Concentration polarization is an important factor that limits the performance of lithium-ion batteries. While a traditional porous separator provides no barrier for salt depletion at the cathode (during discharge), a functional nonporous separator could serve as a gateway for minimizing this depletion while allowing lithium-ion transport. We synthesized a triblock copolymer, poly(styrenesulfonyllithium(trifluoromethylsulfonyl)imide)-b-polyethylene-b-poly(styrenesulfonyllithium(trifluoromethylsulfonyl)imide) (PSLiTFSI-b-PE-b-PSLiTFSI), that self-assembles into nanophase-separated lamellae morphology. Thin membranes obtained through heat pressing this polymer were used as separators in lithium–lithium symmetric cells soaked with liquid electrolytes. Electrochemical experiments on these cells show that our ionic separator slows down salt transport from the cathodic chamber to the anodic chamber by a factor of 4 due to the well-established phenomenon of Donnan exclusion. Our work suggests that ionic separators can serve as a unique gateway to manipulate the concentration polarization, thus providing a potential solution to enable fast charging and discharging of lithium-ion batteries.