Recently, high concentrated electrolyte (HCE, 3 mol L^-1) has been attracting attention as an electrolyte for next-generation batteries. It has been reported that the solvation structure surrounding Li⁺ in HCE is significantly different compared to the electrolyte with a concentration of about 1 mol L^-1 conventionally used in Li-ion batteries. This unique structure gives rise to high decomposition resistance and low vapor pressure. Also, next-generation batteries, such as lithium sulfur batteries, use Li metal as the negative electrode with high theoretical capacity. It is necessary to use HCE as the electrolyte and Li as the electrode to achieve breakthroughs in energy density, and it is important to clarify the deposition and dissolution mechanisms in the vicinity of the electrode in such batteries. However, there are only a few studies from this viewpoint. In this study, mass transfer phenomena associated with electrochemical dissolution reactions were investigated using a sulfolane-based HCE. In addition, the coordination states surrounding Li⁺ were measured by in situ Raman spectroscopy. In this presentation, the relationship between solvation structure and mass transport was investigated.

Experimental

Li metal with a thickness of 200 μm was used for the anode and cathode. The electrolyte was a mixture of LiTFSA (Lithium bis(trifluoromethanesulfonyl)amide) and SL (Sulfolane) at a molar ratio of 1:2. The electrochemical cell was arranged in a quasi-two-dimensional configuration to minimize the effect of natural convection. Constant currents (1.0, 2.5, and 5.0 mA cm-2) were applied and real-time interferometric measurement was performed near the anode during the electrochemical dissolution reaction. In addition, in-situ Raman spectroscopy was used to observe near the anode to investigate changes in the solvation structure surrounding the Li⁺ near the electrode.

Results and Discussion

Interferometric measurements near the anodes during the electrochemical dissolution reaction showed that the concentration of the electrolyte near the anodes increased as soon as the electrolysis started. As the reaction progressed, the diffusion layer extended toward the bulk direction while maintaining parallelism with the anode surface. The concentration at the anode surface increased continuously with the electrolysis time, and the LiTFSA/SL molar ratio increased to 0.62 after 225 s. The relationship between mass transfer phenomena and solvation structure will be discussed in this presentation.