Synthesis and Ionic Conductivity of NASICON-Type Li_1+XFe_XTi_2-X(PO₄)₃(x = 0.1, 0.3, 0.4) Solid Electrolytes Using the Sol–Gel Method

NASICON-type Li_1+XFe_XTi_2-X(PO₄)₃ (x = 0.1, 0.3, 0.4) solid electrolytes for all-solid-state Li-ion batteries were synthesized using a sol–gel method. This study investigated the impact of substituting Fe³⁺ (0.645 Å), a trivalent cation, for Ti⁴⁺ (0.605 Å) on ionic conductivity. Li_1+XFe_XTi_2-X(PO₄)₃ samples, subjected to various sintering temperatures, were characterized using TG-DTA, XRD with Rietveld refinement, XPS, FE-SEM, and AC impedance to evaluate composition, crystal structure, fracture surface morphology, densification, and ionic conductivity. XRD analysis confirmed the formation of single-crystalline NASICON-type Li_1+XFe_XTi_2-X(PO₄)₃ at all sintering temperatures. However, impurities in the secondary phase emerged owing to the high sintering temperature, above 1000 °C, and increased Fe content. Sintered density increased with the densification of Li_1+XFe_XTi_2-X(PO₄)₃, as evidenced by FE-SEM observations of sharper edges of larger quasi-cubic grains at elevated sintering temperatures. At 1000 °C, with Fe content exceeding 0.4, grain coarsening resulted in additional grain boundaries and internal cracks, thereby reducing the sintered density. Li_1.3Fe_0.3Ti_1.7(PO₄)₃ sintered at 900 °C exhibited the highest density among the other conditions and achieved the maximum total ionic conductivity of 1.51 × 10⁻⁴ S/cm at room temperature, with the lowest activation energy for Li ion transport at 0.37 eV. In contrast, Li_1.4Fe_0.4Ti_1.6(PO₄)₃ sintered at 1000 °C demonstrated reduced ionic conductivity owing to increased complex impedance associated with secondary phases and grain crack formation.