Adamantane Self-Assembly Templates Size-Selective Ion-Transport Domains in Solid-State Organic Electrolytes

Ion transport in nanostructured materials is critical to numerous applications. In most solid-state ion conductors, polymers, and conventional ionic liquids, ion mobility is closely linked to the strength of ion coordination site interaction energies, which are dictated by ion size and valence. Here, we investigate how the size and valence of cation solutes impact phase behavior, solvation environment, and ion mobility in adamantane-derived solid organic electrolytes. We find that the ionic liquid 1-(adamant-1-yl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([AdImMe][TFSI]) templates size-selective ion-transport domains that preferentially pass lithium and magnesium over larger alkali and alkaline-earth cations. Our structural and spectroscopic analyses reveal that the incorporation of larger ions induces structural reconstruction within the TFSI domains, disrupting the ion-transport channels and hindering ion mobility, while the local chemical environment around adamantane groups remains unchanged. We further observe that lithium- and magnesium-containing mixtures exhibit surprisingly similar phase behavior and conductivity, particularly when compared to mixtures composed of sodium and potassium solutes. These magnesium-containing mixtures exhibit conductivity on the order of 0.1 mS cm-1 at 75 °C, which is competitive with other solid-state magnesium ion electrolytes under evaluation for use in multivalent batteries. Our discovery that adamantane-templated electrolytes are nonconductive to sodium and potassium ions while exhibiting significant lithium conductivity suggests the potential for these materials to serve as electrochemical membranes for direct lithium extraction from brines. Overall, our results indicate that diamondoid self-assembly can be utilized to template size-selective ion mobility in electrolytes to address emerging challenges in energy storage and resource recovery.