Synthesis and characterization of lithium-ion conductive membranes with low water permeation

Three polymer systems were investigated in an attempt to produce lithium-ion conductive polymers that resist the absorption of water. All were synthesized via ring-opening metathesis polymerization (ROMP) to give a polynorbornene backbone, with each repeat unit bearing a pendent cyclotriphosphazene ring. Each pendent inorganic ring carried hydrophilic, ion conductive 2-(2-methoxyethoxy)ethoxy, and/ or hydrophobic 2,2,2-trifluoroethoxy side groups. The three systems were (a) composite blends of two polymers with all hydrophobic and all lithium ion conductive side groups, (b) homopolymers in which each polymer repeating unit bore both hydrophobic and ion conductive side groups, or (c) copolymers derived from two monomers, one of which bore only hydrophobic side groups and the other with all ion conductive groups. Room-temperature (25 °C) ionic conductivities were measured by incorporating 7 mol % LiBF₄ in each system. Hydrophobicity was estimated from water-contact angles of the polymeric materials with and without LiBF ₄. One of the homopolymer systems with two methoxyethoxyethoxy and three trifluoroethoxy groups on every side group generated conductivities in the range of 1.2 × 10^-5 S/cm at 25 °C in combination with a semihydrophobic surface with a water-contact angle of 77.7 °. The conductivity of this polymer was close to that of the highly hydrophilic, water-soluble poly[bis(2-(2methoxyethoxy)ethoxy)phosphazene] (MEEP), which is one of the most conductive solid polymer electrolytes (2.7 × 10 ^-5 S/cm at 25 °C).⁵