  Solid polymer electrolytes are considered a promising alternative to
traditional liquid electrolytes in energy storage applications because of their
good mechanical properties, and excellent thermal and chemical stability. A
gap, however, still exists in understanding ion transport mechanisms and
improving ion transport in solid polymer electrolytes. Therefore, it is crucial
to bridge composition--structure and structure--property relationships. Here we
demonstrate that size asymmetry, $\lambda$, represented by the ratio of
counterion to charged monomer size, plays a key role in both the nanostructure
and in the ionic dynamics. More specifically, when the nanostructure is
modified by the external electric field such that the mobility cannot be
described by linear response theory, two situations arise. The ionic mobility
increases as $\lambda$ decreases (small counterions) in the weak electrostatics
(high dielectric constant) regime. Whereas in systems with strong electrostatic
interactions, ionomers with higher size symmetry ($\lambda \approx 1$) display
higher ionic mobility. Moreover, ion transport is found to be dominated by the
hopping of the ions and not by moving ionic clusters (also known as
"vehicular'' charge transport). These results serve as a guide for designing
ion-containing polymers for ion transport related applications.
