Ionic Conductivity Measurements of Single-ion Solid Copolymer Electrolytes based on Oxanorbornene Monomers

The need for energy storage both in mobile as well stationary applications continues to grow. Safety issues of lithium-ion batteries have prompted work on polymer-based electrolyte materials with added lithium salts to address flammability but these typically suffer from lower ion conductivity due to ion coupling to polymer dynamics. The resulting ion conductivity of these materials is from both anions and cations where anion conductivity can be a hinderance which has prompted research into single-ion polymers. This report details the synthesis of oxanorbornene-based copolymers where one repeat unit has an oligomeric ethylene oxide (OEO) side chain and the other has a side chain with a sulfonimide type anion and lithium cation (TFSI). These copolymers are synthesized using a Grubbs catalyzed ring-opening metathesis polymerization reaction with varying monomer compositions. The TFSI anion is incorporated in the monomer either before polymerization (graft-through) or after polymerization (graft-to) both via a click reaction. By increasing the OEO content, the glass transition temperature can be lowered although resulting in a lower lithium content. The conductivities of these single-ion copolymers have been measured as a function of temperature using electrochemical impedance spectroscopy. The results demonstrate increased conductivity with decreased glass transition temperature despite lower TFSI content.