Optimizing Ion Transport in Polyether-Based Electrolytes for Lithium Batteries

In this paper, we report on the synthesis of poly(diethylene oxide-alt-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(ethylene oxide) (PEO) and LiTFSI are used as a baseline. The maximum ionic conductivities, σ, of P(2EO-MO) and PEO electrolytes at 90 °C are 1.1 × 10^–3 and 1.5 × 10^–3 S/cm, respectively. This difference is attributed to the T_g of P(2EO-MO)/LiTFSI (-12 °C), which is significantly higher than that of PEO/LiTFSI (-44 °C) at the same salt concentration. Self-diffusion coefficients measured using pulsed-field gradient NMR (PFG-NMR) show that both Li⁺ and TFSI^– 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 σt_+,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. Finally, we posit that this is responsible for the observed differences in the experimentally determined transport properties of P(2EO-MO) and PEO electrolytes.