Electrochemical behavior of electrode reactants dissolved in concentrated lithium bis(trifluoromethanesulfonyl)imide aqueous solutions

In this study, the redox reactions of metal ion complexes and the reduction of oxygen molecules (O2) in highly concentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) aqueous solutions, which have an expanded potential window, were investigated. The electrochemical behavior of the LiTFSI aqueous solution was compared with that of aqueous solutions containing conventional inorganic salts as electrolytes, and the characteristics of the electrode reaction in highly concentrated LiTFSI solutions were clarified. In LiTFSI aqueous solutions, at high concentrations, the solution is hydrophobic, and hydrophobic metal ion complexes can become soluble in concentrated LiTFSI aqueous solutions. It was also observed that electron transfer was promoted as the LiTFSI concentration increased, and it was hypothesized that at high concentrations, TFSI- ions adsorb onto the electrode surface, creating a field that promotes electron transfer. An examination of the diffusion coefficients of the electrode reactants also revealed that in high-concentration LiTFSI aqueous solutions, hydration to O2 no longer occurs, and diffusion occurs only with O2, so the current value of the reduction reaction of O2 was greater than that in normal aqueous solutions.