| Summary: | NASICON-type Li<sub>1+X</sub>Fe<sub>X</sub>Ti<sub>2-X</sub>(PO<sub>4</sub>)<sub>3</sub> (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 Fe<sup>3+</sup> (0.645 Å), a trivalent cation, for Ti<sup>4+</sup> (0.605 Å) on ionic conductivity. Li<sub>1+X</sub>Fe<sub>X</sub>Ti<sub>2-X</sub>(PO<sub>4</sub>)<sub>3</sub> 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<sub>1+X</sub>Fe<sub>X</sub>Ti<sub>2-X</sub>(PO<sub>4</sub>)<sub>3</sub> 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<sub>1+X</sub>Fe<sub>X</sub>Ti<sub>2-X</sub>(PO<sub>4</sub>)<sub>3</sub>, 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<sub>1.3</sub>Fe<sub>0.3</sub>Ti<sub>1.7</sub>(PO<sub>4</sub>)<sub>3</sub> sintered at 900 °C exhibited the highest density among the other conditions and achieved the maximum total ionic conductivity of 1.51 × 10<sup>−4</sup> S/cm at room temperature, with the lowest activation energy for Li-ion transport at 0.37 eV. In contrast, Li<sub>1.4</sub>Fe<sub>0.4</sub>Ti<sub>1.6</sub>(PO<sub>4</sub>)<sub>3</sub> sintered at 1000 °C demonstrated reduced ionic conductivity owing to increased complex impedance associated with secondary phases and grain crack formation.
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