Synthesis and Conductivity Study of Solid Electrolytes Li1 + xAl x Ge2–x(PO4)3 (x = 0–0.65)

A new liquid-phase method has been suggested for synthesis of solid electrolytes with lithium conductivity of composition Li1 + xAl x Ge2–x(PO4)3 (x = 0, 0.1, 0.3, 0.5, 0.6, 0.65) with a NASICON structure. The method is based on the use of an alkaline solution of GeO2 and water-soluble Al(NO3)3 · 9H2O, LiNO3, and (NH4)2HPO4 salts. The synthesized materials have been characterized by X-ray powder diffraction and impedance spectroscopy. The formation of crystalline Li1 + xAl x Ge2–x(PO4)3 products with a NASICON structure is observed at 750–1000°C depending on the degree of substitution of germanium by aluminum. The effect of final powder annealing temperature and ceramics sintering temperature on the phase composition, density, and conductivity of resulting materials has been studied. The material of composition Li1.6Al0.6Ge1.4(PO4)3 produced by sintering the synthesized powder at 900°C exhibits the highest conductivity (3.8 × 10–4 S/cm at a ceramics density of 86%) and the lowest conductivity activation energy (30.5 ± 0.4 kJ/mol in the temperature range 25–250°C).