Ion transport in polymer electrolytes

Transference numbers determined by the classical Hittorf method for liquid polymer electrolytes (solutions of NaSCN etc. in copolymers of ethylene and propylene oxide) yield consistently low values of t+(ca. 0.05). This points to a model in which cations are immobilised by interaction with the polymeric solvent and anions are the principal charge carriers, seemingly at variance with the successful operation of prototype lithium batteries. This paradox is resolved by postulating the ‘transport’ of Li+ ions from anode to cathode via the diffusion of ion pairs down a concentration gradient. A similar mechanism would also explain the higher values of t+(ca. 0.5) determined by the ‘steady-state current’ method and reported elsewhere in the literature.