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 gamma- 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.