Lithium polymer batteries offer a number of advantages to standard lithium ion batteries, including an all-solid state structure, increased safety, and the potential to be combined with lithium metal anodes for increased energy density over lithium intercalation anodes. However, the low-temperature (<80°C) ionic conductivity of polymer electrolytes has remained a major limitation over the past 40 years of academic investigation into polymer electrolytes. Progress in understanding strategies for systematic improvement in ionic conductivity has been dominated a single polymer for elucidating structure-property relationships in ionic conducting polymeric solids, i.e., poly(ethylene oxide) (PEO).

We have synthesized a library of structurally similar polyethers, poly(glycidyl ether)s (PGEs), by anionic polymerization with controlled molecular weight and polydispersity index. Polymer electrolytes were prepared with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a molar ratio of [O]/[Li] = 16. Significantly, no correlation between glass transition temperature or room temperature viscosity and ionic conductivity over the range 20--80°C could be established. This observation indicates that ion transport is not directly correlated to segmental polymer dynamics, and that the polymer dynamics may not be limiting the ionic conductivity in these amorphous polymer electrolytes. Similar to conventional small molecule organic electrolytes, the ionic conductivity increases with the dielectric constant (epsilon) of the parent polymers over the range epsilon = 4.8--6.0. This suggests that synthetic efforts to create highly conductive polymer electrolytes should focus on increasing the dielectric constant of the parent polymeric material.

Next, we examine one of the best performing PGEs, poly(allyl glycidyl ether) PAGE, PAGE was shown to have peak conductivities at [O]/[Li] = 16, with sigma > 3x10-5 S/cm at 25°C and > 5x10 -4 at 80°C. Below 60°C, PAGE has a conductivity that is 10--100 times higher than that of PEO at equivalent salt concentrations with this disparity in conductivities between PAGE and PEO increasing with decreasing temperature. In addition, the synthetic versatility of allyl glycidyl ether as a building block is demonstrated by the preparation and evaluation of various AGE-EO macromolecular architectures that show superior performance to both PAGE and PEO by utilizing copolymerization and thiolene coupling chemistry.

Finally, composite polymer electrolytes combining PGEs with nanoporous polycarbonate membranes were prepared. These systems were found to have increasing ionic conductivities with decreasing pore size, up to 50 times greater than bulk PGEs. This effect was observed in polycarbonate membranes but not porous anodic aluminum oxide, suggesting that the surface chemistry of the membranes plays a strong role and could be further tailored to optimized composite electrolytes for next generation lithium ion batteries.