Solid polymer electrolytes from dilithium salts based on new bis[(perfluoroalkyl)sulfonyl]diimide dianions. Preparation and electrical characterization

Solid polymer electrolytes (SPEs) were prepared from a series of dilithium salts based on new bis[(perfluoroalkyl)sulfonyl]diimide dianions using poly(ethylene oxide) (PEO) as the polymer host. SPE characterization was accomplished using thermal methods, powder X-ray diffraction, proton nuclear magnetic resonance ( 1H-NMR) and electrochemical impedance spectroscopy (EIS). Ionic conductivities for SPEs made using the dilithium salts and also using the monomeric lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI) salt were measured over a temperature range between 120 °C and ambient for materials with EO/Li ratios of 30:1 (dilute) and 10:1 (concentrated). SPEs made using the dimeric salts exhibited ionic conductivities that were consistently low when compared with those from SPEs made using monomeric LiTFSI. This finding is thought to reflect a diminished contribution of the anions in the dimeric salts to the overall SPE conductivity. An unexpected finding of increasing ionic conductivity with increasing fluorine content in the dianions is thought to be the result of two opposing trends. One trend reflects an increase in anion size with increasing fluorine content, which diminishes anion transport and conductivity. Another trend reflects a decrease in anion basicity with increasing fluorine content that results in diminished ion pairing and an enhancement in the number of charge carriers, thereby increasing conductivity.