In-Situ Observation of the Formation and Relaxation Processes of Concentration Gradients in a Lithium Bis(fluorosulfonyl) Amide–Tetraglyme Solvate Ionic Liquid Using Digital Holographic Interference Microscopy

Abstract

Introduction Next-generation Lithium-ion batteries (LIBs) require fast charge–discharge operations, during which a steep concentration gradient (CG) is formed between the two electrodes. The formation of a concentration profile between the electrodes in an LIB with a solvate ionic liquid (SIL) is observed at a distance of less than 1 mm using holographic interferometry. This in situ technique enables the visualization of concentration profile formation near both electrodes during electrolysis, which relaxes after the electrolysis is stopped. The diffusion coefficients near both the electrodes are calculated from the concentrations of transient species near the electrode surfaces. The diffusion coefficient is smaller on the anode side of the cell than that on the cathode side owing to the viscosity of the electrolyte in the diffusion layer. This viscosity effect may have caused the concentration profile to become asymmetrical during the relaxation of the CG. Experimental Li metal was used for the anode and cathode. The distance between the electrodes was 600 μm. The electrolyte was a SIL mixture of LiFSA (lithium bisfluorosulfonylamide) and G4 (tetraglyme) at a molar ratio of 1:1. After 100 seconds of Li electrodeposition and dissolution at a constant current density (3 mA cm-2), the current was stopped, and the relaxation process of the concentration gradient generated by electrolysis was observed with a laser interference microscope (Lyncee Tec) for 1700 seconds. The Li+ concentration near the electrode was calculated from the optical phase difference based on the bulk electrolyte outside the diffusion layer. The electrode surface concentration and transference number of Li+ were used to obtain the diffusion coefficients of Li+ near both electrodes. The time variation of the diffusion layer thickness during and after electrolysis was measured to study the mass transfer phenomenon during the formation-relaxation process of the concentration gradient.