Fast ion conduction in the Li-analogues of Nasicon, Li_1+x [(Ta_1-xGe_x)Al](PO₄)₃

Abstract

Fast lithium ion conducting Nasicon compounds, Li_1+x[(Ta_1-xGe_x)Al](PO₄)₃, x=0-1.0 have been synthesized, characterized by X-ray diffraction and X-ray photoelectron spectroscopy (XPS) and their ionic conductivities were determined as a function of temperature (T) and frequency (f) by the impedance technique. The hexagonal lattice parameters and cell volume showed a systematic change with the germanium dopant, x. The XPS Li(1s) spectra show two peaks with binding energies (BEs) of 55.0 and 55.9 eV for all the Ge-doped compounds, due to lithium ion occupation in the Type I and II lattice sites. The measured σ_ionic at 60°C varies from 2.1-5.5 × 10⁻⁵ S cm⁻¹ for x in the range 0.4-0.8 and E_a and log σ_o go through a broad minimum and maximum respectively. From an analysis of σ_ionicvs.f at various T, the f-independent σ_ionic(dc), the hopping frequency (f_p) of the mobile lithium ions and the exponent n of the ac dispersive regime were determined. The values of the activation energies of the ac and dc conductivity and that of the hopping process were found to be the same, implying that the mobile ion concentration is T-independent. Also, the low-frequency dispersion due to the electrode polarization appears to correlate with that of the f-independent dc conductivity.