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

Next-generation Li-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 a LIB with a solvate ionic liquid 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 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.