Sol-gel synthesis of Al- and Nb-co-doped Li7La3Zr2O12 solid electrolytes

A series of the Li7−y−3xAlxLa3Zr2−yNbyO12 (y = 0.1–0.25, x = 0–0.25) solid electrolytes were synthesized by sol-gel method and sintered at 1150 °C for 1 h. The phase analysis was done using X-ray diffraction method and Raman spectroscopy; the obtained compounds have a cubic structure Ia-3d. The Li6.6Al0.05La3Zr1.75Nb0.25O12 solid electrolyte has the highest total conductivity, 6.3·10−4 S cm−1 at 25 °C. The growth of Al content in Li6.75−3xAlxLa3Zr1.75Nb0.25O12 system leads to conductivity decreasing and activation energy growth. The conductivity descend can be associated with the decreasing of charge carriers (Li+) amount in the Li6.75−3xAlxLa3Zr1.75Nb0.25O12 structure. Li6.9−3xAlxLa3Zr1.9Nb0.1O12 and Li6.8−3xAlxLa3Zr1.8Nb0.2O12 solid electrolytes with a smaller addition of Nb have lower values of lithium-ion conductivity. Thus, it was concluded that high conductivity of Li6.6Al0.05La3Zr1.75Nb0.25O12 ceramic is caused by the optimal ratio of lithium vacancies and the number of charge carriers in the garnet structure. Thus, obtained solid electrolyte can be used in high-energy lithium and lithium-ion batteries.