ION ASSOCIATION IN POLYMER ELECTROLYTES - TRANSPORT AND MATERIALS OPTIMIZATION

The nature of ion association in solid polymer electrolytes is discussed. Measurements of molar conductivity for LiClO4 in poly(ethylene oxide) are presented which indicate the existence of ion pairs, and possibly also ion triples, at concentrations below 0.1 mol dm-3. The theory of d.c. polarization for a fully dissociated, non-ideal, polymer electrolyte is presented and it is shown that the applied potential remains linearly related to the steady state current up to a maximum voltage which depends on the particular electrolyte being considered. In the case of LiClO4 in poly(ethylene oxide) linearity persists beyond the estimated theoretical limit suggesting the presence of mobile neutral clusters at high concentrations (approximately 2 mol dm-3). It is shown that ion association may explain why different values for transport numberss are obtained by different techniques for the same electrolyte. Finally the presence of ion association modifies the strategies which must be adopted in order to synthesize better polymer electrolytes for applications in batteries, smart windows, etc.