Aqueous biphasic systems (ABSs) consist of two immiscible phases composed of only one solvent (water) with the phase separation driven by solutes such as polymers, ionic liquids and salts. Such two-phase systems have proved highly relevant in recent years for applications in electrochemical devices. Indeed, highly concentrated solutions of LiTFSI, so called “Water-in-salt” battery electrolyte, were recently found to form an ABS with LiX (with X=Cl, Br, I) aqueous solutions [1,2]. These LiTFSI-LiX ABSs enable the intercalation at high potential of halides such as Cl- or Br- into graphite, in lieu of the oxidation of water or the evolution of halogenated gas, thus enabling the assembly of efficient dual-ion batteries [3]. Similarly, ABS have been proposed to prevent problematic crosstalk mechanisms as observed in Li-ion/sulfur batteries [4] or to be used to design membraneless redox flow batteries [5]. However for ABS to be widely implemented in electrochemical devices, the ion transfer at liquid/liquid interface is key in obtaining good (dis)charge rate and preventing self-discharge. Thus, it is crucial to first understand the structure and chemistry of these aqueous interfaces.

We studied the LiTFSI-LiCl ABSs first with Fourier transform infrared spectroscopy (FT-IR) and surface tension measurement to assess ion partition and surface tension respectively. Both ion partition and surface tension are found increasing as function of increasing concentration. Such trend of the surface tension is typical of a negative adsorption of ions at the liquid/liquid interface. Using high spatial resolution Raman imaging, we were able to confirm a negative adsorption of ions by assessing the ion concentration profiles at the interface between the two aqueous phases. Indeed, we found concentration profiles of water and ions to be sigmoidal which is characteristic of a negative adsorption. Strikingly, the length of the negative adsorption is ranging from 11 to 2 μm with increasing concentrations and the Raman spectra of water and TFSI anion are continuously changing along the interface from an environment with weak hydrogen bounding network and with anion aggregate to an environment similar to diluted solutions. Moreover, when changing the cation from Li+ to H+, the temperature dependence of the phase diagram is inversed, as we could show by variable temperature nuclear magnetic resonance (VT-NMR) and micro-calorimetry, but the interface is still few microns thick. Thus, we revealed a continuous change in the chemical environment between two aqueous phases at the micrometer scale, which contrast drastically with the interface between two immiscible electrolyte solutions (ITIES) such as oil-water systems where molecularly sharp, nanometer interface are found. Such difference raise question about the impact of the thickness and the chemical composition of the interface on the dynamics of ion and electron transfer at the interface, that we are studying by electrochemical measurements. Furthermore, this work paves the way to compare liquid/liquid and solid/liquid interfaces in order to understand how ion solvation affects the interfacial ion transfer and thus enable a better engineering of the electrolyte and ABS for better electrochemical devices.