STUDIES OF SOME POLY(VINYLIDENE FLUORIDE) ELECTROLYTES

Polymer electrolyte membranes comprised of poly(vinylidene fluoride) ( PVdF), ethylene carbonate (EC), propylene carbonate (PC) and a Li salt (LiX = LiSO3CF3, LiPF6 or LiN(SO2CF3)(2)) were prepared using a therm al extrusion method. The electrolytes containing PVdF-HFP (hexafluopro pylene) copolymer and LiN(SO2CF3)(2) had the broadest composition regi on suitable for isolating dimensionally stable electrolyte films. The properties of the polymer electrolyte membranes, such as crystallinity , thermal stability, conductivity, mechanical strength, electrochemica l stability window and the Li/electrolyte interface stability, were ch aracterized. It was found that plasticizing PVdF with the EC/PC/LiX so lution significantly disordered its crystalline structure. The mechani cal strength of the polymer electrolyte films varied widely and depend ed on the PVdF content. Membranes exhibiting stresses as high as 1000 psi at an elongation-at-break value (strain) of 115% were prepared. Th e conductivity of the electrolytes was influenced by the viscosity of the medium and the concentration of the charge carriers, which are dir ectly related to the weight ratio of PVdF/(EC + PC) and the kind and t he concentration of the Li salt. Room-temperature conductivities as hi gh as 2.2 x 10(-3) S cm(-1) were found for some LiN(SO2CF3)(2)-contain ing electrolytes. Cyclic voltammetry of the LiN(SO2CF3)(2)-containing electrolytes showed anodic stability up to 4.0 V on Al, 4.2 V on Ni an d 4.5 V on stainless steel (SS), as well as cathodic stability down to 0 V on both Ni and SS. The Li/electrolyte interface stability under s tatic condition was found to be good for PVdF-EC/PC-LiN(SO2CF3)(2) ele ctrolytes. (C) 1997 Elsevier Science Ltd.