties were investigated using SEM and optical dilatometry. The densification of the Al-doped sample takes place in two stages through two different shrinkage rates. Using 0.25 mol Al-dopant 94% relative density is achieved at 1100 °C. The effect of Al-doping on electrochemical properties is investigated in detail using AC impedance spectroscopy. The result indicates that the optimum concentration of the Al dopant in this study is 0.25 mol and, exhibits a high bulk conductivity of 5.89 × 10–4 S cm−1 at 25 °C. Also, the grain boundary conductivity for the 0.25 mol Al-doped LLZO sintered at 1100 °C is improved by half an order of magnitude comparing to Al free sample. The effect of a secondary phase formation on the electrochemical response is identified using the distribution of relaxation times (DRT) method. The time constant responses for different resistive components are characterised as a function of frequency and capacitance and the values are then quantified using an equivalent circuit model.