Studies of some poly(vinylidene fluoride) electrolytes

Abstract

Polymer electrolyte membranes comprised of poly(vinylidene fluoride) (PVdF), ethylene carbonate (EC), propylene carbonate (PC) and a Li salt (LiX = LiSO 3CF 3, LiPF 6 or LiN(SO 2CF 3) 2) were prepared using a thermal extrusion method. The electrolytes containing PVdF-HFP (hexafluopropylene) copolymer and LiN(SO 2CF 3) 2 had the broadest composition region suitable for isolating dimensionally stable electrolyte films. The properties of the polymer electrolyte membranes, such as crystallinity, thermal stability, conductivity, mechanical strength, electrochemical stability window and the Li/electrolyte interface stability, were characterized. It was found that plasticizing PVdF with the EC/PC/LiX solution significantly disordered its crystalline structure. The mechanical strength of the polymer electrolyte films varied widely and depended on the PVdF content. Membranes exhibiting stresses as high as 1000 psi at an elongation-at-break value (strain) of 115% were prepared. The conductivity of the electrolytes was influenced by the viscosity of the medium and the concentration of the charge carriers, which are directly related to the weight ratio of PVdF (EC + PC) and the kind and the concentration of the Li salt. Room-temperature conductivities as high as 2.2 × 10 3 ̄ S cm −1 were found for some LiN(SO 2CF 3) 2-containing electrolytes. Cyclic voltammetry of the LiN(SO 2CF 3) 2-containing electrolytes showed anodic stability up to 4.0 V on Al, 4.2 V on Ni and 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 static condition was found to be good for PVdF-EC/PC-LiN(SO 2CF 3) 2 electrolytes.