Synthesis, Characterization, and Ion-Conductive Behavior in an Organic Solvent and in a Polyether of a Novel Lithium Salt of a Perfluorinated Polyimide Anion

Abstract

To achieve highly conductive polymer electrolytes with a controllable ionic transference number, a novel polymeric lithium salt was synthesized and characterized. The novel lithium salt of a perfluorinated polyimide anion, poly(5-oxo-3-oxy-4-trifluoromethyl-1,2,4-pentafluoropentylene sulfonylimide lithium) (LiPPI), has a polyanionic backbone with a repeating unit resembling highly dissociable, thermally and electrochemically stable imide salts, such as lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). The ion-conductive behavior of LiPPI in an organic solvent and in a polyether was extensively studied by using pulse-gradient spin-echo NMR, in addition to differential scanning calorimetry, complex impedance measurement, and dynamic mechanical analysis. Solutions of LiPPI in ethylene carbonate (EC) exhibited a high degree of dissociation and high ionic conductivity, and the self-diffusion coefficient of the anion was lower than that of the cation. Solvent-free polymer electrolytes were prepared by dissolving LiPPI in a matrix polyether to afford a compatible polymer alloy, and the ionic conductivity of the new polymer alloy electrolytes reached ca. 10-5 S cm-1 at 30 °C. Although the lithium ionic transference number in the organic electrolyte solution was approximately the same as that of LiTFSI in EC, the polymer alloy electrolyte gave an apparent transference number higher than 0.7, which was considerably higher than that of LiTFSI in the same polyether.