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(perfluoroethylsulfonyl 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 degreesC, and the complex remains amorphous over a wide temperature range (-100 to 200 degreesC), 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 degreesC. Room temperature conductivities (25 degreesC) 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 greater than or equal to 15.5 V (vs. Li+/Li) on stainless steel electrodes. (C) 2004 The Electrochemical Society.