Highly Conductive PEO-like Polymer Electrolytes

Abstract

Polymer electrolyte membranes comprising poly(vinylidene fluoride)−hexafluoropropene (PVdF−HFP) copolymer plasticized with a solution of LiSO 3 CF 3, LiN(SO 2 CF 3 ) 2, or LiPF 6 in oligomeric poly(ethylene glycol) dimethyl ethers (PEGDME, M w = 250, 400, and 500) were prepared by hot-melt-rolling or solvent-casting techniques. Since the electrolytes containing PEGDME400 and PEGDME500 are “dry” with essentially no volatile components up to 150 °C, we have dubbed them PEO-like. Their thermal stability, mechanical strength, conductivity, electrochemical stability window, and Li/electrolyte interface stability were characterized. Plasticizing PVdF−HFP with the PEGDME/LiX solutions disordered the polymer structure leading to polymer electrolytes having lower crystallinity than the polymer host itself. The mechanical strength of the 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 correlated with the molecular weights of PEGDME as well as the concentration 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-based electrolytes is attributed to the high mobility of the ionic charge carriers. The Li/electrolyte interface stability under open-circuit conditions was found to be good as assessed from the small change in the interfacial impedance for the measured case of the PVdF−PEGDME500−LiN(SO 2 CF 3 ) 2 electrolyte. This electrolyte also showed oxidation stability up to 4.5V versus Li + /Li on Al, Ni, and stainless steel (SS) and reduction stability down to 0.0V versus Li + /Li on both Ni and SS. The applicability of these electrolytes in batteries was demonstrated by the fabrication and testing of Li/oxygen and Li/LiMn 2 O 4 cells.