Lithium extraction is one of the most important concerns in the energy storage field, since the methodologies used play a crucially impactful role in the sustainable development of lithium batteries. Recently in Matter, Ma, Sun, and colleagues developed a COF-based membrane implanted with oligoether side chains to provide a selective lithium ion-diffusion pathway. Thanks to the precise control over the pore structure and surface chemistry, the oligoether functionalized COF membrane allows efficient transport of Li+ while obstructing the pass- through of other ions, leading to an improved Li+/Mg2+ separation factor up to 64. This work highlights the significance of pore-environment engineering for developing customized membrane materials for ion sieving and lithium ion extraction.