Effect of Superstoichiometric Amounts of Sodium and Phosphorus on the Phase Composition and Ionic Conductivity of Zirconium and Sodium Silicophosphates (NASICON)

Using the method of pyrolysis of solutions in a melt, the phase formation of sodium and zirconium silicophosphates Na_1+xZr₂Si_xP_3–xO₁₂ has been studied depending on the concentrations of sodium and phosphorus in the precursors. The influence of the content of these components and calcination conditions on the change in the ionic conductivity of NASICON has been studied. X-ray powder diffraction analysis, scanning electron microscopy, full-profile Rietveld analysis, and electrochemical impedance spectroscopy have been used. The specific values of grain conductivity (σ_b) and grain boundaries (σ_gb) of the samples were calculated. It has been found that the cause for the change in ionic conductivity is a change in the composition of NASICON with increasing concentrations of sodium and phosphorus in the precursor. The main condition for high conductivity of the material is the formation of a crystalline phase corresponding to the composition Na₃Zr₂Si₂PO₁₂, as well as a minimal amount of impurities and glass phase. The conductivity of the NASICON sample (x = 2) under certain processing conditions is ~ 1 × 10^–3 S/cm.