Effect of ion association on transport in polymer electrolytes

Numerous techniques have been employed in an attempt to elucidate details of the conductivity mechanism in polymer electrolytes, and in particular to determine whether cations, anions or both are mobile. Until recently, most of these studies have assumed a ‘strong electrolyte’ model. Here we examine the effect of ion association on the interpretation of different techniques, including Hittorf, concentration cell, centrifugal cell, ratio-tracer, pulsed field gradient n.m.r. and d.c. polarisation methods. The circumstances under which certain of these experiments yield individual ionic transport numbers are described. It is also shown that, in general, measured diffusion coefficients may not be sensibly compared with mobilities from conductivity measurements using the Nernst–Einstein relationship. The d.c. polarisation technique is examined in some detail and the value of the steady-state conductivity in assessing polymer electrolytes for battery applications is described. Finally, some experimental evidence is presented which appears to support the existence of ion association in polymer electrolytes.