Electrical conductivities and Li ion concentration-dependent diffusivities, in polyurethane polymers doped with lithium trifluoromethanesulfonimide (LiTFSI) or lithium perchlorate (LiClO4)

Electrical, thermal and Li transport properties have been measured for polyester polyol and isocyanate-based polyurethanes doped with lithium trifluoromethanesulfonimide (LiTFSI) and lithium perchlorate (LiClO4). Electrical conductivities are estimated at 10−5–10−6 S/cm near 300 K. The conductivities show a Vogel–Tammann–Fulcher (VTF) behavior over wide temperature ranges, characteristic of segmental polymer chain motions, and are approximately an order of magnitude larger for LiTFSI-doped than for perchlorate-doped samples. Differential scanning calorimetry (DSC) shows that Tg does not significantly depend on doping type or concentration. Room-temperature 7Li diffusivities, measured by pulsed gradient NMR, show an unexpected strong, linear increase with LiTFSI doping, but only a weak increase with LiClO4 content. These findings may indicate substantial Li clustering in the LiTFSI-doped polymers, but may also reflect the effects of doping upon interface conduction between hard and soft polymer domains. Charge carrier densities estimated from the Nernst–Einstein relation, using measured NMR diffusivity values and ionic conductivities, range from approximately 8% to 29% of total Li densities for LiTFSI, indicating that a significant fraction of Li is involved in room-temperature ionic conduction in this material. For LiCLO4 the carrier fraction is smaller, implying that Li is more tightly bound to its anion site.