Next-generation electrochemical energy storage (EES) devices, including Li-S and non-aqueous redox flow batteries, require selective membranes capable of allowing lithium transport while blocking crossover of active-species. If not properly controlled, this crossover often leads to capacity fade and cell death. In this presentation, I will discuss our work with a new class of size-selective membranes capable of blocking active-species crossover while allowing high ionic conductivity. These membranes are cast from polymers of intrinsic microporosity (PIMs), which exhibit permanent micropores in the solid-state due to frustrated packing of polymer chains. The design rules and structure-property relationships for achieving high selectivity and long membrane lifetime in this exciting new class of materials will be presented.