Sodium superionic conductors NaTi2(PO4)3 as a solid electrolyte: A combined experimental and theoretical study

Abstract

The natrium superionic conductors (NASICON) family of materials is being widely examined as a solid-state electrolyte for sodium-ion batteries (SIBs). We present a combined experimental and theoretical study of the crystal structure and ionic conductivity of a NASICON NaTi2(PO4)3. Rietveld refinement of the x-ray diffraction pattern reveals that the prepared single phase NaTi2(PO4)3 exhibits rhombohedral symmetry in the R3¯c space group. The elemental analysis by energy dispersive spectroscopy (EDS) confirms the desired composition of NaTi2(PO4)3. The ionic conductivity evaluated from the electrochemical impedance spectroscopy measurements as a function of temperature shows Arrhenius-type relaxation behaviour with activation energy ~0.47(2) eV. A sizable ionic conductivity (~1.16×10-3S/cm at 600K) has been observed for NaTi2(PO4)3. Further, the migration of Na+ via a sigmoidal path in the skeleton of NaTi2(PO4)3 has been investigated by the density functional theory (DFT). The estimated migration energy for the hopping of Na+ between two crystallographic sites is found to be 0.79eV. The calculated conductivity (3.03×10-7S/cm) resulting from the interstitial diffusivity of the Na-ion is consistent with the measured ionic conductivity at room temperature (~1.34×10-7S/cm). While the wide calculated electronic band gap (2.52eV) suggests poor electronic conductivity of NaTi2(PO4)3, it could be beneficial for solid electrolyte applications.