Ions are ubiquitous in nature. They play a crucial role in countless processes, from the function of proteins rendering life possible on earth to the formation of minerals and the regulation of the ocean's acidity. In technology, ions are even more important both as structural elements for composite materials and as charge carriers in energy conversion and storage. Whether in living organisms or in cutting edge batteries, ions occupy a central role in transporting, converting and storing energy. This process usually hinges of charge exchanges that occur at the interface between a solid surface and a liquid in which the ions are dissolved. Because of the small size of most ions, exchange and transport processes at solid-liquid interfaces tend to be dominated by structural and chemical features of the solid such as defects; much like a pillar or a puddle disturbing the natural movement of a crowed in a busy underground passage. It is therefore crucial to be able to follow single ions at the interface with immersed solids in order to fully understand ions' dynamics; any averaged measurement smears out the impact of the dominating surface features of the solid. To date this has not been possible due a lack of experimental technique: most existing approach rely of some form of averaging over many ions in order to derive precise information. The goal of this fellowship is to develop a novel type of microscope able to probe locally and in-situ the dynamics of single ions at the surface of immersed solids with a simultaneous spatiotemporal resolution exceeding 1 nanometre and 50 nanoseconds. This new microscope will subsequently be used uncover the molecular mechanisms enabling certain ions to migrate efficiently through composite materials while preventing others. It will also be used to investigate the dynamics of single ions at model biointerfaces and answer otherwise inaccessible questions for biological systems. It will also be Significantly, this experimental platform will open up the possibility to directly compare experimental results with computer simulations conducted on the same spatial and temporal scales.