Li-based solid electrolytes with the NASICON structure offer promising solutions for developing safer solid-state Li-ion batteries (SSLIBs) as potential alternatives to conventional LIBs that use liquid electrolytes, which are prone to safety risks, leakage, and thermal instability. LATP (Li1+xAlxTi2-x(PO4)3) solid electrolytes belong to the LMP [LiM2(PO4)3: M: Zr, Ti, and Ge] family of materials and are considered promising for large-scale SSLIB applications due to their low cost compared to LGP-based solid electrolytes and superior ionic conductivity relative to LZP-based solid electrolytes. In the present study, highly dense bare LTP and Al3+-doped LTP (LATP) were prepared using the solid-state reaction method, and their structural and transport behaviour were investigated by varying the Al3+ content (x = 0.2 to 0.4) and the sintering temperature (900–1100 °C). Conductivity measurements using impedance spectroscopy showed that Li1+xAlxTi2-x(PO4)3 with an x = 0.3 composition sintered at 1000 °C (0.3-LATP@1000), exhibits the highest ionic conductivity (7.2 × 10−5 S cm−1) at room temperature. The transport results of LATP solid electrolytes, with respect to different sintering temperatures and Al3+ dopant concentrations, were explained in terms of their crystal structure, relative density, and morphology. DC polarization measurements revealed that ions are the major charge carriers, while linear sweep voltammetry tests demonstrated excellent ESW with Li-metal, highlighting the potential of 0.3-LATP@1000 solid electrolyte for SSLIBs. Li stripping and plating measurements using a symmetric cell (Li/0.3-LATP@1000/Li) demonstrated good stability of the solid electrolyte for 500 h at a current density of 0.05 mA cm−2. Finally, SSLIB with the LFP/0.3-LATP@1000/Li configuration was demonstrated, exhibiting a discharge specific capacity of 158 mA h g−1 at 0.1C.