  We present the analysis of the impedance spectra for a binary electrolyte
confined between blocking electrodes with dielectric layers. An expression for
the impedance is derived from Poisson-Nernst-Planck equations in the linear
approximation taking into account the voltage drop on the dielectric layer. The
analysis shows, that characteristic features of the frequency dependence of the
impedance are determined by the ratio of the Debay length and the effective
thickness of the dielectric layer. The impact of the dielectric layer is
especially strong in the case of high concentrated electrolytes, where the
Debay length is small and thus comparable to the effective thickness of the
dielectric layer. To verify the model, measurements of the impedance spectra
and transient currents in a liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB)
confined between polymer-coated electrodes in cells of different thicknesses
are performed. The estimates for the diffusion coefficient and ion
concentration in 5CB obtained from the analysis of the impedance spectra and
the transient currents are consistent and agree with previously reported data.
We demonstrate that calculations of the ion parameters from the impedance
spectra without taking into account the dielectric layer contribution lead in
most cases to incorrect results. Application of the model to analyze violations
of the low-frequency impedance scaling and contradictions in the estimates of
the ion parameters recently found in some ionic electrolytes are discussed.
