Equivalent circuit models and analysis of impedance spectra of solid electrolyte Na0.25Li0.75Zr2(PO4)3

Abstract

Two RC model circuits are connected in series in order to analyze the electrical and dielectric behaviour of mixed alkali Na0.25Li0.75Zr2(PO4)3 NASICON compound. However, the data obtained could best be described by one RC circuit representing the grain boundary resistance () and capacitance () in the temperature and frequency range 300-600 K and 300 kHz to 1GHz, respectively. The values of the grain boundary activation energy obtained by fitting to the Arrhenius equation in a plot is ~ 0.40 eV, which is close to the bulk activation energy for electrical conduction. The maximum conductivity obtained is 0.3 S/m at 590 K. A non Debye character was observed in the dielectric permitivity in its frequency dependence. However, the temperature dependence of followed a linear behaviour at low temperatures and frequencies but decreased at higher temperatures. Complex non linear least squares fitting of impedance data using a composite circuit shows good fitting results with relative standard deviation less than 0.2 for all the free parameters which is indicative of the accuracy of data obtained. Similar good fitting results, using a generic battery model, suggest the applicability of the material in rechargeable lithium ion batteries.