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 (LiClO ₄). 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 T_g 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.