Physiochemical Characterization and Electrochemical Impedance Spectroscopic Analysis of NASICON-Based M1+xAlxTi2−x(PO4)3 Electrolytes for Solid-State Batteries

Abstract

The suitability of NASICON-based M_1+xAl_xTi_2−x(PO₄)₃, where M = Li, Na, and x = 0.5 solid-state electrolytes was investigated. Electrolytes were synthesized using high-temperature annealing of NH₄H₂PO₄, TiO₂, and Al₂O₃ compounds followed by ball milling. Synthesized samples displayed a single crystalline phase like NASICON-type material with space group $\text{R}\overline{3\text{c}}$. X-ray photoelectron spectroscopy and Raman spectrum confirmed the absence of impurities in samples, establishing elemental consistency. Microstructure analysis was performed using field emission scanning electron microscopy; samples displayed a granular surface with agglomeration of these grains in a radius of a few micrometers. Electrochemical impedance spectroscopy study showed that the conductivity of samples increased with increasing working temperature, and the highest conductivity values for Li_1.5Al_0.5Ti_1.5(PO₄)₃ and Na_1.5Al _0.5Ti_1.5(PO₄)₃ at 150°C were found to be 3.5 × 10⁻⁴ and 5.3 × 10⁻⁴ Scm⁻¹, respectively. Findings reinforce the suitability of these electrolytes, offering a basis for future work in solid batteries in general and for lithium-ion and sodium-ion batteries in particular.