When an electrolyte is sandwiched between two electrodes and a voltage is applied, ions accumulate at the electrode interface, forming an electric double layer. In conventional solvent-based electrolytes, ions are solvated by surrounding solvent molecules, and these solvated ions align near the electrode surface to form a Helmholtz layer. In contrast, cations and anions are distributed nearly uniformly in the region between the electrodes.

However, in recent years, increasingly concentrated electrolytes have been used in lithium batteries and other applications. At sufficiently high concentrations, the amount of solvent is greatly reduced, and the interactions among ions become increasingly important. Ionic liquids represent the extreme case of such highly concentrated electrolytes, consisting entirely of cations and anions without a conventional molecular solvent. Under an applied voltage, counterions accumulate near the electrode surface. At these high local concentrations, strong electrostatic repulsion between ions of the same charge competes with strong Coulomb attraction between oppositely charged ions. Consequently, the conventional Helmholtz layer model alone is insufficient to describe the electric double-layer structure, and a new structural principle that explicitly consider ion-ion interactions is required.

In this study, we focus on ionic liquids as a model system for high-density electrolytes and investigate how ions are spatially distributed between two electrodes during the formation of the electric double layer. Through this, we aim to clarify the mechanism governing this spatial distribution and establish new design principles for electrochemical devices utilizing high-density electrolytes.

To dynamically measure ion distribution in a liquid, a measurement method that combines element selectivity with high spatial resolution is required. Therefore, in this study, we used a scanning transmission microscope with a spatial resolution of 50 nm to evaluate the local ion distribution near the electrode, and a soft X-ray transmission microscope with a spatial resolution of 500 nm to measure the ion distribution across the entire sample. The soft X-ray transmission microscope is currently under development. Rather than using X-ray focusing optics, it directly illuminates the sample and combines a scintillator with a visible-light magnification system, enabling wide-field observations and high-speed imaging on the millisecond timescale.

In this study, we have investigated the behavior of ion distribution in response to applied voltage by tracking changes in the concentration distribution of fluorine, oxygen, and nitrogen associated with anions of the ionic liquid.

In this presentation, we will describe in detail the spatial distribution of ions in an electric field as obtained from our measurements and its implications for understanding electric double layers in high concentrated electrolytes.