Conductance of Li-Ionic-Liquid Mixtures in Nanoporous MOFs as Separators for Li-Ion-Batteries*

Metal-organic framework (MOF) based separators in Li-ion-batteries help stabilize the solid electrolyte interphase and strongly affect the battery performance. The mobility and conduction of lithium and organic ionic liquids (ILs) in these materials is crucial and depends on the MOF structures and the IL loading.
Here, we investigate the mobility of Li-IL in the MOF and unveil the detailed conduction mechanism by all-atom molecular dynamics (MD). We observe complex conductivity with loading and composition dependence, as the presence of Li prevents the conductivity collapse at high IL loadings. MD reveals the Grotthuss-like conduction for Li whereas IL exhibits vehicular transport. At small pore fillings, the Li conduction is limited by the large separation between anions. At high pore fillings, the conduction is governed by the bunching of IL. In contrast to the Li-free IL, the bunching effect is attenuated by the formation of charge-neutral Li-anion complexes, which results in a tremendously increased conductivity at maximum filling.
This tuning mechanism may contribtue to development of advanced batteries.