Extended Modeling and Experimental Study of the Li⁺-Ionic Conductivity in Li_1.5+<i>x</i>Al_0.5Ge_1.5Si_<i>x</i>P_3–<i>x</i>O₁₂, <i>x</i> = 0; 0.1

The Li1.5Al0.5Ge1.5(PO4)3 (LAGP) and Li1.6Al0.5Ge1.5P2.9Si0.1O12 (LAGPS) compounds with the NASICON structure have been studied both theoretically and experimentally. The theoretical approach involved analyzing free space in crystal structures (geometrical-topological analysis, GT), calculating ion migration energies using the bond valence site energy (BVSE) method, and determining ionic conductivity at various temperatures through kinetic Monte Carlo simulations. Density functional theory (DFT) calculations were also applied to obtain precise results. While the GT and BVSE analyses revealed similar three-dimensional lithium-ion migration maps for both LAGP and LAGPS structures, DFT calculations indicated a difference in the migration energies: LAGP exhibited a 3D migration energy of 0.84 eV, whereas LAGPS showed a lower energy of 0.57 eV. At the same time, 1D and 2D Li ion diffusions have a lower migration energy. LAGP and LAGPS glass-ceramics were obtained successfully, and their samples were characterized by differential scanning calorimetry. This confirmed the absence of residual glass phases in the samples, as expected. Atomic force microscopy data showed the formation of a continuous texture in both samples. Impedance spectroscopy was used to measure conductivity in the range of −45 to 140 °C. This revealed the absence of any phase transitions in the structures. At various temperatures, the contributions of the bulk and grain-boundary components to the total conductivity were determined. The LAGPS sample showed a higher grain-boundary conductivity and total conductivity compared to those of the LAGP sample, which coincides well with the theoretical results.