  Transport of ions in molecular-scale confined spaces is central to all
aspects of life and technology: into a crack, it may break steel within days;
through a membrane separator, it determines the efficiency of electrochemical
energy conversion devices; or through lipid membranes, it steers neural
communication. Yet, the direct observation of ion mobility and structuring in
sub-nanometer confinement is experimentally challenging and, so far, solely
accessible to molecular simulations. Here, we show quantitative, 3D
molecularly-resolved ion transportation of aqueous ionic liquid and s-block
metal ion solutions, confined to electrochemically-modulated, molecular-sized
slits. Our analysis of atomically resolved solid/liquid interface unveils
generic rules of how enthalpic ion-ion and ion-surface interactions and
entropic confinement effects determine the charge regulation mechanism.
Altering our general understanding, the confined charge regulation may proceed
via fast, kinetically favoured, metastable pathways, followed by slow diffusive
thermodynamic ion reorganization, which has important implications for all
charge-regulated systems.
