This work concerns the elaboration and the characterization of Li1.5Al0.5Ge1.5(PO4)3 (LAGP), to be used as solid electrolyte and ionic conductor additive in composite electrodes of all-solid-state lithium-ion batteries.Large batches of LAGP powders with controlled grain size are prepared by crystallization of the corresponding glass precursor. The influence of different thermal treatments on the nucleation/growth of the crystallites was studied. Optimized crystallization of the glass during thermal treatment is obtained using ground glass.The resulting powder is pelletized by SPS technique before undergoing conductivity measurements. The grain conductivity of the SPS pellet presenting a compactness of 87% corresponds to 1.33 × 10− 4 S·cm− 1 at 20 °C, with an activation energy of 0.38 eV. The conductivities of the SPS pellets and of glass-ceramics are compared.