Glass-ceramics in Li1+xAlxGe2-x(PO4)3 system: The effect of Al2O3 addition on microstructure, structure and electrical properties

Glass-ceramics of the Li1+xAlxGe2-x(PO4)3 series (0 ≤ x ≤ 0.65) was obtained by glass crystallization under the same conditions of heat treatment. The influence of the heating rate (1, 3 and 8 °C min−1) and the different heat treatment times (2, 8 and 12 h) on the lithium-ion conductivity of glass-ceramics were studied. The impact of Al2O3 addition on the phase composition, microstructure, transport properties and molecular structure of lithium germanophosphate was investigated by XRD, SEM, impedance spectroscopy and Raman spectroscopy, respectively. The obtained electrolytes have the NASICON-type structure with space group R-3c. Glass-ceramics is single-phase when 0.3 ≤ x ≤ 0.5, but impurity phases Li4P2O7, AlPO4, and GeO2 appear at x = 0.65. An increase in Al2O3 content leads to the formation of larger crystallites. A significant improvement in ionic conductivity was observed in Al-doped samples. The conductivity of undoped LiGe2(PO4)3 was found to be 4 orders of magnitude lower compared to the Al-doped solid electrolytes. Among the Li1+xAlxGe2-x(PO4)3 glass-ceramics, the composition of Li1·5Al0.5Ge1.5(PO4)3 (x = 0.5) has the highest lithium-ion conductivity, which is 5.03 × 10-4 S cm−1 at 25 °C. The electron conductivity of solid electrolyte at x = 0.5 is 4 × 10−9 S cm−1 at 25 °C. Ionic transference number was found to be close to 0.99.