Optimizing Ion Transport in Polyether-Based Electrolytes for Lithium Batteries

We report on the synthesis of poly(diethylene oxidealt-oxymethylene), P(2EO-MO), via cationic ring-opening polymerization of the cyclic ether monomer, 1,3,6-trioxocane. We use a combined experimental and computational approach to study ion transport in electrolytes comprising mixtures of P(2EO-MO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. Mixtures of poly(ohylene oxide) (PEO) and LiTFSI are used as a baseline. The maximum ionic conductivities, a, of P(2EO-MO) and PEO electrolytes at 90 degrees C are 1.1 x 10(-3) and 1.5 x 10(-3) S/cm, respectively. This difference is attributed to the Tg of P(2EO-MO)/LiTFSI (-12 degrees C), which is significantly higher than that of PEO/LiTFSI (-44 degrees C) at the same salt concentration. Self-diffusion coefficients measured using pulsed-field gradient NMR (PFG-NMR) show that both Li+ and TESI- ions diffuse more rapidly in PEO than in P(2EO-MO). However, the NMR-based cation transference number in P(2EO-MO) (0.36) is approximately twice that in PEO (0.19). The transference number measured by the steady-state current technique, t(+,ss) in P(2EO-MO) (0.20) is higher than in PEO (0.08) by a similar factor. We find that the product at sigma(t+ss), is greater in P(2-EO-MO) electrolytes; thus, P(2EO-MO) is expected to sustain higher steady-state currents under dc polarization, making it a more efficacious electrolyte for battery applications. Molecular-level insight into the factors that govern ion transport in our electrolytes was obtained using MD simulations. These simulations show that the solvation structures around Li' are similar in both polymers. The same is true for TFSI-. However, the density of Li+ solvation sites in P(2EO-MO) is double that in PEO. We posit that this is responsible for the observed differences in the experimentally determined transport properties of P(2EO-MO) and PEO electrolytes.