HIGHLY CONDUCTIVE PEO-LIKE POLYMER ELECTROLYTES

Polymer electrolyte membranes comprising poly(vinylidene fluoride)-hex afluoropropene (PVdF-HFP) copolymer plasticized with a solution of LiS O3CF3, LiN(SO2CF3)(2), or LiPF6 in oligomeric poly(ethylene glycol) di methyl ethers (PEGDME, M-w = 250, 400, and 500) were prepared by hot-m elt-rolling or solvent-casting techniques. Since the electrolytes cont aining PEGDME400 and PEGDME500 are ''dry'' with essentially no volatil e components up to 150 degrees C, we have dubbed them PEO-like. Their thermal stability, mechanical strength, conductivity, electrochemical stability window, and Li/electrolyte interface stability were characte rized. Plasticizing PVdF-HFP with the PEGDME/LiX solutions disordered the polymer structure leading to polymer electrolytes having lower cry stallinity than the polymer host itself. The mechanical strength of th e electrolyte membranes varied depending on the PVdF content. Tensile strength (stress) as high as 420 psi at an elongation-at-break value ( strain) of 75% was observed. The conductivities of the electrolytes co rrelated with the molecular weights of PEGDME as well as the concentra tion of the Li salt, and most of the electrolytes prepared showed room -temperature conductivities of greater than 10(-4) S/cm. The high room -temperature conductivity of these electrolytes compared to PEO-bassd electrolytes is attributed to the high mobility of the ionic charge ca rriers, The Li/electrolyte interface stability under open-circuit cond itions was found to be good as assessed from the,small change in the i nterfacial impedance for the measured case of the PVdF-PEGDME500-LiN(S O2-CF3)(2) electrolyte. This electrolyte also showed oxidation stabili ty up to 4.5V versus Li+/Li on Al, Ni, and stainless steel. (SS) and r eduction stability down to 0.0V versus Li+/Li on both Ni and SS. The a pplicability of these electrolytes in batteries was demonstrated by th e fabrication and testing of Li/oxygen and Li/LiMn2O4 cells.