Complexes of Lithium Imide Salts with Tetraglyme and Their Polyelectrolyte Composite Materials

Complexes of amorphous tetraglyme (G4) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) or lithium bis(perfluoroethyl-sulfonyl)imide (LiBETI) were prepared as pol(yethylene) oxide-type electrolytes. Addition of equimolar amounts of LiTFSI and tetraglyme results in a room temperature ionic liquid with the general formula [Li(G4)]TFSI. Differential scanning calorimetry analysis of [Li(G4)]TFSI reveals that it has a T_g = -61°C, and the complex remains amorphous over a wide temperature range (-100 to 200°C), and has a very low vapor pressure for tetraglyme at room temperature. The corresponding BETI complex, [Li(G4)]BETI, crystallizes upon cooling and displays a T_m = 31°C. Room temperature conductivities (25°C) of [Li(G4)]TFSI and [Li(G4)]BETI were 1.13 and 0.63 mS/cm, respectively. Composite polymer electrolytes were prepared by addition of the complexes to polycations possessing TFSI or BETI anions. The composites afforded thin flexible membranes at polymer concentrations ≥50 mol % polymer with room temperature conductivities greater than 10^-4 S/cm. In general, increased concentrations of BETI anions in these materials resulted in increased mechanical stability but decreased ionic mobility. The complexes and composite polymer electrolytes displayed excellent anodic stability up to +4.5 V (vs. Li⁺/Li) and exhibited breakdown voltages ≥+5.5 V (vs. Li ^-Li) on stainless steel electrodes.