As a typical FAP, tris-2,2,2-trifluoroethyl-phosphate (TFEP) has high solubility toward conventional lithium salts, LiPF₆, LiBF₄, and Li(CF₃SO₂)₂N (LiTFSA). The order of the ionic conductivity of the TFEP solution was LiTFSA>=LiPF₆>LiBF₄ over a wide temperature range. Concentration dependence of Walden’s product (the product of ionic conductivity and viscosity) of the electrolyte solution suggested that the degree of ionic association or ion-pair formation depends on the anionic species in the lithium salt. The IR/Raman spectra of the electrolyte solutions also showed that the ionic structure including the solvent coordination (solvation) of Li⁺ is different among the electrolyte composition. The solvation number of Li⁺ in TFEP was determined by the IR/Raman spectra, and was found to change from 2 to 4, depending on the anion. The DFT (Density Functional Theory) calculation gave the stabilized configuration of the solvated Li⁺ in TFEP that also depends on the anionic species.

An FAP with smaller number of F-atom, bis(2,2,2-trifluoroethy)ethyl phosphate (BTFE), and triethylphosphate (TEP: no F-atom) for comparison were also examined as the solvent of the electrolyte solution. The number of F-atom in the solvent much influenced the solubility of the lithium salt, and hence the conductivity behavior of the resulting electrolyte solution. In conclusion, the molecular structure of FAP influences the ionic structure of the electrolyte solution, which contributes to the electrochemical stability of the electrolyte system in LIB.