Polymer electrolytes have great application potential in energy storage and conversion devices like rechargeable lithium batteries. Polymer electrolytes are inherently safer than traditional liquid electrolytes, because of the superior mechanical performance and low volatility, which results in the suppression of Li dendritic growth, and the prevention of liquid leaks, combustion, and explosion. Consequently there has been extensive research on polymer electrolytes, especially bi-ion conductors obtained by dissolving salt in ion-solvating polymers. For bi-ion conductors, conduction of the cation (often Li⁺ or Na⁺) is of interest. Nevertheless, the anions interacting weakly with the polymer medium usually exhibit faster motion and carry most of the ionic conductivity. These conducting anions rapidly build up at the electrode/electrolyte interface, lowering the electric field that drives translational motion of the cations across the electrolyte. To solve this problem, one approach has been to covalently bond the anion to the polymer, so that the cations become the only conducting ion. We intensively examined ion-conducting mechanism and viscoelastic relaxation dynamics of polymer based single-ion conductors. The conflict between the conductivity and mechanical performance is explained at the molecular scale. Possible pathway to better design of single ion conductor is proposed.