Selective separation of monovalent cationic species from complex mixtures is an industrially relevant procedure necessary for the recovery of many commodity materials, such as lithium from salt brines. Unfortunately, most conventional membranes lack selectivity between monovalent ions, rendering their use in such applications infeasible. One approach to overcoming selectivity limitations is to incorporate ligands into polymer matrices which specifically interact with target cations in an aqueous environment. In this work, using atomistic computer simulations, we assess how incorporating crown ethers, which form host-guest complexes with monovalent cations, into polynorbornene networks impacts the selective partitioning and diffusion of alkali cations. For the case of a 12-Crown-4-functionalized membrane, atomistic molecular dynamics simulations reveal that an unprecedented LiCl/NaCl permeability selectivity of ~ 2.3 arises from strong, favorable interactions between 12-Crown-4 and Na+ ions. These interactions, despite favoring NaCl partitioning into the membrane, sufficiently reduce NaCl diffusivity to permit selective permeation of LiCl. Further simulation studies reveal that a maximum in diffusivity selectivity occurs at an intermediate water volume fraction due to the competition between ion-ligand complexation and free volume size/distribution. Finally, we reveal that ion diffusivities, when scaled by their respective solution diffusivities and free ion fractions, collapse onto an almost universal curve depending on solvent volume fraction. We complement the atomistic simulations with coarse-grained simulations to get a better understanding of the interplay between solubility and diffusivity in ligand functionalized membranes. These results provide critical molecular-level insight into the interplay between membrane chemistry and monovalent ion selectivity, aiding in the rational design of selective membranes for resource recovery.