Concepts and conflicts in polymer electrolytes: The search for ion mobility

Despite four decades of effort the promise of polymer matrices as hosts for highly mobile ionic species has not been realized. Initially a problem caused by the binding of cations to the basic oxygens of polyether type backbones, and partly solved by strategies that yield conductivities some ten orders of magnitude larger than expected from polymer backbone relaxation times at their T_g, the polymer electrolytes still fail to yield the target conductivity of 10⁻² S/cm at ambient temperature, unless used in the gel form in which the ion motion occurs in a liquid medium out of reach of the dilute polymer chains. We review the different attempts made in our lab and others to achieve a complete decoupling of ion motion from segmental relaxation. Concluding that there is some fundamental problem in the original salt-in-polymer solvent (and anionic polymer)physics, due to the ion proximity to the mobility-limiting polymer chains, we look to other ways of employing the strength of polymer materials while supporting ionic conducting phases of high intrinsic conductivity. Our best answer, to date, is an alkali ion-conducting plastic solid phase that can be impregnated into a tough microporous membrane, such as Celgard, of pore dimensions such that the large majority of the alkali cations see only free anions in the course of their migration from cathode to anode, during discharge and recharge of electrochemical devices.