Ionic Structure and Physical Properties of Fluorinated Alkyl-Phosphate Solutions Dissolving Lithium Salts

Abstract

Fluorinated alkylphosphates (FAPs) have been examined as nonflammable organic solvents for lithium-ion battery (LIB) electrolytes. Some research groups have already reported on the contribution of FAPs as either co-solvents or additives to the improvements in the safety of the battery system. There have also been publications concerning FAPs that can improve the battery performance under high-voltage operation by their use as the co-solvents in the electrolyte system. The characteristics of the electrolyte system are strongly dependent on the physico-chemical properties of the electrolyte solution. In the present paper, we have examined several combinations of FAP as the solvent and lithium salts as the electrolyte solute, and investigated the relationship among the solution composition, ionic structure and such physico-chemical properties as viscosity and ionic conductivity.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.