On dielectrics of polymer electrolytes studied by impedance spectroscopy

We present a phenomenological view on dielectric relaxation in polymer electrolytes in the frequency range where conductivity is independent of frequency. Polymer electrolytes are seen as molecular mixtures of an organic polymer and an inorganic salt. The discussion applies also to ionic liquids. The following is based on systems with poly(ethylene oxide) (PEO) comprising the lithium perchlorate salt (LiClO4) and also pure low-molecular PEO. In those systems, dipole-dipole interactions form an association/dissociation equilibrium which rules properties of the system in the low-frequency region. It turns out that effective concentration, cS, of relaxing species provides a suitable variable for discussing electrochemical behavior of the electrolytes. Quantity cS is proportional to the ratio of DC conductivity and mobility. Polymer salt mixtures form weak electrolytes. However, diffusion coefficient and corresponding molar conductivity display the typical (cS)1/2 dependence as well known from strong electrolytes, due to the low effective concentration cS.