Fluid-fluid interfaces allow for the fabrication of very thin films with well-defined thickness, ranging from a few nm to hundreds of nm in thickness. For example, surfactants commonly encountered in real life (1 nm in thickness) can be used to form thin films with large surface area (ranging from tens of cm to m-scale). This suggests that surface-active molecules can be oriented at fluid-fluid interfaces and be used to separate molecules. By controlling the alignment and concentration of the molecules at the fluid-fluid interface, it is possible to control the size of pore between molecules. Recently, we found that when polymers are aligned in two-dimensional space, they exhibit a different structure and size than their free volume in three dimensions, and this can be controlled. Instead of using conventional thin film fabrication methods such as interfacial polymerization or coating techniques, it is expected to lead a new paradigm in membrane fabrication by controlling the adsorption behavior of materials at the interface of two immiscible liquids to produce very thin, large-area separation membranes with high permeability and selectivity. For this reason, we are currently conducting research to fabricate ultra-thin films with well-defined pores using two-dimensional interfacial space to develop battery separators and lithium separation processes, as well as lithium recycling and recovery processes.