Impedance and modulus spectroscopy of polycrystalline solid electrolytes

A method of characterising the electrical properties of polycrystalline electrolytes is described which enables grain boundary (intergranular) and bulk (intragranular) impedances to be separated and identified, by reference to an equivalent circuit which contains a series array of parallel RC elements. In the simple case of the “ideal solid electrolyte”, the equivalent circuit contains a single RC element. The impedance and modulus spectra, i.e. plots of Z″ and M″ versus log ω, are simple “Debye” peaks whose peak maxima coincide at an angular frequency ωmax=(τσ)−1, where τσ is the “conductivity relaxation time” and the complex modulus is the inverse complex perimittivity. For real solid electrolytes there is usually a distribution of relaxation times, in which case the maxima in the impedance and modulus spectra no longer coincide. An assignment of peaks in these more complex spectra is possible in principle, since the modulus spectrum effectively suppresses information concerning grain boundary (and electrode) effects. Experimental results are presented for cold-pressed lithium orthosilicate and germanate, and for sintered β-alumina. Some advantages of this new approach are demonstrated by comparison with conventional impedance and admittance plane methods of analysis.