Conductometric Analysis of Ion Equilibrium in Li+/F− and Mg2+/F− Hybrid Electrolyte Solutions

Fluoride shuttle batteries are expected to be innovative with high energy density superior to lithium-ion ones as on-board power-source for electric vehicles. However, the low solubility of fluoride ion in organic solvents makes it difficult to choose electrolytes. Kawasaki et al. have reported that no precipitation appears in γ-butyrolactone solution of CsF at high concentrations in the presence of excess amounts of Li+ or Mg2+ cation [M. Kawasaki, et al., J. Electrochem. Soc., 169, 110508 (2022).]. Such solutions are called hybrid electrolyte ones. In this study, we performed conductometric titration of Li+- or Mg2+-containing solutions with CsF solutions to elucidate the ion equilibrium. The titration curves were analyzed on models including precipitation of LiF or MgF2 and formation of Li2F+ and LiF2− triple ion or MgF+ associated one to evaluate the corresponding equilibrium constants. The results indicate that no precipitation appears in certain ranges of the F−/Li+ or F−/Mg2+ molar ratio by the triple/associated ion formation. Details of the conductometric analysis are discussed together with some problems involved in the proposed method.